Power distribution network single-phase earth fault transition resistor real-time tracking and fault phase selection method

Abstract

The invention discloses a power distribution network single-phase earth fault transition resistance real-time tracking and fault phase selection method, which is suitable for the technical field of power distribution network protection and control, and comprises the following steps: measuring the zero-sequence voltage of a power distribution network, and calculating the instantaneous value of the fault current of the power distribution network in real time; constructing a single-phase earth fault starting criterion, and judging whether a single-phase earth fault occurs or not; measuring voltages of A, B and C phases to the ground, and drawing a volt-ampere characteristic curve between the fault current and the voltages to the ground of each phase; determining a fault phase based on the distribution characteristics of the volt-ampere characteristic curve; and calculating the transition resistance in real time. According to the method, the influence of non-linear arc resistance instability and power distribution network parameter asymmetry is considered, the fault phase is rapidly and accurately determined, the development of the transition resistance is tracked in real time, and theoretical guidance is provided for correct fault handling and reliable arc extinction.

Description

Technical Field

[0001] The present invention relates to the technical field of distribution network fault detection and protection, and more specifically, to a method for real-time tracking of transition resistance and fault phase selection for single-phase grounding faults in a distribution network. Background Art

[0002] In China, the number of single-phase grounding faults in the distribution network accounts for more than 80% of the total faults, and 80% of them are unstable arcing faults and transient faults. Therefore, the transition resistance is often not a fixed value. It is required to "quickly isolate permanent faults and safely extinguish arcs for transient faults". For permanent faults, the fault point should be quickly disconnected to ensure the safe operation of the power grid; for transient faults, the arc extinguishing device should be quickly put into operation to prevent the fault from deteriorating and reduce the power outage time. Therefore, the prerequisite for fault handling is the quick and accurate identification of the fault type. The fault type and the degree of fault development are directly characterized by the transition resistance. Real-time tracking of the dynamic changes of the transition resistance is the key technology for judging the fault type. With the wide application of active flexible arc extinguishing technology, the quick and accurate input of the arc extinguishing device depends on the accurate discrimination of the fault phase. If the phase selection is incorrect, the fault will deteriorate. Therefore, studying the method for real-time tracking of transition resistance and fault phase selection for single-phase grounding faults in the distribution network is of great significance for the normal and safe operation of the power grid.

[0003] Existing transition resistance measurement technologies mainly utilize steady-state information and are applicable to the identification of transition resistance for permanent stable faults. However, when the transition resistance fluctuates randomly, the existing methods are difficult to track the changes of the transition resistance in real time. Therefore, there is an urgent need for a real-time tracking technology for transition resistance applicable to unstable arcing faults. Traditional phase selection methods are based on the strict symmetry of distribution parameters. However, the distribution parameters of the distribution network are usually asymmetric. Therefore, traditional phase selection criteria often fail in high-resistance grounding faults. In addition, the quick input of the arc extinguishing device requires quick fault phase selection. However, existing phase selection methods often utilize steady-state information such as the change law of zero-sequence voltage trajectory, the amplitude and phase angle changes of the phase-to-ground voltages of each phase before and after the fault, etc., and need to avoid the long transient process, resulting in poor quickness of fault phase selection. Therefore, there is an urgent need to study a quick fault phase selection technology considering the asymmetry of distribution parameters to gain time for fault arc extinguishing.

[0004] The Chinese patent application number is CN202011509050.4, and the application publication number is CN112946414B. The patent name is: A Method for Identifying the Grounding Phase and Transition Resistance of a Distribution Network Based on Zero-Sequence Residual Voltage Suppression. This patent takes the suppression of the neutral point voltage to 0 as the control target, and realizes fault phase selection and transition resistance identification by comparing the change in the injected current of the active flexible device before and after a single-phase grounding fault occurs in the distribution network. This method still has good sensitivity for high-resistance faults, but this method is only applicable to pure resistance faults, lacks rapidity in the fault phase selection method, and requires additional equipment investment. The Chinese patent application number is CN201710360981.4, and the application publication number is CN107192883B. The patent name is: A Method for Identifying the Transition Resistance of High-Resistance Grounding Faults in a Resonant Grounding System. This patent extracts the transient zero-sequence voltage, obtains the transient zero-sequence current at the fault point, and determines the transition resistance through the ratio of the transient zero-sequence voltage to the transient zero-sequence current at the fault point. This method uses transient data and has good rapidity, but this method still cannot be applied to the case where the transition resistance changes randomly, and there are also problems with difficult extraction of transient characteristics. Summary of the Invention

[0005] Technical Problem: With the development of the economic society and the continuous expansion of the scale of the distribution network, the demand for electric energy and the quality requirements are constantly increasing. It is required that the distribution network can instantaneously reflect faults and quickly give fault disposal plans. However, when a fault occurs in the distribution network, the transition resistance has random volatility, and most fault type discrimination and detection methods fail, making it difficult to discriminate and detect arc faults, resulting in operation risks in the distribution network.

