KR100241314B1 - Method and device for diagnosing live cable insulation degradation by measuring AC voltage - Google Patents

Abstract

[purpose]

It provides a method of diagnosing the degree of insulation degradation of high voltage cables without using a measuring power supply during live operation.

[Configuration]

The first and second capacitors 13 and 14 with different values and the first and second resistors 15 and 16 with different values, which are selected by the switching means 12 between the shield end and the ground of the target cable 2. Each selective measurement ground is inserted by an alternating voltage voltmeter 19 which inserts four kinds of selective measurement grounding impedances which are made, and in which there is a third capacitor 17 which is usually always inserted in parallel with one of them. When one of the impedances is selected, the effective voltage value generated at both ends of the ground impedance is measured with an AC voltmeter. As a result, an electromotive current function at the equivalent center noise frequency of the insulator of the measurement target cable is calculated from the four measured voltage values obtained, and the degree of insulation degradation of the cable is determined from the magnitude of the electromotive current function.

Description

Method and device for diagnosing live cable insulation degradation by measuring AC voltage

1 is a circuit diagram showing an embodiment of the present invention.

2 is an equivalent circuit diagram of the embodiment of the present invention shown in FIG.

3 is a circuit diagram showing a conventional method for diagnosing insulation degradation.

4 is an equivalent circuit diagram of FIG.

* Explanation of symbols for main parts of the drawings

1: high voltage busbar 2: measurement target cable

5: anticorrosive layer insulation failure resistance 6: anticorrosive layer local battery

7: ground wire 12: selection switch

13: 1st capacitor 14: 2nd capacitor

15: first resistor 16: second resistor

17, 18: capacitor 19: AC voltmeter

21: insulation degradation diagnostic device

The present invention relates to a method and apparatus for diagnosing the degree of degradation of high-voltage power cable insulation without using a measuring operation during live operation.

Conventionally, as a method for diagnosing the degree of insulation degradation of a high-voltage power cable during live operation, there is no need to contact a grounding device of a high-voltage system, and a method that can be easily performed without having to prepare a signal voltage source for measurement. Known. This method detects the direct current in the ground wire current of the cable to be measured, analyzes its polarity, magnitude and time characteristics, thereby detecting the presence, magnitude, and direction of water tree in the cable insulator. It is to decide whether to continue using the cable. This diagnostic method will be described with reference to FIGS. 3 and 4.

In the third Fig, 1 is a high-voltage bus bar, 2 is a measuring object cable, 3 is the value in looking poor insulation resistance because the through water tree occurs in in the insulator of the subject cable (2) is R _I, 4 is Insulation layer local battery existing in the through-water tree generating portion, the voltage value of E _I , 5 is the corrosion resistance layer insulation poor resistance generated in the exterior of the cable to be measured 2, the value is R _S ,. 6 is an anticorrosive layer local battery present in the corrosion protection layer insulation portion, the voltage value of E _S , 7 is a ground wire for connecting the shielding terminal to the ground, 8 is a DC powder measuring device installed in the middle of the ground wire (7) Measure 4 shows the equivalent circuit of FIG. 3 and shows the inside of the DC powder measuring apparatus 8. As shown in FIG. 9 is an input resistance, the value is _RM , 10 is a micro voltage meter with an amplifier which measures the DC voltage drop of the input resistance 9, and 11 is an AC component bypass capacitor. E _I is the direct current electromotive force which appears only when the cable under measurement is under alternating current high voltage and the through-tree is generated. Therefore, the branch of the direct current function paper ( _{I I)} and the insulation failure resistance value R _I to能肢), and has an input resistance value R _M when creating a closed-circuit, since the input resistance value (R _M) the current (I _M) flowing in is calculated from the voltage measurement by the minute voltage meter 10, through the target cable You will notice that a water tree is occurring.

However, diagnosing the degree of insulation deterioration of the cable with the magnitude of the DC current I _M obtained as described above has the following problems.

