Photovoltaic sending line pilot protection method and system based on dice waveform similarity

Abstract

The application discloses a photovoltaic sending-out line pilot protection method and system based on a Dice waveform similarity, and belongs to the technical field of line pilot protection. The method comprises the following steps: selecting the sampling data window length on both sides of the photovoltaic sending-out line, and continuously sampling the voltage and current on both sides of the line; calculating the Dice similarity of the sampling current of each phase on both sides of the line after transient capacitance current compensation according to the voltage and current sampling values on both sides of the line, wherein the Dice similarity criterion is 1 if the Dice similarity is greater than the setting value, and the Dice similarity criterion is 0 otherwise; and sending the corresponding phase tripping signal according to the Dice similarity criterion, wherein the corresponding phase protection tripping signal is sent if the Dice similarity criterion is 1, and the corresponding phase protection tripping signal is not sent if the Dice similarity criterion is 0, so that the photovoltaic sending-out line pilot protection can correctly act in the internal fault and external fault of the 500kV line. Through the application, the problem of the sensitivity reduction of the photovoltaic sending-out line differential protection can be effectively solved under various transition resistance short-circuit faults and other complex conditions at various positions of the line, and the application has the characteristics of simplicity, reliability, easy operation and strong applicability.

Description

Technical Field

[0001] This invention belongs to the field of longitudinal protection technology for transmission lines, and more specifically, relates to a method and system for longitudinal protection of photovoltaic transmission lines based on Dice waveform similarity. Background Technology

[0002] To address the severe energy crisis and environmental pollution, photovoltaic (PV) power generation has become a widely used renewable energy source in power grid systems. As of the end of November 2022, China's installed PV capacity reached 370 million kilowatts, a year-on-year increase of 29.4%, and PV power plants are increasingly being directly connected to the 500 kV main grid via power plant busbars. Because the short-circuit current characteristics of PV power and other new energy sources differ significantly from those of synchronous motors, and because large-capacity PV provides substantial power, the short-circuit current is close to the system-side short-circuit current. The 500 kV grid is used for large-capacity transmission; if the protection system cannot trip quickly when a PV transmission line fault occurs, it may lead to equipment damage and fault expansion, and also affect the reclosing process. In severe cases, it could impact the power supply security of society, as seen in the US-Canada power outage. Furthermore, the 500 kV grid has a higher transition resistance. Therefore, when an internal fault occurs on a 500 kV line, it is crucial to ensure that the PV transmission line protection operates more sensitively and quickly, which is essential for the stable operation and safety of the power grid.

[0003] As an inverter-type renewable energy source, photovoltaic (PV) power supplies enter a low-voltage ride-through process after a line fault. The phase angle of the short-circuit current fed out by the PV power supply is controlled and related to the degree of voltage drop at the inverter port. During a line fault, the distance protection of the PV transmission line often fails to operate correctly. When a fault occurs within the PV transmission line's coverage area, the phase angle difference between the currents on both sides of the line may exceed 90 degrees, which can lead to a decrease in the sensitivity of the line current differential protection.

[0004] Correspondingly, how to develop a simple, reliable and easy-to-operate longitudinal protection method for photovoltaic power transmission lines has become one of the technical problems that urgently need to be solved in this field. Summary of the Invention

[0005] To address the shortcomings and improvement needs of existing technologies, this invention provides a method and system for longitudinal protection of photovoltaic transmission lines based on Dice waveform similarity, aiming to solve the technical problems of incorrect operation of distance protection and decreased sensitivity of current differential protection caused by photovoltaic transmission line faults.

[0006] To achieve the above objectives, in a first aspect, the present invention provides a method for longitudinal protection of photovoltaic transmission lines based on Dice waveform similarity, comprising the following steps:

[0007] S101. Select the sampling data window length on both sides of the photovoltaic transmission line and continuously sample the voltage and current on both sides of the line;

[0008] S102. Calculate the Dice similarity of the sampled currents of each phase after transient capacitor current compensation on both sides of the line based on the voltage and current sampled values ​​on both sides of the line. If it is greater than the set value, the Dice similarity criterion is 1; otherwise, it is 0.

[0009] S103. Send the corresponding phase trip signal according to the Dice similarity criterion calculated in S102. If it is 1, send the corresponding phase protection trip signal; if it is 0, do not send.

