JPS5967469A - Method for measuring dielectric characteristics and dc resistance of cable - Google Patents

Abstract

PURPOSE:To remove the current flowed into a measuring electrode from a high voltage electrode through a cable end part to make it possible to accurately judge whether a cable is deteriorated or not, by cooling the cable end part. CONSTITUTION:The conductor 1 provided to the terminal end of a cable to be measured is used as a high voltage electrode while the metal sheath of the cable is used as a measuring electrode and an earthed leak current collecting electrode 11 is provided in order to remove a leak current flowed into the measuring electrode from the voltage electrode through the surface of the cable end part when voltage is applied to the high voltage electrode to measure the dielectric characteristics and the DC resistance of the cable. In this case, the cable end part is cooled by a cooling medium pipe 12 to reduce the leak current flowed into the measuring electrode from the high voltage electrode through the inner side of the cable end part.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for measuring the dielectric properties and direct current resistance of a cable, and more particularly to a method for measuring the dielectric properties and direct current resistance of a cable so as to be hardly affected by leakage current.

One of the non-destructive methods for examining whether a cable has deteriorated is a method of measuring dielectric properties and direct current resistance. As the cable deteriorates, the dielectric voltage connection becomes weaker and the DC resistance becomes smaller. Therefore, by tracking the dielectric properties and DC resistance, it is possible to estimate the degree of deterioration and remaining life of the cable. This test is conducted using a normal model cable or a short cable, and the shape of the sample is such that a slit is cut at the end of the cable to separate the guard electrode and the measurement electrode.
The shape is such that the current flowing from the pressure viewing electrode to the measurement electrode through the creeping surface of the terminal can be removed.

However, in order to confirm the presence or absence of deterioration in cables used on actual lines, in the case of OF cables, there are auxiliary methods such as gas analysis of gas in impregnated oil, but usually samples are taken from a certain line. This is done by investigating unique characteristics. In this conventional method, a new cable must be re-introduced after removing the cable, which requires a lot of cost and labor, and since it is not a direct method, there are problems with reliability.

It is theoretically possible to cut a slit on an actual line and separate the measurement electrode from the guard electrode, as in the case of a short sample, but if a surge voltage enters the cable system, it is possible This causes poor insulation at the knot and causes it to break. Furthermore, if the slit portion is provided with insulation to a degree that does not cause discharge due to the intruding surge voltage, the electric field will be disturbed at that portion, which is not good in terms of insulation. Therefore, in actual systems, it is currently impossible to track deterioration.

The present invention has been made in view of the above, and its purpose is to have dielectric properties of a cable that can almost eliminate the influence of leakage current and accurately determine whether or not the cable has deteriorated in a timely manner in an actual system. and to provide a DC resistance measurement method.

A feature of the present invention is that the cable end is cooled to remove the current flowing from the high voltage electrode to the measuring electrode through the inner surface of the cable end.

An embodiment of the method of the present invention will be described in detail below with reference to the drawings.

The figure is a sectional view of a cable terminal portion for explaining an embodiment of the method of the present invention. In the figure, 1 is a cable conductor, 2 is an insulator, 6 is an insulating oil, 4 is a terminal reinforcing paper, 5 is a bell mouth, 6 is a tin-plated copper wire that is in electrical contact with the cable metal sheath 7, and 8 is an insulator. , 9 is the lower metal fitting, 10
11 is the upper metal fitting, 11 is the insulator outer surface leakage current capturing electrode, 12
1 is a refrigerant pipe; 16 is a heat insulating material insulating the refrigerant pipe 12;
4 is a high-voltage electrode power line connected to cable conductor 1, 15
16 is a measurement electrode lead wire connected to the lower metal fitting 9 which is electrically connected to the metal/splash, and 16 is a leakage current capture electrode 11.
is the leakage current ground wire connected to the

