Amplifying Effect of Weak Grid Connection on the Open-Loop Modal Resonance

In Chaps. 2 and 3, the risk of sub/super-synchronous oscillation affected by the weak grid connection of wind turbine generator (WTG) in a single grid-connected WTG (either the PMSG or the DFIG) system is analytically examined. In this chapter, the impact of weak grid connection on the open-loop modal resonance for the WTG to excite the torsional dynamics of the synchronous generator (SG) is studied. In Sect. 6.1, the impact of weak grid connection of a PMSG to cause the torsional sub-synchronous oscillation (SSO) of the SG is examined. In Sect. 6.2, the impact of weak grid connection of a DFIG is studied. Theoretical analysis carried out in this chapter indicates that under the condition of weaker grid connection, stronger open-loop modal resonance may possibly be introduced by a grid-connected WTG (either a PMSG or a DFIG) to excite torsional SSO of an SG in a power system. The weak grid connection, in fact, amplifies the impact of open-loop modal resonance. Subsequently, under the condition of weak grid connection, the torsional oscillations of the SG are more prone to being excited by the grid-connected WTG. In the chapter, evaluation and demonstration by example power systems with a grid-connected WTG are presented.