The Impact of Square Wave Ripples Superimposed on HVDC on Electrical Tree Growth in Epoxy Resin

Power electronic converters are known to introduce harmonics into HVDC transmission systems, so it is critical to understand the effects of these harmonics in polymeric insulation materials. This work considers electrical tree initiation and growth in epoxy resin subjected to +60 kV DC
$\pm 10\text{kV}$
square wave and a simple 10 kV bipolar square wave. The average tree initiation time under the bipolar square wave was 40 min, and this was reduced to 12.5 min under +60 kV DC
$\pm 10\text{kV}$
square wave. Initial tree channel lengths under bipolar square wave extended typically 60
$\mu \mathrm{m}$
, compared to 100
$\mu\mathrm{x}$
under +60 kV DC
$\pm 10\text{kV}$
square wave. For large periods of growth, the HVDC component did not accelerate tree growth, but did lead to more rapid failure. It is argued that the AC component of applied voltage drives tree growth, but ultimately breakdown is controlled by the absolute voltage magnitude.