Flexible DC power grid wide-area safety protection method and system with accessible network construction equipment

Abstract

The invention discloses a flexible direct current power grid wide-area safety protection method and system with accessible network construction equipment, belongs to the field of multi-terminal flexible direct current system protection, and solves the problems that an existing multi-terminal flexible direct current system is complex in protection process, difficult to apply and the like. The flexible direct current power grid comprises a flexible four-end flexible direct current system, two adjacent current converters in the four-end flexible direct current system are connected to form a direct current line, and each direct current line comprises a positive electrode line and a negative electrode line. The method comprises the steps that after a fault occurs, currents of multiple sampling points at the two ends of a positive line and a negative line in each direct-current line in the flexible direct-current power grid are collected, and transverse differential current mutation coefficients of the corresponding sampling points at the two ends of the corresponding positive line and the negative line are obtained through processing; obtaining transverse differential current abrupt change coefficients of the two ends of the corresponding direct-current line according to the transverse differential current abrupt change coefficients of the multiple sampling points of the two ends of the positive line and the negative line in each direct-current line; and according to the transverse differential current abrupt change coefficient of the two ends of each direct current line, whether the corresponding direct current line breaks down is judged, and if yes, fault protection is started.

Description

Technical Field

[0001] This invention relates to the field of multi-terminal flexible DC system protection, and in particular to a method and system for wide-area security protection of flexible DC power grids that can be accessed by grid-building equipment. Background Technology

[0002] With the integration of large-scale flexible DC transmission systems into the grid, the transient characteristics of power grid faults have changed significantly. Because traditional protection methods struggle to clear faults within milliseconds after they occur, short-circuit inrush currents can severely damage power electronic devices in flexible DC systems, even causing converter station shutdowns and system outages. Furthermore, the fault characteristics of multi-terminal flexible DC systems remain poorly understood, making it difficult to quantitatively analyze their impact on the power grid. Therefore, researching new protection principles adapted to multi-terminal flexible DC systems is of significant practical importance for the safe and stable operation of power systems.

[0003] For DC line protection, existing research can be divided into single-ended and double-ended quantity protection based on whether communication is required. Single-ended quantity protection suffers from problems such as small protection range and low judgment accuracy, making it difficult to apply in situations with complex network structures, such as multi-terminal flexible DC systems. In addition, DC converter stations will block out faults within 10ms after a fault, which greatly changes the transient characteristics of DC lines during faults. Existing protection methods cannot balance speed and accuracy, necessitating the development of new protection methods. Summary of the Invention

[0004] Based on the above analysis, the embodiments of the present invention aim to provide a method and system for wide-area security protection of flexible DC power grids that can be accessed by network construction equipment, in order to solve the problems of complex protection process, difficulty in application, and high communication requirements of existing multi-terminal flexible DC systems.

[0005] On one hand, this invention discloses a wide-area security protection method for a flexible DC power grid that can be accessed by network-connected equipment. The flexible DC power grid includes a four-terminal flexible DC system, in which two adjacent converters are connected to form a DC line, and each DC line includes positive and negative lines; the method includes:

[0006] After a fault occurs, the current at multiple sampling points at both ends of the positive and negative poles of each DC line in the flexible DC power grid is collected, and the transient coefficients of the transverse differential current at the corresponding sampling points at both ends of the positive and negative poles are obtained through processing.

[0007] Based on the transient coefficients of the differential current at multiple sampling points at both ends of the positive and negative poles of each DC line, the transient coefficients of the differential current at both ends of the corresponding DC line are obtained.

[0008] Based on the transient coefficient of the differential current at both ends of each DC line, determine whether a fault has occurred in the corresponding DC line. If so, activate the fault protection.

[0009] Based on the above solution, the present invention also makes the following improvements:

[0010] Furthermore, the coefficient of change of the transverse differential current S at terminals 1 and 2 of the DC line 12 S 21 They are represented as follows:

[0011]

[0012] Where, ΔI M12 (m), ΔI M21 (m) represents the transient coefficient of the transverse differential current at the m-th sampling point of the DC line at terminals 1 and 2, respectively, and M represents the number of sampling points.

[0013] Furthermore, based on the transient coefficient of the differential current at both ends of each DC line, it is determined whether a fault has occurred in the corresponding DC line, and the following steps are performed:

[0014] If S is satisfied 12 >S 12-set ,S 21 >S 21-set If a fault occurs in the positive line area, the positive line protection will be activated.

[0015] If S is satisfied 12 <-S 12-set ,S 21 <-S 21-set If a fault occurs in the negative line area, the negative line protection will be activated.

[0016] Otherwise, it indicates an external fault and no action will be taken;

[0017] Among them, S 12-set S 21-set These represent the protection thresholds for terminals 1 and 2 of the DC line, respectively.

[0018] Furthermore, the coefficient of change in transverse differential current ΔI at the m-th sampling point of DC line 1 is... M12 (m) is represented as:

[0019]

[0020] In the formula, These represent the currents at terminals 1 of the positive and negative lines, respectively, during normal operation. These represent the currents at the m-th sampling point of the positive and negative terminals 1 after the fault occurs.

[0021] Furthermore, the coefficient of change in transverse differential current ΔI at the m-th sampling point of the DC line terminal 2 is... M21 (m) is represented as:

[0022]

[0023] in, These represent the currents at the positive and negative terminals of the circuit during normal operation. These represent the currents at the m-th sampling point of the positive and negative terminals of the circuit after the fault occurs.

[0024] On the other hand, this invention discloses a wide-area security protection system for a flexible DC power grid that can be accessed by network-building equipment. The flexible DC power grid includes a flexible four-terminal flexible DC system. In the four-terminal flexible DC system, two adjacent converters are connected to form a DC line, and each DC line includes positive and negative lines. The system includes:

[0025] The data acquisition module is used to collect the current at multiple sampling points at both ends of the positive and negative poles of each DC line in the flexible DC power grid after a fault occurs, and process the data to obtain the transverse difference current mutation coefficient at the corresponding sampling points at both ends of the positive and negative poles.

[0026] The mutation coefficient calculation module is used to obtain the transverse difference current mutation coefficient at both ends of the corresponding DC line based on the transverse difference current mutation coefficient at multiple sampling points at both ends of the positive and negative lines in each DC line.

[0027] The protection discrimination module is used to determine whether a fault has occurred in the corresponding DC line based on the transient coefficient of the differential current at both ends of each DC line. If so, the fault protection is activated.

[0028] Based on the above solution, the present invention also makes the following improvements:

[0029] Furthermore, the coefficient of change of the transverse differential current S at terminals 1 and 2 of the DC line 12 S 21 They are represented as follows:

[0030]

[0031] Where, ΔI M12 (m), ΔI M21 (m) represents the transient coefficient of the transverse differential current at the m-th sampling point of the DC line at terminals 1 and 2, respectively, and M represents the number of sampling points.

