JPH0862280A - Diagnosis of insulation degradation of power cable - Google Patents

Abstract

PURPOSE: To diagnose insulation degradation with high reliability by measuring the time characteristic of a residual voltage under the application of an AC voltage, and diagnosing the degree of the degradation of power cable based on the changing rate of the characteristic in time. CONSTITUTION: A voltage-VDC is applied on a sample 1 from a DC power supply 2 at a time t01 by the switching of a switch SW1 under the shorted state of a switch SW2. The voltage -VDC is discharged through a resistor R0 at a time t02 . The AC-voltage receiving state is set at a time t03 . At a time t04 , the switch SW2 is opened, the application of the AC voltage on the sample 1 from an AC power supply 3 is started at a time 0, the voltage is stepped up to the voltage VAC until a time t1 and the voltage is dropped to zero from a time t2 . The DC voltage is recovered with the time in a capacitor 4 after a time t04 . The voltage is measured with a DC voltmeter 5, and the residual charge can be computed based on the measured value. Here, the voltages at the times 0-t2 are estimated based on the period of the times t04 -0 and the time- change characteristic of the voltages measured after the time t2 . The residual charge is computed with the value corrected by the voltage, and the diagnosis of the degradation is performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for diagnosing insulation deterioration of power cables such as CV cables.

[0002]

2. Description of the Related Art When an AC voltage is applied to a rubber / plastic power cable such as a CV cable in an environment where water is present for a long period of time, voids, foreign matters, protrusions, etc. in an insulator are generated. Moisture is accumulated in the electric field concentrated portion of the above, a minute water void group is formed, and this causes water tree deterioration in which the water void grows in the electric field direction. This water tree lowers the breakdown voltage as it grows, and eventually causes a breakdown of the power cable during operation. In the current diagnosis of insulation deterioration of power cables such as CV cables, the development of a method for reliably detecting this water tree deterioration has become an important issue.

As a method for diagnosing this water tree deterioration, various methods have been studied so far. Among these methods, there is a method of measuring a phenomenon related to dielectric absorption during a grounding period after applying a DC voltage. Generally, when a DC voltage is applied to a composite dielectric made of a plurality of materials having different permittivities and electric conductivities, space charges are accumulated at the interface between different materials. When the DC applied voltage is removed by grounding (short-circuiting the electrodes) after the DC voltage is applied, the space charge starts to move by the space charge electric field formed by itself, and finally disappears by recombination and the like. In the ground circuit after this DC charging,
Even after the instantaneous discharge current based on the geometrical capacity and the instantaneous polarization is attenuated, a direct current flows that decays with time as the space charge moves and disappears. This current is called an absorption current or a reverse absorption current, and the electric charge that becomes the source of the absorption current flowing in this ground circuit is called the residual charge. That is,
Of the space charges generated during DC charging, the components that move and disappear in the insulator during the grounding period correspond to residual charges.

The moving direction and moving speed of the space charge during the grounding period are determined by the direction and magnitude of the space charge electric field, the mobility of the charge, and the like. Generally, the absorption current after DC voltage charging and grounding due to the space charge formed at the interface of the composite dielectric material has a polarity opposite to that of the current when DC voltage is applied. However, when the DC applied electric field is high and space charges are formed due to charge injection from the electrodes, etc., an absorption current of the same polarity as the current during DC charging may be observed.

In the case of water tree-degraded CV cable insulation, cross-linked polyethylene (XLP
Since a group of minute water voids having a large conductivity is formed in E), it has the above-mentioned properties of a composite dielectric, and the number of water trees generated increases with the progress of deterioration. Since the volume and the length of the part also increase, the space charge generated during the direct current charging increases with the progress of deterioration. Therefore, it is possible to use the absorption current and residual charge after grounding the DC voltage as a judgment signal for deterioration diagnosis.However, since the absorption current has a characteristic that it is difficult to handle as a deterioration signal such as decay with time, it is generally used. A value obtained by integrating the absorption current with time and converting it into a residual charge is used as a determination amount for deterioration diagnosis.

As a method of measuring the residual charge after grounding the DC voltage used for diagnosing the water tree deterioration of the CV cable,
"Reverse absorption current measurement" and "residual voltage measurement" are known. In the "reverse absorption current measurement", the absorption current flowing in the ground circuit is continuously measured, and the residual charge is calculated from the time integration. In the "residual voltage measurement", the circuit that has been once grounded is opened again, a high-impedance voltmeter is inserted between the electrodes, and the voltage (called residual voltage) that recovers over time is measured. When this residual voltage is multiplied by the capacitance of the sample, the charge corresponding to the residual charge is measured.

