A power distribution line short-circuit fault nature identification method using double-station inverter power successive injection

Abstract

This invention relates to a method for identifying the nature of short-circuit faults in distribution lines by sequentially injecting power from two inverters, belonging to the field of power system fault identification technology. The method includes: using a grid-connected inverter in the de-energized line to actively inject a power frequency signal controlled by voltage frequency for fault detection; determining whether a strong fault exists in the distribution line based on the composite voltage established by the first inverter injection; if not, calculating the lag injection angle of the second inverter; if a fault exists, outputting a permanent fault and blocking reclosing; controlling the phase of the second inverter to lag by the lag injection angle, and after a short delay, the second inverter injects power frequency energy controlled by voltage frequency lag behind the first inverter; calculating the power correlation of the first inverter before and after the second inverter injection, and completing the identification of the nature of the short-circuit fault in the distribution line based on the correlation. This aims to solve the technical problem in existing technologies where the nature of distribution line faults is difficult to reliably identify, leading to blind reclosing.

Description

Technical Field

[0001] This invention relates to a method for identifying the nature of short-circuit faults in power distribution lines by sequentially injecting power from two inverters, belonging to the field of power system fault identification technology. Background Technology

[0002] The large-scale integration of distributed generation has transformed traditional distribution networks from passive unidirectional networks to active bidirectional networks, placing higher demands on distribution network protection and automatic reclosing technologies. Most overhead line faults in distribution networks are transient, and three-phase reclosing is typically used to improve power supply reliability. However, with the widespread use of cable lines, the proportion of mixed overhead and cable lines is constantly increasing, significantly reducing the success rate of traditional reclosing. Existing automatic reclosing systems in distribution networks generally lack fault nature identification capabilities. If reclosing occurs on a permanent fault, it can cause secondary impacts on the system, threatening the safe and stable operation of the distribution network. Since the distribution network becomes passive after a three-phase trip, there is a lack of electrical quantity information available for fault nature identification. Existing identification methods often require additional equipment or circuit breaker modifications, resulting in complex excitation source operation, high costs, and low sensitivity to high-resistance symmetrical faults. New distribution networks naturally contain inverter-interfaced distributed generation (IIDG), which can serve as excitation sources for fault identification. Therefore, researching a fault nature identification method for distribution lines based on inverter active injection has significant theoretical and engineering application value. Summary of the Invention

[0003] The purpose of this invention is to provide a method for identifying the nature of short-circuit faults in power distribution lines by utilizing the successive power injection of dual inverters, aiming to solve the technical problem in the prior art that it is difficult to reliably identify the nature of faults in power distribution lines, thus leading to blind reclosing.

[0004] To achieve the above objectives, the technical solution of the present invention is: a method for identifying the nature of short-circuit faults in distribution lines using successive power injection from dual inverters, comprising the following steps: S1: When a power distribution line trips due to a fault and loses power, a grid inverter in the power-loss line is used to actively inject a power frequency signal for fault detection by controlling the voltage and frequency. S2: In fault detection, the presence of a strong fault in the power distribution line is determined based on the composite voltage established by the first inverter; wherein, the strong fault includes asymmetrical faults and low-resistance symmetrical faults. S3: If it is determined that there is no major fault in the power distribution line, calculate the injection angle boundary of the secondary inverter based on the output voltage and current of the first inverter, and calculate the lag injection angle of the secondary inverter; if it is determined that there is a major fault in the power distribution line, output a permanent fault and block reclosing. S4: Send a second pulse signal to the two inverters, and generate a synchronous phase signal using the second pulse signal as a clock reference. Control the phase of the secondary inverter to lag according to the lag injection angle. After a short delay, the secondary inverter injects power frequency energy at the voltage frequency control of the first inverter. S5: After the secondary inverter is injected, calculate the correlation coefficient between the active power and reactive power output of the first inverter before and after the secondary inverter is injected. If the correlation is positive, the line fault is a transient fault; if the correlation is negative, the fault is a permanent fault, thus completing the identification of the nature of the short circuit fault in the power distribution line.

[0005] Optionally, S1 specifically includes: When a power distribution line loses power due to a fault trip, the inverter power supply in the lost power line is used to inject power frequency power into the grid under voltage frequency control, thereby establishing voltage for the line. The established voltage is determined by the equivalent load impedance.

