JP2002207060A - Diagnostic method of insulation degradation for power cable - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve reliability of a diagnostic method of insulation degradation for a CV cable that a signal ii (t) with no distortion of a waveform is obtained by inversely converting a measurement signal io (t) of a DC leakage current Id and then is performed based on observing a degradation signal included in the signal ii (t). SOLUTION: A noise included in the signal io (t) and a noise that appears in the signal ii (t) with no distortion of the waveform with emphasis are reduced or eliminated based on a smoothing process.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method of diagnosing insulation deterioration of a power cable, and to a method of diagnosing insulation deterioration of a rubber / plastic insulated power cable, typically a crosslinked polyethylene insulated power cable (hereinafter referred to as "CV cable"). Use to diagnose. In particular, the method for diagnosing insulation deterioration of a power cable according to the present invention is used to diagnose the state of insulation deterioration of a CV cable having a long cable line (one seamless CV cable is long), such as a 22 kVC cable. It is valid.

[0002]

2. Description of the Related Art A water tree generated when a CV cable is used in a flooded or humid state is tied to an electric tree and causes a dielectric breakdown, thereby causing a power transmission accident. Therefore, it is important to know in advance the deterioration state of the insulation of the CV cable based on the presence of a deteriorated portion such as a water tree in order to prevent occurrence of an accident in the transmission line. As a method for diagnosing insulation deterioration of a CV cable for such a purpose, a method described in, for example, IEEJ Technical Report No. 502, page 11, or the like is conventionally known.

[0003] A first example of the conventional method described in IEEJ Technical Report No. 502, p. 11 or the like is that a DC voltage is applied between a cable conductor of a CV cable and an outer periphery of an insulating layer. detecting a sudden variation component of the DC leakage current I _d flowing through the insulating layer (kick), it is diagnostic of deterioration of the insulating layer. That is, when water tree bridging between electrodes in the insulating layer is present, the DC leakage current I _d flowing through the insulating layer is rapidly within a short time as shown in FIG. 2 (e.g., within one second) Change. Therefore, appearing in the DC leakage current I _d, by looking at the sudden change called kick can diagnose deterioration of the CV cable insulation layer by the water tree. Such a diagnostic method is considered to be effective for, for example, diagnosing insulation deterioration of a 6.6 kVC cable, and is partially used.

However, the kick described above is 6.6 kV
A CV with a relatively short cable line length, such as a CV cable
It appears on the DC leakage current I _d of the cable but, 22KVCV
As the cable, the DC leakage current I _d of a long CV Cable Cable line length, as shown in FIG. 3, it does not appear. That is, when the cable line length is as long as several kilometers as in the case of a 22 kVC cable, the capacitance C of the insulating layer increases accordingly. Therefore, the capacitance C and the internal resistance R _{0 of the} DC power supply
The response time constant τ _s (= C · R ₀ ) of the circuit for measuring the DC leakage current I _d is increased. As a result, the DC leakage current _Id is smoothed, and no kick appears as shown in FIG. Therefore, the diagnostic method using this kick cannot diagnose the insulation deterioration state of a long cable such as a 22 kVC cable.

In view of such circumstances, Japanese Patent Application Laid-Open No. 11-27
The 1386 JP, by the magnitude of the DC leakage current I _d to observe gradually irregularly varying fluctuation components over time, a method of diagnosing the deterioration of the insulating layer is described. That is, when a DC voltage is applied to 22kVCV cable insulating layer is deteriorated, the DC leakage current I _d flowing through the insulating layer of the 22kVCV cable, Figure 3
As shown in FIG. The DC leakage current I _d, as shown in FIG. 3, the kick conventional method described above has been used for diagnosis of insulation degradation is not our table, as shown in the enlarged view of FIG. 4, 1 minute There is a minute current fluctuation component of the order of 0.1 μA, that is, a fluctuation component, which fluctuates slowly and irregularly in the period before and after.

