A power distribution network fault-free discrimination method using a photovoltaic power supply to inject a disturbance signal

Abstract

The application provides a power distribution network fault-free discrimination method using a photovoltaic power supply to inject a disturbance signal, and belongs to the technical field of power distribution network relay protection; solves the problem of blind reclosing after phase-to-phase fault of a power distribution network containing a photovoltaic power supply in the prior art; when a fault occurs in the power distribution network, the system side circuit breaker trips three-phase, the photovoltaic grid-connected inverter stops working after the anti-islanding protection acts, the arc at the fault point is extinguished after a fixed delay, and the photovoltaic grid-connected inverter is restarted for additional control. The control strategy adopts a constant voltage / constant frequency double-loop control strategy, through voltage amplitude and frequency adjustment, a constant three-phase different-frequency voltage disturbance signal is output; then the Pearson correlation coefficient values between the three-phase voltages at the line head after phase shifting are calculated, a criterion is constructed, and fault-free discrimination before reclosing is performed; the method can improve the line power supply reliability of the power distribution network containing the photovoltaic power supply.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power distribution network relay protection, and particularly relates to a power distribution network fault-free discrimination method using a photovoltaic power injection disturbance signal. BACKGROUND

[0002] In recent years, photovoltaic power generation has been rapidly developed due to its many advantages such as clean and pollution-free and high efficiency and sustainability. After the photovoltaic power is connected to the grid, the power distribution network changes from a passive network to an active network, and the output of the photovoltaic power has strong uncertainty and volatility, which makes the operation of the power distribution network more complex and is not conducive to the safe and reliable operation of the power system. The first end of the line of the power distribution network is usually equipped with an automatic reclosing device, but the device has a simple logic and recloses after a fixed delay after a fault occurs in the line, which has the risk of blind reclosing and permanent fault and can cause a secondary impact on the system. Therefore, it is of great significance to study the adaptive reclosing technology of the permanent fault recognition capability of the power distribution network containing photovoltaic power.

[0003] At present, the research direction of domestic and foreign scholars is concentrated on the adaptive reclosing technology of high-voltage transmission lines, and the adaptive reclosing research for power distribution networks is relatively less. The methods in the existing literature can be divided into passive detection and active disturbance. Passive detection does not need to increase additional energy injection devices, and usually uses the difference between the line capacitance value and the actual capacitance value to realize fault property discrimination, but this method has a large amount of calculation and is easily affected by the fault location and transition resistance because it relies on the short-time electrical quantity for fault property recognition. The active disturbance method can use the high controllability of power electronic equipment to add an inverter power supply to the distribution transformer, use the relationship between the line wave impedance and the frequency to identify the fault line, or inject a voltage signal through the secondary side of the voltage transformer, and analyze the power distribution line according to the characteristics of the primary side port of the voltage transformer. The characteristic electrical quantity used for detection is actively sent by the power electronic equipment and is not affected by the electrical quantity decay speed. SUMMARY

[0004] In order to solve the problem of blind reclosing of the automatic reclosing device after the phase-to-phase fault of the power distribution network containing photovoltaic power in the prior art, the present application provides a power distribution network fault-free discrimination method using a photovoltaic power injection disturbance signal.

[0005] The technical scheme adopted by the present application is as follows: a power distribution network fault-free discrimination method using a photovoltaic power injection disturbance signal, comprising the following steps:

[0006] Step 1: when a fault occurs in the power distribution line, the outlet circuit breaker trips, and after the anti-islanding protection acts, the photovoltaic grid-connected inverter is restarted after a fixed delay and the additional control strategy is switched, and a three-phase frequency-varying voltage disturbance signal is injected into the power distribution line;

[0007] Step 2: Collect three-phase voltage at the outlet circuit breaker;

[0008] Step 3: Process three-phase voltage data to obtain phase-shifted line head three-phase voltage 、 、 , 、 、 , 、 、 , , ;

[0009] Step 4: If 、 satisfies criterion I and reaches the maximum allowed discrimination time, it is determined that a transient fault occurs, and the reclosing is started; if it does not satisfy criterion I, it is determined that a permanent fault occurs, and the reclosing is blocked; if the maximum allowed discrimination time is not reached, step 5 is performed;

[0010] wherein the expression of criterion I is as follows:

[0011] ;

[0012] In the formula: is the change coefficient of the maximum and minimum values of the phase-phase voltage Pearson correlation coefficient; is the threshold value of the criterion;

[0013] Step 5: After a fixed delay, return to step 4 for execution, and cycle the determination until the maximum allowed discrimination time is reached, and the determination is ended.

