MMC front-end for klystron modulators: an augmented modulation scheme for arm balancing

This paper discusses the control of a Modular Multilevel Converter (MMC) used as a grid-interface for the klystron modulators in the Compact Linear Collider (CLIC). The converter has a DC side load which takes short-duration power pulses, causing high DC side power fluctuations that are not tolerable if seen by the AC grid. The DC-AC power decoupling capability of the MMC enables mitigation of the power ripple on the AC side, guaranteeing compliance with power quality requirements. However, the pulse repetition rate of the CLIC modulators is synchronised the the 50 Hz AC grid and this induces permanent power imbalance in the arms of the MMC, causing voltage deviation and over-modulation unless appropriate balancing strategies are implemented. Unlike existing arm balancing methods that control 50 Hz circulating currents to balance the arm powers, the method proposed in this paper introduces an augmented modulation strategy where modulation signals are redistributed among arms based on the demand from a balancing controller. The resulting controller has lower complexity and its simple structure enables an easier design of the balancing loop, which guarantees predictable dynamics in operation. The effectiveness of the method has been demonstrated in simulation for the full scale CLIC converter ratings and experimentally on a 7kW MMC prototype operating with a 3.3 kA pulsed DC load