JP6215992B2 - Power cable insulation degradation position estimation method and estimation system - Google Patents

Description

  The present invention relates to a technique for accurately estimating an insulation deterioration position in a power cable.

  CV cables are widely used as power cables for indoor and outdoor wiring. It is known that a CV cable is easy to lay and secure, while a specific deterioration phenomenon called a water tree occurs in a crosslinked polyethylene used as an insulator. The water tree is a phenomenon in which a minute amount of water or the like adhering to the insulator permeates into the insulator due to a change with time, greatly reducing the insulation performance of the power cable and causing an insulation breakdown accident.

  In order to prevent an accident caused by insulation deterioration of the power cable, it is necessary to diagnose the installed power cable and replace the power cable in which insulation deterioration is detected. When replacing the power cable in which insulation deterioration is detected, if it is replaced over the entire length, it takes a lot of time and cost, so it is preferable to estimate the position where insulation deterioration occurs and replace only the defective part. .

  For this reason, conventionally, a method for estimating the insulation deterioration position of the power cable has been proposed. FIG. 7 is a diagram illustrating an example of a conventionally proposed method for estimating an insulation deterioration position of a power cable. In the example of this figure, in the power cable 200 having a long length L, it is assumed that insulation deterioration due to the water tree occurs at a position X from one end. Here, it is assumed that the power cable 200 includes an inner conductor 201, an insulator 202 that surrounds the inner conductor 201, an outer conductor 203 that surrounds the insulator 202, and a protective coating 204 that surrounds the outer conductor 203.

  It is known that when a high voltage is applied to the inner conductor 201 of the power cable 200 in which a water tree is present, a discharge called “kick” frequently occurs from the water tree generation position. The pulse-like dischargeable current caused by the kick is propagated by diverting the outer conductor 203 to the charging end side (electric path 1) and the far end side (electric path 2).

  When the respective discharge currents are detected by the detection resistors 320a and 320b and both waveforms are captured by the digital oscilloscope 330, the arrival time of the kick waveform varies depending on the distance difference between the electric circuit 1 and the electric circuit 2.

By observing the rising timing of both waveforms with the digital oscilloscope 330, the arrival time difference Δt can be measured, and the cable propagation length L and the current propagation speed v in the power cable 200 are known.
X = (L−vΔt) / 2
Thus, the insulation deterioration position X can be obtained.

JP 2013-36839 A

  The frequency spectrum of the kick waveform extends to 100 MHz or more, but in general, the pulse propagating through the power cable tends to attenuate as the high-frequency component increases. Become. Furthermore, when the propagation path difference between the electric circuit 1 and the electric circuit 2 is large, there is a difference in attenuation amount for each frequency, and there is a similarity between the waveform observed at the power application side end and the waveform observed at the far end side. It will be lost.

  For this reason, in the measurement of the arrival time difference Δt for comparing the rising timings of both waveforms, the recognition of the rising timing becomes ambiguous and the error is likely to increase, so that the accuracy of the insulation deterioration position estimation may be lowered.

  Therefore, an object of the present invention is to improve the estimation accuracy of the insulation deterioration position in the power cable.

In order to solve the above-described problem, an insulation deterioration position estimation method according to a first aspect of the present invention is an insulation deterioration position estimation method for a power cable having a first conductor and a second conductor separated by an insulator. , Applying a voltage to the end of one conductor, and detecting a discharge current generated at the insulation degradation position due to the voltage and propagating toward both ends of the other conductor to extract a specific frequency Capturing the waveform display device through a narrowband bandpass filter for display, measuring the time difference between the characteristic points of the two approximate sine waveforms displayed on the waveform display device, and measuring the measured time And a step of estimating an insulation deterioration position of the power cable based on the deviation.
Here, prior to the step of applying the voltage, a step of selecting a center frequency of the narrowband bandpass filter from a plurality of candidates based on a length of the power cable may be further included.
In the selecting step, a maximum propagation path difference of the dischargeable current is obtained based on a length of the power cable, and an arrival time difference of the two approximate sine waveforms to the waveform display device in the case of the maximum propagation path difference is determined. A frequency candidate that satisfies a condition not exceeding one period can be selected as the center frequency.
At this time, the highest frequency among the frequency candidates satisfying the condition can be selected as the center frequency.
In either case, the characteristic point of the approximate sine wave can be a zero cross point.
In order to solve the above problems, an insulation degradation position estimation system according to a second aspect of the present invention is an insulation that supports estimation of insulation degradation position of a power cable having a first conductor and a second conductor separated by an insulator. A degradation position estimation system that detects a discharge current that is generated at the insulation degradation position and propagates toward the voltage application side end of the other conductor. A first current detector that extracts an approximate sine waveform of a specific frequency using a narrow band-pass filter, and a discharge current that is generated at an insulation deterioration position and propagates toward the other end of the other conductor; A second current detector for extracting an approximate sine waveform of a specific frequency using a narrow band-pass filter; an approximate sine waveform extracted by the first current detector; and an approximate sine waveform extracted by the second current detector; Same screen Characterized in that it comprises a waveform display device for displaying.
The narrow-band bandpass filter can be constituted by, for example, a ceramic filter or a surface acoustic wave filter.