[0032] Furthermore, based on the transient coefficient of the differential current at both ends of each DC line, it is determined whether a fault has occurred in the corresponding DC line, and the following steps are performed:

[0033] If S is satisfied 12 >S 12-set ,S 21 >S 21-set If a fault occurs in the positive line area, the positive line protection will be activated.

[0034] If S is satisfied 12 <-S 12-set ,S21 <-S 21-set If a fault occurs in the negative line area, the negative line protection will be activated.

[0035] Otherwise, it indicates an external fault and no action will be taken;

[0036] Among them, S 12-set S 21-set These represent the protection thresholds for terminals 1 and 2 of the DC line, respectively.

[0037] Furthermore, the coefficient of change in transverse differential current ΔI at the m-th sampling point of DC line 1 is... M12 (m) is represented as:

[0038]

[0039] In the formula, These represent the currents at terminals 1 of the positive and negative lines, respectively, during normal operation. These represent the currents at the m-th sampling point of the positive and negative terminals 1 after the fault occurs.

[0040] Furthermore, the coefficient of change in transverse differential current ΔI at the m-th sampling point of the DC line terminal 2 is... M21 (m) is represented as:

[0041]

[0042] in, These represent the currents at the positive and negative terminals of the circuit during normal operation. These represent the currents at the m-th sampling point of the positive and negative terminals of the circuit after the fault occurs.

[0043] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0044] The multi-terminal flexible DC power grid protection method provided by this invention establishes an equivalent model of the multi-terminal flexible DC system fault before the converter station is blocked, and constructs a protection criterion based on the differential current mutation coefficient. It has a fast action speed and can clear the fault within 3ms, reducing the impact of the fault short-circuit current on power electronic equipment.

[0045] The protection principle proposed in this invention can accurately and quickly identify single-pole grounding faults inside and outside the area and select the faulty pole before the converter station is locked out, and has good resistance to transition resistance.

[0046] The protection principle proposed in this invention has low requirements for communication devices. The communication devices at both ends of the line only need to transmit real-time logic information and are not affected by the synchronization error at both ends.

[0047] The multi-terminal flexible DC power grid protection system provided by this invention is based on the same principle as the above-mentioned method and has the corresponding technical effects.

[0048] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from the description and drawings, which are particularly pointed out. Attached Figure Description

[0049] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0050] Figure 1 A flowchart of a method for wide-area security protection of a flexible DC power grid that can be accessed by network construction equipment, provided in Embodiment 1 of the present invention;

[0051] Figure 2 This is a schematic diagram of a true bipolar converter station provided in Embodiment 1 of the present invention;

[0052] Figure 3 This is a schematic diagram of a single-pole grounding fault on the DC side of a converter station provided in Embodiment 1 of the present invention;

[0053] Figure 4 The equivalent circuit diagram of MMC provided in Embodiment 1 of the present invention;

[0054] Figure 5 This is a schematic diagram of the transient frequency domain model of the DC side fault of the MMC converter station provided in Embodiment 1 of the present invention;

[0055] Figure 6 This is a schematic diagram of the equivalent model of the positive electrode network of the flexible DC system during normal operation, provided in Embodiment 1 of the present invention.

[0056] Figure 7 This is a schematic diagram of the fault equivalent model for an intra-area fault provided in Embodiment 1 of the present invention;

[0057] Figure 8 The positive electrode line provided in Embodiment 1 of the present invention 13 A schematic diagram of the fault equivalent model during a line fault.

[0058] Figure 9 The positive electrode line provided in Embodiment 1 of the present invention 24 A schematic diagram of the fault equivalent model during a line fault.

[0059] Figure 10 The positive electrode line provided in Embodiment 1 of the present invention 34A schematic diagram of the fault equivalent model during a line fault.

[0060] Figure 11 S when a fault occurs at 50% of the positive DC line provided in Embodiment 2 of the present invention via different transition resistances 12 S 21 Schematic diagram;

[0061] Figure 12 The S-type fault occurring at 50% of the negative DC line provided in Embodiment 2 of the present invention, when a fault occurs through different transition resistances. 12 S 21 Schematic diagram;

[0062] Figure 13 The S-type fault that occurs on an external line with different transition resistances, as provided in Embodiment 2 of the present invention. 12 S 21 Schematic diagram;

[0063] Figure 14 This is a schematic diagram of the structure of the flexible DC power grid wide-area security protection system that the network equipment can access, as provided in Embodiment 3 of the present invention. Detailed Implementation

[0064] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form 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 intended to limit the scope of the present invention.

[0065] Specific embodiment 1 of the present invention discloses a wide-area security protection method for a flexible DC power grid that can be accessed by network-connected devices, flowchart. Figure 1 As shown. To facilitate a better understanding of the solution in this embodiment by those skilled in the art, the implementation principle of the method used in this embodiment will be explained in detail below.

[0066] First, based on the fault transient process of the converter, the fault transient characteristics of the flexible DC converter station are analyzed, and a transient model of the converter station is established. Specifically, according to the fault transient process analysis of the system converter, the formula is derived based on the dynamic characteristics of the MMC DC side and the MMC current inner loop control equation, and the expression of the circulating current of each phase of the MMC is obtained, as explained below.

[0067] Considering that the probability of a single-pole grounding fault is highest in flexible DC systems using overhead lines, the protection proposed in this embodiment mainly targets single-pole grounding faults. A schematic diagram of a true bipolar converter station is shown below. Figure 2 As shown, each pole of the converter station consists of an MMC and a smoothing reactor L. dcThe circuit is connected in series, with the positive and negative poles directly connected and grounded through a grounding electrode. To improve the speed of protection operation, this embodiment will focus on analyzing the characteristics of a single-pole short-circuit fault before the converter is locked out. Taking a single-pole ground fault occurring on the positive pole as an example, the schematic diagram of a single-pole fault on the DC side of the converter station is shown below. Figure 3 As shown.

[0068] MMC equivalent circuit as follows Figure 4 As shown, the positive and negative terminals of the MMC are completely identical. Each terminal consists of three phases and six bridge arms, with a total of 2N submodules in the upper and lower bridge arms of each phase. In the figure, L v U is the reactance value on the AC side of the MMC. dc For DC voltage, I dc For direct current, i vj (j takes a, b, c) as the three-phase current on the AC side of the MMC, u vj (j takes a, b, c) as the output voltage of the MMC AC side, i pj i nj (j takes a, b, c) and u represents the current flowing through the upper and lower arms of the converter, respectively. pj i nj (j takes a, b, c) as the upper and lower bridge arm voltages of the converter, respectively.

