A Transformerless Single-Phase Utility Interface Converter to Attenuate Common-Mode Voltage for DC Microgrid

DC power distribution gains popularity in future residential, renewable energy, transportation and data centers. Such dc system can be connected to ac system through bidirectional interface converter. Compared with isolated converters, transformerless solutions have the advantages of higher power density, efficiency, and lower cost. One concern about the non-isolated connection is the coupled common-mode voltage through ground and the resultant leakage current. In this paper, a two-stage cascaded single-phase ac-dc converter connecting 380 V bipolar dc system and 240 V split-phase ac system is used as an example to discuss the common-mode voltage attenuation. The cascaded converter uses full-bridges for both the ac-dc and the dc-dc stages to achieve decoupled common-mode voltage control. A common-mode equivalent circuit is established to analyze the origin of low-frequency common-mode voltage from the utility and high-frequency noise from the switching behavior of power converters. To attenuate high-frequency noise, a floating common-mode noise filter is connected across the ac and dc ports to contain the EMI noise within the converter. The low-frequency common-mode voltage is controlled by applying a common-mode duty cycle injection method to the full-bridge dc-dc stage. A 5 kW converter prototype is built to validate the attenuation method. The resultant dc bus voltage is symmetric to the ground and suitable for bipolar dc power distribution systems.