Pilot protection method and system for offshore wind power low-frequency cable outgoing line

Abstract

The invention discloses a pilot protection method for an offshore wind power low-frequency cable outgoing line, and belongs to the technical field of power system relay protection, the pilot protection method comprises the following steps: determining a voltage break variable according to acquired voltage information on two sides of a low-frequency cable line, and when the voltage break variable is greater than a preset rated voltage, starting protection and carrying out fault state judgment; according to the maximum value and the minimum value of the current of the current information of the two sides of the low-frequency cable line in the same data window, probability distribution of the current of the two sides of the low-frequency cable line is determined, and a divergence value K of the probability distribution is obtained; comparing the divergence value K value with a setting threshold value, and determining the types of internal and external faults of the low-frequency cable line; and executing corresponding protection operation according to the fault judgment result. According to the method, the current probability distribution difference of the two sides of the low-frequency cable line is extracted, the K-L divergence value K is calculated, and the protection criterion is constructed to perform internal and external fault judgment, so that the accuracy of the judgment result is improved, and the corresponding protection operation is quickly executed.

Description

Technical Field

[0001] The present invention relates to the technical field of relay protection for power systems, and more particularly to a longitudinal protection method and system for offshore wind power low-frequency cable transmission lines. Background Art

[0002] my country boasts abundant wind resources and broad development prospects. By the end of 2023, installed offshore wind power capacity had reached 36.5 million kW. Among offshore wind power transmission methods, low-frequency transmission (LFTS) has become a highly promising grid-connected method for offshore wind power transmission via COSCO Shipping due to its high transmission capacity and low cost. Relay protection, as the first line of defense for the safe operation of power systems, plays a decisive role in their stable operation.

[0003] Existing relay protection technology typically employs a ratio-based differential protection method. This involves collecting current signals from both sides of the low-frequency line in real time using current transformers installed on both sides of the transmission system. A fault is determined based on the relative magnitude of the currents on both sides (the ratio-based braking characteristic). When a fault occurs within the line, the difference in current between the two sides increases significantly, exceeding a preset braking threshold. At this point, the protection device identifies an internal fault. If this is determined to be an internal fault, the protection device quickly issues a trip signal, disconnecting the faulty line.

[0004] In summary, power electronic components are used on both sides of the low-frequency transmission system. Offshore wind farms typically employ permanent magnet synchronous direct-drive wind turbines (PMSGs), while the grid-connected side of the low-frequency cable outgoing line often uses modular multilevel matrix converters (M3Cs) as converters. If a low-frequency cable fails, the influence of the power electronic components on both sides causes the system's electrical quantities to exhibit capacitive characteristics with significant amplitude limitation, phase angle control, nonlinearity, and frequency deviation. This increases the current harmonic content, which in turn affects the current measurement results of traditional ratio-based differential protection, increasing measurement errors, reducing the accuracy of the judgment results, and affecting the accuracy of the protection device's execution when a fault occurs. Summary of the Invention

[0005] In response to the problems existing in the above-mentioned fields, the present invention proposes a longitudinal protection method and system for offshore wind power low-frequency cable transmission lines. The method collects voltage information at both ends of the low-frequency cable line, uses the voltage fault component as the protection startup criterion, collects current information as the original information for fault judgment, and extracts the probability distribution of the current at both ends of the low-frequency cable line. The divergence value K when faults occur inside and outside the submarine cable line area is studied, and the faults inside and outside the area are judged according to the difference in the divergence value K. This protection method can improve the accuracy of the fault judgment results, and thus quickly and targetedly execute protection for the judgment results.

[0006] To solve the above technical problems, the present invention discloses a longitudinal protection method for an offshore wind power low-frequency cable transmission line, comprising the following steps: Obtain current and voltage information on both sides of the low-frequency cable line; According to the voltage information on both sides of the low-frequency cable line, when the voltage mutation is greater than the preset rated voltage, the protection is activated and the fault status is judged; According to the current information on both sides of the low-frequency cable line, by determining the current direction, the data sampling sequence of the current on both sides of the low-frequency cable line in half a cycle is placed in the same data window, and the maximum and minimum values ​​of the current information on both sides of the low-frequency cable line in the same data window are determined; according to the maximum and minimum values ​​of the current, the current data is equally divided into multiple sub-intervals, and the probability distribution of the current on both sides of the low-frequency cable line is determined by calculating the number of current data on both sides in each sub-interval; according to the probability distribution of the current on both sides of the low-frequency cable line, the divergence value K of the probability distribution is obtained; Compare the divergence value K of the probability distribution with the setting threshold. When the divergence value K is higher than the setting threshold, the low-frequency cable line has an internal fault; when the divergence value K is lower than the setting threshold, the low-frequency cable line has an external fault. Execute corresponding protection operations based on the fault identification results.

