Eigenvalue study of torsional interactions between Gravelines generator and IFA2000 HVDC

This paper present eigenvalue stability study of subsynchronous torsional interactions (SSTI) between Gravelines generator and IFA2000 LCC HVDC. Firstly, it presents the state space model development. The procedure for deriving the specific generator and HVDC parameters is also elaborated. The study demonstrates that HVDC may destabilize dominant torsional mode of Gravelines generator for weak French AC grid. This confirms the fidelity of the assembled model since experimental measurements had demonstrated instability for similar conditions. The study is expanded by investigating influence of various AC parameters and HVDC control gains on the damping of Gravelines 6.3 Hz and 12 Hz modes. Torsional interactions are analysed by observing root locus of the torsional modes. Eigenvalue sensitivity study is performed to determine which HVDC gains have dominant impact on the torsional damping. It is concluded that increasing DC current controller and PLL gains can improve damping of the 6.3 Hz mode while deteriorating the 12 Hz mode. By examining participation factors, the study confirms that the control of DC current/DC power is the main cause of torsional interactions. The study also reveals that if HVDC is importing power to France, it can deteriorate damping of the 6.3 Hz mode for particular values of PLL gains. Moreover, participation factors reveal which states could be used as feedback for the development of the supplementary damping controller.