A pilot protection method for multi-terminal flexible DC transmission lines based on transient current ratio

Abstract

The present invention relates to a multi-terminal flexible DC transmission line longitudinal protection method based on transient current ratio, including S1: determining the fault identification criteria inside and outside the zone; S2: determining the fault start criteria; S3: determining the fault pole selection criteria; S4: determining the protection scheme. Current and voltage data are continuously collected on both sides of the line. If the voltage at the DC line protection installation at one end meets the protection start criteria, it is judged that a fault may occur in the DC transmission line, and the protection start element at this end is started. The protection collects the transient current at the protection installation and the transient current at the DC side outlet of the MMC on this side, calculates the ratio, and judges the fault inside and outside the zone, collects the transient voltage of the positive and negative lines, calculates the transient voltage energy ratio, and judges the fault type to select the fault pole of the fault type. The present invention can quickly and reliably identify faults inside and outside the DC line zone, reliably protect the entire length of the line, has strong transient resistance resistance, does not require real-time synchronization of data at both ends of the line, and has a faster operating speed than traditional current differential protection.

Description

Technical Field

[0001] The present invention belongs to the technical field of power system protection and control technology, and specifically relates to a multi-terminal flexible direct current transmission line longitudinal protection method based on transient current ratio. Background Art

[0002] Flexible DC transmission technology based on modular multilevel converter (MMC) can improve the grid connection efficiency of renewable energy power generation, and has many advantages such as fast and independent control of active and reactive power, no commutation failure, and high waveform quality. Therefore, flexible DC transmission technology has become a hot research topic. In order to make full use of the renewable energy in Zhangjiakou area, the State Grid built the Zhangbei four-terminal flexible DC grid demonstration project based on MMC in Beijing and Hebei. The emergence of multi-terminal DC grid (MTDC) has brought new challenges to the existing DC line protection technology. Compared with conventional DC transmission systems, flexible DC transmission systems have smaller damping. After a DC line fault occurs, the power grid will instantly generate a large fault component, causing the converter capacitor to discharge rapidly, and will exceed the tolerance of the power electronic devices of the converter station within a few milliseconds. Therefore, the flexible DC grid requires a shorter protection action time.

[0003] The existing HVDC line protection is divided into two types: double-ended protection and single-ended protection. At present, the research hotspot of double-ended line protection is mainly based on the traveling wave principle and mutation analysis.

[0004] In the prior art, the transient voltage energy ratio across the DC reactor is used to identify internal and external faults. This method has good transient resistance resistance. However, the extraction of high-frequency components is complex and requires high sampling equipment. There are also documents that record longitudinal protection schemes based on wave impedance size and phase, but both schemes require high sampling frequencies and lack applicability. There are also documents that record a DC power grid fault identification scheme based on the traveling wave principle. Fault identification is performed based on the time difference between the fault traveling wave arriving at each converter station. This also requires a high sampling frequency and requires fast communication and information synchronization between stations.

[0005] Smoothing reactors are installed at both ends of the DC transmission line to limit the rapid rise of fault components after the fault. Many single-ended protections use DC reactors as boundary elements. Some literature proposes a single-ended protection scheme based on the voltage of the DC reactor as the protection criterion; some literature proposes that the DC reactor has a good smoothing effect on the voltage fault traveling wave of the out-of-zone fault, and the fault line is selected based on the voltage change rate of the DC line; some literature uses wavelet transform to extract the energy of the high-frequency component of the current as the basis for protection action based on the characteristic that the reactor presents a higher impedance to the high-frequency component, but the data processing is relatively complicated. However, the single-ended protection method is difficult to guarantee the full length of the protection line, and there are problems such as the inability to identify high-resistance faults. Summary of the invention

[0006] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a multi-terminal flexible direct current transmission line longitudinal protection method based on transient current ratio. After a DC line fault occurs in a multi-terminal flexible direct current grid, the MMC will quickly generate a fault current and flow into the fault point along the DC line. Due to the presence of the DC reactor, the impedance of the MMC fault current flowing along the fault line to the fault point is much smaller than the impedance of other paths, so most of the MMC fault current flows directly into the fault point through the fault line; combined with the analysis of the post-fault traveling wave process, the difference in transient characteristics during internal and external faults is theoretically obtained, the transient current ratio at both ends of the fault line is greater than 0.5, and the transient current ratio at one end of the non-fault line must be less than 0.5, and longitudinal protection can be formed according to this feature; the protection scheme has a small amount of calculation, can quickly identify internal and external faults, reliably protect the entire length of the line, and has a strong ability to withstand transient resistance.