[0007] Preferably, the fault status determination specifically includes: By collecting the voltage signals on both sides of the low-frequency cable line, the voltage mutation amount is determined and used as the starting criterion for protection; The startup judgment formula is: In the formula, the voltage mutation amount , is the voltage at the current time minus the voltage one cycle ago; t Indicates the sampling value at the current moment, T is the sampling period, Rated voltage of the system; when When the system is abnormal, the protection starts and enters the fault judgment state; otherwise, it does not start.

[0008] Preferably, determining the maximum and minimum values ​​of the current information on both sides of the low-frequency cable line in the same data window specifically includes: The protection installation location of the low-frequency cable line includes two sampling points, and current information of the two sampling points is obtained, including the current value of the first sampling point W and the current value of the second sampling point M; Taking the current direction from the busbar to the line as positive, the current value on the W side of the first sampling point is taken as positive, and the current value on the M side of the second sampling point is taken as negative; Put the data sampling sequence of the current on both sides of the first sampling point W and the second sampling point M in half a cycle into the same data window, and find the maximum value Ma and the minimum value Mi of the window data.

[0009] Preferably, determining the probability distribution of currents on both sides of the low-frequency cable line specifically includes: According to the maximum value Ma and the minimum value Mi of the window data, the data fluctuation range in the data window is determined to be B=Ma-Mi; Divide B into ten equal subintervals, each with a length of 0.1B and numbered 1 to 10 from low to high. Calculate the number of data points where the current at the first sampling point, side W, and the second sampling point, side M, falls within each subinterval. Calculate the ratio of the number of data points where the current at the first sampling point, side W, and the second sampling point, side M, fall within the subinterval to the total number of sampling points. This is the probability distribution of the current at the first sampling point, side W, and the second sampling point, side M.

[0010] Preferably, obtaining the divergence value K of the probability distribution comprises the following steps: The calculation result of the divergence value K is obtained through the KL divergence calculation formula; The KL divergence calculation formula is: in, and The first sampling point W and the second sampling point M are respectively i The probability distribution corresponding to the current data, N is the number of data in the calculation data window, is the correction factor.

[0011] Preferably, the low-frequency cable line has an in-area fault, specifically comprising: When a low-frequency cable fails within the area, the calculated divergence value K satisfies: Where, is the setting threshold; The setting principle is: Where, To consider the reliability coefficient of the phase angle error on both sides of the low-frequency cable line, including the angle error caused by the current transformer and the angle error caused by the cable capacitance current; To consider the amplitude error caused by the current transformer; is the margin factor.

[0012] Preferably, the divergence value K calculated for the fault outside the low-frequency cable line occurrence area is 0.

[0013] Preferably, it also includes a longitudinal protection system for an offshore wind power low-frequency cable transmission line, comprising: A data acquisition module is used to obtain current and voltage information on both sides of the low-frequency cable line; The protection identification module is used to start the protection and identify the fault status according to the voltage information on both sides of the low-frequency cable line when the voltage mutation amount is greater than the preset rated voltage; The fault identification module is used to place the data sampling sequence of the current on both sides of the low-frequency cable line in a half-cycle in the same data window by determining the current direction based on the current information on both sides of the low-frequency cable line, and determine the maximum and minimum values ​​of the current information on both sides of the low-frequency cable line in the same data window; according to the maximum and minimum values ​​of the current, the current data is equally divided into multiple sub-intervals, and the probability distribution of the current on both sides of the low-frequency cable line is determined by calculating the number of current data on both sides in each sub-interval; according to the probability distribution of the current on both sides of the low-frequency cable line, the divergence value K of the probability distribution is obtained; the divergence value K of the probability distribution is compared with the setting threshold. When the divergence value K is higher than the setting threshold, the low-frequency cable line has an in-zone fault; when the divergence value K is lower than the setting threshold, the low-frequency cable line has an out-of-zone fault; The protection module is used to perform corresponding protection operations according to the fault judgment results.

[0014] Preferably, a computer device is further included, the computer device comprising a memory and a processor, the memory storing a computer program, and when the computer program is executed by the processor, causing the processor to perform the following steps: Obtain current and voltage information on both sides of the low-frequency cable line; According to the voltage information on both sides of the low-frequency cable line, when the voltage mutation is greater than the preset rated voltage, the protection is activated and the fault status is judged; According to the current information on both sides of the low-frequency cable line, by determining the current direction, the data sampling sequence of the current on both sides of the low-frequency cable line in half a cycle is placed in the same data window, and the maximum and minimum values ​​of the current information on both sides of the low-frequency cable line in the same data window are determined; according to the maximum and minimum values ​​of the current, the current data is equally divided into multiple sub-intervals, and the probability distribution of the current on both sides of the low-frequency cable line is determined by calculating the number of current data on both sides in each sub-interval; according to the probability distribution of the current on both sides of the low-frequency cable line, the divergence value K of the probability distribution is obtained; Compare the divergence value K of the probability distribution with the setting threshold. When the divergence value K is higher than the setting threshold, the low-frequency cable line has an internal fault; when the divergence value K is lower than the setting threshold, the low-frequency cable line has an external fault. Execute corresponding protection operations based on the fault identification results.

