JP2001116789A - Method for measuring residual charge of insulator and method for diagnosing insulation deterioration of power cable using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for measuring a residual charge with reliability without using a complicated attenuation vibrating wave voltage for regulating an AC voltage or waveform for causing increases in weight and size of an apparatus for measuring. SOLUTION: A positive or negative pulse voltage Vo is applied to a series connector of a sample insulator 1 and a capacitor 3, a capacitor sharing voltage Vd at this time is observed by a DC voltmeter, and a charge stored in the insulator 1 is detected based on the capacitor sharing voltage retained after the pulse voltage is applied. Thereafter, electrodes of the capacitor 3 are once short-circuited, and then in a state in which the short circuit is opened, a reverse voltage -Vo to a unipolar high voltage Vo first applied is further applied, and the charge is similarly detected. When this is applied to an insulating layer of the CV cable, deteriorating degree based on the residual charge can be known.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a measuring method for detecting space charges accumulated in various insulators, and more particularly to a method for measuring space charges which can be suitably used for diagnosis of insulation deterioration of a power cable.

[0002]

BACKGROUND OF THE INVENTION Crosslinked polyethylene insulated power cables (C
Water tree deterioration is known as an important deterioration factor that determines the long-term life of a rubber / plastic insulated cable such as a V cable. One of the methods of detecting the water tree deterioration is a method of detecting a residual charge. This method is a method in which a high DC voltage is applied to an insulator sample, which is then grounded to remove the DC applied voltage, and the DC charge (residual charge) recovered during the grounding period is measured.

[0003] As a conventional residual charge detection method, FIG.
The methods shown in (a) and (b) are known. FIG. 1A shows a method based on current measurement, in which an instantaneous discharge current at the time of grounding after DC power is applied by the DC high-voltage power applying device 20 (current due to discharge of electric charge on the electrode that has charged the capacitance of the insulator). ) Disappears, a current detection resistor 6a of about 1 to 10 kΩ is inserted into the ground circuit, and the discharge absorption current of the sample 1 (current component generated by the process of moving and eliminating space charge in the insulator) Id (t) =
Vd (t) / Rd detected (actually detected by DC voltmeter 5a)
Then, the time integration value is obtained as a residual charge Q (t).

FIG. 1 (b) shows a method based on voltage measurement.
A DC voltmeter 5b having a high input resistance and a capacitor 6b for charge measurement are connected in parallel between the insulator electrodes, and the DC voltage Vd (t) recovered between the sample electrodes is detected. The residual charge is Q
(t) ≡ (Cd + Cx) Vd (t). FIG.
3A and 3B, Cx and Cd denote capacitances of the sample 1 and the capacitor 6b, 30 denotes a discharge resistor, 40 denotes a changeover switch, and 7b denotes a measurement circuit short-circuit switch.

The source of the residual charge detected by the conventional method shown in FIGS. 1 (a) and 1 (b) is the space charge λ accumulated in the insulator due to the DC application. Are not detected as residual charges. For example, in the current measuring circuit shown in FIG. 1A, even if the space charge λ is accumulated in the insulator, the current Id (t) does not flow through the measuring circuit as long as the charge is stationary. Similarly, in the voltage measuring circuit of FIG.
Is not displaced in the electrode direction, the DC voltage Vd (t) does not change from the initial value Vd (0) = 0 at the start of measurement. That is,
The residual charge detected by the conventional method means that a space charge component that changes spatially or temporally inside the sample insulator is detected during the measurement period after the removal of the DC applied voltage.

[0006] Space charges accumulated in an insulator by application of a DC high electric field include interface charges accumulated at an interface having non-uniform conductivity and dielectric constant, and charges injected from an electrode. If these space charges remain in the insulator after removing the applied DC voltage (that is, after grounding), the space charges themselves form an electric field inside the insulator even after removing the applied voltage. By the action of the space charge electric field, the space charge moves and attenuates so as to return to the state before DC application at the time of grounding. In the measurement by the conventional method, the residual charge saturates to the maximum value when the space charge disappears. That is, the residual charge Q (t) has a time characteristic of recovering from Q (0) = 0 after the start of measurement to a certain saturation value Q (tmax) = Qmax.

