JPH07294590A - Insulation monitoring method for live cable - Google Patents

Abstract

(57) [Summary] [Purpose] Continuation of live wire operation from the early stage of deterioration when no penetration water tree has yet occurred, due to changes in the insulation deterioration degree of the high-voltage power cable that has caused insulation failure due to the water tree. It is intended to provide a method that can be monitored while. [Configuration] A step of applying a low-frequency single-phase AC voltage excluding a commercial frequency between the neutral point and the ground of the first grounding device of the high-voltage system during hot line operation, and the low-frequency single-phase AC voltage is The value obtained by removing the commercial frequency component voltage from the voltage appearing in the low resistance inserted between the shield of the cable to be measured and the ground is measured by using the voltage that attenuates via the impedance of the first grounding device and remains on the high voltage bus as the measurement source. And a step of determining an index value representing a time-series change in insulation deterioration degree of the cable to be measured, based on the measured value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention changes the degree of insulation deterioration of a high-voltage power cable, which has caused insulation failure due to a water tree, from the initial stage of deterioration in which a penetrating water tree has not yet been generated. It concerns a method that can be continuously monitored.

[0002]

2. Description of the Related Art As a conventional technique for monitoring the change status of the insulation deterioration degree of a high voltage power cable during hot line operation by measuring the change of the insulation resistance under the live line of the cable, the DC superposition method shown in FIG. It has been known.

In FIG. 5, 1 is a high-voltage bus bar, 2 is a high-voltage grounding device, and a GPT (instrument grounding transformer) or GTR (grounding transformer) selected for the purpose of injecting a DC signal voltage.
Indicates. A low resistance 3 is inserted between the neutral point of the primary side connection and the ground, and a DC signal voltage (usually positive polarity 50V) is supplied from the DC signal power source 4 through the switch 5 to the grounding device 2
Is sent out to the high voltage bus bar 1 via the winding. On the other hand, the cable 9 to be measured has an insulation failure resistance 10 in its insulating layer. The current obtained by dividing the DC signal voltage by the value of the insulation failure resistance 10 reaches the shield end from the high voltage conductor of the cable 9 to be measured through the insulation failure resistance 10 and returns to the DC signal power supply 4 via the ground through the DC minute current measuring device 12. . The value of the insulation failure resistance 10 of the measurement target cable 9 can be calculated from the leakage current value and the DC signal voltage value read by the DC minute current measuring device 12. In FIG. 5, reference numeral 11 is a capacitor that constitutes a filter for attenuating AC components, and the actual filter circuit is more complicated, but is shown in a simplified form.

Since a high-voltage system often has a plurality of grounding devices, FIG. 5 shows a GPT as a second grounding device other than the device for injecting the DC signal voltage in FIG.
Between the neutral point of the primary side connection and the ground, there is a capacitor 7 inserted for the purpose of preventing a DC signal voltage short circuit.
Reference numeral 8 is a series vibration preventing resistor inserted in series with 7.

[0005]

The insulation failure due to the water tree of the high-voltage power cable can be directly applied to the high-voltage conductor of the DC cable even when the above-mentioned measurement method under the hot line is used or when a power failure occurs. Even if the method of measuring the leakage current is applied to the insulation layer, the insulation resistance does not decrease unless there is a water tree path that penetrates the insulation layer. However, there is a problem in that it cannot be recognized as a poor insulation, although it has decreased. In the case of measurement under power failure, if the applied DC voltage is increased, a through water tree will be forcedly created, and insulation failure that has progressed to a certain extent can be detected.
Then it is the destruction of the insulator, which deviates from the non-destructive test.

Therefore, a high insulation resistance is maintained because the number of penetrating water trees does not yet exist or the number of penetrating water trees is very small, instead of the method of measuring insulation resistance by DC leakage current. From the early stage of such deterioration, there has been a demand for the emergence of a method capable of recognizing the existence of a water tree and monitoring its change status during continuous hot-line operation.