[0015] Preferably, the present invention further comprises a computer-readable storage medium, wherein the computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the processor is caused to perform the following steps: Obtain current and voltage information on both sides of the low-frequency cable line; According to the voltage information on both sides of the low-frequency cable line, when the voltage mutation is greater than the preset rated voltage, the protection is activated and the fault status is judged; According to the current information on both sides of the low-frequency cable line, by determining the current direction, the data sampling sequence of the current on both sides of the low-frequency cable line in half a cycle is placed in the same data window, and the maximum and minimum values ​​of the current information on both sides of the low-frequency cable line in the same data window are determined; according to the maximum and minimum values ​​of the current, the current data is equally divided into multiple sub-intervals, and the probability distribution of the current on both sides of the low-frequency cable line is determined by calculating the number of current data on both sides in each sub-interval; according to the probability distribution of the current on both sides of the low-frequency cable line, the divergence value K of the probability distribution is obtained; Compare the divergence value K of the probability distribution with the setting threshold. When the divergence value K is higher than the setting threshold, the low-frequency cable line has an internal fault; when the divergence value K is lower than the setting threshold, the low-frequency cable line has an external fault. Execute corresponding protection operations based on the fault identification results.

[0016] Compared with the prior art, the present invention has the following beneficial effects: The present invention proposes a longitudinal protection method for offshore wind power low-frequency cable transmission lines. The longitudinal protection method requires the ability to quickly identify faults. Due to the current amplitude limitation of the fault current of the double-sided power electronic source, the present invention determines the voltage mutation amount based on the current and voltage information on both sides of the low-frequency cable line. When the voltage mutation amount is greater than the preset rated voltage, the protection is activated and the fault state is judged. Taking the current information as the original information for judgment, considering that the protection speed decreases when the data window takes the entire cycle, the algorithm taking 1 / 4 cycle will be affected by the transient state of the fault, which seriously affects the calculation results, so the present invention sets the data window to half a cycle. By determining the current direction, the data sampling sequence of the current on both sides of the low-frequency cable line in the half cycle is placed in the same data window, and the maximum and minimum current information of the current information on both sides of the low-frequency cable line in the same data window is determined. The current data is divided into multiple sub-intervals. By calculating the number of current data on both sides in each sub-interval, the probability distribution of the current on both sides of the low-frequency cable line is determined. By studying the divergence value K of the submarine cable line when faults occur inside and outside the zone, the divergence value K of the probability distribution is compared with the setting threshold to distinguish between inside and outside the zone faults, which improves the accuracy of the judgment results and quickly executes the corresponding protection operations according to different fault types. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a flow chart of the longitudinal protection method for offshore wind power low-frequency cable transmission lines proposed by the present invention.

[0018] Figure 2 This is a structural diagram of a low-frequency offshore wind power transmission system based on M3C provided in an embodiment of the present invention.

[0019] Figure 3 The current waveform interval division provided by the embodiment of the present invention.

[0020] Figure 4 The data in each subinterval during normal operation provided by an embodiment of the present invention; wherein (a) is the data distributed in each subinterval at the first sampling point W, and (b) is the data distributed in each subinterval at the second sampling point M.

[0021] Figure 5 The embodiment of the present invention provides an outside f Phase A waveform and K value during 5 ABC faults.

[0022] Figure 6 The area provided by the embodiment of the present invention f 3 A phase waveform and K value when in AG fault.

[0023] Figure 7 The area provided by the embodiment of the present invention f 4Three-phase waveform and K value changes when BC fault occurs.

[0024] Figure 8 The area provided by the embodiment of the present invention f 2 Changes in waveform and K value at different transition resistances in AG.

[0025] Figure 9 The embodiment of the present invention provides an outside f 1 The waveform and K value change when BCG data is abnormal.

[0026] Figure 10 The area provided by the embodiment of the present invention f 2 Comparison of protection performance when phase A is grounded through high resistance; among them, (a) is the performance effect of the corresponding method of the present invention, (b) is the performance effect of the Pearson correlation method, (c) is the performance effect of the KendaⅡ correlation method, and (d) is the performance effect of the cosine similarity. DETAILED DESCRIPTION

[0027] The following is a combination of the embodiments of the present invention Figures 1-10 , the technical solutions in the embodiments of the present invention are clearly and completely described. It should be understood that the terms used in the present invention are only used to describe specific implementation methods and are not intended to limit the present invention.

