Research on harmonic generation principle of series-connected multi-pulse rectifiers and optimization design method for main transformers

To improve the power density of series-connected multi-pulse rectifiers (MPRs), the harmonic generation principle is studied and the optimization design method for main transformers is proposed. According to the topology of series-connected 18-pulse rectifier, its operation waveforms are analysed, and the Fourier series of the waveforms is obtained. Based on the operation waveforms and its Fourier analysis, the principle for harmonic generation of the rectifier is researched, and the phenomenon of autotransformer cannot be applied to series-connected MPRs is researched. According to the characteristics of the transformer, its vector graphics are analysed. Based on the vector graphics, the optimal structure of the transformer with the highest power density is obtained. The results show that the premise of the normal operation of series-connected MPRs is the potential difference between the neutral points of different secondary windings, and only isolation/semi-isolation transformers meet the requirement; the kilovolt-ampere (kVA) rating of the optimal isolation type transformer is 104.6% pu, and the kVA rating of the optimal semi-isolation type transformer is about 71.23% pu.