Partial discharge detection device for intermediate joint of high-voltage cable

Abstract

The invention provides a partial discharge detection device for an intermediate joint of a high-voltage cable. Comprising a first partial discharge sensor directly installed on a high-voltage cable straight-through type intermediate joint, a second partial discharge sensor directly installed on a high-voltage cable insulation type intermediate joint, and a partial amplification signal processing circuit respectively connected with the first partial discharge sensor and the second partial discharge sensor. The invention designs a partial discharge sensor and a detection circuit which are directly mounted on the intermediate joint of the high-voltage cable, and provides a defective joint identification method for the insulated intermediate joint of the high-voltage cable. Since the partial discharge sensor is directly mounted on the cable intermediate joint, the detection sensitivity of the partial discharge sensor is far higher than that of the partial discharge sensor mounted on the cable, and an energy distribution-based high-voltage cable insulation type intermediate joint defect joint identification method is provided, so that the insulation state monitoring effect of cable accessories can be improved.

Description

Technical Field

[0001] The invention belongs to the technical field of partial discharge detection devices for cable joints, and in particular relates to a partial discharge detection device for an intermediate joint of a high-voltage cable. Background Art

[0002] High-voltage cross-linked polyethylene insulated power cables are widely used in urban power grids due to their good electrical performance and convenient laying. When the high-voltage power cable line is long, an intermediate joint is often required. There are two types of intermediate joints: 1. Straight-through intermediate joints must be used due to the need for cable connection; 2. Insulated intermediate joints must be used due to the need for cross-connection and grounding. Since the service life of the cable is 30 years, the cable and accessories are very likely to fail during long-term operation. According to statistics from the State Grid Corporation of China, accessory failures account for more than 70% of cable line failures.

[0003] Partial discharge failure is not only the main cause of cable accessory failure, but also the key characteristic quantity to characterize the status of cable accessories. Therefore, partial discharge monitoring of cable accessories to judge the health status of cable accessories is a top priority of the State Grid Corporation of China's power transmission and transformation equipment operation and maintenance work.

[0004] At present, the local discharge detection of cable accessories often adopts the high-frequency current method, that is, the local discharge current pulse signal on the grounding wire is detected by the high-frequency current transformer inside the cable intermediate joint. This method has the following disadvantages: (1) When the high-frequency current transformer is installed in an inappropriate position, the local discharge signal of the cable intermediate joint cannot be effectively monitored; (2) The cable intermediate joint structure needs to be partially modified, which destroys the original compact insulation structure and may have an adverse effect on the electrical and mechanical properties.

[0005] Xi'an Jiaotong University proposed an indirect cable accessory partial discharge detection method. This method is based on the principle of capacitive coupling, that is, a metal belt is wrapped around the outer semi-conductive layer of the cable near the middle joint. The metal belt and the cable core form a high-voltage capacitor, and the metal belt and the grounded copper mesh form a low-voltage capacitor. The partial discharge signal is measured using the principle of capacitive voltage division. However, the disadvantage of this method is that it uses a cable sensor installed near the middle joint to indirectly respond to the partial discharge of the middle joint, rather than directly detecting the partial discharge signal of the middle joint. This situation causes the detected signal to be one of the partial discharge of the cable, the partial discharge of the cable accessories, the ground potential interference, or a combination of the three. In actual application, the insulation status monitoring effect of the cable accessories is poor. Summary of the invention

[0006] In view of this, the present invention aims to overcome the deficiencies of the above-mentioned problems in the prior art and proposes a partial discharge detection device for an intermediate joint of a high-voltage cable.