[0006] Technical Solution: To solve the above technical problems, the present invention proposes a method for real-time tracking of the transition resistance and fault phase selection for single-phase grounding faults in a distribution network, which specifically includes the following steps:

[0007] Step a: Construct a starting criterion for single-phase grounding faults, collect the zero-sequence voltage of the distribution network in real time, and calculate the fault current in real time. When the absolute value of the fault current at a certain moment is greater than the fault judgment threshold M, it indicates that a single-phase grounding fault has occurred in the distribution network, and proceed to step b; otherwise, continue to monitor the distribution network.

[0008] Step b: Measure the phase-to-ground voltages of each phase of the distribution network, and draw the volt-ampere characteristic curves of phases A, B, and C with the fault current as the abscissa and the three-phase phase-to-ground voltages as the ordinates respectively.

[0009] Step c: Determine the fault phase based on the quadrant where the volt-ampere characteristic curve is located.

[0010] Step d: Calculate the transition resistance R E (t) in real time according to the phase-to-ground voltage of the fault phase and the fault current. The specific expression is:

[0011]

[0012] Among them, u δ (t) is the voltage of the faulty phase to ground, δ = A, B, C, i E (t) is the fault current.

[0013] The single-phase grounding fault starting criterion in step a can be specifically determined by the following steps:

[0014] Step a1: Real-time collect the zero-sequence voltage u 0 (t) of the distribution network, and substitute the zero-sequence voltage into the fault current calculation expression to solve the fault current i E (t) in real time. The specific fault current calculation expression is:

[0015]

[0016] Among them, G Σ and C Σ are the total ground conductance and total ground capacitance of the distribution network respectively, and i bd (t) is the total unbalanced current of the distribution network;

[0017] Step a2: Determine the fault judgment threshold M, take M = 0.1 A. If the absolute value of the fault current is less than the fault judgment threshold M at any time, it indicates that there is no single-phase grounding fault in the distribution network. If the absolute value of the fault current is greater than the fault judgment threshold M at a certain moment, it indicates that there is a single-phase grounding fault in the distribution network, and the time corresponding to when the absolute value of the fault current is first greater than the fault judgment threshold is determined as the fault initial time t 0 .

[0018] The plotting of the volt-ampere characteristic curve in step b can be specifically determined by the following steps:

[0019] Step b1: Starting from the fault initial time t 0 , every other sampling period T, collect the voltages of phases A, B, and C of the distribution network to ground and the fault current, and perform N samplings in total to obtain the three-phase voltage-to-ground voltage sequences U A (n), U B (n), U C (n) and the fault current sequence I(n), where n represents the nth sampling point;

[0020] Step b2: Using the fault current sequence as the abscissa and the three-phase voltage-to-ground voltage sequences as the ordinate, obtain three groups of coordinates (I(n), U A (n)), (I(n), U B (n)), (I(n), U C(n)), place each set of coordinates in the plane rectangular coordinate system respectively, and connect each set of coordinates with a smooth curve. The obtained smooth curve is the three-phase volt-ampere characteristic curve.

[0021] The fault phase selection method in step c can be specifically determined by the following steps:

[0022] Step c1: Multiply the abscissa of each point in the A, B, and C phase volt-ampere characteristic curves by its corresponding ordinate to obtain the instantaneous power P corresponding to each point δ (n):

[0023] P δ (n) = I(n)U δ (n) Equation 3

[0024] where δ = A, B, C;

[0025] Step c2: Record the number of points with instantaneous power greater than 0 in the A, B, and C phase volt-ampere characteristic curves respectively, denoted as Q A 、Q B 、Q C ;

[0026] Step c3: Compare Q A 、Q B 、Q C Among them, the one with the largest quantity indicates that the volt-ampere characteristic curve of this phase is generally located in the first and third quadrants, and this phase is determined as the fault phase.

