Intermediate low voltage direct current (ILVDC) interconnection systems for sparse electrified areas

Abstract

Electricity access is seen as an economic growth enabler and a commodity to improve people's welfare. Hence, electrification is one of the aspects highly regarded by governments worldwide. In Africa, electricity access is still a challenge, this is well illustrated by the low electrification rate of 41%, which is more significant in sub-Saharan region with an electrification rate of 35%. In this part of the continent, on one hand, the reliability of the power supply in electrified areas is still an issue and on the other hand, in areas far from the grid, there are less perspective for grid extension if considered the financial situation and top down model of grid extension used by the utilities. This leads to the extensive use of diesel generators in urban and rural areas. The traditional top down model for grid extension requires huge capitals that are mainly provided by the utility. The return on investment on extended grid determines the approval for extension or not. It is difficult for remote rural areas with no major economic activities like mining to cross the threshold of return on investment acceptable by the utility. Hence new approaches are sought to increase the electrification rate in Africa while maintaining the cost as minimum as possible. Bottom up grid extension using swarm electrification and off-grid solutions such as microgrids are gaining interest of researchers as alternative to the traditional method. It is in this line that this thesis entitled "Intermediate Low Voltage Direct Current (ILVDC) interconnection systems for sparse electrified areas" is looking into combining both approaches by building up a network from the bottom considering the locally available off-grid solutions such as nanogrid diesel generators for microgrids formation. "Olympic rings" microgrids approach is used to extend the electrified areas and anticipate an eventual interconnection with the grid. For this end, nanogrid and microgrid networks are developed, designed and tested. Bi-directional Nanogrid converter are designed with modularized topology to allow easy and economic repair as only affected module should be replaced if needed. Proposed bidirectional converters for nanogrid, microgrid and minigrid are modelled and simulated in PSIM environment. Simulation results show that the design criteria set are met by the converters. Control strategies for power exchange within nanogrids and microgrids, as well as the overall control are proposed. Communication based control strategy algorithms for nanogrids and microgrids are developed. Power line communication (PLC) technique is adopted as a communication system while a DC opto-capacitive coupler for PLC communication system is developed to enhance the safety of the equipment and users. Interconnection of microgrids through ILVDC network is performed with respect to IEC60038 and IEEE 1547. Load flow analysis using DigSilent PowerFactory is performed on individual and interconnected microgrids, with contingency scenario in the latter case. The simulation results proved a successful interaction and improved loading on the machine during the contingency as opposite to results in the Islanded mode.