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Concepts for the Reduction of Low-Frequency Fields near High-Voltage
Overhead Transmission Lines
Thesis · August 2016
DOI: 10.13140/RG.2.2.28293.55525
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Otto-von-Guericke-University Magdeburg
Faculty of Electrical Engineering and Information Technology
Chair for Electromagnetic Compatibility
Master Thesis
Concepts for the Reduction of Low-Frequency Fields near
High-Voltage Overhead Transmission Lines
submitted: August 25, 2016
by:
Sumit Kumar
Mat.No. 204921
born on October 02, 1990
in Bhojpur, India
Abstract
Although there holds no certain mechanism about the occurence of hazards due to the
impact of electromagnetic fields (EMFs) at low frequencies (50 Hz), the adverse effects on
human health has been scientifically shown. The low frequency fields has an interference
with electrical and electronic equipements in it’s close vicinity. There is a need for the
reduction of the magnetic and electric fields of a high voltage line, when it passes through
the dense populated area or whenever there is a chance of interference with nearby devices.
Recently there is an amendment in 26th Federal Immission Control Ordinance, which has
defined the limiting values not only for the fixed frequencies 16 2/3 Hz and 50 Hz, but also
for the continuous frequency range from DC to 10 MHz. These limits have to be observed
by all low-frequency appliances like high-voltage overhead transmission lines.
Transmission line losses must be reduced so that the power system will be more efficient.
The proposed work compares low reactance phasing where the conductors of the two
circuits, from top to bottom, are arranged in reverse order (abc, c’b’a’) with superbundle
phasing, where conductors of the two circuits are arranged in the same order from top to
bottom (abc, a’b’c).
For both configurations the influence of parameters like line imbalances, line impedances
or conductor losses are analyzed and discussed in detail. For numerical configurations
softwares like Matlab, the ElectroMagnetic Transients Program, OHTLC and ATPDraw
are used. The developed worst-case and best-case configuration are compared in a scaled
laboratory demonstration and the advantages of low reactance phasing over superbundle
phasing has been explained in detail.
i
Declaration by the candidate
I hereby declare that this thesis is my own work and effort and that it has not been
submitted anywhere for any award. Where other sources of information have been used,
they have been marked.
The work has not been presented in the same or a similar form to any other testing
authority and has not been made public.
Magdeburg, August 25, 2016
ii
Acknowledgment
It takes a great pleasure for me to express my sincere gratitude towards Prof. Dr.- Ing.
Ralf Vick for providing me the platform to enhance my technical skills in the field of High
Voltage Alternating Current technology & Electromagnetic Compatibility. I would like to
thanks Dr.-Ing. Mathias Magdowski for his invaluable suggestions, insightful technical
information and guidance during the course of this research study. His dedication &
sincerity in his field has been a great source of inspiration for me. I would like to thanks
personally to Gudiya and Vicky for always motivating me to finish this work in due time.
Last but not the least; I would like to thank my family and friends (Ankit, Sowmya, Rujul
and Pavan), who have been of great support during my work on this thesis.
iii
Contents
1 Introduction
1.1 Effects of low frequency fields . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.1 Effects on humans . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.2 Effects on plants . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.3 Effects on animals . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Exposure limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.1 International commission on non-ionizing radiation protection or
(ICNIRP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.2 Institute of electrical and electronics engineers or (IEEE) . . . . . .
1.2.3 American conference of governmental industrial hygienists or (ACGIH)
1.3 Outline of Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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2 Evolution of electric power transmission
2.1 Early transmission . . . . . . . . . .
2.2 Growth of the transmission system .
2.3 Today’s transmission system . . . . .
2.4 Importance of transmission systems .
2.4.1 Reliability . . . . . . . . . . .
2.4.2 Flexibility . . . . . . . . . . .
2.4.3 Economics . . . . . . . . . . .
2.4.4 Competition . . . . . . . . . .
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3 Softwares
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3.1 EMTP-RV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.2 Matlab . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4 Three phase system
23
4.1 Symmetrical components . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.2 Modal transformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
4.2.1 Frequency dependent transmission lines . . . . . . . . . . . . . . . . 26
5 Overhead transmission lines
28
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
5.1.1 Calculations related to AC transmission . . . . . . . . . . . . . . . 28
1
Contents
5.2
5.3
5.4
High voltage cables . . . . . . . . . . . .
5.2.1 Pressure cables . . . . . . . . . .
5.2.2 Screened cables . . . . . . . . . .
5.2.3 Submarine cables . . . . . . . . .
Overhead HVAC transmission lines . . .
5.3.1 Structures . . . . . . . . . . . . .
5.3.2 Insulators . . . . . . . . . . . . .
5.3.3 Conductor types . . . . . . . . .
Overhead lines vs. buried cables . . . . .
5.4.1 Comparison of HVAC and HVDC
5.4.2 Voltage levels . . . . . . . . . . .
5.4.3 Magnetic fields . . . . . . . . . .
6 Results and discussion
6.1 Matlab results . . . . .
6.2 EMTP results . . . . .
6.2.1 Line impedance
6.3 Experimental results .
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7 Summary
68
Bibliography
69
2
Nomenclature
a,A
scalar, also complex valued
~
~a,A
vector, also complex valued
α
Attenuation constant
β
Phase constant
λ
Flux linkage
q
L/C
Surge impedance
a
Radius of the conductor
B
Magnetic field density
C0
Capacitance per unit length
d
Conductor spacing
D1 , D2 , D3 Distance between conductors
f
Frequency
G0
Conductance per unit length
I
Current enclosed by loop
I 0, I 1, I 2
Sequence currents
I a, I b, I c
Unbalanced currents
k0
Fixed constant
kd
Normalized air density factor
ki
Wire irregularity factor
L
Inductance
L0
Inductance per unit length
r
Distance from the center o the wire
3
Nomenclature
R0
Resistance per unit length
SIL
Surge impedance loading
V 0 , V 1 , V 2 Sequence voltages
V a , V b , V c Unbalanced voltages
V0
Line voltage to neutral
ZC
Characteristic impedance
Zs
Series impedance
4
List of Acronyms
EMF
Electromagnetic field
UV
Ultraviolet
ELF
Extremely low frequency
GUI
Graphical user interface
NCRP
National Council of Radiation Protection and Measurements
ACGIH
American Conference of Governmental Industrial Hygienists
OHTLC
Overhead Transmission Line Constants
IRPA/INIRC International Commission on Non-Ionizing Radiation Protection
EMTP
Electromagnetic Transient program
GMD
Geometric mean distance
GMR
Geometric mean radius
5
List of Figures
1.1
Electomagnetic spectrum [1] . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1
Long-Legged Mary Ann type early DC dynamo. [2] . . . . . . . . . . . . . 16
3.1
3.2
Screenshot of EMTP-RV User Interface [3] . . . . . . . . . . . . . . . . . . 21
Graphical interface of the MATLAB workspace [4] . . . . . . . . . . . . . . 22
4.1
4.2
4.3
4.4
Positive sequence components . . . . . . .
Negative sequence components . . . . . . .
Zero sequence components . . . . . . . . .
Three phase balanced/symmetrical system
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5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
5.10
5.11
5.12
Single core conductor channel cable [5] . . . . . . . . . . . . . . . . . . .
Single-core sheath channel [5] . . . . . . . . . . . . . . . . . . . . . . . .
Three-core filler-space channels [5] . . . . . . . . . . . . . . . . . . . . . .
H-type cable [5] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
S L type cable [5] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Comparison of submarine power cables [6] . . . . . . . . . . . . . . . . .
Typical single-circuit 500 kV AC suspension tower (Dimensions in ft.) [7]
Pin-type insulator [8] . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Suspension Insulator [8] . . . . . . . . . . . . . . . . . . . . . . . . . . .
Strain Insulator [8] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Shackle Insulator [8] . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
All Aluminum Conductor(AAC) cable and All Aluminum Alloy Conductor
(AAAC) [9] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Aluminum Conductor Steel Reinforced (ACSR) cable [10] . . . . . . . . .
Comparison of Overhead lines to Underground HTS cables [11] . . . . . .
Comparison of HVAC and HVDC structures [12] . . . . . . . . . . . . . .
General layout of the electricity network in Europe [13] . . . . . . . . . .
Different phasing arrangements of double circuit transmission lines . . . .
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5.13
5.14
5.15
5.16
5.17
6.1
6.2
6.3
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9
Arrangment of conductors in double circuit transmission line . . . . . . . . 50
Magnetic fields for low rectance phasing and superbundle phasing for different voltage levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
Magnetic fields comparison for superbundle phasing and low reactance
phasing with compact conductors . . . . . . . . . . . . . . . . . . . . . . . 54
6
List of Figures
6.4
6.5
6.6
6.7
Line data used for overhead line parameter calculation in ATPDraw . . . .
Simulation model of different phasing arrangements in ATP/EMTP . . . .
Magnitude and phase angle of characteristic impedance of transmission lines
Position of conductors and ground wires for double circuit transmission
lines in OHTLC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.8 Data configurations for double circuit transmission lines in OHTLC . . . .