A) The current value (I _M ) obtained is a direct current (E _I ), because it ignores the presence of another electromotive current functional paper consisting of the value (E _S ) of the anticorrosive layer local battery and the corrosion resistance of the insulation layer (R _S ). It is not possible to distinguish whether it is due to the insulation failure resistance value (R _I ) or whether it is due to E _S or R _S. In general, the electromotive current function due to the voltage value (E _S ) and the corrosion resistance layer insulation resistance value (R _S ) of the anticorrosive layer local battery is divided into the DC electromotive force (E _I ) and the insulation failure resistance value (R _I ). It is much larger than the resulting electromotive current function, and the appearance frequency is also high, which sometimes leads to a misjudgment of a poor insulation cable.

B) Even if the anticorrosive layer local battery voltage is zero, the dividing action of the DC current I _M due to the anticorrosive layer insulation resistance R _S cannot be avoided. The measured direct current I _M only flows in the input resistance 9 as a result of comparing the input resistance value R _M with the anticorrosive layer insulation resistance value R _S. In general, since the value of the input resistance (R _M ) is 10 kΩ to 10 kΩ, and the anticorrosive layer insulation failure resistance (R _S ) is also the same value, an error due to the classification into the R _S branches occurs.

C) If there is no through water tree, even if the amount of water tree is generated in a large amount, the continuous free electron current called DC current (I _M ) cannot flow as a space charge current or the like and is not recognized as an insulation failure.

An object of the present invention is to provide a poor insulation such as a water tree without using a signal voltage power supply for measurement, for example, even when an anticorrosive layer insulation resistance or an anticorrosive layer local battery is present, or whether a through water trimming is present or not. If this occurs, it is to provide a method and apparatus that can recognize it and diagnose the degree of insulation degradation.

The method for diagnosing insulation degradation of the present invention includes four selective measurement ground impedances each consisting of a first measurement capacitor having a different value, a first measurement resistor having a different value from a second measurement capacitor, and a second measurement resistance; Inserting the capacitors in parallel between the shielding ends of the live power cables and the ground respectively; and when selecting at least the first and second measurement resistances, the third capacitors are connected between the shielding ends of the live power cables and the earth, respectively. One of the four selective measurement ground impedances is connected between the shielding and the earth, and the AC effective voltage generated at both ends of the selective measurement ground impedance is measured by an AC voltmeter indicating a true effective voltage excluding the intrusion of DC current. Measuring the four alternating current effective voltage values by performing the measurement on the four selective measurement ground impedances, respectively. Calculate the first electromotive current function at the fundamental frequency of the live cable insulator and the second electromotive current function at the equivalent center noise frequency of N times the fundamental frequency from the measured AC effective voltage values And determining the degree of insulation degradation of the cable from the magnitude of the second electromotive current function.

An apparatus for diagnosing insulation degradation of the present invention is an apparatus used for the method for diagnosing insulation degradation of a live cable, comprising: a third capacitor connected between a shielding end of the live power cable and a ground, a first measuring capacitor having a different value, and a second measuring capacitor; The four selective measurement ground impedances comprising a first measurement resistance and a second measurement resistance different from the measurement capacitor of and one of the four selective measurement ground impedances are selected so as to And switching means for connecting both ends of the selected selective measurement ground impedance to the AC voltmeter connected in series to a DC-current blocking capacitor.

By using the AC voltage field of the fundamental frequency (commercial frequency) actually used, the noise current electromotive current function over a wide frequency naturally occurring according to the degree of insulation failure in the insulation of the cable to be measured is used for the diagnosis of insulation degradation. In order to calculate the second electromotive current function at the equivalent center noise frequency N times the fundamental frequency representing the noise current electromotive current function, the value selected by the switching means between the shielding end of the target cable and the ground is different. In the presence of a third capacitor which inserts four selective measurement ground impedances consisting of first and second capacitors and first and second resistors of different values, and is usually always inserted in parallel with one of them, When one of the selective measurement ground impedances is selected by an AC voltmeter excluding the intrusion of DC current, the effective voltage value generated at both ends of the ground impedance is measured by an AC voltmeter. As a result, an electromotive current function at the equivalent center noise frequency of the insulator of the measurement target cable is calculated from the four measured voltage values obtained, and the degree of insulation degradation of the cable is determined from the magnitude of the electromotive current function.

