A Ground Fault Line Selection Method for Asymmetric Distribution Networks Based on a Flexible Grounding Device

Abstract

A method for selecting the faulty line in an asymmetric distribution network based on a flexible grounding device, which relates to the field of power technology. First, the present invention analyzes the changes in the zero-sequence current of each line before and after the regulation of the zero-sequence voltage at the neutral point, and then uses the degree of discreteness of the zero-sequence equivalent admittance of the faulty and healthy lines under different regulation states to identify the faulty line. The identification method of the present invention has higher accuracy, higher sensitivity in both low- and high-resistance grounding faults, and accurate faulty line selection results, and is suitable for large-scale promotion and application.

Description

Technical Field

[0001] The present invention relates to the technical field of electric power, and particularly relates to a method for selecting a grounding fault line in an asymmetric distribution network based on a flexible grounding device. Background Art

[0002] It is known that a distribution network refers to a power network that receives electric energy from a transmission network or a regional power plant and distributes it locally through distribution facilities or step by step according to voltage levels to various users. It is composed of overhead lines, cables, poles, distribution transformers, disconnect switches, reactive power compensators and some auxiliary facilities, etc., and plays an important role in distributing electric energy in the power network. The distribution network has long reached the user terminals. However, due to the complex wiring structure and usage environment, grounding faults in the distribution network occur frequently. The reliable detection and rapid handling of high-resistance grounding faults are always a major challenge in the construction of global distribution networks. If the faults are not discovered and cleared in time, it may damage the line insulation medium, cause a single-phase grounding fault to develop into an interphase short-circuit fault, resulting in damage to power equipment and reduction of power supply reliability. Rapidly discovering grounding faults is of great significance for the safe and reliable operation of the distribution network. Then, how to provide a method for selecting a grounding fault line in an asymmetric distribution network based on a flexible grounding device has become a long-term technical demand of those skilled in the art. Summary of the Invention

[0003] To overcome the deficiencies in the background art, the present invention provides a method for selecting a grounding fault line in an asymmetric distribution network based on a flexible grounding device. The present invention directly discriminates faults by whether the zero-sequence equivalent admittance changes. Even a very small fault current will make the zero-sequence current change curve become non-linear, thereby causing the zero-sequence equivalent admittance to change, and greatly improving the protection sensitivity, etc.

[0004] To achieve the above-mentioned invention purpose, the present invention adopts the following technical solutions:

[0005] A method for selecting a grounding fault line in an asymmetric distribution network based on a flexible grounding device. The line selection method is to actively change the excitation of the zero-sequence loop, amplify the zero-sequence voltage and current to make them easy to measure, reduce the admittance calculation error, subtract the inherent zero-sequence current of the line during the calculation process to eliminate the influence caused by the asymmetry of the three-phase-to-ground admittance of the distribution network, and use the discrete degree of the zero-sequence equivalent admittance of the line under different regulation states to identify the fault line and the healthy line to improve the criterion margin.

[0006] The described method for selecting the faulty line in an unbalanced distribution network based on a flexible grounding device. The distribution network has n lines. During on-site operation, due to a certain degree of asymmetry in the line-to-ground admittances of the three phases of the distribution network lines, there is an inherent zero-sequence voltage and a certain amount of inherent zero-sequence current in the distribution network. Through theoretical derivation and arrangement, the inherent zero-sequence current of any line i in the normal operation state of the distribution network system is obtained. The calculation formula is:

[0007]

[0008] Where is the three-phase power supply voltage, Y Xi = 1 / R Xi + jωC Xi (X = A, B, C) is the line-to-ground admittance of phase X on line i, is the vector sum of the inherent line-to-ground admittance asymmetry of line i, a is the unit vector operator, a = 1∠120°, and i ∈ [1, 2,..., n].

