Abstract:
To reverse the power flow, line commutated converter
based HVDC transmission systems use polarity reversal.
Insulation is an integral part of any power system, therefore it is
of importance to analyze the performance of insulating material
for better reliability and safety. The paper attempts to address
the performance of insulation under polarity reversals and effect
of these reversals on age/life of insulating material. The aim of
the experiments is to study the ageing of low density polyethylene
and to determine the value of power law coefficient of low density
polyethylene under the effect of polarity reversals. The authors
have conducted a series of experiments under accelerated
thermal conditions to quantify the effect of number of reversals,
voltage levels and time-step of application of polarities and to
determine the ageing coefficient or the power law exponent ‘n’
under the given experimental conditions. The insulation selected
for the experiments is low density polyethylene (LDPE), since it is
believed that polymeric insulation is most susceptible to the
effects of space charge formation. The voltage profile is in the
form of square wave. The time-period and amplitude of the wave
is varied to observe the effect on the number of polarity reversals
occurred before breakdown of LDPE samples. The number of
reversals are recorded and fitted in a statistical model to analyze
the variation of voltage levels and time-gap between reversals.
The number of polarity reversals was recorded before
breakdown of LDPE samples at different voltage levels with
different pulse duration. The value of power law coefficient is
determined for different pulse duration. The discussion includes
impact of dc ageing as well as reversals on the life characteristics
