Islanded Operation of Parallel Inverter-interfaced Microgrids with Photovoltaic Systems

This thesis presents autonomous droop-based control schemes to share power proportionally in inverter-connected islanded microgrids; in which different microgrid structures such as inductive (L-type), resistive (R-type), and resistive-inductive (RL-type) are considered. The main aim of designing new controllers in this dissertation is to distribute the load change among intermittent distributed generators according to their power ratings.A linearized model of microgrids is used to evaluate the change in powers (real-reactive) of the distributed generators due to the load change, but it is noticed that the linearized modeling of microgrids depends on the distribution line parameters. Thus, proportional power sharing cannot be maintained. Improved droop-based algorithms are developed to share power based on droop gains by including a voltage control law; in which the reference values of the generator voltages are kept the same.A compensation-based droop control is provided by adding a power offset to the real power for balancing the inverter output power during the variation in the output of distributed generation. A derivative term is also included in droop algorithms for damping the oscillatory modes of the controllers so that improved dynamic performance can be ensured.This thesis contains eigenvalue analysis to predict the stability of microgrids. However, it is anticipated that this method may not provide satisfactory results in large inverter-dominated systems as it is based on the quasi-static approximation. In order to overcome this shortcoming, an extended dynamic phasors analysis is described in this dissertation in details to find out a suitable stability range of droop gains for the proposed controllers.Finally, the performance of the designed droop-based power sharing controller is verified on a multi photovoltaic source-based islanded test microgrid and superior dynamic performance is obtained compared with the conventional RL-type droop controller