JP3184712B2 - Diagnosis method for insulation deterioration of power cable - Google Patents

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 a power cable such as a CV cable.

[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 matter, protrusions, etc. in an insulator may occur. The water accumulates in the electric field concentration portion of the substrate, and a minute water void group is formed, and this grows in the direction of the electric field, causing water tree deterioration. This water tree, as it grows, lowers the breakdown voltage and ultimately causes a breakdown of the power cable during operation. In the current diagnosis of insulation deterioration of power cables such as CV cables, development of a method for detecting the water tree deterioration with high reliability has become an important issue.

As a method of diagnosing the 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 ground period after DC voltage application. Generally, when a DC voltage is applied to a composite dielectric made of a plurality of materials having different dielectric constants and conductivity, space charges are accumulated at the interface between different materials. When the DC applied voltage is removed by grounding (short between the electrodes) after the completion of the DC application, the space charge starts to move due to the space charge electric field formed by itself and finally disappears due to recombination or the like. The grounding circuit after this DC charging
Even after the instantaneous discharge current based on the geometric capacity and the instantaneous polarization has attenuated, a DC current that attenuates with time flows as the space charge moves and disappears. This current is called an absorption current or a reverse absorption current, and a charge serving as a source of the absorption current flowing through this ground circuit is called a residual charge. That is,
Among the space charges generated during the DC application, the component that moves and disappears in the insulator during the grounding period corresponds to the residual charge.

The direction and speed of movement of space charges during the grounding period are determined by the direction and magnitude of the space charge electric field, the mobility of charges, and the like. Generally, the absorption current after the DC voltage application grounding based on the space charge formed at the interface of the composite dielectric material has the opposite polarity to the current at the time of the DC application. However, when a DC applied electric field is high and a space charge is formed by charge injection from an electrode or the like, an absorption current having the same polarity as the current at the time of DC application may be observed.

[0005] In the case of water tree aged CV cable insulation, a crosslinked polyethylene (XLP) with very low conductivity is used.
Since a group of minute water voids having a large electric conductivity is formed during E), it has the above-mentioned composite dielectric properties, and the number of generated water trees increases with the progress of deterioration. Since the volume and length of the part also increase, the space charge generated during the DC application increases as the deterioration progresses. Accordingly, although the absorption current and residual charge after DC application grounding can be used as a determination signal for deterioration diagnosis, the absorption current has characteristics that are difficult to handle as a deterioration signal such as attenuating with time. The value obtained by integrating the absorption current with respect to time and converting it into the residual charge is used as the determination amount for the deterioration diagnosis.

[0006] As a method of measuring the residual charge after DC charging and grounding used for water tree deterioration diagnosis of 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 once grounded is opened again, a high impedance voltmeter is inserted between the electrodes, and a voltage (residual voltage) which recovers with time is measured. When the residual voltage is multiplied by the capacitance of the sample, a charge corresponding to the residual charge is measured.

In the "reverse absorption current measurement", a current that decays with time is measured. To accurately measure the time characteristic of this current, a measurement having a response much faster than the current decay time constant is required. Circuit is required. If the current detection resistor is carelessly increased, the time constant of the measurement determined by the capacitance of the sample and the detection resistance will increase, and the charge measurement result on the short time after the start of measurement will be smaller than the actual value. It is detected small. One of the reasons that the absorption current is not used for the deterioration diagnosis is that the magnitude of the current depends on such measurement circuit conditions.

[0008] On the other hand, in the "residual voltage measurement", there is a problem of charge leakage from the measurement circuit. Normally, measurement is performed using an electrometer or the like having a high input impedance of about 1013 Ω or more. However, if the capacitance of the sample is small, leakage of electric charge cannot be ignored, and is not suitable for long-time measurement. By the way, movement of space charge in the water tree deteriorated CV cable under the condition where the external electric field is removed after DC application
It takes a long time to disappear. Deterioration of water tree by the present inventors C
In the experience with V-cables, residual charge may still be observed even after grounding for one month at room temperature after DC precharge. Therefore, the residual charge detected within a practical measurement time range of at most about 10 minutes is only a small part of the actually stored space charge, and the detected signal includes a similar phenomenon other than the water tree deterioration part. This makes it difficult to diagnose the water tree deterioration of the CV cable by "measuring reverse absorption current" or "measuring residual voltage".