[0027] Beneficial effects: The present invention proposes a real-time tracking method for the transition resistance of single-phase grounding faults in a distribution network and a fault phase selection method. By using the volt-ampere characteristic curves of each phase drawn by the fault current and the ground voltage of each phase, real-time tracking of the transition resistance after a fault occurs and rapid and accurate fault phase selection are realized, providing a theoretical guidance for subsequent fault disposal schemes. This method is not affected by non-linear transition resistances and parameter asymmetries, can sensitively start faults, and effectively avoids the risks brought by the long-term operation of the distribution network with faults. Brief Description of the Drawings

[0028] Figure 1 is the flow chart of real-time tracking of the transition resistance of single-phase grounding faults and fault phase selection;

[0029] Figure 2 is the schematic diagram of the simulation system in the embodiment;

[0030] Figure 3 is the schematic diagram of fault startup;

[0031] Figure 4 is the three-phase volt-ampere characteristic curve;

[0032] Figure 5 is the real-time tracking diagram of the transition resistance. Detailed implementation manners

[0033] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0034] The present invention proposes a method for real-time tracking of transition resistance and fault phase selection for single-phase grounding faults in a distribution network, as Figure 1 shown, which specifically includes the following steps:

[0035] Step a: Construct a single-phase grounding fault startup criterion, collect the zero-sequence voltage of the distribution network in real time, and calculate the fault current in real time. When the absolute value of the fault current at a certain moment is greater than the fault judgment threshold M, it indicates that a single-phase grounding fault has occurred in the distribution network, and step b is continued; otherwise, the distribution network is continuously monitored.

[0036] Step b: Measure the phase-to-ground voltages of each phase of the distribution network, and draw the volt-ampere characteristic curves of phases A, B, and C with the fault current as the abscissa and the three-phase-to-ground voltages as the ordinates respectively.

[0037] Step c: Determine the fault phase based on the quadrant where the volt-ampere characteristic curve is located.

[0038] Step d: Calculate the transition resistance R E (t) in real time according to the fault phase-to-ground voltage and the fault current. The specific expression is as follows:

[0039]

[0040] where, u δ (t) is the fault phase-to-ground voltage, δ = A, B, C, and i E (t) is the fault current.

[0041] The single-phase grounding fault startup criterion in step a can be specifically determined by the following steps:

[0042] Step a1: Collect the zero-sequence voltage u 0 (t) of the distribution network in real time, and substitute the zero-sequence voltage into the fault current calculation expression to solve the fault current i E (t) in real time. The fault current calculation expression is specifically as follows:

[0043]

[0044] where, G Σ and C Σ are the total ground conductance and total ground capacitance of the distribution network respectively, and ibd (t) is the total unbalanced current of the distribution network;

[0045] Step a2: Determine the fault judgment threshold M, take M = 0.1 A. If the absolute value of the fault current is less than the fault judgment threshold M at any time, it indicates that there is no single-phase grounding fault in the distribution network. If the absolute value of the fault current is greater than the fault judgment threshold M at a certain time, it indicates that there is a single-phase grounding fault in the distribution network, and the time corresponding to when the absolute value of the fault current is first greater than the fault judgment threshold is determined as the fault initial time t 0 .

[0046] The drawing of the volt-ampere characteristic curve in step b can be specifically determined by the following steps:

[0047] Step b1: Starting from the fault initial time t 0 , every other sampling period T, collect the line-to-ground voltages and fault currents of phases A, B, and C of the distribution network, and perform N samplings in total to obtain the three-phase line-to-ground voltage sequences U A (n), U B (n), U C (n) and the fault current sequence I(n), where n represents the nth sampling point;

[0048] Step b2: Using the fault current sequence as the abscissa and the three-phase line-to-ground voltage sequences as the ordinate, obtain three groups of coordinates (I(n), U A (n)), (I(n), U B (n)), (I(n), U C (n)). Place each group of coordinates in a plane rectangular coordinate system respectively, and connect each group of coordinates with a smooth curve. The obtained smooth curve is the three-phase volt-ampere characteristic curve.

[0049] The fault phase selection method in step c can be specifically determined by the following steps:

[0050] Step c1: Multiply the abscissa of each point in the volt-ampere characteristic curves of phases A, B, and C by its corresponding ordinate to obtain the instantaneous power P δ (n) corresponding to each point:

[0051] P δ (n) = I(n)U δ (n) Equation 3

[0052] where δ = A, B, C;

[0053] Step c2: Record the number of points with instantaneous power greater than 0 in the volt-ampere characteristic curves of phases A, B, and C respectively, denoted as Q A , Q B , Q C ;

[0054] Step c3: Compare Q A , Q B , Q C . Among them, the one with the largest quantity indicates that the volt-ampere characteristic curve of this phase is generally located in the first and third quadrants, and this phase is determined as the faulty phase.

