JPH0652281B2 - Water tree current detection method for CV cable - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention is suitable for measuring a water tree current generated due to insulation deterioration of a CV cable (crosslinked polyethylene insulated vinyl sheath cable) in a live state. Regarding the method for detecting the water tree current of the CV cable, more specifically, C
A water tree current detection method for a CV cable in which a stray current is obtained by using an alternating current flowing through a ground wire drawn from a shielded copper of a V cable, and a water tree current is obtained by subtracting the stray current from a total DC component flowing in the ground wire. Regarding

(Prior Art) As shown in FIGS. 4 and 5, for example, in a CV cable 1, a conductor 2 is covered with an inner semiconductive layer 3, and an outer semiconductive layer 4 and an inner semiconductive layer 3 are provided. A cross-linked polyethylene 5 as an insulator is interposed between the outer semiconductive layer 4 and the shielding copper tape 6
The shield copper tape 6 is covered with a shielding cloth 7 and the pressing cloth 7 is wrapped around the shielding copper tape 6, and the pressing cloth 7 is insulated with an insulating vinyl sheath 8.
Is formed by coating. In addition, CV cable 1
As shown in FIG. 6, there are provided three structural bodies in which the shielding copper tapes 6 are integrated, the shielding copper tapes 6 are brought into contact with each other, and the pressing cloth 7 is wound around the three structural bodies. There is also a so-called triplex type CV cable (CVT) which is a CV cable in which the pressing cloth 7 is covered with an insulating vinyl sheath 8. Reference numeral 9 indicates an inclusion.

When the CV cable 1 is insulation deteriorated, a water tree current I _i is generated as shown in FIG. 7. In the example shown in FIG. 7, the shielding copper tape 6 side is + potential, and the conductor 2 side is −potential. Further, the water tree current I _i may flow in the direction opposite to the direction shown in the drawing. This water tree current I _i
As shown in Fig. 8, the high voltage distribution line 10
A ground wire 11 is drawn from the shielded copper tape 6 on the other side of the CV cable 1 having one side connected to and the other side connected to a load, and a water tree current I _i as an insulation deterioration related amount is provided in the middle of the ground wire 11. A measuring device 12 for measuring is connected. The measuring device 12 comprises a detection resistor 13, an amplifier 14 having a filter for removing the alternating current I _AC , and a recording device 15.

However, there is a battery acting electromotive force E _S between the insulating vinyl sheath 8 and the ground, and a commercial frequency electromotive force E _AC due to the unbalance of the system load between the connecting wire 17 of the GPT 16 and the ground, and the GPT 16 is grounded. There is a battery action electromotive force E _E in the part. FIG. 9 shows this state by an equivalent circuit. In FIG. 9, R _i is the insulation resistance of the cross-linked polyethylene 5 portion of the CV cable 1, _RS is the sheath resistance of the insulating vinyl sheath 8, electromotive force E _i , insulation resistance R _i
It is considered that there is a capacitor C _{i in} parallel with, and it is considered that there is a capacitor C _{S in} parallel with the battery action electromotive force E _S and the sheath resistance R _S. When these electromotive forces E _S , E _E , and E _AC are present, stray currents I _S , I _E , and AC current I _AC are generated, and the stray currents I _S and I _E are the DC current component I and the water tree current I _i. Along with that, it flows into the measuring device 12. FIG. 10 shows an equivalent circuit in which the equivalent circuit shown in FIG. 9 is rewritten and expressed by focusing on only the DC current component I.

