WO2016074198A1 - Dc grid current differential protection method and system thereof - Google Patents
Dc grid current differential protection method and system thereof Download PDFInfo
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- WO2016074198A1 WO2016074198A1 PCT/CN2014/091024 CN2014091024W WO2016074198A1 WO 2016074198 A1 WO2016074198 A1 WO 2016074198A1 CN 2014091024 W CN2014091024 W CN 2014091024W WO 2016074198 A1 WO2016074198 A1 WO 2016074198A1
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- fault component
- pole
- remote terminal
- local terminal
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
- H02H7/268—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured for dc systems
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
- H02H7/261—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured involving signal transmission between at least two stations
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
- G01R31/08—Locating faults in cables, transmission lines, or networks
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
- H02H7/261—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured involving signal transmission between at least two stations
- H02H7/263—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured involving signal transmission between at least two stations involving transmissions of measured values
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
- H02H7/265—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured making use of travelling wave theory
Definitions
- the present application relates to a DC grid current differential protection method and a system thereof.
- the main protection for transmission line is mainly based on change rate and amplitude of directional travelling wave front.
- Such protection has an obvious advantage that it only uses local measurements and has very fast operation speed for metal fault.
- this protection is based on the physical feature of smoothing reactor in HVDC system which can slow down the change of current.
- some kind of DC grid systems e.g. some kind of series MTDC system
- the travelling wave by external fault will not flow through the smoothing reactor, the HVDC protection mentioned above based on travelling wave will fail to operate or mal-trip.
- the external DC fault occurs on the line with higher voltage level, its travelling wave front is even larger than that by internal fault. It will bring big trouble to the existing HVDC protection based on travelling wave.
- Fig. 1 is a chart showing travelling wave fronts of internal and external DC fault in LCC DC grid.
- the change rate of the wave fronts from internal fault and external fault are definitely the same at the beginning. And at the same time, the travelling wave front of external fault is even much larger than that of internal fault, because the external line′s voltage level is higher.
- the traditional travelling wave protection device as shown in Fig. 2, three different measurements will start to determinate if the wave has sufficient amplitude for a specified time.
- the first measurement calculates the wave difference between just before the wave front and after 10 samples (0.2 ms) .
- the second and third calculate the wave difference between just before the wave front and after 25 and 35 samples (0.5 and 0.7 ms) . If all three measurements are greater than the threshold, a line fault is detected.
- the backup protection for transmission line is line current differential protection.
- Classical current differential algorithm is used in such protections. It operates when the main protections (travelling wave protection) cannot work (e.g. high resistance fault) .
- I DL is the current of local side
- I DL_FOS is the current of remote side
- the sensitivity of the current differential protection is quite good if with proper setting. But its operation speed is too slow. Its operate time is normally several hundreds of millisecond or even several seconds. The main reason is that the fault transient and charging current will influence this protection algorithm greatly. Thus, a long delay is necessary to ensure reliability.
- Both main protection and backup may be influenced by high impedance faults.
- Z COM is common mode wave impedance
- Z DIF is differential mode wave impedance
- W POLE is pole wave
- W COMM is ground wave.
- I COM is common mode current
- U COM is common mode voltage
- I DIF is differential mode current
- U DIF is differential mode voltage
- the protection detects the wave head by using the change rate of ground wave.
- the DC voltage decreases with a small rate of change when the line grounds to the earth via large impedance, leading to the mis-operations of existing protection based on travelling wave.
- control & protection system will delay to eliminate faults.
- D UT dU dl /dt ⁇ -396kV /ms & U dl ⁇ 200kV, where U dl is line voltage and D UT is the corresponding changing rate.
- Voltage change rate protection will mis-operate caused by small DC voltage decreasing, when the line grounds to the earth via large impedance.
- I SET is fixed current setting which is normally set to 120A
- k is ratio coefficient which is normally set to 0.1.
- one aspect of the present invention provides a DC grid current differential protection method, including the following steps:
- sampling value acquiring step acquiring pole voltage sampling values and pole current sampling values in local terminal and remote terminal of a DC line;
- fault component extracting step calculating fault component pole voltage values according to the pole voltage sampling values of local terminal and remote terminal respectively; and calculating fault component pole current values according to the pole current sampling values of local terminal and remote terminal respectively;
- Bergeron model calculating step acquiring fault component pole current values at selected point on the DC line between the local terminal and the remote terminal, by the calculation of the fault component pole voltage values and the fault component pole current values of local terminal and remote terminal calculated in the fault component extracting step, based on Bergeron model;
- current differential protection judging step judging internal fault if the fault component pole current values at selected point of local terminal and remote terminal acquired in the Bergeron model calculating step meet preset current differential protection criterion.
- the DC grid is of bi-pole and the DC line includes a positive pole DC line and a negative pole DC line
- the local terminal includes a positive pole local terminal and a positive pole remote terminal
- the remote terminal includes a negative pole local terminal and a negative pole remote terminal
- the positive pole DC line electrically connects the positive pole local terminal and the positive pole remote terminal
- the negative pole DC line electrically connects the negative pole local terminal and the negative pole remote terminal
- pole-mode transformation step acquiring fault component mode voltage values for each mode of local terminal and remote terminal by pole-mode transforming said fault component pole voltage value for each of the positive pole local terminal, the positive pole remote terminal, the negative pole local terminal and the negative pole remote terminal, and acquiring fault component mode current values for each mode of the local terminal and remote terminal by pole-mode transforming said fault component pole current values for each of the positive pole local terminal, the positive pole remote terminal, the negative pole local terminal and the negative pole remote terminal;
- the Bergeron model calculating step further comprises:
- the pole voltage sampling values comprise: u LP (t) , i.e. voltage sampling value of positive pole local terminal; u LN (t) , i.e. voltage sampling value of negative pole local terminal; u RP (t) , i.e. voltage sampling value of positive pole remote terminal; u RN (t) , i.e. voltage sampling value of negative pole remote terminal; wherein t means time;
- the pole current sampling values comprise: i LP (t) , i.e. current sampling value of positive pole local terminal; i LN (t) , i.e. current sampling value of negative pole local terminal; i RP (t) , i.e. current sampling value of positive pole remote terminal; i RN (t) , i.e. current sampling value of negative pole remote terminal;
- the fault component pole voltage values comprise: ⁇ u LP (t) , i.e. fault component voltage value of positive pole local terminal corresponding to u LP (t) ; ⁇ u LN (t) , i.e. fault component voltage value of negative pole local terminal corresponding to u LN (t) ; ⁇ u RP (t) , i.e. fault component voltage value of positive pole remote terminal corresponding to u RP (t) ; ⁇ u RN (t) , i.e. fault component voltage value of negative pole remote terminal corresponding to u RN (t) ;
- the fault component pole current values comprise: ⁇ i LP (t) , i.e. fault component current value of positive pole local terminal corresponding to i LP (t) ; ⁇ i LN (t) , i.e. fault component current value of negative pole local terminal corresponding to i LN (t) ; ⁇ i RP (t) , i.e. fault component current value of positive pole remote terminal corresponding to i RP (t) ; ⁇ i RN (t) , i.e. fault component current value of negative pole remote terminal corresponding to i RN (t) ;
- the fault component mode voltage values comprise: ⁇ u L0 (t) i.e. fault component common mode voltage value of local terminal; ⁇ u L1 (t) i.e. fault component differential mode voltage value of local terminal; ⁇ u R0 (t) i.e. fault component common mode voltage value of remote terminal; ⁇ u R1 (t) i.e. fault component differential mode voltage value of remote terminal;
- the fault component mode current values comprise: ⁇ i L0 (t) i.e. fault component common mode current value of local terminal; ⁇ i L1 (t) i.e. fault component differential mode current value of local terminal; ⁇ i R0 (t) i.e. fault component common mode current value of remote terminal; ⁇ i R1 (t) i.e. fault component differential mode current value of remote terminal;
- the fault component travelling wave voltage values comprise: ⁇ u L0+ (t) i.e. fault component common mode forward travelling wave voltage value of local terminal; ⁇ u L0- (t) i.e. fault component common mode inverse travelling wave voltage value of local terminal; ⁇ u L1+ (t) i.e. fault component differential mode forward travelling wave voltage value of local terminal; ⁇ u L0- (t) i.e. fault component differential mode inverse travelling wave voltage value of local terminal; ⁇ u R0+ (t) i.e. fault component common mode forward travelling wave voltage value of remote terminal; ⁇ u R0- (t) i.e. fault component common mode inverse travelling wave voltage value of remote terminal; ⁇ u R1+ (t) i.e. fault component differential mode forward travelling wave voltage value of remote terminal; ⁇ u R1- (t) i.e. fault component differential mode inverse travelling wave voltage value of remote terminal;
- the fault component travelling wave current values comprise: ⁇ i L0+ (t) i.e. fault component common mode forward travelling wave current value of local terminal; ⁇ i L0- (t) i.e. fault component common mode inverse travelling wave current value of local terminal; ⁇ i L1+ (t) i.e. fault component differential mode forward travelling wave current value of local terminal; ⁇ i L1- (t) i.e. fault component differential mode inverse travelling wave current value of local terminal; ⁇ i R0+ (t) i.e. fault component common mode forward travelling wave current value of remote terminal; ⁇ i R0- (t) i.e. fault component common mode inverse travelling wave current value of remote terminal; ⁇ i R1+ (t) i.e. fault component differential mode forward travelling wave current value of remote terminal; ⁇ i R1- (t) i.e. fault component differential mode inverse travelling wave current value of remote terminal;
- the fault component mode current values at the selected point comprise: ⁇ i L0 (x, t) , i.e. fault component common mode current value at the selected point of local terminal; ⁇ i L1 (x, t) , i.e. fault component differential mode current value at the selected point of local terminal; ⁇ i R0 (x, t) , i.e. fault component common mode current value at the selected point of remote terminal; ⁇ i R1 (x, t) , i.e. fault component differential mode current value at the selected point of remote terminal, wherein x is the selected point;
- the fault component pole current values at the selected point comprise: ⁇ i LP (x, t) i.e. fault component pole current value at selected point of positive pole local terminal; ⁇ i LN (x, t) i.e. fault component pole current value at selected point of negative pole local terminal; ⁇ i RP (x, t) i.e. fault component pole current value at selected point of positive pole remote terminal; ⁇ i RN (x, t) i.e. fault component pole current value at selected point of negative pole remote terminal.
- the fault component pole voltage value and the fault component pole voltage value are calculated in the following way:
- T is preset time delay
- the fault component mode voltage value and the fault component mode current value are calculated in the following way:
- the fault component mode travelling wave voltage values are calculated in the following way:
- Z C0 is common mode wave impedance
- Z C1 is differential mode wave impedance
- the fault component mode travelling wave current values are calculated in the following way:
- the fault component mode current values at selected point are calculated in the following way:
- v 0 is the travelling speed of fault component common mode travelling wave
- v 1 is the travelling speed of fault component differential mode travelling wave
- the fault component pole current values at the selected point are calculated in the following way:
- the current differential protection judging step comprises:
- I res is a preset threshold value
- the DC grid is single-pole:
- the Bergeron model calculating step further comprises:
- the fault component pole voltage value and the fault component pole voltage value are calculated in the following way:
- T is preset time delay
- ⁇ i L (t) is the fault component pole current value of local terminal
- ⁇ i R (t) is the fault component pole current value of remote terminal
- ⁇ u L (t) is the fault component pole voltage value of local terminal
- ⁇ u R (t) is the fault component pole voltage value of remote terminal
- i L (t) is the current sampling value of local terminal
- i R (t) is the current sampling value of remote terminal
- u L (t) is the voltage sampling value of local terminal
- u R (t) is the voltage sampling value of remote terminal
- t means time
- the fault component pole travelling wave voltage values are calculated in the following way:
- Z C is wave impedance
- ⁇ u L+ (t) is the fault component pole forward travelling wave voltage value of local terminal
- ⁇ u L- (t) is the fault component pole inverse travelling wave voltage value of local terminal
- ⁇ u R+ (t) is the fault component pole forward travelling wave voltage value of remote terminal
- ⁇ u R- (t) is the fault component pole inverse travelling wave voltage value of remote terminal
- the fault component pole travelling wave current values are calculated in the following way:
- ⁇ i L+ (t) is the fault component pole forward travelling wave current value of local terminal
- ⁇ i L- (t) is the fault component pole inverse travelling wave current value of local terminal
- ⁇ i R+ (t) is the fault component pole forward travelling wave current value of remote terminal
- ⁇ i R- (t) is the fault component role inverse travelling wave current value of remote terminal
- the fault component role current values at selected position are calculated in the following way:
- ⁇ i L (x, t) is the fault component pole current value at selected point of local terminal
- ⁇ i R (x, t) is the fault component pole current value at selected point of remote terminal
- v is the travelling speed of fault component travelling wave.
- the current differential protection judging step comprises:
- I res is a preset threshold value.
- the current differential protection judging step further comprises:
- Another aspect of the present invention provides a computer program comprising computer program code adapted to perform all of the steps of any one of the above when run on a computer.
- a further aspect of the present invention provides a computer program according to the above, embodied on a computer-readable medium.
- Another aspect of the present invention provides a DC grid current differential protection system, comprising the following modules:
- sampling value acquiring module acquiring pole voltage sampling values and pole current sampling values in local terminal and remote terminal of a DC line;
- fault component extracting module calculating fault component pole voltage values according to the pole voltage sampling values of local terminal and remote terminal respectively; and calculating fault component pole current values according to the pole current sampling values of local terminal and remote terminal respectively;
- Step calculating module acquiring fault component pole current values at selected point on the DC line between the local terminal and the remote terminal, by the calculation of the fault component pole voltage values and the fault component pole current values of local terminal and remote terminal calculated in the fault component extracting module, based on Bergeron model;
- current differential protection judging module judging internal fault if the fault component pole current values at selected point of local terminal and remote terminal acquired in the Bergeron model calculating module meet preset current differential protection criterion.
