A high-resistance fault adaptive protection method for distribution network with inverter-based distributed generation

Abstract

This invention proposes an adaptive protection method for high-resistance faults in distribution networks with inverter-type distributed generation, relating to the field of distribution network technology with inverter-type distributed generation. Targeting high-resistance grounding faults in distribution networks with IIDG, it constructs correction coefficients based on SV / GOOSE service and bisection iterative solution in IEC61850. Employing adaptive theory and concepts, it adaptively corrects the zero-sequence current under high-resistance grounding faults to the zero-sequence current under metallic grounding, enabling existing zero-sequence current protection to adapt to IIDG access and high-resistance faults without altering its original setting principles.

Description

Technical Field

[0001] This invention relates to the field of distribution network technology with inverter-type distributed power sources, and in particular to an adaptive protection method for high-resistance faults in distribution networks with inverter-type distributed power sources. Background Technology

[0002] Due to the influence of transmission corridors, natural environment, and overhead line height, high-impedance faults (HIFs) frequently occur in distribution networks, such as when conductors fall on concrete surfaces, grass, or sand. HIF currents are weak, dependent on the medium at the fault point, and can be less than 10% of the normal current. Therefore, traditional overcurrent relay-based protection schemes face difficulties in detecting HIFs. Long-term high-impedance faults can easily lead to fires and electric shock accidents. Furthermore, with the development of renewable and clean energy technologies, distributed generation, especially inverter-interfaced distributed generators (IIDGs), is increasingly connected to medium-voltage distribution systems, potentially rendering existing protection methods inaccurate. Therefore, HIF detection in medium-voltage distribution systems with IIDG integration is currently a challenging and hot research topic. Summary of the Invention

[0003] The purpose of this invention is to address the challenges of high-impedance fault (HIF) detection in medium-voltage power distribution systems increasingly connected to IIDGs, which pose a significant risk of fires and electric shocks due to HIFs. This invention proposes an adaptive protection method for high-impedance faults in power distribution networks with inverter-type distributed power sources.

[0004] To achieve the above objectives, the present invention adopts the following technical solution:

[0005] An adaptive protection method for high-resistance faults in distribution networks with inverter-type distributed power sources includes the following steps:

[0006] S1. The protection adopts the zero-sequence voltage and current over-limit start-up mode, sets the allowable error, and the initial space is [a,b].

[0007] S2. Construct a function based on the line parameters of the protected section and the voltage and current at the beginning and end:

[0008]

[0009] In the formula, Y down(·) Y is the equivalent admittance downstream of the fault point. up(·) Y is the equivalent admittance upstream of the fault point. a(·) The impedance Y of the line upstream of the fault point a(·) Y b(·)For the admittance of the upstream line at the fault point, This refers to the voltage of the M-side bus. The current is the fault line current, and the subscripts (1), (2), and (0) represent the positive and negative zero sequence components, respectively;

[0010] S3. Solve the matrix using the bisection method to determine the fault section;

[0011] S4. Use the bisection method to find the zeros of the function, and then solve the transmission equation of the upstream line of the fault point;

[0012] S5. Solve for the voltage at the fault point using the transmission equation of the upstream line and the voltage and current at the beginning of the line.

[0013] S6. Construct the correction coefficient formula and correct the zero-sequence current;

[0014] S7. Determine whether a fault has occurred in the line based on the existing zero-sequence overcurrent protection.

[0015] As a further description of the above technical solution:

[0016] The steps for solving the matrix using the bisection method are as follows:

[0017] S1. Let c = (a + b) / 2;

[0018] S2. If f(c) = 0, then Y b(0) =c, and output the result; if f(c)≠0, then execute S3;

[0019] S3. If f(a)*f(c)<0, then If f(a)*f(c)>0, then

[0020] S4, Judgment or If true, then

[0021] Y b(0) = (a+b) / 2, otherwise execute S1.

[0022] As a further description of the above technical solution:

[0023] Transmission equation:

[0024]

[0025] In the formula, For the transmission matrix of the line, This refers to the voltage of the M-side bus. Y represents the faulty line current. a(·) Y is the impedance of the line upstream of the fault point. b(·) For the admittance of the upstream line at the fault point, The voltage at the fault point. The fault point current, This represents the upstream current of the fault point.

