JP3879989B2 - Insulation degradation diagnosis method for power cables - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The power cable insulation deterioration diagnosis method according to the present invention is used to diagnose the insulation deterioration state of a crosslinked polyethylene insulated power cable (hereinafter referred to as “CV cable”).
[0002]
[Prior art]
The water tree generated when the CV cable is used in a submerged or humid state is connected to the electric tree, causing a dielectric breakdown and causing a power transmission accident. Therefore, it is important to know in advance the deterioration state of the insulation of the CV cable due to a water tree or the like in order to prevent the occurrence of a transmission line accident. As a method for diagnosing insulation deterioration of CV cables for such purposes, the residual charge method which is the subject of the present invention is disclosed in, for example, “Mitsubishi Cable Industrial Time Report” No. 98 issued by the applicant company in October 2001. It has been described and is conventionally known.
[0003]
FIG. 3 shows a circuit diagram when the residual charge method is carried out. The CV cable 1 to be diagnosed is formed by covering the periphery of the cable conductor 2 with an insulating layer 3 and further covering the periphery of the insulating layer 3 with a shielding layer 4. Of these, the shielding layer 4 is grounded by a ground wire 5. When performing such insulation deterioration diagnosis of the CV cable 1, as shown in FIG. 3, a diagnostic device 11 is connected between the end of the cable conductor 2 and the ground, and the CV cable 1 is grounded. Is configured. First, a DC voltage is applied between the cable conductor 2 and the shielding layer 4 (the insulating layer 3) by the DC power source 7 with the contact of the switch 6 set to the point a. Thereby, electric charges are accumulated between the cable conductor 2 and the shielding layer 4 (the insulating layer 3).
[0004]
Next, by switching the contact of the switch 6 to the point b, the application of the DC voltage is stopped and the cable conductor 2 is grounded by the ground circuit 8. Thereby, the electric charge accumulated between the cable conductor 2 and the shielding layer 4 (the insulating layer 3) is recovered to the ground side through the diagnostic circuit. However, at this time, if a water tree is generated in the insulating layer 3, only the charges accumulated in the water tree remain in the water tree for a long time. It should be noted that the amount of charges (residual charges) remaining in the water tree increases as the degree of deterioration of the insulating layer 3 by the water tree (the degree of growth of the water tree) increases. However, residual charges in the water tree can be easily released from the water tree by applying an AC voltage to the insulating layer 3.
[0005]
Then, an AC voltage is applied between the cable conductor 2 and the shielding layer 4 (the insulating layer 3) by the AC power source 9 with the contact of the switch 6 switched to the point c. As a result, the residual charges in the water tree are discharged from the water tree, and further recovered to the ground side through the diagnostic circuit. Further, while applying an AC voltage as described above, a DC signal (DC current or DC voltage) generated in the diagnostic circuit is measured by the measuring instrument 10 during the time before and after the start of the application of the AC voltage. . As a result, the residual charge recovered to the ground side as described above is detected as the DC signal. The amount of residual charge detected at this time appears as a jump component ΔQ of the DC signal, as shown in FIG. 4 which is a measurement result. The magnitude of the jump component ΔQ increases as the amount of residual charges recovered to the ground side increases, that is, as the degree of deterioration of the insulating layer 3 due to the water tree increases. Therefore, by observing the magnitude of the jump component ΔQ, it is possible to diagnose the degree of deterioration of the insulating layer 3 (ie, the greater the jump component ΔQ is, the more the deterioration is advanced).
[0006]
[Problems to be solved by the invention]
However, when the insulation deterioration diagnosis by the residual charge method as described above is actually performed, an irregular DC noise component is included in the measured value of the DC signal, as shown in FIG. The detection accuracy may be reduced. If the detection accuracy of the jump component ΔQ is reduced in this way, it is difficult to ensure the reliability of the insulation deterioration diagnosis, which is not preferable. On the other hand, various causes for the generation of the DC noise component as described above are conceivable. The main cause is the DC stray current that enters the diagnostic circuit through the grounding system as shown by a chain line in FIG. The presence of ΔI can be mentioned.
In view of the above situation, the power cable insulation deterioration diagnosis method of the present invention can perform reliable insulation deterioration diagnosis by removing the DC noise component included in the measured value of the DC signal. Invented as much as possible.
[0007]
[Means for Solving the Problems]
The power cable which is the subject of the present invention is formed by covering the periphery of the cable conductor with an insulating layer and further covering the periphery of the insulating layer with a shielding layer. The power cable insulation degradation diagnosis method of the present invention applies a DC voltage between the cable conductor and the shielding layer constituting the power cable to be diagnosed in order to perform such insulation degradation diagnosis of the power cable. . Next, the application of the DC voltage is stopped, and the cable conductor and the shielding layer are grounded. Next, an alternating voltage is applied between the cable conductor and the shielding layer, and a direct current generated in the circuit including the power cable and the ground to be diagnosed during the time before and after the start of the application of the alternating voltage. Measure the signal. Then, by evaluating the measured DC signal (observing the magnitude of the jump component appearing in the DC signal), the degree of deterioration of the insulating layer constituting the power cable to be diagnosed is diagnosed (the jump component is It is determined that deterioration is progressing as the value increases.
