JPS61234371A - Decision on insulation capability of cable - Google Patents

Abstract

PURPOSE:To make an accurate decision on defective insulation of a cable to be measured, by judging on whether DC voltage is generated in the cable being measured from four measured values of a voltmeter to allow effective measurement of an insulation resistance value thereof with an exact accuracy. CONSTITUTION:A breaker 2 is opened and with a switch 11 closed, a selector switch 5 is turned to the changerover position (I) to select a specified resistance value. Under such a condition, the switch 11 is opened and DC voltage generated between a cable shield 5 and the ground is measured with a voltmeter 18. Then, the selector switch 15 is turned to the changeover position (II) to select a specified resistance value and the DC voltage is read with the voltmeter 18. Then, the selector switch 15 is returned to the changeover position (I), the switch 11 is closed and subsequently, the breaker 2 is closed while the cable 4 being measured is set under the AC service voltage. Again, the switch 11 is opened and a specified resistance is selected with the selector switch 15 to obtain the DC voltage by reading on the voltmeter 18. Then, the selector switch 15 is turned to the changeover position (II) to select a specified resistance and the DC voltage is obtained by reading on the voltmeter 18.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for determining the insulation performance of a power cable by determining the insulation resistance value of the insulation.

(b) Conventional technology It has been found that high-voltage cross-linked polyethylene insulated cables, which are subject to deterioration due to the water tree phenomenon, generate a significant DC voltage component within the insulation layer of the cable when AC current is applied as the deterioration progresses. . In order to detect this condition under a live line and to know its insulation resistance value, it is basically necessary to send a signal voltage for measurement to the high voltage system superimposed on the AC operating voltage. A low-voltage direct current voltage is used as the signal voltage and is sent through the grounding equipment of the high-voltage system. but,
When there are many grounding devices in a high voltage system, it is impossible to suspend all of these devices from the ground using direct current. Therefore, we give up on applying the signal voltage and use the DC voltage generated in the cable itself as the measurement power source. Conventionally, a method is known in which the voltage appearing between the shield of the cable to be measured and the ground is measured twice under conditions of changing the internal resistance of the voltmeter, and the insulation performance is determined by approximate calculation.

FIG. 5 shows a circuit configuration diagram used in the conventional insulation performance determination method. In FIG. 5, the high voltage bus 1 is connected to a large number of grounding devices (not shown), and is considered to be at ground potential in terms of a DC circuit. A high voltage bus 1 is connected to a cable terminal 3 of a cable 4 to be measured via a breaker 2.

The cable 4 to be measured is connected to the cable shield 5 and the conductor.
A local battery 8 exists in the insulation defective part of the insulation layer of the cable which has undergone considerable water tree deterioration, and its value is defined as E. Between the cable shield 5 and the ground, a shield insulation resistance 9 and a local battery 10, which represent a shield insulation defect, exist, and their values are R8 and R8, respectively. Between the cable shield 5 and the ground, a switch 11 that shorts the cable shield 5 to the ground except during measurement, and a safety arrester 12 are connected in parallel, and a π-type arrester 12 consisting of two capacitors 13 and a choke coil 14 is connected in parallel. F wave circuit is connected. A change-over switch 15 is connected to this π-type furnace wave circuit, and the change-over switch 15 sets either the substantial internal resistance 16 or 17 of the voltmeter 18 to its switching position (1) or (II).
Select by switching. Value R of internal resistance 17
It is assumed that 2' is a fraction of the value R□' of the internal resistance 16, and that the internal resistance of the voltmeter 18 is negligibly higher than R□' or R2'.

FIG. 6 shows an equivalent circuit when FIG. 5 is viewed as a DC circuit. The opening/closing 5s shown in FIG. 6 corresponds to the breaker 2 and is for indicating whether or not the cable 4 to be measured is under an AC working voltage. When the circuit breaker 2 is closed, the local battery 8 is excited and generates a voltage E which increases the value R of the resistor 7.
With this, an electromotive mud wave is formed, and the voltage E8 of the local battery 10 is increased.
is parallelized with another electromotive mud wave configured with the value R8 of the resistor 9. This is equivalent to closing the opening/closing 6s. To measure the voltage, it is necessary to measure the voltage at both ends of the parallel limb with the voltmeter 18, which is a millivolt meter, but since there is a DC resistance component R6 in the choke coil 14, it is necessary to measure the voltage at both ends of the parallel limb. join. That is, to be exact, R becomes R1-R1' + Ro or R2-R2' + Ro.
1. It is necessary to set R2 as the internal resistance of the voltmeter 18.