As a method for solving the above-mentioned problem, a method of applying an AC electric field after DC application grounding, measuring an absorption current under this AC application state, integrating the time, and detecting a residual charge. Has been proposed. The proposers call this method "residual charge measurement", but to be precise, it is a residual charge detection by measuring an absorption current under an AC power supply after a DC power supply is grounded (Japanese Patent Publication No. 5-28350). Gazette).

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

In this "residual charge measurement", an AC voltage is applied after the absorption current is sufficiently attenuated in order to distinguish it from an absorption current in a case where a normal AC charging is not performed after the DC charging and grounding. Then, the absorption current during the AC power application period is continuously measured for a predetermined time, and the magnitude of the residual charge obtained by time-integrating the absorption current, that is, the magnitude of the residual charge at the final measurement time under the AC power application is calculated. It is used for the judgment amount of deterioration diagnosis.

[0012]

In the above-described "residual charge measurement" in which an AC voltage is applied after the DC voltage application and grounding, as in the case of the conventional "reverse absorption current measurement" without using the AC voltage application,
Deterioration diagnosis is performed based on the magnitude of the residual charge at the measurement completion time. However, in the degradation diagnosis using only the magnitude of the degradation signal, for example, 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. As long as there is no characteristic, highly reliable deterioration diagnosis cannot be performed.
Since the residual charge is slightly generated due to the slight water tree deterioration state, it is difficult to distinguish between a large number of small deteriorations and a small number of extreme deteriorations. Problems cannot be solved.

By the way, the time characteristics of the movement and disappearance of the space charge after DC grounding depend on the conductivity and the dielectric constant of the insulator. Therefore, the time characteristic of the detected residual charge can also be a judgment material for water tree deterioration diagnosis. The purpose of the present invention is
In order to solve the above problem, instead of using only the magnitude of the residual charge as the determination signal for the deterioration diagnosis, attention is paid to the time characteristic of the residual charge that appears at the time of AC power application after DC power application grounding. The purpose of this method is to determine the harmfulness of the water tree deterioration part, which is the source of the residual charge, and to perform a highly reliable insulation deterioration diagnosis of the power cable.

[0014]

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

Further, in the insulation deterioration diagnosis of the power cable according to the present invention, it is necessary to accurately measure the time characteristic of the residual charge during the AC power application.
After applying DC power, open the circuit once grounded again, insert a DC voltage measurement circuit between the electrodes with a discharge time constant that can ignore charge leakage during the measurement period, and directly continue the residual charge from the potential difference. Measure.

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

[0017]

(Equation 1)

However, in the method for diagnosing insulation deterioration of a power cable according to the present invention, the magnitude of the residual charge Q (t2) and the absorbed charge Q (t1) appearing before the completion of the voltage boosting and the subsequent AC applied voltage are kept constant.変 化 Q (t2) = Q (t1) +
From the magnitude of ΔQ｝ and Q (t1), an outline of the degradation state of the power cable is diagnosed, and further, the degradation state of the signal source which cannot be determined only by the magnitude of the residual charge such as Q (t1) or Q (t2). Is to be determined from the time variation characteristic of the residual charge represented by ΔQ.

Incidentally, when performing a deterioration diagnosis based on the time change characteristic of the residual charge under such an AC voltage application, it is necessary to accurately measure the true time characteristic. The measurement of the residual charge is basically the same as the measurement of the charge Qx charged in the capacitance Cx. The above-described residual charge detection method based on absorption charge measurement does not directly measure the charge Qx, but measures the current when the electrodes are short-circuited and discharged, and detects the charge from the time integration. . Therefore, if the detection resistor inserted into the discharge circuit is close to zero, there is no particular problem. However, actually, it is necessary to insert a detection resistor Rs having a certain finite size. , Τ = CxRs, that is, a measurement response delay to current detection occurs.

