JP2002340970A - Insulation deterioration diagnostic method for power cable - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve workability of insulation deterioration diagnosis of a cable by canceling a reactive charge signal independent of application of alternating voltage, in the case that the water tree insulation deterioration diagnosis of existing power cables by a residual charge method and clearly displaying forced residual charge by the application of the alternating voltage. SOLUTION: After the insulation layer of a cable to be diagnosed is charged, a cable conductor is earthed once, change of the residual charge of the cable is changed over from an earthed stage to a stage detected by a detection capacity Cd , the alternating voltage Vac is applied in order to accelerate discharge of the residual charge in the detection stage, and the insulation deterioration of the cable is diagnosed from the detection result in the detection stage. A detection signal is canceled by a cancelling voltage source Vx during non- application of the alternating voltage.

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, and more particularly to a method useful for diagnosing water tree deterioration.

[0002]

2. Description of the Related Art As one of causes of insulation deterioration of rubber / plastic power cables, water tree deterioration caused by the interaction between an electric field and moisture is known. This water tree deterioration is a trace of tree-like deterioration, and is generated from irregular portions at the interface between the insulator and the semiconductive layer, or foreign matter or voids in the insulator, and extends over time in the direction of the electric field of the insulator. If left unattended, dielectric breakdown will eventually occur during operation. Therefore, it is necessary to diagnose the progress of the water tree at the time of periodic insulation deterioration diagnosis of the cable. Conventionally, the residual charge method shown in FIGS. It is.

In the residual charge method, a DC voltage or an
As shown in FIG.
Then, charge the cable insulator, and then, as shown in FIG.
Short the cable conductor and the outer shielding layer as
The residual charge Q in the cable)
After that, as shown in FIG. _ac
And switch to a circuit in which capacitor C is connected in series
And AC power supply V_acOf the residual charge Q
Acceleration of emission, and residual charge generated by the accelerated emission
The intermittent change is detected by the capacitor C, and the
Diagnosis of insulation deterioration of cable.

In the above description, even when the AC voltage V _ac is not applied, the discharge of the residual charge proceeds with the time constant determined by the polarization and the electric conduction under the electric stress generated by the residual charge Q. When the battery is charged with a direct current charging time of less than 1 minute,
Even after a lapse of time, the residual charge is not completely released. As described above, under the application of the AC voltage, the residual charge is moved by the applied electric stress and released from the strong restraint, so that the release speed is accelerated, and as a result, the amount of change in the residual charge is increased. , Leading to improved sensitivity.

In the above-mentioned residual charge method, an AC voltage
Ideally, all residual charges are forcibly released by addition
It is. FIG. 5 illustrates this ideal residual charge method.
FIG. 5 (b) shows the electric charge in the ideal residual charge method.
Indicates an intermittent change, that is, a current state._acTo
Period T until application₁In this case, the residual charge
Intrinsic time constant controlled by induction, based on its residual charge
Low current value I due to slow release by electric field_ETo
And the remaining charge is completely removed by applying the AC voltage for Δt time.
Is rapidly forcibly released, and therefore, after the AC voltage application is stopped.
Period T _TwoIn this case, substantially all of the charge is released to zero.
Therefore, the current value is zero. This current state
The corresponding residual charge is obtained by integrating the current I (t) with time.
The change state of the charge Q (t) is shown in FIG.
is there. In (c) of FIG. 5, ΔQ is the AC voltage V_ac
It shows the change in the amount of charge due to only the application, and the rising start point a and saturation
ΔQ can be grasped sufficiently clearly with reference to the end line bb.
You.

However, residual charges accumulated in the cable insulator due to factors unrelated to the deterioration of the cable insulator such as the interface between different insulators in the cable junction box (hereinafter referred to as pseudo residual charges). This pseudo residual charge is released with a unique time constant τ _E , but the residual charge (hereinafter, referred to as the residual charge) which is restrained by the water tree insulation deteriorated portion which has been deteriorated by the water tree and has a weak binding force. Name)
The pseudo-residual charges restrained by the sound insulating portion, which is not degraded and retains the restraining force, are less likely to be field-emitted, and the pseudo-residual charges are preferentially given despite the AC voltage application. If the cable length is short and the residual charge due to the water tree is small,
The residual charge amount becomes smaller than the total charge amount, and the charge amount change ΔQ based on the application of the AC voltage becomes slower. When ΔQ becomes slow in this way, as shown in FIG. 6C, the rising base point a becomes unclear and ΔQ becomes
It becomes more difficult to understand clearly.

