JP2575456B2 - Power cable insulation diagnostic method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of measuring a DC component of a current flowing between a conductor and a shielding layer of a power cable and diagnosing insulation deterioration of the power cable based on the magnitude of the measured DC component. Things.

[Conventional technology and its problems]

It is known that the magnitude of the DC component of the current flowing between the conductor and the shielding layer of the power cable in a live state, that is, the current flowing through the insulating layer, is closely related to the deterioration of the insulating layer. Therefore, by measuring this DC component, it is possible to diagnose how much insulation deterioration of the insulation layer of the CV cable has progressed due to the occurrence of water trees and the like.

Conventionally, the measurement of this current component is performed as shown in FIG. The power cable 11 has a structure in which a shielding layer 13 is provided on the outer periphery of a conductor 12 via an insulating layer (not shown). The conductor 12 is connected to a high-voltage bus 14 and is in a live state. High-voltage bus 14
Is connected to a power transformer (or a ground transformer) 15. In a normal operation state, both ends of the shielding layer 13 are installed, but when measuring a DC component, a current measuring device 17 having an AC grounding capacitor 16 connected in parallel between one end of the shielding layer 13 and the ground is used. Connect and open the ground at the other end. As a result, the shielding layer 13 is insulated from the ground in terms of DC, and is grounded with low impedance in terms of AC.

In this case, the DC component is transferred from the high voltage charging unit to the conductor 12 of the power cable → the same insulating layer → the same shielding layer 13 → the current measuring device 17
It flows along the route of → ground → transformer 15 → high voltage charging section as indicated by the solid line arrow. Therefore, if you read the value of the current measuring device 17,
This means that the DC component of the current flowing through the insulating layer of the power cable can be measured.

However, the shielding layer 13 has a potential 18 due to electromagnetic induction or stray current.
When such a potential 18 is generated, a current as indicated by a dotted arrow is generated.
Therefore, an error may occur in the measured value, and the insulation diagnosis may be erroneously performed.

Therefore, as shown in Fig. 4, the shielding layer of the power cable
There is also proposed a method in which a current measuring device 17 is connected between a neutral point of a power transformer (or a grounding transformer) 15 and the ground, and measurement is performed, while the ground 13 is kept grounded. In this way, since the shielding layer 13 is grounded, it is not necessary to be affected by electromagnetic induction or stray current.

However, the disadvantage of this method is that it is extremely difficult to insert a current measuring instrument in a live state because the power transformer or grounding transformer is generally hard-wired, and there are also safety problems. That's what it means.

[Means for solving the problem and its operation]

The present invention solves the above-described problems, and provides a method for accurately and easily performing an insulation diagnosis. The method measures a DC component of a current flowing between a conductor and a shielding layer of a power cable. In the method of diagnosing power cable insulation deterioration based on the size of the power cable, one end of the reactor is grounded to the conductor of the power cable, and a current measuring device is connected between the other end of the reactor and the ground. The shielding layer is grounded, and a DC current flowing through the current measuring device is measured in this state.

This method makes it possible to easily connect a current measuring device to a live cable conductor by using a reactor, and to eliminate the effects of electromagnetic induction and stray current by grounding a shielding layer, thereby reducing the DC component. Is easily and accurately measured.

If the load is grounded to the power cable and the DC component generated by the load poses a problem, a DC cut filter is connected between the current measuring device and the ground, and the current is measured. It is effective to electrically connect the shield layer of the power cable between the filter and the DC cut filter.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows an embodiment of the present invention, and the same parts as those in FIG. 3 described above are denoted by the same reference numerals. The feature of this method is that one end of the reactor 21 for AC interruption is connected to the conductor 12 of the power cable 11 (via the high voltage bus 14), and the current measuring device 17 is connected between the other end of the reactor 21 and the ground. In addition, the current measurement is performed with the shield layer 13 of the power cable grounded.

In this way, the current measuring device 17 is in the same state as connected to the neutral point of the transformer, and since the shielding layer 13 is grounded, the current measuring device 17 is not affected by electromagnetic induction or stray current. In the device 17, the DC component of the current flowing through the insulating layer of the power cable 11 can be accurately measured. It is convenient if one end of the reactor 21 can be detachably connected to the high-voltage bus 14.

According to experiments, for a 250m 6.6KV CV cable,
As a result of performing current measurement by the method of FIG. 1 and the method of FIG. 3 and FIG. 4, the measured values are the same at 25 nA in the method of FIG. 1 and FIG.
However, it was 31 nA in the method of FIG. Thereby, it was confirmed that the method of FIG. 1 has approximately the same accuracy as the method of FIG. In the method of FIG. 1, a 1.5H reactor was used.

FIG. 2 shows another embodiment of the present invention. Power cable 11
If a load 22 such as a motor is connected to the motor, the DC component generated by the load may affect the measured value.Therefore, this method uses a DC cut filter 23 between the current measuring device 17 and the ground. At the same time, the power cable shielding layer 13 is connected between the current measuring device 17 and the DC cut filter 23.
Are electrically connected to cut off the influence of the DC component generated in the load 22. The other parts are the same as the embodiment of FIG. Note that this method is applicable even when no load is connected to the power cable.

〔The invention's effect〕

As described above, according to the present invention, the DC component can be measured without being affected by electromagnetic induction or stray current, and the use of the reactor allows the power supply transformer and the grounding transformer to be connected without changing the wiring. Wiring work becomes extremely simple because a current measuring instrument can be connected. Also, since it can be connected to either terminal of the reactor power cable,
Measurement work can be performed with a terminal that is more convenient, such as space, and the degree of freedom of work is high.

[Brief description of the drawings]

FIG. 1 and FIG. 2 are wiring diagrams showing an embodiment of a power cable insulation diagnosis method according to the present invention, and FIGS. 3 and 4 are wiring diagrams showing a conventional power cable insulation diagnosis method. 11: power cable, 12: conductor, 13: shielding layer, 14: high-voltage bus,
15: Power transformer or grounding transformer, 16: AC grounding capacitor, 17: Current measuring instrument, 21: Reactor, 22: Load, 23: DC cut filter.

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Hajiyuki Sawada 2-4-3 Okano, Nishi-ku, Yokohama-shi, Kanagawa Prefecture Inside the Yokohama Research Laboratory of Furukawa Electric Co., Ltd. (56) References JP-A-54-140572 (JP, A ) JP-A-51-146871 (JP, A)

Claims (2)

(57) [Claims] 1. A method of measuring a DC component of a current flowing between a conductor of a power cable and a shielding layer and diagnosing deterioration of insulation of the power cable based on a magnitude of the DC component, wherein one end of a reactor is connected to the conductor of the power cable. A power cable insulation diagnosis characterized by connecting a current measuring device between the other end of the reactor and the ground, grounding a shield layer of the power cable, and measuring a DC current flowing through the current measuring device in that state. Method. 2. A method for measuring a DC component of a current flowing between a conductor of a power cable and a shielding layer and diagnosing deterioration of insulation of the power cable based on the magnitude of the DC component, comprising connecting one end of a reactor to the conductor of the power cable; Between the other end of the reactor and the ground, a current measuring device and a DC cut filter are connected in series with the latter on the ground side, and a shielding layer of a power cable is provided between the current measuring device and the DC cut filter. And electrically measuring the DC current flowing through the current measuring device in this state.