In FIG. 10, a stray current I _S as a direct current component,
I _E is shown flowing with the water tree current I _i . The stray currents I _S and I _E are determined by the resistances R _S and R _E and the electromotive forces E _S and E _E acting on the battery. The stray current I _E is the ground line 11a between the measuring device 12 and the ground. It can be removed by sharing with the ground wire 17 of GPT16. Considering the stray current I _S , the electromotive force E _i of the water tree current I _i is usually about several volts 10 V or less, and the battery acting electromotive forces E _S , E _E are about 0.5 V or less. Further, the insulation resistance R _i is several hundred thousand MΩ, the sheath resistance R _S is smaller than the normal insulation resistance, and if the sheath resistance R _S is 200 MΩ or more, the stray current I
_S is 2.5 nanoamps or less, whereas in the deteriorated cable, the water tree current I _i is 10 nanoamps, so it is not necessary to consider the stray current I _S under normal conditions, but the sheath resistance R _S greatly varies depending on environmental conditions and the like, and when the sheath resistance R _S becomes 200 MΩ or less, the ratio of the stray current I _S contributing relatively increases. In FIG. 8, 18 is a power source, 19 is a ground wire pulled out from the shielded copper tape 6 on one side of the CV cable 1, and 20 'is a switch that is opened at the time of measurement.

(Problems to be Solved by the Invention) Therefore, in order to prevent the dielectric breakdown accident due to the insulation deterioration of the CV cable 1 by using the conventional measuring device 12 which detects only the DC component, the insulation deterioration of the CV cable 1 is prevented. CV water tree current detecting method of the cable for detecting a water-tree current I _i based (e.g., JP 59-202075 JP), the measures that the water or tree current I _i is measured stray currents I _S It becomes impossible to identify whether or not there is.

In such a case, it is preferable to have a method capable of measuring the water tree current I _i without being affected by the stray current I _S.

The present invention has been made from the above point of view, and by using an alternating current flowing through a grounding wire drawn from a shielding copper of a CV cable, the stray current is measured to accurately measure the water tree even if the stray current flows. It is an object of the present invention to provide a novel CV cable water tree current detection method capable of measuring current.

(Means for Solving the Problem) A method for detecting a water tree current of a CV cable according to claim 1 of the present invention comprises a maximum portion and a minimum portion of an AC current flowing through a ground wire drawn from a shielding copper of the CV cable. In order to detect the water tree current that is superimposed on, the plurality of current values with respect to the reference zero of the measuring instrument are sampled within at least one cycle of the alternating current while avoiding the maximum part and the minimum part of the alternating current. Then, by arithmetically averaging the plurality of current values, a stray current appearing as a shift of the AC current is obtained, and the stray current is subtracted from the total DC component flowing in the ground line to obtain the water tree current. Characterized by seeking.

According to a second aspect of the present invention, there is provided a method for detecting a water tree current in a CV cable, wherein the water tree current superimposed on a maximum portion and a minimum portion of an alternating current flowing through a ground wire drawn from a shielding copper of the CV cable is detected. For detection, the measuring range of the measuring instrument is changed to saturate the alternating current input to the measuring instrument, and the upper limit value of the measuring range of the alternating current with respect to the reference zero of the measuring instrument is A and the lower limit value is-. A, when the saturating width of the upper AC waveform portion is B with the reference zero as a boundary, the reference zero at the point of giving a saturating width equal to the saturating width B in the lower AC waveform portion with the reference zero as a boundary. determine the current value C for, by the following equation, obtains a stray current I _S which appears as a shift amount of the alternating current, the stray electrostatic from the total DC component flowing through said ground line By subtracting the, and obtains the water tree current.

I _S = (A + C) / 2 (Example) A method for detecting a water tree current of a CV cable according to the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing an embodiment of a water tree current detection method for a CV cable according to claim 1, and in FIG. 1, reference numeral 20 is a measuring instrument. The measuring device 20 has a ground line AC current detecting unit 21 and a DC component current detecting unit 22. The ground line AC current detector 21 has input terminals 21a and 21b, and the DC component current detector 22 has input terminals 22a and 22b. The input terminal 21a is connected to the ground wire 11 drawn out from the shield copper 6, the input terminal 22b is connected to the ground wire 11 via the switch 23, and the input terminals 21b and 22a are grounded via the ground wire 11a. There is.

