JPH08184622A - Method and apparatus for diagnosing insulation deterioration of power cable - Google Patents

Abstract

PURPOSE: To accurately detect the state of the presence or absence of the insulation deterioration of a CV cable by excluding a stray current superposed on the deterioration signal from a water tree. CONSTITUTION: A ground line current flowing to the ground line 5 of the CV cable 3 of an operating state is detected, a sheath insulating resistance is measured, and electromotive force is obtained from the product of the ground line current value and the sheath insulating resistance value. The insulation deteriorated state of the cable to be measured is judged according to the amplitude of the current value obtained by applying the voltage of reverse polarity to the force to the ground line 5 of the cable 3.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of diagnosing insulation deterioration of a power cable under live line to which an AC voltage is applied, and more particularly to a DC component current method. The present invention relates to a method and apparatus for diagnosing insulation deterioration of a power cable that determines deterioration without being affected by a stray current that causes a measurement error. 2. Description of the Related Art Insulating materials, particularly polymer materials, cause a so-called insulation deterioration phenomenon in which the insulating performance gradually decreases due to various causes during use. Degradation of the insulating material differs depending on the type of insulating material used and the place of use. It is extremely important to know the state of deterioration of this insulating material in order to prevent a dielectric breakdown accident in electric power equipment. When a cross-linked polyethylene insulated power cable (hereinafter, referred to as a CV cable) using cross-linked polyethylene as an insulating material is used in a water-rich environment, a water tree is generated in the insulator. This water tree significantly deteriorates the electrical characteristics of the CV cable, causes insulation deterioration, and is a major cause of dielectric breakdown accidents. Therefore, in order to prevent a dielectric breakdown accident due to insulation deterioration of the CV cable, it is important to know the occurrence of this water tree. Conventionally, a method of detecting a water tree of a power cable installed by detecting a direct current component under a hot line has been developed. In the conventional insulation deterioration diagnosis method, a method as shown in FIG. 2 is adopted. The insulation deterioration diagnosing method as shown in FIG. 2 uses the current flowing in the ground wire 5 connected to the shield layer 4 of the CV cable 3 which is the cable to be measured connected to the high voltage busbar 1 via the cable terminal connecting portion 2. The DC component current, which is a deterioration signal from the water tree, is detected from the inside by the measuring device 6, and the CV cable 3 (in FIG. 2, the CV cable 3 is a single-core cable is taken as an example) under the hot line. This is a method for diagnosing a deterioration state (for example, Japanese Patent Publication No. 6-14094). The method shown in this publication is
The DC component current, which is a deterioration signal from the water tree, is detected from the current flowing through the ground wire (or the neutral point ground wire of the grounding transformer) of the power cable, which is the cable under test, under the live line, and the power is detected. This is a method of diagnosing the deterioration state of the cable. That is, the DC component current is detected between the shield layer and the ground of the power cable line that is the target of the insulation deterioration diagnosis in the live line, and its polarity, size, and time characteristics are analyzed, and the presence or absence of the water tree and the water tree The size and the generation direction are detected to determine whether or not the power cable in the live line state can be continuously used. Such a deterioration signal (DC component current) to be detected is very small, and if the insulation resistance of the insulation sheath, which is the outermost layer constituting the power cable, is lowered, the ground signal is generated. A stray current is generated due to the potential difference between the power cable and the shield layer of the power cable, and the stray current is superimposed on the deterioration signal of the insulator of the CV cable. Therefore, the difference between the current value measured by the measuring instrument and the true DC component current (deterioration signal) becomes large, and it appears as a measurement error of the deterioration signal, and the current value measured by the measuring instrument, that is, an error occurs. Since the insulation deterioration diagnosis of the CV cable is performed by the deterioration signal, the proper insulation deterioration diagnosis of the CV cable cannot be performed. A stray current depending on the sheath insulation resistance of the power cable is superimposed on the deterioration signal measured by the measuring device based on the conventional method. Therefore, if the sheath insulation resistance is low, even if a large DC component current is detected by the measuring instrument, this measured value cannot be immediately judged as a deterioration signal, and the deterioration state of the cable must be accurately grasped. I can't. An object of the present invention is to eliminate the stray current superimposed on the deterioration signal from the water tree and to detect with high accuracy the presence or absence of insulation deterioration of the insulator of the CV cable. [0006] When a water tree is generated in the insulator of a CV cable, a direct current component current is generated by this water tree, but this direct current component current does not always show a constant value. None, it is constantly fluctuating. The present invention