[0007]

SUMMARY OF THE INVENTION A first aspect of the present invention is a method for monitoring the insulation of a live-wire cable, which measures and monitors changes in the insulation deterioration degree of the high-voltage power cable during hot-wire operation. Applying a low-frequency single-phase AC voltage excluding the commercial frequency between the neutral point of the first grounding device of the system and the ground, and the low-frequency single-phase AC voltage changes the impedance of the first grounding device. Based on the measured value, the voltage which is attenuated and remains on the high-voltage bus is measured as a measurement source, and the value of the voltage appearing in the low resistance inserted between the shield and ground of the cable to be measured is removed. And a step of determining an index value representing a time-series change in insulation deterioration degree of the cable to be measured.

A second invention is based on a voltage drop of an impedance inserted between a neutral point and a ground of a second grounding device other than the first grounding device for applying the low frequency single-phase AC voltage. Measuring a value proportional to the voltage remaining on the high voltage busbar, and including a correction for correcting the index value based on the state change of the high voltage system based on the measured value and the determined index value. Determining the index value,
Is further included.

A third invention is a ZP whose capacitance is known.
C (capacitor type grounding device) or measuring the voltage drop of the impedance inserted between the neutral point of the short cable and the ground, and obtaining the absolute value of the residual voltage of the high voltage bus based on the measured value. , A step of obtaining a value proportional to the voltage remaining on the high voltage bus bar at the same time as the step of obtaining the absolute value, and a value of the measurement object based on the absolute value and a value proportional to the voltage remaining on the high voltage bus bar. And a step of obtaining an index when the cable is not deteriorated.

In a fourth aspect of the present invention, a third grounding device, which is different from the first and second grounding devices, is inserted between the neutral point and the ground and the measurement is performed on the high voltage bus bar by the null method. Measuring the absolute value of the source voltage, obtaining a value proportional to the voltage remaining on the high voltage bus at the same time as obtaining the absolute value, and proportional to the absolute value and the voltage remaining on the high voltage bus The step of obtaining an index of the cable to be measured when the cable is not deteriorated based on

[0011]

Therefore, according to the first aspect of the invention, the method of measuring the insulation resistance by the direct current leakage current is not used, and the number of penetrating water trees does not yet exist, or even if they exist, the number is extremely small. It is possible to recognize the existence of a water tree and monitor its change during the continuous hot-line operation from the early stage of deterioration where high insulation resistance is maintained.

According to the second aspect of the present invention, even if there is a change in the ground impedance of the high voltage system, it is corrected and an accurate deterioration index can be obtained.

Further, in the third and fourth aspects of the invention, since the deterioration index of the cable to be measured when it is new can be estimated, the deterioration degree at the present time can be presented as a magnification with respect to the original value.

[0014]

Embodiments of the present invention will be described with reference to the drawings.

First, the principle of the present invention will be described.

In the first aspect of the present invention, a low-frequency single-phase AC voltage other than the commercial frequency is injected into the high-voltage bus bar connected to the cable to be measured during live-line operation through the neutral point of the grounding equipment of the high-voltage system. . The current flowing through the insulation layer of the cable to be measured appears as a voltage drop in the low resistance inserted between the shield of the cable and the ground. The residual voltage obtained by attenuating the commercial frequency component voltage with a filter is read from this voltage, and this is used as an index when the change in the degree of insulation deterioration of the cable is approached in time series. This index is a different value for each cable.

Further, in the second aspect of the invention, in preparation for the case where the configuration of the high-voltage system may change, that is, the case where the ground impedance of the high-voltage system changes due to the addition of a new cable, the suspension of some cables, etc. The correction of the index obtained by the invention will be made. Therefore, a value proportional to the measurement source voltage remaining on the high voltage bus was inserted between the neutral point and ground of the second grounding device other than the grounding device used for applying the low-frequency single-phase AC voltage. The voltage drop of the impedance is measured, and the value obtained by dividing the index value obtained by the first invention by this value is used as a new index including correction. This index also has a different value for each cable. Further, in the third and fourth inventions, the purpose is to estimate the origin value of the index of each cable, and therefore the absolute value of the voltage remaining on the high voltage bus bar is measured. However, since this measurement is difficult and it is not practical to carry out it every time, if the proportional constant is obtained from the relationship with the value proportional to the measurement source voltage remaining on the high voltage bus, which is obtained in the second invention,
After that, it is not necessary to measure every time.

