Damping torque analysis of power systems with DFIG wind turbine generators

Renewable energy resources are increasingly being used to meet electric power demands and are gradually replacing conventional generation. In particular, wind generation using variable speed wind turbines (VSWT) which include doublyfed induction generator (DFIG) technology, can not only be adjusted by the amplitude and phase of rotor excitation current but also by active and reactive power independently. The purpose of the former adjustment is to maintain a constant frequency power output which can help to improve system operating efficiency and achieve higher quality power generation, while the latter adjustment can change the power angle in order to keep a stable operation of the generator. An important aspect of instability is the lack of damping to the low-frequency power oscillations in the system, which can be influenced by grid-connected DFIG wind turbine generators. In this paper, damping torque analysis (DTA) is applied to the established Phillips-Heffron model to examine the dynamic interaction between DFIG units and synchronous generators, in particular investigating how DFIGs may affect system oscillation modes. An analysis of the impact of the penetration of DFIG technology on power system stability is presented in the paper and this is exemplified by several interesting application studies and results.