With the interconnection of distributed generation（DG），the distribution system has developed from the traditional passive radial-network to active loop-network， bringing new challenges to system operating security.Several over-voltage problems introduced by the interconnection of DG are discussed in detail.A 10 kV Hangzhou distribution system model is developed to test the proposed theoretical analysis.The over-voltage levels under different conditions are analyzed.Simulation results show that the type and capacity of the DG，the load level， and parameters in the distribution system exert considerable impact on the distribution system over-voltage