Fault phase selection method for photovoltaic power station sending line based on characteristic signal injection

Abstract

The method for fault phase selection of photovoltaic station sending line based on characteristic signal injection includes: constructing a characteristic signal injection starting criterion; dynamically allocating the harmonic current amplitude injected by each photovoltaic unit according to the output proportion of each photovoltaic unit; judging whether it is a three-phase short-circuit fault based on the negative sequence current injection rate; judging whether it is a ground fault and a phase-to-phase fault based on the harmonic zero sequence current amplitude; the two-phase phase-to-phase short-circuit fault and the two-phase phase-to-phase short-circuit ground fault both use the phase harmonic voltage phase difference to distinguish the fault phase; the single-phase ground fault uses the phase harmonic voltage containing rate to distinguish the fault phase. The method uses the active harmonic injection mode, combines the phase voltage phase difference and the voltage containing rate criterion, and improves the accuracy and reliability of fault phase selection.

Description

Technical Field

[0001] This invention relates to the field of fault phase selection technology, and specifically to a fault phase selection method for photovoltaic power plant transmission lines based on feature signal injection. Background Technology

[0002] With the integration of numerous renewable energy power plants, their fault characteristics differ fundamentally from those of traditional power sources. These differences primarily include limited short-circuit current amplitude, phase distortion, and increased non-power frequency components. These features negatively impact traditional power frequency-based protection principles, severely degrading their performance and even affecting the safe and stable operation of the system. Traditional passive protection systems often exhibit false tripping or failure to trip when faced with transmission lines from high-proportion renewable energy power plants, significantly reducing system stability. Therefore, applying active protection to the transmission lines of renewable energy power plants is crucial for resolving false tripping and failure to trip.

[0003] In high-voltage transmission lines, the interruption of three phases has a significant impact on system stability. Therefore, high-voltage lines are typically equipped with single-phase reclosing functionality. Thus, when a fault is determined to be within the designated fault zone, further fault phase selection is required. Traditional phase selection methods usually use current as an indicator, but these methods often exhibit decreased sensitivity in high-impedance scenarios. With the large-scale integration of renewable energy power plants, due to the negative sequence current suppression strategies implemented at these plants, the difference between positive and negative sequence impedances on the power plant side is significant. Traditional phase selection elements based on sequence components and phase current difference abrupt changes lose their phase selection capability on the power plant side, and in severe cases, phase selection may fail, affecting the normal operation of single-phase reclosing and the safe and stable operation of the system.

[0004] To address the above problems, one improvement method is to change the control strategy of the new energy power station during the fault period to make its positive and negative sequence impedances consistent, thereby restoring the traditional phase selection performance, as recorded in the literature [1]: Kang Tao, Weng Hanli, Lin Xiangning, et al. Recovery strategy of phase selection capability of photovoltaic power station transmission line sudden change considering energy storage system control and protection coordination system [J]. Proceedings of the CSEE, 2024, 44(11): 4273-4286. However, this method is essentially a passive adaptation to the existing fault characteristics, and if the characteristics of the power station side continue to change, it will face the risk of failure.

[0005] Another improved method attempts to select phase by injecting characteristic signals, as recorded in reference [2]: Zhu Hongjie, Lan Yuting, He Shigeng, et al. Active phase selection method for high-resistivity grounding fault in distribution network based on characteristic phase difference [J]. Journal of Electric Power System and Automation, 2025, 37(11):24-35. However, it is currently limited to single-phase grounding fault scenarios in distribution networks, and there is no characteristic signal phase selection method applicable to high-voltage transmission lines and covering multiple fault types. Summary of the Invention

[0006] To address the performance degradation of traditional phase selection components under renewable energy integration and improve the accuracy and reliability of fault phase selection in the transmission lines of renewable energy power plants, this invention provides a fault phase selection method for photovoltaic power plant transmission lines based on characteristic signal injection. This method employs active harmonic injection, combined with criteria such as phase voltage phase difference and voltage content rate, to enhance the accuracy and reliability of fault phase selection.