[0026] As a further description of the above technical solution:

[0027] The formulas for calculating the sequence currents at the fault point are as follows:

[0028]

[0029] In the formula: Z 1Σ Z 2Σ Z 0Σ The order equivalent impedance from the short-circuit point to the network, R is the equivalent power source electromotive force. f The resistor at the fault point.

[0030] As a further description of the above technical solution:

[0031] The formulas for calculating the sequence voltages at the fault point are as follows:

[0032]

[0033] As a further description of the above technical solution:

[0034] upstream current at the fault point The calculation formula is as follows:

[0035]

[0036] In the formula, Y up(·) The equivalent admittance is upstream of the fault point.

[0037] As a further description of the above technical solution:

[0038] Zero-sequence current in faulty circuits The calculation formula is as follows:

[0039]

[0040] As a further description of the above technical solution:

[0041] The formula for the correction factor is as follows:

[0042]

[0043] Using this correction factor to correct the zero-sequence current of the faulty line, we have:

[0044]

[0045] In the formula, This is the corrected zero-sequence current, which is consistent with the zero-sequence current of the faulted line during a metallic single-phase ground fault.

[0046] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0047] 1. Traditional zero-sequence current protection has excellent performance in low-resistance grounding faults. Therefore, this method calculates the correction coefficient based on the electrical quantity information at the beginning and end of the fault section, and accurately corrects the zero-sequence current under high-resistance grounding faults to the zero-sequence current under metallic grounding faults. It can be applied to medium-voltage distribution networks connected by IIDG, and can adaptively correct the zero-sequence current under high-resistance grounding faults to the zero-sequence current under metallic grounding faults with high correction accuracy.

[0048] 2. Based on the SV / GOOSE service and bisection iterative solution in IEC61850, it is possible not only to solve the transmission matrix and construct the correction coefficients, but also to effectively distinguish between faulty and non-faulty sections.

[0049] 3. Simulation results demonstrate the effectiveness of this method under different fault locations and transition resistance conditions, and show that it is applicable to branch and segmented lines, IIDG access, arc grounding, and extreme fault conditions. Attached Figure Description

[0050] Figure 1 A schematic diagram of a 10kV low-resistance grounding system containing an IIDG is shown.

[0051] Figure 2 The composite sequence network for a single-phase ground fault is shown;

[0052] Figure 3 The flowchart of the zero-sequence current correction adaptive protection algorithm is shown;

[0053] Figure 4 A 10kV radial distribution network model is shown;

[0054] Figure 5 The effect of zero-sequence current correction is shown;

[0055] Figure 6 The zero-sequence current correction error diagram is shown;

[0056] Figure 7 The simulation results under IIDG access are shown. Detailed Implementation

[0057] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0058] Example 1

[0059] Figure 1 The diagram shows a 10kV low-resistance grounding system with IIDGs. The system consists of n feeders, with each IIDG connected to bus nodes D1 to D2. n T1 is the system main transformer, T0 is the grounding transformer, and R... f R is the transition resistance at the fault point. g The line uses a lumped parameter model to represent the neutral point grounding resistance.

[0060] Among them, the output characteristics of inverter-type distributed generation (IIDG) with voltage source converter (VSC) interface are basically determined by its control strategy. Grid-connected IIDGs generally adopt a PQ control strategy, and also consider low-voltage ride-through control, negative-sequence current elimination control, and maximum current limiting control strategies. Their fault model is equivalent to a current source controlled by PCC positive-sequence voltage, and the fault output current can be expressed as:

[0061]

[0062] In the formula, P represents the fault current of IIDG in a rotating coordinate system. DG This is a reference value for active power. and These are the voltages after and before a fault at the IIDG point of common coupling (PCC); The maximum permissible short-circuit current is equal to twice the rated current; K RS This is the reactive power support ratio coefficient.

[0063] based on Figure 1 The 10kV low-resistance grounding system with IIDG shown has established a single-phase grounding fault composite sequence network as follows: Figure 2 As shown.

