Variable Frequency Operation of DFIG based Wind Farms connected to the Grid through VSC-HVDC Link

High voltage DC transmission using voltage source converters is a quite new technology for the interconnection of offshore wind farms. It facilitates variable frequency operation in the wind farm grid, which can be used for improving the performance of the wind turbines. In this paper a new approach for the coordinated control of voltage source converter based HVDC and wind turbines equipped with doubly-fed induction generators is introduced. Variable frequency can be used to reduce the machine slip. Thus a lower rated converter is sufficient for the same speed range. Alternatively, the slip-limiting operation can be used for extending the speed range while keeping the converter rating at the same level. The basics of variable frequency operation of doubly-fed induction generators are discussed, and the performance is evaluated by a simulation model in MATLAB/Simulink.