A comprehensive method for identifying single-phase open-circuit fault of medium-voltage distribution network

Abstract

The application discloses a kind of medium voltage distribution network single-phase broken line fault comprehensive discrimination method, it is related to power grid fault technical field, the method includes the following steps: S1. the three-phase voltage, current at measurement point is collected;S2. obtain three-phase voltage and current amplitude;S3. whether one phase voltage in three-phase voltage is raised and the remaining two-phase voltage is reduced, if yes, then to step S4, if no, then return to step S1;S4. whether the phase current of voltage raised phase is 0 or the mutation variable of phase current is less than 0, if yes, then to step S5, if no, then return to step S1;S5. line's negative sequence impedance angle is obtained;S6. whether negative sequence impedance angle is in preset range, if yes, then determine that broken line fault occurs and execute step S7, if no, then return to step S1;S7. the ratio of negative sequence current and positive sequence current in line is calculated;S8. based on the ratio result, judge line single-phase broken line fault class.The application can identify the single-phase broken line fault that occurs on branch line in distribution network.

Description

Technical Field

[0001] This invention relates to the field of power grid fault technology, and more specifically, to a comprehensive method for identifying single-phase open-circuit faults in medium-voltage distribution networks. Background Technology

[0002] With the accelerating urbanization process in my country, the scale of power distribution networks is expanding, and the areas covered by their structures are becoming increasingly complex. Power distribution networks are frequently prone to line breakages due to factors such as lightning strikes, ground blasting, mechanical excavation, installation processes, and electrical quality issues. Line breakages are receiving increasing attention because, after a line breakage, there are no obvious overcurrent characteristics, making immediate isolation of the fault impossible. Furthermore, the prolonged descent of the broken conductor to the ground can lead to electric shock accidents. Simultaneously, voltage imbalances can damage electrical equipment. Therefore, accurate and reliable identification of single-phase line breakages has become particularly important.

[0003] Currently, most single-phase open-circuit fault detection methods can only identify cases where the open circuit drops 100% of the load (i.e., open-circuit faults on the main line), and cannot identify partial open-circuit faults (i.e., open-circuit faults on branch lines). This leads to a situation where, when a branch line open-circuit fault occurs in a distribution line, the search for the location of the open-circuit fault still starts from the main line, wasting a lot of time, manpower, and resources. Furthermore, the open-circuit fault cannot be reliably identified and quickly handled after it occurs, which can cause a series of problems such as the expansion of the power outage area or personal safety issues. Summary of the Invention

[0004] To identify single-phase open-circuit faults occurring on branch lines in a distribution network, this invention provides a comprehensive method for identifying single-phase open-circuit faults in medium-voltage distribution networks. The method includes the following steps:

[0005] S1. Real-time acquisition of three-phase voltage and three-phase current at measurement points on the power distribution line to obtain the acquisition results;

[0006] S2. Based on the acquisition results, obtain the amplitude of the three-phase voltage and the amplitude of the three-phase current;

[0007] S3. Calculate the sudden change in the amplitude of the three-phase voltage to obtain the first calculation result. Based on the first calculation result, determine whether there is one phase voltage that increases while the voltage of the remaining two phases decreases. If yes, proceed to step S4; otherwise, return to step S1.

[0008] S4. Obtain the first phase current at time t1 and the second phase current at time t2 of the voltage rise phase in the three-phase voltage, where t1>t2. Calculate the degree of abrupt change from the first phase current to the second phase current to obtain a second calculation result. When the first phase current is within a first preset range, determine whether the second calculation result is greater than a first preset threshold. If yes, proceed to step S5; otherwise, return to step S1. When the first phase current is within a second preset range, determine whether the second calculation result is greater than a second preset threshold. If yes, proceed to step S5; otherwise, return to step S1. When the first phase current is within a third preset range, determine whether the second calculation result is greater than a third preset threshold. If yes, proceed to step S5; otherwise, return to step S1. When the first phase current is within a fourth preset range, determine whether the second calculation result is greater than a fourth preset threshold. If yes, proceed to step S5; otherwise, return to step S1.

[0009] S5. Calculate the positive sequence current, negative sequence voltage and negative sequence current in the power distribution line to obtain the third calculation result, and calculate the negative sequence impedance angle of the power distribution line based on the third calculation result;

[0010] S6. Determine whether the negative sequence impedance angle is within a preset range. If yes, determine that a disconnection fault has occurred at the rear end of the measurement point and execute step S7. If no, return to step S1.

