A method for locating single-phase grounding fault sections in distribution networks based on shielding layer current.

Abstract

This invention discloses a method for locating single-phase grounding fault sections in distribution networks based on shielding current, belonging to the field of power supply reliability technology. It establishes an equivalent model of a three-core cable coupling to analyze the amplitude and phase characteristics of the shielding current in different sections; sets an instantaneous trigger threshold, collects and filters the instantaneous shielding current values ​​to remove noise, and calculates the effective value and fundamental phase when the threshold is continuously exceeded; sets an effective value trigger threshold and counts the number of currents exceeding the threshold within the ring main unit; and determines the fault section based on the number of currents and their phase difference. This invention only requires local shielding current measurement, without the need for voltage transformers, inter-station communication, or topology information, achieving low-cost, highly reliable local fault section location.

Description

Technical Field

[0001] This invention relates to the field of power supply reliability technology, and in particular to a method for locating single-phase grounding fault sections in a distribution network based on shielding layer current. Background Technology

[0002] With the continuous increase in the cable coverage rate of urban power distribution networks, single-phase grounding faults account for more than 80% of all power distribution network faults. Accurate and rapid location of fault sections is of great significance for improving power supply reliability. Existing methods for locating single-phase grounding fault sections in power distribution networks are mainly divided into three categories according to the type of electrical quantities used: phase component method, zero-sequence component method, and shielding layer grounding current method.

[0003] The phase component method directly utilizes the three-phase current conventionally collected by the feeder terminal unit to locate the faulty phase by comparing the transient current change, power frequency change ratio, or transient power direction between the faulty and healthy phases. This method has clear physical meaning and is simple to implement, but it requires the complete installation of three-phase current transformers along the line, resulting in high installation costs, and the criteria are susceptible to interference from distributed power source access.

[0004] The zero-sequence component method obtains the zero-sequence current by using a zero-sequence current transformer or by synthesizing three-phase currents. It then uses characteristics such as the similarity of zero-sequence current waveforms, transient energy distribution, or harmonic phase differences between adjacent detection points to locate specific sections. This method has strong field adaptability, but in practical engineering, installing a zero-sequence current transformer or modifying an existing transformer requires power outages, which limits its widespread application.

[0005] Due to the unique shielding structure of three-core cables, abundant zero-sequence fault characteristics can be induced in the shielding grounding current, and the measuring device can be installed energized. Therefore, fault location based on the shielding grounding current has become a research hotspot in recent years. Existing methods either extract the initial phase difference of the shielding grounding current and combine it with machine learning algorithms for location, or determine the fault section by comparing the imaginary part amplitude of the vector sum of the shielding grounding currents at both ends, or construct a criterion using the amplitude difference between the zero-sequence current and the shielding grounding current. However, these methods generally have the following shortcomings: First, they require centralized analysis and comparison of multiple electrical quantities, placing high demands on synchronous measurement accuracy and communication reliability; second, most methods still require the cooperation of other electrical quantities such as zero-sequence voltage, and cannot independently complete the location based solely on the shielding grounding current; third, they are highly dependent on training samples, have weak interpretability, and are difficult to adapt to topology changes.

[0006] Therefore, how to locate single-phase grounding fault sections in distribution networks using only locally available shielding grounding current without voltage measurement, inter-station communication, or topology information is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] To address the problem that existing methods for locating single-phase grounding fault sections in distribution networks rely on voltage measurement, inter-station communication, or topology information, and cannot achieve highly reliable and low-cost local fault section location using only locally available cable shielding grounding current, this invention proposes a method for locating single-phase grounding fault sections in distribution networks based on shielding current. This method only requires local current measurement to achieve location, thus solving the aforementioned problems.

[0008] This application discloses a method for locating single-phase grounding fault sections in a distribution network based on shielding layer current, comprising the following steps: Using the cables between ring main units as the basic unit for fault location, a coupling equivalent model of three-core cables is established for both intact and faulty sections. Based on the coupling model, the amplitude and phase characteristics of the shielding current in the healthy section, the faulty section, the upstream faulty section, the downstream faulty section, and the healthy branch section are analyzed. Set an instantaneous value trigger threshold, collect the instantaneous values ​​of the current in each cable shield layer using the ring main unit as a unit, and perform filtering and noise reduction. When the instantaneous value of the grounding current of any shield layer continuously exceeds the instantaneous value trigger threshold, use fast Fourier transform to calculate the effective value and fundamental phase of each current cycle. Set an effective value trigger threshold and count the number of currents whose effective value exceeds the effective value trigger threshold among all shielding grounding currents in the ring main unit; The fault section is determined based on the number of currents and the phase difference relationship between different currents.

[0009] Preferably, the instantaneous value trigger threshold is as follows:

[0010] in, For cable-to-ground capacitance, This represents the zero-sequence voltage corresponding to the fault resistance under a single-phase ground fault. The mutual inductance between the cable core and the shielding layer. For the shielding layer impedance, The grounding resistance at the beginning of the shielding layer. This is the grounding resistance at the end of the shielding layer.

[0011] Preferably, the filtering and denoising employs a moving average filtering algorithm and a second-order Butterworth low-pass filtering algorithm. The formula for moving average filtering is as follows:

[0012] in, and For natural numbers, For the filtered first A single instantaneous current value. The first one collected from the original source A single instantaneous current value. This is the length of the filtering window; The transfer function of a second-order Butterworth low-pass filter is as follows:

[0013] in, This is the cutoff angular frequency of the low-pass filter. For the Laplace operator.

