A H control system has a relative complicated structure, for example, the HAdAd controller for the series inverter control is a fourth order system. ... The control block diagram for a UPFC system is shown in Fig. ... control system (CC) and a H ~ controller (RH), where the reference of real power changes from -1000 W to 1000 W and the reactive power reference is kept at zero. For simplicity, the shunt inverter is assumed to be open circuit and the dc link voltage is 1 00 volts. 1000 S 0 0 0 c.