CN117638819A - Multistage cooperative relay protection method suitable for independent micro-grid - Google Patents
Multistage cooperative relay protection method suitable for independent micro-grid Download PDFInfo
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Abstract
The invention discloses a multistage cooperative relay protection method suitable for an independent micro-grid, which comprises the following steps: 1. analyzing the control characteristics of the micro-grid structure and a typical power supply to obtain the voltage and current change rules of the independent micro-grid before and after faults; 2. setting voltage protection criteria at the busbar level of the micro-grid according to the voltage-current change rule, determining that a fault occurs and sending an alarm signal to a control system; 3. aiming at the fault of the double-end power line section, the first measure is as follows: adopting current differential protection to meet the requirement of speediness; and a second measure: impedance measuring elements are arranged at two ends of the line to identify faults in a positive direction area of the line, so that impedance direction longitudinal protection is formed; aiming at the situation that only a load branch circuit with a single-side power supply connected in fails, when the forward direction fails, the voltage of the micro-grid drops, the current of the branch circuit increases, and the protection requirement is met by utilizing overcurrent information on the basis of network-level protection starting. The invention is not affected by the fault type and has better reliability.
Description
Technical Field
The invention belongs to the technical field of micro-grid relay protection, and particularly relates to a multistage cooperative relay protection method suitable for an independent micro-grid.
Background
With the increasing severity of environmental pollution and energy shortage problems worldwide, micro-grid technology has rapidly evolved. Compared with the traditional power grid, the micro-grid supplies electric energy through an internal power supply as a load, the research focus is focused on the aspects of power supply optimal configuration, coordination stability control and the like, and the research on the relay protection level of the micro-grid is relatively less.
Conventional distribution line relay protection is typically multi-segment current protection for single-ended power supply radial lines that are supplied with power from one end power supply to a downstream load feeder. And distributed power sources in the micro-grid are distributed and connected into different positions, so that the possibility of bidirectional flow of system fault current exists. The fault output current of the power electronic interface power supply is far smaller than that of the traditional synchronous power supply, and the load current and the transition resistance have great influence on the fault current. This would make it difficult for conventional distribution line protection to reliably act in a microgrid.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide a multistage cooperative relay protection method which is not influenced by fault types and has better reliability and is suitable for independent micro-grids.
The above object of the present invention is achieved by the following technical solutions:
a multistage cooperative relay protection method suitable for independent micro-grids comprises the following steps:
step 1, analyzing the control characteristics of a micro-grid structure and a typical power supply based on an equivalent model of an energy storage power supply and a photovoltaic power supply in the micro-grid under normal operation conditions and fault conditions to obtain the voltage and current change rules of the independent micro-grid before and after the fault;
step 2, setting voltage protection criteria at the busbar level of the micro-grid according to the voltage and current change rule obtained in the step 1, determining the occurrence of faults, sending alarm signals to a control system, and judging fault lines;
step 3, aiming at the fault of the double-end power supply line section, two protection measures can be selectively adopted, wherein the first protection measure is to adopt current differential protection to meet the requirement of rapidity; the second protection measure is: impedance measuring elements are arranged at two ends of the line to identify faults in a positive direction area of the line; when the impedance value measured at one end enters the range of the action equation, the line positive direction fault can be judged, and a positive direction fault signal is transmitted to the opposite end of the line through the power line carrier channel, so that impedance type direction longitudinal protection is formed; aiming at the situation that only a load branch circuit with a single-side power supply connected in fails, when the forward direction fails, the voltage of the micro-grid drops, the current of the branch circuit increases, and the protection requirement is met by utilizing overcurrent information on the basis of network-level protection starting.
In step 1, the micro-grid system structure comprises an energy storage power supply and a photovoltaic power supply, wherein the energy storage power supply and the photovoltaic power supply are respectively connected to a micro-grid public bus, direct-current electric energy is converted into alternating-current electric energy through a converter, and electric energy requirements are provided for alternating-current loads; when the output power of the photovoltaic power supply is smaller than the load demand, the energy storage power supply outputs power; when the output power of the photovoltaic power supply is larger than the load demand, the photovoltaic power supply inputs power to the energy storage power supply.
