Power distribution network single-phase short-circuit protection method based on characteristic current adaptive injection

Abstract

The present invention relates to the technical field of power system relay protection, in particular to a power distribution network single-phase short-circuit protection method based on characteristic current adaptive injection. The method comprises: collecting a voltage of a power distribution network bus and characteristic frequency currents of feeder outlets; when a zero-sequence voltage in the power distribution network bus is detected to exceed the limit, according to a relationship between the voltage of the power distribution network bus and a threshold voltage, calculating a photovoltaic injection characteristic frequency current control reference value, and photovoltaic active power and reactive current control reference values; latching a control outer loop of a photovoltaic inverter, switching the photovoltaic active power and reactive current control reference values, setting the injection characteristic frequency current control reference value, and starting photovoltaic characteristic current adaptive injection control; and comparing the amplitudes and phases of the characteristic frequency currents of respective feeders, and if there is a certain feeder satisfying the maximum characteristic frequency current amplitude, and this feeder is not the longest feeder or the characteristic frequency current phase thereof lags behind that of other feeders, determining that the feeder is a faulty feeder. The design of the present invention has good economical efficiency, practicability and implementability.

Description

A single-phase short-circuit protection method for distribution network based on characteristic current adaptive injection Technical Field

[0001] The present invention relates to the technical field of power system relay protection, in particular to a single-phase short-circuit protection method for a distribution network based on characteristic current adaptive injection. Background Art

[0002] Driven by the global energy crisis and environmental concerns, renewable energy generation and power electronics technology are developing rapidly. To fully utilize renewable energy sources such as solar energy, distributed photovoltaic power sources are being widely deployed. Existing relay protection systems identify intra- and intra-regional faults based on the current and voltage signals generated by the fault. With the integration of distributed power sources, the grid is experiencing new fault characteristics such as weak infeed, unequal positive and negative sequence impedances, and current frequency deviations. These factors can affect the proper operation of relay protection, leading to a degradation in protection performance and even potential false or ineffective operation.

[0003] To improve the reliability and sensitivity of traditional passive protection, researchers have proposed adding fault information to fault line selection and fault location by actively disturbing and injecting additional characteristic signals. Some researchers have injected special signals into the system through voltage transformers to select fault lines. However, due to the capacity limitations of voltage transformers, the amplitude of the injected signal is limited. For distribution networks with multiple feeders, the faulty feeder may not be effectively identified due to the low current in the feeder. Some researchers have proposed connecting a parallel resistor to the busbar in the substation to inject a disturbance signal into the non-fault phase to locate the distribution network fault. However, this method increases equipment investment, and the connection of the parallel resistor will increase the fault current, which is not conducive to the extinction of the arc at the fault point.

[0004] Since power electronic devices are highly controllable, researchers have proposed using power electronic devices in the power grid to inject signals to reduce the cost of additional devices. It is proposed to use flexible switching devices as a signal injection power source to identify the low-frequency zero-sequence current path between the fault point and the arc suppression coil to determine the fault section in the AC / DC hybrid active distribution network. Some researchers have proposed an early fault location method based on active injection of MMC converters, which injects detection signals through MMC converters to identify single-pole grounding faults. Existing distribution network protection based on active injection of power electronic devices is mostly concentrated in DC distribution networks or AC / DC hybrid distribution networks, and is not suitable for AC distribution networks containing photovoltaics.

[0005] Researchers have proposed a fault handling method that uses an inverter to inject a single characteristic signal into the distribution network. By comparing the characteristic signal energy at each measurement point, the fault path is determined. However, when the transition resistance is large, the capacitor shunt effect significantly weakens the energy flowing through the transition resistance, affecting the accuracy of fault identification. Existing methods use a fixed-amplitude injection rate and do not consider the needs of fault detection, which may lead to insufficient protection sensitivity. In addition, the inverter's power frequency output is coupled with the characteristic frequency output. Existing solutions do not consider the controllability of the photovoltaic inverter itself, which may aggravate the impact of the fault on the inverter and the distribution network.

[0006] In summary, with the integration of distributed power sources (DGs), the controllable, tightly coupled, and multi-morphic nature of fault processes further complicates low-current line selection for single-phase faults. Currently, there is no highly accurate active signal injection fault line selection method that takes into account the controllability of the PV inverter itself. Therefore, accurate and effective fault detection for single-phase grounded PV grid-connected distribution networks has become a pressing challenge for those skilled in the art. In light of this, we propose a single-phase short-circuit protection method for distribution networks based on adaptive characteristic current injection.

