Stability of a d.c. transmission link between weak a.c. systems

The paper discusses the steady-state stability of an h.v. d.c. transmission system linking weak a.c. networks. A low a.c.-system short-circuit ratio complicates matters considerably as the combined a.c. network and filter impedance has a major influence on system stability. The determination of the approximate transfer functions of the combined a.c./d.c. system is achieved through use of modulation techniques based on sinusoidal carrier functions. The results of the stability study show that convertors furnished with voltage-controlled-loscillator pulsing systems are not immune from instability, the worst conditions occuring when the a.c. system short-circuit ratio on the rectifier side is high and on the invertor side is low. Test results from a high-voltage d.c. model indicate that the theory provides acceptable predictions and can therefore be used as a tool for d.c.-system control-circuit design.