[0007] The present invention solves the technical problem by the following technical solutions:

[0008] A multi-terminal flexible direct current transmission line pilot protection method based on transient current ratio, characterized in that the method comprises the following steps:

[0009] S1. Determine the fault identification criteria inside and outside the zone;

[0010] Define K mn To protect P mn Transient current at installation i mn The transient current i of the MMC DC side output MMCm The ratio of

[0011]

[0012] At the beginning of the fault, K mn >0.5, the non-fault line must have one end K mn <0.5, according to this feature, the longitudinal protection is constructed, and the K calculated by the protection at both ends of the fault line is mn Ka and K b The protection action condition is the data window length T after the protection is started. W2 Always satisfied

[0013]

[0014] When T W2 When the value is small, the number of sampling points is small and the reliability of protection is low;

[0015] When T W2 When the value is large, the protection action takes a long time, and the protection principle is more affected by the refraction and reflection of the fault wave between the lines, and the protection reliability is reduced. Considering the speed and reliability of the protection, the data window length T is taken W2 =0.5ms;

[0016] S2. Establish protection start criteria;

[0017] When the system is operating normally, the DC line voltage fluctuates around the rated voltage; when a short circuit occurs, the line voltage drops significantly. A low voltage startup scheme is used to detect the DC voltage to ground at the protection installation. When the time window is long T W1 The sampling voltage amplitude within |U t |Below the set value U set When the protection starts, the protection start setting value is U set =0.8U N , where U N is the DC rated voltage, and the time window length T W1 =0.3ms, the protection start criterion is:

[0018] |U t |<0.8U N

[0019] S3, establish fault pole selection criteria;

[0020] After a single-pole grounding fault occurs, the amplitude of the transient voltage component generated by the coupling effect on the non-fault pole is smaller than that on the fault pole; after a bipolar short-circuit fault occurs, due to the structural symmetry of the symmetrical bipolar MMC converter, the amplitudes of the voltage transient components on both poles are basically the same, and the transient voltage energy E is defined as mn To protect the time window after startup, T W3 Internal protection P mn The integral of the square of the transient voltage u at the installation location is:

[0021]

[0022] Defining β mn To protect P mnThe ratio of the transient voltage energy of the positive and negative lines at the installation location, that is,

[0023]

[0024] In the formula, E mnp and E mnn Protect P mn Transient voltage energy of the positive and negative DC lines at the installation location;

[0025] To increase the reliability of fault selection, take T W3 =0.5ms, with β mn The size of is the basis for selecting the pole, that is

[0026]

[0027] S4. Determine the protection plan;

[0028] When the protection detects that the line voltage amplitude is lower than 0.8U N When the DC transmission line is judged to be faulty, the protection process is started; the protection collects the transient current i at the protection installation location. mn The transient current i at the DC side outlet of the MMC on this side MMCm And calculate K mn , collect the transient voltage of the positive and negative lines and calculate the transient voltage energy ratio β mn ;

[0029] When K mn When >0.5, the protection sends a tripping permission signal to the local end and the opposite end; when the protection receives the tripping permission signal sent by both ends of the line, it is judged as an internal fault;

[0030] When β mn >2, it is judged as positive fault;

[0031] When β mn When <0.5, it is judged as a negative electrode fault;

[0032] When 0.5≤β mn When ≤2, it is judged as a bipolar fault; after selecting the fault line and determining the fault type, the faulty pole DCCB of the faulty line is opened.

[0033] The advantages and beneficial effects of the present invention are:

[0034] 1. The present invention has a small amount of calculation and can reliably and quickly identify faults inside and outside the DC line area and protect the entire length of the line.

[0035] 2. The DC line protection proposed by the present invention has strong transition resistance and noise resistance, is not affected by the distributed capacitance of the line, and can adapt to changes in operating modes.