[0006] Optionally, S2 specifically includes: The high-voltage output power frequency voltage of the first inverter is extracted using Fourier transform, and the voltage imbalance is calculated. The expression is as follows:

[0007] in, The voltage imbalance established after the inverter is injected; This refers to the negative sequence voltage at the high-voltage side output of the inverter. This is the positive sequence voltage output from the high-voltage side of the inverter; If satisfied This indicates the presence of an asymmetrical fault in the line; otherwise, it indicates the absence of an asymmetrical fault. The threshold for determining asymmetric faults; If satisfied If the line has a low-resistance symmetrical fault, then the line has a low-resistance symmetrical fault; otherwise, it indicates that the line does not have a low-resistance symmetrical fault. This is the per-unit value of the high-voltage side output voltage of the first inverter after injection. U TH The per-unit value of the threshold voltage for low-resistance fault identification.

[0008] Optionally, S3 specifically includes: If there are no asymmetrical faults or low-resistance symmetrical faults in the power distribution line, the power factor angle calculated based on the output voltage and current of the first inverter is used as the lower limit of the injection angle for the second inverter. i L And calculate the hysteresis injection angle of the secondary inverter. The expression is:

[0009] in, i H The upper limit of the angle at which the secondary unit lags behind the primary unit is determined by the minimum power factor angle of the distribution network load under low voltage conditions; if i L >θ H This indicates that the fault has disappeared; If a major fault exists, reclosing will be directly blocked.

[0010] Optionally, S5 specifically includes: Calculate the Pearson correlation coefficients of the active and reactive power outputs of the first inverter before and after the injection of the second inverter. Determine whether the Pearson correlation coefficient satisfies ,in, The threshold for the power correlation coefficient; If satisfied If the signal is positive, it indicates a permanent symmetrical weak fault in the power distribution line; otherwise, it indicates a transient fault.

[0011] The beneficial effects of this invention are as follows: Based on the current source characteristics exhibited after the inverter is injected into the distribution line, and considering the impedance differences of each phase in asymmetrical faults and the extremely low impedance of symmetrical low-resistance faults, this invention uses the voltage established by a single inverter injection to identify strong faults; based on the difference between the purely resistive transition resistance and the characteristics of the resistive-inductive load in symmetrical weak faults, this invention uses the power correlation of the first inverter after the injection of the second inverter to identify weak faults. This allows for the rapid identification of multiple faults in the distribution line, and can accurately identify weak faults, regardless of the inverter capacity, without requiring additional excitation source equipment, thus providing a basis for the reclosing of the distribution line. Attached Figure Description

[0012] Figure 1 This is a flowchart of the steps of the present invention; Figure 2 This is a simulation topology diagram of the present invention; Figure 3 A voltage diagram is established for the first inverter of this invention; Figure 4 This is a schematic diagram of the power output of the first inverter during a transient fault according to the present invention. Figure 5 This is a schematic diagram of the power of the first inverter under a permanent symmetrical weak fault according to the present invention. Detailed Implementation

[0013] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0014] Example 1: As Figure 1As shown, a method for identifying the nature of short-circuit faults in distribution lines using sequential power injection from two inverters includes the following steps: S1: When a power distribution line trips due to a fault and loses power, a grid inverter in the power-loss line is used to actively inject a power frequency signal for fault detection by controlling the voltage and frequency. Optionally, after a power distribution line trips due to a fault and loses power, the inverter power supply in the power-loss line is used to control the grid construction and inject power frequency power at voltage-frequency (VF) to establish voltage for the line. It is important to understand that since the capacity of a single inverter is 300kW, its capacity is very small relative to the load. That is, the inverter can be regarded as a current source in both fault and non-fault conditions. Therefore, the voltage established is determined by the equivalent load impedance.

