Flexible straight line longitudinal protection method and system based on inductance parameters

Abstract

The present invention relates to a flexible straight line longitudinal protection method and system based on inductance parameters, which belongs to the technical field of relay protection of power system, and solves the problem that the protection action performance of AC line in the existing flexible DC system is reduced, causing refusal to operate or malfunction. The method includes collecting data after the fault occurs, including: the voltage reference value on the AC side of the converter station, the DC side voltage, the AC current and AC voltage on both sides of the AC line M and N; according to the data, the inductance value on both sides of the AC line M and N is calculated; according to the inductance value on both sides of the AC line, the inductance error coefficient on both sides of the AC line M and N is calculated; it is judged whether the inductance error coefficient on both sides of the AC line M and N meets the fault identification criterion, if it does, it is judged that the AC line has an in-zone fault, and the AC line protection is started, otherwise, it is judged that the AC line has an out-of-zone fault, and the AC line protection does not operate. Accurate and rapid fault identification is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of relay protection of electric power systems, and in particular relates to a flexible straight line pilot protection method and system based on inductance parameters. Background Art

[0002] Compared with traditional AC power grids, flexible direct current (flexible direct current) systems have significant differences in power characteristics, operation modes, and control and protection strategies. Flexible direct current converter stations are composed of a large number of power electronic devices, and the rapid transient characteristics after a fault will have a significant impact on traditional AC line protection. Traditional AC line protection has problems with reduced performance or even incorrect operation. Therefore, it is crucial to study AC line protection methods suitable for flexible direct current system access to ensure the safe operation of the system.

[0003] At present, the AC line protection of flexible DC system is mainly divided into distance protection, zero-sequence current protection and current differential protection. Distance protection calculates the measured impedance by the ratio of the voltage and current at the beginning of the protected line, and then determines the location of the fault point. Distance protection has high sensitivity and can selectively cut off the fault line in a short time. However, since the flexible DC system is characterized by a power electronic controlled source, it cannot provide a stable power frequency fault current, resulting in a reduction in the protection range of traditional distance protection, and refusal to operate may occur. Zero-sequence current protection is an auxiliary criterion in the case of high-resistance grounding faults, with high sensitivity and fast action speed. The converter transformer of the flexible DC system generally adopts star-delta connection, and the zero-sequence current will not flow through the converter station. Therefore, the distribution of zero-sequence current is mainly determined by the zero-sequence impedance of the line and the zero-sequence impedance of the neutral point grounding transformer, and is less affected by the access of the flexible DC system. However, zero-sequence current protection can only identify asymmetric faults, not symmetric faults, and cannot be used as the main protection of the line. Current differential protection uses the amplitude and phase of the current on both sides of the line to identify faults inside and outside the zone through the vector comparison results of the current. It can respond to all fault types, has good adaptability to network topology, and has good selectivity and speed. However, when a high-resistance fault occurs in the area, the amplitude and phase angle of the fault current provided by the converter station side are affected by the control strategy of the flexible DC system itself, which may cause the differential protection to refuse to operate. Summary of the invention

[0004] In view of the above analysis, the embodiments of the present invention aim to provide a flexible direct current line longitudinal protection method and system based on inductance parameter identification, so as to solve the problem that the AC line protection of the existing flexible direct current system is difficult to quickly and reliably identify the fault section.

[0005] On the one hand, an embodiment of the present invention discloses a flexible straight line pilot protection method based on inductance parameters, comprising:

[0006] Collect data after the fault occurs, including: voltage reference value on the AC side of the converter station, voltage on the DC side, AC current and AC voltage on both sides of the AC line M and N;

[0007] Based on the data, calculate the inductance values ​​on both sides of the AC line M and N;

[0008] According to the inductance values ​​on both sides of the AC line, the inductance error coefficient on both sides of the AC line M and N is calculated;

[0009] Determine whether the inductance error coefficient on both sides of the AC line M and N meets the fault identification criteria. If so, it is determined that the AC line has an in-zone fault and the AC line protection is started. Otherwise, it is determined that the AC line has an out-of-zone fault and the AC line protection does not operate.