By utilizing this constantly fluctuating DC component current, it is intended to judge whether or not a water tree is generated in the insulator of the CV cable. According to a first aspect of the invention, there is provided a method for diagnosing insulation deterioration of a power cable, which detects a ground wire current flowing in a ground wire of the power cable in an operating state, measures a sheath insulation resistance, and measures the ground wire current. Electromotive force is obtained from the product of the value and the sheath insulation resistance value, and the insulation deterioration of the cable to be measured depends on the magnitude of the current value obtained by applying a voltage having a polarity opposite to that of the electromotive force to the ground wire of the power cable. It is to determine the state. According to a second aspect of the present invention, there is provided a power cable insulation deterioration diagnosing device for measuring a sheath insulation resistance while detecting a ground wire current flowing through a ground wire connected to a shield layer of a power cable in an operating state. In a device for diagnosing deterioration of a power cable consisting of a measuring instrument and a grounding wire for grounding the measuring instrument, an electromotive force is obtained from a product of a current value obtained by the measuring instrument and a sheath insulation resistance value, and the inverse of the electromotive force is obtained. A DC generator that applies a voltage of polarity to the ground wire of the power cable, and a parallel circuit of a switch that short-circuits the measuring device by bypassing the DC generator via the grounding wire, and the measuring device and the grounding wire. It is configured by being inserted between. BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below. FIG. 1 shows an embodiment of a method and apparatus for diagnosing insulation deterioration of a power cable according to the present invention. In the figure, reference numeral 1 is a high voltage busbar, to which a CV cable 3 (a single-core cable is taken as an example in the present embodiment) which is a cable to be measured is connected via a cable terminal connecting portion 2. An alternating voltage of, for example, 3.8 kV is applied to the CV cable 3 from the high voltage bus bar 1. A ground wire 5 whose other end is grounded is connected to the shielding layer 4 of the CV cable 3, a measuring device 6 is connected to the ground wire 5, and the other end of the measuring device 6 is connected to the grounding wire 5. A parallel circuit of the DC generator 7 and the switch 8 is connected.
The other end of the parallel circuit of the DC generator 7 and the switch 8 is grounded by the ground wire 5. Reference numeral 9 is a sheath coated on the shielding layer 4 of the CV cable 3. The measuring device 6
The ground wire current flowing from the shield layer 4 of the CV cable 3 to the ground wire 5 is measured, and when a predetermined voltage (for example, 5V) is applied to the shield layer 4 of the CV cable 3, the shield layer 4 of the CV cable 3 It has a function of detecting a current (stray current) flowing to the ground side through the sheath 9 of the CV cable 3 and measuring the sheath insulation resistance. The value of the ground wire current flowing from the shield layer 4 of the CV cable 3 to the ground wire 5 measured by the measuring device 6 is multiplied by the sheath insulation resistance value of the sheath 9 of the CV cable 3 to generate the stray current. Calculate power. When measuring the ground wire current flowing through the ground wire 5 by the measuring device 6 and the sheath insulation resistance of the sheath 9 of the CV cable 3, the switch 8 is turned on. The DC generator 7 applies a reverse polarity voltage to the ground line 5 so as to cancel the measured stray current electromotive force based on the calculated stray current electromotive force. When the reverse polarity voltage is applied by the DC generator 7, the switch 8 is turned off. Next, the operation of this embodiment will be described. First, the switch 8 is turned on, and the ground line current (deterioration signal) flowing through the ground line 5 is measured by the measuring device 6 in this state. Then, a predetermined voltage (for example, 5 V) is applied to the shield layer 4 of the CV cable 3 by the measuring device 6, and the shield layer 4 of the CV cable 3 is passed through the sheath 9 of the CV cable 3 by the application of the predetermined voltage. The measuring device 6 detects the value of the electric current (stray current) flowing to the ground side and measures the sheath insulation resistance of the sheath 9 of the CV cable 3. The electromotive force of the ground wire current flowing through the ground wire 5 is calculated from the product of the two values of the ground wire current measured by the measuring device 6 and the sheath insulation resistance. Therefore, the switch 8 is turned off and the measuring device 6
The electromotive force of the ground line current obtained from the two values of the ground line current and the sheath insulation resistance measured by is input to the DC generator 7, and the DC generator 7 cancels the measured electromotive force. A voltage having a polarity opposite to that of electric power is applied to the ground line 5. Then, the ground wire current flowing through the ground wire 5 when the voltage of the opposite polarity is applied is measured by the measuring device 6. As described above, even if the voltage having the opposite polarity to the measured electromotive force is applied to the ground wire 5 by the DC generator 7, the value of the DC component current due to the water tree generated in the insulator of the CV cable 3 varies. Therefore, the ground wire current flowing through the ground wire 5 is “0”.
