Decoupled Control Structure of a Modular Solid State Transformer

This paper proposes a new control strategy for a three phase modular Cascaded-H-Bridge based Solid-State Transformer operating with a wide output voltage range. The control scheme incorporates AC grid side control, output voltage control and distributed cell voltage balancing. The balancing is assisted by feed forward of the oscillating phase powers. The cascaded structure enables highly dynamic and decoupled control of the DC output and AC grid side while limiting the cell voltage deviation. Measurements on a five-level laboratory prototype validate the control concept.