Analysis and Comparison of two Control Strategies for UPFC

In this paper the voltage balance, real power balance and reactive power balance of a UPFC system are analyzed. It is interesting to find that when the UPFC bus voltage is stable as power flows changing any increase/decrease in the transmission line reactive power due to in-phase component of the series voltage injected by the series converter causes an equal increase/increase in the shunt converter reactive power. In short, the shunt converter supplies increase/decrease reactive power in transmission line. So there are two methods to control the UPFC. One is the series converter output voltage control the real/reactive power flows, the shunt converter is controlled to make the dc link voltage stable and generate reactive power to make the UPFC bus voltage constant, the other is that the series converter output voltage is controlled to manage the real power flow and maintain the UPFC bus voltage stable while the shunt converter control the dc link voltage and the reactive power flow. Based on the analysis a comparison of the proposed one with the conventional one is carried out with experiments. It is found that with the proposed control scheme the UPFC can get a better reactive power flow control performance with as small bus voltage ripple as the conventional one. However both control schemes can realize effective power flow control with constant UPFC bus voltage.