The output of the ground line AC current detection unit 21 is input to the stray current calculation unit 25 and the DC component current calculation unit 26 via the A / D conversion unit 24, and the A / D conversion unit 24 and the DC component current calculation unit 26 are connected to each other. A switch 27 is provided between them. The output of the DC component current detection unit 22 is input to the water tree current output unit 29 via the A / D conversion unit 28, and the water tree current output unit 29 outputs the output of the stray current calculation unit 25 and the DC component current calculation unit. 26 outputs and are input.

The ground line AC current detector 21 has an AC current I shown in FIG.
_AC is input. A water tree current I _i appears as a harmonic component in the maximum part and the minimum part of the alternating current I _AC . This harmonic component is asymmetric between the upper AC waveform portion and the lower AC waveform portion. This is because the water tree current I _i is non-linear with respect to the applied voltage and has a difference depending on the polarity. That is, the water tree current I _i increases as the applied voltage rises, and the flowing direction is fixed. On the other hand, the stray current I _S is measured by the measuring device 20.
Appears as a shift of the alternating current I _AC with respect to the reference zero K _Z of. Here, the reference zero K _Z is one in which the measuring device 20 is zero-adjusted and set.

The alternating current I _AC is converted into a digital value by the A / D converter 24 and input to the stray current calculator 25. Stray current calculator
The reference numeral 25 has a function of sampling the current value with respect to the reference zero K _Z of the measuring instrument 20 while avoiding the maximum portion and the minimum portion where the water tree current I _i is superposed within at least one cycle of the alternating current I _AC. . This sampling is performed, for example, by equally time-sharing one cycle of the alternating current I _AC . Here, the sampling value is S
_{1 to} S ₁₆ . The stray current calculator 25 finds the sum of the sampled values S _{1 to} S ₁₆ excluding the maximum part and the minimum part, and divides the sum by the total number to find the average value.

Since the current value including the water tree current I _i is excluded from the average value thus obtained, the obtained average value means the stray current I _S that appears as a shift from the reference zero K _Z. It becomes a value. The value meaning the stray current I _S is input to the water tree current output unit 29.

When the switch 23 is closed, the output of the DC component current detector 22 is supplied to the water tree current output unit 29 by the A / D converter 28.
Be entered via. The DC component current detector 22 has a function of detecting the total DC component including the water tree current I _i and the stray current I _S, and has a filter for removing the _AC component I _AC .

Water tree current output unit 29 subtracts the stray current I _S from the total DC component has a function of obtaining the water-tree current I _i, information indicating this way water trees currents I _i obtained in It is output to the display unit 30.

In this embodiment, the total DC component is detected based on the output of the DC component current detection unit 22, but the switch 23
Is closed and the switch 27 is closed, and the total DC component including the water tree current I _i and the stray current I _S is sampled over the entire cycle of the _AC current I _AC using the DC current component calculator 26. The water tree current I _i may be obtained by subtracting the stray current I _S from the total DC component thus obtained.

If such sampling is repeated over several cycles,
A more accurate water tree current I _i can be obtained.

FIG. 3 is a diagram showing a second embodiment of the water tree current detecting method according to claim 2 of the present invention, which is a CV cable 1
This is an embodiment for accurately measuring the water tree current I _i flowing for each phase.

Since the alternating current I _AC flowing through the grounding line 11 that bundles each phase of the cable 1 flows based on the imbalance of each phase, the alternating current I _AC is, for example, about 30 microamperes at the maximum. The alternating current I _AC flowing in each phase of the CV cable 1 has a maximum of about 30 milliamperes,
This value is much larger than the value of the water tree current I _i is nano-ampere or micro-ampere.
Since the water tree current I _i for each phase was measured by using the method described in 1), the stray current I for each phase of the CV cable 1 was measured.
_S cannot be measured accurately and thus the value of the water tree current I _i obtained is inaccurate.