[0014] Preferably, the formula for calculating the effective value is as follows:

[0015] The formula for calculating the fundamental phase is as follows:

[0016] in, The fundamental complex amplitude, The mode of the fundamental complex amplitude, This is an operation for finding complex angles.

[0017] Preferably, the effective value trigger threshold is determined based on the third effective current value among all current effective values ​​arranged from largest to smallest within the ring main unit, and the calculation formula is as follows:

[0018] in, For reliability coefficient, This is the third effective value of the current in the ring main unit, arranged from largest to smallest.

[0019] Preferably, the fault-determining section includes: If the effective value of no current exceeds If so, then the section of this ring main unit is fault-free.

[0020] If only one current exceeds If the current is within a certain range, then the section to which it belongs is the fault section.

[0021] If two currents exceed If the phase difference is less than 140°, then the section with the phase leading is the fault section.

[0022] If two currents exceed If the phase difference is greater than 140°, then a branch in the downstream section of this ring network cabinet is a fault section.

[0023] Preferably, the phase difference calculation formula is as follows:

[0024]

[0025] in, For the first The phase difference value after normalization of the current, mod(·) is the modulo remainder operation. For the first A current signal For reference signal, For the first Each current phase.

[0026] Preferably, the method further includes: If the faulty section is not effectively located after the fault triggering process is completed, the current data window cache will be automatically cleared and instantaneous value detection will be restarted from the next sampling point after the start of this event.

[0027] The beneficial effects of this invention are: (1) This invention uses only the grounding current of the cable shield layer to locate the single-phase grounding fault section of the distribution network. It replaces multi-point communication with local comparison of local amplitude ratio and phase difference, no longer relying on communication conditions, and does not require voltage transformers, inter-station communication or topology information, thus reducing system cost and meeting the requirements of local deployment.

[0028] (2) The present invention can still accurately extract fault characteristics under high resistance grounding fault conditions, and its fault resistance resistance is better than that of existing methods.

[0029] (3) By establishing an accurate coupling model and dual threshold criteria, this invention can accurately locate faults under complex conditions such as wide transition resistance range, different fault locations and new energy access, and has strong anti-interference ability.

[0030] (4) The present invention has a clear hierarchical logic based on the number of over-threshold currents and phase difference, with a small amount of computation, and is easy to implement in embedded terminals.

[0031] (5) The cyclic re-entry detection mechanism proposed in this invention avoids missed detection caused by signal truncation or instantaneous interference, further improving the positioning reliability, and is compatible with both independent shielded and common shielded three-core cable structures. Attached Figure Description

[0032] Figure 1 This is a flowchart of the method for locating single-phase grounding fault sections in a distribution network based on shielding layer current, according to an embodiment of the present invention. Figure 2 This is a schematic diagram of the ring main unit and cable section structure according to an embodiment of the present invention; Figure 3 This is an equivalent circuit diagram of a three-core cable coupling according to an embodiment of the present invention; Figure 4 This is the equivalent circuit for the coupling of the faulty cable section in an embodiment of the present invention; Figure 5 This is an equivalent circuit diagram of a single-phase ground fault zero sequence according to an embodiment of the present invention; Figure 6 This is a phasor analysis diagram of the current in the first-end fault shielding layer according to an embodiment of the present invention; Figure 7 This is a phasor analysis diagram of the current in the middle and end fault shielding layers of an embodiment of the present invention; Figure 8 This is a phasor analysis diagram of the shielding layer current in the upstream fault section according to an embodiment of the present invention. Figure 9 This is a phasor analysis diagram of the shielding layer current in the downstream fault and healthy branch sections according to an embodiment of the present invention. Figure 10 This is a schematic diagram of a typical fault condition topology in an embodiment of the present invention; Figure 11 This is a phasor diagram of the current in the shielding layer of the monitoring point under typical operating conditions according to an embodiment of the present invention; Figure 12 The working conditions of the embodiments of the present invention f 1. Current waveform diagram of the shielding layer of each ring main unit; Figure 13 The working conditions of the embodiments of the present invention f 1. Current phasor diagram of the shielding layer of each ring main unit; Figure 14 The working conditions of the embodiments of the present invention f 2. Current phasor diagram of the shielding layer of each ring main unit; Figure 15 The working conditions of the embodiments of the present invention f 3. Current phasor diagram of the shielding layer of each ring main unit; Figure 16 The working conditions of the embodiments of the present invention f 4. Current phasor diagrams of the shielding layers of each ring main unit; Figure 17 The working conditions of the embodiments of the present invention f 5. Current phasor diagram of the shielding layer of each ring main unit. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided with reference to the accompanying drawings and embodiments.

[0034] This application discloses a method for locating single-phase grounding fault sections in a distribution network based on shielding layer current. The specific process is as follows: Figure 1 As shown.

[0035] In this embodiment, the grounding current coupling model of the shielding layer is first analyzed.

[0036] like Figure 2As shown, in a typical medium-voltage cable network, the actual distribution network mainly monitors the voltage / current of incoming and outgoing lines through distribution terminal units (DTUs). When locating a fault section, it is necessary to obtain and compare the measurement data of all DTUs on the line. The location method in this embodiment only monitors the grounding current of the shielding layers of all incoming and outgoing cables within the ring main unit. By comparing the amplitude and phase of the shielding current of each cable within the ring main unit, the relative location of the fault point can be determined locally.

