Small signal averaged transfer function model and controller design of modular solid state transformers

The prime objective of this paper is to design and develop simple and efficient closed loop controllers independently for each of the conversion stages of a Modular Solid State Transformer (MSST) for low/medium voltage applications. The controller design is based on small signal averaged transfer function model using topology independent black box approach. Hence it can be universally adopted for any choice of topologies. The independently designed controllers enhance the modularity of the apparatus such that extension and reduction of ac/dc voltage and power levels are possible by addition and removal of modules as and when required. This paves way for easy maintenance and replacement of modules. Further, the cost and complexity in control is reduced to enable large scale market penetration of the apparatus. Time and frequency response plots are illustrated to validate the performance of the individual converters with the designed closed loop controllers. The simple control design adopted for MSST is shown to mitigate power quality issues to a greater extent possible. Simulation and experimental results are presented which substantiate the appropriateness of the small signal model and hence the controllers designed for the MSST.