A method for phase selection of grounding faults in an asymmetric distribution network based on a controllable voltage source

Abstract

A method for phase selection of grounding faults in an asymmetric distribution network based on a controllable voltage source, which relates to the field of power technology. The present invention analyzes in detail the changes in the zero-sequence current of the system before and after the regulation of the zero-sequence voltage at the neutral point, and uses the phase angle relationship between the zero-sequence equivalent admittance and the difference in the pre-fault admittance to the ground for fault phase discrimination. The identification method of the present invention has a higher accuracy rate, is not affected by the unbalance of the three-phase admittances to the ground in the distribution network, and the fault phase selection result is accurate, making it suitable for large-scale popularization 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 the faulty phase of a grounding fault in an asymmetric distribution network based on a controllable voltage source. 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 the electric energy 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 penetrated into 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 faced in the construction of global distribution networks. In the distribution network, due to factors such as the unequal three-phase parameters (including length) of single-core cable lines and the non-transposition of three-phase lines, the situation of unbalanced three-phase admittances to the ground in the distribution network is widespread. Traditional phase selection methods ignore the situation of unbalanced three-phase admittances to the ground in the distribution network, and when a high-resistance grounding fault occurs on the line, the fault information is not obvious, which may lead to incorrect phase selection. Then, how to provide a method for selecting the faulty phase of a grounding fault in an asymmetric distribution network based on a controllable voltage source has become a long-term technical requirement 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 the faulty phase of a grounding fault in an asymmetric distribution network based on a controllable voltage source. The present invention uses the phase angle relationship between the zero-sequence equivalent admittance and the difference in the pre-fault admittance to the ground to discriminate the faulty phase, with relatively high accuracy and sensitivity, and is not affected by changes in the parameters of the distribution network, etc.

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

[0005] A method for selecting the faulty phase of a grounding fault in an asymmetric distribution network based on a controllable voltage source, the phase selection method is to use the change relationship of the zero-sequence equivalent admittance according to the measured relevant parameter values of the distribution network system, combine with the admittance to the ground of the distribution network system, and use the phase angle relationship between the zero-sequence equivalent admittance and the difference in the pre-fault admittance to the ground when the controllable voltage source is used to regulate the zero-sequence voltage of the neutral point by phases to complete the phase selection work of the fault.

[0006] For the method for selecting the faulty phase of a grounding fault in an asymmetric distribution network based on a controllable voltage source, there is a certain degree of asymmetry in the admittances of each phase of the distribution network line to the ground. Therefore, there is an inherent zero-sequence voltage and a certain inherent zero-sequence current in the distribution network. Through theoretical derivation and arrangement, the calculation formula for the inherent zero-sequence current of the distribution network system under normal operating conditions is obtained:

[0007]

[0008] wherein is the sum vector of the inherent system-to-ground admittance asymmetry, a is the unit vector operator, a = 1∠120°, is the three-phase power supply voltage, Y X = 1 / R X +jωC X (X = A, B, C) is the phase-to-ground admittance of phase X.

[0009] For the method for selecting the faulty phase in an unbalanced distribution network grounding fault based on a controllable voltage source, after a single-phase grounding fault occurs in the distribution network, switch S0 is closed at this time, and the neutral zero-sequence voltage is regulated by controlling the controllable voltage source. First, let such that: wherein is the output voltage of the controllable voltage source, is the neutral zero-sequence voltage when the output voltage phase of the controllable voltage source is in the same phase as the power supply phase electromotive force , 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 and the neutral zero-sequence voltage are measured throughout.

[0010] For the method for selecting the faulty phase in an unbalanced distribution network grounding fault based on a controllable voltage source, when regulating the neutral zero-sequence voltage by phases, the difference between the zero-sequence equivalent admittance and the feeder-to-ground admittance before the fault is denoted as (t = A, B, C). When the neutral zero-sequence voltage is in the same phase as the power supply electromotive force of the faulty phase, assuming that the power supply electromotive force phase of the faulty phase is phase C, since the imaginary part is 0, and its phase angle

[0011] For the method for selecting the faulty phase in an unbalanced distribution network grounding fault based on a controllable voltage source, when the neutral zero-sequence voltage is not in the same phase as the power supply electromotive force of the faulty phase, effective analysis is performed on the real and imaginary parts of and . At this time, there is:

[0012]

[0013] For and , the phase angle is obtained as:

[0014]

[0015] where: λ is the controllable voltage source regulation coefficient, R f is the grounding fault transition resistance, both are positive real numbers, for Analysis shows that when \(0 \lt \lambda \leq \frac{1}{2}\), the function is monotonically increasing in this interval. According to the properties of the arctangent function, we have: And for Analysis shows that when \(0 \lt \lambda \leq \frac{1}{2}\), the function is monotonically decreasing. According to the properties of the arctangent function, we have: And is always equal to 0. When \(0 \lt \lambda \leq \frac{1}{2}\), it can be seen that there are significant differences in the phase angles of the faulty phase and the non-faulty phase during different regulation processes. Therefore, the faulty phase can be discriminated based on the phase angle relationship between the zero-sequence equivalent admittance and the pre-fault ground admittance during the phase-separated regulation process.

