HVDC Thesis,2011EEE 079

Long Overhead Electric Power Transmission Line Design With
Assistance of High Voltage Direct Current(HVDC) System
Ahmed Rafique Aziz
June,2011

Department of Electrical & Electronic Engineering,Khulna University of Engineering
&Technology(KUET),Khulna-920300
Long Overhead Electric Power Transmission Line Design with
assistance of High Voltage Direct Current(HVDC) System
This thesis papers is submitted to the Department of Electrical & Electronic Engineering,Khulna
University of Engineering & Technology(KUET),Bangladesh,in partial fulfillment of the requirements
for the Degree of “Bachelor of Science in Electrical &Electronic Engineering”.
Prepared by
Ahmed Rafique Aziz
Roll No:0203079
Thesis Supervisor
Md.SalahUddin Yusuf
Assistant Professor
Department of Electrical & Electronic Engineering
Department of Electrical & Electronic Engineering,Khulna University of Engineering &
Technology(KUET),Khulna-920300,Bangladesh.

Acknowledgement
First and Foremost,I would like to reveal profound gratitude to omniscient Allah who give
me the knowledge for composing this Thesis papers.Then,I would like to thank always inspiring,
enthusiastic and very supportive supervisor Md.SalahUddin Yusuf,Assistant Professor,Department of
Electrical & Electronic Engineering,KUET.He has always been extremly generous with his
time,knowledge and ideas and allowed me great freedom in this research.His enthusiastic approach to
research,his endless excitement for Long Overhead Electric Transmission Line Design with assistance
of HVDC System and his effervescent personality has made this experience all the more enjoyable and
I am greatly appreciative.
I also thank all the teachers of the Department of Electrical & Electronic
Engineering,KUET,who all gave valuable advice and particularly Mr.A.N.M Enamul Kabir,Associate
Professor,EEE,KUET.He had kept attention on the progress of my work and was always available
when I needed to consult with him.His encouragement,motivation and expert guidance has provided a
good basis for my entire thesis work.
Special thanks for Prof.Dr.Md. Abdur Rafiq ,Head of the Dept. of EEE,KUET, for his
valuable advices and guidance and providing the supports and lab facilities for the practical works.
I am indebted to all my class mates of 2k6 batch,EEE,for their cooperation.I am specially
grateful to my maternal uncle for financial cooperations in the whole thesis works.Many thanks to all
those who help to create a nice environment in spite of many obstacles at Khulna University of
Engineering & Technology.
Author
June,2011

Abstract
It is well recognized that direct current and direct voltage offer
special advantages for both land and sea cable systems, both with regard to power
transmission capability, losses, as well as possible transmission length due to no
capacitive currents. As cable systems were used very early in large cities, one of the
first applications considered for HVDC was to use it for city infeed and some
schemes were also built. However, it turned out that the cost for the stations was too
high and that the savings on the cable part were not high enough to justify the high
costs of the converter stations, even considering other possible benefits of the HVDC
techniques such as fast control of active power and almost no contribution to fault
currents.
During the 1990s new HVDC Voltage Source Converters, VSC,
and new HVDC cables with solid insulation have been developed and the relative
cost for the converters has been steadily decreasing. It was, therefore, found justifiable
to reexamine the feasibility of using HVDC, especially based on the new VSC
technique, for long transmission line.
Although the transmission and distribution of electrical power will be
preferably made with conventional AC technique, but HVDC transmission would offer
special advantages for long transmission cable systems with especial requirements
with regard to power flow control, systems with restrictions to short circuit
currents, and other relevant issues. HVDC transmission would have advantages over
the conventional AC solution, simplifying the operation of the system or resulting in a
more economical solution.
The Author has designed a overhead long transmission line from Khulna
to Dhaka district.The Author would like to make a compromise between expensive
HVDC and comparatively inefficient 3-phase AC Transmission.Instance of HVDC,
terminal equipment design is very expensive and sophisticated and complex.On
the other instance,for 3-phase AC transmission considerable demerits arises.
However,3-phase High Voltage (230KV) AC transmission have been existed and
have been operating for few decades.So,HVDC is a certainly new
considerations for transmitting a bulk amount of electric power .
Moreover,HVDC has several considerable advantages over AC and HVDC
has future prospects for new development in Electric Power Sector of Bangladesh.

List of Figures
Page
Figure2.1. The twelve pulse valve group configuration with two converter transformers.
One in star-star connection and the other in star-delta connection. . . . . . 5
Figure 2.2. Example of an HVDC substation. . . . . 7
Figure 2.3. Monopolar and bipolar connection of HVDC converter bridges. . . . . .9
Figure 2.4. Voltage and current waveshapes associated with d.c. converter bridges.. . . 13
Figure 4.1:Mass Impregnated Cable . . . .29
Figure 4.2:Self Contained Fluid –Filled Cables . . . . 29
Figure 4.3:Extruded Cables (XLPE Cables) . . . 30
Figure 5.1 Typical HVDC 500KV Lattice Tower . . . . . 35
Figure6.1:Mass Impregnated Cable . . . . 47
Figure6.2:VSC based HVDC Substation . . . . 48
Figure6.3:VSC based HVDC Converter Arrangement . . . . 48
Figure 6.4:VSC based HVDC Indoor-Outdoor View . . . . 49
Figure 6.5:HVDC transmission with VSC . . . . 49
Figure 6.6 :Control of VSC Based HVDC Transmission . . . . 50
Figure 6.7:HVDC Overall View. . . . 50
Figure 6.8 :Efficiency Vs Transmission System Curve .. . .69
Figure 6.9:Voltage Regulation Vs Transmission System Curve. . . . . 70
Figure 6.10:Transmission Loss Vs Transmission System Curve. . . . . 71

Contents
Acknowledgement . . . . . . . . . . . . . . . . .(i)
Abstract . . . . . . . . . . . . . . . . .(ii)
List of Figures . . . . . . . . . . . . . . . . . (iii)
Chapter 1 Page
Introduction
1.1 General background and overall aim of the study . . . . . . . . . . . . 1
1.2 Contribution in the study . . . . . . . . . . . . 2
1.3 General overview of the thesis . . . . . . . . . . . . 2
Chapter 2
HVDC Overview
2.1 Intoduction . . . . . . . . . . . . . 3
2.2 Why Use DC Transmission . . . . . . . . . . . . . 3
2.3 Configurations . . . . . . . . . . . . . 4
2.4 Twelve Pulse Valve Group . . . . . . . . . . . . . 5
2.5 Thyristor Module . . . . . . . . . . . . . 6
2.6 Substation Configuration . . . . . . . . . . . . . 6
2.7Applications Of HVDC Converters . . . . . . . . . . . . . 8
2.8 HVDC Converter Arrangements . . . . . . . . . . . . . 10
2.9 Environmental Considerations . . . . . . . . . . . . . 11
2.10 D.C Converter Operation . . . . . . . . . . . . . .12
2.11Commutation Failure . . . . . . . . . . . . . 15
2.12 Series Capacitors With D.C. Converter Substations . . . . . . . . . . . . . 16
2.13 Control And Protection . . . . . . . . . . . . . 17
2.14 A.C. Voltage Control . . . . . . . . . . . . . 20
2.15 Special Purpose Controls . . . . . . . . . . . . . 21

Chapter 3 Page
VSC Based HVDC Transmission System
3.1 Introduction . . . . . . . . . . . . 23
3.2 Advantages and Applications for VSC Based HVDC . . . . . . . . . . . . 23
3.3VSC-based HVDC Transmission System Conﬁgurations . . . . . . . . . . . . 25
3.4 Voltage Source Converter . . . . . . . . . . . . 25
3.5 Transformer . . . . . . . . . . . . 25
3.6 Phase Reactor . . . . . . . . . . . . 25
3.7 AC Filter . . . . . . . . . . . . 26
3.8 DC-link Capacitor . . . . . . . . . . . . 26
3.9 DC Cable . . . . . . . . . . . . 26
Chapter 4
HVDC Cables
4.1 Introduction . . . . . . . . . . . 27
4.2 Main Characteristics of an HVDC Cable System . . . . . . . . . . . .28
4.3 Classification of HVDC Cables . . . . . . . . . . . .28
Chapter 5
HVDC Tower
5.1 Construction Process and Costs . . . . . . . . . . . . 31
5.2 Detailed Planning of The Transmission Route . . . . . . . . . . . . 31
5.3 Detailed Design and Execution Drawing . . . . . . . . . . . . 32
5.4 Construction Phases / Time Schedule . . . . . . . . . . . . 32
5.5 Preliminary Work On Construction . . . . . . . . . . . . 32
5.6 Foundations . . . . . . . . . . . . 32
5.7 Pylon Assembly / Switch Yard Erection . . . . . . . . . . . . 33
5.8 Cables Hanging . . . . . . . . . . . . 33
5.9 Tests and Acceptance . . . . . . . . . . . . 34
5.10 Recultivation . . . . . . . . . . . . 34
5.11Values of T & D Lines . . . . . . . . . . . . 34
5.12Typical 500 KV HVDC Lattice Tower . . . . . . . . . . . . 35

Chapter 6 Page
HVDC +/-260 KV Transmission Line Project
6.1Tentative Title . . . . . . . . . 36
6.2Introduction . . . . . . . . . 36
6.3 Long Term Significance . . . . . . . . . 37
6.4 Proposed Plan and Methodology . . . . . . . . . 37
6.5Existing Transmission Network(as on June,2010) . . . . . . . . . 38
6.6 Existing Transmission Lines . . . . . . . . . 38
6.7Existing Power Generation in Ghorasal . . . . . . . . . 39
6.8 Generation Voltage . . . . . . . . . 39
6.9Typical Electrical Parameters for a 230KV Overhead Line . . . . . . . . . 39
6.10 Data of Existing Transmission Line . . . . . . . . . 40
6.11 Typical High Voltage Direct Current (HVDC) Transmission Line Between
Khulna-Ishurdi-Ghorasal . . . . . . . . . 45
6.12 Typical Tower Structure . . . . . . . . .. 46
6.13 HVDC Project Cables . . . . . . . . . 47
6.14 Typical HVDC Circuit Diagram . . . . . . . . . 48
6.15 HVDC +/-260 KV Project Economics . . . . . . . . . 51
6.16 Cost ratios for DC and AC Transmission Line construction . . . . . . . . . 51
6.17 HVDC System Reliability . . . . . . . . . 52
6.18 Cost Structure of Converter Stations . . . . . . . . . 52
6.19 HVDC Current,Voltage,Insulation Level,Power Transmission and Percentage Loss
comparision with HVAC . . . . . . . . . 53
6.20Tower Calculation . . . . . . . . . 55
6.21Preliminary Design of Tower . . . . . . . . . 57
6.22 Corona Loss . . . . . . . . . 58
6.23 MATLAB Program for comparison of HVAC and HVDC . . . . . . . . . 59
6.24 Present HVAC Power Grid of Bangladesh . . . . . . . . . 61
6.25Electrical Design of Typical Existing HVAC Transmission Line . . . . . . . . . 62
6.26 Proposed HVDC Project Electrical Design . . . . . . . . . 67
6.27Typical Performance Curves . . . . . . . . . 69

