Classifications

• • 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
• • 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
• • 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
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
  • H02H3/00—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection
  • H02H3/42—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection responsive to product of voltage and current
• • 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

Definitions

• the present application relates to a DC grid 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.
• Pilot protection can be a good candidate
• Pilot protection based on direction comparison may be one of good candidates for HVDC/DC Grid protection which has many unique advantages such like:
• pilot protection needs communication between terminals, it has much faster speed than current differential protection, because it is based on the initial travelling wave front and will not be influenced by the HVDC control or transient charging current. Generally, its total operation speed may be ⁇ 15 ms including communication delay and the time spend on algorithm may be ⁇ 1 ms.
• the pilot protection can be used for both two-terminal HVDC system and multi-terminal DC grid system including the LCC DC grid, and it is independent of smoothing reactor or DC filter.
• the direction of the fault is determined by the initial travelling wave by both ends, after exchanging the direction information by optical fiber, the pilot direction protection will make the decision to operate or not.
• the pilot direction protection working principle is shown in Fig. 3a, Fig. 3b, Fig. 3c and Fig . 4.
• Fig. 3a shows the situation when internal fault occurs, both Relay A and Relay B will determine the forward fault, the Relay A will send permission signal to Relay B, and Relay B will send permission signal to Relay A.
• the pilot direction protection will operate when local relay determines the forward fault and receives the permit signal from the remote side, as shown in Fig. 4.
• Fig. 3b shows the situation when an external fault occurs at the backside of Relay A, under this condition, Relay A will determine inverse fault, and Relay B determine forward fault, both the relays will not operate.
• Fig. 3c shows the situation when an external fault occurs at the backside of Relay B, under this condition, Relay A will determine forward fault, and Relay B determine inverse fault, both the relays will not operate either.
• Equations 1 and 2 show the existing direction element based on travelling wave:
• ⁇ u′and ⁇ i′ is the voltage and current of initial travelling wave.
• the algorithm above is simple and clear. It can work in DC system in theory generally. When equation 1 is satisfied, forward direction fault is determined, and when equation 2 is satisfied, inverse direction fault is determined.
• the algorithm above has a very obvious disadvantage that it will FAIL to operate for total-reflection boundary.
• the protection cannot detect the fault direction. And as a result, it will fail to operate even for internal fault in theory.
• one aspect of the present invention provides a DC grid protection method, comprising the following steps:
• fault detecting step acquiring fault component travelling wave current value and fault component travelling wave voltage value of a DC line, the fault component travelling wave current value at fault occurring time is ⁇ i and the fault component travelling wave voltage value at fault occurring time is ⁇ u, wherein forward direction is defined extending from the protection to the DC line;
• fault determining step if the fault component travelling wave current value ⁇ i at fault occurring time and the fault component travelling wave voltage value ⁇ u at fault occurring time meet following criterions, performing fault direction determination step, the criterions include:
• fault direction determination step determining the fault as forward fault.
• the forward value is wherein i set is the predetermined threshold which is bigger than zero, and Z C is wave impedance of the line.
• the forward value is wherein k is a constant, 0 ⁇ k ⁇ 1, i set is the predetermined threshold which is bigger than zero, and Z C is wave impedance of the line.
• the sensitivity requirement includes:
• the fault determining step includes:
• the criterion includes:
• k is a constant, 0 ⁇ k ⁇ 1, i set is the predetermined threshold which is bigger than zero, and Z C is wave impedance of the line.
• the predetermined threshold i set is bigger than current noise of the line.
• the protection includes local terminal protection and remote terminal protection, forward direction of the fault component travelling wave current value acquired by the local terminal protection is defined extending from the local terminal protection to the DC line, forward direction of the fault component travelling wave current value acquired by the remote terminal protection is defined extending from the remote terminal protection to the DC line;
• fault direction determination step further includes: :
• the local terminal protection determines the fault as forward fault, sends forward fault detected in local message to the remote terminal protection, and determines internal fault when receives forward fault detected in remote message from the remote terminal protection;
• the remote terminal protection determines the fault as forward fault, sends forward fault detected in remote message to the local terminal protection, and determines internal fault when receives forward fault detected in local message from the local terminal protection.
• 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 protection system, comprising the following modules:
• fault detecting module acquiring fault component travelling wave current value and fault component travelling wave voltage value of a DC line, the fault component travelling wave current value at fault occurring time is ⁇ i and the fault component travelling wave voltage value at fault occurring time is ⁇ u, wherein forward direction is defined extending from the protection to the DC line;
• fault determining module if the fault component travelling wave current value ⁇ i at fault occurring time and the fault component travelling wave voltage value ⁇ u at fault occurring time meet following criterions, performing fault direction determination module, the criterions include:
• fault direction determination module determining the fault as forward fault.