[0016] Adopting the technical solution described above, the present invention has the following advantages:

[0017] The present invention analyzes in detail the changes in the zero-sequence current of the system before and after the regulation of the zero-sequence voltage at the neutral point, and uses the phase angle relationship between the zero-sequence equivalent admittance and the pre-fault ground admittance to discriminate the faulty phase. The identification method of the present invention has a higher accuracy rate, is not affected by the unbalance of the three-phase ground admittances of the distribution network, and the faulty phase selection result is accurate, which is suitable for large-scale promotion and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is the topology diagram of the distribution network grounding fault with a controllable voltage source connected in the embodiment of the present invention;

[0019] Figure 2 is the flow chart of the grounding fault identification in the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0020] The present invention can be more specifically explained through the following embodiments, and the present invention is not limited to the following embodiments;

[0021] Combined with the attached Figures 1-2 An asymmetric distribution network grounding fault phase selection method based on a controllable voltage source, wherein the phase selection method is to complete the phase selection work of the fault according to the measured relevant parameter values of the distribution network system, utilize the change relationship of the zero-sequence equivalent admittance, combine the ground admittance of the distribution network system, and utilize the phase angle relationship between the zero-sequence equivalent admittance and the pre-fault ground admittance when the controllable voltage source regulates the zero-sequence voltage at the neutral point in a phase-separated manner.

[0022] The principle of the faulty phase selection is specifically analyzed as follows:

[0023] During the on-site operation process, due to the fact that the ground admittances of the various phases of the distribution network line are not exactly equal and there is a certain degree of asymmetry, there are inherent zero-sequence voltages and a certain amount of inherent zero-sequence currents in the distribution network. Through theoretical derivation and arrangement, the calculation formula for the inherent zero-sequence current of the distribution network system under normal operating conditions can be obtained:​

[0024]

[0025] wherein is the sum of the inherent asymmetrical vectors of the system's admittance to the ground, a is the unit vector operator, a = 1∠120°, is the three-phase power supply voltage, Y X = 1 / R X + jωC X (X = A, B, C) is the admittance to the ground of phase X.

[0026] After a single-phase grounding fault occurs in the distribution network, at this time, close switch S0, and regulate the zero-sequence voltage of the neutral point by controlling the controllable voltage source. First, make such that: wherein: is the output voltage of the controllable voltage source, is the phase of the output voltage of the controllable voltage source and the phase electromotive force of the power supply when they are in the same phase, the zero-sequence voltage of the neutral point, and 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, measure the zero-sequence current and the zero-sequence voltage of the neutral point throughout.

[0027] Assume that a grounding fault occurs in phase C. When regulating the zero-sequence voltage of the neutral point such that: (t = A, B, C), the corresponding zero-sequence equivalent admittances of the distribution network are:

[0028]

[0029]

[0030]

[0031] Make:

[0032]

[0033] When the zero-sequence voltage of the neutral point is regulated to λ times the electromotive force of the faulty-phase power supply:

[0034]

[0035] Similarly, when the neutral point voltage is regulated to λ times the electromotive force of the non-faulty-phase power supply:

[0036]

[0037] where 1 / R fBoth and λ are positive real numbers. Next, an effective analysis is carried out on the phase angle:

[0038] Taking phase C as the reference phase, when the zero-sequence voltage of the neutral point is in phase with the power supply electromotive force of the fault phase, since the imaginary part is 0, obviously its phase angle

[0039] When the zero-sequence voltage of the neutral point is not in phase with the power supply electromotive force of the fault phase, an effective analysis is carried out on the real and imaginary parts of and At this time, there is:

[0040]

[0041] For and The phase angle is obtained as:

[0042]

[0043] For Analysis shows that when 0 < λ ≤ 1 / 2, the function is monotonically increasing in this interval. According to the properties of the arctangent function, we can get: And for Analysis shows that when 0 < λ ≤ 1 / 2, the function is monotonically decreasing. According to the properties of the arctangent function, we can get: And is always equal to 0. To sum up, when 0 < λ ≤ 1 / 2, it can be seen that during different regulation processes, the phase angles of the fault phase and the non-fault phase are quite different. Therefore, the fault phase can be discriminated according to the phase angle relationship between the difference between the zero-sequence equivalent admittance and the pre-fault ground admittance during the phase-segregated regulation process.