Chapter 7 Page
HVDC Transmission-Opportunities and Challenges
7.1Developments in Energy Policies . . . . . . . . . .72
7.2 Developments in Transmission Networks . . . . . . . . . . 72
7.3Challenges and Opportunities . . . . . . . . . .74
7.3.1Wind Power and Energy Diversity
7.3.2 AC Network Enhancement
7.4 HVDC System Challenges . . . . . . . . . . 75
7.4.1Cost and Value of HVDC
7.4.2 Power Loss
7.4.3 Complexity of HVDC Schemes
7.4.4 Dispatch and Control of HVDC
Scheme
7.4.5 Integration of HVDC Scheme in
AC Network
7.4.6 Harmonics
7.4.7 Operation of HVDC Scheme
With Ground Return
7.4.8 Stability of Network With Multi-
Infeed of HVDC
7.5 Conclusion .. . . . . . . . .78
Chapter 8
Conclusion . . . . . .79-80
REFERENCES

1
Chapter 1
Introduction
1.1 General background and overall aim of the study
It is well recognized that direct current and direct voltage
offer special advantages for both land and sea cable systems, both with
regard to power transmission capability, losses, as well as possible
transmission length due to no capacitive currents.However, it turned out that
the cost for the stations was too high and that the savings on the cable part
were not high enough to justify the high costs of the converter stations, even
considering other possible benefits of the HVDC techniques such as fast
control of active power and almost no contribution to fault currents.
During the 1990´s, with the development of new HVDC
converters using Volt- age Source Converters, VSC, new HVDC cables with
solid insulation and with the relative cost for the converters steadily
decreasing, it was found justifiable to again study the feasibility of using
HVDC, especially based on the new VSC technique, for feeding electrical
power to large cities. This new HVDC-VSC technique will, for instance,
make it possible to control both active and reactive power and will be more
suitable for cable multi-terminal systems.
.From the specific studies performed in close cooperation with
utilities, the major driving forces and evaluating criteria used to decide
whether to rebuilt or expand an existing electrical power or built a complete
new system, were iden- tified. Specific criteria such as thermal security,
voltage security, short circuit current security, reliability of supply, and
capability for power flow control were found to be the major driving forces
in the review of the existing infra- structure.
Each of these criteria was evaluated in a systematic way and a
comparison was made between the existing or expected possible improved AC
technique and an alternative HVDC solution. The comparison was made from
both a technical and an economical point of view.
Finally a more generic study was performed in order to evaluate
the expected break-even distance for a HVDC overhead transmission
system by comparison with an equivalent HVAC transmission. The break-
even distance was in this case the distance in which the saving in capital
cost and lower losses with a DC overhead transmission cable may be enough
to pay for the two converters, one at either end. This distance depends on
several factors, and most of these factors are related to the specific
characteristic of the network. Some parametric study of these factors was also
made in the calculation of the break-even distance.

2
1.2 Contribution in the study
The present study provides the following main contributions:
• A systematic overview of evaluation criteria and values of HVDC solutions
including comparison with the best HVAC alternative.
• Generic conclusions regarding when HVDC could be an alternative for power
transmission to large cities.
• Suggestion and motivation of new Hybrid HVDC topology
• Extension of the concept of ‘break-even distance’ widely mentioned in the literature
when comparing HVDC and HVAC transmission with overhead lines.
1.3 General overview of the thesis
Chapter 2 describes the technical,economical and environmental aspects of HVDC.
Chapter 3 describes the characteristics of HVDC Voltage Source Converters(VSC).
Chapter 4 describes the HVDC cable system.During the last years very cost effective extruded
DC cables have been developed which can fit in the existing cables ducts. These cables have
considerably higher power transmission capability than the corresponding AC cables.
In chapter 5 involves HVDC towers.
In chapter 6 describes the conveniences of using HVDC for bulk amount of electric power
transmission over Ghorasal,Narsinghdi and Khulna district.
In Chapter 7 describes the future challenges and opportunities of HVDC.
Finally in chapter 8 ,generic conclusion is presented for long overhead electric power
transmission.

3
Chapter 2
HVDC Overview
2.1 Intoduction
Electric power transmission was originally developed with direct current. The availability of
transformers and the development and improvement of induction motors at the beginning of
the 20th Century, led to greater appeal and use of a.c. transmission. Through research and
development in Sweden at Allmana Svenska Electriska Aktiebolaget (ASEA), an improved
multi-electrode grid controlled mercury arc valve for high powers and voltages was developed
from 1929. Experimental plants were set up in the 1930’s in Sweden and the USA to
investigate the use of mercury arc valves in conversion processes for transmission and
frequency changing.
D.c. transmission now became practical when long distances were to be covered or where
cables were required. The increase in need for electricity after the Second World War
stimulated research, particularly in Sweden and in Russia. In 1950, a 116 km
experimental transmission line was commissioned from Moscow to Kasira at 200 kV.
The first commercial HVDC line built in 1954 was a 98 km submarine cable with ground
return between the island of Gotland and the Swedish mainland.
Thyristors were applied to d.c. transmission in the late 1960’s and solid state valves became a
reality. In 1969, a contract for the Eel River d.c. link in Canada was awarded as the first
application of sold state valves for HVDC transmission. Today, the highest functional d.c.
voltage for d.c. transmission is +/- 600 kV for the 785 km transmission line of the Itaipu scheme
in Brazil. D.c. transmission is now an integral part of the delivery of electricity in many countries
throughout the world
2.2 Why Use DC Transmission
The question is often asked, “Why use d.c. transmission?” One response is that losses are lower,
but this is not correct. The level of losses is designed into a transmission system and is
regulated by the size of conductor selected. D.c. and a.c. conductors, either as overhead
transmission lines or submarine cables can have lower losses but at higher expense since the
larger cross-sectional area will generally result in lower losses but cost more.
When converters are used for d.c. transmission in preference to a.c. transmission, it is
generally by economic choice driven by one of the following reasons:
1. An overhead d.c. transmission line with its towers can be designed to be less costly per unit
of length than an equivalent a.c. line designed to transmit the same level of
electric power. However the d.c. converter stations at each end are more costly than the
terminating stations of an a.c. line and so there is a breakeven distance above which the
total cost of d.c. transmission is less than its a.c. transmission alternative. The d.c.
transmission line can have a lower visual profile than an equivalent a.c. line and so
contributes to a lower environmental impact. There are other environmental advantages to

4
a d.c. transmission line through the electric and magnetic fields being d.c. instead of ac.
2. If transmission is by submarine or underground cable, the breakeven distance is much less
than overhead transmission. It is not practical to consider a.c. cable systems exceeding 50 km
but d.c. cable transmission systems are in service whose length is in the hundreds of kilometers
and even distances of 600 km or greater have been considered feasible.
3. Some a.c. electric power systems are not synchronized to neighboring networks even though
their physical distances between them is quite small. This occurs in Japan where half the country
is a 60 hz network and the other is a 50 hz system. It is physically impossible to connect the two
together by direct a.c. methods in order to exchange electric power between them. However, if a
d.c. converter station is located in each system with an interconnecting d.c. link between them, it
is possible to transfer the required power flow even though the a.c. systems so connected remain
asynchronous.
2.3 Configurations
The integral part of an HVDC power converter is the valve or valve arm. It may be non-
controllable if constructed from one or more power diodes in series or controllable if constructed
from one or more thyristors in series. Figure 1 depicts the International Electrotechnical
Commission (IEC) graphical symbols for valves and bridges (1). The standard bridge or
converter connection is defined as a double-way connection comprising six valves or valve arms
which are connected as illustrated in Figure 2. Electric power flowing between the HVDC valve
group and the a.c. system is three
phase. When electric power flows into the d.c. valve group from the a.c. system then it is
considered a rectifier. If power flows from the d.c. valve group into the a.c. system, it is an
inverter. Each valve consists of many series connected thyristors in thyristor modules. Figure 2
represents the electric circuit network depiction for the six pulse valve group configuration.
The six pulse valve group was usual when the valves were mercury arc.

5
2.4 Twelve Pulse Valve Group
Nearly all HVDC power converters with thyristor valves are assembled in a converter bridge of
twelve pulse configuration. Figure 3 demonstrates the use of two three phase converter
transformers with one d.c. side winding as an ungrounded star connection and the other a delta
configuration. Consequently the a.c. voltages applied to each six pulse valve group which
make up the twelve pulse valve group have a phase difference of 30 degrees which is utilized
to cancel the a.c. side 5th
and 7th
harmonic currents and d.c. side
6th
harmonic voltage, thus resulting in a significant saving in harmonic filters. Figure 3 also
shows the outline around each of the three groups of four valves in a single vertical stack.
These are known as “quadrivalves” and are assembled as one valve structure by stacking four
valves in series. Since the voltage rating of thyristors is several kV, a 500 kV quadrivalve may
have hundreds of individual thyristors connected in series groups of valve or thyristor
modules. A quadrivalve for a high voltage converter is mechanically quite tall and may be
suspended from the ceiling of the valve hall, especially in locations susceptible to earthquakes.
3 Quadrivalves
Ac Side
a
b
Dc
c Side
a
Figure2.1. The twelve pulse valve group configuration with two converter transformers.
One in star-star connection and the other in star-delta connection.

6
2.5 Thyristor Module
A thyristor or valve module is that part of a valve in a mechanical assembly of series
connected thyristors and their immediate auxiliaries including heat sinks cooled by air,
water or glycol, damping circuits and valve firing electronics. A thyristor module is usually
interchangeable for maintenance purposes and consists of electric components as shown in
Figure 4.
2.6 Substation Configuration
The central equipment of a d.c. substation (2) are the thyristor converters which are
usually housed inside a valve hall. Outdoor valves have been applied such as in the
Cahora Bassa d.c. transmission line between Mozambique and South Africa. Figure 5
shows an example of the electrical equipment required for a d.c. substation. In this
example, two poles are represented which is the usual case and is known as the “bipole”
configuration. Some d.c. cable systems only have one pole or “monopole” configuration
and may either use the ground as a return path when permitted or use an additional cable
to avoid earth currents.
From Figure 5, essential equipment in a d.c. substation in addition to the valve groups
include the converter transformers. Their purpose is to transform the a.c. system voltage
to which the d.c. system is connected so that the correct d.c. voltage is derived by the
converter bridges. For higher rated d.c. substations, converter transformers for 12 pulse
operation are usually comprised of single phase units which is a cost effective way to
provide spare units for increased reliability.
The secondary or d.c. side windings of the converter transformers are connected to the
converter bridges. The converter transformer is located in the switchyard, and if the
converter bridges are located in the valve hall, the connection has to be made through its
wall. This is accomplished in either of two ways. Firstly, with phase isolated busbars

7
where the bus conductors are housed within insulated bus ducts with oil or SF6 as the
insulating medium or secondly, with wall bushings. When applied at d.c. voltages at 400
kV or greater, wall bushings require considerable design and care to avoid external or
internal insulation breakdown.
Harmonic filters are required on the a.c. side and usually on the d.c. side. The
characteristic a.c. side current harmonics generated by 6 pulse converters are 6n +/- 1 and
12n +/- 1 for 12 pulse converters where n equals all positive integers. A.c. filters are
typically tuned to 11th
, 13th
, 23rd
and 25th
harmonics for 12 pulse converters. Tuning to the
5th
and 7th
harmonics is required if the converters can be configured into 6 pulse
operation. A.c. side harmonic filters may be switched with circuit breakers or circuit
switches to accommodate reactive power requirement strategies since these filters
generate reactive power at fundamental frequency. A parallel resonance is naturally
created between the capacitance of the a.c. filters and the inductive impedance of the a.c.
system. For the special case where such a resonance is lightly damped and tuned to a
frequency between the 2nd
and 4th
harmonic, then a low order harmonic filter at the 2nd
or
3rd
harmonic may be required, even for 12 pulse converter operation.
Converter
Dc reactor
and arrester
Dc
Dc surge
capacitor
Converter unit 6
pulse
Converter
transformer
bridge filters
Earth return
transfer
breaker
Metallic return
transfer
breaker
Neutral bus
arrester
Neutral bus surge
capacitor
Ac filter
Earth electrode
and line
Converter unit 12
pulse Midpoint dc
bus
arrester
Dc bus
arrester
Dc bus
arrester
Dc line
arrester
Figure 2.2. Example of an HVDC substation.

