Modelling activity in support of MITICA high voltage system protections

MITICA, the full-scale prototype of ITER Heating Neutral Beam (HNBs), has challenging requirements on its power supplies. In particular its unconventional high voltage system is designed to operate at high power and voltage (up to -1 MV dc and about 54 MW), with many interfaces and high voltage insulation technologies co-existing in a single plant. Moreover, the electrical system must manage frequent breakdowns between the accelerator grids during the operation, creating electrical fast transients characterized by extremely high voltage and current peaks. In the conceptual design phase suitable design provisions were conceived to limit the energy in the grids and to limit the overvoltage on specific components in the case of grids-breakdown. The integration tests of the power supplies showed the capability of the system to generate and sustain voltage as high as 700 kV; at higher voltage the voltage withstanding capability was not confirmed as some breakdowns occurred in the plant. Although the location of the breakdowns has not been identified yet, an impact of such breakdowns was highlighted as a couple of the components of the plant suffered a damage. This paper is focused on the analyses performed for the understanding of the fault occurred on the secondary bushing of the 1 MV insulating transformer dedicated to the supply of the Ion Source and Extraction Power Supply system (ISEPS). An extensive activity of modelling and simulation has been developed for explaining the dynamic occurred. The model is based on the stray parameters of the components, which were not easy to be estimated in the design phase, but which turn out to have relevant effects in local fast transient phenomena. After reaching a full understanding of the phenomena, the models have been used to support the conceptual design of an additional protection system for the transformer, for MITICA but also in view of the ITER installation.