Three-phase short-circuit fault nature identification method based on injection signal

Abstract

The application discloses a three-phase short-circuit fault property identification method based on an injection signal. When a short-circuit fault occurs in a distribution line, a line protection device completes fault removal and fault selection, and sends protection action information and a selection signal to a distributed power supply. After receiving the signal of the line protection device, the distributed power supply injects a tentative signal to the distribution line. The three-phase voltage and three-phase current at the high-voltage side of a grid-connected transformer are continuously detected. The average value of three-phase equivalent resistance is calculated. When the average value of the three-phase equivalent resistance is greater than a set value, the short-circuit fault is determined as a transient fault, otherwise, the short-circuit fault is determined as a permanent fault. The application does not need to increase an additional injection device, is more economical, has high criterion accuracy and better practicability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of short-circuit fault of distribution network, and particularly relates to a method for identifying three-phase short-circuit fault of distribution line by using distributed power injection signal. BACKGROUND

[0002] With the continuous development of China's national economy and the continuous improvement of people's material living standards, the demand for electric power energy is increasing year by year in the current social development, and at the same time, the operation reliability of China's distribution network needs to be further improved. In the three-phase alternating current distribution network, the neutral point grounding mode of China's distribution network is mostly neutral point not grounded, or neutral point grounded through high resistance, or grounded through arc suppression coil. In the distribution network, the most common fault mode is short-circuit fault, which is divided into transient short-circuit fault and permanent short-circuit fault according to the nature. In order to ensure the safe power supply and stable operation of the power system, the traditional automatic reclosing device usually trips the circuit breaker after the fault occurs, and then tries to reclose the circuit breaker after a set time. If the fault is not accurately judged as transient fault or permanent fault, when it recloses on a permanent fault, it will cause greater harm to the power system and electrical equipment. Therefore, if the type of line fault can be correctly distinguished after the circuit breaker trips to determine whether the circuit breaker will act again, it has great significance to ensure the continuity of power supply, reduce the impact of power system accidents, and reduce the loss of fault.

[0003] The existing power distribution line fault property identification technology is mainly divided into active detection and passive detection. For example, Chinese patent application CN202111240161.4 discloses a power distribution line grounding fault property identification and recovery method. After a grounding fault occurs in the power distribution line, a certain phase is reclosed before the power distribution line is energized; the voltage and current waveforms of the reclosed phase are collected and filtered; the derivatives of the voltage and current are calculated; a least squares method is used to identify the capacitance model, and the capacitance in the least squares sense is solved; the model error E(k) of the capacitance model is calculated; a least squares method is used to identify the resistance and inductance model, and the resistance and inductance in the least squares sense are solved; the model error E(k) of the resistance and inductance model is calculated; when E(k) < kE(k), it is a permanent grounding fault, otherwise it is a transient grounding fault. This scheme mainly uses the electrical quantity characteristics of the power distribution line after tripping, without the need for additional equipment to inject signals into the power distribution line, but the available electrical quantity characteristics maintain for a very short time and are unstable, so in the case where the electrical quantity characteristics change is not obvious, the monitoring and judgment effect is usually not good. For example, Chinese patent application CN201420484526.7 discloses a power distribution line phase-to-phase short circuit fault property identification device, which is installed at each switch of the power distribution line and injects a constant frequency small signal into the line sections on both sides of the switch; the full voltage and current signals after the three-phase circuit fault are detected, the signals collected by the injection signal detection circuit are processed and calculated by the DSP, the phase-to-phase short circuit fault property is identified by the ARM, the output of the constant frequency signal injection power supply is controlled, and communication with the line switch controller is completed. However, this scheme has the following problems: the high voltage generated by injection can threaten personal safety, the size of the load is unknown, the output voltage cannot be stabilized, and energy storage equipment is also needed to provide energy for the system, which is high in cost. SUMMARY

[0004] To solve the above problems existing in the prior art, accurately judge the three-phase short circuit fault property of the power distribution line, and prevent the reclosing of the circuit breaker on the permanent fault to cause electrical equipment failure, thereby causing a large range of power outage, the present application discloses a three-phase short circuit fault property identification method based on injection signal.