When measuring the absorption current after the DC voltage is applied and the AC current is applied, 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 considered as an absorption current measurement circuit, Cs is connected in parallel with the capacitance Cx of the sample, and the discharge time constant of the charge is τ
= (Cx + Cs) Rs, and the response time constant of the measurement is Cs
Increase by.

For example, when the current detection resistor Rs is 10 kΩ, the sample capacitance Cx is 0.1 μF, and the parallel capacitor Cs is 200 μF, τ is about 2 seconds. At 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 time integration is Q
(T) = Qx {1-exp (-t / τ)},
In order for the measured charge Q (t) to be equal to the true charge Qx, a delay of about t = 10 seconds occurs. Further, since a low-pass filter or the like for removing AC current is added to the actual DC current measurement 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, so that it becomes difficult to measure a minute current.

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

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 conditions for calculating the response sending time of the absorption current measurement of
Assuming that the current is 00 nC, the magnitude of the detected current signal obtained by the absorption current measurement is 0.30 mV (= 30 n
A) Also, while it is 3.4 μV (= 0.34 nA) in 10 seconds, 0.5 mV is measured in “residual voltage measurement”, and “residual voltage measurement” is considered from the viewpoint of detection sensitivity.
It can be seen that there is no problem in the residual charge detection method by

The biggest problem with the "residual voltage measurement" technique is that
Although there is 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 sufficiently large value compared to the measurement time, the measurement error due to this charge leakage is ignored. it can. For example, if 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
It remains in the measurement circuit without leaking until the measurement completion time after one minute.

Here, the discharge time constant τ of the measuring circuit is Rs
Since it is expressed by (Cx + Cs), the large-capacity capacitor Cs existing in parallel plays a role of suppressing the leakage of electric charge.
That is, as long as there is no problem in the voltage detection sensitivity, an increase in Cs has an advantageous effect on charge measurement. For example, when the capacitance Cx of the sample is 0.1 μF and there is no parallel capacitor Cs, τ = 5
To secure 00 minutes, the resistance Rs of 3 × 1011Ω is required.
However, if a parallel capacitor Cs of 200 μF is present, the resistance Rs is only required to be about 150 MΩ. It is easy to secure the discharge resistance of the measurement system to such a degree, and it is also possible to easily prevent charge leakage from the surface of the electrode end. In the conventional water tree deterioration diagnosis using "residual voltage measurement", measurement was performed for a long time of about 10 minutes without considering the discharge time constant consisting of the capacitance and leakage resistance of this measurement circuit. Leakage of electric charge from the circuit or the end of the electrode resulted in a measurement error.

As described above, the method of diagnosing insulation deterioration of a power cable according to the present invention utilizes the time characteristic of residual charge appearing during AC power application after DC power application grounding as a determination signal for deterioration diagnosis, The residual charge measurement is characterized in that voltage measurement with high input resistance is performed without using the current measurement 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. 1A is a model diagram of space charges formed during DC power application. When a negative DC voltage -VDC is applied to the water tree deterioration insulator, space charge is generated at the interface between the water tree part where the charge easily moves and the healthy part insulator that exists in series and where the charge is difficult to move. Stored. Assuming that the capacitance of the sample including the deteriorated portion is Cx and the ground side electrode is a reference point, in this DC application state, 2 electric charges exist on the sample side electrode of the sample. Inside is the space charge QS
(T) = QSDC exists. Under the DC charging state, this space charge acts to reduce the electric field in the water tree portion. Therefore, in the figure, a positive space charge + QS (t) = + QSDC is applied to the tip of the water tree portion generated from the ground electrode side. Is stored.

[0028]

(Equation 2)

FIG. 1B shows a state in which the DC power supply side electrode is grounded (the electrodes are short-circuited) and the applied DC voltage is removed. At this time, the electric charge -QDC on the electrodes existing during the DC application is instantaneously discharged. However, if the positive space charge + QS (t) exists in the insulator, the electrodes are short-circuited. However, the electric charge on the electrode does not become completely zero, and when the ground electrode is used as a reference point, -Q1
A negative charge (t) is induced.

