Method and device for locating cable grounding fault section

Abstract

The present invention relates to a method and device for locating a section where a cable grounding fault occurs, which is applied to a cable distribution network with an ungrounded neutral point. The method for locating a section where a cable grounding fault occurs comprises the following steps: Step 1: synchronously sampling the zero-sequence voltage of the cable distribution network and the ground current at the beginning and end of each section of the feeder cable, and calculating the corresponding vector values ​​respectively; Step 2: calculating the accommodation calculation value of each section of the feeder cable respectively; Step 3: calculating the fault judgment value of each section of the feeder cable based on the accommodation calculation value and the accommodation estimation value of each section of the feeder cable respectively; Step 4: determining the location of the grounding fault in the cable distribution network based on the fault judgment value of each section of the feeder cable. The device for locating a section where a cable grounding fault occurs comprises a sampling unit and a server. The present invention has a simple principle, is easy to implement, and is convenient to use. It can locate both grounding faults with low fault impedance and grounding faults with high fault impedance.

Description

Technical Field

[0001] The present invention belongs to the field of electric power automation, and in particular relates to a method and device for locating a cable grounding fault occurrence section, which is applicable to a cable distribution network with an ungrounded neutral point. Background Art

[0002] Cable distribution networks are widely used for underground power supply in coal mines and other mines. In recent years, driven by people's pursuit of a beautiful urban environment and the need for higher power supply reliability, as well as by improvements in power cable production technology and reductions in cable application costs, 10kV urban distribution networks are increasingly being converted to three-core cable power supply.

[0003] The copper foil shield and armor of a three-core cable, along with the grounding electrodes of the distribution network, form the grounding grid. When a ground fault occurs in the grid, a regular distribution of ground current forms within this grid. This regularity in ground current distribution provides new principles and methods for identifying ground-fault feeders. Furthermore, because the distribution of ground current within the ground wire is segmented by the grounding electrodes, the current in each segment varies, further facilitating the identification and location of the ground fault.

[0004] Unlike overhead line distribution networks, cable distribution networks have advantages such as fewer faults and are conducive to improving power supply reliability. However, once a ground fault occurs, it is more difficult to find the fault point than in overhead line networks. Therefore, locating the cable section where the ground fault occurs can greatly reduce the search range for the ground fault, greatly reducing the time and workload of finding the ground fault. At the same time, it is also extremely beneficial to improve power supply reliability. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for locating a cable grounding fault occurrence section which has a simple principle, a small amount of calculation and is easy to implement.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A method for locating a section where a cable grounding fault occurs is applied to a cable distribution network with an ungrounded neutral point. The method for locating a section where a cable grounding fault occurs comprises the following steps:

[0008] Step 1: synchronously sampling the zero-sequence voltage of the cable distribution network with an ungrounded neutral point and the ground wire currents at the beginning and end of each feeder cable section where a ground fault occurs, and calculating the corresponding vector values ​​respectively;

[0009] Step 2: Calculating the capacity calculation value of each section of the feeder cable based on the vector value of the zero-sequence voltage and the vector value of the ground current at the beginning and end of each section of the feeder cable;

[0010] Step 3: Calculate the fault judgment value of each feeder cable section based on the calculated capacity value and estimated capacity value of each feeder cable section;

[0011] Step 4: Determine the location of the grounding fault in the cable distribution network with an ungrounded neutral point based on the fault determination amount of each section of the feeder cable.

[0012] In step 1, the phasor value corresponding to the zero-sequence voltage is U 0Re +jU 0Im The phasor values ​​of the ground current at the beginning and end of each section of the feed cable are I siRe +jI siIm , I miRe +jI miIm , where subscript i is the serial number of the feeder cable, subscripts s and m represent the head end and the end end of the feeder cable respectively, and subscripts Re and Im represent the real part and the imaginary part respectively, where:

[0013]

[0014]

[0015]

[0016] u0(k) is the zero sequence voltage, i si (k) is the ground current at the head end of the i-th section of the feeder cable, i mi (k) is the ground current at the end of the i-th section of the feed cable, and N is the number of sampling points in one fundamental wave cycle.

