Speaker
Description
MITICA, the full scale prototype of ITER Heating Neutral Beam (HNB), is under realization at the Neutral Beam Test Facility (Padova, Italy). MITICA Power Supply is a very complex system, composed of several non-standard equipment, beyond the present industrial standard for insulation voltage level (-1MVdc) and dimensions. Procured by European and Japanese Domestic Agencies, it consists of five DC generators, rated for -200kVdc each and series connected to produce -1MVdc acceleration voltage, linked via a SF6 insulated Transmission Line to the beam source, installed inside the vacuum vessel. The Ion Source and Extraction Power Supply system is installed inside a large air insulated Faraday cage and fed by 1MV Insulating Transformer.
Individual components were tested in factory, then voltage withstand tests (up to 1.265MVdc) have been performed at site in five subsequent steps (from 2018 to 2019), according to the installation progress, after connecting equipment belonging to different procurements.
Nevertheless during integrated commissioning up to 1MV, started in 2021, a breakdown occurred somewhere in the HV plant, either in air or in SF6, causing the fault of one diode bridge arm inside 1MV DCG. By means of a fast transient model developed ad hoc, it was possible to explain the failure with the uneven diodes voltage distribution determined by parasitic elements contribution in transient conditions. Moreover, the model allowed verifying the effectiveness of the additional protections presently under detail design phase.
To identify the locations of possible weak insulation points, a new insulation test was performed with enhanced diagnostic on air insulated parts, mainly adding cameras on the visible field. While testing at 1MV, the Insulating Transformer was damaged by an arc suddenly generated at the top of its bushing. Dedicated refined analyses showed that a breakdown in the HV system can generate an impulse overvoltage exceeding the transformer insulation design capability. A way to reduce the stress to the admissible value (interposing an additional RCL passive component) has been identified and is presently under conceptual design.
To identify the possible source of the breakdowns, the detection system has been further enhanced:
- by adding various current and voltage sensors along the plant and recording the typical breakdown patterns via a reduced voltage benchmark campaign;
- identifying with the support of HV experts suitable instruments for DC partial discharge detection, as a first step on the air-insulated part;
- installing a number of cameras at normal and high acquisition frame rate and realizing a microphone network to identify the possible source and propagation of any abnormal acoustic signal.
If such diagnostic system has proven to be effective during the test campaign presently still ongoing, it can be optimized and adopted definitively both in MITICA and in the future HNBs as early breakdown detection system.
Unavoidable schedule delays have been introduced but MITICA has demonstrated its usefulness in identifying and addressing unpredictable problems deriving from the integration of high voltage non-standard components, allowing the definition of corrective actions and tests in due time for the ITER HNBs installations.