[0028] like Figure 1 As shown, the present invention proposes a longitudinal protection method for an offshore wind power low-frequency cable transmission line, comprising the following steps: S1: Obtain the current value of the first sampling point W and the current value and voltage value of the second sampling point M of the low-frequency cable; S2: Determine the voltage mutation amount based on the current value of the first sampling point W and the voltage information of the second sampling point M of the low-frequency cable. When the voltage mutation amount is greater than the preset rated voltage, the protection is activated and the fault status is judged. S3: Based on the current information on both sides of the low-frequency cable line, by determining the current direction, the data sampling sequence of the current on both sides of the low-frequency cable line in half a cycle is placed in the same data window, and the maximum and minimum values ​​of the current information on both sides of the low-frequency cable line in the same data window are determined; according to the maximum and minimum values ​​of the current, the current data is equally divided into multiple sub-intervals, and the probability distribution of the current on both sides of the low-frequency cable line is determined by calculating the number of current data on both sides in each sub-interval; the probability distribution of the current side of the first sampling point W and the current side of the second sampling point M are substituted into the KL divergence formula to obtain the divergence value K of the probability distribution; S4: comparing the divergence value K of the probability distribution with the setting threshold value, when the divergence value K is higher than the setting threshold value, the low-frequency cable line has an internal fault, and when the divergence value K is lower than the setting threshold value, the low-frequency cable line has an external fault; S5: according to the fault discrimination result, performing corresponding protection operation.

[0029] In this step, according to the internal and external fault discrimination result, corresponding protection operation is performed. When the fault state is discriminated, it specifically includes:

[0030] By collecting the voltage signals on both sides of the low-frequency cable line, the voltage mutation is determined, and the voltage mutation is taken as the starting criterion of protection; The starting discrimination formula is: In the formula, the voltage mutation is , the voltage at the current time is subtracted from the voltage one cycle before; t represents the sampling value at the current time, T is the sampling period, is the rated voltage of the system; When the voltage mutation is greater than 0.1 times the rated voltage, that is, , the system is abnormal, the protection starts, and enters the fault discrimination state; otherwise, it does not start.

[0031] The maximum and minimum values of the current in the same data window are determined, specifically including: The protection installation of the low-frequency cable line includes two sampling points, the current information of the two sampling points is obtained, including the current value of the first sampling point W and the current value of the second sampling point M; The current direction of the bus to the line is taken as positive, the current value of the first sampling point W side is taken as positive, and the current of the second sampling point M side is taken as negative; the current of the first sampling point W and the second sampling point M on both sides in the half-cycle data sampling sequence is placed in the same data window, and the maximum value Ma and the minimum value Mi of the current data in the data window are found.

[0032] The probability distribution of the current on both sides of the low-frequency cable line is determined, specifically including: According to the maximum value Ma and the minimum value Mi of the window data, the data fluctuation interval B in the data window is determined as B=Ma-Mi; B is equally divided into ten subintervals, each subinterval is 0.1B long, and is numbered from low to high as 1 to 10; the number of data of the current on the first sampling point W side and the second sampling point M side falling in each subinterval is calculated respectively, and the ratio of the number of data of the current on the first sampling point W side and the second sampling point M side falling in each subinterval to the total sampling points is calculated, that is, the probability distribution of the current on the first sampling point W side and the second sampling point M side.

[0033] Obtaining the divergence value K of the probability distribution includes the following steps: The calculation result of the divergence value K is obtained through the KL divergence calculation formula; The KL divergence calculation formula is: in, and The first sampling point W and the second sampling point M are respectively i The probability distribution corresponding to the current data, N To calculate the number of data in the data window, is the correction factor, take 10 -10 .

[0034] Low-frequency cable line faults occur within the area, including: When a low-frequency cable fails within the area, the calculated divergence value K satisfies: Where, is the setting threshold; The setting principle is: Where, To consider the reliability coefficient of the phase angle error on both sides of the low-frequency cable line, including the angle error caused by the current transformer and the angle error caused by the cable capacitance current; To consider the amplitude error caused by the current transformer; is the margin factor.

[0035] When the low-frequency cable operates normally or an out-of-area fault occurs, the calculated divergence value K is 0.

[0036] The present invention also proposes a longitudinal protection system for an offshore wind power low-frequency cable transmission line, comprising: A data acquisition module is used to obtain current and voltage information on both sides of the low-frequency cable line; The protection identification module is used to start the protection and identify the fault status according to the voltage information on both sides of the low-frequency cable line when the voltage mutation amount is greater than the preset rated voltage; The fault identification module is used to place the data sampling sequence of the current on both sides of the low-frequency cable line in a half-cycle in the same data window by determining the current direction based on the current information on both sides of the low-frequency cable line, and determine the maximum and minimum values ​​of the current information on both sides of the low-frequency cable line in the same data window; according to the maximum and minimum values ​​of the current, the current data is equally divided into multiple sub-intervals, and the probability distribution of the current on both sides of the low-frequency cable line is determined by calculating the number of current data on both sides in each sub-interval; according to the probability distribution of the current on both sides of the low-frequency cable line, the divergence value K of the probability distribution is obtained; the divergence value K of the probability distribution is compared with the setting threshold. When the divergence value K is higher than the setting threshold, the low-frequency cable line has an in-zone fault; when the divergence value K is lower than the setting threshold, the low-frequency cable line has an out-of-zone fault; The protection module is used to perform corresponding protection operations according to the results of internal and external fault judgment.

