To enhance the transmission capacity and increase the utilization rate of transmission line in EHV and UHV grid

an improved hybrid unified-power flow controller（IHUPFC） based on an improved phase shift transformer（IPST） and a small capacity unified power flow controller（UPFC） was developed.After a detail introduction of the structure and voltage regulation performance of IPST

we derived the capacity relationship between the IPST and UPFC.Compared with the existing power flow controllers

the advantages of IHUPFC were analyzed in cost and performances.Besides

we proposed a control strategy for IHUPFC and the effect of capacitance considered explicitly.The simulation results indicate that the proposed IHUPFC and the control strategy regulate the power plow precisely.The power flow control relative error is less than 0.1%.IHUPFC has a simpler structure

lowered carving difficulty

reduced cost

and larger power flow control area than that of existing hybrid power flow controller（HPFC）.The technology of IHUPFC will be widely applied in prospect.