Breakdown identification in MITICA test facility: progress towards power supply integration

The 1MV Neutral Beam Injector MITICA test facility has a very complex and special power supply system, comprising a 1MV dc plant with some parts insulated in air and some other in SF6 gas. The power conversion system, the DC generator, and the High Voltage equipment have been individually commissioned, whereas the integration tests are ongoing. It is a challenging process due to the high voltage involved (up to 1MV dc), to the unconventional application, to the variety of different electrical technologies involved and to the complexity of the interfaces. The integrated tests of the MITICA power supplies have been affected by unexpected breakdowns in the HV part, which did not allow to reach the nominal performance. Moreover, the location of such breakdowns could not be clearly identified. Specific experimental campaigns with improved diagnostics have been performed with the aim of finding the location of the breakdowns. Even though the results do not allow to get a self-consistent explanation of the events and to identify with a high degree of confidence the position of the breakdowns, they have been very useful to exclude the possibility of a breakdown in the air insulated part of the plant. In addition, the tests highlighted the need of improving the diagnostic systems, with particular focus on the monitoring of the SF6 insulation with non-invasive methodology based on partial discharge detectors. The paper reports the results of the investigation aimed at identifying the position of the breakdown. Analyses will be presented, based on new data coming from the improvement of the diagnostic sensors and on simulations performed to support the identification of the breakdown location. The provisions adopted to mitigate the risk of future undesired breakdowns will also be described.