A method for fault line selection based on fault current integral ratio for inter-pole short circuit

Abstract

The present invention discloses a method for selecting a short-circuit line between poles based on a fault current integral ratio, and relates to the field of relay protection for a flexible DC distribution system. First, the fault current characteristics after a short-circuit fault occurs in a DC distribution system line are analyzed, and then the difference in the integral properties of the current curves of the fault line and the non-fault line during the fault is used to distinguish the faults inside and outside the zone. Finally, a method for selecting a short-circuit line between poles in a flexible DC distribution system based on a fault current integral ratio is proposed, which is specifically: reading the line voltage and current data, and then determining the fault initiation criteria, and judging whether there is a fault according to the criteria, and if so, calculating the line current integral ratio, using the integral ratio on both sides of the line to characterize the difference between the fault line and the non-fault line, comparing it with the threshold, determining the fault line, and after selecting the fault inside / outside the zone, a fault signal is issued. The present invention avoids the difficulty of threshold setting caused by noise interference and changes in current amplitude, improves the reliability of the line selection scheme, and can quickly identify the faulty line.

Description

Technical Field

[0001] The invention relates to the field of relay protection of flexible direct current distribution systems, and in particular to a method for selecting a line for an inter-pole short-circuit fault based on a fault current integral ratio. Background Art

[0002] Flexible DC distribution system has the advantages of fewer commutation links, large power supply capacity, low line loss, and support for efficient access of multiple sources and loads. DC distribution network has gradually become an important direction for the development of DC technology and distribution system in the future. However, the low inertia and weak damping characteristics of flexible DC system make the fault current rise fast, and the converter is locked within a few milliseconds after the fault, resulting in large-scale grid shutdown. Therefore, reliable and rapid detection and isolation of faults are the research focus of DC protection technology.

[0003] At present, the protection schemes for DC distribution networks at home and abroad are mainly divided into communication protection and local protection, such as overcurrent, undervoltage, differential protection, etc. Among them, overcurrent protection and undervoltage protection are easily affected by transition resistance and have poor reliability; differential protection requires strict data synchronization, and the distributed capacitance of the DC line will affect its sensitivity; traveling wave protection has low anti-interference ability, high sampling rate requirements, and is generally used in high voltage fields. The above traditional protection schemes have problems such as difficult threshold setting and complex calculation. In addition to the above protection schemes, researchers in related fields at home and abroad have also proposed a series of new protection schemes based on mathematics, artificial intelligence, etc. Some literature proposes to control the modulation wave of the DC / DC converter and identify the fault area based on the control of the harmonics of special frequencies. However, this idea is not applicable in scenarios without DC converters. For multi-terminal flexible DC power grids, some scholars analyzed the fault information contained in the reverse traveling wave and proposed a waveform feature protection scheme based on deep learning. Although it effectively avoids the complex calculation and setting of the threshold value in the protection scheme design process and has good speed, the training process is very complicated and its application in actual engineering needs further consideration. Although the new protection scheme provides a new idea for DC protection, it still has problems such as complex fault feature extraction, large amount of training, and insufficient speed.

[0004] Therefore, in view of the shortcomings of the existing line selection method based on inter-pole short-circuit faults in DC distribution networks, it is of great significance to design a reliable and fast line selection method for inter-pole short-circuit faults in DC distribution systems. Summary of the invention

[0005] In order to solve the problems of inter-pole short-circuit fault line selection method in DC distribution network, overcurrent protection and undervoltage protection are easily affected by transition resistance and have poor reliability; differential protection requires strict data synchronization, and distributed capacitance of DC lines affects its sensitivity, the present invention provides an inter-pole short-circuit fault line selection method based on fault current integral ratio.

