Control Technique for Transformerless Regenerative Testing of Grid-Connected Power Converters

This paper presents a control technique to mitigate the DC circulating current of a transformerless regenerative power converter system. Regenerative or back-to-back configuration of converters is typically used for efficiency characterization and thermal performance evaluation of power electronic converters to preclude the need for high-power electrical source and load in medium or high-power converters. However, this back-to-back setup leads to an undamped system where the presence of any minute DC offsets in nodal voltages lead to considerable circulating current flow and intra-converter interactions. Traditionally, isolation transformers are used between the two converters to remove the DC circulating current. However, the isolation transformer can increase the system cost, especially for high-power applications, and impact the overall system losses. On the other hand, the presence of the circulating current can affect the converter efficiency measurements by introducing more unaccounted losses and can also lead to system instability. Hence to preclude the need for a transformer while also limiting the converter interactions, a circulating current control technique is proposed in this paper for an AC-DC-AC regenerative setup that minimizes the DC circulating current and helps in accurate efficiency measurement. The proposed control technique is evaluated using PLECS simulations and experimentally validated on a 10-kW SiC-based laboratory prototype.