TW202144796A - On-line measuring system for partial discharges signals of cable joints - Google Patents

Abstract

An on-line measuring system for partial discharges signals of cable joints includes two high-frequency current transformers with the same polarity, at least one ground ring and a monitoring unit. The two high-frequency current transformers respectively surround the outer surface of two shoulder portions of the cable joint. The ground ring conductively contacts an outer semiconductive layer of the cable joint. The cable shielding layers of two cables connected to the two ends of the cable joint are crossed from the outside of the two high-frequency current transformers, and are electrically connected to the ground ring. The exposed inner wires of the two cables after removing the cable shielding layer are respectively inserted into the two ends of the cable joint, and pass through the insides of the two high-frequency current transformers simultaneously. Two sensing signals generated by the two high-frequency current transformers are transferred to the monitoring unit for signal processing. The invention can eliminate the external noise of the cable joint when measuring the partial discharges inside the cable joint; and eliminate the internal noise of the cable joint when measuring the partial discharges outside the cable joint.

Description

On-line measurement system of partial discharge signal of cable joint

The present invention relates to an on-line measuring system for partial discharge signals of cable joints, in particular to an on-line measuring system for measuring the internal discharge signals of linear joints in the environment of underground cable ducts.

Under the long-term load of the underground power distribution system, the cable insulation material may deteriorate or even fail. Generally speaking, the underground cable accident can be divided into the cable body and the cable joint according to the location. According to statistics, underground cable accidents mostly occur at cable joints, such as straight joints and terminal joints. Therefore, research on the insulation status of the cable joints of underground cables, and the use of online detection and diagnosis to provide maintenance of the insulation status has become an important project for the maintenance and development of high-voltage power equipment.

At present, for the evaluation of the insulation status of underground cables, partial discharge data is used as the main indicator internationally. If the partial discharge data can be monitored in real time, it can provide a predictable maintenance basis for the initial damage of high-voltage power equipment, and then carry out timely maintenance according to the state of its insulation, so as to avoid the occurrence of unexpected power failures.

Common underground cable joint detection methods can be divided into non-electrical detection and electrical detection. Non-electrical detection methods, such as manual detection of the surface temperature of the cable joint with an infrared searcher, or ultrasonic detection. Electrical testing is usually performed on-site with high-frequency sensors, which are easy to perform quantitative tests and have good sensitivity.

However, manual detection with an infrared searchlight cannot know the partial discharge value of the cable joint, and cannot achieve the purpose of real-time online monitoring. Generally, ultrasonic detection needs to use a sound collector to collect ultrasonic waves, and its sensitivity to the detection of deep partial discharge of insulating materials is very low. It is more suitable for gas sealed switchgear (Gas Insulated Switchgear). The test effect is limited.

When the high frequency current transformer (HFCT) is traditionally used to measure the partial discharge of the cable joint, it can only be installed on the ground wire or the cable body, and the captured signal will be accompanied by many external noise, thus increasing the difficulty of spectrum identification and causing the problem of partial discharge signal attenuation.

One object of the present invention is to provide an online measurement system for partial discharge signals of cable joints, which can eliminate noise generated by external currents and enhance partial discharge signals.

In order to achieve the above object, the present invention provides an online measurement system for partial discharge signals of cable joints, which is suitable for measuring partial discharges generated inside and outside a cable joint. Among them, the straight joint covers the crimping part of the two cables. The basic components of the system include two current sensors, at least one grounding ring and a monitoring unit. The two current sensors have the same polarity and are respectively installed on the two shoulders of the linear connector. When a partial discharge occurs at the linear joint, the two current sensors generate two sensing signals. The grounding ring is arranged between the two current sensors, and can be conductively contacted with the outer semiconducting layer of the linear connector. The respective cable shielding layers of the two cables connected to the two ends of the straight connector are spanned from the outside of the two current sensors and are electrically connected with the grounding ring. The exposed inner layer cables of the two cables after removing the cable shielding layer are inserted into the straight connector, and simultaneously pass through the inner side of one of the two current sensors. The monitoring unit is electrically connected to the two current sensors for performing a signal processing process on the two sensing signals, thereby generating a partial discharge data.

