Effects of voltage sag during starting of an induction motor

Power quality (PQ) has become one of the important issues to be focused with the increasing number of sensitive equipment in industrial distribution system. Voltage sag during large motor starting has becoming the most common PQ problem as induction motor carries about 60 percent of the number of equipment used in the industries and is one of the prevalent source of voltage sag problem. This project aims to study the effect of voltage sag during starting of large induction motors by using three different starting methods i.e. direct online, star-delta and autotransformer. An 11kV induction motor of kiln main drive utilised in a cement factory has been modelled and tested using various motor ratings. The effect of voltage sag during the starting of the motor is observed using different starting methods and the findings are then been compared to ascertain the best starting method of the under test induction motor with least voltage sag problem. Based on the obtained simulation results run on Power System Computer-Aided Design (PSCAD) software, all starting methods shown the linear characteristic on different motor ratings with direct online method produced most significant voltage sag depth while star-delta method gave the least significant voltage sag depth. Thus, the later method is recommended as the best methods of starting based on the NEMA MG-1 standard for variations in voltage and frequency, along with the voltage unbalance by IEEE std. 1159-1995. However, system reliability can be maintained by compensating the reactive power to a certain level using static VAR compensation technique. This technique has been chosen as the mitigation method because it uses semiconductor devices such as thyristor for faster switching of connecting capacitor bank and reactors according to the system requirement with low harmonics interference.