[0055] Embodiment

[0056] Use MATLAB / Simulink simulation to verify the proposed real-time tracking of the transition resistance and fault phase selection method for single-phase grounding faults in the distribution network. The simulation model is a 10 kV medium-voltage distribution network with 3 feeders. The topological structure is as Figure 2 shown. The zero-sequence to ground parameters of each feeder are shown in Table 1. The capacitive current of the simulation model is 72.77 A, the active current is 1.316 A respectively, the asymmetry degree is 1.75%, and the damping rate is 1.809%.

[0057] Table 1 Zero-sequence to ground parameters of each feeder in the distribution network

[0058] Simulation analysis.

[0059] Assume that a high-resistance grounding fault of 3000 Ω occurs in phase A of feeder 1 at 0.13 s, and a Cassie arc model is connected in series with the grounding resistance. The time constant of the arc column is 0.255 ms, and the voltage gradient is 800 V. Let the sampling period T = 0.08 ms. According to the method of the present invention, the zero-sequence voltage of the distribution network is measured in real time, the fault current is calculated in real time and its absolute value is taken. As Figure 3 shown, it can be seen that before 0.13 s, the absolute value of the fault current is less than the fault starting criterion of 0.1, while at 0.13 s, the part exceeding the fault starting criterion appears for the first time. Therefore, it is judged that a single-phase grounding fault has occurred, and the fault time is determined to be 0.13 s. Then, measure the phase-to-ground voltages of phases A, B, and C of the distribution network, and combine with the fault current to draw the volt-ampere characteristic curves of phases A, B, and C. As Figure 4 shown, it can be seen from the characteristic curves that: the volt-ampere characteristic curve of phase A is mainly distributed in the first and third quadrants, while the volt-ampere characteristic curves of phases B and C are distributed in all four quadrants. It can be directly seen from the graph that the number of points on the volt-ampere characteristic curve of phase A located in the first and third quadrants is much larger than that of the other two phases, and the faulty phase can be directly judged as phase A. Finally, divide the phase-to-ground voltage of phase A by the fault current to obtain the real-time calculated value of the transition resistance. As Figure 5 shown, it can be seen that the transition resistance fluctuates with time, and the distribution network is in a state of alternating arc extinction and reignition.

[0060] The results show that the method proposed in the present invention is not affected by the asymmetry of the distribution network parameters and the random fluctuation of the transition resistance, and can accurately and quickly determine the fault phase, track the transition resistance in real time, and provide theoretical guidance for subsequent fault handling.

[0061] The working principle of the present invention.

[0062] Figure 2 Among them, e a , e b , e c are the electromotive forces of the three-phase power supply respectively, and C A , C B , C C are the sums of the distributed capacitances of each phase of all lines A, B, and C to the ground respectively, and G A , G B , G C are the sums of the distributed conductances of each phase of all lines A, B, and C to the ground respectively, and G E (t) is the random conductance of the single-phase grounding fault, and R E (t) is the transition resistance. When the switch K is closed, it means that a single-phase grounding fault occurs in the distribution network.

[0063] Taking the single-phase grounding fault of phase A in the distribution network as an example, after the single-phase grounding fault occurs, according to Kirchhoff's current law:

[0064]

[0065] Among them, And the sum of the three is denoted as the total unbalanced current i bd (t).

[0066] In Equation 4, after the system topology is determined, the unknown quantities are only the fault current and the zero-sequence voltage, and the zero-sequence voltage can be directly measured. Substituting the zero-sequence voltage into Equation 4 can obtain the instantaneous value of the fault current. In addition, the fault current also satisfies:

[0067]

[0068] The fault current is equal to the voltage of the fault phase to the ground divided by the transition resistance. However, both the voltage of the fault phase to the ground and the transition resistance in Equation 5 are unknown. Therefore, the premise of real-time tracking of the transition resistance is fault phase selection. Although the transition resistance is time-varying, all transition resistances in nature are positive. Therefore, the fault current and the voltage of the fault phase to the ground always maintain the same-sign relationship and have the same zero point. That is, when the fault current is used as the abscissa and the voltage of the fault phase to the ground is used as the ordinate, the plotted volt-ampere characteristic curve always falls in the origin, the first, and the third quadrants.