[0037] In medium-voltage power distribution networks, cable shielding layers are directly grounded at both ends. Based on the shielding structure, cables are classified into two categories: independently shielded and commonly shielded. Independently shielded structures include... Figure 2 The structure is shown. In this structure, the shielding layers of each phase are tightly fitted and grounded at both ends. The three-phase shielding layers are electrically interconnected, and the currents of each phase cable core coupled to the shielding layers through electromagnetic induction ultimately flow into the same grounding loop. Therefore, when analyzing the macroscopic fault current path and electromagnetic coupling characteristics, the independent shielding structure can be equivalent to a common shielding structure, and the subsequent analysis is applicable to both types of cables. In order to quantitatively analyze the amplitude and phase characteristics of the grounding current of each cable shielding layer at the ring main unit, this embodiment establishes a coupling equivalent model of the three-core cable for both the intact section and the faulty section.

[0038] The coupling relationship between the three-phase cable core and the shielding body in the section is as follows: Figure 3 As shown in the figure. (Subscript) Represents the head end, which is closer to the busbar. This represents the end closest to the load side. In the diagram, the voltage of the three-phase cable cores at the beginning to ground is... , , The current is , , The voltage at the end to ground is , , The current is , , The currents at the beginning and end of the shielding layer are respectively , The impedance of the three-phase cable core is , , The admittance between each phase cable core and the shielding layer is Mutual induction is The impedance of the shielding layer is The grounding resistances at the beginning and end of the shielding layer are respectively , The reference directions for both voltage and current are shown in the figure.

[0039] The grounding current of a sound three-core cable shield is mainly caused by capacitor leakage current. With electromagnetic induction circulation The layers are stacked together, and the grounding current at the beginning and end of the shielding layer is as shown in the reference direction. , As shown in the following formula: (1) in, This is due to the shunting of the capacitor leakage current caused by the voltage between the cable core and the shielding layer along the beginning of the shielding layer. This refers to the shunting of the capacitor leakage current caused by the voltage between the cable core and the shielding layer along the end of the shielding layer. The following sections discuss the capacitor leakage current. With electromagnetic induction circulation Perform the calculation.

[0040] Due to the impedance of the shielding layer and the grounding resistance at the beginning and end of the shielding layer , The resistance of the cable core is usually much smaller than the capacitive reactance of the cable insulation layer. Therefore, when calculating the total capacitive current injected into the shielding layer from the cable core, the capacitive reactance of the insulation layer plays a dominant role, and the impedance effect of the shielding layer circuit can be ignored. The capacitance leakage current generated by the cable under the action of the cable core voltage is then calculated. for: (2) in, It is the equivalent capacitance between the cable core and the shielding layer.

[0041] Ignoring the voltage drop caused by the cable core impedance: (3) in, It is the zero-sequence voltage.

[0042] Define scale coordinates ,in Corresponding to the beginning, Corresponding to the end. Then from any position The impedance to the first and last grounding points is: (4) in, To start from any position The impedance to the first-end grounding point To start from any position The impedance to the end grounding point.

[0043] Then in the infinitesimal element The injected current is According to the principle of current shunting, the shunted current along the first and second ends of the shielding layer is: (5) in, This is the shunt current of the capacitor current along the first end of the shielding layer. This is the shunt current of the capacitor current along the end of the shielding layer.

[0044] The above equation shows that, under the condition of a constant zero-sequence voltage, if the grounding resistances at the beginning and end are equal, the capacitor current will be evenly distributed along both ends.

[0045] The current flowing through the cable core induces a longitudinal electromotive force in the shielding layer, and gradually attenuates as it propagates along the cable due to continuous leakage to the ground. The origin of the coordinate system is still taken as the beginning of the section. The infinitesimal length at the beginning of the cable Inside, there are: (6) The zero-sequence current of the cable core for: (7) in, This is the zero-sequence current at the beginning.

[0046] Integrating along the segment, the total induced electromotive force of the shielding layer is: (8) The induced circulating current in this section is: (9) According to GB50169-2016, and in light of the actual engineering situation, , Desirable Taking YJV22-8.7 / 15kV-3×50mm² as an example, Approximately The length of a single cable segment in a power distribution network is generally around 5km, but the analysis uses a range of 1 to 8km. The calculated angle range of the induced current lagging behind the zero-sequence current in the cable core is 94.2° to 133.1°.

[0047] Substituting equations (5) and (9) into equation (1), we can obtain the current at the beginning and end of the shielding layer as follows: (10) When a single-phase ground fault occurs in the cable, the coupling equivalent circuit is as follows: Figure 4 As shown. The fault point divides the cable into upstream and downstream sections, and the mutual inductance between the cable cores and shielding layers of the two sections... With insulation resistance Similar to a healthy cable, its upstream and downstream capacitive leakage current and electromagnetic induction circulating current are the same as those of a normal cable. This is the fault resistor.

[0048] Assume the per-unit value of the total length of the faulty section is 1, and the distance from the fault point to the beginning is... ,and In the event of a fault, the current at the beginning and end of the shielding layer... , As shown in the following formula: (11) in, Fault current The component at the beginning of the shielding layer, Fault current The component at the end of the shielding layer is calculated using the following formula: (12) Still with and Pick , Z s = For example, when the fault occurs at the beginning of the section, within a section length l of 1 to 8 km, Advanced The angle is 3.8°~40.2°, and the amplitude is approximately 1.1 1.8 times. When the fault occurs in the middle of the section, and They are equal. When the fault occurs at the end, the amplitude and phase relationship between the two is symmetrical and opposite to that at the beginning.