[0009] In the described method for selecting the faulty line in an unbalanced distribution network based on a flexible grounding device, after a single-phase grounding fault occurs in the distribution network, at this time, switch S0 is closed, and the zero-sequence voltage of the neutral point is regulated by controlling the controllable voltage source. First, let such that: Where: is the output voltage of the flexible grounding device, is the zero-sequence voltage of the neutral point when the output voltage phase of the flexible grounding device is in the same phase as the power supply phase potential , λ is the regulation coefficient. After maintaining this state for a period of time, let such that: After maintaining this state for a period of time, let such that: During the regulation process, the zero-sequence current of each line and the zero-sequence voltage of the neutral point are measured throughout the process.

[0010] In the described method for selecting the faulty line in an unbalanced distribution network based on a flexible grounding device, when the output voltage phase of the flexible grounding device is in the same phase as the power supply phase potential , according to Kirchhoff's law, the zero-sequence current of the faulty line k at this time is obtained, and the expression is:

[0011]

[0012] The calculation formula for the zero-sequence equivalent admittance of line k is as follows:

[0013]

[0014] As can be seen from the above formula, there is an inherent relationship between the zero-sequence equivalent admittance value of the system, the zero-sequence voltage of the neutral point, and the magnitude of the grounding fault transition resistance. The zero-sequence equivalent admittance of the faulty line k will change with the zero-sequence voltage of the neutral point. The remaining non-faulty lines have no faulty branches and the transition resistance does not exist, that is, in the above formula, let Rf tend to infinity. At this time, the zero-sequence current of the non-faulty line j is:

[0015]

[0016] The zero-sequence equivalent admittance of the line is:

[0017]

[0018] That is, the zero-sequence equivalent admittance of the non-faulty line is constantly the sum of the three-phase-to-ground admittances of this line. Similarly, when the output voltage phase of the flexible grounding device is in the same phase as the power supply phase potential respectively at the same phase, the above conclusion still holds. Therefore, by actively regulating the neutral point voltage of the distribution network system, the change characteristics of the relevant parameter values (zero-sequence current, zero-sequence voltage, zero-sequence equivalent admittance of the line, etc.) of the distribution system are effectively measured for fault line selection.

[0019] Adopting the technical solution as described above, the present invention has the following advantages:

[0020] The present invention first analyzes the changes in the zero-sequence current of each line before and after the regulation of the zero-sequence voltage of the neutral point, and then uses the discrete degree of the zero-sequence equivalent admittance of the faulty and healthy lines under different regulation states to identify the faulty line. The identification method of the present invention has a higher accuracy rate, higher sensitivity in both low- and high-resistance grounding faults, and the advantage of accurate fault line selection results, and is suitable for large-scale promotion and application. Description of the Drawings

[0021] Figure 1 is the grounding fault topology diagram of the distribution network with n outgoing lines in the embodiment of the present invention;

[0022] Figure 2 is the grounding fault identification flow chart in the embodiment of the present invention;

[0023] Figure 3 is the diagram of the zero-sequence current amplitude change of each line in the 0.13 kΩ low-resistance grounding fault in the embodiment of the present invention;

[0024] Figure 4 is the diagram of the zero-sequence current phase angle change of each line in the 0.13 kΩ low-resistance grounding fault in the embodiment of the present invention;

[0025] Figure 5 is the diagram of the zero-sequence current amplitude change of each line in the 10 kΩ high-resistance grounding fault in the embodiment of the present invention;

[0026] Figure 6 This is the variation diagram of the zero-sequence current phase angle of each line for the 10 kΩ high-resistance grounding fault in the embodiment of the present invention. Detailed implementation manners

[0027] The present invention can be more specifically explained through the following embodiments, and the present invention is not limited to the following embodiments;

[0028] Combined with the attached Figures 1 to 6 For the method for selecting a grounding fault line in an asymmetric distribution network based on a flexible grounding device, the line selection method is to actively change the excitation of the zero-sequence loop, amplify the zero-sequence voltage and current to make them relatively easy to measure, and reduce the admittance calculation error. During the calculation process, the inherent zero-sequence current of the line is subtracted to eliminate the influence caused by the asymmetry of the three-phase ground admittance of the distribution network, and the discrete degree of the zero-sequence equivalent admittance of the line under different regulation states is used to identify the fault line and the healthy line to improve the margin of the criterion.

