Secondary Frequency and Voltage Control of Islanded Microgrids via Distributed Averaging

In this work we present new distributed controllers for secondary frequency and voltage control in islanded microgrids. Inspired by techniques from cooperative control, the proposed controllers use localized information and nearest-neighbor communication to collectively perform secondary control actions. The frequency controller rapidly regulates the microgrid frequency to its nominal value while maintaining active power sharing among the distributed generators. Tuning of the voltage controller provides a simple and intuitive trade-off between the conflicting goals of voltage regulation and reactive power sharing. Our designs require no knowledge of the microgrid topology, impedances or loads. The distributed architecture allows for flexibility and redundancy, and eliminates the need for a central microgrid controller. We provide a voltage stability analysis and present extensive experimental results validating our designs, verifying robust performance under communication failure and during plug-and-play operation.Comment: Accepted for publication in IEEE Transactions on Industrial Electronic

Design and operation of modular microgrids

textMicrogrids are being considered as a solution for implementing more reliable and flexible power systems compared to the conventional power grid. Various factors, such as low system inertia, might make the task of microgrid design and operation to be nontrivial. In order to address the needs for operational flexibility in a simpler manner, this dissertation discusses modular approaches for design and operation of microgrids. This research investigates Active Power Distribution Nodes (APDNs), which is a storage integrated power electronic interface, as an interface block for designing modular microgrids. To perform both voltage/current regulation and energy management of APDNs, two hierarchical control frameworks for APDNs are proposed. The first framework focuses on maintaining the charge level of the embedded energy storage at the highest available level to increase system availability, and the second framework focuses on autonomous power sharing, and storage management. The detailed design process, control performance and stability characteristics are also studied. The performance is also verified by both simulation and experiments. The control approaches enable application of APDNs as a power router realizing distributed energy management. The decentralized configuration also increases modularity and availability of power networks by preventing single point-of-failures. The advantages of using APDNs as a connection interface inside a power network are discussed from an availability perspective by performing a comparison using Markov-based availability models. Furthermore, the operation of APDNs as power buffers is explored and the application of APDNs enabling modular implementation of microgrids is also studied. APDNs enable the system expansion process—i.e. connecting new loads to the original system—to be performed without modifying the configuration of the original system. The analysis results show that a fault-tolerant microgrid with an open architecture can be realized in a modular manner with APDNs. APDNs also enable simplified selectivity planning for system protection. The effect of modular operation on microgrids is also studied by using an inertia index. The index not only provides insights on how system performance is affected by modular operation of modular microgrids, but is also used to develop a simpler operation strategy to mitigate the effect of plug and play operations.Electrical and Computer Engineerin

A novel compact dq-reference frame model for inverter-based microgrids

The development and the experimental validation of a novel dynamic model of an islanded three-phase Inverter-based Microgrid (IMG) is presented in this paper. The proposed model reproduces the relevant system dynamics without excessive complexity and enough accuracy. The dynamics of the IMG are captured with a compact and scalable dynamic model, considering inverter based distributed generators with d-current droop primary and proportional resonant inner controllers. The complete development of the model, the practical assumptions, and the accurate proportional power sharing of the primary control technique are shown. The accuracy performance was verified in experiments performed at the Aalborg Intelligent Microgrids Laboratory for an islanded IMG case

Challenges, advances and future directions in protection of hybrid AC/DC microgrids

Hybrid microgrids which consist of AC and DC subgrids interconnected by power electronic interfaces have attracted much attention in recent years. They not only can integrate the main benefits of both AC and DC configurations, but also can reduce the number of converters in connection of Distributed Generation (DG) sources, Energy Storage Systems (ESSs) and loads to AC or DC buses. In this paper, the structure of hybrid microgrids is discussed, and then a broad overview of the available protection devices and approaches for AC and DC subgrids is presented. After description, analysis and classification of the existing schemes, some research directions including communication infrastructures, combined control and protection schemes, and promising devices for the realisation of future hybrid AC/DC microgrids are pointed out