First, the principle that the insulation degradation diagnosis method of the present invention is established will be described. In order to measure the insulation resistance of the cable to be measured directly, it is necessary that the insulation resistance is lower than the measurable degree. However, in the recent plastic insulated electric power cable, if a path through the insulator does not occur, the insulation resistance value is very high, no matter how many water trees are generated, and it is practically impossible to measure the value. In addition, even if a through water tree is generated, if the number of errors is small, the insulation resistance value is too high, so that measurement is difficult. Therefore, in the present invention, the measurement method using the direct current is abandoned, and instead, the insulation deteriorated cable is measured by the noise current electromotive current function over a wide frequency naturally occurring under the AC voltage field by the fundamental frequency according to the degree of degradation thereof. The AC effective voltage generated in the capacitor and the resistor having an impedance of 100 kΩ or less inserted between the shielding end of the target cable and the ground was measured to obtain a mistake in the electromotive current function, and the magnitude of the insulation deterioration was judged by the magnitude. The present invention does not require any trouble such as preparing a signal power supply for measurement and connecting the signal voltage to the high voltage system by connecting it between the neutral point of the grounding device of the high voltage system and the earth as in the conventional diagnostic method. The measurement is carried out by simply inserting one of the measuring circuits in the middle of the ground wire of the shielding end of the circuit. In addition, since it handles a relatively large AC current voltage, there is no requirement for the high performance of the measuring instrument, and since the AC effective voltage is measured at low impedance, excluding the DC component, a through pass is generated in the insulator even if it does not occur or is locally generated. Even if the battery is present or absent in the insulator or the anticorrosive layer, it does not become a problem at all, and it is possible to detect from the beginning of insulation deterioration that was not recognized in the prior art using direct current.

In the present invention, the noise current electromotive current function spans a wide frequency range, but using the concept of an equivalent center noise frequency, it is assumed that a noise current electromotive current of a single frequency exists, and the frequency is N of the fundamental frequency. It is assumed to be double. Of course, N is unknown, but there are three other unknowns. One is the anticorrosive layer capacitance of the cable to be measured (shielded metal to ground capacitance (C _S ), and the other two are the electromotive current function (x) at the fundamental frequency (commercial frequency) F _HZ and the equivalent center noise frequency ( NF _HZ ) is the electromotive current function y. The former electromotive current function y should be essentially zero if there is no unbalance in the degree of insulation deterioration and insulation layer capacitance for each cable phase. In practice, however, there are some values due to existing capacitance imbalances and electromagnetic induction phenomena, including information on the imbalance of the degree of insulation deterioration of each phase, which is indistinguishable from other factors. Therefore, this value is used as a reference value and is not used as an index indicating the degree of insulation deterioration. Experience shows that the N value seems to decrease as insulation deterioration progresses, but the change rate is small, and it is not possible to adopt it as an index indicating the degree of deterioration deterioration at this time, and it is also a reference value. C _S ) is also important in the process of calculating and knowing the value, but it is not an indicator of the degree of insulation degradation.

To obtain the four unknowns described above means to establish a four-dimensional system of equations and to interpret them. For this purpose, it is necessary to perform four voltage measurements with different conditions. First, in order to obtain the anticorrosive layer capacitance (CS), the capacitors C ₁ and C ₂ whose values are known and different in value (impedance at each commercial frequency are 100 Ω or less) are measured. By selectively inserting between the shielding stage and the ground, and measuring the effective voltages E _C1 and E _C2 at both ends of the corresponding capacitors, the anticorrosive layer capacitance C _S can be calculated by calculation. At this time, if the cable length to be measured is extremely short, C _S may be calculated as zero (0) or negative (-). As such, the following measurement becomes meaningless, and Co 'is always inserted between the shield and the ground as a dead capacity to avoid this. Therefore, if the obtained capacitance value is Co, Co is Co = Co '+ Cs, not true anticorrosive layer capacitance. In this case, when Co is always calculated to be smaller than the capacitance Co ', it is a result of the measurement error that occurs when the cable to be measured is short. Subsequently, Co = Co' is substituted. The value of the capacitance Co´ should always be several 로는 in the simulation result, but there is a possibility of reducing the number of parts by using C ₁ or C ₂ as the role of Co´ even if Co´ is not prepared separately. To mention.