Since the "reverse absorption current measurement" measures a current that decays with time, in order to accurately measure the time characteristic of this current, a measurement having a response much faster than the decay time constant of the current. A circuit is needed. If the current detection resistance is carelessly increased, the measurement time constant determined by the capacitance of the sample and the detection resistance becomes large, and the charge measurement result on the short time side after the start of measurement is higher than the actual value. It will be detected small. One of the reasons why the absorbed current is not used for deterioration diagnosis is that the magnitude of the current depends on such measurement circuit conditions.

On the other hand, in the "residual voltage measurement", leakage of charges from the measuring circuit becomes a problem. Usually, the measurement is performed using an electrometer or the like having a high input impedance of about 1013 Ω or more, but if the capacitance of the sample is small, leakage of charges cannot be ignored, and it is not suitable for long-time measurement. By the way, the movement of space charge in the water tree deterioration CV cable under the condition that the external electric field is removed after the direct current application.
It takes a long time to disappear. Our Water Tree Degradation C
In the experience with V cables, residual charges may still be observed even after being grounded for one month at room temperature after direct current application. Therefore, the residual charge detected within a practical measurement time range of about 10 minutes at most is a small part of the space charge actually accumulated, and the detection signal shows similar phenomena other than the water tree deterioration part. Since the residual charge and the like due to the above are included, it is difficult to diagnose deterioration of the water tree of the CV cable by "reverse absorption current measurement" or "residual voltage measurement".

As a method for solving the above-mentioned problem, an AC electric field is applied after the DC voltage is grounded, an absorption current is measured under this AC voltage application state, and this is integrated over time to detect a residual charge. Is proposed. Proponents call this method "residual charge measurement", but to be precise, it is residual charge detection by measuring the absorption current under AC voltage after DC voltage grounding (Japanese Patent Publication No. 28350/1993). See the bulletin).

As described above, since the moving speed of the space charge after the DC voltage grounding depends on the magnitude of the electric field inside the insulator, the moving speed of the charge becomes faster if a large electric field is applied from the outside, It becomes possible to measure a large absorption current in a short time. If a direct current is used as the applied electric field to accelerate the movement of the space charge at the time of grounding, it becomes difficult to distinguish the current due to this direct current application and the absorption current to be measured. Then, the absorption current is detected by removing the AC component from the detection current at this time.

In this "residual charge measurement", an AC voltage is applied after the absorption current is sufficiently attenuated in order to distinguish it from the absorption current when normal AC voltage application is not performed after DC voltage application grounding. However, the magnitude of the residual charge obtained by continuously measuring the absorption current during the AC voltage application for a predetermined time and integrating it over time, that is, the magnitude of the residual charge at the final measurement time under AC voltage application, It is used as a judgment amount for deterioration diagnosis.

[0012]

In the above-mentioned "residual charge measurement" in which an AC voltage is applied after the DC voltage is grounded, as in the case of the "reverse absorption current measurement" which does not use the conventional AC voltage,
Deterioration diagnosis is performed based on the magnitude of the residual charge at the time of completion of measurement. However, in the degradation diagnosis using only the magnitude of the degradation signal, the magnitude of the degradation signal generated from each water tree is significantly different depending on the degradation state, for example, the degradation signal is generated only when the degradation is significant. Unless there is a characteristic, reliable deterioration diagnosis cannot be performed.
Since the residual charge is slightly generated depending on a slight water tree deterioration state, it is difficult to distinguish between the occurrence of a large number of slight deteriorations and the occurrence of a small number of extreme deteriorations. Can not solve the problem.

By the way, the time characteristics of the movement and disappearance of the space charge after the DC voltage grounding depends on the conductivity and the dielectric constant of the insulator. Therefore, the time characteristic of the detected residual charge can also be used as a criterion for the water tree deterioration diagnosis. The purpose of the present invention is to
In order to solve the above-mentioned problems, not only the magnitude of the residual charge is used as a determination signal for deterioration diagnosis, but attention is paid to the time characteristic of the residual charge that appears when AC is applied after grounding by applying DC. The purpose is to judge the harmfulness of the water tree deteriorated part, which is the source of the residual charge, and to perform a highly reliable insulation deterioration diagnosis of the power cable.