[0017] In step 2, the calculated capacity of each section of the feeder cable

[0018]

[0019] In step 3, the fault judgment value of each section of the feeder cable is Δ3ωC i =3ωC j.i -3ωC g.i , where 3ωC g.i Provide an estimated capacity for feeder cables for each segment.

[0020] In step 4, if the fault judgment value of each section of the feeder cable has a unique maximum value and the maximum value is greater than a preset setting value, then a ground fault occurs in the section of the feeder cable corresponding to the maximum fault judgment value; if the fault judgment value of each section of the feeder cable is less than the setting value, then a ground fault occurs in the terminal busbar of the feeder cable with the largest ground current at the head end or the terminal end.

[0021] The step 1 also includes a preceding step: determining whether a ground fault occurs in the cable distribution network with an ungrounded neutral point, and if so, executing the step 1;

[0022] In the preceding step, the method for determining whether a ground fault occurs in the cable distribution network with an ungrounded neutral point is as follows: sampling the zero-sequence voltage of the cable distribution network with an ungrounded neutral point, and determining whether the zero-sequence voltage exceeds a set threshold value; if the zero-sequence voltage exceeds the set threshold value, a ground fault occurs in the cable distribution network with an ungrounded neutral point.

[0023] The present invention also provides a device for detecting a ground fault in a cable distribution network with an ungrounded neutral point, wherein:

[0024] A device for locating a section where a cable grounding fault occurs is used in a cable distribution network with an ungrounded neutral point. The device for locating a section where a cable grounding fault occurs comprises:

[0025] A sampling unit, the sampling unit is used to synchronously sample the zero-sequence voltage of the neutral point ungrounded cable distribution network and the ground wire current at the beginning and end of each section of the feeder cable;

[0026] A server is communicatively connected to the sampling unit and is used to respectively calculate the zero-sequence voltage of the cable distribution network with an ungrounded neutral point and the vector values ​​corresponding to the ground current at the beginning and end of each section of the feeder cable; based on the vector values ​​of the zero-sequence voltage and the vector values ​​of the ground current at the beginning and end of each section of the feeder cable, respectively calculate the capacity calculation value of each section of the feeder cable; based on the capacity calculation value and the capacity estimation value of each section of the feeder cable, respectively calculate the fault judgment value of each section of the feeder cable; and based on the fault judgment value of each section of the feeder cable, determine the location of the grounding fault in the cable distribution network with an ungrounded neutral point.

[0027] The sampling unit includes a zero-sequence voltage transformer and a ground current transformer.

[0028] The sampling unit is connected to the server via wireless communication. The sampling unit is connected to the server via a Rola communication system or a NB narrowband communication system.

[0029] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art: the present invention has a simple principle, is easy to implement, and is easy to use, and can locate both low fault impedance ground faults and high fault impedance ground faults. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Attachment Figure 1 This is a three-phase simulation model diagram of a three-core cable distribution network grounding fault.

[0031] Attachment Figure 2This is the zero-sequence equivalent circuit diagram of a grounding fault in a three-core cable distribution network.

[0032] Attachment Figure 3 It is a zero-sequence simulation model diagram of a three-core cable distribution network with a ground fault.

[0033] Attachment Figure 4 is the zero-sequence voltage Ground current distribution diagram under single action.

[0034] Attachment Figure 5 is the fault current Ground current distribution diagram under single action.

[0035] Attachment Figure 6 This is the installation diagram of the ground current transformer. DETAILED DESCRIPTION

[0036] The present invention will be further described below with reference to the embodiments shown in the accompanying drawings.

[0037] Embodiment 1: A method for locating a section where a cable grounding fault occurs in a cable distribution network with an ungrounded neutral point, comprising the following steps:

[0038] Preliminary step: determine whether a ground fault occurs in the cable distribution network with an ungrounded neutral point. If so, proceed to step 1.

[0039] In this pre-sequence step, the method for determining whether a ground fault occurs in the cable distribution network with an ungrounded neutral point is: sampling the zero-sequence voltage of the cable distribution network with an ungrounded neutral point, and determining whether the zero-sequence voltage exceeds a set threshold value; if the zero-sequence voltage exceeds the set threshold value, a ground fault occurs in the cable distribution network with an ungrounded neutral point.

