Abstract
The incorporation of the full dynamics of the different conversion stages of wave energy converters (WECs), from ocean waves to the electricity grid, is essential for a realistic evaluation of the power flow in the drive train. WECs with different power take-off (PTO) systems, including diverse transmission mechanisms, have been developed in recent decades. However, all the different PTO systems for electricity-producing WECs, regardless of any intermediate transmission mechanism, include an electric generator, linear or rotational. Therefore, accurately modelling the dynamics of electric generators is crucial for all wave-to-wire (W2W) models. This paper presents the models for three popular rotational electric generators (squirrel cage induction machine, permanent magnet synchronous generator and doubly-fed induction generator) and a back-to-back (B2B) power converter and validates such models against experimental data generated using three real electric machines. The input signals for the validation of the mathematical models are designed so that the whole operation range of the electrical generators is covered, including input signals generated using the W2W model that mimic the behaviour of different hydraulic PTO systems. Results demonstrate the effectiveness of the models in accurately reproducing the characteristics of the three electrical machines, including power losses in the different machines and the B2B converter.
Highlights
Most research effort in wave energy has been mainly focused on maximising the hydrodynamic power absorption of different types of wave energy converters (WECs) from ocean waves
Power from the ocean waves can be absorbed in many different ways, and the different WEC technologies can be categorised into three main groups based on the working principle of each device, as proposed by [1]: oscillating water column (OWC), oscillating-body and overtopping devices
The inverters of the B2B power converter allow for the control of the speed and air-gap flux in the electric generator and the output power to be delivered into the grid
Summary
Most research effort in wave energy has been mainly focused on maximising the hydrodynamic power absorption of different types of wave energy converters (WECs) from ocean waves. In the case of oscillating-body WECs, hydrodynamic power is given by the multiplication between the device velocity (ẋ), induced by the excitation force (Fexc ), and the PTO force (FPTO ) and can be maximised subject to design limitations by means of different control strategies [6], including advance. Mathematical models that include all of the necessary components of the conversion stages from ocean waves to the electricity grid, known as wave-to-wire (W2W) models, are vital. For several different PTO systems (pneumatic [12], hydraulic [13], or a mechanical transmission system [14] coupled to a rotational generator, or a direct drive system with a linear generator [15]), which have been suggested in the literature for different WECs. The W2W model validated in this paper is a combination of inter-connected sub-models, as shown, where the excitation force is Fexc.
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