Distributed Predictive Secondary Control for Imbalance Sharing in AC Microgrids

This paper proposes a distributed predictive secondary control strategy to share imbalance in three-phase, three-wire isolated AC Microgrids. The control is based on a novel approach where the imbalance sharing among distributed generators is controlled through the control of single-phase reactive power. The main characteristic of the proposed methodology is the inclusion of an objective function and dynamic models as constraints in the formulation. The controller relies on local measurements and information from neighboring distributed generators, and it performs the desired control action based on a constrained cost function minimization. The proposed distributed model predictive control scheme has several advantages over solutions based on virtual impedance loops or based on the inclusion of extra power converters for managing single-phase reactive power among distributed generators. In fact, with the proposed technique the sharing of imbalance is performed directly in terms of single-phase reactive power and without the need for adding extra power converters into the microgrid. Contrary to almost all reported works in this area, the proposed approach enables the control of various microgrid parameters within predefined bands, providing a more flexible control system. Extensive simulation and Hardware in the Loop studies verify the performance of the proposed control scheme. Moreover, the controller’s scalability and a comparison study, in terms of performance, with the virtual impedance approach were carried out.

I. Introduction

Microgrids (MGs) represent an attractive solution in many applications, such as terrestrial, naval or aerospace electrical grids, due to their controllability, capability to include Distributed Generators (DGs) and flexibility [1]. MGs can be classified according to their specific electrical distribution structure as DC [2], AC [3] or Hybrid MGs [3]. Focusing on three phase AC MGs, the control is typically divided into primary, secondary, and tertiary control. Primary control directly regulates each DG’s electrical variables to their desired values, at their point of connection to the MG [3], [4] while tertiary control establishes the power routing across the MG, based on external and internal factors, such as load power demand, DG electrical generation capabilities and cost of energy [3]. Secondary control is responsible for restoring voltage and frequency on the electrical lines and maintaining the desired operation of the MG [3].