Download Power transmission and distribution - A5

CHALMERS
Power transmission and
distribution
Fredrik Johansson
Jonatan Kornhill
Martin Karlsson
Alexander Kazen
08-10-2010
A study on high power transmission from power plant to consumer
Abstract
The power grid on the given island is to be completely rebuilt , and this is a report on how the new
system is going to be built. The project presented in this report is limited as it is focused on the
specific stretches from the two power plants on the island to a given city A. However, since these
stretches are to supply the entire island with electricity factors such as the power consumption of the
island's cities together have been taken into consideration.
When dealing with these kinds of systems, some typical questions arise, such as whether to use AC or
DC, what voltage levels are appropriate, whether to use overhead or underground transmission. If
overhead lines are to be used, answer how to protect them from for instance lightning strokes. Other
factors to take into consideration are the opinions of people around and of course the costs of building
and maintaining the system.
To acquire the information necessary to answer these questions a wide variety of sources such as
books, technical articles, databases and so on have been studied. The information gathered have been
enough to make further calculations from and decide how the system is to be built.
In short, the transmission of power from the plants to the city will occur through airborne power lines
carrying a three-phase alternating current at a voltage of 400kV.
Table of content:
1. Introduction _______________________________________________________________________________ 2
1.1 Project conditions __________________________________________________________ 2
1.2 Method _______________________________________________________________________ 3
2. Results ________________________________________________________________________________________ 3
2.1 AC/DC _________________________________________________________________________ 3
2.1.1 Advantages and disadvantages of AC/DC ________________________ 3
2.1.2 Practical use of high voltage transmission ______________________ 3
2.1.3 The use of three-phase AC transmission _________________________ 4
2.2 Voltage levels ________________________________________________________________ 5
2.3 Overhead- or underground transmission ______________________________ 5
2.3.1 Costs: time, materials and specialized labor ____________________ 5
2.3.2 Public opinion and impact on the environment ________________ 5
2.3.3 Reliability: power outages and repair____________________________ 5
2.3.4 How to protect overhead lines from lightning strokes ________ 5
2.3.5 Summary overhead- or underground transmission ___________ 6
2.4 Components ________________________________________________________________________________ 9
2.4.1 Protective relay ____________________________________________________ 10
2.4.2 Transformer ________________________________________________________ 10
2.4.3 Short circuit breakers ____________________________________________ 10
2.4.4 Shield wires and surge arresters________________________________ 10
2.5 Cables and calculations __________________________________________________________________ 6
2.5.1 Corona Discharge ____________________________________________________ 6
2.5.1.1 The concept of corona discharge _______________________________ 6
2.5.1.2 The use of bundle conductors ___________________________________ 7
2.5.2 Calculations___________________________________________________________ 7
3. Conclusion _________________________________________________________________________________ 11
4. References _________________________________________________________________________________ 11
Introduction
In modern society, there is a constant need for energy to supply our industries and all of our electrical
devices. As cities develop and increase in size and technology, so does the demand for electricity.
This everyday increasing demand requires a functional distribution network that can transmit power in
an optimal way from source to customer over long distances, without major power losses.
The purpose of this project is to model a connection between a city and two power generation plants
on a fictional island and to do this, several factors needs to be taken into account.
The economical aspects are key to decision making when designing a network grid but other factors
such as environmental impact, health concerns and objects in the landscape has to be taken into
consideration as well.
This report will present issues when transmitting high power over long distances and how to solve
them with adequate components and equipment. The main focus will be on the stretches given in the
project conditions.
It will also contain a discussion about AC and DC transmission and which is the most suitable for
different situations and a discussion about the advantages of overhead lines over buried underground
cables.
Project conditions
The stretch from the nuclear/fossil power generation to city A in Figure (1) is 250km long and its path
is partially blocked by a dense forest. It is to be able to transmit power that can supply the entire
island, equal to 1170 MW(peak value).
Figure 1 Map of the island
The other stretch is a 400km long way partially blocked by a mountain and going through grassland
with plenty of farms. This stretch is to deliver power from the environmental friendly power
generation area and needs to be able to support 75% of the islands power consumption.
Method
To obtain an overview of the subject 'transmission and distribution' literature and a variety of different
homepages was used, these regarding similar cases to our specific problems and conditions. In order to
choose cables and estimate the dimensions of these, and what the resistance/capacitance/inductance
values they will have, some simple calculations were done.