- the DC grid is of bi-pole and the DC line includes a positive pole DC line and a negative pole DC line
- the local terminal includes a positive pole local terminal and a positive pole remote terminal
- the remote terminal includes a negative pole local terminal and a negative pole remote terminal
- the positive pole DC line electrically connects the positive pole local terminal and the positive pole remote terminal
- the negative pole DC line electrically connects the negative pole local terminal and the negative pole remote terminal
- pole-mode transformation module acquiring fault component mode voltage values for each mode of local terminal and remote terminal by pole-mode transforming said fault component pole voltage value for each of the positive pole local terminal, the positive pole remote terminal, the negative pole local terminal and the negative pole remote terminal, and acquiring fault component mode current values for each mode of the local terminal and remote terminal by pole-mode transforming said fault component pole current values for each of the positive pole local terminal, the positive pole remote terminal, the negative pole local terminal and the negative pole remote terminal;
- the Bergeron model calculating module further comprises:
- the pole voltage sampling values comprise: u LP (t) , i.e. voltage sampling value of positive pole local terminal; u LN (t) , i.e. voltage sampling value of negative pole local terminal; u RP (t) , i.e. voltage sampling value of positive pole remote terminal; u RN (t) , i.e. voltage sampling value of negative pole remote terminal; wherein t means time;
- the pole current sampling values comprise: i LP (t) , i.e. current sampling value of positive pole local terminal; i LN (t) , i.e. current sampling value of negative pole local terminal; i RP (t) , i.e. current sampling value of positive pole remote terminal; i RN (t) , i.e. current sampling value of negative pole remote terminal;
- the fault component pole voltage values comprise: ⁇ u LP (t) , i.e. fault component voltage value of positive pole local terminal corresponding to u LP (t) ; ⁇ u LN (t) , i.e. fault component voltage value of negative pole local terminal corresponding to u LN (t) ; ⁇ u RP (t) , i.e. fault component voltage value of positive pole remote terminal corresponding to u RP (t) ; ⁇ u RN (t) , i.e. fault component voltage value of negative pole remote terminal corresponding to u RN (t) ;
- the fault component pole current values comprise: ⁇ i LP (t) , i.e. fault component current value of positive pole local terminal corresponding to i LP (t) ; ⁇ i LN (t) , i.e. fault component current value of negative pole local terminal corresponding to i LN (t) ; ⁇ i RP (t) , i.e. fault component current value of positive pole remote terminal corresponding to i RP (t) ; ⁇ i RN (t) , i.e. fault component current value of negative pole remote terminal corresponding to i RN (t) ;
- the fault component mode voltage values comprise: ⁇ u L0 (t) i.e. fault component common mode voltage value of local terminal; ⁇ u L1 (t) i.e. fault component differential mode voltage value of local terminal; ⁇ u R0 (t) i.e. fault component common mode voltage value of remote terminal; ⁇ u R1 (t) i.e. fault component differential mode voltage value of remote terminal;
- the fault component mode current values comprise: ⁇ i L0 (t) i.e. fault component common mode current value of local terminal; ⁇ i L1 (t) i.e. fault component differential mode current value of local terminal; ⁇ i R0 (t) i.e. fault component common mode current value of remote terminal; ⁇ i R1 (t) i.e. fault component differential mode current value of remote terminal;
- the fault component travelling wave voltage values comprise: ⁇ u L0+ (t) i.e. fault component common mode forward travelling wave voltage value of local terminal; ⁇ u L0- (t) i.e. fault component common mode inverse travelling wave voltage value of local terminal; ⁇ u L1+ (t) i.e. fault component differential mode forward travelling wave voltage value of local terminal; ⁇ u L0- (t) i.e. fault component differential mode inverse travelling wave voltage value of local terminal; ⁇ u R0+ (t) i.e. fault component common mode forward travelling wave voltage value of remote terminal; ⁇ u R0- (t) i.e. fault component common mode inverse travelling wave voltage value of remote terminal; ⁇ u R1+ (t) i.e. fault component differential mode forward travelling wave voltage value of remote terminal; ⁇ u R1- (t) i.e. fault component differential mode inverse travelling wave voltage value of remote terminal;
- the fault component travelling wave current values comprise: ⁇ i L0+ (t) i.e. fault component common mode forward travelling wave current value of local terminal; ⁇ i L0- (t) i.e. fault component common mode inverse travelling wave current value of local terminal; ⁇ i L1+ (t) i.e. fault component differential mode forward travelling wave current value of local terminal; ⁇ i L1- (t) i.e. fault component differential mode inverse travelling wave current value of local terminal; ⁇ i R0+ (t) i.e. fault component common mode forward travelling wave current value of remote terminal; ⁇ i R0- (t) i.e. fault component common mode inverse travelling wave current value of remote terminal; ⁇ i R1+ (t) i.e. fault component differential mode forward travelling wave current value of remote terminal; ⁇ i R1- (t) i.e. fault component differential mode inverse travelling wave current value of remote terminal;
- the fault component mode current values at the selected point comprise: ⁇ i L0 (x, t) , i.e. fault component common mode current value at the selected point of local terminal; ⁇ i L1 (x, t) , i.e. fault component differential mode current value at the selected point of local terminal; ⁇ i R0 (x, t) , i.e. fault component common mode current value at the selected point of remote terminal; ⁇ i R1 (x, t) , i.e. fault component differential mode current value at the selected point of remote terminal, wherein x is the selected point;
- the fault component pole current values at the selected point comprise: ⁇ i LP (x, t) i.e. fault component pole current value at selected point of positive pole local terminal; ⁇ i LN (x, t) i.e. fault component pole current value at selected point of negative pole local terminal; ⁇ i RP (x, t) i.e. fault component pole current value at selected point of positive pole remote terminal; ⁇ i RN (x, t) i.e. fault component pole current value at selected point of negative pole remote terminal.
- the fault component pole voltage value and the fault component pole voltage value are calculated in the following way:
- T is preset time delay
- the fault component mode voltage value and the fault component mode current value are calculated in the following way:
- the Bergeron model calculating module comprises:
- the fault component mode travelling wave voltage values are calculated in the following way:
- Z C0 is common mode wave impedance
- Z C1 is differential mode wave impedance
- the fault component mode travelling wave current values are calculated in the following way:
- the fault component mode current values at selected point are calculated in the following way:
- v 0 is the travelling speed of fault component common mode travelling wave
- v 1 is the travelling speed of fault component differential mode travelling wave
- the fault component pole current values at the selected point are calculated in the following way:
- the current differential protection judging module comprises:
- the DC grid is single-pole:
- the Bergeron model calculating module further comprises:
- the fault component pole voltage value and the fault component pole voltage value are calculated in the following way:
- T is preset time delay
- ⁇ i L (t) is the fault component pole current value of local terminal
- ⁇ i R (t) is the fault component pole current value of remote terminal
- ⁇ u L (t) is the fault component pole voltage value of local terminal
- ⁇ u R (t) is the fault component pole voltage value of remote terminal
- i L (t) is the current sampling value of local terminal
- i R (t) is the current sampling value of remote terminal
- u L (t) is the voltage sampling value of local terminal
- u R (t) is the voltage sampling value of remote terminal
- t means time
- the Bergeron model calculating module comprises:
- the fault component pole travelling wave voltage values are calculated in the following way:
- Z C is wave impedance
- ⁇ u L+ (t) is the fault component pole forward travelling wave voltage value of local terminal
- ⁇ u L- (t) is the fault component pole inverse travelling wave voltage value of local terminal
- ⁇ u R+ (t) is the fault component pole forward travelling wave voltage value of remote terminal
- ⁇ u R- (t) is the fault component pole inverse travelling wave voltage value of remote terminal
- the fault component pole travelling wave current values are calculated in the following way:
- ⁇ i L+ (t) is the fault component pole forward travelling wave current value of local terminal
- ⁇ i L- (t) is the fault component pole inverse travelling wave current value of local terminal
- ⁇ i R+ (t) is the fault component pole forward travelling wave current value of remote terminal
- ⁇ i R- (t) is the fault component role inverse travelling wave current value of remote terminal
- the fault component role current values at selected position are calculated in the following way:
- ⁇ i L (x, t) is the fault component pole current value at selected point of local terminal
- ⁇ i R (x, t) is the fault component pole current value at selected point of remote terminal
- v is the travelling speed of fault component travelling wave.
- the current differential protection judging module comprises:
- I res is a preset threshold value.
- the current differential protection judging module further comprises:
- the present invention uses fault component to erase the influence of the load current to differential protection so that the sensitivity is improved.
- Fig. 1 shows a chart showing travelling wave fronts of internal and external DC fault in LCC DC grid
- Fig. 2 shows a measurement schematic view of the traditional travelling wave protection device
- Fig. 3 shows a flow-process diagram illustrating a DC grid current differential protection method in accordance with the present invention
- Fig. 4 illustrates schematically a fault component distributed grid
- Fig. 5 shows the condition when internal fault occur in the line
- Fig. 6 shows the condition when external fault occur in the line
- Fig. 7 shows the simulation model
- Fig. 8 shows the simulation result
- Fig. 9 shows a structural module drawing of a DC grid current differential protection system
- Fig. 10 shows a single pole HVDC system.
- Fig. 3 shows a flow-process diagram illustrating a DC grid current differential protection method in accordance with the present invention, including the following steps:
- step S301 acquiring pole voltage sampling values and pole current sampling values in local terminal and remote terminal of a DC line;
- step S302 calculating fault component pole voltage values according to the pole voltage sampling values of local terminal and remote terminal respectively; and calculating fault component pole current values according to the pole current sampling values of local terminal and remote terminal respectively;
- step S303 acquiring fault component pole current values at selected point on the DC line between the local terminal and the remote terminal, by the calculation of the fault component pole voltage values and the fault component pole current values of local terminal and remote terminal calculated in the step S302, based on Bergeron model;
- step 304 judging internal fault if the fault component pole current values at selected point of local terminal and remote terminal acquired in the step S303 meet preset current differential protection criterion.
- the present invention adopts Bergeron model, so long time-extension is not needed to eliminate the disturb of the distributed charging current, so as to substantially improve the calculating speed in the present invention.
- Step S302 pole voltage sampling values are converted into fault component pole voltage values, and pole current sampling values are converted into fault component pole current values. Therefore, in this step, fault component pole voltage values are separated from the pole voltage sampling values, and fault component pole current values are separated from the pole current sampling values.
- the grid may be divided into no-fault network and fault-component network, so the fault component pole voltage values and fault component pole current values are the pole voltage/current values in the fault-component network.
- the fault component pole voltage values and the fault component pole current values are pole-mode transformed and applied to Bergeron model. That is, the present invention provides a protection which is a current differential protection based on fault component. Therefore, the present invention uses fault component to erase the influence of the load current to differential protection so that the sensitivity is improved.
- the DC grid is of bi-pole and the DC line includes a positive pole DC line and a negative pole DC line
- the local terminal includes a positive pole local terminal and a positive pole remote terminal
- the remote terminal includes a negative pole local terminal and a negative pole remote terminal
- the positive pole DC line electrically connects the positive pole local terminal and the positive pole remote terminal
- the negative pole DC line electrically connects the negative pole local terminal and the negative pole remote terminal
- pole-mode transformation step acquiring fault component mode voltage values for each mode of local terminal and remote terminal by pole-mode transforming said fault component pole voltage value for each of the positive pole local terminal, the positive pole remote terminal, the negative pole local terminal and the negative pole remote terminal, and acquiring fault component mode current values for each mode of the local terminal and remote terminal by pole-mode transforming said fault component pole current values for each of the positive pole local terminal, the positive pole remote terminal, the negative pole local terminal and the negative pole remote terminal;
- step S303 further comprises:
- This embodiment applies the fault component mode voltage and the fault component mode current under the mode-pole transformation to the Bergeron model, so as to particularly achieving the Bergeron model based on the fault component in bi-pole DC grid.
- the pole voltage sampling values comprise: u LP (t) , i.e. voltage sampling value of positive pole local terminal; u LN (t) , i.e. voltage sampling value of negative pole local terminal; u RP (t) , i.e. voltage sampling value of positive pole remote terminal; u RN (t) , i.e. voltage sampling value of negative pole remote terminal; wherein t means time;
- the pole current sampling values comprise: i LP (t) , i.e. current sampling value of positive pole local terminal; i LN (t) , i.e. current sampling value of negative pole local terminal; i RP (t) , i.e. current sampling value of positive pole remote terminal; i RN (t) , i.e. current sampling value of negative pole remote terminal;
- the fault component pole voltage values comprise: ⁇ u LP (t) , i.e. fault component voltage value of positive pole local terminal corresponding to u LP (t) ; ⁇ u LN (t) , i.e. fault component voltage value of negative pole local terminal corresponding to u LN (t) ; ⁇ u RP (t) , i.e. fault component voltage value of positive pole remote terminal corresponding to u RP (t) ; ⁇ u RN (t) , i.e. fault component voltage value of negative pole remote terminal corresponding to u RN (t) ;
- the fault component pole current values comprise: ⁇ i LP (t) , i.e. fault component current value of positive pole local terminal corresponding to i LP (t) ; ⁇ i LN (t) , i.e. fault component current value of negative pole local terminal corresponding to i LN (t) ; ⁇ i RP (t) , i.e. fault component current value of positive pole remote terminal corresponding to i RP (t) ; ⁇ i RN (t) , i.e. fault component current value of negative pole remote terminal corresponding to i RN (t) ;
- the fault component mode voltage values comprise: ⁇ u L0 (t) i.e. fault component common mode voltage value of local terminal; ⁇ u L1 (t) i.e. fault component differential mode voltage value of local terminal; ⁇ u R0 (t) i.e. fault component common mode voltage value of remote terminal; ⁇ u R1 (t) i.e. fault component differential mode voltage value of remote terminal;
- the fault component mode current values comprise: ⁇ i L0 (t) i.e. fault component common mode current value of local terminal; ⁇ i L1 (t) i.e. fault component differential mode current value of local terminal; ⁇ i R0 (t) i.e. fault component common mode current value of remote terminal; ⁇ i R1 (t) i.e. fault component differential mode current value of remote terminal;
- the fault component travelling wave voltage values comprise: ⁇ u L0+ (t) i.e. fault component common mode forward travelling wave voltage value of local terminal; ⁇ u L0- (t) i.e. fault component common mode inverse travelling wave voltage value of local terminal; ⁇ u L1+ (t) i.e. fault component differential mode forward travelling wave voltage value of local terminal; ⁇ u L0- (t) i.e. fault component differential mode inverse travelling wave voltage value of local terminal; ⁇ u R0+ (t) i.e. fault component common mode forward travelling wave voltage value of remote terminal; ⁇ u R0- (t) i.e. fault component common mode inverse travelling wave voltage value of remote terminal; ⁇ u R1+ (t) i.e. fault component differential mode forward travelling wave voltage value of remote terminal; ⁇ u R1- (t) i.e. fault component differential mode inverse travelling wave voltage value of remote terminal;
- the fault component travelling wave current values comprise: ⁇ i L0+ (t) i.e. fault component common mode forward travelling wave current value of local terminal; ⁇ i L0- (t) i.e. fault component common mode inverse travelling wave current value of local terminal; ⁇ i L1+ (t) i.e. fault component differential mode forward travelling wave current value of local terminal; ⁇ i L1- (t) i.e. fault component differential mode inverse travelling wave current value of local terminal; ⁇ i R0+ (t) i.e. fault component common mode forward travelling wave current value of remote terminal; ⁇ i R0- (t) i.e. fault component common mode inverse travelling wave current value of remote terminal; ⁇ i R1+ (t) i.e. fault component differential mode forward travelling wave current value of remote terminal; ⁇ i R1- (t) i.e. fault component differential mode inverse travelling wave current value of remote terminal;
- the fault component mode current values at the selected point comprise: ⁇ i L0 (x,t) , i.e. fault component common mode current value at the selected point of local terminal; ⁇ i L1 (x, t) , i.e. fault component differential mode current value at the selected point of local terminal; ⁇ i R0 (x,t) , i.e. fault component common mode current value at the selected point of remote terminal; ⁇ i R1 (x, t) , i.e. fault component differential mode current value at the selected point of remote terminal, wherein x is the selected point;
- the fault component pole current values at the selected point comprise: ⁇ i LP (x, t) i.e. fault component pole current value at selected point of positive pole local terminal; ⁇ i LN (x, t) i.e. fault component pole current value at selected point of negative pole local terminal; ⁇ i RP (x, t) i.e. fault component pole current value at selected point of positive pole remote terminal; ⁇ i RN (x, t) i.e. fault component pole current value at selected point of negative pole remote terminal.