• the forward value is wherein i set is the predetermined threshold which is bigger than zero, and Z C is wave impedance of the line.
• the forward value is wherein k is a constant, 0 ⁇ k ⁇ 1, i set is the predetermined threshold which is bigger than zero, and Z C is wave impedance of the line.
• the sensitivity requirement includes:
• the fault determining module includes:
• the criterion includes:
• k is a constant, 0 ⁇ k ⁇ 1, i set is the predetermined threshold which is bigger than zero, and Z C is wave impedance of the line.
• fault eliminating module activating protect operation if forward fault is determined.
• the predetermined threshold i set is bigger than current noise of the line.
• the protection includes local terminal protection and remote terminal protection, forward direction of the fault component travelling wave current value acquired by the local terminal protection is defined extending from the local terminal protection to the DC line, forward direction of the fault component travelling wave current value acquired by the remote terminal protection is defined extending from the remote terminal protection to the DC line;
• fault direction determination module further includes: :
• the local terminal protection determines the fault as forward fault, sends forward fault detected in local message to the remote terminal protection, and determines internal fault when receives forward fault detected in remote message from the remote terminal protection;
• the remote terminal protection determines the fault as forward fault, sends forward fault detected in remote message to the local terminal protection, and determines internal fault when receives forward fault detected in local message from the local terminal protection.
• the present invention determines the direction of the fault component travelling wave current value by the condition one: product of the fault component travelling wave current value ⁇ i and the fault component travelling wave voltage value ⁇ u less than a forward value which is bigger than zero and is associating with a predetermined threshold; and activates the protect operation when meet the condition two: absolute value of the fault component travelling wave current value ⁇ i or absolute value of the fault component travelling wave voltage value ⁇ u meets the sensitivity requirement which is determined by the predetermined threshold. Because the fault component travelling wave voltage value is superimposed in the total-reflection situation, so that the present invention can overcome the problem in the prior art and implements the pilot protection in the DC system, especially in the HVDC system.
• 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. 3a illustrates schematically the situation when an internal fault occurs
• Fig. 3b illustrates schematically the situation when an external fault occurs at the backside of Relay A
• Fig. 3c illustrates schematically the situation when an external fault occurs at the backside of Relay B
• Fig. 4 illustrates schematically the pilot protection logic in accordance with the prior art
• Fig. 5 shows a flow-process diagram illustrating a DC grid protection method in accordance with the present invention
• Fig. 6 illustrates schematically the traveling wave of a line
• Fig. 7 illustrates schematically the situation when an inverse fault occurs
• Fig. 8 illustrates schematically the situation when a forward fault occurs
• Fig. 9 illustrates schematically the total-reflection situation
• Fig. 10 shows a structural module drawing of a DC grid protection system
• Fig. 11 illustrates schematically travelling wave based protection in accordance with the prior art
• Fig. 12 illustrates schematically the distributed line capacitance of the line when external fault occurs
• Fig. 13 illustrates schematically the distributed line capacitance of the line when internal fault occurs
• Fig. 14 shows the simulation model
• Fig. 15a shows the simulation result of the fault component travelling wave voltage value, the fault component travelling wave current value and the product of both in local terminal when the internal fault occurs;
• Fig. 15b shows the simulation result of the fault component travelling wave voltage value, the fault component travelling wave current value and the product of both in remote terminal when the internal fault occurs;
• Fig. 16a shows the simulation result of the fault component travelling wave voltage value, the fault component travelling wave current value and the product of both in local terminal when the external fault occurs;
• Fig. 16b shows the simulation result of the fault component travelling wave voltage value, the fault component travelling wave current value and the product of both in remote terminal when the external fault occurs.
• Fig. 5 shows a flow-process diagram illustrating a DC grid protection method in accordance with the present invention, including the following steps:
• step 501 acquiring fault component travelling wave current value and fault component travelling wave voltage value of a DC line, the fault component travelling wave current value at fault occurring time is ⁇ i and the fault component travelling wave voltage value at fault occurring time is ⁇ u, wherein forward direction is defined extending from the protection to the DC line;
• step 502 if the fault component travelling wave current value ⁇ i at fault occurring time and the fault component travelling wave voltage value ⁇ u at fault occurring time meet following criterions, performing fault direction determination step, the criterions include:
• step 503 determining the fault as forward fault.
• the protection detects the fault by any method of the prior art in step 501. But the protection does not determine whether the fault is forward fault or internal fault in step 501.
• the grid may be divided into no-fault network and fault-component network when detected the fault, so the fault component travelling wave current value and the fault component travelling wave voltage value are the travelling wave current/voltage values in the fault-component network.
• the method of dividing the grid into no-fault network and fault-component network can implement with any method of the art.
