Hierarchical Model-Predictive Droop Control for Voltage and Frequency Restoration in AC Microgrids
Authors: Iresha Poonahela; Abdelbasset Krama; Sertac Bayhan; Ugur Fesli; Mohammad B. Shadmand; Haitham Abu-Rub; Miroslav M. Begovic
Abstract:
A hierarchical control structure was introduced to allow the integration of power-electronics based distributed generation into the microgrid in a smart and flexible manner. The main aim of the primary controller in such a structure is to achieve accurate active and reactive power sharing, whereas the secondary control aims to ensure voltage and frequency (V/f) stability. Generally, converter-level secondary controllers utilize classical nested loops that suffer from a slow dynamic response and cumbersome parameters tuning. The existing-model-based and estimation-based secondary controllers are fast, but require complex design methodology, high communication bandwidth, and, consequently, higher data analysis and computational burden.
This article presents a simple predictive-based secondary control for ac microgrid. The proposed control is fast, robust, simple, requires low communication bandwidth and no parameters tuning.
The proposed predictive control is designed to optimally restore voltage and frequency deviations that result from power sharing in the lower control layers. To achieve this, the predicted voltage and current references are utilized to predict the power requirement of the upcoming control cycle. Subsequently, droop characteristics curves are leveraged to estimate V/f deviations based on the present trajectory.
Experimental tests performed with three parallel-connected grid-forming inverters in an islanded operation have been conducted for multiple case studies which included: equal and unequal load power sharing between inverters, a study of parameters mismatch considering the model-based controller, a comparison with linear control in the secondary layer, and an analysis of weighting factor impact on system stability. The results effectively validate that the proposed predictive control can accurately maintain V/f stability, while ensuring active and reactive power sharing between multiple grid-forming inverters.