8
Characteristic d.c. side voltage harmonics generated by a 6 pulse converter are of the
order 6n and when generated by a 12 pulse converter, are of the order 12n. D.c. side
filters reduce harmonic current flow on d.c. transmission lines to minimize coupling and
interference to adjacent voice frequency communication circuits. Where there is no d.c.
line such as in the back-to-back configuration, d.c. side filters may not be required.
D.c. reactors are usually included in each pole of a converter station. They assist the d.c.
filters in filtering harmonic currents and smooth the d.c. side current so that a
discontinuous current mode is not reached at low load current operation. Because rate of
change of d.c. side current is limited by the d.c. reactor, the commutation process of the
d.c. converter is made more robust.
Surge arresters across each valve in the converter bridge, across each converter bridge
and in the d.c. and a.c. switchyard are coordinated to protect the equipment from all
overvoltages regardless of their source. They may be used in non-standard applications
such as filter protection. Modern HVDC substations use metal-oxide arresters and their
rating and selection is made with careful insulation coordination design.
2.7APPLICATIONS OF HVDC CONVERTERS
The first application for HVDC converters was to provide point to point electrical power
interconnections between asynchronous a.c. power networks. There are other applications
which can be met by HVDC converter transmission which include:
1. Interconnections between asynchronous systems. Some continental electric power
systems consist of asynchronous networks such as the East, West, Texas and Quebec
networks in North America and island loads such as the Island of Gotland in the Baltic
Sea make good use of HVDC interconnections.
2. Deliver energy from remote energy sources. Where generation has been developed at
remote sites of available energy, HVDC transmission has been an economical means
to bring the electricity to load centers. Gas fired thermal generation can be located
close to load centers and may delay development of isolated energy sources in the near
term.
3. Import electric energy into congested load areas. In areas where new generation is
impossible to bring into service to meet load growth or replace inefficient or
decommissioned plant, underground d.c. cable transmission is a viable means to
import electricity.
4. Increasing the capacity of existing a.c. transmission by conversion to d.c. transmission.
New transmission rights-of-way may be impossible to obtain. Existing overhead a.c.
transmission lines if upgraded to or overbuilt with d.c. transmission can substantially
increase the power transfer capability on the existing right-of-way.
5. Power flow control. A.c. networks do not easily accommodate desired power flow
control. Power marketers and system operators may require the power flow control
capability provided by HVDC transmission.

9
6. Stabilization of electric power networks. Some wide spread a.c. power system
networks operate at stability limits well below the thermal capacity of their
transmission conductors. HVDC transmission is an option to consider to increase
utilization of network conductors along with the various power electronic controllers
which can be applied on a.c. transmission.
(a) Monopolar configuration
Figure 2.3. Monopolar and
bipolar connection of HVDC
converter bridges.
(b) Bipolar configuration

10
2.8 HVDC Converter Arrangements
HVDC converter bridges and lines or cables can be arranged into a number of
configurations for effective utilization. Converter bridges may be arranged either
monopolar or bipolar as shown in 12 pulse arrangement in Figure 6. Various ways HVDC
transmission is used are shown in simplified form in Figure 7 and include the following:
1. Back-to-Back. There are some applications where the two a.c. systems to be
interconnected are physically in the same location or substation. No transmission line
or cable is required between the converter bridges in this case and the connection may
be monopolar or bipolar. Back-to-back d.c. links are used in Japan for interconnections
between power system networks of different frequencies (50 and 60 Hz). They are also
used as interconnections between adjacent asynchronous networks.
2. Transmission Between Two Substations. When it is economical to transfer electric
power through d.c. transmission or cables from one geographical location to another, a
two-terminal or point-to-point HVDC transmission is used. In other words, d.c. power
from a d.c. rectifier terminal is dedicated to one other terminal operating as an inverter.
This is typical of most HVDC transmission systems.
3. Multiterminal HVDC Transmission System. When three or more HVDC substations
are geographically separated with interconnecting transmission lines or cables, the
HVDC transmission system is multiterminal. If all substations are connected to the
same voltage then the system is parallel multiterminal d.c. If one or more converter bridges
are added in series in one or both poles, then the system is series multiterminal d.c. Parallel
multiterminal d.c. transmission has been applied when the substation capacity exceeds 10%
of the total rectifier substation capacity. It is expected a series multiterminal substation
would be applied when its capacity is small (less than 10%) compared to the total rectifier
substation capacity. A combination of parallel and series connections of converter bridges is
a hybrid multiterminal system. Multiterminal d.c. systems are more difficult to justify
economically because of the cost of the additional substations.
4. Unit Connection. When d.c. transmission is applied right at the point of generation, it
is possible to connect the converter transformer of the rectifier directly to the generator
terminals so the generated power feeds into the d.c. transmission lines. This might be
applied with hydro and wind turbine driven generators so that maximum efficiency of
the turbine can be achieved with speed control. Regardless of the turbine speed, the
power is delivered through the inverter terminal to the a.c. receiving system at its
fundamental frequency of 50 or 60 hz.
5. Diode Rectifier. It has been proposed that in some applications where d.c. power
transmission is in one direction only, the valves in the rectifier converter bridges can be
constructed from diodes instead of thyristors. Power flow control would be achieved at
the inverter, and in the case where the unit connection is used, a.c. voltage control by
the generator field exciter could be applied to regulate d.c. power. This connection may
require high speed a.c. circuit breakers between the generator and the rectifier
converter bridges to protect the diodes from overcurrents resulting from a sustained d.c.
transmission line short circuit.

11
2.9 ENVIRONMENTAL CONSIDERATIONS
The electrical environmental effects from HVDC. transmission lines can be characterized
by field and ion effects as well as corona effects (4), (5). The electric field arises from both
the electrical charge on the conductors and for a HVDC overhead transmission line, from
charges on air ions and aerosols surrounding the conductor. These give rise to d.c. electric
fields due to the ion current density flowing through the air from or to the conductors as
well as due to the ion density in the air. A d.c. magnetic field is produced
by d.c. current flowing through the conductors. Air ions produced by HVDC lines form
clouds which drift away from the line when blown by the wind and may come in contact
with humans, animals and plants outside the transmission line right-of -way or corridor.
The corona effects may produce low levels of radio interference, audible noise and ozone
generation.
Field and corona effects
The field and corona effects of transmission lines largely favor d.c. transmission over a.c.
transmission. The significant considerations are as follows:
1. For a given power transfer requiring extra high voltage transmission, the d.c.
transmission line will have a smaller tower profile than the equivalent a.c. tower
carrying the same level of power. This can also lead to less width of right-of-way for
the d.c. transmission option.
2. The steady and direct magnetic field of a d.c. transmission line near or at the edge of
the transmission right-of-way will be about the same value in magnitude as the earth’s
naturally occurring magnetic field. For this reason alone, it seems unlikely that this
small contribution by HVDC transmission lines to the background geomagnetic field
would be a basis for concern.
3. The static and steady electric field from d.c. transmission at the levels experienced
beneath lines or at the edge of the right-of-way have no known adverse biological
effects. There is no theory or mechanism to explain how a static electric field at the
levels produced by d.c. transmission lines could effect human health. The electric field
level beneath a HVDC transmission line is of similar magnitude as the naturally
occurring static field which exists beneath thunder clouds. Electric fields from a.c.
transmission lines have been under more intense scrutiny than fields generated from d.c.
transmission lines.

12
4. The ion and corona effects of d.c. transmission lines lead to a small contribution of
ozone production to higher naturally occurring background concentrations. Exacting
long term measurements are required to detect such concentrations. The measurements
taken at cross-sections across the Nelson River d.c. lines in Canada failed to
distinguish background from downwind levels (4). While solar radiation influences the
production of ozone even in a rural environment, thereby maintaining its level, any
incremental contribution from a d.c. line source is subject to breakdown, leading to a
resumption of background levels downwind from the line. Investigations of ozone for
indoor conditions indicate that in well mixed air, the half-life of ozone is 1.5 minutes
to 7.9 minutes. Increases in temperature and humidity increase the rate of decay (4).
5. If ground return is used with monopolar operation, the resulting d.c. magnetic field can
cause error in magnetic compass readings taken in the vicinity of the d.c. line or cable.
This impact is minimized by providing a conductor or cable return path (known as
metallic return) in close proximity to the main conductor or cable for magnetic field
cancellation. Another concern with continuous ground current is that some of the
return current may flow in metallic structures such as pipelines and intensify corrosion
if cathodic protection is not provided. When pipelines or other continuous metallic
grounded structures are in the vicinity of a d.c. link, metallic return may be necessary.
2.10 D.C CONVERTER OPERATION
The six pulse converter bridge of Figure 2 as the basic converter unit of HVDC
transmission is used equally well for rectification where electric power flows from the
a.c. side to the d.c. side and inversion where the power flow is from the d.c. side to the
a.c. side. Thyristor valves operate as switches which turn on and conduct current when
fired on receiving a gate pulse and are forward biased. A thyristor valve will conduct
current in one direction and once it conducts, will only turn off when it is reverse biased
and the current falls to zero. This process is known as line commutation.
An important property of the thyristor valve is that once its conducting current falls to zero
when it is reverse biased and the gate pulse is removed, too rapid an increase in the
magnitude of the forward biased voltage will cause the thyristor to inadvertently turn on and
conduct. The design of the thyristor valve and converter bridge must ensure such a
condition is avoided for useful inverter operation.

