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Influence of operating frequency on high-power medium-voltage medium-frequency transformers

This paper provides a fundamental insight into the scaling laws of medium-frequency transformers (MFTs). Economic MFTs are a key enabler for solid-state transformers and viable MVDC solutions. An understanding of the influences on the MFT cost structure, and of the limitations to the reduction of size, mass, and cost, is therefore key. Simplified scaling laws are developed that suggest a potential of an increase in switching frequency to improve the MFT performance metrics. They suggest that for fixed MFT geometry, an increase in frequency can lead to lower losses under constrained inductances. The scaling laws also show that a core-size reduction at a given frequency must always be accompanied by increasing core losses. In practical MFT design, size reduction is only observed up to a certain “scaling saturation” frequency. In the studied examples featuring nanocrystalline cores, it is located near 7 kHz. Insight into the scaling behavior is provided by the analysis of the constraint functions used in the MFT optimization. A constrained loss ratio may limit the designs at all frequencies and inhibit core-size reduction. The “scaling saturation” frequency corresponds to a transition between saturation limit and thermal limit of the core, and can be shifted upward by improved core cooling. This enables an attractive compromise between cost savings and loss increase, and shows the importance of sufficient core cooling.