- This embodiment has the respective calculation for the position and negative poles, so as to achieve the respective differential protections for both of the poles.
- the fault component pole voltage value and the fault component pole voltage value are calculated in the following way:
- T is preset time delay
- the fault component mode voltage value and the fault component mode current value are calculated in the following way:
- step S303 comprises:
- the fault component mode travelling wave voltage values are calculated in the following way:
- Z C0 is common mode wave impedance
- Z C1 is differential mode wave impedance
- the fault component mode travelling wave current values are calculated in the following way:
- the fault component mode current values at selected point are calculated in the following way:
- v 0 is the travelling speed of fault component common mode travelling wave
- v 1 is the travelling speed of fault component differential mode travelling wave
- the fault component pole current values at the selected point are calculated in the following way:
- step S304 comprises:
- the DC grid is single-pole:
- step S303 further comprises:
- This embodiment particularly achieving the Bergeron model based on the fault component in single-pole DC grid.
- step S302 the fault component pole voltage value and the fault component pole voltage value are calculated in the following way:
- T is preset time delay
- ⁇ i L (t) is the fault component pole current value of local terminal
- ⁇ i R (t) is the fault component pole current value of remote terminal
- ⁇ u L (t) is the fault component pole voltage value of local terminal
- ⁇ u R (t) is the fault component pole voltage value of remote terminal
- i L (t) is the current sampling value of local terminal
- i R (t) is the current sampling value of remote terminal
- u L (t) is the voltage sampling value of local terminal
- u R (t) is the voltage sampling value of remote terminal
- t means time
- step S303 comprises:
- the fault component pole travelling wave voltage values are calculated in the following way:
- Z C is wave impedance
- ⁇ u L+ (t) is the fault component pole forward travelling wave voltage value of local terminal
- ⁇ u L- (t) is the fault component pole inverse travelling wave voltage value of local terminal
- ⁇ u R+ (t) is the fault component pole forward travelling wave voltage value of remote terminal
- ⁇ u R- (t) is the fault component pole inverse travelling wave voltage value of remote terminal
- the fault component pole travelling wave current values are calculated in the following way:
- ⁇ i L+ (t) is the fault component pole forward travelling wave current value of local terminal
- ⁇ i L- (t) is the fault component pole inverse travelling wave current value of local terminal
- ⁇ i R+ (t) is the fault component pole forward travelling wave current value of remote terminal
- ⁇ i R- (t) is the fault component role inverse travelling wave current value of remote terminal
- the fault component role current values at selected position are calculated in the following way:
- ⁇ i L (x, t) is the fault component pole current value at selected point of local terminal
- ⁇ i R (x, t) is the fault component pole current value at selected point of remote terminal
- v is the travelling speed of fault component travelling wave.
- step S304 comprises:
- step S304 further comprises:
- the current differential protection method of the present invention seeks to calculate ⁇ i LP (x, t) and ⁇ i RP (x, t) at the selected point x at a particular t, to make the judgment for the positive pole fault, and meanwhile calculate ⁇ i LN (x, t) and ⁇ i RN (x, t) at the selected point x at a particular t, to make the judgment for the negative pole fault.
- the local side 41 and the remote side 42 may be communicated via a communication line, so the local side 41 may acquire all parameter information of the local side 41 and the remote side 42.
- ⁇ i LP (x, t) , ⁇ i RP (x, t) , ⁇ i LN (x, t) and ⁇ i RN (x, t) may be calculated in the following way:
- the fault component is calculated by the following equation (1) :
- T is time delay, which can be set to e. g. 10 ms or 100 ms, depending on the requirements.
- the next step is to do Pole-Mode Transformation to transform the pole quantities to mode quantities.
- the Pole-Mode Transformation matrix for both voltage and current are given in equation (2) .
- fault component traveling wave common and differential mode currents values at the selected point x on the protected line will be calculated based on Bergeron model (travelling wave transmission equation) with the measurements from the two terminals respectively.
- the equation 3 may be used to calculate the common mode forward voltage travelling wave ⁇ u L0+ and inverse voltage travelling wave ⁇ u L0- of the locate side fault component, the differential mode forward voltage travelling wave ⁇ u L1+ and inverse voltage travelling wave ⁇ u L1- of the locate side fault component, the common mode forward voltage travelling wave ⁇ u R0+ and inverse voltage travelling wave ⁇ u R0- of the remote side fault component, and the differential mode forward voltage travelling wave ⁇ u R1+ and inverse voltage travelling wave ⁇ u R1- of the remote side fault component.
- Z C0 is common mode wave impedance
- Z C1 is differential mode wave impedance
- Equation 4 can be used to calculate the common mode forward current travelling wave ⁇ i L0+ and inverse current travelling wave ⁇ i L0- of local side fault component, the differential mode forward current travelling wave ⁇ i L1+ and inverse current travelling wave ⁇ i L1- of local side fault component, the common mode forward current travelling wave ⁇ i R0+ and inverse current travelling wave ⁇ i R0- of remote side fault component, and the differential mode forward current travelling wave ⁇ i R1+ and inverse current travelling wave ⁇ i R1- of remote side fault component.
- the fault component differential mode and common mode current of local terminal and remote terminal at the selected point x can be calculated by using the following equation 5, selected point x calculated by the measurements of local terminal for the fault component differential mode and common mode current of local terminal, selected point x calculated by the measurements of remote terminal for the fault component differential mode and common mode current of remote terminal:
- the positive and negative pole currents at the selected points can be calculated by using Mode-Pole Transformation for both local and remote terminals.
- the transformation matrix is shown in the following equation 6:
- ⁇ u L (t) and ⁇ i L (t) are fault component voltage and current of local terminal
- ⁇ u R (t) and ⁇ i R (t) are fault component voltage and current of remote terminal
- ⁇ i L (x,t) is the current at point x calculated by local measurements
- ⁇ i R (x, t) is the current at point x calculated by remote measurements
- the fault component currents at the selected point ‘x’ will be calculated based on Bergeron model (telegraph equations, travelling wave equation) by the measurements of the two terminals respectively.
- x is the distance between any selected point along the line and the local terminal. For example, if the selected point is the remote terminal, then the distance x is the line length L;
- x is the distance between any selected point along the line and the remote terminal. For example, if the selected point is the remote terminal, then the distance x is zero.
- the method to calculate fault component is:
- u (t) and i (t) are measured pole voltage and current sampling values.
- ⁇ u (t) and ⁇ i (t) are corresponding fault component voltage and current value.
- T is the time delay, can be set to e.g. 10 ms or 100 ms, depending on the requirements. According to this method, the fault component voltage values and current values of both poles and both ends can be calculated as equation 10.
- traveling wave currents at the selected point x on the protected line will be calculated based on Bergeron model (travelling wave transmission equation) with the measurements from the two terminals respectively.
- Equation 11 can be used to calculate the forward voltage travelling wave ⁇ u L+ and inverse voltage travelling wave ⁇ u L-
- equation 12 can be used to calculate forward current travelling wave ⁇ i L+ and inverse current travelling wave ⁇ i L- .
- the current at the selected point x can be calculated by using equation 13.
- v is the travelling speed of travelling wave
- x is any point along the line, it could be middle point, end point, start point, or any other point;
- ⁇ i L (x, t) is the fault component current at the selected point x calculated by the measurements of local terminal
- ⁇ i R (x, t) is the fault component current at the selected point x calculated by the measurements of remote terminal;
- I Local is the local terminal current
- I Remote is the remote terminal current
- Fig. 5 shows the condition when internal fault occur in the line.
- I F is fault current through fault branch as shown in Fig. 5
- I C is the current flow through the distributed capacitance alone the line, normally much higher than zero, especially for the long length transmission line.
- FIG. 6 shows the condition when external fault occur in the line.
- the second method i.e. long time delay (0.5s-1.5s)
- 0.5s-1.5s the response speed is slowed down.
- the present invention is a.
- the present invention is based on travelling wave component calculated by using Bergeron model which already considers the distributed line capacitance.
- the differential current calculated is the fault current I F flow through the fault branch, e.g.
- I F ;
- the differential current calculated by the present invention is zero, e.g.
- 0.
- the operation speed is very important to the protection; it is one most important demand to protection.
- the fast isolation is very beneficial to the system stability and human safe.
- Another two important demands to protection includes reliability and sensitivity.
- a good protection principle must achieve these three advantages: fast operation speed, reliability and sensitivity.
- the operation speed of the present invention is less than 15ms for most cases, while the operation time of classical differential protection is 0.5s-1.5s.
- the algorithm of the present invention can be used as main protection for LCC DC grid as mentioned, and can be used as backup protection for other types of DC grid and gets much higher operation speed than classical differential protection; and it can also be used as main protection for short line whose communication time is short for other types of DC grid system, or point to point HVDC system.
- the present invention has good sensitivity on high resistance faults because it is based on fault component, the influence of load current to the differential protection is erased, while the sensitivity of classical differential protection is decreased by load current.
- the differential protection principle of the present invention is only related to line parameters, it uses the line parameter to calculate the current at point ‘x’ , it has no special requirement on the topology and control of the DC system.
- the present invention can be used as either main protection or backup protection.
- the present invention can be used as backup protection, because the requirement on the operation speed is quite high, usually within 5ms. If the length of the transmission line is short, the time delay caused by the communication can be reduced, and then the present invention can also be used as main protection.
- the direction element of the present invention is based on Bergeron model, the “resonances” is considered inherently by this model, it is not influenced by “resonances” .
- Fig. 7 shows the simulation model
- the ⁇ 800 kV 4-terminal series MTDC consists of two rectifier stations (R1 and R2) and two inverter stations (I1 and 12) .
- Total length of the transmission line is 2000 km, including two branch lines (500 km each) and one backbone lines (1000 km) .
- Each converter station has a configuration with one 12-pulse valve group.
- Each rectifier converter will have a nominal DC voltage of 400 kV across; each inverter converter will have a nominal DC voltage of 373 kV across, and the HV DC line voltage to ground is about 400 kV (for R1 and I1) or about 800 kV (for R2 and I2) .
- the present invention protections, relay 71, 72, are located at the two terminal of the +800kV transmission line as shown in the figure above. And the internal fault is at the end of +800kV line, the external fault is on the +400kV line. And the pole-pole wave impedance Zc is 264 ⁇ in this case.
- Fig. 8 shows the simulation result, an internal fault occurs at 2s and an external fault occurs at 4s.
- -‘IF’ is the actual current flowing through the fault branch
- ‘ldif Bergeron’ shows the differential current that the present invention calculated. It can be observed from Fig. 8 that the calculated differential current after the occurrence of external fault is very small, far less than the calculated differential current under internal fault. That is to say, it can distinguish external fault from internal fault effectively.
- Fig. 9 shows a structural module drawing of a DC grid current differential protection system, comprising the following modules:
- sampling value acquiring module 901 for acquiring the pole voltage sampling values and the pole current sampling values of the local terminal and remote terminal of the DC grid;
- fault component extracting module 902 for calculating fault component pole voltage values according to the pole voltage sampling values in local terminal and remote terminal respectively; and calculating fault component pole current values according to the pole current sampling values in local terminal and remote terminal respectively;
- pole-mode transformation module 903 for acquiring fault component mode voltage values by pole-mode transforming said fault component pole voltage values in local terminal and remote terminal respectively, and acquiring fault component mode current values by pole-mode transforming said fault component pole current voltage values in local terminal and remote terminal respectively;
- Bergeron model calculating module 904 for acquiring fault component pole current values at selected point in local terminal and remote terminal respectively, by the calculation for the fault component mode voltage values and the fault component mode current values in local terminal and remote terminal, based on Bergeron model;
- current differential protection judging module 905 comprising judging internal fault if the fault component pole current values at selected point in local terminal and remote terminal meet preset current differential protection criterion, then sending fault protection command to activate differential protection, otherwise, the differential protection will not be activated.
