JP2003084028A - Hot line diagnostic method for power cable - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot line diagnostic method for a power cable, capable of decreasing a measurement error of a DC component deterioration signal due to variation of stray current in a cable line system, measuring a DC component caused by insulation deterioration of water tree with heightened detection accuracy, and improving determination accuracy of a deterioration signal. SOLUTION: In this hot line diagnostic method for the power cable, one end of a power cable line 1 having a conductor 2, a cross-linking polyethylene insulator and a shielding layer 3 is connected to a ground line 7, and the other end ground line 6 is released from grounding. A DC power supply 10 of bipolarity and a low-pass filter 11 connected in series is disposed between the shielding layer 3 and the ground line 7. Before and after positive DC voltage and negative DC voltage are respectively biased to the shielding layer 3 of the power cable line 1, a stray current flowing through the ground line is measured by the low- pass filter 11 and a DC ammeter 12, and the change rate of the stray current is calculated from the variation. Further, the variation of the stray current which is an error factor is subtracted from the ground line DC current measurement data in biasing the positive and negative DC voltage to obtain an accurate deterioration signal, and detect a water tree deterioration of the cable insulator.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for diagnosing insulation deterioration of a power cable in a live state in which a commercial frequency is applied, and more particularly, a crosslinked polyethylene insulation vinyl sheath power having a conductor, a crosslinked polyethylene insulation and a shielding layer. The present invention relates to a live line diagnosis method for a power cable line using a cable (CV cable).

[0002]

2. Description of the Related Art Conventionally, as a method of diagnosing insulation deterioration in a power cable line such as a CV cable under a live condition, a local CV cable is grounded at one end and positive and negative voltages are biased to a cable shielding layer. There is known a method of measuring a direct current flowing through a ground wire.

Here, looking at the direct current flowing through the ground wire of the CV cable, the direct current is (a) stray current (b) direct current component caused by insulation deterioration of the cable (c) cable shield layer to cable sheath It contains three direct currents that leak through the resistor to the ground.
For this reason, conventionally, the stray current (a) is measured in advance, and then the stray current (a) is subtracted from the ground line DC current when the positive and negative voltages are biased. If found, the direct current (c) component showing a linear response to the positive and negative voltages is offset, and the polarity difference of the direct current component (b) showing a non-linear response to the positive and negative voltages remains as a difference. The deterioration degree was estimated from the size of the deterioration signal.

[0004]

However, according to the conventional method for measuring the DC currents flowing through the ground line when biasing the positive and negative voltages, respectively, in the case of the power cable line in which the fluctuation of the stray current is large, the stray current is Fluctuation of
It appears as an error component of the ground wire DC current measured when a positive or negative voltage is applied to the cable shielding layer, and as a result, a measurement error also occurs with respect to the polarity difference of the DC component, which deteriorates the insulator. Even for a healthy power cable that does not exist, there is a problem of making an erroneous determination that the insulator is deteriorated.

FIG. 3 shows an example in which a measurement error occurs in the ground line DC component polarity difference of a line in which a stray current variation is observed in the conventional cable live line diagnosis method.

In FIG. 3, from the shielding layer of the power cable line where the stray current is observed, to the cross-linked polyethylene insulator,
For example, if a DC voltage of ± 54V is biased,
The DC component polarity difference (nA) at the time of V bias is observed as shown by the curve a. In addition, the stray current I ₀ before biasing
Are observed as shown by the straight line c.

According to FIG. 3, the average value of the curve a of the DC component polarity difference (nA) at the time of ± 54 V bias is normally calculated in the average value calculation range ta in one minute from 4 minutes to 5 minutes after the start of bias. In the case of a power cable line in which the fluctuation of the stray current is large, the fluctuation is in the mean value calculation range ta, and the ground wire at the time of bias is sampled. DC component polarity difference (n
It was a factor causing the measurement error in A).

Therefore, an object of the present invention is to reduce the measurement error of the DC component deterioration signal due to the fluctuation of the stray current in the cable line system and to increase the detection accuracy of the DC component caused by the insulation deterioration of the water tree for the measurement. It is possible,
It is an object of the present invention to provide a method for hot-line diagnosis of a power cable, which can improve the accuracy of determining a deterioration signal.