[0040] Step 1: Synchronously sample the zero-sequence voltage u0(k) of the cable distribution network with an ungrounded neutral point and the ground wire current i at the beginning and end of each section of the feeder cable. si (k), i mi (k), and calculate the corresponding vector values ​​respectively.

[0041] In step 1, the phasor value corresponding to the zero-sequence voltage u0(k) is U 0Re +jU 0Im , the ground current i at the beginning and end of each section of the feed cable si (k), i mi The corresponding phasor values ​​of (k) are I siRe +jI siIm , I miRe +jI miIm , where subscript i is the serial number of the feeder cable, subscripts s and m represent the head end and the end end of the feeder cable respectively, and subscripts Re and Im represent the real part and the imaginary part respectively, where:

[0042]

[0043]

[0044]

[0045] i si (k) is the ground current at the head end of the i-th section of the feeder cable, i mi (k) is the ground current at the end of the i-th section of the feed cable, and N is the number of sampling points in one fundamental wave cycle.

[0046] Step 2: Calculate the capacity calculation value of each section of the feeder cable based on the vector value of the zero-sequence voltage and the vector value of the ground current at the beginning and end of each section of the feeder cable.

[0047] In step 2, the calculated capacity of each section of the feeder cable is 3ωC. j.i for:

[0048]

[0049] Here the subscript j indicates the accommodation calculation value, C j.i The calculated value of the capacitance per phase to ground of the i-th section feed cable.

[0050] Step 3: Set the ground current flowing into the grounding electrode as the reference direction, and calculate the fault judgment value of each section of the feeder cable according to the calculated capacity value and estimated capacity value of each section of the feeder cable.

[0051] In step 3, the fault determination value of each feeder cable section is:

[0052] Δ3ωC i =3ωC j.i -3ωC g.i (2)

[0053] Among them, 3ωC g.i is the estimated capacity of each section of feeder cable, 3ωC g.i Estimated value of the capacitance per phase to ground of the feed cable for the i-th segment.

[0054] Step 4: Determine the location of a ground fault in the cable distribution network with an ungrounded neutral point based on the fault judgment quantification of each feeder cable segment. If the fault judgment quantification of each feeder cable segment has a unique maximum value and this maximum value is greater than a preset setting value, then the feeder cable segment corresponding to the maximum fault judgment quantification has a ground fault. If the fault judgment quantification of each feeder cable segment is less than the setting value, then the terminal busbar of the feeder cable with the largest ground current at the head or tail end has a ground fault.

[0055] Due to the symmetry of the three-core cable core and the contact between the three-phase copper foil shielding layers, there is no induced voltage in the copper foil layer when the power grid is operating normally. Therefore, the cables are all grounded at both ends. Figure 1 It is a simple cable distribution network, and the cable adopts a π-type model. Since the resistance and reactance of the cable core and the reactance of the copper foil shielding layer are not only much smaller than the capacitive reactance of the distributed capacitance of the power grid to the ground and the neutral point grounding impedance, but also much smaller than the resistance of the copper foil shielding layer, they are ignored in the figure to simplify the analysis. For a more complex distribution network, considering the ground wire and grounding electrode, it may have at least dozens of "nodes" and hundreds to thousands of "meshes". It is difficult to directly analyze them. However, when a single-phase grounding fault occurs in the cable distribution network, there is only zero-sequence current in the ground wire current, and all zero-sequence current flows into the grounding network. According to the symmetrical component theory, when a single-phase grounding fault occurs in the three-phase power grid at f, the voltage at the fault point and It can be decomposed into three symmetrical components: positive sequence, negative sequence and zero sequence voltage:

[0056]

[0057] in,:

[0058] Therefore, it can be Figure 1 The three-phase circuit is decomposed into three symmetrical circuits for analysis. Moreover, since the phases of the positive sequence and negative sequence are 120° apart, the sum of the three currents at any moment is equal to zero and does not flow through the fault point f. Therefore, when we only need the distribution of the ground wire current, since there is only zero sequence current in the ground wire current, and no positive sequence and negative sequence current, Figure 1 The three-phase circuit can be simplified into a zero-sequence equivalent circuit, see Figure 2 , the distribution of single-phase ground fault current in the ground wire can be accurately analyzed only in the zero-sequence circuit.