However, the variation of the value of the DC component current due to the water tree is detected in the form of the ground line current. When the insulation deterioration of the insulator of the CV cable 3 is progressing, the stray current cannot be completely canceled even if the counter electromotive force is applied, and the current value is on the order of nA or more. EXAMPLE A cable in which the insulation of the insulator of the CV cable 3 has no insulation deterioration (hereinafter referred to as an undeteriorated cable) and a cable in which the insulation of the CV cable 3 causes water tree deterioration (hereinafter, a deteriorated cable) (Referred to as “)”, a result of measuring the insulation state based on the insulation deterioration diagnosis method according to the present embodiment and a result of measuring the insulation state based on the conventional insulation deterioration diagnosis method are compared. And shown in Table 1. [Table 1] Table 1 shows a comparison of the results of measuring the insulation state of the undeteriorated cable and the deteriorated cable based on the conventional insulation deterioration diagnosis method. In Table 1, for the undeteriorated cable, the sheath insulation resistance is 250 MΩ and the sheath insulation resistance is 2.
Three types of sheaths each having a sheath of 5 MΩ and a sheath having an insulation resistance of 2.5 MΩ were prepared, and for each of them, the ground line current was measured by the insulation deterioration diagnosis method according to the present embodiment, and the ground line was measured by the conventional method. The current is measured and the measured value is shown. As for the deteriorated cable, the sheath insulation resistance is 250 MΩ, the sheath insulation resistance is 25 MΩ, and the sheath insulation resistance is 2.
Three types of sheaths each of which has a coating of 5 MΩ were prepared. For each of them, the ground line current was measured by the insulation deterioration diagnosis method according to the present embodiment and the ground line current was measured by the conventional method. Shows. As is apparent from Table 1, the results of measuring the ground wire current by the conventional method show that in the case of an undeteriorated cable, since the insulator of the CV cable 3 has not deteriorated, the current flowing in the ground wire 5 is The measured value should not show a large value originally. However, looking at the measurement result of the ground wire current by the conventional method in the case of the undeteriorated cable, the ground wire 5 when the sheath insulation resistance of the sheath 9 is 250 MΩ.
The measured value of the current flowing to the terminal is -1.063 nA, and the measured value of the current flowing to the ground wire 5 at 25 MΩ is -11.119 n.
A and the sheath insulation resistance are about 10 times those of 250 MΩ, and the sheath insulation resistance of the sheath 9 is 2.5 M.
When Ω and 250 MΩ are significantly reduced to 1/10 of 250 MΩ, the measured value of the current flowing through the ground line 5 is −118.844 nA, which is about 100 times that when the sheath insulation resistance is 250 MΩ. As described above, in the case of an undeteriorated cable, the measured value of the current flowing through the ground wire 5 should not originally show a large value, but the sheath insulation resistance of the sheath 9 is 250 MΩ, 25 MΩ,
All measured current values of 2.5 MΩ are on the order of nA, and the sheath insulation resistance of the sheath 9 is 250
The measured value of the current flowing through the ground line 5 increases as the resistance decreases to MΩ, 25 MΩ, and 2.5 MΩ. This is because in the case of an undeteriorated cable, the measured value of the current flowing through the ground wire 5 does not increase due to the insulation deterioration of the insulator in the state where there is no insulation deterioration of the insulator of the CV cable 3, but the measurement of the current flowing through the ground wire 5 is performed. This is because the stray current is superposed on the value, and the stray current increases as the sheath insulation resistance of the sheath 9 decreases, and this varying stray current is mainly detected. That is, even in the case of an undeteriorated cable,
The sheath insulation resistance of the sheath 9 is 250 MΩ → 25 MΩ →
As it deteriorates to 2.5 MΩ, the stray current increases,
This stray current is superimposed on the measured value of the current flowing through the ground line 5 and detected as the measured value of the current flowing through the ground line 5. However, looking at the measurement result of the ground wire current by the insulation deterioration diagnosis method of the present embodiment, in the case of an undeteriorated cable, a voltage having a polarity opposite to the electromotive force of the ground wire current flowing through the ground wire 5 is applied. When the ground wire current flowing through the ground wire 5 is measured as shown in Table 1, the measured value of the current flowing through the ground wire 5 when the sheath insulation resistance of the sheath 9 is 250 MΩ is −.
