JPH0614094B2 - Insulation deterioration diagnosis method for power cables - Google Patents

Description

Description: BACKGROUND AND OBJECT OF THE INVENTION The present invention relates to a method for diagnosing insulation deterioration of rubber / plastic insulated power cables, mainly crosslinked polyethylene insulated power cables (hereinafter referred to as “CV cables”). .

It has been revealed that the insulation deterioration of the CV cable is mainly due to the water tree. Therefore, in order to prevent a dielectric breakdown accident due to insulation deterioration of the CV cable, it is important to know the occurrence of this water tree. Conventionally, a DC leakage current method, a residual voltage method, a reverse absorption current method, etc. have been used as a method for detecting this water tree. All of these methods detect the water tree after the track is stopped, and it was not possible to reliably prevent a dielectric breakdown accident due to the water tree on the live track. For this reason, recently, as a cable insulation diagnosis method in a live state, a DC current component (hereinafter referred to as "DC component"), which is a deterioration signal from a water tree, is included in the ground wire current flowing in the ground wire of the cable. A method of detecting and diagnosing the insulation deterioration state of the cable from the polarity and the size thereof is being studied.

By the way, when a water tree is generated in the insulator of the CV cable, a DC component is detected from the ground wire current. At this time, the following can be said.

(1) The generation of the DC component is caused by the generation of the water tree, and when a water tree generated from the inner semiconductive layer side of the cable, a so-called inner water conduction tree is generated, a negative DC component is generated and the cable Water tree generated from the external semiconductive layer side,
When a so-called external water-conducting tree is generated, a direct current component of positive polarity is generated.

(2) The larger the absolute value of the DC component, the larger the volume occupied by the water tree in the cable insulator.

(3) The larger the absolute value of the DC component, the longer the water tree is generated.

From the above, by detecting the polarity and size of the DC component, it is possible to know the presence, size, and direction of the water tree in the cable, and prevent dielectric breakdown accidents during cable operation. You can (Japanese Patent Application Sho 58-76885
See issue. However, there are the following problems when applying this method to an actual cable. In other words, if there is a ground wire in the measured line other than the measurement area, or if the insulation resistance of the cable is reduced due to damage or deterioration of the cable sheath, the metal shield of the ground wire of the measurement area and the cable Closed circuits are formed by layers, ground and other ground lines or sheaths with reduced insulation resistance.As a result, stray currents flow from these ground circuits into the closed circuit, and are measured in combination with the deterioration signal from the water tree. , The deterioration signal cannot be discriminated.

The object of the present invention is to eliminate the above-mentioned drawbacks of the prior art, measure the DC component, which is a deterioration signal from a cable in which a water tree is generated more accurately, and significantly improve the accuracy of cable insulation deterioration detection. To provide a method.

[Summary of the Invention] That is, the gist of the present invention is to detect the DC component from the neutral point and the ground of the grounding transformer of the power cable line, which is the target of the insulation deterioration diagnosis in the live line, and detect its polarity and magnitude. The presence or absence of water trees, their size, and their generation direction are analyzed to determine whether or not the cable can be continuously used.

[Embodiment] An embodiment of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 shows an example of the case where the measured line includes a connection part and the shielding layer is grounded.

In the figure, 1 is a grounding transformer, 2 is a cable terminal connecting portion, 3 is a CV cable, 4 is a cable grounding portion, 5 is a cable grounding wire, 6 is a connecting portion grounding wire, and 7 is a high voltage busbar.

Further, in the figure, ₄ , ₅ , ₆ , and ₇ are stray currents flowing in a closed circuit composed of a ground wire, a cable metal shielding layer, and the ground, and ₁ and ₂ are ground wire currents from the CV cable. ₃ is a current flowing through the ground wire of the grounding transformer 1.

On the other hand, the stray currents ₄ , ₅ , ₆ , ₇ flow in the closed circuit as shown in the figure, and therefore the current ₃ flowing in the ground wire of the grounding transformer 1 is equal to ₁ + ₂ .

Therefore, it is possible to detect the DC component from ₃ and know the presence or absence, size, and generation direction of the water tree in the cable insulator from the size and polarity, and determine whether or not to continue using the cable.

Next, the insulation deterioration diagnosis procedure of the measured line shown in FIG. 1 will be described with reference to FIG.
A switch 9 is installed in the middle of the ground wire 8 of. The switch 9 is
It is closed during non-measurement and released during insulation deterioration diagnosis. Next, the DC component measuring device 10 is connected to both ground wire ends of the switch 9 installation part. DC component measuring device 10
Is a ground fault safety device 11, a filtering circuit 12, and an amplifying circuit 1.
3, an arithmetic circuit 14, and a display device 15. Then, I ₃ flowing from the neutral point of the grounding transformer 1 to the grounding wire 8
The direct current component is detected from the data, the polarity and the size of the direct current component are analyzed, and the presence, size, and direction of the water tree in the cable insulator are detected.

[Effects of the Invention] As described above, by detecting the DC component of the current flowing through the neutral point of the grounding transformer of the present invention, the stray current and the deterioration signal from the cable, which are initially problematic, can be obtained. The DC component can be distinguished, and the cable insulation deterioration condition can be accurately known.

Therefore, it is possible to prevent a cable breakage accident and a power failure accident in advance, and it is possible to significantly reduce damage to electric power consumers.

[Brief description of drawings]

FIG. 1 is an explanatory view of an embodiment of a method for diagnosing insulation deterioration of a power cable of the present invention, and FIG. 1 is an overall explanatory view of a measured line,
The figure is an illustration of an embodiment of the present invention. 1: Grounding transformer, 2: Cable terminal connection part, 3: CV cable, 4: Cable connection part, 5: Cable grounding wire, 6: Connection part grounding wire, 7: High voltage bus bar, 8: Grounding transformer grounding Wire, 9: Switch, 10: DC component measuring device, 11: Ground fault safety device, 12: Filter circuit, 13: Widening circuit, 14: Arithmetic circuit, 15: Display device.

Claims (1)

[Claims] 1. A direct current component is detected from a neutral point of a grounding transformer of a power cable system in an operating state, its polarity and size are analyzed, and the presence or absence of a water tree in the cable insulator and its size. A method for diagnosing insulation deterioration of a power cable, characterized by detecting the direction of occurrence and determining whether or not the cable can be continuously used.