[0006] The reason why the kick and the fluctuation component is generated in the DC leakage current I _d that is observed when the insulation by water trees is a DC voltage is applied to the CV cable deteriorated, the insulation resistance of the water tree section It is considered that it changes unstable with time. That is, the occurrence of electric trees may be observed in the electrode contact portion of the water tree.In such a case, when a discharge occurs in the electric tree portion, the resistance of the discharge portion changes before and after the discharge, and current value of the DC leakage current I _d is varied. Of the fluctuation components, a component having a relatively high frequency becomes a kick, and a frequency component having a remarkably low frequency becomes the fluctuation component. In addition, C having a long cable line
Regardless of the large capacitance C of the insulating layer of the V cable, the above-mentioned fluctuation component appears because the frequency band of this fluctuation component is very close to DC (the period is extremely long), Response time constant τ _s = C · R _o (C
When the length of the cable line of the V cable is about 1 km, the response time constant τ _s is about 10 seconds. ) Is less likely to be affected.

In any case, in the case of the second example of the conventional method described in the above publication, the above-described DC leakage current I _d
Diagnosis of insulation deterioration of the CV cable is performed by using fluctuation components existing in the cable. Specifically, the fluctuation component is observed, and the deterioration state of the insulating layer is determined based on the presence / absence and magnitude of the fluctuation component. That is, based on the presence of the fluctuation component, the existence of the deterioration of the water tree or the like, which leads to the deterioration of the insulating layer, is confirmed, and the deterioration state is confirmed based on the magnitude of the fluctuation component. Note that the relationship between the magnitude of the fluctuation component and the degree of deterioration of the insulating layer is obtained in advance by an experiment. As described above, in the case of the second example of the conventional method, it is also possible to perform a diagnosis of insulation deterioration of a CV cable having a large capacitance of an insulating layer, such as a 22 kVC cable, in which a diagnosis by kick cannot be performed.

[0008] However, in the case of CV cable capacitance C is large, such as the real line, the DC leakage current I _d flowing in the insulating layer of the CV cable, response delay (response time constant tau _s = C · measurement circuit R ₀ ) causes waveform distortion. As a result, fluctuation component of the fluctuation component such as appearing in the DC leakage current I _d becomes small is smoothed, the detection of the fluctuation component is difficult. In addition, the degree to which the fluctuation component is smoothed (decreased) increases as the response delay of the measurement circuit increases. Therefore, the magnitude of the fluctuation component is
Even when the degree of deterioration of the insulating layer is equal, the response time constant τ
_It depends on the magnitude (line length) of the capacitance C of the CV cable to be measured, which determines the magnitude of _s . Therefore, the method of observing the fluctuation component included in the DC leakage current I _d directly, it can not be performed the development of quantitative degradation criterion.

[0009] As a result of the waveform warp occurs in the DC leakage current I _d as described above, immediately after the start of measurement of the DC leakage current I _d is as shown in FIG. 3 described above, the DC leakage current I _d
Appears as the value of decay with time. However, the presence of the attenuated portion hinders detection of a fluctuation component appearing in the portion. In contrast, the value of the DC leakage current I _d is, if you change irregularly in the damping portion is large. Then, this damping part, as compared with the constant part of the DC leakage current I _d, a signal representing the information about the insulation degradation (degradation signal), it is possible to contain more of the fluctuation component. Therefore, in order to perform the insulation deterioration diagnosis reliable power cable, the fluctuation component in the DC leakage current I _d, as well as the constant part, it is preferable to observe in the damping portion. In other words,
The fluctuation component, it is preferable to like can be collected over a wide range of immediately after the start of measurement of the DC leakage current I _d.

[0010]

[Description of the Prior Invention] In order to solve the above problems,
With drift components included in the DC leakage current I _d is prevented from being smoothed in order to eliminate the damping part from the DC leakage current I _d is caused by the response delay of the measuring circuit, the DC leakage current it is necessary to remove the waveform warp of I _d fundamentally. In view of such circumstances, the present inventors have previously caused by response delay of the measurement circuit, it invented a method for removing waveform warp of the DC leakage current I _d. Hereinafter, the present invention will be described.