[0011] S7. Calculate the ratio of negative sequence current to positive sequence current in the power distribution line;

[0012] S8. Based on the calculation result of the ratio of negative sequence current to positive sequence current in the power distribution line, determine the single-phase open circuit fault type of the power distribution line.

[0013] Invention Principle: This invention collects three-phase voltage and current data at measurement points in real time through the power distribution network. The collected three-phase voltage and current are then processed to obtain the power frequency amplitude of the three-phase voltage and current. The abrupt change in the amplitude of the three-phase voltage is calculated to obtain a first calculation result. When it is determined that one phase voltage increases while the remaining two phase voltages decrease (indicating that the abrupt change in the phase voltage of one phase is greater than 0, while the abrupt change in the phase voltage of the remaining two phases is less than 0), the degree of abrupt change in the first phase current at time t1 to the second phase current at time t2 is calculated to obtain a second calculation result. Different judgment thresholds are set for the second calculation result based on the magnitude of the first phase current. Only when the second calculation result exceeds the set judgment threshold is the next judgment performed. Because three-phase load imbalance can also cause fluctuations in phase current, the above method is used to distinguish whether the factor causing the phase current change is a line break fault or load imbalance. The positive-sequence current, negative-sequence current, and negative-sequence voltage in a power distribution line are calculated. The phase difference between the negative-sequence voltage and negative-sequence current is the negative-sequence impedance angle. Therefore, the negative-sequence impedance angle of the power distribution line is obtained through the third calculation results of the negative-sequence voltage and negative-sequence current. It is then determined whether the negative-sequence impedance angle is within a preset range. Since the direction of the negative-sequence current in a faulty line is from the line to the busbar, while the direction of the negative-sequence current in a non-faulty line is from the busbar to the line, the range of values ​​for the negative-sequence impedance angle will differ between faulty and healthy lines. When the negative-sequence impedance angle is determined to be within the preset range, a break-through fault is identified downstream of the measurement point. In a power distribution line with a single-phase break-through fault, the negative-sequence current and positive-sequence current are approximately equal. However, for a healthy power distribution line, the phase current is almost always positive-sequence current. The type (severity) of the single-phase break-through fault in the power distribution line is determined by calculating the ratio of the negative-sequence current to the positive-sequence current. This invention uses phase voltage and phase current fault characteristics as the starting conditions, negative sequence impedance angle as the main criterion, and the ratio of negative sequence current to positive sequence current to reflect the type (severity) of the open circuit fault. It can not only identify 100% open circuit faults (i.e., the main line is broken and 100% of the load is dropped), but also partial open circuit faults (open circuit faults on branch lines). Therefore, different emergency measures can be formulated according to the type of open circuit fault.

[0014] Preferably, step S2.1 is included between steps S2 and S3. Step S2.1 includes: setting a phase current threshold, determining whether at least two phase currents in the three-phase currents are greater than the phase current threshold; if so, proceeding to step S3; otherwise, returning to step S1. Since the positive-sequence current, negative-sequence voltage, and negative-sequence current need to be calculated using phase currents, a phase current threshold needs to be set to ensure calculation accuracy. The circuit break protection judgment is only performed when at least two phase currents in the three-phase currents are greater than the phase current threshold.

[0015] Preferably, the determination of the single-phase open-circuit fault category of the power distribution line based on the calculated ratio of negative-sequence current to positive-sequence current in the power distribution line includes: setting the calculated ratio of negative-sequence current to positive-sequence current in the power distribution line as λ, and presetting a first ratio value as μ, 0<μ<1; when μ<λ<1, the single-phase open-circuit fault category of the power distribution line is determined to be a branch line open-circuit fault; when λ=1, the single-phase open-circuit fault category of the power distribution line is determined to be a main line open-circuit fault.

[0016] In a power distribution line experiencing a single-phase open circuit fault, the negative sequence current and the positive sequence current are approximately equal. However, for a healthy power distribution line, the phase current is almost always the positive sequence current. By calculating the ratio of the negative sequence current to the positive sequence current in the power distribution line, the severity of the single-phase open circuit fault can be determined, i.e., whether the power distribution line experiences a main line open circuit fault or a branch line open circuit fault.

[0017] Preferably, the preset first ratio value μ includes: obtaining the electrical parameters of the power distribution line to obtain the acquisition result, and adjusting μ based on the acquisition result.