Abstract
A DC grid current differential protection method and system are provided. The method includes: sampling value acquiring step (S301): acquiring pole voltage and current sampling values in local and remote terminal of a DC line; fault component extracting step (S302): calculating fault component pole voltage and current values according to the pole voltage and current sampling values of local and remote terminal respectively; Bergeron model calculating step (S303): acquiring fault component pole current values at selected point on the DC line between the local and the remote terminal, by the calculation of the fault component pole voltage and current values of local and remote terminal calculated in the fault component extracting step, based on Bergeron model; current differential protection judging step (S304): judging internal fault if the fault component pole current values at selected point meet preset current differential protection criterion. Because of adopting Bergeron model, it improves the calculating speed.
Description
The present application relates to a DC grid current differential protection method and a system thereof.
BACKGROUND ART
In existing HVDC systems, generally, protection based on travelling wave front of local measurements is used as main protection and classical current differential protection is used as backup protection. But the drawback of them is that, the main protection has bad sensitivity for high resistance fault and may mal-operate in LCC DC grid, and the backup protection has very slow operation speed.
In existing two-terminal HVDC system, the main protection for transmission line is mainly based on change rate and amplitude of directional travelling wave front. Such protection has an obvious advantage that it only uses local measurements and has very fast operation speed for metal fault.
But one of the disadvantages of such protections is that it has very low (bad) sensitivity for high resistance fault. Normally, >200 Ohm fault resistance may lead to failure of operation because the amplitude of the wave front rely heavily on fault resistance. As a result, the high resistance fault has to be cleared by its backup current differential protection with very slow operation speed (e.g. >0.5s) . Obviously, it is not reasonable.
And furthermore, this protection is based on the physical feature of smoothing reactor in HVDC system which can slow down the change of current. In some kind of DC grid systems (e.g. some kind of series MTDC system) , the travelling wave by external fault will not flow through the smoothing reactor, the HVDC protection mentioned above based on travelling wave will fail to operate or mal-trip. For the worst conditions, if the external DC fault occurs on the line with higher voltage level, its travelling wave front is even larger than that by internal fault. It will bring big trouble to the existing HVDC protection based on travelling wave.
Fig. 1 is a chart showing travelling wave fronts of internal and external DC fault
in LCC DC grid.
As shown in the Fig. 1, the change rate of the wave fronts from internal fault and external fault are definitely the same at the beginning. And at the same time, the travelling wave front of external fault is even much larger than that of internal fault, because the external line′s voltage level is higher.
In the traditional travelling wave protection device, as shown in Fig. 2, three different measurements will start to determinate if the wave has sufficient amplitude for a specified time. The first measurement calculates the wave difference between just before the wave front and after 10 samples (0.2 ms) . The second and third calculate the wave difference between just before the wave front and after 25 and 35 samples (0.5 and 0.7 ms) . If all three measurements are greater than the threshold, a line fault is detected.
Considering of the wave front of external fault in Fig. 1 is even larger than that of internal fault and the change rate of both internal & external faults has the same rate, the existing HVDC main protection will mal-trip in LCC DC grid in such cases. In other words, existing HVDC travelling wave protection cannot be used directly in LCC DC grid.
In existing HVDC system, normally, the backup protection for transmission line is line current differential protection. Classical current differential algorithm is used in such protections. It operates when the main protections (travelling wave protection) cannot work (e.g. high resistance fault) .
A typical criterion of current differential protection is shown below,
|IDL-IDL_FOS| >max (120A, 0.1×|IDL + IDL_FOS|/2)
In which IDL is the current of local side, and IDL_FOS is the current of remote side.
Another typical criterion of current differential protection is shown below,
||IDL|-|IDL_FOS|| >90A
Generally, the sensitivity of the current differential protection is quite good if with proper setting. But its operation speed is too slow. Its operate time is normally
several hundreds of millisecond or even several seconds. The main reason is that the fault transient and charging current will influence this protection algorithm greatly. Thus, a long delay is necessary to ensure reliability.
Both main protection and backup may be influenced by high impedance faults.
1) Influence to travelling wave protection
Existing traveling wave criterion is:
|WCOMM| = |ZCOMICOM-UCOM| >350kV
|WPOLE| = |ZDIFIDIF-UDIF| >210kV
wherein ZCOM is common mode wave impedance, ZDIF is differential mode wave impedance, WPOLE is pole wave, and WCOMM is ground wave.
ICOM is common mode current, UCOM is common mode voltage, IDIF is differential mode current, and UDIF is differential mode voltage.
The protection detects the wave head by using the change rate of ground wave.
|dWCOMM/dt| >396kV/ms
The DC voltage decreases with a small rate of change when the line grounds to the earth via large impedance, leading to the mis-operations of existing protection based on travelling wave.
If travelling wave protection mis-operates, control & protection system will delay to eliminate faults.
2) Influence to voltage change rate and low voltage protection
Voltage change rate criterion is:
DUT = dUdl/dt < -396kV /ms & Udl < 200kV, where Udl is line voltage and DUT is the corresponding changing rate.
Voltage change rate protection will mis-operate caused by small DC voltage decreasing, when the line grounds to the earth via large impedance.
3) Influence to current differential protection
The typical criterion of current differential protection is shown below,
|IDL-IDL_FOS| >max (ISET, k×|IDL + IDL_FOS|/2)
wherein ISET is fixed current setting which is normally set to 120A, k is ratio coefficient which is normally set to 0.1.
In order to ensure operation on condition of large impedance fault, setting ISET and k are normally set to a small value. Thus delay time has to be set long enough to avoid mis-operation caused by capacity charging current.
If fast protection (travelling wave protection) mis-operates, backup protection will delay to work. And delay time is too long to guarantee steady operation of power system.
SUMMARY
Accordingly, one aspect of the present invention provides a DC grid current differential protection method, including the following steps:
sampling value acquiring step: acquiring pole voltage sampling values and pole current sampling values in local terminal and remote terminal of a DC line;
fault component extracting step: calculating fault component pole voltage values according to the pole voltage sampling values of local terminal and remote terminal respectively; and calculating fault component pole current values according to the pole current sampling values of local terminal and remote terminal respectively;
Bergeron model calculating step: acquiring fault component pole current values at selected point on the DC line between the local terminal and the remote terminal, by the calculation of the fault component pole voltage values and the fault component pole current values of local terminal and remote terminal calculated in the fault component extracting step, based on Bergeron model;
current differential protection judging step: judging internal fault if the fault component pole current values at selected point of local terminal and remote terminal acquired in the Bergeron model calculating step meet preset current differential protection criterion.
Preferably, the DC grid is of bi-pole and the DC line includes a positive pole DC line and a negative pole DC line, the local terminal includes a positive pole local terminal and a positive pole remote terminal, the remote terminal includes a negative pole local terminal and a negative pole remote terminal, the positive pole DC line electrically connects the positive pole local terminal and the positive pole remote terminal, and the negative pole DC line electrically connects the negative pole local terminal and the negative pole remote terminal, distance from the selected point to the positive pole local terminal is same as distance from the selected point to the negative pole local terminal, and distance from the selected point to the positive pole remote terminal is same as distance from the selected point to the negative pole remote terminal, further comprising:
pole-mode transformation step: acquiring fault component mode voltage values for each mode of local terminal and remote terminal by pole-mode transforming said fault component pole voltage value for each of the positive pole local terminal, the positive pole remote terminal, the negative pole local terminal and the negative pole remote terminal, and acquiring fault component mode current values for each mode of the local terminal and remote terminal by pole-mode transforming said fault component pole current values for each of the positive pole local terminal, the positive pole remote terminal, the negative pole local terminal and the negative pole remote terminal;
the Bergeron model calculating step further comprises:
acquiring fault component mode travelling wave voltage values for each mode of local terminal and remote terminal respectively by the calculation of the fault component mode voltage values and the fault component mode current values for each mode of local terminal and remote terminal based on Bergeron model;
converting the fault component mode travelling wave voltage values of local terminal and remote terminal respectively into fault component mode travelling wave current values of local terminal and remote terminal;
determining fault component mode current values of local terminal and remote terminal at the selected point on the DC line respectively in accordance with the fault component mode travelling wave current values of local terminal and remote terminal;
acquiring fault component pole current values for each of the positive pole local terminal and the negative pole local terminal at the selected point on the DC line by mode-pole transforming the fault component mode current values for each mode of local terminal at the selected point, and acquiring fault component pole current values for positive pole remote terminal and negative pole remote terminal at the selected point by mode-pole transforming the fault component mode current values for each mode of remote terminal at the selected point.
Conveniently, the pole voltage sampling values comprise: uLP (t) , i.e. voltage sampling value of positive pole local terminal; uLN (t) , i.e. voltage sampling value of negative pole local terminal; uRP (t) , i.e. voltage sampling value of positive pole remote terminal; uRN (t) , i.e. voltage sampling value of negative pole remote terminal; wherein t means time;
the pole current sampling values comprise: iLP (t) , i.e. current sampling value of positive pole local terminal; iLN (t) , i.e. current sampling value of negative pole local terminal; iRP (t) , i.e. current sampling value of positive pole remote terminal; iRN(t) , i.e. current sampling value of negative pole remote terminal;
the fault component pole voltage values comprise: ΔuLP (t) , i.e. fault component voltage value of positive pole local terminal corresponding to uLP (t) ; ΔuLN(t) , i.e. fault component voltage value of negative pole local terminal corresponding to uLN (t) ; ΔuRP (t) , i.e. fault component voltage value of positive pole remote terminal corresponding to uRP (t) ; ΔuRN (t) , i.e. fault component voltage value of negative pole remote terminal corresponding to uRN (t) ;
the fault component pole current values comprise: ΔiLP (t) , i.e. fault component current value of positive pole local terminal corresponding to iLP (t) ; ΔiLN (t) , i.e. fault component current value of negative pole local terminal corresponding to iLN (t) ; ΔiRP (t) , i.e. fault component current value of positive pole
remote terminal corresponding to iRP (t) ; ΔiRN (t) , i.e. fault component current value of negative pole remote terminal corresponding to iRN (t) ;
the fault component mode voltage values comprise: ΔuL0 (t) i.e. fault component common mode voltage value of local terminal; ΔuL1 (t) i.e. fault component differential mode voltage value of local terminal; ΔuR0 (t) i.e. fault component common mode voltage value of remote terminal; ΔuR1 (t) i.e. fault component differential mode voltage value of remote terminal;
the fault component mode current values comprise: ΔiL0 (t) i.e. fault component common mode current value of local terminal; ΔiL1 (t) i.e. fault component differential mode current value of local terminal; ΔiR0 (t) i.e. fault component common mode current value of remote terminal; ΔiR1 (t) i.e. fault component differential mode current value of remote terminal;
the fault component travelling wave voltage values comprise: ΔuL0+ (t) i.e. fault component common mode forward travelling wave voltage value of local terminal; ΔuL0- (t) i.e. fault component common mode inverse travelling wave voltage value of local terminal; ΔuL1+ (t) i.e. fault component differential mode forward travelling wave voltage value of local terminal; ΔuL0- (t) i.e. fault component differential mode inverse travelling wave voltage value of local terminal; ΔuR0+ (t) i.e. fault component common mode forward travelling wave voltage value of remote terminal; ΔuR0- (t) i.e. fault component common mode inverse travelling wave voltage value of remote terminal; ΔuR1+ (t) i.e. fault component differential mode forward travelling wave voltage value of remote terminal; ΔuR1- (t) i.e. fault component differential mode inverse travelling wave voltage value of remote terminal;
the fault component travelling wave current values comprise: ΔiL0+ (t) i.e. fault component common mode forward travelling wave current value of local terminal; ΔiL0- (t) i.e. fault component common mode inverse travelling wave current value of
local terminal; ΔiL1+ (t) i.e. fault component differential mode forward travelling wave current value of local terminal; ΔiL1- (t) i.e. fault component differential mode inverse travelling wave current value of local terminal; ΔiR0+ (t) i.e. fault component common mode forward travelling wave current value of remote terminal; ΔiR0- (t) i.e. fault component common mode inverse travelling wave current value of remote terminal; ΔiR1+ (t) i.e. fault component differential mode forward travelling wave current value of remote terminal; ΔiR1- (t) i.e. fault component differential mode inverse travelling wave current value of remote terminal;
the fault component mode current values at the selected point comprise: ΔiL0 (x, t) , i.e. fault component common mode current value at the selected point of local terminal; ΔiL1 (x, t) , i.e. fault component differential mode current value at the selected point of local terminal; ΔiR0 (x, t) , i.e. fault component common mode current value at the selected point of remote terminal; ΔiR1 (x, t) , i.e. fault component differential mode current value at the selected point of remote terminal, wherein x is the selected point;
the fault component pole current values at the selected point comprise: ΔiLP (x, t) i.e. fault component pole current value at selected point of positive pole local terminal; ΔiLN (x, t) i.e. fault component pole current value at selected point of negative pole local terminal; ΔiRP (x, t) i.e. fault component pole current value at selected point of positive pole remote terminal; ΔiRN (x, t) i.e. fault component pole current value at selected point of negative pole remote terminal.
Conveniently, in the fault component extracting step, the fault component pole voltage value and the fault component pole voltage value are calculated in the following way:
in the pole-mode transformation step, the fault component mode voltage value and the fault component mode current value are calculated in the following way:
In the Bergeron model calculating step comprises:
the fault component mode travelling wave voltage values are calculated in the following way:
the fault component mode travelling wave current values are calculated in the following way:
the fault component mode current values at selected point are calculated in the following way:
wherein, v0 is the travelling speed of fault component common mode travelling wave, v1 is the travelling speed of fault component differential mode travelling wave;
the fault component pole current values at the selected point are calculated in the following way:
Conveniently, the current differential protection judging step comprises:
if|ΔiLP(x, t) + ΔiRP(x, t) | >Ires is met, judging the condition as positive internal fault; else, if|ΔiLN(x, t) + ΔiRN (x, t) |>Ires is met, judging the condition as negative internal fault, wherein Ires is a preset threshold value;
otherwise, the differential protection will not be activated.
Preferably, the DC grid is single-pole:
the Bergeron model calculating step further comprises:
acquiring fault component pole travelling wave voltage values of local terminal and remote terminal respectively by the calculation of the fault component pole voltage values and the fault component mode current values of local terminal and remote terminal based on Bergeron model;
converting the fault component pole travelling wave voltage values of local terminal and remote terminal respectively into fault component mode travelling wave current values of local terminal and remote terminal;
determining fault component pole current values of local terminal and remote terminal at the selected point on the DC line in accordance with the fault component pole travelling wave current values of local terminal and remote terminal.
