How HVDC schemes can excite torsional oscillations in turbo-alternator shafts

Recent experience has shown that a variable-frequency drive of modest size produced torsional oscillations in a turbo-alternator shaft which were of such a level as to put the shaft at risk. The paper therefore describes a method by which such subsynchronous frequencies can be identified and calculated. The method used considers the convertor as a type of modulator capable of producing sideband frequencies, some of which are at subsynchronous frequencies. The simple mathematical model developed is used to show that HVDC schemes are also capable of exciting steady-state subsynchronous oscillations in turbo-alternator shafts which are electrically close to the convertor station. Different types of scheme are examined, and it is shown that asynchronous links of both the frequency changer and system interconnector type are potential sources of steady-state shaft oscillations.