In one embodiment, the respective cable shielding layers of the two cables are stripped from the outside, and the stripped two cable shielding layers are combined into a cable jumper shielding wire, and the cable jumper shielding wire is crossed from the outside of the two current sensors. And it is electrically connected with the grounding ring.

In one embodiment, the number of grounding rings is one, and a single grounding lead is used to connect the grounding ring and the cable jumper shielding wire.

In one embodiment, the number of grounding rings is two, and the two grounding rings are respectively installed on two shoulders of the linear joint. The two grounding rings are respectively electrically connected to the cable jumper shielding wire with two grounding leads. Each grounding ring includes two semi-circular conductive sheets, and each semi-circular conductive sheet has a grounding hole. Each grounding lead has a grounding hole with one end tied to one of the semicircular conductive sheets, and the other end is crimped with the cable jumper shielding wire.

In one embodiment, the monitoring unit is used to measure the internal discharge of the linear joint, which includes two oscillator circuits, two surge protection elements, a first multiplexer, a differentiator, an amplifier, and an absolute value hold circuit. , a peak hold circuit, an analog digital converter and a microcontroller. The two oscillating circuits are electrically connected to the two current sensors respectively, the two surge protection elements are electrically connected to the two oscillating circuits respectively, and both are electrically connected to the first multiplexer. The first multiplexer, the differentiator and the amplifier are electrically connected in sequence. The absolute value hold circuit is connected between the amplifier and the peak value hold circuit, and the peak value hold circuit is electrically connected between the absolute value hold circuit and the analog-to-digital converter.

In one embodiment, the monitoring unit further includes an adder and a second multiplexer for measuring the external discharge of the linear connector. The adder and the differentiator form a parallel structure, the parallel structure is electrically connected between the first multiplexer and the second multiplexer, and the second multiplexer is electrically connected between the parallel structure and the amplifier.

In one embodiment, each current sensor is composed of two semi-circular ring-shaped sensing elements, and is suitable for being temporarily installed on a straight joint in an existing underground cable duct in a sandwiching manner.

In one embodiment, each current sensor is a single complete ring-shaped sensing element, which is suitable for being installed at both ends of the linear connector during installation and construction.

The present invention adopts a non-invasive high frequency current transformer (HFCT) to measure the pulse current of partial discharge on the cable joint, and is matched with a signal processing module such as a back-end differential circuit to accurately identify the partial discharge. The discharge signal comes from the inside or outside of the cable joint, and the noise is separated to obtain the relevant data of partial discharge.

The foregoing and other technical contents, features and effects of the present invention will be clearly presented in the following detailed description of a preferred embodiment with reference to the drawings. The directional terms mentioned in the following embodiments, such as: up, down, left, right, front or rear, etc., are only used to refer to the directions of the accompanying drawings. Therefore, these directional terms are only used to illustrate and not to limit the present invention.

FIG. 1 is an embodiment of the on-line measurement system of the partial discharge signal of the cable joint and its use environment of the present invention. The on-line measurement system 10 (hereinafter referred to as “system 10”) for partial discharge signals of cable joints of the present embodiment is suitable for the environment of R, S, T three-phase 25 kV distribution-grade high-voltage underground cable conduits inside one manhole 20 . In the underground cable duct shown in FIG. 1 , each single-phase cable includes a cable joint 30 and cables 31L and 31R connected to the left and right ends thereof. 25 kV distribution level straight joints 30 are generally used for feeder (600A, 500MCM) and branch (200A, #1 AMG) or similar structures to both. For each linear connector 30, the system 10 provides two current sensors 11L and 11R, two grounding rings (see the grounding rings 12L and 12R in FIG. 2) disposed in the housing 122, and a partial discharge monitoring unit (Partial Discharge Monitoring Unit). , PDMU) 13, used to measure the partial discharge that occurs inside the linear joint 30.

