A solid-state smart transformer

Abstract

The Smart Transformer (ST) is a transformer built around power electronic converters (PEC). Due to its improved controllability, besides performing the fundamental task of stepping up and down line frequency voltages, the ST can provide a number on ancillary services to satisfy the evolving needs of the grid. The ST can improve integration of distributed generation (DG) and renewable energy sources (RES) to reduce carbon emissions. Additionally, power quality at both transmission and distribution levels can be improved. This dissertation revolves around the control of a LV/LV Back-to-Back (B2B) inverter in SIMULINK®, using PLECS®. The first section of the dissertation is a review of existing literature on the topic, followed by the design and implementation of the converter in the SIMULINK® model. This involves design of the LCL filters to attenuate harmonic emissions from the converter and design of the relevant current and voltage control loops. Finally, a number of case studies based on load current and line voltage profiles measured on a LV feeder were performed to assess the line and load regulation of the designed converter. From the case studies it was determined that the ST has excellent performance in both line and load regulation. When the load power was varied, the converter was able to maintain a nominal voltage of 230VRMS during steady-state operation. The transient performance of the ST lead to momentary over and under voltages when subjected to sudden load power changes. However, due to the design of the control, these lasted no more than 0.3s. In the line regulation case studies, one can observe that the ST, unlike a conventional power transformer (CPT), is able to completely decouple the power network feeding the transformer from the load being fed by it. Although the RMS grid voltage was varied, the load voltage remained unchanged. This proves the decoupling effect that a ST can have, due to the presence of a DC link. The results show that a ST is able to greatly improve power quality. Due to the potential of the ST concept, more research into the field is warranted to explore the full capability of such a device.