[0015] S2: In fault detection, the presence of a strong fault in the power distribution line is determined based on the composite voltage established by the first inverter; wherein, the strong fault includes asymmetrical faults and low-resistance symmetrical faults. Optionally, due to the unequal impedances of each phase in an asymmetrical fault, the three-phase voltages after constant current injection into the inverter are unequal and contain a large amount of negative sequence voltage; the equivalent impedance of a symmetrical low-resistance fault is extremely low, resulting in an extremely low established voltage. Therefore, the high-voltage output power frequency voltage of the first inverter is extracted using Fourier transform, and the voltage imbalance is calculated. The expression is as follows:

[0016] in, The voltage imbalance established after the inverter is injected; This refers to the negative sequence voltage at the high-voltage side output of the inverter. This is the positive sequence voltage output from the high-voltage side of the inverter; If satisfied This indicates the presence of an asymmetrical fault in the line; otherwise, it indicates the absence of an asymmetrical fault. In this embodiment, the threshold for determining asymmetric faults is... Take 2%; If satisfied If the line has a low-resistance symmetrical fault, then the line has a low-resistance symmetrical fault; otherwise, it indicates that the line does not have a low-resistance symmetrical fault. This is the per-unit value of the high-voltage side output voltage of the first inverter after injection. U TH In this embodiment, the per-unit value of the threshold voltage for low-resistance fault identification is used. U TH Take 0.025 pu.

[0017] S3: If it is determined that there is no major fault in the power distribution line, calculate the injection angle boundary of the secondary inverter based on the output voltage and current of the first inverter, and calculate the lag injection angle of the secondary inverter; if it is determined that there is a major fault in the power distribution line, output a permanent fault and block reclosing. Optionally, if there are no asymmetrical faults or low-resistance symmetrical faults in the power distribution line, the power factor angle calculated based on the output voltage and current of the first inverter is used as the lower limit of the injection angle for the second inverter. i L And calculate the hysteresis injection angle of the secondary inverter. The expression is:

[0018] in, i H The upper limit of the angle at which the secondary unit lags behind the primary unit is determined by the minimum power factor angle of the distribution network load under low voltage. In this embodiment, taking a minimum power factor of 0.9 under low voltage as an example, we take... i H =18.19°; if i L >θ H This indicates that the fault has disappeared, so take... i L =5°; If a major fault exists, reclosing will be directly blocked.

[0019] S4: Send a second pulse signal to the two inverters, and generate a synchronous phase signal using the second pulse signal as a clock reference. Control the phase of the secondary inverter to lag according to the lag injection angle. After a short delay, the secondary inverter injects power frequency energy at the voltage frequency control of the first inverter. Optionally, this embodiment sends a second pulse signal to the two inverters based on a global navigation satellite system (GNSS, such as GPS / BeiDou). Specifically, if there is no strong fault in the line, a synchronization pulse signal is sent to the two inverters based on GNSS, and this is used as a clock reference to generate a synchronization phase signal. The first inverter is controlled to inject the synchronization phase, and the second inverter is controlled to lag the phase of the first inverter. After a short delay, the first inverter is injected with VF control.

[0020] S5: After the secondary inverter is injected, calculate the correlation coefficient between the active power and reactive power output of the first inverter before and after the secondary inverter is injected. If the correlation is positive, the line fault is a transient fault; if the correlation is negative, the fault is a permanent fault, thus completing the identification of the nature of the short circuit fault in the power distribution line.

[0021] Optionally, since the fault transition resistance is purely resistive, the active power of the first inverter increases while the reactive power decreases after the secondary inverter is injected with a lag. However, since the line load is resistive-inductive, both the active and reactive power of the first inverter increase after the secondary inverter is injected. Therefore, the Pearson correlation coefficients for the output active and reactive power of the first inverter before and after the secondary inverter injection are calculated. Determine whether the Pearson correlation coefficient satisfies ,in, The threshold for the power correlation coefficient; If satisfied If the signal is positive, it indicates a permanent symmetrical weak fault in the power distribution line; otherwise, it indicates a transient fault.

[0022] Optionally, in this embodiment, Take -0.7.

[0023] The technical solution of the present invention will be further illustrated below through a specific implementation example.