The current sensors 11L and 11R of the present embodiment use high-frequency current sensors, which are also called high-frequency current comparators. The two current sensors 11L and 11R are installed at the left and right ends of the linear connector 30 to induce the partial discharge pulse current inside the linear connector 30 to generate two sensing signals, and transmit the two detected sensing signals. A differential signal processing process is performed to the partial discharge monitoring unit 13, thereby generating partial discharge data. The ground rings 12L and 12R inside the two casings 122 are installed on the shoulders of the two ends of the straight connector 30, and can be conductively contacted with the outer semiconducting layer of the shoulders, so as to avoid the detection of the two current sensors 11L and 11R. The partial discharge signal is disturbed by noise. The partial discharge monitoring unit 13 can perform subsequent signal processing on the partial discharge signals from the two current sensors 11L and 11R, and transmit the processed data to a computer through the network 14 for insulation diagnosis and analysis.

FIG. 2 illustrates one embodiment of the wiring structure of the system 10 and the straight connector 30 for a single-phase cable. The linear connector 30 has two shoulders 30L, 30R on both ends thereof, and an outer semiconducting layer 30S on its surface. The two ports of the straight connector 30 are respectively connected to the inner layer cables 311L and 311R of the two cables 31L and 31R. When the system 10 is connected to the linear connector 30 , the two current sensors 11L and 11R are respectively installed on the two shoulders 30L and 30R of the linear connector 30 , and are electrically connected to the partial discharge monitoring unit 13 . In this embodiment, the two current sensors 11L and 11R need to have the same polarity.

The grounding ring 12L or 12R of this embodiment is essentially a conductive sheet, which is installed on the inner sidewall of the housing 122 , surrounds the shoulders 30L and 30R of the linear connector 30 and contacts the outer semiconducting layer 30S. In addition to accommodating the grounding ring 12L or 12R, the casing 122 can also accommodate the two current sensors 11L and 11R. It should be noted that the positions of the two grounding rings 12L and 12R must be between the two current sensors 11L and 11R.

2A, each cable 31L (or 31R) has a center conductor 313, and the outer side of the center conductor 313 is sequentially coated with an inner semiconducting layer 314, an insulating layer 315, an outer semiconducting layer 316, and a cable shield. Layer 32L (or 32R) and a skin 312L (or 312R). It is worth noting that, in order to avoid the interference of external noise, a part of the cable shielding layers 32L and 32R of the two cables 31L and 31R are stripped together with the outer skins 312L and 312R, and then the stripped two cable shielding layers 32L and 32R are connected. The shielded wire 32 is combined into a cable jumper. The cable jumper shielding wire 32 is crossed from the outside of the two current sensors 11L and 11R, and then the two grounding leads 15L and 15R are electrically connected to the two grounding rings 12L and 12R, respectively. In one embodiment, the two grounding leads 15L and 15R each have one end connected to a grounding hole 124 of the grounding rings 12L and 12R, and the other end connected to a crimping terminal 33 , and the shielding wire is crossed with the cable through the crimping terminal 33 . 32 crimped together. In this way, the current flowing through the cable shielding layers 32L and 32R does not pass through the inner sides of the two current sensors 11L and 11R, thereby eliminating noise caused by the current flowing through the cable shielding layers 32L and 32R.

After the cable shielding layers 32L and 32R and the outer sheaths 312L and 312R of the cable 31L or 31R are peeled off, an inner layer cable 311L and 311R is exposed. The inner layer cables 311L and 311R include various layers such as a center conductor 313 , an inner semiconducting layer 314 , an insulating layer 315 , and an outer semiconducting layer 316 . When connecting, the inner layer cables 311L and 311R are inserted into the linear connector 30 and passed through the inner sides of the two current sensors 11L and 11R.

