Stochastic Assessment of Voltage Sag in Unbalanced Distribution System with Distributed Generators

The interruption of sensitive equipment due to voltage sag can cause huge economic loss, and thus the study of sag evaluation is of great significance for voltage sag control. In this paper, a stochastic assessment method of voltage sag for unbalanced system with inverter interfaced distributed generator (IIDG) is proposed. The accurate calculation of fault current is a necessary prerequisite for the effective assessment of voltage sag. Different from the traditional synchronous generator model, the fault current of IIDG is affected by the low voltage ride through (LVRT) control strategy, which has a nonlinear relationship with the common coupling point (PCC) voltage. Moreover, three phase coupled component is difficult to decoupled in the unbalanced distribution system. Hence, the traditional short circuit calculation method is not suitable to be applied in the voltage sag assessment of the unbalanced system with IIDG. In the work, the equivalent model of IIDG in fault analysis is analyzed. The stochastic assessment of voltage sag method for unbalanced distribution network is proposed. The effectiveness of the proposed method is verified by the simulation of IEEE 13 node test feeder. The calculation results show that the proposed method is superior to the previous voltage sag method in accuracy.