2. Results
2.1 AC/DC
When transmitting power from power generation to consumer, a choice must be made whether to use
Alternating current or Direct current, both of whom has their advantages and disadvantages.
2.1.1 Advantages and disadvantages of AC/DC
HVDC, or High Voltage Direct Current, have many advantages where mainly one is relevant to this
project, which is its superior efficiency. DC transmission suffers from lower power losses than AC and
over long stretches the difference can be quite large.
According to Siemens' article on “Ultra HVDC Transmission System” the losses are lower than 3%
per 1000 kilometers “as a basic rule of thumb”. A main reason for this is that when using DC the
capacitance of the cables don’t affect the transmission in the same way. When using AC, the current
alternates all the time, meaning the generators has to provide extra power just to charge the
capacitance of the cables, which leads to losses. This also leads to cable length being limited while
using AC, since longer cables means higher capacitance. Another reason for this is that there is no
“skin effect” while using DC. The skin effect occurs in a conductor with an alternating current;
the current will distribute itself more to the skin of the conductor and less to the core, thus much of the
conductor carries little current. This causes the effective resistance in the conductor to increase with
the frequency of the current.
Despite this, AC transmission is the mos commonly used method. The main advantage of this method
is that it is very convenient to transform the voltage. The equipment needed to transform the voltage
from the high voltage used for the transmission down to the low voltage needed in households and
industry is cheap and well-tested.
Another advantage is the high reliability. HVDC have about 97% to 98% availability of full capacity
of the power according to ABB. Also, since AC is a more common way to transmit high power, its
parts are cheap and have high availability. The system in itself is also cheaper than a HVDC system.
With lines that are to supply the whole island with power, a downtime as high as 2-3% is not
acceptable. A worst case scenario has to be considered; that only one of the power plants work and the
other line has to power the island by itself. Then, a fault at this line would be devastating and not
acceptable.
Also, Areva T&D states that HVDC is economically beneficial “when distance between the converter
stations exceeds 600 km for an overhead line”, which is not the case at this island.
Weighing the pros against the cons, it is clear that AC transmission is superior to DC transmission
under the specified conditions for this report.
2.1.2 Practical use of high voltage transmission
An article in Wikipedia [xx] states that to reduce the power losses naturally occurring when
transmitting bulk power, high voltages over the cables is ideal as it leads to significantly more efficient
power transmission.
Assuming there is no phase difference, the power transmitted is equal to the product of the current and
the voltage, described by (1)
𝑃(𝑡) = 𝐼 × 𝑉
(1)
and the power losses in a conductor are a product of the square of the current and the resistance of the
conductor, described by (2)
𝑃(𝐿) = 𝐼 2𝑅
(2)
If the current is doubled there would be a four times greater power loss but with a lower current and
increased voltage, the same amount of power can be transmitted. Also, a lower current means the wire
size can be smaller, which is more economically practical, not only for the wire but the whole
transmission structure. The affects of this lead to why it is advantageous to increase the AC voltage to
higher levels when transmitting power of large amounts.
There are however disadvantages of high voltage usage, mainly the increased insulation requirement.
In power plants lower, more convenient voltage levels are generated, only to be transformed to higher
voltage when transmitting over stretches between plant and consumer, using transformers. See section
2.5 for more detailed information on transformers.
2.1.3 The use of three-phase AC transmission
According to a Wikipedia article [xx three phase electric power] using three-phase system as
transmission method is the most common method of AC electric power transmission. This because it
is in general more economical than an equivalent single- or two phase circuit with the same voltage as
it uses less conductor material.
In a three-phase system, three conductors carrying one AC current each, all of the same frequency (50
or 60 Hz is standard and varies between countries). The currents instantaneous peak values are reached
at different times because of a 120 degree phase shift. This delay between phases enables a constant
power transfer over each complete cycle of the current and also makes it possible to produce a rotating
magnetic field in an electric motor; convenient in many industries.
A three-phase system may have a neutral wire, which allows a system with high voltage still being
able to support single-phase appliances with a lower voltage. Though, it is not common to use a
neutral wire in high voltage distribution situations as the load can be connected between phases.
Properties of three-phase that makes it desirable in electric power systems:


In the case of a linear load, the phase currents tend to cancel out one another, summing to
zero, and thus making it possible to reduce or even eliminate the size of the neutral conductor:
for a balanced load, the currents carried by the phase conductors are all equivalent in size
Power transfer into a balanced load is constant, which helps in reducing vibrations of
generators and motors.
