Microgrids

Architectures and Control

Nikos Hatziargyriou, Nikos Hatziargyriou

• Publisher: Wiley-IEEE Press
• Year: 2013
• ISBN:
  9781118720646
• Edition: 1
• Topic: Tecnología e ingeniería
• Subtopic: Recursos energéticos

Microgrids are the most innovative area in the electric power industry today. Future microgrids could exist as energy-balanced cells within existing power distribution grids or stand-alone power networks within small communities.

A definitive presentation on all aspects of microgrids, this text examines the operation of microgrids – their control concepts and advanced architectures including multi-microgrids. It takes a logical approach to overview the purpose and the technical aspects of microgrids, discussing the social, economic and environmental benefits to power system operation. The book also presents microgrid design and control issues, including protection and explaining how to implement centralized and decentralized control strategies.

Key features:

• original, state-of-the-art research material written by internationally respected contributors
• unique case studies demonstrating success stories from real-world pilot sites from Europe, the Americas, Japan and China
• examines market and regulatory settings for microgrids, and provides evaluation results under standard test conditions
• a look to the future – technical solutions to maximize the value of distributed energy along with the principles and criteria for developing commercial and regulatory frameworks for microgrids

Offering broad yet balanced coverage, this volume is an entry point to this very topical area of power delivery for electric power engineers familiar with medium and low voltage distribution systems, utility operators in microgrids, power systems researchers and academics. It is also a useful reference for system planners and operators, manufacturers and network operators, government regulators, and postgraduate power systems students.

CONTRIBUTORS

Thomas Degner
Aris Dimeas
Alfred Engler
Nuno Gil
Asier Gil de Muro
Guillermo Jiménez-Estévez
George Kariniotakis
George Korres
André Madureira
Meiqin Mao
Chris Marnay
Jose Miguel Yarza
Satoshi Morozumi
Alexander Oudalov
Frank van Overbeeke
Rodrigo Palma Behnke
Joao Abel Pecas Lopes
Fernanda Resende
John Romankiewicz
Christine Schwaegerl
Nikos Soultanis
Liang Tao
Antonis Tsikalakis

1

The Microgrids Concept

Christine Schwaegerl and Liang Tao

1.1 Introduction

Modern society depends critically on a secure supply of energy. Growing concerns for primary energy availability and aging infrastructure of current electrical transmission and distribution networks are increasingly challenging security, reliability and quality of power supply. Very significant amounts of investment will be required to develop and renew these infrastructures, while the most efficient way to meet social demands is to incorporate innovative solutions, technologies and grid architectures. According to the International Energy Agency, global investments required in the energy sector over the period 2003–2030 are estimated at $16 trillion.

Future electricity grids have to cope with changes in technology, in the values of society, in the environment and in economy [1]. Thus, system security, operation safety, environmental protection, power quality, cost of supply and energy efficiency need to be examined in new ways in response to changing requirements in a liberalized market environment. Technologies should also demonstrate reliability, sustainability and cost effectiveness. The notion of smart grids refers to the evolution of electricity grids. According to the European Technology Platform of Smart Grids [2], a smart grid is an electricity network that can intelligently integrate the actions of all users connected to it – generators, consumers and those that assume both roles – in order to efficiently deliver sustainable, economic and secure electricity supplies. A smart grid employs innovative products and services together with intelligent monitoring, control, communication and self-healing technologies.

It is worth noting that power systems have always been “smart”, especially at the transmission level. The distribution level, however, is now experiencing an evolution that needs more “smartness”, in order to

• facilitate access to distributed generation [3,4] on a high share, based on renewable energy sources (RESs), either self-dispatched or dispatched by local distribution system operators
• enable local energy demand management, interacting with end-users through smart metering systems
• benefit from technologies already applied in transmission grids, such as dynamic control techniques, so as to offer a higher overall level of power security, quality and reliability.

In summary, distribution grids are being transformed from passive to active networks, in the sense that decision-making and control are distributed, and power flows bidirectional. This type of network eases the integration of DG, RES, demand side integration (DSI) and energy storage technologies, and creates opportunities for novel types of equipment and services, all of which would need to conform to common protocols and standards. The main function of an active distribution network is to efficiently link power generation with consumer demands, allowing both to decide how best to operate in real-time. Power flow assessment, voltage control and protection require cost-competitive technologies and new communication systems with information and communication technology (ICT) playing a key role.

The realization of active distribution networks requires the implementation of radically new system concepts. Microgrids [5–11], also characterized as the “building blocks of smart grids”, are perhaps the most promising, novel network structure. The organization of microgrids is based on the control capabilities over the network operation offered by the increasing penetration of distributed generators including microgenerators, such as micro-turbines, fuel cells and photovoltaic (PV) arrays, together with storage devices, such as flywheels, energy capacitors and batteries and controllable (flexible) loads (e.g. electric vehicles [12]), at the distribution level. These control capabilities allow distribution networks, mostly interconnected to the upstream distribution network, to also operate when isolated from the main grid, in case of faults or other external disturbances or disasters, thus increasing the quality of supply. Overall, the implementation of control is the key feature that distinguishes microgrids from distribution networks with distributed generation.