[0069] The relationship between the AC side voltage and current of the MMC and the voltage and current of the upper and lower bridge arms of the MMC is as follows:

[0070]

[0071] In the formula, u vj i vj These represent the phase j voltage and current (j = a, b, c) on the AC side of the MMC, respectively, and u pj i pj Represent the j-phase voltage and current of the upper arm of the MMC, respectively, and u nj i nj These are the j-phase voltage and current of the lower arm of the MMC, respectively. dc These represent the voltages between the upper and lower arms of the MMC, L. v Let R0 and L0 be the reactance values ​​on the AC side of the MMC bridge arm, respectively, and let u be the reactance. oo′ This represents the voltage difference between the two ends of the MMC.

[0072] Transforming the above equation into the complex frequency domain, we obtain the mathematical expression characterizing the dynamic characteristics of the MMC on the DC side:

[0073]

[0074] In the formula, i cirj This represents the j-phase circulating current of the MMC, u comjU represents the j-phase common-mode voltage of the MMC. comj (0) represents the initial value of the j-phase common-mode voltage of the MMC, and s represents the complex variable in the complex frequency domain. The specific expression is:

[0075]

[0076] In the formula, u cj u represents the average capacitor voltage of the j-phase submodule of the MMC. pj (0), u nj (0) are the excitation sources corresponding to the initial values ​​of the equivalent inductance and capacitor voltage of the upper and lower bridge arms of phase j, respectively.

[0077] Considering internal circulating current suppression, the inner current control equation of MMC is:

[0078]

[0079] In the formula, These represent the d-axis and q-axis common-mode voltage command values ​​of the MMC, respectively. These represent the d-axis and q-axis circulating current command values ​​of the MMC, respectively; k p1 k i1 These are the proportional and integral coefficients of the MMC control inner loop, respectively; k p2 k i2 These are the proportional and integral coefficients of the outer loop of the MMC control; ω pll This represents the angular frequency of the phase-locked loop output.

[0080] The MMC's current inner loop controller ultimately outputs the upper and lower bridge arm voltage command values. The formula is:

[0081]

[0082] In the formula, This indicates the commanded value of the j-phase common-mode voltage of the MMC; This represents the j-phase differential voltage command value of the MMC.

[0083] The expression for the number of submodules deployed in the upper and lower arms of the MMC is:

[0084]

[0085] round(x) represents taking the integer closest to x.

[0086] Circulating current per phase i cirj The expression can be represented as:

[0087]

[0088] In the formula, I tω represents the peak value of the negative sequence second harmonic of the MMC circulation. s θ represents the angular velocity of the power grid. tj This indicates the j-phase negative sequence second harmonic circulation phase of the MMC.

[0089] Then, based on the above formulas, the relationship between the DC voltage and DC current of the MMC can be derived, and a transient model of the multi-terminal flexible DC system can be established. Combining equations (2)-(9), the relationship between the DC voltage and DC current of the MMC can be obtained as follows:

[0090] U dc =U mmc -Z mmc I dc (10)

[0091]

[0092] In the formula, I dc Z represents the direct current (i.e., the equivalent power supply value of the MMC component). mmc U is the equivalent impedance of MMC. mmc It is the equivalent voltage source for MMC.

[0093] The fault transient frequency domain model of the MMC converter station can be obtained based on the relationship between the DC voltage and DC current of the MMC. (Refer to the appendix.) Figure 5 Then, based on the transient model of the multi-terminal flexible DC system, the normal operation scenario and different fault operation scenarios of the system can be analyzed, and the transient coefficient of the transverse differential current at both ends of the protected line can be calculated accordingly, as detailed below.

[0094] First, based on the analysis of the normal operation scenario of the multi-terminal flexible DC system, the positive and negative lines can be obtained. 12 The current at both ends of the line. When the system is operating normally, the positive equivalent network of the flexible DC system (the positive equivalent diagram of the entire four-terminal flexible DC system) is as follows: Figure 6 As shown, MMC1, MMC2, MMC4, and MMC3 are connected in a clockwise direction. 12 For the line between MMC1 and MMC2, line 13 For the line between MMC1 and MMC3, line 24 For the line between MMC2 and MMC4, line 34 For the line between MMC3 and MMC4, Figure 6 Z in 12 Z 13 Z 24 Z 34 Line 12 ,line 13 ,line 24,line 34 The impedance. The positive direction of the measured current in a DC line is defined as from the busbar to the line.

[0095] According to Kirchhoff's current law, we have:

[0096] I dc =A L I L (13)

[0097] I dc =[I dc1 I dc2 I dc3 I dc4 ] T (14)

[0098] I L =[I 12 I 13 I 24 I 34 ] T (15)

[0099]

[0100] In the formula, I dc The matrix formed by the DC currents of MMC1-MMC4, I L Line 12 ,line 13 ,line 24 ,line 34 The matrix formed by the DC line current, A L This is the current coefficient matrix.

[0101] From the equivalent circuit of the DC side of the multi-terminal flexible DC converter station, we can obtain:

[0102] U dc =U mmc -Z mmc I dc (17)

[0103] U dc =[U dc1 U dc2 U dc3 U dc4 ] T (18)

[0104] U mmc =[U mmc1 U mmc2 U mmc3 U mmc4 ] T (19)

[0105] Z mmc =diag(Z) mmc1 Z mmc2 Z mmc3 Z mmc4 (20)

[0106] In the formula, U dc The matrix formed by the DC voltages of MMC1-MMC4, U mmc Z is the matrix formed by the equivalent voltage sources of MMC1-MMC4. mmc The matrix is ​​formed by the equivalent impedances of MMC1-MMC4.

[0107] according to Figure 6 The relationship between the DC side voltage of the MMC and the line current can be obtained as follows:

[0108] B L U dc =Z L I L (twenty one)

[0109]

[0110] In the formula, B L Z L Let L be the coefficient matrix. dc This indicates the inductance value of the smoothing reactors at both ends of the DC line.

[0111] By combining equations (12), (17), and (21), the positive and negative lines under normal operation can be obtained. 12 Current at both ends:

[0112]

[0113] formula, These are the lines during normal operation. 12 The current across the positive terminal, These are the lines during normal operation. 12 The current across the negative terminal.

