ON THE ISSUE OF END TERMINATIONS OF FLEXIBLE HIGH-VOLTAGE CABLES

V. S. DMITREVSKII, V. A. STRYZHKOV, Yu. N. SHUMILOV

(Presented by the scientific seminar of the Department of Electrical Insulation and Cable Engineering)

Modern flexible high-voltage cables with rubber insulation are widely used for supplying power to excavators and earth-moving machines. Cable termination at the points where it is connected to the machine is carried out by removing the hose sheath and removing from the surface of the insulation of the current-carrying conductor the braid and other coverings over a length of 10–12 cm [1].

Operating experience shows that 25% of all insulation breakdowns are observed at termination points because of ozone cracking of the insulation under the action of atmospheric ozone and ozone generated when corona discharges occur [2]. We conducted studies of the corona-discharge inception voltage \(U_k\) in the end terminations of a KShVT—6 kV cable. The corona-discharge inception voltage was determined with the aid of a partial-discharge indicator [3] and visually in the dark.

Table 1 gives the measurement results, which show that in the end terminations of cables, \(U_k\) for the existing terminations is lower than the phase operating voltage. If one takes into account that, in the cable termination, the insulation operates at line voltage, then in service one may expect the presence of comparatively stable ionization in the terminations. Thus, with the terminations currently in use, corona discharge is not eliminated, i.e., the main cause of breakdown of cable insulation in the terminations is not eliminated.

Table 1

Effect of the type of end termination on the corona-discharge inception voltage

The tests we carried out fully confirmed this proposition. Table 2 gives the results of life tests of the insulation in end terminations.

Table 2

Results of testing cable breakdowns in end terminations
(test time 1000 hours)

The tests were carried out on separate specimens of insulated conductors in the absence of bending and tensile loads. To simulate the current load, a current of 210 A was passed through the current-carrying conductors of the specimens under investigation.

From Table 2 it can be seen that, over the course of 1000 hours, 30–50% of the insulation breakdowns occurred in the end terminations. In all cases the cause of breakdown was cracks developing from the outside to a certain depth. The cracks developed perpendicular to the cable axis. In some cases the cracks were located along the cable axis. The source of mechanical stresses in the insulation in this case is the temperature gradient through the thickness of the insulation, both in the steady-state condition and during the period of heating after the current and voltage are switched on. The lower layers have a higher temperature and greater linear expansion and therefore operate in compression, while the upper layers are stretched because, compared with the lower layers, at a lower temperature they have less linear expansion.

When bending deformations arise at the termination points, one should expect a considerable acceleration of failures in the terminations and an increase in their total number.

In order to increase the corona-discharge inception voltage in the terminations and to protect against ozone cracking, we developed a cable termination whose design is shown in Fig. 1.

The proposed termination is made as follows:

1. The end of the cable is freed from the hose sheath over a length of not less than 30 cm.

2. The semiconducting screen is removed from the insulated conductor in the termination along its entire length, leaving 10 cm from the root of the termination.

3. At a distance of 10 cm from the root of the termination, a conical thickening of raw patching rubber is wound onto each conductor; the dimensions of the thickening are given in Fig. 1.

4. The conical thickenings are vulcanized.

5. The conical thickenings, over half their length on the side of the root of the termination, are covered with a semiconducting compound or semiconducting adhesive tape.

The use of the indicated termination substantially increases the corona-discharge inception voltage (Table 1); on the other hand, the winding is reliable protection against the occurrence of significant bending deformations of the insulation at the most dangerous point of the termination. Service-life tests showed the high effectiveness of using this type of termination. Over the course of several months we did not

no cases of breakdown at the edge were recorded during prolonged voltage testing of the cables when the indicated terminations were used. Thus, the use of special end terminations sharply increases the reliability of cables in service, with comparatively low expenditures for making the end terminations. In addition,

Fig. 1. End termination for one current-carrying conductor of a KShVG cable, 6 kV, 25 mm²: 1—current-carrying conductor; 2—rubber insulation; 3—conical thickening; 4—semiconducting coating

it may be noted that, as the semiconducting coating, an adhesive insulating tape made on the basis of semiconducting polyvinyl-chloride plastic compound may be used; its manufacture at chemical plants is unlikely to encounter serious difficulties.

REFERENCES

1. A. V. Linkov. Excavator Cables. GEI, 1958.

2. A. G. Bezdenezhnykh, V. P. Anokhin. Conditions for the safe operation of excavator cables in the Kuzbass open pits. In: Collection of Articles on Electrical Safety in Coal Mines, “Nedra,” 1966.

3. V. S. Dmitrevskii, Yu. N. Shumilov. The influence of mechanical and thermal effects on the ionization-inception voltage of excavator cables. Cable Technology, No. 37, 1966.