The first method of measuring the absolute value of the measuring source voltage remaining on the high voltage busbar is to measure the counter electromotive force at which the passing current becomes zero by the null method when the impedance of the grounding device is unknown, The grounding equipment is prepared between the neutral point and the ground, and the value of the counter electromotive force is measured. The grounding device in this case is a third grounding device other than the grounding devices used in the first and second inventions.

The second method is Z for which the capacitance value is known.
Use a PC (capacitor type grounding transformer for measuring instrument) or a short cable with a similar capacitance value to measure the voltage drop value of the impedance inserted between the neutral point (shield for cable) and ground. Then, the absolute value of the residual voltage is calculated.

In order to avoid confusion, the frequency of the single-phase AC voltage to be injected into the high voltage system must be the commercial frequency used in the high voltage system.

When a frequency significantly higher than the commercial frequency is selected, the measurement sensitivity is reduced due to the low impedance of the anticorrosion layer capacitance (several μF / km if directly buried) of the cable to be measured.

If a very low frequency close to direct current, which is significantly lower than the commercial frequency, is used, the power supply unit becomes large, and the beat frequency generated between it and the commercial frequency approaches the commercial frequency. Depart from the intent to leave. Therefore, the used frequency is 1/2 of the commercial frequency.
The degree is reasonable.

Next, the voltage value will differ depending on the grounding device used for injection. Since the high voltage system is a resistance grounding system and a large capacity GTR is used and there is almost no voltage drop due to a grounding device when injecting through this, the injection voltage is set to a maximum of 50V. This is because the voltage remaining on the high voltage bus bar is a voltage (60V) that is not regarded as a so-called low voltage.
This is to keep the following).

When the high-voltage system is an ungrounded system, the GTR or small-capacity ZPC having a separate iron core for each phase must be used as the injection device, so most of the injection voltage is the grounding device. Lost in high impedance. In order to keep the residual bus voltage at 50 V at least about 10 V, the injection voltage needs to be several hundred V. But,
Since the voltage itself is applied to the grounding device as so-called V ₀ (zero-phase ground fault voltage), the injection voltage is less than the allowable V ₀ value of the high-voltage system in order to avoid malfunction of the ground detection relay. Must. The value is 86V for a 3KV high-voltage system, assuming a general 4.5% base.
171V for 6KV high voltage system 2 for 10KV high voltage system
It is 86V.

Since the V ₀ level for issuing a ground fault alarm is generally about 30%, it is still possible to double the voltage when the measurement sensitivity is poor.

As the grounding device, the above-mentioned GTR, GPT,
In addition to ZPC, the high-voltage cable itself is also a type of capacitor with a three-phase star connection, so its shield can be used as a neutral point for low-frequency voltage injection. However, there are restrictions such as that the cable is a new or near-new cable, that the absolute value of the anti-corrosion layer capacitance is not large, and that the anti-corrosion layer insulation performance is good. Therefore, a relatively short and substation pit laying cable, such as a cable for an in-house transformer or a cable for a phase-advancing capacitor, is suitable. When a capacitor is used as a detection impedance to be inserted between the shield of the cable to be measured and the ground, it has a low impedance effect on the high frequency component voltage. Therefore, a low resistance should be used except when there is a purpose of DC interruption. Since the cable to be measured originally has a considerable anticorrosion layer capacitance,
The resistance as a detection impedance inserted in parallel with this is preferably as low as the measurement sensitivity allows.

Therefore, the maximum resistance value is the third value indicated in the technical standard.
The seed ground resistance value is set to 100 Ω or less, and the minimum value is set to 10 Ω or more in order to secure the effect of the filter described later.

The voltage drop across the resistor includes the commercial frequency component voltage in addition to the voltage generated by the injected low frequency voltage. Since the cable is essentially equivalent to a capacitor with a three-phase star connection, the commercial frequency component voltage does not appear between the shield and the ground because the vector sum of the charging current of each phase is zero. However, in reality, there is some voltage, which is almost always enough. The reason is that there is an unbalance in the capacitance of each phase of the cable, or there is an unbalance in the capacitance of the entire high-voltage system, and zero-phase voltage is constantly generated. This is because the current flows, and further, the electromagnetic induction voltage is generated in the shield. These are events that are not related to cable insulation degradation, and the commercial frequency component voltage generated due to these causes must be eliminated as much as possible as a noise voltage that causes a measurement error. For this purpose, a filter for selectively eliminating the commercial frequency component voltage is attached to the voltage measuring device as an essential element.

