Control of variable pitch, variable speed wind turbine in weak grid systems

In the case of operating in a weak grid system, when wind power becomes a significant portion of the power system or even the sole energy source, the wind power generator and its converter are expected to help maintain the grid voltage amplitude and frequency, operating like the conventional large synchronous generator (SG). The grid-side converter needs to work as a voltage source to regulate the terminal (grid) voltage amplitude and frequency, which can be achieved by controlling the reactive power and active power flow, respectively. For the direct drive permanent magnet synchronous generator with full power converter, the reactive power can be supplied from the grid-side converter and the active power should be provided by the captured wind power. The power balance between the source (wind) and the grid (load) can be achieved by actively controlling the generator speed and the pitch angle, thus adjusting the wind power input. In the paper, the coordinated control of generator speed and pitch angle are proposed, enabling the generator output power to track the load power. The proposed scheme fully utilizes the system rotational energy by the speed control and the pitch control is used for limiting the speed range and the wind power capture as well. The droop method is used for power sharing among multiple paralleled grid-side wind power converters and the converter dynamics is also considered. The simulation is carried out with different wind speed and load conditions and the results show the proper operation of the proposed control scheme.