A Hybrid Solid State Transformer (HSST) Based on a Two Stage Isolated Solid State Transformer (SST): Control and Modulation

This paper presents the modulation and control for a two stage Solid State Transformer (SST) for use in Hybrid Solid State Transformer (HSST). HSSTs comprise of a traditional line frequency transformer (LFT) and fractional Solid-State Transformer (SST). Since the control range required in typical grid applications is limited, HSSTs present an optimum compromise of cost and controllability, compared to a fully rated SST. A two-stage SST is presented in this application, with a single stage isolated AC/DC followed by a DC/AC inverter. A closed loop system has been developed and presented for the said HSST unit, which has been verified through simulation and experimental results.