Low-Frequency Common-Mode Voltage Control for Systems Interconnected With Power Converters

In this paper, an active common-mode (CM) duty cycle injection method is proposed to control the dc and low-frequency CM voltage in grounded systems interconnected by power converters. Specifically, a 380-V dc nanogrid and a single-phase ac distribution system interfaced with a transformerless ac-dc converter is considered. In such architecture, the ac and dc CM quantities are coupled through the ground; while the high-frequency noise is filtered by passive components, the dc and low-frequency CM voltage need be controlled to generate symmetric dc-bus voltages and effectively mitigate the ground leakage current. Using a two-stage bidirectional ac-dc converter as an example, the operating range of the proposed method is identified under different ac and dc voltages and alternative grounding schemes. Further, a CM equivalent circuit is derived and experimentally validated, and used to design the controller of the proposed control system. Experiments with resistive and constant power loads in rectification and regeneration modes are used to validate the performance and stability of the control method. As a result, the dc-bus voltages are rendered symmetric with respect to ground, and the leakage current is suppressed. Finally, the control method is generalized to three-phase ac-dc converters for larger power systems.