[0007] Summary of the Invention

[0008] The object of the present invention is to provide a single-phase short-circuit protection method for a distribution network based on characteristic current adaptive injection, so as to solve the problems raised in the above background technology.

[0009] To solve the above technical problems, one of the objectives of the present invention is to provide a single-phase short-circuit protection method for distribution network based on characteristic current adaptive injection. The method utilizes photovoltaic power connected to the distribution network busbar to adaptively inject characteristic frequency current according to the fault severity. The method selects the single-phase grounding fault line in the distribution network based on the amplitude and phase characteristics of the characteristic frequency current of each feeder. The method specifically includes the following steps:

[0010] S101, collecting the distribution network bus voltage and the characteristic frequency current of each feeder outlet;

[0011] S102, when it is detected that the zero-sequence voltage in the distribution network bus exceeds the limit, calculating the photovoltaic injection characteristic frequency current control reference value and the photovoltaic active and reactive current control reference values ​​according to the relationship between the distribution network bus voltage and the threshold voltage;

[0012] S103, locking the photovoltaic inverter control outer loop, switching the photovoltaic active and reactive current control reference values ​​and the injection characteristic frequency current control reference value, and starting the photovoltaic characteristic current adaptive injection control;

[0013] S104: Compare the amplitude and phase of the characteristic frequency current of each feeder. If there is a feeder i that meets the fault line selection criterion, proceed to the next step;

[0014] S105: Determine that feeder i is a faulty feeder.

[0015] As a further improvement of the present technical solution, in said S102, when the distribution network bus voltage is greater than the threshold voltage, the photovoltaic injection characteristic frequency current amplitude i inj，f for:

[0016] Among them, Z eql is the amplitude of the system equivalent impedance at the distribution network bus when the distribution network is operating at the maximum load state; Z eq is the magnitude of the system equivalent impedance at the distribution network bus after the fault; The minimum value of the characteristic frequency current amplitude is 4% of the rated current; It is the upper limit of the characteristic frequency current.

[0017] As a further improvement of this technical solution, the photovoltaic injection characteristic frequency current amplitude i′ inj，f In the calculation process, the upper limit of the characteristic frequency current is Calculate by solving the following formula:

[0018] Among them, i′ d，ref is the photovoltaic active power control reference value, i′ q，ref is the reactive current control reference value, i gmax The maximum allowed AC current of the PV to meet the inverter safety constraints.

[0019] As a further improvement of the present technical solution, in S102, when the distribution network bus voltage is greater than the threshold voltage, the photovoltaic active and reactive current control reference values ​​are calculated as follows:

[0020] Among them, P ref is the reference value of photovoltaic active power; U gN and i N is the photovoltaic rated voltage and rated current; U g is the effective value of the distribution network bus voltage; k1 is the support factor of the photovoltaic low voltage ride-through control strategy, which is determined by the grid operation regulations and is usually required to be no less than 1.5.

[0021] As a further improvement of the present technical solution, in said S102, when the distribution network bus voltage is less than or equal to the threshold voltage, the amplitude of the photovoltaic injection characteristic frequency current is calculated as follows: i″ inj，f =0.04i N

[0022] Among them, i inj，fIt is the PV injection characteristic frequency current amplitude when the distribution network bus voltage is less than or equal to the threshold voltage.

[0023] As a further improvement of the present technical solution, in S102, when the distribution network bus voltage is less than or equal to the threshold voltage, the photovoltaic active and reactive current control reference values ​​are calculated as follows:

[0024] Among them, i gmax The maximum allowed AC current of the PV to meet the inverter safety constraints.

[0025] As a further improvement of the present technical solution, in S102, the threshold voltage is calculated by solving the following formula:

[0026] Among them, U gr is the threshold voltage of the distribution network bus.

[0027] As a further improvement of the present technical solution, in S103, the photovoltaic characteristic current adaptive injection control includes two parts: a power frequency control link and a characteristic frequency control link. The power frequency control link is a dual closed-loop control system, in which the current inner loop controls the quadrature and direct axis currents to track the current reference values ​​respectively. The characteristic frequency control link injects the characteristic frequency current through a quasi-PR controller, and the transfer function of the quasi-PR controller is:

[0028] Among them, K p and K r are the proportional coefficient and resonant coefficient of the quasi-PR controller respectively; ω n is the operating frequency of the quasi-PR controller; ω c is the cutoff frequency of the quasi-PR controller, and s represents the Laplace operator.

