Inertial response of variable speed wind turbines

The increasing penetration of variable speed wind turbines in the electricity grid will result in a reduction of the number of connected conventional power plants. This will lead to a reduction of inertia in the grid, as the rotational speed of a variable speed turbine and the grid are decoupled by a power electronic converter. A lower system inertia will result in larger and faster frequency deviations after occurrence of abrupt variations in generation and load.It is possible to implement an additional control loop in the power electronic converter of the turbine, which connects the turbine inertia directly to the grid. It is even possible, by control, to make a ‘virtual inertia’ that is larger than the real inertia. During a frequency drop additional power can be released to the grid with this control. This behaviour will be called inertial response. The additional power is obtained from the kinetic energy that is stored in the rotating mass of the turbine.In this contribution two different control strategies are investigated and compared with each other. A type of control that is equal to the primary frequency control of large power plants shows the best results, both with respect to the power that is needed to limit the frequency drop and with respect to electrical and mechanical stress of the turbine.