[0114]

[0115]

[0116] A2 = 16s 4 L 4 +8s 3 L 3 Z 12 +8s 3 L 3 Z 13+8s 3 L 3 Z 24 +8s 3 L 3 Z 34

[0117] +16s 3 L 3 Z mmc1 +16s 3 L 3 Z mmc2 +16s 3 L 3 Z mmc3 +s 3 16L 3 Z mmc4

[0118] +4s 2 L 2 Z 12 Z 13 +4s 2 L 2 Z 12 Z 24 +4s 2 L 2 Z 13 Z 24

[0119] +4s 2 L 2 Z 12 Z 34 +4s 2 L 2 Z 13 Z 34 +4s 2 L 2 Z 24 Z 34 +4s 2 L 2 Z 12 Z mmc1

[0120] +4s 2 L 2 Z 12 Z mmc2 +4s 2 L 2 Z 13 Z mmc1 +8s 2 L 2 Z 12 Z mmc3

[0121] +8s 2 L2 FROM 13 FROM mmc2 +8s 2 L 2 FROM 12 FROM mmc4 +4s 2 L 2 FROM 13 FROM mmc3

[0122] +8s 2 L 2 FROM 13 FROM mmc4 +8s 2 L 2 FROM 24 FROM mmc1 +4s 2 L 2 FROM 24 FROM mmc2

[0123] +8s 2 L 2 Z24Z mmc3 +4s 2 L 2 FROM 24 FROM mmc4 +8s 2 L 2 Z34Z mmc1

[0124] +8s 2 L 2 FROM 34 FROM mmc2 +4s 2 L 2 FROM 34 FROM mmc3 +4s 2 L 2 FROM 34 FROM mmc4

[0125] +12s 2 L 2 FROM mmc1 FROM mmc2 +12s 2 L 2 FROM mmc1 FROM mmc3 +16s 2 L 2 FROM mmc1 FROM mmc4

[0126] +16s 2 L 2 FROMmmc2 Z mmc3 +12s 2 L 2 Z mmc2 Z mmc4 +12s 2 L 2 Z mmc3 Z mmc4

[0127] +Z 12 Z 13 Z 24 Z 34 +Z 12 Z 13 Z 24 Z mmc3 +Z 12 Z 13 Z 24 Z mmc4

[0128] +Z 12 Z 13 Z 34 Z mmc2 +Z 12 Z 13 Z 34 Z mmc4 +Z 12 Z 24 Z 34 Z mmc1

[0129] +Z 13 Z 24 Z 34 Z mmc1 +Z 12 Z 24 Z 34 Z mmc3 +Z 13 Z 24 Z 34 Z mmc2

[0130] +Z 12 Z 13 Z mmc2 Z mmc3 +Z 12 Z 13 Z mmc2 Z mmc4 +Z 12 Z 13 Z mmc3 Z mmc4

[0131] +Z 12 Z 24 Z mmc1 Zmmc3 +Z 12 WITH 24 WITH mmc1 WITH mmc4 +Z 13 WITH 24 WITH mmc1 WITH mmc3

[0132] +Z 13 WITH 24 WITH mmc1 WITH mmc4 +Z 13 WITH 24 WITH mmc2 WITH mmc3 +Z 12 WITH 24 WITH mmc3 WITH mmc4

[0133] +Z 13 WITH 24 WITH mmc2 WITH mmc4 +Z 12 WITH 34 WITH mmc1 WITH mmc2 +Z 13 WITH 34 WITH mmc1 WITH mmc2

[0134] +Z 12 WITH 34 WITH mmc1 WITH mmc4 +Z 12 WITH 34 WITH mmc2 WITH mmc3 +Z 13 WITH 34 WITH mmc1 WITH mmc4

[0135] +Z 12 WITH 34 WITH mmc3 WITH mmc4 +Z 13 WITH 34 WITH mmc2 WITH mmc4 +Z 24 WITH 34 WITH mmc1 WITH mmc2

[0136] +Z 24 WITH 34 WITH mmc1 WITH mmc3 +Z24 WITH 34 WITH mmc2 WITH mmc3 +Z 12 WITH mmc1 WITH mmc2 WITH mmc3

[0137] +Z 12 WITH mmc1 WITH mmc2 WITH mmc4 +Z 13 WITH mmc1 WITH mmc2 WITH mmc3 +Z 12 WITH mmc1 WITH mmc3 WITH mmc4

[0138] +Z 13 WITH mmc1 WITH mmc2 WITH mmc4 +Z 12 WITH mmc2 WITH mmc3 WITH mmc4 +Z 13 WITH mmc1 WITH mmc3 WITH mmc4

[0139] +Z 13 WITH mmc2 WITH mmc3 WITH mmc4 +Z 24 WITH mmc1 WITH mmc2 WITH mmc3 +Z 24 WITH mmc1 WITH mmc2 WITH mmc4

[0140] +Z 24 WITH mmc1 WITH mmc3 WITH mmc4 +Z 24 WITH mmc2 WITH mmc3 WITH mmc4 +Z 34 WITH mmc1 WITH mmc2 WITH mmc3

[0141] +Z 34 WITH mmc1 WITH mmc2 WITH mmc4 +Z 34 WITHmmc1 WITH mmc3 WITH mmc4 +Z 34 WITH mmc2 WITH mmc3 WITH mmc4

[0142] +2LZ 12 WITH 13 WITH 24 +2LZ 12 WITH 13 WITH 34 +2sLZ 12 WITH 24 WITH 34 +2sLZ 13 WITH 24 WITH 34

[0143] +2LZ 12 WITH 13 WITH 24 +2LZ 12 WITH 13 WITH 34 +2sLZ 12 WITH 24 WITH 34 +2sLZ 13 WITH 24 WITH 34

[0144] +2sLZ 12 WITH 13 WITH mmc2 +2sLZ 12 WITH 13 WITH mmc3 +4sLZ 12 WITH 13 WITH mmc4 +2sLZ 12 WITH 24 WITH mmc1

[0145] +2sLZ 13 WITH 24 WITH mmc1 +4sLZ 12 WITH 24 WITH mmc3 +2sLZ 13 WITH 24 WITH mmc2 +2sLZ 12 WITH 24 WITH mmc4

[0146] +2sLZ 13 WITH 24 WITHmmc3 +2sLZ 13 WITH 24 WITH mmc4 +2sLZ 12 WITH 34 WITH mmc1 +2sLZ 12 WITH 34 WITH mmc2

[0147] +2sLZ 13 WITH 34 WITH mmc1 +2sLZ 12 WITH 34 WITH mmc3 +4sLZ 13 WITH 34 WITH mmc2 +2sLZ 12 WITH 34 WITH mmc4

[0148] +2sLZ 13 WITH 34 WITH mmc4 +4sLZ 24 WITH 34 WITH mmc1 +2sLZ 24 WITH 34 WITH mmc2 +2sLZ 24 WITH 34 WITH mmc3

[0149] +2sLZ 12 WITH mmc1 WITH mmc2 +2sLZ 12 WITH mmc1 WITH mmc3 +2sLZ 13 WITH mmc1 WITH mmc2

[0150] +4sLZ 12 WITH mmc1 WITH mmc4 +4sLZ 12 WITH mmc2 WITH mmc3 +2sLZ 13 WITH mmc1 WITH mmc3

[0151] +2sLZ 12 WITH mmc2 WITH mmc4 +4sLZ 13 WITH mmc1WITH mmc4 +4sLZ 13 WITH mmc2 WITH mmc3

