JPH0442065A - Method for diagnosing hot-line insulation of power cable - Google Patents

Abstract

PURPOSE:To diagnose the deterioration of insulation under the hot-line state by applying a DC voltage to the shielding layer of a power cable, computing the resistance value of the insulator of the power cable, and thereby computing the insulation resistance value of the power cable whose sheath-insulation resistance is low. CONSTITUTION:A grounding line 12 is connected to a sheath 11 of a power cable to be measured 20. An arrestor 5 of a measuring device pat, a capacitor 6 and a measuring device 4 are connected to a shielding layer 10 of a cable insulator through a grounding line 1. Then, a switch 9 is closed, and the insulation resistance Rs of the sheath 11, a DC-component current and a ground potential 13 are measured. Thus, the insulation of the cable 20 is diagnosed. When the insulation resistance Rs is small and the judgment of the DC component is not definite, the switch 9 is opened, and voltages E+ and E+ from DC voltage sources 3 and 3' are applied on the shielding layer 10. A current limiting resistor 2 is inserted so that magnetism is not accumulated in the iron core of a grounding transformer GPT. Then, the current value is measured 4. When the inner insulation resistance Ri of the cable is obtained in this way, the sheath insulating resistance Rs is computed, and the state of the deterioration of the cable can be judged.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a live line insulation diagnosis method that can measure the insulation state of a power cable in a live line state without a power outage.

[Prior Art] Live line insulation diagnosis methods for power CV cables include a method called the so-called DC component method, which estimates the insulation deterioration inside the CV cable from the DC current flowing through the grounding wire without applying voltage; 2 of the so-called DC superposition method, which applies a DC voltage of 6 to 50 V from the neutral point of a grounding transformer (GPT), measures the DC current leaking from the shielding layer, and calculates the insulation resistance. has become the mainstream.

[Problems to be Solved by the Invention] By the way, in the former DC component method, it is possible to easily measure by connecting to the ground wire, but on the other hand, it is possible to measure the potential caused by the potential of the buried part of the ground wire through the sheath. Stray currents may be mixed in and cause problems in determination. The sheath insulation resistance was reduced to 1MΩ by the zero-cross method to prevent this stray current from entering.
It is now possible to accurately correct up to a certain extent, but 10
When the value decreases to about 0Ω, there is a problem that the error becomes large and judgment becomes difficult.

In addition, while the latter DC superimposition method has the advantage of accelerating the deterioration of the power cable and extracting it, it is difficult to inject DC current into the neutral point of the GPT, which is precisely adjusted for ground fault detection. , equipment modification will be required. Furthermore, in the method of applying direct current from the high voltage line side, there is a risk that direct current bias magnetism may occur in the GPT iron core, resulting in malfunction.

The present invention has been made in view of the above points, and aims to provide a live wire insulation diagnosis method that eliminates the drawbacks of the conventional methods described above and allows for a new and more accurate diagnosis. .

[Means for Solving the Problem] In the present invention, a grounding transformer (G
Apply a DC voltage to the shielding layer of the power cable being measured, rather than to the neutral point of the GPT (PT), to prevent a large current from flowing when the DC voltage is applied and causing magnetic accumulation in the GPT core. The judgment is made by inserting a high resistance into the cable to limit the current and then calculating the resistance value of the power cable insulator.

[Function] Therefore, it is possible to calculate the insulation resistance value of the power cable, which cannot be determined from the direct current component due to the low sheath resistance, and it is possible to make a diagnosis by considering this value as well.

