PLL-Based Feed-Forward Control to Attenuate Low-Frequency Common-Mode Voltages in Transformerless LVDC Systems

Low-voltage dc distribution systems have gained significant attention over the last years, since they provide simpler and more efficient integration of distributed generators and storage devices, establishing a pathway to net zero energy buildings. In low-power units, transformerless utility interface is preferred to reduce footprint and losses. However, the lack of galvanic isolation introduces a common-mode (CM) path between the dc distribution bus and the utility service grounding, leading to the presence of high CM voltages at the dc bus feeders. The attenuation of the CM voltage reduces the touch voltage amplitude and leakage current, contributing to a safer environment. In this paper, a feed-forward active compensation of the CM voltage is proposed. It relies on utility voltage parameters, estimated by a phase locked loop, to generate a counter CM voltage; hence, it does not require previous knowledge of the system CM path model or the addition of voltage sensors. Experimental results show that this technique can attenuate the CM voltage to non-dangerous magnitudes without interfering with the microgrid differential mode behavior.