JPH0553230B2 - - Google Patents

Description

[Detailed Description of the Invention] Purpose of the Invention (Industrial Application Field) This invention relates to cables, electrical equipment (for example,
Insulation deterioration-related quantities such as insulation resistance of transformers (transformers), water tree currents, etc. based on cable insulation deterioration are measured in the live state, that is, the state in which power is being supplied to the cable or the state in which power is being supplied to electrical equipment. The present invention relates to an improvement of an insulation deterioration related quantity measuring device that can measure insulation deterioration.

(Prior art) Conventionally, as a means of measuring insulation deterioration-related quantities such as insulation resistance and dielectric loss tangent of a circuit to be measured such as a cable or electrical device, the method is to supply power while the cable or electrical device is in a non-charged state. There is a known method in which an AC or DC high voltage is applied to the circuit to be measured, and insulation deterioration-related quantities such as insulation resistance and dielectric loss tangent are measured.

Next, as a means of measuring insulation deterioration-related quantities in a live line state, for example, in order to measure the insulation resistance of a CV cable in a live line state, a low DC voltage is superimposed on the GPT neutral point, and the CV cable is Measure the direct current component flowing in the grounding wire between the shielding copper and the earth,
Insulation resistance (sheath resistance) as a quantity related to insulation deterioration
There is a device designed to measure . In addition, as an insulation deterioration related quantity measuring device that measures the water tree current of a CV cable as an insulation deterioration related quantity, it applies an AC voltage and detects the DC current component flowing in the grounding wire between the shielding copper and the earth. There are some that are designed to measure

(Problems to be solved by the invention) By the way, as an insulation deterioration related quantity measuring device,
It is desirable to be able to measure CV cables, electrical equipment, etc. in a live state. However, in order to measure the insulation resistance of conventional CV cables in a live state, a low DC voltage is applied to the GPT neutral point. The DC current component flowing through the grounding wire between the shielding copper of the CV cable and the earth is measured, and the insulation resistance (sheath resistance) is measured as an amount related to insulation deterioration. If a stray current based on the cell action and a water tree current due to insulation deterioration occur, the DC measurement current based on the DC low voltage applied to the GPT neutral point will have a stray current due to the cell action and a water tree current due to insulation deterioration. Because of this, there is a problem in that the measured insulation resistance value includes errors due to stray current and water tree current due to battery action.

In addition, in order to measure the water tree current of a CV cable as an amount related to insulation deterioration, stray current is If there is, there will be a problem as explained below.

As shown in Figures 1 and 2, for example, CV
The cable 1 includes a conductor 2 covered with an internal semiconductor layer 3, a cross-linked polyethylene 5 as an insulator interposed between the external semiconductor layer 4 and the internal semiconductor layer 3, and the external semiconductor layer 4 covered with a shielding copper tape 6. A presser cloth 7 is wrapped around the shielding copper tape 6, and the presser cloth 7 is covered with an insulating vinyl sheath 8. The CV cable 1 is provided with three structures including shielding copper tapes 6 as shown in FIG.
It is a so-called triplex type in which the presser cloth 7 is covered with an insulating vinyl sheath 8.
It is a CV cable. There is also a so-called triplex type CV cable (CVT). Reference numeral 9 is an inclusion.

When the insulation of this CV cable 1 deteriorates,
A water tree current I _i is generated as shown in FIG.
In the example shown in FIG. 4, the side of the shielding copper tape 6 is +
The potential on the conductor 2 side is - potential. There are also cases where the potential is opposite. In order to measure this water tree current I _i , as shown in FIG.
Ground wire 1 from shielding copper tape 6 on the other side of cable 1
1 is pulled out, and a measuring device 12 for measuring a water tree current I _i as an insulation deterioration related quantity is connected to the middle of the grounding wire 11. This measuring device 12 is generally composed of a detection resistor 13, an amplifier 14 having a filter, and a recording device 15.

However, between the insulating vinyl sheath 8 and the earth, there is a battery action electromotive force E _S and a ground wire 17 of the GPT 16.
There is a commercial frequency electromotive force E _AC due to the unbalanced system load between the GPT 16 and the ground, and a battery action electromotive force E _E is present at the ground portion of the GPT16. FIG. 6 shows this state using an equivalent circuit. In this FIG. 6, R _i is the crosslinked polyethylene 5 of the CV cable 1.
R _S is the sheath resistance of the insulating vinyl sheath 8, the electromotive force E _i and the insulation resistance R _i
It is thought that there is a capacitor C _i in parallel with the battery action electromotive force E _S and a capacitor in parallel with the sheath resistance R _S
It is thought that there is a _CS . These electromotive forces E _S , E _E ,
When E _AC exists, stray currents I _S and I _E and alternating current I _AC are generated, and the stray currents I _S and I _E flow as a direct current component I together with the water tree current I _i to the measuring device 12. The equivalent circuit shown in Figure 6 is the DC current component I
FIG. 7 shows an equivalent circuit rewritten and expressed by focusing only on the above.

