Comparative Testing of Shield Terminations of High Voltage Cables

In HV systems, cable terminations are one of the weakest links. They are especially stressed by the electric field in free space connections. In this light, several termination types for polyethylene HV cables were tested for dielectric strength and leakage current, down to a pico-ampere level. The tested terminations ranged from simple flush cut to graded insulation using non-linear insulation materials. Procedures and results of the testing are described. The dependencies of leakage current on the applied voltage for different terminations are presented. Visual patterns of breakdown are investigated. The major results are summarized as follows. (1) Flush cut shield may have loose strands and presents a danger of the main insulation denting. (2) Shrink sleeve dominates the ionization phenomena, effectively suppressing the corona discharge. Its influence is much greater at positive polarity of the shield terminus. (3) Shield folded back over an O-ring decreases the electric field, leaves no loose strands and decreases probability of the main insulation damage. It can be recommended for DC applications. (4) Stress grading tapes reduce and greatly stabilize leakage current at a level of 1 nA at 100 kV at room temperature, at positive polarity. They are less effective in leakage suppression at negative polarity. They also increase the breakdown voltage that reaches 130 kV at a 15-cm insulation length, at both polarities.