In step 1, the process of obtaining the voltage and current change rule of the independent micro-grid before and after the fault according to the control characteristic of the energy storage power supply includes:
the energy storage is controlled by V/f, and under the normal operation condition, the voltage of the grid-connected point of the energy storage power supply can correctly track the reference value of the control link, which is expressed as:
U＝U ref (1)
in U ref The reference value of the voltage of the grid-connected point of the energy storage power supply;
when the micro-grid faults, the micro-grid integral voltage drops under the influence of faults and the control capability of an energy storage power supply, namely:
U＜U ref (2)
when the output current of the energy storage power supply reaches the upper limit of the allowable amplitude of the converter, the voltage of the grid-connected point of the energy storage power supply drops; the grid-connected point voltage of the power supply is determined by the maximum output current of the converter, the equivalent impedance of an external system of the power supply and the impedance of the filter; the fault equivalent model of the energy storage power supply is as follows:
in the middle ofFor the voltage of the grid-connected point of the energy storage power supply, < >>For maximum output current of energy storage power supply converter, Z ex Equivalent impedance for external system of energy storage power supply Z w Equivalent impedance of the energy storage power filter;
when the SPWM modulation coefficient of the power supply converter is limited so that the outlet voltage of the converter is limited, the voltage of the energy storage power supply grid-connected point drops; under the condition that the modulation coefficient is limited, the outlet of the converter only contains positive sequence voltage, and the voltage of the grid-connected point of the power supply is determined by the voltage of the outlet of the converter and the voltage drop on the filter impedance; the fault positive and negative sequence equivalent model of the energy storage power supply is as follows:
in the middle ofAnd->Positive and negative sequence voltages of grid-connected points of the energy storage power supply are respectively +.>For the limited value of the energy storage power supply converter outlet voltage, < + >>Outputting positive sequence current for energy storage power supply Z w1 Positive sequence equivalent impedance Z of energy storage power supply outlet filter L2 Is the negative sequence equivalent impedance between the fault point and the power supply grid point, Z s2 Is the internal negative sequence impedance of the energy storage power supply, +.>Is the fault point negative sequence voltage.
In step 1, the process of obtaining the voltage and current change rule of the independent micro-grid before and after the fault according to the control characteristic of the photovoltaic power supply includes:
the photovoltaic power supply is controlled by PQ, and when the micro-grid normally runs or fails and the voltage of the power grid-connected point drops, the output current of the photovoltaic power supply is adjusted to the output power reference value in time, and the output current is changed; for the purpose of improving the output characteristics, the photovoltaic power supply outputs only a positive sequence current component, and the current reference value of the control link can be expressed as:
wherein i is PVd 、i PVq D and q axis components of the output current reference value of the photovoltaic power supply, u PV1d 、u PV1q D and q axis components of positive sequence voltage of photovoltaic power supply grid connection point respectively, M= (u) PV1d ) 2 +(u PV1q ) 2 P and Q are reference values for output active power and reactive power, respectively.
In the step 2, the fault is identified by utilizing the busbar voltage change of the micro-grid; setting voltage protection criteria as follows:
|ΔU φ |≥k d U N (6)
in the formula DeltaU φ Measuring voltage amplitude values for each bus node; k (k) d Taking 0.9 as a voltage starting coefficient; u (U) N Rated voltage of the system; when the bus voltage change condition meets the protection criterion, the occurrence of faults can be determined, an alarm signal is sent to the control system, and fault line judgment is carried out.
In step 3, the current differential protection operation criterion is:
in the method, in the process of the invention,respectively measuring calculated values of currents at two sides of a line; i OP.0 Is the minimum threshold for the action of the vehicle,
I OP.0 ＝K sen I kmin ，K sen to start sensitivity, I kmin Taking the minimum short-circuit current value of the primary side to be cut off; k is the braking coefficient, taking 0.5.
In step 3, when the current differential protection measures that the included angle between the fault phases on both sides of the fault line is greater than 90 °, the power differential protection is set to identify the line fault under the nonmetallic fault condition, and the action equation of the power differential protection is as follows:
|P P +P Q |＞K res |P P -P Q | (8)
wherein P is P And P Q Power values, |P, are measured for both ends of the line, respectively P +P Q I and P P -P Q I can be referred to as the actuation power value and the brake power value, K, of the power differential protection, respectively res Taking 0.5 as a power braking coefficient; when a nonmetallic fault occurs in the micro-grid line, the sum of the measured power values at the two ends of the line is the sum of the line loss power and the transition resistance loss power of the fault point, and the sum is obviously larger than the difference of the measured power values at the two ends of the line.
In step 3, when both ends of the line are judged to be positive-direction faults, faults in the line area can be identified, and a protection action equation of impedance-type direction longitudinal protection is as follows:
z in P And Z Q Respectively measuring impedance values for two sides of the line, Z L K is the line impedance value rel.1 Taking 1.3 as the reliable coefficient of the action equation.