  According to the present invention, since the kick waveform is formed into an approximate sine wave signal having a specific frequency and the time difference between the characteristic points of the approximate sine waveform is measured, it is not necessary to determine the rising timing of the original waveform of the kick waveform. In the approximate sine waveform, the feature point can be clearly identified, so that the error in the arrival time difference measurement can be reduced, and the estimation accuracy of the insulation deterioration position in the power cable can be increased.

It is a figure which shows the structure of the power cable insulation degradation position estimation system which concerns on this embodiment. It is a figure which shows the structural example of the power cable insulation degradation position estimation system in a real field. It is a figure explaining the waveform observed with a digital oscilloscope. It is a flowchart explaining the insulation degradation position estimation procedure in this embodiment. It is a flowchart explaining a center frequency setting process. It is a figure which shows another structure of the power cable insulation degradation position estimation system which concerns on this embodiment. It is a figure which shows an example of the insulation degradation position estimation method of the power cable proposed conventionally.

  Embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram illustrating a configuration of a power cable insulation deterioration position estimation system 100 according to the present embodiment. Also in this example, it is assumed that the power cable 200 to be inspected is composed of the inner conductor 201, the insulator 202, the outer conductor 203, and the protective coating 204. It is assumed that the insulation deterioration due to the water tree occurs at the position of X.

  The power cable 200 is, for example, a CV cable, but can be used as an inspection target if it is a power cable in which different conductors are separated by an insulator. Further, the deterioration of insulation is not limited to the water tree, and the position can be estimated if the insulation is defective, in which partial discharge occurs at the deteriorated portion by applying a high voltage.

  The power cable insulation degradation position estimation system 100 includes a high voltage application unit 110, a first current detection unit 120, a second current detection unit 130, a digital oscilloscope 140, and a blocking coil 150.

  The high voltage application unit 110 is connected to the inner conductor 201 at one end of the power cable 200 and applies a DC high voltage. In the power cable 200, the side to which the DC high voltage is applied is referred to as the charging end side, and the opposite side is referred to as the far end side.

  The first current detection unit 120 is a circuit that detects a current flowing from the power application end side of the external conductor 203, and includes a detection resistor 121, an amplifier 122, and a narrow-band bandpass (BP) filter 123. The voltage generated in the detection resistor 121 is amplified by the amplifier 122, filtered by the narrow band bandpass filter 123, and input to the first channel (Ch 1) of the digital oscilloscope 140.

  The second current detection unit 130 is a circuit that detects a current flowing from the far end side of the outer conductor 203, and includes a detection resistor 131, an amplifier 132, and a narrow band-pass (BP) filter 133. The characteristics of the amplifier 132 and the narrowband bandpass filter 133 are made equal to the characteristics of the amplifier 122 and the narrowband bandpass filter 123 of the first current detection unit 120. The voltage generated in the detection resistor 131 is amplified by the amplifier 132, filtered by the narrow band band pass filter 133, and input to the second channel (Ch 2) of the digital oscilloscope 140.

  The digital oscilloscope 140 is a waveform display device having a storage function. The digital oscilloscope 140 displays the waveforms input from the first channel and the second channel in an overlapping manner on the same screen, so that enlargement / reduction observation is possible. The digital oscilloscope 140 may be composed of a plurality of devices such as a waveform capturing device and a display device.

  The blocking coil 150 is connected between the detection resistor 121 and the detection resistor 131 and the ground line, and is used to reduce common mode noise mixed from the ground line.

