Application of power electronics to the distribution transformer

This dissertation describes the design and prototype construction of a novel Solid State Transformer (SST). Conventional distribution transformers are in many ways relics of turn-of-the-century technology. While efficient and inexpensive, their secondary voltage waveforms are tightly coupled to the applied primary voltage waveforms, and primary currents are dictated by the load currents. This can cause power quality problems for both the source and the load. Conventional distribution transformers also have inherent voltage drop which increases with load. Replacement of conventional distribution transformers with solid-state devices could provide control over the magnitudes and waveforms of the load voltage and input current. Self-protection would be a natural byproduct of the controls. At the same time, a solid-state transformer would perform all the functions of conventional devices. Previous efforts to develop a solid-state transformer, including one recently sponsored by the Electric Power Research Institute (EPRI), have been limited and comparatively crude demonstration projects. They did not provide isolation and have proven to be impractical when applied to utility distribution voltage levels. These efforts did, however, provide the level of interest sufficient to encourage design of the practical solid-state transformer described herein and eventual construction of a prototype. In this dissertation, an SST architecture is set forth that is specifically designed to operate at utility distribution voltages. It provides a sinusoidal secondary voltage regardless of the primary voltage waveform, provides a secondary voltage magnitude that is independent of either load current or primary voltage magnitude, and at the same time draws sinusoidal input current which is in phase with the primary voltage regardless of secondary current or primary voltage waveshapes --Abstract, page iii