A line impedance conditioner to improve zigzag transformer based hybrid Ac/Dc transmission under unbalanced line impedance conditions

Hybrid ac/dc transmission extends the power transfer capacity of existing long ac lines closer to the thermal limit, by superposing dc current onto ac lines through a zigzag transformer. However, this transformer could suffer saturation under unbalanced line impedance conditions. This paper introduces the concept of hybrid line impedance conditioner (HLIC) as an approach to eliminate the line unbalance, which significantly reduces the design cost of the zigzag transformer and could be integrated into the transformer without requiring high voltage insulation. The topology and operation principle are presented, fully utilizing the capability of full bridge converters to generate a hybrid ac/dc voltage as an active impedance compensator. The selection criterion of the line conditioner key components, especially dc link capacitance is also proposed. Simulation and experimental results are provided.