From the customer's point of view, microgrids provide both thermal and electricity needs, and, in addition, enhance local reliability, reduce emissions, improve power quality by supporting voltage and reducing voltage dips, and potentially lower costs of energy supply. From the grid operator's point of view, a microgrid can be regarded as a controlled entity within the power system that can be operated as a single aggregated load or generator and, given attractive remuneration, also as a small source of power or ancillary services supporting the network. Thus, a microgrid is essentially an aggregation concept with participation of both supply-side and demand-side resources in distribution grids. Based on the synergy of local load and local microsource generation, a microgrid could provide a large variety of economic, technical, environmental and social benefits to different stakeholders. In comparison with peer microsource aggregation methods, a microgrid offers maximum flexibility in terms of ownership constitution, allows for global optimization of power system efficiency and appears as the best solution for motivating end-consumers via a common interest platform.

Key economic potential for installing microgeneration at customer premises lies in the opportunity to locally utilize the waste heat from conversion of primary fuel to electricity. There has been significant progress in developing small, kW-scale, combined heat and power (CHP) applications. These systems have been expected to play a very significant role in the microgrids of colder climate countries. On the other hand, PV systems are anticipated to become increasingly popular in countries with sunnier climates. The application of micro-CHP and PV potentially increases the overall efficiency of utilizing primary energy sources and consequently provides substantial environmental gains regarding carbon emissions, which is another critically important benefit in view of the world's efforts to combat climate change.

From the utility point of view, application of microsources can potentially reduce the demand for distribution and transmission facilities. Clearly, distributed generation located close to loads can reduce power flows in transmission and distribution circuits with two important effects: loss-reduction and the ability to potentially substitute for network assets. Furthermore, the presence of generation close to demand could increase service quality seen by end customers. Microgrids can provide network support in times of stress by relieving congestion and aiding restoration after faults.

In the following sections, the microgrid concept is clarified and a clear distinction from the virtual power plant concept is made. Then, the possible internal and external market models and regulation settings for microgrids are discussed. A brief review of control strategies for microgrids is given and a roadmap for microgrid development is provided.

1.2 The Microgrid Concept as a Means to Integrate Distributed Generation

During the past decades, the deployment of distributed generation (DG) has been growing steadily. DGs are connected typically at distribution networks, mainly at medium voltage (MV) and high voltage (HV) level, and these have been designed under the paradigm that consumer loads are passive and power flows only from the substations to the consumers and not in the opposite direction. For this reason, many studies on the interconnection of DGs within distribution networks have been carried out, ranging from control and protection to voltage stability and power quality.

Different microgeneration technologies, such as micro-turbines (MT), photovoltaics (PV), fuel cells (FC) and wind turbines (WT) with a rated power ranging up to 100 kW can be directly connected to the LV networks. These units, typically located at users' sites, have emerged as a promising option to meet growing customer needs for electric power with an emphasis on reliability and power quality, providing different economic, environmental and technical benefits. Clearly, a change of interconnection philosophy is needed to achieve optimal integration of such units.

Most importantly, it has to be recognized that with increased levels of microgeneration penetration, the LV distribution network can no longer be considered as a passive appendage to the transmission network. On the contrary, the impact of microsources on power balance and grid frequency may become much more significant over the years.

Therefore, a control and management architecture is required in order to facilitate full integration of microgeneration and active load management into the system. One promising way to realize the emerging potential of microgeneration is to take a systematic approach that views generation and associated loads as a subsystem or a microgrid.

In a typical microgrid setting, the control and management system is expected to bring about a variety of potential benefits at all voltage levels of the distribution network. In order to achieve this goal, different hierarchical control strategies need to be adopted at different network levels.

The possibility of managing several microgrids, DG units directly connected to the MV network and MV controllable loads introduces the concept of multi-microgrids. The hierarchical control structure of such a system calls for an intermediate control level, which will optimize the multi-microgrid system operation, assuming an operation under a real market environment. The concept of multi-microgrids is further developed in Chapter 5.

The potential impact of such a system on the distribution network may lead to different regulatory approaches and remuneration schemes, that could create incentive mechanisms for distribution system operators (DSOs), microgeneration owners and loads to adopt the multi-microgrid concept. This is further discussed in Chapter 7.

1.3 Clarification of the Microgrid Concept

1.3.1 What is a Microgrid?

In scope of this book, the definition from the EU research projects [7,8] is used:

Microgrids comprise LV distribution systems with distributed energy resources (DER) (microturbines, fuel cells, PV, etc.) together with storage devices (flywheels, energy capacitors and batteries) and flexible loads. Such systems can be operated in a non-autonomous way, if interconnected to the grid, or in an autonomous way, if disconnected from the main grid. The operation of microsources in the network can provide distinct benefits to the overall system performance, if managed and coordinated efficiently.

There are three major messages delivered from this definition...