According to the study of the present inventors, the process of generating space charge in an insulator by applying a high voltage and the process of eliminating space charge after removing an applied voltage generally tend to be irreversible.
In many cases, it takes longer to annihilate than to occur. For example, according to the experimental results of the present inventors using a water tree-degraded CV cable, even when a space charge is formed by applying a DC voltage of less than 1 minute, the residual charge observed after removing the DC voltage is more than 24 hours. It does not reach saturation even after passing. As described above, in the conventional measurement method of FIGS. 1A and 1B, the time until the residual charge reaches the saturation value is long, and a large residual charge cannot be obtained in a practical short-time measurement. From
There has been a problem that it is difficult to distinguish a similar error signal, for example, from a residual charge due to a charge on the insulator surface between the electrodes due to DC application. Therefore, this conventional method is referred to as CV
Even when applied to water tree deterioration diagnosis of cable insulation, highly reliable diagnosis results were not obtained.

As a method for solving this problem, there has been proposed a method of applying an AC voltage when measuring the residual charge after grounding. That is, since the movement of charges is slow only by the space charge electric field at the time of grounding, an AC electric field is applied from the outside and superimposed on the space charge electric field inside the sample to speed up the movement and decay time of the charges. In the measurement method of applying the AC voltage, the method of detecting the residual charge includes a method of measuring current and a method of measuring voltage. FIG. 1 (c) is an example of a circuit for measuring the residual charge under the AC application proposed by the present inventors. In such a measurement circuit, the sample 1
Is connected to an AC high voltage generator 8c, the switch 7c for short-circuiting between the electrodes of the DC charge detection capacitor 6c is opened immediately before the start of AC power application, and the DC voltage component Vd ( t) is detected and the capacitor 6c is detected.
The residual charge Q (t) ≡ (Cd + Cx) Vd is calculated from the product of the total capacitance (Cd + Cx) of the sample 1 and Vd (t).

The reason for using the AC voltage in the conventional residual charge method is that the applied voltage does not include a DC component. That is, if a DC component is contained in the applied voltage, a DC charge for charging the capacitance of the sample is generated on the electrode, which causes a problem that the DC charge cannot be distinguished from the residual charge. In addition, when a positive or negative unipolar voltage is applied, the applied voltage may form a space charge different from the charge generated by the DC charge, and the space charge formed by the DC charge is detected. It was considered unfavorable for the purpose of doing. Therefore, an AC voltage that oscillates positively and negatively is required as a voltage to be applied at the time of measuring the residual charge after the DC application, and a method of applying the AC voltage is to suppress the generation of the DC component by using a voltage stamp ( In other words, a method of continuously raising or lowering the voltage from zero to a predetermined value while avoiding the interruption is adopted.

FIG. 2 is a circuit diagram of FIG.
FIG. 1B is a schematic comparison of the measurement results of the residual charge measurement shown in FIG. 1B and the measurement results of the residual charge method shown in FIG. As shown in the figure, when the AC charging is performed at the time of measuring the residual charge after the DC charging and grounding, the relaxation time of the residual charge of the water tree deteriorated cable is significantly shortened. For example, when the peak value of the AC applied voltage is raised from zero to about the DC applied voltage value or more, the residual charge is saturated to the maximum value in a short time of this boosting. Further, in the example of FIG. 2, as an application method of the AC voltage, a power application method in which the voltage is boosted to the maximum value in a short time and then immediately dropped to zero without holding at the maximum value is shown. When this short-time AC raising / lowering method is adopted, even if a DC error charge for charging the capacitance during the AC application appears, the DC error charge may be ignored and the change in the residual charge before and after the AC voltage application may be read. Therefore, FIG.
2 and the short-time AC application method shown in FIG. 2 significantly improve the reliability of the measurement as compared with the method shown in FIGS. 1 (a) and 1 (b). It is.

However, in order to apply the deterioration diagnosis by the residual charge method to an actual cable line, a large-capacity AC charging device capable of charging the capacitance of the line is required. In this case, the measuring device becomes large and heavy,
Problems arise in measurements in tight spaces. As a method of reducing the size and weight of the device, a method of shortening the AC voltage application time as shown in FIG. 1C as much as possible and securing the current capacity with a small AC power supply device is considered. As the limit, use of an attenuated oscillating wave voltage that oscillates in both polarities can be considered as a voltage waveform that can be substituted for AC. But,
In either case, there is a problem that the control of the applied voltage waveform becomes complicated.

