CHALMERS UNIVERSITY OF TECHNOLOGY
A Study of Power
Distribution to Cities
Technical Communication Course
Group A3: Engström Peter, Fredholm Andreas, Göransson Martin, Göthe Hampus
2010-11-08
Figure 1: Picture of the power grid that supplies the cities of the island with power.
A Study of Power Distribution to Cities
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1
2
3
CONTENT
Introduction ........................................................................................................................ 4
2.1
Background .................................................................................................................. 4
2.2
Problem definition ....................................................................................................... 4
2.2.1
Power distribution ................................................................................................ 4
2.2.2
AC/DC ................................................................................................................... 5
2.2.3
Environmental considerations ............................................................................. 5
2.2.4
Grid components .................................................................................................. 5
2.3
Project scope ............................................................................................................... 5
2.4
Report structure .......................................................................................................... 5
Methods and materials ....................................................................................................... 6
3.1
4
Results ................................................................................................................................. 6
4.1
5
Approach...................................................................................................................... 6
HVAC/HVDC comparison ............................................................................................. 7
4.1.1
History/Introduction ............................................................................................ 7
4.1.2
Advantages and disadvantages of HVDC compared to HVAC transmission ........ 7
4.1.3
Economical aspect regarding HVAC/HVDC .......................................................... 8
4.1.4
Environmental and Health concerns regarding HVAC/HVDC .............................. 8
4.2
Voltage levels ............................................................................................................... 9
4.3
Cable dimensions ....................................................................................................... 10
4.4
Grid components ....................................................................................................... 10
4.4.1
Transmission lines .............................................................................................. 10
4.4.2
Transformers ...................................................................................................... 11
4.4.3
Short circuit breakers ......................................................................................... 11
4.4.4
Lightning protection ........................................................................................... 11
4.4.5
High-Voltage breaker ......................................................................................... 11
4.4.6
Smart grids ......................................................................................................... 11
Discussion.......................................................................................................................... 13
5.1
AC/DC......................................................................................................................... 13
5.2
Voltage levels ............................................................................................................. 13
5.3
Environmental considerations ................................................................................... 13
The Forest ......................................................................................................................... 14
A Study of Power Distribution to Cities
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6
Conclusion ......................................................................................................................... 15
6.1
7
Further study ............................................................................................................. 15
References ........................................................................... Error! Bookmark not defined.
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A Study of Power Distribution to Cities
INTRODUCTION
In the modern western society, the
generation and distribution of electric
power is essential for the daily life. As
populations increase, so must the electric
power grid as an important part of every
developed infrastructure.
The intention of this report is to suggest a
model for a specific part of an electric
power grid, located on an island. The
motivation is to develop a model which
optimizes distribution efficiency and at the
same time minimizes cost and
environmental impact.
2.1
BACKGROUND
Electric power is usually distributed with
help of overhead transmission lines which
are attached to electricity pylons.
Underground cables are used as well, even
though their use is very limited due to the
considerable higher cost.
From the power generation facilities, the
power is distributed along high voltage
lines, reaching up to 800 kV depending on
country. Eventually, the power will be
transformed down to a usable voltage
level which in European homes is 230 V.
For a safe maintenance and long life span,
the grid is equipped with devices, serving
different functions like lightning- and short
circuit protection.
2.2
PROBLEM DEFINITION
The problem is to supply efficient
electrical distribution to City B and City E
through lines 6 and 4 as shown in figure 1
and figure 2. 75% of the power derives
from renewable sources and the
remaining 25% is nuclear power.
Figure 2: Zoomed in section of figure 1.
The task is to determine voltage levels,
power lines/cables etc. so the cities'
power requirements are satisfied, at the
same time considering economical and
environmental aspects.
2.2.1 POWER DISTRIBUTION
The major issue is to supply the cities B
and E with the power they require, using
part 4 and 6 of the power grid. Part 4 is
playing the role as main power supplier
while part 6 acts as a backup, should part
4 malfunction.
To supply city B and E the power lines
need to be able to transmit at least 430
MW.
To achieve the desirable power levels the
transmission lines need to be sized and
built with respect to a lot of different
aspects.
Which voltage level is going to be used for
the most effective transmission,
considering inductance, resistance and
capacitance?
