JP2876322B2 - Diagnosis method for insulation deterioration of CV cable - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for diagnosing insulation deterioration by diagnosing the degree of insulation deterioration of a CV cable due to a water tree or the like under a live line.

[Prior Art] Generally, the insulation performance of an electric insulator is deteriorated due to aging after a power cable is laid. In particular, it is known that, in a CV cable, dendritic cracks are formed in the crosslinked polyethylene insulator, and the occurrence of so-called water trees in which moisture penetrates into the cracks is a main cause of insulation deterioration. Such a decrease in insulation performance may progress if left unattended, leading to a large-scale electrical breakdown accident. Therefore, it is extremely important to grasp the change in the insulation resistance of the cable and to detect the deterioration at an early stage. For this reason, various insulation measurement methods have been conventionally known.In particular, in recent years, several methods for diagnosing in a live state without stopping power transmission at the time of measurement have been proposed. It is attracting attention because of its many advantages.

As a method of performing such constant monitoring, conventionally, for example, as described in Japanese Patent Publication No. 60-8465, a DC current is superimposed on a transmission AC current, and a DC leakage current detected as a result is detected. The so-called DC superposition method in which the insulation resistance of the cable is determined from the components and evaluated, or
As described in JP-A-60-185171, a so-called tan δ method is generally used in which a transmission voltage waveform and a current waveform are measured and a dielectric loss tangent is obtained and evaluated. In particular, in the case of a cv cable, assuming that the water tree has a current rectifying function in JP-A-59-202075, the DC component of the cable leakage current during AC power transmission was measured, and the water tree was measured from its direction and absolute value. A method for estimating the distribution and length and volume of is disclosed.

[Problems to be Solved by the Invention] However, none of the above-mentioned known conventional methods can always sufficiently satisfy the demand for early and accurate detection of insulation deterioration. That is, the direct current superposition method described above is generally considered to have poor detection sensitivity with respect to the degree of deterioration, and has a problem that it is not detected unless the deterioration is considerably severe. Also, a DC superimposed voltage of about several tens of volts is required at the time of measurement, and therefore a DC power supply must be separately prepared.

On the other hand, although the deterioration over the entire cable is detected by the tan δ method, it is known that the detection sensitivity for local deterioration such as water tree is poor.

Further, Japanese Patent Application Laid-Open No. 59-2020 utilizing the rectification function of water trees
In the case of No. 75, as described in the same publication, in the so-called inner water conduction tree generated from the conductor side in the cable insulator and the outer water conduction tree generated from the sheath side, the generated direct currents have opposite polarities. For this reason, when both types of water trees are generated at the same time, the detected direct currents may cancel each other out, making it impossible to perform a sufficient measurement.

[Background of the Invention] As a result of research on the water tree phenomenon, the present inventors have discovered the following new facts. That is, to be measured
When an AC voltage is applied to a CV cable, an electric charge corresponding to the applied AC voltage is induced in the shield layer of the cable by electrostatic coupling with the cable conductor, and due to this time change, the frequency of the applied AC voltage is changed between the ground and the ground. The current (grounding line current) that fluctuates at the same cycle flows. However, when the waveform of the grounding line current is observed, if water trees exist in the insulator of the CV cable, the waveform of the grounding line current will be distorted. Occurs. The distortion of the waveform is greatest near the peak value of the applied AC voltage.

The distorted ground line current contains many harmonic components of the applied AC voltage.

The more severe the water tree deterioration, the larger the amount of harmonic components contained in the ground line current.

In a cable insulator where a water tree exists, from an electrical viewpoint, between the cable conductor and the shielding layer, the capacitance Fa of the sound portion of the insulator and the capacitance Fb of the water tree portion of the insulator Can be considered to exist in parallel in a large number of equivalent circuits to which. When an AC voltage is applied to such a cable, the voltage share of the water tree portion capacitance Fb is forcibly reduced near the peak value of the applied AC voltage, and the voltage share of the sound portion capacitance Fa increases. As a result, a distortion current component including harmonics flows through the insulator. Since the voltage sharing of the sound portion capacitance Fa increases as the volume of the water tree portion increases, it is presumed that the above phenomenon occurs.