[0012]

SUMMARY OF THE INVENTION Accordingly, the present invention provides a reliable residual charge measurement without using an AC voltage that causes the measurement apparatus to be heavy and large, and an attenuated oscillating wave voltage that requires complicated waveform adjustment. The purpose is to provide a method that enables As described above, the conventional residual charge measurement method does not sufficiently consider the irreversibility of the space charge accumulation process due to voltage application and the disappearance process after removal of the applied voltage. Therefore, despite the fact that it takes a long time for space charges formed in a short period of time when a DC voltage is applied to disappear under a grounded state, it is trying to detect a signal based on this movement / elimination process. is there.

According to the present invention, the concept of the conventional residual charge measurement is basically changed, and the movable charge can be easily moved / removed when the applied voltage is removed.
Space charge components that do not disappear are detected as residual charges. In other words, if the space charge does not disappear easily even after the applied voltage is removed, it is better to detect the state in which the space charge is approximately stationary during the measurement period as the residual charge inside the sample. ing. According to this viewpoint, since the signal based on the stationary charge is detected, it is not necessary to perform the AC application to accelerate the movement and disappearance of the charge.

[0014]

According to the present invention, there is provided a method for measuring the residual electric charge of an insulator, comprising applying a positive or negative unipolar high voltage Vo to a series connection of a sample insulator and a capacitor, Is observed with a DC voltmeter, and the electric charge accumulated in the sample insulator is detected based on the capacitor shared voltage remaining after application of the pulse voltage.

In another measurement method of the present invention, after performing the above-described measurement, the electrodes of the capacitor are short-circuited once, and then the short-circuit is opened, and the reverse is applied to the unipolar high voltage Vo applied first. Polarity voltage -Vo is further applied, and then the capacitor sharing voltage at this time is observed with a DC voltmeter,
It is characterized in that charges accumulated in the sample insulator are detected based on a capacitor sharing voltage remaining after application of the pulse voltage.

Further, the present invention can be suitably applied to insulation deterioration diagnosis of a power cable, and the diagnosis method is obtained by the above-described residual charge measurement method, wherein a sample insulator is used as an insulation layer of a power cable and the measurement is performed. The degree of insulation deterioration of the insulating layer is diagnosed based on the residual charge.

[0017]

The principle of measurement according to the present invention will be described below. In the following description, for the sake of simplicity, a case where a space charge is generated in an insulator in a parallel plate electrode by application of a high voltage will be described as an example. In FIG. 3A, reference numeral 1 denotes a sample insulator sandwiched between parallel plate electrodes, 2 denotes a unipolar high voltage generator, and 3 denotes a capacitor for measuring electric charge. Sample insulator 1
A case is considered in which, when a unipolar voltage Vo is applied between points AB in the figure where the high voltage generator 2 and the high voltage generator 2 are connected in series, charges are injected into the insulator from the A-side electrode of the sample 1. Where C
x = εS / d is the capacitance of the sample 1, ε is the dielectric constant of the insulator, d and S are the insulator thickness and electrode area of Cx, Cd is the capacitance of the charge detection capacitor 3, and Vx and Vd
Is the voltage between the electrodes of the sample insulator 1 and the capacitor 3. Reference numeral 4 denotes a switch for opening and closing the electrodes of the capacitor 1, and the switch 4 is always open during a period in which the voltage Vo is applied. For simplicity, the internal resistance of the high voltage generator 2 is assumed to be zero.

Now, in the circuit diagram shown in FIG.
When a voltage Vo is applied between AB, the sample 1 and the capacitor 3
An electric charge is induced on the electrodes. Now, consider a voltage Vd generated between the electrodes of the capacitor 3 in a state in which charges are injected from the A-side electrode of the sample to the surface at the depth d1 to form space charges λ. When the electric charge q is induced to the point M-side electrode of the sample 1, the electric charge -q having the opposite sign is induced to the inner surface of the M-side electrode of the capacitor 3. Further, no outward electric flux appears on the closed curved surface surrounding the outside of the electrode of the sample 1 and the closed curved surface surrounding the outside of the electrode of the capacitor 3. That is, the sum of the charges existing inside these electrodes is zero. Further, the applied voltage Vo between A and B is
Are the voltage Vx between the electrodes of the sample 1 and the capacitor 3 for charge detection.
Is equal to the sum of the inter-electrode voltages Vd.

Vd is obtained based on the above knowledge. Vd = (CxVo + λd1 / d) / (Cx + Cd) (1) This is the voltage observed between the electrodes of the charge detection capacitor 3 when Vo is applied.

