An Input-Series-Output-Series Modular Multilevel DC Transformer With Inter-Module Arithmetic Phase Interleaving Control to Reduce DC Ripples

This paper presents an input-series-output-series modular multilevel dc transformer (ISOS-M2DCT) with an inter-module arithmetic phase interleaving control (IAPIC) for high-voltage dc transmission and medium-voltage dc distribution application. The presented ISOS-M2DCT has several advantages over conventional dual active bridge (DAB)-based dc transformer (DAB-DCT) and M2DCTs. It possesses a higher power capacity, a dc fault tolerant superiority, and a higher power factor in heavy load than DAB-DCT; it simplifies the manufacturing of ac link transformers and averts the problem of circulating currents among different modules compared with M2DCTs. The introduced IAPIC is able to cut down the dc ripples without any rise in volume of passive elements, thereby making it easier to realize the compact design of ISOS-M2DCT. In this paper, the topology, operation principle, modulation strategy, basic control scheme, power characterization, the generating mechanism of dc ripples, and the IAPIC scheme are proposed and analyzed successively. Finally, an ISOS-M2DCT prototype is built, and the proposed method is verified by simulation and experimental results.