JP2742637B2 - Diagnosis method for insulation of CV cable - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a CV cable insulation diagnosis method for diagnosing the degree of insulation deterioration of a CV cable caused by water trees or the like.

[0002]

2. Description of the Related Art In a power cable, insulation deterioration occurs due to aging after installation. In particular, in CV cables, the occurrence of water trees is considered to be the main cause of insulation deterioration.

There is a risk that such insulation deterioration will progress if left unattended and eventually lead to a large insulation breakdown accident. Therefore, the state of deterioration should be accurately grasped at an early stage, and appropriate measures such as cable replacement should be taken. It is necessary. For this reason, various methods for diagnosing insulation deterioration of power cables have been conventionally proposed. In particular, in the case of water tree deterioration in the live line of a CV cable, for example, an AC voltage is applied to a cable conductor, and a DC component in a charging current at this time is measured to estimate a water tree deterioration state. There are known ways to do this.

[0004]

However, in the above-mentioned conventional example, it is difficult to apply a 22 kV CV cable or the like which is grounded at multiple points because it is affected by the ground line current.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned drawbacks and to provide a CV having a grounded shielding layer capable of detecting deterioration of insulation before water trees or the like are extended and insulation is incomplete.
An object of the present invention is to provide a cable insulation diagnosis method.

[0006]

According to the present invention, there is provided a method for diagnosing insulation of a CV cable, comprising the steps of: Applying an AC voltage from a power supply, detecting a current flowing from the AC power supply, removing an AC component, extracting only a specific low frequency current on the low voltage side of the AC power supply, measuring the specific low frequency current, Diagnosis of insulation deterioration of the power cable is performed based on a current value obtained by correcting a current value from a voltage value extracted from the AC voltage through a low-pass filter with a reference voltage value.

[0007]

The insulation diagnostic method for a CV cable having the above-described structure is characterized in that an AC voltage applied between a cable conductor and a shielding layer of a power cable to be measured removes an AC component from a flowing current, and then removes a specific low-frequency component. The current is only extracted on the low voltage side of the power supply and measured. As the water tree increases and its area increases, this current increases.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail with reference to the illustrated embodiments. FIG. 1 is a block diagram for implementing the deterioration diagnosis method of the present invention. In FIG. 1, reference numeral 1 denotes C to be measured.
This is a test power cable such as a V cable, and has a capacitance Ck in an insulator 1c between a conductor 1a and a shielding layer 1b grounded at a terminal. In addition, 1d is a jacket. S is an AC power supply, one end of which is grounded via the impedance element 2, and the other end of which is connected to the conductor 1 a side of the test power cable 1. The shield layer 1b and the impedance element 2 of the test power cable 1 are grounded by a common ground line,
Are applied to the first low-pass filter 3, the resonator 4,
It is connected to a display 6 via a variable gain amplifier 5. Further, a transformer 7, a second low-pass filter 8, and an AC elimination circuit 9 are connected to the AC power supply S without passing through the impedance element 2, and an output of the AC elimination circuit 9 is connected to a gain control input terminal of the variable gain amplifier 5. Connecting. Note that the impedance element 2 may be shared with the capacitor of the first low-pass filter 3.

Here, when a voltage is applied between the conductor 1a of the test power cable 1 and the shielding layer 1b from the AC power supply S,
Even without the water tree, the charging current i flows through the capacitance Ck of the cable 1. The impedance element 2 detects the charging current i, transmits the charging current i to the first low-pass filter 3, removes the 50 Hz or 60 Hz AC component of the AC power source S and the higher frequency component, and extracts only the low frequency pulsating component.

As the first low-pass filter 3, for example, a constant K-type filter is used, and a basic AC component is removed from a charging current i generated by application of an AC voltage of an AC power supply S to the cable conductor 1a. Only the component is output, and this output is input to the narrow-band resonator 4. And the resonator 4
Then, only a specific low frequency component is extracted from the pulsating flow components.

As the resonator 4, various narrow-band resonators can be used. For example, a parallel T-type CR feedback circuit often used in a low frequency band can be used. This parallel T
Type CR feedback amplifier circuit (Twin-T circuit) is a T-type circuit composed of two resistors Ra, Ra and capacitor Cb having the same resistance value, and 2
It is configured using a parallel circuit of a T-type circuit consisting of a plurality of capacitors Ca, Ca of the same capacity and a resistor Rb, and a capacitor Cb and a resistor Rb.
Are values of Cb = 2 · Ca and Rb = Ra / 2, respectively. The parallel T-type CR feedback circuit configured as described above has f = 1 / 2π
It has a frequency characteristic of passing a Ca / Ra frequency signal as a maximum signal, and can select a signal of a frequency f, that is, extract only a signal of a certain specific frequency from a pulsating flow component. Further, this signal is amplified by a variable gain amplifier 5 and transmitted to a display 6 such as a pen graph or an oscilloscope.

At the same time, the voltage waveform of the AC power
, And removes the applied AC component through a second low-pass filter 8. The AC removing circuit 9 extracts the pulse noise contained in the AC voltage except for the AC component, and obtains a variable gain by a signal proportional to this magnitude. The gain of the amplifier 5 is adjusted. That is, if the output of the AC removing circuit 9 is large, the gain of the variable gain amplifier 5 is adjusted to be small, and if it is small, the gain is adjusted to be large. As described above, the ratio between the voltage value obtained by the AC removing circuit 9 and the reference voltage value is obtained, and the current value obtained by the resonator 4 is divided by the variable gain amplifier 5 according to this ratio. The variation of the AC voltage superimposed on the AC voltage can be corrected.