This is because in order to make the potential difference between the electrodes always zero, it is necessary to induce a charge on the electrodes to cancel the potential difference between the electrodes caused by the space charge, and this induced charge −Q 1 (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 on -Q1 (t) on the power-supplying side electrode but also on the ground electrode side, which is referred to as -Q2 (t). Then,-｛Q1 (t) + Q
2 The magnitude of (t)｝ is equal to the magnitude of + QS (t),
In addition, the ratio of the charge induced on both electrodes changes depending on the position of QS (t) existing between the electrodes.

At the same time as grounding, the space charge in the insulator + QS
(T) = + QSDC starts moving due to space charge electric field and density diffusion. Now, ignoring the charge transfer due to density diffusion, the direction of the space charge electric field is reversed at the location where the center of gravity of the space charge exists, so the space charge is either in the direction of the DC charging side electrode or the ground side electrode. However, the amount of charge that moves to the water tree side (the ground electrode side in this figure) having high conductivity is overwhelmingly increased. That is, most of the space charge + QS (t) = + QSDC moves to the ground electrode side in the opposite direction to that during the DC application, and the induced charge -Q1 (t) on the electrode decreases with time. Therefore, an absorption current Ia (t) flows through this ground circuit in a direction opposite to that during the DC application.

FIG. 1C shows a state where the ground is opened again after the DC power application ground of FIG. 1B. After the grounding is released, the circuit for supplying the charge on the DC charging side electrode is cut off. Therefore, the charge -Q1 (t) on the DC charging side electrode is the same as that of the induced charge -Q10 existing at the time of the ground release. It is kept at a constant value and does not depend at all on the movement or disappearance of the space charge QS (t). Therefore, the DC potential VS (t) of the DC charging side electrode in the ground-open state is -Q10 on the charging side electrode.
And the space charge in the insulator + QS (t),
The negative potential component due to -Q10 is -V0, and + QS
Assuming that the positive potential component due to (t) is + VS + (t), the DC potential of the DC charging side electrode is VS (t) =-V0 + VS +
(T).

Since the electric charge -Q10 on the power supply side electrode is induced so that the DC potential difference becomes zero by the space charge + QS (t) when the ground is opened, the DC potential difference VS immediately after the ground is opened. (T) is zero. However, FIG.
Similarly to the case of (B), since the space charge + QS (t) moves to the ground electrode side through the water tree deterioration part with the passage of time, the potential component + VS + by + QS (t) is obtained.
(T) decreases with time, and as a result, the DC potential VS (t) of the power receiving side electrode gradually recovers from zero immediately after the ground is opened to a negative voltage having the same polarity as that at the time of DC power application.

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

By the way, since a capacitance such as a sound insulator exists in parallel with the water tree deterioration part, the DC potential VS
(T) is much smaller than the potential formed by the space charge in the deteriorated portion. Therefore, although the gap between the electrodes is open,
Approximately, there is not much difference from the state where the electrodes are grounded. Therefore, the value obtained by multiplying this VS (t) by the capacitance Cx of the sample is the absorption current ia (t) flowing when the electrodes are grounded.
Is approximately equal to the residual charge Q (t) of the sample when the integration is performed.

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 positive and negative much faster than the moving speed of the space charge QS (t) due to the space electric field is applied, the DC existence position (center of gravity) of the space charge in the insulator changes due to the AC electric field. Therefore, the moving direction of the space charge QS (t) is not different from 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 thus 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 charges QS (t) increases. As a result, the observed DC potential VS (t) arrives at a constant value V0 = Q10 / Cx faster than when no AC electric field is applied. As a result, a larger residual charge Q (t) = VS in a shorter time than when no AC electric field is applied.
Since (t) · Cx can be observed, it is possible to greatly reduce the influence of other error factors.

