An Active Damping Control Strategy for a Multi-Cell AC-DC Solid State Transformer

Cascaded system stability plays a critical role in the proper operation of any multi-stage power electronic based solid state transformer (SST). Hence, it is imperative to develop small-signal models for the SST system, evaluate its stability margins and develop control based solutions for unstable operating conditions to ensure reliable operation. In this respect, the present work elucidates a system level control strategy for a multi-cell AC-DC SST that can be used to interface with a medium voltage (MV) grid. An active damping control strategy based on classical feedback theory is proposed to improve the stability margins of the individual DC link voltages within the multi-cell SST. This is achieved by emulating a virtual resistance at the individual DC links by employing an active damping control strategy. Such a control strategy presents several attractive benefits such as simple control implementation and does not require additional current or voltage sensors. It is demonstrated through extensive circuit simulations that a single active damping controller can stabilize all the individual DC link voltages within the multi-cell SST leading to excellent performance of the SST system.