[0152] +6sLZ 12 WITH mmc3 WITH mmc4 +6sLZ 13 WITH mmc2 WITH mmc4 +2sLZ 13 WITH mmc3 WITH mmc4

[0153] +2sLZ 24 WITH mmc1 WITH mmc2 +6sLZ 24 WITH mmc1 WITH mmc3 +4sLZ 24 WITH mmc1 WITH mmc4

[0154] +4sLZ 24 WITH mmc2 WITH mmc3 +2sLZ 24 WITH mmc2 WITH mmc4 +2sLZ 24 WITH mmc3 WITH mmc4

[0155] +6sLZ 34 WITH mmc1 WITH mmc2 +2sLZ 34 WITH mmc1 WITH mmc3 +4sLZ 34 WITH mmc1 WITH mmc4

[0156] +4sLZ 34 WITH mmc2 WITH mmc3 +2sLZ 34 WITH mmc2 WITH mmc4 +2sLZ 34 WITH mmc3 WITH mmc4

[0157] +8sLZ mmc1 WITH mmc2 WITH mmc3 +8sLZ mmc1 WITH mmc2 WITH mmc4+8sLZ mmc1 Z mmc3 Z mmc4

[0158] +8sLZ mmc2 Z mmc3 Z mmc4

[0159] Where L=L dc .

[0160] Based on the fault scenario analysis within the positive DC line region, the fault conditions within the positive region of a multi-terminal flexible DC system can be calculated. 12 The coefficient of change in transverse current at both ends.

[0161] Positive line 12 When a line fault occurs, the faulty network of the flexible DC system is as follows: Figure 7 As shown.

[0162] According to Kirchhoff's current law, we have:

[0163] I dc =A L I L +I f12 (26)

[0164] I f12 =[0 I f1 0 0] T (27)

[0165] B N u dc =Z L I L -U f12 (28)

[0166] U f12 =[I f1 [sL dc +(1-k)Z 12 ] 0 0 0] T (29)

[0167] In the formula, I f12 For line 12 The matrix formed by the fault current, I f1 positive line 12 The current flowing into the short-circuit point during a fault within the line area; U f12 For line 12 The matrix is ​​formed by the fault voltages; k represents the location where the line fault occurs, with a value of 0 to 1.

[0168] By combining equations (17), (26), and (28), we can obtain the positive electrode line.12 Positive line under fault in the line area 12 Current at the left and right ends of the line It can be represented as:

[0169]

[0170] At this time, the negative electrode network can still operate normally, and the negative electrode line... 12 The current at both ends of the line remains as shown in equation (25).

[0171] Define the coefficient of change of transverse differential current at both ends of a DC line as:

[0172]

[0173] In the formula, These represent a positive circuit fault and normal current, respectively. These represent a fault in the negative circuit and normal current, respectively.

[0174] By combining equations (24), (25), (30), and (31), the line under fault conditions in the positive DC line region can be obtained. 12 The coefficient of change of the transverse differential current ΔI at the left and right ends M12 ΔI M21 for:

[0175]

[0176] In the formula, ΔI M12 ΔI M21 Line 12 The coefficient of change of transverse current at the left and right ends.

[0177] From equation (32), it can be seen that the positive electrode line 12 Under the condition of a single-pole ground fault occurring within the line area, the transient coefficient of the transverse differential current at both ends of the protected line satisfies the following formula:

[0178]

[0179] Based on the analysis of fault scenarios outside the positive DC line area, the line of the multi-terminal flexible DC system can be calculated. 13 Line 24 routes and lines 34 When the line fails 12 The coefficient of change in transverse current at both ends.

[0180] (1) Positive line 13 line

[0181] Assuming positive line 13 When a line fault occurs, the faulty network of the flexible DC system is as follows: Figure 8 As shown.

[0182] According to Kirchhoff's current law, we have:

[0183] I dc =A L I L +I f13 (34)

[0184] I f13 =[0 0 I f2 0] T (35)

[0185] B N U dc =Z L I L -U f13 (36)

[0186] U f13 =[0 I f2 (sL dc +(1-k)Z 13 ) 0 0] T (37)

[0187] In the formula, I f13 U f13 Line 13 Fault current and voltage; I f2 Indicates positive line 13 The current flowing into the short-circuit point when a line fault occurs.

[0188] By combining equations (17), (34), and (36), we can obtain the positive electrode line. 13 In the event of a line fault (outside the area), the positive line... 12 Positive line under fault in the line area 12 The currents at the left and right ends of the line can be expressed as:

[0189]

[0190] At this time, the negative electrode network can still operate normally, and the negative electrode line... 12 The current at both ends of the line remains as shown in equation (25).

[0191] By combining equations (24), (25), (31), and (38), the line under fault conditions in the positive DC line region can be obtained. 12 The coefficients of change of the transverse differential current at the left and right ends are:

[0192]

[0193] From equation (39), it can be seen that the positive electrode line 12 Under the condition of a single-pole ground fault occurring within the line area, the transient coefficient of the transverse differential current at both ends of the protected line satisfies the following formula:

[0194] ΔI M12 ·ΔI M21 <0 (40)

[0195] (2) Positive line 24 line

[0196] Positive line 24 When a line fault occurs, the faulty network of the flexible DC system is as follows: Figure 9 As shown.

[0197] According to Kirchhoff's current law, we have:

[0198] I dc =A L I L +I f24 (41)

[0199] I f24 =[0 0 0 I f3 (42)

[0200] B N U dc =Z L I L -U f24 (43)

[0201] U f24 =[0 0 I f3 (sL dc +(1-k)Z 24 ) 0] T (44)

[0202] In the formula, I f24 U f24 Positive line 24 Fault current and voltage of the line; I f3 Indicates positive line 24 The current flowing into the short-circuit point when a line fault occurs.

[0203] By combining equations (17), (41), and (43), we can obtain the positive electrode line. 12 Positive line under fault in the line area 12 The currents at the left and right ends of the line can be expressed as:

[0204]

[0205] At this time, the negative electrode network can still operate normally, and the negative electrode line... 12 The current at both ends of the line remains as shown in equation (25).

[0206] By combining equations (24), (25), (31), and (45), the line under fault conditions in the positive DC line region can be obtained. 12 The coefficients of change of the transverse differential current at the left and right ends are:

[0207]

[0208] From equation (46), it can be seen that the positive electrode line 12 Under the condition of a single-pole ground fault occurring within the line area, the transient coefficient of the transverse differential current at both ends of the protected line satisfies the following formula:

[0209] ΔI M12 ·ΔI M21 <0 (47)

[0210] (3) Positive line 34 line

[0211] Positive line 34 When a line fault occurs, the faulty network of the flexible DC system is as follows: Figure 10 As shown.