13
X X
Commutation
Rectification or inversion for HVDC converters is accomplished through a process
known as line or natural commutation. The valves act as switches so that the a.c. voltage
is sequentially switched to always provide a d.c. voltage. With line commutation, the a.c.
voltage at both the rectifier and inverter must be provided by the a.c. networks at each
end and should be three phase and relatively free of harmonics as depicted in Figure 8.
As each valve switches on, it will begin to conduct current while the current begins to
fall to zero in the next valve to turn off. Commutation is the process of transfer of current
between any two converter valves with both valves carrying current simultaneously
during this process.
Consider the rectification process. Each valve will switch on when it receives a firing
pulse to its gate and its forward bias voltage becomes more positive than the forward bias
voltage of the conducting valve. The current flow through a conducting valve does not
change instantaneously as it commutates to another valve because the transfer is through
transformer windings. The leakage reactance of the transformer windings is also the
commutation reactance so long as the a.c. filters are located on the primary or a.c. side of
the converter transformer. The commutation reactance at the rectifier and inverter is
shown as an equivalent reactance XC in Figure 8. The sum of all the valve currents
transferred the d.c. side and through the d.c. reactor is the direct current and it is
relatively flat because of the inductance of the d.c. reactor and converter transformer
Rectifier
Id
Inverter
Ivr Ivi
Udr Udi
C
ULr Uvr
Uvi
C
ULi
Commutation voltage at
rectifier
Commutation Voltage
at invert

14
Figure 2.4. Voltage and current waveshapes associated with d.c. converter bridges.
At the inverter, the three phase a.c. voltage supplied by the a.c. system provides the
forward and reverse bias conditions of each valve in the converter bridge to allow
commutation of current between valves the same as in the rectifier. The inverter valve can
only turn on and conduct when the positive direct voltage from the d.c. line is greater than
the back negative voltage derived from the a.c. commutation voltage of the a.c. system at
the inverter.
Due to the line commutation valve switching process, a non-sinusoidal current is taken
from the a.c. system at the rectifier (Ivr in Figure 8) and is delivered to the a.c. system at
the inverter (Ivi in Figure 8). Both Ivr and Ivi are lagging to the alternating voltage. This
non-sinusoidal current waveform consists of the fundamental frequency a.c. component
plus higher harmonics being taken from, and injected into, each a.c. system. The a.c.
filters divert the harmonics from entering the a.c. system by offering a low impedance by-
pass path allowing the commutation voltage to be relatively harmonic free(ULr and ULi in
Figure 8).
Reversal of power flow in a line commutated d.c. link is not possible by reversing the
direction of the direct current. The valves will allow conduction in one direction only.
Power flow can only be reversed in line commutated d.c. converter bridges by changing
the polarity of the direct voltage. The dual operation of the converter bridges as either a
rectifier or inverter is achieved through firing control of the grid pulses.
Short circuit ratio
The strength of the a.c. network at the bus of the HVDC substation can be expressed by
the short circuit ratio (SCR), defined as the relation between the short circuit level in
MVA at the HVDC substation bus at 1.0 per-unit a.c. voltage and the d.c. power in MW.
The capacitors and a.c. filters connected to the a.c. bus reduce the short circuit level. The
expression effective short circuit ratio (ESCR) is used for the ratio between the short
circuit level reduced by the reactive power of the shunt capacitor banks and a.c. filters
connected to the a.c. bus at 1.0 per-unit voltage and the rated d.c. power.
Lower ESCR or SCR means more pronounced interaction between the HVDC substation
and the a.c. network (9), (10). A.c. networks can be classified in the following catagories
according to strength:

15
strong systems with high ESCR: ESCR > 3.0
systems of low ESCR: 3.0 > ESCR > 2.0
weak systems with very low ESCR: ESCR < 2.0
In the case of high ESCR systems, changes in the active/reactive power from the HVDC
substation lead to small or moderate a.c. voltage changes. Therefore the additional
transient voltage control at the busbar is not normally required. The reactive power
balance between the a.c. network and the HVDC substation can be achieved by switched
reactive power elements.
In the case of low and very low ESCR systems, the changes in the a.c. network or in the
HVDC transmission power could lead to voltage oscillations and a need for special
control strategies. Dynamic reactive power control at the a.c. bus at or near the HVDC
substation by some form of power electronic reactive power controller such as a static var
compensator (SVC) or static synchronous compensator (STATCOM) may be necessary
(12). In earlier times, dynamic reactive power control was achieved with synchronous
compensators.
2.11Commutation Failure
When a converter bridge is operating as an inverter as represented at the receiving end of
the d.c. link in Figure 8, a valve will turn off when its forward current commutates to zero
and the voltage across the valve remains negative. The period for which the valve stays
negatively biased is the extinction angle , the duration beyond which the valve then
becomes forward biased. Without a firing pulse, the valve will ideally stay non
conductive or blocked, even though it experiences a forward bias.
All d.c. valves require removal of the internal stored charges produced during the forward
conducting period (defined by period + at the inverter in Figure 8) before the valve
can successfully establish its ability to block a forward bias. The d.c. inverter therefor
requires a minimum period of negative bias or minimum extinction angle for forward
blocking to be successful. If forward blocking fails and conduction is initiated without a
firing pulse, commutation failure occurs. This also results in an immediate failure to
maintain current in the succeeding converter arm as the d.c. line current returns to the
valve which was previously conducting and which has failed to sustain forward blocking
(13).

16
Commutation failure at a converter bridge operating as an inverter is caused by any of the
following reasons:
1. When the d.c. current entering the inverter experiences an increase in magnitude which
causes the overlap angle to increase, the extinction angle is reduced and may reach
the point where the valve is unable to maintain forward blocking. Increasing the
inductance of the d.c. current path through the converter by means of the d.c.
smoothing reactor and commutating reactance reduces the rate of change of d.c.
current. This has the greatest effect on commutation failure onset.
2. When the magnitude of the a.c. side voltage on one or more phases reduces or is
distorted causing the extinction angle to be inadequate as commutation is attempted.
3. A phase angle shift in the a.c. commutating voltage can cause commutation failure.
However, the a.c. voltage magnitude reduction and not the corresponding phase shift is
the most dominant factor determining the onset of commutation failures for single
phase faults.
4. The value of the pre-disturbance steady state extinction angle also effects the
sensitivity of the inverter to commutation failure. A value of = 18O
is usual for most
inverters. Increasing to values of 25O
, 30O
or higher will reduce the possibility of
commutation failure (at the expense of increasing the reactive power demand of the
inverter).
5. The value of valve current prior to the commutation failure also effects the conditions
at which a commutation failure may occur. A commutation failure may more readily
happen if the pre-disturbance current is at full load compared to light load current
operation.
In general, the more rigid the a.c. voltage to which the inverter feeds into and with an
absence of a.c. system disturbances, the less likelihood there will be commutation
failures.
2.12 Series Capacitors With D.C. Converter Substations
HVDC transmission systems with long d.c. cables are prone to commutation failure when
there is a drop in d.c. voltage Ud at the inverter. The d.c. cable has very large capacitance
which will discharge current towards the voltage drop at the inverter. The discharge
current is limited by the d.c. voltage derived from the a.c. voltage of the commutating bus
as well as the d.c. smoothing reactor and the commutating reactance. If the discharge
current of the cable increases too quickly, commutation failure will occur causing
complete discharge of the cable. To recharge the cable back to its normal operating
voltage will delay recovery.
The converter bridge firing controls can be designed to increase the delay angle when
an increase in d.c. current is detected. This may be effective until the limit of the
minimum allowable extinction angle is reached.

17
Another way to limit the cable discharge current is to operate the inverter bridge with a
three phase series capacitor located in the a.c. system on either side of the converter
transformer. Any discharge current from the d.c. cable will pass into the a.c. system through
the normally functioning converter bridge and in doing so, will pass through the
series capacitor and add charge to it. As a consequence, the voltage of the series capacitor
will increase to oppose the cable discharge and be reflected through the converter bridge
as an increase in d.c. voltage Ud. This will act as a back emf and limit the discharge
current of the cable, thereby avoiding the commutation failure.
The proposed locations of the series capacitor are shown in Figure 9 in single line
diagram form (14), (15). With the capacitor located between the converter transformer
and the valve group, it is known as a capacitor commutated converter (CCC). With the
capacitor located on the a.c. system side of the converter transformer, it is known as a
controlled series capacitor converter (CSCC). Each configuration will improve
commutation performance of the inverter but the CSCC requires design features to
eliminate ferroresonance between the series capacitor and the converter transformer if it
should be instigated.
2.13 CONTROL AND PROTECTION
HVDC transmission systems must transport very large amounts of electric power which
can only be accomplished under tightly controlled conditions. D.c. current and voltage is
precisely controlled to effect the desired power transfer. It is necessary therefor to
continuously and precisely measure system quantities which include at each converter
bridge, the d.c. current, its d.c. side voltage, the delay angle and for an inverter, its
extinction angle .
Two terminal d.c. transmission systems are the more usual and they have in common a
preferred mode of control during normal operation. Under steady state conditions, the
inverter is assigned the task of controlling the d.c. voltage. This it may do by maintaining
a constant extinction angle which causes the d.c. voltage Ud to droop with increasing
d.c. current Id as shown in the minimum constant extinction angle characteristic A-B-C-
D in Figure 10. The weaker the a.c. system at the inverter, the steeper the droop.
Alternatively, the inverter may normally operate in a d.c. voltage controlling mode which
is the constant Ud characteristic B-H-E in Figure 10. This means that the extinction angle
must increase beyond its minimum setting depicted in Figure 10 as 18O
.

18
If the inverter is operating in a minimum constant or constant Ud characteristic, than the
rectifier must control the d.c. current Id. This it can do so long as the delay angle is not
at its minimum limit (usually 5O
). The steady state constant current characteristic of the
rectifier is shown in Figure 10 as the vertical section Q-C-H-R. Where the rectifier and
inverter characteristic intersect, either at points C or H, is the operating point of the
HVDC system.
The operating point is reached by action of the on-line tap changers of the converter
transformers. The inverter must establish the d.c. voltage Ud byadjusting its on-line tap
changer to achieve the desired operating level if it is in constant minimum control. If in
constant Ud control, the on-line tap changer must adjust its tap to allow the controlled
level of Ud be achieved with an extinction angle equal to or slightly larger than its
minimum setting of 18O
in this case.
The on-line tap changers on the converter transformers of the rectifier are controlled to
adjust their tap settings so that the delay angle has a working range at a level between
approximately 10O
and 15O
for maintaining the constant current setting Iorder (see Figure
10). If the inverter is operating in constant d.c. voltage control at the operating point H,
and if the d.c. current order Iorder is increased so that the operating point H moves towards
and beyond point B, the inverter mode of control will revert to constant extinction angle
control and operate on characteristic A-B. D.c. voltage Ud will be less than the desired
value, and so the converter transformer on-line tap changer at the inverter will boost its
d.c. side voltage until d.c. voltage control is resumed.
Not all HVDC transmission system controls have a constant d.c. voltage control such as is
depicted by the horizontal characteristic B-H-E in Figure 10. Instead, the constant
extinction angle control of characteristic A-B-C-D and the tap changer will provide the
d.c. voltage control

19
.
Current margin
The d.c. current order Iorder is sent to both the rectifier and inverter. It is usual to subtract a
small value of current order from the Iorder sent to the inverter. This is known as the
current margin Imargin and is depicted in Figure 10. The inverter also has a current
controller and it attempts to control the d.c. current Id to the value Iorder - Imargin but the
current controller at the rectifier normally overrides it to maintain the d.c. current at Iorder.
This discrepancy is resolved at the inverter in normal steady state operation as its current
controller is not able to keep the d.c. current to the desired value of Iorder - Imargin and is
forced out of action. The current control at the inverter becomes active only when the
current control at the rectifier ceases when its delay angle is pegged against its
minimum delay angle limit. This is readily observed in the operating characteristics of
Figure 10 where the minimum delay angle limit at the rectifier is characteristic P-Q. If for
some reason or other such as a low a.c. commutating voltage at the rectifier end, the P-Q
characteristic falls below points D or E, the operating point will shift from point H to
somewhere on the vertical characteristic D-E-F where it is intersected by the lowered P-Q
characteristic. The inverter reverts to current control, controlling the d.c. current Id to the
value Iorder - Imargin and the rectifier is effectively controlling d.c. voltage so long as it is
operating at its minimum delay angle characteristic P-Q. The controls can be designed
such that the transition from the rectifier controlling current to the inverter controlling
current is automatic and smooth.
Voltage dependent current order limit (VDCOL)
During disturbances where the a.c. voltage at the rectifier or inverter is depressed, it will
not be helpful to a weak a.c. system if the HVDC transmission system attempts to
maintain full load current. A sag in a.c. voltage at either end will result in a lowered d.c.
voltage too. The d.c. control characteristics shown in Figure 10 indicates the d.c. current
order is reduced if the d.c. voltage is lowered. This can be observed in the rectifier
characteristic R-S-T and in the inverter characteristic F-G in Figure 10. The controller
which reduces the maximum current order is known as a voltage dependent current order
limit or VDCOL (sometimes referred to as a VDCL). The VDCOL control, if invoked by
an a.c. system disturbance will keep the d.c. current Id to the lowered limit during
recovery which aids the corresponding recovery of the d.c. system. Only when d.c.
voltage Ud has recovered sufficiently will the d.c. current return to its original Iorder level.

