Control, Protection and Breakdown Management in the Acceleration Grid Power Supply of the ITER Heating Neutral Beam Injector

The ITER Heating Neutral Beam Injector (HNB) is designed to deliver 16.5MW of additional power to the plasma with pulse duration of one hour. The beam acceleration is obtained by a multistage accelerator composed of five acceleration grids that are powered by the so-called Acceleration Grid Power Supply (AGPS). The AGPS feeds around 56MW at 1MV dc to the acceleration grids in quasi-steady state and is able to interrupt the power delivery in some tens of microseconds in case of grid breakdown. The operation of the AGPS will be controlled by a dedicated control system, the AGPS Local Control (AGPS-LC). The AGPS-LC will be in charge of coordinating the operation and interlocks of the AGPS, putting in place all necessary actions to protect the AGPS from internal and external faults and limit the amount of energy flowing from the power supply to the grids in case of breakdowns. The AGPS-LC will be interfaced to a central control devoted to the operation of the Neutral Beam Injector and to a central interlock system in charge of coordinating interlocks and protection functions of the overall plant. The coordination of actions and their time synchronization is of paramount importance to guarantee a reliable operation. The complexity of the system and the different operational scenarios require an in-depth analysis to identify the most suitable control and protection strategy during faults and breakdowns. Such analysis is based on the existing experience of the JT-60U negative ion HNB system and on dedicated simulations performed on a detailed model of the power supplies. This paper will present a strategy for the coordination of the AGPS integrated protection during faults and breakdowns. The operational scenario has been simulated by using software tools for circuit analysis and a proposal for the design of the AGPS-LC has been derived.