Design and implementation of a series voltage sag compensator under practical utility conditions

Voltage sags have become one of the most important power quality issues in recent years. According to survey results across the US, voltage sags and short duration power outages account for 92% of power quality problems encountered by industrial customers. Voltage sags often causes undervoltage faults in various sensitive loads and subsequently interrupt manufacturing processes. Such interruptions often inflict severe losses for industries. The existing sag compensation systems accomplish fast response within small fraction of a fundamental cycle by tracking the line voltages closely, and switch on the compensator whenever the voltage waveforms deviate from the normal values. However, utility voltages often contain surges and spikes with amplitudes up to 200% resulting from the switching of power factor correction capacitors and lightning strikes. Such transient disturbances may trigger the sag compensator into operation if the controller is very sensitive. The switching frequency of the compensation inverter is insufficient to compensate the narrow pulses of voltage spikes. Furthermore, the power semiconductors of the inverter may also be damaged by the surges. In this paper, a synchronous reference frame (SRF) based sag compensation scheme are presented. A sag detection mechanism is included in the controller for correct and prompt identification of voltage sags. Disturbances like voltage spikes are attenuated to avoid any false triggering of the compensator. The overall system responds to voltage sags and restore the voltage back to balanced 1.0 pu. for the critical loads within one-eighth to one-fourth of a cycle, which meet the requirement of ITI standard and SEMI-F47 standard. Simulation and laboratory test results will be presented to verify the functionality of the proposed controller.