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

The paper is concerned with the steady-state stability of and h.v.d.c. transmission line linking two strong a.c. networks. The conventional individual-phase pulsing system is considered as no voltage-loop instability is liable to occur under these conditions. A linearised discrete model of the rectifier is used and a model for the invertor under constant-extinction-angle control is developed along the same lines. A rigorous and an approximate lumped representation of the d.c. line are considered in turn. Discrete-data-systems theory, namely the z-transformation in conjunction with frequency-response techniques, is employed in the stability study of the system. The resulting complex transfer functions are evaluated by a digital computer. The calculated stability boundary is in good agreement with test results from an h.v.d.c. simulator. The pioneering study by Busemann of h.v.d.c. system stability in which the rectifier is represented by a constant gain, and the commutation angle as well as the sampling action of the convertors are neglected is shown to be inadequate. In fact, it is shown that all these neglected items profoundly affect the frequency of oscillations and the stability boundaries. It is also shown that the phase difference between the two a.c. networks has negligible effect on the stability of the system.