JP2832737B2 - Method and apparatus for diagnosing 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 CV cables, 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 is measured, and the water tree is determined from its direction and absolute value. A method is disclosed for estimating the distribution and length and the sedimentation of.

[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.

In view of the above problems, the present inventors have conducted intensive studies on the water tree phenomenon, and have found the following new facts. In other words, when an AC voltage is applied to the power cable to be measured and the amplitude of the applied AC voltage is gradually increased from zero, and a quasi-DC component of tens of Hz or less is detected in the ground wire current. Then, when the amplitude of the applied AC voltage reaches a certain value, a pulsating current is detected.

The greater the water tree deterioration, the smaller the amplitude of the AC voltage at which pulsation starts.

The amplitude of the current pulsation monotonically increases with respect to the amplitude of the applied AC voltage.

Based on such new findings, the present inventors have previously proposed a new insulation deterioration diagnosis method (Japanese Patent Application No. 1-167910). However, it has been found that there is the following problem in performing the insulation deterioration diagnosis. That is, the degree of deterioration of the insulator is determined from the magnitude of the pulsating current detected from the ground line current. However, since the pulsating current signal is a minute signal, it is extremely difficult to evaluate. Further, if the pulsating current is simply amplified to prepare for the evaluation, the noise current component is also amplified at the same time, which also makes the evaluation difficult.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a method and an apparatus for diagnosing insulation deterioration of a CV cable, which can more reliably detect insulation deterioration in the insulation deterioration diagnosis method based on the new fact.

Means for Solving the Problems The present invention has been made to achieve the above object, and the gist of the present invention is to apply an AC voltage to a power cable to be measured, and connect a ground wire to the ground cable. Detects the current, detects a pulsating component consisting of a quasi-DC component of tens of Hz or less generated based on the presence of water tree deterioration in the insulator from the grounding line current, and further specifies from the pulsating component. Diagnosis of CV cable insulation deterioration characterized by extracting only the signal of the frequency, amplifying the signal, and performing time analysis of the amplified pulsating component of the specific frequency to detect the degree of deterioration of the cable insulation. A method and a current detecting device for detecting a ground line current from a ground line connecting between a shielding layer of a CV cable and a potential reference, and generating the current based on the presence of water tree deterioration in an insulator from the detected ground line current. Ten A low-pass filter that extracts a pulsating component consisting of a quasi-DC component of several Hz or less, a narrow-band amplifier that is connected to the output of the low-pass filter and extracts only a signal of a specific frequency from the pulsating component and amplifies the signal; A CV cable insulation deterioration diagnosis apparatus, comprising: a waveform display means for displaying an output signal of the narrow band amplifier on a time axis.

[Operation] In the first aspect, by detecting a pulsating component at a frequency of several Hz or less from the ground wire current, it is possible to detect water tree deterioration of the cable insulator based on the above-mentioned new findings. However,
As described above, since the pulsating flow component is an extremely small signal, it is necessary to amplify the pulsating component to make it easy to evaluate. Here, if the pulsating flow component is simply broadband amplified, the noise current component will also be amplified. Therefore, the S / N ratio is improved by extracting and amplifying only the pulsating flow component of a specific frequency. The time analysis of the amplified signal of the specific frequency pulsating component obtained in this way is performed to detect the degree of deterioration of the cable insulator. It should be noted that if the detection of the ground wire current is performed without cutting the ground wire, there are advantages such as being unaffected by ground conditions and being able to measure regardless of the insulation resistance of the sheath.

In the third aspect, a pulsating current component extracted from the ground line current by a low-pass filter is amplified in a very narrow frequency band using a narrow band amplifier, so that one of the detected pulsating current components is detected. Only the pulsating flow component signal of the specific frequency can be selectively amplified.
Therefore, the S / N ratio can be improved and amplified, and the deterioration of water tree can be easily evaluated by displaying the signal on the waveform display means.

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

FIG. 1 is a block diagram for implementing the deterioration diagnosis method of the present invention. In the figure, Ck is a capacitance between a conductor of a test power cable such as a CV cable to be measured and a shielding layer, and a side is a conductor side of the cable and b side is a shielding layer side. E is an AC power supply having one end grounded and the other end connected to the conductor a of the test power cable. Here, when a voltage is applied to the test power cable by the AC power supply E, a charging current flows through the capacitance Ck of the cable, and a ground line current ie flows from the shielding layer b side to the ground through the ground line. Become.
The current detecting device 10 connected between the shield layer b and the ground serves to detect the ground line current ie from the ground line. The ground line current ie detected by the current detector 10 is sent to the low-pass filter 1, where only a pulsating component is extracted from the ground line current ie. The output of the low-pass filter 1 is connected to an amplifier 2 for extracting and amplifying only a signal of a specific frequency from the detected pulsating component, and the output of the amplifier 2 displays the amplified signal on a time axis. For example, a waveform display device 3 such as a pen graph, an oscilloscope or the like is connected.

