JPH0331776A - Diagnostic device for insulation deterioration of cv cable - Google Patents

Abstract

PURPOSE:To prevent a power failure accident beforehand by providing a low- pass filter filtering a quasi-DC current with the frequency lower than several Hz and a waveform display means indicating an output signal of the low-pass filter on a time axis directly or being amplified. CONSTITUTION:The device consists of the low-pass filter 1 and the waveform display device 2 such as a waveform scope consisting of a CRT, pen-recorder, etc., and a timewise variation of the quasi-DC components with the frequency lower than several Hz, which are filtered with the low-pass filter 1 among currents flowing between the input terminals, is displayed on the waveform display device 2. In this case, only the desired quasi-DC components can be well filtered when the low-pass filter 1 having the characteristic shown as the figure is selected. In the waveform display device 2, the variation of the filtered quasi-DC components is displayed on an adequate time axis. The more severe the degree of water-tree deterioration is, the wider an amplitude A of this pulsation increases, so the water-tree deterioration degree for a CV cable C can be estimated by means of obtaining this amplitude A.

Description

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

[Prior Art] Generally, the insulation performance of the electric insulator of a power cable deteriorates due to aging after installation. In particular, it is known that in CV cables, dendritic cracks occur in the crosslinked polyethylene insulator, 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 the 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 performing such constant monitoring, as described in Japanese Patent Publication No. 5Q-8465, etc., a direct current is superimposed on the transmitted evening wave current, and as a result, the direct current of the cable leakage current is detected. The so-called DC 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 Laid-Open Publication No. 185171/1983. The so-called jan δ method for evaluation 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] 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 first-mentioned direct convolution method is generally said to have poor detection sensitivity with respect to the degree of deterioration, and has the problem that only extremely severe deterioration is detected. Also, et when measuring
A DC superimposed voltage of approximately OV is required, and a DC current must be prepared for this purpose.

On the other hand, although the JAN δ method can detect deterioration over 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 direct current generated is different between the so-called internal groove water tree generated from the conductor side of the cable insulator and the external water tree generated from the sheath side. Since they have opposite polarities, 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.

As a result of research on the water tree phenomenon, the present inventors discovered a new 1s fruit of the first order. 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 is detected in the ground line current, (1) A pulsating current is detected when the amplitude of the applied AC voltage reaches a certain value.

(2) Cables with severe water tree deterioration begin to pulsate! The voltage amplitude value is small.

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

SUMMARY OF THE INVENTION An object of the present invention is to provide an insulation deterioration diagnosing device for a CV cable, which eliminates the drawbacks of the conventional example and can accurately detect insulation defects at the time of deterioration based on the above-mentioned new facts.

[Means for Solving the Problems] In order to achieve the above object, in the CV cable insulation deterioration diagnosis device according to the present invention, the shielding layer and potential reference of the power cable to which an alternating current voltage is applied and which is to be measured. A power cable insulation deterioration diagnosing device for diagnosing insulation deterioration of the power cable based on ground wire radio waves flowing from the shielding layer to the potential reference, the power cable insulation deterioration diagnosis device The present invention is characterized by comprising a low-pass filter that filters the following quasi-DC radio waves, and a waveform display means that directly or amplifies the output signal of the low-pass filter and displays it on a time axis.

[Function] The CV cable insulation deterioration diagnostic device having the above configuration filters quasi-DC voltage components of several Hz or less from the ground IQ main current using a low-pass filter, and displays this filtered signal on a time axis using a waveform display means. The amplitude of the pulsation is used as a criterion for water tree deterioration of the power cable being measured.

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

FIG. 1 is a configuration diagram in which the CV cable insulation deterioration diagnostic device of the present invention is connected to a CV cable to be measured.
The circuit configuration is shown for a three-core CV cable C in a live state. That is, it consists of a low-pass filter l whose frequency characteristics are shown in FIG. 2, and a waveform display device M2 such as a waveform scope made of a CRT or a pen recorder, and the number of tJjQ flowing between the input terminals that is filtered by the low-pass filter. The waveform display device 2 is configured to display temporal fluctuations of the quasi-DC component below Hz.

On the other hand, the CV cable to be measured is connected to a three-phase high-voltage bus, and the three-phase high-voltage bus is connected to the secondary side of a transformer with a Y-Δ connection, and Power is being received from a three-phase AC power source that is not connected. Further, the neutral point of the primary winding of the grounding transformer GP-D connected to the three-phase high voltage bus is directly grounded to the earth. As a result of such a connection, a three-phase AC power supply referenced to earth potential becomes cv
The voltage is applied to the cable, and the CV cable C is in a live state.

