JPH0331775A - Diagnostic method for insulation deterioration of cv cable - Google Patents

Abstract

PURPOSE:To accurately discover a defective insulation at the deterioration by detecting quasi-DC components of a grounded line current and analyzing the amplitude and frequency, etc., of a current pulsation. CONSTITUTION:An electrostatic capacitance C between a conductor (a) of power cable to be measured and a shielding layer (b) is connected to an AC power supply S, and an LPF is connected between the layer (b) side and the earth, whereby the quasi-DC components in the current flowing into the earth from the layer (b) side is taken out to supply a voltage proportional to this value to the output ends (c)-(d). The pulsation is recognized when the amplitude of impressed AC voltage exceeds a certain value corresponding to the degree of water-tree deterioration of the cable, and the amplitude of this pulsation has the characteristic such that the more severe the degree of water-tree deterioration is, the wider the amplitude is. The degree of water-tree deterioration can be estimated by means of obtaining this amplitude.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a CV cable insulation deterioration diagnosis method for diagnosing the degree of insulation deterioration of a crosslinked polyethylene power cable called a C■ cable.

[Prior Art] Generally, the insulation performance of the electric insulator of a power cable deteriorates due to aging after installation. especially.

It is known that in CV cables, dendritic cracks occur in the crosslinked polyethylene insulation, and the occurrence of so-called water trees, in which water enters the cracks, is the main cause of insulation deterioration. If left untreated, such deterioration in insulation performance may 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 measurement methods have been known for a long time, but in recent years in particular, several methods have been proposed for diagnosing live wires without stopping power transmission during measurement, making it possible to constantly monitor the condition. It is attracting attention because of its many advantages.

Conventionally, as a method for carrying out 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 as a result, the direct current of the cable leakage current is detected. The so-called direct current external method is used to determine and evaluate the insulation resistance of the cable from the components, or the dielectric loss tangent is determined by measuring the transmission voltage waveform and current waveform as disclosed in Japanese Patent Application Laid-open No. 185171/1983. The so-called jan δ method, which calculates and evaluates, is generally used.

In addition, especially in the case of CV cables, JP-A-59-20
Publication No. 2075 describes a method of measuring the DC component of the cable leakage current during AC power transmission and estimating the distribution, length, and physical strength of the water tree from the direction and absolute value, assuming that the water tree has a current rectifying effect. Disclosed.

[Problems to be Solved by the Invention] By the way, among the above-mentioned conventional techniques, none of the known conventional methods is a method that can fully satisfy the demand for accurately discovering insulation defects at an early stage of deterioration. ,
The DC superimposition method mentioned above is generally said to have poor detection sensitivity for the degree of deterioration, and has the problem that only extremely severe deterioration is detected. Additionally, a DC superimposed voltage of several tens of volts is required during measurement, and a DC power source must be prepared for this purpose.On the other hand, although the Janδ method detects deterioration over the entire cable, it detects localized deterioration such as water trees. It is known that the detection sensitivity for severe deterioration is poor.

Furthermore, Japanese Patent Application Laid-Open No. 59-20 utilizes the rectification effect of water trees.
In the case of Publication No. 2075, the so-called water conducting tree generated from the conductor side of the cable insulator and the external water conducting tree generated from the sheath side are one type of water tree because the generated DC current has opposite polarity to each other. If these 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.

As a result of research on the water tree phenomenon, the present inventors discovered the following new fact. That is, in the process of applying an AC voltage to the power cable to be measured and gradually increasing the amplitude of this AC voltage from zero, if a quasi-DC component of several Hz or less of this ground line current is detected, (1) Pulsating radio waves are detected when the amplitude of the applied AC voltage reaches a certain value.

(2) In cables with severe water tree deterioration, the amplitude value of the AC voltage at which pulsation begins is small.

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

An object of the present invention is to provide a novel 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 1] In order to achieve the above object, in the method for diagnosing insulation deterioration of a C■ cable according to the present invention, an AC voltage is applied to the power cable to be measured, and the ground wire current is measured. Of which number H2
It is characterized by detecting the following temperature-reducing components and performing a time analysis to detect the degree of deterioration due to water trees in the cable insulation.

[Function] In the CV cable insulation deterioration diagnosis method of the present invention, the degree of water tree deterioration in the CV cable is estimated by detecting the quasi-DC component of the grounding wire current and analyzing the amplitude, frequency, etc. of the current pulsation. do.

[Example] The present invention will be explained in detail based on illustrated embodiments.