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A Study of Power Distribution to Cities
When the voltage requirement is known,
the dimensions of the cables have to be
determined in order to minimize losses
due to resistance. Higher voltage levels
give less power loss due to the relation in
Ohm's law, meaning higher voltage equal
less ampere.
2.2.2 AC/DC
Use of AC or DC in the grid is a major
question that needs to be answered. The
advantages and disadvantages of both
types of currents must be considered
when calculating operation and cost
efficiency. In other words, when is the
respective type of transmission
preferable?
2.2.3 ENVIRONMENTAL
CONSIDERATIONS
The power transmission must be adapted
to the environment it is passing through,
which makes it important to determine
the use of either overhead power lines or
regular cables. From point E to B there are
no obstacles in specific. From city B to C
there is a lake, 6 km in width, where the
transmission line is supposed to cross.
From city A to B a dense forest poses an
obstacle for the lines and a solution needs
to be found.
What is the most efficient way to deal
with these environmental obstacles? Is it
necessary to compromise cost for less
environmental impact?
2.2.4 GRID COMPONENTS
This is in a way connected to the previous
question. Considering environmental
aspects is essential in the decision of grid
components.
When is cable or overhead power lines
most desirable considering economical
and environmental aspects?
What other components are needed for
the grid´s maintenance and function?
Which types of transformers are going to
be used?
What kind of lightning protection and
voltage breakers are needed?
2.3
PROJECT SCOPE
The scope of this project is too limited to
the transmission lines 4 and 6. No
considerations have been made
concerning power generation and
distribution to other cities than B and E.
The construction and maintenance of the
transmission line towers will not be
discussed as well.
2.4
REPORT STRUCTURE
Initially, the approach taken to deal with
the problem in question is introduced
(section 2). The report then continues with
a presentation of the results acquired
(section 3) and a discussion regarding
them (section 4). At last, the conclusion
will be exposed (section 5), presenting a
solution of the problem regarded by this
report.
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A Study of Power Distribution to Cities
3
METHODS AND MATERIALS
3.1
APPROACH
In order to solve the problem of power
distribution on the island, the first step
taken was to study the facts and principles
of electrical power engineering. A search
commenced for appropriate information,
mainly on the Internet, but literature
recommended from the teachers was
provided as well. When enough
information was found it was read and
compiled.
For the first draft the text was not
constructed as a scientific report but
instead the facts and information acquired
were presented in a list form. The purpose
of the first draft was to display the group’s
progress for the course teachers, to
display the amount of information
acquired and thus there was no need of a
scientific form.
After receiving the teachers critique
concerning the first draft, the group began
reconstructing the text to fit a more
formal context. It was adapted after the
teachers’ suggestions, and more
information was found to fill in the gaps in
the first draft. Until then, the report had
been written with the different aspects of
the problem regarding power transmission
as just a body, with every aspect under its
own headline. Now, the assignment was
to divide the bodies and transfer the
different parts to their appropriate
headline. E.g. there was a headline named
"Transmission lines" covering the results,
discussions and conclusions regarding the
subject. The headline´s text was divided
and the different parts were put under the
major headlines "Results", "Discussion",
"Conclusion" etc.
The last two weeks, before the hand in of
the second draft, was totally devoted to
create a report written in a proper
scientific form. The handouts received
from classes regarding the framework of a
report were studied and pursued to follow
their recommendation.
A few problems, like transmission line
resistance, required some simple
calculations for a reach of conclusion.
Those calculations were almost the last to
be made, using some basic formulas,
mainly because it was found easier to
perform them when it was known exactly
what had to be calculated.
The last effort was spent making a layout.
The different fonts, reference systems and
table of contents of the example reports
handed out were studied and finally the
group tried to create something equally
competent.
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A Study of Power Distribution to Cities
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RESULTS
DC to AC for commercial and industrial
use. AC has the advantage that it does not
need any converting.
4.1
HVAC/HVDC COMPARISON
There is several different ways to
configure a DC system, from simple backto-back to the more commonly used
bipolar system where two converter
stations are connected by bipolar
conductors with its own ground. There is
also a system called multi-terminal HVDC
transmission where you have more than
two stations that you either connect in
series or parallel. (1) (2)
4.1.1 HISTORY/INTRODUCTION
The first generator ever made was a DC
generator; hence the first power
transmission line used DC. Although DC
was superior to AC in almost every aspect,
DC lost the battle to AC. Mainly because
transporting AC was easier and the voltage
levels were easier to transform. When the
motor was introduced in industries the DC
had no chance since motors only work
with AC. Hence, the commercial users had
to adapt to the industries. This is the main
reason for AC dominance over DC
although DC is still better than AC when it
comes to several aspects due to its
economical, technical and environmental
advantages. Although with a very high
station cost DC is rarely used at shorter
distances.