[0005] To achieve the above purpose, the present application adopts the following technical scheme:

[0006] A three-phase short circuit fault property identification method based on injection signal, characterized in that the three-phase short circuit fault property identification method comprises the following steps:

[0007] (1) When a three-phase short circuit fault occurs in the power distribution line, the line protection device completes fault removal and fault selection, and sends the protection action information and the selected phase signal to the distributed power supply controller;

[0008] (2) After receiving the protection action information and the phase selection signal sent by the line protection device, the distributed power controller changes the grid-connected transformer operating mode after a delay of Δt1, and injects an AC probing signal with a frequency that is not a power frequency or an integer multiple of the power frequency into the distribution line through the grid-connected inverter;

[0009] (3) After the distributed power source injects the AC probing signal, the three-phase voltages U A (ω f ), U B (ω f ), U C (ω f ) and the three-phase currents I A (ω f ), I B (ω f ), I C (ω f ) on the high-voltage side of the grid-connected transformer are continuously detected;

[0010] (4) After the three-phase voltages U A (ω f ), U B (ω f ), U C (ω f ) and the three-phase currents I A (ω f ), I B (ω f ), I C (ω f ) on the high-voltage side of the grid-connected transformer enter a stable stage, i.e., the fluctuations of the amplitudes relative to the center line are less than a preset fluctuation threshold, the equivalent impedance Z(ω f ) of each phase is continuously calculated.

[0011] For phase A:

[0012] For phase B:

[0013] For phase C:

[0014] (5) After the equivalent impedance Z(ω f ) of each phase enters a stable stage, i.e., the fluctuations of the amplitudes relative to the center line are less than a preset fluctuation threshold, the real part Re(Z(ω f )) of the calculated equivalent impedance Z(ω f ) of each phase is taken to obtain the equivalent resistance R a , R b , R c of each phase, and the average value is calculated. If R eq > Rset If R eq <R set permanent fault is determined to occur on the distribution line.

[0015] (6) The distributed power supply controller transmits the fault property identification result to the line protection device, and when the fault property is determined to be transient, the circuit breaker is started to reclose after a set reclosing time is reached, and when the fault property is determined to be permanent, the circuit breaker reclosing instruction is blocked.

[0016] The application further comprises the following preferred schemes:

[0017] In step (2), the time delay Δt1 is in the range of (0.5-1s).

[0018] The frequency ω of the injected test signal f is selected in the range of 100Hz-500Hz.

[0019] In step (2), the duration of the injected signal is controlled in the range of 50ms-100ms.

[0020] In step (5), the preset fluctuation threshold is selected as 3%.

[0021] In step (5), the fault property discrimination resistance set value R set is calculated by multiplying the unit length resistance of the distribution line by the line length.

[0022] The application has the following beneficial technical effects:

[0023] The application fully utilizes the flexible control and fast response speed of the distributed power supply inverter, and can solve the source problem of the injected signal without increasing additional power electronic equipment; secondly, for three-phase short-circuit faults, the criterion of the application is simple to calculate, easy to implement and high in accuracy. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 is the structural block diagram of the application for realizing three-phase short-circuit fault identification by using the existing distributed power supply.

[0025] Figure 2 is the flowchart of the three-phase short-circuit fault property identification method based on the injected signal of the application.

[0026] Figure 3 is the 10kV distribution network simulation model diagram.

[0027] Figure 4 is the identification calculation result of the equivalent resistance of the three-phase short-circuit permanent fault at the end of the overhead line.

[0028] Figure 5 is the equivalent resistance identification calculation result of the three-phase short-circuit transient fault at the end of the overhead line.

