JPS608465B2 - Cable insulation resistance monitoring method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a cable insulation resistance monitoring method for measuring the insulation resistance of a cable during power transmission and monitoring its deterioration state.

The insulation resistance of installed cables may deteriorate due to aging, but with normal means of measuring insulation resistance,
Measurements can only be made during the period during which power transmission is interrupted, and such interruptions in power transmission occur rarely, perhaps once a year.

Therefore, the insulation resistance of a cable can only be measured about once a year, and sometimes only once every few years. Furthermore, measuring insulation resistance required many workers and a lot of effort, such as disconnecting cables, transferring measuring instruments, arranging security personnel and measuring personnel, and restoring power so that power could be transmitted again. . Therefore, the reality is that it is not possible to detect cable insulation deterioration early and prevent electrical breakdown of the cable.

An object of the present invention is to provide a method for easily periodically monitoring the insulation resistance of a cable during power transmission.

Examples will be described in detail below. FIG. 1 is an explanatory diagram of an embodiment of the present invention, where T is a three-phase power transmission and distribution line to which cable 1 and other cables are connected, and the transformer has a Y connection on the primary side and a Δ connection on the secondary side. It is connected to the secondary side of the device Ty.

S is a DC high voltage power supply whose tenth side is normally grounded, and whose one side is connected to an arbitrary line of the power transmission/distribution line T. Ms is an ammeter that measures the current from the DC high voltage power supply S, R3 is a protective resistor, C,
, C2, and C3 are capacitances, L is a choke coil, S is a switch, and Mx is an ammeter that measures the current flowing through the insulation resistance Rx of the cable. The capacitance C3 is for AC grounding the shielding layer at the far end of the cable 1,
Furthermore, the capacitances C, C2 and the choke coil L block the AC component so that the DC leakage current lx can be measured with the ammeter Mx, and are connected to the shielding layer at the near end of the cable 1. .

Of these, the capacitance C, when the switch S is opened, sets the potential of the shielding layer of the cable 1 to the ground potential in an alternating current manner.
Use one with large capacity and low impedance. The above-mentioned configuration is provided for each cable, but a changeover switch, etc. is provided at the K point indicated by an x mark, and the measurement circuit is shared by a configuration that switches between each cable's shielding layer lead wire. You can also.

For example, the DC high voltage power supply S is IK for the ice V distribution line.
For the V and side V distribution lines, select the Fox V DC voltage.

In addition, the line T and the DC high voltage power supply S are directly connected via a protective resistor Rs, but they are connected via a disconnector or cut-out switch, so that a DC high voltage is applied to the line T only during measurement. You can also. Further, the protective resistor Rs is a high resistance such as 20 MQ for the Shigeru V distribution line and 40 MQ for the Shoe V distribution line. For this reason, for lines where many cables are connected and the ground capacitance is large, it will take several minutes to charge, but at the cost of applying steep voltage changes. Furthermore, the final charging voltage for cables with reduced insulation resistance is automatically lowered, so that insulation breakdown does not occur. By applying the DC voltage to any line of the line T, the same DC voltage is applied to each phase through the secondary winding of the power transformer Ty.The current ls flowing through the ammeter Ms is When the insulation resistance to ground of the line T is zero, it is maximum.For example, when the voltage of the DC high-voltage power supply S and the protective resistance Rs are selected to the values described above,
) becomes.

Therefore, in this case, it is sufficient to use a full scale ammeter of 5 A as the ammeter Ms. As mentioned above, the DC voltage is applied from the DC high-voltage power supply S, and after one increment, the switch S is opened, thereby increasing the insulation resistance Rx of the insulation layer of the cable 1.
The DC leakage current lx, which was flowing to the ground via the switch S and the choke coil L, flows to the ammeter Mx via the choke coil L. In this case, an alternating current blocking circuit including capacitances C, , C2 and a choke coil L prevents alternating current from flowing to the ammeter Mk. For example, if a DC voltage of IKV is applied to the shoe V distribution line as described above,
At s, the current ls is 1.0 French A, and at the ammeter Mx, the current lx
is measured as 0.05 French A, then cable 1
The insulation resistance Rx is RX:

It can be seen that 〇×1bi=16000×1, that is, 160 times ○.