6.9 Phasing arrangements for double circuit transmission line in OHTLC . . .
6.10 Devices used for experimental setup during the mesaurement of magnetic field
6.11 Double circuit lines constructed in the EMC laboratory at the chair of electromagnetic compatibility at the Otto-von-Guericke- Universität, Magdeburg
6.12 Magnetic fields comparison for measured values and values obtained from
simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
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61
62
65
66
67
List of Tables
1.1
1.2
1.3
Average magnetic fields of some appliances (µT) [14] . . . . . . . . . . . . 11
Exposure limits for electromagnetic fields [15] . . . . . . . . . . . . . . . . 13
Exposure limits for electric fields [15] . . . . . . . . . . . . . . . . . . . . . 14
2.1
2.2
New transmission line voltages during electrification of the United States [16] 16
Miles of High-Voltage Transmission Lines in the United States [16] . . . . . 17
5.1
5.2
5.3
Comparison of Overhead lines and Underground lines [17] . . . . . . . . . . 43
Different voltage levels for power system network [18] . . . . . . . . . . . . 46
Classification of voltage levels [19] . . . . . . . . . . . . . . . . . . . . . . . 46
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
Position of conductors in overhead lines for different configurations . . .
Current and SIL loading for various voltage levels . . . . . . . . . . . . .
Position of conductors in overhead lines for compact configurations . . .
Comparison of maximum magnetic field for both arrangements . . . . . .
Conductor arrangement and datas for line model drawn in ATPDraw . .
Phase current for different configurations . . . . . . . . . . . . . . . . . .
Maximum current unbalance for different voltage levels . . . . . . . . . .
Symmetrical component current for different configurations . . . . . . . .
Modal parameters for real transformation matrices at 50 Hz obtained from
EMTP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Positive sequence impedance (Per unit values) . . . . . . . . . . . . . . .
Positive sequence impedance (Actual values) . . . . . . . . . . . . . . . .
Zero sequence impedance (Per unit values) . . . . . . . . . . . . . . . . .
Zero sequence impedance (Actual values) . . . . . . . . . . . . . . . . . .
Positioning of conductors for line model drawn in OHTLC . . . . . . . .
Measured magnetic field in the EMC laboratory at the chair of electromagnetic compatibility at the Otto-von-Guericke- Universität, Magdeburg . .
6.10
6.11
6.12
6.13
6.14
6.15
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. 66
1 Introduction
With the increase in science and technology, the systems tend to be more complex further.
Peoples are more curious/concerned about the adverse effects of high voltage transmission
lines (HVTL) on their healths. Due to the exposure of electromagnetic fields (EMF)
occurred by HVTLs, certain risk for diseases like leukemia, breast cancer has been revealed
recently. Electric and magnetic fields exists naturally and also occurs due to human
activity. Occurence of EMFs due to human activity are formed by the use of electric and
magnetic fields. With the increment in industrialization in the society, there tends to be a
drastic change in the level of EMFs.
Figure 1.1: Electomagnetic spectrum [1]
Electric power system frequencies (50 Hz) are placed at the end of electromagnetic
spectrum (see Fig. 1.1) and are sometimes assigned as extremely low frequencies (ELFs).
9
1 Introduction
1.1 Effects of low frequency fields
Now a days electricity has been an essential part of our’s life. Whenever there is a flow of
electricity, the existence of electric and magnetic fields are taken into account. There exists
many sources from which humans are exposed to low frequency fields. Several questions
were raised regarding the concern of extremely low frequency fields:
• Effects of fields on humans, animals and plants?
• What are the exposure limits?
• Can fields be reduced?
1.1.1 Effects on humans
Acute exposure at high levels (above 100 µT) may cause several biological effects that are
illustrated by biophysical mechanisms. Low frequency fields have a great impact on human
body. Low frequency magnetic fields generates electric fields and currents which tends to
stimulate nerve and muscles in human body. Various studies have shown adverse health
effects on humans when exposed to ELF magnetic field. “These includes cancers in adults,
depression, suicide, cardiovascular disorders, reproductive dysfunction, developmental
disorders, immunological modifications, neurobehavioural effects and neurodegenerative
disease.” [20].
1.1.2 Effects on plants
Magnetic fields does have an considerable effect on the growth of plants. According to
Belyavskaya’s research weak electromagnetic fields tends to cause the degradation of
the growth of plants, reduction of cell division and intensified protein synthesis. Some
researchers also claimed that impact of weak EMF shows an increase in the growth of plants,
while others showed there is no change. The disagreement and conflicting results from
the studies illustrates that the effects of magnetic fields on plants may be species-specific
and/or reliant on the characteristics of field exposure such as intensity and duration.
1.1.3 Effects on animals
Some species are more susceptible to low frequency fields:
• Animals that are having electric sense organs.
• Animals having weak defence mechanisms.
10
1 Introduction
The feasible data provided a insufficient basis to determine the hazard of EMF to environmental species. However, besides some local insignificant effects, no critical consequences
of EMF on environmental species were identified.
1.2 Exposure limits
Since the 20th century, the people are getting exposed to electromagnetic fields, but there
was rarely any report of adverse health effects due to exposure. But, with increase in
the demand of electricity, advancment in new technologies, the level of exposure has
also been increased upto a great extent. The main sources of exposures to EMFs are
electrical appliances, the domestic circuits that are used to power them, the lines that
carries electricity to our home (Tab. 1.1). The more time we spend in the vicinity of such
appliances , more will be the exposure to EMFs.
Table 1.1: Average magnetic fields of some appliances (µT) [14]
Distance from source
Appliances
15 cm 30 cm 1.2 m
Iron
0.8
0.1
Dishwasher
2.0
1.0
–
Electric stove element
3.0
0.8
–
Straight-tube fluorescent light 4.0
0.6
–
Electric mixer
10.0
1.0
–
Microwave oven
20.0
1.0
0.2
Circular saw
20.0
4.0
–
Hair dryer
30.0
0.1
–
Vacuum cleaner
30.0
6.0
0.1
Electric can-opener
60.0
15.0
0.2
EMF exposure leads to various adverse health effects on humans, plants and animals.
So, we have to set the limitations on electric and magnetic field exposure. The guidelines
for limiting EMF exposure are established by professional organisations. Tab. 1.2 and
Tab. 1.3 explains the exposure limits of electric and electromagnetic fields. These includes:
• International Commission on Non-Ionizing Radiation Protection or (ICNIRP),
• Institute of Electronical and Electronics Engineers or (IEEE) and
• American Conference of Governmental Industrial Hygienists or (ACGIH).
1.2.1 International commission on non-ionizing radiation protection or (ICNIRP)
The ICNIRP is a scientific association which provides protection against non-ionizing
radiation and helps to determine the exposure limits for low frequency fields. It also
11
1 Introduction
provides guidelines on the adverse consequences of non-ionizing radiation (NIR) to keep
people and surroundings safe from NIR exposure.
Non-ionizing radiation illustrates the section of the electromagnetic spectrum that
covers electromagnetic radiation such as ultraviolet, light, infrared, and radiowaves. In
everyday’s life we came across various sources of non-ionizing radiation such as home
electrical appliances, sun and cellphones.
“The aim of ICNIRP is screen and evaluate scientific knowledge and current findings
toward providing protection guidance on non-ionizing radiation, i.e. radio, microwave and
infrared. In the past, national authorities in more than 50 countries and multinational
authorities such as the European Union have adopted the ICNIRP guidelines and translated
them into their own regulatory framework on protection of the public and of workers from
established adverse health effects caused by exposure to non-ionizing radiation.” [21]
The ICNIRP comprises of:
• a main commission which limits membership upto fourteen in order to provide
assurance of efficiency and
• four Standing Committees containing eight members each which covers various
disciplines of epidemiology, biology, medicine, physics and optical radiation.
“ICNIRP members includes scientists which are hired by universities or radiation protection agencies. Being an independent organisation, ICNIRP connects those communities,
who works for the protection of environment and humans against non-ionizing radiation
across the globe on a larger scale. Before publication, ICNIRP uploads its draft guidelines
online so that public can review and proper suggestions as well as changes are made.” [22]
1.2.2 Institute of electrical and electronics engineers or (IEEE)
The IEEE is the world’s most extensive organization which helps in the advancment of
technology across the globe. IEEE comprises of various professional societies (e.g., Electromagnetic Compatibility Society, Power & Energy Society and Antennas and Propagation
Society ). IEEE has established various guidelines for limiting the exposure of electromagnetic fields on humans, plants and animals. Standards Coordinating Committees (e.g.,
SCC-28, SCC-34) are established by the IEEE-SA Standards Board. They are responsible
for the development of exposure standards associated with subjects that covers more than
one society (e.g., radio frequency safety standards).
International committee on electromagnetic safety
Several exposure standards has been framed by the International Committee on Electromagnetic Safety (ICES) for the proper use (with in the limits) of electromagnetic energy
12
1 Introduction
ranging from 0 Hz to 300 GHz. ICES is funded by the Institute of Electrical and Electronics
Engineers (IEEE) and is governed by its guidelines.
“The aim of ICES is to achieve general agreement among all the shareholders for the
safe use of electromagnetic energy, thereby developing practical standards that are easily
accepted and applied. ICES is driven by a Parent Committee and is further subdivided
into 5 Subcommittees:
• SC1 Techniques, Procedures, and Instrumentation;
• SC2 Terminology, Units of Measurements, and Hazard Communication;
• SC3 Safety Levels with Respect to Human Exposure, 0 - 3 kHz;
• SC4 Safety Levels with Respect to Human Exposure, 3 kHz - 300 GHz; and
• SC5 Safety Levels with Respect to Electro-Explosive Devices.
” [23]
1.2.3 American conference of governmental industrial hygienists or (ACGIH)
The American Conference of Governmental Industrial Hygienists (ACGIH) is a professional
organization consisting of industrial hygienists and experts of related professions, having
headquarters in Cincinnati, Ohio. One of its aim is to ensure protection of workers
by providing all the necessary information (scientific, objective) to occupational and
environmental health professionals.
