JPH03172777A - Diagnostic method for insulation deterioration of cv cable - Google Patents

Abstract

PURPOSE:To accurately detect a defective insulation at the time of cable deterioration by detecting harmonic wave components of a current having the frequency equal to the frequency of AC power source from the earth line current. CONSTITUTION:A current transformer CT is coupled to the earth line 3 pulled out from a shield layer 11 at one end of a CV cable 1 subjected to measurement, and the earth line current (ie) flowing in the earth line 3 is thereby detected. The earth line current (ie) detected by the current transformer CT is sent out to an insulation diagnostic device 2, wherein a fundamental wave component is eliminated by a band elimination filter(BEF) 21 at first, and noise current components, etc., are eliminated by a BPF 22, then the remaining harmonic wave components are displayed by a display device 23 as the absolute values. That is, the decision can be made as, the more the amount of harmonic wave components are detected, the more a water tree deterioration has progressed for the insulator of CV cable. By this procedure, the defective insulation can be accurately detected at the time of cable deterioration.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an insulation deterioration diagnosis method and apparatus for diagnosing the degree of insulation deterioration due to water tree etc. of a CV cable under live wires.

[Prior Art] Generally, the insulation performance of the electric insulator of an electric cable tends to deteriorate due to aging after installation. In particular, it is known that in CV cables, dendritic cracks occur in the cross-linked polyethylene insulation, and the occurrence of so-called water trees, in which water infiltrates into these cracks, is the main cause of insulation deterioration. If left untreated, such deterioration in insulation performance will progress and sooner or later lead to a major dielectric breakdown accident. Therefore, it is extremely important to understand changes in cable insulation resistance and discover deterioration early. For this reason, various insulation d111 determination methods have been known for a long time, but in recent years in particular, several methods have been proposed for diagnosing in a live line state without stopping power transmission during measurement, and it is also possible to constantly monitor the condition. It is attracting attention because it has many advantages such as:

Conventionally, as a method for performing such constant monitoring, as described in Japanese Patent Publication No. 60-8465, etc., a direct current is superimposed on the power transmission alternating current, and the direct current of the cable leakage current detected as a result of this is superimposed on the transmitted alternating current. The so-called direct current superposition method is used to determine and evaluate the insulation resistance of the cable from its components, or the dielectric loss tangent is determined by measuring the transmission voltage waveform and current waveform as described in Japanese Patent Application Laid-open No. 185171/1983. The so-called jan δ method is generally used. In addition, especially in the case of CV cables,
In Publication No. 75, assuming that the water tree has a current rectifying effect, the DC component of the cable leakage current during AC power transmission was measured,
A method for estimating the distribution, length, and volume of a water tree from its direction and absolute value is disclosed.

[Problems to be Solved by the Invention] However, none of the above-mentioned known conventional methods can fully satisfy the demand for early and accurate detection of insulation deterioration.

That is, the first-mentioned DC superimposition method generally suffers from poor detection sensitivity to the degree of deterioration, and has the problem that it cannot be detected unless the degree of deterioration is extremely severe. Furthermore, a DC superimposed voltage of approximately several tens of volts is required during measurement, and therefore a separate DC power source must be prepared.

On the other hand, although the JAN δ method can detect major deterioration in the entire cable, it is known to have a drawback in that it has poor detection sensitivity for localized deterioration such as water trees.

Furthermore, Japanese Patent Application Laid-Open No. 59-20 utilizes the rectification effect of water trees.
In the case of Publication No. 2075, as described in the same publication, the so-called internal groove water tree generated from the conductor side of the cable insulator and the external groove water tree generated from the sheath side, the generated DC currents are opposite to each other. Because of the polarity, if both types of water trees occur at the same time, there is a risk that the detected DC currents will cancel each other out, making it impossible to make sufficient measurements.

[Background of the Invention] As a result of research on the water tree phenomenon, the present inventors discovered the following new fact. In other words, when an AC voltage is applied to a CV cable that is subject to Wjl constant, a charge corresponding to the applied AC voltage is induced in the shielding layer of the cable due to electrostatic coupling with the cable conductor, and due to this time change, the voltage between the cable and the ground increases. A current (grounding line current) that fluctuates at a frequency comparable to the frequency of the applied AC voltage flows through the cable, but when the waveform of the grounding line current is measured a, there is a water tree in the insulation of the CV cable. If it exists,
Distortion occurs in the ground line current waveform (■) The distortion in the above waveform is greatest near the peak value of the applied AC voltage.

■ Distorted ground line current contains many harmonic components of the applied AC voltage.

■ For cables with severe water tree deterioration, the amount of harmonic components contained in the ground wire current increases.

