Behaviour of Surface Electric Field in DC Ship Spacers under Real Operation Conditions: Voltage Transients and Ripple, Temperature Gradient and Pollution

Designing and specifying dc spacers for ship application is not as straightforward and well assessed as for sinusoidal ac supply. In particular, the electric field behaviour in the presence of voltage transients, including energization and voltage polarity inversion, voltage harmonics, voltage ripple, temperature variation and surface contamination, have to be considered as unavoidable conditions in dynamic and innovative electrical assets as those figured out in electric ships. This paper presents a finite element analysis simulation of electric field on the surface of a spacer (stand-off insulator) in dynamic conditions, highlighting the differences between de and ac sinusoidal supply. It is shown that during voltage transients the field is permittivity-driven and then slowly transitions into conductivity-driven as expected in steady state dc. The dielectric properties and the temperature strongly influence the field transition. The potential impact of this dynamic behaviour on spacer design is discussed in the light of contributing to creepage and clearance specifications. The effect of voltage overshoots and ripple on dc spacer is investigated and the feasibility to extend the model to account for the effect of pollution is shown as well.