[0010] Furthermore, the currents on both sides of the line after transient capacitor current compensation are:

[0011]

[0012] Among them, i w_comp and i s_comp These are the compensated currents on both sides of the line, i w and i s These are the measured currents on both sides of the line, u w and u s These are the phase voltages on both sides of the line, u w0 and u s0 These are the zero-mode voltages on both sides of the line, and C1 and C0 are the positive-sequence and zero-sequence capacitances of the line, respectively.

[0013] Furthermore, the Dice similarity of the sampling currents of each phase is:

[0014]

[0015] Where x and y are the currents on both sides of the line after transient capacitor current compensation corresponding to all sampling points within a sampling data window, x n y n The current on both sides of the line corresponding to the nth sampling point within a sampling data window after transient capacitor current compensation, where N is the number of sampling points within a sampling data window, and r Dice (x,y) represents the similarity value of the last sampling point in the sampling data window.

[0016] Furthermore, the sampling data window length is 10ms, and the setpoint is -0.9.

[0017] To achieve the above objectives, in a second aspect, the present invention provides a photovoltaic transmission line longitudinal protection system based on Dice waveform similarity, comprising:

[0018] The sampling module is used to select the sampling data window length on both sides of the photovoltaic transmission line and to continuously sample the voltage and current on both sides of the line.

[0019] The calculation module is used to calculate the Dice similarity of the sampled current of each phase after transient capacitor current compensation on both sides of the line based on the voltage and current sampled values ​​on both sides of the line. If it is greater than the set value, the Dice similarity criterion is 1, otherwise it is 0.

[0020] The processing module is used to send the corresponding phase trip signal according to the Dice similarity criterion calculated by the calculation module. If the value is 1, the corresponding phase protection trip signal is sent; if the value is 0, no signal is sent.

[0021] Furthermore, the currents on both sides of the line after transient capacitor current compensation are:

[0022]

[0023] Among them, i w_comp and i s_comp These are the compensated currents on both sides of the line, i w and i s These are the measured currents on both sides of the line, u w and u s These are the phase voltages on both sides of the line, u w0 and u s0 These are the zero-mode voltages on both sides of the line, and C1 and C0 are the positive-sequence and zero-sequence capacitances of the line, respectively.

[0024] Furthermore, the Dice similarity of the sampling currents of each phase is:

[0025]

[0026] Where x and y are the currents on both sides of the line after transient capacitor current compensation corresponding to all sampling points within a sampling data window, x n y n The current on both sides of the line corresponding to the nth sampling point within a sampling data window after transient capacitor current compensation, where N is the number of sampling points within a sampling data window, and r Dice (x,y) represents the similarity value of the last sampling point in the sampling data window.

[0027] Furthermore, the sampling data window length is 10ms, and the setpoint is -0.9.

[0028] In summary, the above-described technical solutions conceived in this invention can achieve the following beneficial effects:

[0029] (1) This invention does not use the criteria for differential protection of phase current of photovoltaic transmission line, and no longer identifies inrush current by differential current and braking current. Instead, it uses the Dice similarity criterion of current on both sides of photovoltaic transmission line to effectively solve the problems of incorrect operation of distance protection and decreased sensitivity of current differential protection caused by photovoltaic transmission line faults. It can accurately identify internal line faults and high transition resistance short circuits.

[0030] (2) The photovoltaic transmission line longitudinal protection method based on Dice waveform similarity according to the present invention can be applied to photovoltaic power plants of different capacities. It can operate accurately when the photovoltaic transmission line passes through a large transition resistance fault. It has the characteristics of reliability, ease of operation and strong applicability. Therefore, it is especially suitable for large-scale photovoltaic transmission line applications. It can also reduce the safety and stability operation risks of the power grid system caused by the unique short-circuit current characteristics of photovoltaic power plants, which is of great significance. Attached Figure Description

[0031] Figure 1 This is an overall flowchart of the photovoltaic transmission line longitudinal protection method based on Dice waveform similarity constructed according to the present invention;

[0032] Figure 2 This is a topology diagram of the photovoltaic power station and its adjacent power grid;

[0033] Figure 3 These are the instantaneous current waveforms simulating the F3 fault; where (a) is phase A of the AG fault, (b) is phase B of the BC fault, and (c) is phase C of the BC fault.