[0029] As a further improvement of this technical solution, in S104, the fault line selection criterion is set according to the following method:

[0030] When the characteristic frequency current amplitude of feeder i is the largest and this feeder is not the longest feeder, the feeder is determined to be the faulty feeder; or when the characteristic frequency current phase of feeder i lags behind other feeders, the feeder is determined to be the faulty feeder. The judgment criteria are as follows:

[0031] Among them, feeder j is the other feeder in the distribution network except i, and are the characteristic frequency currents at the outlets of feeders i and j respectively; l i and l j are the lengths of feeder i and feeder j respectively, and are the phase angles of the characteristic frequency currents flowing through feeder i and feeder j respectively; when criterion 1 or criterion 2 is met, feeder i is determined to be a fault feeder.

[0032] The second object of the present invention is to provide a single-phase short-circuit protection device for a distribution network, comprising a processor, a memory, and a computer program stored in the memory and running on the processor, wherein the processor is configured to implement the calculation and judgment steps of the above-mentioned single-phase short-circuit protection method for a distribution network based on adaptive injection of characteristic current when executing the computer program.

[0033] The third object of the present invention is to provide a computer-readable storage medium, which stores a computer program. When the computer program is executed by a processor, it implements the calculation and judgment steps of the above-mentioned distribution network single-phase short-circuit protection method based on characteristic current adaptive injection.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] 1. This method for single-phase short-circuit protection in distribution networks based on adaptive characteristic current injection addresses the problem that existing technologies use fault characteristics such as the distribution network's voltage and current to determine the feeder experiencing a single-phase grounding fault. However, due to the weak characteristics of single-phase grounding faults in distribution networks, the line selection sensitivity is often insufficient. The present invention utilizes photovoltaic power to inject characteristic frequency current into the distribution network. The amplitude and phase characteristics of the characteristic frequency current are used to determine the feeder experiencing a single-phase grounding fault, resulting in higher selection sensitivity and a wider range of applications.

[0036] 2. This method for single-phase short-circuit protection in distribution networks based on adaptive characteristic current injection addresses the problem that existing injection-type protection technologies use characteristic signals of constant amplitude, where protection sensitivity is affected by fault severity. This method injects characteristic frequency currents of varying amplitudes into the power grid based on fault severity, maintaining good sensitivity across a wide range of fault locations and severities.

[0037] 3. This single-phase short-circuit protection method for distribution networks based on adaptive characteristic current injection addresses the problem that existing injection protection technologies ignore the interaction between the injection signal magnitude and the low-voltage ride-through of power electronic equipment, potentially causing the amplitude of power electronic equipment to exceed the limit or the injection signal to be difficult to reliably identify. The present invention automatically determines the photovoltaic injection magnitude and fault ride-through reference value based on different fault severity levels, thus addressing both distribution network fault line selection and photovoltaic fault ride-through requirements.

[0038] 4. In the distribution network single-phase short-circuit protection method based on characteristic current adaptive injection, the implementation method is clear, and the required information is only the distribution network bus voltage and the characteristic frequency current of each feeder outlet, which is easy to obtain and simple to implement, ensuring the feasibility of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] FIG1 is a flow chart of an exemplary method for single-phase short-circuit protection of a distribution network based on adaptive characteristic current injection according to the present invention;

[0040] FIG2 is a schematic diagram of an exemplary photovoltaic power distribution network structure according to the present invention;

[0041] FIG3 is one of the information tables of an exemplary simulation embodiment of the present invention;

[0042] FIG4 is a second diagram of an information table of an exemplary simulation embodiment of the present invention;

[0043] FIG5 is a third diagram of an information table of an exemplary simulation embodiment of the present invention;

[0044] FIG6 is a fourth diagram of an information table of an exemplary simulation embodiment of the present invention;

[0045] FIG7 is a fifth diagram of an exemplary information table of a simulation embodiment of the present invention;

[0046] FIG8 is a structural diagram of an exemplary electronic computer platform device in the present invention. DETAILED DESCRIPTION

[0047] The following will provide a clear and complete description of the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0048] Example 1

[0049] As shown in Figure 1, this embodiment provides a single-phase short-circuit protection method for a distribution network based on adaptive characteristic current injection. The method uses photovoltaic cells connected to the distribution network bus to adaptively inject characteristic frequency current according to the severity of the fault. The amplitude and phase characteristics of the characteristic frequency current of each feeder are used to select the single-phase grounding fault line of the distribution network. The method specifically includes the following steps.

