New Control Scheme for a Unified Power-Quality Compensator-Q With Minimum Active Power Injection

Voltage sags are one of the most frequently occurring power-quality problems challenging power systems today. The unified power-quality conditioner (UPQCs) is one of the major custom power solutions capable of mitigating the effect of supply voltage sags at the point of common coupling (PCC) or load. A UPQC-Q employs a control method in which the series compensator injects a voltage that leads the supply current by 90 so that the series compensator at steady state consumes no active power. However, the UPQC-Q has some disadvantages. First, there are limitations in rating when using UPQC-Q for series compensation. Second, there is a phase difference between the input and output voltage in proportion to the severity of voltage sags. As a result, it cannot offer effective compensation for voltage drops. This paper proposes a new control scheme for the UPQC-Q that offers minimum active power injection. The proposed power injection method takes into consideration the limitations of the rated voltage capacity of the series compensator, and the phase difference during voltage sag events. The validity of the proposed control scheme has been investigated by simulation and experimental results for less than 50% voltage sag conditions.