[0037] The proposed method determines the voltage mutation amount based on voltage information on both sides of a low-frequency cable line. When the voltage mutation amount exceeds a preset rated voltage, protection is activated and fault status is determined. The current sampling values ​​on both sides of the low-frequency cable line are used as the raw information for the judgment criterion. By extracting the difference in the current probability distribution on both sides of the low-frequency cable line, its KL divergence value K is calculated. The K value is compared with a set threshold when a fault occurs inside or outside the low-frequency cable line. A protection criterion is constructed to distinguish between inside and outside the fault zone. This method can improve the accuracy of the judgment results and quickly execute corresponding protection operations based on different fault connections.

[0038] Example In order to verify the feasibility of the proposed method, the embodiment provided by the present invention takes a longitudinal protection method for offshore wind power low-frequency cable transmission lines based on KL divergence as an example to verify the longitudinal protection method for offshore wind power low-frequency cable transmission lines proposed by the present invention.

[0039] like Figure 2 As shown, the offshore wind power transmission system structure via low-frequency cable provided in this embodiment includes a direct-drive wind turbine generator set, a wind turbine converter, a step-up transformer (35 / 220 kV), an isolation transformer, a low-frequency cable, and an M3C converter.

[0040] At the generator outlet, wind power is converted to 50 / 3Hz AC power via a back-to-back full-power converter. This power is then transmitted via a step-up transformer and a low-frequency cable to the low-frequency side of the M3C converter. The M3C converter then converts the low-frequency AC power to industrial frequency and integrates it into the onshore power grid. The low-frequency cable includes two sampling points: the first sampling point W and the second sampling point M.

[0041] KL divergence, also known as relative entropy, is a well-known similarity measure in the field of information analysis. It calculates the difference between the information entropy of two probability distributions. u and v ,pass u express v The KL divergence value K is:

[0042] The discrete form is: KL divergence is used to measure the distance between two probability distributions. When the probability distributions are closer, the calculated K value is smaller; when the probability distributions are completely consistent, the calculated K value is 0.

[0043] Since KL divergence is often used in the field of information analysis, its input must be a probability distribution, and the data needs to be preprocessed before calculation.

[0044] Considering that the protection speed will decrease if the data window is taken as a whole cycle, and the algorithm taking 1 / 4 cycle will be affected by the transient state of the fault, which will seriously affect the calculation results, the present invention sets the KL divergence calculation data window to half a cycle (30ms). As the sliding data window advances, the K value is calculated in sequence to form a sequence.

[0045] The current probability distribution is processed as follows Figure 3 As shown, the direction of current flowing from busbar to line is positive. First, Figure 2 Current in Take the negative value and put the data sampling sequence of the current on both sides in half a cycle into the same data window, such as Figure 3 As shown, find the maximum value Ma and the minimum value Mi of the window data, so the data fluctuation range in the data window is B=Ma-Mi. Divide B into ten sub-intervals, each sub-interval is 0.1B long, numbered from 1 to 10 from low to high; calculate respectively and The number of data falling in each subinterval and the corresponding calculation and The ratio of the number of data points falling in the subinterval to the total number of sampling points is the probability distribution, such as Figure 3 As shown, a schematic diagram of data probabilization is shown.

[0046] According to the above processing method, when the low-frequency cable output line is operating normally, the number of data in each subinterval obtained by the above processing under the same data window is as follows: Figure 4 shown.

[0047] Depend on Figure 4It can be seen that when the system is operating normally, the number of data points falling into each subinterval is basically the same. Even if the number of data points is different, the difference is very small. The calculated data probability distribution is shown in Table 1.

[0048] Table 1 Probability distribution of data during normal operation in, and The first sampling point W and the second sampling point M are respectively i The probability distribution corresponding to the interval; in order to avoid the algorithm error caused by the current sampling probability distribution being 0 in a certain interval, the discrete form of the KL divergence value K is corrected and a correction factor is introduced: Where, As the correction factor, the present invention adopts probability distribution for calculation, It is smaller, so the correction factor is taken .

[0049] The K value of the data in Table 1 is calculated using the discrete form formula of the modified KL divergence value K, and K=0.02 is obtained, which is close to 0. This intuitively reflects that the current similarity on both sides is very high when the system is operating normally.

[0050] In summary, during normal system operation, the current amplitudes on both sides are equal, and after processing the current on the M side, the phase angle difference is zero, resulting in a very small calculated K value. However, when a low-frequency cable fault occurs within the area, the currents on both sides differ in amplitude and phase angle, and the K value is no longer close to zero.

[0051] In order to meet the requirements of the relay protection device, the present invention constructs the following criteria including: (1) Protection start criteria The longitudinal protection requires the ability to quickly identify faults. Due to the limitation of the fault current amplitude of the power electronic source on both sides, the present invention uses the voltage mutation as the starting criterion for protection: Where, , t Indicates the sampling value at the current moment, T is the sampling period, which is 60ms in the present invention. The rated voltage of the system. When the system is abnormal, the protection starts and enters the fault judgment state; otherwise, it does not start.