Next, selectively insert the resistors R ₁ and R ₂ whose values are already known and whose values are different from each other at 100 kΩ or less between the shielding end and the ground of the cable to be measured, and at both ends of the corresponding resistances. AC effective voltages E _R1 and E _R2 are measured. As a result, four voltage measurements E _C1 , E _C2 , E _R1 , and E _R2 are solved to solve the system of equations (Co-Co 'is obtained as anticorrosive layer capacitance CS), the fundamental frequency of equivalent center noise frequency. Four unknowns of the electromotive current function x at the magnification ratio N and the fundamental frequency F _HZ can be obtained. In solving the system of equations, the root of which the value of N is 1 or negative is discarded. N = 0 is accepted.

Importantly, in the case of measuring the voltage measured values E _R1 and E _R2 , the capacitive element must be connected in parallel with the resistor R ₁ or R ₂ , and if this is not the case, E _R1 / E _R2 = R ₁ / Only R ₂ does not involve a frequency component, so no answer can be obtained. For this reason, the capacitance Co 'is always inserted. The degree of insulation deterioration of the cable is determined from the magnitude of the electromotive current function y at the equivalent center noise frequency obtained by the calculation, and the treatment is determined. For the time being, we offer the following criteria for 11KV and CV cables.

TABLE 1

Next, an embodiment of the present invention will be described with reference to FIG. 1 is a high voltage bus, 2 is a cable to be measured, 5 is an anticorrosive layer insulation failure resistance present in the cable 2 to be measured, and its values are R _S and 6 is an anticorrosive layer local battery, and its electromotive force is E _S.

20 is the anticorrosive layer capacitance of the cable 2 to be measured, and its value is C _S. The figures do not need to be known in advance to determine the results of later measurements. 7 is a ground wire which connects the shield end of the cable to be measured 2 to the ground. This grounding wire 7 is connected to the selector switch 12 of the insulation deterioration diagnosis apparatus 21 and each end of the capacitor | condenser 17 and 18 in the measurement of insulation diagnostics. Although the selection switch 12 was shown by the rotational switching switch which has a lap mechanism in the drawing, any selection switch 12 may be sufficient as long as it has the same switching function. As the selection position of the selection switch 12, the E position of the direct ground, the PC ₁ position for selecting the first capacitor 13, the PC ₂ position for selecting the second capacitor 14, and the first resistor 15 are always selected. There is a PR ₁ position to select and a PR ₂ position to select the second resistor 16. The other ends of the four types of ground impedances consisting of these first and second capacitors 13 and 14 and the first and second resistors 15 and 16 are collectively grounded. The capacitor 17 is always inserted between the shielding end of the cable to be measured 2 and the ground irrespective of the selection position of the selection switch 12. It is essential that the capacitor 17 exists in parallel with them when the first resistor 15 or the second resistor 6 is selected and measured, but the first capacitor 13 or the second capacitor 14 is selected. Its existence is not essential. However, since it is not preferable to deliberately remove the condenser 17 in order to require an extra mechanism, it is always inserted. An AC voltmeter 19 in which a capacitor 18 is connected in series to cut off a DC component that may invade is used to measure the AC effective value voltage between the shield terminal and the ground of the cable 2 to be measured. Are connected to each other. Since the measurement target of the AC voltmeter 19 is not only a fundamental frequency voltage but a distortion waveform voltage containing high frequency components, a type of so-called true effective voltage can be measured in order to obtain accurate measurement results. A voltmeter of the type which measures the average value and converts it into the effective value which regarded this as a sine wave cannot be adopted.

