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Guest Editorial Special Section on Microgrids
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ICROGRIDS, in particular, and smart grids, in general, are new emerging power distribution infrastructures with prominent potentials among them are realization of demand response and efficient energy consumption, integration of distributed energy resources (DERs), and high-reliability electricity delivery. DERs encompass combined heat and power, small wind turbine, photovoltaic sets, heat or electricity storage, etc. Microgrid is, additionally, attributed by grid-isolated operability bringing about serving energy during upstream network failures, say in disasters. All these benefits are motivating further demands for microgrids on a worldwide basis and in various application areas including commercial markets, industrial complexes, campus environments, military facilities, off-grid operations, community/utility settings, etc. In remote locations, where the main grid connection is either impossible or economically unjustifiable, microgrids are considered as viable alternative for the electrification. In many ways, a microgrid is just a small-scale version of the traditional power grid. Thus, majority of complicated problems in the context of large power systems appears as well in a microgrid. On the hand, the emergence of microgrids in power utility systems has raised additional technical, economical, and regulatory issues since the passivity of distribution sections does not hold any more. The high penetration of scattered power electronic-based resources and demands in microgrids, though actualizes further flexibility in the operation and conditioning, poses technical challenges particularly with regards to their coordinated and hierarchical control. Current research activities, across the world and in academia and industry environments, are not only seeking efficient technical solutions for the microgrid implementation challenges but even exploiting this unprecedented opportunity for the procurement of operational services for the bulk power utilities. Just to name a few, deployment of demand response programs for peak shaving/shifting and congestion management tasks, utilization of energy storage capabilities to compensate the fluctuation of renewable generations, and ancillary service provision through frequency-based demand regulation or time-ahead consumption scheduling. Despite the significant research efforts devoted to the microgrid and smart grid areas, numerous problems related to real world implementations still remain unsolved. The present special section was announced with the objective of addressing and disseminating state-of-the-art R&D results on microgrids to bring together researchers from both academia and industry with the goal of fostering interactions among stakeholders. In response, 190 two-page extended abstracts were received and considered for the first round of reviews. Authors of about 60 selected abstracts were then invited to submit the full papers in the second round and out of them 27 high-quality manuscripts were ultimately approved and included in this special section. These papers are organized according to the following four topics: 1) Microgrid dynamic performance, control/operational strategies, and voltage/frequency regulation Digital Object Identifier 10.1109/TSG.2012.2233071
2) Reliability and power quality aspects of microgrids 3) Fault modeling and protection schemes 4) Others These topics and the papers included are explained in the following sections of this Guest Editorial. I. MICROGRID DYNAMIC PERFORMANCE, CONTROL/OPERATIONAL STRATEGIES, AND VOLTAGE/FREQUENCY REGULATIONS In this section, 14 papers deal with the dynamic modeling and performance of microgrid or its building blocks and propose appropriate control/operational strategies or voltage/frequency regulation approaches. 1) “Neural Network Estimation of Microgrid Maximum Solar Power” by A. Chatterjee and A. Keyhani, proposes an artificial neural network to estimate the optimum tilt angle for maximum solar power in a given location. The developed neural network is trained as well to estimate the optimal yearly irradiation that a PV station is able to receive at that location. 2) “Dynamic Modeling and Operation Strategy for a Microgrid With Wind and Photovoltaic Resources” by S. Bae and A. Kwasinski, presents a dynamic modeling and control strategy for a sustainable microgrid primarily powered by wind and solar energy. A current-source-interface multiple-input dc-dc converter is used to integrate the renewable energy sources to the main dc bus. This study considers both wind energy and solar irradiance changes in combination with load power variations. 3) “Automatic Reconnection From Intentional Islanding Based on Remote Sensing of Voltage and Frequency Signals” by T. M. L. Assis and G. N. Taranto, devises a control strategy for automatic reconnection of a microgrid after an intentional islanding. Remote signals of voltage and frequency at both sides of the coupling circuit breaker are communicated to the generating site, and used as additional signals to the voltage and speed control loops, respectively. 4) “Constrained Potential Function—Based Control of Microgrids for Improved Dynamic Performance” by A. Mehrizi-Sani and R. Iravani, focuses on control and management strategies for integration of distributed energy resources in the power system. This paper conceptualizes a hierarchical framework for the control of microgrids and develops the notion of potential functions for the secondary controller for devising intermediate set points to ensure feasibility of operation. 5) “Secondary Control for Voltage Quality Enhancement in Microgrids” by M. Savaghebi et al., proposes a microgrid hierarchical control structure which consists of primary and secondary levels. The primary control level comprises distributed generators local controllers. The central secondary controller is designed to control voltage harmonics and unbalance compensation. Also, restoration of
