Wind Energy Systems
In conversion systems to supply electrical energy the wind rotor is coupled, generally via a gear box, with an electrical generator in form of an induction machine or a synchronous machine. In variable speed systems power electronic equipment is used to decouple voltages and frequencies of generation and grid side. Wind energy systems may be operated according to the following concepts:
•constant speed (as for mains fed synchronous machines), or almost constant speed (as for the shunt characteristic of mains fed induction machines),
•variable speed (as for generators with frequency decoupling by inverters).
Note that mains fed synchronous machines mentioned here for completeness arenot used in practice. Variable speed systems are found both with induction and synchronous generators
Systems Feeding into the Grid
Systems for feeding into a 50 Hz or 60 Hz network are coupled to a medium voltage or high voltage connecting point. Under normal conditions the frequency may be considered constant, and voltage variations are within specified values, e.g. ±6%.
the Figure shows circuits of typical system concepts. Using an induction generator
IG, the electrical machine can be directly coupled forming a system of almost
constant rotational speed (a). Variable speed systems use a converter to decouple generator speed from grid frequency, either fully fed (b) or with the converter only for slip energy recovery (c). The latter requires a wound-rotor, slip-ring induction machine. Systems with a synchronous generator always work fully fed with converter. the machine may be electrically excited, via slip-rings or brushless (d), or by permanent magnets (e).
Typical concepts for generating electrical power. – using induction generator; (a) direct coupling, (b) fully-fed, (c) doubly fed, – using synchronous generator, fully fed; (d) electrical excitation, (e) PM excitation
A closer look into the concepts which are realized in the vast majority of wind parks gives. Figure 3.15. Part (a) depicts a conventional system, with an induction generator directly connected, driven by the wind turbine via a gear box, where speed ratios of around 100 are common for ratings of 1500kW and above. To avoid high rush-in currents after switching, it is usual to have a soft-starting device, consisting of a phase-controlled power electronic circuit. The figure may easily be amended for a generator with two winding systems of different pole pair number, according to the Danish Concept.
Figure part (b) is typical for systems with a synchronous generator, preferably directly driven which implies a design with a large number of pole pairs. Variants are known where a gear box of only moderate speed ratio of around 10 is used, allowing a smaller generator size. The power is fed to the grid via a converter with intermediate d.c. circuit which must be designed for full load (fully fed).
Figure part (c) is the circuit common for a system with doubly fed slip-ring induction generator. In contrast to part figure (b) the converter rating is typically only 35% of full load to allow for a speed range of 1:2. Power electronic adaptive devices are shown as intermediate circuit converters, but other configurations are also possible Modern equipment uses active front end inverters on the machine side as well as on the grid side, to allow reactive power supply and power factor adjustment