Orientation of corona rings for optimal performance in 132kV double tension string insulators

While composite insulator corona rings were only deemed necessary for ESKOM transmission lines 275kV and above, the necessity for corona rings at lower transmission voltages has gained prominence in recent years. At 132kV, deterioration in the form of blackening was observed on the polymer housing adjacent to the live-end fitting through various field inspections, indicative of damaging corona discharge activity. The implementation of corona rings is an indispensable measure for protecting critical insulator interfaces. Split type corona ring design and their location on the insulator are generally specified by suppliers. Of the different assembly types, strain assemblies were indicated as the worst performing combination with the highest electric fields. Being a dead-end assembly, insulator failures can drop conductors, posing a hazard to line integrity and public safety. The orientation of the splits in the rings of a double insulator strain arrangement was analysed to determine if any optimal arrangement existed. COMSOL was used as the finite element software for electromagnetic studies to study various corona ring scenarios. Simulations were compared with laboratory experiments performed at sea level. Good correlation was observed between the location of high electric fields reported in simulations and tests performed at specified voltages.