Of these two, AC transmission systems is
the most commonly used system, but DC
systems are sometimes preferred over AC,
especially at longer distances where AC
have problems. For example, resistance in
conductors is much higher AC systems
compared to DC. This is because of the
"skin effect" that AC causes. Basically this
means that the density of the conductor is
greater on the surface then at the core.
Therefore the current tends to flow in the
"skin" causing the effective resistance to
go up.
When using High voltage DC transmission
systems there is three basic parts to
consider. The first is a converter station to
convert the AC to DC for transportation.
Secondly, the transmission lines, third, a
second converter to once again convert
4.1.2 ADVANTAGES AND
DISADVANTAGES OF HVDC
COMPARED TO HVAC
TRANSMISSION
The following list of advantages and
disadvantages is taken from (1).
Advantages











Greater power per conductor.
Simpler line construction.
Ground return can be used. Hence
each conductor can be operated as
an independent circuit.
No charging current.
No Skin effect.
Cables can be worked at a higher
voltage gradient.
Line power factor is always unity:
line does not require reactive
compensation.
Less corona loss and radio
interference, especially in foul
weather, for a certain conductor
diameter and rms voltage.
Synchronous operation is not
required.
Hence distance is not limited by
stability.
May interconnect A.C systems of
different frequencies.
A Study of Power Distribution to Cities
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


Low short-circuit current on D.C
line.
Does not contribute to short-circuit
current of an A.C system.
Tie-line power is easily controlled.
Disadvantages




Converters are expensive.
Converters require much reactive
power.
Converters generate harmonic,
require filters.
Multiterminal or network
operation is not easy.
4.1.3 ECONOMICAL ASPECT REGARDING
HVAC/HVDC
When choosing HVDC or HVAC it is very
important to factor in the economical
aspect since they are different between
the different transmission methods. In
order to compare cost between AC/DC all
costs must be compared, main system
elements, capital cost, labor, converter
terminals, filters and so on.
The main difference is between HVAC and
HVDC is the HVDC station, it costs more
than 4 times than the station of a HVAC
system. This is a big deal when electricity
is transmitted shorter distances rather
than long. Although with HVDC being
more efficient when transferring
electricity there is a break-even distance.
There is also cheaper to build transmission
lines for HVDC because you only need two
conductors instead of three conductors
due to the three phase system that HVAC
uses. The break-even distance for
overhead power lines occurs between 500
km to 800 km as illustrated below (1). (2)
Figure 3 This graph illustrates the break-even point for costs
when using HVAC and HVDC transmission. (1)
4.1.4 ENVIRONMENTAL AND HEALTH
CONCERNS REGARDING
HVAC/HVDC
RADIO, TV AND TELEPHONE
INTERFERENCE
There are several components and
phenomena that cause interference in
different frequency spectrums in power
lines. Transient over voltages, corona
discharges and converters etc. That makes
it important to use electromagnetic
shielding of the valve halls.
For a HVDC transmission lines this value is
about 40 dB (μV/m) for 0.5 MHz, 300
meters from the conductor, compared
with HVAC values of 50 db (μV/m). In this
aspect HVDC is also better than HVAC. (1)
CORONA DISCHARGES
Corona discharges occurs when a fluid on
a conductor ionizes because the strength
of an electrical field have exceeded a
certain point, and is not strong enough to
cause complete electrical discharge or
arcing. This process creates noise and
produces Ozone that is a dangerous gas
for people and the environment.
