Variability, Scalability and Stability of Microgrids
2: School of Science Engineering and Information Technology, Federation University Australia, VIC, Australia
3: Department of Energy Technology, Aalborg University Denmark, Aalborg, Denmark
A microgrid is a small network of electricity users with a local source of supply that is usually attached to a larger grid but can function independently. The interconnection of small scale generating units, such as PV and wind turbines, and energy storage systems, such as batteries, to a low voltage distribution grid involves three major challenges: variability, scalability, and stability. It must keep delivering reliable and stable power also when changing, or repairing, any component, or under varying wind and solar conditions. It also must be able to accept additional units, i.e. be scalable. This reference discusses these three challenges facing engineers and researchers in the field of power systems, covering topics such as demand side energy management, transactive energy, optimizing and sizing of microgrid components. Case studies and results provide illustrative examples in each chapter.
Inspec keywords: power system stability; optimisation; fluctuations; distributed power generation
Other keywords: variability; voltage fluctuations; microgrid; frequency instabilities; zero voltage ride through problems; scalability; stability; frequency fluctuations; optimization; low voltage ride through problems
Subjects: Distributed power generation; Power system control; Optimisation techniques; General electrical engineering topics
- Book DOI: 10.1049/PBPO139E
- Chapter DOI: 10.1049/PBPO139E
- ISBN: 9781785616938
- e-ISBN: 9781785616945
- Page count: 623
- Format: PDF
-
Front Matter
- + Show details - Hide details
-
p.
(1)
-
1 Introduction
- + Show details - Hide details
-
p.
1
–14
(14)
Microgrids are a collection of loads and local generations that are also normally connected to the legacy grid. However, the grid connection may be simply for reliability reason, if the local generations and loads match. The grid may just serve as a trading link depending on the mutual need. Considering its many other benefits, microgrid market is growing more rapidly; nowadays, more than 1,840 projects that represent a power capacity of almost 19,280 MW are under development worldwide. According to a research report by the market research and strategy consulting firm, Global Market Insights, Inc, the microgrid market will exceed $19 billion by 2024 [1]. This chapter presents an overview, technical challenges, and future important issues of microgrid developments.
-
2 Microgrid control overview
- + Show details - Hide details
-
p.
15
–71
(57)
A microgrid (MG) is always prone to the uncertainties of its demand variation and generation of its non-dispatchable renewable sources, particularly when operating in the islanded mode. Such events can push voltage and/or frequency of the MG beyond their desired range of operation. This chapter reviews the control and management techniques to retain the voltage and frequency of such MGs within a predefined safe zone. Suitable real time, corrective, and preventive controllers are discussed on the generation and demand side, which aim to satisfy various objectives at different time instances. First, the necessity of such controllers and mechanisms is explained in both grid-tied and islanded modes and during the transition between these modes. Then, islanding detection and its impact on MG management are briefly discussed. Afterwards, the MG's control architecture is outlined, and the existing approaches in the literature are described briefly. Finally, three case studies on different aspects of MG control are reported to show the applicability and criticality of such services for MG operation. The emphasis of the case studies is on the islanded MG operation because frequency and voltage issues are more pronounced for those types of MGs. In particular, a new generalised droop-based controller is explained in Section 2.4.1 as an example of advanced power-sharing strategies for voltage and frequency regulation with the plug-and-play feature. In Section 2.4.2, the primary frequency control problem is tackled from the demand control perspective, where demand response (DR) resources are altered to provide frequency and voltage regulation within a short period of time. Finally, a corrective and preventive controller is outlined and explained in Section 2.4.3. The corrective controller takes action immediately after the occurrence of an event that violates the voltage or frequency by defining the least cost solution among available options. In the preventive controller, generation and load demand forecast are used to predict unexpected events in very short horizons that can lead to voltage/frequency violations and take suitable actions beforehand.
-
3 Requirements analysis in transactive energy management
- + Show details - Hide details
-
p.