[0052] (2) Protection action criteria From the above analysis, when the system is running normally, the calculated K value should be 0. When the zone fault occurs, the fault currents on both sides will show different differences in amplitude and phase angle. At this time, the calculated K >> 0, so it is necessary to set a suitable setting threshold to effectively judge whether the low-frequency cable has failed. When the low-frequency cable has a zone fault, the calculated K value should satisfy:

[0053] In the formula, is the setting threshold.

[0054] Since the K-L divergence can reflect the amplitude and phase angle difference of the currents on both sides, The setting principle is: In the formula, is the reliability coefficient considering the phase angle error of the low-frequency cable line on both sides, generally including the angle error caused by the current transformer (CT) and the angle error caused by the cable capacitance current; is the amplitude error caused by the CT; is the margin coefficient.

[0055] In the power system, the maximum transmission angle error of the current transformer is 8°, and the phase angle deviation caused by the long line capacitance current is generally 7.4° / 100km. The cable length used in the present application is 100km, so the current phase angle error on both sides of the line is 15.4°, and the calculated =0.15; For the amplitude error of the CT measurement, the unilateral error generally does not exceed 10%, considering the most serious case, the transmission error of both sides is 20%, at this time the calculated =0.61; is the margin coefficient, which is 0.4 in the present application; in summary, =1.2. The protection process of the method provided by the embodiment is shown in Figure 1 .

[0056] In this embodiment, a K-L divergence-based longitudinal protection method for offshore wind power low-frequency cable transmission system is used to simulate and analyze the low-frequency power transmission system in PSCAD. The low-frequency simulation system and fault point setting are shown in Figure 2 , and the parameters of the system model are as follows: The wind farm side: equivalent to 50 direct-drive PMSGs, each with a rated capacity of 2MVA, a converter port voltage of 0.69kV, and a system reference power of 100MW; the low-frequency cable transmission line: 100km in length, 220kV in voltage level, 50 / 3Hz in transmission frequency, 0.1213Ω / km in unit length resistance, 0.254mH / km in inductance, 0.1716 / km in capacitance, and 2x30Mvar in parallel high resistance. The M3C side: 220kV in port voltage and 0.5uF in port capacitance. The sampling frequency of the system is 4kHz, the data window used for calculation is 30ms, and the fault occurrence time is 1.2s. In order to reduce the influence of high-order harmonics in external fault, the collected data is subjected to low-pass filtering.

[0057] The simulation verification is carried out on the fault conditions of different positions, types, transition resistances and noises of the low-frequency transmission line, and the protection performance of various types in the case of high-resistance grounding. Figure 5 、 Figure 6 、 Figure 7 、 Figure 8 、 Figure 9 and Figure 10 and Tables 2 and 3.

[0058] wherein, Figure 5 is the A-phase waveform and K value when the ABC fault occurs outside the area, and the fault occurrence time is 1.2s; Figure 6 is the A-phase waveform and K value when the AG fault occurs inside the area; Figure 7 is the three-phase waveform and K value when the BC fault occurs inside the area; Figure 8 and Figure 9 verify the influence of transition resistance and abnormal data on the protection, respectively; Figure 10 is the comparison between the protection provided by the application and the protection principle of similar protection.

[0059] In actual engineering, the transition resistance also has an influence on the performance of the protection. The transition resistance of the grounding fault faced by the 220kV transmission line is generally not more than 100Ω, Figure 8 the protection performance is given when the A-phase grounding fault occurs inside the area through different transition resistances.

[0060] As shown in Figure 8 , with the increase of the transition resistance, the phase angle and amplitude difference of the current waveforms on both sides gradually decrease. However, the protection provided by the application mainly analyzes the relative relationship between the current waveforms on both sides, and is hardly affected by the transition resistance. Even when the grounding fault occurs through a 300Ω transition resistance, the protection can still reliably determine the fault, and the method provided by the application has good transition resistance resistance performance

[0061] During the sampling process of the current transformer, the sampling value may be affected by external interference, and then the sampling value is wrong or out of synchronization, the waveform in the data window is abnormal, and the misjudgment occurs. Figure 9The K value is changed under the condition that one distortion data randomly appears every 1 / 4 cycle of the current on both sides in normal operation, 1ms error is generated in double-end communication, and 10% transformation error is generated.

[0062] By Figure 9 It can be seen that although 4 distortion data are generated in each calculation data window, the data proportion of part of subintervals changes, but the influence on the K value is still small, and the most extreme case is that 4 distortion points all fall in a unified subinterval, so that the probability in the interval only increases by 3%, and the change of the remaining subintervals is also not large. Although the K value increases when 1ms error is generated in double-end communication and transformation error is generated, it is far less than the setting value.

[0063] Table 2 K value under different fault positions As shown in Table 2, when an in-zone fault occurs in the low-frequency cable outgoing line, the K value of the fault phase is much larger than the setting threshold value, and the protection can reliably remove the fault, and the K value of the healthy phase is always less than 1.2, so the protection will not malfunction. When an out-of-zone fault occurs, the calculation results of all phases are less than the setting threshold value, which proves the reliability of the protection. In addition, Table 2 also verifies that the protection has good speed performance and can ensure that the fault is identified within half a cycle after the fault.

