Modular vs. Monolithic Solid-State Transformer: Influence on Transformer Design

The performance of transformers for solid-state transformers is analyzed and compared using a detailed transformer-optimization tool. Transformers for modular and monolithic solid-state-transformer concepts are compared on the basis of cost, mass, volume, and efficiency, varying the power from 200 kVA to 2 MVA and the operating frequency from grid to medium frequency. The results show the beneficial effect of frequency increase on both cost and efficiency and confirm the potential for high efficiency of medium-frequency transformers, which is an important enabler of good solid-state-transformer efficiency. High power per transformer is another key enabler of low cost and high efficiency. This influence is underpredicted by the classical area-product law. The comparison of core materials shows that at low frequencies, the performance of grain-oriented electrical steel is best and further improves up to an optimum frequency. However, it cannot compete with the performance of nanocrystalline material at several kilohertz. By demonstrating that a combination of high power and frequency is advantageous for the transformer, the results show the importance of future investigations into the operational limits of the power-electronic converters that drive the transformer(s).