In the example of this figure, in order to simplify the description, the electrical length from the charging end side of the outer conductor 203 to the digital oscilloscope 140 is equal to the electrical length from the far end side of the outer conductor 203 to the digital oscilloscope 140. Wiring is performed as follows. For this reason, when the arrival time difference Δt of the kick waveform between the first current detector 120 and the second current detector 130 is obtained,
X = (L−vΔt) / 2 (Formula 1)
Thus, the insulation deterioration position X can be estimated.

In the actual field, the kick waveform at the far end is propagated to the charging end side using the conductor portion of another healthy cable 250, and measurement is performed with the digital oscilloscope 140 disposed on the charging end side. become. In this case, if the length of the other cable 250 is Lc,
X = (L + Lc−vΔt) / 2 (Formula 2)
Thus, the insulation deterioration position X can be estimated. When the power cable 200 constitutes a three-phase cable, the other cable 250 can use the inner conductor or the outer conductor of the other-phase power cable. In this case, L + Lc = 2L.

  Returning to the description of FIG. 1, the narrowband bandpass filter 123 and the narrowband bandpass filter 133 extract a specific frequency from the kick waveform, and shape the kick waveform into a sine wave burst signal having a substantially single frequency. For this reason, the narrow band-pass filter 123 and the narrow band band-pass filter 133 select a filter with a very high sharpness Q.

  A filter having a very high sharpness Q can be realized by using a ceramic filter, a surface acoustic wave filter (SAW filter) or the like. For example, a general-purpose ceramic filter having a center frequency of 455 kHz can be used. However, as will be described later, it is desirable to prepare a plurality of types of ceramic filters having different center frequencies and select them according to the maximum value of the arrival time difference Δt based on the length L of the power cable 200.

  FIG. 3 shows an example of a waveform observed by the digital oscilloscope 140 after passing through the narrowband bandpass filter 123 and the narrowband bandpass filter 133 when a kick is generated. The Ch1 waveform on the charging end side and the Ch2 waveform on the far end side are superimposed and displayed on the same time axis. FIG. 3 (a) is a schematic overview and FIG. FIG. When a pulse-like signal is input, the output waveform of the ceramic filter draws a spindle shape with a gentle sine wave envelope as shown in FIG.

  The phase difference between the waveform of Ch1 and the waveform of Ch2 represents the arrival time difference between the waveform on the charging end side and the waveform on the far end side. Therefore, as shown in FIG. 3B, the arrival time difference Δt can be measured by reading the time difference between the characteristic points of the Ch1 waveform and the Ch2 waveform, for example, the zero cross point. That is, it is not necessary to determine the rising timing of the original waveform of the kick waveform when measuring the arrival time difference Δt.

  Since the waveform of Ch1 and the waveform of Ch2 can be regarded as an approximate sine wave by the narrowband bandpass filter 123 and the narrowband bandpass filter 133, the zero-cross point can be clearly identified. For this reason, the error of the arrival time difference Δt measurement can be reduced, and the accuracy of the insulation deterioration position estimation can be increased. In other words, the narrow-band bandpass filter 123 and the narrow-band bandpass filter 133 have a sharpness Q that can extract an approximate sine wave that can clearly identify a zero-cross point.

  The feature point for measuring the arrival time difference Δt is not limited to the zero cross point from negative to positive. For example, it may be a zero cross point from positive to negative, or may be a maximum point or a minimum point. Since both waveforms can be regarded as sine waves having the same single frequency, the section used for time difference measurement can be arbitrarily determined. However, as in section A, a portion with a large amplitude is advantageous in terms of the SN ratio. It is.

  Next, the insulation degradation position estimation procedure in this embodiment will be described with reference to the flowchart of FIG. Here, as an example, a plurality of types of ceramic filters having different center frequencies are prepared as the narrowband bandpass filter 123 and the narrowband bandpass filter 133. If a plurality of types of ceramic filters having different center frequencies are not prepared and the ceramic filter is not selected, the processing (S12) can be omitted.

  First, the characteristics of the power cable 200 to be inspected are acquired (S11). It is assumed that the acquired characteristics include the span L and the propagation speed v. As shown in FIG. 2, when the kick waveform is propagated from the far end side to the charging end side using another cable 250, the length Lc of the other cable 250 is also acquired. In general, the propagation velocity v can be specified by the type of the power cable 200.

  And the center frequency of the ceramic filter used as the narrow-band band pass filter 123 and the narrow-band band pass filter 133 is set (S12). Here, the center frequency setting process (S12) will be described in detail. In the enlarged waveform diagram of FIG. 3B, if the phase difference between the waveform of Ch1 and the waveform of Ch2 is 2π or more, the time difference read based on the deviation of the zero cross point does not match the actual arrival time difference Δt. An error that is an integral multiple of the waveform period occurs.