[0064] Table 3 K value under different signal-to-noise ratios As shown in Table 3, although some harmonics are generated after adding noise, the overall change trend of the collected current on both sides is close, the probability distribution is almost unchanged, the calculated K value is relatively stable, and the influence of noise on the protection performance is small.

[0065] According to Figures 5 to 10 According to the verification results of Table 2 and Table 3, it can be seen that the method of the present application has high sensitivity, good selectivity, fast action speed and high reliability for fault discrimination of the low-frequency cable outgoing line of the offshore wind power, thereby providing reliable relay protection for the power transmission line in the offshore wind power low-frequency power transmission system.

[0066] In summary, the present application uses the current sampling values on both sides of the low-frequency cable as the original information of the criterion, calculates the K-L divergence value K by extracting the probability distribution difference of the current on both sides of the low-frequency cable line, and constructs a protection criterion according to the different K values when the submarine cable has in-zone and out-of-zone faults to discriminate the in-zone and out-of-zone faults.

[0067] The present application is based on the difference between the currents on both ends of the low-frequency cable under different faults, analyzes the corresponding fault characteristics, proposes a criterion based on K-L, and constructs a relay protection which is easy to implement and set, has good selectivity and high sensitivity.

[0068] Compared with the prior art, the method has good rapidity and reliability, and is easier to perform protection action quickly when a fault occurs.

[0069] The above description is only the preferred embodiment of the present application, but the protection scope of the present application is not limited to this, any person skilled in the art can make equivalent replacement or change according to the technical scheme and the inventive concept of the present application within the technical range disclosed by the present application, which should be covered in the protection scope of the present application.

[0070] In addition, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods associated with the documents. In the event of a conflict between the content of the specification and any incorporated document, the content of the specification controls.

Claims

1. A longitudinal protection method for offshore wind power low-frequency cable transmission lines, characterized in that: The following steps are involved: Obtain current and voltage information on both sides of the low-frequency cable line; According to the voltage information on both sides of the low-frequency cable line, when the voltage mutation is greater than the preset rated voltage, the protection is activated and the fault status is judged; According to the current information on both sides of the low-frequency cable line, by determining the current direction, the data sampling sequence of the current on both sides of the low-frequency cable line in half a cycle is placed in the same data window, and the maximum and minimum values ​​of the current information on both sides of the low-frequency cable line in the same data window are determined; according to the maximum and minimum values ​​of the current, the current data is equally divided into multiple sub-intervals, and the probability distribution of the current on both sides of the low-frequency cable line is determined by calculating the number of current data on both sides in each sub-interval; according to the probability distribution of the current on both sides of the low-frequency cable line, the divergence value K of the probability distribution is obtained; Compare the divergence value K of the probability distribution with the setting threshold. When the divergence value K is higher than the setting threshold, the low-frequency cable line has an internal fault; when the divergence value K is lower than the setting threshold, the low-frequency cable line has an external fault. Execute corresponding protection operations based on the fault identification results.

2. The longitudinal protection method for offshore wind power low-frequency cable transmission lines according to claim 1 is characterized in that: The fault status determination specifically includes: By collecting the voltage signals on both sides of the low-frequency cable line, the voltage mutation amount is determined and used as the starting criterion for protection; The startup judgment formula is: In the formula, the voltage mutation amount , is the voltage at the current time minus the voltage one cycle ago; t Indicates the sampling value at the current moment, T is the sampling period, Rated voltage of the system; when When the system is abnormal, the protection starts and enters the fault judgment state; otherwise, it does not start.

3. The longitudinal protection method for offshore wind power low-frequency cable transmission lines according to claim 2 is characterized in that: The determining of the maximum and minimum current values ​​of the current information on both sides of the low-frequency cable line in the same data window specifically includes: The protection installation location of the low-frequency cable line includes two sampling points, and current information of the two sampling points is obtained, including the current value of the first sampling point W and the current value of the second sampling point M; Taking the current direction from the busbar to the line as positive, the current value on the W side of the first sampling point is taken as positive, and the current value on the M side of the second sampling point is taken as negative; Put the data sampling sequence of the current on both sides of the first sampling point W and the second sampling point M in half a cycle into the same data window, and find the maximum value Ma and the minimum value Mi of the window data.

4. The longitudinal protection method for offshore wind power low-frequency cable transmission lines according to claim 3 is characterized in that: Determining the probability distribution of currents on both sides of the low-frequency cable line specifically includes: According to the maximum value Ma and the minimum value Mi of the window data, the data fluctuation range in the data window is determined to be B=Ma-Mi; Divide B into ten equal subintervals, each with a length of 0.1B and numbered 1 to 10 from low to high. Calculate the number of data points where the current at the first sampling point, side W, and the second sampling point, side M, falls within each subinterval. Calculate the ratio of the number of data points where the current at the first sampling point, side W, and the second sampling point, side M, fall within the subinterval to the total number of sampling points. This is the probability distribution of the current at the first sampling point, side W, and the second sampling point, side M.

