JPH07260870A - Method for measuring dc leak current of power cable - Google Patents

Abstract

PURPOSE:To provide a method for measuring the deterioration of insulator in a CV cable by measuring the resistance of an insulator layer based on the measurement of DC leak current in which measurement error due to the leak current flowing on the interface between the insulator layer and an overcoating pipe is eliminated. CONSTITUTION:First, second and third electrodes 4, 5, 6 are disposed sequentially, at an interval, from the conductor side on the surface of a flexible pipe 1 at the terminal joint of a power cable where the electrodes 4, 6 serve as guard electrodes. A voltage is then applied from one terminal of a grounded DC high voltage power supply (P1+P2) having an intermediate joint to the cable conductor. The electrodes 4, 6 on each overcoating pipe 1 are connected with the intermediate joint and a voltage of opposite polarity is applied from the other terminal of the DC high voltage power supply to the electrode 5 thus measuring the DC leak current of an insulator layer in the power cable.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring DC leakage current of a high voltage or extra high voltage CV cable.

[0002]

2. Description of the Related Art In order to inspect the degree of insulation deterioration of a CV cable, an insulation resistance method using a high insulation resistance measuring instrument, a withstand voltage method, a DC leakage current measuring method, a dielectric loss tangent measuring method (tan δ measuring method), a partial discharge measuring method. (Corona measurement method) is used.
The present invention relates to a DC leakage current measuring method among the above measuring methods. First, the conventional CV cable terminal processing method, measuring circuit, and measuring method required for measuring DC leakage current are described. explain. The CV cable basically has a central conductor, and is provided with an inner semiconductive layer, a crosslinked plastic insulator layer, an outer semiconductive layer, a metal shielding layer, and a sheath in this order on the outer circumference of the conductor in cross section. .

[0003]

[Termination of cable] In the case of CV cable, the measurement is to apply a DC voltage between the conductor in the center and the metal shielding layer via the insulator, and to leak through the insulator existing between them. Since only the current is measured, the cable to be inspected should be processed at the time of measurement, and the current that does not pass through the insulating layer should not be mixed up with the measuring circuit instrument due to the configuration of the measuring circuit. is necessary. Therefore, as shown in FIG. 5, in one or both ends of the conventional cable, the shielding layer, the outer semiconducting layer, the insulator layer, and the inner semiconducting layer are peeled off in a stepwise manner from the sheath 17, and the central conductor 1
5, the insulating layer 16, the outer semiconductive layer 13, and the metal shielding layer 14 are exposed to form a terminal portion, and a metal foil is wound around the surface of the insulating layer 16 to form the guard electrode 18.

[0004]

[Measurement Circuit] FIG. 4a shows an example of the measurement circuit. A recorder M for detecting a voltage or the like between the terminals of the shunt D, in which a protection resistor R using a water resistance, a switch S ₁ , a high-voltage DC power supply P, a switch S ₂ , a shunt D, and a microammeter A are connected in series. Are connected in parallel. Protective resistance R at one end of the cable
The conductor 15 is connected to the negative side terminal of the high-voltage power supply P via the, and the shielding layer 14 is connected to the positive side terminal of the power supply P via the shunt D and the microammeter A, and is simultaneously grounded.
The guard electrode 18 is directly connected to the positive terminal side of the high voltage DC power supply P. At the other end, the metal shield layer 14 is grounded, and the guard electrode 18 is directly connected to the positive terminal of the high-voltage DC power supply P using another wire core as shown by the dotted line.

Due to the circuit configuration at the time of the above measurement, the leakage current leaking from the tip of the conductor 15 along the surface of the insulator layer 16 reaches the metal shield layer 14 along the surface of the insulator layer by the guard electrode 18. Can be prevented.

The measuring circuit shown in FIG. 4b is different from the measuring circuit shown in FIG. A in that the guard electrode 18 is set to the ground potential, but the leakage current flowing on the surface of the insulator is excluded. There is no change in that it only measures the leakage current flowing through the device. However, the configuration of a and the configuration of b are selected and applied depending on whether or not the influence of the stray current is exerted. That is, the configuration of a when affected by the stray current,
When not receiving, measure in the configuration of b. The leakage current flowing in the insulator layer can be obtained from the voltage of the recorder M. When applying a DC voltage, in order to exclude the variation of the measurement result due to the partial discharge between the exposed surface of the conductor and the ground due to the partial discharge at the tip of the conductor at the measurement side end and the other end, To prevent this by covering with a synthetic resin cap, see JP-A-57-1861.
No. 77 publication.

[0007]

As described above, by providing the guard electrode on the surface of the insulator layer, it is recognized that the leakage current of the current flowing on the surface of the insulator layer to the metal shielding layer can be excluded. However, when the measurement is repeated using the terminal part equipped with such a guard electrode, the leakage current on the insulator surface of the terminal part cannot be completely reduced to zero, and the measurement error of the DC leakage current is a problem. Sometimes became.

[0008] In the case of a CV cable, the cable terminal is equipped with a terminal connecting portion insulator or a insulator tube at the site installation, and when the DC leakage current is measured in this state, the inside of the insulator or the insulator tube and the cable insulator layer are measured. The leakage current at the interface could not be removed. In most cases, this value is as small as a few nA or less. On the other hand, for example, in the case of a 6.6 kV CV cable, which is significantly deteriorated and a through water tree is generated, the DC leakage current of the cable is several μA or more. Although the slight leakage current shown above does not pose a problem, in the case of a CV cable of 20 kV or more, if the water tree is detected after reaching the penetration, it may cause a breakdown and it will be too late. It is desirable to detect trees. In this case, it is desired that the detection sensitivity of the DC leakage current be nA order or less. When measuring the DC leakage current with such sensitivity, it is necessary to eliminate the influence of the leakage current at the interface between the inside of the flexible tube and the cable.