[0011] Figure 5 shows an equivalent circuit (measurement circuit) showing a state of measuring the DC leakage current I _d. When performing insulation deterioration diagnosis, the sample CV cable 1
A DC voltage is applied between the cable conductor and the outer peripheral surface of the insulating layer by the DC power supply 2 (electromotive force E, resistance value R ₀ of the internal resistor 3). As a result, the DC leakage current _Id flows through the insulating layer. That is, as described above, when a DC voltage is applied to the CV cable 1 (the elapsed time after the application of the DC voltage is represented by t), the sound portion 4 of the insulating layer constituting the CV cable 1 (capacitance C ) Through the charging current i ₄
(T) flows. On the other hand, in the measurement circuit, the sound part 4
A leak current i ₅ (t) of the deteriorated portion flows through a deteriorated portion ₅ (resistance value R ({R ₀ )) of a water tree or the like generated in the insulating layer and existing in parallel with the insulating layer. And these two currents i ₄
(T) and i ₅ (t) are combined and the DC leakage current I _d ｛= i ₄ (t) + i ₅ (t)｝ to be actually measured
Becomes When the DC voltage is applied, the resistance value R of the deteriorated portion 5 fluctuates unstablely with the passage of time t. Then, the resistance value R of the deteriorated part 5 in this manner is based on that fluctuates unstably, the fluctuation components such as kicks and fluctuation component generated during the DC leakage current I _d is as previously described .

First, consider a case where a step-like DC voltage E is applied to the CV cable 1 by the DC power supply 2 in the measurement circuit. In this case, since the measurement circuit has a response delay of a response time constant τ _s (≒ C · R ₀ ), the measurement signal i _o (t) representing the DC leakage current I _d.
Is expressed as in the following equation (1).

(Equation 1) The first term on the right side of the above equation (1) represents the charging current i ₄ (t) flowing through the sound part 4, and the second term on the right side represents the leak current i ₅ (t) flowing through the deteriorated part ₅ , Each is represented.

In the frame α of FIG. 6, the measurement signal i _o (t)
And the voltage V (t) between the electrodes of the CV cable 1. As is clear from the description in the frame α of FIG. 6, the measurement signal i _o (t) is immediately added to the DC voltage E,
An attenuation portion is included in which the value of the measurement signal i _o (t) attenuates with time. Further, the measurement signal i
a _o (t) is, as shown in _part P, is a degraded signal variation component is observed, the fluctuation component is smoothed by the influence of the response delay, small in size Has become. That is, the measurement signal i _o (t) undergoes waveform distortion under the influence of the response delay.

Next, regarding the measuring circuit of FIG. 5, a virtual circuit in which the resistance value R ₀ of the internal resistor 3 is set to zero will be considered.
In the case of such a virtual circuit, the response time constant τ _s (≒ C ·
R ₀ ) becomes zero, and the response delay of the circuit is eliminated. In such a virtual circuit, the above-described CV
When a step-like DC voltage E is applied to the cable 1, a signal i _i representing a DC leakage current I _d flowing through this virtual circuit is obtained.
(T) is represented by the following equation (2).

(Equation 2) In the equation (2), δ (t) is a delta function {δ (t}
0) = 0, δ (t = 0) = ∞, {δ (t) dt = 1}
u (t) is a unit step function ｛u (t <0) =
0 and u (t ≧ 0) = 1｝, respectively. or,
The first term on the right side of the above equation (2) indicates the charging current i ₄ (t) _{R0 = 0} flowing through the sound part 4, and the second term on the right side indicates the deteriorated part leakage current i ₅ (t) flowing through the deteriorated part _5. )
_{R0 = 0} is represented respectively. That is, in the case of the virtual circuit as described above, the charging current i ₄ (t) _{R0 = 0} becomes an impulse waveform in which the value when the DC voltage E is applied (t = 0) becomes infinite, and the degraded portion The leakage current i ₅ (t) _{R0 = 0} has a waveform having a constant value immediately after the application of the DC voltage E.