[0044] Therefore, a phase-selection discrimination function is established. During the process of regulating the zero-sequence voltage of the neutral point in a phase-segregated manner, the zero-sequence current and the zero-sequence voltage of the neutral point are measured at each regulation state. At the same time, combined with the ground insulation parameter value of the distribution network and the inherent zero-sequence current the absolute value of the phase angle of the difference between the zero-sequence equivalent admittance and the pre-fault ground admittance during the phase-segregated regulation process is calculated:

[0045]

[0046]

[0047] When the zero-sequence voltage of the neutral point is regulated to be in the same phase as the electromotive force of the faulty phase, the value of the phase selection discrimination function is 0; when the zero-sequence voltage of the neutral point is regulated to be in the same phase as the electromotive force of the non-faulty phase, the value range of the phase selection discrimination function is

[0048] Considering factors such as the leakage resistance, leakage reactance and excitation impedance of the instrument transformer, there may be certain errors in the measurement of both the zero-sequence voltage and the zero-sequence current, which may further lead to certain errors in the process of solving the phase angle and deviate from the calculated value of the theoretical derivation to a certain extent. Therefore, the following criterion is adopted to select the faulty phase:

[0049] Measure the relevant data and calculate the phase selection discrimination function respectively during the single-phase regulation process and The phase corresponding to the smallest value of the faulty phase discrimination function is the faulty phase.

[0050] When the present invention is specifically implemented, the topology of the distribution network grounding fault is as Figure 1 shown, a controllable voltage source is connected to the neutral point. In the figure, is the three-phase power supply voltage, Y X =1 / R X +jωC X (X = A, B, C) is the admittance to the ground of the X phase, 1 / R X and C X are respectively the conductance to the ground and the capacitance to the ground of the X phase. The total admittance to the ground 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 inherent zero-sequence current of the line. is the zero-sequence voltage of the line neutral point. Assume that a single-phase grounding fault occurs in the C phase of the distribution network system, and R f is the transition resistance at the fault point.

[0051] The flow chart for realizing phase 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 rated phase voltage of the system.

[0052] After it is determined that a fault has occurred in the system, the following distribution network fault phase selection scheme is started: Close the switch S0, and regulate the zero-sequence voltage of the neutral point by controlling the controllable voltage source in phases, 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 of time, and the zero-sequence current of the line and the zero-sequence voltage of the neutral point are measured respectively under each regulation state.Combined with the inherent zero-sequence current of the line in the distribution network under normal operating conditions The zero-sequence equivalent admittance of the line is obtained, and the distribution network ground insulation parameter values are substituted into the formula for calculation respectively and The phase corresponding to the smallest function value is the fault phase

[0053] To verify the feasibility of the phase 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. 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 admittance is set as shown in Table 1 λ takes It is assumed that a fault occurs in phase C of the line at 0.5 s below

[0054] Table 1 System parameters of the simulation scenario

[0055]

[0056] When a single-phase grounding fault occurs in the distribution network, the controllable voltage source starts to act after 0.7 s, 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 line zero-sequence current, and simulate multiple groups of simulation experiments with different fault transition resistances according to this process. The amplitude and phase angle data of the line zero-sequence current and the zero-sequence voltage data of the neutral point recorded by the simulation are shown in Table 2. Calculate the line-to-ground admittance of the fault line through Table 1 and complete the calculation of the fault phase selection function by combining the zero-sequence voltage of the neutral point and the inherent zero-sequence current at the same time. The phase corresponding to the smallest function value is the fault phase

[0057] Table 2 Phase-separated regulation phase selection simulation results

[0058]

[0059]

[0060] 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, comparing the three phases it can be seen that the corresponding to phase C is the smallest among the three phases. According to the criterion, the fault phase is determined to be phase C, which is consistent with the actual situation

[0061] The comparison between the present invention and the prior art is as follows

[0062] 1. Method based on measuring the neutral point voltage offset magnitude and the change amount of the neutral point voltage

[0063] Fault phase identification based on the change amount has good effect when low-resistance grounding faults occur in the distribution network. However, when high-resistance grounding faults occur, due to the insignificant or even no change in the change amount, the sensitivity of this method is relatively low.

[0064] 2. Method for phase selection by constructing a deep learning model based on CNN

[0065] For this method, signal acquisition is relatively simple, with high sensitivity and accuracy, and fast recognition speed. However, this method depends on the historical training library, has poor physical interpretability, and once the distribution network parameters change, the original model may become invalid and need to be retrained, which is time-consuming and laborious.

[0066] The present invention uses the phase angle relationship between the zero-sequence equivalent admittance and the difference between the pre-fault ground admittance to discriminate the fault phase, with both high accuracy and sensitivity, and is not affected by the change of distribution network parameters.