[0014]

A method for diagnosing insulation deterioration of a power cable according to the present invention is similar to the "residual charge measurement" in that a DC voltage VDC is applied to a sample cable and then grounded, and then an AC voltage is applied. The residual charge when VAC is applied to the sample is measured. The AC charging start time is t = 0, the time when the AC voltage is boosted from zero to a predetermined value VAC is t = t1, and the subsequent VAC is Assuming that the time at which the measurement is held and completed is t = t2, the magnitude Q (t1) of the residual charge appearing from time t = 0 to t = t1 and the time t = t1.
Change ΔQ of residual charge during the measurement period from t to t = t2
/ Q (t1) = {Q (t2) -Q (t1)} / Q (t1
), The insulation deterioration diagnosis of the power cable is performed.

Further, in the insulation deterioration diagnosis of the power cable according to the present invention, since it is necessary to accurately measure the time characteristic of the residual charge during AC charging, the residual charge is measured as follows.
After DC voltage application, open the circuit that was once grounded again, insert a DC voltage measurement circuit with a discharge time constant that can ignore the leakage of charge during the measurement period, and connect the residual charge directly from the potential difference. taking measurement.

That is, in the conventional "residual charge measurement", since only the magnitude of the absorbed charge obtained by the AC voltage application is a problem, VAC is held from the AC voltage application start time t = 0 to perform the measurement. Absorption current ia until completion time t = t2
(T) is continuously measured, and the magnitude number 1 of the residual charge obtained by time-integrating the current from t = 0 to t = t2 is used for the deterioration diagnosis.

[0017]

[Equation 1]

However, in the method for diagnosing insulation deterioration of a power cable according to the present invention, the magnitude Q (t2) of the residual charge and the absorbed charge Q (t1) appearing by the time when the voltage boosting is completed and the alternating voltage applied thereafter are constant. The residual charge change ΔQ during the period held at the value is separated into {Q (t2) = Q (t1) +
ΔQ}, Q (t1) magnitude is used to diagnose the outline of the power cable deterioration status, and the deterioration status of the signal source that cannot be identified only by the magnitude of residual charge such as Q (t1) and Q (t2). Is to be determined from the time change characteristics of the residual charge represented by ΔQ.

By the way, in the case of performing the deterioration diagnosis based on the time change characteristic of the residual electric charge under such AC voltage application, it is necessary to accurately measure the true time characteristic. In principle, the measurement of the residual charge is almost the same as the measurement of the charge Qx charged in the electrostatic capacitance Cx. The above-mentioned residual charge detection method by measuring the absorbed charge does not directly measure the charge Qx, but measures the current when the electrodes are short-circuited to discharge this, and the charge is detected from the time integration. . Therefore, if the detection resistance to be inserted into the discharge circuit is close to zero, no particular problem will occur, but since it is actually necessary to insert a detection resistance Rs of a certain finite size, let Cx be the capacitance of the sample. , Τ = CxRs, that is, a discharge time constant, that is, a response delay of measurement to current detection occurs.

By the way, in the case of measuring the absorption current after the DC voltage is grounded under the AC voltage, it is necessary to connect a large capacity capacitor Cs in parallel with the measurement circuit in order to reduce the AC voltage applied to the measurement circuit. However, when this is viewed as an absorption current measuring circuit, Cs is in a state of being connected in parallel with the capacitance Cx of the sample, so the discharge time constant of the charge is τ.
= (Cx + Cs) Rs, and the response time constant of measurement is Cs
Is increased by

For example, when the current detection resistor Rs is 10 kΩ, the capacitance Cx of the sample is 0.1 μF, and the parallel capacitor Cs is 200 μF, τ is about 2 seconds. This time,
The current flowing in the discharge circuit due to the charge Qx charged in Cx is ix (t) = Qx / {τ × exp (-t /
τ)}, and the charge obtained by integrating this over time is Q
Since (t) = Qx {1-exp (-t / τ)},
Since the measured charge Q (t) becomes equal to the true charge Qx, there is a delay of about t = 10 seconds. Moreover, since a low-pass filter for removing an alternating current is added to the actual direct current measuring circuit, the response delay further increases. on the other hand,
If an attempt is made to reduce the detection resistance Rs in order to shorten the response time, the detection signal Vs (t) = Rs.ix (t) becomes small, which makes it difficult to measure a minute current.

On the other hand, in the residual charge measurement by "residual voltage measurement", the charge charged in the electrostatic capacitance is directly detected as the potential difference between the electrodes, so that in principle no response delay occurs. For example, when the charge Qx charged in the electrostatic capacitance Cx is measured as in the absorption current measurement described above, a large-capacity capacitor Cs is connected in parallel with the measurement circuit.
Even if the two are connected, the charge Qx has a potential difference Vs = Q.
It is measured as x / (Cx + Cs) without a response delay. That is, the residual charge at the measurement time is calculated by multiplying the measured potential difference Vs by the sum Cx + Cs of the electrostatic capacitances of the sample and the parallel capacitor.