20
2.14 A.C. Voltage Control
It is desirable to rigidly maintain the a.c. system and commutating bus voltage to a
constant value for best operation of the HVDC transmission system. This is more easily
achieved when the short circuit ratio is high. With low or very low short circuit ratio
systems, difficulties may arise following load changes. With fast load variation, there can
be an excess or deficiency of reactive power at the a.c. commutating bus which results in
over and undervoltages respectively. When the a.c. system is weak, the changes in
converter a.c. bus voltage following a disturbance may be beyond permissible limits. In
such cases, an a.c. voltage controller is required for the following reasons:
1. To limit dynamic and transient overvoltage to within permissible limits defined by
substation equipment specifications and standards.
2. To prevent a.c. voltage flicker and commutation failure due to a.c. voltage fluctuations
when load and filter switching occurs.
3. To enhance HVDC transmission system recovery following severe a.c. system
disturbances.
4. To avoid control system instability, particularly when operating in the extinction angle
control mode at the inverter.
The synchronous compensator has been the preferred means of a.c. voltage control as it
increases the short circuit ratio and serves as a variable reactive power source. Its
disadvantages include high losses and maintenance which add to its overall cost.
Additional a.c. voltage controllers are available and include:
1. Static compensators which utilize thyristors to control current through inductors and switch
in or out various levels of capacitors. By this means, fast control of reactive power is
possible to maintain a.c. voltage within desired limits. The main disadvantage is that it
does not add to the short circuit ratio.
2. Converter control through delay angle control is possible to regulate the reactive power
demand of the converter bridges. This requires that the measured a.c. voltage be used
as a feedback signal in the d.c. controls, and delay angle is transiently modulated to
regulate the a.c. commutating bus voltage. This form of control is limited in its
effectiveness, particularly when there is little or no d.c. current in the converter when
voltage control is required.

21
3. Use of specially cooled metal oxide varistors together with fast mechanical switching
of shunt reactors, capacitors and filters. The metal oxide varistors will protect the
HVDC substation equipment against the transient overvoltages, and the switchings of
reactive power components will achieve the reactive power balance. Its disadvantage is
that voltage control is not continuous, reactive power control is delayed by the
slowness of mechanical switching, and short circuit ratio is not increased.
4. Saturated reactors have been applied to limit overvoltages and achieve reactive power
balance. Shunt capacitors and filters are required to maintain the reactors in saturation.
A.c. voltage control is achieved without controls on a droop characteristic. Short
circuit ratio is not increased.
5. Series capacitors in the form of CCC or CSCC can increase the short circuit ratio and
improve the regulation of a.c. commutating bus voltage.
6. The static compensator or STATCOM makes use of gate turn-off thyristors in the
configuration of the voltage source converter bridge. This is the fastest responding
voltage controller available and may offer limited capability for increased short circuit
ratio.
Since each a.c. system with its HVDC application is unique, the voltage control method
applied is subject to study and design.
2.15 Special Purpose Controls
There are a number of special purpose controllers which can be added to HVDC controls
to take advantage of the fast response of a d.c. link and help the performance of the a.c.
system. These include:
A.c. system damping controls. An a.c. system is subject to power swings due to
electromechanical oscillations. A controller can be added to modulate the d.c. power
order or d.c. current order to add damping. The frequency or voltage phase angle of the
a.c. system is measured at one or both ends of the d.c. link, and the controller is designed
to adjust the power of the d.c. link accordingly.

22
A.c. system frequency control. A slow responding controller can also adjust the power
of the d.c. link to help regulate power system frequency. If the rectifier and inverter are
in asynchronous power systems, the d.c. controller can draw power from one system to
the other to assist in frequency stabilization of each.
Step change power adjustment. A non-continuous power adjustment can be
implemented to take advantage of the ability of a HVDC transmission system to rapidly
reduce or increase power. If a.c. system protection determines that a generator or a.c.
transmission line is to be tripped, a signal can be sent to the d.c. controls to change its
power or current order by an amount which will compensate the loss. This feature is
useful in helping maintain a.c. system stability and to ease the shock of a disturbance
over a wider area.
A.c. undervoltage compensation. Some portions of an electric power system are prone
to a.c. voltage collapse. If a HVDC transmission system is in such an area, a control can
be implemented which on detecting the a.c. voltage drop and the rate at which it is
dropping, a fast power or current order reduction of the d.c. link can be affected. The
reduction in power and reactive power can remove the undervoltage stress on the a.c.
system and restore its voltage to normal.
Subsynchronous oscillation damping. A steam turbine and electric generator can have
mechanical subsynchronous oscillation modes between the various turbine stages and
the generator. If such a generator feeds into the rectifier of a d.c. link, supplementary
control may be required on the d.c. link to ensure the subsynchronous oscillation modes
of concern are positively damped to limit torsional stresses on the turbine shaft.

23
Chapter 3
VSC Based HVDC Transmission System
3.1 Introduction
The HVDC transmission technology can be realized by using current source converters
(CSCs) commutated thyristor switches, known as traditional HVDC or classic HVDC, or
by using voltage source converters (VSC-based HVDC). Due to the rapid development of
power electronic devices with turn-off capability and of DSPs, which are generating the
appropriate ﬁring patterns, the VSC are getting more and more attractive for HVDC
transmission .Usually, the VSCs are using insulated gate bipolar transistor (IGBT) valves
and pulse width modulation (PWM) for creating the desired voltage wave form.
The ﬁrst HVDC transmission using VSC was installed in 1997 in Gotland (Sweden) .
3.2 Advantages and Applications for VSC Based HVDC
By analyzing the operation of both classic HVDC technology and VSC-based HVDC
technology, the main difference between these two technologies can be highlighted: the
controllability. Thus, the controllability in the case of VSC-based HVDC technology is
higher compared with the one of the earlier developed technology. Thereby, if VSCs are
used instead of line-commutated CSCs several advantages can be stated, some of them
being presented below:
(i)VSC converter technology provides rapid and independent control of active and reac-
tive power without needing extra compensating equipment; the reactive power can be
controlled at both terminals independently of the DC transmission voltage level .
On the market, mainly two manufacture refer to the technology of DC transmission using
VSC; these are: ABB under the name HVDC Light R [14], with a power rating from
tenths of megawatts up to over 1000 MW, and the second manufacturer is Siemens under
the name HVDC Plus (”Plus” - Power Link Universal Systems).
(ii) the commutation failures due to disturbances in the AC network can be reduced or even
avoided if VSC-HVDC technology is used.
(iii)the VSC-HVDC system can be connected to a ”weak” AC network or to a network
where no generation source is available (the VSC can work independently of any AC
source), so the short circuit level is low .
(iv)self (forced) commutation with voltage source converters permits black start,
which means that the VSC is used to synthesize a balanced set of three phase voltages
as a virtual synchronous generator.

24
(i)Power supply to insular loads:
new units can be easily added if the expand of the WF is desired .
(iii)Underground/underwater cables:
The use of HVDC cable systems is not constraint by any distance limitations as in the
case of AC cable systems. Moreover, the losses are reduced when an HVDC cable system
is used. The XLPE (Cross Linked Poly-Ethylene) extruded HVDC cables can overcome
RoW constrains and the power transfer capacity is increased at the same time .
(iv)Urban Infeed :
Mainly due to RoW constraints and land use constraints, the compact VSC-based
HVDC technology represents a feasible solution to feed the city centers. Thus, the
underground transmission circuits are placed on already existing dual-use RoWs in or-
der to bring in power as well as to provide voltage support . This process is
realized without compromising reliability and it is an economical way of power supply.
(v)due to its modular, compact and standardized construction, the converter can be easily
and rapidly installed/commissioned at the desired site .
(vi)in comparison with the classic HVDC transmission, the VSCs don not have any
reactive power demand and moreover, they can control their reactive power to regulate
the AC system voltage like a generator .
However, the VSC-based HVDC technology has some drawbacks, which include poten-
tially high power losses and high cost (caused by the converter stations) compared with
traditional HVDC technology.
Because of its advantages, some of them presented above, the VSC-based HVDC
transmission suits very well in certain application. An enumeration of these applications is
presented below:
Due to some of its advantages such as: dynamic voltage control, black start capability or
forced-commutation the VSC-HVDC transmission is capable to supply remote locations
(i.e. islands) using submarine cables and without any need of running expensive local
An example of this application is the Gotland Island System.
(ii)Offshore Application:
The VSC-based HVDC technology represents a very suitable way of transmitting power
from wind farms to the main AC grid. The ability of controlling reactive power as well
as the AC voltage and its contribution to the grid stability makes the VSC-HVDC
technology very popular for such applications. Moreover, the technology is ﬂexible and

