A medium-frequency transformer with multiple secondary windings for grid connection through H-bridge voltage source converters

Although the power output of today´s wind turbine has exceeded 7 MW, the voltage rating of the most common generator is below 700 V. A low-frequency transformer is commonly used to step-up the voltage to the grid voltage level, e.g. 11 kV or 33 kV. These heavy and bulky low-frequency transformers significantly increase the volume and weight of nacelle. To achieve a compact and light nacelle, a medium-voltage converter with series-connected H-bridge (SCHB) topology would be an attractive technology for future wind turbines. However, the SCHB converter requires multiple isolated and balanced DC sources, which makes the application not straightforward. As an alternative approach to generate multiple isolated and balanced sources a prototype transformer with six secondary windings, 1.26 kVA and 10 kHz, is designed and developed for 1 kV five levels SCHB multilevel converters. The experimental results show that the proposed system may be attractive in grid based renewable energy systems.