Three-phase systems ability to produce a magnetic field, rotating in a specified direction, simplifies
the design of electric motors.
2.2 Voltage levels
Presented in table 2.2.1 below are values on Swedish standards concerning transmission and voltage
levels. From this table we find that the standard value for our grid type is 400 kV.
Transmission grid
Sub-transmission grid
Lower distribution grid
Customer grid
400 kV
130 kV
10 kV
400 V
Table 2.2.1 transmission type and voltage levels according to SS 421 05 01, 3:2002
2.3 Overhead- or underground transmission
When deciding about whether to use overhead or underground transmission lines, we need to
take into consideration cost, power outages and repairs, reliability and environmental impact.
2.3.1 Costs: time, materials and specialized labor
According to data from Xcel energy and Tri-State Overhead lines are much more economical than
underground lines; hence the underground lines need to be rerouted from areas that cause problems.
Some examples of such areas are wetlands, unstable slopes, bedrock and other underground
installations; such as water, gas and sewer lines. Another factor that matters when constructing the
lines are roads, railways and bridges where the overhead lines simply are drawn over the obstacles
while the underground lines need expensive construction when faced with these kinds of problems.
More time also needs to be spent designing the underground lines, because of the factors of
specialized construction mentioned above. All these factors contribute to that the cost of building
underground lines can vary from 4 up 15 times more than overhead lines.
2.3.2 Impact on the environment and on society: public opinion, environmental impact
The public opinion on transmission lines is mostly towards the lines itself and the construction sites
(Eugene Research Institute, 1988). A greater number objects on the overhead lines impact on the
surrounding area, this being that they are visible which is not to public consent. The underground lines
are better in this way according to the public opinion. On the other hand when constructing the
different transmission systems overhead lines are more preferred due to the fact that they take less
time to construct, which brings less noise and dust. One simple reason to this is that the construction of
overhead lines typically requires one or more foundations at every structure location, this locations
being from 200 m to more than 400 m apart. This compared to the underground lines construction,
where a continuous trench is required that is at least 1.5 m wide and 1.5 m deep.
2.3.3 Reliability: Power outages and repairs
The overhead lines are exposed to weather-related failures which lead to more power outages than the
buried underground lines. Lightning protection is discussed in 2.3.4 below. On the other hand it is
easier to pinpoint where the overhead lines are damaged and it is just as easy to repair, the time to
repair an overhead line varies from several hours to several days. When an underground line needs to
be repaired an entire section needs to be excavated first and the repairs are often more complicated.
Something worth notation is also that the overhead lines have twice the life expectancy than the
underground lines.
2.3.4 How to protect overhead lines against lightning strokes
A problem that comes up with the use of overhead transmission lines is that they are exposed to
lightning strokes. To avoid costly repairs and to decrease the chances of an impulse flashover and
failures in the system the lines needs to be protected. The most efficient way of doing this is to use
shield wires. See section 2.5.4. Depending on the amount of lightning strokes in the area of our lines
there are different amounts of protection needed. We assume that the GFD value is the standard value
somewhere between 1-3 GFD, and then we need at least two overhead shield wires to protect the lines
well enough.
We are going to use two shielding wires due to the moderate GFD, and the costs will rise with
approximately 10% due to the extra material and work needed.
2.3.5 Summary of advantages and disadvantages of overhead and underground lines
In tab.1 below, a summary of advantages and disadvantages regarding overhead- and underground
lines is presented. From this table it is clear that overhead transmission lines are the most beneficial.
Table 1 Advantages and disadvantages of overhead and underground lines
Overhead Transmission
+ Shorter power outages
-more frequent outages
+ Much more economical
+ Longer life expectancy
+ Easy to repair
+ Shorter time to repair
+ Lesser construction time, dust and
noise
+ Lesser environmental impact
Underground Transmission
+ Much less frequent power outages
- Longer outages
- Much less economical
- Shorter life expectancy
- Complicated to repair
- Longer time to repair
- More construction time, dust and
noise
- Greater environmental impact
2.4 Components
During the report certain components have been mentioned but not explained, below is information on
what they are and what their purpose are. It is also included other components which are needed in the
transmission system.
2.4.1 Protective relay
In high voltage AC transmission there are some issues that might occur, causing problems to the
power flow, such as over-current, over-voltage, reverse power flow and over- and under- frequency. A
protective relay is an electromechanical device designed to detect faults like the ones mentioned above
on an electrical circuit and trip circuit breakers to prevent further damage or power flow errors on the
system. However, digital protective relays will be used as they precede the older-technology-based
electromechanical relays in precision and convenience in application. Digital relays, or numerical
relays as they are also called, also save capital and maintenance cost over electromechanical relays.
