A comparative study on modelling and performances of modular converter based three phase inverters for smart transformer application

Abstract

In this paper, the performance evaluation of a three-phase back-end converter (BEC) of a smart transformer using different modular converters and interleaved multi-carrier phase shift modulation techniques was made. The modular backend converter of the smart transformer feeding a 0.415 kV low voltage distribution system and having a capacity of 50 kVA was designed, modelled, and simulated. Different scenarios were used for critically evaluating the performances of the system and included changes in the modulation index (Mi), changes in frequency, load demand changes, and losses. Performance indicators such as the output voltage and current distortions (THD), the maximum current through and voltage across the submodules, changes in the output voltage and current magnitude, and converter efficiency are used for the evaluation of different BEC topologies. The Piecewise Linear Electrical Circuit Simulation (PLECS) platform is used to model and simulate the circuits in question. When comparing MMC and CHB-based back-end converters having the same number of converter cells, load type, modulation index, output voltage, and current, the results show that the MMC performs better with respect to THD and efficiency. Regarding efficiency, the converter made from SiC MOSFET with part number SCT3017AL yields a higher efficiency (96.63%) than the second SiC MOSFET with part number C3M0015065D. According to semiconductor loss analysis, switching loss outweighs conduction loss. The sub-module in a CHB-based modular converter is exposed to higher current stress in comparison with that used in an MMC topology due to the current division in the upper and lower sub-modules in the case of MMC. As the load demand changes, the device current value also changes, while the voltage remains constant.