In step 3, under the condition that a nonmetallic fault occurs in the line section and the transition resistance of the fault point is large (generally larger than 1Ω in the low-voltage micro-grid line is larger), the negative sequence fault additional electromotive force is not generated by each distributed power supply, and a negative sequence fault component current phase comparison type protection criterion is set as shown in the formula:
in delta m In order to protect the corner of the criteria,negative sequence fault component currents, delta, on both sides of the fault line respectively set The angle is set for the criterion. Considering that the included angle of negative sequence currents at two sides of a line is smaller than 90 degrees, the maximum transfer error of the combined current transformer is not more than15 DEG, setting a criterion angle delta set Taken at 105 °.
Moreover, for the load branch circuit with power supply access on only one side, setting a protection criterion as follows:
I m ＞K rel.2 I Lmax (11)
wherein I is m For measuring current locally in load branch, K rel.2 For the over-current protection reliability coefficient, 1.2, I Lmax The current corresponds to the maximum load current for the present branch.
The invention has the advantages and positive effects that:
the invention provides a micro-grid protection scheme comprising network-level protection, line section protection and branch local protection, which is used for carrying out coverage protection on different types of lines of an independent micro-grid through mutual cooperation of different protection principles, and simultaneously meeting the requirements of protection reliability and rapidity. Further, through PSCAD/EMTDC simulation verification, the rationality of the protection scheme is verified, and the simulation result shows that the provided protection scheme can reliably act and is not influenced by the fault type.
Drawings
FIG. 1 is a flow chart of an independent micro-grid relay protection method based on multi-level network cooperation;
FIG. 2 is a block diagram of a micro-grid system of the present invention;
FIG. 3 is a graph of a micro grid bus voltage (fault # 1) according to an embodiment of the present invention;
FIG. 4 is a graph of current differential protection operation characteristics (failure No. 1) according to an embodiment of the present invention;
fig. 5 is a graph of the power differential protection operation result (failure No. 1) according to an embodiment of the present invention:
FIG. 6 is a graph of an impedance type directional pilot protection measurement (fault # 1);
FIG. 7 is a graph of a negative sequence fault component current phase comparison protection measurement (fault # 1);
FIG. 8 is a graph of the busbar voltage of a second microgrid (fault # 2) according to an embodiment of the present invention;
FIG. 9 is a graph of a second current differential protection operation characteristic (failure No. 2) according to an embodiment of the present invention;
FIG. 10 is a graph of the result of a two-power differential protection operation (failure No. 2) according to an embodiment of the present invention;
FIG. 11 is a graph of pilot protection measurement results (fault # 2) for a second impedance type embodiment of the present invention;
FIG. 12 is a graph of the phase comparison protection measurement of the second negative sequence fault component current (fault # 2);
fig. 13 is a graph of the busbar voltage and load branch current (fault No. 3) of a three micro-grid according to an embodiment of the present invention.
Detailed Description
The structure of the present invention will be further described by way of examples with reference to the accompanying drawings. It should be noted that the present embodiments are illustrative and not restrictive.
The invention is based on an equivalent model of the energy storage power supply and the photovoltaic power supply in the micro-grid under the normal operation condition and the fault condition. Firstly, analyzing the control characteristics of a micro-grid structure and a typical power supply to obtain the voltage and current change rules of an independent micro-grid before and after faults; setting voltage protection criteria at the busbar level of the micro-grid according to the voltage and current change rule to determine fault occurrence and send alarm signals to a control system to judge fault lines; and then, aiming at the double-end power supply line section, adopting current differential protection to meet the requirement of speediness, and further setting power differential protection to identify line faults under the condition of nonmetallic faults. The double-end power supply circuit is used as a micro-grid interconnection circuit and has higher requirements on reliability, so that a second set of protection method is configured, impedance measurement elements are arranged at two ends of the circuit to identify faults in a positive direction area of the circuit, and meanwhile, a negative sequence fault component current phase comparison type protection criterion is arranged to identify nonmetallic faults with larger transition resistance at fault points. For a load branch circuit with a power supply connected to only one side, when a fault occurs in the forward direction, the voltage of the micro-grid drops, the current of the branch circuit increases, and the protection requirement can be met by utilizing overcurrent information on the basis of network-level protection starting.
1. Micro-grid structure and control features
1.1 micro grid System architecture
The micro-grid system structure analyzed by the invention is shown in fig. 2, an energy storage power supply and a photovoltaic power supply are respectively connected to a micro-grid public bus, direct-current electric energy is converted into alternating-current electric energy through a converter, and electric energy requirements are provided for alternating-current loads. When the output power of the photovoltaic power supply is smaller than the load demand, the energy storage power supply outputs power. When the output power of the photovoltaic power supply is larger than the load demand, the photovoltaic power supply inputs power to the energy storage power supply.
1.2 control characteristics of energy storage Power supply
The energy storage power supply is used as a main power supply of the micro-grid, V/f control is adopted, reference voltage and frequency values are set in a control link of the energy storage power supply, and the support function of voltage and frequency is provided for various devices.