Here, in the residual charge measurement by "residual voltage measurement", there is a problem that the magnitude of the detected voltage signal Vs is reduced to Cx / (Cx + Cs) as compared with the case where Cs does not exist. Applying to the measurement circuit condition that calculated the response sending time of the absorption current measurement, the charge Qx is 1
If the value is 00 nC, the magnitude of the detected current signal by the absorption current measurement is 0.30 mV (= 30 n
A), while it is 3.4 μV (= 0.34 nA) in 10 seconds, 0.5 mV will be measured in “residual voltage measurement”, which is “residual voltage measurement” in terms of detection sensitivity.
It can be seen that there is no problem in the residual charge detection method by.

The biggest problem of the "residual voltage measurement" method is that
Although there is a charge leakage during measurement, contrary to the case of current measurement, if the discharge time constant τ of the measurement circuit is set to a value that is sufficiently larger than the measurement time, the measurement error due to this charge leakage will be ignored. it can. For example, assuming that the continuous measurement time of the residual charge is 1 minute, if a time constant of about 500 minutes or more is selected,
99.8% or more of the residual charge generated immediately after the start of measurement is 1
After a minute, it remains in the measurement circuit without leaking until the measurement completion time.

Here, the discharge time constant τ of the measuring circuit is Rs
Since it is represented by (Cx + Cs), the large-capacity capacitor Cs existing in parallel plays a role of suppressing the leakage of charges.
That is, the increase in Cs has an advantage in charge measurement in the range where there is no problem in the voltage detection sensitivity. For example, if the capacitance Cx of the sample is 0.1 μF and there is no parallel capacitor Cs, then τ = 5
To secure 00 minutes, a resistance Rs of 3 × 10 11Ω
However, if the parallel capacitor Cs of 200 μF exists, the resistance Rs is about 150 MΩ. It is easy to secure the discharge resistance of the measurement system to this extent, and it is also possible to easily prevent charge leakage from the surface of the electrode end. In the conventional water tree deterioration diagnosis by "residual voltage measurement", the measurement was performed for a long time of about 10 minutes without considering the discharge time constant consisting of the capacitance and the leakage resistance of this measurement circuit. The charge leakage from the circuit or the electrode end was a measurement error.

As described above, the method for diagnosing insulation deterioration of a power cable according to the present invention utilizes the time characteristic of the residual charge that appears when AC power is applied after grounding DC power, as a determination signal for deterioration diagnosis. Further, the residual charge measurement is characterized by performing voltage measurement with a high input resistance without using the current measuring method used in the conventional “residual charge measurement”.

[0027]

The principle of generation of a deterioration signal in the method for diagnosing insulation deterioration of a power cable according to the present invention will be described below with reference to the drawings. FIG. 1 (A) is a model diagram of space charges formed when direct current is applied. When a negative DC voltage −VDC is applied to the water-tree deteriorated insulator, space charges are generated at the interface between the water-tree portion where the charge easily moves and the healthy portion insulator in which the electric charge is hard to move. Accumulated. Assuming that the capacitance of the sample including the deteriorated part is Cx and the ground side electrode is the reference point, the number of charges of 2 is present on the voltage side electrode of the sample in this state during DC voltage application, and the insulator Space charge QS
(T) = QSDC exists. Since the space charge acts so as to relax the electric field in the water tree portion under the direct current applied state, in the figure, positive space charge + QS (t) = + QSDC at the tip of the water tree portion generated from the ground electrode side. Shows the case where is accumulated.

[0028]

[Equation 2]

FIG. 1B shows a state in which the DC voltage application side electrodes are grounded (the electrodes are short-circuited) and the applied DC voltage is removed. At this time, the electric charge -QDC on the electrodes that was present during DC charging is instantly discharged, but if a positive space charge + QS (t) is present in the insulator, the electrodes are short-circuited. However, the charge on the electrode does not become completely zero, and when the ground side electrode is taken as the reference point, -Q1 appears on the charging side electrode.
A negative charge of (t) is induced.

