Design approach for high power, medium voltage power conversion systems for wind turbines

In this paper, the design approach for the power conversion stage of typical medium voltage (3.3kV to 4.16kV), high power (6-10MW) multi-level converters like those used in offshore wind turbine systems is investigated and evaluated. The back-to-back AC-DC-AC multi-level power conversion stage for offshore wind turbines is usually driven by a Permanent Magnet Synchronous Generator (PMSG) for power ratings of the order of 6-10MW. The design approach, primarily relying on a comprehensive simulation model developed, aims to establish the power processing capability of the power conversion stage under the constraints of the thermal limits of the medium voltage power semiconductor devices, the power quality delivered to the grid (for power factor requirements of 0.9 to unity at the grid) and the overall efficiency. Based on the developed controls for the line-side and the generator-side converter, an instantaneous loss model of the semiconductor devices allows estimation of the instantaneous junction temperatures based on thermal models for the devices. Although the paper provides an overview of the different medium voltage power semiconductor devices, the analysis provided focusses primarily on the Integrated Gate Commutated Thyristor (IGCT). The maximum switching frequencies achievable at the power levels of 6-10MW is then determined based on the thermal constraints. Appropriate selection of the filter elements for the determined switching frequencies then enables quantification of the power quality and the overall system efficiency of the power conversion stage. The achievable switching frequencies at the different power levels of the line-side and the generator-side converters is finally established, along with the achievable power quality and efficiencies at the different operating points.