Voltage Sag Effects on the Process Continuity of a Refinery with Induction Motors Loads

Abstract

Problem statement: Process continuity of industrial plants (like a refinery) is subjected to several shutdowns due to voltage sags causing large induction motors to trip, either by undervoltage or by overcurrent relays, sometimes by the mechanical protection. These unplanned shutdowns cost the plant tens of thousands of dollars per shutdown in addition to material damage costs, restart charges and any penalties due to delay in product delivery and shipping. Approach: The behavior of induction motors during voltage sags was investigated. In addition to the well-known parameters of voltage sags (time and duration), other parameters such as three-phase unbalance, point on the wave of sag occurrence, harmonics, loading percentage and pre-sag voltage were also examined through computer simulations using the MATLAB/SIMULINK toolbox. Motors’ ride-through capability during sags was also studied and guidelines for adjusting the protection relays of the induction motors were highlighted. Results: Results showed that response of induction motor to voltage sags is dependant on sag characteristics, as well as motor and load parameters. Three-phase voltage sags and sags occurring at zero crossing were 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 occurrence. Lightly-loaded motors and motors operated at voltages higher than the nominal voltage are less affected by voltage sags. Conclusion: Manufacturers of induction motors impose strict protection settings to protect their machines from any possible damage. However, the starting process of the motor inherits starting currents and torques much higher than those occurring during voltage sags. Readjusting of the protection relay settings especially the undervoltage relay may be adequate to counteract voltage sags. No conditioning equipment was required.