When the high-voltage system zero-phase voltage contains a large number of high frequency waves of the third order, fifth order, etc., the noise voltage may not be sufficiently removed by the filter corresponding to the fundamental frequency. In that case, the third and fifth frequency filters are further connected. However, it is impossible to completely reduce the noise voltage to zero. To what extent the noise voltage should be removed, it can be said that the measurement can be performed with sufficient accuracy for practical use when the measurement voltage ratio before and after applying the low frequency voltage is 1/5 or less.

The measuring instrument that reads the voltage drop that occurs in the detection resistor via the filter not only reads the low frequency component voltage that is injected, but also the beat frequency that is inevitably generated between this and the commercial frequency, and further the commercial frequency. It is necessary to be able to read much higher frequency components. For this reason, it must be of a type that operates in a wide frequency range and can measure the so-called true RMS voltage. The reason why the present invention is effective is that a cable having a water tree in the insulator has a phenomenon of generating a high frequency noise voltage under a commercial frequency high electric field. To support this phenomenon, we show the following data obtained by actually measuring the voltage drop by changing the cable shield-to-ground impedance with different measuring instruments.

[0031]

[Table 1] The ratio of the 51Ω resistance to the 60 Hz impedance of the 80 μF capacitor is 1.54, but the b / a voltage ratios obtained by both the c and d voltmeters in the table above all exceed that value. That is, the effective impedance of the 80μF capacitor is
It seems that the impedance was much lower than the 60 Hz impedance due to the passage of high-frequency current, and at the same time, the true rms measuring voltmeter of d was used for the measurement with a resistance in which the path measurement effect of the capacitor disappeared. It can be seen that more high frequency component voltage is captured than the total.

Further, as a fundamental phenomenon leading to the invention of the present invention, a cable in which a water tree is generated in an insulator is
Commercial frequency Under a high electric field, there is a phenomenon that more charging current than expected at commercial frequency flows. The following data is shown as proof of this.

Sample cables 1 and 2 are both 10 KV1
× 500mm ² CV cable Removed 29 years after production, effective length of 4m each Experimental method Single-phase 50Hz, 6350V transformer with variable frequency 333V power supply connected in series as a power supply for measurement, 80μF capacitor between shield end and ground , And measure the voltage drop with a DC recording voltmeter via a transducer. Since it is an experimental circuit, there is no concern about the occurrence of a ground fault voltage, so the frequency to be superimposed is a frequency that can be generated with a roaring frequency of 0, 5 to 2 Hz that can respond with a DC recording voltmeter, that is, 49.5 to 48 Hz. As the impedance for detecting the drop, a capacitor whose impedance increases as the frequency becomes lower is used. That is, here
There is no intention to effectively capture the high frequency component voltage, but it is intended to trace the electric field formed under the high electric field of the commercial frequency at a low frequency apart from the commercial frequency.

Experimental Results A voltage measurement recording chart is shown in FIG. 1 for Sample 1 and in FIG. 2 for Sample 2. The peak-peak voltage value and the separately measured DC insulation resistance value are compared and shown in the following table.

[0035]

[Table 2] The calculated predicted voltage is also 4.62 mV in peak-to-peak voltage value. The measured value exceeds the predicted value, except for the beat frequency of 2 Hz, which seems to be due to the response of the recording voltmeter. By the way, not only the sample 1, but also the sample 2, which was judged to be good because the insulation resistance value was at the measurement limit, had a water tree that could be visually confirmed. Both samples are cables that were originally adjacent to each other.

Next, before applying a low frequency power source, 50 H
The voltage drop when z6350V alone is applied is shown as follows from FIG. 1 and FIG. This is an experiment made because the sample is a single core cable, and the case where the electric field configuration and the measurement power supply have the same voltage.

[0037]

[Table 3] Since the predicted voltage calculated is 31.1 mV in this case, 239% for sample 1 and 178% for sample 2
A large charging current reaching up to is flowing.