[0034] Figure 4 It is the Dice similarity of the current in the simulation of F3 fault; where (a) is phase A of AG fault, (b) is phase B of BC fault, and (c) is phase C of BC fault.

[0035] Figure 5 The current Dice similarity of the F3 fault under a simulated 5ms data window is shown; where (a) is phase A of the AG fault, (b) is phase B of the BC fault, and (c) is phase C of the BC fault.

[0036] Figure 6 The current Dice similarity of the F3 fault under a simulated 20ms data window is shown; where (a) is phase A of the AG fault, (b) is phase B of the BC fault, and (c) is phase C of the BC fault.

[0037] Figure 7 The simulation is the Dice similarity of the A-phase current at AG fault F3; where (a) is noise-free and (b) has 20dB SNR noise. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0039] In this invention, the terms "first," "second," etc. (if present) in the invention and the accompanying drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence.

[0040] See Figure 1 This invention provides a method for longitudinal protection of photovoltaic transmission lines based on Dice waveform similarity, comprising the following steps:

[0041] S101. Select the sampling data window length on both sides of the photovoltaic transmission line to continuously sample the voltage and current on both sides of the line.

[0042] In an optional embodiment, the sampling data window length is 10ms.

[0043] S102. Calculate the Dice similarity of the sampled currents of each phase after transient capacitor current compensation on both sides of the line based on the voltage and current sampled values ​​on both sides of the line. If it is greater than the set value, the Dice similarity criterion is 1, otherwise it is 0.

[0044] It should be noted that since there is capacitive current in the line, directly using the voltage and current sample values ​​from both sides of the line for similarity calculation will produce errors. Therefore, a transient capacitive current compensation method is required.

[0045]

[0046] Among them, i w_comp and i s_comp These are the compensated currents on both sides of the line, i w and i s These are the measured currents on both sides of the line, u w and u s These are the phase voltages on both sides of the line, u w0 and u s0 These are the zero-mode voltages on both sides of the line, and C1 and C0 are the positive-sequence and zero-sequence capacitances of the line, respectively.

[0047] The Dice similarity criteria for actions are as follows:

[0048] r Dice (i w_comp i s_comp )>r set

[0049] Where, r set To account for CT transmission error and margin, the protection setting value is set to -0.9.

[0050] In this embodiment, the Dice similarity of the sampling currents of each phase is:

[0051]

[0052] Where x and y are two vectors, representing the currents on both sides of the line after transient capacitor current compensation corresponding to all sampling points within a sampling data window, x n y n The current on both sides of the line corresponding to the nth sampling point within a sampling data window after transient capacitor current compensation, where N is the number of sampling points within a sampling data window, and r Dice (x,y) represents the similarity value of the last sampling point in the sampling data window.

[0053] S103. Send the corresponding phase trip signal according to the Dice similarity criterion calculated in S102. If it is 1, send the corresponding phase protection trip signal; if it is 0, do not send.

[0054] After a fault occurs, the protection system is activated via a starting element. The protection device samples current data from both sides of the line, performs Dice similarity calculation after applying a transient capacitor current compensation algorithm, and if the Dice similarity of three consecutive sampling points is greater than the set value (i.e., the Dice similarity criterion is 1), it is determined to be an internal line fault, and the protection system sends a trip signal. Conversely, if the Dice similarity criterion is less than 1, it is determined to be an external line fault, and the protection system resets.

[0055] The effectiveness of the present invention will be further verified through simulation.

[0056] According to a certain project Figure 2 The photovoltaic power station topology shown was simulated in PSCAD / EMTDC software. The photovoltaic power source was connected to the 35kV bus via a transformer, stepped up to 500kV by the main transformer, and then sent to the external system via the L1 line. To verify the adaptability of the protection method, the operating condition was set to the system's lowest operating mode and the most unfavorable heavy load condition before the photovoltaic fault. Single-phase grounding (AG), two-phase short circuit (BC), two-phase short circuit to ground (BCG), and three-phase short circuit (ABC) faults were respectively set at the photovoltaic side zone outlet (F1), photovoltaic side zone near zone (F2), L1 line midpoint (F3), system side zone near zone (F4), and system side zone outlet (F5). The fault occurrence time was 0.8s, the sampling frequency was 1.2kHz, and the data window was 10ms to verify the advantages and adaptability of the longitudinal protection method proposed in this invention.