Under normal operation conditions, the reference value of the voltage of the energy storage power grid-connected point, which can correctly track the control link, can be expressed as:
U＝U ref (1)
u in ref Is a reference value for the energy storage power supply grid-connected point voltage.
When faults occur in the micro-grid, the voltage or the output current of the grid-connected point of the energy storage power supply is limited, and the control system cannot track the voltage reference value of the control link, so that the voltage of the grid-connected point can be regulated without difference. At this time, the whole voltage of the micro-grid drops under the influence of faults and the control capability of the energy storage power supply, namely:
U＜U ref (2)
when the output current of the energy storage power supply reaches the upper limit of the allowable amplitude of the converter, the voltage of the grid-connected point of the energy storage power supply drops. The grid-connected point voltage of the power supply is determined by the maximum output current of the converter, the equivalent impedance of a system outside the power supply and the impedance of the filter. The fault equivalent model of the energy storage power supply is as follows:
in the middle ofFor the voltage of the grid-connected point of the energy storage power supply, < >>For maximum output current of energy storage power supply converter, Z ex Equivalent impedance for external system of energy storage power supply Z w Is the equivalent impedance of the energy storage power filter.
When the SPWM modulation coefficient of the power supply converter is limited so that the outlet voltage of the converter is limited, the voltage of the grid-connected point of the energy storage power supply drops. Under the condition of limited modulation factor, the converter outlet only contains positive sequence voltage, and the voltage of the power grid-connected point is determined by the voltage drop of the converter outlet and the filter impedance. Therefore, the fault positive and negative sequence equivalent model of the energy storage power supply is as follows:
in the middle ofAnd->Positive and negative sequence voltages of grid-connected points of the energy storage power supply are respectively +.>For the limited value of the energy storage power supply converter outlet voltage, < + >>Outputting positive sequence current for energy storage power supply Z w1 Positive sequence equivalent impedance Z of energy storage power supply outlet filter L2 Is the negative sequence equivalent impedance between the fault point and the power supply grid point, Z s2 Is the internal negative sequence impedance of the energy storage power supply, +.>Is the fault point negative sequence voltage.
1.3 control characteristics of photovoltaic Power supply
The photovoltaic power supply adopts PQ control, a reference power value is set in a control link of the photovoltaic power supply, power requirements are provided for various loads of the micro-grid, and the energy storage power supply can be charged when the loads are smaller.
When the micro-grid normally operates or fails and the voltage of the power grid-connected point drops, the output current of the photovoltaic power supply is adjusted to the output power reference value in time, and the output current is changed. To improve the output characteristics, the photovoltaic power supply outputs only a positive sequence current component, and the current reference value of the control link can be expressed as:
i in PVd 、i PVq D and q axis components of the output current reference value of the photovoltaic power supply, u PV1d 、u PV1q D and q axis components of positive sequence voltage of photovoltaic power supply grid connection point respectively, M= (u) PV1d ) 2 +(u PV1q ) 2 P and Q are reference values for output active power and reactive power, respectively.
2. Protection implementation scheme
2.1 network level protection
When a fault occurs in the micro-grid, the micro-grid needs to be identified from the system level and an alarm signal is sent out. The micro-grid faults cause the change of system impedance parameters, the output current of the energy storage power supply is limited, the supporting effect on the system voltage cannot be realized, and the voltage of each bus of the micro-grid is suddenly changed. Therefore, the fault can be identified by utilizing the busbar voltage change of the micro-grid. Setting voltage protection criteria as follows:
|ΔU φ |≥k d U N (6)
in the formula DeltaU φ Measuring voltage amplitude values for each bus node; k (k) d Taking 0.9 as a voltage starting coefficient; u (U) N Is the rated voltage of the system. When motherWhen the line voltage change condition meets the protection criterion, the fault can be determined, an alarm signal is sent to the control system, and the fault line judgment is carried out.
(2) Line section protection 1
Aiming at the micro-grid double-end power supply Line sections, such as lines Line1 and Line6, which are connected with power supplies at two ends, the flowing direction of Line fault current is difficult to determine under the action of distributed power supplies at two ends. When the line fails, the fault line needs to be accurately judged, no delay is required to cut off the fault, the fault range is prevented from being enlarged, and damage is brought to other equipment of the micro-grid. Here first a current differential protection is employed to meet the requirements of rapidity. The differential protection action criteria are shown in the formula.
In the middle ofRespectively measuring calculated values of currents at two sides of a line; i OP.0 For minimum action threshold, I OP.0 ＝K sen I kmin ，K sen To start sensitivity, I kmin Taking the minimum short-circuit current value of the primary side to be cut off; k is the braking coefficient, taking 0.5.
