OPTIMAL POWER FLOW IN ISLANDED MICROGRIDS

In this thesis, studies about OPF in islanded MGs have been carried out. First, an original formulation and solution approach for the OPF problem in islanded distribution systems is proposed. The methodology is well suited for AC microgrids and can be envisioned as a new hierarchical control structure comprising only two levels: primary and tertiary regulation, the latter also providing iso-frequency operating points for all units and optimized droop parameters for primary regulation. The OPF provides a minimum losses operating point for which voltage drops are limited and power sharing is carried out according to the most adequate physical properties of the infrastructure thus giving rise to increased lifetime of lines and components. Due to the fact that the solution method is based on a numerical approach, the OPF is quite fast and efficient and the operating point can be calculated in times that are comparable to the current secondary regulation level times. Two test systems, 6_bus and 38_bus, have been used. In the different applications. Different scenarios have been investigated to show: - the possibility to solve the OPF in islanded MGs - the possible link between stability of operation and minimum losses. In particular two methods for OPF have been investigated, one based on a numerical approach (Lagrange method) and one based on heuristic optimization (Glow-worm Swarm Optimization, GSO). The latter is a global optimizer that is able to identify multiple optima.Positive and negative aspects of both methods are put into evidence. Numerical optimization indeed can provide stable solutions but cannot deal with a comprehensive formulation able to optimize both active power-to-frequency and reactive power-to-voltage droop coefficient. Also the load with the numerical approach can only be balanced while heuristic optimization allows both balanced and unbalanced loading conditions. Also constraints can be easily considered using a heuristic formulation, while this is not possible using the numerical approach. The thesis is divided as follow: - In the first chapter, the motivation and scientific goals of the thesis have been presented. - In the second chapter, a parametric study changing coefficients of droop control is carried out solving the power flow for balanced and unbalanced three phase different microgrids systems using the Trust Region Method. - In the third chapter, an original formulation and solution approach for the OPF problem in islanded distribution systems based on Lagrange method is proposed. - In the fourth chapter applications of GSO method to solve the optimal power flow problem taking into account the constraints of frequency and line ampacity in three-phase islanded Microgrids with variables are both Kgs and Kds are proposed. The details of optimal results and load flow calculation results are shown in the appendix.