The induction motor (IM) is important equipment among industries. When voltage sag occurs, both the protective device like reclosure mounted in high voltage transmission and distribution lines and the terminal undervoltage tripping of contactor will make the IM disconnect away from the utility, with its terminal voltage changed from voltage sag to interruption. The IM will continue to operate because of inertia. The first-order dynamic model between slip, load torque and terminal voltage is set up based on IM steady-state equivalent circuit. And the universal analytic functions of slip and critical time are deduced from the model which can be used to evaluate the IM low voltage ride through (LVRT) capability, thus providing reference for the designation of IM.

Voltage sag is one of the most critical power quality problems. This paper calculates voltage sag propagation through YNd11 and Dyn11 winding connected transformers in series using symmetrical components, thus obtaining the voltage transfer matrix. Then the voltage sag propagation of single line-to-ground fault is computed using the transfer matrix. Simulation result proves the correctness of the analytical calculation, providing reference for the further study of voltage sag influence and estimation.