As described above, the time characteristic of the residual charge Q (t) is due to the movement and extinction characteristics 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 increases due to, for example, lowering the energy barrier from which the charges have been captured, and the conductivity increases. Considering that the effect of increasing the conductivity due to the AC electric field increases as the deterioration proceeds, it can be considered that the time when the DC potential VS (t) recovers becomes faster as the water tree deteriorates more remarkably. Considering this, as for the time characteristic of the residual charge when an AC electric field is applied after the DC power application and grounding, there is a possibility that the time required for the residual charge to be saturated to a constant value becomes shorter as the deterioration becomes more remarkable.

FIG. 2 shows an 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. It can be considered that the deterioration of the water tree becomes remarkable in the order from (a) to (c), in which the progress of deterioration of the sample is small and the time for recovery is short. By the way, in the above description regarding the polarity of the residual charge, it is stated that the polarity of the observed charge is determined by the moving direction of the space charge after the ground is opened.In the case of normal water tree deterioration, it is the same as the DC applied voltage. Although it was explained that polar residual charge was observed, if the deterioration progressed significantly, even after the transfer of charge through the water tree was completed, it would slowly flow through the healthy insulator in the opposite direction. It is also assumed that the moving electric charge is left. That is, if the water tree deteriorated part is significantly deteriorated during the DC power application, the DC electric field applied to the healthy part insulator existing in series with the water tree part increases significantly, and a part of the space charge is reduced to the sound part insulator side. There is a possibility that the charges may be injected deeply or homo-charges may be injected from the electrode on the healthy part insulator side, and the charges may move in the opposite direction to the above consideration. Therefore, as shown in FIG. 2 (d), when the deterioration is remarkable, there is a possibility that a time characteristic in which the residual electric charge exhibits a maximum value and then attenuates, and the polarity of the electric charge is reversed in some cases. is there.

[0040]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the illustrated embodiments. FIG. 3 shows a measurement 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 a power supply for AC power supply, and a capacitor 4 having a capacitance of CS is connected in series with the power supply for AC power supply. Reference numeral 5 denotes a DC voltmeter having a large input resistance for measuring the residual charge.
And are connected in parallel. SW1 is a changeover switch for applying DC to the sample, grounding the sample, and thereafter applying an AC voltage. SW2 is a capacitor 4
Is a switch for short-circuiting the switch SW2.
Serves to short and open the measurement circuit. Reference numeral 8 denotes a gap arrester for protecting the measurement circuit from an abnormal voltage generated when the sample is destroyed during the measurement.

The magnitude of the capacitance CS of the capacitor 4 is such that when an AC voltage VAC is applied to the sample by an operation described later, the AC voltage Cx VAC /
(Cx + CS) is selected to be a value not more than the allowable input voltage, and the input resistance R of the DC voltmeter 5 is τ = (C
x + CS) The discharge magnetic constant τ of the measurement circuit represented by R is selected so as to be about 500 times or more the voltage measurement time.

In FIG. 3, first, with the switch SW2 short-circuited, the switch SW1 is set to a at time t = t01.
And a DC voltage -VDC from the DC power supply 2 is applied to the sample 1. At time t = t02 after applying DC-VDC to the sample 1 for a predetermined time, the switch SW1 is set to b.
To discharge the DC-VDC through the resistor R0. This resistor R0 is for preventing an abnormal voltage from being generated at the time of DC discharge.

At time t = t03 after the DC voltage -VDC has been completely discharged, the switch SW1 is switched to the c side so that an AC voltage can be applied to the sample 1. Under this state, the DC resistance of the internal impedance of the AC power supply 3 is almost zero, so that when viewed as a DC circuit, the sample 1 is connected in parallel with the large-capacity capacitor 4. , And the sample 1 is grounded because the short-circuit switch SW2 of the measuring circuit is short-circuited.

