A pilot protection algorithm for active distribution network lines taking into account correction coefficients

Abstract

The present invention discloses an active distribution network longitudinal protection method taking into account a correction coefficient, and relates to the field of distribution network line protection technology. The present invention utilizes the positive sequence current fault component on both sides of the line, adds the correction coefficient, constructs a new type of corrected differential current and corrected braking current, and uses cosine similarity to analyze the similarity between them, thereby constructing a line fault degree, and establishing an active distribution network line longitudinal protection criterion to identify short-circuit faults within the area. The present invention can accurately identify short-circuit faults inside and outside the line area, and is not affected by the fault type, fault location, distributed power type, and distributed power penetration rate. It can work reliably in scenarios such as high transition resistance, CT saturation, and signal asynchrony.

Description

Technical Field

[0001] The invention relates to the technical field of power distribution network line protection, and in particular to an active power distribution network line pilot protection algorithm taking into account a correction coefficient. Background Art

[0002] When distributed wind power and distributed photovoltaic power are connected to the traditional distribution network, the power flow changes from unidirectional to bidirectional, and the distribution network also becomes an active distribution network. Distributed power sources contain a large number of power electronic devices. When a fault occurs, its fault characteristics are affected by the topology and control strategy. The access of power electronic devices will increase the harmonic content during the fault, and affect the size of the short-circuit current of the distribution network, which will reduce the sensitivity of the original protection and affect the correct operation of the relay protection equipment. The traditional distribution network is generally a single-power radial network structure, and generally adopts three-stage current protection as line protection, which mainly relies on the setting of the protection threshold and time coordination. After the distributed power source is connected, the distribution network changes from a single power source to multiple power sources, and the flow and short-circuit current change from unidirectional flow to bidirectional flow, and the weak feed problem is prominent. The intermittent, variable and uncertain nature of distributed power sources also pose stability problems for distribution networks with high penetration rates. At this time, traditional protection methods are no longer applicable to active distribution networks.

[0003] Lei Lin, Tang Chengda, Qing Yucheng, et al. (Positive sequence comprehensive impedance longitudinal protection of distribution network containing inverter-type distributed generation [J]. Power System Protection and Control, 2018, 46(18): 149-155) proposed a longitudinal protection method based on positive sequence component comprehensive impedance. This method divides the difference of positive sequence voltage phasors at both ends of the line by the sum of positive sequence voltage phasors at both ends of the line to form the positive sequence component comprehensive impedance. When a fault occurs, the internal and external faults can be distinguished according to the amplitude difference of the positive sequence component comprehensive impedance, which solves the influence of inverter control strategy on the fault characteristics of the distribution network.

[0004] Gao Yan, Li Yongli, Chen Xiaolong, et al. (Principle of adaptive differential protection of active distribution network based on current amplitude ratio [J]. Journal of Electric Power System and Automation, 2021, 33(2): 1-7) The amplitude of the current on both sides of the line was analyzed. When an internal or external fault occurs, the amplitude ratio of the fault component current on both sides of the line is different. Based on this feature, an adaptive current differential protection scheme based on current amplitude ratio is proposed. Different braking coefficient values ​​are set at different fault locations, and adaptive adjustments are made to improve the sensitivity of protection during internal faults.

[0005] Gao Houlei, Li Juan, Zhu Guofang, et al. (Discussion on the application technology of current differential protection in active distribution network [J]. Power System Protection and Control, 2014, 42(05): 40-44) discussed the principle and implementation technology of applying positive sequence fault component current in active distribution network, gave corresponding differential protection action criteria and setting principles according to different feeder structures, and proposed a method of current data self-synchronization based on fault information.

[0006] The invention patent publication number CN117638868A discloses "a method for evaluating the applicable boundaries of directional current protection in distribution networks under distributed photovoltaic access". By checking whether the photovoltaic capacity combination of particles meets the constraint condition 1 of the current protection sensitivity, the constraint condition 2 of the direction criterion that the reverse fault does not cause false operation, or the constraint condition 3 of the direction criterion that the forward fault does not refuse to operate, the individual optimum and the global optimum of the photovoltaic capacity combination are updated according to the situation, and iteration is used until the current optimal photovoltaic capacity combination is obtained.

