JPH0349073B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a cable insulation measurement method for measuring the insulation resistance of a high-voltage power cable under a live line. FIG. 1 is a diagram showing a conventional method of measuring the insulation resistance value of a high-voltage power cable under a live line. It is extremely important for the preventive maintenance of cables to measure the insulation resistance values of the insulating layer and anti-corrosion layer of high-voltage power cables under live lines without stopping the cable's power transmission and to understand the degree of cable deterioration. be. In Fig. 1, 1 is a high-voltage bus, 2 is a grounding transformer, 3 is a DC power supply that sends the measurement voltage to the high-voltage bus 1 through the grounding transformer 2, 4 is a switch, and 5 is a grounding transformer 2. This is a safety grounding circuit inserted between the neutral point and the ground. Although the details of the circuit are not shown in the diagram, normally the neutral point is connected directly to the ground, and during measurement, the grounding with low AC impedance is continued. In terms of direct current, the neutral point is insulated from the ground, and a wave circuit is provided toward the power source to suppress the intrusion of alternating current. On the other hand, the lower end of the cable 6 to be measured is grounded through a safety grounding circuit 7, and is laterally guided through a switching switch 8 to an insulation measuring circuit consisting of two limbs. One limb is a series circuit of a power supply 9 for measuring the insulation resistance of the corrosion protection layer and an ammeter 10 for measuring the insulation resistance of the corrosion protection layer, and the remaining limb is a circuit having an ammeter 11 for measuring the insulation resistance of the insulation layer. A conventional measuring method will be specifically explained with reference to FIG. First, while the switch 4 remains open, that is, no measurement voltage is applied to the high-voltage conductor of the cable 6 to be measured, the direct grounding circuit in the cable safety grounding circuit 7 is opened, and the switching switch 8 is opened. Assuming that is switched to the position shown in the figure, the cable runs from the power supply 9 for measuring the insulation resistance of the corrosion protection layer → the poor insulation resistance of the corrosion protection layer → the earth → returns to the power supply 9 via the ammeter 10 for measuring the insulation resistance of the corrosion protection layer. Creates a closed circuit. Therefore, the corrosion protection layer insulation resistance value can be known from the indication of the ammeter 10. Next, when the switching switch 8 is turned to the opposite side and the switch 4 is closed to send out the voltage for measurement from the DC power supply 3, the voltage for measurement is applied to the high voltage conductor of the cable 6 to be measured through the high voltage bus 1. If there is an insulation resistance defect in the insulation layer of the cable 6 to be measured, the current that appears through the defect returns from the insulation layer insulation resistance measuring ammeter 11 to the DC power supply 3 via the ground. Therefore, the insulation resistance value of the insulation layer can be known from the indication of the ammeter 11. However, in this conventional measurement method, two kinds of power supplies are prepared in advance: a power supply for measuring the insulation resistance of the corrosion protection layer and a power supply for measuring the insulation resistance of the insulation layer. Since it is fixedly wired together with auxiliary equipment, it has the disadvantage that any cable connected to another high voltage bus cannot be measured. Only a limited location and a limited number of cables are to be measured, that is, only cables that are already installed and included in the equipment are to be measured. Furthermore, one set of equipment is fixed equipment for a limited range of cables;
Attempting to widen this range would require an enormous amount of equipment and facility costs, which also had the disadvantage of being uneconomical. The object of the present invention is to provide a live line that eliminates the above-mentioned drawbacks by utilizing the phenomenon that a deteriorated power cable, which has been discovered by the inventor, generates a significant DC voltage component in the insulation layer under a live line. An object of the present invention is to provide a cable insulation measurement method for measuring the insulation resistance of a high-voltage power cable under a live line. The present invention will be described in detail below with reference to the drawings. FIG. 2 is a measurement circuit diagram for each cable to be measured used in the present invention. Although it is shown as a fixed wiring in FIG. 2, it goes without saying that all or part of this may be used as a portable rotational rotation measuring circuit. 1
is the high-voltage busbar, 6 is the cable to be measured that connects to it, and its low end is the safety grounding circuit for the cable 7
grounded through. Although illustration of the details of the safety grounding circuit 7 is omitted, normally the cable cable or shield is connected directly to the ground, and during measurement, the low-impedance grounding is continued for AC, but for DC, the cable is connected directly to the ground. The shield is insulated. That is, a weak grounding state is established due to capacitance. However, this does not prevent the capacitance from being in a grounded state even when not measuring. 14 and 15 have different resistances and are alternately connected to the ammeter 12 by the switching switch 13. In Fig. 2, two resistor limbs are connected to the lower side of the cable, and the ammeter 12 inserted into the ground side is connected to the resistor 14 by the switching switch 13.
Also, although it is shown as being switched and connected to the resistor 15,
Of course, this is the case as described above, i.e., two resistive limbs are inserted on the ground side, and the ammeter 1 is inserted on the cable side.
