Control of a modular multilevel converter with pulsed DC load

Abstract

Abstract-This paper focuses on a Modular Multilevel Converter grid interface for a klystron modulator system that behaves as a pulsed DC load. With such a load, and without mitigating control, the MMC suffers from cell capacitor voltage imbalance between the converter arms which leads to distortion (fluctuation) of the absorbed AC power. This paper proposes an augmented modulation strategy, with a tailored distribution of the modulation signals between the arms within a phase, to ensure low AC power fluctuation. The effectiveness of the method has been verified through simulation and has been experimentally proven on a 7 kW MMC prototype operating with a 3 kA pulsed DC load.

Arm-Balancing Control and Experimental Validation of a Grid-Connected MMC With Pulsed DC Load

“…Using (19) as the control plant, a PI controller can be designed to add a closed loop correction term to N U M A and ensure that the two arms store an equal amount of energy. From (17), the circulating current amplitudes in the different phases depend on the phase shift between the 50 Hz DC link voltage ripple and the corresponding AC voltages and currents.…”

“…Since the pulsed load is a constant source of imbalance among converter arms, the arm balancing controller has to be capable of achieving zero error in the steady state, which is typically guaranteed by PI based controllers, such as those used in [7], [15], [19].…”