Stability enhancement of power system by controlling HVDC power flow through the same AC transmission line

HVDC power transmission employing power electronic device provides a wide range of control in power transmission. Remote generators are sometimes used to supply power to infinite bus through a single circuit long AC transmission line. The system may loose stability after the clearance of a fault if the pre-fault power transfer or the fault clearance time is high. Simultaneous AC-DC power transfer through the transmission line may be used for power flow enhancement. It needs conversion of the line for simultaneous AC-DC power flow to keep the system stable with high values of pre fault power and fault clearance time. During the transient period after the clearance of the fault, AC power supply is kept off and the dc power flow is increased to produce a retarding torque to bring back the generator to its normal speed. HVDC current regulator having very fast control facility may be employed for this purpose. AC circuit breakers are then switched on and the AC power flow is resumed at its pre fault value without any oscillation. The power swing is low and the system is optimally damped at the end of first swing. If the augmentation facility of HVDC power controller is limited, proper governor control of remote generators may be used to reduce the total power flow and to maintain the dc power flow only. After the system becomes stable the AC power supply is switched-on and the power oscillation is minimized with HVDC power controller.