Next, at a time t = t04 after a predetermined time has elapsed from the grounding time t = t02 after the DC power 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. Accordingly, after time t = t04 when the switch SW2 is opened, the capacitance becomes C
Since the DC voltage VS (t) recovers in 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 time t = 0 when a predetermined time has elapsed from time t = t04 when W2 was released, an AC voltage is applied to sample 1 using AC power supply 3. As a method of applying the AC voltage, the time t
= 0, the AC voltage is raised from zero to a prescribed VAC during a time t1 from time t = t1, and after maintaining VAC for a time (t2-t1) from time t = t1 to t = t2, the voltage is again increased. Is reduced to zero.

FIG. 4 is a schematic diagram showing the applied voltage Vx to the sample cable 1 and the measured DC voltage VS (t) when the above operation is performed in the circuit of FIG. Under the condition where no AC power is applied from time t = t04 to t = 0, a DC voltage VS (t) = Vs0 (t) having a large time constant is applied between the electrodes of the capacitor 4 having a capacitance of CS. It gradually recovers. Then, from time t = 0 to t = t1
When the AC voltage is boosted from zero to VAC in a short period of time, the DC voltage VS (t) increases dramatically. Thereafter, during the period from t = t1 to t = t2 in which the AC voltage is held at VAC, the change characteristic of VS (t) differs depending on the deterioration state of the sample cable 1, and as shown in FIG. The rate of change of VS (t) becomes ΔVS / VS as it becomes significant.
(T1) = {VS (t2) -VS (t1)} / VS (t
1) becomes smaller, and when the deterioration is extreme, ΔVS is negative,
That is, VS (t) may start to decrease.

By the way, the DC voltage VS (t) measured at the time of AC power application includes a voltage component VS0 (t) recovered when the AC power application is not performed. The voltage component VS0 (t) includes a deterioration signal generated by the deteriorated portion insulator, but most of the voltage component has no relation to the deterioration state of the insulator, for example, accumulates on the end surface of the sample cable 1. There is a strong possibility that the component is caused by the space charge. That is, the voltage VS (t) between the electrodes of the capacitor 4 when the AC power is applied.
Voltage component V when no AC charging is performed during the measurement result
If S0 (t) is included, it becomes an error factor in 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
VS0 (t) from t = 0 to t = t2 is estimated from the time-varying characteristics of the DC voltage VS (t) = VS0 (t) of the capacitor CS measured under the condition where no AC charging is performed.
The value obtained by subtracting VS0 (t) from the measured value VS (t) during AC power application is used for the deterioration diagnosis. That is, the true measurement result at time t = t1 when the AC voltage is boosted to VAC is given by VS1 = VS
(T1) -VS0 (t1), and the true measurement result at the final measurement time t = t2 where the AC voltage is held at a constant value of VAC is VS2 = VS (t2) -VS0 (t2). From these values, the residual charge Q (t1) = VS1 × (CS + Cx) increased by the AC charging, and Q (t2) = V
S2 × (CS + Cx) is obtained, and the magnitude of Q (t1) and Δ
(T) / Q (t1) = {Q (t2) -Q (t1)} / Q
From the value of (t1), the state of deterioration of the cable is diagnosed. In consideration of the need to normalize the electrode length of the sample cable 1, the residual charge Q (t1) immediately after the AC voltage is boosted is
Q (t1) / Cx divided by capacitance of sample cable 1
[V] is used for deterioration determination.

Table 1 shows an example of the results of measuring the residual charge of a 22 kVCV cable and a 6.6 kVCV cable that have been subjected to water tree deterioration using the above-described measurement method. Samples 1 to 8 have a 22 kVC internal semiconductive layer with an extruded structure.
Sample 9 and Sample 10 are V-cables, and the semiconductive layer is a 6.6 kVCV cable having a tape structure. Sample 1
Is a sample that has hardly deteriorated, and the sample 10 has a water tree in a significantly deteriorated state that has developed between the electrodes to just before bridging.

[0050]

[Table 1]

As shown in Table 1, the magnitude of the residual charge Q (t1) observed immediately after the AC voltage was boosted tends to increase as the AC breakdown voltage decreases, and the residual charge generally increases as the deterioration proceeds. Shows a tendency to increase.
However, there are cases where the breakdown voltage is not significantly reduced even when the residual charge is large, and conversely, when the breakdown voltage is significantly reduced even when the residual charge is small. This is mainly due to a difference in the number of generated water trees existing in the sample cable.