In particular, in the method for diagnosing deterioration of insulation of a power cable according to the present invention, prior to evaluating the DC signal, the DC signal and substantially the same condition (environmental condition) as the power cable to be diagnosed. , Measured in a separate circuit including ground and other power cables (for example, other phase power cables that form the same cable line with the power cable to be diagnosed) Mutual differential operation with the signal is performed. Thereby, a DC noise component included in the DC signal to be evaluated (that is, a DC signal generated in a circuit including the power cable to be diagnosed and the ground) is removed.
[0008]
[Action]
According to the insulation degradation diagnosis method for a power cable of the present invention as described above, a reliable insulation degradation diagnosis can be performed by removing a DC noise component contained in a measured value of a DC signal to be evaluated. That is, as described above, the DC stray current that is the main cause of generation of the DC noise component enters the circuit including the power cable and the ground through the ground system of the power cable. In addition, when considering a plurality of circuits each including a power cable and grounding, the power cables constituting each circuit are laid under substantially the same conditions (environmental conditions, grounding conditions, etc.). If so, the waveforms of the DC stray currents that enter the circuits through the grounding system are almost equal to each other. Further, the waveforms of the DC noise components generated in the circuits based on the presence of the DC stray current are also substantially equal.
[0009]
Therefore, as in the present invention, a DC signal (DC signal generated based on residual charge recovery + DC noise component generated based on the presence of stray current) measured in a circuit including a power cable to be diagnosed and ground. ) And a DC signal (DC stray current) measured in a circuit including other power cables and ground laid under substantially the same conditions (environmental conditions, grounding conditions, etc.) as the power cables to be diagnosed The DC noise components contained in these DC signals can be canceled out by performing a differential operation with each other. In other words, it is possible to remove a DC noise component contained in a DC signal to be evaluated (a DC signal generated in a circuit including a power cable to be diagnosed and ground). Therefore, in the case of the present invention, a highly reliable insulation deterioration diagnosis can be performed.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
1 and 2 show an example of an embodiment of the present invention. In the case of this example, when the insulation deterioration diagnosis of the CV cable 1 to be diagnosed is performed, the CV cable 1a of another phase that forms the same cable line 12 together with the CV cable 1 is used. The shielding layers 4 and 4 constituting each of the CV cables 1 and 1a are collectively grounded by a ground wire 5 respectively.
[0011]
When performing the insulation deterioration diagnosis of the CV cable 1 to be diagnosed, as in the case of the conventional method shown in FIG. 3, the cable conductors constituting the CV cable 1 to be diagnosed as shown in FIG. Diagnostic device 11 between the end of 2 and the ground {however, in FIG. 1, illustration of switch 6, DC power source 7, and ground circuit 8 (see FIG. 3) is omitted, and only AC power source 9 and measuring instrument 10 are provided. Illustrated. } To form a closed circuit including the CV cable 1 and the ground (hereinafter referred to as “first closed circuit”). At the same time, a measuring circuit 10a is connected between the end of the cable conductor 2 constituting the CV cable 1a of the other phase and the ground, and a closed circuit including the CV cable 1a of the other phase and the ground (hereinafter, “ Second closed circuit "). In the state where the first and second closed circuits are configured as described above, the grounding conditions of the CV cables 1 and 1a are substantially equal to each other. Also, a DC signal (DC current or DC voltage) generated in the first closed circuit measured by the measuring device 10 and a signal generated in the second closed circuit measured by the measuring device 10a. A DC signal (DC current or DC voltage) can be input to the differential calculator 13.
[0012]
If the diagnostic circuit as shown in FIG. 1 is configured, next, as in the case of the conventional method shown in FIG. 3, first, the cable conductor 2 constituting the CV cable 1 to be diagnosed and A DC voltage is applied between the shielding layer 4 (insulating layer 3) by the DC power source 7 (see FIG. 3). Thereby, electric charges are accumulated between the cable conductor 2 and the shielding layer 4 (insulating layer 3). Next, the application of the DC voltage is stopped, and the cable conductor 2 constituting the CV cable 1 is grounded by the ground circuit 8 (see FIG. 3). Thereby, the electric charge accumulated between the cable conductor 2 and the shielding layer 4 (insulating layer 3) is recovered to the ground side through the first closed circuit. However, at this time, as described above, the charges accumulated in the water tree generated in the insulating layer 3 remain in the water tree for a long time. Next, an AC voltage is applied by the AC power source 9 between the cable conductor 2 and the shielding layer 4 (insulating layer 3) constituting the CV cable 1. Thereby, the residual charge in the water tree is recovered to the ground side through the first closed circuit.