[0010] On the basis of the above scheme, the present invention also makes the following improvements:

[0011] Furthermore, the equivalent model of the AC side of the converter station in the flexible straight line is:

[0012]

[0013] Where N is the total number of submodules put into use in the upper and lower bridge arms of phase A; U c1 is the submodule capacitor voltage; i va are the A-phase voltage reference value and A-phase current of the AC side of the converter station respectively; R arm , L arm is the equivalent resistance and inductance of any phase bridge arm; L T is the inductance on the AC side of the converter station; u dc is the DC side voltage of the converter station; u ma is the AC A phase output voltage of the converter station; i ma It is the AC A phase output current of the converter station.

[0014] Furthermore, based on the equivalent model, the inductance values ​​on both sides of the AC line M and N are calculated by the following formula:

[0015]

[0016]

[0017] Among them, R fm , L fm are the resistance and inductance of the AC line M side, R fn , L fn are the resistance and inductance values ​​of the N side of the AC line respectively; N is the total number of submodules put into use in the upper and lower bridge arms of the AC line; is the voltage reference value of the AC side of the converter station; u dcis the DC side voltage of the converter station; i m 、i n are the AC currents on both sides of the AC line M and N, respectively, and u m 、u n are the AC voltages on both sides of the AC line M and N, respectively, and e n is the power port voltage outside the N side of the AC line.

[0018] Furthermore, the inductance error coefficient on both sides of the AC line M and N is calculated by the following formula:

[0019]

[0020] Among them, E m 、E n are the inductance error coefficients on both sides of the AC line M and N, H is the number of sampling points in a cycle after the fault occurs, and L fm (k), L fn (k) are the calculated inductance values ​​of the kth sampling point, L m , L n are the equivalent inductances on both sides of the AC line M and N respectively.

[0021] Furthermore, the fault identification criteria include:

[0022]

[0023] Among them, E set is the action threshold;

[0024] If E m 、E n If formula (5) is satisfied, it is determined to be an in-zone fault; otherwise, it is determined to be an out-zone fault.

[0025] Furthermore, according to the fault identification criteria, the fault direction is identified, including:

[0026] When E m 、E n If formula (5) is satisfied, the fault direction is an intra-area fault;

[0027] When E m Formula (5) is not satisfied, and the fault direction is an out-of-zone fault on the M side;

[0028] When E n Formula (5) is not satisfied, and the fault direction is an N-side out-of-zone fault.

[0029] Furthermore, to start the AC line protection, execute:

[0030] Send a trip signal to the protection device of the faulty line.

[0031] Furthermore, the converter station is an MMC converter station, wherein the negative electrode structure is completely consistent with the positive electrode.

[0032] On the other hand, an embodiment of the present invention discloses a flexible straight line pilot protection system based on inductance parameters, including:

[0033] The data acquisition module is used to collect data after a fault occurs, including: voltage reference value on the AC side of the converter station, voltage on the DC side, AC current and AC voltage on both sides of AC lines M and N;

[0034] The inductance calculation module is used to calculate the inductance values ​​on both sides of the AC line M and N according to the data;

[0035] An inductance error coefficient determination module is used to calculate the inductance error coefficients on both sides of the AC line M and N according to the inductance values ​​on both sides of the AC line;

[0036] The fault identification module is used to determine whether the inductance error coefficient on both sides of the AC line M and N meets the fault identification criteria. If so, it is determined that the AC line has an in-zone fault and the AC line protection is started. Otherwise, it is determined that the AC line has an out-of-zone fault and the AC line protection does not operate.

[0037] Based on the further improvement of the above system, the inductance error coefficient on both sides of the AC line M and N is calculated by the following formula:

[0038]

[0039] Among them, E m 、E n are the inductance error coefficients on both sides of the AC line M and N, H is the number of sampling points in a cycle after the fault occurs, and L fm (k), L fn (k) are the calculated inductance values ​​of the kth sampling point, L m , L n are the equivalent inductances on both sides of the AC line M and N respectively.

[0040] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0041] The flexible straight line longitudinal protection method and system based on inductance parameter identification proposed in the present invention can calculate the inductance value and inductance error coefficient on both sides of the AC line based on the data collected after the fault, and accurately identify the internal and external faults according to the calculation results. It has a fast action speed and is not affected by the fault location and transition resistance. It well solves the problem of poor action performance of AC line protection in existing flexible DC systems, which may cause false operation and refusal to operate.