Table 1.2: Exposure limits for electromagnetic fields [15]
1.000 nT
Suggested for private individuals by
the NCRP in 1996, but not yet authorized
10.000 nT
Suggested for workers and their working area
by the NCRP in 1996, but not yet authorized
100.000 nT
Exposure limit in Germany and IRPA/INIRC recommendation
for private individuals(everyday, fixed exposure). Maximum limit as recommended
by the ACGIH for persons having pacemakers or other electronic implants in their body.
500.000 nT
IRPA/INIRC recommendation for workers (everyday, continuous exposure)
1000.000 nT
IRPA/INIRC exposure limit for private individuals (exposure limited for few hours everyday)
5000.000 nT
IRPA/INIRC recommendation for workers (exposure limited for few hours everyday)
1.3 Outline of Thesis
The background of low frequency fields (occurrence, range), their adverse effects on human
health, exposure limits by different associations is briefly described in 1. The chapter wise
objectives and the outline of the thesis is also described in this chapter.
13
1 Introduction
Table 1.3: Exposure limits for electric fields [15]
V
100 m
Recommended in 1996 for “workers” and their work surroundings
by the NCRP, but not yet authorized.
V
1.000 m
Maximum limit as proposed by ACGIH for
persons having pacemakers or other electronic implants in their body.
V
5.000 m
Limitation of current in Germany and recommendation of IRPA/INIRC for
“private individuals”
V
10.000 m
Exposure limit for “workers” as suggested by IRPA/INIRC
V
20.000 m
Exposure limit for “workers” as suggested by ACGIH
V
25.000 m
Exposure limit for “workers” for a period of utmost 2 hours,
as suggested by IRPA/INIRC
Chapter 2 describes the evolution of electric power system, the growth of transmission
system and it’s importance. Chapter 3 describes the various softwares used for the
numerical computation of results during the work.
Chapter 4 describes the advantages of three phase system and desription of symmetrical
components. Chapter 5 describes the overhead lines structures, types of insulators used,
types of conductors, different types of overead lines, different types of voltage cables.
14
2 Evolution of electric power transmission
We all are aware of the difficulties, which we have faced during the outage of power for
few hours. Just imagine our life without power. We can’t imagine such situations in our
life’s. But in the early days there was an existence of such life and in future also, the same
scenario (power outage) is going to happen. In this section we are going to discuss the
evolution of electric power transmission.
2.1 Early transmission
“Before the invention of electricity, there exists several systems which were used for
transmission of power across large distances. Amongst the systems, telodynamic (cable
in motion), pneumatic (pressurized air), and hydraulic (pressurized fluid) transmission
were in great demand during those days.” [24]. During early days, transmission of electric
power over large distance had following barriers:
• Separate generators are needed for separate lines
• Generators had to installed near their loads
Direct Current Beginings
During the early period between 1870’s and 1880s, dc power systems (see Fig. 2.1) ruled
over the entire world. Coal-fired steam engines and hydroelectric power plants were
earlier developed plants in order to power AC and DC generation plants. Most of the
industrialized cities were located besides/nearby waterfalls, hence it was very easy to
convert to hydroelectric power by the means of traditional mill.
High voltage systems are needed in order to transmit DC power over long distances.
This need of transmission led to the discovery of HVDC systems. For transmission of
power over long distances, the HVDC transmission is preferred over other modes, today
it has been used possibly to replace major AC high-voltage modes. The main problem
regarding transfer of power was transmission over long distance. AC power provided the
solution to this problem.
2.2 Growth of the transmission system
During the 19th century, the inventors started to make use of electricity for daily useful
needs by placing small generators adjacent to the machines which requires electricity for
15
2 Evolution of electric power transmission
Figure 2.1: Long-Legged Mary Ann type early DC dynamo. [2]
their working.
Systems which were developed later tends to distribute power using direct current. But
this method suffers from a major drawback. During the distribution of power, the power
plant’s location should be within a range of one mile from the place of the power generation
(load). Later on, it has been realised that the power industry must transform the system
into small power plants operating together so as to serve their respective neighbouring
loads. [16]
Table 2.1: New transmission line voltages during electrification of the United States [16]
Date
Typical
Voltage
1896
11 000
1900
60 000
1912
150 000
1930
240 000
In 1890 a new system of power distribution has been developed by some researchers,
most of them who were employees of Thomas Edison. The most eminent evolution in
power system was the development of high-voltage lines (see Tab. 2.1) using alternating
16
2 Evolution of electric power transmission
current (AC). Transmission of power over long distances turns out to be a major advantage
of the AC systems.
2.3 Today’s transmission system
With the increasing years, our society hs been inclined towards technologies and depends
upon their network as well as on the power plants. Table 2.2 depicts the growth and
feaures of today’s transmission system.
The whole system has been structured into a complex network which comprises of
several plants and lines connected together and operating at diverse voltage levels. Most
of the developing countries laid more emphasis on the transmission system’s capacity to
deliver the power in most reliable and efficient way, till the transmission owners come up
with their plans for their own growth.
Table 2.2: Miles of High-Voltage Transmission Lines in the United States [16]
Voltage
Miles of Transmission
Line
AC
230 kV
60 000
345 kV
150 000
500 kV
240 000
765 kV
2453
Total AC
154 503
DC
250-300 kV
930
400 kV
852
450 kV
192
500 kV
1333
Total DC
3307
TOTAL
157 810
AC/DC
The objective of the current political discussion on transmission is to analyze how
17
2 Evolution of electric power transmission
different technologies and new policies should be combined which can benefit weak areas
of the network up to maximum.
2.4 Importance of transmission systems
The power industry has been transformed up to a great extent and are defined by a number
of factors which comprises of reliability, flexibility, economics, and competition. On a large
scale, a strong transmission system has some distinct features:
• It enhances the reliability of the electric power system
• It tends to make a market (electricity customers) more flexible
• It holds a big impact on power market economically.
• It tends to enable competition among power plants thereby, by providing access of
plants to market on a large scale.
2.4.1 Reliability
The two basic components of electric system reliability, which are being explained by
power system operators are:
(a) Adequacy- the adeptness of the electric system to meet the demands of electrical
requirements and energy of customers always, while expected and unexpected failures
of power lines have to be taken into account.
(b) Security- the capability of the system to sustain an abrupt disruption that tends to
malfunction the whole system.
Some power plants operate continuously; while others are operated only at peak hours
when needed most. Some power plants, such as wind power plants or hydroelectric
units, operate excellently, when there is an adequate amount of wind flow or the flow of
water. Under some circumstances, such as environmental restrictions, there is a closure of
hydroelectric plants. While all power plants are closed on a regular basis for maintenance
and repair. Sometimes there is an unexpected failure of plants and lines which occur due
to weather or natural havoc.
2.4.2 Flexibility
Transmission allows to make proper use of natural reserves such as wind, coal or geothermal
energy, even if the resource’s locations are distant from the electricity users. Wind plants
should be installed at such locations where the occurrence of wind is strong and consistent.
Coal plant locations should be chosen wisely. It should be located near mine’s area because
the transportation of electricity is far cheaper when compared to transportation of coal.
18
2 Evolution of electric power transmission
2.4.3 Economics
The original Edison system which was used to deliver power over a small area has been
more useful for a limited purpose. The customers and the system are bound together by
one company and one generator. There are many power plants which are operating under
different load levels, some produce power cheaper than others across the country.
From the economic side, more effective power plants should be installed and should be
able to use transmission network so as to deliver power completely through the market.
For serving customers at affordable price, low cost generation of power must be ensured
by the robust transmission system. This helps many consumers to get electricity at low
cost even if there is hike in natural gas price or during the occurrence of natural disasters.
2.4.4 Competition
With the recent development (local to regional) in the power system, a lot of scope has
gained contest in the current market. The new competitive generators nowadays are
dependent on unbiased and lawful passage to the transmission system so as to distribute
their commodities to any associated market.
A new term “transmission constrained areas” has been introduced by analysts. They
have only confined capacity to carry out power because the existence of the transmission
system into the limited area is clogged or in the vicinity of its range. The installation of
new transmission lines in the reserved area enables the power generators (within specified
area) to lessen the market prices in order to have a healthy competition with low cost
power.
Apart from having several benefits, transmission is considered to be as an controversial
issue. In order to deliver electricity to its consumers, the role of transmission is considered
equally important as generation. It contributes almost 10% of a consumer’s total electric
bill. Many questions have been raised during the modern development in transmission
systems.
• Need for transmission
• Proper selection of places for transmission lines construction?
19
3 Softwares
Different configurations of overhead transmission lines are discussed, in order to reduce
low frequency fields around overhead lines. During our work we are going to use different
softwares in order to achieve desirable results. These includes:
• EMTP-RV
• Matlab
3.1 EMTP-RV
“EMTP-RV is a full-featured and technically advanced simulation and analysis professional
software for power system transients. The package is an advanced computer program for
the simulation of electromagnetic, electromechanical and control systems transients in
multiphase electric power systems. EMTP-RV is used worldwide as a reference tool by the
main actors of the power system industry (EDF, RTE, Hydro-Québec and many others).
It is suitable for a wide variety of power system studies whether they relate to project,
design and engineering, or to solving problems and unexplained failures.” [3]
Fig. 3.1 shows the user interface of EMTP-RV. EMTP-RV is a specialized software for
the simulation and analysis of transients in power systems. Here is an overview of the
EMTP-RV software:
• “Unique and comprehensive software,
• Interoperable Software linkable to ETAP,
• Large range of phenomenon,
• Advanced model of electrical machines” [25]
20
3 Softwares
Figure 3.1: Screenshot of EMTP-RV User Interface [3]
21
3 Softwares
3.2 Matlab
“MATLAB (matrix laboratory) is a multi-paradigm numerical computing environment and
fourth-generation programming language. A proprietary programming language developed
by MathWorks, MATLAB allows matrix manipulations, plotting of functions and data,
implementation of algorithms, creation of user interfaces (see Fig. 3.2), and interfacing
with programs written in other languages, including C, C++, Java, Fortran and Python.
