Carrier-swapping method to equalize current in a multi-modular current source converter for SMES

Phase-shifted SPWM technique was adopted to control a directly-paralleled multi-modular current source converter for super-conducting magnetic energy storage system. In classic DC current averaging method, the DC side current of this multi-modular current source converter was averaged by adjusting the amplitude and phase angle of the modulation signal of each module individually, such control resulted in the rising of the AC side harmonics. Swapping of the phase-shifted carrier signals between each module was introduced as a way to average the DC currents, the swapping time was selected at the optimal time to reduce the additional switching operation caused by carrier swapping. Simulation and experiments show that the carrier-swapping method can balance the DC side current without adjusting the amplitude and phase angle of the modulation signal of each module individually. The harmonics of the AC current can be reduced compared with the classic current balance control loop. The additional switch operation needs by the carrier swapping is few when swapping at the optimal time.