DC microgrid simulation and control

However, one of the primary challenges in the microgrid is controlling power electronic devices when it is coordinated with the utility grid. The control system in the microgrid has the main objectives, to ensure supply power to the critical loads, to connect and disconnect the load in case of any fault occurring in the microgrid. This thesis investigates different types of strategies used to control and manage a microgrid; also, the report considers inverter control in islanded mode. This thesis explores the ideas for controlling a microgrid in terms of voltage control in both centralized and decentralized configurations. In more detail, simulations were carried out on two control strategies; inverter control, and frequency control. The inverter control method having the ability to bring the stable and efficient electricity to microgrid system has attracted much consideration in recent years. The droop controller had the capability to autonomously perform equal power sharing and maintain stability in islanded mode of operation. There are many existing dc microgrid around the world. Kythnos Island microgrid is one of those existing microgrid, which can supply 12 houses within the island by using the inverter controller, as the utility grid is located far away from the microgrid. However, this island is either ac or dc loads, so some inverters and converters will be involved in the network. The investigation used MATLAB /SIMULINK to simulate the microgrid network. Which done by implementing the controller in the both ac and dc load. The performance measures such as power sharing accuracy between the microgrid and diesel generator will be obtained. The diesel generators used as a backup in the case of any fault occur in the microgrid. The network simulations at different dc and ac loads will be simulating by using Simulink. This then used to calculate the efficiency of the whole network through the Simulink. Then how efficiency is can be used to upgrade the network as future scope. Active and reactive power in the droop control has been commonly reported on and has been shown to work over a series of the condition of the network. The PQ inverter improves the performance of microgrid in islanded mode; however, the result demonstrates that the inverter controller performed well in the simulation of the Kythnos microgrid network. The efficiency of the LVDC indicates that inverter controller with the ac load performance higher efficiency compared to the dc loads

Design And Implementation Of Co-Operative Control Strategy For Hybrid AC/DC Microgrids

This thesis is mainly divided in two major sections: 1) Modelling and control of AC microgrid, DC microgrid, Hybrid AC/DC microgrid using distributed co-operative control, and 2) Development of a four bus laboratory prototype of an AC microgrid system. At first, a distributed cooperative control (DCC) for a DC microgrid considering the state-of-charge (SoC) of the batteries in a typical plug-in-electric-vehicle (PEV) is developed. In DC microgrids, this methodology is developed to assist the load sharing amongst the distributed generation units (DGs), according to their ratings with improved voltage regulation. Subsequently, a DCC based control algorithm for AC microgrid is also investigated to improve the performance of AC microgrid in terms of power sharing among the DGs, voltage regulation and frequency deviation. The results validate the advantages of the proposed methodology as compared to traditional droop control of AC microgrid. The DCC-based control methodology for AC microgrid and DC microgrid are further expanded to develop a DCC-based power management algorithm for hybrid AC/DC microgrid. The developed algorithm for hybrid microgrid controls the power flow through the interfacing converter (IC) between the AC and DC microgrids. This will facilitate the power sharing between the DGs according to their power ratings. Moreover, it enables the fixed scheduled power delivery at different operating conditions, while maintaining good voltage regulation and improved frequency profile. The second section provides a detailed explanation and step-by-step design and development of an AC/DC microgrid testbed. Controllers for the three-phase inverters are designed and tested on different generation units along with their corresponding inductor-capacitor-inductor (LCL) filters to eliminate the switching frequency harmonics. Electric power distribution line models are developed to form the microgrid network topology. Voltage and current sensors are placed in the proper positions to achieve a full visibility over the microgrid. A running average filter (RAF) based enhanced phase-locked-loop (EPLL) is designed and implemented to extract frequency and phase angle information. A PLL-based synchronizing scheme is also developed to synchronize the DGs to the microgrid. The developed laboratory prototype runs on dSpace platform for real time data acquisition, communication and controller implementation