[0036] 3. The present invention does not require data synchronization at both ends of the line, and has low reliability requirements for the communication channel. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a topological structure diagram of a four-terminal flexible DC power grid;

[0038] Figure 2 It is the MMC fault current flow path diagram;

[0039] Figure 3 Additional network diagram for four-terminal flexible DC grid fault;

[0040] Figure 4 is a flow chart of the present invention;

[0041] Figure 5 For line L line23 Simulation experiment diagram of midpoint positive metallic grounding;

[0042] Figure 6 For line L line23 Simulation experiment diagram of midpoint positive high-resistance grounding. DETAILED DESCRIPTION

[0043] The present invention is further described in detail below through specific examples. The following examples are only illustrative and not restrictive, and the protection scope of the present invention cannot be limited thereto.

[0044] The present invention provides a method for longitudinal protection of a multi-terminal flexible direct current transmission line based on transient current ratio. The method uses the transient current ratio between the protection installation location and the MMC DC outlet on the local side after the fault is detected to form an identification criterion. When the fault occurs within the area, the transient current ratios at both ends of the line are greater than 0.5; when the fault occurs outside the area, the transient current ratio at at least one end is less than 0.5. The protection start-up time window of the protection scheme is 0.3ms, and the fault location and fault pole selection are performed simultaneously, with a time window length of 0.5ms. The fault identification is completed within 3ms after the fault occurs.

[0045] The four-terminal flexible DC grid structure is as follows Figure 1 As shown, the four converters all adopt a symmetrical bipolar MMC structure, in which the MMC submodule is a half-bridge submodule, and a DC circuit breaker DCCB and a DC reactor are configured at both ends of the DC transmission line. linemn For DC transmission lines, P mn It is a protection device installed at the beginning and end of each line, BUS m is the DC bus (where m,n=1,2,3,4).

[0046] The half-bridge MMC does not have the ability to self-clear DC faults. When a short-circuit fault occurs in the DC line, the line voltage drops rapidly and is lower than the total voltage of the sub-module capacitors in each phase bridge arm, causing the sub-module capacitors to discharge rapidly to the fault point and the fault current flowing through the MMC to rise rapidly. Before the MMC converter station is locked, the fault transient current is mainly composed of the sub-module capacitor discharge current, and also contains a small amount of AC system feed-in short-circuit current. Taking the DC line positive grounding fault as an example, the fault current path of the fault pole MMC is as follows: Figure 2 As shown, the solid line current path is the submodule capacitor discharge path, the dotted line current path is the AC system feed short-circuit current path, BUS is the DC bus, Z eq is the equivalent impedance from the positive outlet of MMC to the fault point, L arm is the bridge arm inductance, and C is the submodule capacitor. After a fault occurs, the submodule capacitor discharge current flows from the converter to the DC bus, and the AC system feed-in short-circuit current alternately flows between the bridge arms as the AC side voltage changes periodically, and the flow direction is the same as the submodule capacitor discharge current. The transient current detected at the MMC DC side outlet is the superposition of the submodule capacitor discharge current and the AC system feed-in short-circuit current. Its direction always flows from the MMC to the DC bus. The AC system feed-in short-circuit current and the submodule capacitor discharge current are no longer distinguished, and the two are collectively referred to as the MMC fault current.

[0047] Line L line23 Taking the metallic grounding fault at the positive pole F as an example, the fault output model of the MMC converter is equivalent to a time-varying current source. The magnitude of the time-varying current source is the same as the fault current flowing through the MMC. The voltage at the fault point suddenly changes to 0, which is equivalent to introducing a step fault voltage source at the point. The magnitude of the fault voltage source is the opposite value of the operating voltage at the moment before the fault at the point. According to the superposition theorem, the fault additional network of the four-terminal flexible DC grid can be obtained as follows: Figure 3 As shown in the figure, MMCm For MMC m DC side output fault current; i mn To protect P mn Transient current of the line at the installation location; -E is the fault voltage source, E is the operating voltage before the fault at the fault point; Z L is the DC reactor impedance; Z 12 , Z 41 , Z 34 , Z 2F and Z 3F is the line impedance. It is stipulated that the current flowing from the busbar to the line is the positive direction of the line current, and the current flowing from the MMC to the busbar is the positive direction of the MMC DC side outlet current. Definition K mn To protect the transient current i at the installation mn The transient current i of the MMC DC side output MMCm The ratio of

[0048]

[0049] Figure 3 The fault current and node voltage on the line can be regarded as the superposition of the separate effects of the MMC equivalent current source and the fault voltage source at the fault point. The fault voltage traveling wave generated by the separate action of the fault voltage source is counted as the traveling wave W. u and traveling wave V u .