[0018] The reason for setting the first ratio μ is that, depending on the different conditions of different lines, there will be errors when using the ratio of negative sequence current to positive sequence current to judge the severity of single-phase open circuit faults in power distribution lines. However, the impact of such errors can be reduced by adjusting the value of μ. For different conditions of different power distribution lines, the judgment is made by the electrical parameters of the power distribution line. For example, for power distribution lines with small load current during normal operation, even a small negative sequence current may cause some open circuit protection to malfunction. However, in reality, a small negative sequence current will not have a significant impact on the line. Therefore, by increasing the value of μ, some open circuit protection malfunctions caused by line current fluctuations can be avoided.

[0019] Preferably, calculating the abrupt change in the three-phase voltage amplitude to obtain the first calculation result includes calculating the abrupt change in the three-phase voltage amplitude using the following formula:

[0020] ΔU ε =U ε1 -U ε0 ;

[0021] Where ε represents the three-phase separation, ΔU ε U represents the abrupt change in the phase voltage amplitude. ε1 U represents the phase voltage amplitude after the fault. ε0 This represents the phase voltage amplitude before the fault. The above formula can accurately calculate the sudden change in the three-phase voltage amplitude.

[0022] Preferably, calculating the degree of abrupt change in the first phase current to obtain the second calculation result includes calculating the degree of abrupt change in the first phase current using the following formula:

[0023]

[0024] Where δ represents the phase of the voltage rise phase, ΔI δ I represents the degree of abrupt change in the first phase current. δ1 I represents the magnitude of the first phase current. δ0 This represents the amplitude of the second-phase current. The above formula can accurately calculate the degree of abrupt change in the phase current of the voltage-increasing phase.

[0025] Preferably, the calculation of the positive-sequence current, negative-sequence voltage, and negative-sequence current in the power distribution line to obtain the third calculation result includes calculating the negative-sequence current using the following formula:

[0026]

[0027] in, j is the imaginary part. and These represent positive-sequence current, negative-sequence current, and zero-sequence current, respectively. and These represent the three-phase currents respectively;

[0028] The negative sequence voltage is calculated using the following formula:

[0029]

[0030] in, and These represent positive-sequence voltage, negative-sequence voltage, and zero-sequence voltage, respectively. and These represent the three-phase voltages. The negative-sequence current and negative-sequence voltage can be accurately calculated using the above formulas.

[0031] Preferably, the negative sequence impedance angle of the power distribution line is calculated using the following formula:

[0032]

[0033] in, Indicates the negative sequence impedance angle. The angle represents the negative sequence voltage component. The angle represents the negative sequence current component. The negative sequence impedance angle can be accurately calculated using the formula described above.

[0034] Preferably, when Then it is determined that a disconnection fault has occurred at the rear end of the measurement point location; when Then return to step S1.

[0035] When a line break occurs, it's equivalent to superimposing a negative-sequence current source at the break point, flowing in the opposite direction to the pre-fault current. If we define the negative-sequence current flowing from the busbar to the line as positive, the phase difference between the negative-sequence voltage and current upstream of the fault is the phase angle of the equivalent negative-sequence impedance viewed from the fault break point upstream. This phase angle is related to the pre-fault power factor and varies between the second and third quadrants depending on the system's reactive power compensation capability. In this case, the negative-sequence current flows from the line to the busbar in the negative direction. Conversely, the phase difference between the negative-sequence voltage and current downstream of the fault, on healthy branches of the faulty line, and on non-faulty lines is the phase angle of the equivalent negative-sequence impedance viewed from the fault break point downstream of the fault. This phase angle also varies between the first and fourth quadrants depending on the load's reactive power compensation capability. In this case, the negative-sequence current flows from the busbar to the line in the positive direction. Based on the above analysis, the range of negative sequence impedance angle for a healthy line should be -90° to 90°, while the range of negative sequence impedance angle for a faulty line should be 90° to 270°.

[0036] Preferably, the amplitudes of the three-phase voltages and the three-phase currents are obtained using the FFT algorithm. FFT (Fast Fourier Transform) transforms discrete time-domain signals into the frequency domain. In general, time-domain signals are difficult to characterized, but after conversion to the frequency domain, their characteristics become easier to discern. Therefore, the FFT algorithm makes it easier to obtain the amplitudes of the three-phase voltages and three-phase currents.

[0037] One or more technical solutions provided by this invention have at least the following technical effects or advantages:

[0038] This invention uses phase voltage and phase current fault characteristics as the starting conditions, negative sequence impedance angle as the main criterion, and the ratio of negative sequence current to positive sequence current to reflect the type (severity) of the open circuit fault. It can not only identify 100% open circuit faults (i.e., the main line is broken and 100% of the load is dropped), but also identify partial open circuit faults. Thus, different emergency measures can be formulated according to the type of open circuit fault.

