| Summary: | With the advancements in the semiconductor device technologies and computational power, the Solid State Transformer (SST) has emerged as a key research topic due to potential bene ts in power density and controllability when compared to linefrequency transformers. However, in the applications where the power density is a key priority, for example traction and space limited grid applications, the conventional AC to AC SST solution is not suitable because the DC-link capacitors put a constraint on further improvements in the power density. A compact single-stage AC to AC SST solution can be achieved by eliminating the intermediate DC-link stages.
This thesis investigates the feasibility of a new single-stage AC to AC SST based threephase Modular Isolated Matrix Converter (MIMC) topology. The MIMC topology, proposed in this thesis, is obtained by using single-phase Medium Frequency (MF) isolated AC to AC converters as a cell. The arrangement of the cells in the MIMC is derived from the concept of the Direct Matrix Converter. The MIMC achieves structural modularity due to the repetition of the single-phase isolated AC to AC converters. The modularity within the converter is advantageous in terms of hardware implementation, maintenance and it also opens up possibilities of stacking cells based on the required power level. However, as each cell in the MIMC is isolated via a MF transformer, this causes challenges in modulation and commutation.
In this thesis, both scalar and vector type of modulation methods are developed respecting the presence of MF transformers in the MIMC. The standard commutation approach which assumes voltage source at one side and current source at the other is no longer applicable in the presence of practical transformers with non-negligible leakage inductance. Therefore, a leakage tolerant current commutation method which guarantees safe current commutation, without an auxiliary clamp circuit or high band-width measurements, is also proposed. The proposed commutation method is experimentally validated on a single cell and then applied along with the Venturini Modulation method to the MIMC. The experimental results from a 6kW proof of concept prototype validate the proposed operating modes and feasibility of the MIMC.
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