A Single-Pole Grounding Protection Method for Single-Ended MMC DC Distribution Networks Based on Classification Similarity Analysis

Abstract

This invention discloses a single-pole grounding protection method for single-ended MMC DC distribution networks based on classification similarity analysis. The method mainly includes the following steps: First, monitoring the changes in the positive and negative pole-to-ground voltages of the DC distribution network to determine if a single-pole grounding fault has occurred. Second, when a single-pole grounding fault occurs, identifying the faulty pole based on the voltage change characteristics of the positive and negative poles during the fault. Third, if only one of the positive and negative pole similarity coefficients of all feeders is negative, then that feeder is the faulty feeder. Fourth, when two or more feeders have negative positive and negative pole similarity coefficients, calculating the sum of the mutual similarity coefficients of the faulty pole currents of all feeders, and if only one of these values ​​is negative, then that feeder is the faulty feeder. Fifth, calculating the sum of the mutual similarity coefficients of all feeders based on the sum of the corresponding current sampling values ​​of the positive and negative poles, and identifying the feeder with the minimum value as the faulty feeder. This invention can accurately identify feeders and features simple calculation, high accuracy, and ease of implementation. It can effectively improve operation and maintenance efficiency, reduce the scope of power outages, and improve the reliability of DC distribution network operation.

Description

Technical Field

[0001] This invention belongs to the field of power system relay protection, specifically involving a single-pole grounding protection method for single-ended MMC DC distribution networks based on classification similarity analysis. 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. Therefore, it is widely used in DC distribution networks. However, in DC distribution networks, especially AC ungrounded distribution networks, the fault characteristics are not obvious during a single-pole grounding fault, 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 many 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. The imperfections of DC protection equipment and other factors have posed huge constraints and challenges to DC distribution network protection. Therefore, researching a single-pole grounding protection method for DC distribution networks that meets accuracy requirements is an urgent technical problem to be solved in the construction of DC distribution networks. Summary of the Invention

[0004] To address the technical problems mentioned in the background, this invention provides a single-pole grounding protection method for single-ended MMC DC distribution networks based on classification similarity analysis. This method can accurately protect against single-pole grounding faults based on the fault characteristics of the similarity of capacitive currents in each section when a single-pole grounding fault occurs in the DC distribution network, and can trigger an alarm or trip as required.

[0005] To achieve the above-mentioned technical objectives, the technical solution of the present invention is as follows:

[0006] A method for single-pole grounding protection in a single-ended MMC DC distribution network based on classification similarity analysis includes the following steps:

[0007] S1. Number each feeder of the single-ended DC distribution network;

[0008] S2. The current direction is specified according to the flow direction of the busbar and the line;

[0009] S3. By monitoring the direction of current in each line during normal operation of the DC distribution network, determine whether the installation of the instrument transformer is correct;

[0010] S4. Determine the turns ratio n of each current transformer;

[0011] S5. Synchronously sample the positive and negative voltages to ground, the inter-electrode voltages, and the positive and negative currents of each feeder;

[0012] S6. Determine whether a single-pole grounding fault has occurred by measuring the magnitude of the voltage changes between the positive and negative poles and ground.

[0013] S7. When a single-pole grounding fault occurs, the faulty pole can be determined by the magnitude of the voltage between the positive and negative poles and the ground.

[0014] S8. Determine the data length N of the current criterion by analyzing the variation characteristics of the fault electrode voltage;

[0015] S9. Using the sampled values ​​of the positive and negative currents of each feeder, calculate the similarity coefficient p of the positive and negative currents of each feeder. i When there is only one p i When the value is negative, the feeder is a faulty feeder, the fault selection ends, and the relevant protection functions are executed; otherwise, continue to step (10);

[0016] S10. Using the sampled values ​​of the fault pole current of each feeder, calculate the sum P of the mutual similarity coefficients between the fault pole current of each feeder and the fault pole currents of all other feeders. ∑i When there is only one P ∑i If the value is negative, then the feeder is the faulty feeder, the fault selection ends, and the relevant protection functions are executed; otherwise, proceed to step (11).