[0039] This invention ensures the accuracy of calculations for positive-sequence current, negative-sequence voltage, and negative-sequence current by setting phase current thresholds. Only when two of the three-phase currents exceed the phase current thresholds will a line break protection judgment be made. Attached Figure Description

[0040] The accompanying drawings, which are provided to further illustrate embodiments of the invention and constitute a part of this invention, are not intended to limit the scope of the invention.

[0041] Figure 1 This is a schematic diagram of a comprehensive method for identifying single-phase open-circuit faults in a medium-voltage distribution network according to the present invention. Detailed Implementation

[0042] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, where there is no conflict, the embodiments of the present invention and the features thereof can be combined with each other.

[0043] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and therefore the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0044] Example 1

[0045] Please refer to Figure 1 This is a schematic flowchart of a comprehensive method for identifying single-phase open-circuit faults in medium-voltage distribution networks according to the present invention. The method includes the following steps:

[0046] S1. Real-time acquisition of three-phase voltage and three-phase current at measurement points on the power distribution line to obtain the acquisition results;

[0047] S2. Based on the acquisition results, obtain the amplitude of the three-phase voltage and the amplitude of the three-phase current;

[0048] S3. Calculate the sudden change in the amplitude of the three-phase voltage to obtain the first calculation result. Based on the first calculation result, determine whether there is a voltage increase in one phase and a voltage decrease in the remaining two phases. If yes, proceed to step S4; otherwise, return to step S1.

[0049] S4. Obtain the first phase current at time t1 and the second phase current at time t2 of the voltage rise phase in the three-phase voltage, where t1>t2. Calculate the degree of abrupt change from the first phase current to the second phase current to obtain a second calculation result. When the first phase current is within a first preset range, determine whether the second calculation result is greater than a first preset threshold. If yes, proceed to step S5; otherwise, return to step S1. When the first phase current is within a second preset range, determine whether the second calculation result is greater than a second preset threshold. If yes, proceed to step S5; otherwise, return to step S1. When the first phase current is within a third preset range, determine whether the second calculation result is greater than a third preset threshold. If yes, proceed to step S5; otherwise, return to step S1. When the first phase current is within a fourth preset range, determine whether the second calculation result is greater than a fourth preset threshold. If yes, proceed to step S5; otherwise, return to step S1.

[0050] S5. Calculate the positive sequence current, negative sequence voltage and negative sequence current in the power distribution line to obtain the third calculation result, and calculate the negative sequence impedance angle of the power distribution line based on the third calculation result;

[0051] S6. Determine whether the negative sequence impedance angle is within a preset range. If yes, determine that a disconnection fault has occurred at the rear end of the measurement point and execute step S7. If no, return to step S1.

[0052] S7. Calculate the ratio of negative sequence current to positive sequence current in the power distribution line;

[0053] S8. Based on the calculation result of the ratio of negative sequence current to positive sequence current in the power distribution line, determine the single-phase open circuit fault type of the power distribution line.

[0054] The process involves real-time acquisition of three-phase voltage and current at measurement points on the power distribution line. The FFT algorithm is used to obtain the amplitudes of the three-phase (e.g., A, B, and C) voltages and currents. The abrupt changes in phase voltage amplitudes are calculated. For example, if the abrupt change in phase A voltage amplitude is greater than 0, while the abrupt changes in phase B and C voltage amplitudes are less than 0 (i.e., phase A voltage increases while phase B and C voltages decrease), the abrupt change in phase A current amplitude is calculated. If the abrupt change in phase A current amplitude exceeds a set threshold, the positive-sequence current, negative-sequence current, and negative-sequence voltage of the power distribution line are calculated. Based on the calculation results of the negative-sequence voltage and current, the negative-sequence impedance angle is obtained. The negative sequence impedance angle is the phase difference between the negative sequence voltage and the negative sequence current. When the negative sequence impedance angle is between 90° and 270°, it is determined that a break-through fault has occurred at the end of the measurement point. Based on the calculation results of the positive sequence current and the negative sequence current, the ratio of the negative sequence current to the positive sequence current is calculated. Since the values ​​of the negative sequence current and the positive sequence current are approximately equal in the faulty line, the severity of a single-phase break-through fault in the distribution line can be determined by the ratio of the negative sequence current to the positive sequence current. When the ratio of the negative sequence current to the positive sequence current is 1, the distribution line has a 100% break-through fault (the main line loses 100% of its load). When the ratio of the negative sequence current to the positive sequence current is less than 1, the distribution line has a partial break-through fault. This invention uses phase voltage and phase current fault characteristics as the starting conditions, negative sequence impedance angle as the main criterion, and the ratio of negative sequence current to positive sequence current to reflect the type (severity) of the open circuit fault. It can not only identify 100% open circuit faults (i.e., the main line is broken and 100% of the load is dropped), but also identify partial open circuit faults. Thus, different emergency measures can be formulated according to the type of open circuit fault.

