Wind Turbines with Doubly-Fed Induction Generator Systems with Improved Performance due to Grid Requirements

Within more than 20 years of intensive research and development the wind industry has recently superseded the 3 MW size of machines successfully that have been in operation in the early 80
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already. Interestingly the industry has realized that the basic electrical concept of these early day machines was very reasonable. Based now on a very solid background of operational experience with wind turbine generators (WTG) of various sizes and the development of high performing power electronics these latest WTG offer now the required and improved performances that will be necessary for a safe integration into the power systems. While WTG have manly been connected to distribution networks in the past, nowadays project and WTG sizes are calling for a direct connection to the transmission system level via a project specific sub-station. Due to this development an integrated approach is required to organize an optimized sharing of control responsibilities between sub-station control and WTG control. The use of doubly-fed induction generator (DFIG) systems offers a variety of options to implement sophisticated solutions. Different control strategies of the DFIG system are optimized for active and reactive power control (torque and speed control respectively) of WTG especially in steady state conditions. The dynamic control of the magnitude as well as the phase angel of the back-EMF voltage of the DFIG yields to a superior system performance to conventional power generators with grid parallel synchronous machines. Additional hard- and software solutions improve the fault ride through (FRT) capability of DFIG systems in case of transient voltage deviations as a consequence of grid faults. But also asymmetric voltage conditions can be counteracted by an independent control of positive and negative system of the wind turbine grid currents. An adequate control solution for a DFIG has not only to consider the transmission network needs, but also to secure the safe operation of the mechanical system of the WTG. Peak loads, especially at the moment of voltage recovery after transient network faults, have to be carefully analysed and respected in the design phase of a WTG.