[0050] Traveling Wave W u After reaching BUS2, the voltage wave W u Under the action of BUS2, the voltage drops and MMC2 generates a fault current, which is recorded as i MMC2 .i MMC2 There are two paths into the fault point, such as Figure 3 The blue dotted lines show BUS2-F and BUS2-BUS1-BUS4-BUS3-F respectively. The current components flowing into the two paths are calculated separately. 23MMC2 and i 21MMC2 , the impedance Z on the first path B1 =Z L +Z 2F , the impedance of the second path is Z B2 =7Z L +Z 12 +Z 41 +Z 34 +Z 3F The second path has a larger impedance of the line and DC reactor, so it is obvious that Z B1 >Z B2 , so we can get

[0051]

[0052] Fault traveling wave W u When propagating toward BUS1 after refraction, line L line12 There is only a pre-fault traveling wave on the circuit, and the voltage traveling wave amplitude is negative, so the current traveling wave amplitude is negative. 21 The current fault component caused by the installation is taken as i 21W , we can know that 21W <0. According to the superposition theorem, the protection P 21 The detected transient current is composed of the fault current traveling wave and the MMC2 fault current, i 21 =i 21MMC2 +i 21W , according to Kirchhoff's law, we can get i 23 =i MMC2 -i 21 =i 23MMC2 -i21W . Because I 21W <0, we can get K 23 and K 21 Satisfies the following inequality:

[0053]

[0054] Similarly, we can get

[0055]

[0056] Line L line41 The sum of the transient current of the ground capacitance is denoted as ∑i C , it is stipulated that the positive direction is from the line to the earth. According to Kirchhoff's current law, we can get ∑i C =i 14 +i 41 When the fault voltage wave reaches line L line41 When the voltage wave is negative, the line voltage starts to drop. During the drop of line voltage, the ground capacitor discharges, and the sum of the transient current of the ground capacitor ∑i C is less than 0, so i 14 +i 41 <0, then there is i 14 or 41 Less than 0, protect P 14 and protection P 41 K must be measured at one end mn <0.5.

[0057] It can be concluded that after a DC line fault occurs, the fault current generated by the MMC flows into the fault point through two paths. For the MMCs at both ends of the fault line, the impedance of the two paths of the fault current is quite different, so the fault current components flowing to the two paths are quite different, and most of the fault current flows directly through the fault line into the fault point. mn >0.5, the non-fault line must have one end K mn <0.5, longitudinal protection can be constructed based on this feature.

[0058] A method for pilot protection of a multi-terminal flexible direct current transmission line based on transient current ratio comprises the following steps:

[0059] 1. Establish protection start criteria:

[0060] When the system is operating normally, the DC line voltage fluctuates around the rated voltage; when a short circuit occurs, the line voltage drops significantly. The method adopts a low-voltage startup scheme to detect the DC-to-ground voltage at the protection installation. When the time window is long T W1 The sampling voltage amplitude within |U t |Below the set value Uset The protection starts. The low voltage setting value needs to avoid the voltage during normal operation. Considering that the protection can not be started too frequently and can still be started reliably when the transition resistance fails, the protection start setting value is U set =0.8U N , where U N is the DC rated voltage, and the time window length T W1 =0.3ms. The protection start criterion is:

[0061] |U t |<0.8U N (5)

[0062] 2. Establish fault location criteria:

[0063] After the protection is started, measure the transient current i at the protection installation mn The transient current i of the MMC DC side output MMCm And calculate K mn At the beginning of the fault, K mn >0.5, the non-fault line must have one end K mn <0.5, a longitudinal protection can be constructed based on this feature. Assume that the K calculated by the protection at both ends of the line mn K a and K b The protection action condition is the data window length T after the protection is started. W2 Always satisfied

[0064]

[0065] When T W2 When the value is small, the number of sampling points is small and the reliability of protection is low; when T W2 When the value is large, the protection action takes a long time, and the protection principle is more affected by the refraction and reflection of the fault wave between the lines, and the protection reliability is reduced. Considering the speed and reliability of the protection, the data window length T is taken as W2 =0.5ms.

