Effects of Voltage Sags on a Refinery with Induction Motors Loads

Voltage sags are voltage reduction events, followed by restoration of the normal supply conditions after a short duration. Induction Motors (IMs) constitute a large portion of the loads in industrial power systems. The loss of their service in a continuous process plant (like a refinery) may result in a costly shutdown. The basic observed effects of voltage sags on induction motors are speed loss and current and torque transients associated with both voltage reduction and recovery. This paper will focus on the response of the IMs to various parameters of voltage sags; including magnitude, duration, effect of pre-sag voltage, loading percentage, point on the wave of sag occurrence, source harmonics, and three-phase unbalance. Other objectives of this study are to investigate the motors ride through capability during different types of voltage sags, and to establish guidelines for adjusting the protection relays of the IMs. The industrial electrical distribution system of Alexandria National Refining and Petrochemicals Co. (ANRPC) is taken as a case study to investigate such effects through computer simulations using the MATLAB/SIMULINK i toolbox. Validity of the simulations is verified by actual performance.