A single-pole grounding partition protection method for an integrated double-ended MMC DC power distribution network

Abstract

The application discloses a kind of based on integration's double-end MMC direct current distribution network single pole ground partition protection method, mainly include the following steps: direct current distribution network is divided into bus, tie line and feeder;Whether single pole ground fault occurs is judged by monitoring the change of positive and negative electrode voltage to ground.When single pole ground fault occurs, the fault pole is judged according to fault pole voltage;While collecting the fault capacitance current signal of each line of the direct current distribution network.Calculate the similarity coefficient P M·i Of each bus, judge whether it is bus fault, when only the similarity coefficient of a certain bus is positive, then it is judged as bus fault, and the bus is fault bus.The application realizes the based on integration's double-end MMC direct current distribution network single pole ground partition protection method, can accurately judge that bus, tie line or feeder single pole ground fault occurs, improves the protection accuracy high;Can effectively improve operation and maintenance efficiency, reduce the outage range, improve the reliability of direct current distribution network operation.

Description

Technical Field

[0001] This invention belongs to the field of power system relay protection, specifically relating to a method for single-pole grounding zone protection of DC distribution networks based on integrated dual-terminal MMC. Background Technology

[0002] When sampling the DC side of a DC distribution network using a clamping resistor grounding method, it features a small fault current and relatively constant inter-pole voltage during a single-pole grounding fault, making it widely used in DC distribution networks. However, in DC distribution networks, especially AC ungrounded distribution networks, the fault characteristics are not obvious during single-pole grounding faults, and the resistance at the fault point varies significantly, making single-pole grounding fault protection a persistent challenge in relay protection. Because the fault characteristics of single-pole grounding faults in DC distribution networks are complex, with multiple fault types and relatively small fault components, the accuracy of single-pole grounding fault protection is severely affected.

[0003] However, DC distribution network protection is still in the theoretical research stage, and the imperfections of DC protection equipment pose significant constraints and challenges to DC distribution network protection. Therefore, researching accurate single-pole grounding protection methods for DC distribution networks is a crucial technical challenge that needs to be addressed in the construction of DC distribution networks. Summary of the Invention

[0004] To address the technical problems mentioned in the background, this invention provides an integrated dual-terminal MMC DC distribution network single-pole grounding zone protection method. This method accurately protects against single-pole grounding faults based on the similarity of capacitive currents in different sections of the DC distribution network when a single-pole grounding fault occurs, and can trigger alarms or tripping as required.

[0005] To achieve the above technical objectives, the technical solution of this invention is: a method for unipolar grounding zone protection of a DC distribution network based on an integrated dual-terminal MMC, comprising the following steps:

[0006] S1 determines the busbars, tie lines, feeders and their relationships in the DC distribution network, and assigns them numbers; such as busbar M, tie line L, feeder K.

[0007] S2 specifies the direction of the current.

[0008] S3 determines whether the installation of the instrument transformer is correct by monitoring the direction of the current in each line during normal operation of the DC distribution network.

[0009] S4 determines the turns ratio n of each current transformer;

[0010] S5 synchronously samples the positive and negative voltages to ground of the DC distribution network and the positive and negative currents at both ends of each tie line and at the feeder outlet.

[0011] S6 calculates the magnitude and relative voltage of the positive and negative poles to ground to comprehensively determine whether a single-pole grounding fault has occurred.

[0012] When a single-pole grounding fault occurs, S7 determines the faulty pole by the magnitude of the voltage to ground of the positive and negative poles.

[0013] S8 determines the data length N of the current criterion by analyzing the change characteristics of the fault electrode voltage;

[0014] S9 uses the sampled values ​​of the charging and discharging current of the lines connected to each busbar to calculate the similarity coefficient P of the capacitive current of each busbar according to equation (3). M.i When only the similarity coefficient P of a certain generatrix is ​​known... M.i If the result is positive, the fault occurs on that busbar; otherwise, continue to proceed with step (10) to determine the fault.