FIG. 2B shows a configuration in which the inner layer cables 311L, 311R are inserted into the linear joint 30 . In FIG. 2B , the two shoulders 30L and 30R of the straight connector 30 have not yet installed the current sensors 11L, 11R and the grounding rings 12L, 12R, and the shoulder 30R has a hole called a ground eye 124A for binding the ground lead 15R. The straight joint 30 includes an outer semiconducting layer 30S, an insulating layer 301 , an inner semiconducting layer 302 and a conductor connecting sleeve 303 from the outside to the inside. After the two inner layer cables 311L and 311R are inserted into the straight connector 30 , the central conductors 313 of the two inner layer cables 311L and 311R are electrically connected through the conductor connection sleeve 303 . The inner semi-conductive layer 314 , the insulating layer 315 and the outer semi-conductive layer 316 of the two inner-layer cables 311L and 311R are respectively electrically connected to the inner semi-conductive layer 302 , the insulating layer 301 and the outer semi-conductive layer 30S of the straight connector 30 , respectively. . It is worth mentioning that it has been proved by experiments that if the current sensors 11L and 11R are installed in the structure of FIG. 2B but the grounding rings 12L and 12R are not installed, the effect of eliminating noise is far inferior to that after installing the grounding rings 12L and 12R. Effect. Therefore, adding the grounding rings 12L and 12R can greatly improve the measurement accuracy, which is an effect that is difficult for those skilled in the art to expect.

In the embodiment of FIG. 2 , the directions of the external currents flowing through the center conductors 313 of the inner cables 311L and 311R are both leftwards, and the pulse current directions of the internal discharge of the linear joint 30 are one left and one right. The sensing signals generated by the two current sensors 11L and 11R inducing the external current and the pulse current of the internal discharge are subtracted through a differential 134, and the external currents are the same on the two current sensors 11L and 11R. The sensing signals of polarity and magnitude are subtracted to zero after passing through the differential 134 , thereby eliminating the interference caused by the external current flowing through the center conductor 313 to the internal partial discharge signal of the linear connector 30 . On the contrary, the pulse current of the internal discharge will generate sensing signals of the same magnitude but opposite polarities on the two current sensors 11L and 11R. After passing through the differential 134, the subtracted signals will be doubled, thereby enhancing the linear connector 30. the internal partial discharge signal.

3A to 3C are schematic views of the combined structure of the left shoulder 30L of the linear connector 30, the current sensor 11L, the grounding ring 12L and the housing 122 of the embodiment shown in FIG. 2 , wherein FIG. 3A is an exploded view, FIG. 3B is a combined view, Figure 3C is a side view. The inner side of the housing 122 shown in FIGS. 3A to 3C includes the ground ring 12L surrounding the shoulder 30L.

The current sensor 11L of FIG. 3A is a single ring-shaped sensing element, which has a complete ring structure and includes a coil for inducing a pulse current signal and connecting to the partial discharge monitoring unit 13 , which is suitable for installation and construction of the linear joint 30 together. installed at both ends. The left end of the straight connector 30 in FIG. 3A passes through the inside of the high-frequency current sensor 11L. The grounding ring 12L of this embodiment includes two semi-circular conductive sheets, each of which is for example the conductive copper foil 125 in FIG. 3A . The left part of the casing 122 is used for accommodating the current sensor 11L; the right part is designed according to the shape of the conductive copper foil 125 to accommodate the conductive copper foil 125 . The right side of the housing 122 has a hole 124B. The semi-circular conductive copper foil 125 is attached to the inner side wall of the semi-circular shell 122 , and the conductive copper foil 125 is in close contact with the outer semi-conductive layer 30S of the shoulder 30L of the straight connector 30 , and the grounding hole 124 and the hole 124B on the shell 122 and The appearance structure of the grounding eye 124A on the shoulder 30L after alignment and combination is shown in FIG. 3B .

Fig. 3C shows that the semi-circular annular casing 122 provides a plurality of screw holes 123 for aligning with the other semi-circular annular casing 122 and then assembled and fixed as shown in Fig. 3B. After the combination, the grounding hole 124 , the grounding eye 124A and the hole 124B are overlapped, and one end of the grounding lead 15L can be bound, and the other end of the grounding lead 15L can be crimped with the left end of the cable jumper shielding wire 32 .

3D and 3E show the second type of current sensor 110R, which is installed at the right end of the straight connector 30 . The current sensor 110R is composed of two semi-circular ring-shaped sensing elements 112R and 114R, and is suitable for being temporarily installed on the straight joint 30 in the existing underground cable duct in a sandwiching manner.