[0050] S101, collecting the distribution network bus voltage and the characteristic frequency current of each feeder outlet;

[0051] In S101, based on the active distribution network, it is necessary to collect the distribution network bus voltage and the characteristic frequency current of each feeder outlet according to a certain sampling period. Since the specific sampling method and the corresponding sampling limitations are routine capabilities possessed by those skilled in the art, the present invention does not make specific limitations on this.

[0052] S102, when it is detected that the zero-sequence voltage in the distribution network bus exceeds the limit, calculating the photovoltaic injection characteristic frequency current control reference value and the photovoltaic active and reactive current control reference values ​​according to the relationship between the distribution network bus voltage and the threshold voltage;

[0053] In this step, when the distribution network bus voltage is greater than the threshold voltage, the photovoltaic injection characteristic frequency current amplitude i′ inj，f for:

[0054] Among them, Z eql is the amplitude of the system equivalent impedance at the distribution network bus when the distribution network is operating at the maximum load state; Z eq is the magnitude of the system equivalent impedance at the distribution network bus after the fault; The minimum value of the characteristic frequency current amplitude is 4% of the rated current; It is the upper limit of the characteristic frequency current.

[0055] Among them, the photovoltaic injection characteristic frequency current amplitude i′ inj,f In the calculation process, the upper limit of the characteristic frequency current is Calculate by solving the following formula:

[0056] Among them, i′ d,ref is the photovoltaic active power control reference value, i′ q,ref is the reactive current control reference value, i gmax The maximum allowed AC current of the PV to meet the inverter safety constraints.

[0057] Furthermore, when the distribution network bus voltage is greater than the threshold voltage, the photovoltaic active and reactive current control reference values ​​are calculated as follows:

[0058] Among them, P ref is the reference value of photovoltaic active power; U gN and i N is the photovoltaic rated voltage and rated current; U g is the effective value of the distribution network bus voltage; k1 is the support factor of the photovoltaic low voltage ride-through control strategy, which is determined by the grid operation regulations and is usually required to be no less than 1.5.

[0059] Furthermore, when the distribution network bus voltage is less than or equal to the threshold voltage, the amplitude of the photovoltaic injection characteristic frequency current is calculated as follows: inj,f =0.04i N

[0060] Among them, i inj,f It is the PV injection characteristic frequency current amplitude when the distribution network bus voltage is less than or equal to the threshold voltage.

[0061] At the same time, when the distribution network bus voltage is less than or equal to the threshold voltage, the photovoltaic active and reactive current control reference values ​​are calculated as follows:

[0062] Among them, i gmax The maximum allowed AC current of the PV to meet the inverter safety constraints.

[0063] Specifically, in S102, the threshold voltage is calculated by solving the following formula:

[0064] Among them, U gr is the threshold voltage of the distribution network bus.

[0065] S103, locking the photovoltaic inverter control outer loop, switching the photovoltaic active and reactive current control reference values ​​and the injection characteristic frequency current control reference value, and starting the photovoltaic characteristic current adaptive injection control;

[0066] In this step, the photovoltaic characteristic current adaptive injection control includes two parts: the power frequency control part and the characteristic frequency control part. The power frequency control part is a dual closed-loop control system, in which the current inner loop controls the quadrature and direct axis currents to track the current reference value respectively. The characteristic frequency control part injects the characteristic frequency current through a quasi-PR controller. The transfer function of the quasi-PR controller is:

[0067] Among them, K p and K r are the proportional coefficient and resonant coefficient of the quasi-PR controller respectively; ω n is the operating frequency of the quasi-PR controller; ω c is the cutoff frequency of the quasi-PR controller, and s represents the Laplace operator.

[0068] S104, comparing the amplitude and phase of the characteristic frequency current of each feeder, if there is a feeder i that meets the fault line selection criterion, proceed to the next step;

[0069] In this step, the fault line selection criteria are set as follows:

[0070] When the characteristic frequency current amplitude of feeder i is the largest and this feeder is not the longest feeder, the feeder is determined to be the faulty feeder; or when the characteristic frequency current phase of feeder i lags behind other feeders, the feeder is determined to be the faulty feeder. The judgment criteria are as follows:

[0071] Among them, feeder j is the other feeder in the distribution network except i, and are the characteristic frequency currents at the outlets of feeders i and j respectively; l i and lj are the lengths of feeder i and feeder j respectively, and are the phase angles of the characteristic frequency currents flowing through feeder i and feeder j, respectively. When criterion 1 or criterion 2 is met, feeder i is determined to be a fault feeder.

