Trade Study of Isolation Requirements and Magnetic Core Selection for Medium Frequency-Medium Voltage Transformers

A significant emphasis exists on enabling a new generation of medium voltage and medium frequency solid state transformers using ferrite and nanocrystalline alloys for grid-tied applications including renewable integration and electric vehicle charging. Isolation requirements become a critical aspect of the transformer design in such cases, with a key design requirement being the winding-to-winding and core-to-winding spacing requirements. This paper investigates the critical impacts of isolation requirements on medium frequency transformer designs, comparing nanocrystalline and ferrite core materials in such applications. It is found that leakage flux loss becomes a critical factor as isolation distance increases. To illustrate this, a reluctance-based Magnetic Equivalent Circuit (MEC) of a single-phase shell-type transformer constructed with two EE cores is used to estimate leakage flux and associated leakage loss. Experimental results populated from a 10 kHz transformer prototype validate the model and design methodology. This paper also presents the design trade-offs between leakage flux core loss and isolation space, incorporating multi-objective optimization of 50 kW, 10 kHz transformer with ferrite and nanocrystalline cores, based on the NSGA-II algorithm under specific constrained conditions.