FIG. 4 is another embodiment of the wiring structure of the system 10 and the linear connector 30 . Different from the embodiment of FIG. 2 , the number of the grounding ring 12 in the present embodiment is only one, which surrounds the middle portion of the straight connector 30 and is located between the two current sensors 11L and 11R, and the outer semiconducting layer 30S thereof can be Conductive close contact. In addition, the grounding ring 12 and the cable jumper shielding wire 32 are connected by a single grounding lead 15 . Although the structure of FIG. 4 is relatively simplified, it can also eliminate the noise interference caused by the current flowing through the cable shielding layers 32L and 32R and the center conductor 313 originating from the outside of the straight joint 30 , and enhance the internal part of the straight joint 30 . Discharge signal and other functions.

FIG. 5 is a functional block diagram of a partial discharge monitoring unit (Partial Discharge Monitoring Unit, PDMU) according to an embodiment of the present invention. For the combined structure of the two current sensors 11L and 11R and the two grounding rings 12L and 12R installed on each linear connector 30 , the partial discharge monitoring unit 13 provides two oscillator circuits 131 , two surge protection elements 132 , and a multiplexer 133, a differentiator 134, an amplifier 135, an absolute value hold circuit 136, a peak hold circuit 137, an Analog-to-Digital Converter (ADC) 138, and a Microcontroller Unit , MCU) 139. The two oscillator circuits 131 are electrically connected to the two current sensors 11L and 11R, respectively. The two surge protection elements 132 are respectively electrically connected to the two oscillator circuits 131 and both are electrically connected to the multiplexer 133 . Then, the multiplexer 133 , the differentiator 134 and the amplifier 135 are electrically connected in sequence. Particularly, in this embodiment, the absolute value hold circuit 136 is connected between the amplifier 135 and the peak hold circuit 137 , and the peak hold circuit 137 is electrically connected between the absolute value hold circuit 136 and the analog-to-digital converter 138 .

In the underground cable duct, the straight joints 30 of the R, S, and T three-phase cables each generate two sets of sensing signals through the two current sensors 11L and 11R, and the two sensing signals are respectively extended by the two oscillating circuits 131. During the identification time, the two surge protection elements 132 prevent the voltage surge from damaging the signal processing module at the rear end. Next, the measurement phase is selected through the multiplexer 133 , and the differential 134 identifies whether it is the internal signal of the linear connector 30 , and the amplifier 135 amplifies the signal. The signal is converted to a positive value by the absolute value hold circuit 136 , and the maximum value of the signal is taken by the peak hold circuit 137 , and the sample is sent to the microcontroller 139 through the analog-to-digital converter 138 . Finally, the microcontroller 139 transmits the data to the computer 16 for insulation diagnostic analysis.

6 is a functional block diagram of a partial discharge monitoring unit according to another embodiment of the present invention. The difference from FIG. 5 is that the partial discharge monitoring unit 13A has an adder 134A and a second multiplexer 133A. The adder 134A and the differentiator 134 form a parallel structure, the parallel structure is electrically connected between the first multiplexer 133 and the second multiplexer 133A, and the second multiplexer 133A is electrically connected to the parallel structure and the amplifier 135 between. Both the first multiplexer 133 and the second multiplexer 133A are electrically connected to the microcontroller 139 .

The sensing signals generated by the two current sensors 11L and 11R induced by partial discharges generated outside the linear connector 30 are added by the adder 134A, and the external discharges generate the same polarity on the two current sensors 11L and 11R The sensing signal of the sum and the magnitude will be doubled after passing through the adder 134 , thereby enhancing the external discharge signal of the linear connector 30 . On the contrary, the sensing signals of the same magnitude but opposite polarities generated by the internal currents on the two current sensors 11L and 11R are subtracted to zero after passing through the adder 134A. When measuring the external discharge of the linear connector 30, the noise interference caused by the internal current can be eliminated.