Although MATLAB is intended primarily for numerical computing, an optional toolbox
uses the MuPAD symbolic engine, allowing access to symbolic computing abilities. An
additional package, Simulink, adds graphical multi-domain simulation and model-based
design for dynamic and embedded systems.” [26]
Figure 3.2: Graphical interface of the MATLAB workspace [4]
22
4 Three phase system
Power can be tranmitted from generation side to the consumer’s side by the help of single
phase or three phase circuits. Single phase circuit consists of only one phase and one
neutral. So, the transmission of power is limited upto a certain value, when single phase
circuit is used. Three phase circuit is defined as the multiphase system all the phases are
used together to deliver power from the generation side to the distribution stations. Each
one of the phase is having a phase difference of 120°. Since all the phases are involved in
power generation, the power is continuous.
Why three phase is preferred over single phase?
The advantages of three phase systems are listed below:
1. To generate same power the size of 3 phase alternator is small when compared to
single phase alternator. Due to the small size of alternator, less amount of space is
required for installation.
2. The number of conductors in 3 -φ systems is less thereby making the system more
economical.
3. Three phase induction motors have higher power factor and efficiency than that of a
single phase induction motor.
4. The amount of copper and aluminium used for transmission is very low in 3 -φ
systems.
4.1 Symmetrical components
Charles Legeyt Fortescue [27] proposed a new technique called as symmetrical components
so as to solve the set of unbalanced three phase quantities. Faults occuring in three phase
systems are analyzed using this tool.
Fortescue suggested that three phase system contains different independent components
i.e positive, zero and negative for both current and voltage. Positive sequence (see Fig. 4.1)
voltages are always present in the system since these are supplied by generators with in
the system. The second set consistes of phasors which are balanced in nature having equal
magnitude but phase diference of 120° occurs and the sequence rotates in anticlockwise,
is termed as negative sequence (see Fig. 4.2). The third set contains phasors which are
23
4 Three phase system
balanced in nature having equal magnitude and in phase with each other and there is no
rotation of phases and hence termed as zero sequence (see Fig. 4.3).
C
120°
120°
A
120°
B
Figure 4.1: Positive sequence components
B
120°
120°
A
120°
C
Figure 4.2: Negative sequence components
C
B
A
Figure 4.3: Zero sequence components
Unbalanced current and voltage (I a , I b , I c and V a , V b , V c ) can be obtained by the set
of symmetrical components.
24
4 Three phase system
The unbalanced currents can be represented as follows:
Ia = I1 + I2 + I0
(4.1)
I b = a2 I 1 + aI 2 + I 0
(4.2)
I c = aI 1 + a2 I 2 + I 0
(4.3)
The unbalanced voltages can be represented as follows:
Va =V1+V2+V0
(4.4)
V b = a2 V 1 + aV 2 + V 0
(4.5)
V c = aV 1 + a2 V 2 + V 0
(4.6)
VC
IC
IA
IB
VA
VB
VA
VB
VC
3V 1
a2 × V B
VA
a2 × V c
VA
Vc
VB
Figure 4.4: Three phase balanced/symmetrical system
The sequence components (currents or voltages) for 3 phase unbalanced system can be
25
4 Three phase system
obtained as follows: Zero sequence component:
1
I 0 = (I a + I b + I c )
3
1
V 0 = (V a + V b + V c )
3
(4.7)
(4.8)
Positive sequence component:
1
I 1 = (I a + aI b + a2 I c )
3
1
V 1 = (V a + aV b + a2 V c )
3
(4.9)
(4.10)
Negative sequence component:
1
I 2 = (I a + a2 I b + aI c )
3
1
V 2 = (V a + a2 V b + aV c )
3
(4.11)
(4.12)
Here a tends to rotate the vector anti-clockwise by 120° and a2 tends to shift the vector
anti-clockwise by 240°. For a balanced system (see Fig. 4.4) only positive sequence (both
voltage and current) occurs.
4.2 Modal transformation
In order to solve the inacuuracies caused by previous models, proper selection of transmission line model is neccessary. The previus developed modal domain was of no use.
Further it has been discovered that the phase domain models were calculated and then
it’s associated modal domain should be computed for steady state solutions.
4.2.1 Frequency dependent transmission lines
From the physical geometry of the conductors the impedance and admittance matrix
are calculated easily with the help of EMTP. The line parameters are the functions of
frequency, so we have to develop the set of equations in frequency domain first, by using
fitting synthesis. The wave propagation is influenced by the following frequency dependent
parameters:
26
4 Three phase system
• Characteristic impedance (Z c )
Z C (ω) =
v
u 0
u R (ω) + jωL0 (ω)
t
(4.13)
Z C (ω) =
v
u 0
u Z (ω)
t
(4.14)
G0 (ω) + jωC 0 (ω)
Y 0 (ω)
• Propagation constant (γ)
γ(ω) =
q
(R0 (ω) + jωL0 (ω))(G0 (ω) + jωC 0 (ω))
γ(ω) = α(ω) + jβ(ω)
27
(4.15)
(4.16)
5 Overhead transmission lines
Both AC and DC system shares analogous medium for transmission of power. Since the
early days, power transmission over long distances was considered as a big issue. Power
can be transmitted (over long distances) from generation to load side by the means of
transmission lines, underground cables and submarine cables.
5.1 Introduction
The economic growth of any country has a considerable dependency on electrical power.
During the early days, only generation and distribution of electrical power were considered
highly important, as there were several issues and challenges for the transmission of power
over long distances. After the recent development of technologies, the electrical power
transmission is considered as important as generation and distribution.
The power transmission network is a lifelong capital investment with involves a huge
investment. The proportion of this investment relating to transmission and distribution
is about 50% of the total. Half of this, i.e. a quarter of the total amount, is invested for
renewal and expansion of the public power supply cable network.
While selecting the choice of the transmission technology to be used, we have to
emphasize more on proper planning and surveying. Planning is important because every
transmission project is directly dependent upon its geographical conditions, technical
limitations and environmental concerns. During the transmission of electrical energy from
the generation side to consumer the voltage and current levels have to be selected in the
most effective way.
Electrical energy is produced at relatively low voltage and high current at generating
stations. The level of voltage has to be stepped up before transmission within generating
premises to minimize the losses. The energy is transferred to the user through a series of
complex electrical network with safety limitations, practicability and adaptability to the
existing electrical network. There are different types of conduit segments referred to as
bush-bars, distribution lines, cables or transmission lines.
5.1.1 Calculations related to AC transmission
Corona loss
“Due to a corona loss in power system, the efficiency of EHV (extra high voltage lines) has
been significantly reduced. Corona loss occurs when there is an ionization of air molecules
28
5 Overhead transmission lines
near the transmission line conductors. These coronas do not spark across lines, but rather
carry current (hence the loss) in the air along the wire. Corona discharge power losses are
also highly dependent on the weather and temperature” [28].
The corona loss was given by,
"
k0
a
d
P = (f + 25)
V0 − g0 ki akd ln
kd
d
a
r
!#2
X10−5
kW
km
(5.1)
Where k0 is the fixed constant, kd is the normalized air density factor, f is the frequency,
a is the radius of the conductor, d is the conductor spacing, V0 is the line voltage to neutral
and ki is the wire irregularity factor.
Resistive (Skin) loss
During AC transmission, there is a flow of non uniform current through out the conductors
i.e the current density will be more on the surface of the conductor and when we goes deep
inside the conductor the current density will decrease. This phenomenon is termed as skin
effect. In the case of DC system the flow of current is uniform throug out the conductor’s
cross sectional area. There are several factors which affects the skin effect. These includes
the shape of the conductor, frequency, type of material and diameter of the conductors.
At higher frequencies the effective resistance of conductor increases and the cross section
of the conductor is reduced. This phenomena occurs due to skin effect. With the increase
in frequency, there is a decrease in skin effect i.e, at higher frequencies the skin effect is
much lower. A less amount of current is present in the interior of a large conductor. This
makes the use of tubular conductors in order to reduce the cost.
Line Inductance
For transmission line, line inductance is termed as the ratio of flux linkages (Wb-turns) to
the current flowing through the line.
L=
λ
I
(5.2)
The inductance is directly proportional to the spacing between the phases and inversly
proportional to the radius of the conductors in transmission lines. Phases of the overhead
lines are separated at a larger distance in order to ensure proper insulation. So, a higher
voltage line will have more inductance when compared to the low voltage line.
The inductance of overhead lines is given by,
L = 2 × 10-7
GM D
H/m
GM R
29
(5.3)
5 Overhead transmission lines
Where the Geometric Mean Distance (GMD) is defined by
GM D =
q
3
D1 D2 D3
(5.4)
Where D1 ,D2 and D3 are the distances between 3 conductors. The Geometric Mean Radius
is supplied by the manufacturer.
5.2 High voltage cables
Whenever there is a need for power transmission via underground, high voltage cables are
needed. These types of cables are either buried inside the ground or are placed in ducts.
In order to transmit power, conductors are properly insulated which in turn makes the
cables more costlier than overhead lines. Being single cored in nature, high voltage cables
have their additional insulation and sheaths for mechanical protection. For underground
transmission the classification of cables is performed in two ways. These are:
1. insulating material types
2. level of voltage used
Cables are categorized into:
• Low-tension (L.T.) cables — upto 1000 V
• High-tension (H.T.) cables — upto 11.000 V
• Super-tension (S.T.) cables — from 22 kV to 33 kV
• Extra high-tension (E.H.T.) cables — from 33 kV to 66 kV
• Extra super voltage cables — above 132 kV
In the modern day transmission system underground cables are used for transmission
of 3-phase power over short and long distances. For 3-phase transmission the following
cables are used.
