Analysis and Study on Control Strategy for UPFC

Balance among real power, reactive power and voltage of the unified power flow controller (UPFC) system is analyzed. Two important conclusions related to UPFC are obtained. Firstly, it is found that the shunt converter provides all the required reactive power during the change of power flow if the UPFC bus voltage is constant. Secondly, the UPFC bus voltage can be controlled from both the sending side and the receiving side. Based on the analysis, a novel coordinated control strategy for UPFC is proposed. In the proposed strategy, the shunt converter of the UPFC controls the transmission line reactive power flow and the dc link capacitor voltage, and the series converter controls the transmission line real power flow and the UPFC bus voltage. The real/reactive power coordinated control in the UPFC control system can provide good performance during both transient and stable conditions. Experimental work has been carried out to verify the effectiveness of the proposed control strategy.