Overview on the applicability of the ITER/NPP-like technologies to the DEMO Plant Electrical System and promising alternatives

The Plant Electrical System (PES) of the European DEMO, presently under conceptual design, shall supply the loads and deliver net power to the Power Transmission Grid (PTG). The PES subsystems are: Coil Power Supplies (CPS), additional Heating Power Supplies, internal Power Networks and Turbine Generator (TG). CPS includes the power converters, the Switching Network Units providing extra- voltage at plasma start-up and the Fast Discharge Units to protect the superconducting coils. In the present DEMO design, the additional heating function is provided by Electron Cyclotron Resonance Heating (ECRH) only, based on gyrotrons and the related power supplies. Starting from the available requirements coming from the ratings and limits of the coils, plasma scenarios, ECRH power profiles and assumptions on the PTG limits imposed by Transmission System Operator, the applicability of the ITER/NPP-like technologies and design approaches to the DEMO PES components has been evaluated. For some of them, scaling laws suggest that the identified limits could be moved upward with further R&D. For others this is not possible, and a change of technology or the whole philosophy of the circuits is necessary. Severe issues have been identified in the extensive adoption of thyristor-based converters for superconducting coils, since this would imply too huge reactive power to be compensated at ac side. Also, electrical energy storage systems are necessary, to smooth the input active power demand. The technologies adopted in the ITER electrical network have been found applicable but some design approaches need to be revised when scaled to DEMO. The additional stress caused by the DEMO pulsed generation and the potential risk of loss of synchronism with the grid have been evaluated on a NPP-like TG. The results of the survey are presented, and possible ways to cope with the criticalities are identified.