2.4.2 Transformer
One advantage that comes out of using AC is the ease of transforming the voltage. A transformer is an
electrical device used in almost all electrical circuits and is essential in high voltage power
transmission, making long distance transmission economically practical. While they widely range in
design and size the transformers needed for power grids are huge and weight hundreds of tons.
A transformer allows an alternating current voltage to be stepped up or stepped down. It is based on
two principles that allows this: one that an electrical current can produce a magnetic field and two;
electromagnetic induction, which means that changing a magnetic field within a coil of wire induces a
voltage across the ends of the coil.
2.4.3 Short circuit breakers
The security of the lines is very important. When something goes wrong, there must be a system to
minimize the potential damage. A circuit breaker is an automatically-controlled switch that serves to
interrupt fault currents. When a circuit breaker detects overload or short circuit in a circuit it will
interrupt current to protect the circuit and its components from damage. Unlike fuses, which has to be
replaced after interrupting the flow in a circuit, circuit breakers can be reset and resume normal
operation.
Circuit breakers comes in very varying forms and shapes; from small low voltage circuit breakers
commonly used in households to large high voltage circuit breakers which can control the electrical
flow of entire regions.
Also for each phase the circuit breaker that operates on it has an equal number of poles. A circuit
breaker which only operates on one phase is called a “single pole” circuit breaker or “three pole”
circuit breaker if it operates on a three phase system.
High voltage circuit breakers are circuit breakers which handles voltages of 72.5 kV or more.
2.4.4 Shield wires and surge arresters
The use of shield wires was presented section 2.3.4 above; the conclusion was to use shield wires in
combination with surge arresters. A shield wire is shielding wire that runs above the transmission line
and is grounded at every tower. When a lightning strikes on the shield wire the induced surge current
flows through the wire into the structure and through the structure down to the electrical ground. With
this, another problem comes up; flashover. To avoid this surge arresters are needed. These arresters
reduces the structures footing resistance, leading to smaller chances of flashover according to data
collected by Ohio Brass, Northern States Power Co. and Lakeland Electric & Water Utilities (1994).
With a combination of shield wires grounded at every tower and surge arresters mounted at every
structure the transmission lines will be sufficiently protected.
2.5 Cables and calculations
2.5.1 Corona discharge
Grigsby states[xx] even though it is economically practical to transmit power over long distance with
high voltage levels, the overhead lines exposure to atmospheric conditions constantly alters the
conductors surface conditions, causing large variations in the corona activities on the line conductors,
which can lead to discharges. This is causes issues when using three-phase alternating current in
overhead line transmission, known as corona discharges.
2.5.1.1 The concept of corona discharges
A Wikipedia article [xx] explains the concept of corona: A corona discharge is an electrical discharge
brought on by the ionization of a fluid surrounding a conductor, which occurs when the strength of the
electric field or the potential gradient, exceeds a certain value but conditions are insufficient to cause
complete electrical breakdown or arcing.
Rain, dirt or metal protrusions on the conductor increases the local electric field and initiate corona
discharge. The intensity of the discharge further increases under wet weather conditions.
The process of corona is where a current develops from an electrode with high potential in a fluid, in
this case air, ionizing that fluid and creates plasma around the electrode. Eventually the ions generated
will pass charge to nearby areas of lower potential, or form neutral gas molecules by recombining.
This phenomenon is the main source of energy loss on line conductors. Apart from that, it also causes
other unsatisfying conditions:

Audible noise

Electromagnetic interference

Purple glow

Ozone production

Insulation damage
Electromagnetic interference and audible noise along with the energy loss in discharges are the three
main parameters when selecting line conductors. In the early days of electric power transmission
corona power losses were the limiting factor. Later on with the development of EHV (Extra High
Voltage) operating at voltage levels ranging from 300kV to 800kV electromagnetic interferences
became the designing parameters. For UHV (Ultra High Voltage) lines operating at voltages above
800kV, audible noise becomes a larger issue over the other two parameters.
2.5.1.2 The use of bundle conductors
There are different types of corona modes, whom will not be discussed any further in this report as
they are complicated and non-relevant. What is important is that they all cause power loss and needs
to be minimized by adequate control of the surface gradient.
