Novel Frequency-Doubling Modulation and Control Strategy for Three-Level Full Bridge based Power Electronic Traction Transformers

Power electronic traction transformers (PETTs) are widely investigated to substitute bulky line frequency transformers in railway traction system presently. This paper adopts the three-level topology for both grid-side cascaded converters and primary-sides of LLC resonant converters. The cascade number is reduced by half as well as isolated transformers compared to the traditional two-level configuration. Less cascade number of modular converters leaves relatively more space for electric insulation implementation, which is beneficial to PETTs in terms of the limited installation space in the train. A frequency-doubling modulation strategy for LLC resonant converters is proposed, which can enable the operating frequency of isolated transformers two times higher than the switching frequency to further reduce the transformer size without increasing the switching loss. Meanwhile the proposed modulation strategy can realize the capacitor voltage balance autonomously as well. Additionally, the control strategy of cascaded rectifiers and key parameter design guideline of LLC resonant converters are also introduced.