Isolated DC/DC Converters for DC Distribution Grids in Rural Electrification

Abstract

Roughly 840 million people, predominately from rural communities in sub-Sahara Africa, still lack access to electricity. The direct current (DC) microgrid is an emerging grid infrastructure that meshes efficiently with DC based technology such as photovoltaics, batteries, consumer electronics, and electric vehicles. This characteristic of DC microgrids designates them as a preferred solution for new grid infrastructures in rural electrification applications. In order to establish a DC microgrid, power electronic interfaces (PEI) are required for regulating power flow and interconnecting different grid components of the microgrid. In this thesis, the PEI (operating at 200-900 W) connecting a 350 V DC-microgrid to a solar home system (compatible with USB-C) is investigated. A unidirectional half-bridge LLC converter and a bidirectional dual active half-bridge (DAHB) converter (both utilizing gallium-nitride (GaN) transistors and planar transformers (PT)) are designed and tested. A working prototype of the half-bridge LLC converter with a center-tapped secondary is presented and the waveforms at maximum load and no-load conditions are analyzed. A complete design and efficiency approximation for a DAHB converter with a center-tapped secondary and active snubber circuits is included. In addition, simulations (in PLECS) of the designed DAHB converter provide waveform results for both the forward and reverse power flow direction. The results of the work discuss how high frequency operated, half-bridge isolated DC/DC topologies with GaN transistors and planar transformers are an excellent composition of technology for these rural electrification applications. The GaN transistor is most effective in a low voltage (up to 650 V), high performance scheme, and offers inherent benefits which allow for high frequency operation and thus, smaller passive components. Moreover, the effect of current collapse (an adverse effect in GaN transistors) is discussed and analyzed from a design standpoint. The benefits of planar transformers in low-medium power (up to 900 W) rural electrification applications and an in-depth PT design process are presented. Additionally, rural electrification safe extra-low voltage (SELV) standards require that PT designs must have reinforced (or double) isolation between primary and secondary windings. Taking this into consideration, two proposed PT configurations using a U-core and planar E-core, respectively, are compared. The main conclusions of this work aim to bridge the gap for the design and implementation of efficient DC microgrid variable power output converters for use in rural electrification applications.