[0007] To achieve the above object, the technical solution of the present invention is achieved as follows:

[0008] A local discharge detection device for a high-voltage cable intermediate joint, comprising a first local discharge sensor directly mounted on a high-voltage cable straight-through intermediate joint, a second local discharge sensor directly mounted on an insulated intermediate joint of the high-voltage cable, and a local amplification signal processing circuit respectively connected to the first local discharge sensor and the second local discharge sensor;

[0009] The first partial discharge sensor comprises a first wire core, a first outer semiconductive layer and a grounding copper mesh, a first high-voltage electrode, a first 50Ω resistor, a first BNC waterproof connector, and a first coaxial signal cable, the grounding copper mesh of the outer semiconductor layer is removed, a 2mm thick epoxy resin glue is applied on the semiconductive layer, a circular copper electrode with a length less than the cut length of the epoxy resin glue and a thickness of 1mm is applied to the epoxy resin glue, the copper electrode is soldered with a signal lead, the circular copper electrode and the first wire core constitute a high-voltage capacitor, the circular copper electrode and the first outer semiconductor layer and the grounding copper mesh constitute a low-voltage capacitor, and the partial discharge signal detected on the circular copper electrode is sent to the partial discharge signal processing circuit through the first 50Ω resistor, the first BNC waterproof connector and the first coaxial signal cable;

[0010] The second partial discharge sensor includes a second wire core, a second outer semi-conductive layer and a grounding copper mesh, a second high-voltage electrode, a second 50Ω resistor, a second BNC waterproof connector, a second coaxial signal cable, and an insulating flange. The grounding copper mesh of the outer semi-conductive layer is removed, and a circular copper electrode with a length less than the length of the cut grounding copper mesh and a thickness of 1mm is applied to the outer semi-conductive layer. The signal lead is soldered to the copper electrode with solder, and a 2mm thick epoxy resin glue is applied to the copper electrode. Then, the grounding copper mesh is connected to the outer surface of the epoxy resin glue with two semi-circular flanges. The circular copper electrode and the second wire core constitute a high-voltage capacitor, and the circular copper electrode and the second shell of the intermediate connector constitute a low-voltage capacitor. The partial discharge signal detected on the circular copper electrode is sent to the partial discharge signal processing circuit through the second 50Ω resistor, the second BNC waterproof connector and the second coaxial signal cable.

[0011] Furthermore, the first 50Ω resistor is made of two 0.02mm constantan wires, the first BNC connector uses a BNC connector with a waterproof grade of IP65, and waterproof sealing grease is applied between the first BNC connector and the first shell thread, and a radio frequency coaxial signal cable with a characteristic impedance of 50Ω is used to transmit the signal.

[0012] Furthermore, the second 50Ω resistor is made of two 0.02mm constantan wires, the second BNC connector uses a BNC connector with a waterproof grade of IP65, and waterproof sealing grease is applied between the second BNC connector and the second shell thread, and a radio frequency coaxial signal cable with a characteristic impedance of 50Ω is used to transmit the signal.

[0013] Furthermore, the partial discharge signal processing circuit includes a DC blocking capacitor and an active low-pass filter, wherein the amplifier is powered by a regulated power supply.

[0014] Furthermore, the cut-off frequency of the low-pass filter is 300 MHz.

[0015] Furthermore, for the high-voltage cable straight-through connector, the health status of the straight-through connector is determined according to the first partial discharge sensor installed in each phase.

[0016] Furthermore, for a three-phase cable system with insulating joints, the criterion for defective insulating joints is:

[0017] (1) The phase with the smallest amplitude of the detection signal of the second partial discharge sensor installed on the insulating joint is phase X;

[0018] (2) Determine the cross-connection of the X-phase insulation joint to be the X-phase and the Y-phase;

[0019] (3) The defective phase is the Z phase other than the X phase and the Y phase.

[0020] Compared with the prior art, the high-voltage cable intermediate joint partial discharge detection device described in the present invention has the following advantages:

[0021] (1) The local discharge detection device for the intermediate joint of a high-voltage cable designed by the present invention has a higher sensitivity than the current sensor currently in practical use, and has a higher accuracy than the capacitive sensor installed on the cable currently in practical use.