[0069] Assume e b (t)+u 0(t) and i E If (t) has the same sign as i, then e can be obtained b (t) + u 0 (t) and e a (t) + u 0 (t) has the same sign, that is, 2u 0 (t) - e c (t) is always non-positive or always non-negative. Contradicting with 2u 0 (t) - e c (t) is an alternating quantity. So the non-fault phases are not always in the same sign as the fault current. That is, the volt-ampere characteristic curve drawn by the voltage to ground of the non-fault phase and the fault current will be in the second and fourth quadrants. After drawing the three-phase volt-ampere characteristic curve, only the number of points on the curve in the first and third quadrants needs to be determined, and the one with the largest number is determined as the fault phase. After the fault phase is determined, only the transition resistance in Equation 5 is unknown. Dividing the voltage to ground of the fault phase by the fault current gives the instantaneous value of the transition resistance.

Claims

1. A method for real-time tracking of transition resistance and fault phase selection of single-phase grounding fault in distribution network, characterized in that: Calculate the fault current in real time, draw the volt-ampere characteristic curve between each phase-to-ground voltage and the fault current, determine the fault phase according to the distribution characteristics of the volt-ampere characteristic curve, and track the transition resistance in real time, including the following steps: Step a: construct a single-phase grounding fault start criterion, collect the zero-sequence voltage of the distribution network in real time, and calculate the fault current in real time. When the absolute value of the fault current at a certain moment is greater than the fault judgment threshold M, it indicates that a single-phase grounding fault has occurred in the distribution network, and continue with step b, otherwise continue to monitor the distribution network; Step b: Measure the phase-to-ground voltages of the distribution network, and draw the A, B, and C phase volt-ampere characteristic curves respectively, with the fault current as the horizontal coordinate and the three-phase-to-ground voltages as the vertical coordinate; Step c: determining the fault phase based on the quadrant of the volt-ampere characteristic curve; Step d: Calculate the transition resistance R in real time based on the fault phase-to-ground voltage and fault current E (t), the specific expression is: Among them, u δ (t) is the fault relative voltage to ground, δ=A,B,C,i E (t) is the fault current.

2. A method for real-time tracking of transition resistance and fault phase selection of single-phase grounding fault in distribution network according to claim 1, characterized in that: The single-phase ground fault starting criterion in step a can be specifically determined by the following steps: Step a1: Collect the zero-sequence voltage u0(t) of the distribution network in real time, substitute the zero-sequence voltage into the fault current calculation expression, and solve the fault current i in real time. E (t), the fault current calculation expression is as follows: Among them, G Σ and C Σ are the total ground conductance and total ground capacitance of the distribution network, respectively, bd (t) is the total unbalanced current of the distribution network; Step a2: Determine the fault judgment threshold M, take M = 0.1A. If the absolute value of the fault current at any time is less than the fault judgment threshold M, it indicates that there is no single-phase grounding fault in the distribution network. If the absolute value of the fault current at a certain moment is greater than the fault judgment threshold M, it indicates that a single-phase grounding fault has occurred in the distribution network, and the time corresponding to the first time the absolute value of the fault current is greater than the fault judgment threshold is determined as the initial time t0 of the fault.

3. A method for real-time tracking of transition resistance and fault phase selection of a single-phase grounding fault in a distribution network according to claim 1, characterized in that: The volt-ampere characteristic curve drawing in step b can be specifically determined by the following steps: Step b1: Starting from the initial fault time t0, the phase-to-ground voltage and fault current of distribution network A, B, and C are collected every sampling period T, and a total of N samplings are performed to obtain the three-phase-to-ground voltage sequence U A (n), U B (n), U C (n) and fault current sequence I(n), where n represents the nth sampling point; Step b2: Take the fault current sequence as the horizontal coordinate and the three-phase voltage sequence as the vertical coordinate to obtain three sets of coordinates (I(n), U A (n))、(I(n),U B (n))、(I(n),U C (n)), place each set of coordinates in a plane rectangular coordinate system, and connect each set of coordinates with a smooth curve. The resulting smooth curve is the three-phase volt-ampere characteristic curve.

4. A method for real-time tracking of transition resistance and fault phase selection of a single-phase grounding fault in a distribution network according to claim 1, characterized in that: The fault phase selection method in step c can be specifically determined by the following steps: Step c1: Multiply the horizontal coordinate of each point in the A, B, and C phase volt-ampere characteristic curves by its corresponding vertical coordinate to obtain the instantaneous power P corresponding to each point. δ (n): P δ (n)=I(n)U δ (n) Formula 3 Where, δ = A, B, C; Step c2: Record the number of points where the instantaneous power is greater than 0 in the A, B, and C phase volt-ampere characteristic curves, and record them as Q A , Q B , Q C ; Step c3: Compare Q A , Q B , Q C , among which the largest number indicates that the volt-ampere characteristic curve of this phase is generally located in the first and third quadrants, and the phase is determined as the fault phase.

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