[0009]

In order to achieve the above-mentioned object, the present invention provides a live cable diagnosis method for a power cable, which detects insulation deterioration of a power cable line having a conductor, a crosslinked polyethylene insulator and a shielding layer. Under a condition where one end of the power cable line in a live state is connected to a ground line and the other end is released from ground, and positive and negative DC voltages can be biased to the shielding layer of the power cable line, respectively. A first step of measuring a stray current flowing through the ground wire before biasing the positive DC voltage, and biasing the positive DC voltage on the shielding layer of the power cable line to flow to the ground wire A second step of measuring a DC current at a positive DC voltage bias, and after completing the measurement of the DC current at a positive DC voltage bias, and
A third step of measuring a stray current flowing through the ground wire before biasing the negative DC voltage, and biasing the negative DC voltage at the shielding layer of the power cable line to flow to the ground line. A fourth step of measuring a direct current at a negative DC voltage bias, and a fifth step of measuring a stray current flowing through the ground wire after the measurement of the direct current at a negative DC voltage bias is completed. A sixth step of calculating the rate of change of the stray current in the ground line DC current during bias measurement from the measured variation of each stray current, and the change from the polarity difference of the DC current flowing in the ground line. Insulation deterioration of the cross-linked polyethylene insulator of the power cable line is detected by a seventh step of obtaining a deterioration signal by eliminating an error factor of the stray current based on the rate. Providing an active ray diagnostic method of the power cable.

In order to achieve the above object, the present invention is characterized in that the second step and the fourth step are performed by a bipolar DC bias power source inserted in the ground line.
The step of generating the positive DC voltage and the negative DC voltage, and the second step and the fourth step.
The step of includes a step of detecting a direct current at the time of the positive direct current voltage bias and a direct current at the time of the negative direct current voltage bias by a low-pass filter inserted in the ground line. A live line diagnosis method is provided.

In order to achieve the above object, the present invention provides the first step, the third step and the fifth step.
The step of includes a step of measuring a stray current flowing through the ground line by a low-pass filter inserted in the ground line, and the sixth step includes the first step and the third step. Step and fifth
From the variation of each of the stray currents measured by the step, a method of diagnosing the change in the stray current in the ground line DC current during bias measurement is included. provide.

Further, in order to realize the above-mentioned object, the present invention is characterized in that the seventh step includes a DC current at the time of the positive DC voltage bias and a DC current at the time of the negative DC voltage bias respectively detected by a low pass filter. By subtracting the variation of the stray current flowing through the ground line measured in the first step, the third step, and the fifth step from the direct current, the polarity difference of the direct current flowing in the ground line is subtracted. A method for diagnosing a live line of a power cable, the method including the step of obtaining a deterioration signal from

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (Measurement Circuit According to Embodiment) FIG. 1 is an explanatory diagram of a measurement circuit showing a method for diagnosing a live line of a power cable according to an embodiment of the present invention. In FIG.
A CV cable having a conductor 2, a cross-linked polyethylene insulator (not shown), and a shield layer 3 is used as a power cable line 1 in a live state in which an alternating current of 3.8 kV is applied. The conductor 2 has a neutral point G of the grounding transformer 8 (GPT).
It is connected to the power source side 4 which is grounded by the PT earth 9, and further, the ground wire 6 of the shielding layer 3 on the power source side 4 and
The ground wire 7 of the shield layer 3 on the power receiving side 5 is provided.

In the live line diagnosis of the CV cable shown in FIG. 1, the ground wire 6 of the shield layer 3 on the power supply side 4 releases the ground, and is connected in series between the ground wire 7 of the shield layer 3 on the power receiving side 5. The low-pass filter 11 and the DC power source 10 having both polarities are arranged so that the shield layer 3 of the power cable line 1 can be biased with a positive DC voltage and a negative DC voltage, respectively. Further, a DC current connected to the low-pass filter 11 in order to capture a ground line DC current and a stray current flowing through the ground line when the positive DC voltage and the negative DC voltage are biased in the shielding layer 3, respectively. It has a total of 12.

If the cross-linked polyethylene insulator of the power cable line has an insulation deteriorated portion, a DC current flows when the charge accumulated in the insulation deteriorated portion is discharged, and this DC component current is generated by the shield layer 3 to the ground wire. 7 ~ Earth ~ Grounding transformer 8 (G
PT) to the conductor 2 (see the solid line in FIG. 1), and is detected by the DC ammeter 12 through the low-pass filter 11 installed between the ground lines 7 and removing the commercial wave. Further, the stray current at the time of measuring the positive DC voltage bias and the negative DC voltage bias, and the DC component of the sheath leakage current propagate through the shield layer 3 of the power cable line 1 to the ground wire 7 to the ground (dotted line in FIG. 1). Therefore, it is measured by the DC ammeter 12 through the low-pass filter 11 installed between the ground wires 7.

