A New High-Ratio-Transformer-Based Soft Open Point With Mixed-Frequency Modulation Method

This paper presents a new high-ratio-transformer based soft open point (HRT-SOP) topology. By adopting mixed-frequency modulation method, medium-frequency circulating currents are injected into the proposed HRT-SOP, and two circulating current paths are constructed to balance the submodule dc voltage. The use of high-ratio transformer, which step down the medium voltage directly to the low voltage, allows for a significant reduction in the number of submodules. The circulating current models are derived, the control strategies are described and the key parameters are designed. The proposed HRT-SOP is compared with other conventional and state-of-the-art SOP topologies. The comparison shows that the proposed SOP requires least number of semiconductors and capacitors and therefore has the lowest cost. Finally, the correctness and validity of the proposed HRT-SOP are verified by simulation and experimental results.