Protection and Voltage Control of DFIG Wind Turbines during Grid Faults

In recent past doubly fed induction generator (DFIG) wind turbines constitute the mainstream configuration for large wind turbines, and therefore their capability to support the grid during faults is a matter of high priority in the research activity over the world. The behaviour of these generators during grid failure is an important issue, since in case of under voltage of the mains it is not allowed to simply disconnect the turbine but it is mandatory that it keeps on delivering power to the main supply system. The fault ride-through and grid support capabilities of the DFIG wind turbines address primarily the design of DFIG wind turbine control with special focus on power converters protection and on voltage control issues. In contrast to its very good performance in normal operation, the DFIG wind turbine concept is quite sensitive to grid faults and requires special power converter protection. The objective of this paper is to enable and enhance the DFIG wind turbines capability for uninterrupted operation and to support the grid, i.e. to provide voltage recovery assistance, during grid faults. In this paper, the authors have developed the design of the DFIG wind turbine voltage control and protection strategy, which is based on the idea that the both converters of the DFIG (i.e. rotor-side converter and grid-side converter) participate to the grid voltage control in a co-ordinated manner. To protect the rotor side converter, a crowbar at the rotor has to be switched on. The performance of an active crowbar during voltage dips is investigated for several parameter sets of machine, resistor and control. By default the grid voltage is controlled by the rotor-side converter (RSC) as long as it is not blocked by the protection system, otherwise the grid-side converter (GSC) is taking over the voltage control. The behaviour of the system during grid short circuit is mainly affected by resistor value. An aggregated model of a 2MW DFIG wind farm, in the case of super synchronous speed, implemented in the MATLAB simulation, is connected to a power transmission system generic model.In the case of grid faults, the controllability of the DFIG variable speed wind turbine embraces both the wind turbine control for preventing over-speeding of the wind turbine and the control and protection of the power converter during and after grid faults.