[0015] S10 uses the sampled values ​​of the charging and discharging current of the capacitors at both ends of the tie line to calculate the similarity coefficient p of each tie line according to equation (4). L.i When there is only one tie line, the similarity coefficient p of the capacitive currents at both ends is... L.i If the similarity coefficient of the capacitor current at both ends of other tie lines is positive, then the tie line is a faulty line; otherwise, if the fault does not occur in the tie line area, continue to judge according to step (11).

[0016] S11 uses the sampled values ​​of the capacitor current and charging / discharging current at the beginning of all feeders to calculate the similarity coefficient P of each feeder according to equation (5). k.i Then the similarity coefficient P k.i Minimum value P k.i.min The corresponding feeder is the faulty feeder.

[0017] Preferably, the direction of current flow is defined as positive when it flows from the busbar to the line, and negative when it flows from the line to the busbar.

[0018] Preferably, in step S9, the capacitance current similarity coefficient P of each busbar is calculated according to equation (3) using the sampled values ​​of the capacitance current charging and discharging current of the lines connected to each busbar. M.i When only the similarity coefficient P of a certain generatrix is ​​known... M.i If the result is positive, the fault occurs on that busbar; otherwise, continue to proceed to step S10 for judgment.

[0019]

[0020] Where: P M.i The similarity coefficient of the i-th busbar; N is the length of the ground data window calculated in step S8; H is the number of lines connected to the i-th busbar; n s Let i be the current transformer ratio of the s-th line; s.g(k) is the kth sampled value of the fault pole of the sth line.

[0021] Preferably, in step S10, the similarity coefficient p of each tie line is calculated according to equation (4) using the sampled values ​​of the charging and discharging current of the capacitors at both ends of the tie line. L.i When there is only one tie line, the similarity coefficient p of the capacitive currents at both ends is... L.i If the similarity coefficient of the capacitance current at both ends of other tie lines is positive, then the tie line is a faulty line; otherwise, if the fault does not occur in the tie line area, continue to judge according to step S11.

[0022]

[0023] Where: i L.i-1.g (k) represents the k-th sampled value at one end of the i-th connection line; i L.i-2.g (k) is the kth sampled value at the other end of the i-th connection line.

[0024] Preferably, in step S11, the similarity coefficient P of each feeder is calculated according to equation (5) using the sampled values ​​of the capacitor current and charging / discharging current at the beginning of all feeders. k.i Then the similarity coefficient P k.i Minimum value P k.i.min The corresponding feeder is the faulty feeder.

[0025]

[0026] Where: p k.i.s is the similarity coefficient of the fault pole current between the i-th feeder and the s-th feeder; F is the total number of DC distribution network feeders; P k.i Let n be the similarity coefficient of the i-th feeder; k.i n k.s These are the current transformer ratios for the i-th and s-th feeders, respectively; i k.i.g (k), i k.s.g (k) represents the kth sampled value of the fault pole of the i-th and s-th feed lines, respectively.

[0027] The beneficial effects of adopting the above technical solution are as follows: This invention is a novel method for single-pole grounding fault protection that utilizes the similarity of the charging and discharging currents of a line when a single-pole grounding fault occurs on the DC side of a double-ended MMC DC distribution network. This invention effectively solves the problem of single-pole grounding fault protection in double-ended MMC DC distribution networks. Furthermore, this invention features simple calculation and does not require determining the charging and discharging signal frequency. This invention has wide applicability; verification shows that changing the location of the fault point or grid parameters does not affect the accuracy of the protection provided by this invention. This invention can accurately achieve single-pole grounding protection, effectively reduce the scope of power outages, and improve the safety and reliability of DC distribution network operation. Attached Figure Description

[0028] Figure 1 This is a flowchart of the present invention;

[0029] Figure 2 This is a schematic diagram of a dual-ended MMC DC distribution network structure;

[0030] Figure 3 This is a schematic diagram showing a positive grounding fault occurring on bus M3, tie line L2, and feeder K3.

