Nowadays, renewable energies play an important role in electrical power generation. Between different kinds of renewable resources, wind energy has had the fastest growth. Wind farms affect power systems in different manner of conventional synchronous generators. It is because of their different characteristics and dynamic behavior. On the other hand by increasing the wind farms capacity, new grid codes ask them to support power system by providing some ancillary services like voltage and frequency control. In this thesis, DFIG based wind turbines, as one of the most commonly used wind turbine structures, are studied and their impact on power system frequency deviation is evaluated. It is proved that their output active power varies a little after occurrence of active power imbalance in the grid. In the other word they don’t contribute in frequency control of power system. So if conventional synchronous plants are displaced by wind farms, system frequency excursion will increase. On the other hand, DFIG based wind turbines have some features make them potentially capable to participate in frequency control. To use these capabilities effectively, some modifications in the DFIG control system are necessary. Methods make DFIGs capable to participate in grid frequency control are divided into two main groups. The first one is those results in DFIG contribution just in short term frequency control (for few seconds) and the other one is methods make DFIGs capable to provide primary frequency control service for power system. In the short term methods, a supplementary loop is added to active or voltage control loops. The aim of supplementary loop in the active power control loop is to increases DFIG output active power immediately after occurrence of active power imbalance in the grid. The source of this additional active power is kinetic energy stored in wind turbine inertia. The method based on modifying of voltage control loop, tries to manage demand side by controlling of bus voltages. In this way DFIG reduces its bus voltage to reduce the DFIG capability in fast control of reactive power is used and bus voltages are controlled and grid load consequently. These methods usually use frequency deviation (?f) or rate of change of frequency (df/dt) as a criterion of active power imbalance intensity. The impact of using each of these indexes in improving frequency deviations is analyzed. For contribution of wind farms in primary frequency control, they should be equipped with some amount of spinning reserve power. Providing reserve power by a wind farm is different with conventional generation plants because in a wind farm, wind speed determines the available power. So reserve power is changed according to wind condition. The reserve power is provided by Keywords: Frequency control, Wind farm, Doubly-fed induction generators, Frequency deviation index, Rate of chang of frequency index.
