Environmental Engineering Reference
In-Depth Information
the wind generators it was desirable to avoid complex liquid cooled systems.
Extensive thermal and CFD modelling showed that the stator could be easily
cooled by forced air ventilation.
6.4.6 Stator iron losses
The stator design contains a laminated iron core located radially behind then airgap
winding. This serves three purposes:
1.
It provides a means of mechanical support and rigidity close to the coil supports.
2.
It shields external components from stray fl ux.
3.
It provides an easy circumferential path for the fl ux passing behind the airgap
winding, reducing the amount of HTS wire required.
It enhances the fi eld in the active region of the stator winding.
4.
A fully airgap design (type 4 above) machine has a signifi cant component of
magnetic fl ux in the axial direction near to and outside of the straight length of the
machine. This can cause eddy currents to fl ow in the radial and tangential direction
in the laminations, causing a high concentration of loss at the ends. The low frequency
of the generator (<2 Hz) was expected to reduce these losses, but the phenomenon was
still seen as a potential risk at the start of the project.
The conceptual design was modelled using Vector Fields Opera 3D electromag-
netic FE software. The original design had a total end loss due to eddy currents of
55 kW, which may have been possible to remove by cooling, but would have had
a detrimental effect on the effi ciency on a machine of only 8 MW rating. Careful
design of the stator core geometry and further modelling resulted in a design with
this loss reduced to 6 kW.
6.5 The cost-benefi t study
In order to justify the business case for the development of the HTS generator,
Converteam Ltd. commissioned the independent wind turbine consultants BVG
Associates Ltd. to analyse the cost/benefi t of very large offshore turbines employ-
ing HTS direct drive generators and medium voltage power converters [40]. This
study involved the design of a complete, notional, 8 MW, 12 rpm rotor wind tur-
bine, with appropriate foundations by Sheffi eld Forgemasters, and turbine blade
design, control and structural integration by Garrad Hassan Ltd.
Comparisons were made for a typical UK Crown Estate Round 2 offshore
wind farm of 504 MW containing 4 MW geared conventional DFIG turbines
(the baseline), 8 MW geared turbines, 8 MW direct drive PMG turbines, and 8
MW direct drive HTS turbines. To ensure consistent analysis, the study found
fi rst that on monopile foundations, conventional and PMG 8 MW turbines actu-
ally increased the cost of electricity from the wind farm compared to the base-
line 4 MW turbines. Benefi ts of the Converteam design, low mass, 8 MW HTS
generator resulted in an identical cost of energy compared to the baseline, even
at pre-series volume costs.
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