[0017] S11. Using the sampled values ​​of the positive and negative currents of each feeder, calculate the sum of the mutual similarity coefficients P′ between the sum of the sampled values ​​of the positive and negative currents of each feeder and the sum of the sampled values ​​of the positive and negative currents of all other feeders. ∑i Find the minimum value P′ among all. ∑i·min Then P′ ∑i·min The corresponding feeder is the faulty feeder. Once the fault selection is completed, the relevant protection functions will be executed.

[0018] Preferably, the current flowing from the busbar to the line is defined as positive, and the current flowing from the line to the busbar is defined as negative.

[0019] Preferably, in step S9, the similarity coefficient p of the positive and negative currents of each feeder is calculated according to equation (3) using the sampled values ​​of the positive and negative currents of each feeder. i When there is only one p i When the value is negative, the feeder is a faulty feeder, the fault selection ends, and the relevant protection functions are executed; otherwise, continue to step (10);

[0020]

[0021] Where: i i+ (k), i i- (k) represents the kth sampled value of the positive and negative currents of the i-th feeder.

[0022] Preferably, in step S10, using the sampled values ​​of the fault pole current of each feeder, the sum of the mutual similarity coefficients P between the fault pole current of each feeder and the fault pole currents of all other feeders is calculated according to equation (4). ∑i When there is only one P ∑i If the value is negative, then the feeder is the faulty feeder, the fault selection ends, and the relevant protection functions are executed; otherwise, proceed to step (11).

[0023]

[0024] Where: p i·s Let i be the cross-correlation coefficient between the fault pole currents of the i-th and s-th feeders; i i·g (k) represents the k-th sampled value of the fault electrode current of the i-th feeder; n i n s These are the current transformer ratios for the i-th and s-th feeders, respectively.

[0025] Preferably, in step S11: using the sampled values ​​of the positive and negative currents of each feeder, the sum of the mutual similarity coefficients P′ of each feeder based on the sum of the sampled values ​​of the positive and negative currents and the sum of the sampled values ​​of the positive and negative currents of all other feeders is calculated according to equation (5). ∑i Find the minimum value P′ among all. ∑i·min Then P′ ∑i·min The corresponding feeder is the faulty feeder. Once the fault selection is completed, the relevant protection functions will be executed.

[0026]

[0027] Wherein: p′ i·s Let be the cross-correlation coefficient of the positive and negative currents of the i-th and s-th feed lines; i i·p (k) represents the k-th sampled value of the positive current of the i-th feeder, i i·N (k) is the kth sampled value of the negative electrode current of the i-th feeder.

[0028] The beneficial effects of this invention are as follows: This invention is a novel method for single-pole grounding fault protection that utilizes the similarity of the positive and negative currents of the feeders, the similarity of the fault pole currents among the feeders, and the similarity of the sum of the positive and negative currents among the feeders when a single-pole grounding fault occurs on the DC side of a single-ended MMC DC distribution network. This invention effectively solves the problem of single-pole grounding fault protection in single-ended MMC DC distribution networks. Furthermore, this invention features simple calculation, strong anti-interference capability, high accuracy, and ease of implementation. 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 outage area, and improve the safety and reliability of DC distribution network operation. Attached Figure Description

[0029] Figure 1 This is a flowchart of the present invention;

[0030] Figure 2 This is a schematic diagram of a single-ended MMC DC distribution network structure;

[0031] Figure 3 This is a schematic diagram of a positive ground fault occurring on feeder L2;

[0032] Figure 4 This is a waveform diagram of the positive and negative voltages when a positive ground fault occurs on feeder L2;

[0033] Figure 5 This is a waveform diagram of the positive and negative currents of line L1 when a positive ground fault occurs on feeder L2;

[0034] Figure 6 This is a waveform diagram of the positive and negative currents of feeder L2 when a positive ground fault occurs in feeder L2.