[0022] (2) In view of the different structures of the straight-through connector and the insulated connector of the high-voltage cable, partial discharge sensors are designed respectively. The partial discharge sensor installed on the straight-through connector does not change the structural requirements for the connectivity of the grounding copper grid of the straight-through connector, and the partial discharge sensor installed on the insulated connector does not change the structural requirements for the segmented insulation of the insulated connector.

[0023] (3) In view of the dust and water resistance requirements of the high-voltage cable straight-through connector and the insulated connector, a BNC connector is used to ensure that the introduction of the partial discharge sensor will not reduce the dust and water resistance level of the intermediate connector;

[0024] (4) In order to meet the requirements of high-frequency partial discharge signal transmission, non-inductive resistors and radio frequency coaxial cables are used for signal transmission to ensure distortion-free transmission of high-frequency partial discharge signals;

[0025] (5) Based on the simulation analysis and partial discharge test results, a partial discharge signal processing circuit was designed. The partial discharge signal processing circuit consists of a DC blocking capacitor C0 and an active low-pass filter with a cut-off frequency of 300MHz. The amplifier is powered by a regulated power supply. This ensures that all partial discharge signals can pass through the signal processing circuit and filter out higher-frequency interference signals.

[0026] (6) According to the results of multi-conductor transmission line modulus analysis, it is found that the signal amplitude monitored by the three-phase insulation joint shows the characteristics of "two large and one small", and based on this, a defective joint criterion for high-voltage cable insulation joints is proposed. This criterion is simple and effective, and can be conveniently applied to the evaluation of the insulation status of the cross-connected insulation joints of three-phase high-voltage cables. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0028] Figure 1 It is a schematic diagram of installing a first partial discharge sensor of the present invention;

[0029] Figure 2 It is a schematic diagram of installing a second partial discharge sensor of the present invention;

[0030] Figure 3 It is a schematic diagram of a partial discharge signal processing circuit of the present invention;

[0031] Figure 4 It is a schematic diagram of the relationship between the detection impedance current and the axial length of the present invention;

[0032] Figure 5 A schematic diagram of the relationship between the detection impedance current and the radial distance of the present invention;

[0033] Figure 6 A schematic diagram of a three-phase cable cross-connection according to the present invention;

[0034] Figure 7 It is a schematic diagram of the signal amplitude of the three-phase insulating joint sensor of the present invention.

[0035] Description of reference numerals:

[0036] 1-first wire core, 2-first conductor crimping tube, 3-first cross-linked polyethylene insulation, 4-first stress cone, 5-first silicone rubber insulation, 6-first outer semi-conductive layer and grounding copper mesh, 7-first shell, 8-first high-voltage electrode, 9-first 50Ω resistor, 10-first BNC waterproof connector, 11-first coaxial signal cable, 12-second wire core, 13-second conductor crimping tube, 14-second cross-linked polyethylene insulation, 15-second stress cone, 16-second silicone rubber insulation, 17-second outer semi-conductive layer and grounding copper mesh, 18-second shell, 19-second high-voltage electrode, 20-second 50Ω resistor, 21-second BNC waterproof connector, 22-second coaxial signal cable, 23-second insulating flange. DETAILED DESCRIPTION

[0037] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0038] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", and the like are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, features defined as "first", "second", and the like may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, "multiple" means two or more.

[0039] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood by specific circumstances.

[0040] The present invention will be described in detail below with reference to the accompanying drawings and in conjunction with embodiments.