[0072] S105: Determine that feeder i is a faulty feeder.

[0073] Simulation Example

[0074] Figure 2 shows a schematic diagram of the distribution network structure with PV panels. There are four feeders, with lengths of 40 km, 20 km, 10 km, and 5 km, respectively. The positive- and negative-sequence impedances are z1 = 0.031 + j0.029 Ω / km, the positive-sequence capacitance is c1 = 0.338 μF / km, and the zero-sequence impedance is z1 = 0.234 + j0.111 Ω / km, c0 = 0.265 μF / km. Loads are located at the ends of the feeders. The capacities of loads 1, 2, 3, and 4 are 10 MW, 8 MW, 6 MW, and 5 MW, respectively. FTUs are deployed at the PV grid connection point and feeder outlets to collect current and voltage. Fast Fourier transform (FFT) is used to extract the collected signals and determine the characteristic frequency current flowing through each feeder. The frequency of the injected signal is 25 Hz. Different fault locations and fault severity levels are set to verify the effectiveness of the proposed method. Three fault scenarios, f1, f2, and f3, are set as shown in Figure 2.

[0075] To verify the effectiveness of the present invention, a single-phase grounding fault was set on feeder 4, with a fault grounding resistance of 1000Ω. Under different grounding methods, simulations yielded the characteristic frequency current amplitudes and phase angles at different feeder outlets, as shown in Table 1 in Figure 3. The characteristic frequency current amplitude at the outlet of feeder 1 is approximately twice that of feeder 2. The characteristic frequency current amplitude at the outlet of feeder 2 is approximately twice that of feeder 3. The characteristic frequency current amplitude at the outlet of feeder 4 is slightly greater than half that of feeder 3. Under different neutral point grounding methods, the current amplitude flowing through feeder 1 is the largest, but this feeder is the longest feeder in the power grid and does not meet criterion 1 of the proposed protection method. Therefore, the phase angles of the characteristic frequency currents are compared. According to the simulation results, under different neutral point grounding methods, the phase angle of feeder 4 lags behind that of other feeders, meeting criterion 2 of the proposed protection method and correctly selecting feeder 4 as the faulty feeder. Therefore, the proposed method is suitable for fault line selection under different grounding methods.

[0076] To verify the effectiveness of this invention, the fault severity and grounding method remained unchanged while the fault location was varied. A neutral point grounding method was used via an arc suppression coil, and single-phase grounding faults were set on feeders 1, 2, and 4, with a fault grounding resistance of 4000Ω. Simulations yielded the amplitude and phase angle of the characteristic frequency current at different feeder outlets at different fault locations, as shown in Table 2 in Figure 4. At this fault severity, as the fault location changed, the amplitude of the characteristic frequency current flowing through feeder 1, the longest feeder, remained the largest. This does not meet criterion 1 of the proposed protection method. The phase angles of the characteristic frequency currents in each feeder were compared. When the fault point is located on feeder 1 (fault scenario f1), the phase angle of feeder 1 lags behind that of the other feeders. When the fault point is located on feeder 2 (fault scenario f2), the phase angle of feeder 2 lags behind that of the other feeders. When the fault point is located on feeder 4 (fault scenario f3), the phase angle of feeder 4 lags behind that of the other feeders. Combining this characteristic frequency current phase angle information allows the correct selection of the faulty feeder. Therefore, the proposed method is applicable to fault line selection under different fault locations.