[0006] After a short-circuit fault occurs in a flexible DC distribution network, the current integral ratio coefficient at both ends of the fault line is greater than 1, while the current integral ratio coefficient at both ends of the non-fault line is equal to 1. The integral ratio can well characterize the difference between the fault line and the non-fault line. Therefore, the present invention is implemented by the following technical solutions:

[0007] A method for selecting a line for an inter-pole short-circuit fault based on the fault current integral ratio is proposed. First, the fault current characteristics after an inter-pole short-circuit fault occurs in the DC distribution system line are analyzed. Then, the difference in the integral properties of the current curves of the fault line and the non-fault line is used to distinguish the internal and external faults. Finally, a method for selecting a line for an inter-pole short-circuit fault in a flexible DC distribution system based on the fault current integral ratio is proposed. Specifically, the method includes the following steps:

[0008] S1: Analysis of the change trend of inter-pole short-circuit fault current: After a bipolar short-circuit fault occurs in the flexible DC distribution network, if the direction of the fault transient current is the same as that of the normal operating condition, the fault transient current increases in the same direction; if the direction of the fault transient current is opposite to that of the normal operating condition, the fault transient current decreases or increases in the opposite direction compared with the normal operating condition; for example: if an intra-regional fault occurs, the positive and negative pole fault currents of the sending-end converter station (m side) increase in the same direction, while the positive and negative pole currents of the receiving-end converter station (n side) both cross zero and reverse; if an extra-regional fault occurs, the receiving-end and sending-end converter stations both increase in the opposite direction; as shown in the following table:

[0009]

[0010] S2: Analysis of the integral characteristics of the bipolar short-circuit current curve:

[0011]

[0012] Where N represents the number of fault current sampling points corresponding to the integration time window after the protection is started, i(k) represents the instantaneous current value corresponding to the kth sampling point, and j represents the current number;

[0013] The integral S of the fault current curve with respect to the time coordinate axis leads to the following conclusions:

[0014] 1) When a bipolar short circuit occurs in the zone, within the same time i mP The integral value of the current curve with respect to the time axis is greater than i nP The integral value of the current curve with respect to the time axis; similarly, in the same time i mN For the integral value of the time axis greater than i nN For the integral value of the time axis;

[0015] 2) When a bipolar short circuit fault occurs outside the zone, within the same time i mP The integral value of the current curve with respect to the time axis is related to i nPThe integral values ​​of the current curves with respect to the time axis are the same in magnitude but opposite in sign; similarly, within the same time i mN The integral value of the time axis is related to i nN The integral values ​​for the time axis are the same in magnitude but opposite in sign;

[0016] S3: Determination of the integral proportionality coefficient of the bipolar short-circuit current curve:

[0017] make:

[0018]

[0019]

[0020] In the formula, k P and k N Respectively represent the integral proportional coefficients of the positive and negative poles of the line;

[0021] At this time, according to the analysis of formulas (2) and (3), the integral of the fault current curve with respect to the time coordinate axis satisfies:

[0022] (1) When a bipolar short circuit occurs in the zone, within the same time i mP The integral of the current curve with respect to the time axis and i nP The integral proportionality coefficient of the current curve with respect to the time axis is greater than 1, that is, k P >1; Similarly, in the same time i mN For the integral of the time axis and i nN The integral proportionality coefficient for the time axis is also greater than 1, that is, k N >1;

[0023] (2) When a bipolar short circuit fault occurs outside the zone, within the same time i mP The integral of the current curve with respect to the time axis and i nP The integral proportionality coefficient of the current curve with respect to the time axis is equal to 1, that is, k P =1; Similarly, in the same time i mN For the integral of the time axis and i nN The integral proportional coefficient for the time axis is also equal to 1, that is, k N =1;

[0024] S4: Fault start criterion design:

[0025] When a bipolar short circuit fault occurs, the positive and negative voltages of the fault line drop rapidly to 0. This characteristic can be used to provide a fast and sensitive fault start criterion for bipolar short circuit faults. In order to avoid the influence of disturbances, the protection is started when the start criterion is met at three consecutive sampling points, and the fault is identified according to the protection workflow. Therefore, the fault start criterion can be set as:

[0026] |V P |<0.8U N ∩|V N |<0.8U N (4)

[0027] Where V p The voltage to ground at the positive line protection installation location, V n The voltage to ground at the negative line protection installation location; when the above voltage conditions are met, it is determined that a bipolar short circuit fault occurs in the flexible DC distribution line, and then the protection is started to identify the fault interval according to the protection work flow;

[0028] S5: Fault line selection criteria design:

[0029] According to the difference analysis of S3 on the integral proportional coefficient of the current curve during bipolar short-circuit fault, the fault line selection criterion is set as:

[0030]

[0031] In the formula, k set is the set integral proportional coefficient threshold; the integral proportional coefficient value is calculated by formula (2) and formula (3), and compared with the integral proportional coefficient threshold set by formula (4). If it is greater than the threshold, then R t =1, it is determined that a fault has occurred in the area; if not satisfied, then R t =0, it is determined that an out-of-zone fault has occurred.