[0041] like Figure 1-2As shown, the present invention discloses a local discharge detection device for a high-voltage cable intermediate joint, comprising a first local discharge sensor directly mounted on a high-voltage cable straight-through intermediate joint, a second local discharge sensor directly mounted on an insulated intermediate joint of the high-voltage cable, and a local amplification signal processing circuit respectively connected to the first local discharge sensor and the second local discharge sensor;

[0042] The first partial discharge sensor comprises a first wire core 1, a first outer semiconductive layer and a grounding copper mesh 6, a first high-voltage electrode 8, a first 50Ω resistor 9, a first BNC waterproof connector 10, and a first coaxial signal cable 11. The grounding copper mesh of the outer semiconductor layer is removed, and a 2mm thick epoxy resin glue is applied on the semiconductive layer. A circular copper electrode with a length less than the length of the epoxy resin glue cut and a thickness of 1mm is applied to the epoxy resin glue. The copper electrode is soldered with a signal lead by soldering tin. The circular copper electrode and the first wire core 11 form a high-voltage capacitor, and the circular copper electrode and the first outer semiconductor layer and the grounding copper mesh 6 form a low-voltage capacitor. The partial discharge signal detected on the circular copper electrode is sent to the partial discharge signal processing circuit through the first 50Ω resistor 9, the first BNC waterproof connector 10 and the first coaxial signal cable 11. When the first partial discharge sensor is installed, it is connected to the intermediate joint through the first shell 7 of the intermediate joint, the first conductor crimping tube 2, the first cross-linked polyethylene insulation 3, the first stress cone 4 and the first silicone rubber insulation 5;

[0043] Specifically, the first 50Ω resistor 9 is made of 0.02mm constantan wire double wire, the first BNC waterproof connector 10 uses a BNC connector with a waterproof grade of IP65, and waterproof sealing grease is applied at the threads of the first BNC waterproof connector 10 and the first housing 7, and a radio frequency coaxial signal cable with a characteristic impedance of 50Ω is used to transmit signals.

[0044] The second partial discharge sensor comprises a second wire core 12, a second outer semi-conductive layer and a grounding copper mesh 17, a second high-voltage electrode 19, a second 50Ω resistor 20, a second BNC waterproof connector 21, a second coaxial signal cable 22, and an insulating flange 23. The grounding copper mesh of the outer semi-conductive layer is removed, and a circular copper electrode with a length less than the length of the cut grounding copper mesh and a thickness of 1 mm is applied to the outer semi-conductive layer. The signal lead is soldered to the copper electrode with solder, and a 2mm thick epoxy resin glue is applied to the copper electrode. Then, the grounding copper mesh is connected to the outer surface of the epoxy resin glue with two semi-circular flanges. The circular copper electrode and the second wire core 12 constitute a high-voltage capacitor, and the circular copper electrode and the second shell 18 of the intermediate joint constitute a low-voltage capacitor. The partial discharge signal detected on the circular copper electrode is sent to the partial discharge signal processing circuit through the second 50Ω resistor, the second BNC waterproof connector and the second coaxial signal cable. When the second partial discharge sensor is installed, it is connected to the middle joint through the second housing 18, the second conductor crimping tube 13, the second cross-linked polyethylene insulation 14, the second stress cone 15 and the second silicone rubber insulation 16;

[0045] Specifically, the second 50Ω resistor 20 is made of 0.02mm constantan wire double wire, the second BNC waterproof connector 21 uses a BNC connector with a waterproof grade of IP65, and waterproof sealing grease is applied at the threads of the second BNC waterproof connector 21 and the second shell 18, and a radio frequency coaxial signal cable with a characteristic impedance of 50Ω is used to transmit signals.

[0046] like Figure 3 As shown, the partial discharge signal processing circuit is composed of a DC blocking capacitor C0 and an active low-pass filter, and the amplifier is powered by a regulated power supply. After simulation analysis and partial discharge testing, it is found that the spectrum of the middle joint of the high-voltage cable is mainly distributed within 600MHz, and the spectrum has two peaks, which are located at frequencies of 50MHz and 230MHz respectively. The main energy of the partial discharge signal of the middle joint is concentrated in 10MHz to 300MHz. Preferably, the cut-off frequency of the low-pass filter is designed to be 300MHz.