[0212] According to Kirchhoff's current law, we have:

[0213] I dc =A L I L +I f34 (48)

[0214] I f34 =[0 0 0 I f4 (49)

[0215] B N u dc =Z L I L -U f34 (50)

[0216] U f34 =[0 0 0 I f4 (sL dc +(1-k)Z 34 )] T (51)

[0217] In the formula, I f34 U f34 Line 34 Fault current and voltage; I f4 Indicates positive line 34The current flowing into the short-circuit point when a line fault occurs.

[0218] By combining equations (17), (48), and (50), we can obtain the positive electrode line. 12 Positive line under fault in the line area 12 The currents at the left and right ends of the line can be expressed as:

[0219]

[0220] Among them, I f4 Indicates positive line 34 The current flowing into the short-circuit point when a line fault occurs.

[0221] At this time, the negative electrode network can still operate normally, and the negative electrode line... 12 The current at both ends of the line remains as shown in equation (25).

[0222] By combining equations (24), (25), (31), and (52), the line under fault conditions in the positive DC line region can be obtained. 12 The coefficients of change of the transverse differential current at the left and right ends are:

[0223]

[0224] From equation (53), it can be seen that the positive electrode line 12 Under the condition of a single-pole ground fault occurring within the line area, the transient coefficient of the transverse differential current at both ends of the protected line satisfies the following formula:

[0225] ΔI M12 ·ΔI M21 <0 (54)

[0226] Based on the analysis of fault scenarios in negative DC lines, the line faults occurring inside and outside the fault zone of a multi-terminal flexible DC system can be calculated. 12 The coefficient of change in transverse current at both ends.

[0227] Based on the above analysis, it can be concluded that

[0228] In-zone fault:

[0229] When the negative line 12 When a single-pole ground fault occurs within the line area, the current at both ends of the protected line satisfies the following formula:

[0230]

[0231] External fault:

[0232] When the negative line 12 When a single-pole ground fault occurs outside the line zone, the current at both ends of the protected line satisfies the following formula:

[0233] ΔIM12 ·ΔI M21 <0 (56)

[0234] This embodiment is based on the analysis of faults occurring at different locations. The differential current mutation coefficient differs significantly between faults occurring inside and outside the fault zone of a DC line. A protection criterion is constructed based on this difference in the differential current mutation coefficient between faults occurring inside and outside the fault zone. Based on the differential current coefficients for each fault scenario, a discriminant formula for determining whether a fault occurs within the positive or negative pole line zone can be obtained, as detailed below.

[0235]

[0236] Among them, S 12 Represents line 12 The coefficient of change of transverse differential current at the left end (end 1) of the line, S 21 Represents line 12 The coefficient of change in transverse differential current at the right end (end 2) of the line; ΔI M12 (m) represents line 12 The transient coefficient of the transverse differential current at the m-th sampling point of the protection installation location at the left end (end 1) of the line, ΔI M21 (m) represents line 12 The coefficient of change of transverse differential current at the m-th sampling point of the protection installation location at the right end (end 2) of the line.

[0237]

[0238] In the formula, These are the positive lines during normal operation. 12 The current at the left end (end 1) of the line, These are the negative lines during normal operation. 12 The current at the left end (end 1) of the line; positive line 12 The current at the left end (end 1) after a line fault. negative line after the fault 12 The current at the left end (terminal 1) of the line; the positive current direction is from converter station 1 towards line. 12 The current then flows to converter station 2, with the negative current flowing in the opposite direction to the positive current.

[0239]

[0240] In the formula, These are the positive lines during normal operation. 12 The current at the right end (terminal 2) of the line, These are the negative lines during normal operation. 12 The current at the right end (terminal 2) of the line; Positive line after fault12 The current at the right end (terminal 2) of the line, negative line after the fault 12 The current at the right end (end 2) of the line.

[0241] According to equations (33), (40), (47), (54), (55), and (56), the fault criterion in this embodiment can be set as follows:

[0242] If S is satisfied 12 >S 12-set ,S 21 >S 21-set Then the positive line occurs 12 A fault occurred within the line area, triggering the positive line. 12 Line protection; if S is satisfied 12 <-S 12-set ,S 21 <-S 21-set Then the negative line occurs 12 A fault occurred within the line area; the negative line was activated. 12 Line protection; otherwise, it indicates an external fault and will not activate. 12-set S 21-set These are the protection thresholds for the left (end 1) and right (end 2) ends of the protected line, respectively.

[0243] It should be noted that the above uses line 12 The process of analyzing faults within and outside the designated area, using the line as the research object, can be extended to other lines, and the resulting differential current mutation coefficient and protection criteria are the same. Therefore, this embodiment presents a fault identification method applicable to different lines, and based on this, the wide-area security protection method for flexible DC power grids in this embodiment is summarized. (Refer to...) Figure 1 In this embodiment, the flexible DC power grid includes a flexible four-terminal flexible DC system. In this system, two adjacent converters are connected to form a DC line, and each DC line includes positive and negative terminals. Therefore, the method includes:

[0244] Step S1: After the fault occurs, collect the current at multiple sampling points at both ends of the positive and negative lines of each DC line in the flexible DC power grid, and process them to obtain the transverse difference current mutation coefficient at the corresponding sampling points at both ends of the positive and negative lines.

[0245] Step S2: Based on the transient coefficients of the transient current at multiple sampling points at both ends of the positive and negative poles of each DC line, obtain the transient coefficients of the transient current at both ends of the corresponding DC line.

[0246] Step S3: Determine whether a fault has occurred in the corresponding DC line based on the transient coefficient of the differential current at both ends of each DC line. If so, activate the fault protection.

[0247] That is, the above line 12 ,line 13 ,line 24 ,line 34 Each line is treated as an independent DC line for data acquisition, calculation, and protection judgment.

[0248] The coefficient of change of transverse differential current S at terminals 1 and 2 of a DC line 12 S 21 They are represented as follows:

[0249]

[0250] Where, ΔI M12 (m), ΔI M21 (m) represents the transient coefficient of the transverse differential current at the m-th sampling point of the DC line at terminals 1 and 2, respectively, and M represents the number of sampling points.

[0251] The coefficient of change in transverse differential current ΔI at the m-th sampling point of DC line 1 M12 (m) is represented as:

[0252]

[0253] In the formula, These represent the currents at terminals 1 of the positive and negative lines, respectively, during normal operation. These represent the currents at the m-th sampling point of the positive and negative terminals 1 after the fault occurs.

[0254] The coefficient of change in transverse differential current ΔI at the m-th sampling point of the DC line terminal 2 M21 (m) is represented as:

[0255]

[0256] in, These represent the currents at the positive and negative terminals of the circuit during normal operation. These represent the currents at the m-th sampling point of the positive and negative terminals of the circuit after the fault occurs.