[0057] (1) Comparison of existing line differential protection and similarity methods

[0058] Simulations were performed on phase A short-circuited through the transition resistor at point F1 and phases B and C short-circuited through the transition resistor. The results are shown in Tables 1 and 2. To compare the sensitivity of the ratio differential protection and the similarity method, the sensitivity of the three methods is defined as: K diff =I CDφ / 0.75I RDφ K cos =(r cos +0.9) / 0.1, K Dice =(r Dice As shown in Table 2, when a metallic two-phase short-circuit fault occurs within the line area, the hysteresis ratio differential protection has the lowest sensitivity of only 1.061, posing a risk of failure to operate. With the increase of the AG fault transition resistance, the similarity of the fault phase current waveforms on both sides of the line first increases and then decreases. However, with the increase of the BC fault transition resistance, the cosine similarity of the fault phase current decreases, consistent with theoretical analysis. In the case of a 220Ω transition resistance AG fault, the phases of the currents on both sides are close, and the cosine similarity will be lower than the setting value of -0.9, causing ratio differential protection and cosine similarity-based protection to fail to operate. However, the Dice similarity is still -0.462, indicating that Dice similarity-based protection can correctly reflect 700Ω high-resistance faults and performs well. Furthermore, during a two-phase short circuit, the sensitivity of Dice similarity-based protection is the lowest at 7.54, superior to ratio differential protection and cosine similarity-based protection methods.

[0059] Table 1. Sensitivity and accuracy of various methods under different transition resistances in AG faults.

[0060]

[0061]

[0062] Table 2. Sensitivity and accuracy of various methods under different transition resistances in BC fault.

[0063]

[0064] (2) Dice similarity protection performance under different fault locations and types

[0065] Table 3 shows the Dice similarity of the currents on both sides of the line under different fault conditions. Taking F3 with AG and BC metallic faults as an example, the simulated currents on both sides of the line and their Dice similarity are as follows. Figure 3 and Figure 4As shown in the figure. According to the simulation results, the line protection method based on Dice similarity performs well under metallic fault conditions and can correctly identify the fault. The similarity of the three-phase current outside the fault zone and the similarity of the non-faulty phase current inside the fault zone are both close to -1. However, for the similarity of the currents on both sides of the faulty phase inside the fault zone, Dice similarity reflects the difference in the amplitude of the currents on both sides. Its absolute value is small, and it can act within 5ms, showing good speed.

[0066] Table 3. Dice similarity of currents on both sides of the line under various fault conditions.

[0067]

[0068]

[0069] (3) Impact of data window length

[0070] The length of the algorithm's data window affects the response speed of the protection method. This invention sets data window lengths of 5ms, 10ms, and 20ms to verify the protection performance under different data window lengths. Taking an AG and BC metallic fault occurring at F3 as an example, the similarity of the simulated current waveforms on both sides of the line is as follows: Figure 5 and Figure 6 As shown, it is recommended to select a 10ms data window. At this time, the Dice similarity has reached a relatively stable value, and the protection has good speed.

[0071] (4) Impact of line length

[0072] The length of the line affects the magnitude of the short-circuit current. Simulations were set with different line lengths to verify the effectiveness of the proposed method. According to the simulation results, the Dice similarity method proposed in this invention can still correctly identify faults inside and outside the zone and take the correct action under different line lengths.

[0073] Table 4. Cosine similarity of currents on both sides of the line and Dice similarity for various line lengths.

[0074]

[0075]

[0076] (5) Impact of photovoltaic capacity

[0077] Photovoltaic capacity also affects the magnitude of short-circuit current. Simulations were set with different photovoltaic capacities to verify the effectiveness of the proposed method. According to the simulation results, the Dice similarity method proposed in this invention can still correctly identify faults inside and outside the zone and take correct action under different photovoltaic capacities.

[0078] Table 5. Cosine similarity of currents on both sides of the line and Dice similarity under various photovoltaic capacities.

[0079]

[0080] (6) Noise impact

[0081] The data measured by the protection device can be affected by noise. Simulations were performed using Gaussian white noise with different signal-to-noise ratios (SNR) to verify the effectiveness of the proposed method. Generally, an SNR of at least 40 dB is considered acceptable. Taking a metallic fault (AG and BC) at F3 with an SNR of 20 dB as an example, the similarity of the current waveforms on both sides of the simulated line is as follows: Figure 7 As shown, the simulation results combined with the measured current containing Gaussian white noise with different signal-to-noise ratios show that the Dice similarity method proposed in this invention can still correctly identify faults inside and outside the zone and take correct action even when the current signal contains noise with different signal-to-noise ratios.

