Electrical power conversion in arrays of linear direct drive wave converters (WECs)

Wave energy has drawn great interest as a renewable energy technology since it is not been fully explored and is still in its infancy. Over the last few decades several very different types of wave devices have been invented to capture this energy for the generation of clean electricity. Among the wave device technologies, direct drive wave energy converters (WEC), which use linear generators in their power take off (PTO) system, offer a simpler approach for power conversion. As the electricity is produced directly from the reciprocating motion of its translator, this type of power take off can be economically attractive compared to the conventional rotary generator which uses a hydraulic mechanical interface that normally has a high maintenance requirement. Most of the studies that have been carried out by wave energy researchers has focused on designing well -proven and reliable wave devices that can operate in the harsh environment of the open sea. Thus any devices that have low maintenance requirements and long life expectancy would be attractive to investors. The desirable features of a linear generator make it a suitable candidate for use with the direct drive WEC.Direct drive linear generators in wave energy converters provide a simple energy extraction concept by reducing the mechanical intervention in the drive train. Instead of generating constant output power, however, the induced voltage and current generated varies in amplitude and frequency. Thus in order to generate more reliable and stable output power, it is highly recommended that they are implemented in arrays. The hypothesis made in this thesis is that the power quality generated by the linear generator can be improved by connecting several of them in an array, with either a DC link or AC link interconnection.This thesis is concerned about the control strategy of collecting the power from arrays of direct drive WECs using linear generators. Although the study is made for the array, control is applied to each WEC to ensure the optimum power is extracted from the waves. A key requirement to ensure that the power from the WECs is effectively collected is by maintaining the DC link voltage at a constant level. This can be done by controlling the power converter at the grid side. Four different control strategies are proposed for the WEC side control. The results presented validate the performance of the control schemes. The study concludes with a proposal for the configuration and the integration of control system for a multi-array wave park