In such a case, the measuring range of the measuring device 20 is changed to the high-accuracy side to saturate the alternating current I _AC . The upper limit of the measurement range is A and the lower limit is -A. Further, the saturating width of the upper AC waveform portion is B at the reference zero K _Z. The waveform of _AC current I _AC is stray current I
_Since it is expected to be symmetric with respect to _S , the current value C with respect to the reference zero K _Z at the point where the saturating width T equal to the saturating width B is given in the downstream AC waveform portion.
The stray current I _S can be calculated by the following equation.

I _S = (A + C) / 2 By subtracting the stray current I _S thus obtained from the total DC component, the water tree current I _i is obtained.

Note that the water tree current I _i for each phase of the CV cable 1
For example, when measuring the water tree current I _i of the R ₁ phase, the connection between the R ₂ phase and the R ₃ phase and the CV cable 1 is disconnected.

(Effect of the invention) Since the water tree current detection method for a CV cable according to claim 1 of the present invention is the method described above, the water tree current and the stray current superimposed on the AC waveform are separated. The stray current can be measured. Therefore, by subtracting the stray current from the total DC component, the water tree current itself can be accurately measured even in a live state.

Since the method for detecting water tree current of a CV cable according to claim 2 of the present invention is the method described above, it is possible that the alternating current flowing through the ground wire is much larger than the water tree current and the stray current. Also, the stray current can be accurately measured. Therefore, by subtracting the stray current from the total DC component including the water tree current and the stray current, the water tree current itself can be accurately measured even in a live state. is there.

[Brief description of drawings]

FIG. 1 is a circuit diagram for explaining a water tree current detection method for a CV cable according to claim 1 of the present invention, and FIG. 2 is a water tree current detection method for a CV cable according to claim 1 of the present invention. FIG. 3 is a waveform diagram for explaining the water tree current detection method of the CV cable according to claim 2 of the present invention, and FIG. 4 is a C diagram according to the present invention.
FIG. 5 is a side view of the V cable, FIG. 5 is a side view thereof, FIG. 6 is a sectional view of another CV cable according to the present invention, and FIG. 7 is an explanatory view of a water tree current generating mechanism according to the present invention. The figure shows the connection diagram of the conventional measuring instrument to the CV cable.
The figure is an equivalent circuit of the connection diagram shown in FIG. 1 ... CV cable, 6 ... Shielding copper 11 ... Ground wire, 20 ... Measuring instrument 21 ... Ground wire AC current detection unit 22 ... DC component current detection unit, 25 ... Stray current calculation unit I _AC ... AC current, I _i ... Water Tree current I _S … stray current, K _Z … reference zero

Claims (2)

[Claims] Claim: What is claimed is: 1. To detect a water tree current superimposed on a maximum portion and a minimum portion of an alternating current flowing through a grounding wire drawn from a shielded copper of a CV cable, a maximum portion and a minimum portion of the alternating current are detected. Sampling within at least one cycle of the alternating current while avoiding
A plurality of current values with respect to the reference zero of the measuring instrument are obtained, the plurality of current values are arithmetically averaged to obtain a stray current that appears as a shift of the AC current, and the stray is obtained from the total DC component flowing in the ground line. A method for detecting a water tree current in a CV cable, wherein the water tree current is obtained by subtracting the current. 2. A measuring range of a measuring instrument is changed to detect a water tree current superimposed on a maximum portion and a minimum portion of an alternating current flowing through a grounding wire drawn from a shielded copper of a CV cable. Saturating the alternating current input to the measuring instrument, and the upper limit of the measuring range of the alternating current with respect to the reference zero of the measuring instrument is A, the lower limit is -A,
When the saturating width of the upper AC waveform portion is B with the reference zero as a boundary, a current value with respect to the reference zero at a point that gives a saturating width equal to the saturating width B in the lower AC waveform portion with the reference zero as a boundary. The water tree current is obtained by obtaining C, obtaining a stray current I _S that appears as a shift of the AC current by the following formula, and subtracting the stray current from the total DC component flowing in the ground line. The water tree current detection method for CV cable. I _S = (A + C) / 2