• Fig. 6 illustrates schematically the traveling wave of a line.
• the voltage and current value of any location in the line can be expressed by equation (3) according to the principle of electrotechnics:
• Z C is wave impedance
• the u (t) is the travelling wave voltage value at time t of point A and i (t) is the travelling wave current value at time t of point A.
• step 502 the fault component travelling wave current value ⁇ i at fault occurring time and the fault component travelling wave voltage value ⁇ u at fault occurring time should meet the following criterions:
• criterion one: product of the fault component travelling wave current value ⁇ i at fault occurring time and the fault component travelling wave voltage value ⁇ u at fault occurring time less than a forward value which is bigger than zero and is associating with a predetermined threshold;
• the criterion one is used to determine whether the fault component travelling wave current value ⁇ i at fault occurring time is forward or inverse. As flowing from the equipment to the protection is the forward direction of the fault component travelling wave current value ⁇ i at fault occurring time, so ⁇ i>0 when the current flows from the equipment to the protection and ⁇ i ⁇ 0 when the current flows from the protection to the equipment.
• the criterion one includes situation that the product is between 0 to the forward value. Because the forward value is associated with a predetermined threshold which determines the sensitivity requirement in criterion two, so the situation that the product is between 0 to the forward value can be excluded by setting a proper sensitivity requirement.
• the criterion two is used to determine the sensitivity requirement.
• the ⁇ i or ⁇ u meet the sensitivity requirement, it can trigger the step 503.
• the magnitude of ⁇ u will increase or even be doubled according to the traveling wave reflection theory. This can help to accelerate the forward fault determination. And it can implement the pilot protection in DC system, especially in HVDC system.
• the forward value is wherein i set is the predetermined threshold which is bigger than zero, and Z C is wave impedance of the line.
• the criterion one embodies as:
• the forward value is wherein k is a constant, 0 ⁇ k ⁇ 1, i set is the predetermined threshold which is bigger than zero, and Z C is wave impedance of the line.
• the criterion one embodies as: k is a reliability factor, which 0.5 is a typical value. Introducing k as a reliability factor in criterion one can decrease the forward value so that it can detect the forward fault efficiently.
• the sensitivity requirement includes:
• step 502 includes:
• the criterion includes: wherein k is a constant, 0 ⁇ k ⁇ 1, i set is the predetermined threshold which is bigger than zero, and Z C is wave impedance of the line.
• Fig. 7 illustrates schematically the situation when an inverse fault occurs, wherein the protection is relay 71.
• Inverse fault by definition, is the fault 72 occurred at the back side of point A as shown in Fig. 7. If designed properly, the protection relay should not operate under an inverse fault.
• Fig. 8 illustrates schematically the situation when a forward fault occurs, wherein the protection is relay 81.
• Forward fault is the fault 82 occurred at the front side of point A as shown in Fig. 8. If designed properly, the protection relay 81 should operate under a forward fault. In Fig. 8, forward direction is defined extending from the relay 81 to the DC line 83. Therefore we have,
• the protection relay 81 will determine forward fault as long as the second criterion of equation 5 is fulfilled, i.e.
• the predetermined threshold i set is bigger than current noise of the line.
• the setting i set is to ensure the reliability of the relay. If
• the only demand for setting i set is that it should be set larger than noise. So we can know that the setting i set will be a relatively small value, which is helpful to ensure the sensitivity of the direction element.
• the protection includes local terminal protection and remote terminal protection, forward direction of the fault component travelling wave current value acquired by the local terminal protection is defined extending from the local terminal protection to the DC line, forward direction of the fault component travelling wave current value acquired by the remote terminal protection is defined extending from the remote terminal protection to the DC line;
• fault direction determination step further includes: :
• the local terminal protection determines the fault as forward fault, sends forward fault detected in local message to the remote terminal protection, and determines internal fault when receives forward fault detected in remote message from the remote terminal protection;
• the remote terminal protection determines the fault as forward fault, sends forward fault detected in remote message to the local terminal protection, and determines internal fault when receives forward fault detected in local message from the local terminal protection.
• the local terminal protection can communicate with the remote terminal protection in DC system.
• the local terminal protection and the remote terminal protection detected the forward fault, it means the fault is occurring between the local terminal protection and the remote terminal protection. This fault is what means internal fault.
• Both the local terminal protection and the remote terminal protection should activate the internal fault protect operation respectively. And if one of the protections detected the inverse fault, it means the external fault is occurring. And in generally, for the external fault, no protections operate.