Next, the measurement method will be explained. In Fig. 5, the breaker 2 is closed, the switch 11 is closed, and the selector switch 5 is set to the switching position (11) to select the resistance value R□'. From this state, the switch 11 is opened and the cable shield 5 and the ground The DC voltage generated between them is measured by the voltmeter 18, and the voltage is designated as E□'.Since this is the voltage across R□', the voltage changeover switch 15 between the cable shield 5 and the ground is set to the switching position ( Switch to the river, select the resistor R1, get the town as the reading of the voltmeter 18, R2- may be unknown, but empirically assume that E knee is 9 volts, R8 - 0.2 volts, R engineering, R8. It is calculated using the following formula.

Using the above equation, it is possible to separately measure the insulation resistance value R of the insulation layer and the insulation resistance value R8 of the insulation layer without applying a signal voltage for measurement.

ρJ Means for Solving the Problems The conventional cable insulation performance determination method described above had the following drawbacks.

(1) Even if you happen to measure an insulated cable with an insulating layer, there is little chance of being able to measure it with little error, and even a correct constant will be judged as a range of errors and will be dismissed, invalidating the measurement.The reason for this. is the apparent local battery voltage inside the cable that combines E□ and R8.
h-E1E2(R□-R2)/(R□E2-R2E1)
), unless the value reaches 1 volt or more, the calculated value often contains a significant error, so it is dismissed as a large error range.

This is true R8 if the value of E is obtained above 1 volt.
Even if the value varies in the range of O to 0.5 volts, set this to 0.2
Although the error in R and R8 obtained assuming bolts is small, the chances of obtaining such a large E are limited. For example, R□-10MΩ.

In the case of an electric cable with E knee 9V, R8-10 Ω, and L: 8-0.5V, E Akebono is 0.509 V, and R work = 1.
0MΩ, E = l, R8 = 3M, l;? , If the voltage is 0.5 VOIE capacity, it will be E-2.4 sV. In this case, although both cables are defective with the same value of R, the former is rejected as having a large error range because E is less than 1V, and the measurement is invalid.In reality, 2/3 of the cables with poor insulation are defective. Since it is known from experience that the insulation is defective, the chance of finding an insulation defect with little error is 1/3 of that of a defective insulation cable.

(11) Even if the apparent local battery voltage inside the cable is observed to be lower than IV, it may be calculated accurately, but in that case it is also determined to be invalid. If the true R8 value happens to be equal to or close to the assumed value, then the calculated value is correct even if the apparent local battery voltage inside the cable is less than IV. Further, even if the interference insulation resistance value R8 is good, if E is low, the measurement will be invalidated, and the above-mentioned 1/3 chance of being able to measure may also be reduced. For example, R work = 20M
Ω.

II: 9V, R8: IMJ2. Case of l5=0.2V
In the case of jk, it becomes E-0,619v. At this time, calculation using the value of R8 of 0.2 V yields R = 20.1 MΩ, which is correct, but the measurement is invalidated based on the value of E.

(Ill) Even if the actual E-factor deviates from the assumed value, it is a major cause of error, but since calculations are made without being able to confirm this, the measurement accuracy is fundamentally poor ~1゜The purpose of this invention is to It is an object of the present invention to provide a method for determining insulation performance of a cable, which can effectively measure the insulation resistance value of a cable with high accuracy and can accurately determine whether the insulation is defective.

