Sioshansi F.P. Smart Grid: Integrating Renewable, Distributed & Efficient Energy
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Group III to the Fourth Assessment Report of the
Intergovernmental Panel on Climate Change (2007)
Cambridge University Press, Cambridge.
[19] IEA, Energy Technology Perspectives 2010. (2010) IEA,
Paris.
[20] R. Garnaut, The Garnaut Review 2011: Australia in the
Global Response to Climate Change.
http://www.garnautreview.org.au/update-2011/
garnaut-review-2011.html, 2011.
[21] Commonwealth of Australia, Australia's Low Pollution
Future: The Economics of Climate Change Mitigation.
http://www.treasury.gov.au/lowpollutionfuture/report/
default.asp, 2008.
[22] CSIRO, Intelligent Grid – A Value Proposition for
Distributed Energy in Australia. CSIRO Report ET/IR
1152, http://www.csiro.au/resources/IG-report.html,
2009.
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Chapter 8. What Role for Microgrids?
Glenn Platt, Adam Berry, and David Cornforth
Chapter Outline
Introduction 185
Background 186
The Microgrid Concept 188
Key Technologies 192
The Advantages of Microgrids 192
Autonomy of Supply 192
Coordination of Distributed Generation 193
Aggregation of Resources 193
High Penetration of Renewable Supply 193
The Challenges for Microgrid Adoption 194
Control of Microgrids 194
Planning and Design of Microgrids 194
Cost of Microgrids 199
Integrating Renewables into the Microgrid 199
Microgrid Modeling 201
Energy Management 202
Policy, Regulation, and Standards 202
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Existing Installations with Microgrid Features 203
Future Evolution of Microgrids 204
Conclusions 205
Acknowledgment 206
References 206
The microgrid concept offers a stepping stone along the path to autonomous, intelligent low-emissions
electricity systems, by creating a localized low-emissions smart grid that allows advanced control while
being compatible with current electricity infrastructure With such promise, microgrids are of growing
interest to grid operators around the world, as ways of enhancing the performance of both developed
and undeveloped electricity systems The operation of microgrids, however, is not without its
challenges This chapter reviews microgrid technology, detailing worldwide research and developments
of this technology, while also presenting examples of the technical challenges needing to be addressed
before widespread rollout
Microgrids, applications of microgrid, benefits of microgrid
Introduction
While many of the technologies described in this book, from
low-emissions generation sources to new voltage control or
energy storage devices, have the potential to bring great
benefit to electricity systems, they are not without their
challenges. Fundamentally, the complexity associated with
integrating such plant into an already labyrinthine electricity
distribution system is limiting enthusiasm for many of these
technologies. The key to addressing this issue is to minimize
the changes felt at the distribution level by simplifying the
interface to these new resources. Microgrids represent just
such a simplification.
In essence, a microgrid is a collective of geographically
proximate, electrically connected loads and generators. While
microgrids were traditionally viewed as a technology used in
remote area power supplies, they can operate either connected
or disconnected, or “islanded,” from the wider utility grid.
When operating as a member of the wider utility grid,
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microgrids effectively introduce a new level of hierarchy to
the utility grid, where network assets and microgrids
themselves can operate at the leveI of the utility grid;
alternatively they may operate a level below this, internal to
the microgrid, but separated from the wider utility grid
operation. In a microgrid the challenge of controlling large
numbers of distributed resources is reduced to an internal
process, operating solely within the microgrid. By using
microgrids to abstract the challenges of coordinating and
controlling multitudes of distributed resources away from the
wider utility grid, the interoperability challenges facing
today's smart grids are eased significantly—grids can use
conventional command and control techniques, ignorant of
the machinations at lower layers, where the microgrid control
system manages things.
While microgrids are bounded in scope, the challenges of
ensuring stable and reliable operation of these systems should
not be underestimated—particularly in systems with a high
penetration of intermittent renewable supply. Challenges in
this case range from interfacing the microgrid to the wider
electricity system, through to implementing microgrid control
systems without requiring expensive communications and
control infrastructure.
This chapter explores the microgrid concept in the wider
context of the smart grid. Sections “The Microgrid Concept”
and “Key Technologies” detail the microgrid concept and its
benefits, before a discussion of the challenges facing
microgrid operation. The final section of the chapter provides
examples of deployed microgrids that are exploring these
issues in practical detail.
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Background
Traditionally, power networks have been based on a radial
topology, where one generator is attached to many consumers
in a tree-like structure. As shown in Figure 8.1, large,
high-voltage transmission networks act like the trunk of the
tree, carrying electricity long distances away from the
generator and towards the electrical loads. Closer to these
loads, distribution networks act like the branches of the tree,
interconnecting loads and the long-distance transmission
network.
Figure 8.1
Conceptual differences between the traditional grid, which is hierarchical, and a
grid including microgrids.