25
3.3VSC-based HVDC Transmission System Configurations
Such a transmission system consists of: two voltage source converters, transformers,
phase reactors, AC filters, DC-link capacitors and DC cables. In the upcoming paragraphs
each of these components will be briefly discussed.
3.4 Voltage Source Converter
The two VSCs may be seen as the core of this transmission system topology. One of the
VSCs works as rectifier, while the other one works as an inverter, and both of them are
based on IGBT power semiconductors. The two VSC stations are connected through a
DCtransmission line or an overhead line.
Mainly, two basic configurations of VSCs are used on HVDC transmission system.
Theseare the two-level VSC converter, presented in Figure 3.2(a), and the three-level
VSC converter, which is presented Figure 3.2(b) .
The two-level VSC, also known as the three phase, two level, six-pulses bridge, is the
simplest configuration suitable for HVDC transmission. Such a converter consists of six
valves (each valve consist of an IGBT and an anti-parallel diode) and is capable of
generating two voltage levels −0.5 · UDCn and +0.5 · UDCn .
In high power applications, the three-level VSC configuration , repre-
sents a reliable alternative to the two-level VSC configuration, because the phase
potentials can be modulated between three levels, −0.5·UDCn , 0 and +0.5·UDCn . In
this configuration,one arm of the converter consists of four valves.
3.5 Transformer
The transformers are used to interconnect the VSC with the AC network. The
main function of the transformers is to adapt the voltage level of the AC network to a
voltage level suitable to the converter. This voltage level can be controlled using a tap
changer, which will maximize the reactive power flow.
3.6 Phase Reactor
The phase reactors, known also as converter reactors, are used to continuously control the
active and reactive power flow.The phase reactors have three main functions:
the last function is to limit the short-circuit currents.
Typically, the short-circuit voltage of the phase reactor is 15%.
the second function is to provide active and reactive power control; the active and
reactive power flow between the AC and the DC side is defined by the fundamental
frequency voltage across the reactors.
• the first one is to provide low-pass filtering of the PWM pattern in order to provide the
desired fundamental frequency voltage,

26
3.7 AC Filter
3.8 DC-link Capacitor
As presented in Figure , on the DC side, there are two capacitor stacks of the same power
rating. The main goal of the DC-link capacitor is to provide a low-inductance path for the
the harmonics ripple on the DC voltage.
Depending on the size of the DC side capacitor, DC voltage variations caused by distur-
bances in the system (e.g. AC faults) can be limited .
3.9 DC Cable
the self contained fluid filled(oil filled, gas pressurized) cables, the solid cables and XLPE
polymer extruded cables. Lately, the last mentioned type seems to be the the preferred
choice for VSC-based HVDC transmission system, because of their mechanical strength,
flexibility and low weight
The main goal of the AC filters is to eliminate the harmonic content - which was created
by using the PWM technique - of the output AC voltage. Otherwise, if these harmonic
components are not eliminated or reduced, malfunctioning in the AC grid will appear.
Typical requirements for AC filters are: individual harmonic distortion level (Dh ≈ 1%),
total harmonic distortion (THD) level may vary between 1.5% and 2.5% and telephone
influence factor (TIF) between 40 and 50% .
Depending on the desired filter performances or requirements, the filter configuration is
varying from application to application. In a typical HVDC Light scheme, the AC filter
consists of two or three grounded /ungrounded tuned filter branches .
Mainly, three types of DC cables are suitable for HVDC transmission systems.
These are:

27
Chapter 4
HVDC Cables
4.1 Introduction
Firsts of all, Cables are used when Overhead Line(that are simple and cheap but
with a significant impact on ambient) cannot be built for environmental reasons or when
power shall be transmitted underwater (through sea, lakes or rivers). In first case we have
the so called Underground High Voltage Cable systems, in the second case Submarine
Cable systems. In general the power is transmitted using Alternating Current (AC) by
simply connecting the two networks. The two networks must be SYNCHRONOUS:
same frequency, same phasing(different voltages can be managed with transformers).
Disturbances are also transmitted between the two networks.
A cable under AC voltage is subject to a capacitive current that is proportional to
the frequency f[Hz], to the voltage V[V], to the unitary capacitance C [μF/km] and
to the cable length L[km]:I = 2·π· f · C · V · L
Cables for HV-AC transmission typically have a capacitance of the order of
0,2-0,3 [μF/km] therefore require capacitive currents of 10 to 25 [A/km],
depending on system voltage and frequency.
For short lengths (few kilometers) this is not a problem, but for long lengths,
e.g. above 60-80 km the capacitive current become similar in magnitude (even if in
quadrature) to the active current that the cable is asked to transmit: losses
are very much increased and consequently actual cable rating is reduced. With
DC transmission, the things for the cable system are much simpler: f = 0;
Consequently, capacitive current and main effects relevant to reactances are
eliminated.Only conductor resistance plays the major role.
Transmission (Joule) losses are:W [W] = R · L · I
2
(+ W Earth Return)
and Voltage Drop:ΔV [V] = R · L · I(+ ΔV Earth Return)
Practically, there are no limits for the Transmission Length, quite independently
from transmission Voltage and Power.
However, systems are operated in AC; therefore DC transmission requires Converter
Stations at both ends to convert AC to DC at sending point and DC to AC at receiving
end. The two networks are not required to be syncronised; they can have different
frequency and voltage. The power flow is simply controlled by voltage drop. The
system, overall, acts like a Generating Power Station that is injecting power into the
receiving network. Conventional High-Power Converters use Tyristors (controlled
Diodes): the current must flow in one direction only. Therefore, when the power flow is
reversed, also the polarity on the HVDC cable is reversed:

28
4.2 Main Characteristics of an HVDC Cable System
In general, an HVDC system can be composed by various sections, sometime
including OHL lines, land and submarine cable. The HVDC Cable system is typically
made by:
(i)Cables ,(ii)Intermediate Joints and (iii)End Terminations.
In the Land (Underground) sections, Installation is generally done from large
drums, in excavated trenches, being the cable directly buried or pulled in
plastic pipes.
For Submarine Cables, the Installation is done by laying the cable on the sea bottom by
using suitable Ships, that can accomodate large quantity of cable on board, stored on
rotating platforms.
Very often, the cable is protected on the sea bottom against
possible damages caused by fishing tools and anchors by various
methods.
4.3 Classification of HVDC Cables
Cables used for HVDC transmission are mainly of three types:
1. MI: Insulated with special paper, impregnated with high viscosity compound.
2. SCFF: Insulated with special paper, impregnated with low viscosity oil
3.Extruded: Insulated with extruded polyethylene-based compound
These HVDC cables are briefly explained as follows:
1.Mass Impregnated Cables :
Mass Impregnated Cables are the most used; they are in service for more than
40 years and have been proven to be highly reliable. At present used for
Voltages up to 500 kV DC.Conductor sizes up to 2500 mm2.
Typical Manufacturing Flow Diagram of a Mass Impregnated Cables.

29
Figure 4.1:Mass Impregnated
Cable
2.Self Contained Fluid-Filled Cables :
Self Contained Fluid-Filled Cables are used for very high voltages (they are
qualified for 600 kV DC) and for short connections, where there are no
hydraulic limitations in order to feed the cable during thermal transients; at
present used for Voltages up to 500 kV DC.Conductor sizes up to 3000 mm2.
Figure 4.2:Self Contained Fluid –Filled Cables

30
3. Extruded Cables :
Extruded Cables for HVDC applications are still under development; at present
they are used for relatively low voltages (up to 150 kV DC), mainly associated
with Voltage Source Converters, that permit to reverse the power flow without
reversing the polarity on the cable.
In fact, an Extruded Insulation (generally PE based) can be subjected to an
uneven distribution of the charges, that can migrate inside the insulation due to
the effect of the electrical field.
It is therefore possible to have an accumulation of charges in localised areas inside the
insulation( space charges) that, in particular during rapid polarity reversals, can give
rise to localised high stress and bring to accelerated ageing of the insulation.
Figure 4.3:Extruded Cables (XLPE Cables)

31
Chapter 5
HVDC Tower
5.1 Construction Process and Costs
One of the major problems of the lines are the using of the area below. The first step for
the project is to define according to the possible consequences the best routing in term of
costs and result for the crossed areas.
Use of the area below an overhead line is restricted because objects must not come too
close to the energized conductors. Overhead lines and structures may shed ice, creating a
hazard. Radio reception can be impaired under a power line, due both to shielding of a
receiver antenna by the overhead conductors, and by partial discharge at insulators and
sharp points of the conductors which creates radio noise.
In the area surrounding overhead lines it is dangerous to risk interference; e.g. flying kites
or balloons, using ladders or operating machinery. In add some studies are showing that
life of organism can be influenced by the electrical field. The view of the lines can also
be another difficulty due to the tourism presence and real estate area in the vicinity.
Overhead distribution and transmission lines near airfields are often marked on maps, and
the lines themselves marked with conspicuous plastic reflectors, to warn pilots of the
presence of conductors.
All these subjects shall be anticipated in the first phase of the project, then we can
summarize as follow taking account of quality and reliability requirements:
Rough determination of the route, taking account of the following criteria:
Environmental compatibility
Low impact on nature
Most cost-effective construction possible
Efficient operation (small losses)
Consideration of natural or man-made obstacles (e.g. lakes, mountains and mountain
ranges, cities, conservation areas, etc.)
Possible locations of transformer substations
Possible locations of assembly yards
Maintenance costs in the operating phase
5.2 Detailed Planning of The Transmission Route
For the detailed planning, routing is carried out – an operation which involves recording
and assessing the features of the terrain in particular. This routing is carried out in stages,
in ever more detail.

32
5.3 Detailed Design and Execution Drawing
Taking account of the results of the routing, a detailed execution plan for
the overhead line is worked out. Besides a detailed geological survey (soil testing), this
also includes the design planning of the pylons. This essentially depends on topological
conditions (minimum clearances from objects and trees), scenic aspects (low mast height
in built-up areas wherever possible) and meteorological effects (influence of wind, ice
load, avalanche hazard), as well as on the number of conductor systems.
In order to ensure the highest possible level of operational safety, a thorough study of
wind conditions is carried out along the entire route. Individual wind zones are
established in the course of this study and the pylons are dimensioned accordingly.
Particular attention must be paid to critical sections in which the topography is such that
it can give rise to "funnel effects"characterised by high wind speeds. The crossing of
mountain tops is also to be regarded as critical.
Sectioning into ice-load zones (if there are) is likewise carried out. In Europe this is based
on a pan-European standard with individual national appendices. In critical areas, the
design of pylons and conductors should be reinforced.
In areas at risk from avalanches, pylons must be provided with special protection (e.g. by
means of avalanche wedges, intended to steer the avalanche forces around the pylon).
A project flowchart is drawn up for the realisation of the project. With longer lengths of
transmission lines, the project as a whole is divided into individual lots (e.g. 20 km).
5.4 Construction Phases / Time Schedule
In this chapter we will focus only on the T&D lines itself erection, the
construction of other element as substations are well-known.
5.5 Preliminary Work On Construction
Once the detailed planning has been carried out and the approval process completed, a
start can be made with the actual on-site construction work. However, considerable
preliminary work is needed before the actual work of erecting overhead lines can begin.
This preliminary work
includes:
Tree-felling work on routes running through forests
Road building work
Site facilities (usually about every 20 km)
5.6 Foundations
Foundations for tower structures may be large and costly, particularly if the ground
conditions are poor, such as in wetlands. Each structure may be considerably
strengthened by the use of guy wires to resist some of the forces due to the conductors.
In case the earth is extremely aggressive, special concrete must be used to avoid damage
in the foundation. In extreme climatic circumstances a foundation must be stronger and
bigger. If you are to build closely to the coast, you must consider that the wind conditions
are stronger there than in the middle of the land mass. Where you are, determines the
terrain class. The size of the pylon is also an important factor in the evaluation of the load

