JP3756084B2 - Insulation degradation diagnosis method for power cables - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an insulation deterioration diagnosis method for power cables, and is particularly useful for diagnosis against water tree deterioration.
[0002]
[Prior art]
One of the causes of insulation deterioration of rubber / plastic power cables is known to be water tree deterioration caused by the interaction between electric field and moisture.
This water tree deterioration is a tree-like deterioration trace, which is generated from irregular portions at the interface between the insulator and the semiconductive layer, or from foreign matter or voids in the insulator, and extends in the direction of the electric field of the insulator over time. If left unattended, it will eventually lead to dielectric breakdown 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, as the water tree deterioration diagnosis method, the residual charge method shown in FIGS. It is.
[0003]
In this residual charge method, the cable insulator is charged as shown in FIG. 4A by applying a voltage such as a DC voltage or an impulse voltage, and then the cable conductor and the outside are shown in FIG. A circuit in which the shield layer is short-circuited (the cable conductor is grounded) and the residual charge Q is accumulated in the cable, and then the AC power supply Vac and the capacitor C are connected in series as shown in FIG. , The discharge of the residual charge Q is accelerated by the application of the AC voltage by the AC power source Vac, the temporal change of the residual charge caused by the accelerated discharge is detected by the capacitor C, and the insulation deterioration of the cable is detected from the detection result Diagnosing the condition.
[0004]
In the above, even if there is no charge of the AC voltage Vac, the release of the residual charge proceeds with the time constant determined by the polarization and electrical conduction under the electrical stress generated by the residual charge Q. For example, when charging is performed with a direct current charge of less than 1 minute, residual charges are not completely released even after 24 hours.
Thus, as described above, under the application of an AC voltage, the residual charge is moved up by the applied electrical stress and released from strong restraint, resulting in an increase in the release rate and consequently an increase in the amount of change in the residual charge. , Leading to improved sensitivity.
[0005]
In the residual charge method, it is ideal to forcibly release all of the residual charges by applying an alternating voltage. FIG. 5 shows this ideal residual charge method.
(B) in FIG. 5 shows the temporal change of charge in this ideal residual charge method, that is, the current state, and in the period T1 until the AC voltage Vac is applied, the residual charge is caused by polarization or electrical conduction. It exhibits a low current value IE because it is slowly released by the electric field based on its residual charge with its inherent time constant controlled, and all of the remaining charge is rapidly forcedly released by applying an AC voltage for Δt time, In the period T2 after the AC voltage application is stopped, the electric current value is zero because substantially all of the charges are released and zero.
The residual charge corresponding to this current state is obtained by integrating the current I (t) over time, and the change state of the charge Q (t) is shown in FIG.
In FIG. 5C, ΔQ indicates a change in the amount of charge due to only application of the AC voltage Vac, and ΔQ can be grasped sufficiently clearly on the basis of the rising start point a and the saturation end line bb.
[0006]
[Problems to be solved by the invention]
However, there is residual charge accumulated in the cable insulator due to factors unrelated to cable insulator deterioration such as the interface between different insulators in the cable connection box (hereinafter referred to as pseudo residual charge). This pseudo-residual charge is released with a specific time constant τE, but remains in the water tree insulation deterioration part that is deteriorated by the water tree and its binding force is weakened (hereinafter referred to as this residual charge). ), The pseudo-residual charge restrained by the sound insulation part that is not affected by the restraining force is less likely to be emitted, and the pseudo-residual charge is preferential in spite of the AC voltage application. Therefore, when the cable length is short and the residual charge amount due to the water tree is small, the residual charge amount is small with respect to the total charge amount, and the change in charge amount ΔQ based on the application of the AC voltage is small. It just becomes slower.
When ΔQ becomes slow in this way, as shown in FIG. 6C, the above-described rising base point a becomes unclear and it becomes more difficult to clearly understand ΔQ.
[0007]
It is an object of the present invention to perform a water tree insulation deterioration diagnosis of an existing power cable by a residual dragon load method (after charging and charging the insulation layer of the cable to be diagnosed, grounding the cable conductor once, Switching to the stage where the change in charge is detected, applying an alternating voltage to accelerate the release of the residual charge in this detection stage, and diagnosing cable insulation deterioration from the detection result in the detection stage) The purpose is to improve the workability of the cable insulation deterioration diagnosis by clearly displaying the forced residual charge change due to the application of the alternating voltage.
[0008]
[Means for Solving the Problems]
According to the method for diagnosing degradation of power cable insulation according to the present invention, the cable conductor is temporarily grounded after charging and charging the insulation layer of the cable to be diagnosed. In a method of diagnosing cable insulation deterioration by applying an alternating voltage to accelerate discharge and measuring a temporal change in residual charge by a detector, an invalid signal cancellation voltage source Vx and its voltage Vx are detected in a detection circuit. A variable resistor Rx for adjusting the discharge time constant is provided, the voltage source Vx is switched on almost simultaneously with switching to the detection stage, and the detection signal from the switching time to the application of the alternating voltage is canceled. The discharge time constant is adjusted, and the alternating voltage is applied under this adjusted state .