[0015] Within the framework β in FIG. 6 shows a signal i _i (t) representative of the DC leakage current I _d flowing through the virtual circuit, and a voltage V (t) between the electrodes the CV cable 1. The As is apparent from the description of the framework β in FIG. 6, to the signal i _i (t), the value of the signal i _i (t) is not included in the damping part for damping over time. Further, this signal i _i
A (t) is, as shown in P ₂ parts, a deterioration signal variation component is clearly manifest. That is, in the case of the virtual circuit, since there is no response delay of the circuit, fluctuation components shown in the P ₂ parts is smoothed (its value becomes smaller) that no, appears as a clear deterioration signal. In other words, the signal i _i (t) is a signal having no waveform distortion due to a response delay.

Therefore, when observing the signal i _i (t) representing the DC leakage current I _d flowing through the virtual circuit, the signal i _i (t)
_The fluctuation component, which is a degraded signal appearing in _i (t), can be collected over a wide range immediately after the start of measurement (application of the DC voltage E). For this reason, highly reliable insulation deterioration diagnosis can be performed.
Further, the above-mentioned fluctuation component clearly appears regardless of the magnitude of the capacitance C of the CV cable 1 to be measured. Because of this,
This makes it possible to establish a deterioration determination standard that is not affected by the line length of the CV cable 1.

However, in actual measurement, it is impossible to make the resistance value R ₀ of the internal resistor 3 zero, so that the DC leakage current I _d itself corresponding to the signal i _i (t) is
It cannot be measured by the measurement circuit shown in FIG. Therefore, in the case of the prior invention, the signal i _i (t) is obtained from the actually measured DC leakage current I _d {measurement signal i _o (t)} by the following method.

That is, the signal i _i represented by the above equation (2)
From (t), the measurement signal i _o expressed by the above equation (1) is obtained.
The conversion to (t) {i _i (t) → _io (t)} can be expressed by a convolution (convolution integral) by an impulse response. Specifically, the following equation (3) holds.

(Equation 3) However, in the equation (3), the impulse response function h (t)
Is represented by the following equation (4).

(Equation 4)

On the other hand, from the measurement signal i _o (t) represented by the above equation (1), the signal i _o represented by the above equation (2) is obtained.
Conversion to _i (t) (inverse conversion) ｛i _o (t) → i _i
(T)｝ can be represented by the inverse convolution relation to the above convolution. Specifically, the following equation (5) holds.

(Equation 5)

Therefore, in the case of the prior invention, when the insulation deterioration diagnosis of the CV cable 1 is performed, first, as shown in FIG.
In step 1, a DC leakage current I _d {measurement signal i _o (t)} flowing through the measurement circuit is measured by a measurement device (not shown) provided in the measurement circuit shown in FIG. And
In the next step 2, the response time constant τ _s of the measurement circuit is obtained (estimated) from the waveform of the measurement signal i _o (t).
Then, in the next step 3, these measurement signals i _o (t)
The operation (inverse conversion operation) of the above equation (5) is executed by the arithmetic unit incorporated in the measuring device using the response time constant τ _s and the signal i representing the DC leakage current I _d flowing through the virtual circuit. _i (t) is obtained. Then, in the last step 4, the fluctuation component in the signal i _i (t) obtained in this way is observed.
By observing the fluctuation component in the signal i _i (t) in this way, it is possible to perform a reliable insulation deterioration diagnosis as described above, and to establish a deterioration judgment standard independent of the line length of the CV cable 1. Becomes

[0021]

However, in many cases, the CV cable 1 embodying the above-mentioned prior invention is a so-called real line that is actually used for power transmission over a long period of time. When the above-described invention is implemented for such an actual line, the measurement signal i _o (t) includes not only a fluctuation component that becomes a degradation signal but also an insulation signal as shown in a frame α ′ in FIG. In some cases, a fluctuation component (noise) irrelevant to the information on deterioration is superimposed. Such noise is generated based on, for example, a contact failure of the power supply lead wire, and appears in the measurement signal i _o (t) without being affected by the response delay of the measurement circuit. On the other hand, in the case of carrying out the destination invention, as described above, the measurement signal i _o (t)
It is necessary to obtain (estimate) the response time constant τ _s of the measurement circuit from the waveform of Therefore, as described above, the measurement signal i _o
If (t) to the noise is superimposed is wrong estimation of the response time constant tau _s (estimated value of the response time constant tau _s deviates significantly from the actual value) is likely. When the response time constant τ _s is incorrectly estimated as described above,
The signal i _i (t) after the inverse conversion based on the calculation of the equation may have a problem such as remaining waveform distortion, and may not be able to perform a reliable insulation deterioration diagnosis.