Because the fault output current amplitude of the distributed photovoltaic power supply is smaller, under the condition that load current and fault point transition resistance are larger, the fault phase current included angle at two sides of a fault line measured by the current differential protection is possibly larger than 90 degrees, so that brake current is larger than action current, and the differential protection is difficult to reliably act. Therefore, the power differential protection is further arranged to identify line faults in nonmetallic fault conditions, and the power differential protection action equation is as follows:
|P P +P Q |＞K res |P P -P Q | (8)
p in the formula P And P Q Power values, |P, are measured for both ends of the line, respectively P +P Q I and P P -P Q I can be referred to as the actuation power value and the brake power value, K, of the power differential protection, respectively res For the power brake factor, 0.5 is taken. When a nonmetallic fault occurs in the micro-grid line, the sum of the measured power values at the two ends of the line is the sum of the line loss power and the transition resistance loss power of the fault point, and the sum is obviously larger than the difference of the measured power values at the two ends of the line.
(3) Line section protection 2
Considering that the double-ended power supply line is generally used as a connecting line of the micro-grid, the requirement on the reliability of relay protection is high. The second set of protection methods is further configured here, specifically set forth below.
Impedance measurement elements are provided at both ends of the line to identify faults occurring in the forward direction region of the line. When the impedance value measured at one end enters the range of the action equation, the line positive direction fault can be judged, and a positive direction fault signal is transmitted to the opposite end of the line through the power line carrier channel, so that the impedance type direction pilot protection is formed, and when the two ends of the line are judged to be positive direction faults, the faults in the line area can be identified. The protection action equation of impedance type direction longitudinal protection is as follows:
z in P And Z Q Respectively measuring impedance values for two sides of the line, Z L K is the line impedance value rel.1 Taking 1.3 as the reliable coefficient of the action equation.
Because the impedance characteristic of the distributed power supply of the power electronic interface in the micro-grid is difficult to determine and the output current is far smaller than that of the synchronous generator, under the conditions that nonmetallic faults occur in the line section and the transition resistance of fault points is large, the reliability of the action is difficult to ensure by the impedance type directional pilot protection. Considering that each distributed power supply does not generate negative sequence fault additional electromotive force, the phase comparison type protection criterion of the negative sequence fault component current is further set as shown in the formula:
delta in m In order to protect the corner of the criteria,negative sequence fault component currents, delta, on both sides of the fault line respectively set The angle is set for the criterion. Considering that the included angle of negative sequence currents on two sides of a line is smaller than 90 degrees, the maximum transmission error of a combined current transformer is not more than 15 degrees, and setting a criterion angle delta set Taken at 105 °.
(4) Branch local protection
For a load branch circuit with a power supply connected to only one side, when a fault occurs in the forward direction, the voltage of the micro-grid drops, the current of the branch circuit increases, the protection requirement can be met by utilizing overcurrent information on the basis of network-level protection starting, and the protection criterion is set as follows:
I m ＞K rel.2 I Lmax (11)
in which I m For measuring current locally in load branch, K rel.2 For the over-current protection reliability coefficient, 1.2, I Lmax The current corresponds to the maximum load current for the present branch.
3. Simulation analysis
And constructing a micro-grid system containing an energy storage and photovoltaic power supply by using PSCAD/EMTDC software, and performing simulation analysis on the protection scheme as shown in figure 2. The system voltage level was 380V. The line parameter is R 1 ＝R 2 ＝0.336Ω/km，X 1 ＝X 2 ＝0.08Ω/km，R 0 ＝3.36Ω/km，X 0 =0.28 Ω/km. Each line length was 0.1km. The rated power of the energy storage power supply is 200kVA, and the rated power of the photovoltaic power supply is 160kW.
3.1 case one
An AB two-phase short circuit (fault No. 1) occurs when 50% of the length of the double-ended power supply Line1 is at 0.5 s. The P busbar waveforms before and after failure are shown in fig. 3. Because the output current of the energy storage power supply reaches the limit value and cannot achieve the voltage support target, the busbar voltage of the micro-grid is suddenly changed, the grid-level protection identifies the fault and sends an alarm signal, and the line section protection starts to judge the fault line.
The current differential protection operation characteristic of the faulty line is shown in fig. 4. When a metallic fault occurs, the fault phase operation current of the current differential protection is larger than the braking current, the protection is reliable, and the fault line section can be accurately identified.
The operation result of the power differential protection is shown in fig. 5. Before the fault occurs, the power values at the two sides of the line are reversed equally, and the power action amount is |P P +P Q And I is zero. After the fault occurs, the power values at the two sides of the line change, but the power action quantity is still smaller than the power braking quantity, and the metallic fault cannot be identified by the visible power differential protection.
