Frequency-domain modelling of interharmonics in HVDC systems

A fast and accurate method for the direct calculation of noncharacteristic frequencies occurring around HVDC links is presented. The technique calculates small signal linearised frequency crosscoupling interrelationships for both converters about a base operating point, modelling the coupling with matrix transfers. The full system, including AC and DC systems, is modelled by connecting all individual transfers together with appropriate system equations and nodal analysis. The resulting equation set is rearranged and solved directly for the unknown interharmonic quantities. Returned subsynchronous frequencies and the effect of switching instant variation are discussed. The method is validated against time-domain simulation, for two different asynchronous links, with excellent results.