[0014] Further, after the island protection action in step 1, the photovoltaic grid-connected inverter is restarted and the additional control strategy is switched after a delay of 0.5s-1s to avoid the arc extinction time of the fault point.

[0015] Further, the additional control strategy adopts a constant voltage / constant frequency double-loop control strategy.

[0016] Further, the processing of three-phase voltage data in step 3 includes: respectively delaying the time window length of B-phase and C-phase voltage by 、 , is the disturbance voltage period length, to obtain the phase-shifted line head three-phase voltage 、 、 .

[0017] Further, the three-phase different-frequency voltage disturbance signal injected by the photovoltaic grid-connected inverter into the power distribution line after restarting in step 1 is:

[0018] The voltage disturbance signal has a voltage amplitude of 35-150V and a constant frequency of 75Hz-150Hz, and the injection duration of the voltage disturbance signal is 350ms-400ms.

[0019] Further, the frequency of the injected three-phase different-frequency voltage disturbance signal is selected as an inter-harmonic.

[0020] Further, the voltage amplitude of the injected three-phase different-frequency voltage disturbance signal meets the minimum working voltage of the detection accuracy in the power distribution network.

[0021] Further, the maximum error of 10% is taken as the setting threshold value, that is, .

[0022] The application has the beneficial effects relative to the prior art: after the fault system side breaker trips in the power distribution network, the photovoltaic grid-connected inverter is used for additional control, the disturbance signal is actively injected, the Pearson correlation coefficient value between the two-phase voltages of the line head after phase shifting is calculated, and the criterion is constructed for fault-free discrimination before reclosing. Based on the flexibility and high controllability of the photovoltaic grid-connected inverter, no additional device needs to be added, and the investment cost is small. For the power distribution network line containing photovoltaic power supply, the method of the application can improve the power supply reliability, and has certain application value. BRIEF DESCRIPTION OF DRAWINGS

[0023] The application will be further described below in combination with the drawings:

[0024] Figure 1 is a work flow chart of a fault-free discrimination method for a power distribution network using a photovoltaic power supply to inject a disturbance signal provided by an embodiment of the application;

[0025] Figure 2 is a timing diagram of a fault-free discrimination method for a power distribution network using a photovoltaic power supply to inject a disturbance signal provided by an embodiment of the application;

[0026] Figure 3 is a schematic diagram of a disturbance signal injected into a power distribution network in a fault-free discrimination method for a power distribution network using a photovoltaic power supply to inject a disturbance signal provided by an embodiment of the application;

[0027] Figure 4 is an additional control strategy diagram of a disturbance signal injected in a fault-free discrimination method for a power distribution network using a photovoltaic power supply to inject a disturbance signal provided by an embodiment of the application;

[0028] Figure 5Is a kind of three-phase voltage waveform diagram of line head before and after phase shift in the power distribution network fault-free discrimination method provided by the application using photovoltaic power injection disturbance signal;

[0029] Figure 6 Is a kind of single-phase ground fault composite sequence network structure diagram in the power distribution network fault-free discrimination method provided by the application using photovoltaic power injection disturbance signal;

[0030] Figure 7 Is a kind of positive sequence network structure diagram in the power distribution network fault-free discrimination method provided by the application using photovoltaic power injection disturbance signal. DETAILED DESCRIPTION

[0031] As Figures 1 to 7 shown, the application provides a kind of power distribution network fault-free discrimination method using photovoltaic power injection disturbance signal, comprising the following steps:

[0032] Step 1: when the power distribution line fails, the three-phase breaker trips, after the action of anti-islanding protection, photovoltaic grid-connected inverter is restarted after fixed time delay and additional control strategy is switched, three-phase frequency voltage disturbance signal is injected to the power distribution line;