[0042] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0044] Figure 1 It is a flow chart of the flexible straight line longitudinal protection method based on inductance parameters;

[0045] Figure 2 It is a schematic diagram of the structure of the flexible DC system;

[0046] Figure 3 It is the basic structure inside the MMC converter station;

[0047] Figure 4 It is the equivalent model of the AC side of MMC;

[0048] Figure 5 It is the fault network diagram when the fault occurs in the area;

[0049] Figure 6 This is the fault network diagram when the fault occurs outside the M side area;

[0050] Figure 7 This is the fault network diagram when the fault occurs outside the N side area;

[0051] Figure 8 Schematic diagram of the structure of the flexible straight line longitudinal protection system based on inductance parameters;

[0052] Fig. 9 is the inductance value when a phase A ground fault occurs at f1 through different transition resistances; where, Fig. 9 (a) and (b) represent L fm , L fn The change curve of

[0053] Fig.10 is the inductance error coefficient when a phase A ground fault occurs at f1 through different transition resistances; where, Fig.10 (a) and (b) represent E m 、E n The change curve of

[0054] Fig.11 is the inductance value when AB phase fault occurs at different positions in the AC line area; where, Fig.11(a) and (b) represent L fm , L fn The change curve of

[0055] Fig.12 is the inductance error coefficient when AB phase fault occurs at different positions in the AC line area; where, Fig.12 (a) and (b) represent E m 、E n The change curve of

[0056] Fig.13 is the inductance error coefficient when a phase A ground fault occurs at f2 through different transition resistances; where, Fig.13 (a) and (b) represent E m 、E n The change curve of

[0057] Fig.14 is the inductance value when the transition resistance at f2 is 0 and the A phase is grounded; where, Fig.14 (a) and (b) represent L fm , L fn The change curve of

[0058] Fig.15 is the inductance error coefficient when a two-phase AB grounding fault occurs at f3 through different transition resistances; where, Fig.15 (a) and (b) represent E m 、E n The change curve of

[0059] Fig.16 is the inductance value when the transition resistance at f3 is 0 and the AB two-phase grounding fault occurs; where, Fig.16 (a) and (b) represent L fm , L fn The change curve. DETAILED DESCRIPTION

[0060] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0061] Example 1

[0062] A specific embodiment 1 of the present invention discloses a flexible straight line longitudinal protection method based on inductance parameters, the flow chart is as follows: Figure 1 As shown, the following steps are included:

[0063] Step S1: Collecting data after the fault occurs, including: voltage reference value on the AC side of the converter station, voltage on the DC side, AC current and AC voltage on both sides of AC lines M and N;

[0064] Step S2: Calculate the inductance values ​​on both sides of the AC line M and N according to the data;

[0065] Step S3: Calculate the inductance error coefficient on both sides of the AC line M and N according to the inductance values ​​on both sides of the AC line;

[0066] Step S4: Determine whether the inductance error coefficients on both sides of the AC line M and N meet the fault identification criteria. If so, determine that the AC line has an in-zone fault and start the AC line protection. Otherwise, determine that the AC line has an out-of-zone fault and the AC line protection does not operate.

[0067] Compared with the prior art, the flexible straight line longitudinal protection method based on inductance parameters proposed in this embodiment calculates the inductance value and inductance error coefficient at both ends of the line based on the data collected after the fault, and accurately identifies internal and external faults according to the calculation results. It has a fast action speed and is not affected by the fault location and transition resistance. It well solves the problem of reduced action performance or even incorrect action of AC line protection in existing flexible DC systems.

[0068] In order to facilitate those skilled in the art to better understand the formation process of the scheme in this embodiment, Figure 2 Taking the flexible DC system shown in FIG. 1 as an example, the working principle of the flexible straight line longitudinal protection method based on inductance parameter identification provided in this embodiment is described as follows:

[0069] Figure 3 This is the basic structure inside the bipolar MMC converter station, where the negative pole structure is exactly the same as the positive pole. Assume that the positive pole DC voltage is u dc / 2, the negative DC voltage is -u dc / 2, the voltage difference between the positive and negative electrodes is u dc ,according to Figure 3 The relationship between the upper and lower bridge arms of phase A in the positive converter station can be obtained as follows:

[0070]

[0071]

[0072] i va =i pa -i na (3)

[0073] Among them, u va 、i va are the voltage and current of phase A on the AC side of the converter station respectively; u pa 、una are the equivalent capacitor voltages of the upper and lower bridge arms of phase A respectively; i pa 、i na are the upper and lower bridge arm currents of phase A respectively; R arm , L arm are the equivalent resistance and inductance of any phase bridge arm respectively; u dc is the DC side voltage of the converter station.

