Fault Behavior of Wind Turbines

Synchronous generators have always been the dominant generation type in the grid. This fact affected both planning and operation of power systems. With the fast increase of wind power share in the grid in the last decade, the situation is changing. In some countries wind power represents already a consistent amount of the total generation. Wind turbines can be classified as non-synchronous generation and they behave differently than synchronous generation under many circumstances. Fault behavior is an important example. This thesis deals with the behavior of wind turbines during faults in the grid. The first part focuses on the fault currents delivered by wind turbines with Doubly-Fed Induction Generators (DFIG). The second part investigates the impact of faults below the transmission level on wind turbine grid fault ride-through and the voltage support that wind turbines can provide in weak grids during faults. A wide theoretical analysis of the fault current contribution of DFIG wind turbines with crowbar protection is carried out. A general analytical method for fault current calculation during symmetrical and unsymmetrical faults in the grid is proposed. The analytical method can be used to find the maximum fault current and its AC or DC components without the need to actually perform detailed simulations, which is the method used today. DFIG wind turbines may also be protected using a chopper resistance on the DC-link. A method to model the DC-link with chopper as an equivalent resistance connected to the generator rotor during symmetrical grid faults is presented. This allows to calculate the short-circuit currents of a DFIG with chopper protection as an equivalent DFIG with crowbar protection. This is vi useful since fault current calculation methods for DFIG with crowbar are available in the literature. Moreover, power system simulation tools include standard models of DFIG wind turbines with crowbar protection, but often not with chopper protection. The use of an aggregate model to represent the fault current contribution of a wind farm has been analyzed through simulations. It has been found that the aggregate model is able to reproduce accurately the total fault current of the wind farm for symmetrical and unsymmetrical faults. The use of aggregate models simplifies simulation models and saves simulation time. The Swedish grid code requires wind turbines at all voltage levels to ride through faults at the transmission network. For faults at voltage levels below transmission level fault clearing times are often longer and …