Improved Hierarchical Multi-loop Voltage Control based on droop control for Inverter-based DG Units in an Islanded Microgrid

In this paper, a multi-loop hierarchical controller for a renewable DG based microgrid system is proposed in order to control the voltage of point of common coupling and also to perform accurate active and reactive power sharing. Inner current and voltage loops that are designed based on the dynamic model of microgrid system are used to obtain appropriate switching functions for interfaced inverters and the reference values for currents of DG units. Also, a modified droop controller is introduced to enhance the performance of designed voltage control loop. In order to improve the dynamic operation of the proposed controller in supplying demanded power for harmonically distorted loads, the extraction of harmonic components of PCC voltages is accomplished. Upper and lower limits of voltage amplitude and frequency droops are analyzed using capacity curves of each DG unit. The effect of renewable power generation uncertainty on DC link voltage variations are also compensated using an additional control loop in hierarchical structure. The main contributions of this paper are the above-mentioned multi-objective control system along with a droop control based on capacity curves. The MATLAB/SIMULINK simulation shows a proper steady state and transient performance during changes in generation and nonlinear harmonically distorted loads.