JPH09292434A - Detection method for harmful defect with reference to impulse voltage - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture a high-reliability cable by a method, wherein a breakdown precursor pulse in a state that an AC voltage is applied after an impulse voltage is superposed, is detected and a breakdown starting point with reference to the impulse voltage is extracted. SOLUTION: With reference to a cable 1 to be tested, a three-point gap 10 at an impulse generator 7 is discharged by the assembly of a phase shifter 8 and a pulse generator 9. At this time, in order to protect a testing transformer 6 and the generator 7, a control resistance 12 and a blocking gas 13 are installed at an apparatus. In addition, by a surge voltage at a time when an impulse voltage is applied, the output of the shifter 8 is taken into a high-speed circuit breaker 5 so as not to operate the circuit breaker 5, and the high-speed circuit breaker 5 is turned off only at 1/2 to several cycles for which the impulse voltage is applied. By this circuit mechanism, an electric tree whose generation is started by the impulse voltage is developed by an AC voltage, a breakdown precursor pulse at this time is detected by a partial-discharge detection electrode 3 and a detector 4, the high-speed circuit breaker is operated by its signal, and the output of the transformer 6 is cut off instantaneously.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting harmful defects against an impulse voltage by extracting a breakdown starting point for a lightning impulse voltage in an insulator of a power cable, especially a CV cable.

[0002]

2. Description of the Related Art In order to extract the starting point of breakdown against AC voltage,
As shown in FIG. 2, the combination of the high-speed power cutoff device 5 for the high-voltage power supply and the partial discharge pulse detection device 4 using a balanced circuit captures the partial discharge pulse generated from the breakdown starting point due to the progress of the electrical tree, and changes the power supply voltage. There is a pre-breaking method in which the breaking starting point is extracted before the entire path of the cable insulator is broken by instantaneous breaking. In FIG. 2,
Reference numeral 1 is a test cable that is a target of harmful detection; CV cable, 2 is a terminal for charging, 3 is an electrode for detecting partial discharge, 6 is a transformer for testing, and 11 is a current limiting reactor.

On the other hand, with respect to the impulse voltage, the destruction phenomenon is early (on the order of μS), and therefore the above-mentioned precursor cutoff method cannot be applied. Therefore, at present, first, a destructive test is carried out by applying an overvoltage with an impulse voltage alone, and a precursor test under the above-mentioned AC voltage is carried out with the residual sample from which the breaking point has been removed, and its performance is measured with an AC single voltage. When it is significantly lower than the breakdown value, the breakdown starting point is considered to be the influence of the lightning impulse voltage applied earlier, and the breakdown starting point for the lightning impulse voltage is extracted.

[0004]

The above-described conventional technique; the method of detecting the starting point of the breakdown against the impulse voltage detected by the residual sample after the breakdown test by the impulse by applying the overvoltage has the following problems. The weakest point in the test sample has already broken, and the original starting point of failure is unknown. Due to the application of overvoltage, the original fracture strength of the extracted fracture starting point is unknown. It is unclear whether the extracted breakdown starting point is simply due to the lightning impulse voltage or the abnormal voltage at the time of breaking.

As described above, in the method of detecting the starting point of breakdown in the residual sample after the overvoltage impulse voltage breakdown test, there remains a problem in evaluating the extracted starting point of breakdown.

Therefore, the present invention has an object to provide a method for extracting a breaking starting point which is the weakest point against a lightning impulse voltage existing in a sample under test.
In addition, by extracting the starting point of breakdown, weaknesses that caused the breakdown, for example, the size and shape of foreign matter; voids, etc., can be identified. By evaluating the local electric field at these weaknesses, the electrical tree in the cable can be evaluated. To be able to calculate the generated electric field. This result has a problem in that it is possible to clarify a weak point that is harmful in cable manufacturing and to provide a method that makes it possible to manufacture a more reliable cable by implementing a countermeasure for removing the weak point. .

[0007]

[Means for Solving the Problems] Means of the present invention provided to solve the above-mentioned problems; a harmful defect detection method for impulse voltage is to detect an impulse voltage in a power cable under a voltage application of an AC breakdown voltage or less. By superimposing and detecting the destruction precursor pulse under AC voltage after superimposing,
It consists of detecting the harmful level defect in the cable insulation by extracting the starting point of breakdown for the impulse voltage.

In a preferred embodiment, the AC pre-charge voltage to be applied is the ground voltage of the AC cable,
The pre-AC voltage to be applied is the breakdown value of the AC single voltage of 3
There is provided a method of reducing the voltage to 0% or less, and a method of electrically disconnecting the high-speed cutoff device when an impulse voltage is applied to avoid the operation by the impulse voltage.

The above-mentioned breakage starting point is a portion which becomes a weak point such as a foreign substance in the insulator, a protrusion, a void. Then, the method of the present invention detects the advance pulse of the electrical tree, that is, the precursor precursor pulse, which has begun to be generated from the breakdown starting point by the impulse voltage, and instantaneously cuts off the AC voltage, thereby destroying before the whole road destruction. The starting point is extracted.

