JP2000074980A - Method for diagnosing insulation deterioration of power cable - Google Patents

Abstract

PROBLEM TO BE SOLVED: To evaluate the water tree deterioration of a long cable by impressing an a.c. voltage of a predetermined pattern from a lower voltage to a higher voltage for a plurality of number of times, and using as an index as the ratio of a residual electric charge at a first time of the impression and a value after the impression of a predetermined number of times. SOLUTION: In diagnosing the water tree deterioration of a CV power cable or the like, for example, a test cable is grounded after a d.c. is impressed thereto, and then a predetermined a.c. voltage Va is impressed as a first a.c. charging while a step-up time is set to, for instance, 60 sec.. A second charging is similarly carried out with a predetermined voltage Vb higher than the Va after charging in a predetermined pattern. A ratio Q2/Q1 of a quantity Q1 of residual electric charges measured at the first charging of the low voltage and a quantity Q2 of residual electric charges at the second charging of the high voltage increases in correspondence with a degree of water tree deterioration. A method of continuously impressing an a.c. of two levels as the a.c. current-charging and comparing measured values of residual electric charges of the levels is also carried out to obtain information on the deterioration in the longitudinal direction or the like.

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 deterioration of insulation of a power cable, particularly, deterioration of water tree of a CV cable.

[0002]

2. Description of the Related Art When a CV cable is used under wet conditions, water trees may be formed in an insulator. The generation of water trees is a factor that lowers insulation performance. However, there is no established insulation diagnostic method for water tree degraded cables.

Many methods have been proposed for diagnosing the water tree deterioration.

As a method of diagnosing insulation deterioration capable of detecting water tree deterioration of a CV cable of 22 KV class or higher, there is a residual charge measurement. Conventionally, residual charge measurement is performed by applying a DC voltage, then grounding and removing the DC applied voltage, and then applying a single AC charging voltage to detect the resulting DC current component. I do. The residual electric charge is calculated by integrating the obtained current signal. This method has a good correlation with the degree of water tree deterioration and breakdown voltage, and is attracting attention as an insulation diagnostic method.

[0005] The mechanism of generation of residual charges in the water-tree-degraded insulator is explained as follows.

[0006] When water trees are present in the insulator, space charge is accumulated at the interface between the water tree portion and the sound insulator due to direct current application. If the DC applied voltage is removed by grounding after DC application, space charge remains inside the insulator,
This starts to move by the electric field (space charge electric field) formed by itself, and finally disappears.

When the space charge is moved and attenuated, if a ground (short circuit) is maintained, a direct current (reverse absorption current) that attenuates exponentially with time flows to the ground circuit. When the ground is opened, the DC voltage (residual voltage) recovers between the opened electrodes so that the DC voltage (residual voltage) saturates to a certain value with time. The generation of the reverse absorption current and the residual voltage is caused by a response delay of space charge inside the insulator with respect to a sudden change of an externally applied voltage such as grounding after DC application.

[0008] The residual charge recovered after the grounding of the DC power application can also be used for determining the water tree deterioration signal. However, the time required to move and extinguish the space charge in the water tree deteriorated part is extremely long, so the time required for the residual charge observed in the measurement circuit to recover is significantly slowed. I can't.

However, when an AC voltage is applied during the measurement of the residual charge, the moving speed of the space charge existing in the water tree deterioration part is increased, so that the residual charge caused by the deterioration can be realized in a short time. This is the residual charge measurement using AC charging.

[0010]

The residual charge measurement method described above has a good correlation with the degree of water tree deterioration and breakdown voltage, and has attracted attention as an insulation deterioration diagnosis method. Is to evaluate the residual charge, and the total charge detected from the entire cable is to be evaluated. For this reason, when the minute deterioration is uniform and when the extreme deterioration is locally present, the breakdown voltage is lower in the latter case,
The degree of deterioration may be determined to be equivalent. Therefore, this evaluation method has a limit as a method of diagnosing insulation deterioration of a long cable.

The present invention has been made to solve such a problem, and is a method for diagnosing insulation deterioration of a cable having deteriorated water tree, and a method capable of evaluating the degree of water tree deterioration in a long cable. It is intended to provide.

