Medium Voltage High-Power Converter Topology for 10 MW Wind Generation with the Large Permanent Magnet Wind Generator Systems

This paper proposes a modular, medium voltage, highpower converter topology for the large permanent magnet wind generator system, eliminating the gridside step-up transformer, which is desirable for both onshore and offshore wind turbines. The power conversion systems for large wind turbines are facing a great challenge as today’s wind turbine power outputs approach 5 MW and above. The conventional low voltage power conversion system will suffer from a high transmission current, which significantly increases losses and cost of the cables as well as voltage drop. The converter modules are cascaded to achieve medium voltage output. Each converter module is fed by a pair of generator coils with 90 phase shift to get the stable dc-link power. At the gridside, H-bridge inverters are connected in series to generate multilevel medium voltage output and the voltageoriented vector control scheme is adopted to regulate the converter active and reactive power transferred to the grid. The power factor correcn (PFC) circuit enables the generator to achieve unity power factor operation and the generator armature inductance is used as ac-side PFC boost inductance. Simulation results with a 2-MW wind turbine system and experimental results with a down-scaled 3-kW system validate the proposed topology and control methods. The proposed system can successfully deliver power from the wind generator to the grid. INTRODUCTION Today, the most popular large variable-speed wind turbines are rated around 1.5–3 MW. Nevertheless, 7MW wind turbines have recently appeared and even larger wind turbines, e.g., 10MW, are under development in order to reduce the unit cost of wind power generation [1], [2]. Wind turbines equipped with direct-drive permanent magnet generators (PMGs) and full power converters are generally favored due to simplified drive train structure and thus higher reliability, especially for offshore applications, compared with the doubly fed induction generatorbased system. Most of the present wind generator and power converter systems are based on the 690Vand two-level voltage–source or current–source converters are normally used [3], [4]. The continuous increase in wind turbine power ratings will generate larger current, e.g., from 1673 A for 2-MW system to 8810 A for 10-MW system. Power converters are therefore connected in parallel to handle the increasing current [5], [6]. Meanwhile, large current transfer results in a parallel connection of multiple power cables going down through the tower and causes substantial losses ( ), voltage drop, as well as high cost of cables, switchgears, and terminal connections [7]. These disadvantages can be offset by placing the step-up transformer (e.g., 690 V/33 kV) into the nacelle. However, the bulky and heavy transformer occupies the limited space of the nacelle and increases the mechanical stress of the tower. Therefore, a mediumvoltage power conversion system (e.g., 10 kV) would be more desirable for large wind power conversion by reducing the current level and associated cable cost and losses, as well as improving the system power density. The benefits of adopting medium-voltage power conversion technology have been proved in motor drive applications, where medium-voltage (3–33 kV) configuration is generally used when the system power rating is higher than 1 MW [8]. Table I shows the current rating of an exemplar 5and 10-MW systems with 690-V and 10-kV voltage level for comparison. As seen, transferring from low voltage