[0007] The technical solution adopted in this invention is as follows: The method for selecting the phase of a photovoltaic power plant's outgoing line based on feature signal injection includes the following steps: Step 1: Construct the feature signal injection initiation criterion; Step 2: Dynamically allocate the amplitude of the injected harmonic current to each photovoltaic unit according to the output ratio of each photovoltaic unit; Step 3: Based on the negative sequence current injection rate, determine whether it is a three-phase short circuit fault; Step 4: Based on the amplitude of the harmonic zero-sequence current, determine whether it is a ground fault or a phase-to-phase fault; Step 5: For both two-phase phase-to-phase short circuit faults and two-phase phase-to-phase short circuit to ground faults, the phase harmonic voltage phase difference is used to identify the faulty phase; for single-phase ground faults, the phase harmonic voltage content rate is used to identify the faulty phase.

[0008] In step 1, based on the instantaneous change in current value... Second difference from the instantaneous current value Calculate weighting factors As a triggering criterion; specifically including: S1.1: Calculate the instantaneous change in current value Second difference from the instantaneous current value : (1); (2); In the above formula, This indicates the instantaneous change in the current value at the current sampling moment; This represents the second-order difference of the instantaneous current value at the current sampling moment; This represents the instantaneous current value at the current sampling moment; This represents the instantaneous current value at the previous sampling time; express The instantaneous value of the current at the previous sampling moment; Indicates the previous sampling time; express The previous sampling time; Indicates the current sampling time; This represents the time window between the current sampling time and the previous sampling time.

[0009] S1.2: The instantaneous change in current value in S1.1 Second difference from the instantaneous current value Perform normalization and calculate its weighting factors. : (3); In equation (3), This represents the current weighting factor at the current sampling time; Indicates the reference current; Indicates the proportionality coefficient; Indicates the power frequency angular frequency; S1.3: Instantaneous weighting factor The value is compared with a set threshold. If the condition is met, the injection is initiated. (4); In equation (4), This represents the current weighting factor at the current moment; This represents the threshold of the current weighting factor.

[0010] Step 2 includes the following steps: S2.1: Calculate the percentage of rated operating power output for each photovoltaic unit. : (5); In equation (5), This indicates the percentage of rated operating power output of each photovoltaic unit; This indicates the rated active power output of each photovoltaic unit. This indicates the rated active power output of all photovoltaic units. The values ​​1, 2, ... can be used to represent the 1st, 2nd, ...th photovoltaic unit, respectively; S2.2: According to Calculation of the total harmonic current amplitude of the line based on the rated current of the line: (6); In equation (6), Indicates the rated current amplitude of the transmitting line. This indicates the amplitude of the total harmonic current injected into the transmitting line.

[0011] S2.3: Based on the output ratio of each photovoltaic unit in S2.1 Calculate the amplitude of the corresponding injected harmonic current: (7); In equation (7), This indicates the amplitude of the harmonic current injected into each photovoltaic unit.

[0012] Step 3 includes the following steps: S3.1: Calculate the 8th harmonic negative sequence current injection rate : (8); In equation (8), This indicates the 8th harmonic negative sequence current on the station side. This indicates the 8th harmonic positive sequence current on the station side. This indicates the 8th harmonic zero-sequence current on the station side. , , These represent the corresponding proportionality coefficients.

[0013] S3.2: Construct the fault symmetry identification criterion as shown in equation (9): (9); In equation (9), The threshold for negative sequence current injection rate is used; if equation (9) is satisfied, it is determined to be a three-phase short circuit fault; otherwise, the fault type and fault phase will be further determined. In step 4, the ground fault and phase-to-phase fault are determined based on the amplitude of the 8th harmonic zero-sequence current on the station side, as shown in equation (10): (10); In equation (10), This indicates the amplitude of the 8th harmonic zero-sequence current on the station side; The threshold value of the zero-sequence current of the 8th harmonic on the station side; if it satisfies equation (10), it is judged as a two-phase short circuit fault, otherwise it is judged as a ground fault.

[0014] Step 5 includes: 5.1: Phase identification of single-phase grounding faults based on the 8th harmonic voltage content: If the fault is determined to be ground fault according to equation (10), based on the characteristic that the amplitude of the 8th harmonic voltage of the fault phase is much lower than that of the 8th harmonic voltage of the non-fault phase when a single-phase ground fault occurs, firstly, the phase 8th harmonic voltage content rate is defined as shown in equation (11), and the single-phase ground fault phase selection criteria are established as shown in equations (12), (13) and (14). (11); (12); (13); (14); In the above formula, , and These represent the 8th harmonic voltages of phases A, B, and C on the station side, respectively. , and These represent the voltage content of the 8th harmonic of the three phases A, B, and C, respectively. This represents the proportionality coefficient, with a value of 0.5.