[0257] Then, based on the transient coefficient of the differential current at both ends of each DC line, it can be determined whether a fault has occurred in the corresponding DC line, and the following steps can be taken: If S is satisfied... 12 >S 12-set ,S 21 >S 21-set If a fault occurs in the positive line area, the positive line protection will be activated; if S is satisfied... 12 <-S 12-set ,S 21 <-S 21-setIf a fault occurs within the negative line zone, the negative line protection will activate; otherwise, if the fault occurs outside the zone, it will not activate. Wherein, S 12-set S 21-set These represent the protection thresholds for terminals 1 and 2 of the DC line, respectively.

[0258] It should be noted that, regarding Line 12 Based on the differential current coefficients of the above-mentioned fault scenarios, the protection criterion threshold can be set for the line, as explained in detail below.

[0259] According to equations (32), (39), (46), and (53), under the condition of a single-pole ground fault occurring within the protected line area, the farther the fault location is from the protection, the smaller the short-circuit current, i.e., k is taken as 1 and the transition resistance R... f When the Ω is 300Ω, |ΔI 12 When the value of | is minimized, S is at this point. 12 Let it be S 1a The expression is as follows:

[0260]

[0261] Similarly, when k is 0 and the transition resistance R is... f When the Ω is 300Ω, |ΔI 21 When the value of | is minimized, S is at this point. 21 Let it be S 2a The expression is as follows:

[0262]

[0263] When a single-pole ground fault occurs outside the protected area, and the fault location is far from the protection device, a dead zone is easily created because the current measured at the protection device installation point is relatively small. Therefore, a lower limit needs to be set for the protection threshold. This embodiment sets the threshold as follows:

[0264]

[0265] Therefore, this embodiment will set the operating threshold value of the protection at both ends of the DC line using the following formula:

[0266]

[0267] S 1a When there is a fault in the area, line 12 The minimum differential current change coefficient at the left end (end 1) of the line, S 2a When there is a fault in the area, line 12 The minimum differential current change factor at the right end (ends 2) of the line; S 1b When the fault is outside the zone (line) 24 (Line fault) 12The minimum differential current change coefficient at the left end (end 1) of the line, S 2b When the fault is outside the zone (line) 13 (Line fault) 12 The minimum differential current change factor at the right end (ends 2) of the line; S 1c When the fault is outside the zone (line) 34 (Line fault) 12 The minimum differential current change coefficient at the left end (end 1) of the line, S 2c When the fault is outside the zone (line) 34 (Line fault) 12 The smallest transverse current mutation coefficient at the right end (end 2) of the line.

[0268] In addition, it should be noted that when it is necessary to determine the range of action threshold values ​​for the protection at both ends of other lines, the corresponding lines can be rotated to... Figure 6 line 12 The parameters A1 and B1-B4 used in formulas (63)-(66) can also be directly replaced according to the changes in parameters (such as line impedance) before and after the line rotation to obtain the corresponding line parameters, and thus calculate the corresponding line parameter S. 1b ,S 1c ,S 1a ,S 2b ,S 2c ,S 2a The calculation is performed to obtain the range of the operating threshold values ​​of the protection at both ends of the corresponding line. Or, more generally, if the multi-terminal flexible DC system is completely symmetrical and all impedances are exactly the same, the calculation formulas of the aforementioned parameters A1 and B1-B4 can be simplified. The simplified parameter calculation results are directly applicable to each line, and formula (67) can be directly used as the range of the operating threshold values ​​of the protection at both ends of each line.

[0269] Example 2

[0270] To verify the correctness of the above scheme, Embodiment 2 of the present invention uses a multi-terminal flexible straight-line system model built on the RT-LAB simulation platform for simulation verification. The structure of the multi-terminal flexible straight-line system is as follows: Figure 5 As shown in Table 1, the main parameters of the system are as follows: the sampling frequency is selected as 20kHz, the selected frequency is 500Hz, and the protection action threshold value is set to 50. This article uses line... 12 Simulation experiments were conducted on the protection at both ends for different types of faults inside and outside the protection zone.

[0271] Table 1 Main parameters of the multi-terminal flexible line system

[0272]

[0273]

[0274] In the positive DC line of the multi-terminal flexible DC system 12 A single-pole ground fault with varying transition resistances is set at 50% of the fault location, ranging from 0 to 300Ω. The S value under this fault condition is... 12 S 21 like Figure 11 (a) Figure 11 As shown in (b).

[0275] Depend on Figure 11 It can be seen (corresponding to Equation 58 above) that under different fault conditions, the smaller the transition resistance and the longer the fault time, the more S... 12 S 21 The larger the value, the more... Figure 11 The simulation results are consistent.

[0276] When a single-pole ground fault occurs in the positive line area, the fault transition resistance is 300Ω, and the fault time t = 0.1ms, S 21 It has a minimum value of 109.24, S 12 There is a minimum value of 111.65, both of which are greater than the threshold value, so the protection can operate correctly.

[0277] In the negative DC line of the multi-terminal flexible DC system 12 A single-pole ground fault with varying transition resistances is set at 50% of the fault location, ranging from 0 to 300Ω. The S value under this fault condition is... 12 S 21 like Figure 12 (a) Figure 12 As shown in (b).

[0278] Under different fault conditions, the smaller the transition resistance and the longer the fault time, the better. 12 S 21 The smaller the value, the more... Figure 13 The simulation results are consistent.

[0279] When a single-phase ground fault occurs in the negative line area, the fault transition resistance is 300Ω, and the fault time t = 0.1ms, S 21 It has a maximum value of -114.68, S 12 It has a maximum value of -111, both of which are less than the threshold value.

[0280] outside the area line 24 The line is configured with a single-pole ground fault via varying transition resistances, ranging from 0 to 300 Ω. The S value under fault conditions is... 12 S 21 The following are respectively: Figure 13 (a) Figure 13 As shown in (b).

[0281] It can be seen that when a single-phase ground fault occurs on a line outside the positive pole line area, S 12 S 21 Unable to satisfy S 12 >S 12-set ,S 21 >S 21-set or S 12 <-S 12-set ,S 21 <-S 21-set The protection system identified the fault as occurring outside the protection zone. This data indicates that when a fault occurs outside the protection zone, the protection system can correctly identify the fault, is less affected by transition resistance, and has high sensitivity.

[0282] In summary, this protection method is less affected by the size of the transition resistance when facing different types of faults occurring outside the DC line zone. It can accurately identify faults outside the zone for different sizes of transition resistances and different fault types, and has high sensitivity and high resistance capability.