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The radial nature of modern power distribution networks is
one of the core challenges in modernizing the control and
operation of contemporary power systems. For example,
adding automated protection devices to existing centralized
control schemes places a heavy load on data communication
networks. Similarly, as the network becomes more complex
the processing power of the central control system becomes a
challenging factor [1]. In addition to these challenges of
computing power, such centralized control schemes are
challenged by “point-of-attack” issues where system-wide
failures can occur if the central controller fails.
Another challenge to the reliable operation of current power
systems comes from the growing prevalence of distributed
generation—electricity generation sources that are typically
much smaller than conventional power stations, and located
close to electrical loads in the network. As discussed in
Chapter 7, these distributed generators have a number of
benefits that are driving their uptake, with advocates claiming
benefits such as curtailment of transmission and distribution
losses, greater robustness in the face of extreme weather
events or attack, improved reactive power support, and
decreased deployment time.
Importantly, distributed generators are often based on low or
zero-emission generation sources—from highly efficient gas
turbines through to renewable energy systems. Moreover,
their close proximity to loads means that the efficiency of
distributed generators can be further improved by their waste
heat—for example to heat or cool nearby buildings. Though
the range and depth of distributed generation technologies are
great, their use is not without significant challenges.
Fundamentally, it is expected that localized generators will be
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able to supply power back into the grid when local supply
exceeds local demand. This challenges the traditional
assumption of a unidirectional power distribution network, as
power can now flow in both directions, and it may be quite
common for power to flow locally, in varying directions,
between distributed generators and nearby consumers [2].
With the growth in such devices, system planning and
operation studies that assumed unidirectional power flow are
no longer accurate, and the reliable operation of power
systems incorporating large amounts of distributed generation
becomes increasingly problematic [2].
While their environmental benefits are clear, as detailed in
Chapter 6, the introduction of renewable energy sources such
as wind and solar generation can exacerbate the challenges of
distributed generation. The intermittency in the power supply
available from such generation (caused by, for example, wind
gusts or clouds passing overhead) means that not only is
power flow bidirectional, but the power being fed into the
system from such sources at any one time can vary randomly,
depending on local environmental conditions. In the European
Union, the increasing penetration of distributed generation
such as wind farms is beginning to cause problems that, if
unmitigated, will challenge the integrity and security of the
electricity system [2].
The Microgrid Concept
Fundamentally, many of the challenges discussed in the
previous section are due to the inflexible way power systems
are traditionally operated. In particular, their centralized,
radial design limits the dynamic reconfiguration that is
possible in the system. Though some reconfiguration can
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occur at the edges of the “tree,” the core structure remains in
place, is difficult to change in-situ, and is at risk of failure.
Considering this issue, contemporary power systems research
has been increasingly focused on how best to integrate
distributed energy resources such as renewable generation
devices into the larger electricity grid. Of the various
methodologies available, grouping distinct distributed
resources so they represent a single generator or load to the
wider electricity system is the technique that least affects
existing infrastructure. When such loads and generators are
located within close geographical proximity of each other,
such a system is often referred to as a microgrid. More
specifically, in this work we define a microgrid as a collection
of controllable and physically proximate distributed generator
and load resources, where there are multiple sources of AC
power and at least one of these is based on a renewable
energy technology such as wind or solar energy.
1
1
In some literature the term “minigrid” is also used in reference to groups
of distributed generators and loads. In this work “minigrid” is considered
a synonym of “microgrid.”
A microgrid may or may not be connected to the wider
electricity grid. Here, an isolated microgrid is defined as a
microgrid that is not connected to the utility grid in any way,
shape, or form; it is a distinct island for which no point of
common coupling (PCC) exists. A connected microgrid is
defined as a microgrid that may be connected to the utility
grid; it may operate as a distinct island, but features a point of
common coupling (PCC) that allows interaction with the
utility grid, most typically to facilitate import/export of
power.
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There are a variety of reasons why microgrids are gaining
more interest from the wider research and deployment
community. On the one hand, microgrids represent a way of
coordinating the growing number of sites with local on-site
generation. For example, a university campus with
roof-mounted solar cells and a diesel-powered backup
generator can be transformed into a microgrid by adding
intelligent control systems to the generators and linking these
to load controllers to form a dynamic self-contained energy
system. On the other hand, microgrids represent an entirely
new way of powering remote or rural communities—rather
than one centralized, often diesel-powered generating station,
these communities can be powered by a large number of
low-emissions generators, linked with appropriate load
control.
The spectrum of possibilities between these two examples is
quite wide, and, across the range of references at the end of
this chapter, some ambiguity certainly lies in the wider
community regarding this term. To assist, Table 8.1 gives our
particular vision for what features a microgrid should have,
and later sections provide examples of deployments with
these characteristics, while Table 8.2 provides some examples
of what, in this chapter, is not considered to be a microgrid.
Table 8.1
A Vision for Microgrids—Key Characteristics
Quality Explanation
Intelligent
Capable of sensing system conditions and reconfiguring device
operation and system topology given goals such as reliability of
supply, cost of operation, and emissions minimization.
Efficient
Based primarily on clean renewable generation sources and intelligent
energy-efficient loads.
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