Dynamic Models of Wind Turbines

Doktorsavhandling, 2008

The impact of wind power generation in the power system is no longer negligible. Therefore, there is an urgent need for wind turbine models that are capable of accurately simulating the interaction between wind turbines or wind farms and the power system. One problem is that no standardized model of wind turbines for power system stability studies is currently available. In response to this problem, generic dynamic models of wind turbines for stability studies are proposed in this thesis. Three wind turbine concepts are considered; fixed-speed wind turbines (FSWTs), doubly fed induction generator (DFIG) wind turbines and full converter wind turbines (FCWTs). The proposed models are developed for positive-sequence phasor time-domain dynamic simulations and are implemented in the standard power system simulation tool PSS/E with a 10 ms time step. Response accuracy of the proposed models is validated against detailed models and, in some cases, against field measurement data. A direct solution method is proposed for initializing a DFIG wind turbine model. A model of a dc-link braking resistor with limited energy capacity is proposed, thus a unified model of an FCWT for a power system stability analysis can be obtained. The results show that the proposed models are able to simulate wind turbine responses with sufficient accuracy. The generic models proposed in this thesis can be seen as a contribution to the ongoing discourse on standardized models of wind power generation for power system stability studies. Aggregated models of wind farms are studied. A single equivalent unit representation of a wind farm is found to be sufficient for most short-term voltage stability investigations. The results show that non-linearities due to maximum power tracking characteristics and saturation of electrical controllers play no important role in characterizing wind farm responses. For a medium-term study, which may include wind transport phenomena, a cluster representation of a wind farm provides a more realistic prediction. Different influencing factors in designing dynamic reactive power compensation for an offshore wind farm consisting of FSWTs are also investigated. The results show that fault ride-through capability of the individual turbines in the wind farm utilizing an active stall control significantly reduces the requirement for the dynamic reactive power compensation.