To sum up, the present invention utilizes two sets of non-intrusive high-frequency current sensors of the same polarity to be installed at both ends of the cable joint, and a grounding ring is provided outside the high-frequency current sensor, which is fixed on the two ends of the cable joint. At the shoulder, the shielding layer of the cable is connected to the outer semiconducting layer of the cable splice across the high frequency current sensor using a ground ring. Compared with the case where the grounding ring is not provided, the present invention can effectively measure the polarity of the partial discharge pulse current signal inside the cable joint, and then filter the noise from the external circuit by the rear differential circuit; The polarity of the partial discharge pulse current signal outside the cable joint is determined, and the noise from the inside of the joint is filtered out by the back-end adder. After that, the amplifier circuit, absolute value circuit, peak hold circuit, and analog signal sampling circuit are sent to the microcontroller to obtain the internal or external partial discharge evolution map, including the discharge amount, voltage phase, and pulse time. Finally, the measurement signal is integrated into the The networking technology is transmitted to the cloud for real-time monitoring. With the on-line measurement system of the partial discharge signal of the cable joint of the present invention, the partial discharge signal can be accurately identified from the inside or outside of the cable joint, and the noise can be separated, thereby obtaining the relevant data of the partial discharge.

However, the above are only preferred embodiments of the present invention, and should not limit the scope of the present invention, that is, any simple equivalent changes and modifications made according to the scope of the patent application of the present invention and the contents of the description of the invention, All still fall within the scope of the patent of the present invention. In addition, it is not necessary for any embodiment of the present invention or the claimed scope of the present invention to achieve all of the objects or advantages or features disclosed in the present invention. In addition, the abstract section and the title are only used to aid the search of patent documents and are not intended to limit the scope of the present invention.

10: On-line measurement system of partial discharge signal of cable joint

11L, 11R, 110R: Current sensor

112R, 114R: semi-circular ring sensing element

12, 12L, 12R: grounding ring

122: Shell

123: screw hole

124: ground hole

124A: Ground Eye

124B: Hole

125: Conductive copper foil

13, 13A: Partial discharge monitoring unit

131: Oscillation circuit

132: Surge protection components

133: Multiplexer (first multiplexer)

133A: Second Multiplexer

134: Differential

134A: Adder

135: Amplifier

136: Absolute value hold circuit

137: Peak hold circuit

138: Analog to Digital Converter

139: Microcontroller

14: Internet

15, 15L, 15R: ground lead

16: Computer

20: Manhole

30: Straight Connector

30L, 30R: Shoulder

30S: Outer semiconducting layer

301: Insulation layer

302: Inner semiconducting layer

303: Conductor connection sleeve

31L, 31R: cable

311L, 311R: inner cable

312L, 312R: outer skin

313: Center conductor

314: Inner semiconducting layer

315: Insulation layer

316: Outer semiconducting layer

32: Cable jumper shield wire

32L, 32R: Cable shielding layer

33: Crimp terminal

FIG. 1 is a schematic diagram of the structure of an on-line measurement system for partial discharge signals of an underground power cable joint according to an embodiment of the present invention.

FIG. 2 is a schematic diagram of a partial discharge signal measurement wiring diagram of an underground power cable joint according to an embodiment of the present invention.

FIG. 2A is a schematic diagram of the inner layer structure of a cable according to an embodiment of the present invention.

2B is a schematic diagram of the internal structure of a linear joint according to an embodiment of the present invention.

3A to 3C are schematic diagrams of the combined structure of a linear connector, a high-frequency current sensor and a grounding ring according to an embodiment of the present invention.

FIG. 3D and FIG. 3E are schematic structural diagrams of another type of high-frequency current sensor.

FIG. 4 is a schematic diagram of the measurement wiring of the partial discharge signal of the underground power cable joint according to another embodiment of the present invention.

FIG. 5 is a functional block diagram of a partial discharge monitoring unit (Partial Discharge Monitoring Unit, PDMU) according to an embodiment of the present invention.

6 is a functional block diagram of a partial discharge monitoring unit according to another embodiment of the present invention.

