Wind Farm Configuration and Energy Efficiency Studies - Series DC versus AC Layouts

Doctoral thesis, 2006

In this thesis, the design and control of a wind farm utilizing series-connected wind turbines with a DC output has been evaluated. The advantage is that a suitable DC voltage level, appropriate for transmission of the generated power directly, without using a large centralized DC/DC converter, can be obtained. This is achieved by series-connecting a number of wind turbines. In addition, the energy production using various wind turbines and wind park layouts have been investigated. Furthermore, the energy production costs have been determined. Finally, the limiting factors for the installation of smaller wind parks have been evaluated. For instance, when dynamic power pulsations have to be considered, from a power quality point of view, when a wind energy installation is to be connected to the grid. The results found are that the uneven power production from the individual wind turbines creates design as well as control difficulties for the wind farm with series-connected wind turbines. A control scheme for the wind turbines is proposed and investigated in this thesis. It is found that the proposed control scheme manages to safely operate the wind farm, even when large deviations in the individual power production of the turbines exist. A down-scaled prototype has been built, representing one wind turbine unit, and the base current control ability was verified experimentally. Moreover, it is found that it is necessary to design the individual wind turbine converters for a voltage level of about 35 % higher than the nominal voltage. Otherwise there will be a substantial energy loss due to the uneven power production from the individual wind turbines that occurs in actual installation. In addition, it is found that the series dc wind park has a good economical potential, since it eliminates the need for an offshore platform in the wind park. Finally, the electrical limiting factors for the installation of wind farms are determined using field experimental data. For instance, it is shown that for a wind park of about 10 wind turbines, the power pulsations are "smoothened" sufficiently so that the flicker emission never will be the limiting factor, even for fixed-speed turbines, when a wind energy installation is connected to a network. Worth mentioning is that it was found that the summation formula for flicker given in IEC 61400-21 can give a flicker prediction that is too low.