Control and Supervision of Wind Energy Conversion Systems

This paper is about a PhD thesis and includes the study and analysis of the performance of an onshore wind energy conversion system. First, mathematical models of a variable speed wind turbine with pitch control are studied, followed by the study of different controller types such as integer-order controllers, fractional-order controllers, fuzzy logic controllers, adaptive controllers and predictive controllers and the study of a supervisor based on finite state machines is also studied. The controllers are included in the lower level of a hierarchical structure composed by two levels whose objective is to control the electric output power around the rated power. The supervisor included at the higher level is based on finite state machines whose objective is to analyze the operational states according to the wind speed. The studied mathematical models are integrated into computer simulations for the wind energy conversion system and the obtained numerical results allow for the performance assessment of the system connected to the electric grid. The wind energy conversion system is composed by a variable speed wind turbine, a mechanical transmission system described by a two mass drive train, a gearbox, a doubly fed induction generator rotor and by a two level converter.

PRICE TRANSMISSION ON THE MILK PORTUGUESE MARKET

This paper aims to analyze the milk prices transmission along the food chain in Portugal, temporally and spatially. The results show that the volatility on retail prices is small but happens after 2008. The farm gate price does not change when the price of package milk changes. In mainland, price transmission does not happen but for Azores the transmission is effective. In the intensive systems, the risk to collapse is bigger than in the extensive systems, where the volatility of prices reflects the process of markets adjustment.