[0029] During the on-site operation process, since the ground admittances of the three phases of the distribution network lines are not completely equal and there is a certain degree of asymmetry, there is an inherent zero-sequence voltage and a certain inherent zero-sequence current in the distribution network. Through theoretical derivation and arrangement, assuming that the distribution network has n lines, the calculation formula for the inherent zero-sequence current of any line i in the normal operation state of the distribution network system can be obtained:

[0030]

[0031] Where is the three-phase power supply voltage, Y Xi =1 / R Xi +jωC Xi (X = A, B, C) is the ground admittance of phase X on line i, is the vector sum of the asymmetry of the inherent ground admittance of line i, a is the unit vector operator, a = 1∠120°, i ∈ [1, 2,..., n].

[0032] After a single-phase grounding fault occurs in the distribution network, at this time, close the switch S0, and regulate the zero-sequence voltage of the neutral point by controlling the controllable voltage source. First, make such that: Where: is the output voltage of the flexible grounding device, is the zero-sequence voltage of the neutral point when the phase of the output voltage of the flexible grounding device is in the same phase as the phase electromotive force of the power supply λ is the regulation coefficient. After maintaining this state for a period of time, make such that: After maintaining this state for a period of time, make such that: During the regulation process, the zero-sequence current of each line and the zero-sequence voltage of the neutral point are measured throughout the process.

[0033] Specifically, taking the case where the output voltage phase of the flexible grounding device is in the same phase as the phase electromotive force of the power supply as an example, the expression of the zero-sequence current of the faulty line k at this time is obtained by Kirchhoff's law as follows:

[0034]

[0035] The calculation formula for the zero-sequence equivalent admittance of line k is as follows:

[0036]

[0037] It can be seen from this formula that there is an inherent relationship between the zero-sequence equivalent admittance value of the system, the zero-sequence voltage of the neutral point, and the magnitude of the grounding fault transition resistance. The zero-sequence equivalent admittance of the faulty line k will change with the zero-sequence voltage of the neutral point. The remaining non-faulty lines have no fault branches and the transition resistance does not exist, that is, in formula (4), let R f tend to infinity, and the zero-sequence current of the non-faulty line j at this time is:

[0038]

[0039] The zero-sequence equivalent admittance of the line is:

[0040]

[0041] That is, the zero-sequence equivalent admittance of the non-faulty line is constantly the sum of the three-phase-to-ground admittances of this line. Similarly, when the output voltage phase of the flexible grounding device is in the same phase as the phase electromotive force of the power supply respectively, the above conclusion still holds. Therefore, by actively regulating the neutral point voltage of the distribution network system, the change characteristics of relevant parameter values (zero-sequence current, zero-sequence voltage, zero-sequence equivalent admittance of the line, etc.) of the distribution system can be effectively measured for fault line selection.

[0042] To effectively quantify the change of the zero-sequence equivalent admittance during the regulation process, the zero-sequence equivalent admittance of the line under different regulation states is taken, and after the regulation ends, the variance is calculated with the amplitude of the zero-sequence equivalent admittance of the line under each regulation state as the sample. The line with a higher variance value is the faulty line.

[0043] For the convenience of calculation and explanation, a fault line selection function is established During the process of regulating the zero-sequence voltage of the neutral point in phases, the zero-sequence current of each line is measured in real time and the zero-sequence voltage of the neutral point At the same time, combined with the inherent zero-sequence current of each line under normal operating conditions we get: where: during the process of regulating in phases, the phase angle of the output voltage of the flexible grounding device and the phase electromotive force of the power supply The same, where t represents one of the three-phase power supply electromotive forces and serves as a marker for the phase-separated regulation state, with values of A, B, and C.

