Grid support with wind turbines by the provision of ancillary services

In several countries, the wind power penetration increased tremendously in the last years. This increase in wind power penetration raises concerns about the secure and reliable operation of the power system. The intermittency of the wind causes fluctuations in the output power of wind turbines. In order to balance the grid, conventional units have to maintain additional power reserves. Furthermore, as the current wind turbines generally do not participate in ancillary services such as frequency control, reserve provision, voltage control, etc., this may compromise the proper functioning of the power system. However, many of the aformentioned problems can be solved by using an appropriate control strategy for the wind turbines. Modern wind turbines are equipped with a power-electronic converter to inject the generated power in the grid. By using different control strategies for this converter, a broad range of ancillary services can be provided by wind turbines. These turbines with a power converter are flexible and have a short response time compared to directly coupled units. A short overview of the different ancillary services that can be provided by wind turbines will be presented. As wind turbines are connected to the grid by means of a power converter, the rotational speed is decoupled from the grid frequency. Consequently, when a frequency deviation occurs, contrary to conventional synchronous generators, no inertial response is obtained, which deteriorates the frequency response. By adding an additional control loop to the wind turbines, a similar inertial response as for synchronous generators can be obtained. Furthermore, by operating wind turbines slightly below their maximum power point, power reserves can be maintained to participate in the (primary) frequency control. Wind turbines are also perfectly suitable for the provision of voltage control. When the turbine is not operating at its rated power, the converter is underutilized and has the ability to inject reactive power in the grid. This reactive power injection can be used to regulate the voltage at the terminal of the wind turbine. Moreover, as it is difficult to transport reactive power over long distances, voltage issues should be tackled locally. In the case of small and medium wind turbines, they can provide local voltage control, as they are distributed generation units. Finally, the power converter of the wind turbine can be used to enhance the power quality. This can be achieved by using selective compensation of harmonics or by controlling the voltage to mitigate voltage dips and flicker. Also, unbalance can be decreased by controlling the power injection independently for the three phases. As is the case for voltage control, the converter rating has to be respected. Contrary to frequency control, voltage control and power quality enhancement can be provided together with the active power production, without reducing the power yield. Frequency control with wind turbines comes at the cost of a lower power yield, unless storage facilities are used