73
–97
(25)
This chapter focusses on the effective usage of transactive energy (TE) and the importance of developing an economical TE-management (TEM) process. TE is a concept that can play a vital role in improving the efficiency and reliability of a power system. This notion is promising for the energy industry in providing an intelligent and interactive future. This concept initiates various requirements for power distribution and transmission that works efficiently and is totally reliable. This leads to the exploration of requirements engineering (RE) approaches which can play a vital role in the development of TE and management process. This chapter explains the usage of RE models in relation to micro-grid and smart grid development. The wide-ranging development of smart grid systems demands supplementary software models so that its full potential can be explored and utilised. It only makes sense that consolidation of extensive usage of distributed energy and renewable energy sources is important in relation to the future of smart grid to bring about an economical and reliable functioning of a power system. An innovative approach in the form of TE towards the future smart grid is highly beneficial for the power-system operations. This novel approach has been extensively researched in recent years around the world. Within this chapter, we are outlining a goal-oriented RE (GORE) approach to structure TEM system. The main objective of this chapter is to perform reasoning and impact of nonfunctional requirements (NFRs) on the TEM. This reasoning will help decision makers in getting the desired outcomes from an efficient and reliable power system.
-
4 Transformation of microgrid to virtual power plant
- + Show details - Hide details
-
p.
99
–142
(44)
In this chapter, the problem of microgrid (MG) and virtual power plant (VPP) concepts are considered as the most promising solutions to integrate distributed generation resources into the electric power system. Currently, the main drawback of distributed energy resource (DER) is that many of them are operating individually without any coordination with other electrical power sources, which leads to decrease of main grid reliability. MG, as a group of DER and interconnected loads, can connect and disconnect from the grid to enable it to operate in both grid-connected or standalone/isolated mode. With emerging of advanced communication and information technology, the small DER units would be aggregated then monitored, controlled and treated as a single large power plant. Bidirectional power flows, stability issues, uncertainty are the challenges to MGs. These can be overcome by integrating MGs as VPP. This concept can strengthen and diversified generation structure through decentralized dispatching and distribution energy networks. Additionally, a DER prosumers grouped together, controlled and coordinated by VPP may become significant part of the wholesale market. There are many approaches to the VPP concept. In this chapter, the concept of VPP for DER integration with power system is introduced and discussed. The focus will be paid to the modelling of proposed concept by MATLAB® SimulinkTM software and the results of the simulation in different operation scenarios have been presented.
-
5 Operations of a clustered microgrid
- + Show details - Hide details
-
p.
143
–174
(32)
Microgrids (MGs) are referred to as isolated and self-sufficient electricity supply systems that well suit remote areas. These systems are generally composed of nondispatchable and dispatchable energy resources to reduce the electricity production cost. Emergencies such as overloading, faults and shortfalls can result in difficulty for the smooth operation of MGs. The main aim of this study is to discuss the operation of MGs by presenting a power transaction management scheme. It focuses on the scenario when MGs are provisionally coupled to resolve the emergency situation and termed clustered MGs. For this purpose, power transaction is taken as an instance of purchasing or selling of electricity amongst healthy and problem MGs. The key objective of a suitable power transaction technique should then be regulating the power amongst the provisionally coupled MGs by adjusting the suitable power generation from all available dispatchable sources. An optimization problem is formulated for achieving this purpose, and its main purpose is to minimize the costs and technical impacts while focusing on the above-considered parameters. Genetic algorithm which is a heuristic optimization technique is used to solve the formulated optimization problem, and the performance of the suitable power transaction strategy is evaluated by several numerical analyses.
-
6 Distributed energy network using nanogrid
- + Show details - Hide details
-
p.
175
–219
(45)
With the guidance of local governments and the concerns of the environment, more and more distributed energy resources (DERs) and distributed energy storage units (DESUs) are installed by subscribers in remote villages, outskirts and mountainous areas. The DERs and DESUs form an easy, self-controlled nanogrid. Nanogrid can be seen as smaller and technologically simpler islanding microgrids. When more and more renewable sources are interfaced to the nanogrid, a power-management issue is important for this system to supply smooth power to the customers. Therefore, some reviews for nanogrid and some representative control strategies for renewable sources in nanogrid are presented in this book chapter.
-
7 Sizing of microgrid components
- + Show details - Hide details
-
p.