[0055] In this invention, because the phase current of the distribution line will also change abruptly when a three-phase load imbalance occurs, and the degree of change is affected by the magnitude of the phase current, this invention sets a corresponding threshold for the degree of phase current change based on the magnitude of the phase current in order to distinguish whether the change is caused by a line break or a three-phase load imbalance. Only when the degree of change in phase current exceeds the set threshold is it considered to be a phase current change caused by a line break, thus proceeding to the next step of judgment. The interval between time t1 and time t2 should be as short as possible. This invention sets the interval between time t1 and time t2 to 1 second. At this time, the difference between the first phase current and the second phase current is calculated first, then the ratio of this difference to the first phase current is calculated, and finally the ratio is converted into a percentage form to obtain the second calculation result. When the first phase current is less than 30A, determine if the second calculation result is greater than 60%. If yes, proceed to the next step; otherwise, return to step S1. When the first phase current is between 30 and 60A, determine if the second calculation result is greater than 40%. If yes, proceed to the next step; otherwise, return to step S1. When the first phase current is between 60 and 100A, determine if the second calculation result is greater than 20%. If yes, proceed to the next step; otherwise, return to step S1. When the first phase current is greater than 100A, determine if the second calculation result is greater than 10%. If yes, proceed to the next step; otherwise, return to step S1. For example, if the first phase current is 100A and drops to 70A after 1 second, the sudden change in the first phase current is 30%, which is greater than the set 20%. In this case, proceed to the next step.

[0056] The invention includes a step S2.1 between steps S2 and S3. Step S2.1 involves setting a phase current threshold and determining whether at least two phase currents in the three-phase currents exceed the phase current threshold. If so, step S3 is executed; otherwise, the process returns to step S1. This invention requires calculating positive-sequence current, negative-sequence voltage, and negative-sequence current based on phase currents. However, this method only identifies single-phase open-circuit faults; the other two phases should be operating normally. To ensure calculation accuracy, a phase current threshold needs to be set. Open-circuit protection is only detected when at least two phase currents exceed the phase current threshold. The preferred phase current threshold is 10A. This is because when the phase current of the distribution line is below 10A, three-phase load imbalance can cause large fluctuations in phase current. Furthermore, an open-circuit fault can cause sudden changes in phase current. In such cases, it is impossible to distinguish whether the sudden change in phase current is due to an open-circuit fault or three-phase load imbalance, ultimately affecting the judgment of the open-circuit fault.

[0057] The determination of the single-phase open-circuit fault category in the power distribution line, based on the calculated ratio of negative-sequence current to positive-sequence current, includes: setting the calculated ratio of negative-sequence current to positive-sequence current as λ, with a preset first ratio of μ, where 0 < μ < 1; when μ < λ < 1, the single-phase open-circuit fault category is determined to be a branch line open-circuit fault; when λ = 1, the single-phase open-circuit fault category is determined to be a main line open-circuit fault. When an open-circuit fault occurs in the power distribution line, a significant negative-sequence current will appear in the faulty line, and the amplitude of the negative-sequence current is equal to that of the positive-sequence current. Therefore, when the line has a 100% open-circuit fault, the ratio of negative-sequence current to positive-sequence current is λ = 1. If the line can identify 50% open-circuit faults, in the lines with open-circuit faults, the ratio of negative-sequence current to positive-sequence current is λ = 1, while the phase currents in the remaining circuits without open-circuit faults are all positive-sequence currents. At this time, the ratio of negative-sequence current to positive-sequence current in the entire power distribution line is 0.33. Set the first ratio μ to 0.33. When μ < λ < 1, it is determined that the line has a partial line breakage fault (i.e., the branch line breaks and drops off part of the load). The closer it is to 1, the more serious the line breakage is. When λ = 1, it means that the line has a 100% line breakage fault.