5. The longitudinal protection method for offshore wind power low-frequency cable transmission line according to claim 4 is characterized in that: Obtaining the divergence value K of the probability distribution comprises the following steps: The calculation result of the divergence value K is obtained through the KL divergence calculation formula; The KL divergence calculation formula is: in, and The first sampling point W and the second sampling point M are respectively i The probability distribution corresponding to the current data, N is the number of data in the calculation data window, is the correction factor.

6. The longitudinal protection method for offshore wind power low-frequency cable transmission lines according to claim 5, characterized in that: The low-frequency cable line has an in-area fault, specifically including: When a low-frequency cable fails within the area, the calculated divergence value K satisfies: Where, is the setting threshold; The setting principle is: Where, To consider the reliability coefficient of the phase angle error on both sides of the low-frequency cable line, including the angle error caused by the current transformer and the angle error caused by the cable capacitance current; To consider the amplitude error caused by the current transformer; is the margin factor.

7. The longitudinal protection method for offshore wind power low-frequency cable transmission lines according to claim 6, characterized in that: The divergence value K calculated for the fault outside the low-frequency cable line occurrence area is 0.

8. A longitudinal protection system for offshore wind power low-frequency cable transmission lines, characterized in that: include: A data acquisition module is used to obtain current and voltage information on both sides of the low-frequency cable line; The protection identification module is used to start the protection and identify the fault status according to the voltage information on both sides of the low-frequency cable line when the voltage mutation amount is greater than the preset rated voltage; The fault identification module is used to place the data sampling sequence of the current on both sides of the low-frequency cable line in a half-cycle in the same data window by determining the current direction based on the current information on both sides of the low-frequency cable line, and determine the maximum and minimum values ​​of the current information on both sides of the low-frequency cable line in the same data window; according to the maximum and minimum values ​​of the current, the current data is equally divided into multiple sub-intervals, and the probability distribution of the current on both sides of the low-frequency cable line is determined by calculating the number of current data on both sides in each sub-interval; according to the probability distribution of the current on both sides of the low-frequency cable line, the divergence value K of the probability distribution is obtained; the divergence value K of the probability distribution is compared with the setting threshold. When the divergence value K is higher than the setting threshold, the low-frequency cable line has an in-zone fault; when the divergence value K is lower than the setting threshold, the low-frequency cable line has an out-of-zone fault; The protection module is used to perform corresponding protection operations according to the fault judgment results.

9. A computer device, characterized in that: The computer device includes a memory and a processor, wherein the memory stores a computer program, and when the computer program is executed by the processor, the processor performs the following steps: Obtain current and voltage information on both sides of the low-frequency cable line; According to the voltage information on both sides of the low-frequency cable line, when the voltage mutation is greater than the preset rated voltage, the protection is activated and the fault status is judged; According to the current information on both sides of the low-frequency cable line, by determining the current direction, the data sampling sequence of the current on both sides of the low-frequency cable line in half a cycle is placed in the same data window, and the maximum and minimum values ​​of the current information on both sides of the low-frequency cable line in the same data window are determined; according to the maximum and minimum values ​​of the current, the current data is equally divided into multiple sub-intervals, and the probability distribution of the current on both sides of the low-frequency cable line is determined by calculating the number of current data on both sides in each sub-interval; according to the probability distribution of the current on both sides of the low-frequency cable line, the divergence value K of the probability distribution is obtained; Compare the divergence value K of the probability distribution with the setting threshold. When the divergence value K is higher than the setting threshold, the low-frequency cable line has an internal fault; when the divergence value K is lower than the setting threshold, the low-frequency cable line has an external fault. Execute corresponding protection operations based on the fault identification results.

10. A computer-readable storage medium, characterized in that The computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the processor performs the following steps: Obtain current and voltage information on both sides of the low-frequency cable line; According to the voltage information on both sides of the low-frequency cable line, when the voltage mutation is greater than the preset rated voltage, the protection is activated and the fault status is judged; According to the current information on both sides of the low-frequency cable line, by determining the current direction, the data sampling sequence of the current on both sides of the low-frequency cable line in half a cycle is placed in the same data window, and the maximum and minimum values ​​of the current information on both sides of the low-frequency cable line in the same data window are determined; according to the maximum and minimum values ​​of the current, the current data is equally divided into multiple sub-intervals, and the probability distribution of the current on both sides of the low-frequency cable line is determined by calculating the number of current data on both sides in each sub-interval; according to the probability distribution of the current on both sides of the low-frequency cable line, the divergence value K of the probability distribution is obtained; Compare the divergence value K of the probability distribution with the setting threshold. When the divergence value K is higher than the setting threshold, the low-frequency cable line has an internal fault; when the divergence value K is lower than the setting threshold, the low-frequency cable line has an external fault. Execute corresponding protection operations based on the fault identification results.

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