JPS6036026B2 - Insulation monitoring method for resistance grounding cables - Google Patents

Description

[Detailed description of the invention] The present invention relates to a method for monitoring insulation of resistance grounding cables.

Attempts have been made to measure the main body insulation resistance of a cable under charging without stopping the cable's power transmission, but such attempts have been made in the so-called high voltage area used as an ungrounded system, i.e. Alternatively, it was intended for Iso V cables, and could not be applied to insulation monitoring of cables selected for resistance grounding systems. Figure 1 illustrates the conventional method, in which the secondary side of a power transformer 1 is ungrounded, which is shown as having a primary star-shaped secondary triangular connection for dropping from an extra high voltage to a high voltage. A system high-voltage bus 2 is connected to the high-voltage bus 2, and high-voltage cables 6, 7, etc. to be monitored are connected to the high-voltage bus 2, and there is a connection between the high-voltage bus 2 and the ground for the purpose of facilitating the detection of single-line ground faults. Grounding transformer 3 with primary star type secondary open triangular connection
is connected, and the neutral point of the primary side connection of this transformer 3 is normally dropped directly to the ground, but here it is grounded through a large capacity capacitor 4 for the purpose of measuring the insulation resistance of the cable. A DC power supply device 5 is connected in parallel with this capacitor 4.
Alternatively, the DC current measuring device 8 is connected, while the DC current measuring device 8 or the DC power supply device 5 is connected between the edge of the cable 6 under measurement and the earth, and the cable not under measurement is connected. The seven pillars will fall to the earth.

If the DC power supply device 5 is connected to the power transformer side and the DC current measuring device 8 is connected to the cable side, the DC measurement voltage is converted to high voltage by the DC power supply device through the grounding transformer 3. A direct current measuring device 8 connected between the busbar 2 and the ground and the current applied to the conductors of the busbar 2 and all the cables connected thereto and leaking through the insulation of the measurement cable to the cable end. On the other hand, if the DC current measuring device 8 is connected to the power transformer side and the DC power supply device 5 is connected to the cable side, the DC measurement voltage by the DC power supply device is The current applied to the cable to be measured and leaked through the cable's insulator via the high-voltage bus 2 and the grounding transformer 3 is connected to the neutral point of the primary connection of the grounding transformer and the earth. It is read with a DC current measuring device 8 connected between the two. In the two cases mentioned above, the positions of the DC power supply device 5 and the DC current measuring device 8 are swapped, but in both cases, it is still possible to measure a current that corresponds to the size of the main body insulation resistance of the cable to be measured. Both the DC power supply and the DC current measuring device are actually equipped with a safety arrester to prevent the intrusion of AC, so that they can safely fulfill the purpose of measurement while being charged. There is. After measuring cable 6, drop the sheath of cable 6 to the ground and remove cable 7.
Then, connect a DC current measuring device or DC power supply between the bridge and the ground and perform the same measurements, and then perform the same measurements on other cables in sequence. The above conventional method is intended for ungrounded systems, but in this case, even if a single line ground fault occurs on a cable running into the system or on a high-voltage bus, the ground fault current is generally small, about 10 to 20 A or less. There are many.

Therefore, the grounding transformer 3 only needs to have a small capacity, and the capacitor 4 connected between its primary neutral point and the earth does not need to have a large capacitance or withstand voltage. do not. On the other hand, in resistance-grounded power transmission and distribution systems, which are commonly used in special high-voltage systems, a power transformer with a large capacity and a star-shaped secondary side is usually used, and the secondary side neutral point and A grounding resistor is inserted between the earth and
The current during a single-line ground fault is 100 to 200 A or more, and inserting a capacitor in series with a grounding resistor would result in an enormous increase in size and cost in terms of required capacitance and withstand voltage performance. This means that it is practically impossible to implement. For this reason, it has not been possible to apply the conventional method to special high-voltage systems and use them to perform insulation monitoring during power transmission of cables connected thereto. SUMMARY OF THE INVENTION An object of the present invention is to provide an economical and highly safe insulation monitoring method that can monitor the insulation of a resistance grounding cable during power transmission.

