JPS6036025B2 - Voltage application method for power cable insulation monitoring - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for applying a DC measurement voltage necessary for measuring and monitoring the main body insulation resistance of a power cable connected to an ungrounded high-voltage distribution system under live wire conditions. An improved method of applying voltage to a primary neutral point is provided.

FIG. 1 shows a conventional method of applying a voltage for monitoring power cable insulation.

A high voltage bus 2 is connected to the secondary triangular connection of the power transformer 1 . The neutral point 0 of the primary star-shaped wire of the grounding transformer (G.P.T.) 3 is normally dropped directly to the ground, but for the purpose of measuring and monitoring the insulation resistance of the cable, it is grounded through the capacitor 4. , and a safety arrester 5 is connected in parallel with this. A DC power supply device is further connected in parallel to the capacitor 4 . This DC power supply device includes a DC power supply 6, an output current limiting resistor 7,
It is composed of an output ammeter 8, a power switch 9, a multiplier resistor 10 for measuring output voltage, and an output voltmeter (microammeter) 11. The impedance of the capacitor 4 must be low enough to allow the current to flow in the event of a single-line ground fault and not to degrade the ground fault detection function of the grounding transformer 3, so its capacitance must be sufficient. It is a big one.
The output current limiting resistor 7 functions to limit the output current from the DC power supply 6 to a certain value or less even when the ground insulation resistance of the high voltage bus 2 becomes extremely low, thereby reducing the voltage actually applied to the high voltage bus 2. do. The resistor 10 and voltmeter 11 are used to directly measure the voltage actually applied to the high voltage bus via the voltage drop of the resistor 7, and the current consumption can be read by the ammeter 8. This is extremely small compared to the output current from 6. The power supply switch 9 is always open, and although not shown, it is normal to operate with both ends of the capacitor 4 short-circuited, but the high voltage bus 2
When measuring the insulation resistance of the cable connected to the high-voltage bus, the short-circuit of the capacitor 4 is removed, the power switch 9 is closed, and the DC power 6 is applied to the high-voltage bus through the grounding transformer 3. The high-voltage busbar leads to the conductor of each cable, so if you insert an ammeter (not shown) between the target cable base and the ground, you can read the current value leaking through the insulation resistance of that cable. Cable insulation resistance can be measured. however,
The conventional voltage application method as described above has the following drawbacks.

In other words, the high-voltage bus itself has a fairly large capacitance7, including the ground capacitance value of the many power cables connected to it7, and the capacitance of the capacitor 4 is also connected in parallel to the ground. However, since the power supply 6 is applied through the limiting resistor 7, the charging time constant is large and it takes time to charge. On the other hand, since it takes time to discharge after charging, the capacitor 4 cannot be short-circuited immediately after the insulation measurement of one cable is completed. Since the ground insulation resistance value of the high-voltage bus 2 can vary greatly due to factors other than the insulation resistance value of the cable to be measured, the applied voltage may not be constant, and the voltage may drop significantly and the measurement sensitivity may decrease. Also, since the applied voltage is not fixed, the current leaking through the cable insulation can be transferred to the edge of the cable, and if an ammeter is inserted between the cable and the ground to read it, the ammeter should be marked with a resistance scale. It is not possible to directly read the insulation resistance value. moreover,
A high-performance capacitor with a large capacity that can withstand abnormal voltage is required, and a short-circuit mechanism is required to protect it when not measuring, making it expensive. In addition, the impedance or capacitance value of the capacitor is set so that it does not affect its ground detection function when inserted in series with a grounding transformer, and also to ensure that the large current that flows in the event of a ground fault falls within its withstand voltage performance. The decision must be made with sufficient consideration so that it can be passed on. An object of the present invention is to provide an improved method that eliminates the drawbacks of the conventional methods as described above.

FIG. 2 shows a method of applying a measuring voltage according to the invention.

Reference numbers 1-3 are similar to those in FIG. The neutral point 0 of the primary side star connection of the bushland transformer 3 is the normally closed switch 1
4 are all grounded to the earth by a grounding resistor 12.
On the other hand, an arrester 5 is directly inserted between the neutral point 0 and the earth. The switch 14 and the arrester 5 measure the current being supplied to the high voltage bus by inserting the ammeter 13 between the neutral point 0 and the point 0' (non-grounded side of the grounding resistor 12) as shown in the figure. This is provided in case there is a need for reading. Therefore, when there is no need for reading, the switch 14 and the arrester 5 are unnecessary, and the neutral point 0 and the point 0' are directly connected. Earth resistance 12
The resistance value is usually selected to be 100 or less. Since this value is extremely low, there is no need for a switch to constantly short-circuit it and open it during measurement, and this resistor can be left connected at all times. The output of a DC voltage regulator 15 driven by an AC low voltage power supply is supplied to a resistor 12 . The output voltmeter 16a and the output ammeter 15b are built into the DC voltage regulator 15. This AC input switch 16 is turned on when measuring the insulation resistance of the cable.
Do N. The resistance value of resistor 12 is 100, and the DC voltage regulator 1
When the output current of the resistor 12 is 5A, a voltage of 5V is generated across the resistor 12. That is, since a stable DC voltage of 5 to the earth is obtained at point 0', this voltage is used as a power source via ammeter 13, and voltage is directly applied from neutral point 0 to high voltage bus 2. Note that it is necessary to insert the ammeter 13 and read the applied current when you want to measure the insulation resistance to ground of the entire high-voltage bus system, and this ammeter is scaled with resistance values instead of current values. You can also leave it there.

The method of the present invention as described above has the following advantages.

An inexpensive resistor is sufficient without the need for an expensive large-capacity capacitor, and since the burden voltage in the event of a single-line ground fault is low and no high voltage is generated, it can be kept on and off at all times, and therefore short-circuit switches and arresters can be used. does not require. In addition, since the applied voltage is constant regardless of the insulation resistance value of the entire high-voltage bus system, there is no decrease in sensitivity when measuring cable insulation resistance. By adding a resistance scale to the ammeter, you can directly read the insulation resistance value of each cable. Furthermore, the time constant of charging and discharging is several orders of magnitude smaller than that of conventional methods, and voltage application and discharging can be performed instantaneously, so that measurement time can be shortened. Furthermore, since the value of the grounding resistance is selected to be extremely low, there is no need to worry about deterioration of the ground fault detection function of the grounding transformer.

[Brief explanation of drawings]

FIG. 1 shows a conventional voltage application method for power cable insulation monitoring, and FIG. 2 shows a voltage application method according to the present invention. 1... Power transformer, 2... High voltage bus, 3... Grounding transformer, 4... Capacitor, 5... Arrester, 6...
- DC power supply, 7... Output current limiting resistor, 8... Output ammeter, 9... Power switch, 10... Multiplier resistance for output voltage measurement, 11... Output voltmeter, 12... Earthing resistance, 13... Ammeter, 14... Switch, 15.
...DC voltage regulator, 15a...Output voltmeter, 15b
... Output ammeter, 16... AC input switch. Figure 1 Figure 2

Claims (1)

[Claims] 1 In order to measure and monitor the insulation resistance of a cable connected to an ungrounded high-voltage bus under live wire conditions, a grounding By inserting a resistor between the primary neutral point of the transformer and the ground, and connecting a DC constant voltage power source to both ends of this resistor, the voltage generated across this resistor can be used as a power source for leakage current measurement. 1. A method for applying a voltage for monitoring power cable insulation, characterized in that the method is used as a power cable insulation monitoring method.