It is an object of the present invention to perform a water tree insulation deterioration diagnosis of an existing power cable by a residual dragging method (after charging and charging an insulating layer of a cable to be diagnosed, once grounding a cable conductor,
Switch from the grounding stage to a stage for detecting a change in the residual charge of the cable, apply an alternating voltage in this detection stage to accelerate the release of the residual charge, and determine the insulation deterioration of the cable from the detection result in the detection stage. In this case, the change in the residual charge caused by the application of the alternating voltage is clearly displayed, thereby improving the workability of the cable insulation deterioration diagnosis.

[0008]

According to the method for diagnosing deterioration of insulation of a power cable according to the present invention, the insulation layer of the cable to be diagnosed is charged by charging, and then the cable conductor is once grounded. A method for diagnosing cable insulation deterioration from the detection result in the detection step, wherein an alternating voltage is applied to accelerate the release of the residual charge in the detection step, The present invention is characterized in that a detection is performed in the above detection step by applying a canceling current or a canceling voltage so as to cancel a detection signal during application of the alternating voltage.

According to another method of diagnosing insulation deterioration of a power cable according to the present invention, after charging and charging an insulating layer of a cable to be diagnosed, a cable conductor is once grounded, and from this grounding step, a change in residual charge of the cable is performed. In the method of diagnosing cable insulation deterioration from the detection result in the detection step, the alternating voltage is applied to accelerate the discharge of the residual charge in the detection step. From the time change Q (t) of the charge thus obtained, Q ′ _E (t) approximate to the time change of the charge without application of the alternating voltage is obtained, and the curve Q (t) and this curve Q ′ _E (t ) Is calculated by calculating the differential [Q (t) −Q ′ _E (t)].

[0010]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.
An embodiment will be described. FIG. 1 is used in the present invention.
Of an example of a power cable insulation deterioration diagnostic device
I have. In FIG. 1, reference numeral 1 denotes a cable to be diagnosed,
Shielding layer is grounded, usually rubber / plastic power
Cables are targeted. V_dcIs filled with cable insulator d.
DC power supply for power supply.
A pulse power supply can also be used. 21 is a charging connection
Point, 22 is the contact for grounding the cable conductor, 23 is the alternating voltage mark
This is a contact for addition and detection. R_eIs a grounding resistor. 11
Is a movable contact for the contacts 21 to 23,
Connected to the conductor 10. V_acIs to the cable insulator d
A power supply for applying an alternating voltage.
The accumulated residual charge can be removed without accumulating new residual charge.
Applied to force release and expedite release,
An alternating voltage or a damped oscillating voltage can be used. C_dIs a police box
Voltage source V_acAnd a ground-connected detection
And the detection capacitor C_dDetection end from both ends
Is configured. SW1 is a capacitor C_dConnect in parallel to
Switch. V_xIs the invalid detection that appears at the detection end.
An invalid signal canceling voltage source for canceling outgoing signal, SW2 is
This is a switch for the canceling voltage source. R_xIs the invalid signal canceling voltage
And the time constant τ_E≒ C _dR_xIs a variable resistor to give
You.

It is assumed that water tree deterioration has occurred in the cable 1. According to the present invention, in order to diagnose insulation deterioration of this cable, first, a three-point switch is turned on to a charging contact 21 in FIG. The cable insulator d is charged with the DC power supply _Vdc , and the three-point switch is connected to the grounding contact 22.
And the cable conductor 10 is held at the ground potential,
A substantial short circuit occurs between the cable conductor and the cable outer shielding layer. Further, the three switches are switched to the alternating voltage application / detection contact 23. Even after this switching, if the switch SW1 is on, the ground state is maintained. Next, the switch SW1 is turned off to switch to the step of detecting a change in the residual charge of the cable.

