A new control strategy for three-phase four-wire UPQC when voltage fluctuating on its DC side

As comprehensive equipment compensating for power quality, when the three-phase four-wire UPQC works under unbalance and nonlinear conditions, its supply voltage and load current would be unbalance and distortion. Such characteristics of the supply voltage come from the power grid voltage changes, in contrast, that of the load current are subject to changes in the load, which mean that both of them are uncertainty and non-relevance, and lead to the voltage fluctuations on DC side. These fluctuations will directly affect the series and parallel converters of UPQC transforming normally. In this paper, causes and the inevitability of fluctuation of voltage on DC side are discussed in detail, considering the three-phase four-wire UPQC are working on its stable state when the supply voltage and load current are unbalance and distort. In order to eliminate the affect of voltage fluctuations on DC side on the series and parallel converter, the concept of DC voltage regulation coefficient was proposed, which then used as a suppression measures of voltage fluctuation on DC side in the direct current control strategy with load voltage feedforward for parallel converter and the double-closed-loop control strategy for serial converter. Experimental results show that using these control strategy, can inhibit the impact of voltage fluctuations on DC side on the series and parallel compensator. Compared with the control strategy without the DC voltage regulation coefficient, UPQC compensation performance has been improved, indicating that the control strategies are effective.