Assuming that Vo = 0 in the above equation (1), the state of the capacitor 1 after the applied voltage Vo has disappeared to zero is obtained.
The voltage observed between the electrodes is obtained. In the method according to the first aspect of the present invention, the value obtained by detecting Vd after the disappearance of the applied voltage and multiplying the detected value by the capacitance (Cx + Cd) is defined as the residual charge. That is, when the space charge λ accumulates in the insulator due to the application of the unipolar voltage Vo and does not move or disappear in the insulator even after Vo disappears to zero, it is expressed by the following equation (2). Residual charge Q is detected. Q≡ (Cx + Cd) Vd = λd1 / d (after Vo has disappeared to zero) (2) That is, when the space charge λ remains on the d1 surface in the insulator of the sample 1, the applied voltage Even after Vo disappears to zero, the voltage Vd between the electrodes of the DC charge detection capacitor 3 does not become zero. The residual charge Q obtained by multiplying this Vd by the capacitance (Cx + Cd) is proportional to the size λ of the space charge remaining inside the sample and the depth d1 of the existing point.

Next, the difference between the present invention and the conventional method will be described.
The residual charge that is to be detected by the present invention cannot be detected by the conventional current measurement technique. The reason for this is that the present invention attempts to detect a charge that is approximately in a stationary state during the measurement period even after the applied voltage has disappeared, and no current flows if the charge is stationary.

In addition, comparing the method of the present invention with the conventional voltage measurement method, the method of the present invention shows that the applied voltage Vo
Is different from the conventional method in that the electrodes of the charge detection capacitor 3 are not short-circuited by the switch 4 after the disappearance of. In the conventional method, the residual charge measurement is started after the electrodes of the sample 1 are once grounded in order to remove the applied voltage. When the operation of the conventional method is applied to FIG. 3A, the initial conditions at the start of the measurement are such that Vx = 0 on the electrode of the sample 1 and Vd = 0 on the electrode of the capacitor 3. The charge is induced to be When the space charge λ accumulates on the surface of the sample at the depth d1 in the sample, the charge is induced on the electrode of the sample 1, but there is no space charge in the capacitor 3; No charge is induced on top. This is the initial condition at the time of the residual charge measurement according to the conventional method, and forms the state Vx = −Vd = 0 where no potential difference appears outside.
Therefore, in the conventional method, the space charge λ moves spatially,
Alternatively, a signal of Vx = −Vd ≠ 0 is detected only when the condition of equilibrium with the induced charge on the electrode is broken due to a change in size with time or the like. The difference between the first aspect of the present invention and the conventional method has been described above.

The method of the present invention is the same as the conventional method.
, And will be described below. FIG.
In (a), first, a unipolar voltage + Vo is applied to the sample 1 and the switch 4 is closed to short-circuit the electrodes of the sample 1 and the capacitor 3 for charge measurement in the same manner as in the conventional method.
As described above, when the space charge + λ is injected from the A-side electrode of the sample to the depth d1 of the insulator and accumulated by application of the voltage Vo, a charge is induced on the electrode of the sample 1 due to the short circuit of the switch 4. Is done. Next, switch 4 is opened and + Vo
When a unipolar voltage -Vo of the opposite polarity to-is applied across AB, the electric field inside the sample due to -Vo returns the space charge + [lambda] due to the previous + Vo application to the A-side electrode of the sample so that the space charge + [lambda] disappears. Works.

For the sake of simplicity, a new space charge is not formed by application of the reverse polarity voltage -Vo, and the first + V
Considering the case where the space charge + λ formed by the application of o returns to the A-side electrode side by Δd, after −Vo has decayed to zero, the space charge moves to the electrodes of the sample 1 and the capacitor 3 due to the movement of the space charge. A charge has been induced. As a result,
A voltage of Vd = − △ q / Cd = −λd1 / d × 1 / (Cx + Cd) is generated between the electrodes of the charge detection capacitor 3, and the residual charge Q = (Cx + Cd) Vd = −λd1 / d is detected. Is done.