[0012]

SUMMARY OF THE INVENTION A method of diagnosing power cable insulation deterioration according to the present invention measures the residual charge by applying a DC voltage or an impulse voltage to a power cable, grounding the AC cable, and then applying an AC voltage. In the insulation deterioration diagnosis method,
As the AC voltage application, an AC voltage application consisting of an application pattern of a predetermined boosting time and a constant voltage application time was performed a plurality of times in order from the low voltage side to the high voltage side, and was measured during the first power application. diagnosing residual charge Q ₁ and a degree of insulation degradation ratio P _n = Q _n / Q ₁ between the measured residual charge amount Q _n as an index to the n-th division electrostatic characterized.

[0013] Further, in the insulation deterioration diagnosis method, a DC voltage or an impulse voltage is applied to a power cable, then the power cable is grounded, and then an AC voltage is applied to measure a residual charge.
As the AC voltage Division conductive, the AC voltage V _a of the two levels, V _x (V _a <V _x) was continuously applied, the AC voltage V _a at the residual charge side value and the AC voltage V _x
Is characterized in that the degree of insulation deterioration is diagnosed from comparison with the residual charge side constant value in (1).

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 5 is a block diagram of a measurement circuit for implementing the present invention. In FIG. 5, first, the changeover switch 5 is connected to the contact 5a side, and the DC voltage _VDC from the DC power supply device 2 is applied to the test cable 1 for a predetermined time. Thereafter, the changeover switch 5 is switched to the contact 5b, and the test cable 1 is grounded. Next, the changeover switch 5 is switched to the contact 5c to be connected to the test transformer 3 for AC power application, thereby establishing a circuit state in which measurement of residual charge and AC power application are possible. Incidentally, in the primary winding side of the test transformer 3 is connected to the induced voltage regulator 4, thereby changing the output voltage of the secondary side of the test transformer 3 from zero to V _AC. A residual charge measuring device 6 is connected between the low voltage terminal V of the test transformer 3 and the shielding layer of the test cable 1 via ground.

Next, an AC voltage is applied to the test cable 1 under the above-described residual charge measurement state. Voltage application of the AC voltage is boosted by 60 seconds before voltage application voltage V _AC from zero.

A problem in performing the insulation diagnosis of the cable by the above-described residual charge method is a difference in the degree of deterioration in the longitudinal direction of the cable.

Therefore, three types of samples A, B, and C in the state of water tree deterioration shown in FIG.

FIG. 1 shows the measurement results. In the figure, in order to pay attention to the waveform of the residual charge signal, the residual charge signal on the vertical axis is normalized at the maximum of each signal.

FIG. 1 shows the following characteristics accompanying the progress of water tree deterioration.

1) The rising voltage of the signal, that is, the voltage at which the residual charge signal is generated, increases as the deterioration proceeds (see the arrow in the figure).

2) The residual charge signal generated at a high voltage increases as the deterioration proceeds (see the arrow in the figure).

As the deterioration of the water tree progresses, the deterioration (oxidation, etc.) of the wall surface inside the water tree progresses, and the electric charge accumulated by the DC voltage application is captured by a deeper trap, and the energy required for release increases. For this reason, it is estimated that the AC charging voltage is increased. In addition, it is considered that the residual charge signal component at a high voltage increases due to an increase in deep traps as the deterioration proceeds.

Therefore, it is possible to evaluate the progress of water tree deterioration by quantitatively evaluating the change in the waveform.

According to the first aspect of the present invention, two AC charging operations are performed as a quantitative evaluation of the waveform. FIG. 2 shows the power application pattern. Calculate the ratio P = Q ₂ / Q ₁ of the residual charge Q ₁ measured during the first low voltage application to the residual charge Q ₂ measured during the second high voltage application. .

Table 1 shows the evaluation results of the test cables (6.6 KV cables) A, B, and C in which the water tree deterioration shown in FIG. In addition, FIG. 4 shows an evaluation result showing a correspondence between each P and an equivalent years of 60 Hz in the accelerated deterioration test period of the sample.

[0026]

[Table 1] In the samples B and C, although the water tree deterioration is clearly progressing, the residual charge amount between the samples B and C does not show a large difference in the conventional method for measuring the residual charge using a single voltage. on the other hand,
In the method of the present invention, P is clearly increased in accordance with the degree of water tree deterioration as shown in FIG.

Next, as a method of quantitatively evaluating the change in the waveform shown in FIG. 1 according to the second aspect of the present invention, grounding is performed after the above-described DC voltage is applied, and then the AC voltage is applied.
Two levels of AC voltage, for example, V ₁ , V ₂ (V ₁ <V
₂₎ was continuously applied, is to compare the residual charge side value I _b in residual charge side value I _a and V ₂ in V _1.