• Fig. 10 shows a flow-process diagram illustrating a DC grid protection method in accordance with the present system, comprising the following modules:
• fault detecting module 1001 acquiring fault component travelling wave current value and fault component travelling wave voltage value of a DC line, the fault component travelling wave current value at fault occurring time is ⁇ i and the fault component travelling wave voltage value at fault occurring time is ⁇ u, wherein forward direction is defined extending from the protection to the DC line;
• fault determining module 1002 if the fault component travelling wave current value ⁇ i at fault occurring time and the fault component travelling wave voltage value ⁇ u at fault occurring time meet following criterions, performing fault direction determination module, the criterions include:
• fault direction determination module 1003 determining the fault as forward fault.
• the present invention uses the travelling wave based direction element in DC system.
• the method of the present invention can avoid mai-operation due to lightning disturbance by introducing time delay or differential mode.
• travelling waves will appear in the line when a lightning occurs in or somewhere near the line.
• the protection should not mal-operate because of travelling wave.
• One usual method to avoid mal-operate is to use the time delay, for example, existing travelling wave protection which has been used in HVDC lines for years uses time delay to avoid mai-operation from lightning, whose operation characteristics is shown as Fig. 11, which will operate only when four voltage sampling points are larger than the threshold.
• the present invention’s algorithm can also use such delay to make sure the reliability considering of the lightning wave is very short.
• the fault occurs in the line because of lightning.
• the protection should operate to isolate the fault.
• the fault caused by lightning may be metal fault or fault with high resistance.
• the present invention s method has good sensitivity on high resistance faults because it is based on fault direction determination rather than amplitude determination.
• While the present invention s method has high tolerance for high impedance because it is based on fault direction determination, not based on the amplitude of travelling wave. Even when a fault with high resistance occurs, the fault direction in both line ends can be determined clearly.
• ‘1’ refers the positive direction
• ‘0’ is the negative direction, i.e. only one bit is transferred in the communication channel, so small bandwidth is needed.
• the direction information used in the present invention has little requirement on synchronization. From this point of view, the principle is immune to channel asymmetry.
• the direction element the present invention is only related to line parameter Z C and the relationship between travelling wave voltage and travelling wave current. It has no special requirement on the topology and control of the DC system.
• the protection principle of the present invention is based on the initial travelling wave; therefore the typical operation time for direction element will be very fast, normally less than 1ms. Considering the communication time between 1ms-20ms depending on the line length, the pilot protection will operate within 2ms-21ms.
• pilot protection as main protection or backup protection according to the requirement on the operation speed of different DC systems.
• pilot protection can be used as either main protection or backup protection.
• pilot protection can be used as backup protection, because the requirement on the operation speed is quite high, usually within 5ms. Ifthe length of the transmission line is short, the time delay caused by the communication can be reduced, and then the present invention’s pilot protection can also be used as main protection.
• pilot protection 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.
• Fig. 12 shows the situation when an external fault occurs.
• an external fault occurs, there is current flowing through the distributed line capacitance, which will increase the differential current. Because of complex charge and discharge process of line capacitance and inductance, there will be large amount of high frequency components appears in the current.
• Fig. 13 shows the situation when an internal fault occurs, similar to external fault, the differential current also consists rich high frequency components which will exist for a relatively long time. Therefore it also needs time to calculate differential current with acceptable accuracy, which increases the operation time as well.
• the present invention s protection principle is based on travelling wave process during the transient state after fault inception.
• the complete distributed transmission line model is considered to calculate the traveling wave.
• direction element is based on travelling wave, which uses the feature of charge and discharge process of line capacitance and inductance. It is not influenced by “resonances” .
• aLCC DC Grid system is used to demonstrate the present invention method.
• pilot protection can be used for both complex system like DC Grid (LCC or VSC) and simple system like two-terminal HVDC system (HVDC classic and VSC HVDC) .
• the ⁇ 800 kV 4-terminal series MTDC consists of tow rectifier stations R1, R2 and tow inverter stations I1, I2. 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.
• the lower converter stations R1, I1 have been connected to earth electrodes normally.
• the upper converter stations R2, I2 also have earth electrodes but disconnected from the transmission line normally.
• 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 (refer to Section 2.5) , 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 protection relay 141 is located at one terminal of the +400kV transmission line as shown in the figure 15. And the internal fault is at the end of +400kV line, the external fault is at the beginning of the +800kV line. And the pole-pole wave impedance Zc is 264 ⁇ in this case.
• Relay 141 is the local terminal protection communicated with the relay 142 which is the remote local terminal protection.
• the pilot protection will detect the external fault and will not trip reliably.

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• 2014
  • 2014-11-13 EP EP14905956.0A patent/EP3218978A4/de not_active Withdrawn
  • 2014-11-13 WO PCT/CN2014/091027 patent/WO2016074199A1/en active Application Filing
  • 2014-11-13 CN CN201480078461.9A patent/CN106463950B/zh active Active

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