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, ...

The standard method for controlling an IGBT inverter (or any VSC inverter for that matter) is by vector current control. This control system consists of two cascaded control loops. One possible realisation of the outer controller is to control the DC bus voltage such that no more power is taken off the DC bus than is available. This creates a current reference, which is fed into the inner current controller. The inner current controller then regulates the current passing through the IGBT such that the desired power is dispatched onto the grid. Whilst most research treats the grid connection as a simple RL circuit, there is little consistency on the method by which the gains of the inner current controller are selected. Internal model control, modulus optimum and root locus methods are just a few of the methods used to find the gains. However, it is not clear which of these methods yields the best performance of the inner current controller. This work suggests that tuning on phase margin or manually tuning may not achieve the best results.

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