Reactive power generation by DFIG based wind farms with AC grid connection

Balancing reactive power within a grid is one of the fundamental tasks of transmission system operators. With increasing portion of wind power, wind turbines have to contribute to reactive power generation during steady state as well as during transient conditions. First, this paper provides an overview about the available options to supply reactive power by wind farms typically connected to the grid by AC cables. Then the Fault-ride Through (FRT) sequence of the Doubly-Fed Induction Generators (DFIG) is discussed in order to explain the reactive power generation capability of this type of wind turbines during low voltage periods. Under steady state conditions the utilization of var sources represents an optimization task. It is shown in the paper that generation of reactive power by WT may be a favorable option under economic aspects. FRT with DFIG is possible even if the grid voltage drops to zero. To protect the converter against overcurrent and overvoltage DFIG are equipped with crowbar and sometimes with chopper. When the crowbar is switched on the machine is a var consumer. However the Line Side Converter (LSC) can be controlled to supply up to 50% of the required reactive current. When the crowbar is not activated the DFIG can supply reactive power from the rotor side through the machine as well as through the LSC. For illustration, simulation results for an exemplary fault are shown and elucidated.