JPH0679046B2 - Method for measuring DC component of power cable - Google Patents

Description

Description: TECHNICAL FIELD In the present invention, the insulation performance and diagnosis of a power cable are performed under a hot line (with an AC voltage applied). Therefore, the DC component of the current flowing between the conductor and the shield layer of the power cable is detected. It concerns a method of measuring.

[Prior art]

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 under the live line, 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 detect the water tree or the like generated in the insulating layer of the CV cable.

Conventionally, this DC component is measured as shown in FIG. The power cable 1 has a structure in which a shield layer 3 is provided on the outer periphery of a conductor 2 via an insulating layer (not shown), and the conductor 2 is connected to a high voltage bus bar 4 and is in a live state. Further, for example, a GPT (grounding type instrument transformer) 5 is connected to the high voltage bus bar 4, and its neutral point is grounded. Both ends of the shield layer 3 are grounded in a normal operating state, but when measuring a DC component, the AC grounding capacitor 6 and the DC current measuring device 7 are connected between one end of the shield layer 3 and the ground. Connect a parallel circuit and open the ground at the other end. As a result, the shield layer 3 is in a state of being insulated from the ground in terms of direct current and being low impedance grounded in terms of alternating current.

By doing this, the DC component flows from the high voltage charging section through the route of the conductor 2 of the power cable → the same insulating layer → the same shielding layer 3 → DC current measuring device 7 → ground → GPT5 → high voltage charging section as indicated by the one-dot chain line arrow. It will be. Therefore, if the value of the DC current measuring device 7 is read, the DC component of the current flowing through the insulating layer of the power cable can be measured.

When the high-voltage bus 4 is connected to a grounding transformer or a neutral point grounding type transformer, the neutral point grounding part of the transformer may be used instead of the GPT5.

〔problem〕

However, in the case where a load 8 is connected to the power cable 1 as shown in the figure, when a direct current component is generated from the load 8, the direct current component flows through the ground as shown by a dotted arrow, and the direct current measuring device 7 It will affect the measured value. Therefore, when a load is connected to the power cable, an error may easily occur in the measured value of the DC component, and the insulation performance may be erroneously diagnosed.

[Means for solving problems and their effects]

In view of the above-mentioned problems of the prior art, an object of the present invention is to provide a transformer (GPT, grounding transformer, neutral point grounding transformer, single-phase transformer) which is grounded at one end with a neutral point. Neutral point grounding transformer combined with a stand) provides a method for accurately measuring the DC component of the current flowing between the conductor and the shield layer of a power cable with a load connected to the other end. .

In order to achieve this object, the present invention provides a series connection of a parallel circuit of a capacitor for AC grounding and a DC current measuring device and a DC cutting filter between the neutral point of the transformer and the ground, with the latter as the ground side. In addition, the shield layer of the power cable is electrically connected between the parallel circuit and the DC cut filter, and the DC current flowing through the DC current measuring device is measured in that state.

By doing so, the DC component generated in the load does not flow into the DC current measuring device, and the DC component of the power cable can be accurately measured.

〔Example〕

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

In FIG. 1, the same parts as those in FIG. 2 are designated by the same reference numerals. This measurement method is different from the conventional one in that the parallel circuit of the AC grounding capacitor 6 and the DC current measuring device 7 and the DC cutting filter 9 are provided between the neutral point of the GPT5 and the ground.
The measurement is performed while the DC cut filter 9 is connected in series with the ground side and the shield layer 3 of the power cable 1 is connected by the electric wire 10 between the parallel circuit and the DC cut filter 9. The DC cut filter 9 is the same as the AC grounding capacitor 6 and cuts the DC component invading from the ground side and acts as a low impedance grounding element for AC.

In this way, the current flowing between the conductor 2 and the shield layer 3 of the power cable 1 flows as indicated by the one-dot chain line arrow, and the DC component thereof flows to the DC current measuring device 7. On the other hand, the current generated in the load 8 flows as indicated by the dotted arrow, but its DC component is cut by the DC cut filter 9, so it does not flow to the DC current measuring device 7. Therefore, in the DC current measuring device 7, the conductor 2 and the shielding layer 3 of the power cable 1
Only the DC component of the current flowing between them can be accurately measured.

In the above embodiment, the AC voltage was applied to the power cable via the high voltage bus, but the AC voltage may be applied to the power cable by directly connecting the AC power supply device to the power cable. Further, in the drawing, for convenience, the power cable 1
Although only one (for one phase) is shown, it goes without saying that the power cables are similarly connected to the other phases. The power cable may be three single-core power cables or one three-phase, three-core power cable.

〔The invention's effect〕

As described above, according to the present invention, the transformer grounded at the neutral point on one end side is measured for the DC component of the current flowing between the conductor and the shield layer for the power cable to which the load is connected on the other end side. In this case, it is possible to measure only the DC component of the current flowing between the conductor and the shield layer of the power cable without being affected by the DC component of the current generated by the load, improving the measurement accuracy and improving the insulation performance. The reliability of diagnosis can be improved.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a DC component measuring method of a power cable according to an embodiment of the present invention, and FIG. 2 is a circuit diagram showing a conventional measuring method. 1: Power cable, 2: Conductor, 3: Shielding layer, 4: High voltage busbar, 5: GP
T, 6: AC grounding capacitor, 7: DC current measuring device, 8:
Load, 9: DC cut filter, 10: electric wire.

Claims (1)

[Claims] 1. A method for measuring a DC component of a current flowing between a conductor and a shield layer in a power cable having a neutral point grounded at one end and a load connected at the other end, Between the neutral point of the transformer and the ground, a parallel circuit of a capacitor for AC grounding and a DC current measuring device and a DC cut filter are connected in series with the latter on the ground side, and the parallel circuit and the DC cut filter are connected. A method for measuring a direct current component of a power cable, characterized in that the shield layer of the power cable is electrically connected between the two, and the direct current flowing through the direct current measuring device is measured in that state.