11L, 11R: Current sensor

12L, 12R: grounding ring

122: Shell

13: Partial discharge monitoring unit

134: Differential

15L, 15R: Ground lead

30: Straight Connector

30L, 30R: Shoulder

30S: Outer semiconducting layer

31L, 31R: cable

311L, 311R: inner cable

312L, 312R: outer skin

32: Cable jumper shield wire

32L, 32R: Cable shielding layer

33: Crimp terminal

Claims (11)

An online measurement system for partial discharge signals of cable joints, which is suitable for measuring partial discharges occurring inside and outside a straight joint, wherein the straight joint is used to cover the crimping place of two cables, and has two shoulders and an outer Semi-conductive layer, each of the cable has a cable shielding layer and an inner layer cable, the inner layer cable includes a central conductor, the cable shielding layer is wrapped outside the inner layer cable and is connected with the central conductor Electrically insulating, the system includes: Two current sensors, with the same polarity, are respectively installed on the two shoulders of the linear connector, when partial discharge occurs in the linear connector, the two current sensors generate two sensing signals; at least one ground ring disposed between the two current sensors and conductively contacting the outer semiconducting layer of the straight connector, wherein each of the cable shielding layers spans from the outside of the two current sensors and electrically connected to the ground ring, each of the inner layer cables is inserted into the straight connector and passes through the inner side of one of the two current sensors; and A monitoring unit is electrically connected to the two current sensors for performing a signal processing process on the two sensing signals. The on-line measurement system for partial discharge signals of cable joints as described in claim 1, wherein the respective cable shielding layers of the two cables are stripped from the outside, and the stripped two cable shielding layers are combined into a cable A jumper shield wire, the cable jumper shield wire spans from the outside of the two current sensors and is electrically connected with the ground ring. The on-line measurement system for partial discharge signals of cable joints as described in claim 1, wherein the number of the grounding ring is one, and includes a single grounding lead connected between the grounding ring and the cable jumper shielding wire . The on-line measurement system for partial discharge signals of cable joints as described in claim 1, wherein the number of the grounding rings is two, and the two grounding rings are respectively installed on the two shoulders of the straight joint. The on-line measurement system for partial discharge signals of cable joints as described in item 4 of the patent application scope, further comprising two grounding leads, wherein each of the grounding leads is connected to one of the two grounding rings and the cable jumper shielding wire between. The on-line measurement system for partial discharge signals of cable joints as described in claim 4, wherein each of the grounding rings includes two semi-circular conductive sheets, and each of the semi-circular conductive sheets has a grounding hole. The on-line measurement system for partial discharge signals of cable joints as described in claim 6, wherein each of the grounding leads has the grounding hole with one end tied to one of the semi-circular conductive sheets, and the other end connected across the cable Shielded wire crimp. The on-line measurement system for partial discharge signals of cable joints as described in claim 1, wherein the monitoring unit comprises two oscillator circuits, two surge protection elements, a first multiplexer, the differential, and an amplifier , an absolute value hold circuit, a peak hold circuit, an analog digital converter and a microcontroller, the two oscillation circuits are respectively electrically connected to the two current sensors, and the two surge protection elements are electrically connected to the The two oscillator circuits are both electrically connected to the first multiplexer, the first multiplexer, the differentiator and the amplifier are electrically connected in sequence, characterized in that the absolute value holding circuit is connected to the amplifier and the peak hold circuit, the peak hold circuit is electrically connected between the absolute value hold circuit and the analog-to-digital converter. The online measuring system for partial discharge signals of cable joints as described in claim 8, wherein the monitoring unit comprises an adder and a second multiplexer, wherein the adder and the differential form a parallel structure , the parallel structure is electrically connected between the first multiplexer and the second multiplexer, and the second multiplexer is electrically connected between the parallel structure and the amplifier. The on-line measurement system for partial discharge signals of cable joints as described in claim 1, wherein each of the current sensors is composed of two semi-circular ring-shaped sensing elements. The on-line measurement system for partial discharge signals of cable joints as described in claim 1, wherein each of the current sensors is a complete ring-shaped sensing element.

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