[0031] Figure 4 This is a waveform diagram of the positive and negative current at each detection point when a positive ground fault occurs on bus M3;

[0032] Figure 5 This is a waveform diagram of the positive and negative currents of each tie line when a positive ground fault occurs in tie line L2;

[0033] Figure 6 This is a waveform diagram of the positive and negative currents of each feeder when a positive ground fault occurs in feeder k3. Detailed Implementation

[0034] The present invention will now be described in further detail with reference to the accompanying drawings.

[0035] This invention proposes a method for protecting a single-pole grounding fault in a DC distribution network grounded via a clamping resistor, such as... Figure 1 As shown, it includes the following steps:

[0036] A method for unipolar grounding zone protection of DC distribution networks based on integrated dual-terminal MMC includes the following steps:

[0037] S1 determines the busbars, tie lines, feeders and their relationships in the DC distribution network, and assigns them numbers; such as busbar M, tie line L, feeder K.

[0038] S2 specifies that the current direction is positive when it flows from the busbar to the line, and negative when it flows from the line to the busbar.

[0039] S3 determines whether the installation of the instrument transformer is correct by monitoring the direction of the current in each line during normal operation of the DC distribution network.

[0040] S4 determines the turns ratio n of each current transformer;

[0041] S5 synchronously samples the positive and negative voltages to ground of the DC distribution network and the positive and negative currents at both ends of each tie line and at the feeder outlet.

[0042] S6 calculates the magnitude and relative voltage of the positive and negative poles to ground to comprehensively determine whether a single-pole grounding fault has occurred.

[0043] When equation (1) is satisfied, it is determined that a single-pole grounding fault has occurred.

[0044] <(UP≤0.4.0.5.UPN.eor UN ≤0.4.0.5.UPN.e(1)

[0045] lUPN≥0.8.UPN.e

[0046] Wherein: UP, UN, UPN, and UPN.e are the rated values ​​of positive-to-ground voltage, negative-to-ground voltage, inter-electrode voltage, and inter-electrode voltage, respectively;

[0047] When a single-pole ground fault occurs (S7), the faulty pole is determined by the magnitude of the voltage between the positive and negative poles and ground; when U... P ≥|U N If the fault occurs, it is determined to be a negative electrode fault; otherwise, it is determined to be a positive electrode fault.

[0048] S8 determines the data length N of the current criterion by analyzing the change characteristics of the fault electrode voltage;

[0049] Start from t=1 and proceed according to equation (2). Stop the calculation when equation (2) is satisfied. At this time, the value of t is the length N of the data window.

[0050]

[0051] Where: T is the number of sampling points in 5ms.

[0052] ug(t) is the value of the voltage between the fault electrode and ground at the t-th sampling point;

[0053] S9 uses the sampled values ​​of the charging and discharging current of the line connected to each busbar to calculate the similarity coefficient PM.i of the charging and discharging current of each busbar according to the formula (3); when only the similarity coefficient PM.i of a certain busbar is positive, the fault occurs on that busbar; otherwise, continue to judge according to step (10).

[0054] The similarity coefficient PM.i of the i-th busbar is calculated as shown in equation (3):

[0055]

[0056] Where: PM.i is the similarity coefficient of the i-th busbar; N is the length of the ground data window calculated in step (8); H is the number of lines connected to the i-th busbar; n s is the current transformer ratio of the s-th line; is.g(k) is the k-th sampled value of the fault pole of the s-th line.

[0057] S10 uses the sampling values ​​of the charging and discharging current of the capacitors at both ends of the tie line to calculate the similarity coefficient pL.i of each tie line according to formula (4); when only one tie line has a positive similarity coefficient pL.i of the capacitors at both ends of the tie line, and the similarity coefficients of the capacitors at both ends of other tie lines are negative, the tie line is a faulty line; otherwise, the fault has not occurred in the tie line area, and the judgment is continued according to step (11); the calculation method of the similarity coefficient pL.i of the i-th tie line is shown in formula (4).