[0044] Let

[0045] Calculate:

[0046]

[0047] Considering that in the actual measurement process, factors such as the leakage resistance, leakage reactance, and exciting impedance of the current transformer may cause certain errors in the measurement of zero-sequence voltage and zero-sequence current, the following criterion is used to select the faulty line:

[0048] Measure the relevant data and calculate the fault line selection function for each line respectively And compare them. The line corresponding to the largest function value is the faulty line.

[0049] During specific implementation, the grounding fault topology of a distribution network with n outgoing lines is as Figure 1 shown, with the neutral point connected to a flexible grounding device. In the figure, is the three-phase power supply voltage, and Y Xi = 1 / R Xi + jωC Xi (X = A, B, C) is the admittance to ground of phase X on line i, and 1 / R Xi and C Xi are respectively the conductance to ground and capacitance to ground of phase X on line i. The total admittance to ground of line i is is the neutral point grounding admittance. When the neutral point is grounded through an arc suppression coil, L is the inductance value of the arc suppression coil. is the zero-sequence current of line i. is the zero-sequence voltage of the line neutral point. Assume that a single-phase grounding fault occurs on line k, and R f is the transition resistance at the fault point.

[0050] The flow chart for realizing the line selection in an unbalanced distribution network is as Figure 2 shown. When the distribution network is operating normally, the zero-sequence voltage of the distribution network neutral point is monitored in real time. Generally, it is determined that a grounding fault has occurred in the system when the change in the zero-sequence voltage of the neutral point is greater than 3% of the system rated phase voltage.

[0051] After determining that a fault has occurred in the system, start the distribution network fault line selection scheme as follows: Close switch S0, and control the phase-separated regulation of the zero-sequence voltage of the neutral point through the flexible grounding device, so that the zero-sequence voltage values of the distribution network system neutral point are successively: where: (0 < λ ≤ 1 / 2, t = A, B, C), each regulation state lasts for several cycles, and the zero-sequence current of each line is measured respectively under each regulation state and the zero-sequence voltage of the neutral point At the same time, combined with the inherent zero-sequence current of each line in the distribution network under normal operating conditions The zero-sequence equivalent admittance of each line is obtained, and the values of the fault line selection functions of each line are calculated respectively Among them, the line corresponding to the largest function value is the fault line; the flow chart of the distribution network fault line selection is as Figure 2 shown

[0052] To verify the feasibility of the line selection method described in the present invention, a single-phase grounding fault model of a 10 kV distribution network as shown in Figure 1 is built in PSCAD / EMTDC. A total of 3 outgoing lines are set, the neutral point is grounded through an arc suppression coil, and the over-compensation operation mode is adopted. The detuning degree is set to -5%, the inductance of the arc suppression coil is 0.162 H, and the line-to-ground admittances of the three lines are set as shown in Table 1. The line-to-ground admittance of Line 1 is symmetrical, and the remaining lines are all set to the three-phase asymmetrical state. When the distribution network system is working normally, the inherent zero-sequence currents of the three lines are measured as λ takes It is assumed that a fault occurs in Phase C of Line 3 at 0.5 s below

[0053] When a single-phase grounding fault occurs in the distribution network, after 0.7 s, the flexible arc suppression device starts to act, changing the zero-sequence voltage of the line neutral point, that is, making the neutral point and maintain for 0.1 s in turn, record the amplitude and phase angle of the zero-sequence current of each line, and simulate multiple groups of simulation experiments with different fault transition resistances according to this process. The amplitude and phase angle data of the zero-sequence current of each line recorded by the simulation are shown in Table 3. The amplitude and phase angle of the zero-sequence current for faults with transition resistances of 0.13 kΩ and 10 kΩ are respectively as Figures 3 to 6 shown

[0054] Table 1 Line parameters of the simulation scenario

[0055]

[0056]

