JP2665935B2 - Measuring method of dielectric loss of cable - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for measuring a dielectric loss of a cable in a live state.

(Technical Background of the Invention and its Problems) The dielectric loss of the insulator of the power cable is one of the important factors that determine the performance of the cable. Generally, the dielectric loss of a cable is evaluated using tan δ. The tan δ of the cable before shipment that has not been laid yet is measured using a sharing bridge or the like. On the other hand, the measurement of tan δ for a live cable is performed as follows.

 FIG. 2 shows an explanatory view of the measuring method.

The cable 1 shown in the figure has one end 2 a of a conductor 2 connected to an AC power supply 3 and the other end 2 b connected to a load 4.
The one end 2a is grounded via two voltage-dividing resistors 5, 6 connected in series. On the other hand, this cable 1
Of the cable sheath 7 is directly grounded at one end via a ground wire 8. Normally, when both ends of the cable sheath 7 are directly grounded, a large induced current circulates in the cable sheath 7, and thus such a grounding method is often adopted for an actual line. A current transformer 9 for measuring a current is mounted on the ground line 8.

A voltage measuring terminal 10 is provided at a connection point 11 between the two voltage dividing resistors 5 and 6, and a charging current measuring terminal 12 is provided at both ends of a coil 9a of the current transformer 9.

With this connection, only the charging current based on the capacitance of the cable 1 flows through the ground wire 8 because the cable sheath 7 is grounded at one end. Therefore, the output voltage of the voltage measuring terminal 10 is the alternating voltage E applied to the conductor 2 of the cable 1.
The output current of the charging current measuring terminal 12 is proportional to the charging current I of the cable 1. tan δ can be obtained from the phase angle between the alternating voltage E and the charging current I. Generally, a vector product of the alternating voltage E and the charging current I is divided by a scalar product of the two to obtain tan δ.

It should be noted that, in actuality, a live line ta in which a detection unit 13 surrounded by a chain line
An nδ measuring device is already commercially available (Show Checker TD: Showa Electric Cable Co., Ltd. trademark).

By the way, the voltage dividing resistor 5 of the detecting unit 13 indicated by a broken line in the drawing.
It is necessary to connect one end to the conductor 2 of the cable 1 for measurement. However, depending on the termination configuration of the cable line, there is a case where a high-voltage portion such as a cable conductor is not exposed at all.

Therefore, for such a cable line, it is difficult to measure the live line tan δ as it is.

(Object of the Invention) The present invention has been made in view of the above points, and has as its object to provide a method for measuring a dielectric loss in a live state of a cable in which a conductor is not exposed. is there.

(Summary of the Invention) A method for measuring a dielectric loss of a cable according to the present invention measures an alternating voltage applied to a conductor of the cable via an electrode for voltage detection provided on a cable line, while charging the cable. By measuring the current, the apparent dielectric loss is determined from the relationship between the alternating voltage and the charging current, and the dielectric loss based on the apparent dielectric loss is determined in advance and based on the presence of the power detection electrode portion. A net dielectric loss of the cable is obtained by subtracting a loss error.

(Embodiment of the Invention) Hereinafter, the present invention will be described in detail with reference to an embodiment shown in the drawings.

FIG. 1 is a longitudinal sectional view of a cable connecting portion in which a method for measuring a dielectric loss of a cable according to the present invention is performed.

In the figure, the end of a cable 1 is connected to an electric device such as a switch (not shown) on the left side of the figure. In the figure, a conductor 20 of the cable 1 is connected to a compression terminal 21. The compression terminal 21 is connected to a terminal 22 on the equipment side by a terminal tightening bolt.
It is tightened and fixed by 23.