[0015] If equation (11) is satisfied, it is determined to be a phase A ground fault; if equation (12) is satisfied, it is determined to be a phase B ground fault; if equation (13) is satisfied, it is determined to be a phase C ground fault. 5.2: Phase identification of two-phase phase-to-phase short circuits and two-phase phase-to-phase short-circuit-to-ground faults based on the phase difference of the 8th harmonic voltage: 1) If the ground fault is determined according to equation (10), and equations (12), (13) and (14) are not satisfied, then it is determined to be a two-phase short-circuit ground fault; Considering that the phases of the 8th harmonic voltages of the two faulted phases are close when a phase-to-phase short circuit or a phase-to-phase short circuit to ground fault occurs, while the phases of the 8th harmonic voltages of the faulted phase and the non-faulted phase are significantly different, the phase difference of the 8th harmonic voltages is defined as shown in equation (15): (15); In equation (15), This indicates the phase difference of the 8th harmonic voltage; This indicates the phase of the 8th harmonic voltage of phase X; This indicates the phase of the 8th harmonic voltage in the Y phase; Pick or or ,correspond Pick or or .

[0016] Further determine the fault phase based on equations (16), (17), and (18): (16); (17); (18); Considering the phase selection sensitivity setting, If equation (16) is satisfied, it is determined to be an AB phase-to-phase short-circuit ground fault; if equation (17) is satisfied, it is determined to be a BC phase-to-phase short-circuit ground fault; if equation (18) is satisfied, it is determined to be a CA phase-to-phase short-circuit ground fault. 2) If the fault is determined to be a two-phase short circuit fault according to equation (10), the fault phase is determined according to equations (16), (17) and (18). If equation (16) is satisfied, the fault is determined to be an AB phase-to-phase short circuit fault; if equation (17) is satisfied, the fault is determined to be a BC phase-to-phase short circuit fault; if equation (18) is satisfied, the fault is determined to be a CA phase-to-phase short circuit fault.

[0017] This invention provides a method for fault phase selection in the outgoing lines of photovoltaic power plants based on feature signal injection, with the following technical advantages: 1) The fault phase selection method based on feature signal injection in this invention gets rid of the dependence on traditional power frequency fault characteristics and is not affected by the negative sequence current suppression strategy and low voltage ride-through control strategy of new energy power plants. It has excellent phase selection performance in the scenario of new energy access.

[0018] 2) The method of the present invention has strong resistance to transition resistance and can still correctly select the phase when the line is grounded by a single phase through a 300-ohm transition resistor at the end of the line.

[0019] 3) The principle of the method of the present invention is clear. It only needs to utilize the electrical quantities on the station side and does not require additional testing equipment, which is conducive to the practical application in engineering. Attached Figure Description

[0020] The present invention will be further described below with reference to the accompanying drawings and examples; Figure 1 This is a flowchart of the phase selection process for faults in the transmission lines of new energy power plants based on characteristic signal injection.

[0021] Figure 2 This is the action graph for initiating the criteria.

[0022] Figure 3 A topology diagram of power transmission lines for new energy sources.

[0023] Figure 4(a) shows the negative sequence current injection rate curve during a three-phase short-circuit fault. Figure 4(b) shows the amplitude curve of the zero-sequence current of the 8th harmonic.

[0024] Figure 5(a) shows the negative sequence current injection rate curve during a two-phase short-circuit fault (AB phases). Figure 5(b) shows the amplitude curve of the 8th harmonic zero-sequence current during a two-phase short-circuit fault (AB phases).

[0025] Figure 6(a) shows the phase selection results for a two-phase short circuit fault (phase of the 8th harmonic voltage of each phase). Figure 6(b) shows the phase selection result of the two-phase short circuit fault (phase difference of the 8th harmonic voltage).

[0026] Figure 7(a) shows the negative sequence current injection rate curve during a metallic ground fault in phase A. Figure 7(b) shows the amplitude curve of the 8th harmonic zero-sequence current during a phase A metallic ground fault.

[0027] Figure 8 This is a graph showing the content rate of the 8th harmonic voltage in each phase during a metallic ground fault in phase A.