Therefore, although the conventional method determines that the insulation is good, this experiment shows that there is a possibility that the degree of insulation failure of the cable in which the water tree exists can be detected. However, the experiment was performed on a very short length of single-core cable using a test power supply. In order to avoid the influence of the commercial frequency noise voltage that may exist in the actual system, it is essential to eliminate the commercial frequency component voltage (including the third harmonic and the fifth harmonic fraction in some cases) from the measured value. For this purpose, it is necessary to attach a filter for selectively removing the commercial frequency component voltage to the voltage drop measuring circuit and at the same time inject a single-phase AC voltage of another frequency.

The first embodiment of the present invention will be described with reference to FIG.

FIG. 3 is roughly divided into a measurement power supply section A, a measurement cable section B, a correction-included index measurement section C and a proportional constant measurement section D.

That is, in the measurement power supply unit A, 1 is a high voltage bus bar, 2 is
Is a grounding device, and is shown by GTR in the case of a resistance grounding system in FIG. A low resistance 3 is connected between the neutral point of the primary winding of the grounding device 2 and the ground. There is an injection low-frequency single-phase AC power supply 13 in parallel with the low resistance 3, and when the switch 5 is closed, the output of the injection low-frequency single-phase AC power supply 13 is consumed by the low resistance 3 and the grounding device 2 Is sent out to the high voltage bus bar 1 through the winding impedance of.

In the measurement cable portion B, 14 is a cable selected as a measurement target, and is shown as a single-core cable in FIG. 3, but of course, a 3-core cable may be used. Reference numeral 15 indicates a defective insulation portion in which a water tree is generated in the insulating layer. It does not matter whether there is a penetrating water tree in this part. 16 is 1
Low resistance for voltage drop detection inserted between the shield end of 4 and the ground, 17 is an AC voltmeter for reading the voltage drop generated at 16, and 18 is a filter for removing the commercial frequency component voltage from the voltage drop at 16. The insertion position is not limited to the illustrated example, and the series element and the parallel element can be arranged in various shapes. If this is further required, the third and fifth commercial frequencies
A subharmonic filter is also attached.

Cable 1 to be measured at measurement cable section B
In order to measure the index indicating the change in the deterioration degree of 4, the noise voltage drop before operating the injection low-frequency single-phase AC power supply 13 is read with the AC voltmeter 17 as a reference, and then the switch 5 is turned on. When the output of the injection low-frequency single-phase AC power supply 13 is closed and sent to the high-voltage bus bar 1, the residual low-frequency voltage attenuated by the winding impedance of the grounding device 2 remains on the high-voltage bus line 1. The AC voltmeter 17 reads the voltage drop at the low resistance 16 due to the current passing through the insulating layer of the cable 14 using this voltage as the measurement source voltage. If this value is 5 times or more of the noise voltage drop read previously, it indicates that the measurement is sufficiently reliable. This value is used as an index that indicates the change in insulation deterioration of the cable.

Of course, the larger the value, the larger the degree of insulation deterioration, but it cannot be compared with other cables. Even if the cables of the same type and the same size are compared with each other, the meaning of the defective insulation portion in the length direction of the cables will be different for each cable, so it is meaningless. That is, in principle, this index is applied only to the cable, and it is significant to regard this as a time-series change.

The index value obtained at the measurement cable portion B should be basically proportional to the magnitude of the low frequency voltage remaining on the high voltage bus bar, and the magnitude of the residual voltage depends on the magnitude of the entire high voltage series at that time. It is almost proportional to the ground impedance. However, there is no guarantee that this impedance value will be constant over a long period of time. Therefore, in order to ensure the accuracy of grasping the manner in which the index changes in time series, a value proportional to the low frequency voltage remaining on the high voltage bus bar may be obtained each time as a correction factor for the index value.