SUMMARY OF THE INVENTION An object of the present invention is to solve the drawbacks of the conventional method and to provide a new insulation deterioration diagnosis method for CV cables that can accurately detect insulation failure at the time of deterioration based on the new fact described above.

[Means for Solving the Problems] The 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 shielding layer of a CV cable to which an AC power supply voltage is applied and a reference potential. The degree of deterioration of the CV cable insulator is detected by detecting a third harmonic component of a current having a frequency equal to the frequency of the AC power supply from the ground wire current.

[Operation] In the present invention, the third harmonic component of the AC power supply frequency is detected from the ground line current to diagnose the insulation deterioration of the CV cable based on the above-mentioned new knowledge. In other words, it can be determined that the water tree deterioration of the insulator of the CV cable is progressing as the detected harmonic component amount is larger.

Embodiment An embodiment of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is an example of a configuration diagram for embodying the insulation deterioration diagnosis method of the present invention. In the figure, Ck is the capacitance between the conductor and the shielding layer of the CV cable to be measured, with the X side being the conductor side of the cable and the Y side being the shielding layer side. E is an AC power supply having one end grounded and the other end connected to the conductor X side of the cable. When the present invention is carried out when the cable is in a live state, the applied line voltage may be used as it is, and the AC power supply E is not required.

Here, when a voltage is applied to the test CV cable by the AC power supply E, a charging current flows through several capacitances Ck of the cable, and a ground wire current ie flows from the shielding layer Y side to the ground. As described above, the ground line current ie is a current that fluctuates mainly in the same cycle as the frequency of the applied AC voltage. When the frequency of the AC power supply E is 50 Hz, the ground line current ie also has a 50 Hz current component as the fundamental wave. Become.

As shown in FIG. 2, when the waveform of the above-mentioned ground line current ie is observed, in the cable having no water tree, although a slight superposition of a noise current component is recognized, the ground line current ie is almost a sine wave. (Indicated by the middle dotted line) is detected, but in the case of a cable with a water tree, the waveform is particularly distorted near the peak value of the applied AC voltage (indicated by the dashed line in the diagram) as indicated by the solid line in the diagram. The ground wire current ie is observed.

The deterioration diagnosis is performed by detecting the third harmonic component from the ground line current ie. In this embodiment, first, the ground line current ie is detected by the ground line current detector 20 from the ground line, and the band elimination is performed. The filter (hereinafter referred to as BEF) 21 removes the fundamental wave component (a current component having the same frequency as the applied AC voltage) from the ground line current ie, and then removes unnecessary current components such as a noise current component from the current. In this example, a case where the signal is passed through a band-pass filter (hereinafter referred to as BPF) 22 to detect only the third harmonic component of the fundamental wave is illustrated. FIG. 3 shows an output current waveform of the BPF 22. The current is only the third harmonic component of the fundamental wave from which the fundamental current component and the noise current component are removed from the ground line current ie. Various known filter circuits can be employed as the BEF21 and the BPF22, and an appropriate filter circuit may be appropriately selected and used.

Finally, the harmonic components detected by the filtering of the BEF 21 and the BPF 22 are subjected to processing such as rectification in the harmonic component amount display device 23 to display the magnitude of the absolute amount of the harmonic components. As described above, since the amount of harmonic components increases as the water tree deteriorates more severely, water tree deterioration diagnosis can be performed by measuring the amount of harmonic components with the harmonic component amount display device 23.

FIG. 4 shows the method of diagnosing deterioration according to the present invention in the state of a live line.
It is a figure showing the case where it is applied to a phase three-wire system line. A current transformer CT is coupled to the grounding wire 3 extending from the shield layer 11 at one end of the CV cable 1 to be measured.
The ground line current ie detected by the current transformer CT, which is configured to detect the ground line current ie flowing in the ground line 3, is sent to the insulation diagnostic device 2, and firstly in the device 2, the BEF 21 , The noise component and the like are removed by the BPF 22, and the remaining harmonic component is displayed as an absolute amount on the display device 23. When the measurement is actually performed, the ground wire 30 is also drawn from the shielding layer 12 at the other end of the CV cable 1, a switch S is provided at an intermediate portion thereof, and the switch S is closed at the time of measurement to perform the measurement with one end grounded.