FIG. 2A shows an alternating voltage V at the detection stage.
The state of _ac application is shown, and the point t ₀ is the time when the switch SW1 is turned off. At this point in time t ₀ , the residual charge (main residual charge) accumulated in the water tree deteriorated portion of the cable insulator and the residual charge (pseudo residual charge) accumulated due to factors unrelated to the degradation of the interface between the dissimilar insulators of the junction box and the like. ) Is left in the cable insulator.
In the period from the time point t _O to the start time t _l of the alternating voltage application, a part of these residual charges Q _E is released based on the inherent time constant (hereinafter, referred to as self-emission), and remains due to the application of the alternating voltage. Although a significant portion Q _s of the charge amount is forcibly accelerated release part is left, alternating voltage application stop after, the time remaining part charges Q _'E is the specific constant τ
Self-release based on _E.

In the conventional residual charge method, a part of the residual charge Q
_EIs self-emitted based on the inherent time constant.
Current generated by the application of the alternating voltage
Minute Q _sCurrent caused by forced release
Charge Q '_EIs self-emitted based on the intrinsic time constant
The current generated by the detection capacitor C_dso
Detected, voltage output and displayed. As a result, FIG.
As shown in the figure, the charge remaining after the forced release (the aforementioned
Q '_E) For the self-emission based on the time constant
Makes it difficult to recognize the saturation end line bb of the residual charge
In addition, this residual charge is relatively small
When the change in charge amount ΔQ is weak due to addition,
The starting point a also becomes unclear and the alternating voltage V_acAlso apply
However, it is difficult to clearly grasp the change in charge
It is on the street.

However, the power cable according to the present invention is inferior.
In the diagnostic method, the residual charge before the forced release (the Q_E) Is solid
Electricity generated by self-emission based on a given time constant
Charge after flow and forced discharge (Q ′_E) Is unique
The current generated by self-emission based on the time constant
Handled as invalid detection signal, invalid signal canceling voltage source V _x
Is canceled by. That is, the switch of the apparatus shown in FIG.
Turn off switch SW1 to check for changes in cable residual charge.
Switch SW2 almost simultaneously when switching to the detection stage
And turn on the resistor R_xBy adjusting
Effective signal canceling voltage V_xAnd the time constant τ_E≒ C_dR _xGiving
Invalidation signal is canceled by the canceling voltage source created by
Then, the alternating voltage source V_acActivate
Thus, an alternating voltage is applied to the cable insulator d.

By the application of the alternating voltage, the residual charge Q _s (the main residual charge and the pseudo residual charge) is forcibly moved to expedite the discharge, and a rapid change of the residual charge, that is, a large current is generated. Since the current flows, a rapid change of the residual charge, that is, a large current signal appears at the detection terminal, and a temporal change in the residual charge due to the application of the alternating voltage is output to the output terminal of the detection capacitor _Cd . However, since the fluctuation of the residual charge based on the inherent time constant τ _E during the application of the alternating voltage is not output to the charge detection end, only the temporal fluctuation of the residual charge due to the application of the alternating voltage can be accurately and easily grasped. .

FIG. 2B shows the residual charge amount change curve Q _s (t) displayed and output to the detection unit in the present invention, and the display and output to the detection unit in the conventional method in which the invalid detection signal is not canceled. And a residual charge amount change curve based on self-emission during application of no alternating voltage, that is, a residual charge amount change curve based on an invalid detection signal is denoted by Q _E (t). Then

[Number 1] Q (t) = is in the relation of _{Q s (t) + Q E} (t), the amount of charge change ΔQ based on forced emission by the alternating voltage applied, grasp the conventional method displays the curve Q (t) Although it is difficult, it can be understood that the display curve Q _s (t) according to the present invention can be clearly understood.

The application of the alternating voltage by the alternating voltage source Vac is performed in order to forcibly release the existing residual charge accumulated by the DC voltage application without generating a new residual charge. It is effective to carry out abruptly with time, and it is usually preferable to set the elevating time to 20 seconds or less.

