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Control Strategy Analysis of a Novel Transformerless Unified Power Quality Controller Based on Ultracapacitor and TRIAC

Unified Power Quality Controller (UPQC) can deal with most kinds of power quality problems perfectly, yet its high economic cost hinders its large-scale promotion. Hence, this paper adopts a cost-effective transformerless UPQC based on ultracapacitor and triode for alternating current (TRIAC) with fewer IGBT modules to deal with voltage and current quality problems together. At first, the topologies of the traditional and improved UPQC are compared, and then the LCL and LC filters are designed to be effective for both voltage and current compensation considering the voltage drop and current control precision. Meanwhile, the inverter control strategies for both the uninterruptible power supply (UPS) and active power filter (APF) mode are designed with high dynamic performance and control accuracy, the series-connected ultracapacitor modules used as energy backup enhance the fault ride-through capability of the improved UPQC. Finally, offline simulations and controller hardware in the loop tests were carried out to verify the feasibility of the proposed control strategy of the improved UPQC.