[0029] Figure 6 is the equivalent resistance identification calculation result of the three-phase short-circuit permanent fault at the end of the cable line.

[0030] Figure 7 is the equivalent resistance identification calculation result of the three-phase short-circuit transient fault at the end of the cable line. DETAILED DESCRIPTION

[0031] In order for those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor should belong to the scope of protection of the present application.

[0032] With the vigorous development of the "distributed photovoltaic whole county promotion" plan in China, the high penetration of distributed photovoltaic power supply in the distribution network will challenge the traditional distribution network protection and fault handling mode.

[0033] The fault property identification of the distribution line has been a big problem in the field of distribution network protection for a long time. The effective identification of the fault property on the distribution line with high penetration of distributed power supply is of great significance to improve the safety of reclosing and ensure the safety of equipment and power supply reliability. At the same time, with the wide access of distributed power supply in the distribution network, the existing distributed power supply provides the possibility for the identification of three-phase short-circuit fault.

[0034] As shown in the accompanying Figure 1 , the distributed power supply is connected to the distribution line through a step-up transformer. In the present application, the step-up transformer, the inverter step-up transformer and the grid-connected transformer have the same meaning. The distributed power supply controller in the present application is also called the inverter controller, which is used to control the working mode of the step-up transformer and the distributed power supply, and adjust the output voltage and power signal of the distributed power supply.

[0035] As shown in the accompanying Figure 2 , the present application is a method for identifying the fault property of the distribution line by using the injection signal of the distributed power supply. The present application comprises the following steps:

[0036] (1) When a short-circuit fault occurs in the distribution line, the line protection device completes fault removal and fault selection, and sends the protection action information and the selection signal to the distributed power supply through the 5G network.

[0037] (2) After receiving the signal from the line protection device, the distributed power source delays by Δt1 (0.5~1s), then changes the inverter operating mode and injects a test signal into the distribution line. The frequency of the injected test signal is ω. f Select a frequency range of 100Hz to 500Hz, and ensure that it is not an integer multiple of the power system frequency of 50Hz. The duration of the injected signal should be controlled between 50ms and 100ms.

[0038] (3) After the distributed power source enters the injection test signal mode, the three-phase voltage U on the high-voltage side of the grid-connected transformer is continuously monitored. A (ω f ), U B (ω f ), U C (ω f ) and three-phase current I A (ω f ), I B (ω f ), I C (ω f ).

[0039] (4) The three-phase voltage U on the high-voltage side of the grid-connected transformer A (ω f ), U B (ω f ), U C (ω f ) and three-phase current I A (ω f ), I B (ω f ), I C (ω f Once the amplitude fluctuation relative to the center line reaches a stable phase, i.e., after it is less than the preset fluctuation threshold, the equivalent impedance Z(ω) of each phase continues to be measured. f )calculate.

[0040] For phase A:

[0041] For phase B:

[0042] For phase C:

[0043] This invention is not applicable to two-phase short circuits or single-phase grounding faults.

[0044] (5) In each phase equivalent impedance Z(ω) f After the value enters a stable phase, i.e., the fluctuation of the amplitude relative to the center line is less than the preset fluctuation threshold, the calculated equivalent impedance Z(ω) of each phase is then... f Take the real part Re(Z(ω)f )) to obtain the equivalent resistance R a , R b , R c , and calculate the average value If R eq >R set , it is determined that the fault on the distribution line is transient; if R eq <R set , it is determined that the fault on the distribution line is permanent. R set is calculated by multiplying the unit length resistance of the distribution line (determined according to the line cross-sectional area and the type of conductor, which can be obtained by looking up the table) by the length of the line. In the preferred embodiment of the present application, the preset fluctuation threshold is that the fluctuation of Z(ω f ) is less than 3%.

[0045] (6) The distributed power supply returns the fault property identification result to the line protection device through the 5G network, and the line protection device performs reclosing or blocking reclosing action according to the fault property identification result.