This is because in order to make the potential difference between the electrodes always zero, it is necessary to induce charges on the electrodes to cancel the potential difference between the electrodes caused by the space charge. This induced charge -Q1 (t ) Is the space charge in the insulator + QS
The polarity is opposite to that of (t), and its magnitude depends on the magnitude of the space charge + QS (t) in the insulator and the position where it exists in the insulator. That is, although not shown in the figure, the induced charge due to the space charge + QS (t) appears not only at -Q1 (t) on the charging side electrode but also at the ground electrode side, and this is -Q2 (t). Then,-{Q1 (t) + Q
2 (t)} is equal to + QS (t),
In addition, the ratio of charges induced on both electrodes changes depending on the position of QS (t) existing between the electrodes.

Space charge + QS in the insulator at the same time as grounding
(T) = + QSDC starts moving due to space charge electric field or density diffusion. Now, ignoring the movement of charges due to density diffusion, the direction of the space charge electric field is reversed at the place where the center of gravity of the space charge exists, so the space charge is either in the direction of the DC voltage application side electrode or the ground side electrode. Also contains a component that moves, but the amount of electric charge that moves to the water tree side (the ground electrode side in this figure) having a large conductivity is overwhelmingly large. That is, most of the space charge + QS (t) = + QSDC moves to the side of the ground electrode in the direction opposite to that at the time of direct current application, and the induced charge -Q1 (t) on the electrode decreases with time. Therefore, the absorption current Ia (t) flows in the direction opposite to that at the time of direct current application in the ground circuit.

FIG. 1C shows a state in which the grounding is opened again after the DC voltage grounding of FIG. 1B. After the ground is opened, the circuit that supplies the electric charge to the DC voltage application side electrode is cut off. Therefore, the charge -Q1 (t) on this DC voltage application side electrode is equal to the induced charge -Q10 existing at the time when the ground is opened. It is maintained at a constant value and becomes completely independent of the movement and disappearance of the space charge QS (t). Therefore, the DC potential VS (t) of the DC voltage application side electrode in the ground open state is -Q10 on the voltage application side electrode.
And the space charge in the insulator + QS (t),
The negative potential component due to -Q10 is -V0, and + QS
If the positive potential component due to (t) is + VS + (t), the DC potential of the DC voltage application side electrode is VS (t) =-VO + VS +
(T).

The charge -Q10 on the voltage-applying side electrode is induced by the space charge + QS (t) when the ground is opened so that the DC potential difference becomes zero, and therefore the DC potential difference VS immediately after the ground is opened. (T) is zero. However, Figure 1
As in the case of (B), the space charge + QS (t) moves to the ground electrode side via the water tree deterioration portion with the passage of time, so the potential component + VS + due to + QS (t)
(T) decreases with time, and as a result, the DC potential VS (t) of the voltage-applying side electrode gradually recovers from zero immediately after the ground is opened to a negative voltage having the same polarity as when DC power is applied.

When all the space charges QS (t) finally move to the electrodes and disappear, only the charges -Q1 (t) =-Q10 on the DC voltage application side electrode remain. The charge -Q10 is a value proportional to the size QS (t) of the space charge when the ground is opened, and the DC potential VS (t) is from zero to a certain -V.
The phenomenon of recovering to the value of 0 faithfully reflects the process of space charge migration. This DC potential VS
(T) is called residual voltage.

By the way, since a capacitance such as a sound insulator exists in parallel with the water tree deterioration portion, the DC potential VS
(T) is much smaller than the potential formed by the space charge in the deteriorated portion. Therefore, although the electrodes are open,
Approximately, there is no great difference from the state where the electrodes are grounded, so the value obtained by multiplying VS (t) by the capacitance Cx of the sample is the absorption current ia (t) that flows when the electrodes are grounded.
Is almost equal to the residual charge Q (t) of the sample when is integrated.

Next, the behavior of the space charge QS (t) when an AC electric field is applied between the electrodes in the state of FIG. 1C will be described. When an AC electric field that changes positively and negatively much faster than the moving speed of the space charge QS (t) due to the space charge electric field is applied, the direct current position (center of gravity) of the space charge in the insulator changes depending on the AC electric field. Therefore, the moving direction of the space charge QS (t) is the same as that in the case where only the space charge electric field exists. However, the AC electric field effectively reduces the energy barrier in which the charges are trapped and increases the mobility of the charges, and therefore has the effect of increasing the moving speed of the space charges.

Therefore, when an AC electric field is applied between the electrodes without discharging the charge Q1 (t) on the electrodes in the state of FIG. 1C, the moving speed of the space charge QS (t) increases. As a result, the observed DC potential VS (t) reaches a constant value V0 = Q10 / Cx faster than when the AC electric field is not applied. As a result, a large residual charge Q (t) = VS in a shorter time than when the AC electric field is not applied.
Since (t) · Cx can be observed, the influence of other error factors can be significantly reduced.