For this purpose, a second grounding device separate from the grounding device 2 used for injecting the low-frequency voltage is required. If this does not exist, it is preferable to install a new three-phase reactor for this purpose. Instead of this, it is possible to use a ZPC or an appropriate cable, but since these may deteriorate, it is necessary to use the same grounding device for a long time because of the second invention. Not suitable for

In FIG. 3, in the correction-included index measuring section C,
A second grounding device 36 is used to obtain the correction element, typically a GPT. Reference numeral 7 is a capacitor inserted between the neutral point and the ground, and 8 is a series resistor for preventing vibration. What is measured now is the injected low frequency component voltage appearing between the neutral point of the grounding device 36 and the ground. This is not the low frequency voltage itself remaining on the high voltage busbar 1,
Then, the voltage drop in the primary winding of the grounding device 36 is further subtracted. Since the commercial frequency component voltage is inevitably mixed in, the filter for removing it is indicated by 20 and the reference numeral 19 is an AC voltage measuring device. Filter 2
The insertion position of 0 is not limited to the illustrated example, and the series element and the parallel element can be arranged in various shapes. If necessary, third-order and fifth-order harmonic component filters of the commercial frequency are also added to this.

In the correction-included index measuring section C, in order to measure the correction-included index, the noise voltage drop before operating the injection low-frequency single-phase AC power supply 13 is read by the AC voltage measuring device 19 for reference. Then, when the switch 5 is closed and the output of the injection low-frequency single-phase AC power supply 13 is sent out, the output of the injection low-frequency single-phase AC power supply 13 is consumed by the low resistance 3 and the first grounding device is used. It is delivered to the high voltage busbar 1 through the winding impedance of 2. Reference numeral 21 indicates the capacitance to ground of the entire high-voltage system, and the voltage based on the ratio of the impedance of the winding inductance of the grounding device 2 and the capacitance of the capacitance to ground 21 at the injection low frequency is high-voltage busbar. The voltage remains at 1.

Here, the voltage value read by the AC voltage measuring device 19 is a value proportional to the residual voltage and corresponds to a change in the capacitance of the ground capacitance 21 with each measurement. A value obtained by dividing the index for each measurement of each cable obtained from the measurement cable portion B by this value is used as a correction-included index that represents the change in the insulation deterioration degree of the cable.

The measurement of the cable portion B (first and second inventions) is not limited to when the cable is new, and many years have already passed since the cable was used. In most cases. Therefore, what is lacking in the measurement at the measurement cable section B is that there is no information on how much the indexes of the respective cables when they were new were integrated.

The proportional constant measuring section D (third invention) aims to solve this deficiency and to provide a means for estimating the index of a new product. The proportional constant measuring unit D uses a new cable whose capacitance value is known as the absolute value of the high voltage residual low frequency voltage of the high voltage bus, which is an unknown value in the measurement in the measurement cable unit B and the correction included index measuring unit C. ZP
The case where measurement is performed using C is shown.

In the proportional constant measuring section D, 26 is a cable in which the value of the electrostatic capacitance 27 for the three phases is accurately known.
A filter 29 for selectively passing the injected low-frequency component current, an AC ammeter 31, and a multiplier resistor 30 are inserted in series between the shield end and the ground. Magnification resistance 3
The resistance value of 0 is sufficiently lower than the impedance of the anticorrosion layer capacitance 28 of the cable.

The measurement by the proportional constant measuring section D is such that if the value of the AC ammeter 31 is read while the output of the injection low-frequency single-phase AC power supply 13 is being sent to the high-voltage bus bar 1, the known multiplying resistance is obtained. The bus residual voltage can be calculated from the impedance values of 30, the filter 29 and the three-phase capacitance 27.

Since it is not practical to carry out such a measurement for each measurement, the reading of the AC ammeter 31 is obtained and at the same time, the correction-included index measuring section C (second invention) is used for the second grounding. From the relationship between the primary neutral point obtained by the device 36 and the voltage drop at the ground insertion impedance (the series impedance of the capacitor 7 and the resistor 8 in the figure) (the ratio of the two), the second grounding device 36 Obtain the constant of proportionality, ie the value by which the residual voltage on the bus is multiplied by the measured voltage drop. After that, by multiplying the voltage drop value for each measurement (measurement by the second grounding device) by this proportional constant, the bus residual voltage at each time can be obtained. If the residual voltage of the bus bar is obtained at any time point, the insulation degradation degree of zero, that is, an index for a new product can be obtained by using the designed value or the measured capacitance of each cable.

It should be noted here that the anticorrosion layer capacitance value 28 is usually unknown and this acts as an error factor.

A second embodiment of the present invention will be described with reference to FIG.

