A modified phase shift control of the dual active bridge-based modular power electronic transformer to minimize the LVdc side voltage ripples under unbalanced load conditions

Power Electronic Transformers (PETs) are considered as an emerging solution for power conversion due to their benefits over the low frequency power transformers (LFPTs) such as lower size, multi-port structure and different control functionalities. However, unbalanced load conditions may limit the reduction of size due to using bulky capacitors to absorb the resulting 2nd harmonic voltage oscillations. This paper aims to provide a solution to this problem in the Dual Active Bridge (DAB)-based Modular Multilevel Converter Power Electronic Transformers (MMC-PET). In this study, a modified phase shift control strategy for the DAB isolation stage is presented. In this strategy, the duty cycle is allowed to change sinusoidally around the operating point. This consequently transfers the power oscillations to the HV side where their effect is less significant. The system is simulated using Matlab-Simulink under different unbalanced load conditions. Besides, the theoretical analysis and the simulation results are experimentally validated using a laboratory prototype. According to the obtained results, the proposed controller succeeded to reduce the voltage ripples from 7.5% to 0.2% and this helps in replacing large Aluminum electrolytic capacitors with low size film capacitors and consequently reducing the system overall size and volume which is a great benefit.