[0007] The methods proposed in the above documents have their own shortcomings. They only consider a certain type of distributed power source and cannot adapt to the access of multiple distributed power sources to the distribution network. When the distributed power sources with high penetration rate are connected to the distribution network, their protection algorithms may fail. In order to solve this problem, the present invention proposes a longitudinal protection method for active distribution network lines based on taking into account the correction coefficient. Summary of the invention

[0008] The object of the present invention is to provide an active distribution network line pilot protection method based on taking into account a correction coefficient to solve the problems raised in the background technology.

[0009] In order to achieve the above object, the present invention adopts the following technical solutions:

[0010] An active distribution network pilot protection method taking into account a correction coefficient comprises the following steps:

[0011] S1, collect the current on the bus m side of line mn. When the current mutation exceeds the starting threshold, the protection is started and the positive sequence fault component current on the m side is calculated. Similarly, the current on the bus n side of line mn is collected to calculate the positive sequence fault component current on the n side: And proceed to the next step;

[0012] S2, using the positive sequence fault component current on both sides of line mn obtained in S1, calculate the corrected braking current of line mn respectively Corrected differential current

[0013]

[0014] Where, N represents the correction coefficient, N is a constant, N>1;

[0015] S3, calculate the corrected braking current of the line in the time window before each sampling moment after the fault occurs obtained in S2 With the corrected differential current Cosine similarity S between arrays:

[0016]

[0017] The value range of S is [-1,1];

[0018] S4. Based on the cosine similarity S obtained in S3, construct the line fault degree d:

[0019]

[0020] The value range of d is [0,1];

[0021] S5. According to the line fault degree d, establish the active distribution network line fault protection criterion:

[0022] d>D set (4)

[0023] Where D set is the protection threshold;

[0024] S6. Based on the active distribution network line fault protection criterion obtained in S5, when the line protection is activated for three consecutive moments, it is determined that a short circuit fault occurs in the line area; otherwise, it is a normal line.

[0025] Preferably, the corrected braking current of the line in S2 is With the corrected differential current The phase characteristics analysis is as follows:

[0026] According to the different magnitudes of the positive sequence current fault components on both sides of the line, the corresponding correction braking current is constructed. With the corrected differential current Analysis and correction current and The phase angle difference between

[0027] When the system is operating normally or an out-of-zone fault occurs, According to formula (1), we get:

[0028]

[0029] Because N>1, at this time and The phase is the same, and the angle between them is θ rd is 0;

[0030] For formula (1), when hour, It is expressed as:

[0031]

[0032] and The angle θ between rd It is expressed as:

[0033]

[0034] Where F represents the amplitude ratio of the positive sequence fault component current on both sides of the line, θ mn Represents the angle between the positive sequence current fault components on both sides of the line;

[0035] Since the right side of the equal sign in equation (6) is about θ mn An even function of θ rd The size of θ mn The positive and negative of θ is irrelevant, so mn Defined as the angle;

[0036] when hour, It is expressed as:

[0037]

[0038] and The angle θ between rd It is expressed as:

[0039]

[0040] When the downstream of the line is connected to an inverter-type distributed power source (IIDG) and an in-area fault occurs, when the correction coefficient N is a different constant, according to formula (6), the variable F, θ mn The angle θ under different values ​​within the range of variation rd The value of the angle θ is obtained rd are all obtuse angles; as the correction coefficient N increases, the angle θ rd The minimum value of becomes smaller; when F is small, θ rd Small value;

[0041] When the downstream of the line is connected to a motor-type distributed generation (MTDG) and an in-area fault occurs, when the correction coefficient N is a different constant, according to formula (8), the variables F, θ are obtained by calculation. mn The angle θ under different values ​​within the range of variation rd The value of the angle θ is obtained rd are all obtuse angles; as the correction coefficient N increases, the angle θrd The minimum value of becomes larger accordingly; when F is 1, θ rd Small value;

[0042] In summary, as the correction factor N increases, when the downstream is connected to an inverter-type distributed power source (IIDG), θ rd Gradually decreases, when the downstream is connected to the motor-type distributed power source (MTDG) rd Gradually getting bigger.