In this test method, the impulse voltage charging is performed by assembling the phase adjuster and the pulse generator, and the AC voltage zero crossing point without the influence of the AC voltage or the AC breakdown voltage is the impulse breakdown performance. It is good to carry out under the application of the AC voltage of 50% or less of the AC expected breakdown voltage, which does not affect the above. To explain the grounds, the breakdown of the insulator is caused by weak points in the insulator; foreign matter; projections; voids, and electrical trees are generated, which rapidly progress and lead to all-way breakdown. After all-paths breakdown, all the electrical energy injected into the sample flows to the breakdown path, so the weak point that became the breakdown starting point is burned and disappears. The pre-shutoff method is a method of instantaneously shutting off the power source and extracting the starting point before all the roads are destroyed under AC voltage. However, the impulse voltage is
This is a phenomenon of S order. Therefore, the impulse voltage is applied under the AC voltage of the extent (voltage value, voltage phase) that does not affect the impulse breakdown performance, and the electrical tree that starts to be generated by the impulse voltage is AC-charged. It is possible to identify the weak point in the cable insulator that is destroyed by the impulse voltage by advancing with a voltage, detecting the destruction precursor pulse at this time, and extracting the starting point of the destruction.

[0011]

1 is a test circuit diagram in a harmful detection method for impulse voltage according to a preferred embodiment of the present invention. The test method of the present embodiment is a method in which an impulse voltage superposition circuit is used in combination with the breakdown precursor interruption test circuit under AC voltage application shown in FIG.

The phase adjuster 8 corresponding to the phase of the AC voltage is applied to the test cable 1 composed of the CV cable so that the impulse voltage can be applied in an arbitrary phase of the AC voltage.
The pulse generator 9 is assembled to discharge the three-point gap 10 of the impulse generator 7. At this time, in order to protect the test transformer 6 and the impulse generator 7, each device is provided with a control resistor 12 and a blocking gap 13. Furthermore, the output of the phase adjuster 8 is taken into the high-speed interrupting device 5 so that the high-speed interrupting device 5 does not operate due to the surge voltage during impulse voltage application, and the high-speed interrupting is performed only for a half cycle to several cycles where the impulse voltage is applied. Turn off the circuit.

With the above-mentioned circuit mechanism, the electrical tree which has begun to be generated by the impulse voltage is developed by the AC voltage, and the destruction precursor pulse at this time is detected by the partial discharge emission electrode 3 and the detector 4, and the high speed signal is used by this signal. The breaker is activated to cut off the test transformer output instantaneously.

FIG. 3 is a graph showing the results of an AC superposed lightning impulse breakdown test using a 6 mm-insulated CV cable. When the applied AC voltage and the lightning impulse voltage have opposite polarities, when the AC pre-charge voltage becomes higher, A decrease in impulse breakdown voltage is observed. In this case, the lightning impulse performance under AC superposition is not the original value. Therefore, in the case of this voltage assembly, the AC applied voltage is 30% of the AC single breakdown voltage.
Must be:

According to the harmful defect detection method for impulse voltage of the present embodiment as described above, when the test cable (CV cable) is a removal cable that has deteriorated over time, and the AC voltage is a ground voltage. It is possible not only to withstand the surge voltage of the deteriorated cable, but also to evaluate the deterioration factor, for example, the harmfulness of "the size of the water tree".

[0016]

According to the present invention as described above, there is provided a method for extracting the breakdown starting point which is the weakest point against the lightning impulse voltage existing in the sample under test; By doing so, it is possible to specify the weak points that caused the destruction, for example, the size and shape of foreign matter; voids, etc., and by evaluating the local electric field at these weak points, the electric tree generated electric field in the cable can be calculated. This result shows that weak points that are harmful in cable manufacturing can be clarified, and by implementing measures to eliminate them, it is possible to manufacture more reliable cables, which are harmful defects for impulse voltage. The intended task of providing a detection method can be achieved.

[Brief description of drawings]

FIG. 1 is a test circuit diagram in a harmful defect detection method for impulse voltage according to an embodiment of the present invention.

FIG. 2 is a circuit diagram of a pre-breakdown breaking circuit as a conventional example.

FIG. 3 is a graph showing AC superimposed lightning impulse characteristics of a CV cable.

[Explanation of symbols]

 1 Test cable 2 Terminal for voltage application 3 Partial discharge detection electrode 4 Partial discharge detector 5 High speed circuit breaker 6 Test transformer 7 Impulse generator 8 Phase adjuster 9 Pulse generator 10 3 point gap 11 Current limiting reactor 12 Braking Resistance 13 blocking gap

Claims (4)

[Claims] 1. In a power cable, an impulse voltage is superimposed under a voltage below the AC breakdown voltage, and a breakdown precursor pulse under the AC breakdown after the superposition is detected to detect a breakdown starting point for the impulse voltage. A harmful defect detection method for impulse voltage, which detects a harmful level defect in the cable insulation by doing so. 2. The detection method according to claim 1, wherein the applied AC pre-charge voltage is the ground voltage of the cable, and the harmful defect is detected with respect to the impulse voltage. 3. The detection method according to claim 1, wherein the applied pre-AC voltage is 30% or less of the breakdown value at an AC single voltage. 4. The detection method according to claim 1, wherein the high-speed interruption device is electrically disconnected when an impulse voltage is applied,
A harmful defect detection method for impulse voltage that avoids operation by impulse voltage.