(Measurement of DC Current of Ground Line and Measurement of Stray Current when Biased According to Embodiment) In the measuring method of the present invention, shielding is performed by the DC power supply 10 having both positive DC voltage and negative DC voltage. The layer 3 is biased with a positive DC voltage and a negative DC voltage, respectively, and grounded before and after detecting a ground line DC current at the time of positive DC voltage bias and a ground line DC current at the time of negative DC voltage bias. Each of the stray currents flowing in the line is measured.

The measurement of the stray current and the detection of the ground line DC current at the time of positive and negative DC voltage bias of the embodiment are performed by the following procedures (a) to (e), respectively. (A) Stray current A (b) Ground line DC current (+ IdcY) at positive voltage bias
= Leakage current at positive bias + DC component at positive bias + Stray current B (c) Stray current C (d) Ground line DC current at negative voltage bias (-IdcZ)
= Leakage current at negative bias + DC component at negative bias + Stray current D (e) Stray current E

(Calculation of change rate of stray current / calculation of grounding wire DC current at bias) Grounding wire at positive DC voltage and negative DC voltage bias Grounding wire measured before and after detection of DC current respectively The rate of change of the stray current is calculated from the variation of the stray current flowing through (stray current A + stray current C + stray current E). Next, by subtracting the fluctuation component of the stray current flowing through the ground line from the detected ground line DC current at the time of positive and negative DC voltage bias, and obtaining the deterioration signal from the polarity difference of the DC current flowing through the ground line. , The fluctuation of the stray current, which causes an error, is removed from the ground line DC current. As a result, an accurate deterioration signal is obtained by eliminating the stray current that causes an error from the polarity difference of the DC current flowing through the ground line when biasing the positive and negative DC voltages, and based on this deterioration signal, cross-linked polyethylene insulation is obtained. It is possible to detect water tree deterioration of the body.

(Example of calculation of rate of change of stray current, example of calculation of ground line DC current when biased) An example of the equation for calculating the rate of change of the stray current and the equation for calculating the polarity difference of the DC current is as follows. is there. t1 = Elapsed time after start of positive voltage bias (s) t2 = Elapsed time after start of negative voltage bias (s) T = Bias measurement time (s) Mia = Stray current A average value Mib = Stray current B average value Mic = stray current C average value Ipx (t1) = ground line DC current at positive voltage bias (before elimination of stray current) Imx (t2) = ground line DC current at negative voltage bias (before elimination of stray current) Ipy (t1) = Ground line DC current during positive voltage bias (after elimination of stray current) Imy (t2) = Ground line DC current during negative voltage bias (after elimination of stray current) where Ipx (t1), Imx (t2), Ipy (t
1) and Imy (t2) are functions of the times t1 and t2, and the time required to measure the DC component at the positive voltage and the negative voltage bias is T. Ipy (t1) = Ipx (t1)-{Mia + (Mib
−Mia) × t1 / T} Imy (t2) = Imx (t2) − {Mib + (Mic
-Mib) x t2 / T}

[Equation 1]

(Measurement example according to the embodiment) FIG. 2 shows the measurement error due to the ground line DC component polarity difference of the line in which the stray current fluctuation is observed in the power cable live line diagnosis method of the embodiment of the present invention. It is explanatory drawing which shows the correction example.

In FIG. 2, when a DC voltage of ± 54 V is biased on the power cable line in which the fluctuation of the stray current is observed, the polarity difference (nA) of the DC component at the time of ± 54 V bias is applied.
Are observed as shown by the curve a. The stray current I ₀ before the DC voltage bias is observed as shown by the straight line c.
Also, the stray current I ₁ at the start of ± 54 V bias is ± 5
A stray current simulated line b connecting the stray current I ₂ at the end of the 4V bias is as shown in FIG. The average value calculation range ta in FIG. 2 is sampled 5 times / sec for 1 minute from 4 minutes to 5 minutes after the start of bias, and the average value of 300 samples is calculated and used for determination.

According to FIG. 2, the variation of the stray current was found from the observed stray current I ₀ (straight line c) before bias, stray current I ₁ at the start of bias and stray current I ₂ at the end of bias. Then, by the difference between the DC component polarities (average value of the curve a) at the time of ± 54 V bias and the stray current variation,
An accurate deterioration signal can be obtained from the average value after subtraction, and water tree deterioration of the crosslinked polyethylene insulator can be detected based on this deterioration signal.

In the embodiment of the present invention, the water tree deterioration of the insulator can be detected even when a DC voltage of ± 27 V is biased instead of the DC voltage of ± 54 V. Further, instead of biasing the cable shielding layer, even if a DC voltage is applied to the outer conductive layer or the outer semiconductive layer of the cable, the water tree deterioration of the insulator can be similarly detected. .