In a cable insulator in which a water tree exists, from an electrical point of view, between the cable conductor and the shielding layer, the capacitance Fa of the healthy part of the insulator and the capacitance IFb of the part of the water tree are separated. It can be considered that there are many equivalent circuits connected in parallel. When an AC voltage is applied to such a cable, the voltage share of the water tree section capacitance Fb is forcibly reduced near the peak value of the applied AC voltage, so the voltage share of the healthy section capacitance Fa increases. As a result, distorted current components containing harmonics flow through the insulator. Since the voltage share of the healthy part capacitance Fa increases as the volume of the water pan increases, it can be inferred that the above phenomenon occurs.

An object of the present invention is to provide a new method for diagnosing insulation deterioration of a Cv cable, which eliminates the drawbacks of the conventional method and can accurately detect insulation defects at the time of deterioration, based on the above-mentioned new facts.

[Means for Solving the Problems] A method for diagnosing insulation deterioration of a CV cable according to the present invention detects a ground line current from a ground line connecting between a reference potential and a shielding layer of a CV cable to which an AC power supply voltage is applied. The present invention is characterized in that the degree of deterioration of the CV cable insulator is detected by detecting harmonic components of a current having a frequency equal to the frequency of the AC power source from the ground line current.

[Operation] In the present invention, based on the above new knowledge (C
This is to diagnose insulation deterioration of V cable. That is, as the amount of harmonic components detected increases, it can be determined that water tree deterioration of the insulator of the CV cable progresses.

[Example] An example of the present invention will be described in detail below based on the drawings.

FIG. 1 is an example of a configuration diagram for embodying the insulation deterioration diagnosing method of the present invention. In the figure, Ck is the capacitance between the conductor and shielding layer of the CV cable to be measured, and
The side is the conductor side of the cable, and the Y side is the shielding layer side. E
is an AC power source, one end of which is grounded, and the other end connected to the conductor X side of the cable. Note that when the present invention is implemented when the cable is in a live state, the applied line voltage may be used as is, and the AC power source E is not required.

Here, when a voltage is applied to the test Cv cable from the AC power source E, a charging current flows through the capacitance ff1ck of the cable, and a ground line current ie flows from the shielding layer Y side to the ground. As mentioned above, the grounding line current ie is a current that mainly fluctuates at a period comparable to the frequency of the applied AC voltage, and when the frequency of the AC power source E is 50Hz, the grounding line current ie also has a 50Hz current component as a fundamental wave. Become.

As shown in Fig. 2, the waveform of the grounding line current ie is 1t.
When performing II measurements, although some noise current components are superimposed on cables without water trees, a ground line current ie (indicated by a dotted line in the figure) that is close to a sine wave is detected; For existing cables, the applied AC voltage is indicated by the solid line in the figure (indicated by the dashed line in the figure).
The ground line current ie whose waveform is particularly greatly distorted near the peak value of riA is measured.

Deterioration diagnosis is performed by detecting harmonic components from the grounding line current ie. In this embodiment, first, the grounding line current ie is detected from the grounding line by the grounding line current detector 20, and then a band elimination filter is applied. (hereinafter referred to as BEF)2
1, remove the fundamental wave component (current component with the same frequency as the applied AC voltage) from the grounding line current ie, and then remove unnecessary current components such as noise current components from this current to obtain harmonic components of the fundamental wave. A case is illustrated in which the signal is passed through a Pandpass Filter (hereinafter referred to as BPF) 22 in order to detect only the PPF. FIG. 3 shows the output current waveform of the BPF 22, and the current consists of only the high frequency component of the fundamental wave from which the fundamental wave current component and the noise current component are removed from the ground line current ie.

Various known filter circuits can be employed as the BEF 21 and BPF 22, and any suitable filter circuit may be selected and used as appropriate.

Finally, BEF21. The harmonic components detected by the filtering of the BPF 22 are processed, for example, by rectification, in the harmonic component amount display device 23, and the magnitude of the absolute amount of the harmonic components is displayed.

As mentioned above, the amount of harmonic components increases as the cable undergoes more severe water tree deterioration, so water tree deterioration diagnosis can be performed by measuring the amount of harmonic components with the harmonic component amount display device 23.

FIG. 4 is a diagram showing a case where the deterioration diagnosis method of the present invention is applied to a three-phase three-wire line in a live line state. A current transformer CT is coupled to the grounding wire 3 drawn out from the shielding layer 11 at one end of the CV cable 1 to be measured, so as to detect the grounding wire current ie flowing through the grounding wire 3. being done. Earthing wire current ie detected by current transformer CT
is sent to the insulation diagnosis device 2, in which the fundamental wave component is first removed by the BEF 21 as described above, and
Noise current components and the like are removed by the BPF 22, and the remaining harmonic components are displayed as absolute quantities on the display device 23. When actually performing measurements, the grounding wire 30 is also drawn out from the shielding layer 12 at the other end of the CV cable 1, and a switch S is provided in the middle thereof, and the measurement is performed with the switch S closed and one end grounded.