This is a problem for both HVAC and HVDC
systems because of the already high
pollution of ozone cities have. Ozone
A Study of Power Distribution to Cities
9
naturally has a concentration of
approximately 50 ppb (parts per billion) in
clean air, while in cities this value may
reach 150 ppb. When the value reaches
about 180-200 ppb it becomes health risk
for people, so it is important to keep these
values down. (1) (3)
TRANSIENT OVER VOLTAGES
Transient over voltages is a problem,
basically it is a peak voltage that can reach
up to three times the crest voltage in the
conductor for HVAC systems but only 1.7
times for HVDC. Voltage peaks is very
dangerous for machines and devices so it
is crucial that they are correctly handled,
so they do not cause any damage. (1) (4)
ELECTRIC FIELDS
EMF is not proven to be dangerous but
still is a sensitive topic and not to be taken
lightly. There is ongoing research and
much public debate on how this affects
humans.
The voltage levels in lines 4 and 6 depend
on how much power city B and E need.
The cities need 430 MW to operate at the
moment but to be able to develop the
lines must be able to transmit
approximately 500 MW. As seen in table 3
the voltage required to reach 500 MW is
between 345 kV and 500 kV. Therefore
400 kV will be used to achieve this in both
line 4 and 6. 400 kV power lines are used
in many countries power grid, including
the Swedish grid. (5)
To calculate the current level in the power
lines the equation
is used.
The losses in the lines can easily be
calculated by a similar equation:
.
The equations above display that the use
of a higher voltage level decreases the
current level, resulting in a lower power
loss. Using an even higher voltage will
increase the cost to construct and
maintain the cables, which will be
unfavorable in an economical aspect.
Everything that is conductive and gets
electrically charged emits a EMF field.
Potential effects of EMF vary widely on
Intensity and frequency. And since power
lines is a big source of EMF, it is something
that should be taken into consideration. In
this aspect HVDC is superior due to the
fact that it does not use steady-statedisplacement that HVAC uses. (1) (2)
= short-circuit current [kA] during
time
.
= short-circuit current rating during
1 second. See the 1 second value in
Table 1 for the conductor and in
Table 2 for the metal screen.
NOISE
A transformer is the main source of noise.
The converter transformers that is a must
for HVDC operation is approximately 10 to
20 dB higher than regular transformers. In
this aspect, noise is the only limiting factor
for future and existing HVDC lines. (1) (2)
4.2
VOLTAGE LEVELS
= short-circuit duration (sec)
(6) (7)
A Study of Power Distribution to Cities
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Table 1 (relevant parts from Table 14 (6).
Max short-circuit current on the screen
during 1 s, kA.
lighter and cost approximately three times
less than copper. (8)
Table 3 Some common voltage levels and their natural load.
Aluminium
conductor
Aluminium
conductor
mm
65°C
90°C
1600
166
151
per mm2
0.104
0.0945
Cross section
Voltage
132
230
345
500
700
43
130
300
830
1600
(kV)
Natural
load
(MW)
Table taken from (1)
Table 4 Resistivity and density of some metals.
Material
Table 2 (relevant parts from Table 15 (6).
(20°C)
Max. short-circuit current on the screen
during 1 s, kA.
Metallic
Metallic
screen
screen cross
temperature
section, mm
before the
short-circuit
Metallic
screen
temperature
before the
short-circuit
Copper
screen
50°C
70°C
95
16
15
per mm2 Cu
0.165
0.153
4.3
The cable dimension depends on which
material is used as the conductor in the
power lines. The most common
conductors used are aluminum and
copper. According to table 4, silver has the
better resistivity than both aluminum and
copper but it is also heavier and more
expensive, and is therefore not used.
Aluminum is approximately three times
Density
Kg/m3
(20°C)
Silver
1,59·10-8
10 500
Copper
1,6730·10-8
8 960
Aluminum
2,6548·10-8
2 698
Iron
9,71·10-8
7 874
Information in table 4 is taken from (9)
4.4
CABLE DIMENSIONS
Resistivity
Ohm/m
GRID COMPONENTS
4.4.1 TRANSMISSION LINES
There are two main types of electrical
transmission lines, overhead power lines
and ground cables. The clearly dominant
type is the overhead power line, and one
of the major reasons is economical since
the cost for cables is 15 to 45 times the
cost for overhead power lines. However,
ground cables may be preferable to use on
a few occasions. In vicinity of cities, cables
might be used on an esthetic base and
when crossing larger water masses there
are no viable options other than using
cables.
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A Study of Power Distribution to Cities
The cross-section dimension of the lines
depends on the voltage and current they
are required to withstand. The
transmission lines dealt with in this report
are required to endure 1250 A and 400 kV.