[0074] Combining equations (1)-(3) we can get:

[0075]

[0076] Assuming that the submodule capacitor voltages of the same bridge arm are the same, the bridge arm voltage can be represented by the sum of the switching function and the total bridge arm capacitor voltage, as follows:

[0077]

[0078]

[0079] Among them, S pa , S na are the switch functions of the upper and lower bridge arms, respectively, indicating the input ratio of the submodules in the bridge arms; is the sum of the total sub-module capacitor voltages of the upper bridge arm, is the sum of the total submodule capacitor voltages of the lower bridge arm; and the sum of the upper and lower bridge arm capacitor voltages is expressed as:

[0080]

[0081] Depend on Figure 3 The relationship between the MMC DC side voltage and the capacitor voltage of each submodule is obtained as follows:

[0082] u dc =NU c1 (8)

[0083] Where N is the total number of submodules put into use in the upper and lower bridge arms of phase A; U c1 is the submodule capacitor voltage.

[0084] At any time, the number of submodules put into use in the upper and lower bridge arms of phase A are:

[0085]

[0086]

[0087] in, It is the voltage modulation wave of phase A on the AC side of the converter station, that is, the voltage reference value.

[0088] According to the definition of the bridge arm switching function of the converter station, and combined with equations (9) and (10), the bridge arm switching function can be obtained as follows:

[0089]

[0090] Combining equations (5), (6), (7), (8) and (11), we can obtain:

[0091]

[0092] Combining equation (4) and equation (12), we can get:

[0093]

[0094] according to Figure 3 The AC phase A output voltage of the converter station is:

[0095]

[0096] Among them, L T is the inductance of the converter AC side, i ma It is the AC A phase output current of the converter station.

[0097] Combining equation (13) and equation (14), we can get:

[0098]

[0099] According to the above analysis, the equivalent model of the AC side of the flexible DC system converter station can be obtained, as follows: Figure 4 As shown. m is the equivalent inductance of the converter station, and its value is (L arm +2L T ) / 2; R m is the equivalent resistance of the converter station, and its value is R arm / 2;U sm It is the equivalent voltage source of the converter station, which is affected by the voltage modulation wave and DC current of the converter station control system.

[0100] When a fault occurs in the AC line area, the fault network diagram is as follows Figure 5 In the figure, x is the distance between the fault point and the busbar on the M side, and d is the total length of the MN line.

[0101] Depend on Figure 5 It can be seen that when an intra-area fault occurs, the equivalent model of the converter station on the M side remains unchanged. Based on formula (15), the AC voltage on the M side can be obtained as follows:

[0102]

[0103] Among them, u m is the AC voltage on the M side of the AC line, is the voltage reference value of the AC side of the converter station, i m is the AC current on the M side of the AC line.

[0104] In order to eliminate the influence of the converter station control system on the AC voltage when there is a fault in the area, the equivalent voltage source in the AC voltage on the M side can be eliminated. Therefore, the inductance value expression on the M side is defined as:

[0105]

[0106] Among them, R fm , L fm are the resistance and inductance values ​​of the M side respectively.

[0107] Combining equation (16) and equation (17), we can get:

[0108]

[0109] It can be seen from formula (18) that when a fault occurs in the AC zone, the inductance value of the M side is a known constant value, which is the sum of half of the bridge arm inductance and the transformer inductance.

[0110] The AC voltage on the N side is:

[0111]

[0112] Among them, u n is the AC voltage on the N side of the AC line, e n is the power port voltage outside the N side of the AC line, i n is the AC current on the N side of the AC line, R n , L n are the equivalent resistance and equivalent inductance on the N side of the AC line respectively.

[0113] The N-side calculated inductance expression is defined as:

[0114]

[0115] Among them, R fn , L fn are the resistance and inductance of the N side of the AC line respectively.

[0116] Combining equation (19) and equation (20), we can get:

[0117]

[0118] It can be seen from formula (21) that when a fault occurs in the AC zone, the inductance value on the N side is a known constant, which is the equivalent inductance of the AC system.

[0119] From the above analysis, it can be seen that when a fault occurs in the AC line area, the inductance values ​​on both sides of the line are constant.

[0120] When an AC M side fault occurs outside the zone, the fault network diagram is as follows: Figure 6 As shown. Figure 6 It can be seen that when an AC fault occurs outside the M side, the AC voltage on the M side is:

[0121]

[0122] In the formula, R l , L l They are the resistance and inductance of the MN segment line respectively.