[0009]
According to another insulation degradation diagnosis method for a power cable according to claim 2, in the other insulation degradation diagnosis method for the power cable according to claim 1, an operation for canceling the detection signal during non-application of the alternating voltage is canceled or canceled. This is performed by applying a voltage.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows an example of an insulation deterioration diagnosis device for a power cable used in the present invention.
In FIG. 1, reference numeral 1 denotes a cable to be diagnosed, with an external shielding layer grounded, and is usually a rubber / plastic power cable.
Vdc is a DC power supply for charging the cable insulator d, and an impulse power supply can be used instead of the DC power supply. Reference numeral 21 is a charging contact, 22 is a cable conductor grounding contact, and 23 is an alternating voltage application / detection contact. Re is a grounding resistor. Reference numeral 11 denotes a movable contact for the contacts 21 to 23, which is connected to the cable conductor 10.
Vac is a power source for applying an alternating voltage to the cable insulator d. As will be described later, Vac is applied to accelerate the discharge by forcibly moving the accumulated residual charge without accumulating new residual charge. AC voltage or damped oscillation voltage can be used. Cd is a detection capacitor connected in series between the alternating voltage source Vac and the ground, and a detection end is constituted by both ends of the detection capacitor Cd. SW ₁ is a switch connected in parallel to the capacitor Cd.
Vx is invalid signal canceling voltage source for canceling the invalid detection signal appearing at the detection end, SW ₂ is a switch for canceling a voltage source. Rx is a variable resistor for giving the time constant τE≈CdRx to the invalid signal cancellation voltage.
[0011]
The cable 1 is assumed to have water tree deterioration, and in order to diagnose insulation deterioration of the cable according to the present invention, in FIG. d is charged by the DC power source Vdc, and then the three-point switch is switched to the ground contact 22 so that the cable conductor 10 is held at the ground potential, and the cable conductor and the cable outer shielding layer are substantially short-circuited. .
Further, the three-point switch is switched to the alternating voltage application / detection contact 23. Even after this switching, if the switch SW1 is on, the grounding state is maintained.
Next, the switch SW1 is turned off to switch to a stage where a change in the residual charge of the cable is detected.
[0012]
In Figure 2 (b) shows the state of application of the alternating voltage Vac at the detection stage, the point t ₀ is the time of turn off the switch SW _1. At this time t ₀ , the residual charge (main residual charge) accumulated in the water tree degradation portion of the cable insulator and the residual charge (pseudo residual charge) accumulated due to factors irrelevant to the degradation of the dissimilar insulator interface of the junction box, etc. ) Remains in the cable insulation.
In FIG. 2A, a part Q _E of these residual charges is released based on the inherent time constant in a period from the time t _O to the start of alternating voltage application t ₁ (hereinafter referred to as self-release). ) By applying the alternating voltage, a considerable portion Qs of the remaining charge amount is forcibly accelerated and released, but a part is left, and after the alternating voltage application is stopped, the remaining partial charge Q ′ _E becomes the inherent characteristic. Self-released based on the time constant τ _E of
[0013]
In a conventional residual charge method, caused by the current generated by a portion of the residual charge Q _E are self-released based on the specific time constant, corresponding parts Qs of residual charges by the application of the alternating voltage is forced release The current generated by the self-discharge of the current and a part of the charge Q ′ _E based on the inherent time constant is all detected by the detection capacitor Cd and displayed as a voltage.
As a result, as shown in FIG. 5, for the self-discharge based on the time constant of the charge (Q ′ _E ) remaining after the forced discharge, the saturation termination line bb of the remaining charge is recognized by applying an alternating voltage. Further, when the residual charge is relatively small and the change in charge amount ΔQ is weak due to the application of the alternating voltage, the rising start point a becomes unclear and the alternating voltage Vac is applied. As described above, it is difficult to clearly grasp the change in the charge amount.
[0014]
However, in the method for diagnosing power cable degradation according to the present invention, the residual charge before forced discharge (Q _E ) is self-discharged based on a specific time constant and the remaining charge after forced discharge (the above-mentioned Q ′ _E ) is treated as an invalid detection signal by the self-discharge based on the inherent time constant, and is canceled by the invalid signal cancellation voltage source Vx.
That is, to turn almost simultaneously switch SW ₂ is switched to the stage to turn off the switch SW ₁ of the apparatus shown in FIG. 1 to detect a change in the remaining charge of the cable, invalid signal cancellation voltage by adjusting a further resistor Rx An invalid detection signal is canceled by a canceling voltage source created by applying a time constant τ _E ≈C _d R _x to Vx, and the alternating voltage source Vac is operated under this canceling state to provide the cable insulator d. An alternating voltage is applied.