When the measurement is completed, the switch S is closed to set the shielding layer of the cable 1 to the ground potential, and measurements on other cables are performed.

The insulation resistance of other cables can be measured while the power is being transmitted by measuring the DC electric current AS flowing through the DC high-voltage power supply S and the current lx flowing through the insulating layer of the cable as described above. When the measurement of the insulation resistance of all the cables is completed, the operation of the DC high-voltage power supply S is stopped or the cable is disconnected from the line T, thereby completing the complete stripping. FIG. 2 is an explanatory diagram of an embodiment in which a grounding transformer CPT is connected to a line T, and a DC high-voltage power supply S is connected to the primary winding of the grounding transformer GPT via a protective resistor s. Connected to neutral point.

Note that since this neutral point 0 is normally connected directly to the ground, it is desirable to construct an automatic insertion circuit that is in the connection state shown in FIG. 2 only during measurement. It is also necessary to connect a capacitance Co between the neutral point ○ and the ground to form a low impedance path for the ground fault current during measurement. If this ground fault current increases further, the ground fault current is detected on the secondary side of the grounding transformer GPT and the capacitance C is increased. For safety reasons, it is desirable to short-circuit the capacitance Co or to connect the arrester Ay in parallel to the capacitance Co as shown in the figure. Since the embodiment shown in FIG. 2 is a case where the neutral point of the line T is grounded,
This shows that it can also be applied to the case where the secondary winding of the power transformer Ty is Y-connected.

Figure 3 shows an equivalent circuit when measuring insulation resistance using the monitoring method according to the present invention. In the present invention, there is no need to directly measure it.

Then, the voltage E of the DC high voltage power supply S, and the current ls flowing through it
, the protective resistance Rs, and the current lx flowing through the insulating layer, the insulation resistance Rx of the insulating layer can be found as RX:Rx.

Figure 4 shows the DC leakage current lx [mountain A
] and insulation resistance Rx [MQ], where the voltage E of the DC high-voltage power supply S is IKV and the protective resistance Rs is 200M.
Q, and the curve of ls=lx is Rp=
The case of gong is shown.

The insulation resistance Rx can be measured in a range of about 10MQ to 20,000MQ, and it is practically sufficient to measure insulation resistance of this level with a normal high-voltage power cable. For example, in the Hina VCV cable, 10 plates MQ-K
The standard for diagnosing insulation resistance deterioration is that if the insulation resistance falls below this limit, there is a high risk of electrical breakdown in the near future, and such cables must be replaced immediately. Then, the above performance is sufficient for performing such a diagnosis. As explained above, the present invention can measure the insulation resistance of a cable even when the cable is in a power transmission state, making it easy to perform periodic measurements over a relatively short period of time, for example, about one week. By understanding changes in cable insulation resistance, it is possible to detect cable insulation deterioration early and prevent major accidents.

In addition, the DC voltage applied during measurement can be lower than the AC working voltage, and since it is applied to the cable via a high-resistance protective resistor, the applied voltage gradually increases over time. In this situation, there is no screen that can accelerate the deterioration of the cable and the transformer connected to it.

The present invention is applicable not only during power transmission but also during power transmission interruption.
Of course, insulation resistance can also be measured.

[Brief explanation of the drawing]

Figures 1 and 2 are explanatory diagrams of different embodiments of the present invention, Figure 3 is an equivalent circuit during measurement, and Figure 4 is a leakage current l.
It is a relationship curve diagram between x and insulation resistance Rx. S is a DC high voltage power supply, Rs is a protection resistor, and C. , C,, C2, C3 are capacitances, T is transmission and distribution line, Ms
, Mx is an ammeter, now a choke coil, S is a switch,
1 [Well, it's a cable. O1 figure O2 crab O3 figure. 4 spots of juice

Claims (1)

[Claims] 1. A DC high voltage power source is connected between any line or neutral point of the power transmission/distribution line to which the cable is connected and the earth via a protective resistor, and between the shielding layer of the cable and the earth. A DC pass filter circuit is connected to the DC high voltage power supply, and the DC current flowing through the protective resistor and the DC leakage current flowing from the cable to the ground through its insulating layer and the DC pass filter circuit are measured. A method for monitoring cable insulation resistance, characterized in that the insulation resistance of the cable is determined based on the relationship between voltage and the protective resistance.