Active control of low frequency common mode voltage to connect AC utility and 380 V DC grid

Abstract

DC system is promising in data centers and future homes. It can be interconnected with ac system through power converters. One typical case is connecting the 380 V dc grid to the single-phase ac utility through a transformerless two-stage ac/dc converter. In such configuration, the ac and dc system common mode (CM) quantities are coupled through the common ground. While the high frequency noise is filtered by passives, the asymmetric dc and low frequency CM voltage need to be controlled actively for the connection of bipolar dc systems. In this paper, a passive floating filter is used to contain the high frequency noise within the converter. An active CM duty cycle injection method is proposed to suppress the low frequency (e.g. double line frequency) CM voltage ripple and generate symmetric dc bus voltage to the ground. The operation range is identified and the impact from bus voltage and grounding scheme is analyzed. Detailed CM circuit is modeled. The CM transfer function is derived and verified by measurement to complete the control loop design. Design trade-off is provided. Experimental results validate the performance and stability of the CM voltage control loop. The control method is also generalized to three-phase cascaded converters.