33
on the foundation and consequently the size of the foundation.
For this purpose the following methods can be applied:
The earth is dug up normally and in keeping with the size of the foundation, after which
the foundation is cast.
Bunging/ Sheet piling method is applied in narrow spaces. Interlocking sheets of steel are
pressed down at all four corners and the cast of the foundation starts step by step from
there. The earth will not fall into the pit during the dig, since it is held by plates.
Piling method this method is used for building an especially strong foundation. The
method is suitable for places where the ground does not have a strong adhesion (sandy
earth).Concrete piles are thrust into the ground into e.g. 10 metres depth with
approximately half a meter to one meter above the ground. The upper part of the concrete
pile is then blasted off and the iron inside the pile bent into the top layer of the
foundation, which is being cast on top. Thus the foundation is anchored in the best
possible way into the ground and has great static carrying capacity. The time frame
depends on the size of the foundation, but it typically takes one week to cast a foundation
of 5x5 metres.
The drying of foundation depends on the time of the year and the weather. In Summer the
foundation is ready for use after 1 – 2 weeks, whereas in Winter the foundation dries for
about 3 – 4 weeks.
5.7 Pylon Assembly / Switch Yard Erection
Whereas concrete and round steel masts are supplied complete, lattice pylons are
usually delivered in individual pieces and assembled into segments on site – on the
ground. The pylon segments and arms are then fixed together (pylon assembly).
Depending on the local conditions, this is done using either cranes or – especially in
rough terrain – helicopters.
According to the type and size of the elements the preassembling is scheduled.
The location of the T&D line and weight of the elements can drive to a mixed solution.
5.8 Cables Hanging
After the erection of the steel structure and the fitting of the surge arresters, isolators, and
cable reels are preassembled on the ground then they are attached to the pylon. The cable
reels allow the pilot rope, pulling rope and conductors to be installed.
Parallel to this, the cable-drum and winch sites are constructed and anchored
appropriately. The cable reels and cable winches are then fastened onto them. The usual
and simple method is the use of drawing machine tool.
Where the transmission route crosses transportation routes such as motorways or railway
lines,safety scaffolding is set up in the crossing area to prevent danger to the traffic
running below in the event of any cables falling.
These pilot ropes are up to 6 km long and are used for attaching the cable pulling ropes.
It will allow initiating the drawing of the conductor at its place.
After the pulling rope, finally the (operating) conductors and, depending on the voltage
level and lightning protection, one or two earthing conductors are hoisted up.
The pulling rope is a steel-wire rope with enough tensile strength to be able to hoist up
the final conductor and the earthing conductor (lightning protection cable).
The pilot ropes (usually nylon ropes 10-15 mm in diameter) are then hoisted up, using helicopters.

34
The cables are then adjusted. This involves tensioning the cables to the relevant tension
and adjusting to provide the necessary sag. The cables are then braced in the case of
angle pylons and clamped in the case of support pylons.
The final work consists of fitting the spacers of the individual conductor bundles (field
spacers),installing the bird warning and aircraft warning spheres, and attaching the cable
loops on the pylons.
5.9 Tests and Acceptance
operation following a precisely specified start-up programme.
5.10 Recultivation
.
5.11Values of T & D Lines
Investment cost. A high-voltage, direct current (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.
The here below scheme summarize the cost comparison between DC and AC line. It
appear that some technical trend, such as material, diameters, and other parameters can
influence the diagram, but as they are linked to the mechanical characteristics of the
materials, the choice can be driven through the global parameters as mentioned. This fact
explains partially the big differences which can occur between price of tow projects.
The test phase is very important, as it should simulate every possible operating condition. Besides
visual and mechanical inspections (clamped and screwed connections), earth-fault tests are also
carried out, as well as technical tests in the transformer stations. The line section is then taken into
Once all the work has been completed, the relevant road removal, reforestation and recultivation
work is carried out.

35
5.12. Typical 500 KV HVDC Lattice Tower:
Transmission Line Quick Facts
(All numbers are typical and approximate, and will vary with final route and design.)
Total length: 500 kilometres
Total towers: 1500
Span between towers: 365 metres (1200
feet) Tower height: 39 metres (128
feet)
Tower width (at arms): 27 to 29 metres (89 to 95
feet) Max. tower base (square): 13 metres (43 feet)
Min. conductor height: 12 metres (39 feet)
Total wires: 2 sets of 4 conductor wires,
1 set of 2 neutral return wires,
2 sets of overhead shield
wires Right-of-way width: 55 to 60 metres (180 to 197
feet) Total right-of-way: 2750 hectares (6800 acres)

36
Chapter 6
HVDC +/-260 KV Transmission Line Project
6.1Tentative Title:
Long Overhead Electric Power Transmission Line Design with assistance of High Voltage
Direct Current (HVDC) System.
6.2Introduction:
HVDC transmission has been in use for more than 50 years.It has proved to be a reliable
and valuable transmission media for electrical energy and has a number of
technical advantages compared with HVAC transmission. Nonetheless, a comprehensive
HVDC/HVAC system planning approach is not commonly found within utilities, and
therefore full advantage is not being taken of the HVDC technology. of electrical power
transmission.Recent developments in energy policies and stronger environmental lobbies have
a significant impact on the design and construction networks, and could provide a number
of opportunities for HVDC transmission. However, HVDC transmission is perceived to
be expensive, difficult to integrate in an ac network, to require highly skilled personnel to
operate and maintain, and to have high power losses. In today electricity industry, in view of
the liberalisation and increased effects to conserve the environment, HVDC solutions have
become more desirable for the following reasons:
1.Environmental advantages
2.Economical (cheapest solution)
3.Asynchronous interconnections
4.Power flow control
5.Added benefits to the transmission (stability, power quality etc.)
High voltage DC (HVDC) Transmission system consists of three basic parts: 1) converter
station to convert AC to DC 2) transmission line 3) second converter station to convert back
to AC. HVDC transmission systems can be configured in many ways on the basis of
cost,flexibility and operational requirements.The simplest one is the back-t-back
interconnection and it has two converters on the same site and there is no transmission line.This
type of connection is used as an inter tie between two different AC transmission systems.The
monopolar link connect two converter stations by a single conductor line and earth or,sea is
used as a returned path.The most common HVDC link is bipolar ,where two converter stations
are connected by bipolar conductors and each conductor has its own ground return.The multi-
terminal HVDC transmission systems have more than two converters stations which could be
connected is series or,parallel.

37
6.3 Long Term Significance:
There are noteable advantages of HVDC transmission which are as follows:
Advantages:
1.Greater power per conductor.
2.Simpler line construction.
3.Ground return can be used.
4.Hence each conductor can be operated as an independent circuit.
5.No charging current.
6.No Skin effect.
7.Cables can be worked at a higher voltage gradient.
8.Low short- Line power factor is always unity: line does not require reactive
compensation.
9.Less corona loss and radio interference, especially in foul weather, for a certain
conductor diameter and rms voltage.
10.Synchronous operation is not required.
11.Hence distance is not limited by stability.
12.May interconnect A.C systems of different frequencies.
circuit current on D.C line.
13.Does not contribute to short-circuit current of a A.C system.
14.Tie-line power is easily controlled.
However,there are unavoidable disadvantages of HVDC system which are as follows:
Disadvantages:
1.Converters are expensive.
2.Converters require much reactive power.
3.Converters generate harmonic, require filters.
4.Multiterminal or network operation is not easy.
Considering advantages,HVDC is a preferable method for transmission of bulk amount of
power over long distances.HVDC is reliable method.For plenty of advantages and technical
and economical reasons,HVDC provides long standing potential of enhancing the
compensating of the rapidly growth demand of electric power in Bangladesh undoubtedly .In
far future,the electric power sector of Bangladesh will enjoy the technical and economical
advantages for employing the HVDC transmission instead of HVAC.
6.4 Proposed Plan and Methodology:
The Author has observed the inefficient existing HVAC transmission with respect to the
growing demand of electric power under the lack of generation of electric power
corresponding to load in Bangladesh.The electric energy shortage has been existing for 20
years in Bangladesh.One of the possible solution of this problem is adoption of HVDC
transmission.Obviously,we know that due to considerable amount of line loss,total generated
power is not entirely transmitted to the receiving load center.So,we have lossed huge amount
of power for transmission line.But,if we implement the HVDC system for transmission of
electric power,technically ,in transmission line less power loss will be
occurred.Therefore,extra power would be added to the receiving load center.Thes added extra

38
power will serve a load which are deprieved of service instance of HVAC.However,HVDC
transmission is economically quite cheap.
Experimentally,the Author has proposed long overhead HVDC transmission line from
Ghorasal –Ishurdi-Khulna.It is a typical model for transmission in Bangladesh.Generated
power in Ghorasal will be converted to DC by a converter and then transmitted through the
bipolar DC line including ground return and in Khulna converted to AC by a
converter.So,AC generation,DC transmission and AC distribution is adopted.The Author has
typically proposed for a +/-260KV,350KM,500MW Bipolar Earth Return overhead HVDC
transmission system between Ghorasal and Khulna.This is Voltage Source Converter (VSC)
based overhead HVDC system.
6.5Existing Transmission Network(as on June,2010):
PGCB owns and operates the high voltage transmission network throughout
Bangladesh.The national gfrid operates at 230KV,132KV and 66KV and controls and
manages the second to second operation of electricity transmission system ,balancing
electricity generation to meet the demand.
Salient features of PGCB and BPDB is transmission network is:
Grid Substations Capacity:16749 MVA
Total No. of substations :108 Nos.(7 nos.BPDB,PGCB 88 nos and DPDC 13 nos.)
6.6 Existing Transmission Lines:
*230 KV Transmission Lines:
Serial No. Name of Lines Length in
Route ,KM
Length in
CKT,KM
No. of Circuits Conductor
1. Ghorasal-
Ishurdi
178.00 356.00 Double Mallard,795MCM
2. Khulna-Ishurdi 185.00 370.00 Double Twin
AAAC,37/4.176mm
*132 KV transmission Lines:
Serial No. Name of Lines Length in Route
,KM
Length in
CKT,KM
No. of Circuits Conductor
1. Goalpara-
Ishurdi
169.00 338.00 Double AAAC,804MCM
2. Khulna-
Khulna
9.00 18.00 Double Twin
AAAC,37/4.176
mm

39
6.7Existing Power Generation in Ghorasal:
1.Ghorasal Unit -1,2:2X55MW(Installed Capacity)85 MW (Present Capacity)
2.Ghorasal Unit-3,4,5,6 :4X210MW(Installed Capacity)760 MW (Present Capacity)
3.Ghorasal 100MW (Aggreko)HSD QRPP:100MW(Installed Capacity)100MW(Present
Capacity)
4.Ghorasal 45MW(Aggreko)HSD QRPP:45MW(Installed Capacity)45MW(Present Capacity)
5.Ghorasal 78.5 MW(Max.)Gas QRPP
Proposed Power Plant:
1.Ghorasal 200-300MW Gas Turbine Peaking Power Plant Project
2.Invitation Notice for Ghorasal 100+/-10% MW Gas Fired Power Project At
Ghorasal,Narsingdi,Bangladesh.
6.8 Generation Voltage:
Terminal voltage of different generators are 11KV,11.5KV and 15.75KV.
Bangladesh Power Development Board Installed Capacity:
As on June,2010,the total installed capacity including IPP consists of the following mix:
*Hydro-230MW(3.95%)
*Steam-2638MW(45.31%)
*Gas Turbine-1466MW(25.18%)
*Combined Cycle-1263MW(21.69%)
*Diesel-226MW(3.87%)
Total-5823MW(100%)
6.9Typical Electrical Parameters for a 230KV Overhead Line:
Parameters Quantity
R[ohm/KM] 0.050
XL[ohm/KM] 0.488
Bc[micros/KM] 3.371
α [nepers/KM] 0.000067
β [rad/KM] 0.00128
Z0 [ohm] 380
SIL[MW] 140
Charging[MVA/KM] 0.18

