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Unified power flow controller(UPFC) is the most advance flexible ac transmission system(FACTS). Active/reactive power balance for UPFC is analyzed in this paper when it is working in power flow control mode, and an improved real time power coordinate control strategy is proposed in paper. In the parallel converter control loops, an improved real-time dynamic feed forward method is added to reduce over voltage caused by imbalance power. The results of hardware-in-loop simulation based RT-LAB platform show that this new coordinate control strategy，comparing with the conventional one, can decrease excessive voltage excursions of the send terminal ac voltage and DC-link voltage and improve the control performance during power flow changing or system occurring fault.