[0032] In the above process, the selection of the threshold value is very important for the fault line selection criterion. If the threshold value is set too large, it will be necessary to extend the protection integral sampling time, which will inevitably prolong the protection action time and is not conducive to rapid protection action; if the value is too small, the amount of fault information obtained within the integral sampling time will be reduced, which may cause the protection device to malfunction. In order to ensure that the system completes the fault removal before the converter is locked, avoid the expansion of the power outage scope, and obtain sufficient fault transient information to enable reliable protection action, the integral calculation time and protection action time are comprehensively considered, and the fault current within 1ms (i.e. 20 sampling points) after the protection is started is selected for integration, and k is set. set The threshold value can be determined by those skilled in the art according to the state of the circuit in actual situations. The threshold value can be determined by the principle that satisfies the above conditions.

[0033] Compared with the prior art, the present invention has the following beneficial effects: the present invention provides a method for selecting a line for an inter-pole short-circuit fault based on a fault current integral ratio, which does not require the use of boundary elements to construct boundary characteristics, has strong resistance to transition resistance, noise, and transmission delay, and is not affected by AC side faults. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 The figure is a schematic diagram of the topological structure of a flexible DC distribution network involved in a specific embodiment of the present invention.

[0035] Figure 2 It is a schematic diagram of the inter-pole short-circuit fault current in a specific embodiment of the present invention.

[0036] Figure 3 It is a flow chart of the fault line selection scheme involved in a specific embodiment of the present invention. DETAILED DESCRIPTION

[0037] The present invention is further described below in conjunction with specific embodiments.

[0038] Typical DC distribution system structure is as follows: Figure 1 As shown. The AC system is connected to the MMC converter through a connecting transformer, and the medium-voltage DC distribution system adopts a pseudo-bipolar wiring method. In view of the fact that the protection scheme used in this embodiment is designed based on the current at both ends of the positive and negative lines, protection devices are installed at both ends of the positive and negative poles of the DC lines L1-L6, and f1-f5 are possible grounding fault points. This embodiment adopts the master-slave control strategy commonly used in medium-voltage DC distribution systems, assuming that MMC1 adopts fixed DC voltage and fixed AC voltage control, PV adopts MPPT control, and MMC2 adopts fixed active power and fixed AC voltage control.

[0039] The fault current equation is as follows:

[0040]

[0041] Where: i mP (0) and U dc (0) represent the initial values ​​of the current and voltage of the DC system before the fault; the specific meanings of other factors are as follows:

[0042]

[0043] By differentiating the fault current equation (1), we can get its differential expression as follows:

[0044]

[0045] According to formula (3), the fault current curve in the capacitor discharge stage is a convex function. The differential value of the fault current is the largest at the beginning of the fault. Assuming that the fault occurs at t=0+, the maximum value of the current differential at this time is:

[0046]

[0047] Considering that the DC current and voltage in the flexible DC distribution network differ by more than two orders of magnitude, and the equivalent resistance of the fault circuit R eqVery small, so the maximum current differential value at t = 0+ is:

[0048]

[0049] The specific verification and implementation process of this embodiment is as follows: Figure 3 As shown:

[0050] S1: Analysis of the change trend of inter-pole short-circuit fault current: According to the analysis in the table below, after a bipolar short-circuit fault occurs in the flexible DC distribution network, if the direction of the fault transient current is the same as that of the normal operating condition, the fault transient current increases in the same direction; if the direction of the fault transient current is opposite to that of the normal operating condition, the fault transient current decreases or increases in the opposite direction compared with the normal operating condition;

[0051]

[0052] S2: Analysis of the integral characteristics of the bipolar short-circuit current curve:

[0053]

[0054] Where N represents the number of fault current sampling points corresponding to the integration time window after the protection is started, i(k) represents the instantaneous current value corresponding to the kth sampling point, and j represents the current number;