[0049] Fault current Related to the neutral grounding method and fault resistance, in a low-resistance grounding system, the fault current is composed of the resistive current flowing through the neutral grounding resistor and the ground capacitance current of the entire system except for this section. (13) in, For the flow through the neutral point grounding resistance The resistive current, This is the sum of the system's capacitance to ground. Fault Section capacitance to ground, It is the sum of the capacitive currents of the entire system excluding the faulty section.

[0050] Electromagnetic induction circulation The zero-sequence current varies depending on the fault location; the zero-sequence current in this section is divided by the fault point. (14) in, This is the zero-sequence current at the end.

[0051] Integrating along the segment, the total induced electromotive force of the shielding layer in that segment is: (15) The electromagnetic induction circulating current is: (16) Substituting into equation (11), the grounding current of the first and last shielding layers is: (17) Next, the current characteristics of the faulty section and the non-faulty section are compared.

[0052] Based on the shielding layer grounding current coupling model, Figure 5 Taking the zero-sequence equivalent circuit shown as an example, we analyze the zero-sequence current distribution after a fault.

[0053] In the diagram, a single-phase ground fault occurs in section 13, sections 11 and 12 are upstream of the fault, section 14 is downstream of the fault, and sections 15 and 16 are healthy branch sections. The system's zero-sequence voltage generates a capacitive current to ground in each section and a resistive current at the neutral point grounding resistance. All zero-sequence currents are injected into the fault point through the ground and then shunt from the fault point to each section. Because the resistive component amplitude is much larger than the capacitive component, the overall fault current is resistive. The zero-sequence current flowing through the upstream section of the fault point is the vector sum of the resistive current and the downstream line capacitive current, with a relatively large amplitude; the downstream section and healthy branch sections only experience downstream capacitive current to ground, with a smaller amplitude. The amplitude and phase characteristics of the shielding layer grounding current in each section are analyzed as follows: For the faulty section, since the fault location affects the shunting of the fault current at the beginning and end of the shielding layer and the magnitude of the induced circulating current, three typical cases are considered: the fault is located at the beginning, middle, and end of the section. Using the reference phase, the phasor analysis diagram of the shielding current during a fault at the beginning of the section is plotted as follows: Figure 6 As shown in the figure. The fault current... Due to neutral point resistive current The sum of the capacitive currents of the entire system excluding the faulty section Synthesized, its phase is slightly ahead. . and Divide the current according to equation (12). The zero-sequence current of the cable core is shown in equation (14), when When it is close to 0, The main component is the capacitive current flowing downstream of the fault, and... Compared to smaller amplitudes, the generated induced circulating current It is also relatively small. Therefore and Mainly depends on and ,Right now Advanced The angle ranges from 11.4° to 56.9°, and the amplitude is approximately... 1.1 2.5 times.

[0054] Similarly Using the reference phase, the phasor analysis diagram of the fault shielding layer current in the middle of the section is plotted as follows: Figure 7 As shown in (a), the phasor analysis diagram of the current in the end-fault shielding layer is as follows: Figure 7 As shown in (b).

[0055] When the fault location is in the middle of the section The flow is evenly distributed along the beginning and end. and for Half of, direction and on the contrary. Lag Lag Approximately 90°~140°, according to formula (11) and and synthesis and .at this time Still lagging behind At a certain angle, the amplitude is greater than .

[0056] When the fault is located at the end of the section Advanced The angle is between 21° and 57°, and the amplitude is approximately 1.2 2.5 times, Lag The amplitude is approximately 90°~140°, and is larger than when the fault occurs in the middle of the section. It is still calculated according to formula (11). and synthesis and .at this time Still lagging behind At a certain angle, but the amplitude is the same as near.

[0057] In summary, regardless of where the fault is located in the cable, All lagged behind At a certain angle, The phase is concentrated between -140° and -170°. The phase is concentrated between -170° and 140°.

[0058] For the upstream section of the fault From the fault point through the upstream section to the busbar, the cable cores of the upstream section are allowed to flow through this section. In addition, it also carries significant amplitude. . Strong induction in the shielding layer Its amplitude is much greater than that of this section. ,become and The dominant component, whose phasor diagram is as follows Figure 8 As shown in the figure, Induced circulating current Phase lag Approximately 90°~140°. The grounding current of the shielding layer is affected by this. The phase is concentrated between 50° and 90°. The phases are concentrated between -90° and -140°, with similar amplitudes and a phase difference of approximately 170°. For the downstream section of the fault and the intact branch section, if the downstream section of the fault is the end of the feeder, its zero-sequence current... Only for the capacitor current of this section If there are still downstream lines, then the capacitive current of each downstream section is also included. , Figure 9 Phasor diagrams for two typical cases are given.

[0059] Figure 9 (a) indicates the end of the feeder. Advanced The induced circulation at nearly 90° Lag Approximately 90°~140°, the two are vectored together according to equation (11). and . Advanced Approximately 90°, with an amplitude ratio of approximately 1.5, and both are ahead. .

[0060] Figure 9 (b) represents the non-feeder end, an extreme case where the downstream section is much longer than the current section. In this case, the zero-sequence current of the cable core includes the capacitive current of the current section and all downstream sections. The corresponding amount is relatively large. and The amplitude and phase are mainly determined by The two amplitudes are similar, and the phase difference is close to 170°.

[0061] comprehensive Figure 9 In (a) and (b), the downstream section of the fault The phase is concentrated between 90° and 140°. The phase is concentrated between -50° and 90°.

