The coexistence of ac and dc subgrids in a hybrid microgrid is likely given that modern distributed sources can either be ac or dc. Linking these subgrids is a power converter, whose topology should preferably be not too unconventional. This is to avoid unnecessary compromises to reliability, simplicity, and industry relevance of the converter. The desired operating features of the hybrid microgrid can then be added through this interlinking converter. To demonstrate, an appropriate control scheme is now developed for controlling the interlinking converter. The objective is to keep the hybrid microgrid in autonomous operation with active power proportionally shared among its distributed sources. Power sharing here should depend only on the source ratings and not their placements within the hybrid microgrid. The proposed scheme can also be extended to include energy storage within the interlinking converter, as already proven in simulation and experiment. These findings have not been previously discussed in the literature, where existing schemes are mostly for an ac or a dc microgrid, but not both in coexistence.

Autonomous Control of Interlinking Converters with Energy Storages in Hybrid AC-DC Microgrids

Due to inherent advantages of DC system over AC system such as compatibility with renewable energy sources, storage devices and modern loads, Direct Current Microgrid (DCMG) has been one of the key research areas from last few years. The power and energy management in the DCMG system has been a challenge for the researchers. MG structure and control strategies are the integrated part of the power and energy management system. This paper covers all the aspects of the control of DCMG, whether it is DC bus voltage, power or energy related. Different MG Structures with their comparative analysis has been given in this paper. Various control schemes: Basic control schemes like centralized, decentralized and distributed control and multilevel control scheme such as hierarchal control has been discussed. The Power management in grid-connected, Islanded mode and transition mode has been presented. Different energy management strategies have been presented as energy management plays very important role in optimizing the size and rating of energy storage system and their maximum utilization. The energy management of a battery and super capacitor based HESS in all configurations has also been discussed and finally, future trends in further research are presented.

A review on overall control of DC microgrids

With the rapid development of power electronics technology, microgrid (MG) concept has been widely accepted in the field of electrical engineering. Due to the advantages of direct current (DC) distribution systems such as reduced losses and easy integration with energy storage resources, DC MGs have drawn increasing attentions nowadays. With the increase of distributed generation, a DC MG consisting of multiple sources is a hot research topic. The challenge in such a multi-source DC MG is to provide voltage support and good power sharing performance. As the control strategy plays an important role in ensuring MG’s power quality and efficiency, a comprehensive review of the state-of-art control approaches in DC MGs is necessary. This paper provides an overview of the primary and secondary control methods under the hierarchical control architecture for DC MGs. Specifically, inner loop and droop control approaches in primary control are reviewed. Centralized, distributed, and decentralized approach based secondary control is discussed in details. Key findings and future trends are also presented at last.

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Primary and secondary control in DC microgrids: a review

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.

/pdf/distributed-control-strategies-for-microgrids-an-overview-b0opeslrl1.pdf

Distributed Control Strategies for Microgrids: An Overview

Networked microgrids (NMGs) provide a promising solution for accommodating various distributed energy resources (DERs) and enhancing the system performance in terms of reliability, resilience, flexibility, and energy efficiency. With the penetration of MGs, the communication-based distributed control is playing an increasingly important role in NMGs for coordinating a multitude of heterogeneous and spatially distributed DERs, which feature enhanced efficiency, reliability, resilience, scalability, and privacy-preserving as compared with conventional centralized control. This article aims to provide a comprehensive survey of distributed control and communication strategies in NMGs. We provide thorough discussions and elaborate on: 1) Essential merits of MGs and NMGs, and their practical implementations; 2) Distributed communication network characteristics and specific operation objectives of NMGs; 3) Classifications of distributed control strategies in NMGs and their salient features; 4) Communication reliability issues concerning data timeliness, data availability, and data accuracy, and the development of countermeasures; 5) Advancements in several promising communication and computation technologies and their potential applications in NMGs.

Distributed Control and Communication Strategies in Networked Microgrids

The microgrid concept is gaining popularity with the proliferation of distributed generation. Control techniques in the microgrid are an evolving research topic in the area of microgrids. A large volume of survey articles focuses on the control techniques of the microgrid; however, a systematic survey of the hierarchical control techniques based on different microgrid architectures is addressed very little. The hierarchy of control in microgrid comprises three layers, which are primary, secondary, and tertiary control layers. A review of the primary and secondary control strategies for the ac, dc, and hybrid ac–dc microgrid is addressed in this paper. Furthermore, it includes the highlights of the state-of-the-art control techniques and evolving trends in the microgrid research.