[0066] 3. Establish fault selection criteria:

[0067] After a single-pole ground fault occurs, the amplitude of the transient voltage component generated by the coupling effect on the non-fault pole is smaller than that on the fault pole; after a bipolar short circuit fault occurs, due to the structural symmetry of the symmetrical bipolar MMC converter, the amplitudes of the voltage transient components on the two poles are basically the same. Definition of transient voltage energy E mn To protect the time window after startup, T W3 Internal protection P mn The integral of the square of the transient voltage u at the installation location is:

[0068]

[0069] Defining β mn To protect P mn The ratio of the transient voltage energy of the positive and negative lines at the installation location, that is,

[0070]

[0071] In the formula, E mnp and E mnn Protect P mn The transient voltage energy of the positive and negative DC lines at the installation location. To increase the reliability of fault selection, take T W3 =0.5ms, with β mn The size of is the basis for selecting the pole, that is

[0072]

[0073] 4. Protection process:

[0074] The protection method of the present invention is as follows Figure 4 As shown, when the protection detects that the line voltage amplitude is lower than 0.8U N When the DC transmission line is judged to be faulty, the protection process is started. mn The transient current i at the DC side outlet of the MMC on this side MMCm And calculate K mn , collect the transient voltage of the positive and negative lines and calculate the transient voltage energy ratio β mn When K mn When >0.5, the protection sends a tripping permission signal to the local end and the other end. When the protection receives the tripping permission signal sent by both ends of the line, it is judged as an internal fault.

[0075] When β mn >2, it is judged as positive fault;

[0076] When β mn When <0.5, it is judged as a negative electrode fault;

[0077] When 0.5≤β mn When ≤2, it is judged as a bipolar fault. After selecting the fault line and determining the fault type, the faulty pole DCCB of the faulty line is opened.

[0078] Simulation Verification

[0079] A Zhangbei four-terminal flexible DC power grid model was built in the PSCAD / EMTDC simulation platform for simulation verification. Its topology is as follows: Figure 1As shown, MMC1-MMC4 converter stations are Fengning, Beijing, Zhangbei and Kangbao converter stations respectively. Zhangbei, Beijing and Kangbao stations adopt fixed active power control and fixed reactive power control, Fengning station adopts fixed reactive power control and fixed DC voltage control, and the rated DC voltage of the DC grid is ±500KV. The overhead lines between the converter stations adopt the frequency-dependent line model with positive and negative poles on the same tower. Its length is as shown in Figure 1 As shown, the sampling frequency is 20kHz.

[0080] Metallic ground fault simulation

[0081] DC line L line23 The simulation results of a metallic positive grounding fault at the midpoint are as follows Figure 5 shown. Figure 5 a shows the positive and negative voltages at both ends of the fault line. 0.35ms after the fault occurs, the fault wave is transmitted to the protection installation positions at both ends of the line. The positive voltage drops rapidly and reaches the protection start setting value. After 0.3ms, the protection starts. The negative line voltage fluctuates slightly due to coupling, and its voltage change is much smaller than the positive voltage change. After the protection starts, the transient voltage energy ratio β of the two-pole line is calculated. mn , we get β 23 =33,β 32 =69, so it can be determined as a positive fault. Although the negative line voltage is close to the protection start threshold, due to the presence of the fault selection element, the negative protection can be reliably locked and will not malfunction.

[0082] Calculate the transient current i at the protection installation on the positive line mn The transient current i at the MMC outlet on this side MMCm The ratio K mn , the K measured by the protection at both ends of the same line mn Plotted in the same coordinate system, the waveform is as follows Figure 5 As shown in b, the time corresponding to the dotted line on the horizontal axis is the data window T W2 , no longer protect the K before the start mn For analysis, plot it as 0. Analysis Figure 5 b shows that the protection at both ends of the faulty line first senses the fault component and starts, so the protection P 23 and P 32 First enter the data window T W2 Sampling and calculating K mn The farther the protection is from the fault point, the later the protection starts. Within 2.5ms after the fault occurs, K mn All of them are greater than 0.5, and the non-fault line must have one end K mn <0.5.

