Abstract
Nowadays, solid state transformer (SST) is been considered as an alternative option in smart grid where additional ancillary services that cannot be provided by conventional line frequency transformer (LFT) are required. However, the power transmission capability of SST is limited to that of its semiconductor switches which are power electronics devices. As such SST needs a new way of dealing with overload. This paper presents an investigation into the overloading scenario in SST by taking the advantage of the galvanic isolation (provided by the three stage configuration) between the source and load sides to extend the operating region of the load side converter without exceeding the thermal limits of the power semiconductor switches. The overloading is achieved by modifying the SST control algorithm to include a current limiting controller that modulates the output current as a function of junction temperature of the semiconductor devices. The proposed control method was validated in a Simulink model of a three stage SST in MATLAB (R) using Simulink (R) and PLECS (R) toolboxes. Simulation results show that the SST can provide long term overload when operated within its wide range of electrical and thermal operating regions and can also provide additional short term overload when operated at the margins of these regions.