(10)
4.4.2 TRANSFORMERS
Transformers are an absolute necessity for
the use of electrical power in homes and
industry. They transform the electricity
from the transmission lines to a higher or
lower voltage, so that it can be used for
general usage. For example, you use
several transformer stations to lower the
voltage in the transmission lines coming
from the power plant. In Sweden, the
voltage is transformed down from the
hundreds of kV in the transmission lines to
the 230 Volts used in homes. (11)
4.4.3 SHORT CIRCUIT BREAKERS
Short circuit breakers are essential in a
power transmission net. They are used for
protection against short circuits and
power surges. When they occur, the
breakers task is to break the connection
between the power lines. When breaking
the lines, it sometimes occurs that the
current continues to flow in an arc
between the breaking points. The arcs
temperature can exceed 50,000 °C and
obviously, it is lethal for anyone coming
close. If there were no short circuit
breakers in the net, then, if a short circuit
would occur, it would result in massive
currents, with potential to destroy the
lines, stations and transformers (12) (13)
overhead shield wires and surge arresters.
Overhead shield wires are the main
protection for stations and for overhead
transmission lines in areas with high
lightning activity. They are wires usually
placed above the phase conductors. The
backside with shield wires are the almost
10 percent higher line construction cost
and energy waste due to induction from
the power lines. They are quite effective
though, offering a 95% protection rate
against lightning strikes.
Surge arresters are constructions
connected across insulators at high risk
locations along a transmission line, e.g.
towers etc. Their function is to minimize
power interruption due to lightning
strikes. Early models were made of
porcelain and were very heavy. They were
hard to mount on power-lines and towers
and there was a risk for falling porcelain.
But nowadays, when they have gotten
lighter and smaller, they are a viable
option to overhead shield wires. They
have a very high rate of success and do
not induce voltage, thus saving
energy. (14)
4.4.5 HIGH-VOLTAGE BREAKER
High-Voltage breakers are used as a kind
of on and off switch to isolate high voltage
electrical equipment. This is mainly for the
purpose of maintenance, so that personal
safely can access power lines and
equipment. It´s breaking procedure works
similarly to the short circuit breaker, but is
controlled manually, while the short
circuit breaker breaks automatically. (12)
(13)
4.4.4 LIGHTNING PROTECTION
4.4.6 SMART GRIDS
There are mainly two types of lightning
protection for wires and power stations,
The main problem with electricity is that it
cannot be stored efficiently. Hence, all
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A Study of Power Distribution to Cities
electricity must be used at the same time
as it is produced. This has as a
consequence, that the energy production
must be generated after energy
consumption assumptions. Sometimes this
may not be in correlation with the actual
consumption, leading to either energy
wastage or a lack of energy. So, to manage
this problem, there has been a
development of smart grids, which
function is as a kind of regulating terminal.
Sensors are placed at key locations to feed
the system with data of consumption and
production of electricity. This makes it
possible to regulate the in- and output
from the power stations. Smart grid
systems also give the opportunity to
prioritize "green" energy over fossil and
nuclear energy. For instance, when
windmills are producing energy the grid
always uses that "green" energy instead of
energy produced from less green sources
like fossil fueled power plants.
Compared to the presently used
transmission conductors, the smart grid is
far more advanced. Today power
transmission is a one-way transport from
the power plants to transmission lines
and, lastly, the consumers. The smart grid
system relies mostly on reusable power
sources, which quite often are
geographically spread out. This makes it
necessary for the smart grid to be able to
direct the transmission to the consumers.
The reliability of "green" sources can be
seen as a disadvantage. When the grids
are overloaded, it might be necessary for
the consumers to cut down on their
consumption, creating interruption for
people watching TV, working on
computers etc. (15) (16) (17)
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A Study of Power Distribution to Cities
5
DISCUSSION
5.1
AC/DC
AC offers a better rate of efficiency than
DC and is thus economically favorable. DC
is a more efficient way of power
transmission during normal conditions
only at distances greater than 600 km.
Since the longest distance to consider,
part 6 of the grid, only is 160 km, AC
would be the most favorable choice. DC
has an advantage over AC considering
underwater transmission performance,
especially over distances longer than 30
km. This is an interesting property, since a
lake is crossing part 6 of the transmission
grid. The said lake is only 6 km in width,
which makes the transformation from AC
to DC and back again less cost effective
than using AC all the way.