[0058] The preset first ratio value μ includes: obtaining the electrical parameters of the power distribution line and adjusting μ based on the obtained results. The electrical parameters include phase voltage, current limit, line voltage, line current, and other parameters. The obtained electrical parameters are used to distinguish the line condition. For example, when the line is operating normally and the three-phase load is balanced and there is no open circuit fault, λ = 0. In this case, a smaller proportion of partial open circuit faults can be identified by reducing the value of μ (this is more effective for lines with balanced three-phase loads and large load currents). Let μ = 0.11. When μ < λ < 1 and λ is close to μ, it indicates that the line may have an open circuit, resulting in a 20% load loss. Let μ = 0.25. When μ < λ < 1 and λ is close to μ, it indicates that the line may have an open circuit, resulting in a 40% load loss. When the three-phase load is unbalanced during normal operation and there is no open circuit fault, a negative sequence current is generated in the distribution line due to the load imbalance. The ratio λ of the negative sequence current to the positive sequence current is greater than 0. Therefore, it is necessary to increase μ to avoid partial false tripping of the open circuit protection. Assuming μ = 0.67, when μ < λ < 1 and λ is close to μ, it indicates that the line may experience an open circuit, resulting in 80% load loss. For lines with small load currents during normal operation, even a small negative sequence current may trigger partial false tripping of the open circuit protection. Therefore, increasing μ can also prevent false tripping of the open circuit protection caused by line current fluctuations.

[0059] The calculation of the abrupt change in the three-phase voltage amplitude to obtain the first calculation result includes calculating the abrupt change in the three-phase voltage amplitude using formula (1):

[0060] ΔUε =U ε1 -U ε0 (1);

[0061] Where ε represents the three-phase separation, ΔU ε U represents the abrupt change in the phase voltage amplitude. ε1 U represents the phase voltage amplitude after the fault. ε0 This represents the phase voltage amplitude before the fault. The sudden change in phase voltage amplitude can be accurately calculated using formula (1). ε represents the three phases, namely phases A, B, and C in the power system. For example, for phase A, U... ε1 This indicates that the phase voltage amplitude of phase A after the fault is 7kV, U ε0 This indicates that the phase voltage amplitude of phase A before the fault was 6kV, ΔU ε This indicates that the sudden change in the phase voltage amplitude of phase A is 1 kV. The above data is only for explanation of formula (1), and the actual data can be adjusted according to the specific situation. This invention does not impose any specific limitations.

[0062] The calculation of the degree of abrupt change in the first phase current to obtain the second calculation result includes calculating the degree of abrupt change in the first phase current using formula (2):

[0063]

[0064] Where δ represents the phase of the voltage rise phase, ΔI δ I represents the degree of abrupt change in the first phase current. δ1 I represents the magnitude of the first phase current. δ0 This represents the amplitude of the second phase current. Formula (2) can accurately calculate the abrupt change in the phase current of the voltage-increasing phase. For example, if the voltage-increasing phase is phase A, I... δ1 This indicates that the amplitude of the first phase current is 50A, I δ0 This indicates that the amplitude of the second phase current is 20A, ΔI δ This indicates that the abrupt change in the phase current of phase A is 60%, which is greater than the set 40%. At this point, the next step of judgment is performed. The above data is only for explanation of formula (2). The actual data can be adjusted according to the specific situation. This invention does not impose any specific limitations.

[0065] The calculation of the positive-sequence current, negative-sequence voltage, and negative-sequence current in the power distribution line to obtain the third calculation result includes calculating the negative-sequence current using formula (3):

[0066]

[0067] in j is the imaginary part. and These represent positive-sequence current, negative-sequence current, and zero-sequence current, respectively. and These represent the three-phase currents respectively;

[0068] The negative sequence voltage is calculated using formula (4):

[0069]

[0070] in and These represent positive-sequence voltage, negative-sequence voltage, and zero-sequence voltage, respectively. and These represent the three-phase voltages respectively. The positive sequence, negative sequence, and zero sequence are used to analyze the asymmetry of voltage and current in the system, decomposing the asymmetrical components of the three phases into symmetrical components (positive and negative sequences) and a zero sequence component in the same direction. For AC power systems, there are generally three phases: A, B, and C. The positive sequence, negative sequence, and zero sequence components of the power system are determined according to the order of the three phases A, B, and C. Positive sequence: Phase A leads Phase B by 120 degrees, Phase B leads Phase C by 120 degrees, and Phase C leads Phase A by 120 degrees; Negative sequence: Phase A lags Phase B by 120 degrees, Phase B lags Phase C by 120 degrees, and Phase C lags Phase A by 120 degrees; Zero sequence: Phases A, B, and C are in the same phase. The formulas (3) for calculating negative sequence current and (4) for calculating negative sequence voltage are derived based on this principle and can accurately calculate the positive sequence, negative sequence, and zero sequence current and voltage in the distribution line. The specific data for the three-phase current in formula (3) and the three-phase voltage in formula (4) can be obtained based on the actual collected data. This invention does not impose any specific limitations.

