Recent results on the stabilizing effect of a d.c. transmission link

It is well known that power system transient stability can be improved by utilizing the effects of the power flow in a d.c. transmission link in a parallel a.c - d.c. power system. The literature describes various control schemes that have been used for this purpose. In a recent experimental study it was determined that a sudden increase in the d.c. power level after the clearing of an a.c. fault, even if only for a short time interval, can have significant effects in maintaining, power system stability. In this paper, two control schemes, neither of which requires angle measurements or communication links between the ends of the d.c. link are described. A procedure for the formulation of the swing equation in terms of the bus voltages is also recommended. The post fault voltage dips are taken into account to estimate the critical clearing time for any two busses in a power system with parallel operation of an a.c. and a d.c. transmission link.