5.2
VOLTAGE LEVELS
Transmission line 4 and 6 must each be
able to transfer 500 MW. A higher voltage
level produces a higher level of efficiency,
thus saving money. We are going to follow
the Swedish standard in voltage levels for
the net backbone, using 400 kV over the
transmission lines connecting the cities.
5.3
TRANSMISSION LINES
Considering the higher cost of
underground cables, the main part of the
power distribution is going to be through
overhead power lines. On two occasions,
across the lake and in the vicinity of the
cities, are cables going to be used. The
lake, 6 km in width, makes it impossible to
use overhead power lines, and they are
very impractical and ugly to use in a city.
The esthetic aspect has been found more
important than cost in this case.
Insulated wires are going to be used when
the transmission lines are built through
the dense forest. This is to avoid short
circuit from falling branches and debris.
The material used for the conductor is
aluminum, since it is cheaper and lighter
then copper.
The cable drawn across the lake is going to
be a XLPE-type cable, since the material
has many advantages.
It is maintenance-free, has low electrical
losses and is environmentally friendly.
5.4
ENVIRONMENTAL
CONSIDERATIONS
Concerning the lake, the alternatives are
to build an underwater power line or to
build overhead power lines around the
lake. The forest poses other difficulties but
have similar solutions. You can either build
transmission lines around the forest or
clear a path through it. The added cost of
clearing a path large enough to construct
an overhead power line would be less
than building around the forest.
THE LAKE
While DC is normally preferred for
submarine cables, the distance does not
make it economically worthwhile in our
situation due to transformation costs. The
distance across the lake we have to take in
consideration is very manageable for AC,
which is a major concern when using AC
for submarine cables. (18)
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A Study of Power Distribution to Cities
THE FOREST
In order to create a clear path through the
forest for the power lines there is a
number of issues that needs to be
addressed. The first concern that needs to
be addressed is the width of clearing
around the power lines for a reliable
transmission. Another important issue is
the maintenance of the area around the
power-lines, there should never be any
trees tall enough to fall on top of the
power-lines to break them or create a
short circuit. There should be no tree
vegetation around the power line area and
about 6 to 10 meters around it. An
additional area of about 20 meters should
be pruned to create a less dense area to
make it more reliable and easier to
maintain. When pruning said area, pine
trees should be saved over leaf trees
because of the fact that pine trees are less
damaging to the power-lines. If the
pruning has been done properly there
should be little concern for another major
action to the pruned area for about 2
decades. While the owner of the forest is
responsible for the maintenance, free help
is available from the energy distributor in
order to assure a safe felling of the trees
and to prevent the distribution form
failing. (7)
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A Study of Power Distribution to Cities
CONCLUSION
Considering the purpose of this report, to
optimize the power distribution with
minimal environmental impact and cost,
some conclusions can be made.
It is preferable to use AC rather than DC,
in both an economical and distribution
efficient view. Due to the larger cost, use
of underground cable is limited to just a
few occasions; at the bottom of the lake
part 6 of the grid is crossing and in the
vicinity of the cities. The voltage of the
transmission lines has been chosen to 400
kV, according to Swedish standard values.
This voltage level is ideal for the power
lines in question as it decreases power loss
compared to lower voltages, deliver the
power the cities require and at the same
time leaves room for a future increase of
load as the population expands.
The environmental impact derived from
the transmission lines should not pose a
problem. The disforestation and building
of a power line through the forest could
be a concern, but it has to be remembered
that the area affected is limited and the
impact on animal life is acceptable.
The cable dimension is set to be 1600
mm2, according to the document “XLPE
Land Cable Systems” (6)
The inductance for the cables used has
been estimated to 0.38 mH/km and the
capacitance has been estimated to
0.20µF/km according to the XLPE Land
Cable Systems document (6).
Even though expensive, some grid
components are essential. Thus, the
transmission lines will be equipped with
short circuit breakers, high voltage
breakers, transformers in the vicinity of
the cities and overhead shield wires.
6.1
FURTHER STUDY
Some aspects not dealt with in this report
could be the target for further study. The
building of transmission line towers is such
an aspect. The generation of
electromagnetic fields is another topic
worth exploring, especially the fields
health impact on humans and wildlife.
16
7
A Study of Power Distribution to Cities
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A Study of Power Distribution to Cities
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