[0055] The integral S of the fault current curve with respect to the time coordinate axis yields the following conclusions: Figure 2 As shown:

[0056] 1) When a bipolar short circuit occurs in the zone, within the same time i mP The integral value of the current curve with respect to the time axis is greater than i nP The integral value of the current curve with respect to the time axis; similarly, in the same time i mN For the integral value of the time axis greater than i nN For the integral value of the time axis;

[0057] 2) When a bipolar short circuit fault occurs outside the zone, within the same time i mP The integral value of the current curve with respect to the time axis is related to i nP The integral values ​​of the current curves with respect to the time axis are the same in magnitude but opposite in sign; similarly, within the same time i mN The integral value of the time axis is related to i nN The integral values ​​for the time axis are the same in magnitude but opposite in sign;

[0058] S3: Determination of the integral proportionality coefficient of the bipolar short-circuit current curve:

[0059] make:

[0060]

[0061]

[0062] In the formula, k P and k N Respectively represent the integral proportional coefficients of the positive and negative poles of the line;

[0063] At this time, according to the analysis of formulas (2) and (3), the integral of the fault current curve with respect to the time coordinate axis satisfies:

[0064] (1) When a bipolar short circuit occurs in the zone, within the same time i mP The integral of the current curve with respect to the time axis and i nP The integral proportionality coefficient of the current curve with respect to the time axis is greater than 1, that is, k P >1; Similarly, in the same time i mN For the integral of the time axis and i nN The integral proportionality coefficient for the time axis is also greater than 1, that is, k N >1;

[0065] (2) When a bipolar short circuit fault occurs outside the zone, within the same time i mP The integral of the current curve with respect to the time axis and i nP The integral proportionality coefficient of the current curve with respect to the time axis is equal to 1, that is, k P =1; Similarly, in the same time i mN For the integral of the time axis and i nN The integral proportional coefficient for the time axis is also equal to 1, that is, k N =1;

[0066] S4: Fault start criterion design:

[0067] When a bipolar short circuit fault occurs, the positive and negative voltages of the fault line drop rapidly to 0. This characteristic can be used to provide a fast and sensitive fault start criterion for bipolar short circuit faults. In order to avoid the influence of disturbances, the protection is started when the start criterion is met at three consecutive sampling points, and the fault is identified according to the protection workflow. Therefore, the fault start criterion can be set as:

[0068] |V P |<0.8U N ∩|V N |<0.8U N (9)

[0069] Where V p The voltage to ground at the positive line protection installation location, V n The voltage to ground at the negative line protection installation location; when the above voltage conditions are met, it is determined that a bipolar short circuit fault occurs in the flexible DC distribution line, and then the protection is started to identify the fault interval according to the protection work flow;

[0070] S5: Fault line selection criteria design:

[0071] According to the difference analysis of S3 on the integral proportional coefficient of the current curve during bipolar short-circuit fault, the fault line selection criterion is set as:

[0072]

[0073] In the formula, k set is the set integral proportional coefficient threshold; the integral proportional coefficient value is calculated by formula (2) and formula (3), and compared with the integral proportional coefficient threshold set by formula (4). If it is greater than the threshold, then R t =1, it is determined that a fault has occurred in the area; if not satisfied, then R t =0, it is determined that an out-of-zone fault has occurred.

[0074] In this embodiment, if the threshold value is set too large, the protection integral sampling time will need to be extended, which will inevitably extend the protection action time and is not conducive to rapid protection action; if the threshold value is set too small, the amount of fault information obtained within the integral sampling time will be reduced, which may cause the protection device to malfunction. In order to ensure that the system completes fault removal before the converter is locked, avoid the expansion of the power outage range, and obtain sufficient fault transient information to ensure reliable protection action, the integral calculation time and protection action time are comprehensively considered, and the fault current within 1ms (i.e. 20 sampling points) after the protection is started is selected for integration, and k is set. set The threshold value can be determined by those skilled in the art according to the state of the circuit in actual situations, and the threshold value determination principle only needs to meet the above conditions.