[0046] In order to ensure the implementation effect of the present application, for the selection of the distributed photovoltaic power supply for injecting the signal, a distributed photovoltaic power supply with a capacity of 500kW or more and directly connected to the grid through an inverter booster transformer should be selected, and a distributed roof photovoltaic connected to a low-voltage distribution line should not be used (note: the installation position of the distributed roof photovoltaic is usually located at the end of the low-voltage distribution line, and the electrical distance from the 10kV distribution line is far, and the shunt effect of the low-voltage line impedance and the low-voltage load will adversely affect the effectiveness of the injected signal), in order to avoid the adverse effects of the low-voltage load and the line impedance on the fault property identification.

[0047] Figure 3 The 10kV distribution network simulation model is shown, and the distribution network system parameters are shown in Table 1.

[0048] Table 1 Distribution network system parameters

[0049]

[0050] In Table 1, P k represents the short-circuit loss, U k represents the short-circuit voltage, P o represents the no-load loss, I o represents the no-load voltage, and Dyn11 represents the connection mode of the distribution transformer.

[0051] The system occurs three-phase short-circuit fault at 0s, the fault point is at the end of the line, the line protection will cut off the fault line, assuming that the distributed photovoltaic power installed at the beginning of the line implements the injection signal, at 0.06s, the distributed power enters the injection signal mode, at 0.08s, the stable output is a three-phase symmetrical voltage signal with a peak value of 100V and a frequency of 230Hz.

[0052] Figure 4 The equivalent resistance parameter identification results of overhead line L1 in permanent fault are given. After a short transient process, the injected voltage and current signals are quickly stabilized. In the steady state, the effective values of the three-phase voltages U A , U B , U C output by the fault identification device are about 21.61V, the effective values of the three-phase fault phase currents I A , I B , I C are about 1.47A, and the average value of the positive sequence resistances of the three phases A, B and C identified after stabilization R eq is 2.382Ω. At the same time, the positive sequence resistance value under the condition of three-phase short-circuit fault converted from the actual distribution line parameters should be the sum of half of the line impedance 0.138*10 and the fault transition resistance R f , which is 2.380Ω. It can be seen that the error between the calculated value and the actual value is 0.002Ω, and the error percentage is 0.084%.

[0053] Figure 5 The resistance parameter identification results of overhead line L1 in transient fault are given. In the steady state, the effective values of the three-phase voltages U A , U B , U C output by the fault identification device are about 811.88V, the effective values of the three-phase currents IA, IB and IC are about 0.83A. The average value of the positive sequence resistances of the three phases A, B and C identified after stabilization R eq is 69.18Ω. The positive sequence resistance value converted from the actual distribution line parameters should be the sum of the line resistance (0.138*10)Ω, the load resistance 0.1053*(10 / 0.4) 2 Ω and the equivalent resistance of the distribution transformer 2.7563Ω, which is 69.7Ω. It can be seen that the error between the calculated value and the actual value is 0.52Ω, and the error percentage is 0.75%.

[0054] Similarly, permanent and transient faults are set on cable line L3 respectively, and the positive sequence resistance values calculated by using the voltage and current data injected by the distributed power on the distribution line impedance parameters are shown in Figure 6 and Figure 7 . The average value of the positive sequence resistances of the three phases A, B and C identified after stabilization R eq1.396Ω and 6861Ω. The positive sequence resistance value under the three-phase short-circuit fault condition of A, B, C should be the sum of the line impedance (0.079*5)Ω and the fault transition resistance R f The sum of half of the values is 1.395Ω. It can be seen that the error between the calculated value and the actual value is 0.001Ω, and the error percentage is 0.072%. The loop positive sequence resistance value under the transient fault condition converted from the actual distribution line parameters should be the sum of (0.079*5)Ω, the load resistance 0.1053*(10 / 0.4) 2 Ω and the equivalent resistance of the distribution transformer 2.5Ω, which is 68.71Ω. It can be seen that the error between the calculated value and the actual value is 0.1Ω, and the error percentage is 1.75%.