221
–262
(42)
A microgrid (MG) is a distinct energy system consisting of distributed energy resources (DERs) and loads having the ability to operate in parallel with, or independently from, the main power grid. MGs, which were initially introduced to ensure smooth operation and control of DERs in distribution networks, offer unprecedented economic and reliability benefits to electricity consumers with minimal carbon emission. These benefits, however, must be analysed and compared with the capital investment cost of the MG to ensure a complete return on investment and to justify the MG deployment. The biggest obstacle for the widespread and rapid deployment of MGs is the high capital investment cost of MGs. A true assessment of MGs economic benefits is a challenging task due to the significant uncertainties involved in the assessment. These uncertainties may include the intermittency of the renewable generation, the varying states of charge (SoC) of battery energy storage system (BESS), the uncertain demands, the varying market price, the probability of the MG islanding, the level of developer's risk-aversion and the unpredictably of the user preferences in the smart load management system. Moreover, some of the assessment metrics, such as the measure of reliability improvements are difficult to comprehend for consumers when represented in terms of the supply availability. Thus, efficient and optimum planning models are required to ensure the economic feasibility of MG deployments and to justify the investments based on cost-to-profit analysis under uncertain conditions. This chapter demonstrates a detailed model for the optimum sizing of MG components under the uncertainties involved in the system. The proposed model is validated with the simulation of several case studies conducted on a system depicting a similar MG in a medium-voltage (MV)-distribution system derived from electricity network of a power utility in New South Wales, Australia. The results from the case studies demonstrate the efficacy of the proposed model for the optimum sizing of the MG components to justify the MG deployment.
-
8 Optimal sizing of energy storage system
- + Show details - Hide details
-
p.
263
–289
(27)
Energy storage system (ESS) as a growing technology plays a substantial role in operation and planning of microgrids. It is a viable option in smoothing the power of renewable energy sources (RESs), peak load mitigation, voltage control, frequency regulation and reliability enhancement of microgrid. Sizing of energy storages in microgrids is still a challenge. A non-optimal small capacity of storage system cannot ensure the proper operation of a microgrid. On the other side, a non-optimal large capacity may increase the cost and power losses in the system. In this chapter, different types of energy storages with their characteristics such as capacity, cost and efficiency are introduced. The necessity of energy storages in microgrids is explained. Optimal sizing of an ESS in a microgrid is discussed. The objective function (OF), system constraints and operation conditions are addressed.
-
9 Microgrid communications - protocols and standards
- + Show details - Hide details
-
p.
291
–326
(36)
The recent advancements in the Internet of Things (IoT) and telecommunication infrastructure have significantly increased the reliability and effectiveness of communication protocols in microgrid environment. Nowadays, the equipment in a smart microgrid not only exchange information with one another much faster but also control, monitor and diagnose faults much faster and more reliably. Despite the benefits of new technologies, they are tailored to satisfy specific requirements, and therefore, they can be only used for certain applications. The microgrid communication model consists of a three-layer architecture, where the energy management system (EMS) sits in the top layer and controls the overall operations of the island of microgrids. The middle layer includes the local controllers (LCs) that regulate operations within the local grid. The bottom layer includes IoT devices, such as smart meters, fault recorders and protective relays, which continuously capture and transmit the stream of sensed data. Such hierarchical architecture introduces specific computation and latency requirements for each layer of microgrid communication. To meet these requirements, each layer must use different communication equipment and protocols. This chapter provides an insight into communication requirements, system architecture, standards, protocols and tools used in microgrid communications. The chapter concludes with a case study, where wireless technology is utilised for reliable and optimal operations in a microgrid.
-
10 Voltage stability of microgrids
- + Show details - Hide details
-
p.
327
–376
(50)
This chapter focuses on voltage stability of microgrids. Some control schemes to maintain voltages of all the buses of a microgrid at acceptable levels in normal conditions and after being subjected to disturbances will be presented. Voltage stability enhancement and reactive power sharing in inverter-based microgrids are discussed, including droop-control techniques.
-
11 Frequency stability and synthetic inertia
- + Show details - Hide details
-
p.
377
–394
(18)
Due to low inertia, stochastic nature of renewable energy sources (RESs) and sudden load changes, nearly all the modern microgrids are associated with the dynamic frequency stability issues. Thus it restricts the maximum number of the renewable energy systems that can be penetrated to the microgrid. In order to increase the penetration of low-inertia sources to the microgrid, the frequency stability issues need to be addressed. The frequency stability issues of a typical microgrid are addressed by the addition of extra inertial support from the power sources using power converters and appropriate control loop. In this chapter, the readers are introduced with the basic concepts of the synthetic inertia support for the dynamic frequency stability issues of a hydro-photovoltaic (PV) microgrid. The details of the frequency-power-response-based synthetic inertial support and current control loops are elaborated using a simulation example. The deviation in system's frequency is usually compensated by sourcing or absorbing the active power by the inertial support loop, so by utilizing the concept of inertial loop, the maximum number of RESs integration can be enhanced.