[0064] In the picture, Z is the electromotive force of the system power supply; S(·) The equivalent impedance of the system; This refers to the voltage of the bus on the M side; Y represents the faulty line current. a(·) Y b(·) These are the upstream line impedance and admittance of the fault point, respectively; Lg For the zero-sequence inductance of the grounding transformer; Z Kj(·) (j=1,…n,j≠i) is the equivalent impedance of the non-faulty line; These are the upstream and downstream currents of the fault point, respectively. These are the voltage and current at the fault point, respectively. For the voltage and current at the end of the line; Z load(·) The load impedance is denoted as . In this invention, the subscripts (1), (2), and (0) represent the positive and negative zero-sequence components, respectively.

[0065] In detail, the sequence currents at the fault point can be obtained as follows (formulas for calculating the sequence currents at the fault point):

[0066]

[0067] In the formula: Z 1Σ Z 2Σ Z 0Σ The order equivalent impedance from the short-circuit point to the network; The equivalent power source electromotive force;

[0068] Furthermore, from equation (2), the sequence voltages at the fault point can be obtained as follows:

[0069]

[0070] Based on equations (2) and (3), the zero-sequence current upstream of the fault point can be obtained.

[0071]

[0072]

[0073] In the formula, Y up(·) The equivalent admittance is upstream of the fault point.

[0074] In further detail, the transmission equations of the lumped parameter model of the line are shown below:

[0075]

[0076] In equation (5), This is the transmission matrix for the line.

[0077] Combining equations (3), (4), and (5), the zero-sequence current of the faulty line can be obtained. As shown below:

[0078]

[0079] Based on equations (3) and (6), the correction coefficients proposed in this invention are as follows:

[0080]

[0081] At the same time, by using this correction factor to correct the zero-sequence current under high-resistance grounding in equation (7), we have:

[0082]

[0083] In equation (7), The zero-sequence current is the corrected zero-sequence current, which is completely consistent with the zero-sequence current of the fault line during a metallic single-phase ground fault.

[0084] like Figure 2 As shown, to solve for the correction coefficients, it is also necessary to solve for the transmission matrix of the upstream line at the fault point and the boundary conditions for a single-phase ground fault, which have the following mathematical expressions:

[0085]

[0086] In equation (9): Y down(·) The equivalent admittance downstream of the fault point; neglecting the negative-sequence capacitance to ground and the zero-sequence impedance of the faulted line. The mathematical expression is as follows:

[0087]

[0088] In equation (10): L i The length of the faulty line. These are the negative-sequence equivalent impedance and zero-sequence equivalent admittance downstream of the fault section, respectively.

[0089] In addition, the upstream line impedance Y at the fault point a(·) Admittance Y b(·) The following constraints exist:

[0090]

[0091] In equation (11): z (·) c (·) The impedance per unit length of the line and the capacitance to ground are given.

[0092] Line parameters, start and end voltage and current construction functions:

[0093]

[0094] Combining equations (5), (11), (12), and (13), we can obtain Y b(0) The equation is given by unknowns, and it is transformed into a problem of finding the zeros of the function.

[0095] This invention employs the bisection method to solve for the zeros of the aforementioned function. Based on the mathematical concept of infinite approximation, the bisection method continuously divides the interval containing the zero of the function into two parts, gradually bringing the two endpoints of the interval closer to the zero, thus obtaining an approximate value for the zero. After solving for the transmission matrix T, the positive and negative zero-sequence voltages at the fault point can be calculated based on the electrical quantities at the beginning of the fault section, and a zero-sequence current correction coefficient can be constructed.

[0096] like Figure 3 As shown, the zero-sequence current correction adaptive process proposed in this invention is as follows:

[0097] S1. The protection adopts a zero-sequence voltage and current over-limit start-up method;

[0098] S2. Construct the function shown in equation (12) based on the line parameters of the protected section and the voltage and current at the beginning and end.

[0099] S3. Determine the faulty section based on the basic conditions of the bisection method;

[0100] S4. Use the bisection method to find the zeros of the function, and then solve for the transmission matrix of the upstream line of the fault point;

[0101] S5. Solve for the fault point voltage based on equation (5) and the voltage and current at the beginning of the line;

[0102] S6. Construct the correction coefficient as shown in equation (7), and correct the zero-sequence current according to equation (8);

[0103] S7. Determine whether a line fault has occurred based on the existing zero-sequence overcurrent protection.