[0009]

The present invention employs a method in which a guard electrode is formed and a leakage current is escaped by the guard electrode so as not to enter the measuring circuit as in the case of measuring with a measuring instrument shown in the figure.
The number of electrodes provided is not one. That is, the first, second, and third electrodes are provided on the surface of the cable terminating connection portion insulator or the surface of the flexible tube at certain intervals in order from the conductor side, and the first and third electrodes are used as guard electrodes, and the conductor is connected to the second electrode. A voltage having a polarity opposite to the polarity applied to is applied to weaken the electric field strength in the longitudinal direction of the cable to directly prevent the leakage current and measure the leakage current passing only through the insulating layer.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an example of a cable termination connecting portion to which the present invention is applied. In the figure, reference numeral 1 denotes a spiral tube at a terminal end of the cable, and 2 denotes a power cable which is inserted into the flexible tube 1 from below and has a conductor terminal 3 formed by exposing a conductor only at an upper end portion. Tube 1 that goes slightly below the conductor terminal 3
The metal foil is closely wound around the gentle annular surface of the pleated outer wall, the first electrode 4 is provided, and the second electrode 5 is similarly provided on the lower side of the first electrode 4 and further on the lower side. A third electrode 6 is provided, and the first and third electrodes are used as guard electrodes.

[0011]

[Measurement of Leakage Current] In order to measure the DC leakage current, the conductor terminals are cut off at both ends of the power cable connected to the distribution system, and as shown in FIG. Measurement is performed by applying a DC high voltage source as follows to a power cable having three electrodes provided on the surface at intervals along the length direction. FIG. 2 explains the measuring method, but as shown in the figure, two high-voltage DC power supplies P ₁ and P ₂ are used as power supplies, which are connected in series and grounded at the intermediate connection point. In the figure, a negative voltage is applied to the cable conductor, but a positive voltage may be applied to the cable conductor. A high voltage of negative polarity is applied to the cable conductor from the power source P ₁ + P ₂ negative polarity side terminal, and the first and third guard electrodes 4 and 6 of both insulators or tubes are located between the power sources P ₁ and P _2. It is connected to the connection point N and has a ground potential. Further, a positive voltage opposite to the polarity applied to the conductor is applied to both the second electrodes 5 from the positive terminal of the power source P ₁ + P ₂ . Shunt resistance 10 in the shielding layer 14 of the power cable
Is connected to a voltage recorder 11 and is grounded via a shunt resistor 10. That is, a cable conductor, a cable insulator layer, an external semiconductive layer, a shielding layer, and a shunt resistance 10 are connected in series on the negative side of the power source P ₁ , and a circuit in which leakage current is measured via ground is formed. become. Then, the leakage current is obtained from the voltage applied to the shunt resistance 10. Switch power supply P ₁ + P ₂
The measurement can be started by turning on at S ₁ , but in the operating state, the first, second, and
The arrangement of the electrodes 3, 5, 6 of FIG. 3, that is, the equipotential surface in the case of the electrode structure of the present invention is shown in FIG. 3a, but a voltage having the opposite polarity to the voltage applied to the conductor is applied to the second electrode. By this, the electric field in the interface direction at the terminal, that is, the electric field in the cable longitudinal direction can be weakened, and the interface current between the inside of the tube and the insulator can be reduced. Note that FIG. 3b shows an equipotential surface in the case of a normal guard electrode structure.

[0012]

According to the present invention, the leakage current at the interface between the porcelain bushing, the flexible tube and the cable insulator is reduced, and the DC leakage current of the cable can be measured with high accuracy.

[Brief description of drawings]

FIG. 1 shows an example of terminal processing of a CV cable to which the present invention is applied.

FIG. 2 shows an explanatory diagram of the measuring method of the present invention.

FIG. 3A shows equipotential lines when the electrode structure of the present invention and electrical connection are used, and b shows equipotential lines when a normal guard electrode and electrical connection are used.

4A and 4B respectively show a conventional CV cable DC leakage current measuring circuit.

FIG. 5 shows a cable end according to conventional processing.

[Explanation of symbols]

 1 Spiral tube 2 Power cable 3 Conductor terminal 4 First electrode 5 Second electrode 6 Third electrode 10 Shunt resistance 11 Voltage recorder 14 Shielding layer 16 Insulating layer

Claims (1)

[Claims] 1. A first, a second, and a third electrode are provided on a surface of an insulator or a porcelain tube at both ends of a power cable in order from the conductor side, and have connecting points in the middle. Further, a direct current high voltage power supply in which the connection point is grounded is used, and a voltage is applied between one terminal of the power supply and the intermediate connection point between the cable conductor and the shielding layer of the cable to connect the first and third electrodes. As a guard electrode, it is connected to an intermediate connection point of the grounded power supply, and a voltage having a polarity opposite to that applied from the conductor is applied to the second electrode from the other terminal of the power supply to leak current of the cable insulator. A method for measuring DC leakage current in a power cable, which is characterized by measuring