[0058]

[0059] Where: iL.i-1.g(k) is the kth sampled value at one end of the i-th tie line; iL.i-2.g(k) is the kth sampled value at the other end of the i-th tie line.

[0060] S11 uses the sampled values ​​of the capacitor current and charging / discharging current at the beginning of all feeders to calculate the similarity coefficient Pk.i of each feeder according to Equation (5); then the feeder corresponding to the minimum value of the similarity coefficient Pk.i, Pk.i.min, is the faulty feeder; the calculation method of the similarity coefficient Pk.i of the i-th feeder is shown in Equation (5).

[0061]

[0062] Where: p k.i.s is the similarity coefficient of the fault pole current between the i-th feeder and the s-th feeder; F is the total number of DC distribution network feeders; P k.i Let n be the similarity coefficient of the i-th feeder; k.i n k.s These are the current transformer ratios for the i-th and s-th feeders, respectively; i k.i.g (k), i k.s.g (k) represents the kth sampled value of the fault pole of the i-th and s-th feed lines, respectively.

[0063] like Figures 1 to 6 The diagram shows a dual-terminal MMC DC distribution network structure. The AC side uses an ungrounded neutral point, and the rated voltage on the DC side is [voltage value missing]. + The voltage level is 10kV; the sampling frequency is 20kHz; the power grid has four busbars M1 to M4. There are three tie lines L1 to L3, with lengths of 5km, 4km, and 3km respectively, and current transformer ratios of 200 / 5, 200 / 5, and 200 / 5 respectively. There are six feeders k1 to k6, with lengths of 4km, 3km, and 6km, and 3km, 6km, and 5km respectively, and current transformer ratios of 100 / 5 for all of them. Assuming the inter-pole voltage variation is within ±20% of the rated voltage (distinguishing between inter-pole short circuits), a single-pole ground fault is considered to have occurred if the voltage drop between the positive or negative pole and ground exceeds 20%.

[0064] (1) When a single-pole ground fault occurs at the positive terminal of bus M3, the charging and discharging waveforms of the capacitor current at each line detection point are as follows: Figure 4 As shown in the figure; their similarity coefficients are shown in Table 1.

[0065] Table 1. Similarity coefficient values ​​when M3 experiences a single-pole grounding fault.

[0066] name name N PM1 PM2 PM3 PM4 PL1 PL2 PL3 PK1 PK2 PK3 PK4 PK5 numerical values numerical values 46 -0.90 -0.88 0.81 -0.92 — — — — — — — —

[0067] As shown in Table 1, when a single-pole ground fault occurs on bus M3, the similarity coefficient of M3 is positive, while the similarity coefficients of the other buses are all negative. Therefore, it is determined that the fault is on bus M3, and the judgment ends.

[0068] (2) When a single-pole ground fault occurs at the positive terminal of tie line L2, the charging and discharging waveforms of the capacitor current at each line detection point are as follows: Figure 5 As shown in the figure; their similarity coefficients are shown in Table 2.

[0069] Table 2 shows the similarity coefficient values ​​when a single-pole grounding fault occurs in L2.

[0070] name N PM1 PM2 PM3 PM4 PL1 PL2 PL3 PK1 PK2 PK3 PK4 PK5 PK6 numerical values 47 -0.91 -0.76 -0.71 -0.89 -0.94 0.90 -0.92 — — — — — —

[0071] As shown in Table 2, when a single-pole ground fault occurs on tie line L2, the similarity coefficients of all busbars are negative, therefore the fault is not on the busbar. Among the tie lines, only the similarity coefficient of tie line L2 is positive, while the others are negative, so L2 is correctly identified as the faulty line.

[0072] (3) When a single-pole ground fault occurs at the positive terminal of feeder K3, the charging and discharging waveforms of the capacitor current at each line detection point are as follows: Figure 6 As shown in the figure; their similarity coefficients are shown in Table 3.