2 is a resistor 14 or a resistor 15 by a switching switch 13
It may be configured such that the connection is switched between the two. Here, the ammeter 12 will operate as a voltmeter with the resistor 14 or the resistor 15 as its multiplier resistance.
It is assumed that the cable 6 to be measured has a defective part in the insulation layer and a defective part in the insulation layer in the anticorrosion layer, and this area is divided into 16
When magnified as shown in the circle, it is 17
18 is a high voltage conductor, 18 is a resistor, 19 is a resistance with poor insulation in the insulation layer, 20 is a DC power supply generated in the defective part of the insulation layer, 21 is a resistance with poor insulation in the anti-corrosion layer, 22
is a local battery occurring in an area with poor insulation of the corrosion protection layer. To further refer to 20 and 22 above, 2
0 is CV where insulation deterioration has progressed due to water tree generation
It is a direct current power source that appears prominently in the insulation of a cable under live wires, and its voltage reaches several volts with polarity such that the soft side has a positive potential and the conductor side has a negative potential. 22 is 0 to 0.5V, which makes the weak metal side that occurs in the insulation defective part of the corrosion protection layer approximately positive potential.
The local battery normally does not reach voltages above 1V. FIG. 3 is a diagram showing an equivalent circuit of the region 16 where insulation defects exist. FIG. 3a shows an equivalent circuit of region 16. R _I is the insulation failure resistance of the insulation layer corresponding to 19, E _I is the battery corresponding to the DC power supply 20 generated in the insulation layer insulation failure part, R _S is the corrosion protection layer insulation failure resistance corresponding to 21, E _S is a battery corresponding to the local battery 22 occurring in the corrosion protection layer insulation defect area. In terms of a DC circuit, the high voltage conductor 17 is at the same potential as the ground, so a circuit with poor insulation layer consisting of R _I and E _I and a circuit with poor insulation layer with corrosion protection layer consisting of R _S + E _S are connected to the ground 18 and the ground. (conductor 17) and are completely parallel to each other, resulting in an equivalent circuit as shown in FIG. 3a. In terms of an electrical circuit, the circuit of FIG. 3a can be expressed as being equivalent to the circuit of FIG. 3b. That is, a circuit consisting of an apparent internal resistance R and an apparent internal power source E exists between the cable cable and the ground. In the present invention, the measurement circuit shown in FIG. 2 is used to measure the apparent internal resistance R and the apparent internal power source E shown in the equivalent circuit of FIG. After first knowing the values, approximate values of the insulating layer insulation failure resistance R _I and the anticorrosion layer insulation failure resistance R _S shown in FIG. 3a are determined by the method described in detail below. The measuring method of the present invention will be specifically described below. In FIG. 2, if the cable to be measured 6 is directly grounded to the ground inside the cable safety grounding device 7, first remove it.
The reading of the ammeter 12 when the switching switch 13 is turned to the side of the resistor 14 with a resistance value R _I , and the reading of the ammeter ₁₂ when it is turned to the side of the resistor 15 of a resistance value R ₂ .
Take E ₂ . As mentioned earlier, the ammeter 12 is connected to the resistor 1
4 and the resistor 15 are multiplier resistors of different values, so the values of reading E ₁ and E ₂ can be read out as voltage values (volts), respectively. Since this circuit has a large capacitance and a large specific number, it is necessary to take enough time to wait for the voltage value to saturate before taking a reading on the voltmeter. Reading E ₁ and E ₂ ,
The following relationship exists between E and R. E ₁ = R ₁ / R + R ₁ E, E ₂ = R ₂ / R + R ₂ E E ₁ / E ₂ = R ₁ / R + R ₁ / R ₂ / R + R ₂ By rearranging the above equation, R and E can be found as follows. It will be done. R=R ₁ R ₂ (E ₁ −E ₂ )/E ₂ R ₁ −E ₁ R ₂ E=E ₁ ×R+R ₁ /R ₁ =E ₂ ×R+R ₂ /R _2. Next, give specific numbers. When calculated, R ₁ =
If E ₁ = 1.40V and E ₂ = 0.09V are obtained when 2MΩR ₂ = 0.1MΩ, R = 2×0.1 (1.40−0.09)/0.09×2−1.40×0.1=
6.55 (MΩ) E = 1.40 x 6.55 + 2/2 = 5.99 (V) Now we know the values of the apparent internal resistance R and the apparent internal power source E of the equivalent circuit shown in Figure 3b. Next, find the values of the insulation failure resistances R _I and R _S.
First, let the ratio of R _S and R _I be α. R _S /R _I = α∴R _S = αR _I R = R _I R _S /R _I +R _S = α/1+αR _I R _I =1+α/αR R _S = (1+α)R Therefore, it was found that if the ratio α is determined, it can be decomposed into R _I and R _S from the value of R. Next, applying the total voltage theorem, E _I /R _I +E _S /R _I α 1/R _I +1/R _I α=E E _I α+E _S α+1=E Rearranging the above equation, α=E− E _S /E _I −E is obtained. That is, the ratio α can be determined by appropriately assuming E _I and E _S. If we consider the voltage range of the local battery E _S here, we can assume a range starting from zero volts and theoretically up to about 1 volt, but according to the inventor's knowledge, most voltages are between 0 and 0.5 volts. It fits. According to the inventor's knowledge, the voltage value of the DC power source E _I is set at 9 volts. It has been found that this value is independent of the working voltage of the high voltage power cable. Now that the theoretical preparations are complete, we can move on to actual calculations.
Now, the apparent internal power supply E and the apparent internal resistance R
Assume that the values of are known, and as in the calculation example above, E=5.99 (V) and R=6.55 (MΩ) are obtained. At this time, we do not know how many volts the value of E _S is, but it is almost certain that it is between 0 and 0.5 V, so (Case 1) E _S = 0 V (Case 2) E _S = 0.2 V (Case 3) Find α for the three cases of E _S = 0.5V, then calculate R _I and
Calculate R _S.