[0082] Table 6. Dice similarity of currents on both sides of the line under different signal-to-noise ratios.

[0083]

[0084]

[0085] Comprehensive simulation results show that, regardless of various fault locations, fault types, transition resistances, line lengths, photovoltaic capacity, and noise levels, the protection system based on Dice waveform similarity for longitudinal protection of photovoltaic transmission lines can operate correctly. Furthermore, the method proposed in this invention outperforms the cosine similarity method under high transition resistance conditions. Using a 10ms data window, the protection system can operate within half a cycle, therefore the criterion proposed in this invention does not affect the speed of protection operation.

[0086] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for longitudinal protection of photovoltaic transmission lines based on Dice waveform similarity, characterized in that, Includes the following steps: S101. Select the sampling data window length on both sides of the photovoltaic transmission line and continuously sample the voltage and current on both sides of the line; S102. Calculate the Dice similarity of the sampled currents of each phase after transient capacitor current compensation on both sides of the line based on the voltage and current sampling values ​​on both sides of the line. If it is greater than the set value, the Dice similarity criterion is 1; otherwise, it is 0. The Dice similarity of the sampled currents of each phase is: in, x , y This represents the current on both sides of the line after transient capacitor current compensation, corresponding to all sampling points within a sampling data window. x n , y n For a sampling data window, the first n The current on both sides of the line corresponding to each sampling point after transient capacitor current compensation N The number of sampling points within a sampling data window. This is the similarity value of the last sample point in the sampling data window; S103. Send the corresponding phase trip signal according to the Dice similarity criterion calculated in S102. If it is 1, send the corresponding phase protection trip signal; if it is 0, do not send.

2. The photovoltaic transmission line longitudinal protection method based on Dice waveform similarity according to claim 1, characterized in that, The currents on both sides of the line after transient capacitor current compensation are: in, i w_comp and i s_comp These are the compensated currents on both sides of the line. i w and i s These are the measured currents on both sides of the line. u w and u s These are the phase voltages on both sides of the line. u w0 and u s0 These are the zero-mode voltages on both sides of the line. C 1 and C 0 represents the positive-sequence and zero-sequence capacitances of the circuit, respectively.

3. The photovoltaic transmission line longitudinal protection method based on Dice waveform similarity according to claim 1, characterized in that, The sampling data window length is 10ms, and the setpoint is -0.

9.

4. A photovoltaic transmission line longitudinal protection system based on Dice waveform similarity, characterized in that, include: The sampling module is used to select the sampling data window length on both sides of the photovoltaic transmission line and to continuously sample the voltage and current on both sides of the line. The calculation module is used to calculate the Dice similarity of the sampled currents of each phase after transient capacitor current compensation on both sides of the line based on the voltage and current sampled values ​​on both sides of the line. If it is greater than the set value, the Dice similarity criterion is 1; otherwise, it is 0. The Dice similarity of the sampled currents of each phase is as follows: in, x , y This represents the current on both sides of the line after transient capacitor current compensation, corresponding to all sampling points within a sampling data window. x n , y n For a sampling data window, the first n The current on both sides of the line corresponding to each sampling point after transient capacitor current compensation N The number of sampling points within a sampling data window. This is the similarity value of the last sample point in the sampling data window; The processing module is used to send the corresponding phase trip signal according to the Dice similarity criterion calculated by the calculation module. If the value is 1, the corresponding phase protection trip signal is sent; if the value is 0, no signal is sent.

5. The photovoltaic transmission line longitudinal protection system based on Dice waveform similarity according to claim 4, characterized in that, The currents on both sides of the line after transient capacitor current compensation are: in, i w_comp and i s_comp These are the compensated currents on both sides of the line. i w and i s These are the measured currents on both sides of the line. u w and u s These are the phase voltages on both sides of the line. u w0 and u s0 These are the zero-mode voltages on both sides of the line. C 1 and C 0 represents the positive-sequence and zero-sequence capacitances of the circuit, respectively.

6. The photovoltaic transmission line longitudinal protection system based on Dice waveform similarity according to claim 4, characterized in that, The sampling data window length is 10ms, and the setpoint is -0.9.

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