The Acceleration Grid Power Supply of the MITICA test facility in Padova
(Italy) is currently under commissioning. 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 unconventional application, to the variety of different electrical
technologies involved and to the complexity of the interfaces. During the
integrated tests of the power supplies the achievement of 700kV stable
operation has been demonstrated for the first time in a Neutral Beam Injector,
but an unexpected event occurred, most likely a breakdown in the HV part, which
resulted in a fault of the DC generator. A subsequent test using an auxiliary
power supply was performed to check the voltage withstanding capability of the
HV plant, but another breakdown occurred at around 1MV. This paper describes
the activity performed to identify the location of the breakdowns affecting the
integrated tests. A test campaign has been devised with increased diagnostic
capabilities and specific strategy conceived to trigger intentional breakdowns
in specific locations and collect measurement patterns for different cases. The
results of the campaign will be presented and the current understanding of the
issue will be described, with a view on future tests and further improvements
of diagnostics