At 0.419 nA, the measured value of the current flowing through the ground line 5 at 25 MΩ is −0.257 nA, and the measured value of the current flowing at the ground line 5 at 2.5 MΩ is −0.800 nA. That is, even if the sheath insulation resistance value of the sheath 9 deteriorates from 250 MΩ to 25 MΩ to 2.5 MΩ and the stray current increases, if the insulator of the CV cable 3 does not deteriorate, the ground wire 5 is connected to the ground wire 5. It can be seen that the measured values of the flowing ground wire current are all in the pA order, and there is no influence of the stray current. Further, as is apparent from Table 1, when the ground wire current measurement result by the conventional method is seen, in the case of a deteriorated cable, the water tree deterioration occurs in the insulator of the CV cable 3, so that the ground wire 5 The measured value of the current flowing through should be originally large, and the sheath insulation resistance of the sheath 9 is 250
The measured value of the current flowing through the ground wire 5 when MΩ is -40.1
The measured value of the current flowing through the ground line 5 at 31 nA and 25 MΩ is −45.332 nA, and the measured value of the current flowing through the ground line 5 at 2.5 MΩ is −132.071 nA. However, as is clear from a comparison between the conventional example and the present embodiment in the case of an undeteriorated cable, a stray current is superposed on these values. Looking at these three results, in the measurement result of the ground wire current by the conventional method, in the case of an undeteriorated cable in which the insulator of the CV cable 3 is not deteriorated and in the insulator of the CV cable 3, water tree deterioration is observed. There is no difference of about 14 to 38 nA from the case of the deteriorated cable that is occurring, and in the method of measuring the ground wire current according to the conventional method, it is determined whether the water tree deterioration occurs in the insulator of the CV cable 3. It is substantially difficult to judge whether or not there is a change of about 14 to 38 nA in the measured value of the ground line current. In the case of a deteriorated cable, the ground wire current flowing in the ground wire 5 is applied based on the insulation deterioration diagnosis method of this embodiment by applying a voltage having a polarity opposite to the electromotive force of the ground wire current flowing in the ground wire 5. As shown in Table 1, when the sheath insulation resistance of the sheath 9 is 250 MΩ, the measured current value flowing through the ground wire 5 is −4.927 nA, and when the sheath insulation resistance of the sheath 9 is 25 MΩ, the measured current value flowing through the ground wire 5 is -7.439 nA,
Furthermore, the measured value of the current flowing through the ground wire 5 at 2.5 MΩ is -18.297 nA. Stray currents due to a decrease in the sheath insulation resistance of the sheath 9 are not superimposed on these detected values, as is clear from a comparison with the conventional method in the case of an undeteriorated cable. However, since the water tree is deteriorated in the insulator of the CV cable 3, the measured value of the current flowing through the ground wire 5 is in the order of nA.