[0047] When the radial radius of the annular copper electrode is 18 mm, the detection impedance Z L The relationship between the current at both ends and the axial length is as follows Figure 4 As shown. Figure 4 It can be seen that as the axial length increases, the detection impedance Z L The current at both ends gradually increases. When the axial length is kept at 20 mm and the radial distance of the annular copper electrodes is changed, the impedance Z is detected. L The relationship between the current at both ends and the radial distance is as follows Figure 5 As shown. Figure 5 It can be seen that as the radial distance increases, the detection impedance Z LThe current at both ends gradually decreases. Under the premise of ensuring that the signal is not distorted, the axial length of the capacitive sensor can be appropriately increased and the radial distance can be reduced. Preferably, an axial length of 20 mm and a radial distance of 18 mm are selected as the optimal design parameters of the annular copper electrode.

[0048] For high-voltage cable straight-through joints, the health status of the straight-through joints can be judged based on the partial discharge sensors installed in each phase; for high-voltage cable insulated joints, since they are three-phase cable cross-connection joints, their grounding copper mesh is isolated by insulating flanges and connected with medium-voltage cross-linked polyethylene cables. The core and grounding copper mesh of the medium-voltage cross-linked polyethylene cable are respectively connected to the grounding copper mesh lead-out wires at both ends of the high-voltage cable, and then interconnected in the cross-connection box. Therefore, the partial discharge signal detected by the insulated joint partial discharge sensor installed at the cross-connection position is not only affected by the partial discharge of the insulated joint of the current phase, but also by the partial discharge of the insulated joints of the other two phases. For three-phase cable systems with insulated joints, the modulus transformation theory of multi-conductor transmission lines can be used to analyze their patterns. Assume that the characteristic admittance of the three-phase cable on the side where partial discharge occurs is Y C1 , the characteristic admittance of the three-phase cable away from the partial discharge side is Y C2 , according to the cross-permutation order:

[0049] Y c2 =M T Y c M (1)

[0050] where Y C is the characteristic admittance when arranged in normal order, and M is the cross-permutation matrix. Taking the permutation order of AB, BC, and CA as an example, M is:

[0051]

[0052] The refractive index at the cross-connection is deduced as:

[0053] α=E+(M T Y c MY c )(M T Y c M+Y c ) -1 (3)

[0054] The ratio of the sensor signals on the cross-connected insulation joints can be derived by formula (3). Assuming that partial discharge occurs on the insulation joint of phase A, the ratio of the sensor signals on the three-phase insulation joints is 1+2x:1:1+2x, where x is the ratio of the external characteristic admittance to the internal characteristic admittance of the three-phase cable. In other words, partial discharge occurs on the insulation of phase A, and the three-phase transposition order is as follows: Figure 6As shown, when the three-phase transposition order is AB, BC, and CA, the signal amplitude of the three-phase insulation joint sensor is as follows: Figure 7 As shown in the figure, the partial discharge signal amplitudes of the A-phase insulation joint and the C-phase insulation joint are greater than those of the B-phase insulation joint, and the signal amplitudes show the characteristics of "two large and one small". The criterion for defective insulation joints is:

[0055] (1) The phase with the smallest amplitude of the detection signal of the partial discharge sensor installed on the insulating joint is the X phase;

[0056] (2) Determine the cross-connection of the X-phase insulation joint to be the X-phase and the Y-phase;

[0057] (3) The defective phase is the Z phase other than the X phase and the Y phase.

[0058] The present invention designs a partial discharge sensor and a detection circuit directly installed on the middle joint of a high-voltage cable, and proposes a defective joint identification method for an insulated middle joint of a high-voltage cable. Since the partial discharge sensor is directly installed on the middle joint of the cable, its detection sensitivity is much higher than that installed on the cable. In addition, a defective joint identification method for an insulated middle joint of a high-voltage cable based on energy distribution is proposed, which can improve the insulation status monitoring effect of cable accessories.

