Case study of bird streamer caused transient earth faults on a 275KV transmission grid.

Abstract

This thesis discusses the results of an investigation that was initiated in January

1996 to determine the root cause of the increasing fault trend in respect of

transient earth faults on the 275 kV transmission grid in KwaZulu-Natal, South

Africa. Historically it was thought that the persistently poor performance of this

network was caused by pollution faults. This network was reinsulated with

silicone composite insulators, and cane fire as well as veld fire management

programmes were introduced. These projects did not result in a consistently

decreasing fault trend on this 275 kV transmission grid.

The burn marks caused by the power arcs, which were identified in this study,

appeared to indicate that air gap breakdown was occurring. Birds were also

observed in close proximity to the faulted towers. Consequently it was thought

that bird streamers caused the transmission line faults. Welded rod bird guards

designed to prevent bird streamer faults were installed on eighteen 275 kV

transmission lines. The accumulative length of these transmission lines is

932 km. The implementation of this initiative coincided with a 73% reduction in

the total number of transient earth faults. This improvement in performance

indicates a strong statistical correlation showing that a large number of the

transient earth faults on the transmission grid are related to bird streamers.

Bird streamer induced faults were identified by means of the following diagnostic

techniques:

• Burn mark analysis

• Time-of-day analysis Bird streamer line faults have been observed on I string, V string and strain

jumper assemblies on the 275 kV power lines. However, on the 400 kV power

lines bird streamer faults have only been observed on V string assemblies.

Experimental work involved simulated bird streamers and determining the

minimum flashover distance for AC system voltages. Electric field measurements

by means of a capacitive probe were undertaken at the ground plane. The

electric field measurements at the ground plane under bird streamer intrusion

confirm that if the streamer is moved away from the live tower hardware, the

electric field enhancement at the ground plane decreases below the background

streamer propagation field.

This case study determined that in order to prevent bird streamer faults the bird

streamer must be moved away from the live tower hardware. The distance it

must be moved is at least 900 mm for 275 kV power lines and 1 100 mm for

400 kV lines.