[0104] It is worth noting that in equation (10), the equivalent impedance / admittance downstream of the fault section needs to be given. This impedance / admittance will not change before and after a single-phase ground fault occurs in the previous section, so the real-time requirements for communication are not high. IEC61850 is currently the communication protocol with the best interoperability and the most promising application prospects in power systems. Typically, the IEC61850 protocol includes three services: Manufacturing Message Specification (MMS) service, General Object-Oriented Substation Event (GOOSE) service, and Sampled Value (SV) service. Among them, the SV service is used to propagate high-speed, real-time analog and digital sampled values ​​from IED to other devices in the substation network. The GOOSE service aims to achieve fast and reliable point-to-point communication while meeting the stringent power system control and automation requirements. Therefore, this patent uses the SV / GOOSE service to obtain the required equivalent impedance / admittance.

[0105] Example 2

[0106] like Figure 4As shown, a 10kV radial distribution network model was built using MATLAB / Simulink. The system includes a power source, transformers, grounding resistors, and four feeders. Three IIDGs are connected to the grid via switches S1, S2, and S3, respectively, each with a rated power of 2MW. The neutral point grounding resistance R... g =10Ω.

[0107] The line parameters are shown in Table 1:

[0108] Table 1:

[0109]

[0110] 1) Single-line fault testing without IIDG

[0111] First, we are in feeder segment l 31 The algorithm was verified by setting up single-phase grounding faults with different fault locations and transition resistances. Based on the proposed algorithm, the correction effect of the zero-sequence current is as follows: Figure 5 As shown.

[0112] refer to Figure 5 In the diagram, the green curve represents the zero-sequence current under a transition resistance of 1kΩ, the red curve represents the zero-sequence current under a metallic grounding fault, and the blue curve represents the corrected zero-sequence current. It can be seen that the zero-sequence current under a high-resistance grounding fault is weak, and after correction, its amplitude is almost indistinguishable from the zero-sequence current under a metallic grounding fault, demonstrating a good correction effect.

[0113] refer to Figure 6 The paper presents the relative error of the zero-sequence current correction. It can be seen that the correction error is larger at the beginning of the line, but the overall correction error does not exceed 0.5%. Moreover, the correction error hardly changes with the increase of the transition resistance, indicating that the zero-sequence current correction effect of this algorithm is almost unaffected by the transition resistance.

[0114] 2) Fault testing of branch and segment lines without IIDG

[0115] Next, we verified the effectiveness of the protection algorithm on branch and segmented lines. The simulation results are shown in Table 2. Here, "D" represents the distance from the fault point to the beginning of the fault section, "ε" represents the relative error of the zero-sequence current correction, "T" indicates tripping, and "NT" indicates no tripping.

[0116] Different fault sections were selected. 12 l 13 l 14 Under different fault distances and transition resistances, it can be observed that the zero-sequence current can be accurately corrected to a metallic grounding condition under high-resistance grounding. This proves that the proposed protection algorithm is still applicable in branch and segmented lines.

[0117] Table 2. Fault Test Results of Branch Section Lines

[0118]

[0119] 3) Testing under IIDG access

[0120] like Figure 7 As shown, to verify the effectiveness of the protection algorithm with IIDG access, we connected three IIDGs and observed the zero-sequence current, corrected zero-sequence current, and correction error at different fault distances in different fault sections with a transition resistance of 1kΩ. The simulation results are as follows. Figure 7 As shown, (a) and (b) represent the fault section l, respectively. 14 and l 41 The simulation results.

[0121] After the IIDG was connected, the zero-sequence current correction error did not increase significantly. This is because the method proposed in this paper focuses on the distribution of sequence voltage and current, and is almost unaffected by the magnitude of the values. Since the grid-connected transformer of the IIDG does not provide a zero-sequence path or a negative-sequence elimination control strategy, the IIDG only exists in the positive-sequence network and does not change the distribution of sequence voltage and current. Therefore, the method proposed in this invention is well applicable to distribution networks containing IIDG.

[0122] 4) Comparison with existing methods

[0123] To further verify the effectiveness of the proposed method, we compared it with the method in reference [2].