The current differential protection is able to identify faults within the line segment when a metallic fault occurs and the power differential protection is unable to reliably identify faults within the line segment. The two are combined to form the line section protection to function correctly.
The impedance measurement results of the impedance-oriented longitudinal protection are shown in fig. 6. And the measuring results of the impedance elements at the two sides of the line are all in the impedance circle, and the positive fault is judged. That is, in the event of a metallic fault, the impedance-oriented pilot protection is able to correctly identify the fault within the line segment.
The current phase calculation result of the negative sequence fault component current phase comparison type protection is shown in fig. 7. After the fault occurs, the measured values of the current negative sequence components at the two sides of the line are suddenly changed, the phase angles of the negative sequence current fault components are 91.26 degrees and 121.09 degrees respectively, the included angle is about 29.83 degrees, and under the condition of metallic fault, the phase comparison type protection of the current negative sequence fault components can accurately identify and cut off the fault line.
It can be seen that both the impedance-type directional pilot protection and the negative sequence fault component current phase comparison protection can identify faults within the line segment when a metallic fault occurs. The two are combined to form the line section protection to function correctly.
3.2 case two
An AB two-phase ground fault occurs when 50% of the length of the double-ended power supply Line6 is at 0.5s, and the transition resistance is 0.3 Ω (No. 2 fault). The P busbar waveforms before and after failure are shown in fig. 8. Because the output current of the energy storage power supply is limited after the fault occurs, the busbar voltage of the micro-grid is suddenly changed, the grid protection identifies the fault and sends an alarm signal, and the line section protection starts to judge the fault line.
The operation characteristics of the fault line current differential protection are shown in fig. 9. When nonmetallic faults occur, the fault A-phase action current of the current differential protection is smaller than the braking current, and the phenomenon is gradually serious along with the increase of the asymmetric transition resistance. The micro-grid has small line impedance and load impedance, and the current differential protection is extremely easy to be influenced by the transition resistance, so that the braking coefficient is difficult to adjust, and the protection cannot reliably operate.
The power differential protection operation result is shown in fig. 10. Before the fault occurs, the power values at the two sides of the line are reversed equally, and the power action amount is |P P +P Q And I is zero. After the fault occurs, the power values at the two sides of the line change, the power action quantity is rapidly increased and is larger than the power braking quantity due to the existence of transition resistance at the fault point, and the power differential protection can accurately identify nonmetallic faults and determine the fault line.
The current differential protection is unable to identify the fault within the line segment when a nonmetallic fault occurs and the power differential protection is able to identify the fault within the line segment. The two are combined to form the line section protection to function correctly.
The impedance measurement results of the impedance-oriented vertical slice protection are shown in fig. 11. When nonmetallic faults occur, under the action of power supply fault currents at two sides of a fault line, the measured results of impedance elements at two sides of the line fall outside an impedance circle, and the situation that faults occur in the positive direction of a protection measuring point can not be judged, so that the impedance type directional pilot protection can not accurately identify faults in a line section.
The current phase calculation result of the negative sequence fault component current phase comparison type protection is shown in fig. 12. After the fault occurs, the measured values of the negative sequence current components at the two sides of the line are suddenly changed, the phase angles of the negative sequence current fault components are 81.94 degrees and 142.23 degrees respectively, the included angles are about 60.29 degrees, and the fault in the line section is correctly identified. When nonmetallic faults occur, the protection method for phase comparison of the negative sequence fault component currents can still correctly identify fault areas.
It can be seen that the impedance-type directional pilot protection cannot identify faults in the line section when nonmetallic faults occur, and the negative sequence fault component current phase comparison protection can identify faults in the line section. The two are combined to form the line section protection to function correctly.
3.3 case three
When 50% of the length of the load branch Line7 is 0.5s, an AB two-phase ground fault occurs, and the transition resistance is 0.3 omega (fault No. 3). The P busbar voltage before and after the fault and the current protection measurement waveform at the load branch point 8 are shown in fig. 13. After the fault occurs, the busbar voltage of the micro-grid is suddenly changed, the micro-grid level protection identifies the fault, the grid level protection identifies the fault and sends an alarm signal, and the local protection of the branch circuit starts to judge the fault line. The load branch Line7 current increases beyond the maximum load current, and the overcurrent protection can reliably identify and cut off the fault Line.
