Fuzzy Control of WT with DFIG for Integration into Micro-grids

Few years ago Power Systems consisted mainly of large generation plants supplying distant loads through the utility grids. The last years, though, a number of factors lead this structure to change gradually. Small generators of some MW have been already dispersed (DGs) throughout the transmission grid. The distribution of smaller generation units throughout the distribution system as near as possible to the consumer loads has already begun. The DG, up to now, does not provide auxiliary services such as back-up power, voltage support and reliability of supply and its operation is kept under constant power factor equal to 1 at all times. During network disturbances, the DGs up to the present are disconnected until normal operation is reestablished. When the distributed generation penetration is high this may lead to system instability. Therefore, the DG has to change from passive appendage of primary energy supplier to active sources remaining connected to the grid and offering ancillary services. In some countries legislation changed, so that DG remains connected during disturbances and supports the grid. Under this operating philosophy, DGs must support the grid during local disturbances, as central generation stations support high voltage systems in the transient period. This can be achieved through the control of the DGs electronic interface to the main grid and the energy storage plants. These controllable (centrally or distributed) DGs (<100kWe) connected to the distribution grid, together with the local energy storage devices and the local controlled loads comprise an active distribution grid, the so-called micro-grid. In other words, the micro-grid can be seen as a miniature of a large interconnected grid that can provide the demanded power and can also change from interconnected to islanded mode of operation (Soultanis, 2008) and vice-versa. The micro-grid concept is the effective solution for the control of grids with high level of DG penetration (Brabandere et al., 2007). The control of the micro-grid has to be reliable, flexible and according to system specifications. The DGs of the micro-grid have to cooperate in order to cover the local load needs for active and reactive power either under local disturbances or under islanding operation mode. So, in order to achieve the full benefits from the operation of the controllable distributed generation, an hierarchical control system architecture comprising three control levels can be envisaged in a future micro-grid. The Micro source Controller (MC) is the controller of the first layer (peer-to-peer (Meiqin et al., 2008)) and uses local information to control the voltage and the frequency of the micro-grid in transient conditions. This way, any DG can be integrated into the micro-grid operating in «plug and