[0028] Figure 9(a) shows the negative sequence current injection rate curve during a metallic short-circuit ground fault in phase AB. Figure 9(b) shows the amplitude curve of the 8th harmonic zero-sequence current during a metallic short-circuit ground fault in phase AB.

[0029] Figure 10 This is a graph showing the content rate of the 8th harmonic voltage in each phase during a metallic short-circuit ground fault in phases AB.

[0030] Figure 11(a) shows the phase selection results for a two-phase short-circuit ground fault (phase of the 8th harmonic voltage of each phase). Figure 11(b) shows the phase selection result of the two-phase short-circuit ground fault (phase 8th harmonic voltage phase difference). Detailed Implementation

[0031] A method for fault phase selection in photovoltaic power plant transmission lines based on characteristic signal injection, utilizing the instantaneous change in current value... Its second difference Calculate weighting factors The method involves several steps: determining whether an 8th harmonic negative-sequence current signal is injected from the photovoltaic power station into the transmission line; dynamically adjusting the amplitude of the injected harmonics for each photovoltaic unit based on its output; calculating the negative-sequence current injection rate to determine if it is a three-phase short-circuit fault; further, determining whether it is a ground fault or a phase-to-phase fault based on the amplitude of the zero-sequence harmonic current on the power station side; using the phase harmonic voltage phase difference to identify both two-phase phase-to-phase short-circuit faults and two-phase phase-to-ground short-circuit faults, and using the phase harmonic voltage content rate to identify single-phase ground faults. This method solves the problem of decreased phase selection performance or even failure of traditional phase selection elements due to negative-sequence current suppression strategies when connected to new energy power stations. Specifically, it includes the following steps: Step 1: Construct the feature signal injection initiation criterion: When a fault occurs in the transmission line, the instantaneous current value on the station side will change abruptly. To quickly and reliably identify the fault and initiate harmonic injection, this invention constructs a weighted factor initiation criterion based on the instantaneous current value change and the second-order difference, as follows: (1) Real-time calculation of instantaneous current change Second difference from the instantaneous current value : (1); (2); In the formula, This represents the instantaneous current value at the current sampling moment. Indicates the previous sampling time. express The previous sampling time, This represents the instantaneous change in the current value at the current sampling moment. This represents the second-order difference of the instantaneous current value at the current sampling moment.

[0032] (2) Sudden change in instantaneous current value Second difference from the instantaneous current value Perform normalization and calculate its weighting factors.

[0033] (3); In the formula, This represents the current weighting factor at the current sampling time. Indicates the reference current. This represents the proportionality coefficient. The value is 0.8. It represents the power frequency angular frequency.

[0034] (3) Instantaneous weighted mutation factor The value is compared with a set threshold. If the condition is met, the injection is initiated. (4); In the formula, This represents the current weighting factor threshold. Considering both anti-interference capability and sensitivity, it is recommended... The value is 20.

[0035] Step 2: Harmonic Co-injection of Photovoltaic Units: To ensure reliable injection of the characteristic signal (8th harmonic) while taking into account the operating status of each photovoltaic unit, the harmonic injection amplitude is dynamically allocated according to the output ratio of each photovoltaic unit.

[0036] (1) Calculate the percentage of rated operating power of each photovoltaic unit.

[0037] (5); In the formula, This indicates the percentage of rated operating power output of each photovoltaic unit. This indicates the rated active power output of each photovoltaic unit. This indicates the rated active power output of all photovoltaic systems. The values ​​1, 2, ... can be used to represent the 1st, 2nd, ...th photovoltaic cells, respectively.

[0038] (2) According to Calculation of line rated current and total harmonic current amplitude (6); In the formula, Indicates the rated current amplitude of the transmitting line. This indicates the amplitude of the total harmonic current injected into the transmitting line.

[0039] (3) Calculate the corresponding injected harmonic current amplitude based on the output ratio of each photovoltaic unit in step 1. (7); In the formula, This indicates the amplitude of the harmonic current injected into each photovoltaic unit.