FIGS. 3A and 3B show another embodiment of the method for diagnosing insulation deterioration of a power cable according to the present invention. In this alternative embodiment, a normal diagnostic device in which the canceling current source Ix or the canceling voltage source Vx is omitted is used for the above-described diagnostic device, and the insulating layer of the cable to be diagnosed is charged according to a normal residual charge method. After charging, the cable conductor is once grounded, and then switched from this grounding stage to a stage for detecting a change in cable residual charge, as shown in FIG.
The alternating voltage Vac is applied to display a charge temporal change curve Q (t) shown in FIG. This curve Q (t)
Is a signal in which a detection signal based on the application of the alternating voltage and a detection signal (ineffective detection signal) not based on the application of the alternating voltage are superimposed.
When _E (t), the residual charge amount of change due to the alternating voltage applied, it can be calculated from _{Q (t) -Q E (t} ). Thus,
Curve Q based on detection signal not depending on alternating voltage application
_E (t) is, for example, the function is a quadratic function at ² + bt
+ C or [1 / (a't ² + b't + c ')]-1 /
c 'or exponential function a "[1-exp (-b" t)]
And a, b or a ', b', c ', or a ", such that three or two points of the curve Q (t) are satisfied.
b 'or by chain-linking the curve portions of the curve Q (t) when no alternating voltage is applied. This approximate function can be obtained by Q' _E (t)
If, by calculating the _{Q (t) -Q 'E (} t), as shown in (b) of FIG. 3, it is clearly display the changes in the residual charge due to the alternating voltage applied Q's (t) Can be.

[0019]

According to the method for diagnosing insulation deterioration of a power cable according to the first and second aspects of the present invention, the residual charge change ΔQ is slow because the amount of the residual charge that is easily discharged is relatively small, or As shown in FIG. 2B, since the alternating voltage is relatively low and the change ΔQ of the residual charge based on the forced emission due to the application thereof is slow, the residual charge based on the self-emission before the application of the alternating voltage is applied. rising start point of the boundary between the variation Q _E or is obscured, and or terminated saturation line is unclear because of the residual charge amount changes Q _'E based on self discharge after forced discharge, forced by the conventional method Even when it is difficult to grasp the residual charge change ΔQ due to the emission from the residual charge change curve Q (t), the present invention provides the residual charge change Q _E based on the self-emission before applying the alternating voltage and the self-emission after the forced emission. Charge variation Q based on Charge amount change curve Q _E consisting of _E
(T) is treated as an ineffective charge change, and the residual charge change Q
_{Since S} (t) is displayed, it is possible to clearly grasp the residual charge amount change ΔQ based on the forced release.

In the method for diagnosing insulation deterioration of a power cable according to a third aspect of the present invention, as shown in FIG.
From (t) 'calculated to _E (t), the curve Q' approximation of the curve Q in the invalid charge amount change curve Q _E (t) wherein the _E (t) curve Q
(T) and the charge amount change curve Q ′ _s (t)
, The change in the residual charge amount based on the forced release can be clearly grasped as described above.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an example of a diagnostic device used in a method for diagnosing insulation deterioration of a power cable according to the present invention.

FIG. 2 is a view used to explain a method of diagnosing insulation deterioration of a power cable according to the present invention using the apparatus of FIG. 1;

FIG. 3 is a diagram showing a method for diagnosing deterioration of insulation of a power cable according to claim 3;

FIG. 4 is a drawing showing the principle of the residual charge method.

FIG. 5 is a drawing used to explain an ideal mode of the residual charge method.

FIG. 6 is a drawing used to explain another embodiment of the residual charge method.

[Explanation of symbols]

1 Diagnostic cable 11, 21, 22, 23 Switch contact V _dc DC power supply R _e Ground resistance C _d capacitor R _x Variable resistance V _ac alternating voltage source V _x Negative voltage source S Detector

Claims (3)

[Claims] And a charging step of applying an alternating voltage to accelerate the discharge of the residual electric charge in the grounding step, wherein the cable conductor is grounded once. A method for diagnosing cable insulation deterioration from the detection result in the above, wherein the detection in the detection step is performed under an operation of canceling the detection signal while the alternating voltage is not applied. Deterioration diagnosis method. 2. A method for diagnosing insulation deterioration of a power cable according to claim 1, wherein the operation of canceling the detection signal while the alternating voltage is not applied is performed by applying a canceling current or a canceling voltage. 3. After charging the insulating layer of the cable to be diagnosed by charging, the cable conductor is grounded once, and then switched from this grounding step to a step of detecting a change in the residual charge of the cable. In a method of diagnosing cable insulation deterioration based on the detection result in the detection step, an alternating voltage is applied in order to accelerate the discharge of charges, and the temporal change Q (t) of the charge detected and displayed in the detection step Q ′ _E (t) approximate to the temporal change of the electric charge without application of the alternating voltage is obtained, and the difference [Q (t) −Q ′) between the curve Q (t) and the curve Q ′ _E (t) is obtained. _E (t)], and a method for diagnosing deterioration of the power cable.