The presence of static convertors involves a power system in frequent waveform-dependent topological changes. A general model is described in this paper to analyse the dynamic interaction between convertors and power systems. An efficient state-space formulation is used, with isolation of the nodal equations affected by the variable topology. The degree of system representation accepted by the model is optional, but detailed representation is recommended for the convertor and a.c. related plant components, such as convertor transformers, filters and synchronous machines. The versatility of the model is illustrated by typical cases of normal and disturbed power-system operation.