Consider a case where a residual charge signal I _X is generated at a certain AC voltage V _X. At this time, the AC voltage V
A residual charge signal I _X obtained by voltage application to _X, the AC voltage V
_a (V _a <V _X) and the residual charge signal measured when the V _X becomes two levels of voltage was continuously applied (I _a, I
Between the ^{_{x *), I X = I}} a + I X * the relationship is established.

For example, there is a possibility that the result is the same as the result of performing the ordinary residual charge measurement in the following two cases, that is, when evaluating the total residual charge amount of the cable sample.

1) A case where a region extremely deteriorated locally and a minute deterioration are mixed.

2) A case where minute deterioration is uniformly present.

At this time, the two levels of charges (V
_a , V _X , V _a <V _X ), the residual charge is continuously applied, so that the residual charge due to the minute deterioration is released at the time of Va, and as _a result, the AC section of V _X is performed subsequently. The residual charge signal measured by the electric charge is only the one caused by extreme deterioration.

In this way, it is possible to confirm the presence or absence of extreme deterioration.

As shown in these measurement results, information on the degree of water tree deterioration in the longitudinal direction of the cable can be obtained, and highly reliable insulation deterioration diagnosis can be performed.

[0035]

As described above, according to the power cable insulation deterioration diagnosis method of the present invention, the following effects are obtained.

1) In the residual charge measurement method, the degree of progress of water tree deterioration can be grasped more accurately than the conventional method by focusing on the residual charge signal. Therefore, it can be utilized as a highly reliable insulation diagnosis method for water tree deteriorated CV cables.

2) Since the residual charge method is a simple method for measurement, a simple and appropriate deterioration evaluation can be performed by combining the method with the method of the present invention.

3) It is possible to specify a water tree deterioration region caused by insulation breakdown, and to perform more accurate insulation deterioration diagnosis.

4) When the residual charge measurement is performed, it is possible to determine whether the signal is from the cable portion or a signal irrelevant to other deterioration, and can be used as verification of measurement reliability.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a change in a residual charge signal depending on the degree of water tree deterioration according to the present invention.

FIG. 2 is an explanatory diagram showing a power application method according to claim 1 of the present invention.

FIG. 3 is a graph showing a water tree deterioration state of a sample.

FIG. 4 is an explanatory diagram showing a cumulative time (year) converted from a commercial frequency (60 Hz) from evaluation results _{Pn of} samples A, B, and C;

FIG. 5 is a measurement circuit diagram for implementing the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Test cable 2 DC charging device 3 AC charging transformer 4 Induction voltage regulator 5 Changeover switch 6 Residual charge measuring device

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Manabu Yoshida 42-1 Shintomi, Futtsu City, Chiba Prefecture Inside Fujikura Futtsu Factory (72) Inventor Kazuo Watanabe 42-1 Shintomi, Futtsu City, Chiba Prefecture Fujikura Futtsu Factory Co., Ltd. (72) Inventor Tsuneo Toda 1 Higashi-Shinmachi, Higashi-ku, Nagoya-shi, Aichi Prefecture Inside Chubu Electric Power Co., Ltd. 2G015 AA27 CA05 CA06 CA08 CA20

Claims (2)

[Claims] 1. A method for diagnosing insulation deterioration, comprising: applying a DC voltage or an impulse voltage to a power cable, grounding the power cable, and then applying an AC voltage to measure a residual charge. The AC voltage application consisting of the voltage application pattern of the boosting time and the constant voltage application time is performed a plurality of times in order from the low voltage side to the high voltage side, and the residual charge Q ₁ measured at the first time application and the n-th application the measured insulation deterioration diagnosis method for a power cable, which comprises diagnosing the degree of insulation degradation ratio P _n = Q _n / Q ₁ of the residual electric charge amount Q _n as an index at the time of electrodeposition. 2. A method for diagnosing insulation deterioration, comprising: applying a DC voltage or an impulse voltage to a power cable, grounding the power cable, and then applying an AC voltage to measure a residual charge. level of the AC voltage V _a, V _x (V _a <V _x) was continuously applied, the AC voltage V _a at the residual charge side value and the AC voltage V _x
A method for diagnosing the degree of insulation deterioration from a comparison with a fixed value on the residual charge side of the power cable.