[0057] Through Figures 3 to 6 It can be seen that during the period of 0.7 s to 1 s, the flexible grounding device regulates the neutral point voltage when accessing the distribution network system. Whether the line has a low-resistance grounding fault or a high-resistance grounding fault, the amplitude of the zero-sequence current will change greatly with the change of the output phase of the flexible grounding device, while the amplitude of the zero-sequence current of the non-fault line remains relatively constant. From Equation Sum formula It can be inferred that the zero-sequence equivalent admittance of the faulty line will fluctuate greatly with the change of the neutral point voltage, while the zero-sequence equivalent admittance of the non-faulty line remains constant, verifying the proposed faulty line selection principle.

[0058] Table 2 Simulation results of single-phase grounding fault line selection with phase-separated regulation

[0059]

[0060]

[0061] As can be seen from Table 2, when single-phase grounding faults of 0.13 kΩ, 1 kΩ, 5 kΩ, and 10 kΩ occur in the distribution network respectively, by comparing the fault line identification function z i of the three lines, the line corresponding to the largest function value is the faulty line. In all four cases, it can be accurately determined that line L3 is the faulty line. It can be seen that this method can effectively eliminate the adverse effects of the asymmetry of the insulation parameters of each phase of the distribution network on the selection of grounding fault lines, and can also sensitively identify the faulty line under the condition of a relatively high grounding transition resistance.

[0062] The beneficial effects of the present invention are as follows:

[0063] (1) The single-phase grounding fault line selection method proposed by the present invention can sensitively judge whether a grounding fault occurs in a line by whether the zero-sequence equivalent admittance changes, and has a relatively high identification accuracy.

[0064] (2) The unequal parameters of the three-phase lines to the ground and the relatively high grounding transition resistance will not affect the final identification result of the present invention, and there are fewer interference factors, etc.

[0065] Compared with the prior art, the main problem is that under the condition of high-resistance grounding faults, the fault characteristics are weak and difficult to detect. Traditional methods are all passive detections, while the present invention can actively amplify the fault characteristics and identify faults through zero-sequence equivalent admittance.

[0066] 1. Traditional measurement of neutral point voltage and change in neutral point voltage

[0067] Due to factors such as unequal three-phase parameters (including length) of single-core cable lines and non-transposition of three-phase lines, the situation of unbalanced three-phase admittances to the ground in the distribution network is common. Traditional line selection methods ignore the situation of unbalanced three-phase admittances to the ground in the distribution network, and the fault information may not be obvious when a high-resistance grounding fault occurs in the line, which may lead to incorrect line selection.

[0068] 2. Damping ratio

[0069] For a low-resistance grounding fault, the fault current is relatively large, and the damping ratio changes significantly before and after the fault. At this time, it is easy to judge the fault by measuring the damping ratio. However, for a high-resistance grounding fault, the fault current is very small, the active current between the line and the ground changes little, and the damping ratio basically remains unchanged, making it difficult to identify whether a fault has occurred and resulting in low protection sensitivity.

[0070] The present invention directly discriminates faults by whether the zero-sequence equivalent admittance changes. Even a very small fault current will make the zero-sequence current change curve become non-linear, thereby causing the zero-sequence equivalent admittance to change and improving the protection sensitivity, having certain advantages over the method of damping ratio.

[0071] The present invention is applied to the identification of single-phase grounding faults in a distribution network. It analyzes in detail the changes in the zero-sequence current of each line before and after the regulation of the zero-sequence voltage at the neutral point, and uses the discreteness of the zero-sequence equivalent admittance of the faulty and healthy lines under different regulation states to identify the faulty line. Compared with the traditional method of measuring the magnitude of the neutral point voltage offset and the change amount of the neutral point voltage, the identification method of the present invention has a higher accuracy rate, higher sensitivity in both low- and high-resistance grounding faults, and accurate fault line selection results.

[0072] Parts not described in detail in the present invention are prior art.