  In the power cable 200, the arrival time difference Δt is the largest when the propagation path difference between the power application end side kick waveform and the far end side kick waveform is maximized. Assuming that the maximum propagation path difference is DLmax, in the case of FIG. 1, when a kick occurs at either end of the power cable 200, the maximum propagation path difference DLmax = L. In the case of FIG. 2, when a kick occurs at the power application end of the power cable 200, the maximum propagation path difference DLmax = L + L1.

  In order to prevent the phase difference between the waveform of Ch1 and the waveform of Ch2 from exceeding 2π, the maximum arrival time difference (= maximum propagation path difference DLmax / propagation speed v) is one cycle even in the case of the maximum propagation path difference DLmax. The frequency f not exceeding (= 1 / f) may be set as the center frequency f0.

That is,
f <v / DLmax
The frequency f satisfying the above may be set as the center frequency f0.

For example, for the sake of simplicity, assuming that Lc = L and the maximum propagation path difference DLmax = 2L in FIG. 2, when a 455 kHz ceramic filter is used, if the power cable 200 has a length of less than 198 m, the phase difference is 2π. Less than. The propagation speed of the kick is 1.80 × 10 ⁸ m / s. When a 45 kHz ceramic filter is used, the phase difference is less than 2π if the power cable 200 has a length of less than 2 km.

  As described above, by lowering the center frequency of the ceramic filter, it becomes possible to cope with the power cable 200 having a longer length.

  On the other hand, if the center frequency of the ceramic filter is lowered, the extracted wavelength becomes longer and the waveform is extended in the time axis direction, so the measurement time must be lengthened. If the sampling period of the digital oscilloscope 140 is left as it is, the amount of data becomes excessive, so it is necessary to lengthen the sampling period to lower the measurement resolution, and the range of estimated positions (position resolution) becomes coarse. For this reason, it is preferable that the center frequency of the ceramic filter is high from the viewpoint of measurement resolution.

  Therefore, in this embodiment, the center frequency is set according to the procedure shown in the flowchart of FIG. First, the frequency f at which the phase difference is 2π is calculated from the maximum propagation path difference DLmax and the propagation velocity v acquired in the process (S11) (S121). The maximum propagation path difference DLmax can be calculated by the length L in the case of FIG. 1, the length L + the length L1 in the case of FIG. 2, and the frequency f can be calculated by f = v / DLmax. it can.

  Then, among the plurality of types of ceramic filters having different center frequencies, the largest ceramic filter having a center frequency equal to or less than the calculated frequency f is selected (S122). Thereby, it is possible to estimate the insulation deterioration position with as high a measurement resolution as possible while preventing the phase difference between the waveform of Ch1 and the waveform of Ch2 from becoming 2π or more. However, if it is clear that noise components in a specific frequency band are mixed in due to the surrounding environment or the like, avoid using that frequency band as the center frequency.

  Note that the error in the arrival time difference Δt that occurs when the phase difference is 2π or more is an integral multiple of the waveform period as described above. Therefore, even when the ceramic filter center frequency is not selected, the insulation deterioration with a plurality of positions as candidates is possible. Position estimation is possible.

  Returning to the description of the flowchart of FIG. 4, when the center frequency of the ceramic filter is set (S12), the set ceramic filter is used as the narrowband bandpass filter 123 and the narrowband bandpass filter 133, and the high voltage applying unit 110 is used. A high DC voltage is applied to the outer conductor 203 of the power cable 200 (S13). As a result, a kick phenomenon occurs in an insulation deterioration portion such as a water tree.

  The generated kick waveform propagates in the direction of both ends of the outer conductor 203 of the power cable 200, is detected by the first current detection unit 120 and the second current detection unit 130, is captured by the digital oscilloscope 140, and is displayed on the screen. (S14). In this example, a DC high voltage is applied to the inner conductor 201 and the outer conductor 203 propagates a kick current. However, a DC high voltage is applied to the outer conductor 203 and the inner conductor 201 applies a kick current. You may make it propagate.

  Then, the measured person enlarges and observes the displayed waveform, measures the arrival time difference Δt (S15), and estimates the insulation deterioration position X according to the above (Equation 1) or (Equation 2) (S16). The digital oscilloscope 140 may be added with a function of automatically recognizing the zero cross points of both waveforms, measuring the arrival time difference Δt, and calculating the insulation deterioration position X.