[0040] Step 3: Three-phase short-circuit fault identification based on negative sequence current injection rate: After injecting the characteristic signal, the first step is to determine whether the fault is a symmetrical fault (three-phase short circuit): (1) Extract the positive sequence, negative sequence and zero sequence current components of the 8th harmonic on the station side. , , Calculate the negative sequence current injection rate

[0041] (8); In the formula, This indicates the 8th harmonic negative sequence current on the station side. This indicates the 8th harmonic positive sequence current on the station side. This indicates the 8th harmonic zero-sequence current on the station side. , , These represent the corresponding proportionality coefficients. When a three-phase short-circuit fault occurs, since only the 8th harmonic negative sequence current exists, at this time... proportionality coefficient and None of them will be to This has an impact. When an asymmetrical short-circuit fault occurs, due to the existence of... and One or both exist, therefore a scaling factor is set. and This will increase the denominator, thus making Decrease. and Relative values The larger the value, the more likely it is to cause asymmetric faults. The smaller the value, the better. Therefore, to ensure a clear difference between symmetrical and asymmetrical faults, this paper suggests taking... , , The values ​​are 0.2, 0.4, and 0.4 respectively.

[0042] (2) The fault symmetry identification criterion is constructed as shown in equation (9).

[0043] (9); In the formula, Theoretically, it should be a number close to 1, but in order to reduce the influence of the small amplitude of the 8th harmonic negative sequence current in the few milliseconds after the injected signal, The value can be appropriately reduced. Considering the difference from asymmetric faults, this paper takes... If equation (9) is satisfied, it is determined to be a three-phase short circuit fault; otherwise, the fault type and fault phase will be further determined.

[0044] Step 4: Criteria for identifying ground faults and phase-to-phase faults based on the amplitude of the 8th harmonic zero-sequence current: The determination of whether a ground fault is present is based on the amplitude of the 8th harmonic zero-sequence current on the station side, as shown in equation (10): (10); In the formula, This indicates the amplitude of the 8th harmonic zero-sequence current on the station side. This represents the threshold value of the 8th harmonic zero-sequence current amplitude on the station side. As the transition resistance increases and the fault location moves further away from the station side... Gradually decreasing, therefore, The line should be set according to the high-resistance grounding at the end of the line and a certain margin should be reserved. According to the line parameters in this paper, the final value is 0.005kA. If equation (10) is satisfied, it is judged as a two-phase short circuit fault; otherwise, it is judged as a ground fault.

[0045] Step 5: Single-phase ground fault phase identification based on the 8th harmonic voltage content: If the fault is determined to be ground fault according to equation (10), based on the characteristic that the amplitude of the 8th harmonic voltage of the fault phase is much lower than that of the 8th harmonic voltage of the non-fault phase when a single-phase ground fault occurs, the phase 8th harmonic voltage content rate is first defined as shown in equation (11), and the single-phase ground fault selection criteria are established as shown in equations (12), (13) and (14).

[0046] (11); (12); (13); (14); In the formula, , and These represent the 8th harmonic voltages of phases A, B, and C on the station side, respectively. , and These represent the 8th harmonic voltage content of phases A, B, and C, respectively, taking into account high-resistance grounding at the end of the line and phase selection sensitivity settings. The value is 0.5.

[0047] If equation (11) is satisfied, it is determined to be a phase A ground fault; if equation (12) is satisfied, it is determined to be a phase B ground fault; if equation (13) is satisfied, it is determined to be a phase C ground fault.

[0048] Step 6: Determine the phase of a two-phase short circuit and a two-phase short circuit to ground fault based on the phase difference of the 8th harmonic voltage. (1): If the ground fault is determined according to equation (10), and none of them satisfy equations (12), (13) and (14), then it is determined to be a two-phase short-circuit ground fault.

[0049] Considering that the phases of the 8th harmonic voltages of the two faulted phases are close when a phase-to-phase short circuit or a phase-to-phase short circuit to ground fault occurs, while the phases of the 8th harmonic voltages of the faulted phase and the non-faulted phase are significantly different, the phase difference of the 8th harmonic voltages is defined as shown in equation (33): (15); In the formula, Desirable , , ,correspond Desirable , , .

[0050] Further, the fault phase is determined based on equations (16), (17) and (18).

[0051] (16); (17); (18); Considering the phase selection sensitivity setting, in the formula If equation (16) is satisfied, it is determined to be an AB phase-to-phase short-circuit ground fault; if equation (17) is satisfied, it is determined to be a BC phase-to-phase short-circuit ground fault; if equation (18) is satisfied, it is determined to be a CA phase-to-phase short-circuit ground fault.