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IEEE TRANSACTIONS ON SMART GRID, VOL. 3, NO. 4, DECEMBER 2012
point of common coupling voltage amplitude and microgrid frequency to the rated values is managed by this controller. “Active Power Management of Multihybrid Fuel Cell/Supercapacitor Power Conversion System in a Medium Voltage Microgrid” by A. Ghazanfari et al., develops an effective active power management strategy using fuel cell as a main power source and supercapacitor as a complementary source, for multi-hybrid power source in a medium-voltage islanded microgrid. In this paper, a suitable and fast power sharing control strategy is presented for a distributed generation composed of fuel cell and supercapacitor units. “Real-Time Energy Management Algorithm for Mitigation of Pulse Loads in Hybrid Microgrids” by A. Mohamed et al., presents a real-time energy management algorithm for hybrid ac/dc microgrids involving sustainable energy and hybrid energy storage system consisting of supercapacitors for ultra-fast load matching beside lithium-ion batteries for relatively long term load buffering. The energy management algorithm aims mainly at managing the energy within the system such that the effect of pulsed (short duration) loads on the power system stability is minimized. “A Wavelet/PSO Based Voltage Regulation Scheme and Suitability Analysis of Copper- and Aluminum-Rotor Induction Machines for Distributed Wind Power Generation” by X. Lu et al., presents a comprehensive suitability analysis of commercially available niche copper-rotor induction motor and conventional aluminum-rotor induction motor to be used as induction generators in the autonomous wind distributed generation. “Intelligent Frequency Control in an AC Microgrid: Online PSO-Based Fuzzy Tuning Approach” by H. Bevrani et al., addresses a new online intelligent approach by using a combination of the fuzzy logic and the particle swarm optimization techniques for optimal tuning of the most popular existing proportional-integral based frequency controllers in the ac microgrid systems. “Improving the Integration of Wind Power Generation Into AC Microgrids Using Flywheel Energy Storage” by G. O. Suvire et al., presents the design and implementation of a novel high performance power controller of the flywheel energy storage system to enhance the integration of wind generation into emerging grid-interactive ac microgrids. “Flywheel Energy Storage Systems for Ride-Through Applications in a Facility Microgrid” by R. Arghandeh et al., aims at developing a tool to demonstrate the use of FES units for securing critical loads during a utility outage in a microgrid environment. It illustrates how an FES can help improve the load serving capability and provide a highly reliable ride-through capability for critical loads during a utility disturbance. “Hierarchical Structure of Microgrids Control System” by A. Bidram and A. Davoudi, reviews the status of hierarchical control strategies applied to microgrids. The control structure consists of primary, secondary, and tertiary levels, and is a versatile tool in managing stationary and dynamic performance of microgrids while incorporating economical aspects. “Centralized Control for Parallel Operation of Distributed Generation Inverters in Microgrids” by K. T. Tan et al., presents a centralized control system that coordinates parallel operations of different distributed generation inverters
within a microgrid. The control design for the DG inverters employs a new model predictive control algorithm that allows faster computational time for large power systems by optimizing the steady-state and the transient control problems, separately. 14) “Real-Time Central Demand Response for Primary Frequency Regulation in Microgrids” by S.A. Pourmousavi and M. H. Nehrir, develops a DR strategy to provide frequency regulation within a smart islanded microgrid considering minimum possible load manipulation, intermittent wind power generation, and communication delay. A cost-benefit analysis is additionally fulfilled to demonstrate the economic benefits of the DR approach compared to conventional automatic generation control and operating reserve. II. RELIABILITY
POWER QUALITY ASPECTS MICROGRIDS
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In this section, seven papers about the microgrid reliability assessment techniques, impact of DERs on microgrid reliability indices, and the microgrid power quality analysis are presented. 1) “Integration of High Reliability Distribution System in Microgrid Operation” by M. E. Khodayar et al., evaluates the role of high reliability distribution system in microgrid operations. Looped distribution networks, storage systems, and local DERs are accounted for enhancing the microgrid reliability while offering hourly ancillary services and demand response for reducing operation costs. 2) “Availability Evaluation of Micro-Grids for Resistant Power Supply During Natural Disasters” by A. Kwasinski et al., discusses how micro-grids availability during natural disasters and in their aftermath can be assessed. The analysis focuses on two critical groups of components that allow micro-grids to improve power supply availability: distributed generators and local energy storage. 3) “Microgrid Generation Capacity Design With Renewable and Energy Storage Addressing Power Quality and Surety” by A. Nasiri et al., presents a modeling approach for highly-renewable energy penetrated microgrid systems including the source and demand variations. The system is monitored by a number of power quality measures. The proposed modeling approach is used to determine the capacity requirements for distribution generations to meet the power quality criteria in both grid-connected and islanded modes. 