[0035] Figure 7 This is a waveform diagram of the positive and negative currents of line L3 when a positive ground fault occurs in feeder L2;

[0036] Figure 8 This is the waveform of the positive and negative currents of line L4 when a positive ground fault occurs on feeder L2. Detailed Implementation

[0037] To further illustrate the technical details and advantages of the present invention, the present invention will be described in further detail below with reference to the accompanying drawings.

[0038] like Figures 1 to 8 As shown, a single-pole grounding protection method for a single-ended MMC DC distribution network based on classification similarity analysis includes the following steps:

[0039] S1 numbers each feeder of the single-ended DC distribution network;

[0040] 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.

[0041] 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.

[0042] S4 determines the turns ratio n of each current transformer;

[0043] S5 synchronously samples the positive and negative voltages to ground, the inter-electrode voltages, and the positive and negative currents of each feeder;

[0044] S6 determines whether a single-pole grounding fault has occurred by measuring the magnitude of the change in voltage between the positive and negative poles and the ground.

[0045] When any value of the positive or negative pole voltage to ground is less than 80% of its rated voltage to ground, and the inter-pole voltage is greater than 80% of the rated inter-pole voltage, it is determined that a single-pole grounding fault has occurred; as shown in equation (1):

[0046]

[0047] Among them: U P U N U PN U PN·e These are the rated values ​​of positive-to-ground voltage, negative-to-ground voltage, inter-electrode voltage, and inter-electrode voltage, respectively.

[0048] When a single-pole ground fault occurs, the faulty pole is determined by the magnitude of the positive and negative pole voltages to ground; when U is satisfied... P ≥|U N If the fault occurs at the negative pole, it is determined that a single-pole grounding fault has occurred; otherwise, it is determined that a single-pole grounding fault has occurred at the positive pole.

[0049] S8 determines the data length N of the current criterion by analyzing the change characteristics of the fault electrode voltage;

[0050] The calculation starts from t=1 and proceeds according to equation (2). The calculation stops when equation (2) is satisfied. At this point, the value of t is the length N of the data window.

[0051]

[0052] Where: T is the number of sampling points in 5ms.

[0053] u g (t) represents the value of the voltage to ground at the t-th sampling point of the fault electrode;

[0054] S9 uses the sampled values ​​of the positive and negative currents of each feeder to calculate the similarity coefficient p of the positive and negative currents of each feeder according to equation (3). i When there is only one p i When the value is negative, the feeder is a faulty feeder, the fault selection ends, and the relevant protection functions are executed; otherwise, continue to step (10);

[0055]

[0056] Where: i i+ (k), i i- (k) represents the kth sampled value of the positive and negative currents of the i-th feeder, respectively;

[0057] S10 uses the sampled values ​​of the fault pole current of each feeder to calculate the sum of the mutual similarity coefficients P between the fault pole current of each feeder and the fault pole current of all other feeders according to equation (4). ∑i When there is only one P ∑iIf the value is negative, then the feeder is the faulty feeder, the fault selection ends, and the relevant protection functions are executed; otherwise, proceed to step (11).

[0058]

[0059] Where: p i·s Let i be the cross-correlation coefficient between the fault pole currents of the i-th and s-th feeders; i i·g (k) represents the k-th sampled value of the fault electrode current of the i-th feeder; n i n s The transformation ratios of the current transformers for the i-th and s-th feeders are respectively, and M is the total number of all feeders in the distribution network;

[0060] S11 uses the sampled values ​​of the positive and negative currents of each feeder to calculate the sum of the mutual similarity coefficients P′ of each feeder based on the sum of the sampled values ​​of the positive and negative currents and the sum of the sampled values ​​of the positive and negative currents of all other feeders, according to equation (5). ∑i Find the minimum value P′ among all. ∑i·min Then P′ ∑i·min The corresponding feeder is the faulty feeder. Once the fault selection is completed, the relevant protection functions will be executed.