Comparing this with the ground wire current value flowing in the ground wire 5 based on the insulation deterioration diagnosis method of the present embodiment in the case of an undeteriorated cable in which the water tree deterioration does not occur in the insulator of the CV cable 3, the sheath 9 Sheath insulation resistance value is 250M
Even if the cable deteriorates from Ω to 2.5 MΩ, in the case of an undeteriorated cable, the measured values of the ground wire current flowing in the ground wire 5 are all in the pA order, and water tree deterioration occurs in the insulator of the CV cable 3. In addition, according to the insulation deterioration diagnosis method of the present embodiment, the measured value of the ground wire current flowing through the ground wire 5 changes from the pA order value to the nA order value, so that the deterioration state can be easily detected. . As is apparent from Table 1, the ground wire current flowing through the ground wire 5 increases with the change in the insulation resistance of the sheath 9 for both the undeteriorated cable and the deteriorated cable. In particular, when the insulation resistance of the sheath 9 is remarkably lowered to 2.5 MΩ, the ground wire current flowing through the ground wire 5 sharply increases. Therefore,
Based on the insulation deterioration diagnosis method of the present embodiment, the voltage of the reverse polarity of the electromotive force of the ground wire current flowing through the ground wire 5 is applied and the ground wire current flowing through the ground wire 5 is measured. , The measured current value becomes small and becomes a pA order value. However, in the case of a deteriorated cable, a high current value can be measured even if a voltage having a polarity opposite to the electromotive force of the ground wire current flowing through the ground wire 5 is applied. That is, even if a voltage having a polarity opposite to the electromotive force of the ground line current flowing through the ground line 5 is applied, the measured current value is not corrected to the pA order. Therefore, when the insulation resistance of the sheath 9 decreases and the stray current increases, a voltage having a polarity opposite to the electromotive force of the ground wire current flowing through the ground wire 5 is applied to measure the ground wire current flowing through the ground wire 5. However, if the measured value is large, it can be determined that the water tree deterioration has occurred in the insulator of the CV cable 3. When the undeteriorated cable in which the insulator of the CV cable 3 is not deteriorated by the water tree is placed in various environments, generally, the insulator of the CV cable 3 is not deteriorated by the water tree. Even if the sheath 9 which is the outermost layer of the CV cable 3 is deteriorated and the insulation resistance is reduced, when the insulator of the CV cable 3 is deteriorated by the water tree, the outermost layer of the CV cable 3 is deteriorated. It does not mean that the insulation resistance does not change without deterioration of a certain sheath 9. Therefore, initially, when the CV cable 3 covering the sheath 9 having an insulation resistance of 250 MΩ is laid, even if the insulator of the CV cable 3 is not deteriorated by the water tree, the sheath 9 which is the outermost layer of the CV cable 3 is provided. May deteriorate and the insulation resistance may decrease to 25 MΩ or 2.5 MΩ. In this case, the value of −11.119 nA or −118.844 nA is detected in the measurement result of the ground line current by the conventional method, but the ground line current is detected based on the insulation deterioration diagnosis method of the present embodiment. When the voltage of the opposite polarity to the electromotive force of the ground line current flowing in 5 is applied and the ground line current flowing in the ground line 5 is measured, it is -0.257 nA or -0.800 n.
A value of pA is detected for both A and current value. However, initially, when the CV cable 3 covering the sheath 9 having an insulation resistance of 250 MΩ is laid, if the insulator of the CV cable 3 deteriorates due to the water tree, C
The sheath 9 which is the outermost layer of the V cable 3 may deteriorate and the insulation resistance may decrease to 25 MΩ or 2.5 MΩ. In this case, the ground line current measured by the conventional method shows that
A value of −45.332 nA or −132.071 nA is detected, the detected current value is both on the order of nA, and there is no significant difference in the numerical values from the case where the insulator of the CV cable 3 is not deteriorated, and therefore the insulation of the CV cable 3 is not generated. It is not possible to easily detect that the body has deteriorated due to a water tree. In addition, based on the insulation deterioration diagnosing method of the present embodiment, when a voltage having a polarity opposite to the electromotive force of the ground line current flowing through the ground line 5 is applied and the ground line current flowing through the ground line 5 is measured, it is -7. 439n
A or -18.297 nA and the current value are both detected in the nA order, which facilitates the CV cable 3
Since the insulator of No. 1 is not deteriorated and the numerical values are in pA order, a large difference appears, and it is possible to easily detect that the insulator of the CV cable 3 is deteriorated by the water tree. Further, from the results of Table 1, according to the measurement by the DC component current method (the measurement of the ground wire current by the conventional method), the insulation of the CV cable 3 is deteriorated by the water tree, and the sheath insulation resistance is lowered. (2.5 MΩ), the influence of the stray current on the ground wire current becomes large, and the ground wire current when the insulator deteriorates is -132.071 nA.
