The drawbacks of conventional Modular Multilevel Converters (MMC) are described in terms of short-circuits. The configuration of MMC-transformer-MMC proves to handle short-circuits at the input and output without the need for additional DC or AC circuit breakers for protection. The transformer can be operated with a higher frequency, reducing therefore the size of the transformer and the capacitors considerably. A multitude of input and output configurations are possible, AC/AC, AC/DC and DC/DC. In particular a DC/DC structure is analysed, serving as DC circuit breaker and voltage elevation interface at the same time. The control strategy of the DC/DC converter is described and an experimental prototype validates the proposed structure and control.