This can be done using bundle conductors, which consist of several conductors connected by nonconducting spacers. This method of line structure reduces the corona loss and in particular its
interference with communication systems caused by electromagnetic interference.
2.5.2 Calculations
Looking at an example in [xx4.4.5 transmissionlines] (unfinished) you see a single-phase equivalent
circuit to a three-phase circuit. The phases are shifted by 0°, 120° and 240° respectively. Figure75 is a
representation of the phase with 0° shift.
This equivalent circuit carries one third of the total power transmitted is supplied by the line-to-neutral
voltage.
In the numerical example [xx4.5 transmissionlines] (unfinished) they’ve used the “bluebird”
conductor, a typical ACSR (Aluminum Cable Steel Reinforced) conductor. Its outer strands are
aluminum, a material with great conductivity, low cost and low weight. Because aluminum will
deform and sag under temperature shifts, the center strand of the conductor is of steel to strengthen the
conductor.
The following calculations are based upon the same equations as in the “bluebird” example, using a
different, yet similar conductor; the Kiwi. The same lattice tower, and values on distances between
phases etc are used. This way it is easy to compare how the different parameters affect the end results
of conductor choice.
Here follow the calculations for stretch 1, the line from a nuclear/fossil power generation:
Line data
The line voltage 𝑽𝑳𝒊𝒏𝒆 = 𝟒𝟎𝟎 𝒌𝑽
The line length
𝑳𝑳𝒊𝒏𝒆 = 𝟐𝟓𝟎 𝒌𝒎
The operating line frequency 𝒇 = 𝟔𝟎 𝑯𝒛
𝝎 = 𝟐𝝅𝒇
The number of conductors in each bundle 𝒏 = 𝟑
The distance between conductors in the bundle 𝒅 = 𝟒𝟓. 𝟕𝟐 𝒄𝒎
The distance between phases 𝑫 = 𝟗. 𝟕𝟔 𝒎
Following data was obtained from the table 4.1[xxtransmission lines](unfinished)
The diameter of the conductor 𝑫𝑴𝑳𝒊𝒏𝒆 = 𝟒. 𝟒 𝒄𝒎
The radius of the conductor 𝒓𝒄 = 𝑫𝑴𝟐𝑳𝒊𝒏𝒆 = 𝟐. 𝟐 𝒄𝒎
The line geometric mean radius (For line inductance calculations) 𝑮𝑴𝑹𝒄 = 𝟏. 𝟕 𝒄𝒎
𝛀
The resistance of the line per miles at 75° C 𝑹𝟕𝟓 = 𝟎. 𝟎𝟓𝟓𝒎𝒊
−𝟗
𝜺𝟎 = 𝟏𝟎𝟑𝟔𝝅
Universal constants
𝑭
𝒎
𝝁𝟎 = 𝟒𝝅 × 𝟏𝟎−𝟕
𝑯
𝒎
The geometric mean radius is given by (3):
𝑮𝑴𝑹 = √𝒅 × 𝑮𝑴𝑹𝒄 = 𝟎. 𝟎𝟖𝟖 𝒎
(3)
The equivalent radius of the three-conductor bundle is given by (4):
𝟑
𝒓𝒆𝒒𝒖 = √𝒅𝟐 × 𝒓𝒄 = 𝟎. 𝟒𝟔 𝐦
(4)
The geometric mean distance(GMD) between the phase conductor is given by (5):
𝟑
𝑮𝑴𝑫 = √𝑫𝟑 × 𝟐 = 𝟏𝟐. 𝟐𝟖𝟗 𝒎
(5)
Line reactance per miles is given by (6):
𝝁𝟎
𝛀
𝑿𝑳 = 𝝎 × 𝟐𝝅
𝐥𝐧 𝑮𝑴𝑫
=𝟎.𝟒𝟑𝟒
𝑮𝑴𝑹
𝒎𝒊
(6)
The total reactance of the line is given by (7):
𝑿𝒍𝒊𝒏𝒆 = 𝑿𝑳 × 𝑳𝑳𝒊𝒏𝒆 = 𝟔𝟕. 𝟒𝟔 𝛀
(7)
The lines resistance of the three-conductor bundle is given by (8):
𝑹𝑳𝒊𝒏𝒆 = 𝑹𝒏𝟕𝟓 ×𝑳𝑳𝒊𝒏𝒆 = 𝟐. 𝟖𝟓 𝛀
(8)
The overall impedance of the line is given by (9):
𝒁𝑳𝒊𝒏𝒆 = 𝑹𝑳𝒊𝒏𝒆 +𝒋𝑿𝑳𝒊𝒏𝒆 = 𝟐. 𝟖𝟓 + 𝒋𝟔𝟕. 𝟒𝟔 𝛀
(9)
The line capacitance-to-ground per miles is given by (10):
𝑪𝑳𝒊𝒏𝒆 =
𝟐𝝅×𝜺𝟎
𝐥𝐧 𝒓𝑮𝑴𝑫
= 𝟏𝟔. 𝟔 𝒏𝑭
𝒎𝒊
(10)
𝒆𝒒𝒖
Peak values of the current are given by (11) at 100% of the total power, divided by three:
𝐏
𝟑
=
𝐕𝐦𝐈𝐦
𝟐
𝐏×𝟐
⇒ 𝐈𝐦 = 𝐕𝐦×𝟑 = 𝟏. 𝟗𝟓 𝐤𝐀
(11)
For stretch 2, the line from an environment friendly power generation, it is safe to assume that the line
has a lower temperature, given that the line stretches over a mountain and on a higher altitude. Also,
the lines have a greater length.
