Recently, DG (distributed generation) is rapidly developed around the world. But there are several problems in the grid-connected operation. The paper focuses on the selection of proper transformers to meet the grounding requirements in the distribution networks. Through theory analysis and simulation, optimal selection methods are proposed. Yg/Â¿-connection transformer is suitable for small resistance grounding distribution networks while Â¿ /Yg and Â¿ / Â¿ connection transformers are fit for small current neutral grounding networks. The conclusions are useful for the DG-connected power system.