In general, when a double-ended power supply line or a single-ended power supply load branch breaks down, the busbar voltage of the micro-grid breaks down, and the grid-level protection can identify the fault. When a double-end power supply circuit has a metallic fault or a nonmetallic fault, the current differential protection and the power differential protection are matched with each other, the impedance type direction longitudinal protection and the negative sequence fault component current phase comparison type protection are matched with each other, so that two sets of micro-grid circuit section protection are formed, and the fault circuit section can be reliably identified. The load branch current increases beyond the maximum load current, and the overcurrent protection can reliably identify the load branch fault as a local protection. Therefore, when different types of faults occur to different types of lines of the micro-grid, the protection scheme can accurately identify and cut off the faulty lines.
Although the embodiments of the present invention and the accompanying drawings have been disclosed for illustrative purposes, those skilled in the art will appreciate that: various substitutions, changes and modifications are possible without departing from the spirit of the invention and the appended claims, and therefore the scope of the invention is not limited to the embodiments and the disclosure of the drawings.
Claims (10)
1. The multistage cooperative relay protection method suitable for the independent micro-grid is characterized by comprising the following steps of:
step 1, analyzing the control characteristics of a micro-grid structure and a typical power supply based on an equivalent model of an energy storage power supply and a photovoltaic power supply in the micro-grid under normal operation conditions and fault conditions to obtain the voltage and current change rules of the independent micro-grid before and after the fault;
step 2, setting voltage protection criteria at the busbar level of the micro-grid according to the voltage and current change rule obtained in the step 1, determining the occurrence of faults, sending alarm signals to a control system, and judging fault lines;
step 3, aiming at the fault of the double-end power supply line section, two protection measures can be selectively adopted, wherein the first protection measure is to adopt current differential protection to meet the requirement of rapidity; the second protection measure is: impedance measuring elements are arranged at two ends of the line to identify faults in a positive direction area of the line; when the impedance value measured at one end enters the range of the action equation, the line positive direction fault can be judged, and a positive direction fault signal is transmitted to the opposite end of the line through the power line carrier channel, so that impedance type direction longitudinal protection is formed; aiming at the situation that only a load branch circuit with a single-side power supply connected in fails, when the forward direction fails, the voltage of the micro-grid drops, the current of the branch circuit increases, and the protection requirement is met by utilizing overcurrent information on the basis of network-level protection starting.
2. The multi-stage cooperative relay protection method suitable for independent micro-grids according to claim 1, wherein: the micro-grid system structure comprises an energy storage power supply and a photovoltaic power supply, wherein the energy storage power supply and the photovoltaic power supply are respectively connected to a micro-grid public bus, direct-current electric energy is converted into alternating-current electric energy through a converter, and electric energy requirements are provided for alternating-current loads; when the output power of the photovoltaic power supply is smaller than the load demand, the energy storage power supply outputs power; when the output power of the photovoltaic power supply is larger than the load demand, the photovoltaic power supply inputs power to the energy storage power supply.
3. The multi-stage cooperative relay protection method suitable for independent micro-grids according to claim 2, wherein: in step 1, the process of obtaining the voltage and current change rule of the independent micro-grid before and after the fault according to the control characteristic of the energy storage power supply comprises the following steps:
the energy storage is controlled by V/f, and under the normal operation condition, the voltage of the grid-connected point of the energy storage power supply can correctly track the reference value of the control link, which is expressed as:
U＝U ref (1)
in U ref The reference value of the voltage of the grid-connected point of the energy storage power supply;
when the micro-grid faults, the micro-grid integral voltage drops under the influence of faults and the control capability of an energy storage power supply, namely:
U＜U ref (2)
when the output current of the energy storage power supply reaches the upper limit of the allowable amplitude of the converter, the voltage of the grid-connected point of the energy storage power supply drops; the grid-connected point voltage of the power supply is determined by the maximum output current of the converter, the equivalent impedance of an external system of the power supply and the impedance of the filter; the fault equivalent model of the energy storage power supply is as follows:
in the middle ofFor the voltage of the grid-connected point of the energy storage power supply, < >>For maximum output current of energy storage power supply converter, Z ex Equivalent impedance for external system of energy storage power supply Z w Equivalent impedance of the energy storage power filter;
when the SPWM modulation coefficient of the power supply converter is limited so that the outlet voltage of the converter is limited, the voltage of the energy storage power supply grid-connected point drops; under the condition that the modulation coefficient is limited, the outlet of the converter only contains positive sequence voltage, and the voltage of the grid-connected point of the power supply is determined by the voltage of the outlet of the converter and the voltage drop on the filter impedance; the fault positive and negative sequence equivalent model of the energy storage power supply is as follows:
in the middle ofAnd->Positive and negative sequence voltages of grid-connected points of the energy storage power supply are respectively +.>For the limited value of the energy storage power supply converter outlet voltage, < + >>Outputting positive sequence current for energy storage power supply Z w1 Positive sequence equivalent impedance Z of energy storage power supply outlet filter L2 Is the negative sequence equivalent impedance between the fault point and the power supply grid point, Z s2 Is the internal negative sequence impedance of the energy storage power supply, +.>Is the fault point negative sequence voltage.