Conveniently, in the fault component extracting step, the fault component pole voltage value and the fault component pole voltage value are calculated in the following way:
In the Bergeron model calculating step comprises:
the fault component pole travelling wave voltage values are calculated in the following way:
the fault component pole travelling wave current values are calculated in the following way:
the fault component role current values at selected position are calculated in the following way:
Conveniently, in the current differential protection judging step comprises:
if |ΔiL(x, t) + ΔiR (x, t) | >Ires is met, judging the condition as internal fault, wherein Ires is a preset threshold value.
Conveniently, the current differential protection judging step further comprises:
If the condition is judged as internal fault, then sending fault protection command to activate differential protection, otherwise, the differential protection will not be activated.
Another aspect of the present invention provides a computer program comprising computer program code adapted to perform all of the steps of any one of the above when run on a computer.
A further aspect of the present invention provides a computer program according to the above, embodied on a computer-readable medium.
Another aspect of the present invention provides a DC grid current differential protection system, comprising the following modules:
sampling value acquiring module: acquiring pole voltage sampling values and pole current sampling values in local terminal and remote terminal of a DC line;
fault component extracting module: calculating fault component pole voltage values according to the pole voltage sampling values of local terminal and remote terminal respectively; and calculating fault component pole current values according to the pole current sampling values of local terminal and remote terminal respectively;
Bergeron model calculating module: acquiring fault component pole current values at selected point on the DC line between the local terminal and the remote terminal, by the calculation of the fault component pole voltage values and the fault component pole current values of local terminal and remote terminal calculated in the fault component extracting module, based on Bergeron model;
current differential protection judging module: judging internal fault if the fault component pole current values at selected point of local terminal and remote terminal acquired in the Bergeron model calculating module meet preset current differential protection criterion.
Preferably, the DC grid is of bi-pole and the DC line includes a positive pole DC line and a negative pole DC line, the local terminal includes a positive pole local terminal and a positive pole remote terminal, the remote terminal includes a negative pole local terminal and a negative pole remote terminal, the positive pole DC line electrically connects the positive pole local terminal and the positive pole remote
terminal, and the negative pole DC line electrically connects the negative pole local terminal and the negative pole remote terminal, distance from the selected point to the positive pole local terminal is same as distance from the selected point to the negative pole local terminal, and distance from the selected point to the positive pole remote terminal is same as distance from the selected point to the negative pole remote terminal, further comprising:
pole-mode transformation module: acquiring fault component mode voltage values for each mode of local terminal and remote terminal by pole-mode transforming said fault component pole voltage value for each of the positive pole local terminal, the positive pole remote terminal, the negative pole local terminal and the negative pole remote terminal, and acquiring fault component mode current values for each mode of the local terminal and remote terminal by pole-mode transforming said fault component pole current values for each of the positive pole local terminal, the positive pole remote terminal, the negative pole local terminal and the negative pole remote terminal;
the Bergeron model calculating module further comprises:
acquiring fault component mode travelling wave voltage values for each mode of local terminal and remote terminal respectively by the calculation of the fault component mode voltage values and the fault component mode current values for each mode of local terminal and remote terminal based on Bergeron model;
converting the fault component mode travelling wave voltage values of local terminal and remote terminal respectively into fault component mode travelling wave current values of local terminal and remote terminal;
determining fault component mode current values of local terminal and remote terminal at the selected point on the DC line respectively in accordance with the fault component mode travelling wave current values of local terminal and remote terminal;
acquiring fault component pole current values for each of the positive pole local terminal and the negative pole local terminal at the selected point on the DC line by mode-pole transforming the fault component mode current values for each mode of local terminal at the selected point, and acquiring fault component pole current values for positive pole remote terminal and negative pole remote terminal at
the selected point by mode-pole transforming the fault component mode current values for each mode of remote terminal at the selected point.
Conveniently, the pole voltage sampling values comprise: uLP (t) , i.e. voltage sampling value of positive pole local terminal; uLN (t) , i.e. voltage sampling value of negative pole local terminal; uRP (t) , i.e. voltage sampling value of positive pole remote terminal; uRN (t) , i.e. voltage sampling value of negative pole remote terminal; wherein t means time;
the pole current sampling values comprise: iLP (t) , i.e. current sampling value of positive pole local terminal; iLN (t) , i.e. current sampling value of negative pole local terminal; iRP (t) , i.e. current sampling value of positive pole remote terminal; iRN(t) , i.e. current sampling value of negative pole remote terminal;
the fault component pole voltage values comprise: ΔuLP (t) , i.e. fault component voltage value of positive pole local terminal corresponding to uLP (t) ; ΔuLN (t) , i.e. fault component voltage value of negative pole local terminal corresponding to uLN (t) ; ΔuRP (t) , i.e. fault component voltage value of positive pole remote terminal corresponding to uRP (t) ; ΔuRN (t) , i.e. fault component voltage value of negative pole remote terminal corresponding to uRN (t) ;
the fault component pole current values comprise: ΔiLP (t) , i.e. fault component current value of positive pole local terminal corresponding to iLP (t) ; ΔiLN (t) , i.e. fault component current value of negative pole local terminal corresponding to iLN (t) ; ΔiRP (t) , i.e. fault component current value of positive pole remote terminal corresponding to iRP (t) ; ΔiRN (t) , i.e. fault component current value of negative pole remote terminal corresponding to iRN (t) ;
the fault component mode voltage values comprise: ΔuL0 (t) i.e. fault component common mode voltage value of local terminal; ΔuL1 (t) i.e. fault
component differential mode voltage value of local terminal; ΔuR0 (t) i.e. fault component common mode voltage value of remote terminal; ΔuR1 (t) i.e. fault component differential mode voltage value of remote terminal;
the fault component mode current values comprise: ΔiL0 (t) i.e. fault component common mode current value of local terminal; ΔiL1 (t) i.e. fault component differential mode current value of local terminal; ΔiR0 (t) i.e. fault component common mode current value of remote terminal; ΔiR1 (t) i.e. fault component differential mode current value of remote terminal;
the fault component travelling wave voltage values comprise: ΔuL0+ (t) i.e. fault component common mode forward travelling wave voltage value of local terminal; ΔuL0- (t) i.e. fault component common mode inverse travelling wave voltage value of local terminal; ΔuL1+ (t) i.e. fault component differential mode forward travelling wave voltage value of local terminal; ΔuL0- (t) i.e. fault component differential mode inverse travelling wave voltage value of local terminal; ΔuR0+ (t) i.e. fault component common mode forward travelling wave voltage value of remote terminal; ΔuR0- (t) i.e. fault component common mode inverse travelling wave voltage value of remote terminal; ΔuR1+ (t) i.e. fault component differential mode forward travelling wave voltage value of remote terminal; ΔuR1- (t) i.e. fault component differential mode inverse travelling wave voltage value of remote terminal;
the fault component travelling wave current values comprise: ΔiL0+ (t) i.e. fault component common mode forward travelling wave current value of local terminal; ΔiL0- (t) i.e. fault component common mode inverse travelling wave current value of local terminal; ΔiL1+ (t) i.e. fault component differential mode forward travelling wave current value of local terminal; ΔiL1- (t) i.e. fault component differential mode inverse travelling wave current value of local terminal; ΔiR0+ (t) i.e. fault component common mode forward travelling wave current value of remote terminal; ΔiR0- (t) i.e. fault
component common mode inverse travelling wave current value of remote terminal; ΔiR1+ (t) i.e. fault component differential mode forward travelling wave current value of remote terminal; ΔiR1- (t) i.e. fault component differential mode inverse travelling wave current value of remote terminal;
the fault component mode current values at the selected point comprise: ΔiL0 (x, t) , i.e. fault component common mode current value at the selected point of local terminal; ΔiL1 (x, t) , i.e. fault component differential mode current value at the selected point of local terminal; ΔiR0 (x, t) , i.e. fault component common mode current value at the selected point of remote terminal; ΔiR1 (x, t) , i.e. fault component differential mode current value at the selected point of remote terminal, wherein x is the selected point;
the fault component pole current values at the selected point comprise: ΔiLP (x, t) i.e. fault component pole current value at selected point of positive pole local terminal; ΔiLN (x, t) i.e. fault component pole current value at selected point of negative pole local terminal; ΔiRP (x, t) i.e. fault component pole current value at selected point of positive pole remote terminal; ΔiRN (x, t) i.e. fault component pole current value at selected point of negative pole remote terminal.
Conveniently, in the fault component extracting module, the fault component pole voltage value and the fault component pole voltage value are calculated in the following way:
in the pole-mode transformation module, the fault component mode voltage value and the fault component mode current value are calculated in the following
way:
In the Bergeron model calculating module comprises:
the fault component mode travelling wave voltage values are calculated in the following way:
the fault component mode travelling wave current values are calculated in the following way:
the fault component mode current values at selected point are calculated in
the following way:
wherein, v0 is the travelling speed of fault component common mode travelling wave, v1 is the travelling speed of fault component differential mode travelling wave;
the fault component pole current values at the selected point are calculated in the following way:
Conveniently, the current differential protection judging module comprises:
if |ΔiLP (x, t) + ΔiRP (x, t) | >Ires is met, judging the condition as positive internal fault; else, if |ΔiLN (x, t) + ΔiRN (x, t) |>Ires is met, judging the condition as negative internal fault, wherein Ires is a preset threshold value;
otherwise, the differential protection will not be activated.
Preferably, the DC grid is single-pole:
the Bergeron model calculating module further comprises:
acquiring fault component pole travelling wave voltage values of local terminal and remote terminal respectively by the calculation of the fault component pole voltage values and the fault component mode current values of local terminal and remote terminal based on Bergeron model;
converting the fault component pole travelling wave voltage values of local terminal and remote terminal respectively into fault component mode travelling wave
current values of local terminal and remote terminal;
determining fault component pole current values of local terminal and remote terminal at the selected point on the DC line in accordance with the fault component pole travelling wave current values of local terminal and remote terminal.
Conveniently, in the fault component extracting module, the fault component pole voltage value and the fault component pole voltage value are calculated in the following way:
In the Bergeron model calculating module comprises:
the fault component pole travelling wave voltage values are calculated in the following way:
the fault component pole travelling wave current values are calculated in the
following way:
the fault component role current values at selected position are calculated in the following way:
Conveniently, in the current differential protection judging module comprises:
if |ΔiL (x, t) + ΔiR (x, t)| >Ires is met, judging the condition as internal fault, wherein Ires is a preset threshold value.
Conveniently, the current differential protection judging module further comprises:
If the condition is judged as internal fault, then sending fault protection command to activate differential protection, otherwise, the differential protection will not be activated.
Bergeron model bases on distributed parameters and telegraph equations (wave equations) . Therefore, the present invention adopts Bergeron model, so long time-extension is not needed to eliminate the disturbance of the distributed charging current, so as to substantially improve the calculating speed in the present invention.
Meanwhile, the present invention uses fault component to erase the influence of the load current to differential protection so that the sensitivity is improved.
Fig. 1 shows a chart showing travelling wave fronts of internal and external DC fault in LCC DC grid;
Fig. 2 shows a measurement schematic view of the traditional travelling wave protection device;
Fig. 3 shows a flow-process diagram illustrating a DC grid current differential protection method in accordance with the present invention
Fig. 4 illustrates schematically a fault component distributed grid;
Fig. 5 shows the condition when internal fault occur in the line;
Fig. 6 shows the condition when external fault occur in the line;
Fig. 7 shows the simulation model;
Fig. 8 shows the simulation result;
Fig. 9 shows a structural module drawing of a DC grid current differential protection system;
Fig. 10 shows a single pole HVDC system.
DETAILED DESCRIPTION OF THE EMBODIMENTS
Hereinafter, the present invention is further introduced in detail by the particular embodiments in combination with the figures.
Fig. 3 shows a flow-process diagram illustrating a DC grid current differential protection method in accordance with the present invention, including the following steps:
step S301: acquiring pole voltage sampling values and pole current sampling values in local terminal and remote terminal of a DC line;
step S302: calculating fault component pole voltage values according to the
pole voltage sampling values of local terminal and remote terminal respectively; and calculating fault component pole current values according to the pole current sampling values of local terminal and remote terminal respectively;
step S303: acquiring fault component pole current values at selected point on the DC line between the local terminal and the remote terminal, by the calculation of the fault component pole voltage values and the fault component pole current values of local terminal and remote terminal calculated in the step S302, based on Bergeron model;
step 304: judging internal fault if the fault component pole current values at selected point of local terminal and remote terminal acquired in the step S303 meet preset current differential protection criterion.
Bergeron model bases on distributed parameters and telegraph equations (wave equations) . Thereby, the distributed charging current during the fault transient is taken into account inherently and accurately by this method in theory.
Therefore, the present invention adopts Bergeron model, so long time-extension is not needed to eliminate the disturb of the distributed charging current, so as to substantially improve the calculating speed in the present invention.
Meanwhile, in Step S302, pole voltage sampling values are converted into fault component pole voltage values, and pole current sampling values are converted into fault component pole current values. Therefore, in this step, fault component pole voltage values are separated from the pole voltage sampling values, and fault component pole current values are separated from the pole current sampling values. In this case, when the fault occurs in the grid, the grid may be divided into no-fault network and fault-component network, so the fault component pole voltage values and fault component pole current values are the pole voltage/current values in the fault-component network. In the subsequent steps S303 and S304, the fault component pole voltage values and the fault component pole current values are pole-mode transformed and applied to Bergeron model. That is, the present invention provides a protection which is a current differential protection based on fault component. Therefore, the present invention uses fault component to erase the influence of the load current to differential protection so that the sensitivity is improved.