[0015]
By applying this alternating voltage, the residual charge Qs (main residual charge and pseudo residual charge) is forcibly moved to expedite the emission, and a sudden change of the residual charge, that is, a large current flows. A sudden change of the residual charge, that is, a large current signal appears at the end, and a temporal variation of the residual charge due to application of the alternating voltage is output to the output end of the detection capacitor Cd.
However, since the fluctuation of the residual charge based on the inherent time constant τE during no 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 grasped accurately and easily.
[0016]
FIG. 2B shows the residual charge amount change curve Qs (t) displayed and output to the detection unit in the present invention and the residual charge amount displayed and output to the detection unit in the conventional method without canceling the invalid detection signal. Q (t) is a change curve Q (t), and Q _E (t) is a residual charge amount change curve based on self-emission while no alternating voltage is applied, that is, a residual charge amount change curve based on an invalid detection signal.
[Equation 1]
Q (t) = Qs (t) + Q _E (t)
Whereas it is difficult to grasp the change in charge amount ΔQ based on forced discharge by applying alternating voltage with the display curve Q (t) of the conventional method, according to the display curve Qs (t) of the present invention, Can understand clearly.
[0017]
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. In general, it is preferable to set the elevating time to 20 seconds or less.
[0018]
FIG. 3B shows an example of a temporal change curve Q ( t ) of the residual charge when the alternating voltage V ac is applied, as shown in FIG.
This curve Q (t) is obtained by superimposing a detection signal based on alternating voltage application and a detection signal (invalid detection signal) not based on alternating voltage application. 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, the curve QE (t) based on the detection signal not applied with the alternating voltage, for example, can be expressed as a quadratic function at ² + bt + c or [1 / (a′t ² + b′t + c ′)] − 1 / c ′, or an exponential function a ″ [1-exp (−b ″ t)], a, b or a ′, b ′, so as to satisfy three or two points of the curve Q (t). It can be obtained approximately by determining c ′ or a ″, b ″, or by linking the curve portions of the curve Q (t) when no alternating voltage is applied.
If this approximate function is Q ′ _E (t), Q (t) −Q ′ _E (t) is calculated, and as shown in FIG. Q's (t) can be displayed clearly.
[0019]
【The invention's effect】
In the insulation degradation diagnosis method for power cables according to claims 1 and 2 of the present invention, the residual charge change ΔQ is slow because the amount of residual charge that is easily forcibly released is relatively small, or the alternating voltage is relatively low. Since the residual charge amount change ΔQ based on the forced discharge due to the application is low, the residual charge amount change Q _E based on the self-release before applying the alternating voltage as shown in FIG. The start point of the boundary becomes unclear, or the end saturation line becomes unclear due to the residual charge amount change Q ′ _E based on self-discharge after forced discharge. Even in the case where it is difficult to grasp the quantity change ΔQ from the residual charge change curve Q (t), in the present invention, the residual charge quantity change Q _E based on the self-release before applying the alternating voltage and the residual charge quantity based on the self-release after the forced release are applied. consisting of a change Q _'E Treated as load volume change curve Q _E (t) invalid charge amount changed, because they are displayed this residual charge amount under invalid charge amount cancel variations in operation change Q _S (t), the residual based on forced release Ru can clearly grasp a change in the charge amount ΔQ.
[Brief description of the drawings]
FIG. 1 is a drawing showing an example of a diagnostic device used in a method for diagnosing degradation of power cable insulation according to the present invention.
FIG. 2 is a drawing used to explain a method for diagnosing power cable insulation degradation according to the present invention using the apparatus of FIG. 1;
FIG. 3 is a diagram illustrating an example of residual charges in a method for diagnosing insulation deterioration of a power cable.
FIG. 4 is a diagram showing the principle of a residual charge method.
FIG. 5 is a drawing used to explain an ideal embodiment of the residual charge method.
FIG. 6 is a drawing used to explain another embodiment of the residual charge method.
[Explanation of sign]
1 Diagnosis cable 11, 21, 22, 23 Switch contact Vdc DC power supply Re Ground resistance Cd Capacitor Rx Variable resistance Vac Alternating voltage source Vx Canceling voltage source S Detector

Claims (1)

After charging and charging the insulation layer of the cable to be diagnosed, the cable conductor is temporarily grounded, and in the detection stage, detection is performed by applying an alternating voltage to accelerate the discharge of residual charges after a predetermined alternating voltage non-application time. In a method for diagnosing cable insulation deterioration by measuring a temporal change in residual charge, a detection circuit is provided with a variable resistance Rx for adjusting an invalid signal cancellation voltage source Vx and a discharge time constant of the voltage Vx. The voltage source Vx is switched on almost simultaneously with switching to the detection stage, and the discharge time constant is adjusted so as to cancel the detection signal from the switching to the application of the alternating voltage. A method for diagnosing deterioration of power cable insulation , wherein the alternating voltage is applied in step ( b).

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