Study on the SSO caused by HVDC link in hybrid AC-DC power systems

Subsynchronous oscillation (SSO) in hybrid AC-DC power systems caused by highly non-linear HVDC devices can lead to shaft failure and electrical instability of a turbo-generator at oscillation frequencies lower than the normal system frequency. With more and more HVDC transmission systems installed in electrical power systems, analysis and control of such SSO becomes a great concern of power engineers in power system design and operations. In this paper, a sevennode hybrid AC-DC system with different operating modes is to be investigated where EMTDC/PSCAD based digital simulation system with detailed multi-block generator model and a HVDC transmission link based on the IEEE first benchmark system model is built to investigate the SSO caused by the HVDC link in a hybrid AC-DC power system. The impact of the HVDC system on damping the SSO will be presented. Also, some operating proposals aiming at damping or even eliminating the SSO in the hybrid AC-DC power system are presented and verified using digital simulations.