[0062] Based on the above shielding layer grounding current model and characteristic analysis, typical fault conditions based on the relative position of the ring main unit and the fault are obtained. The ring main unit can simultaneously acquire the shielding layer end current of the incoming cable and the shielding layer beginning current of each outgoing cable, providing favorable conditions for local fault location. According to the position of the fault point relative to the ring main unit, the fault scenario can be divided as follows: Figure 10 The five typical operating conditions shown are: ① Upstream section fault f 1; ② Incoming line fault f 2; ③ Outgoing line fault f 3; ④ Downstream section fault f 4; ⑤ Other branch circuit faults f 5.

[0063] The phasor diagrams of the shield currents of the cables connected to the monitoring points under various operating conditions are as follows: Figure 11 As shown in the figure, the incoming line segment is segment 3, and the outgoing segments are segments 4, 5, 6, and 7. To monitor the current at the end of the shielding layer of the incoming line section of the ring main unit. , , , This refers to the current at the beginning of the shielding layer in the outgoing section. Specific analyses for each operating condition are as follows: f 1 and f 5. In the event of a fault, since the monitoring point ring main unit is not directly electrically connected to the fault point and is located outside the fault current path, the fault current will not flow through the ring main unit. Therefore, only capacitive current flows through the shielding layer of the cable connected to ring main unit 2. and its induced circulation The amplitude is much smaller than the fault current. Combining Figure 9 Phasor analysis of the shielding currents in the downstream fault section and the intact branch section shows that the outgoing current phase leads the incoming current. Therefore, under both operating conditions, the current amplitudes of the shielding currents of each cable in the monitoring point ring main unit are relatively small. The amplitude is relatively large and the phase lag is significant. , , , The phase is leading and relatively concentrated, such as Figure 11 As shown in (a).

[0064] f 2. In the event of a fault, the fault point is located on section 3 of the incoming cable of the ring main unit at the monitoring point, and the fault current is... The current flows through section 3. Therefore, the fault current component flows through the cable shielding layer of section 3. and and its induction The amplitude is relatively large. Only the cable in each outgoing section flows through... and its induced circulation The amplitude is much smaller than Analysis of the faulty section Figure 6 , 7 It can be seen that the phase of the faulty incoming current is quite close to that of the healthy outgoing current. Therefore, under this operating condition, Amplitude significantly greater than , , , And the phases of the currents are close, such as Figure 11 As shown in (b).

[0065] f 3. In the event of a fault, the fault point is located on section 7 of the outgoing line of the monitoring point ring main unit. Section 3 is the upstream section of the fault, and the remaining outgoing line sections are intact. The fault current flows through sections 3 and 7, combined with... Figure 6 , 7 Analysis 8 shows that the shielding layer of the faulty outgoing section simultaneously flows through... and its induction The incoming line section shielding layer flows through Inductive The shielding layer current amplitude is relatively large. Only a small amount of current flows through the intact outgoing wire. and its induced circulation The amplitude is much smaller than Under this operating condition, and Amplitude significantly greater than , , , Phase lags behind Approximately 90°~140° , , The phase is between the two, such as Figure 11 As shown in (c).

[0066] f 4. In the event of a fault, the fault point is located in the common branch downstream of the monitoring point ring network cabinet. Sections 3 and 7 are both upstream of the fault, while the remaining outgoing sections are intact. The cable shielding layer of both sections 3 and 7 is filled with [the following]. Inductive The amplitude is relatively large; the well-maintained section only flows through and its induction The amplitude is much smaller than the fault current. Figure 6 , 7As shown in section 8, the phase of the current at the end of the incoming line shield layer in the upstream section of the fault lags behind the current at the beginning of the outgoing line by approximately 170°, while the phase of the current in the healthy outgoing line is between the two. That is, under this operating condition... and Amplitude significantly greater than , , , Phase lags behind Approximately 170°, such as Figure 11 As shown in (d).

[0067] Based on the above analysis, the shielding layer current characteristics at the ring main unit node under five operating conditions during a single-phase ground fault are summarized in Table 1: Table 1. Characteristics of grounding current of shielding layer under various operating conditions

[0068] Based on the current characteristics summarized in Table 1, a local fault location process can be developed, the specific logic of which is as follows: Figure 1 As shown.

[0069] Set instantaneous value trigger threshold To ensure sensitive triggering, the minimum value of the shielding layer grounding current during a high-resistance grounding fault can be used. For example, the minimum possible value of the shielding layer current under a fault resistance of 5kΩ and a cable length of 1km can be calculated using the following formula: (18) in, The capacitance to ground of a 1km cable. This is the zero-sequence voltage corresponding to a 5kΩ fault resistor under a single-phase ground fault.

[0070] Instantaneous current values ​​are collected using ring main units as units, and noise reduction is performed using a moving average filtering algorithm and a second-order Butterworth low-pass filtering algorithm.

[0071] The formula for moving average filtering is as follows: (19) in, and For natural numbers, For the filtered first A single instantaneous current value. The first one collected from the original source A single instantaneous current value. The filter window length is set to 1 / 4 to 1 / 2 of the cycle number of sampling points in this embodiment.

[0072] The transfer function of a second-order Butterworth low-pass filter is shown in the following equation: (20) in, The cutoff angular frequency for the low-pass filter is 100π rad / s in this embodiment. For the Laplace operator.

[0073] When any current is continuous The instantaneous value exceeds At that time, the effective value of each current per cycle is calculated using Fast Fourier Transform. In this embodiment... Take the number of sampling points corresponding to 1 / 4 cycle.

[0074] The formula for Fast Fourier Transform is as follows: (twenty one) in, For the first Second harmonic complex amplitude This represents the number of sampling points per cycle. For harmonic order, the fundamental frequency is taken as... .

