Measuring a possible HVDC insulation killer: fast charge pulses

The increase of rated voltages for modern HVDC polymeric cables up to about 400 kV, thanks to high performance insulating materials and higher design fields, may raise long-term reliability concerns due to space charge accumulation. In particular, heterocharge build up can enhance locally the electric field at the electrode/insulation interface. Use of Voltage Source Converters (VSC), which do not permit polarity inversions, allows a limited amount of injected charge to be tolerated, but only if it leads to homocharge accumulation. However, if injected charge migrates towards the opposite electrode and extraction is delayed, heterocharge can build up, thus affecting eventually electrical insulation. This paper shows that at electric field values of 20-40 kV/mm, i.e. in the range of cable design fields, small charge packets, injected from the electrodes and crossing the insulation very fast as coherent charge pulses, can lead to significant heterocharge accumulation in the presence of a partially-blocking electrode. Even in the absence of a noticeable heterocharge accumulation, the electric field perturbation caused by each charge pulse can provide a non-negligible life reduction of cable insulation. Therefore, the development of proper techniques able to detect and analyze these fast pulses is of utmost importance not only for scientific reasons, but also for design and reliability issues.