FIG. 2 shows an example of a circuit configuration diagram of the block diagram shown in FIG. In the figure, E denotes an AC power supply. 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 becomes unnecessary. Reference numeral 4 denotes a test CV cable, 41 denotes its conductor, 42 denotes an insulator, and 43 denotes a shielding layer. A current transformer CT (corresponding to the current detection device 10 in FIG. 1) is coupled to a ground line drawn from the shielding layer 43 to the ground, and a ground line current ie flowing through the ground line
Is detected by the current transformer CT.

Other methods of detecting the ground wire current ie from the ground wire include a means for inserting a resistor in the ground wire and using the voltage appearing at both ends of the resistor, and a clamp-type current detector using an iron core and a Hall element. And the like. However, in the case of detection using a resistor, the ground wire must first be cut and a resistor inserted, and a potential difference is generated between the CV cable to be measured and the ground. As a result, the cable-to-ground capacitance is reduced. It is not preferable because it adversely affects the measurement. On the other hand, in the current transformer CT, the clamp-type current detector, and the like (that is, current detecting means that does not disconnect the ground line), the above-described problem of the capacitance need not be considered and the ground line may be disconnected. Since there is no need to perform any processing such as stripping, there are various advantages such as easy adaptation to the existing line, and furthermore, there is no influence from the local battery.

The low pass filter 1, and a capacitor C ₀ between the shield layer 43 and the ground as shown, thereby the coil L _1, a capacitor C ₁ and the coil L _2, a 2-stage comprising a capacitor C ₂ Ijo The case where a K-type filter is used is illustrated. The low-pass filter 1 outputs only a pulsating current component from the ground line current ie generated by the application of the AC power supply E to the cable conductor 41, and the output is input to the narrow-band amplifier 2.

Various narrow-band amplifiers can be used as the amplifier 2. In this embodiment, a case where a feedback amplifier using a parallel T-type CR circuit 20, which is often used in a low frequency band, as a feedback circuit is used. ing.

The parallel T-type CR circuit (Twin-T circuit) 20 is composed of a T-type circuit comprising two resistors Ra, Ra having the same resistance value and a capacitor Cb, and two capacitors Ca, Ca having the same capacitance and a resistor Rb. It consists of a parallel circuit of a T-type circuit, with a capacitor Cb and a resistor Rb
Are values of Cb = 2Ca and Rb = Ra / 2, respectively. The parallel T-type CR circuit 20 configured as described above has f ₀ = 1 / 2π · Ca
- has a frequency characteristic of passing a maximum signal frequency signals Ra, the use of the said circuit in the feedback circuit of the negative feedback amplifier, the negative feedback occurs at a frequency other than the f _0, of the signal of frequency f ₀ Selective amplification, that is, it is possible to extract and amplify only a signal of a certain specific frequency from the pulsating flow components. G in the figure indicates an amplifier. Then, the output of the amplifier 2 is sent to the waveform display device 3, and the specific frequency signal of the pulsating component is displayed on the time axis.

FIG. 3 is an example of a graph showing the time change of the output of the low-pass filter 1 in the above-described device. The horizontal axis represents time t, and the vertical axis represents current I. When an AC voltage is applied to the test CV cable 4, a charge corresponding to the applied AC voltage is induced in the shielding layer 43 by electrostatic coupling with the conductor 41. Current that fluctuates at a cycle similar to the frequency of the applied AC voltage (ground line current i
e) flows, but in addition to this, if there is water tree deterioration in the insulator 42, the above-mentioned pulsating current is superimposed. The pulsating current refers to a current other than the current that depends on the applied AC voltage, and is a frequency current equal to or lower than the frequency of the applied AC voltage. Normally, the frequency of the applied AC voltage is 50 Hz or 60 Hz. For example, if the circuit of the low-pass filter 1 is designed to pass a frequency signal of 50 Hz or 60 Hz or less, only the pulsating flow component as shown in FIG. Ground wire current ie
It can be detected from inside. The frequency of the pulsating 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.

Since the amplitude A of the pulsating flow component increases as the degree of water tree deterioration increases, the degree of water tree deterioration can be estimated by obtaining the amplitude A. However, the output current of the low-pass filter 1 is extremely small, and it is difficult to accurately determine the magnitude of the amplitude A with this output. Although the pulsating flow component has a narrow frequency band, the pulsating flow component is composed of a large number of frequency components, and further contains many noise components.

Thus, by amplifying the output current by the feedback amplifier 2 described above, an output as shown in FIG. 4 in which only a signal of a specific frequency of the pulsating component is amplified is obtained. At this amplified output, its amplitude A 'is larger than said amplitude A and S /
Since the N ratio is good and the frequency is a single frequency, waveform observation by the waveform display device 3 becomes easy, and the degree of water tree deterioration can be found more accurately.