As described above, the time characteristic of the residual charge Q (t) is due to the movement and extinction characteristic of the space charge in the insulator, and this time change depends on the conductivity and the inductivity of the water tree deteriorated portion. Dependent. When an AC electric field is applied, the effective charge mobility is increased by lowering the energy barrier where the charges are captured, and the conductivity is increased. Considering that the effect of increasing the conductivity due to the AC electric field increases in the deteriorated portion where the deterioration progresses, it can be considered that the time when the DC potential VS (t) recovers becomes faster as the water tree deteriorates significantly. Considering this, as the time characteristics of the residual charge when an AC electric field is applied after the DC voltage is grounded, the time until the residual charge saturates to a constant value may become shorter as the deterioration becomes more remarkable.

FIG. 2 shows the estimation of the relationship between the time characteristic of the residual charge and the deterioration state of the sample when an AC electric field is applied. In the case where the residual charge recovers slowly (a). It can be considered that the state of deterioration of the sample is small, and that the water tree deterioration is remarkable in the order of (a) to (c) where the recovery time is short. By the way, in the above explanation of the polarity of the residual charge, it is stated that the polarity of the observed charge is determined by the movement direction of the space charge after the ground is opened. It was explained that a polar residual charge was observed, but when the deterioration progressed significantly, even after the transfer of the charge through the inside of the water tree was completed, the charge slowly moved in the opposite direction in the healthy insulator part. It is also assumed that the charge that moves with is left. That is, if the water tree deteriorated portion is remarkably deteriorated during DC power application, the DC electric field applied to the sound portion insulator existing in series with the water tree portion is remarkably increased, and a part of the space charge is on the sound portion insulator side. There is a possibility that charges may move in the opposite direction to the above consideration due to deep injection or homo injection from the electrode on the insulator side of the sound part. Therefore, as shown in (d) of FIG. 2, when the deterioration is significant, there is a possibility that a residual charge shows a maximum value and then decays, and in some cases, a time characteristic such that the polarity of the charge is reversed. is there.

[0040]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below based on the illustrated embodiments. FIG. 3 shows a measuring circuit for diagnosing insulation deterioration of a power cable according to the present invention. In the figure, 1 is a sample cable having a capacitance of Cx, and 2 is a DC power supply. Reference numeral 3 denotes an AC power supply for power supply, and a capacitor 4 having a capacitance of CS is connected in series with this AC power supply for power supply. 5 is a DC voltmeter with a large input resistance for measuring the residual charge. This DC voltmeter 5 is a capacitor 4
And are connected in parallel. SW1 is a changeover switch for applying a direct current to the sample, grounding it, and then applying an alternating voltage thereto. SW2 is the capacitor 4
This switch SW2 is for short-circuiting
Plays the role of shorting and opening the measuring circuit. Reference numeral 8 is a gap arrester for protecting the measurement circuit from an abnormal voltage generated when the sample is broken during measurement.

The magnitude of the electrostatic capacity CS of the capacitor 4 is such that when the AC voltage VAC is applied to the sample by the operation described later, the AC voltage Cx VAC /
(Cx + CS) is selected to be a value below the allowable input voltage, and the input resistance R of the DC voltmeter 5 is τ = (C
The electromagnetic radiation constant τ of the measuring circuit represented by x + CS) R is selected to be a value about 500 times the voltage measuring time or more.

In FIG. 3, first, with the switch SW2 short-circuited, the changeover switch SW1 is turned on at time t = t01.
, And a DC voltage −VDC is applied to the sample 1 from the DC power supply 2 for power supply. At time t = t02 after applying DC-VDC to the sample 1 for a predetermined time, the changeover switch SW1 is turned on.
Switching to the side, DC-VDC is discharged through the resistor R0. The resistor R0 is for the purpose of preventing abnormal voltage generation during DC discharge.

At time t = t03 after the DC voltage -VDC is completely discharged, the changeover switch SW1 is changed over to the c side so that the AC voltage can be applied to the sample 1. In this state, the DC resistance of the internal impedance of the AC power supply 3 is close to zero, so that when viewed as a DC circuit, the sample 1 is connected in parallel with the large-capacity capacitor 4. In addition, since the short-circuit switch SW2 of the measuring circuit is short-circuited, the sample 1 is grounded.

