Steady-state stability of parallel h.v. a.c.-d.c. power-transmission systems

A number of authors have shown that appropriate utilisation of the fast controllability of the power flow in d.c. transmission link can significantly damp oscillations in associated a.c. networks. The improved damping has been interpreted to mean that stability will be improved over the whole range of system operation. The results of this study show that the effect on system stability of a d.c.-link reference-current controller depends largely on the system operating point, and a control system which improves stability at one operating point may have a detrimental effect at other system operating conditions. A system consisting of a single machine, feeding over a parallel a.c.-d.c. transmission link into an infinite busbar, is investigated using a mathematical model of the d.c. link which is consistent with the accepted steady-state model of an a.c. system. The effects of various regulators on the stability limits are given.