[0022] Also, the (5) in the calculation of the equation, for carrying out differential operation on the measurement signal i _o (t), if the noise is superimposed on the measurement signal i _o (t) as described above As shown in a frame β ′ in FIG.
_{During i} (t), the noise appears greatly enhanced.
As a result, there is a possibility that the fluctuation component which becomes a degraded signal is buried in the noise emphasized as described above and becomes difficult to detect, or in a case where the fluctuation component is remarkable, the detection becomes impossible. In view of the above-described circumstances, the power cable insulation deterioration diagnosis method of the present invention emphasizes the noise superimposed on the measurement signal i _o (t) and the signal i _i (t) after the inverse conversion. The present invention has been invented in order to enable a reliable insulation deterioration diagnosis by smoothing the noise that has appeared.

[0023]

According to the method of diagnosing insulation deterioration of a power cable according to the present invention, similarly to the above-mentioned invention, when a DC voltage is applied to a power cable whose insulation layer has been deteriorated, the power cable is diagnosed. evaluating the DC leakage current I _d flowing through the insulating layer, for diagnosing the degree of deterioration of the insulating layer. Specifically, a signal i _o representing the DC leakage current I _d
From (t), the response time constant τ _s of the circuit for measuring the DC leakage current I _d is obtained. At the same time, the (inverse) conversion equation i _{i
(T) = τ s · di} o (t) / dt + i o (t) [ where, t> 0] based on the above signal i _o a (t), the signal i obtained by removing the influence of response delay with the said circuit _i (t). Then, the signal i _i after this (inverse) conversion
By observing a fluctuation component in which the magnitude of (t) changes irregularly with the passage of time t, the degree of deterioration of the insulating layer is diagnosed.

Further, in the case of the method for diagnosing insulation deterioration of a power cable according to the present invention, when observing the above-mentioned fluctuation component,
After performing at least one of the following processes, the degree of deterioration of the insulating layer is diagnosed. A process of smoothing a variation component (noise) other than a variation component serving as a signal representing information related to the deterioration of the insulating layer among the variation components included in the signal i _o (t) representing the DC leakage current I _d . A process of smoothing a variation component (noise) other than a variation component serving as a signal representing information relating to the deterioration of the insulating layer among the variation components included in the signal i _i (t) after the (inverse) conversion.

[0025]

The basic operation of the above-described method of diagnosing insulation deterioration of a power cable according to the present invention is the same as that of the above-described prior invention. In particular, according to the power cable insulation deterioration diagnosis method of the present invention, the noise included in the signal i _o (t) representing the actually measured DC leakage current I _d and the signal i _i (t) after (inverse) conversion. Can be reduced or eliminated by smoothing, respectively. Therefore, the task of estimating the response time constant tau _s of signal i _o (t) representing the DC leakage current I _d, the variation component as a degradation signal from (opposite) the converted signal i _i (t) detected Work can be performed accurately. For this reason, highly reliable insulation deterioration diagnosis can be performed.

[0026]

FIG. 1 shows a first embodiment of the present invention.
An example is shown. The feature of the present invention is that the DC leakage current I
measurement signal i _o (t) representing _d and signal i _i after inverse transformation
Variation components (noise) irrelevant to the information related to insulation deterioration included in (t) may be reduced or eliminated by smoothing, respectively. In addition, the basic operation (flow) in performing the insulation deterioration diagnosis is the same as that of the above-described prior invention. For this reason, the description of the overlapping parts will be omitted or simplified, and the following description will focus on the characteristic parts of the present invention.