[0055] The above analysis can prove that, based on the distributed power injection characteristic frequency signal condition, the positive sequence resistance value under the three-phase short-circuit fault condition and the non-fault condition of the distribution line can be accurately obtained according to the voltage and current information of the detected distributed power output, and then the fault type is judged, so as to reflect the operation state of the line.

[0056] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, but not to limit it. Although the present application has been described in detail with reference to the above examples, those skilled in the art should understand that the specific embodiments of the present application can be modified or replaced by the same, without departing from the spirit and scope of the present application. Any modification or equivalent replacement, which does not depart from the spirit and scope of the present application, should be covered within the protection scope of the claims of the present application.

Claims

1. A method for identifying the nature of a three-phase short-circuit fault based on an injected signal, characterized in that, The three-phase short-circuit fault property identification method comprises the following steps: (1) When a three-phase short-circuit fault occurs in a distribution line, a line protection device completes fault removal and fault phase selection, and sends protection action information and a phase selection signal to a distributed power supply controller; (2) After receiving the protection action information and the phase selection signal sent by the line protection device, the distributed power supply controller changes the working mode of a grid-connected transformer after a delay of Δt1, and injects a three-phase symmetrical AC trial signal with a frequency of non-power frequency and an integer multiple of non-power frequency into the distribution line through the grid-connected transformer; (3) After the distributed power enters the AC injection signal, the three-phase voltage U A (ω f ), U B (ω f ), U C (ω f ) and the three-phase current I A (ω f ), I B (ω f ), I C (ω f ) on the high voltage side of the grid-connected transformer are continuously detected; (4) The three-phase voltage U A (ω f ), U B (ω f ), U C (ω f ) and the three-phase current I A (ω f ), I B (ω f ), I C (ω f ) enter the stable stage, that is, the amplitude fluctuation relative to the center line is less than the preset fluctuation threshold, and the equivalent impedance Z(ω f ) of each phase is continuously calculated. For phase A: For phase B: For phase C: (5) the phase equivalent impedance Z(ω f ) values enter a stable phase, i.e. the fluctuation of the amplitude relative to the center line is less than a preset fluctuation threshold, then the real part Re(Z(ω f )) of each phase equivalent impedance Z(ω f ) calculated is taken to obtain the phase equivalent resistance R a , R b , R c , and the average value If R eq > R set , it is determined that a transient fault occurs on the distribution line; if R eq < R set , it is determined that a permanent fault occurs on the distribution line. (6) The distributed power supply controller transmits the fault property identification result to the line protection device, and when the fault property is judged to be a transient fault, starts a circuit breaker to reclose after a set reclosing time is reached, and when the fault property is judged to be a permanent fault, locks the circuit breaker reclosing instruction.

2. The three-phase short-circuit fault property identification method based on an injected signal according to claim 1, characterized in that: In step (2), the delay Δt1 is in the range of 0.5-1 s.

3. The three-phase short-circuit fault property identification method based on an injected signal according to claim 1, characterized in that: The frequency ω of the injected probe signal f The selection is in the range 100 Hz - 500 Hz.

4. The three-phase short-circuit fault property identification method based on an injected signal according to claim 1 or 3, characterized in that: In step (2), the duration of the injected signal is controlled to be in the range of 50-100 ms.

5. The three-phase short-circuit fault property identification method based on an injected signal according to claim 1, characterized in that: In step (5), the preset fluctuation threshold is selected to be 3%.

6. The three-phase short-circuit fault property identification method based on an injected signal according to claim 1 or 5, characterized in that: In step (5), the failure property discrimination resistance set value R set It is calculated by multiplying the resistance per unit length of the distribution line by the length of the line.

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