The effect of a floating conductor on the breakdown strength of a DC gap at both polarities

This paper presents a preliminary investigation into the effect of a floating conductor on the breakdown strength of a rod-plane gap energised by both positive and negative polarities. This experiment models the scenario of live-line maintenance, therefore providing results for a research into HVDC live-line maintenance, with special reference to the existing HVDC lines and proposed future projects in Southern Africa. The experimental work makes use of a floating conductor comprised of a copper sphere and a rod protrusion on the anodic side of the sphere, based on conclusions from previous research. The experimental work is split into two main parts with positive and negative DC voltages. The experimental work determines the U₅₀ breakdown voltage of a rod-plane gap as the floating conductor is moved within the gap. A comparison of the change in the breakdown strength of the gap is made against the base case of a rod-plane gap with no object and a gap size which is equal to the split gap size. The U₅₀ breakdown voltage is then presented as a function of the primary gap (energised rod to floating conductor) size. From the positive polarity results, it is evident that the presence of a floating conductor raises the DC breakdown strength of the gap, with the breakdown strength increasing almost linearly as the primary gap is increased. A completely different effect is observed with the presence of a floating conductor in a negative DC gap. The presence of a floating conductor immediately lowers the breakdown strength of the gap, and there is a sharp decline in the breakdown strength as the object approaches the ground plane. This result suggests that the implementation of live-line maintenance on HVDC lines would have to consider two completely separate guidelines to deal with positive and negative HVDC lines respectively.