[0013]
In the case of this example, the AC voltage is applied as described above, and the DC voltage generated in the first closed circuit by the measuring instrument 10 during the time before and after the start of the application of the AC voltage. The signal is measured by the measuring instrument 10a for each DC signal generated in the second closed circuit. Then, the DC signals measured by both the measuring devices 10 and 10a are input to the differential arithmetic unit 13, respectively. Then, the differential arithmetic unit 13 performs a differential operation between these DC signals and outputs the result.
[0014]
In the case of this example, the DC signal measured by the measuring instrument 10 (generated in the first closed circuit) passes through the CV cable 1 to be diagnosed as in the case of the conventional method shown in FIG. A DC signal generated on the basis of the recovery of the electric charge accumulated between the cable conductor 2 and the shielding layer 4 (insulating layer 3) to the ground side and the first closed circuit enters through the ground system. A combined signal with a DC noise component generated based on the presence of the DC stray current ΔI is as shown by a solid line α in FIG. On the other hand, the DC signal (generated in the second closed circuit) measured by the measuring instrument 10a is based on the presence of the DC stray current ΔI ′ that enters the second closed circuit through the ground system. Only the generated DC noise component is obtained, as shown by a broken line β in FIG. In the case of this example, the CV cables 1 and 1a constituting the first and second closed circuits are laid under substantially the same conditions (environmental conditions, grounding conditions, etc.). For this reason, the waveforms of the DC stray currents ΔI and ΔI ′ that enter the first and second closed circuits through the grounding system are substantially equal to each other (ΔI≈ΔI ′), and the DC stray currents ΔI, The waveforms of the DC noise components generated based on the presence of ΔI ′ are also almost equal to each other.
[0015]
Therefore, as described above, the DC signal measured by the measuring device 10 (generated in the first closed circuit) and the DC signal measured by the measuring device 10a (generated in the second closed circuit). If the differential operation with the signal is performed by the differential operation unit 13, the DC noise component contained in both the DC signals can be canceled out. In other words, the DC noise component included in the DC signal (generated in the first closed circuit) measured by the measuring device 10 can be removed. As a result, as an output of the differential arithmetic unit 13, as shown by a solid line γ in FIG. 2, a DC signal that does not include a DC noise component {the cable conductor 2 and the shielding layer constituting the CV cable 1 to be diagnosed 4 (the DC signal generated based on the recovery of the electric charge accumulated between the layers 4 and the insulating layer 3 to the ground side) can be obtained. In the DC signal obtained in this way, a jump component ΔQ that becomes an index when evaluating the degree of deterioration of the insulating layer 3 clearly appears. For this reason, in this example, a highly reliable insulation deterioration diagnosis can be performed.
[0016]
【The invention's effect】
As described above, according to the insulation deterioration diagnosis method for power cables of the present invention, it is possible to perform a reliable insulation deterioration diagnosis by removing the DC noise component included in the measured value of the DC signal to be evaluated. .
[Brief description of the drawings]
FIG. 1 is a diagnostic circuit diagram showing an example of an embodiment of the present invention.
FIG. 2 is a diagram showing a measurement result of a DC signal.
FIG. 3 is a diagnostic circuit diagram when a conventional residual charge method is performed.
FIG. 4 is a diagram showing a measurement result of a DC signal.
FIG. 5 is a diagram showing a measurement result of a DC signal including a DC noise component.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1, 1a CV cable 2 Cable conductor 3 Insulating layer 4 Shielding layer 5 Grounding wire 6 Switch 7 DC power supply 8 Grounding circuit 9 AC power supply 10, 10a Measuring device 11 Diagnosis device 12 Cable line 13 Differential computing unit

Claims (1)

In order to perform insulation deterioration diagnosis of a power cable in which the periphery of the cable conductor is covered with an insulating layer and the periphery of the insulating layer is covered with a shielding layer, the cable conductor constituting the power cable to be diagnosed and the shielding layer A DC voltage is applied to the cable conductor, and then the application of the DC voltage is stopped and the cable conductor and the shielding layer are grounded. Then, an AC voltage is applied between the cable conductor and the shielding layer, and During the time before and after the start of voltage application, the DC signal generated in the circuit including the power cable to be diagnosed and the ground is measured, and the power to be diagnosed is evaluated by evaluating the measured DC signal. Prior to evaluating the DC signal in the method for diagnosing the degree of deterioration of the insulation layer constituting the cable, the DC signal is evaluated before the DC signal is evaluated. By performing a differential operation on each other with a DC signal measured in another circuit including ground and another power cable laid under substantially the same conditions as the power cable to be diagnosed A method for diagnosing deterioration of power cable insulation, comprising: removing a DC noise component contained in the DC signal to be evaluated.

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