[0059] In the figure, i=1~n is the serial number of the grounding electrode from the main substation to the user side grounding electrode; j=1~m is the serial number of the ground wire connected to the i-th grounding resistor, and m is the total number of ground wires connected to the i-th grounding resistor; is the current in the jth ground wire connected to the i-th ground electrode; R Cui.j 、R Fei.j and C i.j are the copper foil shielding layer (three phases), armor layer resistance and distributed capacitance of each phase of the jth cable between the i-th and i+1-th cables; R ei.j is the resistance of the i-th grounding electrode; and are the three-phase power supply potentials respectively.

[0060] Figure 2 middle It is the zero-sequence voltage of the three-phase grid when a single-phase ground fault occurs at point F; is the fault current flowing through the fault point f (three times the zero-sequence current). To make analysis easier, according to the circuit's "substitution theorem", if the voltage and current of the connection port of two one-port networks in the circuit are known, then one of the one-port networks can be replaced by a voltage source with a voltage equal to the connection port or a current source with a current equal to the connection port. Split into and and make Since the voltage source Current and voltage source The current is the same as Then use the current source Alternative voltage source get Figure 3 Zero-sequence model for analyzing ground current distribution during single-phase ground fault in cable distribution network.

[0061] According to the superposition principle of linear circuits, Figure 3 It can be decomposed into two circuits, and the ground current can be analyzed and calculated independently in the two circuits and Then, add the two analysis results together, that is: You can accurately obtain the image Figure 1 Ground current distribution when a single-phase ground fault occurs in a three-phase distribution network. and They are: ground wire current and zero sequence voltage of the jth branch ground wire of the i-th grounding electrode Ground current and fault current under separate action Ground current under single action.

[0062] Zero-sequence voltage Ground current distribution under single action: Figure 3 middle, (open circuit), forming zero sequence voltage Circuit under single action Figure 4 The grid conductor and the earth (E) are subjected to zero-sequence voltage. Under this condition, the zero sequence current changes from Starting from the ground, it passes through the distributed capacitance of the cable and electrical equipment to the ground wire, then enters the earth E, and returns to the zero-sequence voltage Depend on Figure 4It can be seen that for all cable segments without ground faults, the sum of the ground currents at the cable headend and end (assuming the reference direction of the ground current at both ends is set to flow positively toward the ground pole) equals the three-phase-to-ground capacitance current of that cable segment. Dividing this sum by the zero-sequence voltage gives the three-phase-to-ground total capacitance of that cable segment. The actual estimated capacitance (estimated capacitance) of that cable segment is subtracted from the three-phase-to-ground total capacitance (calculated capacitance) of that cable segment, and this difference is used as the fault determination variable. If the estimated capacitance is accurate, the difference is theoretically equal to zero. Taking into account the estimation error of the actual estimated capacitance (for a given cable network, the conductor cross-section, type, and length of the cable segment are fixed and known, allowing for capacitance estimation), measurement errors of the current transformers, and calculation errors of the monitoring devices, a set value is set as the threshold for the fault determination variable to prevent false trips (false alarms). This fault determination variable is a constant that is independent of the fault impedance at the ground fault point. Therefore, it can locate both low-fault-impedance and high-fault-impedance ground faults. This is a significant advantage of this location method. The inverse of the high fault impedance value that needs to be responded to can be set as the setting value of the fault judgment amount. If the fault judgment amount is less than the setting value of the fault judgment amount, there must be no ground fault in this section of the cable.