An internal semiconductive layer 24 is formed so as to surround the high-voltage portion including the compression terminal 21, and a molding insulating layer 25 made of rubber or the like surrounds the entire connection portion including the internal semiconductive layer 24. . On the outer peripheral surface of the insulating layer 25, an external semiconductive layer 27 electrically connected to the semiconductive layer 26 of the cable 1 is formed. The outer semiconductive layer 27 is cut off in a ring shape in the longitudinal direction at a position located just on the outer peripheral portion of the terminal tightening bolt 23. An annular voltage detection electrode 28 is embedded in this portion. When a normal cable is used, a conductive band-shaped cover 29 is attached so as to be in direct contact with and surround the outer peripheral surface of the detection electrode 28. The cover 29 is selected to have a width that completely covers the annularly cut portion of the outer semiconductive layer 27 so as to maintain the electrical continuity of the outer semiconductive layer 27 in the longitudinal direction.

On the other hand, when the method for measuring dielectric loss according to the present invention is performed, the cover 29 is removed to expose the electrode for power detection. The detection electrode is originally provided for measuring the applied voltage without directly contacting the conductor 20 of the cable 1.

In the method of the present invention, the voltage measuring terminal 10 of the tan δ measuring device 30 is electrically connected to the voltage detecting electrode 28. On the other hand, a ground wire 8 is connected to the outer conductor 7 of the cable 1 as shown in FIG. 2, and one end of a coil of a current transformer 9 attached to the ground wire 8 is connected to a charging current measuring terminal 12.
It is connected to the.

After performing the measurement preparation as described above, the method of the present invention is performed as follows.

First, if the connection as shown in FIG. 1 is performed, the alternating voltage applied to the conductor 20 of the cable 1 via the voltage detection electrode 28 and the cable 1 are connected in exactly the same manner as described with reference to FIG.
The predetermined dielectric loss can be measured from the relationship with the charging current. However, this dielectric loss actually includes a dielectric loss error based on the presence of the voltage detection electrode 28 in the net dielectric loss of the cable.

Therefore, according to the method of the present invention, tan
The net dielectric loss of the cable is obtained by subtracting the dielectric loss error from the so-called apparent dielectric loss measured using the δ measuring device 30.

Incidentally, the dielectric loss error is obtained as follows.

First, a connection portion as shown in FIG. 1 is formed using a normal cable 1 whose dielectric loss has been accurately measured in advance.
Then, the dielectric loss is determined by the connection shown in FIG. Since the dielectric loss thus determined is an apparent dielectric loss including the cable 1 and the voltage detection electrode 28, the net dielectric loss of the cable 1 is subtracted therefrom. Thus, a dielectric loss error based on the presence of the detection electrode 28 is obtained. The dielectric loss error obtained in this way is recorded, and when a certain insulation deterioration occurs after the cable 1 is used and it becomes necessary to measure the dielectric loss, the following equation is used as described above. To determine the net dielectric loss of the cable.

Net dielectric loss of cable = apparent dielectric loss−dielectric loss error The present invention is not limited to the above embodiments.

In the above-described embodiment, an annular power detection electrode was provided at the connection between the cable and the device, and tanδ was measured using this power detection electrode. In addition, the cable conductor was exposed. In the case where there is no electrode, any type of power detection electrode may be used as long as the voltage of the conductor can be indirectly measured using the electric field formed by the conductor.

(Effect of the Invention) According to the above-described method for measuring the dielectric loss of a cable according to the present invention, it is possible to measure the dielectric loss in a live state of a cable line in which a conductor is not exposed, and maintain the cable And its long-term reliability can be ensured.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a cable connecting portion in which a method of measuring a dielectric loss of a cable according to the present invention is performed, and FIG. 2 is a schematic diagram for explaining a method of measuring a dielectric loss of a conventional general cable. 1 ... cable, 7 ... cable sheath, 8 ... ground wire, 9 ... current transformer, 10 ... voltage measurement terminal, 12 ... charging current measurement terminal, 20 ... conductor, 28 ... power detection Electrode, 30 …… tanδ measuring instrument.

Claims (1)

(57) [Claims] An AC voltage applied to a conductor of the cable via an electrode for voltage detection provided on a cable line is measured, and a charging current of the cable is measured, and the AC voltage and the charging current are measured. Obtain the apparent dielectric loss from the relationship, subtract the previously determined dielectric loss error based on the presence of the voltage detection electrode portion from the apparent dielectric loss, the net dielectric of the cable A method for measuring a dielectric loss of a cable, wherein the loss is determined.