[0043] Preferably, the characteristics of the similarity S and the fault degree d in S3 and S4 are analyzed as follows:

[0044] Assume that A and B are two arrays of the same length: A=[a1,a2,…,a k ],B=[b1,b2,…,b k ], k is the length of A and B arrays;

[0045] The inner product formula between array A and array B is expressed as:

[0046] A·B=|A|B|cosθ (9)

[0047] According to formula (9), substituting into arrays A and B, the cosine similarity S between the two arrays is expressed as cosθ:

[0048]

[0049] In formula (10), the range of cosine similarity is between [-1, 1]. The closer its value is to 1, the higher the cosine similarity between the two arrays. On the contrary, the closer it is to -1, the lower the cosine similarity between the two arrays.

[0050] The corrected braking current in the time window before each sampling moment of the line With the corrected differential current The cosine similarity between rd From [-1, 1] to [0, 1], the distribution network line fault degree d is constructed as:

[0051]

[0052] It can be seen from formula (11) that when the line is operating normally or an out-of-area fault occurs, θ rd is 0°, cosθ rd is 1, the fault degree d of the line is 0; when an internal fault occurs, θ rd is an obtuse angle, cosθ rd is close to -1, and the fault degree d of the line is close to 1, which is used to distinguish between faults inside and outside the line area.

[0053] Preferably, the source distribution network line fault protection criterion in S5 specifically includes the following contents:

[0054] 1) Using the positive sequence current fault component on both sides of the line and According to formula (1), the corrected current at each sampling time after the fault occurs is calculated: and Instantaneous value; for each moment t, take the corrected current in the time window before that moment on both sides of the line and The instantaneous value is used to form the modified braking current array and the modified differential current array at that moment; then, using formula (10), the cosine similarity value cosθ between the modified braking current and the modified differential current array is calculated: rd ;

[0055] 2) The cosine similarity value cosθ rd Substituting into equation (11) we can get the line fault degree d.

[0056] 3) When the fault degree d satisfies formula (4), the line is judged to be faulty and the line protection is activated once.

[0057] Compared with the prior art, the present invention provides an active distribution network pilot protection method taking into account the correction coefficient, which has the following beneficial effects:

[0058] The present invention proposes a longitudinal protection method for active distribution network taking into account correction coefficients. The positive sequence current fault components on both sides of the line are used and correction coefficients are added to construct new corrected differential current and corrected braking current containing correction coefficients. Analysis shows that the angle between the corrected differential current and the corrected braking current is close to 180° when the fault occurs in the area. The cosine similarity is used to analyze the similarity between them, thereby constructing a line fault degree, establishing a line longitudinal protection criterion for the active distribution network, being able to distinguish short-circuit faults inside and outside the line area, and better adapting to the fault detection and protection of the active distribution network line containing IIDG and MTDG in the downstream. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 The present invention provides a flow chart of active distribution network longitudinal protection taking into account the correction coefficient;

[0060] Figure 2 For the present invention and Vector diagram of angle;

[0061] Figure 3 When the downstream of the present invention is connected to IIDG and Angle θ rd The change curve of

[0062] Figure 4 When the downstream of the present invention is connected to MTDG and Angle θ rd The change curve of

[0063] Figure 5 This is the active distribution network model diagram mentioned in Embodiment 1 of the present invention;

[0064] Figure 6 is the positive sequence current fault component curve on both sides of the line AB1 mentioned in Embodiment 1 of the present invention;

[0065] Figure 7 The modified differential current and modified braking current curves of the line AB1 mentioned in the first embodiment of the present invention;

[0066] Figure 8 It is the fault degree curve of line AB1 when a two-phase ground short circuit fault occurs at f1 mentioned in Embodiment 1 of the present invention. DETAILED DESCRIPTION

[0067] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0068] See also Figure 1 , a longitudinal protection method for active distribution network lines taking into account the correction coefficient, the steps of which include:

[0069] Step 1: Collect the current and voltage on the bus m side of line mn. When the current mutation exceeds the start threshold, start the protection and calculate the positive sequence fault component current on the m side. Similarly, the current on the bus n side of line mn is collected to calculate the positive sequence fault component current on the n side:

[0070] Step 2: Using the positive sequence fault component currents on both sides of line mn obtained in step 1, calculate the corrected braking current of line mn respectively. Corrected differential current

[0071]

[0072] Wherein, N is the correction coefficient, its value is a constant, N>1;

[0073] Corrected braking current With the corrected differential current The characteristic analysis of the phase difference includes the following:

[0074] In order to analyze the corrected braking current in formula (1.1) more intuitively With the corrected differential current The structure of Figure 2 Lieutenant General and Expressed in vector form; where N = 1.6;

[0075] Depend on Figure 2 See, when the vector The angle θ mn From small to big, When the corrected braking current is constructed by formula (1.1) With the corrected differential current The angle θ between rd They are all obtuse angles, close to 180°;

[0076] When a short circuit occurs in the line mn, the positive sequence fault component current on both sides of the line It has the following features:

[0077] 1) When a short circuit occurs in the distribution network line, the IIDG output current is ≤1.2~2pu, and the positive sequence current fault component on the power supply side The load side 3 to 30 times of the phase difference θ mn From 0 to 180°; as the fault point gradually moves away from the system power supply side, the amplitude ratio gradually decrease;

[0078] 2) When a short-circuit fault occurs in the distribution network line, the fault current on both sides of the line is provided by the system power supply and MTDG respectively. The short-circuit current provided by MTDG is 4~6pu, θ mn ∈(0,18°), the amplitude ratio F of the positive sequence current fault component on both sides is between 0.5 and 3;

[0079] When an intra-zone short-circuit fault occurs with an MTDG downstream, different fault locations correspond to a certain amplitude ratio range. As the fault location α increases, the amplitude ratio F gradually decreases.

[0080] According to the different magnitudes of the positive sequence current fault components on the left and right sides of the line, the corresponding correction braking current is constructed. With the corrected differential current In this analysis, two currents are constructed and The phase difference;

[0081] When the system is operating normally or an out-of-zone fault occurs, According to formula (1.1), we can get: Because N>1, so at this time and Same phase.

[0082] For formula (1.1), when hour, It is expressed as:

[0083]

[0084] and The angle θ rd It is expressed as:

[0085]

[0086] Where F is the amplitude ratio of the positive sequence fault component current on both sides of the line, θ mn is the angle between the positive sequence current fault components on both sides of the line. Since the right side of the equal sign in equation (1.3) is about θ mn An even function, θ rd The size of θ mn The sign of θ is irrelevant. mn Defined as the angle.

[0087] when hour, It is expressed as:

[0088]

[0089] and The angle θ between rd It is expressed as:

[0090]

[0091] When the line is connected to IIDG downstream and an internal fault occurs, the amplitude ratio F of the positive sequence current fault component on both sides of the line is about 3 to 30, and the angle θ mn It varies between 0 and 180°. When N is a different constant, according to formula (1.3), the current is calculated and The angle θ between rd With the variables F, θ mn The change curve is as Figure 3 shown.

[0092] Depend on Figure 3 It can be seen that when the downstream is connected to IIDG, the current and Angle θ rd In most cases, it is an obtuse angle. As the correction factor N increases, the angle θ rd The minimum value of will also become smaller. When F is small, θrd When N=1.5, θ rd The minimum value of θ is 139.46°; when N = 3, θ rd The minimum value of is 90.04°. N should not be too large.

[0093] When the MTDG is connected downstream of the fault in the line area, the amplitude ratio F of the positive sequence current fault component on both sides of the line is close to 1, greater than 1, and less than 1. When F is between 0.5 and 3, the angle θ mn Small, between 0 and 18°, close to 0°. When N is a different constant, according to formula (1.5), calculate the current and The angle θ between rd With the variables F, θ mn The change curve is as Figure 4 shown.

[0094] Depend on Figure 4 It can be seen that when MTDG is connected downstream, the current and Angle θ rd In most cases, it is an obtuse angle. As the correction factor N increases, the angle θ rd The minimum value of becomes larger accordingly. When F is 1, θ rd When N = 1.5, θ rd The minimum value of θ is 140.41°; when N = 3, rd The minimum value of is 158.18°. N should not be too small.

[0095] Based on the above analysis, as the correction coefficient N increases, when the downstream is connected to IIDG, θ rd Gradually decreases; when the downstream is connected to MTDG, θ rd Therefore, it is more appropriate to take the correction coefficient N as 1.6 (the angle θ rd The curve is similar to that of N=1.5).

[0096] Step 3: Use the corrected braking current of the line in the time window before each sampling moment after the fault occurs calculated in step 2 With the corrected differential current Array, calculate the cosine similarity S between them:

[0097]

[0098] The characteristic analysis of similarity S includes the following:

[0099] If A and B are two arrays: A=[a1,a2,a3…a n ],B=[b1,b2,b3…b n], n is the length of the array.

[0100] The inner product formula between array A and array B is expressed as:

[0101] A·B=|A||B|cosθ (1.7)

[0102] According to formula (1.7), by substituting arrays A and B, we can get the cosine similarity of the two arrays as follows:

[0103]

[0104] The range of cosine similarity in formula (1.8) is between [-1,1]. The closer the value is to 1, the higher the cosine similarity between the two arrays; conversely, the closer it is to -1, the lower the cosine similarity between the two arrays.

[0105] Step 4: Based on the similarity S obtained in step 3, construct the line fault degree d:

[0106]

[0107] Step 5: Establish line fault protection criteria according to the line fault degree d, as follows:

[0108] d>D set (1.10)

[0109] Among them, D set Indicates the protection threshold. Considering the impact of data asynchrony, when the data has a 4 ms delay error, the minimum value of d is 0.74, and the protection threshold can be set to 0.5.

[0110] The specific protection determination method for line short circuit fault is as follows:

[0111] 1) Using the positive sequence current fault component on both sides of the line and According to formula (1.1), the corrected current at each moment after the fault is calculated and Instantaneous value. For each moment i, take the corrected current in the time window T before that moment on both sides of the line and Instantaneous value, forming two corrected current arrays. The time window length T here is 10ms. Then use formula (1.8) to calculate the cosine value cosθ between them rd .

[0112] 2) The cosine value cosθ rd Substitution In the equation, we get the line fault degree d.

[0113] 3) When the fault degree d satisfies formula (1.10), it is determined that there is a fault in the line, the line protection is activated once, and the process goes to step 6.

[0114] Step 6: According to the line short-circuit fault protection criterion obtained in step 5, when the line protection is activated for three consecutive moments, it is determined that the line has an internal short-circuit fault, and the active distribution network line is protected; otherwise, it is a normal line.

[0115] Based on the above operations, the specific example content is as follows:

[0116] Embodiment 1:

[0117] In order to verify the effectiveness of the protection proposed in this paper, the PSCAD / EMTDC simulation software was used to build the following Figure 5 The 10kV active distribution network model is shown in Figure 1. The base voltage is 10.5kV, the transformer capacity is 50MVA, the line parameters are (0.17+j0.34)Ω / km, the length is 4km, the IIDG capacity is 2.5MW, the MTDG capacity is 5MW, and the load active and reactive power are 0.8MW and 0.4MVA respectively.

[0118] 1) Short circuit fault test

[0119] use Figure 5 The active distribution network model is selected, and the time window length T is selected as 10ms. It is assumed that a two-phase grounding short circuit occurs at f1 (α = 5%) of line AB1 and is grounded through a 0.01Ω transition resistor. The waveforms of the positive sequence current fault component and the correction current on both sides of line AB1 are as follows: Figure 6 , Figure 7 As shown in Figure 2, the line fault degree curve is as follows: Figure 8 shown.

[0120] Depend on Figure 6 , Figure 7 It can be seen that the corrected braking current With the corrected differential current The angle between them is large, close to 180°. The average value of the fault degree is about 0.996, which is greater than the protection threshold of 0.5, and the protection can operate correctly.

[0121] Due to the correction of the braking current With the corrected differential current The angle at the moment of fault and at each subsequent moment is close to 180°, and the fault degree d curve presents a step state when and after the fault occurs.

[0122] The simulation results of different fault types under 0.01Ω transition resistance at f2 and f3 are shown in Table 1 and Table 2.

[0123] Table 1 Fault simulation results under 0.01Ω transition resistance at f2 in line B1C1 (IIDG)

[0124]

[0125] Table 2 Fault simulation results under 0.01Ω transition resistance at f3 in line B1C2 (MTDG)

[0126]

[0127] It can be seen from Table 1 and Table 2 that when faults occur at f2 and f3, the fault type and fault location have little effect on the fault degree. The fault degree d is close to 1, which is much larger than the protection threshold of 0.5, and the protection can operate reliably.

[0128] 2) Impact of transition resistance on protection

[0129] In order to verify the influence of transition resistance on the protection algorithm, 100Ω transition resistance is set at f1, f2 and f3. The simulation results when a fault occurs are shown in Tables 3 to 5.

