A process for optimizing both the design and operation of the generator for a large offshore vertical axis wind turbine (VAWT) is developed. The objectives of the optimization process are to minimize additional costs and losses in the generator to allow for a fair evaluation of the impact of the VAWT environment on the powertrain. A spectrum of torque control strategies was tested based on the ratio, q, of the allowed electrical torque variation to the inherent mechanical torque variation. Equations relating q to the generator losses were established. The effect of q on the energy extracted by the rotor was also investigated and incorporated into the optimization process. This work shows that a variable q strategy with respect to wind speed can improve turbine performance across the range of operational wind speeds depending on the torque loading from the rotor blades. In turn, this also allows for the torque rating of the generator to be reduced from the peak torque rating that would otherwise be expected, creating an opportunity to downscale the generator size, reducing costs. The optimization of powertrain design and operation should be carried out at as high level as is possible, ideally using the fully factored cost of energy (COE) to guard against unexpected losses because of excessive focus in one COE factor (for example reducing upfront cost but in turn reducing availability). KEYWORDS drivetrain, optimization, permanent magnet generator, vertical axis turbine, wind, wind-direct drive | INTRODUCTIONThis work proposes new torque control strategies for generators of vertical axis wind turbines (VAWTs) and demonstrates how these strategies influence the costs and efficiency of the powertrain and energy capture of the VAWT. It uses a nested optimization process on both the powertrain design and operation and demonstrates that to minimize cost of energy, these torque control strategies should vary with wind speed; which in addition, allows the powertrain to be rated for a lower torque value than the peak mechanical torque. List of symbols: Ae, iron loss eddy current loss coefficient; Ah, iron loss hysteresis loss coefficient; bBFe, iron flux density; b B0, baseline iron flux density = 1.5 T 9 ; b, number of rotor blades; Cp, power coefficient; Ep, induced emf; Fm, magnet MMF; f e, electrical frequency; f 0, baseline angular frequency = 50 Hz 9 ; f y, profit function; h, torque harmonic number; I, generator R.M.S. current; i, iron segment; J, moment of inertia of generator; j, square root of −1; k T , generator torque constant; m, mass; P Cu , power loss from copper; P Fe , power loss from iron; P Fe0h , hysteresis losses in iron at 1.5 T and 50 Hz (per unit mass) 9 ; P Fe0e , eddy current losses in iron at 1.5 T and 50 Hz (per unit mass) 9 ; p, number of pole pairs of generator; q, torque control factor (ratio of electrical torque variation to mechanical torque variation); qLM, loss minimization q strategy; qOPT, optimal q strategy; R, phase resistance; R, magnetic reluctance; Telec, electrical torque; Tmech, ...

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This case study investigates the generator of a large offshore Vertical Axis Wind Turbine (VAWT). In particular this case study focuses on the effect of the inherent cyclic torque loading of the VAWT on the generator in terms of the costs and losses experienced. A spectrum of torque control strategies were created based on the ratio q of the allowed electrical torque variation to the mechanical torque variation. Equations relating the copper and iron losses of the generator to this q factor were established and investigated for a test turbine. In addition the relationship between the allowed variation in electrical torque and the cost of the active materials of the generator was explored.