Next, by the application of the reverse polarity voltage -Vo ', the space charge + λ due to the first + Vo application returns to the A-side electrode of the sample and disappears, and at the same time, the space charge -λ' of the opposite polarity is newly applied to the sample. Considering the case where the voltage is injected from the A-side electrode to the d1 surface and this remains after -Vo has been attenuated to zero, the voltage between the electrodes of the charge detection capacitor 3 is Vd = − (Λd1 / d + λ′d1 / d) / (Cx + Cd), and the residual charge is detected as Q = (Cx + Cd) Vd = − (λd1 / d + λ′d1 / d). That is, if the reverse polarity voltage -Vo 'applied later has a sufficient time width for the disappearance of the previously formed space charge + λ and the formation of a new space charge -λ', these unipolar voltages are individually applied. The sum of the residual charges obtained when the voltage is applied is detected.

[0026]

FIG. 3B is an example of a circuit for measuring residual charges according to the present invention. In the drawing, reference numeral 1 denotes a sample insulator, and 2 denotes a unipolar high voltage generator, which generates an impulse voltage Vo in this example. 2a is a capacitor which is charged to a predetermined voltage from another DC high voltage generator. 2d
Is a discharge gap, and a unipolar impulse voltage is applied to the sample 1 from the capacitor 2a by discharging in the discharge gap 2d. 2b is a resistor for determining a charging time constant of the impulse voltage, and 2c is a resistor for determining a discharging time constant of the impulse voltage. Reference numeral 3 denotes a DC charge detection capacitor whose capacitance Cd is set to a value much larger than the capacitance Cx of the sample 1. Reference numeral 4 denotes a switch for short-circuiting the electrodes of the capacitor 3, and reference numeral 5 denotes a DC voltmeter for measuring a voltage Vd between the electrodes of the capacitor. The input resistance of the DC voltmeter 5 is, for example, 100 MΩ in order to prevent leakage of electric charge during the measurement period.
It is a high value of about the above. In FIG. 3B, an impulse waveform is used as the applied voltage, but any unipolar voltage such as a square wave, a triangular wave, and a sin half waveform may be used. In FIG. 3B, the connection point between the low-voltage electrode of the sample 1 and the charge detection capacitor 3 is connected to ground, but the connection point between the discharge resistor 2c and the charge detection capacitor 3 on the high voltage side is connected to ground. (The polarity of the detection voltage Vd of the charge detection capacitor 3 is inverted by changing the ground point).

When the gap 2d of the high voltage generator is discharged while the switch 4 is open, an impulse voltage Vo is applied to the sample 1 and the charge detection capacitor. At this time, the voltage V between the electrodes of the charge detection capacitor 3
d is measured by the DC voltmeter 5. FIG. 4 is a schematic diagram of an observed waveform of the inter-electrode voltage Vd. Impulse voltage-Vo
Is applied, no space charge is formed in the insulator of the sample 1 due to -Vo, or if space charge disappears at the same time as -Vo disappears even if space charge is formed due to -Vo, -Vo With the disappearance of Vd, Vd disappears to zero. On the other hand, a space charge is formed by -Vo, and-
If space charge remains in the insulator and remains stationary after Vo disappears, Vd maintains a constant value + ΔVd and does not become zero.
The value obtained by multiplying + △ Vd by the sum of the capacitance of the sample and the charge detection capacitor (Cx + Cd) is the residual charge Q formed by the application of the impulse voltage −Vo.

Next, when the switch 4 is short-circuited, the voltage Vd between the electrodes of the charge detection capacitor 3 becomes zero. At this time, if the space charge in the sample insulator is stationary without disappearing, a charge having a sign opposite to that of the space charge is induced on both electrodes of the sample by the space charge. Even if the short circuit of the switch 3 is opened in this state, if the space charge in the sample insulator is stationary, no voltage appears between the electrodes of the capacitor and the capacitor is kept at zero. During the short-circuit period of the switch 3, a DC voltage having a polarity opposite to the first applied voltage is applied to the capacitor 2a. Finally, the switch 3 which has been short-circuited is opened again, and thereafter, the impulse voltage + Vo having the opposite polarity is applied. Is applied. At the time of this + Vo application, the space charge formed at the time of the first -Vo application disappears, and at the same time, a space charge of the opposite polarity is newly formed, and after the + Vo disappears, the space charge remains in the insulator. Then, a voltage of -2 Vd is detected between the electrodes of the charge detection capacitor. The value obtained by multiplying this voltage by the sum of the capacitance of the sample and the charge detection capacitor (Cx + Cd) is the impulse voltage −V
This is the residual charge when a voltage + Vo of the opposite polarity is applied after o is applied.