[0024] Specifically, such as Figure 2 As shown, with L 4. An incident occurred 8km from the head. R f Taking a symmetrical weak fault of 40Ω as an example, a PSCAD / EMTDC simulation model was built, and fault detection was performed by actively injecting fault signals from two inverters on the line. The simulation step size was 3. m Each inverter has a rated capacity of 300kW, and the inverter reference phase is provided by GNNS. L 4. Remaining load after fault trip S 3 = 4 + j1.9372MVA, take the voltage and current measurement points on the high-voltage side of the inverter output. The specific implementation steps are as follows: Step 1: After the power distribution line trips due to a fault and loses power, a grid inverter in the power-loss line is used to actively inject power frequency signals for fault detection by controlling the voltage and frequency. Specifically, a three-phase short-circuit ground fault occurs on the distribution line at 0.4s. Circuit breaker QF3 trips at 0.5s. After a 0.5s delay to release residual charge, IIDG1 is activated via zero-start boost mode under VF control at 1s. The per-unit voltage values ​​injected into the first inverter during transient and permanent faults are as follows: Figure 3 As shown.

[0025] Step 2: In fault detection, determine whether there is a strong fault in the power distribution line based on the composite voltage established by the first inverter. Specifically, during transient / permanent faults, the three-phase voltage of a single inverter after injection is symmetrical, which is obtained through calculation. The circuit is determined to be free of asymmetrical faults. The voltage per unit value is 0.0265 for transient faults and 0.207 for permanent faults, both of which are greater than the preset threshold of 0.025. Therefore, it is determined that there are no low-resistance symmetrical faults in the distribution line.

[0026] Step 3: If it is determined that there is no major fault in the power distribution line, calculate the injection angle boundary of the secondary inverter based on the output voltage and current of the first inverter, and calculate the lag injection angle of the secondary inverter; if it is determined that there is a major fault in the power distribution line, output a permanent fault and block reclosing. Specifically, for transient faults, Fourier transform is used to extract the power frequency voltage and current information on the high-voltage side of the first inverter and calculate the power factor angle to obtain the lower limit of the injection angle. i L1 =25.6397°> i H Then, taking the lower limit of the angle as 5°, we can further obtain the hysteresis injection angle Δ. i 1 = 11.5950°; For permanent faults, the lower limit of the injection angle is obtained based on voltage and current information. i L2 =5.3521°, further yielding the hysteresis injection angle Δ i 2 = 11.7711°.

[0027] Step 4: Send a second pulse signal to the two inverters based on GNSS, and generate a synchronization phase signal using the second pulse signal as a clock reference. Control the phase of the secondary inverter to lag according to the lag injection angle. After a short delay, the secondary inverter injects power frequency energy at the voltage frequency control of the first inverter. Specifically, GNNS sends a second pulse signal to the two inverters and generates an inverter synchronization phase signal using the second pulse signal as a clock reference. The first inverter injects power according to the synchronization phase, and controls the second inverter to lag the first inverter by 11.5950° in phase. The second inverter starts injecting power at zero voltage boosting control at 2s according to VF control.

[0028] Step 5: After the secondary inverter is injected, calculate the correlation coefficient between the active power and reactive power output of the first inverter before and after the secondary inverter is injected. If the correlation is positive, the line fault is a transient fault; if the correlation is negative, the fault is a permanent fault, thus completing the identification of the nature of the short circuit fault in the power distribution line. Specifically, for transient faults, the power of the first inverter after the injection of the secondary inverter is as follows: Figure 4 As shown, the output power of the first inverter within a time window of Δt=0.5s before and after the injection of the second inverter is extracted, and the power correlation coefficient is calculated. =0.91524>-0.7, indicating that the power distribution line fault has disappeared, and is judged as a transient fault. For permanent faults, the power of IIDG1 is as follows: Figure 5 As shown, the correlation coefficients of active and reactive power of the first inverter within the Δt time window before and after IIDG2 injection are... =-0.9999<-0.7, thus correctly identifying it as a permanent weak fault.

[0029] In summary, since distribution networks are typically equipped with three-phase automatic reclosing, blind reclosing after a line trips due to a fault may close the circuit onto a permanent fault, causing secondary impacts on the system and threatening the safe and stable operation of the distribution network. Furthermore, after a power outage, the distribution line becomes a passive network, lacking robust electrical quantity information to determine the presence of a fault. Therefore, this invention analyzes the equivalent impedance of each phase in faulty and non-faulty circuits, as well as the power distribution difference between two inverters. It uses the composite voltage established by the injection from the first inverter to identify strong faults and non-faulty circuits, and utilizes the power correlation of the first inverter after the injection from the second inverter to identify weak faults and non-faulty circuits. This invention can accurately identify the nature of various short-circuit faults in distribution lines, effectively solving the technical problem of traditional automatic reclosing operating blindly due to its inability to distinguish fault types.