(a) Belted cables — upto 11 kV
(b) Screened cables — from 22 kV to 66 kV
(c) Pressure cables — beyond 132 kV.
30
5 Overhead transmission lines
5.2.1 Pressure cables
The presence of voids in solid type cables increases the chances of breakdown of insulation,
which in turn makes the use of cables unreliable for voltages above 66 kV. Pressure cables
are used when the operating voltages exceed 66 kV.
Voids are easily removed bt the application of an extreme amount of pressure on
compound. The pressure cables are classified into following types:
• Oil-filled cables
• Gas pressure cables
Oil-filled cables
In such types, ducts are provided with in the core or nearby in order to ensure proper
circulation under pressure. To supply oil continuously to the channel, external reservoirs
are installed at appropriate distances along the path of the cable. Eradication of voids
makes the oil-filled cables suitable for use ranging from 66 kV up to 230 kV.
Oil-filled cables are of three types:
(i) single-core conductor channel
(ii) single-core sheath channel
(iii) three-core filler-space channels
• Single-core conductor channel
Figure 5.1: Single core conductor channel cable [5]
Fig. 5.1 shows a typical single core conductor channel cable. It consists of a hollow
conductor which behaves as an oil channel.
External reservoir helps to supply the oil under pressure to the channel. The different
layers of paper insulation have been compressed by oil under pressure and is forced
31
5 Overhead transmission lines
to eliminate the voids that may have formed between the layers. Increase in cable
temperature causes the oil to expand and the extra oil that is going to flow out will
be stored in the reservoir. When there is fall in the temperature, the stored oil tends
to flow back in the reservoir.
• Single-core sheath channel
Figure 5.2: Single-core sheath channel [5]
The installation is very easy because of the location of channels at earth potential
(see Fig. 5.2).
• Three-core filler-space channels
Figure 5.3: Three-core filler-space channels [5]
In the 3-core oil-filler cable, the filler spaces are used to store the oil ducts. Fig. 5.3
describes the layout of three-core filler space channel cable. The oil-filled cables have three
principal advantages.
(i) No voids formation and ionisation
(ii) Increase of temperature range and dielectric strength
(iii) The cause of leakage and defects are easily tracked.
High initial cost and complex layout structure are the major drawbacks of these types
of cables.
32
5 Overhead transmission lines
5.2.2 Screened cables
These type of cables have an operating voltage ranging from 22 kV to 33 kV. In some
exceptional cases, they can be used upto 66 kV. There are two types of screened cables
which are listed and explained below:
• H type cables
• S.L. type cables
Hochstadter or H type Cable
Figure 5.4: H-type cable [5]
Fig. 5.4 shows the general layout of H type cable. Each one of the conductor is properly
insulated by a perforated paper and is surrounded by a metallic screen which is in contact
with each of the attached conducter’s screen. The arrangement of the cores is done in
such a way that power dissipation will be minimised. All screens are grounded in order to
make the potential value zero at the sheaths. The screens are so thin that there is rarely
any current induced in them. Thermal expansion leads to the separation of the cores, but
the core separation cannot introduces stress in the dielectric thereby reducing the losses.
For such cables, the cores are not provided with an additional lead covering. If they are
placed in ground for transmission of power, then they must be safeguarded with additional
coverings. Heat is dissipated to the outer surroundings by the use of metallised screens.
Their usage is limited up to 66 kV.
S.L. type cable
S.L. cable refers to separate lead screened cables. The general layout of S.L cable is shown
in Fig. 5.5. In such type of cables the core is properly insulated and provided with an
additional layer of lead. Screens are used to reduce the possibility of stress occurrence
between core and sheath. An additional sheath per core makes these cables to bend
according to their requirements, while this property was absent in H type cables. H-type
cables and S.L. type cables shares same electrical and thermal advantages. The lack of
33
5 Overhead transmission lines
Figure 5.5: S L type cable [5]
oil in the filler reduces the occurrence of hazards which may occur due to oil drainage,
thereby making these cables suited for hilly tracks. Due to simplification of terminating
equipment, the S.L. type construction is beneficial for short runs.
34
5 Overhead transmission lines
5.2.3 Submarine cables
In 1811 first submarine power cable was installed in Germany that was insulated with
natural rubber. Submarine power cables are used to connect power grids across bays,
estuaries, rivers, straits, etc. Fig. 5.6 shows a comparison of different types of submarine
cables.
(a) 115 kV single conductor submarine power
cable - 1962
(b) Modern submarine power cable
Figure 5.6: Comparison of submarine power cables [6]
35
5 Overhead transmission lines
5.3 Overhead HVAC transmission lines
HVAC transmission circuit is 3 phase system consisting of exposed conductors which are
supported by a chain of transmission structures. The number of conductors are limited in
the range of one to four for each phase. Insulators are used to isulate the phases from
the ground. Transmission structures makes the use of “single circuit” or “double circuit”
which contains three phase and six phase respectively. The overhead lines are described in
a better way by following:
5.3.1 Structures
Transmission structures are used to support overhead lines. For the construction of
structures, several materials are used based on the classification:
• For low voltage tranmsission lines, wooden structures are used, but their use is
limited for high voltage lines.
• For high voltage lines tubular steel and lattice structures are used because of their
durability, ease of maintenance and installation.
The transmission structures are designed based on the purpose and their functionality
in the transmission lines. Here is the overview of transmission structures:
(i) Tangent or Suspension Type Structures
(ii) Strain Angle Type Structures
(iii) Dead-end Type Structures
Suspension type structures (see Fig. 5.7) are most commonly used structures and their
usage is limited upto straight part of the lines. As the conductors are placed in a straight
line, their handling capacity is limited from 0 to 2°.
When there is a change in the direction of line conductors, angle type structures are
used. Due to change in the direction, extra force will be implemented on the lines. In
order to withstand such forces angle type structures are used. When there is a large
change in the angle of transmission line conductors, dead end structures are used. When
compared to other types of structures, dead end structures are most stronger, larger in
terms of loading capacity. Climatic factors such as wind, ice and snow plays a key role
while designing and implementing the structures in transmission line.
5.3.2 Insulators
In order to ensure proper functioning of transmission lines i.e, to avoid any leakage of
current, overhead conductors should be provided with proper insulation. Proper insulation
36
5 Overhead transmission lines
Figure 5.7: Typical single-circuit 500 kV AC suspension tower (Dimensions in ft.) [7]
can be ensured by the use of insulators, which tends to prevent leakage current passing
from conductor to earth. Porcelain is the most used material for insulators but several
composition materials like glass and steatite are also used. Insulator’s selection plays a key
role in proper functioning of overhead lines. The properties which every insulator material
should have are listed below:
(a) Insulators should have immense mechanical strength so as to endure conductor load,
wind load etc.
(b) They should have a higher electrical resistance so as to to avoid the possibility of
the leakage current passing to the earth.
(c) They should have a higher relative permittivity.
(d) They should be free from impurities.
37
5 Overhead transmission lines
Pin type insulators
Pin insulator (see Fig. 5.8)was the first developed overhead insulator, but still they are in
demand for transmission and distribution up to 33 kV system. For 11 kV system one part
type insulator is used. As the name suggests, one part type insulator contains only one
piece of glass or porcelain. This type of insulator is mounted on the pole structures with
the help of bolts. They are used for straight transmission lines. Above 33 kV systems, the
insulators become too heavy thereby making the system more complex and costly.
Figure 5.8: Pin-type insulator [8]
Suspension type insulators
Above 33 kV pin type insulators are not used due to it’s oversize and overweight. It is
very difficult to handle and replace bigger size insulators. In order to overcome these
problems, suspension insulator (see Fig. 5.9) was developed. Discs are connected in series.
Each disc is associated with a low voltage. Working voltage has a direct dependency upon
the number of discs in series. These type of insulators are more economical and they
replace pin insulators thereby reducing the area and cost of the system. They consists of
interconnected discs, with each disk designed to support a particular voltage. If one disc
is damaged amongs the sets, the whole string will not be useless, infact it can be replaced
by a good one.
Strain insulators
The transmission lines are subjected to a large tension when there is a dead end or curve
in the line. Strain insulators are used in order to reduce the excessive tension. For low
voltage levels (below 11 kV) shackle insulators are used, but for higher voltage lines a
group of suspension type insulators are used together as strain insulator. They are used to
manage the mechanical stress by reducing the pressure from a conductor at the corner of a
38
5 Overhead transmission lines
Figure 5.9: Suspension Insulator [8]
transmission line. A strain insulator (Fig. 5.10) should posses higher mechanical strength
and proper insulation.
Figure 5.10: Strain Insulator [8]
Shackle insulators
For low voltage distribution networks, shackle insulators (Fig. 5.11) are used. With the
increased use of underground cables for distribution, the use of such types of insulators
has been decreased rapidly. These insulators can be used horizontally or vertically. They
can be fixed to the pole with the help of bolts.
