JPH0616069B2 - Insulation deterioration detector for power cable - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] Crosslinked polyethylene power cables often cause dielectric breakdown accidents due to water trees.

The present invention relates to a measuring device for detecting insulation deterioration of the cable under a live line.

[Prior Art] A technique for measuring a resistance of a cable insulator by an AC / DC superposition method has been conventionally known. An example of this is shown in FIG.
-34977 gazette).

10 is a cross-linked polyethylene power cable to be measured (Fig. 3), 12 is a cable conductor, and 20 is a cable insulator 1.
4 is a resistance, 16 is a cable shielding layer, and 22 is an insulation resistance of the cable sheath 18.

The power cable 10 is connected to the trunk line 24 having a high voltage or higher, the switch 26 is closed and the cable shielding layer 16 is grounded during normal times (except during measurement).

At the time of measurement, the switch 26 is opened, and the DC voltage is superimposed on the cable insulator 14 through the neutral point of the grounding transformer 30 by the DC power source 28, and the leakage current is connected to the cable shield layer 16 by the DC ammeter 32. Measured by
The value of the insulator resistance 20 is obtained.

[Problems to be Solved by the Invention] (1) In the above method, the sheath insulation resistance 22 causes an error as compared with the resistance of the circuit from the cable shielding layer 16 through the ammeter 32 to the power supply 28. It needs to be high enough not to occur.

However, in general, the insulation resistance value of a defective cable sheath is often several kΩ to several tens kΩ. Therefore, in such a case, in order to obtain the true value of the cable insulator resistance 20, it is necessary to first obtain the insulation resistance of the sheath 18 and then add the correction by calculation.

(2) Further, when the insulation resistance of the cable sheath 18 is lowered, a DC voltage 34 is generated due to the action of a local battery, which becomes an obstacle to measurement.

For example, at a measurement voltage of 50 V, 2000 MΩ insulator resistance 2
In order to measure 0, a current of 25 nA must be measured, but the local battery of the sheath has a maximum of about 0.5 V, so if the insulation resistance 22 of the sheath drops to 100 kΩ, the measured current will be 25 nA. On the other hand, a current of about 200 times about 5 μA will flow.

Therefore, various adjusting mechanisms for compensating the above local current have been considered, but if the sheath insulation resistance 22 reaches several kΩ, there is a possibility that measurement becomes impossible.

[Means for Solving the Problems] (1) Based on the phenomenon that the cable sheath 18 side has a positive polarity, the DC current generated when the sheath insulation resistance 22 decreases from the measurement in the actual line, As shown in FIG. 1, the DC power supply 40 in the loop circuit including the DC power supply 40 and the DC ammeter 42 and the power cables 10A, 10
The first rectifying element 4 in series with the DC ammeter 42 between
4 in the forward direction with respect to the DC power source 40 and in the reverse direction with respect to the DC current generated in the cable shielding layer 16, (2) In addition, the DC power source 40 and the power cables 10A, 10B The second rectifying element 50 with respect to the DC power source 40 between the loop circuit and the ground at a position closer to the DC power source 40 side than both the DC ammeter 42 and the first rectifying element 44. And (3) DC isolation between the loop circuit and the ground between the loop circuit and the ground at a point other than the rectifying element 50 to solve the above problems. planned.

[Description thereof] FIG. 1 illustrates a case where the power cables 10A and 10B are connected to the main line 24.

A filter 35 includes a choke coil 36 and a capacitor 3.
It consists of 8 and blocks alternating current.

40 is a DC power source, 42 is a micro DC ammeter, and 44 and 50 are rectifying elements, which are made of semiconductors or selenium.

45 is also a filter, a choke coil 46 and a capacitor 4
8 and.

52 is a changeover switch.

[Operation] A case of measuring the power cable 10A will be described.

(1) The switch 26A is opened to float the measurement loop circuit including the insulator resistance 20A, the DC power supply 40, the DC ammeter 42 and the like from the ground.

By applying the voltage from the DC power supply 40, the current is measured by the DC ammeter 4
2 → rectifier element 44 → choke coil 46 → insulator resistance 2
It flows in the direction of 0A → the main line 24 → the neutral point of the grounding transformer 30 → the choke coil 36 → the power source 40.

Therefore, the cable insulator resistance 20A can be obtained from the reading of the ammeter 42.