[0033] Step 2: three-phase voltage 、 、 at the outlet breaker is collected

[0034] Step 3: three-phase voltage data is processed: the time window length of B phase and C phase voltage is respectively advanced 、 , is the length of disturbance voltage period, and the three-phase voltage 、 、 of line head after phase shift is obtained. Pearson correlation coefficient value 、 、 between each other is calculated 、 、 , and the maximum value , minimum value is obtained

[0035] Step 4: if 、 meet criterion I and reach maximum allowed discrimination time, it is determined that transient fault occurs, and reclosing is started;If criterion I is not met, it is determined that permanent fault occurs, and reclosing is blocked;If maximum allowed discrimination time is not reached, step 5 is carried out

[0036] Step 5: After a fixed delay, return to Step 4 for execution, and make a loop judgment. Until the maximum allowed discrimination time is reached, the judgment ends.

[0037] The step 1 includes the following specific implementation process:

[0038] Step 1.1: In order to avoid the influence of line residual voltage on the judgment result, delay 0.5s-1s to avoid the arc extinction time of the fault point, and then use a three-phase different frequency voltage disturbance signal for fault discrimination.

[0039] Step 1.2: The additional control strategy adopts a constant voltage / constant frequency (V / F) double-loop control strategy.

[0040] The step 4 includes the following specific implementation process:

[0041] Step 4.1: Calculate the Pearson correlation coefficient value between the line head voltages after phase shifting. When a transient fault occurs, after the fault disappears, because the waveforms of the three-phase voltages are consistent, theoretically .

[0042] Step 4.2: Use , to construct a fault-free identification criterion I. If criterion I is met within the maximum allowed discrimination time, it is determined to be a transient fault, and the reclosing is started; if criterion I is not met, it is determined to be a permanent fault, and the reclosing is blocked.

[0043] The expression of criterion I is as follows:

[0044] (1);

[0045] In the formula: is the change coefficient of the maximum and minimum values of the phase-phase voltage Pearson correlation coefficient; is the threshold value of the criterion, and the maximum error is taken as 10% as the setting threshold value, that is .

[0046] According to the above method, in step 1, the photovoltaic power supply finally outputs a three-phase different frequency voltage disturbance signal with an amplitude of 35-150V and a frequency of 75Hz-150Hz, and the signal injection duration is 350ms-400ms.

[0047] The application will be further described below according to specific embodiments.

[0048] As shown in Figure 1 , the power distribution network fault-free discrimination method using a photovoltaic power supply to inject a disturbance signal proposed by the application includes the following specific steps:

[0049] Step 1: The power distribution line fails, the outlet circuit breaker trips three-phase, after the anti-islanding protection action, in order to avoid the residual power of the line affecting the judgment result, delay 0.5s-1s to avoid the fault point arc extinction time, switch the additional control strategy, inject three-phase frequency voltage disturbance signal to the power distribution line, the working timing of the process is shown in Figure 2 .

[0050] Among them, the additional control adopts the V / F double-loop control strategy shown in Figure 4 , mainly considering the selection of the frequency, amplitude and injection time of the three-phase frequency voltage disturbance signal. The frequency of the three-phase frequency voltage disturbance signal should not be too high, and too high frequency will interfere with the power carrier communication. The narrowband range of power communication carrier communication in China is 3kHz-500kHz, and the broadband range is 1MHz-30MHz. The switching frequency of photovoltaic grid-connected inverter is usually 10kHz-20kHz. If the frequency of the three-phase frequency voltage disturbance signal is too high, more than half of the switching frequency, then the mixed signal will be generated. At the same time, the frequency of the three-phase frequency voltage disturbance signal should be selected as the inter-harmonic, which can distinguish it from other interference waves and ensure that the injected voltage disturbance signal is not covered.

[0051] The amplitude of the three-phase frequency voltage disturbance signal is mainly selected according to the detection accuracy of the measuring equipment and the overcurrent capacity of the power electronic equipment. For example, in a 10kV distribution network, a voltage transformer with a detection accuracy of 0.5 level is configured (i.e. the maximum allowable error under rated conditions: voltage amplitude error (ratio difference) ≤±0.5%; phase error (angle difference) ≤±20 minutes (about 0.33°)), the minimum working voltage of the phase voltage to meet the detection accuracy is 2% of the rated primary voltage, i.e. ; At the same time, the current flowing through the converter should not be higher than 1.5 times the maximum allowable overcurrent capacity of the rated current, so the short-circuit voltage percentage of the photovoltaic power (PV) side step-up transformer under the injection of three-phase frequency voltage disturbance signal is:

[0052] (2);

[0053] In the formula: is the frequency of the disturbance voltage signal; is the rated frequency; is the short-circuit voltage percentage.

[0054] Therefore, the amplitude selection range of the three-phase frequency voltage disturbance signal is:

[0055] (3);

[0056] In the formula: is the ratio of the line first end voltage to the injected three-phase frequency voltage disturbance signal; ​For the primary side rated voltage; For the voltage amplitude of fault identification using photovoltaic power supply.

[0057] The photovoltaic power output constant amplitude is 45V, and the frequency is 125Hz three-phase different frequency voltage disturbance signal.

[0058] Step 2: Collect the characteristic quantity line head outlet circuit breaker three-phase voltage after injecting three-phase different frequency voltage disturbance signal for 200-300ms 、 、 ; Process the three-phase voltage data, respectively, phase shift 120°, 240° for B phase and C phase voltage, get the phase shifted line head three-phase voltage 、 、 . It can be seen from Figure 5 that the three-phase voltage waveform at the head end still has a large difference when the fault occurs, and the three-phase voltage waveform is similar after the fault disappears.

[0059] Step 3: Pearson correlation coefficient is shown in formula (4), which is a way to measure waveform similarity.

[0060] (4);

[0061] In the formula: 、 is the different phase head voltage, wherein , ; 、 is the average of different phase head voltage; 、 is the standard deviation of variable 、 ; is the sample size; is the Pearson correlation coefficient between two phase voltages, which represents the quotient of the covariance and standard deviation of different phase voltages, the value range is [-1, 1], the greater, the higher the correlation between and . Pearson correlation coefficient can be used to reflect the difference between the waveforms of the line head voltage of different fault types, and construct fault identification criterion.

[0062] The Pearson correlation coefficient values 、 、 between each other are calculated in turn 、 、 , and the maximum value and the minimum value ​.

[0063] Step 4: using , , to construct a fault-free identification criterion I. If criterion I is met within the maximum allowed discrimination time, it is determined that the fault is transient, and the recloser is started; if criterion I is not met, it is determined that the fault is permanent, and the recloser is blocked; if the maximum allowed discrimination time is not reached, step 5 is performed;

[0064] Step 5: after a fixed delay, return to step 4 for execution and cyclic determination. Until the maximum allowed discrimination time is reached, the determination is ended.

[0065] The principle of the present application is that when a fault occurs in the power distribution network, the system-side circuit breaker trips three-phase, the anti-islanding protection acts, and the photovoltaic grid-connected inverter stops working. After a fixed delay, the arc at the fault point is extinguished, and the photovoltaic grid-connected inverter is restarted for additional control. The additional control strategy adopts a constant voltage / constant frequency double-loop control strategy, which adjusts the voltage amplitude and frequency of the photovoltaic power source to output a constant frequency three-phase voltage disturbance signal.

[0066] According to the equivalent sequence network as shown in Figure 6 , the distance of the fault point from the system-side circuit breaker accounts for the proportion of the line length from the PCC (Point of Common Coupling) to the system-side circuit breaker , is the disturbance voltage injected by the photovoltaic power source; , are the positive sequence and negative sequence voltages at the fault point, respectively; , are the positive sequence and negative sequence currents at the fault point, respectively; , are the positive sequence and negative sequence impedances from the PCC to the system-side circuit breaker, respectively; , are the positive sequence and negative sequence impedances of the line from the PCC to the load, respectively; , are the line load equivalent impedances, respectively.