[0073] Table 3 shows the similarity coefficient values ​​when a single-pole grounding fault occurs in K3.

[0074] name N PM1 PM2 PM3 PM4 PL1 PL2 PL3 PK1 PK2 PK3 PK4 PK5 PK6 number 4 -0.8 -0.7 -0.8 -0.9 -0.9 +0.9 -0.9 +2.6 +2.5 -4.6 +2.4 +2.19 +2.01 value 8 9 7 0 1 4 0 2 7 6 2 1

[0075] As shown in Table 3, when a single-pole grounding fault occurs on feeder K3, the similarity coefficients of all buses are negative, so the fault is not on the bus; the similarity coefficients of all tie lines are also negative, so the fault is not on the tie line either; among all feeder similarity coefficients, only feeder K3 has the smallest similarity coefficient, so the judgment that the fault occurred on feeder K3 is correct.

Claims

1. A method for unipolar grounding zone protection of DC distribution networks based on integrated dual-terminal MMC, characterized in that: Includes the following steps: S1 determines and numbers the busbars, tie lines, feeders, and their relationships in the DC distribution network; busbar M, tie line L, feeder K; S2 specifies the positive direction of the current; S3 determines whether the installation of the instrument transformer is correct by monitoring the direction of the current in each line during normal operation of the DC distribution network. S4 determines the turns ratio n of each current transformer; S5 synchronously samples the positive and negative voltages to ground of the DC distribution network and the positive and negative currents at both ends of each tie line and at the feeder outlet. S6 calculates the magnitude and relative voltage of the positive and negative poles to ground to comprehensively determine whether a single-pole grounding fault has occurred. When a single-pole grounding fault occurs, S7 determines the faulty pole by the magnitude of the voltage to ground of the positive and negative poles. S8 determines the data length N of the current criterion by analyzing the change characteristics of the fault electrode voltage; S9 uses the sampled values ​​of the charging and discharging current of the lines connected to each busbar to calculate the similarity coefficient P of the capacitive current of each busbar according to equation (3). M.i When only the similarity coefficient P of a certain generatrix is ​​known... M.i If the result is positive, the fault occurs on that busbar; otherwise, continue to proceed to step S10 for judgment. Where: P M.i The similarity coefficient of the i-th bus is given by n; N is the data length calculated in step S8; H is the number of lines connected to the i-th bus; n s Let i be the current transformer ratio of the s-th line; s.g (k) represents the k-th sampled value of the fault pole of the s-th line; S10 uses the sampled values ​​of the charging and discharging current of the capacitors at both ends of the tie line to calculate the similarity coefficient p of each tie line according to equation (4). L.i When there is only one tie line, the similarity coefficient p of the capacitive currents at both ends is... L.i If the similarity coefficient of the capacitance current at both ends of other tie lines is positive, then the tie line is a faulty line; otherwise, if the fault does not occur in the tie line area, continue to judge according to step S11. Where: i L.i-1.g (k) represents the k-th sampled value at one end of the i-th connection line; i L.i-2.g (k) represents the k-th sampled value at the other end of the i-th connection line; S11 uses the sampled values ​​of the capacitor current and charging / discharging current at the beginning of all feeders to calculate the similarity coefficient P of each feeder according to equation (5). k.i Then the similarity coefficient P k.i Minimum value P k.i.min The corresponding feeder is the faulty feeder; Where: p k.i.s is the similarity coefficient of the fault pole current between the i-th feeder and the s-th feeder; F is the total number of DC distribution network feeders; P k.i Let n be the similarity coefficient of the i-th feeder; k.i n k.s These are the current transformer ratios for the i-th and s-th feeders, respectively; i k.i.g (k), i k.s.g (k) represents the k-th sampled value of the fault pole of the i-th and s-th feed lines, respectively.

2. The method for single-pole grounding zone protection of DC distribution network based on integrated dual-terminal MMC as described in claim 1, characterized in that: The current direction is defined as positive when it flows from the busbar to the line, and negative when it flows from the line to the busbar.

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