Modelling and Control Design of Pitch-Controlled Variable Speed Wind Turbines

MOLINA, MARCELO GUSTAVO; MERCADO, PEDRO ENRIQUE

Wind Turbines

In-Tech Education and Publishing

Lugar: Viena; Año: 2011; p. 373 - 402

In the past decade, many problems related to energy factors (oil crisis), ecological aspects (climatic change), electric demand (significant growth) and financial/regulatory restrictions of wholesale markets have arisen worldwide. These difficulties, far from finding effective solutions, are continuously increasing, which suggests the need of technological alternatives to assure their solution. Under these circumstances, distributed or dispersed generation (DG) arises as the technological alternative with the ability of giving an effective solution to such difficulties. Distributed generation consists of generating electricity as near as possible of the consumption site, in fact like it was made in the beginnings of the electric industry, but now incorporating the advantages of the modern technology. Here it is consolidated the idea of using clean non-conventional technologies of generation that use renewable energy sources (RESs) that do not cause environmental pollution, such as wind, photovoltaic, wave, hydraulic, and more sophisticated systems based on hydrogen. The main advantages of using RESs as DG systems are the elimination of harmful emissions and inexhaustible resources of the primary energy (Heier, 2006).Medium to large grid-connected wind turbine generators (WTGs) are particularly becoming today the most important and fastest growing form of electricity generation among the renewable technologies. They attract interest as one of the most cost-effective ways to generate electricity from RESs (Guerrero, et al., 2010). Indeed, this RES technology started in the eighties with a few tens of kW power capacity to date with multi-MW size wind turbines that are being installed. This trend is expected to be increased in the near future, sustained by the cost competitiveness of wind power technology and the development of new power electronics technologies, new circuit topologies and control strategies. These profits include the strong support provided by governments of different countries, as investment subsidies and incentives that impact directly on the commercial acceptance of wind turbine generators. The growing number of distributed generators, particularly based on wind power systems, brings new challenges to the operation and management of the power distribution system, especially when the intermittent energy source constitutes a significant part of the total system capacity (Rahman, 2003). Under this scenario, the power electronic technology plays an important role in the integration of DG into the electrical grid since the DG system is subject to requirements related not only to the RES itself but also to its effects on the power system operation. The use of power electronic converters enables wind turbines to operate at variable (or adjustable) speed, and thus permits to provide more effective power capture than the fixed speed counterparts (Timbus et al., 2009). In fact, variable speed wind turbines have demonstrated to capture 8-15% more energy than constant speed machines. In variable speed operation, a control system designed to extract maximum power from the wind turbine and to provide constant grid voltage and frequency is required. As well as becoming larger, wind turbine designs were progressing from fixed speed, stall-controlled and with drive trains with gear boxes to become pitch controlled, variable speed and with or without gearboxes.Among variable speed wind turbine generators, direct-in-line systems and doubly-fed induction generator (DFIG) systems have increasingly drawn more interests to wind turbine manufactures due to their advantages over other variable speed wind turbines and currently have the most significant potential of growth. Direct-in-line systems consists of a direct-driven (without gearbox) permanent magnet synchronous generator (PMSG) grid-connected via a full-scale power converter, while DFIG systems are built with a common induction generator with slip ring and a partial-scale converter connected to the rotor windings. Both modern pitch-controlled variable speed wind turbines technologies are emerging as the preferred technologies and have become the dominating type of yearly installed wind turbines in recent years.