[0123] Combining equation (17) and equation (22), we can get:

[0124]

[0125] Depend on Figure 6 We can get:

[0126]

[0127]

[0128] Among them, u f is the fault point voltage, i f is the fault point current.

[0129] Combining equations (23), (24) and (25), we can obtain:

[0130]

[0131] It can be seen from formula (26) that when a fault occurs outside the AC zone on the M side, the inductance value on the M side is affected by the fault current and its value deviates from the constant value.

[0132] The AC voltage on the N side is:

[0133]

[0134] Combining equation (20) and equation (27), we can get:

[0135]

[0136] It can be seen from formula (28) that when a fault occurs outside the AC zone on the M side, the inductance value on the N side is a known constant, which is the equivalent inductance of the AC system.

[0137] From the above analysis, it can be seen that when a fault occurs outside the AC line area on the M side, the inductance value on the M side deviates from the constant value due to the influence of the fault current, while the inductance value on the N side remains a constant value.

[0138] When an AC N-side fault occurs outside the zone, the fault network diagram is as follows: Figure 7 As shown. Figure 7 It can be seen that when an AC N-side fault occurs outside the zone, the AC voltage on the M side is:

[0139]

[0140] It should be noted that, when a fault occurs in the MN area or outside the N side area, the converter station model on the M side remains unchanged, so equation (29) is the same as equation (16).

[0141] Combining equation (17) and equation (29), we can get:

[0142]

[0143] It can be seen from formula (30) that when a fault occurs in the AC zone, the inductance value of the M side is a known constant value, which is the sum of half of the bridge arm inductance and the transformer inductance.

[0144] The AC voltage on the N side is:

[0145]

[0146] Combining equation (20) and equation (31), we can get:

[0147]

[0148] It can be seen from formula (32) that when a fault occurs outside the AC zone on the N side, the inductance value on the N side is affected by the fault current and its value deviates from the stable value.

[0149] From the above analysis, it can be seen that when a fault occurs outside the AC line area on the N side, the inductance value on the M side remains constant, while the inductance value on the N side deviates from the constant value due to the influence of the fault current.

[0150] According to the above analysis, when a fault occurs in the AC line area, the inductance value of the protection at both ends of the line is a fixed value. When a fault occurs outside the reverse zone, the inductance value will deviate from the fixed value. Therefore, this embodiment uses the inductance parameters at both ends of the line to identify the fault, and the calculation formula of the inductance error coefficient is:

[0151]

[0152] Among them, E m 、E n is the inductance error coefficient at both ends of the line, H is the number of sampling points in a cycle after the fault occurs, and L fm (k), L fn (k) are the inductance values ​​of the kth sampling point calculated by the protection on both sides of the line.

[0153] It should be noted that the value of H is determined by the sampling frequency, and the inductance error coefficient adopts the averaging method, which can reduce the impact of the inductance value at a certain moment on the overall, improve the reliability of the protection criterion, and reduce the probability of false operation and refusal.

[0154] Preferably, one cycle is set to 5 ms.

[0155] Fault identification criteria include:

[0156]

[0157] Among them, E set is the action threshold, considering the influence of calculation error, noise interference and other factors, E set Set to 0.3.

[0158] If the fault criterion is met, it is determined to be an in-zone fault; otherwise, it is determined to be an out-of-zone fault.

[0159] According to the fault identification criteria, the protection on both sides of the AC line identifies the fault direction, including:

[0160] When E m 、E n If formula (34) is satisfied, the fault direction is an intra-area fault;

[0161] When E m Formula (34) is not satisfied, and the fault direction is an out-of-zone fault on the M side;

[0162] When E n Formula (34) is not satisfied, and the fault direction is an N-side out-of-zone fault.

[0163] Therefore, after a fault occurs, identification and judgment are performed based on the inductance values ​​on both sides of the AC line MN. When the inductance values ​​on both sides of MN are less than the action threshold, an intra-zone fault occurs, and the protection on both sides of the AC line transmits the fault direction as "internal fault", and sends a tripping signal to the protection device of the faulty line; when the inductance value on the M side is greater than or equal to the action threshold, an M-side out-of-zone fault occurs, and the protection on both sides of the AC line only transmits the fault direction as "M-side out-of-zone fault" and does not operate; when the inductance value on the N side is greater than or equal to the action threshold, an N-side out-of-zone fault occurs, and the protection on both sides of the AC line only transmits the fault direction as "N-side out-of-zone fault" and does not operate.