[0071] The negative sequence impedance angle of the power distribution line is calculated using formula (5):

[0072]

[0073] in Indicates the negative sequence impedance angle. The angle represents the negative sequence voltage component. This refers to the angle of the negative sequence current component. The impedance angle is the phase difference between phase voltage and phase current in an AC circuit. The negative sequence impedance angle is the phase difference between the negative sequence voltage and negative sequence current, specifically the difference between the angles of the negative sequence voltage component and the negative sequence current component. For example... The value should be 30°. The above data is only for explaining formula (5). The actual calculation data can be adjusted according to the specific situation. This invention does not make any specific limitations.

[0074] Among them, when Then it is determined that a disconnection fault has occurred at the rear end of the measurement point location; when Then return to step S1. When a line break occurs, it is equivalent to superimposing a negative sequence current source in the opposite direction to the current before the fault at the break location. If the negative sequence current flowing from the bus to the line is defined as the positive direction, the phase difference between the negative sequence voltage and the negative sequence current on the upstream side of the fault is the phase angle of the equivalent negative sequence impedance viewed from the fault break point to the upstream side of the fault. The phase angle of the negative sequence impedance is related to the power factor before the fault and varies between the second and third quadrants depending on the magnitude of the reactive power compensation capability on the system side. At this time, the negative sequence current flows from the line to the bus in the negative direction. On the downstream side of the fault, the phase difference between the negative sequence voltage and the negative sequence current on the healthy branches and non-faulty lines is the phase angle of the equivalent negative sequence impedance viewed from the fault break point to the downstream side of the fault. It also varies between the first and fourth quadrants depending on the magnitude of the reactive power compensation capability on the load side. At this time, the negative sequence current flows from the bus to the line in the positive direction. Based on the above analysis, the range of the negative sequence impedance angle for a healthy line should be -90° to 90°, while the range for a faulty line should be 90° to 270°. Therefore, when Then it is determined that a disconnection fault has occurred at the rear end of the measurement point location. Then, the three-phase voltage and three-phase current at the measurement points on the distribution line will continue to be collected in real time.

[0075] Specifically, the amplitudes of the three-phase voltages and three-phase currents are obtained using the FFT algorithm. FFT (Fast Fourier Transform) transforms discrete time-domain signals into the frequency domain. While time-domain signals are generally difficult to analyze, their characteristics become clearer after conversion to the frequency domain. Therefore, by employing the FFT algorithm to perform spectral analysis on the three-phase voltage and current signals, the amplitudes of the three-phase voltages and currents can be obtained relatively easily.

[0076] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0077] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A comprehensive method for identifying single-phase open-circuit faults in medium-voltage distribution networks, characterized in that, The method includes the following steps: S1. Real-time acquisition of three-phase voltage and three-phase current at measurement points on the power distribution line to obtain the acquisition results; S2. Based on the acquisition results, obtain the amplitude of the three-phase voltage and the amplitude of the three-phase current; S3. Calculate the sudden change in the amplitude of the three-phase voltage to obtain the first calculation result. Based on the first calculation result, determine whether there is a voltage increase in one phase and a voltage decrease in the remaining two phases. If yes, proceed to step S4; otherwise, return to step S1. S4. Obtain the first phase current at time t1 and the second phase current at time t2 of the voltage rise phase in the three-phase voltage, where t1>t2. Calculate the degree of abrupt change from the first phase current to the second phase current to obtain a second calculation result. When the first phase current is within a first preset range, determine whether the second calculation result is greater than a first preset threshold. If yes, proceed to step S5; otherwise, return to step S1. When the first phase current is within a second preset range, determine whether the second calculation result is greater than a second preset threshold. If yes, proceed to step S5; otherwise, return to step S1. When the first phase current is within a third preset range, determine whether the second calculation result is greater than a third preset threshold. If yes, proceed to step S5; otherwise, return to step S1. When the first phase current is within a fourth preset range, determine whether the second calculation result is greater than a fourth preset threshold. If yes, proceed to step S5; otherwise, return to step S1. S5. Calculate the positive sequence current, negative sequence voltage and negative sequence current in the power distribution line to obtain the third calculation result, and calculate the negative sequence impedance angle of the power distribution line based on the third calculation result; S6. Determine whether the negative sequence impedance angle is within a preset range. If yes, determine that a disconnection fault has occurred at the rear end of the measurement point and execute step S7. If no, return to step S1. S7. Calculate the ratio of negative sequence current to positive sequence current in the power distribution line; S8. Based on the calculation result of the ratio of negative sequence current to positive sequence current in the power distribution line, determine the single-phase open circuit fault type of the power distribution line.