2 shows an equivalent circuit of the embodiment of FIG. In FIG. 2, the above two electromotive current functions are provided. One is the first electromotive current function having the electromotive force function (x) at the fundamental frequency (commercial frequency) F _HZ , and the other is the electromotive force function (y) at the frequency N times the fundamental frequency, that is, NF _HZ . It is a second electromotive current branch having. None of the electromotive current functions is equivalent to the constant current generator of infinite internal resistance because it transmits a constant current regardless of the change in the impedance of the external circuit. Co represents the total capacitance value of the capacitors 17 and 20 shown in FIG. 1, that is, Co '+ Cs. The current sent from the DC noise electromotive current branch consisting of the electromotive force E _S of the anticorrosive layer local battery and the anticorrosive layer insulation resistance value R _S is blocked by the capacitor 18 and measured by the AC voltmeter 19. Measurement voltage at selection of PC ₁ position (E _C1 ), measurement voltage at selection of PC ₂ position (E _C2 ), measurement voltage at selection of PR ₁ position (ER ₁ ), measurement voltage at selection of PR ₂ position (E _R2 ) does not affect. In addition, even if there is another DC noise electromotive current function branch composed of DC electromotive force E ₁ and poor insulation resistance R ₁ inside the insulator of the measurement target cable 2, the influence does not appear in the measured value.

Next, the actual measurement procedure will be described. First, it connects via the insulation deterioration diagnostic apparatus 21 in the middle of the ground line 7 of the shielding end of the cable 2 to be measured. At this time, the selection position of the selection switch 12 is set in advance to the E position, that is, the position of the direct ground. Next, the AC effective value voltages E _C1 , E _C2 , E _R1 , when each of the PC ₁ position, the PC ₂ position, the PR ₁ position, and the PR ₂ position are selected (the order of selection may be arbitrary regardless of this order). Measure each E _R2 . In the circuit of the insulation diagnostic measuring device 21, a large time constant exists directly in DC, but the measurement can be performed within a short time regardless of the measurement of AC voltage. When the measurement in four positions is complete | finished, the insulation deterioration diagnostic apparatus 21 is removed from the ground wire 7, and it moves to the measurement of another measurement object cable.

The four alternating current effective voltages measured from the equivalent circuit of FIG. 2 are represented by the following four expressions.

Solving the simultaneous equation by substituting the measured AC effective voltage value into the above equation, it is possible to obtain the values of the total capacitance value (Co), multiples (N), electromotive current functions (x) and (y). All three values except for the electromotive current function (y) are retained as reference values, and the insulation deterioration is diagnosed by the magnitude using the electromotive current function (y) at a frequency N times the fundamental frequency. Determine. When the total capacitance Co is always calculated to be smaller than the capacitance Co ', Co = Co'. Roots where N is 1 or negative are invalid and other roots are found. N = 0 is valid. Further value capacitance in the PC ₁ position (C _1), the capacitance value (C _2), the resistance (R ₂₎ of the resistance value (R _1), PR ₂ in the PR ₁ in the PC ₂ position The specific value of must be less than 100 kW in terms of safety, since it is the four ground impedances of the cable to be measured. For example, C ₁ = 80 ms, C ₂ = 240 ms, R ₁ = 51 ms, and R2 = 24 ms.

Next, FIG. 2 shows a specific example in which the present invention is applied to a practical 11KV, CV cable line. The four ground impedances use the numerical values given above. The other fundamental frequency (F) is F = 60 Hz, and the capacitance value is Co'3.45 kHz.

TABLE 2

On the other hand, the results of the investigation by the prior art of measuring the insulation resistance by superimposing DC 50V on an AC field under these cables lively are shown in Table 3.