[0130] Table 3 Fault simulation results under 100Ω transition resistance at f1 in line AB1 (IIDG, MTDG)

[0131]

[0132] Table 4 Fault simulation results under 100Ω transition resistance at f2 in line B1C1 (IIDG)

[0133]

[0134] Table 5 Fault simulation results under 100Ω transition resistance at f3 in line B1C2 (MTDG)

[0135]

[0136] From Tables 3 to 5, it can be seen that when different types of faults occur at f1, f2, and f3 and are grounded through a high transition resistance of 100Ω, the fault degree d is still close to 1 in most situations. When a two-phase grounding short circuit occurs at f3 and the fault degree d is 50%, it has a small value of 0.873, which is greater than the threshold value of 0.5. It can be seen that the protection algorithm is not affected by the transition resistance, and the protection can still operate reliably under high resistance.

[0137] 3) Impact of CT saturation on protection

[0138] The CT saturation of the current transformer will cause the current waveform on the secondary side to be distorted, and the current on the primary side cannot be transmitted, which will affect the current differential protection. When a fault occurs at f2 and f3 respectively and the CT saturation of the current transformer occurs, the simulation results are shown in Tables 6 and 7.

[0139] Table 6 Simulation results of CT saturation fault under 0.01Ω transition resistance at f2 in line B1C1 (IIDG)

[0140]

[0141] Table 7 Simulation results of CT saturation fault under 0.01Ω transition resistance at f3 in line B1C2 (MTDG)

[0142]

[0143] According to Tables 6 and 7, compared with the case where CT saturation does not occur, the magnitude of the fault degree d has no significant change, and the fault degree d is close to 1, which is greater than the threshold value of 0.5. It can be seen that the protection algorithm is not affected by CT saturation, and the protection can operate correctly when CT saturation occurs.

[0144] 4) Impact of data asynchrony on protection

[0145] In order to verify the proposed protection algorithm's resistance to the impact of data asynchrony, different types of faults are set at f1. Taking the phase of the positive sequence current fault component at the power supply side protection as the reference, the position of the positive sequence current fault component data at the load side protection is adjusted so that the positive sequence fault component current data on both sides are asynchronous. When the data on both sides have an asynchronous error of 2ms, 3ms, and 4ms (about 86.4°), the simulation results are shown in Tables 8, 9, and 10.

[0146] Table 8 Fault simulation results (IIDG, MTDG) when the data has a 2ms asynchronous error under the 0.01Ω transition resistance at f1 in line AB1

[0147]

[0148] Table 9 Fault simulation results (IIDG, MTDG) when the data has a 3ms asynchronous error under the 0.01Ω transition resistance at f1 in line AB1

[0149]

[0150]

[0151] Table 10 Fault simulation results (IIDG, MTDG) when the data has a 4ms asynchronous error under the 0.01Ω transition resistance at f1 in line AB1

[0152]

[0153] Tables 8 to 10 show that when the fault data in the fault area is not synchronized, the fault degree d of the fault line decreases, and the d of the normal line increases. When a two-phase ground short circuit occurs at 95% of line AB1 and there is a 4ms error delay, the fault degree d of line AB1 decreases slightly to 0.743, which is greater than the threshold value of 0.5. At this time, the fault degree d of the normal line increases slightly to 0.434, which is less than the threshold value of 0.5. The fault section can still be identified, so that the protection can be reliably operated. This method can tolerate an asynchronous error of 4ms.