2. The comprehensive method for identifying single-phase open-circuit faults in medium-voltage distribution networks according to claim 1, characterized in that, Between steps S2 and S3, there is also step S2.1, which includes: setting a phase current threshold, determining whether there are at least two phase currents in the three phase currents that are greater than the phase current threshold, if yes, then proceed to step S3, if no, then return to step S1.

3. The comprehensive method for identifying single-phase open-circuit faults in medium-voltage distribution networks according to claim 1, characterized in that, Based on the calculated ratio of negative-sequence current to positive-sequence current in the power distribution line, the single-phase open-circuit fault category of the power distribution line is determined as follows: the calculated ratio of negative-sequence current to positive-sequence current in the power distribution line is set as... And preset the first ratio to be μ, 0 < μ < 1; when μ < When <1, the single-phase open-circuit fault of the power distribution line is determined to be a branch line open-circuit fault; when When =1, the single-phase open circuit fault of the power distribution line is determined to be a main line open circuit fault.

4. The comprehensive method for identifying single-phase open-circuit faults in medium-voltage distribution networks according to claim 3, characterized in that, The preset first ratio value μ includes: obtaining the electrical parameters of the power distribution line to obtain the acquisition result, and adjusting μ based on the acquisition result.

5. The comprehensive method for identifying single-phase open-circuit faults in medium-voltage distribution networks according to claim 1, characterized in that, The calculation of the abrupt change in the three-phase voltage amplitude to obtain the first calculation result includes calculating the abrupt change in the three-phase voltage amplitude using the following formula: ； Where ε represents the three-phase separation, This represents the abrupt change in the phase voltage amplitude. Indicates the phase voltage amplitude after the fault. This indicates the phase voltage amplitude before the fault.

6. The comprehensive method for identifying single-phase open-circuit faults in medium-voltage distribution networks according to claim 1, characterized in that, The calculation of the degree of abrupt change from the first phase current to the second phase current to obtain the second calculation result includes calculating the degree of abrupt change from the first phase current to the second phase current using the following formula: ； in, Indicates the phase of the voltage increase phase. The degree of abrupt change indicates the extent of the transition from the first-phase current to the second-phase current. This indicates the amplitude of the first phase current. This indicates the amplitude of the second-phase current.

7. The comprehensive method for identifying single-phase open-circuit faults in medium-voltage distribution networks according to claim 1, characterized in that, The calculation of the positive-sequence current, negative-sequence voltage, and negative-sequence current in the power distribution line yields a third calculation result, including the calculation of the negative-sequence current using the following formula: ； in, , j is the imaginary part. , and These represent positive-sequence current, negative-sequence current, and zero-sequence current, respectively. , and These represent the three-phase currents respectively; The negative sequence voltage is calculated using the following formula: ； in, , and These represent positive-sequence voltage, negative-sequence voltage, and zero-sequence voltage, respectively. , and These represent the three-phase voltages respectively.

8. The comprehensive method for identifying single-phase open-circuit faults in medium-voltage distribution networks according to claim 7, characterized in that, The negative sequence impedance angle of the power distribution line is calculated using the following formula: ； in, Indicates the negative sequence impedance angle. The angle represents the negative sequence voltage component. The angle represents the negative sequence current component.

9. A comprehensive method for identifying single-phase open-circuit faults in medium-voltage distribution networks according to claim 8, characterized in that, When 90° < If the angle is ≤270°, it is determined that a wire breakage fault has occurred at the rear end of the measurement point; if -90° ≤ If the angle is ≤90°, return to step S1.

10. The comprehensive method for identifying single-phase open-circuit faults in medium-voltage distribution networks according to claim 1, characterized in that, The amplitudes of the three-phase voltages and the amplitudes of the three-phase currents are obtained using the FFT algorithm.

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