A modified control strategy for unified power flow controller

This paper presents a modified coordination control scheme for a UPFC. The basic control strategy is such that the shunt converter of the UPFC controls the transmission line reactive power and the DC link capacitor voltage. The series converter controls the transmission line real power flow and the UPFC bus voltage. Real, reactive power and voltage balance of the UPFC system is analyzed. Two important results related to UPFC control have been obtained. First, it is indicated that the shunt converter almost provides all the required reactive power during the power flow changes if the UPFC bus voltage is constant. Second, the UPFC bus voltage can be controlled both from the sending side and from the receiving end side. Inclusion of the real power coordination controller in the UPFC control system avoids excessive DC link capacitor voltage excursions and improves its recovery during transient conditions, MATLAB-SIMULINK simulations have been conducted to verify the effectiveness of the proposed control strategy.