REGULATION DROOP COEFFICIENTS IN ISLANDED MICROGRIDS

In this thesis, a novel non-linear droop control is proposed. The droop curve is generated as a piecewise linear curve from the optimized operating points of the system under consideration for different values of the supplied load. For each Optimal Power Flow (OPF), the droop coefficients are generated and these create small piecewise linear parts of the final droop curve. The operation is constrained by frequency and voltage limits. It is well known that the load flow problem solution can provide infinite operating points if the generators set point is not fixed. For each loading condition, the solution to the OPF problem outputs a single solution. Such solution, in case of microgrids with inverter-interfaced units, is an expanded state of the system comprising the operating frequency and voltage. In this case, all microgrids’ components are modeled as frequency and voltage dependent elements. The OPF is solved off-line for various loading conditions so as to construct the new droop as a piecewise linear curve. The novelty of this approach is that the droop coefficients in the f-P plane and V-Q plane are set to provide, together with frequency and voltage regulation during transients, also the lowest value of the energy losses in the microgrid, both during a load variation and in steady-state, without any other adjustment in the set-points of the generators.