FIG. 4 is roughly divided into a measurement power supply section A, a measurement cable section B, a correction-included index measurement section C, and a proportional constant measurement section D '.

In FIG. 4, the same symbols as those in FIG. 3 indicate the same components as those in FIG.

Compared with the first embodiment described above, the second embodiment is characterized in that the zero method is used in the proportional constant measuring unit, and the other points are the same as in the first embodiment.

That is, the measurement cable portion B and the correction-included index measurement portion C represent the index indicating the change in the insulation deterioration degree of the cable 14 and the change in the insulation deterioration degree of the cable 14 by the same operation as in the first embodiment. The corrected index is measured.

In the second embodiment, the absolute value of the high voltage residual low frequency voltage of the high voltage bus is measured by the zero method, and 46 in the proportional constant measuring section D'is a third grounding device used for this purpose. Is. This may not be GPT but ZPC or a small cable. A filter 42 for selectively passing an injected low-frequency component current between the neutral point and the ground, and an output of an AC ammeter 43 and a potentiometer 44 are inserted in series with the filter 42. The input of the potentiometer 44 is connected to the output of the injection low-frequency single-phase AC power supply 13. Reference numeral 45 is an AC voltmeter that measures the variable output voltage of the potentiometer 44.

This proportional constant measurement unit D'measures when the switch 5 is closed and the output of the injection low-frequency single-phase AC power supply 13 is sent to the high-voltage bus bar 1 through the first grounding device 2. In addition, by adjusting the potentiometer 44, the AC voltmeter 45 when the reading of the AC ammeter 43 shows zero or the minimum value
It becomes the high voltage bus residual voltage that the target of reading.

The output system of the injection low-frequency single-phase AC power supply 13 as the counter electromotive force does not have to be the potentiometer system shown in the figure, and the phase adjustment is performed in order to bring the reading of the AC ammeter 43 as close to zero as possible. There may be vessels.

Since it is not practical to carry out such a measurement every time, the reading of the AC voltmeter 45 is obtained, and at the same time, the correction-included index measuring section C (second invention) is used for the second grounding. From the relationship between the primary neutral point obtained by the device 36 and the ground-to-ground insertion impedance, the voltage drop in the series impedance of the capacitor 7 and the resistor 8 in the figure (ratio of the two),
A proportional constant of the second grounding device 36, that is, a value for obtaining the bus residual voltage by multiplying the measured voltage drop is obtained. After that, by multiplying the voltage drop value for each measurement (measurement by the second grounding device) by this proportional constant, the bus residual voltage at each time can be obtained. If the residual voltage of the bus bar is obtained at any time point, the insulation degradation degree of zero, that is, an index for a new product can be obtained by using the designed value or the measured capacitance of each cable.

Here, filters 42 and 29 for selectively passing the injected low frequency component current shown in FIGS. 3 and 4 are used.
If there is a parallel element in and it falls to the ground, the error increases. When the insulation constant degree of zero, that is, the index value when the cable is new, is obtained by the proportional constant measurement unit D or D ′ (the third invention or the fourth invention), the measurement cable unit B and the correction-included index measurement unit C (First invention and second invention)
Unlike the case of the independent execution, it becomes possible to judge the deterioration degree of the cable by the ratio of the current index to the original index. However, the cable does not show the same degree of water tree deterioration over the entire length, and exhibits various strengths of deterioration depending on the location. Therefore, since the average deterioration aspect of the cable as a whole is grasped, it is not possible to apply the deterioration index of high magnification as obtained in the experiment. Therefore, the following deterioration determination threshold is provisionally set.

Magnification 1.0-1.1 Less than 1.1-1.2 Less than 1.2 Good Judgment Light caution Heavy caution Treatment Continued use under supervision Replacement preparation Replacement The above table accumulates empirical data It is a work that fits together and should be modified appropriately.