2) Means for Solving the Problems The method of the present invention provides a method for solving the problems in a configuration in which the electromotive current limbs of the cable to be measured are connected in parallel, and a DC-grounded bus bar and the cable to be measured are connected in parallel. Open the circuit breaker to stop the application of AC voltage to the cable, measure the DC voltage appearing between the shield and the ground twice with different internal resistances of the voltmeter, close the circuit breaker and stop applying the AC voltage to the cable. An AC voltage is applied to the cable from the bus bar, and the DC voltage appearing between the shield and the ground is measured twice using the different internal resistances of the voltmeter to obtain four measured voltage values.

(e) Effect The insulation resistance value of the insulation layer is calculated from the four measured voltage values of the voltmeter obtained by the above-mentioned measurements, the known different internal resistance values of the voltmeter, and the known simulated internal resistance value. RI The insulation resistance value R8 can be determined separately, and the voltage value E present in the cable to be measured,
There is no need to assume E8.

(f) Embodiment FIG. 1 shows a configuration diagram of a measurement circuit used in this invention,
The same parts as in FIG. 5 are given the same reference numerals, and their explanation will be omitted. In FIG. 1, an electromotive current limb consisting of a simulated local battery 19 and a resistor 20 constituting the internal resistance of this battery 19 is added in parallel between the cable shield 5 and the ground. Normally, one dry battery is used as the simulated local battery 19, and its value is set as E. Let the value of the internal resistance 20 be RA.

FIG. 2 is an equivalent circuit diagram of the measurement circuit shown in FIG. 1 as a DC circuit. In the open state for 5 seconds, which is equivalent to the state in which the contacts of the circuit breaker 2 are open, the voltage across the parallel circuit of the electromotive current limb consisting of Es and R8 and the electromotive current limb consisting of E, RA. is measured. In addition, in the closed state of opening/closing 6s, which is equivalent to the state in which the contacts of the breaker 2 are closed, both ends of the total parallel circuit in which the electromotive current limbs consisting of the E and R circuits are added in parallel to the above parallel circuit. voltage is measured. In this case, since there is a DC resistance R6 in the choke coil 14, the internal resistance of the voltmeter 18 is R□-R,
'+Ro or R2=R2'+Ro.

Next, the measurement method of the present invention will be explained.

In Fig. 1, open the breaker 2, close the switch 11, and set the selector switch 5 to the switching position (11) to select the resistance value R□'. From this state, open the switch 11 and connect the cable shield 5 and the ground. The DC voltage generated between the cable shield 5 and the ground is measured with the voltmeter 18, and the value is set as e□'.Since this is the voltage across R□', the voltage selector switch 15 between the cable shield 5 and the ground is set to the switching position ( II), select the resistor R2' to obtain 02' as the reading on the voltmeter 18, then return the changeover switch 15 to the changeover position (1), close the switch 11, and then close the breaker 2. The cable 4 to be measured is placed under an AC operating voltage.At this time, the presence or absence of a load current flowing through the cable 4 to be measured, and its magnitude, are independent of each other.Open the switch 11 again, and select the resistor R1' with the selector switch 15. Obtain a reading of 03' on the voltmeter 18, then switch the selector switch 15 to the switching position (TIJ and select the resistor R', obtain a reading of 04' on the voltmeter 18, and end the process. The four voltage values θ11e
21 θ3. The conditions for e4 are organized as shown in the table below.

The insulation resistance value R1 of the insulation layer and the insulation resistance value R8 of the insulation layer are calculated as follows. That is, the equivalent circuit when e□, e2 is measured is shown in Figure 3 (7!), and the same circuit is
When rewritten using the composite apparent resistance R8' of RA and the composite apparent voltage 88' of .1:rao, it becomes as shown in FIG. 3(B). Since in Fig. 3 holds true, we obtain. Also, e3. The equivalent circuit when e4 is measured is shown in Fig. 4 (A), and the circuit is divided into a composite apparent resistance of Rs, RA, and R, and a composite apparent resistance of R8, EA, and E. When rewritten using the voltage E ′ of
). In Figure 4, since holds true, we obtain.

As shown in formulas (2) and (4), R8, R engineering is R8, lice,
There is no need to assume these values since they are determined independently of E-engine. Note that since measurements always involve measurement errors, (2
) and (4), calculate R' and R8' using equations (1) and (3), and check that these values are not negative values and that R' ≤ R8'. You need to make sure that the conditions are also met at the same time.