[0077] To verify the effectiveness of the invention and further verify the correctness of the proposed method under different fault levels, the fault transition resistance was changed to 1Ω, 100Ω, 500Ω, 2000Ω and 5000Ω in the fault scenarios f1, f2 and f3, respectively. The neutral point was grounded through an arc suppression coil. Under the three fault scenarios f1, f2 and f3, the amplitude and phase angle of the characteristic frequency current at different feeder outlets under different transition resistances can be obtained through simulation, as shown in Table 3 in Figure 5, Table 4 in Figure 6 and Table 5 in Figure 7. As can be seen from Table 3, under the fault scenario f1, as the transition resistance changes, the amplitude of the characteristic frequency current in feeder 1 is the largest and the phase angle of the characteristic frequency current lags behind that of other feeders. It can be correctly judged that feeder 1 has a fault. As can be seen from Table 4, under the fault scenario f2, as the transition resistance gradually increases, the feeder with the largest amplitude of the characteristic frequency current changes. According to the amplitude characteristics and phase angle characteristics of the characteristic frequency current, it can be determined that feeder 2 has a fault. Table 5 shows that in the f3 fault scenario, as the transition resistance gradually increases, the magnitude relationship of the characteristic frequency current in each feeder changes. Similarly, based on the phase angle characteristics of the characteristic frequency current, it can be determined that feeder 4 has a fault. Therefore, the proposed method is applicable to single-phase fault line selection under different transition resistances.

[0078] The present invention discloses a single-phase short-circuit protection method for a distribution network based on adaptive characteristic current injection. The method collects the distribution network bus voltage and the characteristic frequency current at the feeder outlet. When it is detected that the zero-sequence voltage in the distribution network bus exceeds a limit, the method calculates a photovoltaic injection characteristic frequency current control reference value and a photovoltaic active and reactive current control reference value based on the relationship between the distribution network bus voltage and a threshold voltage. The photovoltaic inverter control outer loop is locked, the photovoltaic active and reactive current control reference values ​​and the injection characteristic frequency current control reference value are switched, and the photovoltaic characteristic current adaptive injection control is started. The amplitude and phase of the characteristic frequency current of each feeder are compared. If a feeder exists that satisfies the maximum characteristic frequency current amplitude and is not the longest feeder or the characteristic frequency current phase lags behind other feeders, the feeder is determined to be a faulty feeder.

[0079] As shown in FIG8 , this embodiment further provides a single-phase short-circuit protection device for a power distribution network. The device includes a processor, a memory, and a computer program stored in the memory and running on the processor.

[0080] The processor includes one or more processing cores, and the processor is connected to the memory via a bus. The memory is used to store program instructions. When the processor executes the program instructions in the memory, the calculation and judgment steps of the above-mentioned distribution network single-phase short-circuit protection method based on characteristic current adaptive injection are implemented.

[0081] Alternatively, the memory may be implemented by any type of volatile or non-volatile memory device or a combination thereof, such as static random access memory (SRAM), electrically erasable programmable read-only memory (EEPROM), erasable programmable read-only memory (EPROM), programmable read-only memory (PROM), read-only memory (ROM), magnetic memory, flash memory, magnetic disk or optical disk.

[0082] In addition, the present invention also provides a computer-readable storage medium, which stores a computer program. When the computer program is executed by a processor, it implements the calculation and judgment steps of the above-mentioned distribution network single-phase short-circuit protection method based on characteristic current adaptive injection.

[0083] Optionally, the present invention also provides a computer program product containing instructions, which, when running on a computer, enables the computer to execute the calculation and judgment steps of the above-mentioned single-phase short-circuit protection method for distribution network based on adaptive injection of characteristic current.

[0084] Those skilled in the art will appreciate that the process of implementing all or part of the steps of the above embodiments may be accomplished by hardware, or may be accomplished by instructing the relevant hardware through a program, and the program may be stored in a computer-readable storage medium, and the above-mentioned storage medium may be a read-only memory, a disk, or an optical disk, etc.

[0085] The above shows and describes the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely preferred examples of the present invention and are not intended to limit the present invention. Various changes and improvements may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and improvements fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A single-phase short-circuit protection method for distribution network based on characteristic current adaptive injection, characterized in that: The photovoltaic power connected to the distribution network busbar is used to adaptively inject characteristic frequency current according to the severity of the fault. The single-phase grounding fault line selection of the distribution network is realized through the amplitude and phase characteristics of the characteristic frequency current of each feeder. The specific steps include: S101, collecting the bus voltage of the distribution network and the characteristic frequency current of each feeder outlet; S102, when it is detected that the zero-sequence voltage in the distribution network bus exceeds the limit, according to the relationship between the distribution network bus voltage and the threshold voltage, the photovoltaic injection characteristic frequency current control reference value and the photovoltaic active and reactive current control reference value are calculated; S103, locking the photovoltaic inverter control outer loop, switching the photovoltaic active and reactive current control reference values ​​and the injection characteristic frequency current control reference value, and starting the photovoltaic characteristic current adaptive injection control; S104, comparing the amplitude and phase of the characteristic frequency current of each feeder, if there is a feeder i that meets the fault line selection criterion, executing the next step; S105. Determine that feeder i is a faulty feeder.