[0063] Fault current I at fault point f k Ground current distribution under single action: Figure 3 In the equation, since the capacitive reactance of the distributed capacitance is much larger than the grounding electrode resistance and the ground wire resistance, the distributed capacitance is regarded as an open circuit, and the following equation is made: The fault current I forming the fault point f k A circuit acting alone, such as Figure 5 . Fault current I k It does not flow through the conductor, but only through the grounding electrode and the ground wire. Moreover, for all cable sections without ground faults, this current simultaneously passes through the ground wire current transformers at the beginning and end of the cable, which does not affect the calculated value of formula (2). For cable sections with ground faults, the fault current I k It flows into the ground current transformer at the beginning and end of both sides and flows into the earth through the fault point. This current is related to the zero-sequence voltage. Under the action of a single cable, the ground current of each cable section has opposite directions. Therefore, for the cable section where the ground fault occurs, the calculated value 3ωC is accommodated. i -I k / U0, the fault judgment value of this cable is equal to I k / U0, therefore, if there is only one cable section with a fault quantity of Δ3ωC i If it is greater than the set value, then this cable section must be the cable section where the ground fault occurs.

[0064] from Figure 5It can also be seen that if the ground fault does not occur on the cable, but on the busbar (inside the switch cabinet, or on the busbar, etc.), then the fault current I k The ground current transformers at the beginning and end of all cable segments are passed through, so the fault quantity of all cable segments is Δ3ωC i However, it is obvious that the ground current of the cable section with the ground fault point toward the power supply side (regardless of the current transformer at the beginning or end) is the largest. Therefore, if the fault judgment value Δ3ωC of all cable sections is i If the amplitudes are all smaller than the set value, then there is a ground fault at the end busbar of the corresponding feeder cable segment where the ground current at the beginning or end of the cable is the largest.

[0065] The device for locating the section where a cable grounding fault occurs, which implements the above-mentioned method for locating the section where a cable grounding fault occurs, includes a sampling unit and a server. The sampling unit is used to synchronously sample the zero-sequence voltage of the cable distribution network with an ungrounded neutral point and the ground wire current at the beginning and end of each section of the feeder cable. The server is used to respectively calculate the vector values ​​corresponding to the zero-sequence voltage of the cable distribution network with an ungrounded neutral point and the ground wire current at the beginning and end of each section of the feeder cable; based on the vector value of the zero-sequence voltage and the vector value of the ground wire current at the beginning and end of each section of the feeder cable, respectively calculate the capacity calculation value of each section of the feeder cable; based on the capacity calculation value and the capacity estimation value of each section of the feeder cable, respectively calculate the fault judgment value of each section of the feeder cable; based on the fault judgment value of each section of the feeder cable, determine the location of the grounding fault in the cable distribution network with an ungrounded neutral point.

[0066] The sampling unit includes a zero-sequence voltage transformer (zero-sequence voltage acquisition device) and a ground current transformer (ground current acquisition device). The zero-sequence voltage transformer is installed in the substation, and a zero-sequence voltage monitoring device is installed at its location. Each line current transformer is installed at the beginning and end of each section of the feeder cable, and a ground current monitoring device is also configured. The installation of the ground current transformer is as follows: Figure 6 , the ground current transformer uses the current flowing into the ground pole as the reference direction.

[0067] Since the zero-sequence voltage and local line current collection devices are not in the same location, the sampling unit is generally connected to the server through wireless communication (Rola communication system or NB narrowband communication system or other wireless communication system), so that the collected data is transmitted to the server, and then the feeder cable segment where the ground fault occurs is calculated and determined.

[0068] The above provides a new method for locating a ground fault feeder cable segment in any cable distribution network with an ineffectively grounded neutral point. While cable distribution networks offer advantages such as fewer faults and improved power supply reliability, once a ground fault occurs, it is more difficult to find the fault point than in overhead power lines. Therefore, locating the cable segment where the ground fault occurs can significantly narrow the search range for the ground fault, significantly reducing the time and workload required to find the ground fault, while also significantly improving power supply reliability. The fault determination variable is a constant that is independent of the fault impedance at the ground fault point. This method is particularly advantageous in that it can locate both low-fault-impedance and high-fault-impedance ground faults.

[0069] The above embodiments are intended only to illustrate the technical concepts and features of the present invention. Their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. They are not intended to limit the scope of protection of the present invention. Any equivalent changes or modifications made in accordance with the spirit of the present invention are intended to be covered by the scope of protection of the present invention.