The invention will now be described with reference to FIG. 2, which shows an example of a circuit that makes it possible to carry out the invention.

In FIG. 2, the power transformer 1' is shown as having a star-shaped connection on the primary and secondary sides and a triangular connection on the tertiary side. 2' is connected to this high voltage bus 2', and as in the case of FIG.
7', etc., and a grounding resistor 9 of tens to hundreds of digits is connected between the neutral point of the secondary star-shaped connection of the power transformer 1' and the ground. 10' indicates a low-resistance resistor inserted between the ground side terminal of the grounding resistor 9 and the ground, and its resistance value is selected to have a value that does not significantly affect the resistance value of the grounding resistor 9. Therefore, a value one order of magnitude lower than that of resistor 9 is appropriate.

Instead of using a resistor additionally inserted between the ground side terminal of the grounding resistor 9 and the ground as this resistor 10, a resistor with an intermediate tap is used as the grounding resistor 9, and the intermediate tap of the intermediate tapped resistor is used as the earthing resistor 9. Resistance 1 is the part between the earth and
It may be used as 0, and in this way,
The connection of the resistor 10 and the selection of its value relative to the resistance value of the resistor 9 are facilitated. However, for convenience of explanation, the following description will proceed assuming that the resistor 10 is additionally inserted. A DC power supply 11 or a DC current measuring device 8' is connected in parallel to the resistor 10, while a DC current measuring device 8' or A DC power supply device 11 is connected. resistance 10'
At first glance, the parallel-connected DC power supply device 11 seems to have similar contents to the DC power supply device 5 in FIG.
In reality, the contents of the two are different. That is, in the case of the DC power supply device 5, the output current flows directly to the high voltage bus, but in the case of the DC power supply device 11, most of the output current of the device flows through the resistor 10', and the voltage drop across the resistor 10' is used as the power source. As a result, the measurement voltage is applied to the extra high voltage bus 2' through the grounding resistor 9 and the power transformer 1'.
While it is a small output current type power source of about 5 hA, the DC power supply device 11 is a large output current type power source with a maximum value of 10A, for example. This is because the current value of the DC power supply 11 becomes such a value when the resistance value of the resistor 10 is low and an applied voltage value necessary for measurement, for example, 50 V is to be obtained. It is also conceivable to insert the DC power supply device 11 directly between the grounding resistor 9 and the earth instead of the resistor 10,
In practice, if this is done, the internal resistance of the DC power supply device 11 becomes a problem, and furthermore, there is a risk that the DC power supply device 11 will be damaged in the event of a single line ground fault, so it cannot be adopted. Although it is not preferable to increase the resistance value of the resistor 10 for the same reason, it is advantageous in that the output current value of the DC power supply device 11 can be reduced. Therefore, in carrying out the present invention, the resistance value of the resistor 10 is selected with appropriate tradeoffs, and is selected to be the value described above. In addition, a DC current measuring device 8 and a resistor 1 are connected in parallel to the capacitor 5.
0' This is different from the DC current measuring device 8' which is connected in parallel.In the case of the DC current measuring device 8, the measured current flows directly into the measuring device, but in the case of the DC current measuring device 8', the current flows directly into the measuring device. It is bypassed by the resistor 10 with a low resistance value, and in other words, what is measured by the measuring instrument is the minute voltage drop caused by the resistor 101 due to the minute measurement current. It is necessary to use measurement instruments that are much more sensitive than 8. Here again, the DC current measuring device 8'
It is preferable that the resistance value of the resistor 10 that provides a bypass path between the input terminals of the input terminal is high because it does not require increasing the sensitivity of the measuring instrument, but this increases the risk of damage to the measuring instrument in the event of a single-line ground fault. After all, the resistance value of the resistor 10 cannot be made high.
It is also possible to insert a direct current measuring device 8' instead of the resistor 10 directly between the grounding resistor 9 and the earth, but in practice this would reduce the ability to flow a large current in the event of a single-line ground fault. This is impossible because the DC current measuring device 8' does not have it at all. In this case, even if an arrester is connected in parallel for protection, the risk of damage to the DC current measuring device 8' cannot be eliminated. Next, how to implement the method of the present invention will be explained. In Fig. 2, the end of the cable 7' that is not under measurement is dropped to the ground, and the DC current measuring device 8' or Connect the DC power supply 11. The DC current measuring device 8' connected to this cable 6' side is substantially the same as the DC current measuring device 8 in Fig. 1, unlike the case where one resistor is connected in parallel. That's fine. If the DC power supply device 11 is connected to the power transformer side and the DC current measuring device 8' is connected to the cable side, the voltage for DC measurement is specially connected through the grounding resistor 9 and the power transformer 1'. Applied to the high voltage bus 2' and the cable conductor connected thereto,
The current leaking through the insulation of the cable to be measured is read by a DC current measuring device 8' connected between the cable sheath and the ground. When the DC current measuring device 8' is connected to the cable side and the DC power supply 11 is connected to the cable side, the DC measurement voltage is applied to the edge of the cable to be measured, and is transmitted through the insulator of the cable. The DC current measuring device 8' bypasses the current leaking through the special high voltage bus 2', the power transformer 1', and the grounding resistor 9 with the series low resistance 10 connected between the grounding resistor 9 and the ground. I read it. In the above two cases! ', the positions of the DC power supply device 11 and the DC current measuring device 8' are switched, but in either case, it is still possible to measure the current according to the magnitude of the main body insulation resistance of the cable to be measured. After measuring cable 6', drop the sheath of cable 6' to the ground and connect cable 7.
', connect the DC current measuring device 8' (or DC power supply 11) between the bridge and the earth, and perform measurements in the same way for the other cables. As can be understood from the above explanation, in the method of the present invention, there is no difference in wiring or equipment from the conventional method on the cable side, only the wiring and equipment on the power transformer side are different, and the method used therein is Compared to capacitors, resistors are cheaper, have a smaller volume, and have less risk of breakage, so they do not involve any particular increase in cost or risk, and are superior in economy and safety, and are suitable for resistive grounding systems. This makes it possible to measure the insulation resistance of the cable during power transmission.

