Operation and control of isolated AC micro-grids for rural electrification in developing countries

While many advances have been done in terms of energy distribution, there is still a percentage of the population lacking access to electricity. In order to provide access to the missing sectors in the most economic way, the use of micro-grids has been proposed. The capability of micro-grids to operate disconnected from the main grid make them attractive for rural areas which are the most affected sectors. Due to the fact that these micro-grids are islanded, they have to set their own bus voltage and frequency. Different control schemes have been proposed in order to coordinate the voltage and frequency of the distributed generators (DG) which form the micro-grid. This thesis discusses two problems. First, the control methods for the distributed generators in a Low voltage (LV) grid are explored. Second, the mitigation of voltage unbalance caused by single phase loads is discussed. This thesis focuses on two control schemes that do not require communication between converters. These are the "traditional" P.F/Q.V droop control and the "opposite" P.V/Q.F droop control. First, the improvement of using a virtual impedance is shown in each control scheme. Afterwards, through different tests their performance is assessed. The obtained results shown that the opposite droop control has a better performance than the traditional droop in mainly resistive lines such as the ones used in LV micro-grids. Single phase loads can create a voltage unbalance. Due to the nature of the micro-grid, it can be difficult to reduce this unbalance. If left unattended the voltage unbalance will greatly decrease the power quality across the micro-grid. This thesis tests the effectiveness of a negative sequence compensation mechanism. This negative sequence compensation scheme is aimed at compensating voltage unbalance at the point of common coupling (PCC), but requires a low level of communication among DGs. This thesis explores the possibility of using this negative sequence compensation mechanism without any type of communication, and the effectiveness of such methods at a non-PCC location.