4) “Improving Reliability of Islanded Distribution Systems With Distributed Renewable Energy Resources” by H. E. Brown et al., defines and solves an optimization problem called the feeder addition problem which aims at determining potential locations for adding interties between feeders in a legacy radial distribution system to improve the reliability in the islanded mode of operation—a desired feature under the Smart Grid Initiative. 5) “Frequency/Sequence Selective Filters for Power Quality Improvement in a Microgrid” by M. Illindala and G. Venkataramanan, proposes improved methods of filtering signals in three-phase, three-wire power systems, especially when there is a narrow gap between the desired and undesired frequencies. The proposed techniques contain both complex bandpass and bandstop sections and are designed on three-phase space-vector quantities. 6) “Distributed Energy Resources Impact on Distribution System Reliability Under Load Transfer Restrictions” by
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A. M. Leite da Silva et al., presents an impact analysis of distributed energy resources integration on distribution systems, focusing mainly on reliability aspects. An interesting algorithm to correctly determine the amount of capacity that may be transferred to other feeders is presented and discussed, taken into consideration the presence of distributed generation. 7) “Reliability-Constrained Optimal Sizing of Energy Storage System in a Microgrid” by S. Bahramirad et al., develops a model for calculating the optimal size of a storage system in a microgrid considering reliability criteria. An expansion planning problem is proposed to consider the investment cost of the storage system, as well as operating cost of the microgrid. III. FAULT MODELING AND PROTECTION SCHEMES This section consists of four papers focusing on series fault modeling in dc microgrids, overvoltage protection of photovoltaic generators, and protection schemes tailored for the microgrid operational characteristics. 1) “A DC Arc Model for Series Faults in Low Voltage Microgrids” by F. M. Uriarte et al., analyzes the voltage-current signatures of series-fault arcs in dc microgrids and presents a new simple-yet practical- model for this type of faults. The proposed model is independent of the power system’s time constants since they are rarely exactly known in practice. 2) “Online Overvoltage Prevention Control of Photovoltaic Generators in Microgrids” by P. Zhang et al., proposes a novel online active power control method for large gridconnected PV power plants to maintain PV terminal voltages within specified range while maximizing the PV energy yields. The proposed method is based on precise active power limit prediction using dynamic equivalence. 3) “Optimizing the Roles of Unit and Non-unit Protection Methods Within DC Microgrids” by S. D. A. Fletcher et al., presents a detailed analysis of dc microgrid behavior under fault conditions, illustrating the challenging protection requirements, and demonstrating the shortcomings of non-unit approaches for these applications. The culmination of this work is the proposal of a flexible protection scheme design framework for dc microgrid applications which enables the required levels of fault discrimination to be achieved whilst minimizing the associated installation costs. 4) “A Communication-Assisted Protection Strategy for Inverter-Based Medium-Voltage Microgrids” by M. A. Zamani et al., proposes a communication-assisted protection strategy implementable by commercially available microprocessor-based relays for protection of medium-voltage microgrids. The paper also introduces the structure of a relay that enables the proposed protection strategy.
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IV. OTHERS Two practical papers about the impact of microgrids on the planning of the medium-voltage distribution networks and characterization of the Vanadium Redox battery are presented. 1) “Impact of MV-Connected Microgrids on MV Distribution Planning” by R. J. Millar et al., looks at microgrids from the point of view of network layout, protection, operation, their impact on the income of the distribution system operator and delaying network reinforcement. Another significant contribution this paper brings is a simple but sufficiently accurate way to model microgrids for long-term network planning purposes. 2) “Performance Prediction of a Vanadium Redox Battery for Use in Portable, Scalable Microgrids” by J. D. Guggenberger et al., focuses on characterizing Vanadium Redox battery parasitic loads such as pump, heating, ventilation, and air conditioning energy requirements to determine system performance based on known climatic operating conditions and load requirements. Stochastic analysis of variable ambient temperature and load requirements were used during this characterization. ACKNOWLEDGMENT The Guest Editorial Board would like to thank the IEEE Power and Energy Society, the authors for their contributions to this special section, and the reviewers for their generous and prompt help and valuable remarks and suggestions. Special thanks to Prof. M. Shahidehpour, IEEE TRANSACTIONS ON SMART GRID Editor-in-Chief, and Cheryl Koster, Power and Energy Society Publications Administrator, for their constant help and support. M. FOTUHI-FIRUZABAD, Guest Editor Sharif University of Technology Tehran, 15556-34414 Iran
[email protected] R. IRAVANI, Guest Editor University of Toronto Toronto, ON M5S 1A1 Canada F. AMINIFAR, Guest Editor Sharif University of Technology Tehran, 15556-34414 Iran N. HATZIARGYRIOU, Guest Editor National Technical University of Athens Athens, 10682 Greece M. LEHTONEN, Guest Editor Aalto University Helsinki, 00076 Finland