[Table] That is, the objective was achieved, with the insulation layer insulation failure resistance R _I =9.98 (MΩ) and the anticorrosion layer insulation failure resistance R _S =19.08 (MΩ). Since the values obtained between Cases 1 and 3 had a deviation of less than a few percent, we used the arithmetic mean to find the average value. If so, it would be appropriate to use the geometric mean for the average value. Such a case occurs when the value of E is observed to be low, and the maximum deviation rate is approximately 38% when the minimum value of E is 1V.
reach. Therefore, it can be said that the present invention has practical significance only when a voltage value exceeding 1V is observed as an apparent internal power supply voltage value. An advantage of the present invention is that since it is not necessary to install a specific measurement power supply equipment on a specific high-voltage bus in advance, any high-voltage cable connected to any high-voltage bus can be measured. The rotation measuring device according to the present invention can be applied to high-voltage cables, has a simple configuration, can be manufactured at low cost, and has easy measurement operation.If you carry one device around, you can understand the deterioration state of many cables, and there is no need for a fixed power source. Therefore, it is possible to economically detect the insulation deterioration state of a live cable. The field of application of the present invention is high voltage and extra high voltage power cables, mainly high voltage CV cables.

[Brief explanation of drawings]

Fig. 1 is a diagram showing a conventional method of measuring the insulation resistance value of a high-voltage power cable under live wires, Fig. 2 is a measurement circuit diagram for each cable to be measured used in the present invention,
FIG. 3 is a diagram showing an equivalent circuit of the region 16 where insulation defects exist. 1: High voltage busbar, 2: Grounding transformer, 3: DC power supply, 4: Switch, 5, 7: Safety grounding circuit, 6: Cable to be measured, 8, 13: Switching switch, 9: Power supply, 10, 11, 12: Ammeter, 17: High voltage conductor, 18: Resistance, 19, R _I : Resistance with poor insulation in the insulation layer, 20, E _I : DC power supply, 21, R _S : Resistance with poor insulation in the anti-corrosion layer, 22 , E _S : Local battery.

Claims (1)

[Claims] 1 Measure the voltage twice with different multiplier resistances between the edge of the live cable and the ground, and from the results, determine the apparent internal power supply voltage and apparent internal resistance that exist between the edge of the live cable and the ground. , assuming that the apparent internal power supply voltage is 1 volt or more, the respective voltage values of the DC power supply generated in the defective insulation layer and the local battery generated in the defective insulation layer of the corrosion protection layer. A method for measuring cable insulation under a live wire, characterized in that the apparent internal resistance is decomposed to determine the insulation failure resistance of the insulation layer and the insulation failure resistance of the corrosion protection layer.