Control for Bipolar DC Microgrid and DC/DC Bidirectional Converter in Energy Access Context

Abstract

Rural electrification remains a significant challenge for engineers, given its intricate relationship with multiple issues, including geographical constraints and underdeveloped infrastructural provisions. As a result, with the development of Solar Home Systems (SHS) and direct current (DC) microgrids, DC systems for energy access have emerged as a promising solution.

In this context, a bidirectional DC/DC converter serves as a bridge connecting the SHS and the grid. Thus, it is important to design a converter's control mechanism to ensure a smooth and efficient exchange of power in both directions. Beyond this aspect, the system perspective involves the coordination of photovoltaic (PV), battery storage, and grid power supply priorities within the SHS. Consequently, this thesis centers its attention on two objectives: the control of the bidirectional DC/DC converter and the system-level control of the DC microgrid.

To ensure high efficiency under variable loads and smooth transitions between two directional operations, this thesis proposes and evaluates a novel control methodology. Transfer functions are derived through mathematical modeling and PI parameters are determined by analyzing Bode-Plots. This work also shows the simulation and testing results. Both results demonstrate that the proposed control method achieves a high efficiency under light load conditions, maintaining a stable output voltage despite load power dramatic fluctuations.

Analysis of energy exchange between the DC microgrid and SHS involves a bipolar DC microgrid model based on an established grid in Matlab/Simulink. Employing droop control, which is a widely used decentralized strategy, for each converter within the grid. To ensure priority order of power supply in the system, a system-level decentralized control methodology is developed, assuming internal communication within the SHS. Through simulation, the priority order of PV, battery, and grid is verified, along with the impact of dynamic shifts between scenarios on the load. However, given the absence of communication devices in the existing SHS, an alternate decentralized coordinated control strategy is developed and simulated, without direct communication interfaces. Simulation results demonstrate that the SHS bus voltage remains stable even with power supply changes.