[0067] The advantages of the present invention are as follows:

[0068] (1) The single-phase grounding fault phase selection method for the distribution network proposed by the present invention uses the change relationship of the zero-sequence equivalent admittance, combines the system ground admittance of the distribution network, and uses the phase angle relationship between the zero-sequence equivalent admittance and the difference between the pre-fault ground admittance when the neutral point zero-sequence voltage is regulated by the controllable voltage source in a phase-separated manner to complete the phase selection work for the fault, with high sensitivity and accurate identification results. The sensitivity is high and the identification result is accurate.

[0069] (2) Neither a high fault grounding transition resistance nor an unbalanced three-phase line ground admittance will affect the final identification result of the present invention, and it has good stability.

[0070] The present invention is applied to the identification of single-phase grounding faults in the distribution network, analyzes in detail the changes in the system zero-sequence current before and after the regulation of the neutral point zero-sequence voltage, and uses the phase angle relationship between the zero-sequence equivalent admittance and the difference between the pre-fault ground admittance to discriminate the fault phase; compared with the traditional method of measuring the neutral point voltage offset magnitude and the change amount of the neutral point voltage, the identification method of the present invention has a higher accuracy rate, is not affected by the unbalanced three-phase ground admittances of the distribution network, and the fault phase selection result is accurate.

[0071] The parts not detailed in the present invention are the prior art.

[0072] The embodiments selected herein for disclosing the invention purpose of the present invention are currently considered appropriate. However, it should be understood that the present invention is intended to include all changes and improvements of all embodiments belonging to the concept and scope of the present invention.

Claims

1. A ground fault phase selection method for an asymmetric distribution network based on a controllable voltage source, characterized in that: The phase selection method is to complete the phase selection of the fault according to the measured relevant parameter values of the distribution network system, utilize the variation relationship of the zero-sequence equivalent admittance, combine the shunt admittance of the distribution network system to the ground, and use the phase angle relationship between the zero-sequence equivalent admittance and the shunt admittance to the ground before the fault when the neutral zero-sequence voltage is regulated phase by phase with a controllable voltage source; There is a certain degree of asymmetry in the shunt admittance of each phase of the distribution network line to the ground. Therefore, there is an inherent zero-sequence voltage and a certain inherent zero-sequence current in the distribution network. Through theoretical derivation and arrangement, the calculation formula for the inherent zero-sequence current of the distribution network system under normal operating conditions is obtained: Among them is the sum of the inherent system's asymmetrical vector of the admittance to the ground, a is the unit vector operator, a = 1∠120°, is the three-phase power supply voltage, Y X = 1 / R X + jωC X (X = A, B, C) is the admittance to the ground of phase X.

2. The method for selecting the phase of a grounding fault in an asymmetric distribution network based on a controllable voltage source according to claim 1, wherein: 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, let such that: where is the output voltage of the controllable voltage source, is the zero-sequence voltage of the neutral point when the phase of the output voltage of the controllable voltage source 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, let such that: After this state is maintained for a period of time, let such that: The zero-sequence current and the zero-sequence voltage of the neutral point are measured throughout the regulation process.

3. The phase selection method for grounding faults in an asymmetric distribution network based on a controllable voltage source according to claim 1, characterized in that: The difference between the zero-sequence equivalent admittance and the pre-fault feeder-to-ground admittance when regulating the zero-sequence voltage of the neutral point by phase separation is denoted as When the phase of the zero-sequence voltage of the neutral point is the same as that of the fault-phase power supply electromotive force, assuming that the phase of the fault-phase power supply electromotive force is the C phase, because the imaginary part is 0, and its phase angle 4. The method for selecting the phase of a grounding fault in an asymmetric distribution network based on a controllable voltage source according to claim 1, characterized in that: When the zero-sequence voltage of the neutral point is not in phase with the electromotive force of the faulty-phase power supply, effectively analyze the real and imaginary parts of and . At this time, there is: For and The phase angle is obtained as follows: where: λ is the controllable voltage source regulation coefficient, R f is the grounding fault transition resistance, both are positive real numbers. For analysis shows that when 0 < λ ≤ 1 / 2, the function is monotonically increasing in this interval. According to the properties of the arctangent function, we have: And for analysis shows that when 0 < λ ≤ 1 / 2, the function is monotonically decreasing. According to the properties of the arctangent function, we have: And is always equal to 0. When 0 < λ ≤ 1 / 2, it can be seen that there are significant differences in the phase angles of the fault phase and the non-fault phase during different regulation processes. Therefore, the fault phase can be discriminated according to the phase angle relationship between the difference between the zero-sequence equivalent admittance and the pre-fault shunt admittance during the split-phase regulation process between them.

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