FIG. 2 is a graph of the output of the variable gain amplifier 5 with respect to the applied voltage. The horizontal axis represents the applied voltage, and the vertical axis represents the current. When an AC voltage is applied to the test power cable 1, an electric charge corresponding to the applied AC voltage is induced in the shielding layer 1b by electrostatic coupling with the conductor 1a. The charging current i that fluctuates at about the same cycle flows, but in addition to this, the insulator 1
If there is water tree degradation in c, a pulsating current will be superimposed. The pulsating current is a low-frequency current other than the AC current that depends on the applied AC voltage as described above, and the pulsating current is generated in the deteriorated portion due to the pulse noise superimposed on the AC voltage of the AC power supply S. It is considered that the current has a lower frequency than the frequency of the applied AC voltage.

Normally, the frequency of the applied AC voltage is 50 Hz or 60 Hz. Therefore, if the circuit of the first low-pass filter 3 is designed to pass a frequency signal of, for example, less than 50 Hz or 60 Hz, only the pulsating component is charged. It can be detected from the current i. The frequency of the pulsating flow component to be detected is arbitrary, but the higher the frequency, the more disadvantageous in terms of ground capacity. Therefore, the frequency is preferably about 5 Hz or less, and for example, 1 Hz is suitable.

If a water tree is present, the DC impedance of the water tree is high, but the AC impedance is low. For example, a current flows through the water tree. The magnitude of the pulsating flow component increases as the degree of water tree deterioration increases, and the degree of water tree deterioration can be estimated by obtaining this magnitude. For example, in a sound test power cable 1 in which a water tree does not exist in the cable insulator 1c, the magnitude of the current obtained by the display 6 is substantially constant as shown in FIG. On the other hand, when the water tree is present, the current value becomes B with increasing voltage.
It becomes larger as shown in FIG.

In this case, when the gain of the variable gain amplifier 5 is fixed, the value obtained by the display 6 is related to the influence of the magnitude of the pulse noise superimposed on the AC power supply S. The measured value C of FIG. 2 is obtained. The measured value C does not always clearly represent the size of the water tree, but is the amount of transmission of the pulse noise in the AC power supply S, so that the degree of the water tree is difficult to determine accurately.

Then, when the magnitude of the pulse noise directly obtained from the AC power supply S, that is, the gain of the variable gain amplifier 5 is adjusted by the signal passed through the second low-pass filter 8, a measured value B is obtained. The measured value B is nothing but a result of calibrating the measured value C according to the magnitude of the pulse noise included in the AC power supply S, and the degree of deterioration of the insulator 1c can be diagnosed more accurately.

In the method shown in this embodiment, the AC power supply S
Since the current flowing from the impedance element 2 is taken out as a voltage in the impedance element 2, it can be applied to a multipoint grounded 22 kV CV cable or the like without being affected by the ground line current. At this time, cable 1
Since other characteristics are also measured, a power source that charges only the cable 1 instead of the hot wire is used. Further, even if a DC current does not flow, deterioration is detected by a change in capacitance, so that early detection is possible.

In the above-described embodiment, the first and second low-pass filters 3 and 8 are provided separately. However, these can be commonly used by switching with a switch. Further, in the present embodiment, a case where the present invention is applied to a single-core power cable having a small charging current due to the cable capacitance Ck is used as an example, but the present invention is applicable to a three-core batch power cable or a three-core batch measurement of a single-core three-wire power cable line. Is similarly applicable.

[0020]

As described above, the method for diagnosing insulation of a CV cable according to the present invention enables measurement with the shield layer of the line being grounded at multiple points, and is free from the influence of stray currents and reduces insulation deterioration. It can be found early.

[Brief description of the drawings]

FIG. 1 is a block circuit configuration diagram for implementing the present invention.

FIG. 2 is a graph showing measurement results.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Test power cable 1a Conductor 1b Shielding layer 1c Insulator 1d Jacket 2 Impedance element 3, 8 Low-pass filter 4 Resonator 5 Variable gain amplifier 6 Display 7 Transformer 9 AC removal circuit S AC power supply i Charging current

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Takao Sanada 3-4-1 Marunouchi, Chiyoda-ku, Tokyo Mitsubishi Cable Industries, Inc. Tokyo Office (72) Inventor Akihiro Fukumoto 3-4-1 Marunouchi, Chiyoda-ku, Tokyo No. Mitsubishi Cable Industries, Ltd. Tokyo office (56) References JP-A-64-35281 (JP, A) JP-A-3-31776 (JP, A)

Claims (1)

(57) [Claims] 1. An AC voltage is applied from an AC power supply between a cable conductor of a power cable to be measured having a shielded layer grounded and the shielded layer, a current flowing from the AC power supply is detected, and an AC component is removed and specified. Extract only the low-frequency current on the low-voltage side of the AC power supply, measure the specific low-frequency current ,
The current value is converted from the AC voltage through a low-pass filter.
The voltage corrected by the ratio of the extracted voltage value to the reference voltage value
A method of diagnosing insulation deterioration of the CV cable based on a flow value .