The effect of a floating conductor on the breakdown performance of a non-uniform air gap under HVDC stress at both polarities
Date
2013-07-19
Authors
Hart, Craig L
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
A model for predicting the e ect of a Floating Conductor (FC) on the breakdown
voltage is presented and critically analysed in conjunction with results from laborat-
ory work. Firstly, the scenario of live-line maintenance on HVDC lines is modelled
by a FC within a rod-plane gap. The use of a rod-plane gap is to produce a non-
uniform eld distribution, representing the non-uniform eld that would surround
the conductor of a transmission line. This test object is then further simpli ed into
simple gap geometries such that the breakdown voltage of the two gaps separated by
the FC can be numerically calculated and the breakdown voltage of the entire test
object predicted. The DC breakdown voltage of the rod-plane gap is then presented
as a function of the position of the FC in the gap. The simulations are performed
for both positive and negative DC voltages. The laboratory work uses a similar test
object, with the U50 breakdown voltage recorded as a function of the position of
the FC within the rod-plane gap. The simulation model is then critically analysed
by comparing the laboratory and simulation results. It is concluded that although
the model predicts the change in the breakdown strength from the starting to the
ending points, the developed model has omitted detail that leads to discrepancies in
the predicted results when compared to the laboratory results. This omitted detail
includes the e ect of corona discharge from the FC, additional parameters a ecting
the potential of the FC, the oversimpli cation of the gap geometry for numerical
calculation, and the lack of a dynamic model for the changing electrostatic scenario
as an applied voltage increases towards the breakdown voltage. The results also show
that the position of a FC a ects the breakdown strength of a non-uniform gap. This
e ect is due to the polarity dependent nature of breakdown in a non-uniform gap
and in particular, the di erence in the breakdown strength of a quasi-symmetrical
(rod-rod) and asymmetrical (rod-plane) non-uniform gap. The e ects of charged
particles and their interaction with the
oating conductor in the gap is also deemed
to have a signi cant e ect on the breakdown strength, with their behaviour also
in
uenced by the sustained electric eld that is a result of HVDC stress.