On the other hand, {Q (t2) -Q (t1)} / Q
The time characteristic of the residual charge under the alternating current application represented by (t1) shows an extremely good correlation with the decrease in the breakdown voltage of the sample, and as the breakdown voltage decreases, ΔQ (t2) -Q
(T1)｝ / Q (t1) decreases, and in the case of the sample 10 in a remarkably deteriorated state, {Q (t2) −Q (t1)} / Q
(T1) has changed to a negative value. That is, the time change characteristic of the residual charge under the AC application accurately represents the deterioration state of the sample, and if this characteristic is used, highly reliable water tree deterioration diagnosis can be performed.

In the "residual charge measurement" for obtaining the residual charge from the conventional absorption current measurement, the residual charge Q (t2
) Is used in the deterioration determination, but in the results shown in Table 1, {Q (t2) -Q (t1)} / Q (t1)
A larger value indicates that the deterioration has not progressed, so that the deterioration diagnosis based on the magnitude of Q (t2) has lower diagnostic accuracy than the deterioration diagnosis based on the magnitude 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, considering that an earlier time after the start of the AC power application is preferable.

Although the DC application voltage VDC has a negative polarity in this detailed description, there is no problem even if it has a positive polarity. Further, in the example of the measurement circuit of FIG. 3, an example is shown in which a DC voltage measurement circuit (capacitor 4 and DC voltmeter 5) is inserted between the AC power supply power source 3 and the ground.
Even if it is changed to a circuit inserted between the ground and the ground, the polarity of the observed voltage is apparently reversed, and there is essentially no problem at all.

[0055]

As described above, the method for diagnosing deterioration of insulation 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 power application grounding. By using the time change ratio of the residual charge as a 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 after the elapse of a certain time are used. A highly detectable and reliable water tree deterioration diagnosis of the power cable can also be achieved.

Further, as a method for detecting the residual charge under the AC application after the DC application grounding according to the second aspect, the DC voltage between the electrodes of the sample cable is charged while the electrodes are open DC. Measurement within a negligible time range, thereby greatly eliminating the response delay of the measurement circuit, which is a problem with the conventional method of detecting residual charge by measuring the absorption current flowing through the grounding circuit, and It is possible to accurately detect the time characteristic of the residual charge below.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram relating to residual charge and generation of absorption current after DC application.

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

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

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

──────────────────────────────────────────────────続 き Continuing on the front page (72) Naka Sakamoto, Inventor 1008 Niibori, Kumagaya-shi, Saitama Mitsubishi Electric Works Co., Ltd. Kumagaya Works (56) References JP-A-62-127676 (JP, A) JP-B 5-28350 (JP, B2) JP-B 62-41345 (JP, B2) (58) Fields surveyed (Int. Cl. ⁷ , DB name) G01R 31/12

Claims (2)

(57) [Claims] 1. A method of diagnosing insulation deterioration of a power cable by applying a DC voltage to a power cable, grounding the power cable, and thereafter measuring a residual charge generated by the DC power supply in a state where an AC voltage is applied. And measuring the time characteristic of the residual charge under the application of an AC voltage, and diagnosing the degree of insulation degradation of the power cable based on the time change ratio of the residual charge. 2. A method for diagnosing insulation deterioration, comprising: applying a DC voltage to a power cable and then grounding the AC cable, and then measuring a residual charge generated by the DC application in a state where the AC voltage is applied. A capacitor is connected in series with the AC power application device to be applied, and a DC voltage component appearing between the electrodes of the capacitor is detected.The DC voltage component detected between the electrodes of the capacitor is added to the capacitance of the sample cable and the capacitor. A method for diagnosing insulation deterioration of a power cable, wherein a value obtained by multiplying the sum of the capacitance and the residual capacitance is used as a residual charge at the time of AC power application.