[0075] The effective value of each current cycle and the phase of the fundamental wave are calculated based on the fundamental wave component, using the following formula: (twenty two) (twenty three) in, This is the effective value of the fundamental current. The fundamental complex amplitude, The mode of the fundamental complex amplitude, The phase of the fundamental current wave. This is an operation for finding complex angles.

[0076] Set valid value trigger threshold Considering that at most two lines within the same ring main unit carry fault current, this embodiment uses the third most significant current value arranged from largest to smallest within the ring main unit. Multiply by the reliability coefficient As As shown in the following formula: (twenty four) In this embodiment Take 10, but the specific value can be adjusted according to the system parameters and topology.

[0077] Using the maximum current as a reference signal, calculate the phase of other currents: (25) in, For the first A current signal For reference signal, For the first Each current phase.

[0078] Because the power frequency phase of a power system has a 360° periodicity, directly calculating the phase difference is prone to cross-cycle jump errors (e.g., the phase difference calculation result shows a 350° difference). The 10° difference (essentially the same phase difference, with inconsistent positive and negative values) leads to errors in lead / lag discrimination and misjudgment of faulty sections. Therefore, the original phase difference is calculated first, and then phase difference normalization is performed to uniformly constrain any phase difference value to [ ]. Within the interval [180°, 180°], the normalization formula is as follows: (26) in, For the first The phase difference value after normalization of the current takes values ​​in the range of [-180°, 180°], and mod(·) is the modulo operation.

[0079] According to the ring main unit, more than The number of currents is used to determine the fault section: If the effective value of no current exceeds If so, then the section of this ring main unit is fault-free.

[0080] If only one current exceeds If the current is within a certain range, then the section to which it belongs is the fault section.

[0081] If two currents exceed If the phase difference is less than 140°, then the section with the phase leading is the fault section.

[0082] If two currents exceed If the phase difference is greater than 140°, then a branch in the downstream section of this ring network cabinet is a fault section.

[0083] If the faulty section is not effectively located after the fault triggering process is completed, the current data window cache will be automatically cleared, and the instantaneous value detection of the entire process will be restarted from the next sampling point of the current event start point. This avoids the problem of missed fault detection caused by signal truncation and instantaneous interference, realizes cyclic re-entry detection, and ensures the reliability of positioning.

[0084] In one specific embodiment, the effectiveness and reliability of the method proposed in this application are verified through simulation. A system is built in PSCAD / EMTDC as follows: Figure 10The simulation model shown is as follows. The system power supply voltage is 121kV, frequency is 50Hz, the main transformer has a turns ratio of 121kV / 10.5kV and a rated capacity of 45MVA, and the system is grounded through a 10Ω small resistor. The photovoltaic transformer is connected to the distribution network in a delta connection. The total load of feeder 1 is 1.92MW, and the renewable energy penetration rate is 56.63%. The lengths of sections 1 to 12 are 3km, 1km, 4km, 1km, 1.5km, 2km, 2km, 1km, 5km, 3km, 1km, and 2km respectively. The total length of feeder 1 is 28km, and the total length of the remaining feeders is 60km. The cables used in the simulation are YJV22-8.7 / 15kV-3*50mm² three-core cables of actual specifications. Five typical fault cases are set up to correspond to five operating conditions, as shown in Table 2. Table 2 Typical Fault Case Setting Table

[0085] Figure 12 The working conditions are given. f The grounding current waveforms of the shielding layers of four ring main units are shown below. Using a 5kHz sampling frequency and a 20ms data window, the fundamental amplitude and phase of the shielding currents of the four ring main units are extracted using Fast Fourier Transform. The phasor diagram is plotted using the current with the largest amplitude as the reference phase. Figure 13 As shown. For the operating conditions. f 2~ f 4. Using the same processing methods, draw the phasor diagram as follows: Figures 14-17 As shown.

[0086] Figure 13 In the ring main unit 1, only the current amplitude of section 1 was significantly higher than normal, reaching 36.54A, and section 1 was determined to be a faulty section. The currents of the other ring main units were all less than 0.8A, and they were determined to be fault-free sections.

[0087] Figure 14 In ring main unit 1, the currents in sections 3 and 1 are significantly higher than normal, at 5.46A and 2.08A respectively, with a phase difference of 129°. Section 3, with the phase leading, is determined to be a faulty section. In ring main unit 2, only section 3 has a significantly higher current, at 3.54A, so section 3 is also determined to be a faulty section. All currents in ring main units 3 and 4 are less than 0.1A, therefore, ring main units 3 and 4 are determined to have no faulty sections.

[0088] Figure 15In ring main unit 1, the currents in section 3 and section 1 are significantly higher than normal, at 1.56A and 1.26A respectively, with a phase difference of 177°. Therefore, the downstream branch of ring main unit 1 is identified as a faulty section. In ring main unit 2, the currents in section 7 and section 3 are significantly higher than normal, at 1.81A and 1.61A respectively, with a phase difference of 100.9°. Therefore, section 7 is identified as a faulty section. In ring main unit 3, only section 7 has a significantly higher current, at 1.90A. Therefore, section 7 is identified as a faulty section. All currents in ring main unit 4 are less than 0.1A, therefore, ring main unit 4 has no faulty sections.