As another method of detecting the ground line current ie, a resistor R is inserted into the ground line 3 'as shown by a dotted line in the figure, and the insulation diagnostic device 2' is operated using a voltage appearing at both ends of the resistor R. You can also. However, in the case of detection by the resistor R, a potential difference occurs between the CV cable 1 and the ground, and as a result, the capacitance Cs between the cable and the ground adversely affects the measurement. It is desirable to detect the ground line current ie by the current transformer CT, which causes almost no capacitance to occur. Further, in the detection by the current transformer CT, there is no need to apply any processing to the ground wire 3, so that it is easy to apply to an existing line, and the detection is not affected by a local battery. Is also excellent.

In the present invention, the third harmonic is selected as the detected harmonic. To detect only this third harmonic, use BP
What is necessary is just to set the circuit condition of F22 suitably.

The reason for using the third harmonic in the present invention is as follows. Generally, as a transformer connection in a special high-voltage line of about 66 kV or more, the YY connection is advantageous in that neutral grounding is possible in various types and there is no phase displacement,
In an actual extra high voltage line, the Δ connection is used as a tertiary winding of a transformer. This is the third in the case of YY connection.
Since the harmonic exciting current cannot flow, the induced electromotive force is not a sine wave but a distorted waveform including the third harmonic. Therefore, a return path for the third harmonic is provided by using a Δ connection transformer, and the sine wave induced electromotive force is generated. It generates power. Therefore, in the extra high voltage line, the third harmonic is positively excluded, and the line itself has very few third harmonics.

Therefore, when the present invention is implemented, if only the third harmonic of the fundamental wave of the ground line current ie is detected, the harmonics of the line itself are not affected. The evaluation of the line characteristics, that is, the water tree deterioration diagnosis can be performed. Furthermore, in this case BPF2
Since the bandwidth for filtering in 2 can be narrowed, it is possible to further reduce the mixing of noise components.

The above-mentioned insulation diagnostic device 2 may be individually provided for all cables to be diagnosed and used constantly for monitoring as a stationary type, or may be a portable type incorporated in a housing (not shown) which is easy to carry, and a plurality of types. It may be commonly used for cables.

Further, in the present embodiment, the case of application to a single-core three-wire power cable is illustrated, but the present invention is similarly applicable to a three-core batch power cable or a single-core power cable line.

[Effects] According to the method for diagnosing insulation deterioration of a CV cable according to the present invention as described above, there is a novel method for performing water tree deterioration diagnosis by detecting the third harmonic component of the ground line current. Compared to the hot wire insulation diagnosis method, diagnosis can be performed simply without the need to prepare a separate DC power supply. In addition, if there is a deteriorated part due to water tree in the insulator, its deterioration can be detected.Thus, in the past, only those with severe deterioration such as through water tree could be detected. Can be detected. Therefore, according to the method of the present invention, it is possible to simply detect a water tree in which deterioration has not progressed much, and economically prevent a major accident from occurring.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an example of a method for diagnosing deterioration of a CV cable according to the present invention, FIG. 2 is a graph showing a waveform of a ground line current, and FIG. FIG. 4 shows the method of the present invention in three phases and three phases.
The circuit diagrams illustrating the case where the present invention is applied to a line type line are shown. DESCRIPTION OF SYMBOLS 1 ... CV cable, 11,12 ... Shielding layer, 2 ... Insulation diagnostic device, 20 ... Ground wire current detector, 21 ... Band emission filter, 22 ... Band pass filter, 23
…… Display device, 3,30 …… Ground line, CT …… Current transformer, ie…
Ground wire current, E: AC power supply

Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G01R 31/12

Claims (2)

(57) [Claims] 1. A ground line current is detected from a ground line connecting between a shielding layer of a CV cable to which an AC power supply voltage is applied and a reference potential, and a frequency of a frequency equal to the frequency of the AC power supply is detected from the ground line current. A CV cable insulation deterioration method characterized by detecting a degree of deterioration of a CV cable insulator by detecting a third harmonic component of a current. 2. The method of claim 1, wherein the detection of the ground line current from the ground line is performed by a current transformer.