[0061]

[0062] Wherein: p′ i·s Let be the cross-correlation coefficient of the positive and negative currents of the i-th and s-th feed lines; i i·p (k) represents the k-th sampled value of the positive current of the i-th feeder, i i·N (k) is the kth sampled value of the negative electrode current of the i-th feeder;

[0063] like Figure 2 The single-ended MMC DC distribution network structure diagram shown depicts an AC side with an ungrounded neutral point and a DC side rated voltage of ±10kV; the sampling frequency is 24kHz. Z The power grid has four feeders, L1 to L4, with lengths of 8km, 5km, 3km, and 6km respectively. Their current transformer ratios are 200 / 5, 200 / 5, 150 / 5, and 100 / 5 respectively. A single-pole ground fault is considered to have occurred when the voltage drop between the positive or negative pole and ground exceeds 20% of the rated voltage to ground.

[0064] like Figure 3 As shown, when a single-pole ground fault occurs at the positive terminal of feeder L2, the electrical waveforms of the positive and negative terminals of each feeder are as follows: Figures 4 to 8 As shown in the figure; its correlation coefficient is shown in Table 1. Since only p2 is negative, the faulty line can be accurately selected through step (9).

[0065] Table 1. Conclusions of Classification Similarity Coefficient Calculation

[0066] name <![CDATA[p1]]> <![CDATA[p2]]> <![CDATA[p3]]> <![CDATA[p4]]> <![CDATA[p ∑1 ]]> <![CDATA[p ∑2 ]]> <![CDATA[p ∑3 ]]> <![CDATA[p ∑4 ]]> <![CDATA[p′ ∑1 ]]> <![CDATA[p′ ∑2 ]]> <![CDATA[p′ ∑3 ]]> <![CDATA[p′ ∑4 ]]> Criteria 0.87 -0.95 0.87 0.92 0.96 -2.74 1.02 0.94 0.88 -2.98 0.97 1.06

[0067] After adding random interference signals with 50% amplitude and phase to the sampling signals of the positive and negative currents of each feeder, the calculated values ​​are shown in Table 2. Since p1 and p2 are both negative, the faulty line cannot be determined according to step (9). ∑1 p ∑2 All values ​​are negative, therefore the faulty circuit cannot be identified according to step S10, p′ ∑ Only p′ ∑2 Since the value is negative, step S10 determines that L2 is a faulty line. At the same time, changing the interference signal can still accurately determine the faulty line.

[0068] Table 2. Conclusions on the calculation of classification similarity coefficients after additional interference.

[0069] name <![CDATA[p1]]> <![CDATA[p2]]> <![CDATA[p3]]> <![CDATA[p4]]> <![CDATA[p ∑1 ]]> <![CDATA[p ∑2 ]]> <![CDATA[p ∑3 ]]> <![CDATA[p ∑4 ]]> <![CDATA[p′ ∑1 ]]> <![CDATA[p′ ∑2 ]]> <![CDATA[p′ ∑3 ]]> <![CDATA[p′ ∑4 ]]> Criteria -0.11 -0.57 0.64 0.58 -0.71 -1.89 0.93 0.78 0.69 -2.06 1.17 1.10