[0164] Example 2

[0165] Specific embodiment 2 of the present invention discloses a flexible straight line longitudinal protection based on inductance parameters, the structural diagram of which is shown in FIG. Figure 8 As shown, including:

[0166] The data acquisition module is used to collect data after a fault occurs, including: voltage reference value on the AC side of the converter station, voltage on the DC side, AC current and AC voltage on both sides of AC lines M and N;

[0167] The inductance calculation module is used to calculate the inductance values ​​on both sides of the AC line M and N according to the data;

[0168] An inductance error coefficient determination module is used to calculate the inductance error coefficients on both sides of the AC line M and N according to the inductance values ​​on both sides of the AC line;

[0169] The fault identification module is used to determine whether the inductance error coefficient on both sides of the AC line M and N meets the fault identification criteria. If so, it is determined that the AC line has an in-zone fault and the AC line protection is started. Otherwise, it is determined that the AC line has an out-of-zone fault and the AC line protection does not operate.

[0170] Furthermore, the inductance error coefficient on both sides of the AC line M and N is calculated by the following formula:

[0171]

[0172] Among them, E m 、E n are the inductance error coefficients on both sides of the AC line M and N, H is the number of sampling points in a cycle after the fault occurs, and L fm (k), L fn (k) are the calculated inductance values ​​of the kth sampling point, E set is the action threshold, L m , L n are the equivalent inductances on both sides of the AC line M and N respectively.

[0173] The present system embodiment and the above method embodiment are implemented based on the same principle, and their related parts can refer to each other and can achieve the same technical effect.

[0174] The specific implementation process of this system embodiment can be referred to the above method embodiment, and this embodiment will not be repeated here. Since the principle of this system embodiment is the same as that of the above method embodiment, this system also has the corresponding technical effects of the above method embodiment.

[0175] Example 3

[0176] System structure Figure 2 As shown. Among them, the DC line adopts the frequency-dependent line model. The main parameters of the multi-terminal flexible DC system are shown in Table 1. The sampling frequency is 10kHz, and the fault occurrence time is zero time.

[0177] Table 1 Main parameters of flexible DC system

[0178]

[0179] The scenario 1 set in this example is: Figure 2 Phase A ground fault occurs at point f1. The inductance values ​​under different transition resistance fault conditions are as follows: Fig. 9 As shown, the inductance error coefficient is Fig.10 shown.

[0180] exist Fig. 9 (a) Fig. 9 In (b), when a phase A ground fault occurs inside the AC line, the inductance values ​​on both the M and N sides of the line can be reliably identified under different transition resistances, and their values ​​are consistent with the theoretical calculated values. As the transition resistance increases, the inductance value fluctuates less and is close to the theoretical value. Fig.10 (a) Fig.10 In (b), as the transition resistance increases, the inductance error coefficient at both ends of the line gradually increases. In the case of a fault with a transition resistance of 300Ω, the inductance error coefficient E on the M side is m The maximum value is 0.0335 at 4.2ms. N-side inductance error coefficient E n The maximum value is 0.0647 at 7.8ms. m 、E n Both of them are smaller than the threshold value, so it can be determined that an in-area fault occurs on line MN.

[0181] According to the above analysis, this method can still accurately identify high-resistance faults in the AC line area, and has high reliability and strong ability to tolerate transition resistance.

[0182] Scenario 2 in this example is: suppose an AB phase fault occurs at different locations in the AC line area, and the fault resistance is 300Ω. The inductance value in this fault situation is as follows: Fig.11 As shown, the inductance error coefficient is Fig.12 shown.

[0183] exist Fig.11 (a) Fig.11 In (b), when an AB phase fault occurs at different positions of the AC line, the inductance values ​​on both the M and N sides of the line can be reliably identified, and their values ​​are consistent with the theoretical calculated values. Fig.11 (a) It can be seen that when a fault occurs at the head end of the M side, the inductance value fluctuates in the initial stage of the fault, but the deviation is not large, and gradually approaches the theoretical value 5ms after the fault. Fig.11 (b) It can be seen that when faults occur at different locations, the inductance value on the N side fluctuates very little, which is consistent with the theoretical value. Fig.12 (a) Fig.12 In (b), when the M side has a head-end fault, the M side inductance error coefficient E mThe maximum value is 0.0525 at 1.1ms. In the case of a fault at the N-side head end, the N-side inductance error coefficient E n The maximum value is 0.0105 at 0.1ms. m 、E n Both are less than the threshold value, and it can be determined that line MN has an in-area fault.