FIG. 1 is a circuit diagram for implementing the method according to the present invention. C is the capacitance between the conductor and the shielding layer of the power cable to be measured, with the a side being the conductor side and the b side being the shielding layer side. S is an AC power supply, one end of which is grounded and the other end connected to the conductor a side. In addition, a low-pass filter LPF is connected between the shield layer side and the ground, extracts the quasi-DC component of the current flowing from the shield layer side to the ground, and applies a voltage proportional to this value to the output terminals c-d. give. As shown in Figure 1, the low-pass filter LPF specifically connects a capacitor CO between the shielding layer side and the ground, and connects the coil Ll, capacitor CI, coil L2, capacitor C
A conceivable configuration is such that a voltage output is obtained through a two-stage so-called predetermined type filter consisting of 2' and a resistor H at the end. Furthermore, a pen recorder, for example, is connected to the output terminals c-d to record the voltage output on the time axis, and the time change of the obtained output signal is grasped.

FIG. 2 is an example of a graph of the time change of the quasi-DC component obtained by the method described above, in which the horizontal axis shows time t and the vertical axis shows current flow. If the amplitude of the applied AC voltage exceeds a certain value depending on the degree of water tree deterioration of the cable,
As shown in FIG. 2, pulsations below pHZ are observed in the current r. As described above, the amplitude A of this pulsation has a characteristic that it increases as the degree of water tree deterioration becomes more severe, so by determining this amplitude A, the degree of water tree deterioration can be estimated. In this method, since the AC power source S can be used, monitoring in a live state is always possible.

On the other hand, when measuring a stopped cable,
In Figure 1, the AC power source S is an AC variable power source S with variable amplitude.
', and by gradually increasing the amplitude of the output voltage of this AC variable power supply S° from zero, even more accurate diagnosis is possible. The time change of the direct current component in this case is as shown in the graph of the m3 diagram, for example. Note that the horizontal axis represents time (or power supply voltage amplitude V proportional to this), and the vertical axis represents current flow. As mentioned above, when the amplitude of the applied AC voltage is increased, pulsations are observed in the quasi-DC component starting from the amplitude value vO as shown in Fig. 3, and this pulsation starting voltage vO is due to water tree deterioration. The more intense it gets, the lower it gets.

Therefore, even if this pulsation start voltage vO is determined, it is possible to estimate the degree of water tree deterioration.

As a guideline for making a diagnosis using the pulsation start voltage vO, Fig. 4 shows a graph of the capacitance C of the cable to be measured and the pulsation start voltage vO.The horizontal axis is the capacitance C. is shown on a logarithmic scale, and the vertical axis shows the pulsation start voltage vO. In Fig. 4, a state point
exists approximately on the solid line B-B', whereas in a cable with water tree deterioration, the pulsation starting voltage vO at the same capacitance C decreases, and for example, as shown by the arrow, the pulsation starting voltage vO
' will deviate from the solid line B-B'.

Therefore, based on the deviation of the state point X° from the solid line BB',
It becomes possible to quantitatively evaluate the degree of water tree deterioration.

In any of the cases described above, if there is an output fluctuation such as a miscellaneous power supply in the L power supply, this fluctuation can be measured separately to cancel it out, or the frequency band of the fluctuating waveform can be clearly identified. In such a case, it is preferable to use a so-called No. 7 filter to remove the influence of fluctuations.

Further, in the embodiment described above, a single-phase cable was described, but diagnosis can be similarly performed for a three-phase cable.

[Effects of the Invention] As explained below, the CV cable insulation deterioration diagnosis method according to the present invention measures a novel characteristic quantity for water tree deterioration, which is the pulsation of the quasi-DC component of the cable grounding line current. It is possible to perform accurate diagnosis at an early stage of deterioration of power cables whether they are in a live state or a stopped state.

[Brief explanation of drawings]

The drawings show an embodiment of the method for diagnosing insulation deterioration of CV cables according to the present invention, in which Fig. 1 is a circuit configuration diagram, Fig. 2 is a graph of the 11111 waveform during measurement in a live line state, and Fig. 3 is a diagram of the 11111 waveform when the cable is stopped. 4th graph of observed waveforms in measurements under
The figure is a graph of diagnostic criteria for cables in a stopped state. The symbol S is an AC power source, So is an AC a'f transformer power source, C is the capacitance of the cable, and LPF is a low-pass filter.

Claims (1)

[Claims] 1. By applying an AC voltage to the power cable to be measured, detecting the pulsating current component of several Hz or less of the ground wire current, and performing a time analysis, it is possible to detect deterioration due to water trees in the cable insulation. CV characterized by detecting the degree of
Cable insulation deterioration diagnosis method.