[0052] (2): If it is determined to be a two-phase short circuit fault according to equation (10), the fault phase is determined according to equations (16), (17) and (18). If equation (16) is satisfied, it is determined to be an AB phase-to-phase short circuit fault; if equation (17) is satisfied, it is determined to be a BC phase-to-phase short circuit fault; if equation (18) is satisfied, it is determined to be a CA phase-to-phase short circuit fault.

[0053] Step 7: Simulation Verification: Building a photovoltaic power plant transmission line model in PSCAD, such as Figure 2As shown. The photovoltaic power station has a capacity of 250MW, the transformer ratio on the photovoltaic power station side is 38.5 / 242kV, the connection group is YND11, the capacity is 300MW, the transmission line WS is 100km long, and the positive sequence equivalent impedance of the line is... The zero-sequence equivalent impedance is The equivalent impedance of the system is F2, F3, and F4 are faults located within the system area at distances of 75km, 50km, and 25km from the system side, respectively. F1 is an external fault on the line depot side, and F5 is an external fault on the line system side. The system frequency is 50Hz, and the sampling frequency is 5kHz.

[0054] A three-phase short circuit occurs at F3 in the line area at t=3.5s. The negative sequence current injection rate is calculated according to formula (8). The results are shown in Figure 4(a) and Figure 4(b). Since the calculated value is within the set threshold range after the fault, it is determined to be a three-phase short circuit fault.

[0055] A two-phase metallic short-circuit fault occurs at F3 in the line area at t=3.5s. The negative sequence current injection rate is calculated according to equation (8), and the result is shown in Figure 5(a). Since the calculated value is outside the set threshold range after the fault, it is first determined to be a non-three-phase short-circuit fault. The amplitude of the zero-sequence current of the 8th harmonic is calculated as shown in Figure 5(b). According to Figure 5(b), the amplitude of the zero-sequence current of the 8th harmonic after the fault satisfies equation (10), so it is determined to be a two-phase short-circuit fault. The phase of the 8th harmonic voltage of each phase is extracted and the phase difference of the 8th harmonic voltage of each phase is calculated as shown in Figure 6(a) and Figure 6(b). According to Figure 6(a) and Figure 6(b), the phase of the 8th harmonic voltage of the two faulty phases A and B is close after the fault, while the healthy phase C has a large phase difference with the two faulty phases A and B. Therefore, the phase difference between the two phases A and B approaches 0 and is lower than the set threshold. Therefore, it is determined to be a two-phase short-circuit fault between A and B, so the faulty phase can be accurately identified.

[0056] A metallic ground fault of phase A occurred at F3 in the line area at t=3.5s. The negative sequence current injection rate was calculated according to equation (8), and the result is shown in Figure 7(a). Since the calculated value was outside the set threshold range after the fault, it was first determined to be a non-three-phase short circuit fault. The amplitude of the zero-sequence current of the 8th harmonic was calculated as shown in Figure 7(b). According to Figure 7(b), the amplitude of the zero-sequence current of the 8th harmonic on the station side does not satisfy equation (10), so it is determined to be a ground fault. The voltage content of the 8th harmonic of each phase was further calculated as follows: Figure 8 As shown. By Figure 8 It can be seen that the 8th harmonic voltage content rate of each phase after the fault satisfies equation (11), therefore it is determined to be a phase A ground fault.

[0057] A two-phase metallic short-circuit ground fault occurred at F3 in the line area at t=3.5s. The negative sequence current injection rate was calculated according to equation (8), and the result is shown in Figure 9(a). Since the calculated value was outside the set threshold range after the fault, it was initially determined to be a non-three-phase short-circuit fault. The amplitude of the 8th harmonic zero-sequence current was calculated, as shown in Figure 9(b). According to Figure 9(b), the amplitude of the 8th harmonic zero-sequence current on the station side did not satisfy equation (10), therefore it was determined to be a ground fault. Further calculations were made of the 8th harmonic voltage content of each phase, as shown in Figure 9(b). Figure 10 As shown. By Figure 10 It can be seen that the content of the 8th harmonic voltage of each phase after the fault does not satisfy equations (12), (13) and (14). Therefore, the phase of the 8th harmonic voltage of each phase is extracted and the phase difference of the 8th harmonic voltage of each phase is calculated as shown in Figure 11(a) and Figure 11(b). According to Figure 11(a) and Figure 11(b), the phase of the 8th harmonic voltage of the two faulty phases A and B is close after the fault, while the healthy phase C has a large phase difference with the two faulty phases A and B. Therefore, the phase difference between phases A and B approaches 0 and is lower than the set threshold. Therefore, it is determined to be a short-circuit ground fault between phases A and B, so the faulty phase can be accurately identified.