[Example] Hereinafter, the present invention will be described in detail with reference to the drawings. 1st
FIG. 1 is a circuit diagram showing the configuration of an apparatus applied to the method for diagnosing live insulation of a power cable according to the present invention. Electrical case to be measured
The sheath 11 of the pull 20 is grounded by a wire 12, the grounding wire 1 is connected to the shielding layer 10 of the cable insulator, and the measuring device (
ammeter) 4 is connected. To this measuring device 4, superimposed voltage sources 3 and 3' whose polarity can be reversed are connected in parallel via switches, and a high-resistance superimposed current-limiting resistor 2 is connected in series. Then, insulation diagnosis of the power CV cable 20 is performed using a device in which a superimposed part capacitor 7, a superimposed part arrester 8, and a switch 9 are connected in parallel to this heavy part voltage source and are grounded by a grounding wire 1. This is performed by first closing the switch 9 and measuring the insulation resistance value R1 of the sheath 11, the DC component current, and the earth potential 13, which is the sheath potential (2). If the deterioration state of the power cable can be determined based on the data at this time, the determination is made as is, but if the sheath insulation resistance value R is as small as several hundred ohms, the switch 9 is opened and the voltage from the DC voltage sources 3 and 3' is E. , E- are applied and the current value is measured by the ammeter 4.

That is, the current flowing through the insulator of the power cable 20 due to the electric potentials E, E- of the weight voltage sources 3, 3' is 1. resistance 11
The current flowing through the sheath due to the earth potential V is 1.1
When the current flowing through the sheath due to the potentials Ey and E- of the superimposed voltage sources 3 and 3' is , the equivalent circuit is as follows:
As shown in the figure, the current value measured by the ammeter 4,
When there is no deterioration in the power cable, I, =L +1. +I.

becomes.

On the other hand, the current flowing through the insulator 1. Since this is when the insulation resistance value of the cable shielding layer is R8, the internal insulation resistance value R of the cable is determined by R.

Insulation resistance value R of the power cable 2o obtained in this way
1, the low range of the sheath insulation resistance value R6 is determined.

What should be kept in mind when measuring this type of power cable using a live bell disconnection device is that the ground wire l must be prevented from becoming open or becoming a high impedance ground. Now, when the power supply grounding resistance R1 is set to 100MΩ, the shielding layer 10 will be close to open if it is only this, so a capacitor 7 (c8
Insert ゜μF). In this state, the potential of the shielding layer 1° is such that the charging T4 current with a charging current of 20 mA flows for 20 hours.
X10-"÷(2XπX50X10X10-') #6
．． Since the voltage is only about 5V, there is no danger.A capacitor 6 of about 30 μF is inserted in parallel with the ammeter 4, which is a measuring device, but this is also about 2V, so there is no problem.Even if a ground fault occurs during measurement. If this occurs, first the arrester 8 is closed, then the arrester 5 is closed, and the grounding wire 1 is brought to a complete state within 200 microseconds, making it safe.

[Effects of the Invention] As explained above, according to the power cable live line diagnosis method of the present invention, it is possible to calculate the insulation resistance value of a power cable whose sheath insulation resistance is low and cannot be determined from the DC component. Diagnosis can be made by considering the value.

In addition, it is generally known that the current value increases when a negative DC voltage is applied to a deteriorated part of the water tree. Judgment is made by changing . That is, for example, if there is an insulation deteriorated part such as corona discharge in the sheath 11, a large current will flow when the switch 3' is on and the switch 3 is off. Conversely, when switch 3 is on and switch 3' is off, no current flows. Also, if there is a water tree etc. in the insulator, current will flow when switch 3 is on and switch 3' is off, and conversely, current will flow when switch 3 is off and switch 3' is on. If we can obtain a measurement result that shows that there is no flow, and that the balance between the two is great, we can conclude that the power cable at that time has deteriorated, and this is an excellent method for making a reliable diagnosis. be.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a device applied to the live wire insulation diagnosis method of the present invention, and FIG. 2 is an equivalent circuit diagram of FIG. 1. 1... Grounding wire 2... Superimposed current limiting resistor 3.3'... Weight voltage 4... Measuring device (ammeter) 5... Measuring device arrester 6... Measuring device capacitor 7 ... Weight part capacitor 8 ... Weight part arrester 9 ... Switch 0 ... Insulation resistance of cable insulator 1 ... Cable sheath insulation resistance 2 ... Ground wire 3 ... Earth potential No. figure

Claims (1)

[Claims] A method for diagnosing live wire insulation of a power cable, characterized by diagnosing the insulation state of the power cable by connecting a high resistance, an ammeter, and a polarity reversible DC power source in series with the grounding wire of the power cable.