Another method for detecting the grounding wire current ie is to insert a resistor R into the grounding wire 3° as shown by the dotted line in the figure, and operate the insulation diagnostic device 2° using the voltage that appears across this resistor R. You can also write it down. However, in the case of detection using the resistor R, a potential difference occurs between the Cv cable 1 and the ground, and as a result, the cable-ground capacitance MCs
Since this does not have a sudden effect on the measurement, it is desirable to detect the grounding line current ie using a current transformer C'F, which does not generate a capacitance such as 76 between the cable and the ground.

Detection using a rough transformer C' does not require equivalent processing on the grounding wire 3, so it is easy to apply to existing lines, and detection using a resistor R is not affected by local batteries. It is also superior in these respects compared to the case of .

The harmonics to be detected may be the harmonics of all persons, or only one specific large harmonic, and these selections can be made by appropriately setting the circuit conditions of the BPF22. can be done.

By the way, in general, Y-Y connection is advantageous for connecting transformers in special high-voltage lines of about 66 kV or higher because it allows neutral point grounding in various ways and there is no phase shift, but in actual special cases In high-voltage lines, the delta connection is used as the tertiary winding of the transformer. This is because in the case of Y-Y connection, the third harmonic excitation current cannot flow, so the induced electromotive force is not a sine wave but a distorted waveform that includes the third harmonic. The third harmonic circulation path is provided to generate a sinusoidal induced electromotive force.

Therefore, the third harmonic is actively excluded from the extra high voltage line, and the line itself contains extremely little third harmonic.

Therefore, when implementing the present invention, if only the third harmonic of the fundamental wave of the grounding line current ie is detected, it will be more accurate since it will not be affected by the harmonics of the line itself. It is also possible to evaluate the characteristics of railway lines, in other words, to diagnose water damage and deterioration. Roughly, in this case B
Since the filtering bandwidth in the PF 22 can be narrowed, it is possible to further reduce the mixing of noise components.

The above insulation diagnostic device 2 may be installed individually on all cables to be diagnosed and used for constant monitoring as a stationary type, or
Alternatively, it may be of a portable type built into a casing (not shown) that is easy to transport, and used commonly for a plurality of cables.

Further, in this embodiment, application to a single-core three-wire cable cable is exemplified, but the present invention can be similarly applied to a three-core bundled power cable or a single-core cable cable line.

[Effects] According to the method for diagnosing insulation deterioration of Cv cables of the present invention as explained above, it is a novel method of diagnosing water tree deterioration by detecting harmonic components of the grounding line current. Compared to the insulation diagnosis method, diagnosis can be performed simply without the need to prepare a separate DC power supply. In addition, if there is a site of deterioration caused by water trees in the insulator, the deterioration can be detected. Even a single defect can be detected. According to the method of the present invention, it is possible to detect water trees in a simple manner in which deterioration has not progressed much, and it is possible to economically prevent major accidents.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an example of implementing the CV ripple deterioration diagnosis method of the present invention, Fig. 2 is a graph showing the waveform of the grounding line current, and Fig. 3 is the output of the bandpass filter. Figure 4 is a graph showing the time change of the method of the present invention.
A circuit diagram illustrating a case where the circuit is applied to a phase three-wire line is shown. 1...CV cable, 11.12...shielding layer, 2.
...Insulation diagnostic device, 20...Grounding wire current detector, 21
...Band illumination filter 22...
Bandpass filter, 23...Display device, 3.30
...Grounding wire, CT...Current transformer, IE...Grounding wire current, E...AC power supply patent applicant Mitsubishi Cable Industries, Ltd. Representative Director Yuki Yuki

Claims (3)

[Claims] (1) Detect the grounding line current from the grounding line connecting between the shielding layer of the CV cable to which the AC power supply voltage is applied and the reference potential, and from the grounding line current, detect a current with a frequency equal to the frequency of the AC power supply. By detecting harmonic components, CV
A method for diagnosing insulation deterioration of a CV cable, the method comprising detecting the degree of deterioration of a cable insulator. (2) C according to claim (1), characterized in that the grounding line current from the grounding line is detected by a current transformer.
Method for diagnosing insulation deterioration of V cable. (3) Claim (1) characterized in that when detecting the harmonic component, only the third harmonic component is detected.
Method for diagnosing insulation deterioration of CV cables as described in Section 1.