2. The single-phase short-circuit protection method for distribution network based on characteristic current adaptive injection according to claim 1 is characterized in that: In S102, when the distribution network bus voltage is greater than the threshold voltage, the photovoltaic injection characteristic frequency current amplitude i′ inj,f for: Among them, Z eql The amplitude of the system equivalent impedance at the distribution network bus when the distribution network is operating at the maximum load state; Z eq is the magnitude of the system equivalent impedance at the distribution network bus after the fault; It is the minimum value of the characteristic frequency current amplitude, which is 4% of the rated current; It is the upper limit of the characteristic frequency current.

3. The single-phase short-circuit protection method for distribution network based on characteristic current adaptive injection according to claim 2 is characterized in that: The photovoltaic injection characteristic frequency current amplitude i′ inj,f In the calculation process, the upper limit of the characteristic frequency current is Calculate by solving the following equation: Among them, i′ d,ref is the photovoltaic active power control reference value, i′ q,ref is the reactive current control reference value, i gmax The maximum permissible PV AC current that meets the inverter safety constraints.

4. The single-phase short-circuit protection method for distribution network based on characteristic current adaptive injection according to claim 3 is characterized in that: In S102, when the distribution network bus voltage is greater than the threshold voltage, the photovoltaic active and reactive current control reference values ​​are calculated as follows: Among them, P ref is the photovoltaic active power reference value; U gN and i N is the photovoltaic rated voltage and rated current; U g is the effective value of the distribution network bus voltage; k1 is the support coefficient of the photovoltaic low voltage ride-through control strategy, which is determined by the grid operation procedures.

5. The single-phase short-circuit protection method for distribution network based on characteristic current adaptive injection according to claim 4 is characterized in that: In S102, when the distribution network bus voltage is less than or equal to the threshold voltage, the amplitude of the photovoltaic injection characteristic frequency current is calculated as follows: i″ inj,f =0.04i N Among them, i″ inj，f It is the characteristic frequency current amplitude of photovoltaic injection when the bus voltage of the distribution network is less than or equal to the threshold voltage.

6. The single-phase short-circuit protection method for distribution network based on characteristic current adaptive injection according to claim 5 is characterized in that: In S102, when the distribution network bus voltage is less than or equal to the threshold voltage, the photovoltaic active and reactive current control reference values ​​are calculated as follows: Among them, i gmax The maximum permissible PV AC current that meets the inverter safety constraints.

7. The single-phase short-circuit protection method for distribution network based on characteristic current adaptive injection according to claim 6 is characterized in that: In S102, the threshold voltage is calculated by solving the following formula: Among them, U gr is the threshold voltage of the distribution network bus.

8. The single-phase short-circuit protection method for distribution network based on characteristic current adaptive injection according to claim 1 is characterized in that: In S103, the photovoltaic characteristic current adaptive injection control includes two parts: the power frequency control link and the characteristic frequency control link; wherein the power frequency control link is a double closed-loop control system, and the current inner loop controls the AC and DC axis currents to track the current reference values ​​respectively; the characteristic frequency control link injects the characteristic frequency current through the quasi-PR controller, and the transfer function of the quasi-PR controller is: Among them, K p and K r are the proportional coefficient and resonant coefficient of the quasi-PR controller respectively; ω n is the operating frequency of the quasi-PR controller; ω c is the cutoff frequency of the quasi-PR controller, and s represents the Laplace operator.

9. The single-phase short-circuit protection method for distribution network based on characteristic current adaptive injection according to claim 1, characterized in that: In S104, the fault line selection criterion is set according to the following method: When the characteristic frequency current amplitude of feeder i is the largest and the feeder is not the longest feeder, the feeder is determined to be a faulty feeder; or when the characteristic frequency current phase of feeder i lags behind other feeders, the feeder is determined to be a faulty feeder, and the judgment criteria are as follows: Among them, feeder j is the other feeder in the distribution network except i, and are the characteristic frequency currents at the outlets of feeder i and j respectively; I i and I j are the lengths of feeder i and feeder j respectively, and are the phase angles of the characteristic frequency currents flowing through feeder i and feeder j respectively; when criterion 1 or criterion 2 is met, feeder i is determined to be a faulty feeder.

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