[Brief explanation of the drawing]

FIG. 1 shows an example of a circuit implementing a conventional cable insulation monitoring method, and FIG. 2 shows an example of a circuit implementing a cable insulation monitoring method according to the present invention. 1'...power transformer, 2'...resistance grounding system special high voltage bus, 6', 7'...cable, 8'...DC current measuring device,
9... Earthing resistor, 10... Low resistance, 11... DC power supply device. Hair - Illustration 2

Claims (1)

[Claims] 1. In a resistance-grounded power transmission and distribution system, a DC power supply device or a DC Connect a current measuring device, connect a DC current measuring device or a DC power supply between the end of the cable under measurement among the cables connected to the system and the earth, and A method for monitoring the insulation of a resistance grounding cable, the method comprising measuring the current flowing through the insulating body of a cable to the DC current measuring device. 2. In the method according to claim 1, a part of the grounding resistance is connected between the ground side of a grounding resistor connected between the neutral point of the secondary side of the power transformer and the ground. A method for monitoring the insulation of a resistance grounding cable, characterized by comprising an additionally inserted resistor. 3. In the method according to claim 1, a part of the grounding resistance is connected between an intermediate tap of a grounding resistor with an intermediate tap connected between a neutral point on the secondary side of a power transformer and the earth, and the earth. 1. A method for monitoring the insulation of a resistance grounding cable, characterized in that the insulation monitoring method comprises a portion belonging to a resistance grounding system.