[0067] Taking a two-phase interphase fault in the BC phase as an example, combined with the two-phase interphase short-circuit fault boundary conditions , , and the zero-sequence current , the sequence components of the fault point current are:

[0068] (5);

[0069] In the formula: is the sum of the positive sequence impedances; is the sum of the negative sequence impedances; is the transition resistance; is the impedance of the injected disturbance voltage source and the impedance of the photovoltaic boost transformer;

[0070] The fault point voltage sequence component is:

[0071] (6);

[0072] The head voltage measured at the system side circuit breaker is determined by the voltage at the fault point as:

[0073] (7);

[0074] If the system side circuit breaker trips due to transient fault and the fault is eliminated before reclosing, the positive sequence network without fault after the fault is eliminated can be obtained as shown in Figure 7 The current provided by the photovoltaic power supply is:

[0075] (8);

[0076] The head voltage measured at the system side circuit breaker is:

[0077] (9).

[0078] In the case where the injected disturbance voltage signal is unchanged, there is a significant difference between the voltage amplitude and phase of the line head of transient and permanent faults. If the B-phase and C-phase voltages are shifted by 120° and 240° respectively, since the voltages of each phase are symmetrical in the period without fault, the three-phase voltage waveforms coincide after phase shift, and the remaining fault waveforms still have a large difference. Therefore, the fault type can be distinguished by using the difference between the voltage waveforms of each phase after phase shift.

[0079] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement to part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for fault identification of power distribution network with injection of disturbance signal by photovoltaic power source, characterized in that: The method comprises the following steps: Step 1: when a power distribution line fails, an outlet circuit breaker trips in three phases, after the action of anti-islanding protection, the photovoltaic grid-connected inverter is restarted and the additional control strategy is switched after a fixed delay, and a three-phase frequency-varying voltage disturbance signal is injected into the power distribution line; Step 2: three-phase voltages at the outlet circuit breaker are collected; Step 3: process the three-phase voltage data to obtain the line head three-phase voltage after phase shifting 、 、 , calculate the Pearson correlation coefficient value between each two in turn 、 、 , and obtain the maximum value 、 、 and the minimum value of the Pearson correlation coefficient value ; Step 4: If , Criterion I is met and the maximum allowed discrimination time is reached, it is determined that a transient fault has occurred and the reclosing is started; if Criterion I is not met, it is determined that a permanent fault has occurred and the reclosing is blocked; if the maximum allowed discrimination time is not reached, Step 5 is performed; The expression of criterion I is as follows: ； In the formula: is the variation coefficient of the maximum and minimum values of the phase-phase voltage Pearson correlation coefficient; is the threshold value of the criterion; Step 5: after a fixed delay, step 4 is returned to and executed, and the cycle is determined until the maximum allowed determination time is reached, and the determination is ended.

2. The method of claim 1, wherein the method further comprises: After the action of anti-islanding protection in step 1, the photovoltaic grid-connected inverter is restarted and the additional control strategy is switched after a delay of 0.5-1 s to avoid arc extinction time at the fault point.

3. The method of claim 1, wherein the method further comprises: determining whether the power grid is in a fault state based on the comparison. The additional control strategy adopts a constant voltage / constant frequency double-loop control strategy.

4. The method of claim 1, wherein the method further comprises: The processing of the three-phase voltage data in step 3 includes: respectively advancing the time windows of the B-phase and C-phase voltages by , , is the length of the disturbance voltage period, and the phase-shifted line head three-phase voltages , , are obtained.

5. The method of claim 2, wherein the method further comprises: The three-phase frequency-varying voltage disturbance signal injected into the power distribution line after the photovoltaic grid-connected inverter is restarted in step 1 is as follows: The voltage disturbance signal is a constant voltage disturbance signal with a voltage amplitude of 35-150 V and a frequency of 75-150 Hz, and the injection time of the voltage disturbance signal is 350-400 ms.

6. The method of claim 5, wherein the method further comprises: The frequency of the injected three-phase frequency-varying voltage disturbance signal is selected as an inter-harmonic.

7. The method of claim 5, wherein the method further comprises: The voltage amplitude of the injected three-phase frequency-varying voltage disturbance signal meets the minimum working voltage of detection accuracy in the power distribution network.

8. The method of claim 1, wherein the method further comprises: determining whether the power grid is in a fault condition based on the injected disturbance signal. Take the maximum error of 10% as the setting threshold value, that is .

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