[0058] To further verify the proposed protection scheme's ability to withstand transition resistance, the phase selection results were verified under single-phase ground fault, two-phase short circuit, and two-phase short circuit-to-ground scenarios, respectively. The results are listed in Tables 1, 2, and 3. Simulation results show that this method can adapt to phase selection for faults in new energy transmission lines.

[0059]

[0060]

[0061]

[0062] The present invention proposes a method for fault phase selection of photovoltaic power plant transmission lines based on feature signal injection, which has the following characteristics: ①: This method solves the problem that the phase selection performance of traditional phase selection elements is reduced or even fails due to the negative sequence current suppression strategy when connected to new energy power stations.

[0063] ②: It has strong resistance to transition resistance and can still correctly select the phase when a single-phase grounding occurs at the end of the line through a 300-ohm transition resistance.

[0064] ③: It can achieve correct phase selection under different fault locations and fault scenarios.

Claims

1. A method for selecting phase faults in photovoltaic power plant transmission lines based on feature signal injection, characterized in that... Includes the following steps: Step 1: Construct the feature signal injection initiation criterion; Step 2: Dynamically allocate the amplitude of the injected harmonic current to each photovoltaic unit according to the output ratio of each photovoltaic unit; Step 3: Based on the negative sequence current injection rate, determine whether it is a three-phase short circuit fault; Step 4: Based on the amplitude of the harmonic zero-sequence current, determine whether it is a ground fault or a phase-to-phase fault; Step 5: For both two-phase phase-to-phase short circuit faults and two-phase phase-to-phase short circuit to ground faults, the phase harmonic voltage phase difference is used to identify the faulty phase; for single-phase ground faults, the phase harmonic voltage content rate is used to identify the faulty phase.

2. The method for selecting phase faults in photovoltaic power plant transmission lines based on feature signal injection according to claim 1, characterized in that: In step 1, based on the instantaneous change in current value... Second difference from the instantaneous current value Calculate weighting factors As a triggering criterion; specifically including: S1.1: Calculate the instantaneous change in current value Second difference from the instantaneous current value : （1）； （2）； In the above formula, This indicates the instantaneous change in the current value at the current sampling moment; This represents the second-order difference of the instantaneous current value at the current sampling moment; This represents the instantaneous current value at the current sampling moment; This represents the instantaneous current value at the previous sampling time; express The instantaneous value of the current at the previous sampling moment; Indicates the previous sampling time; express The previous sampling time; Indicates the current sampling time; This represents the time window between the current sampling time and the previous sampling time; S1.2: The instantaneous change in current value in S1.1 Second difference from the instantaneous current value Perform normalization and calculate its weighting factors. : （3）； In equation (3), This represents the current weighting factor at the current sampling time; Indicates the reference current; Indicates the proportionality coefficient; Indicates the power frequency angular frequency; S1.3: Weighting factors The value is compared with the set threshold. If the condition is met, the injection is initiated. （4）； In equation (4), This represents the current weighting factor at the current moment; This represents the threshold of the current weighting factor.

3. The method for fault phase selection of photovoltaic power plant transmission lines based on feature signal injection according to claim 2, characterized in that: Step 2 includes the following steps: S2.1: Calculate the percentage of rated operating power output for each photovoltaic unit. : （5）； In equation (5), This indicates the percentage of rated operating power output of each photovoltaic unit; This indicates the rated active power output of each photovoltaic unit. This indicates the rated active power output of all photovoltaic units. The values ​​1, 2, ... can be used to represent the 1st, 2nd, ...th photovoltaic unit, respectively; S2.2: According to Calculation of the total harmonic current amplitude of the line based on the rated current of the line: （6）； In equation (6), Indicates the rated current amplitude of the transmitting line. This indicates the total harmonic current amplitude injected into the transmitting line; S2.3: Based on the output ratio of each photovoltaic unit in S2.1 Calculate the amplitude of the corresponding injected harmonic current: （7）； In equation (7), This indicates the amplitude of the harmonic current injected into each photovoltaic unit.