Next, at a time t = t04 after a lapse of a predetermined time from the grounding time t = t02 after the DC voltage application, the short circuit switch SW2 of the measuring circuit is opened. In this state, when viewed as a DC circuit, the sample 1 is connected in parallel with the large-capacity capacitor 4 and the electrodes are open. Therefore, after the time t = t04 when the switch SW2 is opened, the electrostatic capacitance becomes C
Since the DC voltage VS (t) recovers to the S capacitor 4 with the passage of time, if this voltage is measured by the DC voltmeter 5, the residual charge can be obtained as VS (t) × (CS + Cx).

In the above measurement state, the short-circuit switch S
At a time t = 0 when a predetermined time has elapsed from the time t = t04 when W2 is opened, an AC voltage is applied to the sample 1 using the AC power supply 3 for electric power supply. As the method of applying the AC voltage, time t
= 0 to time t = t1 during t1 time, the AC voltage is boosted from zero to the specified VAC, and then VAC is held for a time (t2-t1) from time t = t1 to t = t2 and then the voltage is restored again. Step down to zero.

FIG. 4 is a schematic diagram showing the applied voltage Vx to the sample cable 1 and the DC voltage VS (t) measured when the above-mentioned operation is performed in the circuit of FIG. Under the condition that the AC voltage is not applied from time t = t04 to t = 0, the DC voltage VS (t) = Vs0 (t) having a large time constant is applied between the electrodes of the capacitor 4 having the capacitance CS. It gradually recovers. After that, from time t = 0 to t = t1
If the AC voltage is boosted from zero to VAC in a short time, the DC voltage VS (t) increases dramatically. After that, during the period from t = t1 to t = t2 in which the AC voltage is held at VAC, the change characteristic of VS (t) varies depending on the deterioration state of the sample cable 1, and as shown in FIG. As it becomes more remarkable, the change rate of VS (t) ΔVS / VS
(T1) = {VS (t2) -VS (t1)} / VS (t
1) becomes small and ΔVS is negative when the deterioration is extreme,
That is, VS (t) may start to decrease.

By the way, the DC voltage VS (t) measured during AC voltage application includes a voltage component VS0 (t) which is recovered when AC voltage is not applied. This voltage component VS0 (t) includes a deterioration signal generated by the deteriorated insulator, but most of it is irrelevant to the deterioration condition of the insulator, for example, accumulated on the end surface of the sample cable 1. There is a strong possibility that it is a component that is caused by the space charge. That is, the voltage VS (t) between the electrodes of the capacitor 4 when the AC voltage is applied.
Voltage component V when AC voltage is not applied during the measurement result of
If S0 (t) is included, it becomes an error factor in the deterioration diagnosis, so it is necessary to remove this voltage component VS0 (t) from the measurement result VS (t).

Therefore, in the insulation deterioration diagnosis according to the present invention, the period from time t = t04 to t = 0 and t = t2.
Estimating VS0 (t) from t = 0 to t = t2 from the time change characteristic of the DC voltage VS (t) = VS0 (t) of the capacitor CS measured under the condition that the alternating current is not applied thereafter,
A value obtained by subtracting VS0 (t) from the measured value VS (t) during AC power supply is used for deterioration diagnosis. That is, the true measurement result at time t = t1 when the AC voltage is boosted to VAC is VS1 = VS
(T1) -VS0 (t1), and the true measurement result at the final measurement time t = t2, when the AC voltage was held at a constant value of VAC, was VS2 = VS (t2) -VS0 (t2). , The residual charge Q (t1) = VS1 × (CS + Cx) increased by AC charging from these values, and Q (t2) = V
S2 × (CS + Cx) is calculated, and the magnitude of Q (t1) and Δ
(T) / Q (t1) = {Q (t2) -Q (t1)} / Q
The deterioration condition of the cable is diagnosed from the value of (t1). In consideration of the need to normalize the electrode length of the sample cable 1, the residual charge Q (t1) immediately after boosting the AC voltage is
Q (t1) / Cx divided by the capacitance of sample cable 1
It is used for deterioration determination as [V].

Table 1 shows an example of the results of measuring the residual charges of the 22 kVCV cable and the 6.6 kVCV cable which have been deteriorated by the water tree using the above-mentioned measuring method. Samples 1 to 8 are 22 kVC in which the internal semiconductive layer has an extruded structure.
The V cables, and Samples 9 and 10 are 6.6 kVCV cables in which the semiconductive layer has a tape structure. Sample 1
Is a sample that is not substantially deteriorated, and in sample 10, there is a water tree in a significantly deteriorated state that has developed between the electrodes just before the bridging.

