JPS60225072A - Diagnosis of deterioration in insulation for power cable - Google Patents

Abstract

PURPOSE:To elevate the detecting accuracy of deterioration in the insulation of a cable by detecting DC component from current flowing to the neutral point of a grounding transformer to measure the DC component more accurately from the cable in which a water tree is generated. CONSTITUTION:Currents I1 and I2 refer to earth wire currents from a CV cable while current I3 to that flowing through the earth wire of a groundin transformer 1. On the other hand, as the current I3 flowing through the earth wire of the grounding transformer 1 equals I1+I2 as stray current I4-I7 flow through a closed circuit. Therefore, DC component is detected from the current I3 to find the presence, size and direction of generation concerning water tree in a cable insulator and thus the propriety of continuous use of the cable can be determined.

Description

[Detailed Description of the Invention] [Background and Objectives of the Invention] The present invention relates to a method for diagnosing insulation deterioration of rubber/plastic insulated power cables, mainly cross-linked polyethylene insulated power cables (hereinafter referred to as "Cv cables"). It is.

It has become clear that insulation deterioration of C■ cables is mainly caused by water trees. Therefore, in order to prevent insulation breakdown accidents due to insulation deterioration of CV cables, it is important to know the occurrence of this water tree. Conventionally, as methods for detecting this water tree, a DC leakage current method, a residual voltage method, a reverse absorption current method, etc. have been used. All of these methods detect water trees after the line has stopped, and cannot reliably prevent insulation breakdown accidents caused by water trees on live lines. For this reason, recently, as a method for diagnosing cable insulation in a live line state, the DC component, which is a deterioration signal from the water tree, is detected from the grounding line current flowing in the grounding line of the cable, and its polarity, size, and time are detected. A method of diagnosing the state of cable insulation deterioration based on its characteristics is being considered. However, when this method is applied to an actual installed cable, there are the following problems. In other words, if there is a grounding wire other than the measuring point in the line under test, or if the insulation resistance of the cable has decreased due to damage or deterioration of the cable sheath, the grounding wire at the measuring point or the metal shield of the cable Closed circuits are formed by the reduced insulation resistance of the ground, ground, and other ground wires or sheaths, and as a result, stray currents flow into these closed circuits from the ground and are measured by superimposing them with the deterioration signal from the water tree. , it becomes impossible to distinguish the degraded signal.

An object of the present invention is to provide a novel method that eliminates the drawbacks of the prior art described above, more accurately measures the DC component that is a deterioration signal from a cable where water trees have occurred, and significantly improves the accuracy of detecting cable insulation deterioration. Our goal is to provide the following.

[Summary of the Invention] That is, the gist of the present invention is to detect the DC component between the neutral point of the grounding transformer of the electric power cable line that is the target of insulation deterioration diagnosis in the live line and the ground, and to determine the polarity and magnitude of the DC component. The purpose of this method is to analyze the water tree's temperature and time characteristics, detect the presence, size, and direction of occurrence of water trees, and determine whether or not the above-mentioned cable can be continued to be used.

[Embodiment] Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 shows an example in which the line to be measured includes a connection part and its shielding layer is grounded.

In the figure, 1 is a grounding transformer, 2 is a cable end connection, 3 is a C cable, 4 is a cable connection, 5 is a cable ground wire, 6 is a connection ground wire, and 7 is a high voltage bus bar. .

Also, 14 in the figure. Is, IS, Iヮ are stray currents flowing in a closed circuit consisting of the ground wire, cable metal shielding layer, and earth, and If, 12 is the ground wire current from the C■ cable. 3 is the current flowing through the grounding wire of the grounding transformer 1.

On the other hand, stray current 14. Since Is, Is, 1.7 flows in a closed circuit as shown in the figure, the grounding transformer 1
The current I3 flowing through the ground wire is equal to I++I2.

Therefore, it is necessary to detect the DC current from within the IS, determine the presence, size, and direction of water tree in the cable insulation from its magnitude, polarity, and time characteristics, and determine whether or not the cable can be continued to be used. I can do it.

Next, referring to FIG. 2, the insulation deterioration diagnosis procedure for the line under test shown in FIG. 1 will be explained. First, the grounding transformer 1
A switch 9 is installed in the middle of the grounding wire 8. The switch 9 is
It is closed when not measuring, and opened when diagnosing insulation deterioration. Next, a DC component measuring device 10 is connected to both ends of the grounding wire at the location where the switch 9 is installed. The DC component measuring device 10 is
It consists of a ground fault timing device 11, a filter circuit 12, an amplifier circuit 13, an arithmetic circuit 14, and a display device 15. However,
The DC current component flowing from the IS flowing from the neutral point of the grounding transformer 1 to the grounding wire 8 is detected, its polarity, magnitude, and time characteristics are analyzed, and the existence and size of water trees in the cable insulation are determined.
Detect the direction of occurrence.

[Effects of the Invention] As described above, by detecting the DC component of the current flowing to the neutral point of the grounding transformer of the present invention, stray current etc. and deterioration signals from the cable, which were originally of the same title, can be detected. It is possible to distinguish between the direct current component and accurately determine the status of cable insulation deterioration.

Therefore, cable breakage accidents and power outage accidents can be prevented, and damage to power consumers can be greatly reduced.

[Brief explanation of drawings]

The figures are explanatory diagrams of one embodiment of the insulation deterioration diagnosis method for cylindrical cables according to the present invention.
The figure is a detailed explanatory diagram of the present invention. 1: Grounding transformer, 2: Cable terminal connection, 3: CV
Cable, 4: Cable connection part, 5: Cable ground wire,
6: Connection ground wire, 7: High voltage bus, 8: Grounding transformer grounding wire, 9: Switch, 10: DC component measuring device, 11: All equipment for ground fault, 12: Filter circuit, 13: Width amplification Circuit, 14:
Arithmetic circuit, 15: Display device.

Claims (1)

[Claims] Detect the DC component from the neutral point of the grounding transformer of the power cable system in operation, analyze its polarity, size, and time characteristics, and analyze the presence, size, and occurrence of water trees in the cable insulation. A method for diagnosing insulation deterioration of an electric power cable, the method comprising detecting the direction and determining whether or not the cable can be continued to be used.