[0154] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. An active distribution network pilot protection method taking into account a correction coefficient, characterized in that: The specific steps include: S1, collect the current on the bus m side of line mn. When the current mutation exceeds the starting threshold, start the protection and calculate the positive sequence fault component current on the m side. Similarly, the current on the bus n side of line mn is collected to calculate the positive sequence fault component current on the n side. And proceed to the next step; S2, using the positive sequence fault component current on both sides of line mn obtained in S1, calculate the corrected braking current of line mn respectively Corrected differential current Where, N represents the correction coefficient, N is a constant, N>1; S3, calculate the corrected braking current of the line in the time window before each sampling moment after the fault occurs obtained in S2 With the corrected differential current Cosine similarity S between arrays: The value range of S is [-1,1]; S4. Based on the cosine similarity S obtained in S3, construct the line fault degree d: The value range of d is [0,1]; S5. According to the line fault degree d, establish the active distribution network line fault protection criterion: d>D set (4) Where D set is the protection threshold; S6. According to the active distribution network line fault protection criterion obtained in S5, when the line protection is activated for three consecutive moments, it is determined that the line has an internal short circuit fault; otherwise, it is a normal line; The characteristics of the similarity S and the fault degree d described in S3 and S4 are analyzed as follows: Assume that A and B are two arrays of the same length: A=[a1,a2,…,a k ],B=[b1,b2,…,b k ], k is the length of arrays A and B; The inner product formula between array A and array B is expressed as: A·B=A||Bcos0(9) According to formula (9), substituting into arrays A and B, the cosine similarity S between the two arrays is expressed as cosθ: In formula (10), the range of cosine similarity is between [-1, 1]. The closer its value is to 1, the higher the cosine similarity between the two arrays. On the contrary, the closer it is to -1, the lower the cosine similarity between the two arrays. The corrected braking current in the time window before each sampling moment of the line With the corrected differential current The cosine similarity cosθrd between them is transformed from [-1, 1] to [0, 1], and the distribution network line fault degree d is constructed as follows: It can be seen from formula (11) that when the line is operating normally or an out-of-area fault occurs, θrd is 0° and cosθ rd is 1, the fault degree d of the line is 0; when an internal fault occurs, θrd is an obtuse angle, cosθ rd is close to -1, and the fault degree d of the line is close to 1, which is used to distinguish between faults inside and outside the line area.

2. The active distribution network pilot protection method taking into account the correction coefficient according to claim 1, characterized in that: Corrected braking current of the circuit in S2 With the corrected differential current The phase characteristics analysis is as follows: According to the different magnitudes of the positive sequence current fault components on both sides of the line, the corresponding correction braking current is constructed. With the corrected differential current analyze and The phase angle difference between When the system is operating normally or an out-of-zone fault occurs, According to formula (1), we can get: Because N>1, at this time and The phases are the same, and the angle θrd between them is 0; For formula (1), when hour, It is expressed as: and The angle θrd between them is expressed as: Where F represents the amplitude ratio of the positive sequence fault component current on both sides of the line, θ mn Represents the angle between the positive sequence current fault components on both sides of the line; Since the right side of the equal sign in equation (6) is about θ mn An even function, θ rd The size of θ mn The positive and negative of θ is irrelevant, so mn Defined as the angle; when hour, It is expressed as: and The angle θ between rd It is expressed as: When the downstream of the line is connected to an inverter-type distributed power source and an in-area fault occurs, when the correction coefficient N is a different constant, according to formula (6), the variable F, θ mn The angle θ under different values ​​within the range of variation rd The value of the angle θ is obtained rd are all obtuse angles; as the correction coefficient N increases, the angle θ rd The minimum value of becomes smaller; when F is small, θ rd Small value; When the downstream of the line is connected to a distributed power source and an in-area fault occurs, when the correction coefficient N is a different constant, according to formula (8), the variables F, θ are obtained by calculation. mn The angle θ under different values ​​within the range of variation rd The value of the angle θ is obtained rd are all obtuse angles; as the correction coefficient N increases, the angle θ rd The minimum value of becomes larger accordingly; when F is 1, θ rd Small value; Based on the above analysis, as the correction coefficient N increases, when the downstream is connected to an inverter-type distributed power source, θ rd Gradually decreases, when the downstream is connected to a distributed power source rd Gradually getting bigger.

3. The active distribution network pilot protection method taking into account the correction coefficient according to claim 1, characterized in that: The source distribution network line fault protection criteria described in S5 specifically include the following: 1) Using the positive sequence current fault component on both sides of the line and According to formula (1), the corrected current at each sampling time after the fault occurs is calculated: and Instantaneous value; for each moment t, take the corrected current in the time window before that moment on both sides of the line and The instantaneous value is used to form the modified braking current array and the modified differential current array at that moment; then, using formula (10), the cosine similarity value cosθ between the modified braking current and the modified differential current array is calculated: rd ; 2) The cosine similarity value cosθ rd Substituting into formula (11) we get the line fault degree d; 3) When the fault degree d satisfies formula (4), it is determined that there is a fault in the line, the line protection is activated once, and the process enters S6.

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