[0067]

As described above, the method of monitoring the insulation of a live-line cable according to the present invention is a commercial frequency system in which a noise frequency field formed by a high frequency electric field over a wide frequency is injected into a high voltage system through a grounding device of the high voltage system. With a low-frequency single-phase AC voltage different from the frequency, the voltage appearing in the resistance inserted between the shield end of the cable to be measured and the ground is traced by measuring while excluding the commercial frequency component,
Consider the change in this value as the change in insulation deterioration,
Furthermore, since a means for correcting the change in the impedance to ground of the entire high-voltage system and knowing the absolute value of the injected low-frequency voltage remaining on the high-voltage bus was also provided, even if water trees were generated in the insulating layer, the insulation resistance could be reduced. It is possible to trace and monitor changes in the degree of insulation deterioration under the hot line, including initial defects that cannot be captured by shape.

Further, it is possible to eliminate intrusion of measurement error due to an element unrelated to insulation deterioration.

Further, even if there is a change in the impedance to ground of the high-voltage system, it can be corrected and an accurate deterioration index can be obtained. Furthermore, since the deterioration index of the cable to be measured when it is new can be estimated, the deterioration degree at the present time It can be presented as a ratio to the original value, and it has the effect of being able to monitor all three phases at once, which is not possible with the tan δ method. It is of great benefit to the industry as an insulation monitoring method for live-wire cables that is superior to conventional technology. large.

[0070]

[Brief description of drawings]

[0071]

FIG. 1 is a diagram showing an experimental result for Sample 1. FIG.

[0072]

FIG. 2 is a diagram showing an experimental result for Sample 2.

[0073]

FIG. 3 is a configuration diagram of a first embodiment of the present invention.

[0074]

FIG. 4 is a configuration diagram of a second embodiment of the present invention.

[0075]

FIG. 5 is an explanatory diagram of a conventional technique.

[0076]

[Explanation of symbols]

 A measurement power supply section B measurement cable section C correction index measurement section D, D'proportional constant measurement section 1 high voltage bus bar 2, 6, 36, 46 grounding equipment 3, 8, 16, 30 resistance 5 switch 7 capacitor 13 injection Low-frequency single-phase AC power supply for measurement 14 Cable to be measured 15 Insulation defective part 17, 45 AC voltmeter 18, 20, 29, 42 Filter 26 Cable 31, 43 AC ammeter

Claims (4)

[Claims] Claim: What is claimed is: 1. A method for monitoring the insulation of a live-wire cable for measuring and monitoring a change in the degree of insulation deterioration of a high-voltage power cable during hot-wire operation, comprising: Applying a low-frequency single-phase AC voltage excluding the commercial frequency between the characteristic point and the ground, and the low-frequency single-phase AC voltage is attenuated via the impedance of the first grounding device A step of measuring the value obtained by removing the commercial frequency component voltage from the voltage appearing in the low resistance inserted between the shield and ground of the cable to be measured as a measurement source, and the time-series insulation of the cable to be measured based on the measured value. A method of monitoring the insulation of a live-wire cable, comprising: a step of determining an index value that represents a change in the degree of deterioration. 2. A high voltage bus bar based on a voltage drop of an impedance inserted between a neutral point and a ground of a second grounding device other than the first grounding device for applying the low-frequency single-phase AC voltage. A step of measuring a value proportional to the residual voltage, and determining a correction-included index value for correcting the index value based on the state change of the high-voltage system based on the measured value and the determined index value. The method for monitoring insulation of a live cable according to claim 1, further comprising: 3. A step of measuring a voltage drop of an impedance inserted between a neutral point and a ground of a ZPC (capacitor-type grounding device) or a short cable having a known capacitance, and based on the measured value. Obtaining an absolute value of the residual voltage of the high voltage bus, obtaining a value proportional to the voltage remaining on the high voltage bus at the same time as the step of obtaining the absolute value, the absolute value and the voltage remaining on the high voltage bus The step of obtaining an index when the cable to be measured is in a non-deteriorating state based on a value proportional to
The method for monitoring the insulation of a live-wire cable according to paragraph. 4. An absolute value of a measuring source voltage remaining on a high voltage bus bar by a null method inserted between a neutral point and ground of a third grounding device different from the first and second grounding devices. Measuring the value, obtaining a value proportional to the voltage remaining on the high voltage bus at the same time as the step of obtaining the absolute value, and a value proportional to the absolute value and the voltage remaining on the high voltage bus. The step of obtaining an index when the cable to be measured is in a non-deteriorating state based on the above-mentioned.
The method for monitoring the insulation of a live-wire cable according to paragraph.