Anti-windup and load shedding for smooth transitions in multi-loop power management of islanded single unit-sourced microgrids

Abstract

The huge development of distributed generation (DG) systems and their insertion in the main grid has made the concept of microgrids the target of much research. Renewable energy sources (RES) like photovoltaic (PV) and wind systems represent a vast part of DG units once they have been developed to replace non-clean energy sources. Still, it has been a challenge to make possible RES operations in islanded microgrid environments, especially those formed by PV systems. The intermittent behavior of RES and their lack of power reserve have provoked many problems like voltage and frequency deviations. For this reason, energy storage devices such as battery energy storage systems (BESS) represent an important solution for power support in islanded systems due to their fast response to outages. Several strategies have been studied to provide standalone microgrid environments with power management for maintaining the balance between generation power and load demand. However, critical situations where the microgrid is composed of only one generation system have been little addressed. In these cases, the physical limitation of BESS instigates crucial decisions for keeping the grid balance such as generation power curtailment, when BESS charging limits are reached, and load-shedding, when it reaches discharging limits. This work studies a multi-loop power management strategy for a three-phase islanded microgrid formed by one PV/BESS-based generation unit with grid-forming droop control for the Voltage Source Inverter (VSI), including a secondary frequency/voltage regulation and Proportional-Resonant control loops. To better understand the unit behavior, the operation is divided into states. The transition among the states is performed by the multi-loop strategy whenever the BESS has reached any limit. However, it is extremely necessary to ensure smooth state transitions in order to avoid harmful current and voltage variations. As the strategy is based on Proportional-Integral controllers, the physical limitation of the BESS may cause constant steady-state errors and consequently integration windup. This problem is extremely undesirable, because it may cause decision delays and/or undesired transients during the transition among operation states. This work proposes the use of integration anti-windup techniques in important PI controllers for improving state transitions. Two well-known anti-windup techniques are compared in order to assess smoother behavior. In addition, this work provides an improved load-shedding strategy based on the unit DC-link voltage to keep the power balance in cases of low generation power availability. The results showed that anti-windup methods are essential to avoid undesired transients, decision delays, and current/voltage harmful oscillations during operation state transitions. Two anti-windup methods were tested: clamping and back-calculation. Both showed satisfactory performances. However, the back-calculation technique showed a better performance in a situation when the BESS state-of-charge (SoC) has to be regulated. This technique presented a better response by avoiding under and overshoots during the transitions. Finally, the work was developed through Matlab/Simulink simulations.