The present invention is applicable regardless of whether the CV cable whose water tree deterioration is to be detected is hot or not. At the time of a live line, the AC voltage applied to the line is used as it is as the AC power source E in FIGS. 1 and 2,
As described above, water tree deterioration can be found from the magnitude of the amplitude of the pulsating flow component. In the case of a non-live line, it is necessary to prepare an AC power supply separately.In this case, a more accurate power supply can be provided by using a power supply capable of varying the applied voltage and gradually increasing the applied voltage to the cable from zero. Deterioration diagnosis becomes possible. The time change of the pulsating component in this case is
For example, a graph shown in FIG. 5 is obtained. The horizontal axis represents time t, and the vertical axis represents current I. As described above, when the amplitude of the applied AC voltage increases, pulsation is observed from a certain amplitude value or V ₀ as shown in FIG. 5, and the pulsation start voltage V ₀ becomes lower as the water tree deterioration is more severe. Become. Thus even seeking the pulsating start voltage V _0, it is possible to estimate the degree of water tree degradation.

As a guideline for determining when performing diagnosis using a pulsating start voltage V _0, shows a graph of the capacitance F of the measurement target cable pulsating start voltage V ₀ in FIG. 6. The horizontal axis indicates the capacitance F on a logarithmic scale, shows a pulsating start voltage V ₀ and the vertical axis. In FIG. 6, the state point X indicating the capacitance F and the pulsation onset voltage V ₀ for a healthy cable without water tree deterioration.
There 'while present on, in cable water tree degradation occurs pulsating start voltage V ₀ which at the same electrostatic capacity F is reduced, the state point X' generally solid B-B is the solid line as indicated by an arrow B-
It will deviate from B '. Therefore, it is possible to quantitatively evaluate the degree of water tree deterioration based on the shift of the state point X 'from the solid line BB'.

Furthermore, the above-described insulation deterioration diagnosis apparatus for CV cables according to the present invention may be provided individually for all cables to be diagnosed and used as a stationary type for constant monitoring, or a housing (not shown) that is easy to transport. And may be used in common for a plurality of cables.

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

[Effects] According to the method and apparatus for diagnosing insulation deterioration of a CV cable according to the present invention as described above, a pulsating component in a cable ground wire current is detected, and a single frequency signal is further detected from the pulsating component. This method is to analyze the output signal that has been extracted and amplified, and to analyze the time of water tree degradation by time analysis. Since reading can be performed with high accuracy and the S / N ratio is good, more reliable deterioration diagnosis can be performed.

Further, as means for extracting and amplifying only a signal of a specific frequency from the pulsating flow component, a narrow-band amplifier, for example, a feedback amplifier using a parallel T-type CR circuit having high selectivity in a low-frequency band as a feedback circuit is used. As a result, the pulsating flow component can be amplified with high selectivity and the S / N ratio can be improved, so that more reliable and accurate deterioration diagnosis can be performed.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an example for carrying out a method for diagnosing deterioration of a CV cable according to the present invention, FIG. 2 is a circuit configuration diagram showing an example of a specific circuit configuration of the block diagram shown in FIG.
FIG. 3 is a graph showing the time change of the output of the low-pass filter, FIG. 4 is a graph showing the time change of the output of the feedback amplifier, FIG. 5 is a graph of the display waveform in the diagnosis of the stop line, FIG. Represents graph diagrams of diagnostic criteria for stop lines. 1 low pass filter, 10 current detection device, 2
Amplifier, 20 ... parallel T-type CR circuit, 3 ... waveform display device,
4 CV cable under test, 41 Conductor, 42 Insulator, 43
…… Shielding layer, ie …… ground line current, E …… AC power supply, CT…
…Current transformer

Continuation of front page (56) References JP-A-63-135876 (JP, A) JP-A-62-91868 (JP, A) JP-A-62-9277 (JP, A) JP-A-3-31775 (JP) , A) (58) Field surveyed (Int. Cl. ⁶ , DB name) G01R 31/12

Claims (3)

(57) [Claims] 1. An AC voltage is applied to a power cable to be measured, a ground wire current is detected from a ground wire of the power cable, and a dozen or so generated based on the presence of water tree deterioration in an isolated body from the ground wire current. Detects a pulsating component consisting of a quasi-DC component of less than Hz, further extracts only a signal of a specific frequency from the pulsating component, amplifies the signal, and performs time analysis of the amplified pulsating component of the specific frequency. A method for diagnosing insulation deterioration of a CV cable, wherein the method detects a degree of deterioration of the cable insulation. 2. The method according to claim 1, wherein the detection of the ground line current from the ground line is performed without disconnecting the ground line. 3. A current detecting device for detecting a ground line current from a ground line connecting between a shielding layer of a CV cable and a potential reference, and detecting the presence of water tree deterioration in an insulator based on the detected ground line current. A low-pass filter for extracting a pulsating component consisting of a quasi-DC component of tens of Hz or less, and a narrow-band filter connected to an output of the low-pass filter for extracting only a signal of a specific frequency from the pulsating component and amplifying the signal; An apparatus for diagnosing deterioration of insulation of a CV cable, comprising: a band amplifier; and waveform display means for displaying an output signal of the narrow band amplifier on a time axis.