  As mentioned above, although embodiment of this invention was described, this invention is not restricted to these embodiment, A various deformation | transformation can be performed. For example, in the example illustrated in FIG. 1, each current detection unit is configured by a detection resistor, an amplifier, and a narrow-band bandpass filter. However, as illustrated in FIG. 6, the first current detection unit 120 is configured as a detection resistor. 121, a preamplifier 124, a narrowband bandpass filter 125, a main amplifier 126, and a narrowband bandpass filter 127. The second current detection unit 130 includes a detection resistor 131, a preamplifier 134, a narrowband bandpass filter 135, and a main amplifier. 136 and a narrow-band bandpass filter 137. Here, the narrowband bandpass filter 127 and the narrowband bandpass filter 137 arranged in the subsequent stage are mainly used for internal noise reduction.

  In this case, the narrowband bandpass filter 125, the narrowband bandpass filter 127, the narrowband bandpass filter 135, and the narrowband bandpass filter 137 all have the same characteristics. Further, the characteristics of the preamplifier 124 and the preamplifier 134 are made the same, and the characteristics of the main amplifier 126 and the main amplifier 136 are made the same so that the phases of both waveforms do not shift during amplification.

DESCRIPTION OF SYMBOLS 100 ... Power cable insulation degradation position estimation system 110 ... High voltage application part 120 ... 1st electric current detection part 121 ... Detection resistance 122 ... Amplifier 123 ... Narrow band-pass filter 124 ... Preamplifier 125 ... Narrow band band-pass filter 126 ... Main amplifier 127: Narrowband bandpass filter 130 ... Second current detector 131 ... Detection resistor 132 ... Amplifier 133 ... Narrowband bandpass filter 134 ... Preamplifier 135 ... Narrowband bandpass filter 136 ... Main amplifier 137 ... Narrowband bandpass filter 140 ... Digital oscilloscope 150 ... Blocking coil 200 ... Power cable 201 ... Internal conductor 202 ... Insulator 203 ... External conductor 204 ... Protective coating

Claims (6)

A method for estimating an insulation deterioration position of a power cable having a first conductor and a second conductor separated by an insulator,
Selecting a center frequency of a narrowband bandpass filter for extracting a specific frequency from a plurality of candidates based on the length of the power cable;
Applying a voltage to the end of one conductor;
The discharge current generated at the insulation degradation position due to the voltage and propagating in the direction of both ends of the other conductor is detected, and the waveform display device passes through the narrow-band bandpass filter of the selected center frequency. Steps to capture and display,
Measuring a time lag between feature points of two approximate sine waveforms displayed on the waveform display device;
Estimating an insulation deterioration position of the power cable based on the measured time difference; and
An insulation degradation position estimation method characterized by comprising: The step of selecting includes
Find the maximum propagation path difference of the discharge current based on the length of the power cable,
According to claim 1, the arrival time difference to the waveform display device of the two approximate sinusoidal waveforms in the case of maximum propagation path difference and selecting a center frequency satisfying candidate frequencies not exceeding 1 cycle Insulation degradation position estimation method. 3. The insulation degradation position estimation method according to claim 2 , wherein the highest frequency among the frequency candidates satisfying the condition is selected as a center frequency. Feature points of the approximate sine wave, insulation degradation position estimating method according to any one of claims 1 to 3, characterized in that a zero-crossing point. An insulation deterioration position estimation system that supports estimation of an insulation deterioration position of a power cable having a first conductor and a second conductor separated by an insulator,
A selection unit that selects a narrowband bandpass filter from a plurality of narrowband bandpass filters having different center frequencies based on the length of the power cable;
A voltage application unit for applying a voltage to the end of one conductor;
A discharge current generated at the insulation degradation position and propagated toward the voltage application side end of the other conductor is detected, and an approximate sine waveform of a specific frequency is extracted using the selected narrow-band bandpass filter. A current detector;
Second current detection for detecting a discharge current generated at the insulation degradation position and propagating toward the other end of the other conductor, and extracting an approximate sine waveform of a specific frequency using the selected narrow-band bandpass filter And
A waveform display device that captures the approximate sine waveform extracted by the first current detection unit and the approximate sine waveform extracted by the second current detection unit and displays them on the same screen;
An insulation deterioration position estimation system comprising: 6. The insulation degradation position estimation system according to claim 5 , wherein the narrow-band bandpass filter is configured by a ceramic filter or a surface acoustic wave filter.

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