In practicing the present invention, first, in step 1, the DC leakage current _Id is measured by the measuring circuit shown in FIG. The measurement signal i _o (t) representing the DC leakage current I _d has not only the fluctuation component serving as the deterioration signal but also has no relation to the information regarding the insulation deterioration, as shown in the above-mentioned box α ′ in FIG. A fluctuation component (noise) may be included. The noise generated due to a poor contact of the power supply lead wire or the like is often a steep and short-cycle fluctuation of a pulse or the like as shown in the frame α 'in FIG.

Therefore, in the case of this example, the following step 2
Then, a smoothing process of noise included in the measurement signal i _o (t) is performed. In the case of this example, as the noise smoothing processing method in step 2, a simple moving average processing that is effective for reducing steep and short-period noise as described above is employed. That is, the average value of the current values of the measurement signal i _o (t) at time t and at times t ± Δt before and after the time t is again calculated as the current value of the measurement signal i _o (t) at time t. A process is performed to set the value {displayed with y (i) in the illustrated example}. Note that, in order to sufficiently smooth only the noise without substantially smoothing the fluctuation component serving as the deterioration signal by this processing, Δt (interval time) is set as Δt (interval time). It is sufficient to select an appropriate time that is sufficiently shorter than the cycle. That is, the period of the fluctuation component (about 1 minute) which is the deterioration signal is sufficiently longer than the period of the noise (about several seconds). If this is done, it is possible to sufficiently smooth only the above-mentioned noise without almost smoothing the fluctuation component that becomes a degraded signal.

When the noise-smoothed measurement signal i _o (t) is obtained as described above, in the next step 3, the waveform of the noise-smoothed measurement signal i _o (t) is calculated as follows: The response time constant τ _s of the measurement circuit is estimated. Then, when the estimated value of the response time constant τ _s is obtained, the next step 4
Then, the measurement signal i _o (t) is inversely converted into a signal i _i (t) free from waveform distortion by performing the operation (inverse conversion operation) of the above equation (5). In the above-described inverse transform operation, the measurement signal i _o (t) after noise smoothing is used.
Perform differentiation operation on. For this reason, in the signal i _i (t) after the inverse conversion, the noise (which has not been completely eliminated by the smoothing processing in step 2) included in the measurement signal i _o (t) before the inverse conversion is included. Appears with emphasis.

Therefore, in the case of this example, the following step 5
Then, a smoothing process is performed on the noise that has been emphasized and appeared in the signal i _i (t) after the inverse conversion. In the case of this example, a method of executing convolution and integration (convolution) on the signal i _i (t) after the above-described inverse transformation is adopted as the noise smoothing processing method in step 5. That is, by executing the convolution integral with respect to the signal after the inverse transform i _i (t), gives the response delay (response time constant tau ₀₎ in the signal after the inverse transform i _i (t), this Inverted signal i
_The noise included in _i (t) is smoothed.

It should be noted that the above-mentioned convolution integral is used for execution.
Response time constant τ₀ Is the signal i after the inverse conversion._i 
The response of the measurement circuit in order not to impair the characteristics of (t)
Time constant τ _s Much smaller than (τ_o ≪τ_s ) Set
There is a need. In the case of this example, execute the above convolution integral
Response time constant τ₀ Was set to 2 seconds. This
Is because the signal i after the inverse conversion is_i (T) about 2 seconds
Response delay τ₀ Even if the
The amplitude is hardly changed, and furthermore, the signal after the above inverse conversion
i_i Smooth out noise included in (t) to make it less noticeable
To achieve this, a response delay τ of at least about 1 to 2 seconds₀ Is necessary
Is confirmed by experiments and the like.

If the noise included in the inversely transformed signal i _i (t) has been smoothed as described above, the final step 6
Then, the signal i _i (t) after the noise is smoothed (in the illustrated example, indicated by i (t)) is used to observe a fluctuation component that becomes a degraded signal to diagnose the insulation deterioration of the CV cable. Perform

According to the method for diagnosing insulation deterioration of a power cable of the present invention as described above, the measurement signal i _o of the DC leakage current I _d is measured.
(T) and noise included in the signal i _i (t) after the inverse conversion can be reduced or eliminated by smoothing, respectively. Therefore, an operation of estimating a response time constant τ _s from the measurement signal i _o (t) (smoothed noise),
The signal i _i (t) after the above-mentioned inverse conversion is obtained by smoothing noise. The operation of detecting a fluctuation component that becomes a deterioration signal from the signal i (t)} can be accurately performed. Because of this,
A reliable insulation deterioration diagnosis can be performed.