[0059] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A partial discharge detection device for a high-voltage cable intermediate joint, characterized in that: It includes a first partial discharge sensor directly installed on a high-voltage cable straight-through intermediate joint, a second partial discharge sensor directly installed on an insulated intermediate joint of the high-voltage cable, and a local amplification signal processing circuit connected to the first partial discharge sensor and the second partial discharge sensor respectively; The first partial discharge sensor comprises a first wire core, a first outer semiconductive layer and a grounding copper mesh, a first high-voltage electrode, a first 50Ω resistor, a first BNC waterproof connector, and a first coaxial signal cable, the grounding copper mesh of the outer semiconductor layer is removed, a 2mm thick epoxy resin glue is applied on the semiconductive layer, a circular copper electrode with a length less than the cut length of the epoxy resin glue and a thickness of 1mm is applied to the epoxy resin glue, the copper electrode is soldered with a signal lead, the circular copper electrode and the first wire core constitute a high-voltage capacitor, the circular copper electrode and the first outer semiconductor layer and the grounding copper mesh constitute a low-voltage capacitor, and the partial discharge signal detected on the circular copper electrode is sent to the partial discharge signal processing circuit through the first 50Ω resistor, the first BNC waterproof connector and the first coaxial signal cable; The second partial discharge sensor includes a second wire core, a second outer semi-conductive layer and a grounding copper mesh, a second high-voltage electrode, a second 50Ω resistor, a second BNC waterproof connector, a second coaxial signal cable, and an insulating flange. The grounding copper mesh of the outer semi-conductive layer is removed, and a circular copper electrode with a length less than the length of the cut grounding copper mesh and a thickness of 1mm is applied to the outer semi-conductive layer. The signal lead is soldered to the copper electrode with solder, and a 2mm thick epoxy resin glue is applied to the copper electrode. Then, the grounding copper mesh is connected to the outer surface of the epoxy resin glue with two semi-circular flanges. The circular copper electrode and the second wire core constitute a high-voltage capacitor, and the circular copper electrode and the second shell of the intermediate connector constitute a low-voltage capacitor. The partial discharge signal detected on the circular copper electrode is sent to the partial discharge signal processing circuit through the second 50Ω resistor, the second BNC waterproof connector and the second coaxial signal cable.

2. A high-voltage cable intermediate joint partial discharge detection device according to claim 1, characterized in that: The first 50Ω resistor is made of 0.02mm constantan wire double wire, the first BNC waterproof connector uses a BNC connector with a waterproof grade of IP65, and waterproof sealing grease is applied between the first BNC waterproof connector and the first shell thread, and a radio frequency coaxial signal cable with a characteristic impedance of 50Ω is used to transmit signals.

3. A high voltage cable intermediate joint partial discharge detection device according to claim 1, characterized in that: The second 50Ω resistor is made of 0.02mm constantan wire double wire, the second BNC waterproof connector uses a BNC connector with a waterproof grade of IP65, and waterproof sealing grease is applied between the second BNC waterproof connector and the second shell thread, and a radio frequency coaxial signal cable with a characteristic impedance of 50Ω is used to transmit signals.

4. A high-voltage cable intermediate joint partial discharge detection device according to claim 1, characterized in that: The partial discharge signal processing circuit comprises a DC blocking capacitor and an active low-pass filter, wherein the amplifier is powered by a regulated power supply.

5. A high-voltage cable intermediate joint partial discharge detection device according to claim 4, characterized in that: The cut-off frequency of the low-pass filter is 300 MHz.

6. A high-voltage cable intermediate joint partial discharge detection device according to claim 1, characterized in that: For a high-voltage cable through-type joint, the health status of the through-type joint is determined according to a first partial discharge sensor installed in each phase.

7. A high-voltage cable intermediate joint partial discharge detection device according to claim 1, characterized in that: For a three-phase cable system with insulating joints, the criteria for defective insulating joints are: (1) The phase with the smallest amplitude of the detection signal of the second partial discharge sensor installed on the insulating joint is phase X; (2) Determine the cross-connection of the X-phase insulation joint to be the X-phase and the Y-phase; (3) The defective phase is the Z phase other than the X phase and the Y phase.

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