Jenkins N., Strbac G., Ekanayake J. Distributed Generation

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Preface

Public electricity supply was originally developed in the form of local generation

feeding local loads, the individual systems being built and operated by independent

companies. During the early years, up to around 1930, this proved quite sufficient.

However, it was then recognised that an integrated system, planned and operated by

a specific organisation, was needed to ensure an electricity supply that was both

reasonably secure and economic. This led to large centrally located generators

feeding the loads via transmission and distribution systems.

This trend may well have continued but for the need to minimise the envir-

onmental impact of energy use (particularly CO

emissions) and concern over the

security of supply of imported fossil fuels. Consequently, governments and energy

planners are now actively developing alternative and cleaner forms of energy

production, these being dominated by renewables (e.g. wind, solar, biomass), local

CHP plant and the use of waste products. In many countries this transformation of

electricity supply is managed through energy markets and privately owned, regu-

lated transmission and distribution systems.

The economics and the location of sustainable energy sources have meant that

many of these generators have had to be connected into distribution networks rather

than at the transmission level. A full circle has therefore evolved with generation being

distributed round the system rather than being located and dispatched centrally.

It is now recognised that the levels of information and control of the state of

distribution networks are inadequate for future low-carbon electricity supply sys-

tems and the SmartGrid concept has emerged. In addition to much greater use of

ICT (Information and Control Technology), a SmartGrid will involve load custo-

mers much more in the operation of the power system. The distribution network

will change its operation from passive to active, and the distributed generators will

be controlled to support the operation of the power system.

Over several decades, models, techniques and application tools have been

developed for central generation with traditional transmission and distribution

networks and there are many excellent texts that relate to and describe the assess-

ment of such systems. However, some very specific features of distributed gen-

eration, namely that large numbers of relatively small generators are distributed

around the system, often connected into relatively weak distribution networks,

mean that existing tech niques and practices have had to be reviewed and updated to

take these features into account. The questions raised by large numbers of dis-

tributed generators and their control within a deregulated commercial environment

(virtual power plants), active network management and microgrids are also

addressed.

This book is the outcome of the authors’ experience in teaching courses on

Distributed Generation to undergraduate and MSc students in the United Kingdom,

the United States and Sri Lanka. Many universities are now offering courses on

Renewable Energy and how sustainable, low-carbon generation can be integrated

effectively into the distribution system. There is a great demand from employers for

graduates who have studied such courses but, in almost all countries, a critical

shortage of students with a strong education in Electrical Engineering and Power

Systems. Hence the book has four tutorial chapters (with examples and questions)

to provide fundamental material for those without a strong electrical engineering

background. Non-specialists may then benefit from the main body of the text once

they have studied the tutorial material.

Finally, no book, including this, could be produced in isolation, and we are

indebted to all those colleagues and individuals with whom we have been involved

in university, industry and in a number of professional organisations. Particular

thanks are due to TNEI Ltd for the use of IPSA and the Manitoba HVDC Centre for

the PSCAD/EMTDC simulation programs used in the examples in Chapters 3 and 4.

RWE npower renewables and Renewable Energy Systems (RES) generously made

available a number of photographs. Also we would like to thank Beishoy Awad for

all his help with the drawings.

xii Distributed generation

About the authors

Nick Jenkins was at the University of Manchester (UMIST) from 1992 to 2008. He

then moved to Cardiff University where he is now Professor of Renewable Energy.

His previous career included 14 years industrial experience, of which five years

were spent in developing countries. He is a Fellow of the IET, IEEE and Royal

Academy of Engineering. In 2009 and 2010 he was the Shimizu Visiting Professor

at Stanford University where he taught a course on ‘Distributed Generation and the

Grid Integration of Renewables’.

Janaka Ekanayake joined Cardiff University as a Senior Lecturer in June 2008

from the University of Manchester, where he was a Research Fellow. Since 1992 he

has been attached to the University of Peradeniya, Sri Lanka and was promoted to

Professor of Electrical and Electronic Engineering in 2003. He is a Senior Member of

IEEE and a Member of IET. His main research interests include power electronic

applications for the power system, and renewable energy generation and its

integration. He has published more than 25 papers in refereed journals and has co-

authored two books.

Goran Strbac is Professor of Electrical Energy Systems at Imperial College,

London. He joined Imperial College in 2005 after 11 years at UMIST and 10 years

of industrial and research experience. He led the development of methodologies

that underpinned the new UK distribution and transmission network operation, plus

design standards and grid codes to include distributed generation. He has conducted

impact assessments of large-scale penetration of renewables on the UK generation,

transmission and distribution networks for the UK Energy Review and also for the

last two Energy White Papers.