Control Techniques in AC, DC, and Hybrid AC–DC Microgrid: A Review

A multi-microgrid (MMG) system comprises of a group of self-sufficient microgrids capable of operating in grid-connected, islanded, or interconnected modes. A three-microgrid-based MMG system consisting of a solar photovoltaic system, doubly-fed induction generator-based wind generation, and static synchronous compensator as distributed energy sources are considered. Each microgrid includes a battery interfaced with a bidirectional DC–DC converter. As there are multiple battery units in the MMG system, different states-of-charge (SoC) are evident in the batteries. Hence, there is a need to charge/discharge by following the SoC of the batteries. In this study, an intelligent scheme employing fuzzy logic is applied to monitor the SoC of the batteries and determine the reference current for current control through the bidirectional DC–DC converter in an MMG system. Microgrid frequency control in the MMG system is carried out using the proposed five-membership function-based fuzzy logic control. During sudden transient events, the proposed intelligent frequency control strategy shows an improved transient response as compared to conventional PI control. A multi-input and multi-output fuzzy logic-based power management algorithm is proposed to handle multiple sources, batteries, and loads in the system. The MMG system with the proposed control is simulated using real-time digital simulator OP4510 from OPAL-RT.

Battery state-of-charge-based control and frequency regulation in the MMG system using fuzzy logic

With the increasing growth of renewable energy sources, it is important to understand and analyze its effect on the overall power system. The real-time simulation is a platform that allows virtual plant and controller to be thoroughly investigated before they can be implemented in an actual system. In a valid real-time simulation, the virtual model closely resembles its physical counterpart. It promotes flexibility of operation, faster simulation with no risk of component failure under any contingency analysis. This paper presents modeling and simulation of an AC microgrid with a solar photovoltaic system, in a real time environment using HYPERSIM from Opal-RT. The photovoltaic system is connected to a 15 node test distribution network to examine its interaction with the AC grid. Various case studies like steady state operation, step changes in irradiation, generation outage, and three-phase to the ground fault have been carried out in the real time simulator, HYPERSIM. This preliminary real-time electrical transients simulation with the modeled AC microgrid can be useful for beginners in this field, using HYPERSIM.

Modeling and real-time simulation of an AC microgrid with solar photovoltaic system

In recent years, there has been significant growth in wind energy based power generation. This has encouraged research on the various aspects and effect of the installation of wind power plants. Wind turbine emulators (WTE) are being employed in research laboratories as they reproduce the torque developed by a wind turbine at a given wind speed. Commonly, DC motors or induction motors are employed for wind turbine emulation. However, DC motors require frequent maintenance, are expensive and bulky. With better controllability and lesser maintenance costs, doubly fed induction motor (DFIM) can be useful for wind turbine emulation. This paper presents modeling and control of DFIM as a WTE in MATLAB/SIMULINK environment. Further, a start-up sequence using direct torque control (DTC) is proposed for the DFIM.

Wind turbine emulation using doubly fed induction motor

In this study, a ring-type multi-microgrid (MG) system comprising of three MGs with solar energy-based generation, wind energy-based generation, and static synchronous compensator as primary sources along with batteries as emergency back-up is proposed. The voltage-source converters at the individual MG level serve as the interface between the neighbouring MGs and the grid. An augmented proportional sliding-mode-control-based approach is proposed in the primary control level to improve the power quality of the system during balanced and unbalanced loading conditions. A negative-sequence controller is also included to mitigate the negative-sequence voltage from the inverter output voltage. The multi-MG (MMG) system is capable of operation in grid-connected, islanded, or interconnected mode with balanced and unbalanced loads. The bidirectional DC -DC converters interfacing the batteries at the DC link is controlled by a hysteresis-function-based sliding-mode control. A comparison of the proposed inverter primary control with proportional -integral controller, quasi-proportional -resonant controller, and hysteresis-band current controller is carried out. The comparison reveals that the total harmonic distortion and voltage unbalance factor improves by employing the proposed control when compared with conventional control. The proposed MMG system with its control strategies is modelled in MATLAB/Simulink environment and is experimentally verified using OP4510 from OPAL-RT.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-gtd.2018.6620

Saroja Kanti Sahoo

Papers

Control Techniques in AC, DC, and Hybrid AC–DC Microgrid: A Review

Battery state-of-charge-based control and frequency regulation in the MMG system using fuzzy logic

Modeling and real-time simulation of an AC microgrid with solar photovoltaic system

Wind turbine emulation using doubly fed induction motor

Multi-mode control and operation of a self-sufficient multi-microgrid system