[0024]

According to the method for diagnosing a live line of a power cable of the present invention, positive and negative DC voltages are biased to a cable shielding layer of a power cable line in a live state before and after a ground line DC current is measured. In addition, the stray current flowing in each ground line is measured, and the change rate of the stray current in the ground line DC current during bias measurement is obtained from the fluctuations, and the ground line DC current measurement data during positive and negative DC voltage bias is measured. Therefore, since the fluctuation component of the stray current is subtracted, the factor of the measurement error due to the fluctuation of the stray current can be eliminated, and the DC component polarity difference in which the error of the deterioration signal is reduced can be obtained. As a result, even in a cable line system where a stray current is observed, it is possible to increase the accuracy of detection of the DC component caused by insulation deterioration of water trees, etc., and to determine the deterioration signal of the DC component caused by insulation deterioration. There is an effect that it is possible to provide a live cable diagnosis method of a power cable with improved accuracy.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a measuring circuit showing a live cable diagnosing method for a power cable according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a variation example of a stray current and a correction error of a measurement error due to a ground line DC component polarity difference when a DC voltage is biased, in a method of diagnosing a live line of a power cable according to an embodiment of the present invention.

FIG. 3 is an explanatory diagram showing an example in which an error occurs in a ground line DC component polarity difference of a line in which a stray current variation is observed in a conventional cable hot line diagnosis method.

[Explanation of symbols]

1 Power cable line (CV cable) 2 Conductor 3 Shielding layer 4 Power supply side 5 Power receiving side 6 Ground wire (power supply side) 7 Ground wire (power receiving side) 8 Grounding transformer (GPT) 9 Ground (for GPT) 10 DC power supply (Bipolar) 11 Low-pass filter 12 DC ammeter I ₀ Stray current (before bias) I ₁ Stray current (at bias start) I ₂ Stray current (at bias end) a DC component polarity difference curve (± 54V bias) b Stray Current simulation straight line (line connecting I ₁ and I ₂ ) c Stray current I 0 straight line (before bias) ta Average value calculation range

Claims (6)

[Claims] 1. A method of hot-line diagnosis of a power cable, which detects insulation deterioration of a power cable line having a conductor, a cross-linked polyethylene insulator, and a shielding layer, wherein one end of the power cable line in a live state is connected to a ground line. Then, while the other end is released from the ground and the shielding layer of the power cable line can be biased with a positive and a negative DC voltage, respectively, the current flows through the ground line before the positive DC voltage is biased. A first step of measuring a stray current, and a second step of biasing a positive DC voltage on the shielding layer of the power cable line to measure a DC current at a positive DC voltage bias flowing through the ground line. And measuring the stray current flowing through the ground line after the measurement of the direct current at the positive DC voltage bias is finished and before the negative DC voltage is biased. And a fourth step of biasing a negative DC voltage to the shielding layer of the power cable line to measure a DC current when a negative DC voltage bias flows in the ground line, the negative step After the measurement of the DC current at the time of the DC voltage bias is finished, the fifth step of measuring the stray current flowing through the ground line, and the variation of the measured stray current, the ground line DC during the bias measurement. A sixth step of calculating the rate of change of the stray current in the current, and a seventh step of obtaining the deterioration signal by deleting the error factor of the stray current based on the rate of change from the polarity difference of the DC current flowing through the ground line. The method for diagnosing the live line of a power cable is characterized by detecting insulation deterioration of the crosslinked polyethylene insulator of the power cable line. 2. The second step and the fourth step include a step of generating the positive DC voltage and the negative DC voltage by a bipolar DC bias power source inserted in the ground line. The method for diagnosing a live line of a power cable according to claim 1, which is characterized in that. 3. In the second step and the fourth step, a DC current when the positive DC voltage bias and a DC current when the negative DC voltage bias are applied are controlled by a low-pass filter inserted in the ground line. The method of diagnosing a live line of a power cable according to claim 1, further comprising a detecting step. 4. The first step, the third step and the fifth step each include a step of measuring a stray current flowing through the ground line by a low-pass filter inserted in the ground line. The method for diagnosing a live line of a power cable according to claim 1, which is characterized in that. 5. The sixth step is based on the fluctuation of the stray currents measured in the first step, the third step and the fifth step, respectively. The method for diagnosing a live line of a power cable according to claim 1, further comprising the step of calculating a change rate of the stray current. 6. The seventh step is based on the direct current when the positive DC voltage bias is detected and the direct current when the negative DC voltage bias is detected by a low-pass filter. Step and fifth
2. The step of obtaining a deterioration signal from the polarity difference of the DC current flowing in the ground line by subtracting the fluctuation amount of the stray current flowing in the ground line measured in each step of Method for diagnosing live line of the described power cable.