Chapter 1

Introduction

Plateau Arde

chois Wind Farm, Rho

ne Alpes, France (6.8 MW)

850 kW Doubly fed induction generator wind turbines [RES]

1.1 The development of the electrical power system

In the early days of electricity supply, each town or city would have its own small

generating station supplying local loads. However, modern electrical power systems

have been developed, over the past 70 years, mainly following the arrangement

indicated in Figure 1.1. Large central generators of ratings up to 1000 MW and

voltages of around 25 kV feed electrical power up through generator transformers

to a high voltage interconnected transmission network operating at up to 400 kV in

most of Europe and 750 kV in North America and China. The transmission system is

used to transport the electrical power, sometimes over considerable distances, and it

is then extracted and passed down through a series of distribution transformers to

final circuits for delivery to the customers. The transmission and distribution circuits

are mainly passive with control of the system provided by a limited number of large

central generators [1,2]. From around 1990, there has been a revival of interest in

connecting generation to the distribution network and this has come to be known as

distributed generation (DG) or the use of distributed energy resources (DER).

The name Distributed Generation can be considered to be synonymous and

interchangeable with the terms embedded generation and dispersed generation,

which are now falling into disuse. In some countries a strict definition of distributed

generation is made, based either on the rating of the plant or on the voltage level to

which it is connected. However, these definitions usually follow from particular

national, technical documents used to specify aspects of the connection or operation

of distributed generation and not from any basic consideration of its impact on the

power system. This book is concerned with the fundamental behaviour of generation

connected to distribution systems and so a rather broad description of distributed

generation is preferred to any particular limits based on plant size, voltage or prime

mover type.

Distributed generation may be connected at a number of voltage levels from

120/230 V to 150 kV. Only very small generators may be connected to the lowest

voltage networks, but large installations of som e hundreds of megawatts are con-

nected to the busbars of high vol tage distribution systems (Figure 1.2).

Central power

station

Central power

station

Central power

station

Distribution network

Distribution networkTransmission network

Figure 1.1 Conventional large electri c power system

The term distributed energy resources includes both distributed generation and controllable load.

2 Distributed generation

Extra high voltage

transmission network

High voltage

transmission network

Medium voltage

distribution network

Low voltage

distribution network

GGGG

DGDGDG

Small scale

Medium size

Large

scale

GGGG

Figure 1.2 Connection of distributed generators

Connecting generation to distribution networks leads to a number of challenges

as these circuits were designed to supply loads with power flows from the higher to

the lower voltage circuits. Conventional distribution networks are passive with few

measurements and very limited active control. They are designed to accommodate

all combinations of load with no action by the system operator.

Also, as electrical supply and demand must be balanced on a second-by-second

basis, the injection of power from distrib uted generation requires an equivalent

reduction in the output of the large central generators. At present, central genera-

tion provides electrical energy but also ancillary services, such as voltage and

frequency control, reserve and black-start, which are essential for the operation and

stability of the power system. As its use increases, distributed generation will need

to provide the ancillary services that are needed to keep the power system func-

tioning with fewer central generators being operated.

1.2 Value of distributed generation and network pricing

For distributed generation to compete successfully with central generation in a

competitive environment, network pricing arrangements are critically important.

The challenge for realising the value and calculating the impact of generation

connected at different levels in the network is illustrated in Figure 1.3 by the value

chain from power generation to consumption.

Figure 1.3 shows the price of electricity produced by central generation and

sold in the wholesale markets to be around 2–3 p/kWh (the price of wholesale

Introduction 3

electricity). By the time electricity reaches the end consumer, the relative ‘value’ of

electricity has increased, and the price, depending on the voltage level in the net-

work, is now 4–12 p/kWh (the retail price of electricity). This increase in the value

of electricity up to the point of consumption is driven primarily by the added cost of

network transmission and distribution services required to deliver power from

central generators to customers elsewhere in the network.

Distributed generation is located closer to the consumer and has fewer

requirements for the transport services of the transmission and distribution net-

works. In essence, distributed generation is delivering power direct to demand at an

equivalent value of 4–12 p/kWh, i.e. the costs avoided by not using the network.

However, this network cost reduction, generated by the favourable location of some

generators, is not fully recognised within the present commercial and regulatory

framework. As a consequence, non-conventional generation invariably competes

with conventional generation in the wholesale markets at a price (2–3 p/kWh) that

may be significantly lower than the true value of electricity delivered from a

location close to demand.

The same example is also relevant for the treatment of demand. Customers

taking energy from the network at the right location in the network (i.e. close to

generation ) have less requirement for network services. Yet for most consumers, all

the power they receive is priced at the fixed retail rate, occasionally reflecting

differences in time of use but never fluctuating in response to location and the

distance between generation and the consumer.