If R' and R8' are negative values, check the voltmeter 18.
This is an error caused by the existence of a negative error in
It is better to correct the indicated value by shifting the indicated value of the voltmeter 18 by a voltage value corresponding to the resolution in an attempt to obtain a positive value as the calculated value.

In the case of R'-R8', or el-e3 or
In the case of 2-e4, it is assumed that the cable to be measured is a good cable that does not generate a DC voltage component E in the insulation layer under the AC working voltage, and it can be easily determined whether the DC voltage component E□ is generated. Also, if R<R8' is obtained, E
It can be determined that there is a crack.

In the above embodiment, first the breaker 2 was opened and θ□' and θ' were measured with no AC working voltage applied, and then the breaker 2 was closed and 03' and 04' were measured. , first, e', 04' with the breaker 2 closed.
You may measure 81' and 02' by opening the breaker and then measuring 81' and 02'. Also, θ□' and e' or 03' and 04
The order of measurement of ' is also different.

(g) Effects The present invention has the following advantages compared to conventional insulation performance determination methods.

(1) The measurement will not be rejected as invalid, and will always be a valid measurement.In other words, the apparent local battery voltage value of the oil inside the cable does not matter, and the polarity does not matter, so the insulation layer can be accurately measured in any case. The insulation resistance value R and the insulation resistance value R8 can be measured separately.

(II) Measurement accuracy can be improved. That is, since the local battery voltage value is not assumed, no hypothetical error occurs.

Measurement accuracy depends on the resolution of the voltmeter.

(Irrespective of whether or not DC voltage is generated due to poor insulation, it is easy to determine whether or not DC voltage is generated within the insulation layer.)

Furthermore, in this invention, the time required to measure the voltage (e□, e2) without applying the cable AC working voltage is
There is no need to remove the cable terminal from the busbar or drop it to the ground, so the pure measurement time is only a few minutes, which is an advantage.

[Brief explanation of the drawing]

FIG. 1 is a measurement circuit diagram of an embodiment of the present invention. Figure 2 is the first
Figure 3 is the equivalent circuit diagram of Figure 1 when no AC working voltage is applied, Figure 4 is the equivalent circuit diagram of Figure 1 when AC working voltage is applied, Figure 5 is the equivalent circuit diagram of Figure 1. is the measurement circuit diagram used in the conventional insulation performance judgment method, and Figure 6 is the measurement circuit diagram used in the conventional insulation performance judgment method.
FIG. DESCRIPTION OF SYMBOLS 1... High voltage bus, 2... Breaker, 3... Cable terminal, 4... Cable to be measured, 5... Cable shield, 6... Conductor, 7... Insulating layer insulation resistance , 8
...Local battery, 9...Shiyahei insulation resistance, 10...
・Local battery, 11... switch, 12... arrester, 13... capacitor, 14... choke coil,
15... Selector switch, to 1 17... Internal resistance, 1
8... Voltmeter, 19... Simulated local battery, 20...
internal resistance. (5 other people) Figure 3

Claims (1)

[Scope of Claims] In a state where an electromotive force limb consisting of a simulated local battery and a simulated internal resistance is connected in parallel between the edge of the cable to be measured and the ground, a DC-grounded bus bar and the cable to be measured are connected in parallel. opening a shield disconnector between the cable and stopping the application of AC voltage to the cable, and measuring the DC voltage appearing between the shield and the ground twice with different internal resistances of the voltmeter; Closing the shield breaker and applying AC voltage from the bus to the cable to be measured, and measuring the DC voltage appearing between the shield and ground twice using the different internal resistances of the voltmeter; From the four measured voltage values of the voltmeter, the known internal resistance value of the voltmeter, and the known simulated internal resistance value, it is determined whether or not a DC voltage component is generated inside the insulation layer of the cable to be measured. A method for determining the insulation performance of a cable, characterized by making a judgment and separately determining the insulation resistance value of the insulation layer and the insulation resistance value of the insulation layer.