In the above-described example, the noise smoothing process is performed on both the measurement signal i _o (t) of the DC leakage current I _d and the signal i _i (t) after the inverse conversion. Was explained. However, in the case of the present invention, the measurement signal i _o
When the noise included in (t) is small, only one of the measurement signal i _o (t) and the signal i _i (t) after the inverse conversion is smoothed. It is also possible to perform a conversion process.

[0035]

Since the method for diagnosing insulation deterioration of a power cable according to the present invention is constructed and operates as described above, the insulation independent of the line length of the target power cable is performed in the same manner as in the above-mentioned prior invention. Deterioration criteria can be formulated.
In particular, in the case of the present invention, the operation of estimating the response time constant τ _s from the measurement signal i _o (t) and the signal i _i after the inverse conversion are performed.
The operation of detecting the deteriorated signal included in (t) can be performed accurately. Therefore, the reliability of the insulation deterioration diagnosis can be improved.

[Brief description of the drawings]

FIG. 1 is a flowchart showing an example of an embodiment of the present invention in the order of steps.

[Figure 2] diagram showing a DC leakage current I _d that occurs when a DC voltage is applied to the diagnosis of insulation degradation of CV cable having a relatively short cable line.

[Figure 3] diagram showing a DC leakage current I _d that occurs when a DC voltage is applied to the diagnosis of insulation degradation of CV cable having a long cable line.

FIG. 4 is a diagram showing a state in which an X portion of FIG. 3 is taken out and a current value is amplified.

Figure 5 is an equivalent circuit showing a state of measuring the DC leakage current I _d.

FIG. 6 is a flowchart showing the insulation deterioration diagnosing method of the present invention in the order of steps;

[7] signal i _o representative of the DC leakage current I _d as measured
(T) contains noise and the signal i _i after the inverse transformation
The figure which shows the state in which the noise is emphasized in (t).

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 CV cable 2 DC power supply 3 Internal resistance 4 Sound part 5 Deterioration part

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Iwao Otaka 1008 Niibori, Kumagaya-shi, Saitama Mitsubishi Cable Industry Co., Ltd. Kumagaya Works (72) Inventor Masafumi Ueno 1-3-1 Uchisaiwaicho, Chiyoda-ku, Tokyo Tokyo Within Electric Power Company (72) Inventor Satoru Koizumi 1-3-1 Uchisaiwaicho, Chiyoda-ku, Tokyo Tokyo Electric Power Company F-term (reference) 2G014 AA15 AB33 AC00 2G015 AA27 BA04 CA01

Claims (1)

[Claims] 1. A evaluates DC leakage current I _d that when a DC voltage is applied to the power cable insulating layer is deteriorated flowing through the insulating layer of the power cable, in order to diagnose the degree of degradation of the insulating layer, the signal i representative of the DC leakage current I _d
The response time constant τ _s of the circuit for measuring the DC leakage current I _d is obtained from _o (t), and the conversion equation i _i (t) = τ _s
The signal i _o (t) is converted into a signal i _i (t) from which the influence of the response delay of the circuit has been removed based on di _o (t) / dt + _io (t) [where t> 0]. When observing a fluctuation component in which the magnitude of the converted signal i _i (t) changes irregularly with the passage of time t,
A method for diagnosing the degree of deterioration of the insulation layer after performing at least one of the processes. A process of smoothing, among the fluctuation components included in the signal i _o (t) representing the DC leakage current I _d , fluctuation components other than the fluctuation component serving as a signal representing information on the deterioration of the insulating layer. A process of smoothing, among the fluctuation components included in the converted signal i _i (t), the fluctuation components other than the fluctuation component serving as a signal representing information relating to the deterioration of the insulating layer.