[0073] The embodiments selected herein for disclosing the object of the present invention are considered to be suitable at present. However, it should be understood that the present invention is intended to include all changes and improvements of all embodiments within the scope of this concept and invention.

Claims

1. An asymmetric distribution network grounding fault line selection method based on a flexible grounding device, characterized in that: The line selection method is to actively change the excitation of the zero-sequence loop, amplify the zero-sequence voltage and current to make them easy to measure, reduce the calculation error of admittance, subtract the inherent zero-sequence current of the line during the calculation process to eliminate the influence caused by the asymmetry of the three-phase-to-ground admittance of the distribution network, and use the discrete degree of the zero-sequence equivalent admittance of the line under different regulation states to identify the faulty line and the healthy line to improve the margin of the criterion; The distribution network has n lines. During on-site operation of the three-phase-to-ground admittance of the distribution network, due to a certain degree of asymmetry in the line-to-ground admittances of the three phases of the distribution network lines, there are inherent zero-sequence voltages and a certain amount of inherent zero-sequence current in the distribution network. After theoretical derivation and arrangement, the inherent zero-sequence current of any line i in the normal operating state of the distribution network system has the following calculation formula: Among them is the three-phase power supply voltage, Y Xi = 1 / R Xi + jωC Xi (X = A, B, C) is the shunt admittance to ground of phase X on line i, is the sum vector of the inherent shunt admittance asymmetry to ground of line i, a is the unit vector operator, a = 1∠120°, i ∈ [1, 2, …, n].

2. The method for selecting a faulty line in an asymmetric distribution network based on a flexible grounding device according to claim 1, characterized in that: After a single-phase grounding fault occurs in the distribution network, switch S0 is closed at this time, and the zero-sequence voltage of the neutral point is regulated by controlling the controllable voltage source. First, make such that: where: is the output voltage of the flexible grounding device, is the zero-sequence voltage of the neutral point when the phase of the output voltage of the flexible grounding device is in the same phase as the phase electromotive force of the power supply λ is the regulation coefficient. After this state is maintained for a period of time, make such that: After this state is maintained for a period of time, make such that: During the regulation process, the zero-sequence current of each line and the zero-sequence voltage of the neutral point are measured throughout the process.

3. The method for selecting the faulty line in an asymmetric distribution network based on a flexible grounding device according to claim 1, characterized in that: The output voltage phase of the flexible grounding device and the phase electromotive force of the power supply When they are in the same phase, the zero-sequence current of the faulty line k at this time is obtained according to Kirchhoff's law The expression is as follows: The calculation formula for the zero-sequence equivalent admittance of line k is as follows: It can be seen from the above formula that there is an inherent relationship between the zero-sequence equivalent admittance value of the system, the zero-sequence voltage of the neutral point, and the magnitude of the grounding fault transition resistance. The zero-sequence equivalent admittance of the faulty line k will change with the zero-sequence voltage of the neutral point. The remaining non-faulty lines have no faulty branches and the transition resistance does not exist, that is, in the above formula, let R f tend to infinity. At this time, the zero-sequence current of the non-faulty line j is: The zero-sequence equivalent admittance of the line is: That is, the zero-sequence equivalent susceptance of the non-faulty line is constantly the sum of the three-phase susceptances to ground of this line. Similarly, when the output voltage phases of the flexible grounding device are in the same phase as the phase electromotive forces of the power source respectively, the above conclusion still holds. Therefore, by actively regulating the neutral point voltage of the distribution network system, the change characteristics of relevant parameter values (zero-sequence current, zero-sequence voltage, line zero-sequence equivalent susceptance, etc.) of the distribution system are effectively measured for fault line selection. When they are in the same phase, the above conclusion still holds. Therefore, by actively regulating the neutral point voltage of the distribution network system, the change characteristics of relevant parameter values (zero-sequence current, zero-sequence voltage, line zero-sequence equivalent susceptance, etc.) of the distribution system are effectively measured for fault line selection.

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