5.3.3 Conductor types
The transmission line is considered as the backbone of the power system. The conductor
materials play a key role in estimating the cost and span of transmission lines. Copper
is most suited for the choice of the material for conductor due to its high conductivity
and immense tensile strength. The higher cost incurred turns out to be the drawback of
39
5 Overhead transmission lines
Figure 5.11: Shackle Insulator [8]
copper’s use. The material used for power transmission has a dependency on following
factors:
1. Required electrical properties
2. Required mechanical strength
3. Local conditions
4. Cost of material
There isn’t any predefined guideline for the design of transmission and distribution
lines. The conductors electrical and mechanical properties has a great impact on the cost
incurred during the design of power lines. In order to serve the stated purpose, following
are the conductors used:
1. AAC - All Aluminum Conductor
2. AAAC - All Aluminum Alloy Conductor
3. ACSR - Aluminum Conductor Steel Reinforced
• AAC
AAC (Fig. 5.12a) doesn’t fits for applications like transmission lines and rural
distribution where long spans are utilized, due to its relatively poor strength-toweight ratio. However, it is extensively used in urban and coastal regions due to its
excellent conductivity and corrosion resistance.
• AAAC
They used for distribution near ocean areas where there is a maximum chance
of corrosion while using other conductors. The advantages of AAAC (Fig. 5.12b)
conductors are listed below:
40
5 Overhead transmission lines
(a) AAC
(b) AAAC
Figure 5.12: All Aluminum Conductor(AAC) cable and All Aluminum Alloy Conductor
(AAAC) [9]
- Low loss of power when compared to ASCR conductor
- Very good corrosion resistance
- Higher resistance to abrasion
• ACSR
Having higher strength to weight ratio and being economical, ACSR conductors
are preferred over others for transmission lines having long spans. As the name
suggests itself, ACSR (Fig. 5.13) conductors are combination of aluminium and steel.
They merge the light weight and good conductivity of aluminium with high tensile
strength along with the ruggedness of steel.
Figure 5.13: Aluminum Conductor Steel Reinforced (ACSR) cable [10]
5.4 Overhead lines vs. buried cables
In order to transmit electric power from generation point to consumer side, we can use
either overhead lines or underground cables. Each of these methods has certain pros and
cons. Fig. 5.14 and Tab. 5.1 shows a comparison between overhead lines and underground
cables.
41
5 Overhead transmission lines
Figure 5.14: Comparison of Overhead lines to Underground HTS cables [11]
42
5 Overhead transmission lines
Table 5.1: Comparison of Overhead lines and Underground lines [17]
Overhead lines
Underground lines
1. Conducter size is small
2. Conductor size is quite large
2. Less insulation needed because
2. Very high insulation is required be-
overhead lines are open to surroundings
cause the underground system is located
and essential insulation is provided by
inside the ground thereby making the
air.
area quite compact.
3. Dissipation of heat is easy as
3. Heat dissipation is difficult due to
overhead lines are exposed to
non exposure of underground lines to
atmosphere.
the atmosphere.
4. The absence of insulation coating
4. Higher cost is incurred as the system
over conductors makes the system very
needs more number of insulation layers.
cheap.
5. Detection of faults is very easy.
5. Faults cannot be detected easily because of the complex nature of the system.
6. System maintenance work is very
6. Maintenance turns out to be uneasy.
easy.
7. Power is transmitted over long
7. Used for shorter distance.
distance.
8. Safety of public is at stake.
8. More safety is assured.
43
5 Overhead transmission lines
5.4.1 Comparison of HVAC and HVDC systems
Investment cost
The transmission of electric power through DC systems
needs lesser number of conductors when compared to AC
systems. The use of lesser conductors (Fig. 5.15) makes
HVDC transmission lines cheaper than an HVAC line.
Over a certain distance, termed as break-even distance,
HVDC lines are cheaper than HVAC. For overhead lines,
the break-even distance is around 600 km.
Figure 5.15: Comparison of HVAC and HVDC structures [12]
Losses
In DC systems, there is no skin effect and also the corona
losses are very low when compared to AC systems. This
makes favourable for the use of HVDC systems over HVAC
systems.
Asynchronous connection
There exists diferent standardization for AC power grids.
In some countries 50 Hz is taken, whereas in some countries
60 Hz is used. It is highly impossible to connect two
different power grids operating at different frequencies.
But a HVDC link makes it possible , thereby use of HVDC
over long distance is preferrable.
44
5 Overhead transmission lines
5.4.2 Voltage levels
Whenever there is a flow of current through network, loss in power takes place. As the
power loss is directly proportional to the square of the current, transmission line operates
at high voltage and low current. More the power will be transmitted, more will be the
voltage. So, there is a need for standardisation of voltage levels across the globe.
Figure 5.16: General layout of the electricity network in Europe [13]
Power system comprises of generating stations, transmission systems and distribution
systems. To deliver a vast amount of power from generating stations to distribution centres,
transmission systems are used. The transmission voltages (Tab. 5.2) are dependent on
following factors:
• the distance upto which power is to be delivered
• the amount of power to be transmitted
• stability of the system
Distribution stations are used to deliver power to the consumers. Fig. 5.16 shows a
general layout of the electricity network in Europe. Tab. 5.3 shows the different voltage
levels and their associated risks.
45
5 Overhead transmission lines
Table 5.2: Different voltage levels for power system network [18]
Power system network
Voltage levels
Generating stations
6.6 kV, 11 kV, 21 kV, 31 kV
Transmission systems
110 kV, 220 kV, 132 kV, 400 kV
Distribution systems
11 kV, kV, 3.3 kV, 230 V
Table 5.3: Classification of voltage levels [19]
IEC voltage range
AC
DC
Defining risk
High voltage (supply system)
> 1000 VRMS
> 1500 V
electrical arcing
Low voltage (supply system)
50 – 1000 VRMS
120 - 1500 V
electrical shock
Extra-low voltage (supply system)
< 50 VRMS
<120 V
low risk
5.4.3 Magnetic fields
Whenever there is a use of electricity, electric current is involved and where there is a flow
of electric current, magnetic fields comes into existence. They are generated by electric
current. Magnetic fields around overhead lines tends to induce current in the human
body, which leads to the concern of health issues when exposed directly to magnetic fields.
So, it is recommended to determine the exact amount of the fields in vicinity of home,
working place and surroundings. When there is a flow of electron inside the conductor,
magnetic fields are produced. The magnitude of a magnetic field is directly dependent
on the current flowing inside the loop or wire, not on the voltage across them. Magnetic
fields are measured in either Gauss (G) or Tesla (T).
Methods to reduce magnetic fields around overhead lines
Although it has not been mentioned anywhere across the globe that direct or indirect
exposure to magnetic fields causes any types of diseases like cancer, leukemia etc, one
should avoid direct or indirect exposure to magnetic fields. The exposure limits has been
set up by various organizations like ICNIRP, IEEE and WHO. Magnetic fields around
overhead lines can be reduced by following methods:
• Height of the towers: With the increase in the height of the tower (structure),
magnetic fields can be significantly reduced. But this approach has a certain
drawback. When the height of the towers will be increased , the space allotted for
designing tower will also increase and the capital cost in setting up the structure
will also shoot up to a great extent.
46
5 Overhead transmission lines
• Compact conductors: For this method the bundles are optimized by a certain factor,
but this in turn causes other instabilities.
• Swapped Phases: By interchanging the top and bottom phases of the transmission
lines, magnetic fields can be reduced.
According to Ampere’s law, the integral of B around any closed path is equal to µ times
the current enclosed in the area.
I
B · dl = µ0 I
(5.5)
Where, the line integral is over any arbitrary loop , I is the current enclosed by that
loop and r is the distance from the center of the wire The magnetic field generated by a
single wire is obtained by following equation.
~ = µ0 I a~ϕ
B
2πr
(5.6)
|B| = (Bx 2 + By 2 ) 2
1
Bx =
By =
n
X
µ0 I j y j
2
2
j=1 2π(xj + y j )
n
X
µ 0 I j xj
2
2
j=1 2π(xj + y j )
|B| =
q
(Bx 2 + By 2 )
(5.7)
(5.8)
(5.9)
(5.10)
The magnetic fields along x-axis and y-axis are calculated using (5.8) and (5.9) respectively. The resultant magnetic fields along the axes are calculated using (5.10).
Surge Impedence Loading
The SIL can be calculated by the formula:
|V |2
SIL = q
L/C
47
(5.11)
5 Overhead transmission lines
|V |
|I L | = √ q
3 L/C
(5.12)
q
Where, L/C is the surge impedance in Ω, |V | is the line voltage in kV, |I L | is the line
current in A and SIL is the surge impedance loading in MW.
Surge impedance values lies in the range of 400 to 600Ω for overhead lines. Short
transmission lines (<50 miles) are used to transmit load upto 1.2 to 1.5 times SIL.
Characteristic impedance of overhead lines
The characteristic impedance of a two conductor line is given by the formula:
d
Z = 120 ln
r
!
(5.13)
Where, d is the separation of conductors and r is the radius of conductors.
√
The resistance increases relatively with increase in frequency by a factor of f , where f
is frequency. On other hand, with the increase in f , the inductance tends to decrease.
1. For a low frequency (f f c ), where f c is the critical frequency
Z = R + jωL, R = Rdc =
µ0
ρ
,L =
S
8π
(5.14)
2. For a high frequency (f f c )
√
√
ω µ0 ρ
R
1
Z = (1 + j)R, R = √
∝√
∝ ωL =
ω
ω
2 2πrf
s
Z0 =
√
Z
ω
1
∝
=√
Y
ω
f
With the increase in frequency, the characteristic impedance tends to decrease.
48
(5.15)
(5.16)
5 Overhead transmission lines
(a) Superbundle phasing
(b) Low reactance phasing
Figure 5.17: Different phasing arrangements of double circuit transmission lines
Fig. 5.17 shows the different phasing arrangements of double circuit transmission lines.
