A transformerless interface converter for a distributed generation system

Integrating various interface converters of small-rated distributed generators into the power system has become a promising solution to rapidly increasing demand of premium electric power. The output voltage of distributed generators, such as PVs, is usually so low that a low-frequency coupling transformer is required to place between the interface inverter and the grid for proper power delivery. However, this transformer may impede practical installation due to its large weight and size. This paper proposes a transformerless interface converter for a distributed generation system. Instead of the low-frequency transformer, a coupling capacitor is installed between the inverter and the grid to sustain a part of the fundamental voltage. By dynamically adjusting inverter output voltage vector, the power can be converted between the inverter and the grid without any low-frequency transformer. Based on this scheme, the low-voltage generating source would be directly able to deliver available power into the high-voltage grid with unity power factor operation, or to supply reactive power for grid voltage regulation. The proposed method can also suppress power system harmonics due to its voltage control nature. Operation principles and design issues of the coupling capacitor are explained in detail, and computer simulation results are provided to validate the effectiveness of the proposed approach.