(2) Further, the current in the reverse direction due to the positive potential generated due to the decrease in the sheath insulation resistance 22A is blocked by the rectifying element 44 and does not flow to the ammeter 42.

(3) Further, since this measurement circuit is a loop circuit independent of the ground, a measurement error due to the decrease of the sheath insulation resistance 22A does not occur.

(4) Also, because it is a closed circuit independent of the ground, it is not affected by the stray current of the ground.

(5) Further, the current flowing through the insulator resistance 20B of the power cable 10B that is not the measurement target flows from the power supply 40 through the rectifying element 50 and does not flow through the ammeter 42, so that no measurement error occurs.

(6) If there is no rectifying element 50, the current flowing through the insulator resistance 20B of the cable 10B passes through the sheath insulation resistance 22A of the cable 10A. For that purpose, the DC ammeter 42 uses the resistances 20A and B of the cable insulators.
It is the sum of the currents that pass through, and there is a risk of making an incorrect deterioration determination.

(7) If the rectifying element 50 is not used and the grounding is directly performed, the above danger can be eliminated, but the applied voltage becomes a resistance partial pressure of the insulator resistance 20A and the sheath insulation resistance 22A, and the rectification is performed. Below the operating voltage of element 44,
It becomes impossible to measure.

[Advantages of the Invention] (1) A loop circuit using the rectifying element 44 is formed, and the DC power source 40 and the power cables 10A and 10 in the loop circuit are formed.
A first rectifying element 44 is assembled in series with a DC ammeter 42 between B and B in the forward direction with respect to the DC power source 40 and in the reverse direction with respect to the DC current generated in the cable shielding layer 16. Since this has occurred, it is possible to completely eliminate the influence of the generated potential due to the reduction of the insulation resistance 22 of the sheath, and it is not necessary to equip a compensating circuit for balancing.

(2) Further, the DC power source 40 and the power cable 10A,
10B, and at a position closer to the DC power source 40 side than any of the DC ammeter 42 and the first rectifying element 44, a second rectifying element 50 is provided between the loop circuit and the ground. Since it is assembled in the forward direction with respect to the DC power supply 40, even if the insulation resistance 22 of the sheath is reduced to several kΩ, no measurement failure occurs (in many real lines, insulation is degraded). ).

(3) Since the loop circuit and the ground are galvanically isolated at a point other than the rectifying element 50 and floated from the ground,
Not affected by stray current in the earth.

(4) In addition, an erroneous current due to deterioration of the cable that is not measured does not flow to the DC ammeter 42.

[Brief description of drawings]

 FIG. 1 is an explanatory diagram of an embodiment of the present invention, FIG. 2 is an explanatory diagram of a conventional technique, and FIG. 3 is an explanatory diagram of a power cable 10. 10: Power cable, 12: Cable conductor 14: Cable insulator, 16: Cable shielding layer 18: Cable sheath 20: Resistance of cable insulator 22: Insulation resistance of sheath, 24: Trunk line 26: Switch, 30: Transformer for grounding 35: Filter, 40: DC power supply 42: DC ammeter, 44: Rectifying element 45: Filter, 50: Rectifying element

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Minor Yamamoto 1-5-1, Kiba, Koto-ku, Tokyo Fujikura Electric Line Co., Ltd. (72) Inventor Takashi Shingen 1-1-5, Kiba, Koto-ku, Tokyo Fujikura Electric Wire Co., Ltd.

Claims (1)

[Claims] 1. A power cable comprising a loop circuit including a DC power supply (40) and a DC ammeter (42) connected in series to a cable conductor (12) and a cable insulator (14) of the power cable. In an insulation deterioration detecting device for a crosslinked polyethylene power cable, wherein a DC voltage of the DC power supply (40) is superimposed on an AC voltage applied to (10), the DC power supply (40) and the power cable in the loop circuit. Between
The first rectifying element (4) is connected in series with the DC ammeter (42).
4) is installed in the forward direction with respect to the DC power supply (40) and in the reverse direction with respect to the DC current generated in the cable sheath 18, and between the DC power supply (40) and the power cable. A second rectifying element between the loop circuit and the ground at a position closer to the DC power source (40) than both the DC ammeter (42) and the first rectifying element (44). (50) was installed in the forward direction with respect to the DC power supply (40), and the loop circuit and the ground were galvanically isolated at points other than the second rectifying element (50). An insulation deterioration detecting device for a power cable, which is characterized in that: