Embed Size (px)
Citation preview
UNIVERSITY OF NAIROBI
FACULTY OF ENGINEERING
DEPARTMENT OF ELECTRICAL AND INFORMATION ENGINEERING
DESIGN OF HIGH VOLTAGE DIRECT CURRENT (HVDC) LINE FROM ETHIOPIA TO ISINYA (KENYA)
PROJECT INDEX: PRJ 062
BY
CHARLES JULIUS KILONZI
F17/1756/2006
SUPERVISOR: DR. C.WEKESA
EXAMINER: MR.N.S WALKADE
PROJECT REPORT SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR THE AWARD OF THE DEGREE
OF
BACHELOR OF SCIENCE IN ELECTRICAL AND ELECTRONIC ENGINEERING OF THE UNIVERSITY OF NAIROBI 2011
SUBMITTED ON: 18TH MAY, 2011
ii
DEDICATION
I dedicate this project work to my father Charles, mother Jane and grandmother Margaret for
continually inspiring me to work hard in my academics, my brothers Dr. Mulwa, Nicodemus and
Robert and my sisters Margaret, Mwikali, Tabitha, Syombua and Elizabeth for their prayers and
encouragement. Thank you.
iii
ACKNOWLEDGEMENTS
My sincere gratitude goes to Dr. Cyrus Wekesa, my project supervisor for his support, guidance,
helpful suggestions and continued monitoring of my progress during the project work. Much
appreciation goes to my lecturers in the Department of Electrical and Information Engineering
for the worthy knowledge they have imparted on me for the last five years. Am very grateful to
Dr. Mativo, Eng. Samson Akuto and Eng. Githinji all of KETRACO Company Limited for the
helpful information they provided to me with regards to this project.
I would like to thank all my friends especially Martha Kyule, Robert Muema and Clyde Omurayi
for their encouragement and support during my project work. Special thanks to Victor Kyalo my
proof reader.
Most importantly I thank God for keeping me healthy during the project period and also for
insight in what I was undertaking.
God bless us all.
iv
DECLARATION AND CERTIFICATION
Except where indicated and acknowledged, I certify that the information presented in this report
is my original effort and has not been presented before for a degree award in this or any other
university to the best of my knowledge.
…………………………………..............
CHARLES JULIUS KILONZI
F17/1756/2006
This report has been submitted to the Department of Electrical and Information Engineering,
University of Nairobi with my approval as supervisor:
………………………………
Dr. Cyrus Wekesa
Date: ……………………
v
TABLE OF CONTENTS CONTENTS PAGE DEDICATION ........................................................................................................................... ii
ACKNOWLEDGEMENTS ...................................................................................................... iii
DECLARATION AND CERTIFICATION ................................................................................iv
TABLE OF CONTENTS ............................................................................................................ v
LIST OF FIGURES ................................................................................................................ viii
LIST OF TABLES .....................................................................................................................ix
ACROYNMS AND ABBREVIATIONS .................................................................................... x
ABSTRACT ..............................................................................................................................xi
CHAPTER ONE: INTRODUCTION .......................................................................................... 1
1.1 Brief Background ..................................................................................................................1
1.2 Statement of the Problem ......................................................................................................1
1.3 Objective of the project .........................................................................................................1
1.4 Project Organization ..............................................................................................................2
CHAPTER TWO: LITERATURE REVIEW .............................................................................. 3
2.1 Advantages of HVDC versus AC Transmission .....................................................................3
2.1.1 Technical Advantages .............................................................................................................. 3
2.1.2 Economic Advantages ............................................................................................................. 4
2.2 Disadvantages of HVDC Transmission over AC Transmission ..............................................6
2.3 Types of DC Links ................................................................................................................6
2.3.1 Monopolar DC Link .................................................................................................................. 6
2.3.2 Bipolar DC Link ........................................................................................................................ 7
2.3.3 Homopolar DC Link ................................................................................................................. 8
CHAPTER THREE: GENERAL HVDC TRANSMISSION LINE DESIGN .............................. 9
3.1 General Considerations .........................................................................................................9
3.1.1 Location of the Line (Line routing) ........................................................................................... 9
3.1.2 Transmission Voltage .............................................................................................................. 9
3.1.3 Conductor Type and Size ....................................................................................................... 10
3.1.4 Line Supports and Cross-arms ............................................................................................... 10
3.1.5 Span, Conductor Configuration, Spacing and Clearance ......................................................... 11
3.1.6 Sag and Tension .................................................................................................................... 12
3.1.7 Insulation .............................................................................................................................. 20
vi
3.1.8 Ground Wire ......................................................................................................................... 21
3.2 Earth Electrode/ Station Earth ............................................................................................. 21
3.2.1 Choice of Material for Earth Electrode................................................................................... 21
3.2.2 Design of Earth Electrode ...................................................................................................... 22
3.2.3 Shape of Earth Electrode ....................................................................................................... 22
3.3 Converter Stations ............................................................................................................... 22
3.3.1 Converter Space Requirements ............................................................................................. 22
3.3.2 Pole Level .............................................................................................................................. 22
3.3.3 Converter Transformers ........................................................................................................ 23
3.3.4 Smoothing Reactors .............................................................................................................. 23
3.3.5 HVDC Switchyard .................................................................................................................. 23
3.3.6 HVDC Filters .......................................................................................................................... 24
3.3.7 HVAC Filters .......................................................................................................................... 24
3.4 Protection Considerations .................................................................................................... 24
CHAPTER FOUR: ETHIOPIA-KENYA HVDC LINE DESIGN.............................................. 25
4.1 Design Specifications .......................................................................................................... 25
4.1.1 HVDC Interconnector Routing ............................................................................................... 25
4.1.2 Choice of Transmission Voltage ............................................................................................. 28
4.1.3 Determination of Conductor Type and Size ........................................................................... 29
4.1.4 Choice of Span, Conductor Configuration and Clearance ....................................................... 32
4.1.5 Sag and Tension Analysis ....................................................................................................... 34
4.1.6 HVDC Insulation Design ......................................................................................................... 38
4.1.7 Choice of Earth/Ground Wire ................................................................................................ 41
4.1.8 Design of Line Supports and Cross-arms ................................................................................ 41
4.2 Design of Converter Stations ............................................................................................... 44
4.2.1 Converter Space Requirements ............................................................................................. 44
4.2.2 Pole Level .............................................................................................................................. 44
4.2.3 Converter Transformers ........................................................................................................ 45
CHAPTER FIVE: DESIGN RESULTS AND ANALYSIS ....................................................... 46
5.1 Results and Discussions....................................................................................................... 46
5.1.1 Routing Adopted ................................................................................................................... 46
5.1.2 Results for Sag Analysis ......................................................................................................... 46
5.1.3 Insulation Length .................................................................................................................. 47
5.1.4 Other Design Aspects Results ................................................................................................ 47
vii
5.1.5 Converter Station Results ...................................................................................................... 48
5.2 Efficiency and Cost Estimation of the Designed HVDC Line .............................................. 49
5.2.1 Efficiency Calculation ............................................................................................................ 49
5.2.2 Cost Estimation ..................................................................................................................... 50
CHAPTER SIX: CONCLUSION AND FUTURE WORK ........................................................ 52
6.1 Conclusion .......................................................................................................................... 52
6.2 Recommendation for Future Work ...................................................................................... 53
APPENDICES .......................................................................................................................... 54
Appendix 1: Schematic diagrams different converter transformer configurations....................... 54
Appendix 2: Comparison of different transformer configurations .............................................. 55
Appendix 3: A map of the area to be affected by the HVDC interconnector [8] ......................... 56
Appendix 4: Conductor surface voltage gradients for a +/- 500kV line [11]. .............................. 57
Appendix 5: Current/power carrying capacity for different conductor types [10] ....................... 58
Appendix 6: Joule Losses Cost [10] .......................................................................................... 59
Appendix 7: Standard insulation levels for range II. (From IEC 60071-1), [11] ......................... 60
Appendix 8: Clearances to withstand lightning over-voltages, EN 50341-1, [11] ....................... 61
Appendix 9: Clearances to withstand switching overvoltages, EN 50341-1, [11] ....................... 62
Appendix 10: The Complete HVDC/HVAC Transmission System [8] ...................................... 63
Appendix 11: Unit Prices for Main Equipment [8] .................................................................... 64
Appendix 12: Cost Estimation of the Complete Project [8]. ....................................................... 64
REFERENCES ......................................................................................................................... 67
viii
LIST OF FIGURES
PAGE
FIGURE 2.1 COST COMPARISON BETWEEN AC AND DC TRANSMISSION .............................................5
FIGURE 2.2 MONOPOLAR DC LINK WITH GROUND RETURN PATH .....................................................7
FIGURE 2.3 BIPOLAR DC LINK WITH A DEDICATED LVDC METALLIC RETURN CONDUCTOR ...............7
FIGURE 2.4 HOMOPOLAR DC LINK WITH TWO HVDC CONDUCTORS AND GROUND AS RETURN ...........8
FIGURE 3.1 PARABOLIC FORM OF A CONDUCTOR BETWEEN SUPPORTS AT SAME LEVEL .................. 12
FIGURE 3.2 ICE COATED CONDUCTOR .......................................................................................... 14
FIGURE 3.3 A CONDUCTOR SUSPENDED BETWEEN SUPPORTS AT DIFFERENT LEVELS ...................... 18
FIGURE 4.1 HVDC 500 KV BIPOLAR LINE CORRIDOR FOR NON-POPULATED AREAS ......................... 33
FIGURE 4.2 HVDC 500 KV BIPOLAR LINE CORRIDOR FOR POPULATED AREAS ................................. 33
FIGURE 4.3 SHIELDING ANGLE VERSUS SHIELD-WIRE AVERAGE HEIGHT ....................................... 40
FIGURE 4.4 SELF-SUPPORTING LATTICE TOWER DESIGN FOR A HVDC 500 KV BIPOLAR LINE .......... 43
FIGURE 5.1 CIRCUITRY DESIGN OF EACH THYRISTOR VALVE ........................................................ 49
FIGURE 5.2 CIRCUITRY DESIGN OF EACH WYE-DELTA PART OF THE THYRISTOR VALVE ................. 49
ix
LIST OF TABLES
PAGE
TABLE 4.1 DETAILS FOR SECTION 1, PART 1: WOLAYTA SODO - ARBA MINCH ............................. 25
TABLE 4.2 DETAILS FOR SECTION 1, PART 2: ARBA MINCH – MEGA ............................................ 26
TABLE 4.3 DETAILS FOR SECTION 2, PART 1: MEGA TO TURBI ..................................................... 26
TABLE 4.4 DETAILS FOR SECTION 2, PART 2: TURBI TO MARSABIT............................................... 27
TABLE 4.5 DETAILS FOR SECTION 3, PART 1: MARSABIT TO ISIOLO .............................................. 27
TABLE 4.6 DETAILS FOR SECTION 3, PART 2: ISIOLO TO ISINYA .................................................... 28
TABLE 4.7 LINE MINIMUM COSTS AND JOULE LOSSES IN CASE OF 50% POWER TRANSFER .............. 31
TABLE 4.8 MINIMUM CLEARANCE STANDARDS FOR 500KV HVDC LINE ........................................ 34
TABLE 4.9 SHIELD-WIRE SAGS FOR DIFFERENT SECTIONS ............................................................. 40
TABLE 5.1 ROUTING SECTIONS AND THEIR RESPECTIVE DISTANCES .............................................. 46
TABLE 5.2 CONDUCTOR SAGS FOR DIFFERENT SECTIONS .............................................................. 46
TABLE 5.3 OBTAINED INSULATION LENGTHS ............................................................................... 47
TABLE 5.4 RESULTS FOR OTHER DESIGN ASPECTS ........................................................................ 47
TABLE 5.5 DESCRIPTIONS OF THE CHOSEN THYRISTORS AND CONVERTER TRANSFORMERS ............ 48
TABLE 5.6 COST ESTIMATE FOR THE PROJECT WHEN FULLY COMPLETED ..................................... 50
TABLE 5.7 COST ESTIMATE FOR THE PROJECT USING HVAC SYSTEM ........................................ 51
x
ACRONYMS AND ABBREVIATIONS
AC Alternating Current
DC Direct Current
KETRACO Kenya Electricity Transmission Company
HVDC High Voltage Direct Current
HVAC High Voltage Alternating Current
EEPCo Ethiopian Electric Power Corporation
ROW Right Of Way
UHV Ultra High Voltage
EHV Extra High Voltage
KPLC Kenya Power Lighting Company
OPGW Optical Ground Wire
ACSR Aluminium Conductor Steel Reinforced
AAAC All Aluminium Alloy Conductor
CIGRE ‘Conseil International des Grands Réseaux Électriques’(French)
IEC International Electro-technical Commission
BIL Basic Insulation Level
SR Synchronous Reactor
AC Alternating Current Filter
DCF Direct Current Filter
EL Electrode Line
KV Kilo Volt
MVA Mega Volt-Ampere
MW Mega Watt
kA Kilo Ampere
xi
ABSTRACT
HVDC transmission is preferred over HVAC transmission for power transmission over long
distances. This is due to the many advantages associated with HVDC transmission as explained
in chapter two of this project. This project thus involved the design of a HVDC transmission line
to transmit power between Ethiopia and Kenya.
The design involved choice of the most feasible routing which relied to much extend on the
feasibility study done by KETRACO Company Limited (Kenya). The entire route was
subdivided into five sections. A bipolar dc link configuration was chosen with a standard
transmission voltage of +/- 500kV. The most economical solution for the conductor type from
the design was ACSR Pheasant, four conductors per pole. The conductor size was 726.79mm
cross-sectional area and 35.10mm diameter.
For each of the five sections subdivided during routing, sags were calculated assuming an
erection tension of 63709N obtained calculations involving the physical conditions of the area
during worst probable conditions and during erection time. The largest sag obtained from the
calculations was 13.42m. Insulation design was then done taking into account the effects of
pollution, lightning and switching over-voltages.
The support towers were also design taking into considerations the values of transmission
voltage, sag obtained, insulation lengths and minimum clearance of the line conductors from the
ground. The designed transmission line required two converter stations; one at Wolayta/Sodo in
Ethiopia and another at Isinya in Kenya.
1
CHAPTER ONE: INTRODUCTION
1.1 Brief Background
The transmission and distribution of electrical energy started with direct current (DC) in the late
19th century, but it was inefficient due to the power loss in conductors since the low voltages
generated then could not be stepped up. Alternating current (AC) offered much better efficiency,
since it could easily be transformed to higher voltages, with far less loss of power. AC
technology was soon accepted as the only feasible technology for generation, transmission and
distribution of electrical energy. The power systems all over the world have been expanded
vastly during the past few decades. Now-a-days large blocks of power are transmitted over long
distances from the remotely located power stations to urban load centres. The main limitations of
long distance power transmission using AC are: voltage regulation problems and system stability
problems. The world's first commercial HVDC transmission link, was built in 1954 between the
Swedish mainland and the island of Gotland, with a rating of 20 MW, 200 A and 100 kV. This
was a monopolar dc link which made use of sea return.
1.2 Statement of the Problem
Electric power transmission can be done using either direct current or alternating current
transmission systems. In both systems several design specifications are followed in order to
come up with the most feasible and reliable system. For long distance power transmission
HVDC systems are preferred over ac systems. As a result the Kenya-Ethiopia Interconnector
adopted a HVDC design approach. A detailed design of the HVDC transmission line for Kenya-
Ethiopia interconnector would bring out the design criteria for the HVDC transmission lines and
their advantages over the AC ones.
1.3 Objective of the project
The objective of this project was to achieve the design specifications of a High Voltage Direct
Current (HVDC) transmission line from Ethiopia to Isinya (Kenya). The design specifications
considered in the design included:
1. The most feasible routing.
2. The standard transmission voltage.
2
3. The type and size of conductor required.
4. The required span, conductor configuration, spacing and clearance.
5. The optimum sags and tension on the conductors.
6. The insulation design required.
7. The choice of the ground/earth wires.
8. The type and requirements of the line supports and cross-arms.
1.4 Project Organization
This project is organised in six chapters:
• Chapter one gives the introduction to the project, the problem statement and the project’s
objective.
• Chapter two describes the advantages and disadvantages of the HVDC transmission
systems compared to the ac systems. It also gives the different types of dc links.
• In chapter three the general design specifications for a HVDC transmission line are
explained in detail.
• Chapter four gives the procedures followed in meeting the project’s objective.
• The results obtained and their analyses are done in chapter five, this chapter also presents
the efficiency of the designed transmission line and its cost as an estimate.
• Chapter six gives the conclusion and the recommendations for future work. The project
ends by outlining the appendices and references used.
3
CHAPTER TWO: LITERATURE REVIEW
2.1 Advantages of HVDC versus AC Transmission
The reasons behind a choice of HVDC instead of AC to transmit power in a specific case are often
numerous and complex. Each individual transmission project displays its own set of reasons justifying the
choice. Some of the advantages of HVDC transmission over the AC one are explained below:
2.1.1 Technical Advantages
1. Less Reactive Power Requirement (Long Distance Water Crossing)
In a long distance AC transmission, the reactive power flow due to the large conductor
capacitance will limit the maximum transmission distance. With HVDC there is no such
limitation thus, for long conductor links; HVDC is the most viable technical alternative [1].
2. Asynchronous Connection (No System Stability Problems)
It is sometimes difficult or impossible to connect two AC networks due to stability reasons. This
is because an ac system must remain in synchronism and to maintain stability under transient
conditions, the length of a 50Hz uncompensated ac line must be less than about 500km, though
the value can increase in case of series compensated lines. The dc transmission has no such
limitations and thus no stability problems [2].
3. Limited Short Circuit Current
A HVDC transmission does not contribute to the short circuit current of the interconnected AC
system. However, the interconnection of two ac systems by an ac line increases the short-circuit
current in the system [3].
4. Increased Controllability (Independent Control of AC Systems )
The ac systems interconnected by a dc line can be controlled independently. The two systems
can be completely independent as regards frequency, system control, short circuit rating and
future expansion [1].
5. Fast Change of Energy Flow
The change in the direction of energy flow can be achieved faster when two ac systems are
interconnected by a dc line. This phenomenon allows for a more flexible coordination of system
4
control at the two ends, a more economical use of cheap power generation in either of the two ac
systems and reduction of system reserve and standby capacity [2].
6. Less corona Loss and Radio Interference
Especially during bad weather conditions the corona loss and radio interference are lower for a
dc line as compared to those in an ac line of same conductor diameter and voltage [3].
7. Greater Reliability
In a two-conductor bipolar dc line in the event of a fault on one conductor, the other conductor
can continue to operate with ground return; this phenomenon makes the bipolar dc line more
reliable than a 3-conductor 3-phase ac line. Also a monopolar dc line with ground return is
simpler than a 3-phase ac line and is equally reliable [2].
8. Less Potential Stress
The potential stress on the insulation in case of dc systems is () times of that in ac system for
the same working voltage [4]. Hence for same working voltage less insulation is required. Also
underground cables can be used because of less potential stress and negligible dielectric loss.
2.1.2 Economic Advantages
Dc transmission offers many economic advantages over an ac transmission especially for long
distance bulk power transmission which include:
a. Less Investment Cost
A HVDC transmission line costs less than an AC line for the same transmission capacity.
However, the terminal stations are more expensive in the HVDC case due to the fact that they
must perform the conversion from AC to DC and vice versa. On the other hand, the costs of
transmission medium (overhead lines and cables), land acquisition/right-of-way costs are lower
in the HVDC case. Moreover, the operation and maintenance costs are lower in the HVDC case.
Initial loss levels are higher in the HVDC system, but they do not vary with distance. In contrast,
loss levels increase with distance in a high voltage AC system [3]. Above a certain distance, the
so called "break-even distance", the HVDC alternative will always give the lowest cost.
5
Figure 2.1 gives the comparison of the cost for dc and ac transmission with reference to distance
of transmission.
Figure 2.1: Cost comparison between ac and dc transmission
b. Simple and Cheap
A dc line is simpler and cheaper as compared to an ac line. The towers of a dc line are also
simpler, cheaper and narrower as compared to the towers of ac lines. The right of way
requirement of a dc line is about 20-40 % lesser than that of an ac line of the same power
transmission capacity. Also in this scheme power can be transmitted through one conductor by
using earth as returning conductor, hence much copper is saved [3].
c. Lower Losses
An optimized HVDC transmission line has lower losses than AC lines for the same power
capacity. The losses in the converter stations have of course to be added, but since they are only
about 0.6 % of the transmitted power in each station, the total HVDC transmission losses come
out lower than the AC losses in practically all cases [3].
d. Stage Construction
An HVDC transmission line can be built in stages. Initially the line can be built as a monopolar
line with ground return. Later on the line can be converted into a bipolar line. This aspect
becomes very important when there is a long period between the successive stages because the
investment for the future extension is not required to be made initially [2].
6
e. Environment Friendly
The land coverage and the associated right-of-way cost for a HVDC overhead transmission line
is not as high as for an AC line. This reduces the visual impact. It is also possible to increase the
power transmission capacity for existing rights of way. There are, however, some environmental
issues which must be considered for the converter stations, such as: audible noise, visual impact,
electromagnetic compatibility and use of ground or sea return path in monopolar operation [3].
2.2 Disadvantages of HVDC Transmission over AC Transmission
Some of the disadvantages of power transmission using High Voltage Direct Current systems
over the Alternating Current systems are as listed below:
1. The converters are expensive. This makes the terminal costs of HVDC systems quite high
compared to those of ac systems.
2. The converters require much reactive power for the commutation of current from one
phase to the other. This reactive power varies with the transmitted power but is
independent of line length. It has been found that for lines greater than 400km in length, a
dc line requires less reactive power than an ac line [2].
3. Converters generate some harmonics. As a result filters should be installed to filter the
main harmonics.
4. Multi-terminal or network operation is not easy.
2.3 Types of DC Links
2.3.1 Monopolar DC Link
This type of dc link consists of one high voltage conductor, usually of negative polarity and uses
ground or sea water as the return conductor. The negative polarity is preferred on overhead lines
due to low radio interference [2]. This scheme is advantageous from an economic point of view,
but is prohibited in some countries because the ground current causes corrosion of pipe lines and
other buried metal objects. In most cases this type of dc link is used for submarine crossings.
Figure 2.2 shows a monopolar dc link with ground return path.
7
Figure 2.2: Monopolar dc link with ground return path
2.3.2 Bipolar DC Link
This type consists of two high voltage conductors, one with positive and the other negative
polarity. At each terminal two converters of equal rated voltages are connected in series on the
dc side. The neutral points (the junctions between converters) are grounded at one or both ends.
If both neutrals are grounded, the two poles operate independently [2]. If there are restrictions to
use of ground electrodes or if the transmission distance is relatively short, a dedicated low
voltage direct current (LVDC) metallic conductor is used as the return path [5]. The rated voltage
of this dc link is expressed as say +/-500KV. Figure 2.3 shows this type of a dc link with a
dedicated LVDC metallic conductor as return path.
Figure 2.3: Bipolar dc link with a dedicated LVDC metallic conductor as return path.
8
2.3.3 Homopolar DC Link
It has two (or more) conductors all having the same polarity (usually negative) and always
operates with ground as the return conductor. If a fault develops at one conductor the converter
equipment can be reconnected so that the healthy conductors (having some overload capacity)
can supply more than half the rated power.
Figure 2.4 shows a homopolar dc link with two conductors and ground as the return path.
Figure 2.4: Homopolar dc link with two HVDC conductors and ground as return path.
9
CHAPTER THREE: GENERAL HVDC TRANSMISSION LINE DESIGN
3.1 General Considerations
3.1.1 Location of the Line (Line routing)
Thorough investigations need to be done before the routing of a HVDC transmission line is
determined. Before coming up with the most desirable and practical route the following main
factors should be considered:
1. Cost of construction of the desired line.
2. Cost of clearing the path where the line is to pass through.
3. Cost of maintenance of the HVDC transmission line.
4. The accessibility costs of the line.
The above costs should be as low as possible so as to ensure economic viability of the whole
HVDC system.
3.1.2 Transmission Voltage The line voltage determines to a great extent the cost and performance of a transmission line. A
high voltage is usually adopted for transmission due to the following reasons:
• The size of the conductor is reduced with increase in transmission voltage which in turn
reduces the cost of the supporting structure materials.
• With the increase in transmission voltage line current is reduced which results in
reduction of line losses hence higher efficiencies.
• At high transmission voltages the low line currents result in low voltage drops in the lines
leading to better voltage regulation.
An empirical formula for the optimum transmission voltage for line more than 30km long has
been provided in the Cable Research Hand Book to avoid complications and labour involved [2]
= . .
+
(3.1)
Where
V = line voltage in Kilovolts.
L = transmission distance in kilometres.
P = power transmitted in kilowatts.
10
A standard voltage nearest to this value should be adopted. The above formula gives only a
preliminary estimate. Once the preliminary estimate is available, a detailed study of both
technical and economical aspects can be done in order to come up with the best choice of the
transmission voltage.
3.1.3 Conductor Type and Size
In selection of the conductor size several methods can be employed. One of the methods is
based on the steady temperature which is reached when theIR losses are equal to the heat
losses due to both convection and radiation. This criterion for selection of conductor size is
applicable for lines up to 220KV [2]. Kelvin’s law may also be employed [4]. For High Voltage
(HV), Extra High Voltage (EHV) and Ultra High Voltage (UHV) transmission lines, the
considerations of corona and radio frequency interference have to be taken into account.
Conductors selected on the basis of corona and radio frequency interference considerations
would normally be thicker than the ones selected from the point of view of current carrying
capacity. The most commonly used conductors are copper, aluminium and steel. Silver is a better
conductor than copper; but its mechanical weakness and high cost eliminate it as a practical
conductor [6].
3.1.4 Line Supports and Cross-arms
The supports for the overhead HVDC line chosen must be able to carry the load due to the
chosen conductors and the insulations. This load includes the ice and wind loads on the
conductors together with the wind load on the support itself. The most commonly used line
supports are wooden poles, Reinforced Cement Concrete (RCC) poles, steel tube poles (which
are all used for voltages up to 33KV) and steel towers (which are used for voltages of 66KV and
above) [2]. Thus for HVDC transmission only steel towers are used. These towers are fabricated
from painted or galvanized steel angle sections which are usually transported separately and
erection done on site. Their advantages include:
• Ability to withstand very severe weather conditions.
• They have a very long life.
• They have a high degree of reliability.
• The risk of service interruptions due to damage of insulators is greatly reduced since
large spans can be used.
11
• They are very suitable for double circuit lines.
They also posses some disadvantages which include:
• They are very costly.
• They are bulky and have to be assembled on site.
The height of the tower depends on the line voltage and the span length. The legs of the towers
are set in special foundations. Overhead HV, EHV and UHV lines mostly use self supporting
steel towers fabricated from galvanized steel angle sections. There are mainly two common
configurations of steel towers; tangent towers and angle (deviation) towers.
In design, the shape and length of cross-arms depend on the line voltage and the desired
conductor configuration. Both vertical and horizontal loads are taken into account when
designing cross-arms. Normally, an allowance is made for the loads imposed on the cross-arms
due to change in the line profile especially in hilly areas [2].
3.1.5 Span, Conductor Configuration, Spacing and Clearance
Longer span results to use of few towers but the towers are taller and more costly. So as to
reduce the high cost of insulators the higher the operating voltage, the span should be long. Since
insulators constitute the weakest part of the transmission line, reduction in their use results to
increased reliability of the line. Modern high voltage lines have spans between 200m and 400m.
For river and ravine crossings, exceptionally long spans of up to 800m are employed [2].
Both symmetrical and unsymmetrical conductor configurations are used in practice. Practically,
a flat horizontal configuration results in a lesser tower height but a wider right of way. On the
other hand, a vertical configuration results in taller towers and thus increased lightning hazards.
During transmission line design, adequate spacing between conductors should be kept in order to
avoid conductors from coming within sparking distance of each other even while swinging due
to wind. The spacing of aluminium conductors can be calculated by:
= √+
(3.2)
Where; S=line sag in meters.
V=line voltage in kilovolts. [2]
12
3.1.6 Sag and Tension
Sag and tension analysis of a conductor is an important consideration in overhead transmission
line design. The continuity and quality of electric service depend largely on whether the
conductors have been properly installed.
3.1.6.1 Sag between Supports at the Same Level
Figure 3.1 is used to analyze the sag and tension for a conductor between supports at the same
level and with the same height [2].
Figure 3.1: Parabolic form of a conductor between supports at same level.
The conductor is assumed to be flexible and sags below the level AB due to its weight. The exact
shape of the conductor is that of a catenary. With an exception of conductors with very long
spans and very large sags, the shape of the conductors is assumed to be that of a parabola
(where a equals the constant for a given conductor with O as the origin) and this
assumption provides ease of calculation with sufficient accuracy.
Letting; l = length of span, in meters.
S = sag at mid span, in meters.
T= conductor tension in Newtons.
w= conductor weight, in Newtons per meter.
y= S, when
From (3.3)
13
Hence, =
(3.4)
Thus, =
Giving =
(3.5)
Considering the equilibrium of half line OA and assuming the conductor to be almost horizontal
and taking moments about A,
=
And =
(3.6)
Substituting these values of S:
=
(3.7a)
=
(3.7b)
3.1.6.2 Sag and Tension Analysis Taking Into Account the Effects of Ice and Wind
A transmission line conductor is also subject to wind pressure in addition to its own weight. A
coating of ice may also be formed on the conductors of the lines in hilly areas during severe
winter conditions. The two factors should thus be taken into account when calculating the sag or
tension of the conductor.
A one meter length conductor is considered having the following parameters:
d= diameter of conductor, in meters.
t= radial thickness of ice on the conductor, in meters.
Figure 3.2 shows the conductor in consideration [2].
14
Figure 3.2: Ice coated conductor.
The overall diameter of ice covered conductor as seen from Figure 3.2 is given by:
D d 2t Meters (3.8)
Thus volume of ice per meter length of conductor is given as;
V π
π
m
V π
D dm (3.9)
The weight of ice is approximately 8920N/m. Thus the weight of ice per meter length of
conductor is then given as,
w 8920 π
D dN/m (3.10)
w 2230πd 2t dN/m
w 2230πd 4t 4dt dN/m
w 8920πdt tN/m
w 8920πtd tN/m (3.11)
For design purposes, the wind pressure is assumed to act horizontally on the projected area of the
ice covered conductor. From Figure 3.2 it is seen that this projected area is D square meters per
15
meter length of the conductor. Thus for a wind pressure of p Newtons per square meter of
projected area, wind load is given by:
F = pD N/m (3.12)
With the wind velocity specified, the wind pressure p is taken as 0.059vN/m [2]
Where, v = wind velocity in km/hr.
Therefore the total force acting on the conductor per meter length is given by:
F = w + w + F .N/m (3.13)
This force Ft lies in the new plane of conductor and is inclined to the vertical at an angle given
by:
γ = tan
(3.14)
Equations (3.5) and (3.7) are still valid if all the measurements are made in the new plane of
conductor and with w replaced by. If T is the limiting tension and Ft is the total force per
meter on the conductor under worst probable conditions, then using Equation (3.6), the sag in the
new plane is given as:
S =
meters (3.15)
Thus the vertical sag is given by:
S = S cos γ (3.16)
When the effect of ice is absent then Equation (3.13) becomes;
F = w + F.
N/m (3.17)
And Equation (3.14) changes to:
γ = tan
(3.18)
16
3.1.6.3 Determining Total Length of Conductor
Considering a small right angled triangle on the parabola = in the Figure 3.1, it gives:
ds = dx + dy (3.19)
Dividing both sides by dx :
= 1 +
= 1 + 4ax
This gives,
ds = 1 + 4ax.dx≅ 1 + 2axdx (3.20)
Integrating Equation (3.20),
s = x +
+ k
When x = 0, s = 0. Therefore, k = 0
Thus, s = x +
(3.21)
s is the curved distance from O to P(x, y).
With z is the total length of the conductor line, then = in the Equation (3.21). Substituting the
value of from Equation (3.7a), the length of half the conductor line is determined, that is.
Thus,
=
+ ×
×
× =
+
This gives,
z = +
(3.22)
Substituting the value of S in the above equation,
z = (1 +
) (3.23)
3.1.6.4 Effect of Erection Conditions on Sag and Tension Analysis
In practice, the ambient conditions during line erection are very different from the worst
probable conditions. The regulations require that sag must be calculated for the worst probable
conditions and minimum ground clearance should be maintained for these conditions. According
17
to the regulations, the maximum tension in the conductor should not exceed half the breaking
load [7]. This is maintained so as to allow a factor of safety of two. The erection tension should
be such that if loading conditions subsequently occur there will be no infringement on the factor
of safety.
If the quantities under the worst probable conditions are denoted by subscript 1 and those under
the erection conditions denoted by subscript 2; then z, T, θandFdenote the conductor
length, tension, temperature and total load per meter under the worst probable conditions.
z, T, θandF denote the same quantities under the erection conditions. A relationship
between the two conditions is derived so as to show the interdependence between the two set of
conditions.
An increase in tension from TtoT elongates the conductor by
meters,
Where, A is the conductor cross-sectional area, and
E is the Young’s modulus of elasticity.
The increase of temperature from θ to θ increases the conductor length by
αzθ − θ
Where; α is coefficient of linear expansion.
Thus, z = z +
+ αθ − θ (3.24)
Substituting forz and z from Equation (3.23), then the above equation becomes,
1 +
= 1 +
+
+ α(θ − θ) (3.25)
Using the binomial expansion and neglecting second order terms Equation (3.25) reduces to,
()
−
= ()
+ α(θ − θ) (3.26)
After rearranging the terms in the above equation it can be written as,
T T − T − αAEθ − θ−
=
(3.27)
18
This results to a cubic equation which can be solved graphically or analytically. Using this
derived equation erection tension T1 is determined such that the tension T2 (that is, tension under
worst probable conditions) will not exceed the safe limit of tension. The sag for erection
conditions is then calculated using this value of tension.
3.1.6.5 Sag and Tension Analysis with Supports at Different Levels
In several occasions, especially in hilly areas, the two supports of a span are at different levels.
Figure 3.3 shows a section of a conductor suspended between two supports B and C which are at
different levels.
Figure 3.3: A conductor suspended between supports at different levels
The curve BOCA is the complete parabola with A and B at the same level. The actual line BOC
is a part of this complete parabola.
If is the actual span (that is horizontal distance between B and C),
is the span of the complete parabola from B to A,
And (x, y are the coordinates of point C.
Then,x (3.28)
Since the equation of parabola that is y ax is valid for both the curve BOCA and BOC,
Equation (3.6) still holds.
Substituting the coordinates of point C in Equation (3.7b),
y S h
19
Or
=
(3.29)
Substituting the values of S and x1 from equations (3.6) and (3.28) into equation (3.29),
=
(3.30)
Re-arranging the terms in the equation (3.30),
= +
(3.31)
Since h, T, andw are known, can be found out. The formula given by equation (3.31) is
valid even when the two supports B and C fall on the same side of origin O (that is, spanl is less
than
).
3.1.6.6 Factors Affecting Sag
As seen from the above analysis, sag is mainly affected by the following factors:
1) Weight of the conductor - In general, sag is directly proportional to weight per unit length
of the conductor. Ice and wind loads also increase the conductor line sag.
2) Span (distance between adjacent supports) - Sag is proportional to the square of the span,
thus a longer span results in an increase in sag. An increase of 25% in span is seen to increase
the sag by 56.25%.
3) Conductor tension - Sag is inversely proportional to the conductor tension. However, an
increase in conductor tension causes more stresses in the conductor and more load on
insulators and towers.
4) Ground clearance - Electricity rules specify minimum clearance between the ground and the
conductors. To maintain this minimum clearance when a high value of sag is desired, tall
towers are used.
5) Temperature - Sag increases with increase in temperature and a decrease in temperature
reduces the sag. However, when a decrease in temperature is accompanied by snow and ice,
the sag may increase due to the effect of the snow and ice.
20
3.1.7 Insulation
To determine whether or not an insulator can be used, both its mechanical strength and electrical
properties are considered. The suspension insulators are the ones commonly used in high
voltages [6]. The line insulation chosen should be sufficient to take care of the following three
kinds of over-voltages:
3.1.7.1 Switching Over-Voltages
In HVDC lines, these over-voltages can be initiated when energizing and reclosing, during fault
interruption, dropping of lines or load rejections. These surges have infinite wave shapes. Surges
are affected by meteorological conditions such as relative air density, humidity, rain and wind.
Insulation design can be done in two ways:
a) Handbook method - in this method, a certain high value of surge strength and some
meteorological conditions are selected and the insulation designed for these conditions.
b) Reliability method - here an effort is made to access the relative frequency of occurrence
of all the combinations of electrical and meteorological events. The effect of insulation
increase on the line performance is evaluated and the insulation strength decided on the
basis of satisfactory performance and reasonable investment [3].
3.1.7.2 Temporary Power Over-Voltages
These are usually due to sudden loss of load. Flashovers caused by these over-voltages are
common during morning hours when dew or fog is present. This is due to an interaction of a thin
layer of contaminant on the insulator surface with a deposit of moisture. If the insulation design
of the line is adequate for switching surges then it can cater for the temporary over-voltages.
3.1.7.3 Lightning Over-Voltages
Lightning performance of a line is not easily predicted due to the difficulty in estimating the
most probable number of outages due to lightning per 100km per year. The lightning outage of a
line depends on many probabilities which include: probability of a thundercloud coming over a
line, probability of a lighting stroke from the thunderstorm to the line, the probability that the
stroke will land at a particular spot on the line (tower or mid span), the probability that the
magnitude of current due to lightning stroke will be beyond a certain magnitude and the
probability of occurrence of a flashover [2].
21
3.1.8 Ground Wire
Ground wires are used to shield the current carrying conductors from the lightning strokes by
dissipating the lightning currents to the ground. They may be one or two. These wires are placed
above the current carrying conductors and are grounded at each or alternate tower. These wires
are commonly made from galvanized steel. In design of high voltage lines, a shielding angle of
30° is considered adequate; however this shielding angle increases to 45° for high voltage lines
in areas with low lightning hazards. For extra high voltage lines the shielding angle is kept at
about 20° [2]. In design, the ground wires should have lesser sags as compared to the current
carrying conductors. A ground wire should have sufficient mechanical strength and be able to
carry the maximum expected lightning current without overheating. The size of ground wire is
generally chosen on the basis of mechanical strength since experience has shown that if this is
satisfied, the wire can carry the maximum lightning current without overheating [2].
3.2 Earth Electrode/ Station Earth
To allow for safe operation after implementation, suitable ground electrodes have to be installed
at each terminal station of the HVDC transmission line. The terminal station is connected to the
earth electrode through an insulated cable known as the earth electrode line. An array of
interconnected conductors referred to as an earth mat or grid is placed in earth at each terminal to
protect the equipment from over-voltages and for safety of personnel. This is done by ensuring
that all the equipment’s earth terminals and the neutral of the convertor transformers are
connected to this earth mat. There is no direct metallic link between earth electrodes and earth
grid.
3.2.1 Choice of Material for Earth Electrode
Since the direction of earth current may reverse due to change in power transmitted, any of the
two electrodes may act as an anode. Thus, the electrolytic corrosion of the anode is an important
configuration in selection of material and design of earth electrode. Iron has a high rate of
corrosion as compared to graphite and hence it is not used. Also, when graphite is directly buried
in earth there is significant loss of material due to corrosion. To reduce this loss of material,
graphite electrodes buried in a pit filled with crushed coke are used and thus the electrode
transfers the current to the coke which distributes it to the earth.
22
3.2.2 Design of Earth Electrode
The following design aspects govern the design of an earth electrode [2]:
a) The current density at the electrode surface should not exceed 1.5A/m.
b) The temperature of the electrode and its surrounding should be limited to 60 .
c) The earth resistance should be as low as possible.
d) The step voltage on the ground surface above the electrode should be within safe limits.
3.2.3 Shape of Earth Electrode
There are usually three commonly used configurations for earth electrodes, which include:
1. Straight electrodes.
2. Ring electrodes.
3. Radial star electrode. [2]
3.3 Converter Stations
For a HVDC link, two converter stations are required, one at each end of the HVDC transmission
line. In operation, one station operates as a rectifier station while the other operates as an inverter
station. In most cases, each of the two stations is bi-directional in that it can perform both
rectification and inversion depending on the power flow.
3.3.1 Converter Space Requirements
The space requirements for converters are determined according to the power capacity and
voltage of the DC link. The main components at the converter site are outdoor DC switchgear,
control building, valve halls, rectifier transformers, smoothing reactors, DC switchyard, DC
filters and AC filters [8].
3.3.2 Pole Level
In line with today's industrial standard for a HVDC system, two six-pulse valve groups are
connected in series on the DC side to form a 12-pulse group. Each 6 pulse group is connected to
a separate secondary winding of the converter transformer. Secondary winding connections in
delta and star arrangements are provided for a 30° phase shift, thus eliminating lower order
harmonics [5]. Consequently, AC filters are used for the filtering of the high harmonics.
23
3.3.3 Converter Transformers
The converter transformers are used to step down the ac voltages to suitable values for feeding
the converters. Generally, there are three basic transformer arrangements for connecting the 12-
pulse valve groups to the three-phase HVAC system:
a) Two three-phase two-winding transformers.
b) Three single-phase three-winding transformers.
c) Six single-phase two-winding transformers.
Appendix 1 shows schematic diagrams of the three alternative connections, while Appendix 2
gives a comparison of the three configurations, citing some advantages and disadvantages of
each of them. Selection of the converter transformer bank is primarily dependent on the
maximum weight that can be transported to the site and the spare part policy [8].
3.3.4 Smoothing Reactors
DC smoothing reactors are used in the high voltage side of each pole. These help to reduce
current ripple on the DC system, protect the valves against steep-fronted surges caused by
atmospheric over-voltages and limit DC fault currents. Smoothing reactors are available as air-
cooled / air-insulated reactors or as oil-immersed reactors. In most designs air-cooled reactors are
used because they are generally cheaper than oil-immersed reactors and require less
maintenance, but need larger clearances as compared to other installations [9].
3.3.5 HVDC Switchyard
The HVDC switchyard incorporates all equipment required for conversion, operation, protection
and control of the DC power. It is located next to the valve hall, separated from the HVAC side.
The equipment here includes switches to isolate lines, valves and electrodes from each other as
well as associated grounding switches. Circuit breakers are considered in the neutral circuit of
each pole. Apart from switching equipment, metering and measuring transducers form an
essential part of the DC switchyard since all controls and protection rely on directly measured
values of the actual currents and voltages before and after the smoothing reactors, across the
valves and on the earth electrode.
24
3.3.6 HVDC Filters
To avoid interference with telephone lines running parallel to the DC overhead lines, DC filters
are required to absorb the harmonic currents. Active DC filters are recommended, because they
can reduce harmonics to lower levels than passive filters. Furthermore, they are cheaper and need
less space for installation as compared to passive filters [5].
3.3.7 HVAC Filters
The HVAC filters are designed to filter the main harmonics and to provide reactive power for the
converter station. The HVDC converter station requires reactive power for the commutation of
current from one phase to the other. The reactive power demand depends on the trigger delay
angle, angle of overlap and on the operating current and voltage of the HVDC transmission [8].
3.4 Protection Considerations
The following principles are considered in protection of HVDC systems:
• Every fault condition must be detectable by at least two different protection systems.
• Spurious trips of protection systems must be avoided as they can – especially in HVDC
transmission link - give rise to serious disturbances in both receiving and sending AC
systems.
• Fast protection systems are designed by providing overlapping zones of protection for
each equipment item. Coordination between protection on the AC and DC sides is
required in order to ensure optimum DC power transfer following DC or AC network
disturbances.
• Testing of one of the duplicated protection equipments must be possible without affecting
the equipment in service and without introducing any operational constraints.
The protection systems for the HVDC transmission link may act in one of the following ways:
1. Converter blocking.
2. Forced retard - forcing the converter control system to operate at a firing angle
corresponding to full inverter operation.
3. Opening the pole neutral switch, thus resulting to complete pole isolation.
4. Disconnecting the pole from the HVDC line, that is complete line isolation.
25
CHAPTER FOUR: ETHIOPIA-KENYA HVDC LINE DESIGN
4.1 Design Specifications The complete transmission link was to be from Gibe III in Ethiopia to Isinya in Kenya. The
distance was approximated to be about 1200 km. From the power system analysis and the
preliminary results of the economic/financial evaluation done by KETRACO, a preference for a
solution which was fully or partially based on a long-distance HVDC link was proposed [8]. The
design specifications below cover the design of a fully based HVDC link between the two
terminals:
4.1.1 HVDC Interconnector Routing
A desktop survey and site visits were performed in close cooperation with the Engineering and
Transmission Line Departments of KETRACO, EEPCo and KPLC. On Ethiopian territory, the
choice of the routing relied mostly on a detailed survey provided by EEPCo on the most
promising route from Wolayta/Sodo to Mega. On Kenyan territory, a desktop survey was
performed for the entire line route and site detailed surveys done to support the optimum location
of the HVDC/HVAC terminal. The general overview map for the whole project area showing the
line route is given in Appendix 3.
The most economical and feasible route for the interconnector was divided into three sections,
whose details are given in Table 4.1 to Table 4.6.
4.1.1.1 Section 1: Wolayta/Sodo to Mega
The total length for section 1 was 382.2 km, described briefly in Table 4.1 and Table 4.2.
Table 4.1-Details for Section 1, part 1: Wolayta Sodo - Arba Minch
Aspect Description
1. Line route. Preliminary line route was about 110km
2. Geo-
morphology
Gradually the line route comes down on hilly areas of about 2000m above
sea level close to Damota mountains. Over a 30km length the altitude
becomes 1200m. The line route continues at this altitude or slightly higher
up to Arba Minch which is at about 1500m above sea level.
3. Hydrology Approximately 60 km was to be close to the Lake Abaya, passing a lot of
26
small rivers. The terrain was seen to be suitable for agriculture.
4. Restrictions No restrictions.
5. Maintenance Line route followed the main road Sodo-Arba Minch-Mega, to facilitate its
construction and maintenance.
6. Vicinity Villages: Sodo, Tebela, Korga, Birbir, Lante, Arba Minch.
Table 4.2-Details for Section 1, part 2: Arba Minch – Mega
Aspect Description
1. Line route Preliminary line route length was about 271 km
2. Geo-
morphology
Arba Minch is located West of Sodo city, at about 1500m above the sea
level. The first 35 km line passes mountain areas. After that, the line crosses
mountain and hilly areas at altitudes between 800 m and 1200 m above sea
level up to north side of the Mega village.
3. Hydrology Approximately 35 km line was to be close to the Lake Shamo, passing a lot
of small rivers. The terrain was suitable for agriculture.
4. Restrictions No restrictions.
5. Maintenance The line route followed the main road Sodo-Arba Mianch-Mega, to facilitate
its construction and maintenance.
6. Vicinity
On the first 40 km, Lake Shamo is located on the East side and mountains
area in West side. In the proximity of the line were villages: Arba Minch,
Arguba, Gata, Bekawile, Elwega, Hidiale, Yabelo, and Mega.
4.1.1.2 Section 2: Mega to Marsabit
The total length of the line within this section was 237.7 km. A description for section 2 is given
in Table 4.3 and Table 4.4.
Table 4.3-Details for Section 2, part 1: Mega to Turbi
Aspect Description
1. Line route The straight connection was about 107 km
2. Geo-
morphology
The line route passed a flat area with small hills for the first around 25km
and gradually came down from 1200m to around 500 m above sea level. The
route crossed the border between Ethiopia and Kenya West of Moyale
27
village.
3. Hydrology The line route crossed a lot of small rivers. The terrain was suitable for
agriculture.
4. Restrictions No restrictions.
5. Maintenance Maintenance and constructions is more difficult.
6. Vicinity No village and roads were in the proximity of the line.
Table 4.4-Details for Section 2, part 2: Turbi to Marsabit
Aspect Description
1. Line route The preliminary line route length was about 131 km.
2. Geo-
morphology
The line route met flat and hilly areas at the extremities of the section and
mountainous area at middle. The altitudes were between 800 m and 1500 m
above the sea level.
3. Hydrology The line route crossed a lot of small rivers. The terrain was suitable for
agriculture.
4. Restrictions
The restrictions were related to the Marsabit National Reserve. The
possibility to avoid the crossing of the National Reserve was a target for the
detailed line survey.
5. Maintenance The line route followed the main road Sodo- Mega- Moyale-Isiolo, to
facilitate construction and maintenance.
6. Vicinity In the proximity there were a few villages, more important was Marsabit.
4.1.1.3 Section 3: Marsabit to Isinya
The description of section 3 is given in Table 4.5 and Table 4.6.
Table 4.5-Details for Section 3, part 1: Marsabit to Isiolo
Aspect Description
1. Line route The preliminary line route length was about 209km The revised line route
was to pass Isiolo on the East to avoid the Isiolo town.
2. Geo-
morphology
The area was flat, with minimum altitude variations. The altitude was
approximately 1500m above sea level.
3. Hydrology The line route crossed a lot of small rivers. The terrain was suitable for
28
agriculture.
4. Restrictions The restrictions were in Losai National Reserve and Buffalo Springs Game
Reserve areas.
5. Maintenance The line route followed the main road Sodo-Mega- Moyale-Isiolo, to
facilitate construction and maintenance.
6. Vicinity In the proximity of the line route were few villages; more important were
Loglogo and Isiolo.
Table 4.6-Details for Section 3, part 2: Isiolo to Isinya
Aspect Description
1. Line route The preliminary line route length was about 289 km.
2. Geo-
morphology
The relief was a mountainous area, altitudes between 1500m and 2500m
above the sea level. The line route followed the East side of Mount Kenya.
3. Hydrology The line route crossed a lot of small rivers and avoided Lake Thika. The
terrain was suitable for agriculture.
4. Restrictions The restrictions were Mount Kenya, Meru and Nairobi National Parks, and
Mwea National Reserve.
5. Maintenance The line route crossed many roads to facilitate construction and maintenance.
6. Vicinity In the proximity of the line were many villages, more important being Isiolo,
Meru, Chuka, Embu, Thika, Athi River and Isinya.
4.1.2 Choice of Transmission Voltage
The HVDC line designed was to have a capacity of 2000MW. A bipolar scheme was adopted for
the line with each conductor having to accommodate 1000MW. Using the empirical formula
given in Equation (3.1) the transmission voltage was approximated to be:
V = 5.5 .
+
.
= 570.25KV (4.1)
A standard +/- 500 kV HVDC bipolar scheme was chosen for the HVDC link so as to ease the
choice of standard rated power equipment during installation.
29
4.1.3 Determination of Conductor Type and Size
The HVDC line design included the economic optimization of the conductors, considering
applicable international standards. The lines were to be protected against lightning discharges by
two earth wires, that is, one ground wire and one OPGW type. The +/-500 kV, 1200km long
Bipolar HVDC Line from Wolayta/Sodo to Isinya was designed without metallic return.
Conductors used had to satisfy the following:
• Transfer of a maximum design power of 2000 MW at +/- 500 kV nominal voltages on the
bipolar line without neutral conductor.
• Transfer continuously the specified maximum continuous overload for the pole operation
over ground return, assumed 1500 MW.
• Provide safety of the line, considering the mechanical loads from wind.
• Provide satisfactory radio interference (RI), audible noise (AN) and corona loss
performances.
The optimal conductor selection process was complex with the choice of suitable conductor (and
sub-conductors) depending on the operating voltage, the power to be transmitted and the
acceptable voltage drop and losses in the conductor. Thereafter, radio interference, audible noise
and corona losses needed to be evaluated to find the optimum conductor bundle.
4.1.3.1 Voltage Drop Considerations
For a single conductor configuration, maximum power to be transferred assuming a certain
percentage drop restriction voltage is given by Equation (4.2): [8]
=
%× × (4.2)
Where: V = Sending end voltage, pole to ground, in our case 500 kV.
% = Percentage drop in voltage.
= DC resistance of the conductor in Ω / km.
L = Distance in km.
Thus, to transfer 1500 MW per one pole, with maximum 10% drop voltage, the pole resistance
needed to be less than: =
× ( )× (4.3)
Thus, =
× × = . / (4.4)
30
Assuming four conductors per bundle (per pole), then the resistance of a single strand used to
make the four bundled conductor was obtained by multiplying the overall calculated resistance
of the bundled conductor by four which gave:
= . × = . / (4.5)
Accordingly, conductors with electrical resistances lower than 0.055556Ω could be used in this
project. The conductors with this property were: Finch, Bunting, Grackle, Bittern, Pheasant,
Dipper, Martin, Bobolink, Plover, Nuthatch, Lapwing, Falcon, Chukar– ACSR types and
ASTER 851 – AAAC type [10]. The next procedure was to compare the given conductor types
on the basis of their radio interference (RI), audible noise (AN) and corona loss performances.
4.1.3.2 Radio Interference (RI), Audible Noise (AN) and Corona Loss Performances
This involved determining the Radio Interference, Audible Noise and corona loss performances
of the conductors. These qualities depend on the surface voltage gradients of the candidate
conductors. This process was done taking into account different bundle arrangements. The basis
for this comparison was to get a conductor type with a surface voltage gradient lower than the
acceptable surface voltage gradient for a long transmission line which is 22kV/cm [10].
The following acceptable line geometry for the 500KV HVDC transmission line was assumed:
• 14 m pole spacing.
• 11 m conductor above ground level.
From Appendix 4, assuming a four-bundled line, all the conductors being compared were found
to have surface voltage gradients lower than 22kV/cm, which is the recommended value [8].
4.1.3.3 Corona Loss for 500 kV HVDC Line
Practically, a direct current transmission line exhibits a much smaller change in corona loss than
comparable alternating current lines in the transition from fair to rainy weather. Typically corona
losses per pole for direct current transmission lines are usually in the range of 1.5-2 KW/km. For
the HVDC line in consideration the maximum corona loss was approximated to be:
2KW/km× 1200km=2400Kw=2.4MW
4.1.3.4 Power Carrying Capacity Considerations
The power carrying capacities per line of the above chosen conductor types were then considered
at a wind velocity of 28.8m/s. This was done using the data given in Appendix 5. From this data,
31
Finch, Bunting, and Grackle types of conductors were ruled out since their power carrying
capacities were far below the expected 1500MW during overloads. Bittern and Pheasant
conductor types had power carrying capacities of 1474MW which was considered sufficient
since the 50% power overload assumed was quite high. Dipper, Martin, Bobolink, Plover,
Nuthatch, Lapwing, Falcon, Chukar– ACSR types and ASTER 851 – AAAC types had their
carrying capacities higher than 1500MW.
4.1.3.5 Current Carrying Capacity Consideration
This was the next step involved in determining conductor size for this project. The required
current carrying capacity of the designed line was given by;
Currentcarryingcapacityforline, I
3kA (4.6)
For the four bundled conductor line chosen, each conductor was to carry a current given by:
Maximumcurrentineachstrand, I
750A (4.7)
From the data given in appendix 5, Bittern and Pheasant conductor types had current carrying
capacities of 737A which was considered still sufficient since the overload condition considered
in the calculation was higher than the recommended 30%. Dipper, Martin, Bobolink, Plover,
Nuthatch, Lapwing, Falcon, Chukar– ACSR types and ASTER 851 – AAAC types had their
carrying capacities higher than 750A.
4.1.3.6 Economical Considerations
The conductors which were not ruled out in the previous stages were then compared in terms of
their costs. Table 4.7 gives the line minimum costs and Joule losses cost differences in case of
50% power transfer for the conductors with 4 x ACSR Finch as the reference.
Table 4.7: Line minimum costs and Joule losses in case of 50% power transfer [10].
32
Appendix 6 gives the joule loss costs in case of half power transfer, as compared to 4 x ACSR
FINCH. Considering full power transfer, the economical solution became the conductor with low
electrical resistance and lower line minimum costs, among which 4 x Pheasant remained the
most attractive. 4 x Falcon and 4× ASTER 851 were not selected due their higher mass, making
them more difficult to transport and install in mountainous areas [10].
4.1.3.7 Selected Conductor
ACSR Pheasant, four conductors per pole, was the most economical solution for the HVDC line.
The line costs were estimated to be 246,000 Euro/km, based on commodity prices by the second
quarter 2008. To avoid corrosion and reduce losses, ACS wire types instead of inner steel wires
could be used as the other option, subject to final decision on cost basis only, since all other
characteristics were the same [10].
Therefore, 4xACSR Pheasant was chosen as the main option and ACS Pheasant as an alternative.
4.1.4 Choice of Span, Conductor Configuration and Clearance
In this design, a span of 400m was chosen; this was the practical value for the transmission
voltage involved [3]. A flat horizontal conductor configuration was adopted because it would
result in a lesser tower height though a wider right of way would be required. According to the
general guidelines for the environmental treatment of transmission lines by CIGRE Study
Committee B2 [10], the Right Of Way (ROW) for the HVDC bipolar 500 kV line was as
follows:
• ROW of 50m was chosen in non-populated areas as shown in Figure 4.1.
• ROW of 65m was chosen in populated areas as illustrated in Figure 4.2.
33
Figure 4.1: HVDC 500 kV bipolar line corridor for non-populated areas and forests [8].
Trees with heights lower than 4 m were allowed inside a 20 m line corridor and trees shorter than
8 m were allowed for the rest of the line corridor.
Figure 4.2: HVDC 500 kV bipolar line corridor for populated areas [8].
34
The minimum clearances between the conductors and ground at their maximum sags were also
considered in this design. Table 4.8 gives some of the clearances standards used in this project.
Table 4.8: Minimum clearance standards for 500KV HVDC line from common features [10]
Crossing obstacle Minimum clearance
1. Ground 10m
2. Cross country and secondary roads 10m
3. Streets 11m
4. Residential areas 11m
5. Highway, motor ways 11m
6. Over pipelines 11m
7. Power and telecommunication lines 7m
8. Sport places and school courts 10m
9. Fences/Walls, Constructions with fire proof roofs 7m
In the design of the towers a minimum clearance of 11m was used since this was the largest
among the clearances.
4.1.5 Sag and Tension Analysis
During the design of the HVDC transmission line, sag(s) was calculated for each section of the
whole line. This was done using the altitude figures obtained at different points during routing
studies and assuming uniform gradients between these points.
The erection tension was first determined from Equation (3.27) which took into consideration the
worst probable physical conditions and the physical conditions at erection time.
The following data for the ACSR Pheasant was used in the calculations: [10]
Ø Breaking load (B.L) =200000 (N)
Ø Modulus of elasticity (E) =79500N/mm
Ø Co-efficient of linear expansion () =1.96× 10C
Ø Conductor unit mass () =2.3kg/m
Ø Conductor overall diameter (D) =35.1mm
Ø Conductor cross-sectional area =726.79mm
35
Maximum wind velocity was 28.6m/s
To calculate the erection tensionT, equation 3.27 was used, that is:
T T − T − αAEθ − θ−
=
, (4.8)
Where physical quantities at worst probable conditions were given as:
• T = .
=
= 100000N (Assuming a safety factor of two)
• θ = − 5 (Taken as the lowest temperature noted in the area)
• F = √[w + F ]
w= 2.3 × 9.81 = 22.563N/m
v = 28.6m/s = 28.6(
) = 102.96km/hr
Windpressure, p = 0.059102.96 = 625.44N/m
F = pD = 625.4435.1 × 10= 21.953N/m
F = √22.563 + 21.953 = 31.48N/m
Physical quantities at erection time were taken as:
• θ = 36 (Taken as the mean temperature of the region)
• F = w = 22.563N/m (Assuming still condition)
• α = 1.96 × 10C
• A = 726.79 × 10m
• E = 79.5 × 10N/m
Thus,
αAEθ − θ= (1.96 × 10726.79 × 10(79.5 × 10)36 − − 5= 46431.85
= .(.× ).
= 38172.82
= .× .× .
= 1.961 × 10
Therefore,
TT − 100000 − 46431.85 − 38172.82= 1.961 × 10
TT − 15395.33= 1.962 × 10
T − 15395.33T − 1.961 × 10= 0
The solution of above equation gives the erection tension as:
T = 63709.26471 ≅ 63709N
36
The conductor sags at erection time were then calculated at each section using the determined
tension at erection.
4.1.5.1 Sag between Wolayta /Sodo and Arba Minch
For the first 30km the altitude was between 2000m and 1200m.
The elevation of the land is:
θ = tan[
] = 1.5275°
Using a span of 400m:
h = 400 sin1.5275= 10.66m
Thus the span of the complete parabola becomes:
l = l +
= 400 + × × ..×
= 550.5m
S =
= .× .
× = 13.42m
Assuming the remaining distance (110-30) =80km to be fairly level,
S =
= .×
× = 7.08m
4.1.5.2 Sag between Arba Minch and Mega
For the first 35km the altitude was between 1500m and 800m.
The elevation of the land:
θ = tan
= 1.1458°
h = 400 sin(1.1458) = 8m
l = l +
= 400 + × × .×
= 512.94m
S =
= .× .
× = 11.65m
For the next about (271-35) =236 km the altitude was between 800m and 1200m.
θ = tan
= 0.097 °
h = 400 sin0.097= 0.68m
l = l +
= 400 + × × ..×
= 409.57m
S =
= .× .
× = 7.43m
37
4.1.5.3 Sag between Mega and Turbi
For the first 25km the route was flat. Thus,
S =
= .
× = 7.08m
For the next (107-25) =82km the altitude was between 1200m and 800m. Thus,
θ = tan
= 0.28°
h = 400 sin0.28= 1.95m
l = l +
= 400 + ..×
= 427.55m
S =
= .× .
× = 8.09m
4.1.5.4 Sag between Turbi and Marsabit
The entire route of about 131km had altitudes between 800m and 1500m. Thus,
θ = tan
= 0.31°
h = 400 sin(0. 31) = 2.14m
l = l +
= 400 + × × ..×
= 430.18m
S =
= .× .
× = 8.19m
4.1.5.5 Sag between Marsabit and Isiolo
The line route was approximately flat for the entire course, thus erection sag was calculated as
given;
S =
= .×
× = 7.08m
4.1.5.6 Sag between Isiolo and Isinya
The length of the route was approximately 289km with a mountainous relief of altitudes between
1500m and 2500m. The erection sag was calculated as follows:
θ = tan[
] = 0.1983°
h = 400 sin0.1983= 1.384m
l = l +
= 400 + × × ..×
= 419.54m
S =
= .× .
× = 7.79m
38
As it can be seen from the above calculations the erection tension Twas approximately 63709
Newtons. The largest value of the sag required for during erection was thus, 13.42m. The height
of the tower designed had to be able to provide adequate ground clearance even during the
largest value of sag.
4.1.6 HVDC Insulation Design
4.1.6.1 Choice of Insulation Configuration
Due to the high value of the dc voltage to be transmitted the suspension towers were chosen to be
used in the design with suspension insulator strings (I-strings for the +/-500kV lines) and the
tension towers (angle-tension and dead-end towers) with tension insulator strings.
The insulation design was done based on its three main dangers: pollution, lightning and
switching over-voltages.
4.1.6.2 Pollution
The main environmental danger considered was airborne pollution of the insulator surfaces.
The pollution requirements considered are defined in IEC 60815, [11]:
“Pollution Level “Heavy pollution“ with a creepage distance related to the line max. voltage
500kV is 4.325mm/kV r. m. s.”
Thus a creepage length of 4.325 mm/kV was chosen, that is:
4.325mm× 500 =2162.5 mm=2.1625m.
The selected insulator strings had also to satisfy the requirements of lightning impulse stresses.
4.1.6.3 Over-Voltages
4.1.6.3.1 Lightning Over-Voltages
The standard lightning impulse withstand voltage for the insulator sets was determined in
accordance with the requirements of IEC 60071-1 [11].
Thus for the ± 500kV HVDC system, a Basic Insulation Level BIL of 1300 kV (lightning
impulse withstand voltage) was chosen as given in Appendix 7 (standard insulation levels for
range II).
From IEC 60071-2, [11], the altitude correction factor for lightning impulse at 2000m was
calculated as follows:
39
= (.× )
= (.× ) = . (4.9)
The lightning withstand voltage at 2000 m was thus given by:
1300 x 1.1445 = 1487kVmax.
From EN 50341-1, [11] (that is, using the data given in Appendix 8), the clearance
corresponding to a lightning withstand voltage of 1487KV was 3.0m.
4.1.6.3.2 Temporary and Switching Over-Voltages
According to IEC 60071-1 [11], the 500 kV HVDC system was designed based on Switching
Impulse Level of 1050kV as can be seen in Appendix 7.
From IEC 60071-2, [11] the altitude correction factor for switching impulse at an altitude of
2000m was determined as follows:
= (.× )
= (.× ) = . (4.10)
The switching withstand voltage at 2000 m was thus:
1050 x 1.1874= 1246 kV max.
From EN 50341-1, [11] (that is, using the data given in Appendix 9), the minimum electrical
distance for a switching withstand voltage of 1246KV was 4.30 m.
Thus, the minimum clearance between the live parts of the insulator strings and the grounded
members of the tower was chosen to be the largest between the two distances; this was 4.30m.
4.1.6.4 Direct Lightning Stroke Shielding
If no lightning stroke shielding is provided, the current carrying conductors will be exposed to
lightning strikes, which would cause a considerable number of over-voltages to be set up by the
lightning current flowing through the surge impedance of the line. There would also be burning
at the strike points on conductors. Following the Guide for EHV Line Shielding Angles proposed
by Armstrong and Whitehead, which is: “the shielding angle for an average shield wire height
above soil of more than 35m and vertical type of towers is to be less or equal to 0 ° ”. Figure
4.3 shows a graph of the shielding angle versus shield-wire average height.
40
Figure 4.3: Shielding angle versus shield-wire average height [8].
Therefore, so as to ensure freedom from shielding failure, two earth-wires subtending an angle
approximated to 0° (zero degrees) to the current carrying conductors at the tower were
incorporated in the design. The minimum distance between live conductors and shield wire
needed to be at least 9m for both suspension angle towers and terminal towers [11]. In this
design the clearance between the live conductors and the shield wire was thus chosen to be 9m
as shown in Figure 4.4. The sag of shield wire should be 10% less than the sag of current
carrying conductor. Thus the sags of the shield wire were designed to be not more than 90% of
the sag of the conductors at their respective sections. The shield-wire sags for the sections under
considerations were calculated based on the respective live conductor sags at these sections and
the results tabulated in Table 4.9.
Table 4.9: Shield-wire sags for different sections.
Section under consideration Sag of the shield-wire
1. Wolayta /Sodo - Arba Minch 90% 13.42m . (for the first 30km)
90% 7.08m . (for the next 80km)
2. Arba Minch – Mega 90% 11.65m . (for the first 35km)
90% 7.43m . (for the next 236km)
41
3. Mega to Turbi 90% × 7.08m = . (for the first 25km)
90% × 8.09m = . (for the next 82km)
4. Turbi to Marsabit 90% × 8.19m = . (for the 131km)
5. Marsabit to Isiolo 90% × 7.08m = . (for the 209km)
6. Isiolo to Isinya 90% × 7.79m = . (for the 289km)
4.1.6.5 Insulator Set Designs
In order to meet the specific leakage path requirements listed above, each leg of the I-insulator
sets was to comprise of one unit, composite type or the three long rod porcelain type. Tension
insulator sets were to comprise of two strings of insulators with sag adjusters fitted at the earth
end of each string. Each pair of strings of one composite type was proposed to be connected
together by means of yoke plates at the earth end. Low duty tension sets comprised double
strings of insulators fitted with sag adjusters at the earth end.
4.1.7 Choice of Earth/Ground Wire
As stated earlier, to provide a good lightning performance of the designed transmission line, a
shielding angle of zero degrees was required for the two earth wires: one shield-wire and one
OPGW, which were to be attached to the cross-arms. The normal shield wire chosen was of
Galvanized Steel Wire type. The earth wires were chosen with regard to their mechanical
capability since as stated earlier, an earth wire chosen on the basis of the mechanical
requirements automatically has the required electrical properties. Thus an OPGW of 48 fibres
was seen to have adequate mechanical and electrical requirements. For the OPGW, ACS wires
were chosen to be used as armouring. Due to the thick aluminium coating of the ACS wires, they
would provide an excellent capability of resistance against corrosion. The service life of the ACS
strands was assumed to be the same as that of the conductor. As stated earlier, the shield-wire
sags were maintained at values not more than 90% of current carrying conductor sags.
4.1.8 Design of Line Supports and Cross-arms
To ensure increased reliability, self-supporting lattice towers were designed for the bipolar
500KV HVDC line. According to the general guidelines for the environmental treatment of
transmission lines by CIGRE Study Committee B2, “all HVAC and HVDC lines should have two
earth wires, one OPGW and one ground wire, on tower earth wire peaks”. The design figure for
42
the towers was according to the design parameters obtained earlier in this project. The cross-
arms had to ensure that the current carrying conductors were at least 4.30m from the grounded
tower parts. A greater length than this was used in the design so as to ensure that the live
conductors do not come within sparking distance with the tower body. The formula given in
Equation (3.2) gave;
Spacing = √13.42 +
= 7m
Thus, seven-meter long cross-arms were considered in this design. The insulation strings used
were five meters long.
The following parameters based on previous results were also considered:
• Maximum conductor sag of 13.42m. During the tower design this maximum conductor
sag was approximated to 14m.
• Conductor clearance of 11.00m from the ground.
• Spacing of 9m between the live conductors and the shield wires.
Thus, the total tower height was 34m, that is:
14m + 11m + 9m = 34m
Since a span of 400m was adopted for this project, the total number of towers required for the
whole HVDC interconnector was estimated as follows:
Numberoftowers ≅
(4.11)
Numberoftowers ≅
=
= 3000
The actual number of towers was expected to be slightly lower than this because the span chosen
had to increase where the HVDC line crosses rivers, tributaries, streets or main roads.
Figure 4.4 shows the outline of the self-supporting lattice towers designed for this HVDC
transmission line.
43
Figure 4.4: Self-supporting lattice tower design for HVDC 500 kV bipolar line
44
4.2 Design of Converter Stations Two converter stations were designed, one at each end of the 500kV HVDC transmission line.
One of the converters was to be located at Wolayta/Sodo in Ethiopia and the other at Isinya in
Kenya. Under normal operation, the rectifier station was assumed to be the Wolayta/Sodo
converter station and the inverter station at Isinya.
4.2.1 Converter Space Requirements
The space requirements for the 500 kV DC converters were estimated at approximately 500m x
500m [5]. In estimation of the space requirements the following main components were
considered for converter site: 500 kV outdoor DC switchgear, control building, valve halls,
converter transformers, smoothing reactor, DC switchyard, DC filters and AC filters.
4.2.2 Pole Level
This project suggests use of the modern state of the art technology of direct light-triggered
thyristors (LTT) with integrated overvoltage protection eliminating the need for electronic logic
at high potential [9].
The same valve design was to be adopted for the rectifier and inverter stations. In
the design, the thyristor valves were arranged in three twin towers for each pole. One twin tower
represented one quadrivalve comprising of the four valves connected to the same ac phase. Each
of the four valves in one quadrivalve structure consisted of two and a half modular units. Thus
one tower consisted of 10 modular units. Each valve modular unit in turn included two valve
sections connected in series and each valve section comprised of 15 thyristor levels. Therefore, a
thyristor valve for Ethiopia-Kenya HVDC project with two and a half modular units was
comprised of five valve sections with 75 thyristors connected in series. Thus, the design ensured
that each pole consisted of two six-valve groups connected in series. Each six valve group was
connected to a separate secondary winding of the converter transformer. Secondary winding
connections for each pole were designed to be in delta and star arrangements which provided for
a 30° phase shift, thus eliminating lower order harmonics [5].
The procedure outlined below was used to get the rating of the thyristors required:
The dc line conductor current assuming an overload of 50% overload was 3kA as given by
Equation 4.6
Thus the maximum current expected at each pole twin tower was given by,
45
I = ×
=
= 500A (4.12)
This was the maximum current expected to pass through the series thyristors when they were in
conduction mode.
The voltage across each thyristor valve was
= 250KV
Since each thyristor valve had 75 thyristors connected in series, the voltage across a single
thyristor was given by:
V = ×
= 1.67KV (4.13)
The firing angle, of the thyristors was calculated using Equation (4.6).
Vo = √( )
πcos α
(4.14)
cos α = π√( )
(4.15)
For each thyristor valve,
Vo =
= 250KV
V( ) ≅ 220KV
cos = ()√()
= 0.84146
Thus, = cos 0.84146 = 32.71° (4.16)
4.2.3 Converter Transformers
The three single-phase, three-winding transformer configuration was chosen for the use as the
converter transformers due to its factors as given in Appendix 2. Though configuration required
three single-phase, three-winding transformers per pole, a fourth single-phase, three-winding
transformer was suggested as a spare. This made the total number of such transformers required
to be sixteen, since each pole was designed to have two separate valve sections. The converter
transformers chosen for this project had to:
a) Accommodate the power capacity of each pole section, which was 500MW.
b) Step down the ac line voltage from 400KV to a reasonable value chosen to be 220KV.
c) Enough insulation to withstand the expected voltage stresses. To determine the insulation
levels, a Standard Lightning Impulse Withstand Voltage for 400KV was chosen as shown
in Appendix 7. This gave a value of 1300KV.
46
CHAPTER FIVE: DESIGN RESULTS AND ANALYSIS
5.1 Results and Discussions
5.1.1 Routing Adopted
The routing adopted was divided into three sections whose part-names and respective
estimated distances are given in Table 5.1.
Table 5.1: Routing sections and their respective distances
Name Estimated distance (km)
1. Wolayta /Sodo - Arba Minch 110
2. Arba Minch – Mega 271
3. Mega to Turbi 107
4. Turbi to Marsabit 131
5. Marsabit to Isiolo 209
6. Isiolo to Isinya 289
From Table 5.1, the total estimated distance is 1117km. To cater for any discrepancies incurred,
an approximated line length of 1200km was chosen for design purposes.
5.1.2 Results for Sag Analysis
Table 5.2 gives the results of the conductor sag calculations:
Table 5.2: Conductor sags for different sections.
Section under consideration Conductor sag obtained
1. Wolayta /Sodo - Arba Minch 13.42m (for the first 30km)
7.08m (for the last 80km)
2. Arba Minch – Mega 11.65m (for the first 35km)
7.43m (for the last 236km)
3. Mega to Turbi 7.08m (for the first 25km)
8.09m (for the last 82km)
4. Turbi to Marsabit 8.19m (for the 131km)
5. Marsabit to Isiolo 7.08m (for the 209km)
6. Isiolo to Isinya 7.79m (for the 289km)
47
From Table 5.2, the largest sag obtained for the design was 13.42m. This was the value used in
determining the heights of the designed HVDC line towers. This value was approximated to 14m
during tower design.