[0124] In this test, in section l 14 The simulation results of setting up a single-phase ground fault, changing the fault location and transition resistance, are shown in Table 3. It can be seen that compared with this method, the method in reference [2], after correcting the zero-sequence current, allows the zero-sequence current protection to operate, but the correction error of the zero-sequence current is very large. Moreover, for the non-faulty section l... 12 and l 11 This method can ensure effective differentiation. However, reference [2] only uses zero-sequence voltage and current over-limit start-up, and when a ground fault occurs in the downstream section, the zero-sequence current protection of the upstream section will malfunction.

[0125] Table 3 shows the comparison results with existing methods.

[0126]

[0127]

[0128] 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 equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. An adaptive protection method for high-resistance faults in a distribution network containing inverter-type distributed power sources, characterized in that, Includes the following steps: S1. The protection adopts the zero-sequence voltage and current over-limit start-up mode, sets the allowable error, and the initial space is [a,b]. S2. Construct a function based on the line parameters of the protected section and the voltage and current at the beginning and end: In the formula, Y down(·) Y is the equivalent admittance downstream of the fault point. up(·) Y is the equivalent admittance upstream of the fault point. a(·) Y is the impedance of the line upstream of the fault point. b(·) For the admittance of the upstream line at the fault point, This refers to the voltage of the M-side bus. The current is the fault line current, and the subscripts (1), (2), and (0) represent the positive and negative zero sequence components, respectively; S3. Solve the matrix using the bisection method to determine the fault section; S4. Use the bisection method to find the zeros of the function, and then solve the transmission equation of the upstream line of the fault point; S5. Solve for the voltage at the fault point using the transmission equation of the upstream line and the voltage and current at the beginning of the line. S6. Construct the correction coefficient formula and correct the zero-sequence current; S7. Determine whether a fault has occurred in the line based on the existing zero-sequence overcurrent protection. Transmission equation: In the formula, For the transmission matrix of the line, This refers to the voltage of the M-side bus. Y represents the faulty line current. a(·) Y is the impedance of the line upstream of the fault point. b(·) For the admittance of the upstream line at the fault point, The voltage at the fault point. The fault point current, This represents the upstream current of the fault point.

2. The adaptive protection method for high-resistance faults in a distribution network containing inverter-type distributed power sources according to claim 1, characterized in that, The steps for solving the matrix using the bisection method are as follows: S11. Let c = (a + b) / 2; S12. If f(c) = 0, then Y b(0) =c, and output the result; if f(c)≠0, then execute S3; S13. If f(a)*f(c)<0, then c→a′; if f(a)*f(c)>0, then c→b′. S14. Determine whether |a′-b| < ε or |ab′| < ε. If true, then Y b(0) = (a+b) / 2, otherwise execute S11.

3. The adaptive protection method for high-resistance faults in a distribution network with inverter-type distributed power sources according to claim 2, characterized in that, The formulas for calculating the sequence currents at the fault point are as follows: In the formula: Z 1Σ Z 2Σ Z 0Σ The order equivalent impedance from the short-circuit point to the network, R is the equivalent power source electromotive force. f The resistor at the fault point.

4. The adaptive protection method for high-resistance faults in a distribution network containing inverter-type distributed power sources according to claim 3, characterized in that, The formulas for calculating the sequence voltages at the fault point are as follows:

5. The adaptive protection method for high-resistance faults in a distribution network containing inverter-type distributed power sources according to claim 4, characterized in that, upstream current at the fault point The calculation formula is as follows: In the formula, Y up(·) The equivalent admittance is upstream of the fault point.

6. The adaptive protection method for high-resistance faults in a distribution network containing inverter-type distributed power sources according to claim 5, characterized in that, Zero-sequence current in faulty circuits The calculation formula is as follows:

7. The adaptive protection method for high-resistance faults in a distribution network containing inverter-type distributed power sources according to claim 6, characterized in that, The formula for the correction factor is as follows: Using this correction factor to correct the zero-sequence current of the faulty line, we have: In the formula, This is the corrected zero-sequence current, which is consistent with the zero-sequence current of the faulted line during a metallic single-phase ground fault.

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