[0184] From the above analysis, it can be seen that this protection scheme can achieve fast and accurate protection action at different fault locations in the area. When a high-resistance fault occurs at the end of the line, it still has high sensitivity.

[0185] The scenario 3 set in this example is: Figure 2 Phase A ground fault occurs at f2. The inductance error coefficient under different transition resistance fault conditions is as follows: Fig.13 shown.

[0186] When a phase A ground fault occurs at f2, the inductance value calculated by the M-side protection will deviate from the theoretical value, and the calculated inductance error coefficient is greater than the threshold value. Fig.13 (a) It can be seen that as the transition resistance increases, the inductance error coefficient E m Gradually increase. m When t=9.2ms and the transition resistance is 0Ω, there is a minimum value of 0.971, which is still greater than the threshold value. The inductance value on the N side of the line is consistent with the theoretical value, and the inductance error coefficient E n is less than the threshold value. Therefore, it can be determined that a fault outside the reverse zone occurs on the M side. Among them, the inductance values ​​on both sides of the line when the transition resistance is 0Ω are as follows: Fig.14 As shown. Fig.14 It can be seen that under this fault condition, the inductance value on the M side of the line will deviate from the theoretical value, with a large difference, while the inductance value on the N side of the line is consistent with the theoretical value, which is consistent with the theoretical analysis. Therefore, under different transition resistance fault conditions, the fault can still be reliably identified as an out-of-zone fault, and the protection will not be malfunctioned.

[0187] The scenario 4 set in this example is: Figure 2 In the figure, a two-phase AB grounding fault occurs at f3. The inductance error coefficient under different transition resistance fault conditions is as follows: Fig.15 shown.

[0188] When a two-phase AB ground fault occurs at f3, the inductance value calculated by the N-side protection will deviate from the theoretical value, and the calculated inductance error coefficient is greater than the threshold value. The inductance value on the M side of the line is consistent with the theoretical value, and the inductance error coefficient E m is less than the threshold value. Fig.15 (b) It can be seen that as the transition resistance increases, the N-side inductance error coefficient E n Gradually decrease. nWhen t=1.5ms and the transition resistance is 0Ω, it has a minimum value of 0.692. m The maximum value is 0.0431 when t=1.2ms and the transition resistance is 100Ω. The inductance values ​​on both sides of the line when the transition resistance is 0Ω are as follows: Fig.16 As shown. Fig.16 It can be seen that under this fault condition, the inductance value on the N side of the line will deviate from the theoretical value, with a large difference. The inductance value on the M side of the line is consistent with the theoretical value, which is consistent with the theoretical analysis.

[0189] According to the above analysis, under different transition resistance conditions, this method can reliably determine that the fault is outside the AC zone on the N side, and the protection does not malfunction. Moreover, only the judgment result of the fault direction needs to be transmitted between the two sides of the line, and no electrical quantity information needs to be transmitted, which is less affected by data synchronization errors.

[0190] The above scenario results show that the present invention has the following characteristics:

[0191] (1) This method can accurately and reliably identify various types of faults and is not affected by the fault location and transition resistance. It still has high sensitivity in the case of high-resistance faults.

[0192] (2) This protection method uses time domain information to solve the inductance, and can accurately identify the fault within 5ms after the fault, with a fast action speed.

[0193] (3) This method does not require strict synchronization of data at both ends of the line. It only needs to transmit logical quantity information. The sampling frequency requirement is low. It has a strong ability to tolerate transition resistance and can reliably identify various types of faults.

[0194] Those skilled in the art will appreciate that all or part of the processes of the above-mentioned embodiments can be implemented by instructing related hardware through a computer program, and the program can be stored in a computer-readable storage medium, wherein the computer-readable storage medium is a disk, an optical disk, a read-only storage memory, or a random access memory, etc.

