Subsynchronous oscillatory stability analysis of an AC/DC transmission system

A formal analysis of torsional interactions -a form of subsynchronous resonance (SSR)- in series-compensated systems featuring voltage source converter (VSC) based high-voltage direct-current (HVDC) links are investigated in this paper. Two ac/dc systems are studied. The first one consists of the IEEE First Benchmark Model for SSR studies which has been upgraded with a point-to-point VSC-based HVDC link. The second system represents a simplified yet upgraded future Great Britain power system, where reinforcement is achieved through onshore series compensation and offshore submarine VSC-HVDC transmission. Simulation of the two systems has been carried out in PSCAD, with full dynamic system modelling being done in MATLAB. Detailed state-space representations of the systems under study have been constructed. Stability assessments have been carried out using eigenanalysis, with results agreeing well with those obtained through time-domain simulations.