[0089] Figure 16 In ring main unit 1, the currents in sections 3 and 1 are significantly higher than normal, at 5.10A and 4.15A respectively, with a phase difference of 177.2°, indicating a downstream branch fault in ring main unit 1. Similarly, in ring main unit 2, the currents in sections 3 and 7 are significantly higher than normal, at 5.27A and 2.81A respectively, with a phase difference of 172.4°, also indicating a downstream branch fault in ring main unit 2. In ring main unit 3, the currents in sections 9 and 7 are significantly higher than normal, at 10.89A and 2.90A respectively, with a phase difference of 117.7°, indicating that section 9, with its leading phase, is the faulty section. All currents in ring main unit 4 are less than 0.1A, indicating no faulty sections in ring main unit 4.

[0090] Figure 17 In ring main unit 1, the currents in section 10 and section 1 are significantly higher than normal, at 343.21A and 149.21A respectively, with a phase difference of 129.6°, indicating section 10 is a faulty section. In ring main unit 4, only section 10 has a significantly higher current, at 296.86A, indicating section 10 is also a faulty section. The currents in ring main units 2 and 3 are all below the threshold, indicating no faulty sections in ring main units 2 and 3.

[0091] In summary, for the five typical operating conditions described above, the amplitude and phase characteristics of the shielding current in each ring main unit are consistent with the theoretical criteria, and the fault location is accurate. The results preliminarily verify the effectiveness of the method proposed in this application under different fault locations, transition resistances, and new energy access conditions.

[0092] To further examine the adaptability of the proposed method under extreme conditions, based on Figure 10 The distribution network topology shown is used to set up 100 simulation examples. Fault locations cover five typical operating conditions in this embodiment, with fault distances set at 1%, 50%, and 99% of the total segment length, and fault resistances at 0Ω, 100Ω, 500Ω, 1000Ω, 3000Ω, and 5000Ω. The impact of photovoltaic (PV) grid connection is superimposed at the extreme locations of the highest resistance (5000Ω) and fault distances of 1% and 99%, and the fault phases are randomly assigned. Location criteria and procedures are used to independently identify fault segments in each set of simulation data. Results show that in all 100 examples, each ring main unit can accurately identify the fault segment without any missed or false identifications. The performance of the proposed method is analyzed below from the dimensions of fault resistance, fault distance, and new energy grid connection.

[0093] Taking a fault in the middle of section 7 as an example, the fault resistance is taken as 0~5000Ω. Under this condition, ring main unit 2 is directly connected to the fault section, and ring main unit 1 is adjacent to the upstream node. The current amplitude of both is significant and includes over-threshold current, which can reflect the response characteristics of the fault section and the upstream. The current amplitude of the other ring main units is small and none of them trigger the criteria. The current data of ring main units 1 and 2 are shown in Table 3.

[0094] Table 3. Shielding current characteristics of the fault in the middle of section 7 under different fault resistances.

[0095] As shown in Table 3, as the fault resistance increases from 0Ω to 5000Ω, the amplitude of the shielding grounding current in both ring main unit 1 and ring main unit 2 decreases significantly. The maximum current in ring main unit 2 drops from 288.85A to 0.93A, an attenuation of more than 300 times. However, the phase difference of the significant currents in the two ring main units remains stable: the phase difference in ring main unit 1 remains stable at around 177°, which is consistent with the typical characteristics of the upstream section of the fault; the phase difference in ring main unit 2 fluctuates within the range of 121° to 135°, and is always less than the criterion threshold of 140°.

[0096] To investigate the impact of fault distance on the shielding current, a metallic grounding fault in section 3 was used as an example, with the fault located at the beginning, middle, and end of the section. Fault distance primarily alters the shunting ratio of the fault current at both ends of the section, thus affecting the current amplitude of the ring main units on both sides, while having a relatively small impact on the phase difference. The current data extracted from the ring main units on both sides are shown in Table 4.

[0097] Table 4. Shielding current characteristics of section 3 fault at different fault distances.

[0098] As shown in Table 4, when the fault location moves from the beginning to the end of the section, the maximum current amplitude of ring main unit 1 gradually decreases, from 403.42A to 333.01A; while the maximum current amplitude of ring main unit 2 shows an upward trend, from 274.84A to 299.36A. This phenomenon is consistent with the change in the shunting ratio of the fault current at the beginning and end of the shielding layer with the fault distance. The phase difference between the two over-threshold currents in ring main unit 1 fluctuates slightly between 123.1° and 126.6°, with a change range of less than 4°, and does not show a significant shift due to the change in fault distance.

[0099] To investigate the impact of distributed photovoltaic (PV) grid connection on the shielding layer current characteristics, a high-resistance fault at the beginning of section 7 was used as an example, with a fixed fault distance of 1% and a fault resistance of 5000Ω. Under this condition, only ring main unit 1 and ring main unit 2 experienced over-threshold currents, while the currents of the other ring main units did not trigger the criterion. The extracted current data for the two ring main units are shown in Table 5.

[0100] Table 5. Current characteristics of the shielding layer for high-resistivity faults at the beginning of section 7 before and after photovoltaic grid connection.

[0101] As shown in Table 5, after the photovoltaic grid is connected, the maximum current amplitude and phase difference of ring main unit 1 remain unchanged; the maximum current amplitude of ring main unit 2 increases from 0.89A to 0.96A, and the phase difference decreases from 122.1° to 120.8°, with a change of less than 2°.

[0102] In summary, the increased fault resistance mainly causes a decrease in current amplitude, while the phase relationship does not shift significantly. Changes in fault distance primarily affect the current amplitude distribution at both ends of the segment, while the phase difference remains relatively stable. The impact of photovoltaic (PV) grid connection on both current amplitude and phase difference is not significant.