[0050]

[Table 1]

As shown in Table 1, the magnitude Q (t1) of the residual charge observed immediately after boosting the AC voltage tends to increase as the AC breakdown voltage decreases. Shows an increasing tendency.
However, it is recognized that the breakdown voltage does not always decrease remarkably even when the residual charge is large, or conversely, the breakdown voltage significantly decreases even when the residual charge is small. This is mainly due to the difference in the number of water trees existing in the sample cable.

On the other hand, {Q (t2) -Q (t1)} / Q
The time characteristic of the residual charge under AC voltage application, which is represented by (t1), shows a very good correlation with the decrease of the breakdown voltage of the sample. As the breakdown voltage decreases, {Q (t2) -Q
(T1)} / Q (t1) decreases, and in the case of the sample 10 in a significantly deteriorated state, {Q (t2) -Q (t1)} / Q
(T1) has changed to a negative value. That is, the time-varying characteristic of the residual charge under AC voltage accurately represents the deterioration state of the sample, and by using this characteristic, highly reliable water tree deterioration diagnosis can be performed.

In the "residual charge measurement" in which the residual charge is obtained from the conventional absorption current measurement, the residual charge Q (t2 at time t2
) Is used for deterioration determination, the results shown in Table 1 show that {Q (t2) -Q (t1)} / Q (t1).
The larger the value, the more the deterioration has not progressed, which means that the deterioration diagnosis based on the size of Q (t2) is worse than the deterioration diagnosis based on the size of Q (t1). Therefore, in the deterioration diagnosis according to the present invention, Q (t1) is adopted as the deterioration judgment based on the magnitude of the residual charge because it is preferable that the time is short after the start of the AC voltage application.

In this detailed description, the DC applied voltage VDC has a negative polarity, but it does not matter if it has a positive polarity. Further, in the measurement circuit example of FIG. 3, an example in which a DC voltage measurement circuit (capacitor 4, DC voltmeter 5) is inserted between the AC power supply 3 for power supply and the ground is shown.
Even if the circuit inserted between the ground and the ground is changed, the polarity of the observed voltage is only apparently reversed and there is essentially no problem.

[0055]

As described above, the method for diagnosing insulation deterioration of a power cable according to claim 1 accurately measures the time characteristic of the residual charge when an AC voltage is applied after the DC voltage is grounded. By using the time change rate of the residual charge as the determination signal for the deterioration diagnosis, a small number of extreme deteriorations that could not be determined by the conventional deterioration diagnosis using only the magnitude of the residual charge at a certain time elapsed time can be performed. It is also possible to achieve reliable and reliable water tree deterioration diagnosis of power cables.

Further, as a method of detecting the residual charge under the AC voltage application after the DC voltage application and grounding according to claim 2, the DC voltage between the electrodes of the sample cable is charged with the DC voltage between the electrodes being opened. The leakage of electric current is measured within a time range that can be ignored, and this significantly eliminates the response delay of the measuring circuit, which is a problem with the conventional method for detecting residual charge by measuring the absorption current flowing in the ground circuit, and AC It can be possible to accurately detect the time characteristic of the residual charge under.

[Brief description of drawings]

FIG. 1 is an explanatory diagram regarding generation of a residual charge and an absorption current after a DC voltage is applied.

FIG. 2 is an explanatory diagram of a time characteristic of a residual charge due to DC voltage application when an AC electric field is applied.

FIG. 3 is a measurement circuit diagram for measuring residual charge under AC voltage application after DC voltage application grounding.

FIG. 4 is a time chart schematically showing a procedure of applying a voltage to a sample and a measured DC voltage.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inaka Sakamoto 1008 Shinbori, Kumagaya, Saitama Prefecture Mitsubishi Cable Industries, Ltd. Kumagaya Manufacturing Co., Ltd. (72) Inventor Masayoshi Nakagawa 1008, Shinbori, Kumagaya, Saitama Mitsubishi Cable Industries, Ltd. Within

Claims (2)

[Claims] 1. A method of diagnosing insulation deterioration of a power cable by grounding after applying a DC voltage to the power cable and then measuring the residual charge generated by the DC voltage in the state of applying an AC voltage. A method for diagnosing insulation deterioration of a power cable, which comprises measuring a time characteristic of a residual charge under application of an AC voltage and diagnosing a degree of insulation deterioration of the power cable based on a time change rate of the residual charge. 2. In a state in which the electrodes are opened in a DC circuit,
The method of diagnosing insulation deterioration of a power cable according to claim 1, wherein the DC voltage between the electrodes of the sample cable is measured within a time range significantly shorter than the discharge time constant of the measurement circuit.