[0195] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A flexible straight line pilot protection method based on inductance parameters, characterized in that: include: Collect data after the fault occurs, including: voltage reference value on the AC side of the converter station, voltage on the DC side, AC current and AC voltage on both sides of the AC line M and N; Based on the data, the inductance values ​​on both sides of the AC line M and N are calculated; Calculating the inductance error coefficients on both sides of the AC line M and N according to the inductance values ​​on both sides of the AC line; Determine whether the inductance error coefficients on both sides of the AC line M and N meet the fault identification criteria. If so, it is determined that the AC line has an in-zone fault and the AC line protection is activated. Otherwise, it is determined that the AC line has an out-of-zone fault and the AC line protection does not operate. The inductance error coefficient on both sides of the AC line M and N is calculated using the following formula: Among them, E m 、E n are the inductance error coefficients on both sides of the AC line M and N, H is the number of sampling points in a cycle after the fault occurs, and L fm (k), L fn (k) are the calculated inductance values ​​of the kth sampling point, L m , L n are the equivalent inductances on both sides of the AC line M and N respectively.

2. The flexible straight line pilot protection method based on inductance parameters according to claim 1 is characterized in that: The equivalent model of the AC side of the converter station in the flexible straight line is: Where N is the total number of submodules put into use in the upper and lower bridge arms of phase A; U c1 is the submodule capacitor voltage; i va are the A-phase voltage reference value and A-phase current of the AC side of the converter station respectively; R arm , L arm is the equivalent resistance and inductance of any phase bridge arm; L T is the inductance on the AC side of the converter station; u dc is the DC side voltage of the converter station; u ma is the AC A phase output voltage of the converter station; i ma It is the AC A phase output current of the converter station.

3. The flexible straight line pilot protection method based on inductance parameters according to claim 2 is characterized in that: Based on the equivalent model, the inductance values ​​on both sides of the AC line M and N are calculated using the following formula: Among them, R fm , L fm are the resistance and inductance of the AC line M side, R fn , L fn are the resistance and inductance values ​​of the N side of the AC line respectively; N is the total number of submodules put into use in the upper and lower bridge arms of the AC line; is the voltage reference value of the AC side of the converter station; u dc is the DC side voltage of the converter station; i m 、i n are the AC currents on both sides of the AC line M and N, u m 、u n are the AC voltages on both sides of the AC line M and N, respectively, and e n is the power port voltage outside the N side of the AC line.

4. The flexible straight line pilot protection method based on inductance parameters according to claim 3 is characterized in that: The fault identification criteria include: Among them, E set is the action threshold; If E m 、E n If formula (5) is satisfied, it is determined to be an in-zone fault; otherwise, it is determined to be an out-zone fault.

5. The flexible straight line pilot protection method based on inductance parameters according to claim 4 is characterized in that: According to the fault identification criterion, the fault direction is identified, including: When E m 、E n If formula (5) is satisfied, the fault direction is an intra-area fault; When E m Formula (5) is not satisfied, and the fault direction is an out-of-zone fault on the M side; When E n Formula (5) is not satisfied, and the fault direction is an N-side out-of-zone fault.

6. The flexible straight line pilot protection method based on inductance parameters according to claim 1 is characterized in that: To start the AC line protection, execute: Send a trip signal to the protection device of the faulty line.

7. The flexible straight line pilot protection method based on inductance parameters according to claim 2 is characterized in that: The converter station is an MMC converter station, wherein the structure of the negative electrode is completely consistent with that of the positive electrode.

8. A flexible straight line pilot protection system based on inductance parameters, characterized in that: include: The data acquisition module is used to collect data after a fault occurs, including: voltage reference value on the AC side of the converter station, voltage on the DC side, AC current and AC voltage on both sides of AC lines M and N; An inductance calculation module, used to calculate the inductance values ​​on both sides of the AC line M and N according to the data; An inductance error coefficient determination module, used to calculate the inductance error coefficients on both sides of the AC line M and N according to the inductance values ​​on both sides of the AC line; The fault identification module is used to determine whether the inductance error coefficients on both sides of the AC line M and N meet the fault identification criteria. If so, it is determined that the AC line has an in-zone fault and the AC line protection is activated. Otherwise, it is determined that the AC line has an out-of-zone fault and the AC line protection does not operate. The inductance error coefficient on both sides of the AC line M and N is calculated using the following formula: Among them, E m 、E n are the inductance error coefficients on both sides of the AC line M and N, H is the number of sampling points in a cycle after the fault occurs, and L fm (k), L fn (k) are the calculated inductance values ​​of the kth sampling point, L m , L n are the equivalent inductances on both sides of the AC line M and N respectively.

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