The method for measuring residual charge according to the present invention can be suitably applied to a diagnosis of insulation deterioration of a power cable. In this case, the above-mentioned insulator sample may be replaced with an insulating layer of a power cable to be diagnosed, and a unipolar high voltage may be applied using a cable conductor. Table 1 is an example in which the result of measuring the residual charge of a water tree deteriorated CV cable sample using the method of the present invention is compared with the result of the conventional method. A is undegraded 22kV
B is an 11 kV CV cable in which long and long internal water trees frequently occur. The impulse voltage waveform has a crest length of about 50 μsec and a crest length of about 50 ms
And the magnitude of the applied voltage was -10 kV. In the conventional method, the residual charge was measured in a state in which an alternating current of 12.7 kVrms was applied after applying a direct current of -30 kV. As is clear from the table, the residual charge measuring method according to the present invention detects the difference in the deterioration state of the cable with no difference as compared with the conventional method.

[0030]

[Table 1]

[0031]

According to the method for measuring the residual charge of an insulator of the present invention as described above, a method is employed in which a space charge component which does not disappear or move even after the applied DC voltage is removed is detected as a residual charge. This eliminates the need for an AC voltage application facility that tends to increase the size of the measuring device, and enables highly reliable measurement of the residual charge. Therefore, when the residual charge measurement method according to the present invention is applied to, for example, insulation deterioration diagnosis of a power cable, insulation deterioration diagnosis based on the residual charge can be performed with high accuracy and without requiring an increase in the size of the measurement device. This is an excellent effect.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a conventional method for measuring residual charges, wherein (a) shows a current measuring method, (b) shows a voltage measuring method, and (c) shows an AC voltage applying method.

FIG. 2 is a graph showing a time characteristic of a residual charge detected by a conventional residual charge measuring method.

3A and 3B are circuit diagrams showing a method for measuring residual charges according to the present invention, wherein FIG. 3A is a circuit diagram showing the principle thereof, and FIG. 3B is a circuit diagram showing an example of the embodiment.

FIG. 4 is a schematic diagram for performing the measurement of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Sample insulator 2 Unipolar high voltage generator 3 Capacitor for charge measurement 4 On / off switch

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[Procedure amendment]

[Submission date] December 21, 1999 (1999.12.
21)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Claims

[Correction method] Change

[Correction contents]

[Claims]

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0014

[Correction method] Change

[Correction contents]

[0014]

According to the present invention, there is provided a method for measuring the residual electric charge of an insulator, comprising applying a positive or negative unipolar high voltage Vo to a series connection of a sample insulator and a capacitor, Is observed with a DC voltmeter, and the electric charge accumulated in the sample insulator is detected based on the capacitor shared voltage remaining after the application of the unipolar voltage.

[Procedure amendment 3]

[Document name to be amended] Statement

[Correction target item name] 0015

[Correction method] Change

[Correction contents]

In another measurement method of the present invention, after performing the above-described measurement, the electrodes of the capacitor are short-circuited once, and then the short-circuit is opened, and the reverse is applied to the unipolar high voltage Vo applied first. Polarity voltage -Vo is further applied, and then the capacitor sharing voltage at this time is observed with a DC voltmeter,
The electric charge accumulated in the sample insulator is detected on the basis of the capacitor shared voltage remaining after the application of the unipolar voltage.

Claims (3)

[Claims] 1. A positive or negative unipolar high voltage Vo is applied to a series connection of a sample insulator and a capacitor, the capacitor sharing voltage at this time is observed with a DC voltmeter, and the capacitor sharing voltage remaining after the pulse voltage is applied. A method for measuring the residual charge of an insulator, comprising detecting a charge accumulated in a sample insulator based on the method. 2. After the measurement of claim 1, the capacitor is short-circuited once and the short-circuit is released, and then the voltage -Vo of the opposite polarity to the unipolar high voltage Vo applied first is applied. Further applying the voltage, then observing the capacitor sharing voltage at this time with a DC voltmeter, and detecting the charge accumulated in the sample insulator based on the capacitor sharing voltage remaining after the pulse voltage is applied. Charge measurement method. 3. The residual charge measuring method according to claim 1, wherein the sample insulator is used as an insulating layer of a power cable, and a degree of insulation deterioration of the insulating layer is diagnosed based on the residual charge obtained by the measurement. A method for diagnosing power cable insulation deterioration.