In Fig. 5.17a the conductors are arranged in same order from top to bottom i.e, (ABC,
A’B’C’). In Fig. 5.17b the top and bottom conductors positions are swapped and the
order of the conducters will be in the form of (ABC, C’B’A’). The swapping of conductors
turns out to be advantageous over previous phasing arrangements. When compared to
superbundle phasing low reactance phasing has following advantages:
• Low resistive loss
• Low magnetic field
• Low shield wire losses
49
6 Results and discussion
6.1 Matlab results
Figure 6.1: Arrangment of conductors in double circuit transmission line
Fig. 6.1 shows the arrangement of conductors in double circuit transmission lines. The
position of the conductors are given as input in the Matlab program. We can calculate
the current and surge impedance loading for different voltage levels using Eq. (5.11) and
(5.12). From Tab. 6.2 we can see that with an increase in voltage, SIL also increases which
inturn leads to the increase in current value. As magnetic field is directly dependent on
current, we can say that more the current value more will be the magnetic field.
Fig. 6.2 shows the magnetic fields comparison for both phasing arrangements. The magnetic fields comparison (compact conductors) for both phasing arrangements is illustrated
in Fig. 6.3. From the above graphs it can be shown that swapping of phase conductors
50
6 Results and discussion
Table 6.1: Position of conductors in overhead lines for different configurations
Superbundle phasing
Low reactance phasing
dimensions (in m)
dimensions (in m)
h1
14
14
h2
26.7
26.7
h3
34
34
a11
-2.88
-7.08
a12
-2.38
-6.58
a21
2.38
6.58
a22
2.88
7.08
b11
-10.0
-10.0
b12
-9.50
-9.50
b21
9.50
9.50
b22
10.0
10.0
c11
-7.08
-2.88
c12
-6.58
-2.38
c21
6.58
2.38
c22
7.08
2.88
Conductors notation
Table 6.2: Current and SIL loading for various voltage levels
Voltage levels kV
SIL loading (MW)
Current (A)
154
88.93
334
275
283.59
596
380
541.5
822
500
937.5
1082
leads to a reduction of magnetic field around transmission lines. Tab. 6.1 and Tab. 6.3
shows the positioning of conductors and compact conductors of overhead lines for different
configurations respectively.
51
6 Results and discussion
Table 6.3: Position of conductors in overhead lines for compact configurations
Superbundle phasing
Low reactance phasing
(compact) dimensions (in m)
(compact) dimensions (in m)
h1
14
14
h2
24.3
24.3
h3
29.2
29.2
a11
-2.40
-5.76
a12
-1.90
-5.26
a21
1.90
5.26
a22
2.40
5.76
b11
-8.1
-10.0
b12
-7.6
-9.50
b21
7.6
9.50
b22
8.1
10.0
c11
-5.76
-2.40
c12
-5.26
-1.90
c21
5.26
1.90
c22
5.76
2.40
Conductors notation
52
·10−5
5
Low reactance phasing
4.5
4
Superbundle phasing
3.5
3
2.5
2
1.5
1
0.5
0
−100−80 −60 −40 −20 0 20 40 60 80 100
4
Magnetic flux (T)
Magnetic flux (T)
6 Results and discussion
·10−5
Low reactance phasing
Superbundle phasing
3.5
3
2.5
2
1.5
1
0.5
0
−100−80 −60 −40 −20 0
Distance (m)
Distance (m)
(a) 500 kV
(b) 380 kV
2
1.6
Low reactance phasing
Superbundle phasing
Magnetic flux (T)
Magnetic flux (T)
·10−5
2.5
1.5
1
0.5
0
−100−80 −60 −40 −20 0
20 40 60 80 100
·10−5
Low reactance phasing
Superbundle phasing
1.4
1.2
1
0.8
0.6
0.4
0.2
20 40 60 80 100
0
−100−80 −60 −40 −20 0
Distance (m)
20 40 60 80 100
Distance (m)
(c) 275 kV
(d) 154 kV
Figure 6.2: Magnetic fields for low rectance phasing and superbundle phasing for different
voltage levels
Table 6.4: Comparison of maximum magnetic field for both arrangements
Phase arrangements
Voltage level (kV)
Maximum magnetic field (T)
Superbundle phasing
154
1 × 10−5
275
1.8 × 10−5
380
2.5 × 10−5
500
3.25 × 10−5
154
0.75 × 10−5
275
1.3 × 10−5
380
1.7 × 10−5
Low reactance phasing
500
2.2 × 10−5
The maximum magnetic field for low reactance phasing and superbundle phasing is shown
in Fig. 6.4.
53
·10−5
4.5
LR phasing
4
Superbundle phasing
3.5
3
2.5
2
1.5
1
0.5
0
−100−80 −60 −40 −20 0 20 40 60 80 100
·10−5
Magnetic flux (T)
Magnetic flux (T)
6 Results and discussion
3
LR phasing
Superbundle phasing
2.5
2
1.5
1
0.5
0
−100−80 −60 −40 −20 0
Distance (m)
Distance (m)
(a) 500 kV
2
(b) 380 kV
·10−5
·10−5
1.4
LR phasing
Superbundle phasing
Magnetic flux (T)
Magnetic flux (T)
2.5
20 40 60 80 100
1.5
1
0.5
0
−100−80 −60 −40 −20 0
20 40 60 80 100
Distance (m)
1.2
LR phasing
Superbundle phasing
1
0.8
0.6
0.4
0.2
0
−100−80 −60 −40 −20 0
20 40 60 80 100
Distance (m)
(c) 275 kV
(d) 154 kV
Figure 6.3: Magnetic fields comparison for superbundle phasing and low reactance phasing
with compact conductors
54
6 Results and discussion
6.2 EMTP results
Figure 6.4: Line data used for overhead line parameter calculation in ATPDraw
Table 6.5: Conductor arrangement and datas for line model drawn in ATPDraw
Phase no.
Rin (cm)
Rout (cm)
DC Resistance (Ω/km)
Horizontal (m)
Vtower (m)
1
0.37
1.4795
0.0597
-4.12
30.24
2
0.37
1.4795
0.0597
-4.47
24.12
3
0.37
1.4795
0.0597
-5.75
18
4
0.37
1.4795
0.0597
4.12
30.24
5
0.37
1.4795
0.0597
4.47
24.12
6
0.37
1.4795
0.0597
5.75
18
7
1e − 6
0.815
0.22227
-6.75
36.36
8
1e − 6
0.815
0.22227
6.75
36.36
Fig. 6.4 and Tab. 6.5 illustrates the phase, radius of conductors and position of conductors
as well as the line model used in ATPDraw.
The maximum difference between the currents occuring in different phases is given in
55
6 Results and discussion
Table 6.6: Phase current for different configurations
Phase a
Phase b
Phase c
Voltage
Phasing arrangements
I (kA)
Angle (degree)
I (kA)
Angle (degree)
I (kA)
Angle (degree)
500 kV
Superbundle phasing
25.86
-78.73
29.88
157.76
25.07
29.69
Low reactance phasing
27.82
-81.43
30.08
156.63
27.7
34.72
Superbundle phasing
15.21
-75.4
17.06
158.8
15.40
30.10
Low reactance phasing
16.64
-81.33
17.11
158.35
16.53
33.75
Superbundle phasing
8.91
-77.56
9.6
158.8
8.74
30.11
Low reactance phasing
9.71
-80.6
9.80
157.98
9.48
32.33
275 kV
154 kV
Table 6.7: Maximum current unbalance for different voltage levels
500 kV
275 kV
154 kV
Difference (kA)
Ratio (%)
Difference (kA)
Ratio (%)
Difference (kA)
Ratio (%)
Superbundle phasing
4.38
17.85
1.85
11.64
0.86
9.46
Low reactance phasing
2.38
8.34
0.58
3.46
0.32
3.3
Table 6.8: Symmetrical component current for different configurations
Positive Seq.
Voltage
500 kV
275 kV
154 kV
Zero Seq.
Negative Seq.
I1 (kA)
Angle (degree)
I0 (kA)
Angle (degree)
I2 (kA)
Angle (degree)
Superbundle phasing
26.83
-83.62
0.607
-63.6
2.98
38.48
Low reactance phasing
28.51
-83.37
0.24
-15.88
3.94
34.98
Superbundle phasing
15.79
-82.16
0.49
-33.40
1.74
43.93
Low reactance phasing
16.73
-83.06
0.33
-70
0.611
53.74
Superbundle phasing
9.04
-82.81
0.33
-57.84
0.82
42.66
Low reactance phasing
9.64
-83.41
0.32
-58.55
0.41
43.63
56
6 Results and discussion
(a) Superbundle phasing.
(b) Low reactance phasing.
Figure 6.5: Simulation model of different phasing arrangements in ATP/EMTP
Tab. 6.7. The difference has been computed by substracting the smallest current from the
largest current of each of the phases in the Tab. 6.6. The ratio has been calculated by
the ratio of the difference in current to the average current of each of the three phases.
From the Tab. 6.6, it has been clearly shown that the superbundle phasing causes the
largest unbalance of the phase currents and low reactance phasing causes less unbalance
when compared to superbundle phasing. The symmetrical components of phase currents
is listed in Tab. 6.8. Fig. 6.5 shows the simulation model of both phasing arrangements
(Superbundle and Low reactance phasing) in ATP/EMTP.
57
6 Results and discussion
Table 6.9: Modal parameters for real transformation matrices at 50 Hz obtained from
EMTP
R’(Ω/km)
)
L’( mH
km
G’(Ω/km)
µF
)
C’( km
Zc (Ω)
Phase(°)
0.0407
2.940
2.5 × 10−10
9.46 × 10−3
582
-11.8
0.0171
0.83
2.1 × 10−10
1.46 × 10−2
260
-16.5
0.0159
0.632
2.04 × 10−10
1.76 × 10−2
214
-19.3
Modal transformation


0.0267 + j0.0483 0.0098 + j0.0266 0.0095 + j0.0241



Z s = 0.0098 + j0.0266 0.0254 + j0.0477 0.0089 + j0.0273


0.0095 + j0.0241 0.0089 + j0.0273 0.0248 + j0.0496
(6.1)
The series impedance matrix (phase domain) is calculated by EMTP and is given by
Eq. (6.1). The matrix comprises of self impedance and mutual impedance values. After
modal transformation we got the values of overhead line parameters (Tab. 6.9.
