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

Voltage sags have become one of the most important power quality concerns 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 cause undervoltage faults in various sensitive loads and subsequently interrupt the manufacturing processes. Such interruptions often inflict severe losses for industries. In Taiwan, ROC, most high-tech manufacturers use uninterruptible power supplies to avoid interruptions, but the cost effectiveness of such an approach remains unclear. As the utility grid continues to improve the reliability of electric power, the inverter-based voltage sag compensator has become a viable solution to prevent production interruptions resulting from voltage sags. The existing sag compensation systems accomplish a fast response within a 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, the utility voltages often contain transient spikes with amplitudes up to 200% resulting from switching of power-factor-correction capacitors, circuit breakers switchings, lightning strikes, and so on. Such transient disturbances may trigger the sag compensator into operation if its controller is very sensitive. The switching frequency of the sag compensator inverter is inadequate to compensate the narrow pulses of voltage spikes. Furthermore, the power semiconductor devices (like insulated gate bipolar transistors) of the inverter may also be damaged due to overvoltage by the surges. In this paper, a brief overview of power quality issues of a high-tech industry park in Taiwan is provided to validate the need for ride-through technologies. A synchronous-reference-frame-based controller for the inverter-based voltage sag compensator is also 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 restores the voltage back to balanced 1.0 pu for critical loads within one-eighth to one-fourt- h of a cycle, which meet the requirement of industry standards like the SEMI-F47 standard. Simulation and laboratory test results are presented to verify the functionality of the proposed system.