FIG. 7 shows a state in which stray currents I _S and I _E as direct current components flow together with water tree current I _i . These stray currents I _S , I _E are resistors R _S ,
The stray current I E is determined by R _E and the battery action electromotive force E _{S and} E _E , but the stray current I _E can be reduced by sharing the ground wire 11a between the measuring device 12 and the earth with the ground wire 17 of the GPT 16. Can be removed. Therefore, the stray current
Considering I _S , the electromotive force E _i of the water tree current I _i
is usually less than several tens of volts, and the battery action electromotive force E _S ,
E _E is about 0.5 volt or less. Also, insulation resistance
R _i is several thousand MΩ, the sheath resistance R _S is usually smaller than the insulation resistance, and when the sheath resistance R _S is 200 MΩ or more, the stray current I _S is less than 2.5 nanoamperes, whereas in a deteriorated cable, the water tree current Since I _i is approximately several tens of nanoamperes or more, there is no need to consider the stray current I _S under normal conditions, but the sheath resistance R _S fluctuates greatly depending on environmental conditions and other factors, and the sheath resistance R _S When the value is less than 200 MΩ, the contribution of the stray current I _S becomes relatively large, making it impossible to distinguish whether the stray current I _S or the water tree current I _i is being measured. In such cases, the sheath resistance can be reduced without being affected by stray currents.
If R _S can be measured, the water tree current I _i can be accurately measured, so from this point of view, insulation resistance as an insulation deterioration related quantity can be measured without being affected by stray current, which is a DC component current flowing in the circuit to be measured. (Sheath resistance is also used to mean insulation resistance.)
It is desirable to be able to measure both the stray current and the stray current. In addition, in FIG. 5, 18 is a power supply;
9 is a grounding wire drawn out from the shielding copper tape 6 on the other side of the CV cable 1, and 20 is a switch that is opened during measurement.

Therefore, this invention was made in consideration of the above circumstances, and its purpose is to avoid being affected by stray current, which is a DC current component, in the circuit to be measured. An object of the present invention is to provide an insulation deterioration related quantity measuring device that can measure insulation resistance and stray current at the same time.

(Means for Solving the Problems) In order to achieve this object, the present invention provides a circuit to be measured in which insulation deterioration-related quantities are measured in such a way that the influence on the alternating current impedance component of the circuit to be measured is negligible. An extremely low frequency voltage applying section that applies a low frequency voltage, a DC component current detecting section that detects a DC component current flowing through the circuit to be measured based on the extremely low frequency voltage, and the DC component current detecting section. a calculation unit that calculates insulation resistance based on a detection output of the circuit; and a calculation unit that controls the DC component current detection unit before the ultra-low frequency voltage is applied to the circuit to be measured to detect a DC component flowing in the circuit to be measured. After the current is detected by the DC component current detection section, positive and negative ultra-low frequency voltages are alternately applied to the circuit to be measured by controlling the ultra-low frequency voltage application section to detect the direct current flowing in the circuit to be measured. A component current is detected by a DC component current detection section, and the insulation resistance is determined from the difference in detection output of the DC component current detection section before and after the ultra-low frequency voltage is applied to the circuit to be measured and the ultra-low frequency voltage. The invention is characterized in that the insulation deterioration related quantity measuring device includes a timing control section that controls the arithmetic operation control section so as to cause the arithmetic operation section to calculate .

(Embodiment) Hereinafter, a first embodiment of the insulation deterioration related quantity measuring device according to the present invention will be described with reference to FIGS. 8 to 10.

In FIG. 8, 30 is a circuit to be measured in which an insulation deterioration related quantity is measured. This circuit to be measured 3
0 is the CV cable 1 here, but it may also be a transformer or other electrical equipment. Reference numeral 31 denotes an insulation deterioration related quantity measuring device for measuring the insulation deterioration related quantity of the circuit 30 to be measured. The insulation deterioration related quantity measuring device 31 can measure even when the circuit 30 to be measured is in a live line state, and the insulation deterioration related quantity measuring device 31 includes an extremely low frequency voltage applying section 70 and a DC component current detecting section. 71, a calculation section 72, a timing control section 73, an insulation resistance recording section 74, and a DC component current recording section 75.

The very low frequency voltage applying section 70 has a function of applying a rectangular wave voltage V as a very low frequency voltage to the circuit to be measured 30. Here, the frequency of the rectangular wave voltage V is set to such an extent that the influence on the AC impedance component of the circuit to be measured 30 can be ignored, for example, 0.02
A rectangular wave voltage with a frequency of about Hz or less is used. When the rectangular wave voltage V is applied to the circuit to be measured 30, a DC component current flows through the circuit to be measured 30. The DC component current detection section 71 detects the measurement target circuit 3.
The calculation unit 72 has a function of calculating the DC component current and the insulation resistance based on the detection output of the current detection unit, and the timing control unit 73 has a function of controlling the ultra-low frequency voltage application section 70, the DC component current detection section 71, and the calculation section 72.

Next, measurement using the insulation deterioration related quantity measuring device according to the present invention will be explained with reference to FIG.

A DC component current originally flows through the circuit to be measured 30 in a live state, and first, in the state before applying the rectangular wave voltage V in section I, the DC component current ₁ flowing in the circuit to be measured 30 is Measure. Next, a positive square wave voltage in the interval
Apply _V2 . At this time, the DC component current flowing through the circuit to be measured 30 is assumed to be ₂ . Let R _S2 be the resistance obtained by measuring this section, for example, the sheath resistance. At this time, the following relational expression holds true between the sheath resistance R _S2 , the DC component currents ₁ and ₂ , and the rectangular wave voltage _V2 .

R _S2 = V ₂ / ₂ - ₁ Next, the application of the rectangular wave voltage V is stopped in the section, the DC component current ₃ in the section is measured, and then the negative rectangular wave voltage V ₄ is applied in the section.
Apply. The DC component current flowing through the circuit to be measured 30 at this time is assumed to be ₄ . The resistance obtained by measuring this section, for example, the sheath resistance.
Let it be R _S4 . At this time, the following relational expression holds true between the sheath resistance R _S4 , the DC component currents ₃ and ₄ , and the rectangular wave voltage _V4 .

R _S4 = V ₄ / ₄ − ₃ Therefore, the average sheath resistance R _S is determined as R _S = R _S2 + R _S4 /2.

The timing control unit 73 controls the DC component current ₁ ,
₂ , ₃ , ₄ , sheath resistance R _S2 in each section,
In order to obtain R _S4 and the average sheath resistance R _S , the extremely low frequency voltage applying section 70, the DC component current detecting section 71, and the calculating section 72 are controlled. The sheath resistance R _S determined in this way is used as explained below. For example, the sheath resistance R _S is artificially changed _, at least three sets of the sheath resistance R _S and the corresponding DC component current are obtained, and the tenth Obtain the relationship curve shown in the figure and calculate the sheath resistance
By estimating the extreme DC component current on the increasing side of R _S , the water tree current _i can be measured even when the stray current _S remains included.

(Effects of the Invention) Since the insulation deterioration related quantity measuring device according to the present invention uses ultra-low frequency as explained above, it can be used even when a stray current, which is a direct current component current, flows in the circuit to be measured. This has the effect that the insulation resistance can be measured simultaneously with the stray current with high accuracy without being affected by the stray current.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a CV cable according to the present invention.
Fig. 2 is a side view thereof, and Fig. 3 is related to this invention.
4 is an explanatory diagram of the generation mechanism of the water tree current according to the present invention; FIG. 5 is a diagram of the connection of a conventional measuring device to the CV cable; FIGS. 6 and 7 are its diagrams. Equivalent circuit of the connection diagram shown in Figure 5, No. 8
The figure is a block circuit diagram of the insulation deterioration related quantity measuring device according to the present invention, FIG. FIG. 4 is a relationship curve diagram for explaining an example in which a water tree current is estimated and obtained by simultaneously measuring sheath resistance and a DC component current using the method. 30... Circuit to be measured, 31... Insulation deterioration related quantity,
70... Very low frequency voltage application section, 71... DC component current detection section, 72... Arithmetic section, 73... Timing control section.

Claims (1)

[Scope of Claims] 1. An ultra-low frequency voltage application unit that applies an ultra-low frequency voltage to a circuit to be measured whose insulation deterioration-related quantity is to be measured, the effect of which is negligible on the alternating current impedance component of the circuit to be measured; a DC component current detection unit that detects a DC component current flowing through the circuit to be measured based on the ultra-low frequency voltage; and a calculation unit that calculates insulation resistance based on the detection output of the DC component current detection unit. , before the ultra-low frequency voltage is applied to the circuit to be measured, controlling the DC component current detection unit to cause the DC component current detection unit to detect the DC component current flowing through the circuit to be measured; By controlling the ultra-low frequency voltage application section, positive and negative ultra-low frequency voltages are alternately applied to the circuit to be measured, and the DC component current flowing through the circuit to be measured is detected by the DC component current detection section; The arithmetic control unit is configured to cause the arithmetic unit to calculate the insulation resistance from the difference between detection outputs of the DC component current detection unit before and after the ultra-low frequency voltage is applied to the circuit to be measured and the ultra-low frequency voltage. An insulation deterioration related quantity measuring device, comprising: a timing control section for controlling an insulation deterioration related quantity.