Voltage Sag Effects on a Refinery with Induction Motors Loads

Process continuity of industrial plants (like a refinery) is subjected to several shutdowns due to voltage sags causing large induction motors (IM) to trip. In this paper, the response of induction motors to voltage sags is investigated through computer simulations using the MATLAB/SIMULINK toolbox. The simulations are verified by actual performance. The basic observed effects of voltage sags on IM are speed loss and current and torque transients associated with both voltage reduction and recovery. Results have also showed that three-phase voltage sags and sags occurring at zero crossing are the most severe events. Transient currents occurring at the instants of voltage sag and voltage recovery are directly proportional to the voltage drop, not to the remaining voltage magnitude. Transient currents and torques induced at the instant of sag recovery are higher than those induced at the instant of sag starting. Unloaded motors and motors operated at voltages higher than the nominal voltage are less affected by voltage sags. Conservative protection settings may cause IM to trip leading to plant unnecessary shutdown. Readjusting the motor protection relays based on the procedure proposed in this paper may be adequate to ride-through most of voltage sags. No compensation equipment are required