Abstract: There are many fault distributions along the actual distribution network lines，but the existing

voltage sag stochastic assessment methods only consider the uniform fault distribution. According to the different

fault distribution characteristics along the line，a stochastic assessment method of voltage sag was proposed，which considers the influence of fault distribution along boundary line of vulnerable area. The critical points of system lines were calculated according to the given voltage sag threshold and the vulnerable area of bus which connected sensitivity load was formulated. The distribution of boundary lines was analyzed within the determined vulnerable area and three fault distribution（uniform distribution，exponential distribution and normal distribution）along the boundary line were simulated to assess the expected sag frequency. Finally，the proposed method was verified valid by taking the IEEE30 bus system as an example.

      Key words: voltage sag；stochastic assessment；boundary line；fault distribution；vulnerable area