In a preferable embodiment of the present invention, in particular, the DC grid is of bi-pole and the DC line includes a positive pole DC line and a negative pole DC line, the local terminal includes a positive pole local terminal and a positive pole remote terminal, the remote terminal includes a negative pole local terminal and a negative pole remote terminal, the positive pole DC line electrically connects the positive pole local terminal and the positive pole remote terminal, and the negative pole DC line electrically connects the negative pole local terminal and the negative pole remote terminal, distance from the selected point to the positive pole local terminal is same as distance from the selected point to the negative pole local terminal, and distance from the selected point to the positive pole remote terminal is same as distance from the selected point to the negative pole remote terminal, further comprising:
pole-mode transformation step: acquiring fault component mode voltage values for each mode of local terminal and remote terminal by pole-mode transforming said fault component pole voltage value for each of the positive pole local terminal, the positive pole remote terminal, the negative pole local terminal and the negative pole remote terminal, and acquiring fault component mode current values for each mode of the local terminal and remote terminal by pole-mode transforming said fault component pole current values for each of the positive pole local terminal, the positive pole remote terminal, the negative pole local terminal and the negative pole remote terminal;
the step S303 further comprises:
acquiring fault component mode travelling wave voltage values for each mode of local terminal and remote terminal respectively by the calculation of the fault component mode voltage values and the fault component mode current values for each mode of local terminal and remote terminal based on Bergeron model;
converting the fault component mode travelling wave voltage values of local terminal and remote terminal respectively into fault component mode travelling wave current values of local terminal and remote terminal;
determining fault component mode current values of local terminal and remote terminal at the selected point on the DC line respectively in accordance with the fault component mode travelling wave current values of local terminal and remote
terminal;
acquiring fault component pole current values for each of the positive pole local terminal and the negative pole local terminal at the selected point on the DC line by mode-pole transforming the fault component mode current values for each mode of local terminal at the selected point, and acquiring fault component pole current values for positive pole remote terminal and negative pole remote terminal at the selected point by mode-pole transforming the fault component mode current values for each mode of remote terminal at the selected point.
This embodiment applies the fault component mode voltage and the fault component mode current under the mode-pole transformation to the Bergeron model, so as to particularly achieving the Bergeron model based on the fault component in bi-pole DC grid.
In one embodiment:
the pole voltage sampling values comprise: uLP (t) , i.e. voltage sampling value of positive pole local terminal; uLN (t) , i.e. voltage sampling value of negative pole local terminal; uRP (t) , i.e. voltage sampling value of positive pole remote terminal; uRN (t) , i.e. voltage sampling value of negative pole remote terminal; wherein t means time;
the pole current sampling values comprise: iLP (t) , i.e. current sampling value of positive pole local terminal; iLN (t) , i.e. current sampling value of negative pole local terminal; iRP(t) , i.e. current sampling value of positive pole remote terminal; iRN(t) , i.e. current sampling value of negative pole remote terminal;
the fault component pole voltage values comprise: ΔuLP (t) , i.e. fault component voltage value of positive pole local terminal corresponding to uLP (t) ; ΔuLN(t) , i.e. fault component voltage value of negative pole local terminal corresponding to uLN (t) ; ΔuRP (t) , i.e. fault component voltage value of positive pole remote terminal corresponding to uRP (t) ; ΔuRN (t) , i.e. fault component voltage value of negative pole remote terminal corresponding to uRN (t) ;
the fault component pole current values comprise: ΔiLP (t) , i.e. fault component current value of positive pole local terminal corresponding to iLP (t) ; ΔiLN (t) , i.e. fault component current value of negative pole local terminal corresponding to iLN (t) ; ΔiRP (t) , i.e. fault component current value of positive pole remote terminal corresponding to iRP (t) ; ΔiRN (t) , i.e. fault component current value of negative pole remote terminal corresponding to iRN (t) ;
the fault component mode voltage values comprise: ΔuL0 (t) i.e. fault component common mode voltage value of local terminal; ΔuL1 (t) i.e. fault component differential mode voltage value of local terminal; ΔuR0 (t) i.e. fault component common mode voltage value of remote terminal; ΔuR1 (t) i.e. fault component differential mode voltage value of remote terminal;
the fault component mode current values comprise: ΔiL0 (t) i.e. fault component common mode current value of local terminal; ΔiL1 (t) i.e. fault component differential mode current value of local terminal; ΔiR0 (t) i.e. fault component common mode current value of remote terminal; ΔiR1 (t) i.e. fault component differential mode current value of remote terminal;
the fault component travelling wave voltage values comprise: ΔuL0+ (t) i.e. fault component common mode forward travelling wave voltage value of local terminal; ΔuL0- (t) i.e. fault component common mode inverse travelling wave voltage value of local terminal; ΔuL1+ (t) i.e. fault component differential mode forward travelling wave voltage value of local terminal; ΔuL0- (t) i.e. fault component differential mode inverse travelling wave voltage value of local terminal; ΔuR0+ (t) i.e. fault component common mode forward travelling wave voltage value of remote terminal; ΔuR0- (t) i.e. fault component common mode inverse travelling wave voltage value of remote terminal; ΔuR1+ (t) i.e. fault component differential mode forward travelling wave voltage value of remote terminal; ΔuR1- (t) i.e. fault component differential mode inverse travelling
wave voltage value of remote terminal;
the fault component travelling wave current values comprise: ΔiL0+ (t) i.e. fault component common mode forward travelling wave current value of local terminal; ΔiL0- (t) i.e. fault component common mode inverse travelling wave current value of local terminal; ΔiL1+ (t) i.e. fault component differential mode forward travelling wave current value of local terminal; ΔiL1- (t) i.e. fault component differential mode inverse travelling wave current value of local terminal; ΔiR0+ (t) i.e. fault component common mode forward travelling wave current value of remote terminal; ΔiR0- (t) i.e. fault component common mode inverse travelling wave current value of remote terminal; ΔiR1+ (t) i.e. fault component differential mode forward travelling wave current value of remote terminal; ΔiR1- (t) i.e. fault component differential mode inverse travelling wave current value of remote terminal;
the fault component mode current values at the selected point comprise: ΔiL0 (x,t) , i.e. fault component common mode current value at the selected point of local terminal; ΔiL1 (x, t) , i.e. fault component differential mode current value at the selected point of local terminal; ΔiR0 (x,t) , i.e. fault component common mode current value at the selected point of remote terminal; ΔiR1 (x, t) , i.e. fault component differential mode current value at the selected point of remote terminal, wherein x is the selected point;
the fault component pole current values at the selected point comprise: ΔiLP (x, t) i.e. fault component pole current value at selected point of positive pole local terminal; ΔiLN (x, t) i.e. fault component pole current value at selected point of negative pole local terminal; ΔiRP (x, t) i.e. fault component pole current value at selected point of positive pole remote terminal; ΔiRN (x, t) i.e. fault component pole current value at selected point of negative pole remote terminal.
This embodiment has the respective calculation for the position and negative poles, so as to achieve the respective differential protections for both of the poles.
In one embodiment:
In the step S302, the fault component pole voltage value and the fault component pole voltage value are calculated in the following way:
in the pole-mode transformation step, the fault component mode voltage value and the fault component mode current value are calculated in the following way:
In step S303 comprises:
the fault component mode travelling wave voltage values are calculated in the following way:
the fault component mode travelling wave current values are calculated in the following way:
the fault component mode current values at selected point are calculated in the following way:
wherein, v0 is the travelling speed of fault component common mode travelling wave, v1 is the travelling speed of fault component differential mode travelling wave;
the fault component pole current values at the selected point are calculated in the following way:
In one embodiment:
in the step S304 comprises:
if |ΔiLP (x, t) + ΔiRP (x, t) | >Ires is met, judging the condition as positive internal fault; else, if |ΔiLN (x, t) + ΔiRN (x, t) | >Ires is met, judging the condition as negative internal fault, wherein Ires is a preset threshold value;
otherwise, the differential protection will not be activated. In one embodiment, the DC grid is single-pole:
step S303 further comprises:
acquiring fault component pole travelling wave voltage values of local terminal and remote terminal respectively by the calculation of the fault component pole voltage values and the fault component mode current values of local terminal and remote terminal based on Bergeron model;
converting the fault component pole travelling wave voltage values of local terminal and remote terminal respectively into fault component mode travelling wave current values of local terminal and remote terminal;
determining fault component pole current values of local terminal and remote terminal at the selected point on the DC line in accordance with the fault component pole travelling wave current values of local terminal and remote terminal.
This embodiment particularly achieving the Bergeron model based on the fault component in single-pole DC grid.
In one embodiment, in step S302, the fault component pole voltage value and the fault component pole voltage value are calculated in the following way:
in step S303 comprises:
the fault component pole travelling wave voltage values are calculated in the following way:
the fault component pole travelling wave current values are calculated in the following way:
the fault component role current values at selected position are calculated in the following way:
In one embodiment, in step S304 comprises:
if |ΔiL (x, t) + ΔiR (x, t) | >Ires is met, judging the condition as internal fault.
In one embodiment, step S304 further comprises:
If the condition is judged as internal fault, then sending fault protection command to activate differential protection, otherwise, the differential protection will not be activated.
Bi-pole DC grid
In a preferable embodiment of the present invention, as shown in Fig. 4 illustrating schematically a fault component distributed grid, the current differential protection method of the present invention seeks to calculate ΔiLP (x, t) and ΔiRP (x, t) at the selected point x at a particular t, to make the judgment for the positive pole fault, and meanwhile calculate ΔiLN (x, t) and ΔiRN (x, t) at the selected point x at a particular t, to make the judgment for the negative pole fault. The local side 41 and the remote side 42 may be communicated via a communication line, so the local side 41 may acquire all parameter information of the local side 41 and the remote side 42. In particular, ΔiLP (x, t) , ΔiRP (x, t) , ΔiLN (x, t) and ΔiRN (x, t) may be calculated in the following way:
Fault component currents and voltages calculation
The fault component is calculated by the following equation (1) :
wherein, T is time delay, which can be set to e. g. 10 ms or 100 ms, depending on the requirements.
Pole-Mode Transformation
After getting the fault component values currents and voltages Δ iLP (t) , ΔiLN (t) , ΔuLP (t) , ΔuLN (t) , ΔiRP (t) , ΔiRN (t) , ΔuRP (t) and ΔuRN (t) by equation (1) , the next step is to do Pole-Mode Transformation to transform the pole quantities to mode quantities. The Pole-Mode Transformation matrix for both voltage and current are given in equation (2) .
Differential current calculation based on Bergeron model
In this step, fault component traveling wave common and differential mode currents values at the selected point x on the protected line will be calculated based on Bergeron model (travelling wave transmission equation) with the measurements from the two terminals respectively.
(1) calculation for fault component mode travelling wave voltage value
The equation 3 may be used to calculate the common mode forward voltage travelling wave ΔuL0+ and inverse voltage travelling wave ΔuL0- of the locate side fault component, the differential mode forward voltage travelling wave ΔuL1+ and inverse voltage travelling wave ΔuL1- of the locate side fault component, the common mode forward voltage travelling wave ΔuR0+ and inverse voltage travelling wave ΔuR0- of the remote side fault component, and the differential mode forward voltage travelling wave ΔuR1+ and inverse voltage travelling wave ΔuR1- of the remote side fault component.
wherein ZC0 is common mode wave impedance, ZC1 is differential mode wave impedance.
(2) calculation for fault component mode travelling wave current value
(3) fault component mode current value at selected position
Based on the travelling wave principle, the fault component differential mode and common mode current of local terminal and remote terminal at the selected point x can be calculated by using the following equation 5, selected point
x calculated by the measurements of local terminal for the fault component differential mode and common mode current of local terminal, selected point x calculated by the measurements of remote terminal for the fault component differential mode and common mode current of remote terminal:
(4) Mode-Pole Transformation
In this step, the positive and negative pole currents at the selected points can be calculated by using Mode-Pole Transformation for both local and remote terminals. The transformation matrix is shown in the following equation 6:
The criterion for activation of current differential protection:
If the following equation 7 is met:
|ΔiLP (x, t) + ΔiRP (L-x, t) | >Ires (7)
then the condition is judged as “positive pole internal fault” , so fault protection command is sent, and the controlling of differential protection is activated.
If the following equation (8) is met:
|ΔiLN (x, t) + ΔiRN (L-x, t) |>Ires (8)
then the condition is judged as “negative pole internal fault” , so fault protection command is sent, and the controlling of differential protection is activated.
Single-pole DC grid:
In a preferable embodiment of the present invention, as shown in Fig. 10, where ΔuL (t) and ΔiL (t) are fault component voltage and current of local terminal, ΔuR (t) and ΔiR (t) are fault component voltage and current of remote terminal, ΔiL (x,t) is the current at point x calculated by local measurements, ΔiR (x, t) is the current at point x calculated by remote measurements,
As shown in the figure above, the fault component currents at the selected point ‘x’ will be calculated based on Bergeron model (telegraph equations, travelling wave equation) by the measurements of the two terminals respectively.
For current component calculated by local measurement, x is the distance between any selected point along the line and the local terminal. For example, if the selected point is the remote terminal, then the distance x is the line length L;
For current component calculated by remote measurement, x is the distance between any selected point along the line and the remote terminal. For example, if the selected point is the remote terminal, then the distance x is zero.
In the following sections, the calculation steps will be introduced in details.
Fault component currents and voltages calculation
The method to calculate fault component is:
In equations 9, u (t) and i (t) are measured pole voltage and current sampling values. Δu (t) and Δi (t) are corresponding fault component voltage and current value. T is the time delay, can be set to e.g. 10 ms or 100 ms, depending on the requirements. According to this method, the fault component voltage values and current values of both poles and both ends can be calculated as equation 10.
Differential current calculation based on Bergeron model
In this step, traveling wave currents at the selected point x on the protected line will be calculated based on Bergeron model (travelling wave transmission equation) with the measurements from the two terminals respectively.
Traveling wave voltage component calculation
Equation 11can be used to calculate the forward voltage travelling wave ΔuL+ and inverse voltage travelling wave ΔuL-
Traveling wave current component calculation
And then, equation 12 can be used to calculate forward current travelling wave ΔiL+ and inverse current travelling wave ΔiL-.
Traveling wave current component calculation at selected point
Based on the traveling wave principle, the current at the selected point x can be calculated by using equation 13.
Where,
v is the travelling speed of travelling wave;
t is time;
x is any point along the line, it could be middle point, end point, start point, or any other point;
ΔiL (x, t) is the fault component current at the selected point x calculated by the measurements of local terminal;
ΔiR (x, t) is the fault component current at the selected point x calculated by the measurements of remote terminal;
Differential current calculation
Calculate the differential current by the pole currents, and compare with the threshold, as illustrated in equation 14. If the differential current is larger than restrain current, it means internal fault. Otherwise, it means external fault. The criterion for detecting internal fault is shown below.
|ΔiL (x,t) + ΔiR (x,t) | >Ires (14).