TABLE 3

Summarizing the results of Tables 2 and 3, feeder 1 can only recognize the presence of poor insulation layer resistance even in the prior art, but it is clearly recognized in the method of the present invention. The feeder 2 is judged to be good in the prior art and similarly distinguished from the feeder 3 which is judged to be good, but the difference from the feeder 3 is clear in the method of the present invention. Probably, feeder 2 is considered to be a cable in an initial state of insulation deterioration, which is difficult to recognize because there is no pass through the insulating layer or the number thereof is very small, and the insulation resistance value is extremely high. In other words, it can be seen that the method of the present invention is more sensitive than the conventional art by direct current, and thus, has a great power in recognizing the initial state of the insulation deterioration with little or no insulation layer through-pass.

As described above, the active cable insulation degradation diagnosis method and apparatus according to the present invention have the following advantages. In other words,

A) No need to make contact with the earthing device of the high-voltage system or special preparation of the signal voltage power supply for measurement, and it can be economically inexpensive and technically easy and safe at any time to diagnose insulation degradation.

B) The size of the cable to be measured does not matter.

C) Whether or not there is a local battery in the insulation layer of the cable to be measured or a local battery in the corrosion protection layer

D) Do not ask for the presence or absence of a path through the insulating layer. Therefore, it is possible to recognize the change in the degree of deterioration of the insulating layer when the insulating layer insulation resistance is high enough to be impossible to measure in the conventional art.

E) Selective measurement Because the ground impedance is very small, it is not affected by the magnitude of the insulation layer insulation resistance.

F) Since the insulation deterioration diagnosis device is not affected by the DC time constant, the measurement time is extremely short.

The above-mentioned advantages are suitable for detecting the progress of insulation degradation earlier than the state of cable insulation deterioration recognized by the current technology, and especially in the diagnosis of deterioration of special high-pressure plastic insulation cable, for which no firm insulation degradation diagnosis technology is currently established. Most suitable.

Claims (2)

Four selective measurement ground impedances consisting of a first measuring capacitor having a different value and a second measuring capacitor having a different value and a first measuring resistor having a different value, and a third capacitor with a shielding end of the live power cable. The four selective measurement ground impedances in parallel between the earths and in the state in which the third capacitor is connected between the shield terminal of the live power cable and the earth when at least the first and second measurement resistances are selected. Is selected between the shielding and the earth, and the AC effective voltage generated at both ends of the selected measurement ground impedance is measured by an AC voltmeter indicating a true effective voltage excluding the intrusion of DC current. Measuring four AC effective voltage values for each of the two selective measurement ground impedances, and measuring the four AC effective voltage values. Calculates the first electromotive current function at the fundamental frequency of the live cable insulator and the second electromotive current function at the equivalent center noise frequency of N times the fundamental frequency. A live cable insulation deterioration diagnostic method according to an alternating current 4-voltage measurement, comprising the step of determining the degree of insulation deterioration of the cable from the magnitude. When selecting at least the first and the second measurement resistance, in the state where the third capacitor is connected between the shield end of the live power cable and the earth, one of the four selective measurement ground impedances is selected, and the shield and the earth are selectively connected. The AC effective voltage generated at both ends of the selective measurement ground impedance is measured by an AC voltmeter indicating a true effective voltage excluding the intrusion of DC current, and the measurement is performed for each of the four selective measurement ground impedances. The AC effective voltage value is measured, and from the measured four AC effective voltage values, the first electromotive current function at the fundamental frequency of the insulator of the live cable and the second at the equivalent center noise frequency of N times the fundamental frequency A live cable that calculates the pre-current function of the cable and determines the degree of insulation degradation of the cable from the magnitude of the second pre-current function. An insulation deterioration diagnostic apparatus for use in diagnosing a single insulation deterioration, comprising: a third capacitor connected between a shield end of the live power cable and a ground, a first measurement capacitor having a different value, and a second measurement capacitor having a different value; Selecting the four selective measurement ground impedances of the other first and second measurement resistances and one of the four selective measurement ground impedances, and connecting the shielding of the live cable to the earth; And means for simultaneously connecting both ends of the selected selective measurement ground impedance to the AC voltmeter connected in series to a capacitor for DC current interruption.