When voltage is applied from the high voltage electrode power line 14 to the cable conductor 1 at the end of the cable, reinforcing paper is applied to the barrier layer around which the tin-plated copper wire 6, which is in electrical contact with the metal sheath 7 which is the measurement electrode, is wound. A current flows through the inner surfaces of the insulator 4 and the insulator 8. Further, a current flows through the outer surface of the insulator 8 to the measurement electrode lead wire 15 . The presence of currents flowing through these creeping surfaces can cause large errors or make measurements difficult when measuring the dielectric properties, DC resistance, of the insulator 2. Therefore, these currents are removed by the following method. That is, the leakage current on the outer surface of the insulator 8 flows from the leakage current capture electrode 11 that is in contact with the outer surface of the insulator 8 at a position above the bell mouth 5 to the earth through the leakage current grounding wire 16, and then flows to the earth through the leakage current grounding wire 16. to prevent it from entering. Furthermore, the current flowing into the measurement electrode wire 15 through the creeping surface of the terminal reinforcing paper 4 and the inner creeping surface of the insulator 8 is controlled by flowing liquid nitrogen, for example, into the refrigerant pipe 12 installed on the outer surface of the insulator 8 above the bell mouth 5. Try to reduce this by cooling the area around it. The insulation resistance of the insulating oil and the creeping surface of the solid dielectric increases as the temperature decreases.
The resistance at the interface with the insulator 8 increases by about one order of magnitude every time the temperature decreases by about 25°C. Therefore, if the temperature of the above-mentioned portion falls below a certain temperature, the above-mentioned creeping portion can be considered to be electrically insulated. Here, there is a concern that the insulating oil 6 may freeze during cooling and cause corona discharge, causing deterioration of the insulating oil 6, but the voltage required to judge the deterioration of the cable itself is such that corona discharge occurs. There is no need to apply a high voltage; a voltage below the corona starting voltage is sufficient. Furthermore, in this embodiment, measurement can be performed with high precision even at a temperature of about -30'C on the creeping portion, so measurement can be performed in a temperature range where the insulating oil 6 does not freeze.

Please note that the measurement accuracy is affected by the fact that the cable section near the terminal is also cooled, so errors may occur with extremely short cables, but valid measurements can be obtained with cables of approximately 10 m or longer, which are normally used for characteristic tests. In the actual system, the error can be completely ignored.

As described above, according to the embodiments of the present invention, it is possible to accurately measure the dielectric characteristics and DC resistance of a cable in an actual system, and it is possible to timely determine the degree of deterioration of the cable. There is also the possibility of automated continuous monitoring of cable deterioration through automation of test equipment.

It goes without saying that the creeping current of the terminal reinforcing paper 4 and the insulator 8 may be eliminated by replacing the insulating oil 6 present in the insulator 8 with pre-cooled insulating oil.

As explained above, according to the present invention, since the influence of leakage current can be almost completely eliminated, the presence or absence of cable deterioration can be accurately determined in a timely manner in an actual system.

[Brief explanation of the drawing]

The figure is a sectional view of a terminal portion of a cable for explaining an embodiment of the method for measuring dielectric characteristics and direct current resistance of a cable according to the present invention. 1: Cable conductor, 2: Insulator, 6: Insulating oil, 4: Terminal reinforcement paper, 5: Bell mouth, 7: Cable metal sheath, 8: Insulator, 9: Lower metal fitting, 11: Insulator outer surface leakage current capture electrode ,1
2: Refrigerant pipe, 14: High voltage electrode power line, 15: Measurement electrode lead wire, 16: Fallen current grounding wire.

Claims (1)

[Claims] 1. The conductor at the end of the cable to be measured is used as a high voltage electrode, the metal sheath of the cable is used as a measurement electrode, and when a voltage is applied to the high voltage electrode, the entire creepage of the end of the cable is passed from the high voltage electrode to the measurement electrode. In the measurement method of measuring the dielectric properties and direct current resistance of the cable by providing a grounded leakage current capturing electrode for removing leakage current flowing into the cable, the cable end is completely cooled and the cable is passed through the inner surface of the cable end. A method for measuring dielectric properties and direct current resistance of a cable, characterized in that leakage current flowing from the high voltage electrode to the mJ measuring electrode is reduced.