Distributed Control Strategies for Microgrids: An Overview

There is an increasing interest and research effort focused on the analysis, design and implementation of distributed control systems for AC, DC and hybrid AC/DC microgrids. It is claimed that distributed controllers have several advantages over centralised control schemes, e.g., improved reliability, flexibility, controllability, black start operation, robustness to failure in the communication links, etc. In this work, an overview of the state-of-the-art of distributed cooperative control systems for isolated microgrids is presented. Protocols for cooperative control such as linear consensus, heterogeneous consensus and finite-time consensus are discussed and reviewed in this paper. Distributed cooperative algorithms for primary and secondary control systems, including (among others issues) virtual impedance, synthetic inertia, droop-free control, stability analysis, imbalance sharing, total harmonic distortion regulation, are also reviewed and discussed in this survey. Tertiary control systems, e.g., for economic dispatch of electric energy, based on cooperative control approaches, are also addressed in this work. This review also highlights existing issues, research challenges and future trends in distributed cooperative control of microgrids and their future applications

A Control Algorithm Based on the Conservative Power Theory for Cooperative Sharing of Imbalances in 4-Wire Systems

© 1986-2012 IEEE. A cooperative control scheme based on the conservative power theory (CPT) is proposed, which can share imbalances in three-phase four-wire droop-controlled systems. By utilizing the CPT, the balanced, unbalanced, and distorted components of the currents and powers in a microgrid can be identified. Using control loops based on virtual impedances and implemented in the stationary a-b-c frame, the imbalances and harmonics are shared between the different four-leg inverters in the microgrid. A secondary control loop is implemented to regulate the maximum voltage imbalance/distortion at the point of common coupling or any other point in the microgrid. The theoretical background of the method is presented, and experimental validation is demonstrated using a laboratory-scale microgrid with two inverters operating at 5 kW

Continuous Set Model Predictive Control for Energy Management of Modular Multilevel Matrix Converters

The Modular Multilevel Matrix Converter is an AC-AC power converter proposed for high power applications such as motor drive and wind energy conversion systems. The M 3C has 9 clusters, allowing 4 circulating currents for converter energy management. Control of the M 3C is frequently divided into Different Frequency Mode (DFM) and Equal Frequency Mode (EFM). EFM is more challenging, because of the larger capacitor voltage oscillations that can be produced. Conventional energy management control strategies for EFM/DFM are usually based on 8 energy control loops used to define four circulating current references composed of several predefined frequencies and positive/negative sequences. The control schemes are typically different for EFM/DFM operation increasing the complexity. In this paper, a Continuous-Control-Set Model Predictive Control (CCS-MPC) for energy management of the M 3C is proposed. The control scheme is based on solving an equality constrained quadratic programming problem, using a state variable model of the M3C, where the optimal solution is analytically obtained. The result is a single and simple control law to obtain circulating current references during EFM/DFM, ensuring a good performance with optimal current specifications. The proposed strategy is experimentally validated using a down-scaled M3C prototype composed of 27 power cells

A design methodology of multi-resonant controllers for high performance 400Hz ground power units

In aerospace applications, a Ground Power Unit (GPU) has to provide balanced and sinusoidal 400 Hz phase-to-neutral voltages to unbalanced and non-linear single-phase loads. Compensation of high-order harmonics is complex, as the ratio between sampling frequency and compensated harmonics can be very small. Thus multiple superimposed resonant controllers or PI nested controllers in multiple dq frames are not good alternatives. The first approach cannot ensure stability, while the second cannot track sinusoidal zero-sequence components, typically present in unbalanced system, and unachievable high bandwidth at the inner current control loop is typically required. In this paper, a simple methodology for designing a single-loop, multiple resonant controller for simultaneous mitigation of several high-order harmonics, ensuring stability, is presented. Experimental results, based on a 6kW four-leg NPC converter, validates the proposed controller design, showing excellent steady state and transient performance

Cyber-attacks in modular multilevel converters

Distributed control of modular multilevel converter (MMC) submodules offers several potential benefits such as flexibility and modularity. In this approach, low-level control tasks, such as capacitor voltage balancing, can be distributed amongst controllers placed in the SMs. This decreases the computational burden for the central control system that performs high-level control tasks. Distributed control architecture requires a cyber-physical network (CFN) through which local controllers share all the information necessary to perform their respective control loops. To date, none of the reported works in this field have paid attention to potential imperfections in the CFN. Indeed, previous works are based on the assumption that the network always provides correct information to the local controllers. However, erroneous measurements in the CFN may degrade the distributed control scheme operation, leading to suboptimal or even unstable operation. These events can occur in the presence of cyber-attacks. This paper is the first to investigate the impacts of cyber-attacks in MMCs. The effects of a specific cyber-attack, named false data injection attack (FDIA), on a consensus-based distributed control strategy are studied. Additionally, a method for detecting FDIAs is proposed, along with a countermeasure strategy, to ensure the safe operation of the MMC. The proposals reported in this paper are validated using simulation and experimental results