During measurement, one end of the input terminal of the insulation deterioration diagnostic device having the above-described configuration is connected to the shielding layer S of the CV cable C, and the other end is grounded. At this time, a charge corresponding to the applied AC voltage is generated in the shielding layer S of the CV cable C due to electrostatic coupling with the internal conductor, and due to this time change, the shielding layer S has the same frequency as the applied AC voltage. This results in a wave of current that fluctuates with a certain period. In addition to this, when the CV cable C has water tree deterioration, the above-mentioned pulsating current is taken into consideration, and the low-pass filter 1 extracts only the pulsating current. That is, since the frequency of the applied AC voltage is usually 50H2 or 60Hz, by selecting the low-pass filter l having the characteristics illustrated in FIG. 2, it is possible to filter only the desired quasi-DC component well. In the waveform display device 2, the third
As shown in the figure, the fluctuation of the filtered quasi-DC component is displayed on an appropriate time axis. The examiner can easily determine the amplitude of the pulsating current by observing this displayed waveform, and since the amplitude A of this pulsation increases as the degree of water tree deterioration becomes more severe, by determining this oscillation @A, This means that the degree of water tree deterioration of CV cable C can be estimated.

In the above embodiment, the output of the low-pass filter l is connected to #1 between the low-pass filter 1 and the waveform display device 2.
By inserting the amplifier 3 that increases the amplitude, good observation becomes possible even if the amplitude of the AC voltage applied to the CV cable C is small.

Furthermore, even when the CV cable is of the single-core, three-wire cable group C', diagnosis can be made using the same device. That is,
In this case, as shown in the partial diagram of FIG. 4, if the shielding layer S° of each cable constituting the cable group C° is connected to the input terminal of the device in parallel, the method described above is exactly the same. On the other hand, if any one of the shielding layers S' of each cable constituting the cable group C' is connected to the input terminal of the device, it is possible to independently diagnose the water tree deterioration of each cable.

The example shown in Fig. 4 is circuit-wise equivalent to diagnosing a live cable C'' with a single-phase power source P whose one end is grounded, as shown in the configuration diagram of Fig. 5, and only three-phase AC is used. Of course, it is also possible to diagnose live cables using single-phase alternating current.

Furthermore, it is also possible to diagnose a stopped cable with a similar device. In this case, a separate power supply unit that applies an AC voltage is required, but in this case, an AC variable power source S' whose amplitude of applied AC voltage is variable is used, and the output voltage amplitude of this AC variable power source S' is set to zero. A more accurate diagnosis can be made by gradually increasing the number from . The time change of the quasi-DC component in this case is as shown in the graph of FIG. 6, for example. Note that the horizontal axis represents time or the 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 from the amplitude value vO shown in FIG. It gets lower. 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. 7 shows a graph of the capacitance F of the cable to be measured and the pulsation start voltage vO.The horizontal axis is the capacitance F. is shown on a logarithmic scale, and the vertical axis shows the pulsation start voltage vO. In Fig. 7, state point
is generally on the solid line B-B', whereas in a cable with water tree deterioration, the pulsation start voltage vO at the same capacitance F decreases, and the state point X° is on the solid line B as shown by the arrow.
-B' will deviate from the equation. Therefore, real 1IB-
Based on the deviation of the state point X° from B', it becomes possible to quantitatively evaluate the degree of water tree deterioration.

Furthermore, the CV cable insulation deterioration diagnosis device according to the present invention described above may be installed individually on all the cables to be diagnosed and used for constant monitoring as a stationary type, or it may be installed in a case (not shown) that is easy to transport. It may be built into a portable type and used commonly for multiple cables.

In any of the above cases, in practical use, a dividing means is required to separately measure and offset the output fluctuations such as noise of the AC voltage applied to the cable, and a signal with a frequency of the fluctuation waveform is required. It is desirable to provide a notch filter or the like to remove this.

[Effects of the Invention] As explained above, in the CV cable insulation deterioration diagnosis device according to the present invention, the low-pass filter filters the quasi-DC component of the ground line current of the power cable, and the waveform display means displays this filtered component on a time axis. It is possible to easily estimate the degree of water tree deterioration of the power cable from the amplitude of the pulsation of this filtered component, which can prevent cable insulation breakdown accidents, stoppages, and car accidents. It is possible to significantly reduce damage to electricity consumers.

[Brief explanation of drawings]

The drawings show an embodiment of the insulation deterioration diagnosis device for a C■ cable according to the present invention. Fig. 1 shows circuit a factors when applied to live-line diagnosis of a three-core power cable, and Fig. 2 shows a low-pass filter. Figure 3 is a graphical representation of the displayed waveform during measurement in a live line state, Figure 4 is a partial configuration diagram for live line diagnosis of a single-core 3-wire power cable, and Figure 5 is a single-phase live line state. FIG. 6 is a graph of displayed waveforms in diagnosis of a stopped line, and Figure m7 is a graph of diagnostic criteria for a stopped line. Symbol l is a low-pass filter, 2 is a waveform display device, 3 is an amplifier, C, C', C'' are power cables, s, s',
s'' is a shielding layer.

Claims (1)

[Claims] 1. Connect the shielding layer of the power cable to which an AC voltage is applied and which is the measurement target, and a potential reference, and measure the insulation deterioration of the power cable based on the ground line current flowing from the shielding layer to the potential reference. A power cable insulation deterioration diagnostic device to be diagnosed, comprising a low-pass filter that filters quasi-direct current of several Hz or less from the ground wire current, and a waveform that directly or amplifies the output signal of the low-pass filter and displays it on a time axis. 1. An insulation deterioration diagnostic device for a CV cable, comprising: a display means.