A Study of High Frequency Voltage Effects in Medium Voltage Cable Terminations

Abstract

High-power voltage sourced converters(VSC’s) are becoming increasingly prevalent in modern transmission systems. These systems primarily use switching schemes generating kHz range harmonics, which may be magnified by one or more system resonances. Despite the high frequency harmonics, VSC systems widely use insulated equipment designed for operation at power frequencies; this includes critical substation components such as medium voltage polymeric cables and terminations. The stress grading systems of non-geometric (compact) cable terminations are susceptible to insulation degradation and eventual flashover failure, under high frequency harmonic stresses. As such, the present work studies high frequency voltage effects in cross-linked polyethylene cable terminations, and their relationship to stress grading (SG) design and material properties. Finite element modeling (FEM) has been used to analyze electric field and resistive heating in termination designs, in response to parametric variations in SG material properties. Experimental studies investigate thermal behaviour in a variety of commercial termination designs, using a high voltage, high frequency test setup developed to replicate conditions of high frequency harmonic resonance in a VSC system.

The study results show that high frequency voltage application increases the electric field, resistive heating, and surface temperature rise, in non-geometric (compact) termination designs using field-dependant stress grading materials. Geometric (stress cone) designs are insensitive to high frequency harmonics; however, they have disadvantages compared to compact designs, making them a less practical long-term solution for high frequency applications. Among non-geometric designs, the field-dependent electrical conductivity σ (E), the permittivity ε, and the temperature dependencies of σ (E) and ε strongly influence the termination electrical and thermal behaviour under high frequency stress. Since thermal hotspots in cable termination SG areas may lead to material degradation and eventual failure, recommendations are made for an optimal non-geometric stress grading design, for terminations operating in environments where high frequency harmonics may be present.