[0083] High resistance ground fault simulation

[0084] In the theoretical analysis of metallic faults, this method considers both the MMC fault current and the fault current traveling wave generated by the fault voltage source. In theory, the introduction of transition resistance will only change the magnitude of the transient current flowing through the line, and will not change the distribution characteristics of the transient current. This method uses the transient current ratio after the fault as the action criterion, which is less affected by the change in the magnitude of the transient current. Therefore, in theory, the protection action criterion is still applicable to high-resistance faults. Assume that line L line23 A positive ground fault occurs at the midpoint, and the transition resistance is 300Ω. The K mn Waveform Figure 6 After adding the transition resistor, K 23 and K 32 The waveform is similar to that when a metallic fault occurs. The protection at both ends of the faulty line can operate reliably, and the protection of the non-faulty line will not malfunction, indicating that the transition resistance has little effect on the protection principle.

[0085] By analyzing the distribution characteristics of transient current on the line when the DC line fails, the present invention realizes fault location based on the ratio of transient current of the DC line to the MMC fault current, cooperates with the fault start criterion and pole selection criterion, considers the fault traveling wave transmission speed and the inter-station communication time, and can quickly locate the fault pole of the fault line within 3ms after the DC line fails. This protection scheme only needs to transmit the tripping permission signal, does not require data synchronization, has low requirements on the reliability of communication, and has strong resistance to transition resistance and noise, and is not affected by the distributed capacitance of the line.

[0086] Although the embodiments and drawings of the present invention are disclosed for illustrative purposes, those skilled in the art will appreciate that various substitutions, changes and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A method for pilot protection of a multi-terminal flexible DC transmission line based on transient current ratio, characterized in that: The steps of the method are: S1. Determine the fault identification criteria inside and outside the zone; Define K mn To protect P mn Transient current at installation i mn The transient current i of the MMC DC side output MMCm The ratio of At the beginning of the fault, K mn >0.5, the non-fault line must have one end K mn <0.5, according to this feature, the longitudinal protection is constructed, and the K calculated by the protection at both ends of the fault line is mn K a and K b The protection action condition is the data window length T after the protection is started. W2 Always satisfied Take the data window length T W2 =0.5ms; S2. Establish protection start criteria; When the system is operating normally, the DC line voltage fluctuates around the rated voltage; when a short circuit occurs, the line voltage drops significantly. A low voltage startup scheme is used to detect the DC voltage to ground at the protection installation. When the time window is long T W1 The sampling voltage amplitude within |U t |Below the set value U set When the protection starts, the protection start setting value is U set =0.8U N , where U N is the DC rated voltage, and the time window length T W1 =0.3ms, the protection start criterion is: |And t |<0.8U N Step 3: Establish fault pole selection criteria; After a single-pole grounding fault occurs, the amplitude of the transient voltage component generated by the coupling effect on the non-fault pole is smaller than that on the fault pole; after a bipolar short-circuit fault occurs, due to the structural symmetry of the symmetrical bipolar MMC converter, the amplitudes of the voltage transient components on the two poles are basically the same, and the transient voltage energy E is defined as mn To protect the time window after startup, T W3 Internal protection P mn The integral of the square of the transient voltage u at the installation location is: Defining β mn To protect P mn The ratio of the transient voltage energy of the positive and negative lines at the installation location, that is, In the formula, E mnp and E mnn Protect P mn Transient voltage energy of the positive and negative DC lines at the installation location; To increase the reliability of fault selection, take T W3 =0.5ms, with β mn The size of is the basis for selecting the pole, that is Step 4: Determine the protection plan; When the protection detects that the line voltage amplitude is lower than 0.8U N When the DC transmission line is judged to be faulty, the protection process is started; Protection collection protection installation location transient current i mn The transient current i at the DC side outlet of the MMC on this side MMCm And calculate K mn , collect the transient voltage of the positive and negative lines and calculate the transient voltage energy ratio β mn ; When K mn When >0.5, the protection sends a tripping permission signal to the local end and the opposite end; when the protection receives the tripping permission signal sent by both ends of the line, it is judged as an internal fault; When β mn >2, it is judged as positive fault; When β mn When <0.5, it is judged as a negative electrode fault; When 0.5≤βmn≤2, it is judged as a bipolar fault; after selecting the fault line and determining the fault type, the faulty pole DCCB of the faulty line is opened.

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