5.1.3 Insulation Length
The obtained results of the insulation lengths based on the different aspects which result to
failure of insulation systems on HVDC overhead transmission conductors were as shown in
Table 5.3.
Table 5.3: Obtained insulation lengths
Aspect considered Insulation length (m)
1. Pollution 2.1625
2. Lightning over-voltages 3.0000
3. Switching over-voltages 4.3000
From Table 5.3, the insulation length considered during the design was that obtained from
switching over-voltages since it was the largest, with a value of 4.3m. To ensure that there would
be no chance of the live conductors coming into sparking distance with the tower body, a value
of seven meters was used in the design of the cross-arms.
5.1.4 Other Design Aspects Results
The results obtained for other considered design aspects were tabulated in Table 5.4.
Table 5.4: Results for other design aspects
Design aspect Results obtained
1. Transmission voltage 500Kv
2. Conductor type and size 4xACSR Pheasant, 726mm
3. Conductor span 400m
4. Conductor configuration Horizontal
5. Right Of Way (ROW) 50m for less-populated areas
65m for dense populated areas
6. Clearance 11m
48
7. Earth wires one OPGW and one Galvanized Steel
7. Line supports self-supporting lattice towers
8. Estimated number of towers 3000
The transmission voltage was determined according to the expected power capacity of the line
and the transmission distance. The chosen conductor type and size gave the best qualities with
regard to power loss and economic considerations. A span of 400m was chosen with a horizontal
conductor configuration. This ensured reasonable tower heights. The largest expected clearance
was 11m which was used in the design of the towers. One OPGW and one galvanized steel wire
were used for the earth/ground wires in accordance with the general guidelines for the
environmental treatment of transmission lines by CIGRE Study Committee B2 [10]. Self-
supporting lattice steel towers were used to ensure durability and reliability.
5.1.5 Converter Station Results
Table 5.5 gives the summarised results for the required thyristors and converter transformers in
this project.
Table 5.5 Descriptions of the chosen thyristors and converter transformers
Particular Description
1. 600 Thyristors
Rated Current: 500A
Rated Voltage: 1.67KV≅ 2KV
Firing angle: 32.71°
2. Eight Converter transformers
500MW rated
400KV/220KV
1300KV insulation level (LIWL)
The results were analysed and found to be reliable since there were thyristors and transformers in
market having the obtained descriptions and ratings.
The general design diagram for the whole HVDC system was as shown in Appendix 10.
Figure 5.1 gives the details of each of the 500MW thyristor valve shown in Appendix 10.
49
Figure 5.1: Circuitry design of each thyristor valve.
The connections for the wye-delta part of each of the 500MW thyristor valve were done as
illustrated Figure 5.2.
Figure 5.2: Circuitry design of each wye-delta part of the thyristor valve.
5.2 Efficiency and Cost Estimation of the Designed HVDC Line
5.2.1 Efficiency Calculation
Under normal operation, the power capacity of the designed HVDC transmission line was
1000MW per pole. Assuming corona losses of two kilowatts per kilometre per pole, the corona
losses were estimated as follows:
2KW/km 1200km 2400KW ≅ 2.4MW
The IR losses were then calculated as follows:
Conductorcurrent, I , ,
(5.1)
50
I =
=
= 2000A = 2kA (5.2)
R = 0.013889Ω/km × 1200km = 16.6668Ω
Thus,
IR = 2000A × 16.6668Ω = 33333.6W ≅ 33.33KW (5.3)
Totalconductorlosses, P = coronalosses + IRlosses
P = 2400KW + 33.33KW = 2433.33KW ≅ 2.433MW (5.4)
Hence efficiency of the designed HVDC transmission line was given by:
Efficiency, η = , , ,
(5.5)
That is,
η = .
× 100% = 99.75% (5.6)
5.2.2 Cost Estimation
In giving the cost estimation of the project implementation, a fully completed design was
assumed. In accordance with the guidelines given by KETRACO Company Limited, the whole
project was to be done in two phases. The unit prices for the main equipment of the HVDC
transmission line are given in Appendix 11, whereas the cost break-down of the full HVDC
alternative by project phases, countries and landing points, is shown in Appendix 12. The
summarised cost estimation as represented in the two appendixes is given in Table 5.6.
Table 5.6: Cost estimation for the HVDC project when fully completed
Country Kenya Ethiopia
Costs in Billion Ksh.
Phase 1 41.95332 29.51016
Phase 2 12.648 11.5584
Country’s Total costs (Billion Ksh.) 54.60132 41.06856
Overall Total Costs (Billion Ksh.) 95.66988
So as to show the economic advantage of the use of HVDC transmission system over the HVAC
transmission system, the cost estimation of the project when implemented using HVAC system
51
was done and the two costs compared. Table 5.7 gives the cost estimation of the HVAC system.
The figures were provided by KETRACO from the conceptual study conducted [8].
Table 5.7: Cost estimation for the project using HVAC system.
Country Kenya Ethiopia
Costs in Billion Ksh.
Phase 1 53.51832 38.54676
Phase 2 28.69752 21.15024
Country’s Total costs (Billion Ksh.) 82.21584 59.697
Overall Total Costs (Billion Ksh.) 141.91284
Table 5.6 and Table 5.7 clearly show that the adoption of HVDC transmission system rather
than an HVAC one cuts costs. The total cost saved by using HVDC transmission system was
calculated as:
Costsaved = BillionKsh. (141.91284 − 95.66988 = BillionKsh. 46.24296 (5.7)
Thus, Ksh.46,242,960,000 would be saved by using HVDC transmission system instead of using
HVAC system in this project.
52
CHAPTER SIX: CONCLUSION AND FUTURE WORK
6.1 Conclusion The design of a HVDC transmission line from Ethiopia to Kenya was done successfully
following the required design criteria. The most feasible routing of the line was determined, and
it was divided into three sections having a total of six parts which were: Wolayta/Sodo – Arba
Minch – Mega – Turbi – Marsabit – Isiolo – Isinya. The transmission voltage chosen was
500KV. This was arrived at using the formula given in equation 3.1. The choice of conductor
type and size was done on the basis of minimum power losses, power and current carrying
capacity and economic considerations. A 4× Pheasant ACSR conductor was found to be the best
suited as pertains to the above considerations.
The design assumed a span of 400m with horizontal conductor configuration whose ROW was
50m for the less populated areas and 65m for the densely populated areas. The chosen clearance
of the live conductors was 11m. After calculating the sag for each of the six sub-sections, the
maximum sag obtained was 13.42m; thus an approximate value of 14m was used for the design
of the support towers. The suspension insulators were adopted for this design. The calculations
done with reference from the IEC tables gave a recommended insulation length of 4.3m. An
insulation length of seven meters was adopted in the design to ensure a substantial safety factor.
Two earth wires were employed in the design; their sag was maintained at 90% of the conductor
sags. A distance of nine meters was maintained between the earth-wires and the conductors.
Thus the total tower height was 34m.
The design of the converter stations was also done though not to detail. The use of direct LTT
was adopted. Each thyristor valve comprised of five valve sections with 15 thyristor levels each,
thus making a total of 75 thyristors connected in series. The firing angle for the thyristors was
calculated and found to be 32.71°. The current and voltage ratings of the thyristors chosen were
1kA and 4KV respectively. The converter transformer ratings were 500MW, 400KV/220KV and
1300KV LIWL. The efficiency of the design HVDC transmission line was found to be 99.75%
and the cost of implementing the complete HVDC system was estimated
atBillionKsh. 95.66988. Thus the objective of the project was successfully achieved.
53
6.2 Recommendation for Future Work
Though the objective of the project was achieved, some areas were not completely tackled and
others were not tackled at all due to time limitation. Hence future work can be covered in these
areas which include:-
1. Detailed design of the converter stations: - this includes design of ground electrodes
and their associated wires, AC and DC filters, smoothening reactors, control rooms and
the HVDC switchyards.
2. HVDC system protection: - this includes both the protection of the HVDC transmission
line and the converter stations. This is usually done on the ac side of the converter station
and thus calls for a detailed converter station design as pointed in the first
recommendation.
54
APPENDICES
Appendix 1: Schematic diagrams different converter transformer configurations
55
Appendix 2: Comparison of different transformer configurations
56
Appendix 3: A map of the area to be affected by the HVDC interconnector [8]
57
Appendix 4: Conductor surface voltage gradients for a +/- 500kV line [11].
58
Appendix 5: Current/power carrying capacity for different conductor types [10]
59
Appendix 6: Joule Losses Cost [10]
60
Appendix 7: Standard insulation levels for range II. (From IEC 60071-1), [11]
61
Appendix 8: Clearances to withstand lightning over-voltages, EN 50341-1, [11]
62
Appendix 9: Clearances to withstand switching overvoltages, EN 50341-1, [11]
63
Appendix 10: The Complete HVDC/HVAC Transmission System [8]
64
Appendix 11: Unit Prices for Main Equipment [8]
Appendix 12: Cost Estimation of the Complete Project [8].
65
66
67
REFERENCES
[1] D.M Larruskain & Others, “Transmission and Distribution Networks: AC versus DC,”
University of the Basgue Country-Bilbao (Spain)
[2] B.R Gupta, Power system analysis and design, © 2000 S. Chand & Company Ltd (An ISO
9001:2000 Company) 7361, Ram Nagar, New Delhi-110 055
[3] Kala Meah, Student Member, IEEE, and Sadrul Ula, Senior Member, IEEE “Comparative
Evaluation of HVDC and HVAC Transmission Systems,” from IEEE Xplore
[4] Anthony J. Pansini, “Power Transmission and distribution”, Second Edition @2005 By the
Fairmount Press Inc.
[5] Siemens Power Transmission and Distribution High Voltage Division, HVDC proven
Technology, Order No. E60001-U131-A92-V2-7600, Printed in Germany Diepo-Stele 30000
[6] H. Wayne Beaty, “Electrical Power Distribution Systems” @1998 by PeunWell Publishing
Company.
[7] D. Das, Electrical power systems, ©2006 New Age International (P) Limited, Publishers.
New Delhi.
[8] Dr. Popescu, “HVDC Converter Station and Facilities and Basic design on transmission lines
report,,” Final feasibility report on Ethiopia-Kenya Power Systems interconnection project,
vol. 2, March 2009,
[9] J. Holweg, H.P. Lips, Q.B. Tu, M. Uder, Peng Baoshu, Zhang Yeguang, “Modern HVDC
Thyristor Valves for China's Electric Power System," in Proc. 2002 IEE-PES/CSEE
International Conference on Power Systems Conf.
[10] Ethiopia-Kenya Power System Interconnection Project Feasibility Study –Inception Report,
Fichtner GmbH, November 2007
[11] www.iec.ch/standards