Fig. 6.6 shows the variation of magnitude and phase of Zc with respect to frequency.
58
6 Results and discussion
Impedance (Ω)
750
Zc vs. Frequency
700
650
600
550
500
450
0
5
10 15 20 25 30 35 40 45 50
Frequency (Hz)
(a) Magnitude of Zc
−4
Zc vs. Phase
−6
Phase (°)
−8
−10
−12
−14
−16
−18
−20
0
5
10 15 20 25 30 35 40 45 50
Frequency (Hz)
(b) Phase of Zc
Figure 6.6: Magnitude and phase angle of characteristic impedance of transmission lines
59
6 Results and discussion
6.2.1 Line impedance
(a) Superbundle phasing (Circuit1)
(b) Superbundle phasing (Circuit2)
Figure 6.7: Position of conductors and ground wires for double circuit transmission lines
in OHTLC
Table 6.10: Positive sequence impedance (Per unit values)
Voltage levels (kV)
Phasing arrangements
%R
%X
% R+jX
%Z
500 kV
Low reactance phasing
0.0434
0.4976
0.0434+j0.4976
0.4995
Superbundle phasing
0.0432
0.5003
0.0432+j0.5003
0.5022
Table 6.11: Positive sequence impedance (Actual values)
Voltage levels (kV)
Phasing arrangements
R(Ω)
X
R+jX(Ω)
Z(Ω)
500 kV
Low reactance phasing
1.0838
12.44
1.0838+j12.44
12.4871
Superbundle phasing
1.0793
12.5086
1.0793+j12.5086
12.5551
The Overhead Transmission Line Constants Program has been used to calculate the
line impedances of the overhead lines. Fig. 6.7 and Fig. 6.8 shows the position of phase
conductors, ground wires , type of ground wires and the distance over which the line
60
6 Results and discussion
(a) Low reactance phasing (Circuit1)
(b) Low reactance phasing (Circuit2)
Figure 6.8: Data configurations for double circuit transmission lines in OHTLC
Table 6.12: Zero sequence impedance (Per unit values)
Voltage levels (kV)
Phasing arrangements
%R
%X
% R+jX
%Z
500 kV
Low reactance phasing
0.6626
2.0970
0.6626+j2.0970
2.1992
Superbundle phasing
0.6615
2.0987
0.6615+j2.0987
2.2005
Table 6.13: Zero sequence impedance (Actual values)
Voltage levels (kV)
Phasing arrangements
R (Ω)
X
R+jX (Ω)
Z (Ω)
500 kV
Low reactance phasing
16.5656
52.42
16.5656+j52.42
54.98
Superbundle phasing
16.5381
52.46
16.5381+j52.46
55.123
61
6 Results and discussion
Figure 6.9: Phasing arrangements for double circuit transmission line in OHTLC
Table 6.14: Positioning of conductors for line model drawn in OHTLC
Phase no.
Horizontal distance (m)
Vtower distance (m)
1
6.5
15
2
9.5
20
3
5.5
30
4
-6.5
15
5
-9.5
20
6
-5.5
30
7
4.5
35
8
-4.5
35
62
6 Results and discussion
impedance is calculated. The output positive sequence and zero sequence transmission line
impedances are calculated for both phasing arrangements (superbundle and low reactance
phasing) and both the results have been compared. Fig. 6.9 and Tab. 6.14 shows the
position of conductors that has been used as input data for calculation of impedances
in OHTLC. Tab. 6.10, Tab. 6.12, Tab. 6.11 and Tab. 6.13 shows the calculated values
of impedances for superbundle and low reactance phasing. It has been evident that low
reactance phasing yields lower value of impedances when compared to superbundle phasing.
63
6 Results and discussion
6.3 Experimental results
In order to evaluate and compare magnetic fields for both phasing arrangements, a physical
model has been setup in the laboratory at the chair of electromagnetic compatibility at
the Otto-von-Guericke-Universität Magdeburg). The following devices has been used for
the computation of results:
• Field probes
• Gigaport HD+ box
• Power supply
• Oscilloscope
For physical setup we have used two wooden board (rectangular) having equal size. In
order to fix both boards, a wooden pole structure has been used. A program has been
written in Matlab which controls the output channels of the HD+ box. For different
positions and different heights, the magnetic fields are computed and compared or both
phasing arrangements. Fig. 6.10 shows the different types of devices that are being used
during the measurement of magnetic field around overhead lines. Tab. 6.15 shows the
measured values of magnetic fields for low reactance phasing and superbundle phasing.
Double circuit lines (Fig. 6.11) have been constructed with the help of copper enamulated
wire in the EMC laboratory at the chair of electromagnetic compatibility at the Otto-vonGuericke-Universität, Magdeburg.
Tab. 6.15 shows the measured magnetic field of both phasing arrangements for different
positions. Fig. 6.12a shows the measured magnetic field and Fig. 6.12b shows the simulation
results of the magnetic field. It has been clearly shown that measured values are much
lower when compared to the simulation values because of the large difference in current.
At centre (distance=0 cm or 0 m, the field is maximum/stronger because of it’s closeness
to the source, when we move far away from the source , the magnetic field tends to be
lower/weaker. The simulation is performed at 500 kV and 1000 A, while the measured
values are taken at 0.90 V and 0.43 A. Since magnetic field is directly proportional to
current and it decreases with the distance from the source, the measured values are lower
than simulation values. It has been clearly shown that the magnetic field is relatively low
for low reactance phasing when compared to superbundle phasing. The magnetic field
tends to decrease with the increase in distance from the source. When the phases of the
overehead lines are interchanged i.e from superbundle phasing to low reactance phasing
there is an approximately decrease in magnetic field by a factor of 15 - 18% for measured
values, while there is a decrease in the magnetic field by a factor of 20 - 30% for simulated
values.
64
6 Results and discussion
(a) Power supply
(b) Gigaport HD+ box
(c) Oscilloscope
(d) Magnetic field probe
Figure 6.10: Devices used for experimental setup during the mesaurement of magnetic
field
65
6 Results and discussion
Table 6.15: Measured magnetic field in the EMC laboratory at the chair of electromagnetic
compatibility at the Otto-von-Guericke- Universität, Magdeburg
Phasing arrangements
Distance from the centre
Magnetic field
Superbundle phasing
0 cm
0.622 µT
15 cm
0.215 µT
30 cm
36.1 nT
45 cm
15.3 nT
57 cm
10.6 nT
76 cm
6.8 nT
0 cm
0.542 µT
15 cm
0.159 µT
30 cm
34.2 nT
45 cm
13.2 nT
57 cm
9.3 nT
76 cm
4.9 nT
Low reactance phasing
Figure 6.11: Double circuit lines constructed in the EMC laboratory at the chair of electromagnetic compatibility at the Otto-von-Guericke- Universität, Magdeburg
66
6 Results and discussion
·10−7
7
Superbundle phasing
Low reactance phasing
5
4
3
Magnetic flux (T)
Magnetic field (T)
6
2
1
0
0
10
20
30
40
50
Distance (cm)
60
70
80
·10−5
5
Low reactance phasing
4.5
4
Superbundle phasing
3.5
3
2.5
2
1.5
1
0.5
0
−100−80 −60 −40 −20 0 20 40 60 80 100
Distance (m)
(a) Measured values at 0.91 V
(b) Simulation results at 500 kV
Figure 6.12: Magnetic fields comparison for measured values and values obtained from
simulation
67
7 Summary
Humans are exposed to electromagnetic fields directly or indirectly i.e by the means of
natural sources like thunderstorms or by the means of manmade (transmission lines and
electrical apparatus). Continuous exposure to these fields leads to various types of diseases
like cancer, leukemia etc. The low frequency fields has an interference with electrical and
electronic equipement in it’s close vicinity. So, we have to reduce the elctromagnetic fields
around the environment, when it passes through dense populated areas. This work has
presented electromagnetic field reduction techniques i.e by the use of different phasing
arrangements magnetic fields have been reduced upto a certain level. We have calculated
the magnetic fields for the following arrangements:
• Superbundle phasing
• Low reactance phasing
A Matlab program has been written for the calculation of magnetic fields for both
arrangements. In order to calculate the magnetic fields we have given the number of
conductors, position of conductors and the heights of the conductors as input. From the
results it has been shown that low reactance phasing produces a lower magnetic field when
compared to superbundle phasing. The conductors are compacted by a factor of 15% and
again the both phasing arrangements for compact conductors are studied and the results
produce a lower magnetic field while compared to the original position of conductors. But
compaction of conductors leads to structural instabilities and other problems.
For both phasing arrangements the line impedances have been calculated with the help
of OHTLC program and the results indicates a lower impedance value for low reactance
phasing than superbundle phasing. The phase currents have been calculated with the help
of EMTP and the results clearly indicates a lower current imbalance for low reactance
phasing as compared to superbundle phasing.
For both phasing arrangements a physical setup has been implemented in the EMC laboratory at the chair of electromagnetic compatibility at the Otto-von-Guericke-Universität,
Magdeburg. The lines have been constructed and with the help of field probes, the
magnetic fields has been computed and compared. The measured values clearly depict
that there will be a lower magnetic field when there is change of phasing from superbundle
to low reactance.
68
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