Stability enhancement of power system by simultaneous ac-dc power transmission

Generators may supply power to infinite bus through a single circuit long ac transmission line. The system becomes unstable after the clearance of a fault if the pre fault power transfer or the fault clearance time is high. Conversion of the line for simultaneous ac-dc power flow may 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 flow is switched off and the dc power flow is augmented to produce a retarding torque to bring back the generator to its normal speed. Fast control of dc current regulator may be used for this purpose. The circuit breakers are then reclosed 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.