[0075] The scope of protection claimed by the present invention is not limited to the above specific embodiments. For those skilled in the art, the present invention may have various deformations and changes. Any modifications, improvements and equivalent substitutions made within the concept and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for selecting line for inter-pole short-circuit fault based on fault current integral ratio. Features: The steps include: S1: Analysis of the variation trend of inter-pole short-circuit fault current: After a bipolar short-circuit fault occurs in the flexible DC distribution network, if the direction of the fault transient current is the same as that of the normal operating condition, the fault transient current increases in the same direction; If the direction of the fault transient current is opposite to that of the normal operating condition, the fault transient current decreases or increases in the opposite direction compared with the normal operating condition; S2: Analysis of the integral characteristics of the bipolar short-circuit current curve: Where N represents the number of fault current sampling points corresponding to the integration time window after the protection is started, i(k) represents the instantaneous current value corresponding to the kth sampling point, and j represents the current number; The integral S of the fault current curve with respect to the time coordinate axis leads to the following conclusions: 1) When a bipolar short circuit occurs in the zone, within the same time i mP The integral value of the current curve with respect to the time axis is greater than i nP The integral value of the current curve with respect to the time axis; Similarly, in the same time i mN For the integral value of the time axis greater than i nN For the integral value of the time axis; 2) When a bipolar short circuit fault occurs outside the zone, within the same time i mP The integral value of the current curve with respect to the time axis is related to i nP The integral values ​​of the current curves with respect to the time axis are the same in magnitude but opposite in sign; similarly, within the same time i mN The integral value of the time axis and i nN The integral values ​​for the time axis are the same in magnitude but opposite in sign; S3: Determination of the integral proportionality coefficient of the bipolar short-circuit current curve: make: In the formula, k P and k N Respectively represent the integral proportional coefficients of the positive and negative poles of the line; At this time, according to the analysis of formulas (2) and (3), the integral of the fault current curve with respect to the time coordinate axis satisfies: (1) When a bipolar short circuit occurs in the zone, within the same time i mP The integral of the current curve with respect to the time axis and i nP The integral proportionality coefficient of the current curve with respect to the time axis is greater than 1, that is, k P >1; Similarly, in the same time i mN For the integral of the time axis and i nN The integral proportionality coefficient for the time axis is also greater than 1, that is, k N >1; (2) When a bipolar short circuit fault occurs outside the zone, within the same time i mP The integral of the current curve with respect to the time axis and i nP The integral proportionality coefficient of the current curve with respect to the time axis is equal to 1, that is, k P =1; Similarly, in the same time i mN For the integral of the time axis and i nN The integral proportional coefficient for the time axis is also equal to 1, that is, k N =1; S4: Fault start criterion design: When a bipolar short circuit fault occurs, the positive and negative voltages of the fault line drop rapidly to 0. This characteristic is used to provide a fast and sensitive fault start criterion for bipolar short circuit faults. In order to avoid the influence of disturbances, the protection is started when the start criterion is met at three consecutive sampling points, and the fault is identified according to the protection workflow. Therefore, the fault start criterion is set as: |V P |<0.8U N ∩|V N |<0.8U N (4) Where V p is the voltage to ground at the positive line protection installation location, V n The voltage to ground at the negative line protection installation location; when the above voltage conditions are met, it is determined that a bipolar short circuit fault occurs in the flexible DC distribution line, and then the protection is started to identify the fault interval according to the protection work flow; S5: Fault line selection criteria design: According to the difference analysis of S3 on the integral proportional coefficient of the current curve during bipolar short-circuit fault, the fault line selection criterion is set as: In the formula, k set is the set integral proportional coefficient threshold; the integral proportional coefficient value is calculated by formula (2) and formula (3), and compared with the integral proportional coefficient threshold set by formula (5). If it is greater than the threshold, then R t =1, it is determined that a fault has occurred in the area; if not satisfied, then R t =0, it is determined that an out-of-zone fault has occurred.

2. A method for selecting a line for an inter-pole short-circuit fault based on a fault current integral ratio according to claim 1, Features: Taking into account the integral calculation time and protection action time, the fault current within 1ms after the protection is started is selected for integration, that is, 20 sampling points.

3. The method for selecting a line for an inter-pole short-circuit fault based on a fault current integral ratio according to claim 1, Features: Integral proportional coefficient threshold k set is 1.5.

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