4. The multi-stage cooperative relay protection method suitable for independent micro-grids according to claim 2, wherein: in step 1, the process of obtaining the voltage and current change rule of the independent micro-grid before and after the fault according to the control characteristic of the photovoltaic power supply comprises the following steps:
the photovoltaic power supply is controlled by PQ, and when the micro-grid normally runs or fails and the voltage of the power grid-connected point drops, the output current of the photovoltaic power supply is adjusted to the output power reference value in time, and the output current is changed; for the purpose of improving the output characteristics, the photovoltaic power supply outputs only a positive sequence current component, and the current reference value of the control link can be expressed as:
wherein i is PVd 、i PVq D and q axis components of the output current reference value of the photovoltaic power supply, u PV1d 、u PV1q D and q axis components of positive sequence voltage of photovoltaic power supply grid connection point respectively, M= (u) PV1d ) 2 +(u PV1q ) 2 P and Q are reference values for output active power and reactive power, respectively.
5. The multi-stage cooperative relay protection method suitable for independent micro-grids according to claim 1, wherein: in the step 2, the identification of faults is realized by utilizing the busbar voltage change of the micro-grid; setting voltage protection criteria as follows:
|ΔU φ |≥k d U N (6)
in the formula DeltaU φ Measuring voltage amplitude values for each bus node; k (k) d Taking 0.9 as a voltage starting coefficient; u (U) N Rated voltage of the system; when the bus voltage change condition meets the protection criterion, the occurrence of faults can be determined, an alarm signal is sent to the control system, and fault line judgment is carried out.
6. The multi-stage cooperative relay protection method suitable for independent micro-grids according to claim 1, wherein: in step 3, the current differential protection action criteria are:
in the method, in the process of the invention,respectively measuring calculated values of currents at two sides of a line; i OP.0 For minimum action threshold, I OP.0 ＝K sen I kmin ，K sen To start sensitivity, I kmin Taking the minimum short-circuit current value of the primary side to be cut off; k is the braking coefficient, taking 0.5.
7. The multi-stage cooperative relay protection method suitable for independent micro-grids according to claim 6, wherein: in step 3, under the condition that the fault phase current included angle at two sides of the fault line measured by the current differential protection is larger than 90 degrees, setting a power differential protection to identify the line fault under the nonmetallic fault condition, wherein the power differential protection action equation is as follows:
|P P +P Q |＞K res |P P -P Q | (8)
wherein P is P And P Q Power values, |P, are measured for both ends of the line, respectively P +P Q I and P P -P Q I can be referred to as the actuation power value and the brake power value, K, of the power differential protection, respectively res Taking 0.5 as a power braking coefficient; when a nonmetallic fault occurs in the micro-grid line, the sum of the measured power values at the two ends of the line is the sum of the line loss power and the transition resistance loss power of the fault point, and the sum is obviously larger than the difference of the measured power values at the two ends of the line.
8. The multi-stage cooperative relay protection method suitable for independent micro-grids according to claim 1, wherein: in step 3, when both ends of the line are judged to be positive direction faults, faults in the line area can be identified, and a protection action equation of impedance type direction longitudinal protection is as follows:
z in P And Z Q Respectively measuring impedance values for two sides of the line, Z L K is the line impedance value rel.1 Is movingAnd (3) taking 1.3 as a reliable coefficient of an equation.
9. The multi-stage cooperative relay protection method suitable for independent micro-grids according to claim 8, wherein: in step 3, under the condition that nonmetallic faults occur in the line section and the transition resistance of the fault point is large, the distributed power supplies are considered to not generate negative sequence fault additional electromotive force, and a negative sequence fault component current phase comparison type protection criterion is set as shown in the formula:
in delta m In order to protect the corner of the criteria,negative sequence fault component currents, delta, on both sides of the fault line respectively set Setting an angle for a criterion; considering that the included angle of negative sequence currents on two sides of a line is smaller than 90 degrees, the maximum transmission error of a combined current transformer is not more than 15 degrees, and setting a criterion angle delta set Taken at 105 °.
10. The multi-level cooperative relay protection method suitable for independent micro-grids according to claim 1, wherein for load branches with power access on only one side, a protection criterion is set as follows:
I m ＞K rel.2 I Lmax (11)
wherein I is m For measuring current locally in load branch, K rel.2 For the over-current protection reliability coefficient, 1.2, I Lmax The current corresponds to the maximum load current for the present branch.
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