𝛀
Choosing 𝑹𝟓𝟎 = 𝟎. 𝟎𝟓𝟏𝟏𝒎𝒊
from table 4.1[xx transmission lines]
And the having the length 𝑳𝑳𝒊𝒏𝒆 = 𝟒𝟎𝟎 𝒌𝒎
The following results are produced:
Total reactance of the line is given by (7):
𝑿𝒍𝒊𝒏𝒆 = 𝑿𝑳 × 𝑳𝑳𝒊𝒏𝒆 = 𝟏𝟎𝟕. 𝟖𝟕 𝛀
The lines resistance of a the three-conductor bundle is given by (8):
𝑹𝑳𝒊𝒏𝒆 = 𝑹𝒏𝟓𝟎 ×𝑳𝑳𝒊𝒏𝒆 = 𝟒. 𝟐𝟑 𝛀
The overall impedance of the line is given by (9):
𝒁𝑳𝒊𝒏𝒆 = 𝑹𝑳𝒊𝒏𝒆 +𝒋𝑿𝑳𝒊𝒏𝒆 = 𝟒. 𝟐𝟑 + 𝒋𝟏𝟎𝟕. 𝟖𝟕 𝛀
The peak value of the current is given by (11) at 75% of the total power, divided by three:
𝑷×𝟎.𝟕𝟓
𝟑
=
𝑽𝒎𝑰𝒎
𝟐
⇒ 𝑰𝒎 =
𝑷×𝟎.𝟕𝟓×𝟐
𝑽𝒎×𝟑
= 𝟏. 𝟒𝟔 𝒌𝑨
3. Conclusion
In regards to the conditions stated in this report, it’s been concluded that HVAC is the better way to
transmit high power for both stretches in both a financial perspective as well as in availability. HVAC
power transmission is usually done with a three-phase system network.
It is normal to transform voltages up to high levels but to have a lower current when transmitting
large bulk power since it results in lower energy loss and the need for wire size is decreased.
The power is to be transmitted in overhead lines, as they are superior to underground cables in
almost every way, hence why underground cables are mainly used in high populated areas where
there is no possibility for overhead lines to be built.
With the choice of overhead lines followed the need to support them with the adequate components
and to protect them from lightning. For a balanced system with efficient current control there needs
to be protective relays, short circuit breakers and transformers. As lightning protection shield wires
with surge arresters are essential.
4. References
(1)ABB, 2010. HVDC Classic reliability and availability (hitting the headlines article) [online]
Available at:
<http://www.abb.com/industries/ap/db0003db004333/eb8b4075fb41252ec12574aa00409424.aspx>
[Accessed 8 November 2010]
(2)Areva T&D, 2008. HVDC Point-To-Point Inland Only (hitting the headlines article) [online]
Available at: <http://www.areva-td.com/solutions/US_386_HVDC+Point-toPoint+Inland+Only.html> [Accessed 8 November 2010]
(3)Grigsby, Leonard L. Electric Power Generation, Transmission and Distribution .2007. [E-book]
CRC Press. Available through: < http://www.lib.chalmers.se/> [Accessed 8 November 2010]
(4)Heartland Transmission Project, 2009. Underground and Overhead Transmission Lines (hitting the
headline article) [online] Available at:
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Unfinished due to time
Transmission line pdf article from Thomas hammarstöm… something
…
…