4. The method for fault phase selection of photovoltaic power plant transmission lines based on feature signal injection according to claim 3, characterized in that: Step 3 includes the following steps: S3.1: Calculate the 8th harmonic negative sequence current injection rate : （8）； In equation (8), This indicates the 8th harmonic negative sequence current on the station side. This indicates the 8th harmonic positive sequence current on the station side. This indicates the 8th harmonic zero-sequence current on the station side. , , These represent the corresponding proportionality coefficients; S3.2: Construct the fault symmetry identification criterion as shown in equation (9): （9）； In equation (9), The threshold for negative sequence current injection rate is denoted as ; if equation (9) is satisfied, it is determined to be a three-phase short circuit fault; otherwise, the fault type and fault phase will be further determined.

5. The method for selecting phase faults in photovoltaic power plant transmission lines based on feature signal injection according to claim 4, characterized in that: In step 4, the ground fault and phase-to-phase fault are determined based on the amplitude of the 8th harmonic zero-sequence current on the station side, as shown in equation (10): （10）； In equation (10), This indicates the amplitude of the 8th harmonic zero-sequence current on the station side; The threshold value of the zero-sequence current of the 8th harmonic on the station side; if it satisfies equation (10), it is judged as a two-phase short circuit fault, otherwise it is judged as a ground fault.

6. The method for selecting phase faults in photovoltaic power plant transmission lines based on feature signal injection according to claim 5, characterized in that: Step 5 includes: If the fault is determined to be ground fault according to equation (10), based on the characteristic that the amplitude of the 8th harmonic voltage of the fault phase is much lower than that of the 8th harmonic voltage of the non-fault phase when a single-phase ground fault occurs; firstly, the phase 8th harmonic voltage content rate is defined as shown in equation (11), and the single-phase ground fault phase selection criteria are established as shown in equations (12), (13) and (14). （11）； （12）； （13）； （14）； In the above formula, , and These represent the 8th harmonic voltages of phases A, B, and C on the station side, respectively. , and These represent the voltage content of the 8th harmonic of the three phases A, B, and C, respectively. This represents the proportionality coefficient.

7. The method for fault phase selection of photovoltaic power plant transmission lines based on feature signal injection according to claim 6, characterized in that: If equation (11) is satisfied, it is determined to be a phase A ground fault; if equation (12) is satisfied, it is determined to be a phase B ground fault; if equation (13) is satisfied, it is determined to be a phase C ground fault.

8. The method for fault phase selection of photovoltaic power plant transmission lines based on feature signal injection according to claim 7, characterized in that: Step 5 further includes: if the ground fault is determined according to formula (10), and formulas (12), (13) and (14) are not satisfied, then it is determined to be a two-phase short-circuit ground fault; Considering that the phases of the 8th harmonic voltages of the two faulted phases are close when a phase-to-phase short circuit or a phase-to-phase short circuit to ground fault occurs, while the phases of the 8th harmonic voltages of the faulted phase and the non-faulted phase are significantly different, the phase difference of the 8th harmonic voltages is defined as shown in equation (15): （15）； In equation (15), This indicates the phase difference of the 8th harmonic voltage; This indicates the phase of the 8th harmonic voltage of phase X; This indicates the phase of the 8th harmonic voltage in the Y phase; Pick or or ,correspond Pick or or .

9. The method for fault phase selection of photovoltaic power plant transmission lines based on feature signal injection according to claim 8, characterized in that: The fault phase is determined according to equations (16), (17), and (18): （16）； （17）； （18）； Considering the phase selection sensitivity setting, If equation (16) is satisfied, it is determined to be an AB phase-to-phase short-circuit ground fault; if equation (17) is satisfied, it is determined to be a BC phase-to-phase short-circuit ground fault; if equation (18) is satisfied, it is determined to be a CA phase-to-phase short-circuit ground fault.

10. The method for selecting phase faults in photovoltaic power plant transmission lines based on feature signal injection according to claim 9, characterized in that: If the fault is determined to be a two-phase short circuit according to equation (10), the fault phase is determined according to equations (16), (17) and (18). If equation (16) is satisfied, the fault is determined to be an AB phase-to-phase short circuit fault; if equation (17) is satisfied, the fault is determined to be a BC phase-to-phase short circuit fault; if equation (18) is satisfied, the fault is determined to be a CA phase-to-phase short circuit fault.

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