A new zero steady-state error control strategy used in three-phase four-wire UPQC

For three-phase four-wire UPQC, when source voltage or load current are unbalance or distort, conventional proportional-integral controller on the p-q-r axis can not eliminate steady-state error. To solve this problem, an AC equivalent proportional-integral controller in p-q-r axis is proposed. Its principle of eliminating steady-state error to zero is analyzed and a new control strategy by using of it with conventional proportional-integral controller is presented. A principle analysis of the proposed control strategy is described in particular. Meanwhile the control formulas on the improved p-q-r coordinate are deduced in detail. The control schematic diagram based on these formulas is presented. Experiment result shows that UPQC efficiently carry out its compensation function while adopting the proposed control strategy.