Investigation into the effect of rotationally shifted arcing horna on a sub-transmission 132kV system.

Abstract

Insulators perform a vital role in a high voltage transmission system as they are expected to withstand normal operating voltages as well as external overvoltages such as those caused by lightning strikes. These arcing horns are primarily fitted to protect the insulator against arc damage in the event of a flashover occurring. In addition to this, they perform a role in the insulation level of the sub-transmission system by providing coordinated protection from backflashover events that are caused by direct strikes to transmission towers.

Currently, the eThekwini Municipality maintains the need of placing arcing horns on their 132kV insulators within a certain span length of a nearby substation. The arcing horns are subject to rotational shifts in the event of adverse weather conditions and this leads to unscheduled maintenance and replacement of the insulator arcing horn arrangement. This upkeep is both costly and time consuming and is a process which may not be necessary. The rotational shift leads to a longer flashover distance and higher breakdown strength and implies that the system will be better protected against flashover and backflashover. However the integrity of the insulation co-ordination of the system is compromised in the process.

This work investigates the effect of the rotation of the arcing horns on both the protection of the insulator as well as the sub-transmission system by means of an insulation co-ordination study implemented in ATP/EMTP. The study was used to determine the probability of a lightning strike causing back flashover.

The model in ATP/EMTP was dependent on a leader progression model and an experiment, conducted on 22kV and 88kV insulators, to validate the breakdown mechanism was undertaken. It was noted that different break- down mechanisms exist for varying spark gap distances which could influence the trend of the results; a correlation between the model and the experiment was derived.

The rotation of arcing horns on the insulators do not require immediate maintenance or replacement of the unit. The leader progression model yielded results in conjunction with the experiment which indicate the lowest possible breakdown voltages for each rotated arrangement. While allowing the arcing horns to rotate result in greater system protection by increasing the basic insulation level of the sub-transmission system and thereby decreases the occurrence of backflashover and the associated earth fault. The degree through which they rotate should be monitored as they could potentially pose an issue to the insulation co-ordination of the system.