[0103] To verify the robustness of the proposed method, Gaussian white noise with signal-to-noise ratios of 50dB, 40dB, 30dB, and 20dB was added to the simulated shielding current signal, and the same procedure was used for fault location. The results are shown in Table 6.

[0104] Table 6 Noise Robustness Test Results

[0105] As shown in Table 6, the accuracy of the method is no less than 95% at noise levels of 20dB and above, indicating that the method has good noise immunity. Analysis of the error results revealed that the current threshold, set based on high-resistance grounding faults, is too low. Under low signal-to-noise ratio conditions, noise can easily trigger low-resistance faults such as metallic grounding prematurely, leading to a shift in the location window. This, in turn, causes errors in the calculation of the criteria based on amplitude ratio and phase difference, resulting in location failure. In practical applications, the threshold can be appropriately increased to achieve a balance between noise immunity and high-resistance fault detection sensitivity.

[0106] To further verify the overall performance of the proposed method, it was compared with existing typical segment localization methods, and the results are shown in Table 7.

[0107] Table 7 Comparison of the method in this application with existing segment location methods

[0108] As shown in Table 7, regarding multi-point communication dependency, existing methods all rely on multi-point communication to complete information collection and comparison. However, the method proposed in this application replaces multi-point communication with local comparison of amplitude ratio and phase difference, eliminating reliance on communication conditions. Regarding topology information dependency, existing methods generally require obtaining the topological connection relationships between segments, while the method proposed in this application can complete fault identification without this information. Regarding fault resistance withstand capability, the highest fault resistance withstand capability in existing methods is 3000Ω, while the method proposed in this application can reach 5000Ω, demonstrating better adaptability to high-resistance faults.

[0109] Based on the systematic analysis of the equivalent model and fault characteristics of the shielding layer grounding current in each section of the cable, this application proposes a method for locating single-phase grounding fault sections in distribution networks based on the shielding layer current. First, a coupled equivalent model of the shielding layer grounding current is established and its amplitude and phase characteristics are analyzed. Five typical fault conditions are classified according to the fault location, and section location criteria are proposed. Finally, the effectiveness and reliability of the proposed method are verified through simulation.

[0110] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for single-phase earth fault section location in a power distribution network based on shielded layer current, characterized in that, Includes the following steps: Using the cables between ring main units as the basic unit for fault location, a coupling equivalent model of three-core cables is established for both intact and faulty sections. Based on the coupling model, the amplitude and phase characteristics of the shielding current in the healthy section, the faulty section, the upstream faulty section, the downstream faulty section, and the healthy branch section are analyzed. Set an instantaneous value trigger threshold, collect the instantaneous values ​​of the current in each cable shield layer using the ring main unit as a unit, and perform filtering and noise reduction. When the instantaneous value of the grounding current of any shield layer continuously exceeds the instantaneous value trigger threshold, use fast Fourier transform to calculate the effective value and fundamental phase of each current cycle. Set an effective value trigger threshold and count the number of currents whose effective value exceeds the effective value trigger threshold among all shielding grounding currents in the ring main unit; The fault section is determined based on the number of currents and the phase difference relationship between different currents.

2. The method for single-phase earth fault section location in a power distribution network based on shield wire current according to claim 1, characterized in that, The instantaneous value trigger threshold is as follows: wherein, C is the cable-to-ground capacitance, V is the zero sequence voltage corresponding to the fault resistance under single-phase ground fault, M is the mutual inductance between the cable core and the shield layer, Z is the shield layer impedance, R is the shield layer head-end grounding resistance, R is the shield layer tail-end grounding resistance.

3. The method of claim 2, wherein, The filtering and denoising process employs a moving average filtering algorithm and a second-order Butterworth low-pass filtering algorithm. The formula for moving average filtering is as follows: wherein, with being a natural number, is the filtered i-th current instantaneous value, is the i-th current instantaneous value of the original acquisition, is the filtered i-th current instantaneous value, is the i-th current instantaneous value of the original acquisition, is the filter window length; The transfer function of a second-order Butterworth low-pass filter is as follows: wherein is a low-pass filter cut-off angular frequency, is a Laplacian operator.

4. The method of claim 3, wherein, The formula for calculating the effective value is as follows: The formula for calculating the fundamental phase is as follows: wherein is the fundamental complex amplitude, is the modulus of the fundamental complex amplitude, is the complex angle evaluation operation.

5. The method of claim 4, wherein, The effective value trigger threshold is determined based on the third effective current value arranged from largest to smallest among the effective current values ​​in the ring main unit. The calculation formula is as follows: wherein, is a reliability coefficient, is the third current effective value arranged from large to small among the current effective values in the ring main unit.

6. The method of claim 5, wherein, The fault-determining section includes: If the current effective value does not exceed , then the ring main unit has no fault section; If only one current exceeds then the section to which this current belongs is the faulty section; If two currents exceed If the phase difference is less than 140°, then the section with the phase leading is the fault section; If two currents exceed If the phase difference is greater than 140°, then a branch in the downstream section of this ring network cabinet is a fault section.

7. The method for locating single-phase grounding fault sections in a distribution network based on shielding layer current according to claim 6, characterized in that, The formula for calculating the phase difference is as follows: in, For the first The phase difference value after normalization of the current, mod(·) is the modulo remainder operation. For the first A current signal For reference signal, For the first Each current phase.

8. The method for locating single-phase grounding fault sections in a distribution network based on shielding layer current according to claim 7, characterized in that, The method further includes: If the faulty section is not effectively located after the fault triggering process is completed, the current data window cache will be automatically cleared and instantaneous value detection will be restarted from the next sampling point after the start of this event.

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