[0283] Example 3

[0284] Embodiment 2 of the present invention discloses a wide-area security protection system for a flexible DC power grid that can be accessed by network-connected equipment, as shown in the schematic diagram below. Figure 14 As shown, the system includes:

[0285] The data acquisition module is used to collect the current at multiple sampling points at both ends of the positive and negative poles of each DC line in the flexible DC power grid after a fault occurs, and process the data to obtain the transverse difference current mutation coefficient at the corresponding sampling points at both ends of the positive and negative poles.

[0286] The mutation coefficient calculation module is used to obtain the transverse difference current mutation coefficient at both ends of the corresponding DC line based on the transverse difference current mutation coefficient at multiple sampling points at both ends of the positive and negative lines in each DC line.

[0287] The protection discrimination module is used to determine whether a fault has occurred in the corresponding DC line based on the transient coefficient of the differential current at both ends of each DC line. If so, the fault protection is activated.

[0288] Since the system in this embodiment and the method in Embodiment 1 are related and can be referenced from each other, this description is redundant and will not be repeated here. Because this system embodiment shares the same principle as the above method embodiment, it also possesses the corresponding technical effects of the above method embodiment.

[0289] Those skilled in the art will understand that all or part of the processes of the methods described in the above embodiments can be implemented by a computer program instructing related hardware, and the program can be stored in a computer-readable storage medium. The computer-readable storage medium may be a disk, optical disk, read-only memory, or random access memory, etc.

[0290] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for wide-area security protection of a flexible DC power grid to which network-connected equipment can be accessed, characterized in that, The flexible DC power grid includes a flexible four-terminal flexible DC system. In this system, two adjacent converters are connected to form a DC line, and each DC line includes positive and negative terminals. The method includes: After a fault occurs, the current at multiple sampling points at both ends of the positive and negative poles of each DC line in the flexible DC power grid is collected, and the transient coefficients of the transverse differential current at the corresponding sampling points at both ends of the positive and negative poles are obtained through processing. Based on the transient coefficients of the differential current at multiple sampling points at both ends of the positive and negative poles of each DC line, the transient coefficients of the differential current at both ends of the corresponding DC line are obtained. Based on the transient coefficient of the differential current at both ends of each DC line, determine whether a fault has occurred in the corresponding DC line. If so, activate the fault protection.

2. The method for wide-area security protection of flexible DC power grids accessible to network equipment according to claim 1, characterized in that, The coefficient of change of transverse differential current S at terminals 1 and 2 of a DC line 12 S 21 They are represented as follows: Where, ΔI M12 (m), ΔI M21 (m) represents the transient coefficient of the transverse differential current at the m-th sampling point of the DC line at terminals 1 and 2, respectively, and M represents the number of sampling points.

3. The method for wide-area security protection of flexible DC power grids accessible to network equipment according to claim 2, characterized in that, Based on the transient coefficient of the differential current at both ends of each DC line, determine whether a fault has occurred in the corresponding DC line, and execute: If S is satisfied 12 >S 12-set ,S 21 >S 21-set If a fault occurs in the positive line area, the positive line protection will be activated. If S is satisfied 12 <-S 12-set ,S 21 <-S 21-set If a fault occurs in the negative line area, the negative line protection will be activated. Otherwise, it indicates an external fault and no action will be taken; Among them, S 12-set S 21-set These represent the protection thresholds for terminals 1 and 2 of the DC line, respectively.

4. The method for wide-area security protection of flexible DC power grids accessible to network equipment according to claim 3, characterized in that, The coefficient of change in transverse differential current ΔI at the m-th sampling point of DC line 1 M12 (m) is represented as: In the formula, These represent the currents at terminals 1 of the positive and negative lines, respectively, during normal operation. These represent the currents at the m-th sampling point of the positive and negative terminals 1 after the fault occurs.

5. The method for wide-area security protection of flexible DC power grids accessible to network equipment according to claim 4, characterized in that, The coefficient of change in transverse differential current ΔI at the m-th sampling point of the DC line terminal 2 M21 (m) is represented as: in, These represent the currents at the positive and negative terminals of the circuit during normal operation. These represent the currents at the m-th sampling point of the positive and negative terminals of the circuit after the fault occurs.

6. A wide-area security protection system for a flexible DC power grid that can be accessed by network-connected equipment, characterized in that, The flexible DC power grid includes a flexible four-terminal flexible DC system. In this system, two adjacent converters are connected to form a DC line, and each DC line includes positive and negative terminals. The system includes: The data acquisition module is used to collect the current at multiple sampling points at both ends of the positive and negative poles of each DC line in the flexible DC power grid after a fault occurs, and process the data to obtain the transverse difference current mutation coefficient at the corresponding sampling points at both ends of the positive and negative poles. The mutation coefficient calculation module is used to obtain the transverse difference current mutation coefficient at both ends of the corresponding DC line based on the transverse difference current mutation coefficient at multiple sampling points at both ends of the positive and negative lines in each DC line. The protection discrimination module is used to determine whether a fault has occurred in the corresponding DC line based on the transient coefficient of the differential current at both ends of each DC line. If so, the fault protection is activated.

7. The wide-area security protection system for flexible DC power grids that the network equipment can access according to claim 1, characterized in that, The coefficient of change of transverse differential current S at terminals 1 and 2 of a DC line 12 S 21 They are represented as follows: Where, ΔI M12 (m), ΔI M21 (m) represents the transient coefficient of the transverse differential current at the m-th sampling point of the DC line at terminals 1 and 2, respectively, and M represents the number of sampling points.

8. The flexible DC power grid wide-area security protection system that the network equipment can access according to claim 7, characterized in that, Based on the transient coefficient of the differential current at both ends of each DC line, determine whether a fault has occurred in the corresponding DC line, and execute: If S is satisfied 12 >S 12-set ,S 21 >S 21-set If a fault occurs in the positive line area, the positive line protection will be activated. If S is satisfied 12 <-S 12-set ,S 21 <-S 21-set If a fault occurs in the negative line area, the negative line protection will be activated. Otherwise, it indicates an external fault and no action will be taken; Among them, S 12-set S 21-set These represent the protection thresholds for terminals 1 and 2 of the DC line, respectively.

9. The flexible DC power grid wide-area security protection system that the network equipment can access according to claim 8, characterized in that, The coefficient of change in transverse differential current ΔI at the m-th sampling point of DC line 1 M12 (m) is represented as: In the formula, These represent the currents at terminals 1 of the positive and negative lines, respectively, during normal operation. These represent the currents at the m-th sampling point of the positive and negative terminals 1 after the fault occurs.

10. The flexible DC power grid wide-area security protection system that the network equipment can access according to claim 9, characterized in that, The coefficient of change in transverse differential current ΔI at the m-th sampling point of the DC line terminal 2 M21 (m) is represented as: in, These represent the currents at the positive and negative terminals of the circuit during normal operation. These represent the currents at the m-th sampling point of the positive and negative terminals of the circuit after the fault occurs.

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