For a grid-connected converter, the output filter is probably one of the heaviest and bulkiest components. As the power rating continues to rise, the switching frequency is quite limited and the size of the output L filter can increases a lot. It is challenging to design the output filter with small size. It has been a consensus for researchers and engineers that high-order filters are more suitable than the conventional L-type filter because they exhibit higher attenuations of switching harmonics even at low switching frequencies. However, among so many kinds of high-order filters, it can still be unclear which one is more suitable for high-power converters with low switching frequencies. Hence, in this paper, three high-order output filters (i.e., LCL, LLCL, LCL-LC) are selected and designed for 4 MW grid-connected inverter, by considering the variations of filter parameters. Especially, in the view of minimizing the total stored energy, the comparisons of different high-order filters are emphasized. It is shown that a higher-order filter may not always have a lower value of stored energy and the LLCL filter can realize the lowest stored energy among the discussed high-order filters.
Design and Comparison of High-Order Output Filters for Grid-Connected Converters with Low Switching Frequency Hao Qian, Jinming Xu, Yuan Hu and Shaojun Xie