Performance analysis
Classical differential protection in HVDC line
The criterion of typical classical differential protection is shown below:
|ILocal + IRemote| >ISet (9)
Where, ILocal is the local terminal current, IRemote is the remote terminal current.
Fig. 5 shows the condition when internal fault occur in the line. For internal fault, we have
|ILocal + IRemote| = IF + IC (10)
Where, IF is fault current through fault branch as shown in Fig. 5, and IC is the current flow through the distributed capacitance alone the line, normally much higher than zero, especially for the long length transmission line. Compared with Equation (9) we can observe that the protection principle can operate correctly.
However for external fault, problem will occur due to the capacitor current. Fig. 6 shows the condition when external fault occur in the line. For external fault, we have
|ILocal + IRemote| = IC (11)
From equation (11) we know that in order to avoid mai-operation under external fault, the setting ISet must be set higher than IC. Since IC only exists temporarily after fault, another method to avoid mai-operation is to keep ISet to a normal value, but use long time delay to wait until the transient process disappear.
Normally in practical application the second method, i.e. long time delay (0.5s-1.5s) , is used in order not to hurt the sensitivity of the protection criterion under high impedance fault. But consequently, the response speed is slowed down.
The present invention
The present invention is based on travelling wave component calculated by using Bergeron model which already considers the distributed line capacitance.
Because of this, the accurate differential current can be calculated, which excludes the charging current for the distributed capacitors:
-When an internal fault occurs, the differential current calculated is the fault current IF flow through the fault branch, e.g. |ΔiLP (x, t) + ΔiRP(x, t)| = IF;
-When an external fault occurs, the differential current calculated by the present invention is zero, e.g. |ΔiLP (x, t) + ΔiRP (x, t) | = 0.
This allows the present invention to be not influenced by the distributed capacitance along the line so as to ensure the operation speed.
Quick operation speed
The operation speed is very important to the protection; it is one most important demand to protection. When a fault occurs, the system stability and human safe is threatened, the fast isolation is very beneficial to the system stability and human safe. Another two important demands to protection includes reliability and sensitivity. A good protection principle must achieve these three advantages: fast operation speed, reliability and sensitivity.
Because of capacitive current, classical differential protection can’t operate quickly, but wait until transient period passed, so its speed is limited. The present invention is not influenced by distributed capacitance along the line, which unlike classical differential protection, so it could achieve faster operation speed. And also because it is not influenced by capacitive current, it can use lower current threshold and can achieve higher sensitivity.
Considering communication time which depends on line length and communication routing, the operation speed of the present invention is less than 15ms for most cases, while the operation time of classical differential protection is 0.5s-1.5s. The algorithm of the present invention can be used as main protection for LCC DC grid as mentioned, and can be used as backup protection for other types of DC grid and gets much higher operation speed than classical differential protection; and it can also be used as main protection for short line whose communication time is short for other types of DC grid system, or point to point HVDC system.
Good sensitivity on high resistance faults
The present invention has good sensitivity on high resistance faults because it is based on fault component, the influence of load current to the differential protection is erased, while the sensitivity of classical differential protection is decreased by load current.
Extensive adaptability
In this section, the adaptability will be analyzed from two aspects: working principle and operation speed.
Working principle vs. adaptability
From above analysis, it can be obtained that the differential protection principle of the present invention is only related to line parameters, it uses the line
parameter to calculate the current at point ‘x’ , it has no special requirement on the topology and control of the DC system.
Operation speed vs. adaptability
We know from that the operation speed of the present invention is less than 15ms for most cases.
Therefore, we can choose to configure the present invention protection as main protection or backup protection according to the requirement on the operation speed of different DC systems.
For example, for point to point DC line or DC grid based on LCC technology, and point to point DC line with VSC technology, the present invention can be used as either main protection or backup protection.
For DC grid based on VSC technology, the present invention can be used as backup protection, because the requirement on the operation speed is quite high, usually within 5ms. If the length of the transmission line is short, the time delay caused by the communication can be reduced, and then the present invention can also be used as main protection.
It should be pointed out that the performance of the present invention, when configured as backup protection, is much better than the existing backup protection based on differential current, of which the operation time is usually longer than hundreds milliseconds.
Resonances influence
There is distributed line capacitance along the line, because the HVDC line is very long, the line capacitance is large. When a fault occurs, large oscillation in voltage and current appears ( “resonances” ) , some traditional protection principle will be influenced heavily, such as traditional current differential protection, low voltage protection, etc.
But the direction element of the present invention is based on Bergeron model, the “resonances” is considered inherently by this model, it is not influenced by “resonances” .
Simulation
Simulation model
Fig. 7 shows the simulation model, the ±800 kV 4-terminal series MTDC consists of two rectifier stations (R1 and R2) and two inverter stations (I1 and 12) . Total length of the transmission line is 2000 km, including two branch lines (500 km each) and one backbone lines (1000 km) . Each converter station has a configuration with one 12-pulse valve group. Each rectifier converter will have a nominal DC voltage of 400 kV across; each inverter converter will have a nominal DC voltage of 373 kV across, and the HV DC line voltage to ground is about 400 kV (for R1 and I1) or about 800 kV (for R2 and I2) .
The present invention protections, relay 71, 72, are located at the two terminal of the +800kV transmission line as shown in the figure above. And the internal fault is at the end of +800kV line, the external fault is on the +400kV line. And the pole-pole wave impedance Zc is 264 Ω in this case.
Fig. 8 shows the simulation result, an internal fault occurs at 2s and an external fault occurs at 4s. In Fig. 8,
-‘IF’ is the actual current flowing through the fault branch;
-‘ldif Bergeron’ is the differential current calculated by the protection principle based on Bergeron model;
-‘ldif classical’ is the one calculated by the classical differential protection principle.
Internal fault analysis
As shown in Fig. 8, an internal fault occurs at 2s, with fault resistance of 3000ohm.
It should be noted that when internal fault occurs, ‘ldif Bergeron’ is not completely as same as ‘IF’ , the reason lies that the principle based on Bergeron model calculates differential current using fault component, and fault component in this simulation only exist 50ms, so these two current differs after 50ms from fault inception. But during time from fault inception (2s) to 2.05s, the differential calculated
by principle based on Bergeron model is close enough to actual fault current ‘IF’ .
From Fig. 8 it can also be obtained that the differential current classical protection calculated is also near the actual fault current ‘IF’ , but with large waves.
External fault analysis
As shown in Fig. 8, an external fault occurs at 4s.
When an external fault occurs, there should be no fault current theoretically. However the differential current calculated by classical differential protection ( ‘ldif classical’ ) is quite large, even higher than the differential current when internal fault occurs at 2s. So we can observe that the classical differential current protection can’ t distinguish external fault from internal fault in transient process, it must wait until transient process disappears.
‘ldif Bergeron’shows the differential current that the present invention calculated. It can be observed from Fig. 8 that the calculated differential current after the occurrence of external fault is very small, far less than the calculated differential current under internal fault. That is to say, it can distinguish external fault from internal fault effectively.
In a nutshell, the simulation results show that the differential protection based on Bergeron model is less influenced by distributed line capacitance compared with classic differential protection.
Fig. 9 shows a structural module drawing of a DC grid current differential protection system, comprising the following modules:
sampling value acquiring module 901, for acquiring the pole voltage sampling values and the pole current sampling values of the local terminal and remote terminal of the DC grid;
fault component extracting module 902, for calculating fault component pole voltage values according to the pole voltage sampling values in local terminal and remote terminal respectively; and calculating fault component pole current values according to the pole current sampling values in local terminal and remote terminal respectively;
pole-mode transformation module 903, for acquiring fault component mode voltage values by pole-mode transforming said fault component pole voltage values
in local terminal and remote terminal respectively, and acquiring fault component mode current values by pole-mode transforming said fault component pole current voltage values in local terminal and remote terminal respectively;
Bergeron model calculating module 904, for acquiring fault component pole current values at selected point in local terminal and remote terminal respectively, by the calculation for the fault component mode voltage values and the fault component mode current values in local terminal and remote terminal, based on Bergeron model;
current differential protection judging module 905, comprising judging internal fault if the fault component pole current values at selected point in local terminal and remote terminal meet preset current differential protection criterion, then sending fault protection command to activate differential protection, otherwise, the differential protection will not be activated.
The above-identified embodiments are only used for representing several examples of the present invention, which are illustrated in detail, but shall not be understood to limit the protection scope of the present patent. It should be noted that, several modifications and/or improvements may be made for the skilled in the art, without going beyond the technical concept of the present invention, all of which fall into the protection scope of the present invention. Therefore, the protection scope of the present invention is dependent on the accompanied Claims.
Claims (20)
- A DC grid current differential protection method, including the following steps:sampling value acquiring step: acquiring pole voltage sampling values and pole current sampling values in local terminal and remote terminal of a DC line;fault component extracting step: calculating fault component pole voltage values according to the pole voltage sampling values of local terminal and remote terminal respectively; and calculating fault component pole current values according to the pole current sampling values of local terminal and remote terminal respectively;Bergeron model calculating step: acquiring fault component pole current values at selected point on the DC line between the local terminal and the remote terminal, by the calculation of the fault component pole voltage values and the fault component pole current values of local terminal and remote terminal calculated in the fault component extracting step, based on Bergeron model;current differential protection judging step: judging internal fault if the fault component pole current values at selected point of local terminal and remote terminal acquired in the Bergeron model calculating step meet preset current differential protection criterion.
- The method according to claim 1, wherein the DC grid is of bi-pole and the DC line includes a positive pole DC line and a negative pole DC line, the local terminal includes a positive pole local terminal and a positive pole remote terminal, the remote terminal includes a negative pole local terminal and a negative pole remote terminal, the positive pole DC line electrically connects the positive pole local terminal and the positive pole remote terminal, and the negative pole DC line electrically connects the negative pole local terminal and the negative pole remote terminal, distance from the selected point to the positive pole local terminal is same as distance from the selected point to the negative pole local terminal, and distance from the selected point to the positive pole remote terminal is same as distance from the selected point to the negative pole remote terminal, further comprising:pole-mode transformation step: acquiring fault component mode voltage values for each mode of local terminal and remote terminal by pole-mode transforming said fault component pole voltage value for each of the positive pole local terminal, the positive pole remote terminal, the negative pole local terminal and the negative pole remote terminal, and acquiring fault component mode current values for each mode of the local terminal and remote terminal by pole-mode transforming said fault component pole current values for each of the positive pole local terminal, the positive pole remote terminal, the negative pole local terminal and the negative pole remote terminal;the Bergeron model calculating step further comprises:acquiring fault component mode travelling wave voltage values for each mode of local terminal and remote terminal respectively by the calculation of the fault component mode voltage values and the fault component mode current values for each mode of local terminal and remote terminal based on Bergeron model;converting the fault component mode travelling wave voltage values of local terminal and remote terminal respectively into fault component mode travelling wave current values of local terminal and remote terminal;determining fault component mode current values of local terminal and remote terminal at the selected point on the DC line respectively in accordance with the fault component mode travelling wave current values of local terminal and remote terminal;acquiring fault component pole current values for each of the positive pole local terminal and the negative pole local terminal at the selected point on the DC line by mode-pole transforming the fault component mode current values for each mode of local terminal at the selected point, and acquiring fault component pole current values for positive pole remote terminal and negative pole remote terminal at the selected point by mode-pole transforming the fault component mode current values for each mode of remote terminal at the selected point.
- The method according to claim 2, wherein the pole voltage sampling values comprise: uLP (t) , i.e. voltage sampling value of positive pole local terminal; uLN (t) , i.e. voltage sampling value of negative pole local terminal; uRP (t) , i.e. voltage sampling value of positive pole remote terminal; uRN (t) , i.e. voltage sampling value of negative pole remote terminal; wherein t means time;the pole current sampling values comprise: iLP (t) , i.e. current sampling value of positive pole local terminal; iLN (t) , i.e. current sampling value of negative pole local terminal; iRP (t) , i.e. current sampling value of positive pole remote terminal; iRN (t) , i.e. current sampling value of negative pole remote terminal;the fault component pole voltage values comprise: ΔuLP (t) , i.e. fault component voltage value of positive pole local terminal corresponding to uLP (t) ; ΔuLN (t) , i.e. fault component voltage value of negative pole local terminal corresponding to uLN (t) ; ΔuRP (t) , i.e. fault component voltage value of positive pole remote terminal corresponding to uRP (t) ; ΔuRN (t) , i.e. fault component voltage value of negative pole remote terminal corresponding to uRN (t) ;the fault component pole current values comprise: ΔiLP (t) , i.e. fault component current value of positive pole local terminal corresponding to iLP (t) ; ΔiLN (t) , i.e. fault component current value of negative pole local terminal corresponding to iLN (t) ; ΔiRP (t) , i.e. fault component current value of positive pole remote terminal corresponding to iRP (t) ; ΔiRN (t) , i.e. fault component current value of negative pole remote terminal corresponding to iRN (t) ;the fault component mode voltage values comprise: ΔuL0 (t) i.e. fault component common mode voltage value of local terminal; ΔuL1 (t) i.e. fault component differential mode voltage value of local terminal; ΔuR0 (t) i.e. fault component common mode voltage value of remote terminal; ΔuR1 (t) i.e. fault component differential mode voltage value of remote terminal;the fault component mode current values comprise: ΔiL0 (t) i.e. fault component common mode current value of local terminal; ΔiL1 (t) i.e. fault component differential mode current value of local terminal; ΔiR0 (t) i.e. fault component common mode current value of remote terminal; ΔiR1 (t) i.e. fault component differential mode current value of remote terminal;the fault component travelling wave voltage values comprise: ΔuL0+ (t) i.e. fault component common mode forward travelling wave voltage value of local terminal; ΔuL0- (t) i.e. fault component common mode inverse travelling wave voltage value of local terminal; ΔuL1+ (t) i.e. fault component differential mode forward travelling wave voltage value of local terminal; ΔuL0- (t) i.e. fault component differential mode inverse travelling wave voltage value of local terminal; ΔuR0+ (t) i.e. fault component common mode forward travelling wave voltage value of remote terminal; ΔuR0- (t) i.e. fault component common mode inverse travelling wave voltage value of remote terminal; ΔuR1+ (t) i.e. fault component differential mode forward travelling wave voltage value of remote terminal; ΔuR1- (t) i.e. fault component differential mode inverse travelling wave voltage value of remote terminal;the fault component travelling wave current values comprise: ΔiL0+ (t) i.e. fault component common mode forward travelling wave current value of local terminal; ΔiL0- (t) i.e. fault component common mode inverse travelling wave current value of local terminal; ΔiL1+ (t) i.e. fault component differential mode forward travelling wave current value of local terminal; ΔiL1- (t) i.e. fault component differential mode inverse travelling wave current value of local terminal; ΔiR0+ (t) i.e. fault component commonmode forward travelling wave current value of remote terminal; ΔiR0- (t) i.e. fault component common mode inverse travelling wave current value of remote terminal; ΔiR1+ (t) i.e. fault component differential mode forward travelling wave current value of remote terminal; ΔiR1- (t) i.e. fault component differential mode inverse travellingwave current value of remote terminal;the fault component mode current values at the selected point comprise: ΔiL0 (x, t) , i.e. fault component common mode current value at the selected point of local terminal; ΔiL1 (x, t) , i.e. fault component differential mode current value at the selected point of local terminal; ΔiR0 (x, t) , i.e. fault component common mode current value at the selected point of remote terminal; ΔiR1 (x, t) , i.e. fault component differential mode current value at the selected point of remote terminal, wherein x is the selected point;the fault component pole current values at the selected point comprise: ΔiLP (x, t) i.e. fault component pole current value at selected point of positive pole local terminal; ΔiLN (x, t) i.e. fault component pole current value at selected point of negative pole local terminal; ΔiRP (x, t) i.e. fault component pole current value at selected point of positive pole remote terminal; ΔiRN (x, t) i.e. fault component pole current value at selected point of negative pole remote terminal.