-
12 Microgrid protection
- + Show details - Hide details
-
p.
395
–461
(67)
Power-system protection encompasses the interrelated concerns of assurance of human safety, mitigation of equipment damage and provisions for reliable transmission and distribution of electrical power to end users. For conventional power delivery systems-from national power grids to household electrical distribution- the design methods and engineering design solutions for protection have benefited from over a century of experience. However, as renewable energy penetration into the grid is increasing, long-held practices for protective system design and associated hardware and controls are proving to be inadequate. This is particularly the case for microgrids. The microgrid has emerged as an efficient means for introduction of locally supplied renewable energy source(s) (RESs), such as solar photovoltaic (PV) and wind, and a diversity of local generation sources, such as national gas generation, into existing grid structures. With the inclusion of energy storage systems (ESSs), the microgrid provides a means for improving grid resiliency and achieving truly energy secure systems. Throughout this chapter, distributed generation (DG) or distributed generators (DGs) refers to any distributed source of power, including RESs or more conventional natural gas generators and back-up supply diesel generators. The term distributed energy resource (DER) refers generally to DGs and ESSs. The inclusion of ESS is implied by the use of the term DER, whereas DG or DGs refer only to power generation. The microgrid concept effectively integrates and manages simultaneously generation, energy storage and load demand. The microgrid also provides a viable means for electrification of energy poor areas. Although technically not a microgrid, electrified transportation power-distribution systems share many common traits with microgrids. From the beginning, the achievement of accurate fault discrimination within a microgrid has been an issue. In the case of AC microgrids, this protection challenge has not prohibited implementations because the microgrid can be integrated into existing protective structures, albeit with suboptimal behavior during fault events.
-
13 Black start and islanding operations of microgrid
- + Show details - Hide details
-
p.
463
–495
(33)
This chapter describes different control techniques applied in microgrids (MGs) with responsive distributed energy resources (DER) for intentional and automatic islanding, deployment of a local black start strategy and including the reconnection to the upstream distribution network. The coordination of MG local resources, achieved through an appropriated network of controllers and communication system, enables some cells of the distribution system, with sufficient autonomy, to operate interconnected to the upstream network or autonomously in some circumstances. In this case, the potentialities of DER can be truly realized if the islanded operation is allowed and bottom-up black start functionalities are implemented. In particular, black start will be addressed for medium voltage (MV) MGs, made of several low voltage (LV) MGs, leading to the concept of a multi-MG (MMG). The chapter will then describe the black start sequence that can be implemented in such a grid considering also the contribution from electric vehicle storage.
-
14 Microgrid feasibility study and economics
- + Show details - Hide details
-
p.
497
–532
(36)
The microgrid concept is a promising approach to enable smarter energy grids. Microgrids are capable of managing and coordinating distributed generation, storage devices and loads in a more decentralized way reducing the need for centralized coordination and management entities. The increased control over communication network and digitization makes microgrids increasingly active, i.e. they can generate, sense, compute, communicate and actuate. This will also enable a more proactive role of consumers. Hence, efficient optimization and control of microgrid operation is extremely important. This chapter discusses how advanced control and optimization techniques can be used to realize the technical, economical and environmental potential of microgrids and facilitate more active demand management. Several aspects related to the feasibility study with particular focus on the control and operational aspects are discussed. The principal sources of uncertainty (e.g. energy demand, renewable generation) are taken into account to render microgrid operation less sensitive to unforeseen events. Simulation and experimental results show the potential economic and environmental benefits of the illustrated operational strategies.
-
15 Power electronics-microgrid interfacing
- + Show details - Hide details
-
p.
533
–571
(39)
Power electronics is the key enabling technology for modern power systems. Power converters are increasingly used in a wide range of applications from generation to consumption levels. Due to the significant importance of power electronics in modern power systems, this book chapter presents the possible structure of the future power systems employing the microgrid (MG) technologies. Different MG topologies and converter structures are introduced. Moreover, the control and operation principles of power electronic converters in MGs are discussed. Two case studies are provided in order to show the importance of power electronics in operation and control of modern power systems. Finally, the chapter is summarized with highlighting challenges encountering modern power electronics-based power systems.
-
Back Matter
- + Show details - Hide details
-
p.
(1)