Claims

1. A method for locating a cable grounding fault section, applied to a cable distribution network with an ungrounded neutral point, characterized by: The method for locating a cable ground fault section comprises the following steps: Step 1: synchronously sampling the zero-sequence voltage of the cable distribution network with an ungrounded neutral point and the ground wire currents at the beginning and end of each feeder cable section, and calculating the corresponding phasor values ​​respectively; Step 2: Calculating the capacity calculation value of each section of the feeder cable based on the phasor value of the zero-sequence voltage and the phasor value of the ground wire current at the beginning and end of each section of the feeder cable; Step 3: Calculate the fault judgment value of each feeder cable section based on the calculated capacity value and estimated capacity value of each feeder cable section; Step 4: determining the location of the grounding fault in the cable distribution network with an ungrounded neutral point based on the fault judgment amount of each section of the feeder cable; In step 1, the phasor value corresponding to the zero-sequence voltage is U 0Re +jU 0Im The phasor values ​​of the ground current at the beginning and end of each section of the feed cable are I siRe +jI siIm , I miRe +jI miIm , where subscript i is the serial number of the feeder cable, subscripts s and m represent the head end and the end end of the feeder cable respectively, and subscripts Re and Im represent the real part and the imaginary part respectively, where: u0(k) is the zero sequence voltage, i si (k) is the ground current at the head end of the i-th section of the feeder cable, i mi (k) is the ground current at the end of the i-th section of the feeder cable, and N is the number of sampling points in one fundamental wave cycle; In step 2, the calculated capacity of each section of the feeder cable In step 3, the fault judgment value of each section of the feeder cable is Δ3ωC i =3ωC j.i -3ωC g.i , where 3ωC g.i Estimated capacity of feeder cables for each section; In step 4, if the fault judgment value of each section of the feeder cable has a unique maximum value and the maximum value is greater than a preset setting value, then a ground fault occurs in the section of the feeder cable corresponding to the maximum fault judgment value; if the fault judgment value of each section of the feeder cable is less than the setting value, then a ground fault occurs in the terminal busbar of the feeder cable with the largest ground current at the head end or the terminal end.

2. The method for locating a cable ground fault section according to claim 1, characterized in that: The step 1 also includes a preceding step: determining whether a ground fault occurs in the cable distribution network with an ungrounded neutral point, and if so, executing the step 1; In the preceding step, the method for determining whether a ground fault occurs in the cable distribution network with an ungrounded neutral point is as follows: sampling the zero-sequence voltage of the cable distribution network with an ungrounded neutral point, and determining whether the zero-sequence voltage exceeds a set threshold value; if the zero-sequence voltage exceeds the set threshold value, a ground fault occurs in the cable distribution network with an ungrounded neutral point.

3. A device for locating a cable grounding fault section, applied to a cable distribution network with an ungrounded neutral point, for implementing the method for locating a cable grounding fault section as claimed in claim 1, characterized in that: The device for locating the section where the cable grounding fault occurs comprises: A sampling unit, the sampling unit is used to synchronously sample the zero-sequence voltage of the neutral point ungrounded cable distribution network and the ground wire current at the beginning and end of each section of the feeder cable; A server is communicatively connected to the sampling unit and is used to respectively calculate the zero-sequence voltage of the cable distribution network with an ungrounded neutral point and the phasor values ​​corresponding to the ground current at the beginning and end of each section of the feeder cable; based on the phasor values ​​of the zero-sequence voltage and the phasor values ​​of the ground current at the beginning and end of each section of the feeder cable, respectively calculate the capacity calculation value of each section of the feeder cable; based on the capacity calculation value and the capacity estimation value of each section of the feeder cable, respectively calculate the fault judgment value of each section of the feeder cable; and based on the fault judgment value of each section of the feeder cable, determine the location of the grounding fault in the cable distribution network with an ungrounded neutral point.

4. The device for locating a cable ground fault section according to claim 3, characterized in that: The sampling unit includes a zero-sequence voltage transformer and a ground current transformer.

5. The device for locating a cable ground fault section according to claim 3, characterized in that: The sampling unit is connected to the server via wireless communication.

6. The device for locating a cable ground fault section according to claim 5, characterized in that: The sampling unit is connected to the server via a Rola communication system or a NB narrowband communication system.

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