- The method according to claim 3, wherein in the fault component extracting step, the fault component pole voltage value and the fault component pole voltage value are calculated in the following way:in the pole-mode transformation step, the fault component mode voltage value and the fault component mode current value are calculated in the following way:In the Bergeron model calculating step comprises:the fault component mode travelling wave voltage values are calculated in the following way:the fault component mode travelling wave current values are calculated in the following way:the fault component mode current values at selected point are calculated in the following way:wherein, v0 is the travelling speed of fault component common mode travelling wave, v1 is the travelling speed of fault component differential mode travelling wave;the fault component pole current values at the selected point are calculated in the following way:
- The method according to claim 3, wherein the current differential protection judging step comprises:if |ΔiLP (x, t) +ΔiRP (x, t) |>Ies is met, judging the condition as positive internal fault; else, if |ΔiLN (x, t) +ΔiRN (x, t) |>Ires is met, judging the condition as negative internal fault, wherein Ires is a preset threshold value;otherwise, the differential protection will not be activated.
- The method according to claim 1, wherein the DC grid is single-pole:the Bergeron model calculating step further comprises:acquiring fault component pole travelling wave voltage values of local terminal and remote terminal respectively by the calculation of the fault component pole voltage values and the fault component mode current values of local terminal and remote terminal based on Bergeron model;converting the fault component pole travelling wave voltage values of local terminal and remote terminal respectively into fault component mode travelling wave current values of local terminal and remote terminal;determining fault component pole current values of local terminal and remote terminal at the selected point on the DC line in accordance with the fault component pole travelling wave current values of local terminal and remote terminal.
- The method according to claim 6, wherein in the fault component extracting step, the fault component pole voltage value and the fault component pole voltage value are calculated in the following way:In the Bergeron model calculating step comprises:the fault component pole travelling wave voltage values are calculated in the following way:the fault component pole travelling wave current values are calculated in the following way:the fault component role current values at selected position are calculated in the following way:
- The method of according to claim 7, wherein in the current differential protection judging step comprises:if |ΔiL (x, t) +ΔiR (x, t) |>Ires is met, judging the condition as internal fault, wherein Ires is a preset threshold value.
- The method according to any one of claim 1 to 8, wherein the current differential protection judging step further comprises:If the condition is judged as internal fault, then sending fault protection command to activate differential protection, otherwise, the differential protection will not be activated.
- A computer program comprising computer program code adapted to perform all of the steps of any one of the above when run on a computer.
- The computer program according to claim 6, embodied on a computer-readable medium.
- A DC grid current differential protection system, comprising the following modules:sampling value acquiring module: acquiring pole voltage sampling values and pole current sampling values in local terminal and remote terminal of a DC line;fault component extracting module: calculating fault component pole voltage values according to the pole voltage sampling values of local terminal and remote terminal respectively; and calculating fault component pole current values according to the pole current sampling values of local terminal and remote terminal respectively;Bergeron model calculating module: acquiring fault component pole current values at selected point on the DC line between the local terminal and the remote terminal, by the calculation of the fault component pole voltage values and the fault component pole current values of local terminal and remote terminal calculated in the fault component extracting module, based on Bergeron model;current differential protection judging module: judging internal fault if the fault component pole current values at selected point of local terminal and remote terminal acquired in the Bergeron model calculating module meet preset current differential protection criterion.
- The system according to claim 12, wherein the DC grid is of bi-pole and the DC line includes a positive pole DC line and a negative pole DC line, the local terminal includes a positive pole local terminal and a positive pole remote terminal, the remote terminal includes a negative pole local terminal and a negative pole remote terminal, the positive pole DC line electrically connects the positive pole local terminal and the positive pole remote terminal, and the negative pole DC line electrically connects the negative pole local terminal and the negative pole remote terminal, distance from the selected point to the positive pole local terminal is same as distance from the selected point to the negative pole local terminal, and distance from the selected point to the positive pole remote terminal is same as distance from the selected point to the negative pole remote terminal, further comprising:pole-mode transformation module: acquiring fault component mode voltage values for each mode of local terminal and remote terminal by pole-mode transforming said fault component pole voltage value for each of the positive pole local terminal, the positive pole remote terminal, the negative pole local terminal and the negative pole remote terminal, and acquiring fault component mode current values for each mode of the local terminal and remote terminal by pole-mode transforming said fault component pole current values for each of the positive pole local terminal, the positive pole remote terminal, the negative pole local terminal and the negative pole remote terminal;the Bergeron model calculating module further comprises:acquiring fault component mode travelling wave voltage values for each mode of local terminal and remote terminal respectively by the calculation of the fault component mode voltage values and the fault component mode current values for each mode of local terminal and remote terminal based on Bergeron model;converting the fault component mode travelling wave voltage values of local terminal and remote terminal respectively into fault component mode travelling wave current values of local terminal and remote terminal;determining fault component mode current values of local terminal and remote terminal at the selected point on the DC line respectively in accordance with the fault component mode travelling wave current values of local terminal and remote terminal;acquiring fault component pole current values for each of the positive pole local terminal and the negative pole local terminal at the selected point on the DC line by mode-pole transforming the fault component mode current values for each mode of local terminal at the selected point, and acquiring fault component pole current values for positive pole remote terminal and negative pole remote terminal at the selected point by mode-pole transforming the fault component mode current values for each mode of remote terminal at the selected point.
- The system according to claim 13, wherein the pole voltage sampling values comprise: uLP (t) , i.e. voltage sampling value of positive pole local terminal; uLN (t) , i.e. voltage sampling value of negative pole local terminal; uRP (t) , i.e. voltage sampling value of positive pole remote terminal; uRN (t) , i.e. voltage sampling value of negative pole remote terminal; wherein t means time;the pole current sampling values comprise: iLP (t) , i.e. current sampling value of positive pole local terminal; iLN (t) , i.e. current sampling value of negative pole local terminal; iRP (t) , i.e. current sampling value of positive pole remote terminal; iRN (t) , i.e. current sampling value of negative pole remote terminal;the fault component pole voltage values comprise: ΔuLP (t) , i.e. fault component voltage value of positive pole local terminal corresponding to uLP (t) ; ΔuLN (t) , i.e. fault component voltage value of negative pole local terminal corresponding to uLN (t) ; ΔuRP (t) , i.e. fault component voltage value of positive pole remote terminal corresponding to uRP (t) ; ΔuRN (t) , i.e. fault component voltage value of negative pole remote terminal corresponding to uRN (t) ;the fault component pole current values comprise: ΔiLP (t) , i.e. fault component current value of positive pole local terminal corresponding to iLP (t) ; ΔiLN (t) , i.e. fault component current value of negative pole local terminal corresponding to iLN (t) ; ΔiRP (t) , i.e. fault component current value of positive pole remote terminal corresponding to iRP (t) ; ΔiRN (t) , i.e. fault component current value of negative pole remote terminal corresponding to iRN (t) ;the fault component mode voltage values comprise: ΔuL0 (t) i.e. fault component common mode voltage value of local terminal; ΔuL1 (t) i.e. fault component differential mode voltage value of local terminal; ΔuR0 (t) i.e. fault component common mode voltage value of remote terminal; ΔuR1 (t) i.e. fault component differential mode voltage value of remote terminal;the fault component mode current values comprise: ΔiL0 (t) i.e. fault component common mode current value of local terminal; ΔiL1 (t) i.e. fault component differential mode current value of local terminal; ΔiR0 (t) i.e. fault component common mode current value of remote terminal; ΔiR1 (t) i.e. fault component differential mode current value of remote terminal;the fault component travelling wave voltage values comprise: ΔuL0+ (t) i.e. fault component common mode forward travelling wave voltage value of local terminal; ΔuL0- (t) i.e. fault component common mode inverse travelling wave voltage value of local terminal; ΔuL1+ (t) i.e. fault component differential mode forward travelling wave voltage value of local terminal; ΔuL0- (t) i.e. fault component differential mode inverse travelling wave voltage value of local terminal; ΔuR0+ (t) i.e. fault component common mode forward travelling wave voltage value of remote terminal; ΔuR0- (t) i.e. fault component common mode inverse travelling wave voltage value of remote terminal; ΔuR1+ (t) i.e. fault component differential mode forward travelling wave voltage value of remote terminal; ΔuR1- (t) i.e. fault component differential mode inverse travelling wave voltage value of remote terminal;the fault component travelling wave current values comprise: ΔiL0+ (t) i.e. fault component common mode forward travelling wave current value of local terminal; ΔiL0- (t) i.e. fault component common mode inverse travelling wave current value of local terminal; ΔiL1+ (t) i.e. fault component differential mode forward travelling wave current value of local terminal; ΔiL1- (t) i.e. fault component differential mode inverse travelling wave current value of local terminal; ΔiR0+ (t) i.e. fault component common mode forward travelling wave current value of remote terminal; ΔiR0- (t) i.e. fault component common mode inverse travelling wave current value of remote terminal; ΔiR1+ (t) i.e. fault component differential mode forward travelling wave current value of remote terminal; ΔiR1- (t) i.e. fault component differential mode inverse travelling wave current value of remote terminal;the fault component mode current values at the selected point comprise: ΔiL0 (x, t) , i.e. fault component common mode current value at the selected point of local terminal; ΔiL1 (x, t) , i.e. fault component differential mode current value at the selected point of local terminal; ΔiR0 (x, t) , i.e. fault component common mode current value at the selected point of remote terminal; ΔiR1 (x, t) , i.e. fault component differential mode current value at the selected point of remote terminal, wherein x is the selected point;the fault component pole current values at the selected point comprise: ΔiLP (x, t) i.e. fault component pole current value at selected point of positive pole local terminal; ΔiLN (x, t) i.e. fault component pole current value at selected point of negative pole local terminal; ΔiRP (x, t) i.e. fault component pole current value at selected point of positive pole remote terminal; ΔiRN (x, t) i.e. fault component pole current value at selected point of negative pole remote terminal.
- The system according to claim 14, wherein in the fault component extracting module, the fault component pole voltage value and the fault component pole voltage value are calculated in the following way:in the pole-mode transformation module, the fault component mode voltage value and the fault component mode current value are calculated in the following way:In the Bergeron model calculating module comprises:the fault component mode travelling wave voltage values are calculated in the following way:the fault component mode travelling wave current values are calculated in the following way:the fault component mode current values at selected point are calculated in the following way:wherein, v0 is the travelling speed of fault component common mode travelling wave, v1 is the travelling speed of fault component differential mode travelling wave;the fault component pole current values at the selected point are calculated in the following way:
- The system according to claim 14, wherein the current differential protection judging module comprises:if |ΔiLP (x, t) +ΔiRP (x, t) |>Ires is met, judging the condition as positive internal fault; else, if |ΔiLN (x, t) +ΔiRN (x, t) |>Ires is met, judging the condition as negative internal fault, wherein Ires is a preset threshold value;otherwise, the differential protection will not be activated.
- The system according to claim 12, wherein the DC grid is single-pole:the Bergeron model calculating module further comprises:acquiring fault component pole travelling wave voltage values of local terminal and remote terminal respectively by the calculation of the fault component pole voltage values and the fault component mode current values of local terminal and remote terminal based on Bergeron model;converting the fault component pole travelling wave voltage values of local terminal and remote terminal respectively into fault component mode travelling wave current values of local terminal and remote terminal;determining fault component pole current values of local terminal and remote terminal at the selected point on the DC line in accordance with the fault component pole travelling wave current values of local terminal and remote terminal.
- The system according to claim 17, wherein in the fault component extracting module, the fault component pole voltage value and the fault component pole voltage value are calculated in the following way:In the Bergeron model calculating module comprises:the fault component pole travelling wave voltage values are calculated in the following way:the fault component pole travelling wave current values are calculated in the following way:the fault component role current values at selected position are calculated in the following way:
- The system of according to claim 18, wherein in the current differential protection judging module comprises:if |ΔiL (x, t) +ΔiR (x, t) |>IRes is met, judging the condition as internal fault, wherein Ires is a preset threshold value.
- The system according to any one of claim 12 to 19, wherein the current differential protection judging module further comprises:If the condition is judged as internal fault, then sending fault protection command to activate differential protection, otherwise, the differential protection will not be activated.
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