~8–12 p/kWh

~5–8 p/kWh

~4–5 p/kWh

~2–3 p/kWh

>1 GW

~100 MW

~10 MW

<1 MW

Wholesale

electricity price

Domestic

consumer supply

price

HV/MV

industrial

consumer

supply price

Transmission

LV Distr i b u t i o n

Wholesale

energy market

Generation size

Energy market &

network level

Relative price of wholesale & retail

electricity

Figure 1.3 Value chain for electricity from central generation to LV distribution.

Prices are for relative comparison only and can also be considered to

be in US cents or Euro cents

4 Distributed generation

The full value of generation in these terms will obviously be dependent on a

number of factors, from time of use and location in the system to density of

penetration of similar generation and timing of system peaks with output. In some

instances renewable and distributed generators may cause more costs than benefit,

but the principle of realising the full value of generation still holds. Ignoring these

particular features (time of use and location) results in sub-optimal network

development because the full and true impact of the user on the network is not

represented. Ultimately, this prevents new low-carbon generation (and demand)

from competing with incumbent generation and traditional network solutions to

guide optimal network development, with the result that the system must resort to

increasingly expensive and unnecessary network reinforcement, and sub-optimal

network support solutions.

1.3 SmartGrids

Throughout the world, energy policy is developing rapidly with the aims of pro-

viding electrical energy supplies that are

1. Low or zero-carbon to reduce the production of greenhouse gases and mitigate

climate change.

2. Secure and not dependent on imported fossil fuel.

3. Economic and affordable by industry, commerce and all sections of society.

These objectives of energy policy converge in the use of distributed generation;

renewables and cogeneration (combined heat and power, CHP). Recently, the name

SmartGrids [3] has become common to describe the future power network that will

make extensive use of modern information and communication technologies to

support a flexible, secure and cost-effective de-carbonised electrical power system.

SmartGrids are intelligently controlled active networks that facilitate the integration

of distributed generation into the power system. It is hard to envisage a de-

carbonised electrical power system, supplied from renewable energy sources and

constant output generation (perhaps nuclear and fossil plant with carbon capture and

storage) without greater involvement of the load in its operation. Hence an impor-

tant aspect of the SmartGrid concept is demand side participation.

Demand side participation is an important potential means of increasing flex-

ibility and controllability in the power system. Controllable loads such as charging

electric vehicles and heat pumps with thermal storage will allow increased utili-

sation of renewable energy. More radically, it is anticipated that the battery energy

storage of electric vehicles may be used to inject power into the system at times of

high generation cost or to facilitate islanded operation of distribution networks. The

role of smart metering and how the customers will wish to control their loads and

take part in the operation of the power system is an important topic of investigation

with trials being undertaken in many countries at present.

The European Technology Platform for SmartGrids has published a definitio n

of a SmartGrid [3].

Introduction 5

‘‘A SmartGrid is an electricity network that can intelligently integrate the

actions of all users connected to it – generators, consumers and those that

do both – in order to efficiently deliver sustainable, economic and secure

electricity supplies.

A SmartGrid employs innovative products and services together with

intelligent monitoring, control, communication and self-healing technol-

ogies to:

● better facilitate the connection and operation of generators of all sizes

and technologies;

● allow electricity consumers to play a part in optimising the operation

of the system;

● provide consumers with greater information and choice of supply;

● significantly reduce the environmental impact of the total electricity

supply system and

● deliver enhanced levels of reliability and security of supply.’’

Smart meters are an important element of a SmartGrid as they have the potential to

provide much greater visibility of network power flows and voltages, particularly

on low voltage networks where the number of measurements is presently very

limited. Details of how the SmartGrids are to be implemented have still to be

worked out and will probably vary from country to country. However, there is little

doubt of the important consequences of the SmartGrid concept for distributed

generators of all types and ratings [4].

1.4 Reasons for distributed generation

The conventional arrangement of a modern large power system (illustrated in

Figure 1.1) has a number of advantages. Large generating units can be made effi-

cient and operated with only a relatively small staff. The interconnected high vol-

tage transmission network allows the most efficient generating plant to be

dispatched at any time, bulk power to be transported large distances with limited

electrical losses and generation reserve to be minimised. The distribution networks

can be designed simply for uni-directional flows of power and sized to accom-

modate customer loads only.

However, particularly in response to climate change, many governments have

set ambitious targets to increase the use of renewable energy and to reduce green-

house gas emissions from electricity generation. Examples of these policy initia-

tives include the 2007 European Union requirement to provide 20% of all energy

used in Europe from renewable sources by 2020 and the California Renewable

Portfolio Standard that calls for 33% of electr ical energy to be from renewables

by the same year. More radically, many climate scientists and policymakers in

developed countries consider that an 80% reduction in greenhouse gas emissions by

2050 is necessary if average global temperature rises of more than 2



C are to be

avoided.

6 Distributed generation