[0030] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for identifying the nature of short-circuit faults in distribution lines using successive power injection from dual inverters, characterized in that, The method includes the following steps: S1: When a power distribution line trips due to a fault and loses power, a grid inverter in the power-loss line is used to actively inject a power frequency signal for fault detection by controlling the voltage and frequency. S2: In fault detection, the presence of a strong fault in the power distribution line is determined based on the composite voltage established by the first inverter; wherein, the strong fault includes asymmetrical faults and low-resistance symmetrical faults. S3: If it is determined that there is no major fault in the power distribution line, calculate the injection angle boundary of the secondary inverter based on the output voltage and current of the first inverter, and calculate the lag injection angle of the secondary inverter; if it is determined that there is a major fault in the power distribution line, output a permanent fault and block reclosing. S4: Send a second pulse signal to the two inverters, and generate a synchronous phase signal using the second pulse signal as a clock reference. Control the phase of the secondary inverter to lag according to the lag injection angle. After a short delay, the secondary inverter injects power frequency energy at the voltage frequency control of the first inverter. S5: After the secondary inverter is injected, calculate the correlation coefficient between the active power and reactive power output of the first inverter before and after the secondary inverter is injected. If the correlation is positive, the line fault is a transient fault; if the correlation is negative, the fault is a permanent fault, thus completing the identification of the nature of the short circuit fault in the power distribution line.

2. The method for identifying the nature of short-circuit faults in distribution lines using successive power injection from dual inverters as described in claim 1, characterized in that, Specifically, S1 is: When a power distribution line loses power due to a fault trip, the inverter power supply in the lost power line is used to inject power frequency power into the grid under voltage frequency control, thereby establishing voltage for the line. The established voltage is determined by the equivalent load impedance.

3. The method for identifying the nature of short-circuit faults in distribution lines using successive power injection from dual inverters, as described in claim 1, is characterized in that... Specifically, S2 is: The high-voltage output power frequency voltage of the first inverter is extracted using Fourier transform, and the voltage imbalance is calculated. The expression is as follows: ； in, The voltage imbalance established after the inverter is injected; This refers to the negative sequence voltage at the high-voltage side output of the inverter. This is the positive sequence voltage output from the high-voltage side of the inverter; If satisfied This indicates the presence of an asymmetrical fault in the line; otherwise, it indicates the absence of an asymmetrical fault. The threshold for determining asymmetric faults; If satisfied If the line has a low-resistance symmetrical fault, then the line has a low-resistance symmetrical fault; otherwise, it indicates that the line does not have a low-resistance symmetrical fault. This is the per-unit value of the high-voltage side output voltage of the first inverter after injection. U TH The per-unit value of the threshold voltage for low-resistance fault identification.

4. The method for identifying the nature of short-circuit faults in distribution lines using successive power injection from dual inverters as described in claim 1, characterized in that, Specifically, S3 is: If there are no asymmetrical faults or low-resistance symmetrical faults in the power distribution line, the power factor angle calculated based on the output voltage and current of the first inverter is used as the lower limit of the injection angle for the second inverter. θ L And calculate the hysteresis injection angle of the secondary inverter. The expression is: ； in, θ H The upper limit of the angle at which the secondary unit lags behind the primary unit is determined by the minimum power factor angle of the distribution network load under low voltage conditions; if θ L > θ H This indicates that the fault has disappeared; If a major fault exists, reclosing will be directly blocked.

5. The method for identifying the nature of short-circuit faults in distribution lines using successive power injection from dual inverters as described in claim 1, characterized in that, Specifically, S5 is: Calculate the Pearson correlation coefficients of the active and reactive power outputs of the first inverter before and after the injection of the second inverter. Determine whether the Pearson correlation coefficient satisfies ,in, The threshold for the power correlation coefficient; If satisfied If the signal is positive, it indicates a permanent symmetrical weak fault in the power distribution line; otherwise, it indicates a transient fault.

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