In contrast, the ground wire current in the case of an undeteriorated cable is -11
It is almost the same as 8.844 nA, which means that proper insulation deterioration diagnosis of the insulator of the CV cable 3 cannot be performed. However, according to the insulation deterioration diagnosing method of the present embodiment in which the electromotive force of the ground wire current flowing in the ground wire 5 and the voltage of the opposite polarity are applied, the insulation of the CV cable 3 is deteriorated by the water tree, and the sheath insulation resistance is increased. Decreases (2.5 MΩ),
Even if the stray current becomes large, a voltage (counter electromotive force) having a polarity opposite to that of the ground line current flowing through the ground line 5 is applied to cancel the stray current. Therefore, only by the magnitude of the current value,
The insulation deterioration diagnosis of the CV cable can be performed. According to the first aspect of the present invention, the ground wire current flowing through the ground wire of the power cable in the operating state is detected, and the sheath insulation resistance is measured to obtain the ground wire current value. The electromotive force is obtained from the product of the sheath insulation resistance value, and a voltage having a polarity opposite to that of the electromotive force is applied to the ground wire of the power cable, so that the deterioration from the water tree even if the sheath insulation resistance is 2.5 MΩ or less. By eliminating the stray current superimposed on the signal, it is possible to determine with high accuracy whether or not insulation deterioration has occurred in the cable to be measured, based on the magnitude of the obtained current value. According to the second aspect of the present invention, the ground wire current flowing in the ground wire connected to the shield layer of the power cable in the operating state connected to the high voltage bus bar through the cable terminal connecting portion is detected, and the sheath is detected. In a device for insulation deterioration diagnosis of a power cable comprising a measuring instrument for measuring insulation resistance and a grounding line for grounding the measuring instrument, a grounding wire is obtained from the product of the grounding wire current value and the sheath insulation resistance value obtained by the measuring instrument. A DC generator that obtains an electromotive force and applies a voltage having a polarity opposite to that of the electromotive force to the ground wire of the power cable; and a switch that short-circuits the measuring instrument by bypassing the DC generator and via the ground wire. Since the parallel circuit is inserted between the measuring instrument and the ground wire, the sheath insulation resistance is 2.5.
Even at MΩ or less, it is possible to eliminate the stray current that is superimposed on the deterioration signal from the water tree, and to determine with high accuracy whether or not insulation deterioration has occurred in the cable to be measured depending on the magnitude of the obtained current value.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of an insulation deterioration diagnosing method and apparatus according to the present invention.

FIG. 2 is a circuit diagram showing a conventional insulation deterioration diagnostic device.

[Explanation of symbols]

1 …………………………………………………… High voltage bus 2 …………………………………………………… Cable terminal connection 3… ………………………………………………… CV cable 4 …………………………………………………… Shield layer 5 ……………… …………………………………………… Grounding wire 6 …………………………………………………… Measuring instrument 7 ………………………… ………………………………… DC generator 8 …………………………………………………… Switch 9 …………………………………… …………………sheath

Claims (2)

[Claims] 1. An electromotive force is obtained from the product of the ground wire current value and the sheath insulation resistance value while detecting the ground wire current flowing in the ground wire of the power cable in the operating state and measuring the sheath insulation resistance. , A diagnosis of insulation deterioration of a power cable, characterized in that the insulation deterioration state of the cable to be measured is determined by the magnitude of the current value obtained by applying a voltage having a polarity opposite to that of the electromotive force to the ground wire of the power cable. Method. 2. A measuring instrument which is connected to a shield layer of an electric power cable in an operating state, detects a grounding line current flowing in a grounding line and measures a sheath insulation resistance, and a grounding line which grounds the measuring instrument. In an insulation deterioration diagnosis device for a power cable, an electromotive force is obtained from the product of a current value obtained by the above measuring instrument and a sheath insulation resistance value, and a DC voltage having a polarity opposite to the electromotive force is applied to the ground wire of the power cable. A power cable insulation deterioration diagnosis device in which a parallel circuit of a generator and a switch that short-circuits the measuring device by bypassing the direct current generator via a ground line is inserted between the measuring device and the ground line. .