Claims

1. A method for single-pole grounding protection in a single-ended MMC DC distribution network based on classification similarity analysis, characterized in that: Includes the following steps: S1. Number each feeder of the single-ended DC distribution network; S2. The current direction is specified according to the flow direction of the busbar and the line; S3. By monitoring the direction of current in each line during normal operation of the DC distribution network, determine whether the installation of the instrument transformer is correct; S4. Determine the turns ratio n of each current transformer; S5. Synchronously sample the positive and negative voltages to ground, the inter-electrode voltages, and the positive and negative currents of each feeder; S6. Determine whether a single-pole grounding fault has occurred by measuring the magnitude of the voltage changes between the positive and negative poles and ground. S7. When a single-pole grounding fault occurs, the faulty pole can be determined by the magnitude of the voltage between the positive and negative poles and the ground. S8. Determine the data length N of the current criterion by analyzing the variation characteristics of the fault electrode voltage; S9. Using the sampled values ​​of the positive and negative currents of each feeder, calculate the similarity coefficient p of the positive and negative currents of each feeder. i When there is only one p i When the value is negative, the feeder is a faulty feeder, the fault selection ends, and the relevant protection functions are executed; otherwise, continue to step (10); S10. Using the sampled values ​​of the fault pole current of each feeder, calculate the sum P of the mutual similarity coefficients between the fault pole current of each feeder and the fault pole currents of all other feeders. ∑i When there is only one P ∑i If the value is negative, then the feeder is the faulty feeder, the fault selection ends, and the relevant protection functions are executed; otherwise, proceed to step (11). S11. Using the sampled values ​​of the positive and negative currents of each feeder, calculate the sum of the mutual similarity coefficients P′ between the sum of the sampled values ​​of the positive and negative currents of each feeder and the sum of the sampled values ​​of the positive and negative currents of all other feeders. ∑i Find the minimum value P′ among all. ∑i·min Then P′ ∑i·min The corresponding feeder is the faulty feeder. Once the fault selection is completed, the relevant protection functions will be executed.

2. The single-pole grounding protection method for single-ended MMC DC distribution networks based on classification similarity analysis according to claim 1, characterized in that: The current is defined as positive when it flows from the busbar to the line, and negative when it flows from the line to the busbar.

3. The single-pole grounding protection method for single-ended MMC DC distribution networks based on classification similarity analysis according to claim 1, characterized in that: In step S9, using the sampled values ​​of the positive and negative currents of each feeder, the similarity coefficient p of the positive and negative currents of each feeder is calculated according to equation (3). i When there is only one p i When the value is negative, the feeder is a faulty feeder, the fault selection ends, and the relevant protection functions are executed; otherwise, continue to step (10); Where: i i+ (k), i i- (k) represents the kth sampled value of the positive and negative currents of the i-th feeder.

4. The method for single-pole grounding protection of single-ended MMC DC distribution networks based on classification similarity analysis according to claim 1, characterized in that: In step S10, using the sampled values ​​of the fault pole current of each feeder, the sum of the mutual similarity coefficients P between the fault pole current of each feeder and the fault pole currents of all other feeders is calculated according to equation (4). ∑i When there is only one P ∑i If the value is negative, then the feeder is the faulty feeder, the fault selection ends, and the relevant protection functions are executed; otherwise, proceed to step (11). Where: p i·s Let i be the cross-correlation coefficient between the fault pole currents of the i-th and s-th feeders; i i·g (k) represents the k-th sampled value of the fault electrode current of the i-th feeder; n i n s These are the current transformer ratios for the i-th and s-th feeders, respectively.

5. The method for single-pole grounding protection of single-ended MMC DC distribution networks based on classification similarity analysis according to claim 1, characterized in that: In step S11: Using the sampled values ​​of the positive and negative currents of each feeder, calculate the sum of the mutual similarity coefficients P′ of each feeder based on the sum of the sampled values ​​of the positive and negative currents and the sum of the sampled values ​​of the positive and negative currents of all other feeders according to equation (5). ∑i Find the minimum value P′ among all. ∑i·min Then P′ ∑i·min The corresponding feeder is the faulty feeder. Once the fault selection is completed, the relevant protection functions will be executed. Wherein: p′ i·s Let be the cross-correlation coefficient of the positive and negative currents of the i-th and s-th feed lines; i i·p (k) represents the k-th sampled value of the positive current of the i-th feeder, i i·N (k) is the kth sampled value of the negative electrode current of the i-th feeder.

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