40
6.10 Data of Existing Transmission Line
Table 7.1:Transmission Line Data
ID Database ID Status From
Bus
To
Bus
KV
Level
Length Length
Unit
R1
[P.U]
X1
[P.U]
B1
[P.U]
R1*
[P.U]
2010_2020_1 Ghorasal_Ishurdi ON 2010 2020 230 178 KM .00015 .00077 .0004 0
2020_2032_1 Ishurdi_Khulna
New_1
ON 2020 2032 230 172 KM .00008 .00055 .0006 0
New_2
ON 2020 2032 230 172 KM .00008 .00055 .0006 0
ID Database ID R0[P.U] X0[P.U] B0[P.U] LOADING
LIMIT[A]
EMERGENCY
LOADING
LIMIT[A]
No. of
Conductor
Per phase
Tower
Structure
N1
Neutral
Status
2010_2020_1 Ghorasal_Ishurdi .00065 .0023 .0009 753.07 1129.61 1 Double
Circuit_Lattice
Eliminated
New_1
.0006 .0021 .0003 1500 2250 1 Double
Circuit_Lattice
Eliminated
New_2
.0006 .0021 .0003 1500 2250 1 Double
Circuit_Lattice
Eliminated
Table 7.2 :Static Load
ID DataBase ID Status Duplic Duplic Info From Bus P load
[MW]
Q Load
[MVAR]
Ghora_Sl 1 Ghora_SL1 ON 100 Combined
Data
1130 1.8 0.9
Ghora_Sl2 Ghora_Sl2 ON 100 Combined
Data
2010 40.8 20
Ghora 1 Ghora 50 ON 100 Combined
Data
Ghor1 24.67 12.3
Ghora2 Ghora25 ON 100 Combined
Data
Ghora2 12.335 6.1
Ishurdi 1 Ishurdi10 ON 100 Combined
Data
Ishurdi1 5.333 2.5
Ishurdi 2 Ishurdi 10 ON 100 Combined Ishurdi 2 5.333 2.5

41
Data
Ishurdi 3 Ishurdi 10 ON 100 Combined
Data
Ishurdi 3 5.333 2.5
KhulnaC1 KhulnaC48 ON 100 Combined
Data
KhulnaC1 20 6.5
KhulnaC2 KhulnaC48 ON 100 CombinedData KhulnaC2 20 6.5
KhulnaC3 KhulnaC48 ON 100 Combined
Data
KhulnaC3 20 6.5
7.3 Three Winding Transformer:
Location Status Duplic Duplic Info From Bus Secondary Bus
ID
Tertiary Bus
ID
Ghorasal ON 1 N/A 2010 1130 4
Ghorasal ON 1 N/A 2010 1130 4
Ishurdi ON 1 N/A 2020 1401 9
Ishurdi ON 1 N/A 2020 1401 9
Ishurdi ON 1 N/A 2020 1401 9
Khulna ON 1 N/A 2032 1332 11
Khulna ON 1 N/A 2032 1332 11
Location Total min(%) Total max(%) Primary
Voltage(KV)
Secondary
Voltage(KV)
Tertiary
Voltage(KV)
Primary
(MVA)
Ghorasal -2 2 230 132 33 125
Ghorasal -2 2 230 132 33 125
Ishurdi -2 2 230 132 33 125
Ishurdi -2 2 230 132 33 125
Ishurdi -2 2 230 132 33 125
Khulna -2 2 230 132 33 125
Khulna -2 2 230 132 33 125
Location Secondary
(MVA)
Tertiary(MVA) Primary
Winding
Secondary
Winding
Tertiary
Winding
Z1 P_S[P.U]
Ghorasal 125 25 YG YG D .077

42
Ghorasal 125 25 YG YG D .077
Ishurdi 225 25 YG YG D .0585
Khulna 225 25 YG YG D .0585
Khulna 225 25 YG YG D .0585
Location Z1 P_T[P.U] Z1 S_T[P.U] X/R Positive
P_S
X/R Positive
P_T
X/R positive
S_T
Z0 P_S
Ghorasal .24 .16 50 50 50 .077
Ghorasal .24 .16 50 50 50 .077
Ishurdi .0762 .0516 42 50 42 .0585
Ishurdi .0762 .0516 42 42 42 .0585
Ishurdi .0762 .0516 42 42 42 .0585
Khulna .0762 .0516 42 42 42 .0585
Khulna .0762 .0516 42 42 42 .0585
Location Z0 P_T[P.U] Z0 S_T[P.U] X/R zero P_S X/R Zero P_T P_S Phase shift P_T Phase
Shift
Ghorasal .24 .16 50 50 0 -30
Ghorasal .24 .16 50 50 0 -30
Ishurdi .0762 .0516 42 42 0 -30
Ishurdi .0762 .0516 42 42 0 -30
Ishurdi .0762 .0516 42 42 0 -30
Khulna .0762 .0516 42 42 0 -30
Khulna .0762 .0516 42 42 0 -30
Location No. of taps Loading
Limiting[MVA]
Emergency
loading
limit[MVA]
V min Tap[%] V max tap[%] Control Bus
voltage
Ghorasal 17 125 150 90 110 132
Ghorasal 17 125 150 90 110 132

43
Ishurdi 17 225 275 90 110 132
Ishurdi 17 225 275 90 110 132
Ishurdi 17 225 275 90 110 132
Khulna 17 225 275 90 110 132
Khulna 17 225 275 90 110 132
7.4 Fixed tap transformer
ID Status Duplic Duplic Info. From bus To Bus Rated S
[MVA]
1130_GHORA_1 ON 1 N/A 1130 GHOR_1 100
1130_GHORA_1 ON 1 N/A 1130 GHOR2 100
1130_GHORA5U ON 1 N/A GHOR1U132 100 100
1130_GHORA6U ON 1 N/A GHOR2U132 100 100
1302_KHULN_0 ON 1 N/A 1302 KHULNAC2 100
1401_ISHUR_0 ON 1 N/A 1401 ISHURDI1 100
ID Primary [KV] Secondary[KV] Primary
Winding
Secondary
Winding
Phase Shift Z1[P.U]
1130_GHORA_1 132 33 D YG 30 .152
1130_GHORA_1 132 33 D YG 30 .152
1130_GHORA5U 132 33 D YG 30 .1429
1130_GHORA6U 132 33 D YG 30 .1429
1302_KHULN_0 132 33 D YG 30 .1187
1302_KHULN_1 132 33 D YG 30 .1187
1302_KHULN_2 132 33 D YG 30 .1187
1401_ISHUR_0 132 33 D YG 30 .7662
1401_ISHUR_1 132 33 D YG 30 .7662
1401_ISHUR_2 132 33 D YG 30 .7662

44
ID Z0[P.U] X/R Positive X/R Zero Type Loading
limit[MVA]
Emergency
Loading
limit[MVA]
Primary
tap[%}
1130_GHORA_1 .116 42 42 Shell 75 90 100
1130_GHORA_1 .116 42 42 Shell 41 49 100
1130_GHORA5U .3761 999.9 999.9 Core 115 140 100
1130_GHORA6U .3761 999.9 999.9 Core 115 140 100
1302_KHULN_0 .0906 42 42 Shell 64 76.8 100
1302_KHULN_1 .0906 42 42 Shell 64 76.8 100
1302_KHULN_2 .0906 42 42 Shell 64 76.8 100
1401_ISHUR_0 .4361 42 42 Core 13.3 16 100
1401_ISHUR_1 .4361 42 42 Core 13.3 16 100
1401_ISHUR_2 .4361 42 42 Core 13.3 16 100

45
6.11 Typical High Voltage Direct Current (HVDC) Transmission Line Between Khulna-
Ishurdi-Ghorasal:
Scheme Ratings(Typical)
Commissioning Year 2011(Suppose)
Power Transmitted ,MW 500
Direct Voltage,KV +/-260
Configuration Bipole,Ground Return
Converter Type Force Commutated Voltage Source Converter
Converter Transformer One star/star at rated 203 MVA and 415/111.5
KV;One star/delta at rated 203 MVA and
415/111.5 KV.
Converter per stations 2
Direct Voltage per converter,KV 260
Direct Current,A 40
Reactive Power Supply Capacitors,Synchronous Condensers
Converter Station Location and AC Grid
Voltage
Goalpara,260 KV and Ghorasal ,260KV
132/230 KV
Length of Overhead DC Line 350 KM
Cable Arrangement 2 Cable,ground return
Cable Route Length 350 KM
Grounding of the DC Circuit Full current in two ground electrode stations
AC grids at both ends Synchronous
Control Constant Power in either Direction
Emergency change of power flow On manual or,automatic order to preset value
Main reason for choosing HVDC System Overhead Transmission of bulk amount of
power.

46
6.12 Typical Tower Structure
Typical +/- 260 KV Transmission Line Tower Structure:(All numbers are typical and
approximate and will vary with final route and design)
Tower entities Typical Ratings
Total Length 350 KM
Total towers 1050
Space between towers 365 metres(1200 feet)
Tower height 39 metres(128 feet)
Tower width (at arms) 27 to 29 metres( 89 to 95 feet)
Max. tower base (square) 40 metres ( 135 feet)
Min. conductor height 12 metres( 39 feet)
Total wires 1 sets of 2 conductor wires
Right of way width 55 to 60 metres(180 to 197 feet)
Total right of way 2750 hectares(6800 acres)
Insulator arrangement Conventional cross arms
Typical foundation dimensions 4 off,4X4 m square pad,1 m deep (above
ground) 0.9 m diameterX4m deep pier(below
ground)pad protrudes 400mm above ground.

47
6.13 HVDC Project Cables:
Figure6.1:Mass Impregnated Cable
Mass Impregnated Cables are the most used; they are in service for more than
40 years and have been proven to be highly reliable. At present used for
Voltages up to 500 kV DC.Conductor sizes up to 2500 mm2.
Typical Weight= 30 to 60 kg/m
Typical Diameter = 110 to 140 mm

48
6.14 Typical HVDC Circuit Diagram
Figure6.2:VSC based HVDC Substation
Figure6.3:VSC based HVDC Converter Arrangement

49
Figure 6.4:VSC based HVDC Indoor-Outdoor View
Figure 6.5:HVDC transmission with VSC

50
Figure 6.6 :Control of VSC Based HVDC Transmission
Figure 6.7:HVDC Overall View.

51
6.15 HVDC +/-260 KV Project Economics:
HVDC cost values given in year 2011 US $/KW (both ends inclusive) for one valve group per
pole.For Bipole +/-260 KV,500 MW Khulna_Ishurdi_Ghorasal Project,costs are typically as
follows:
Schemes Cost (Typical),US $
Valve Groups 21
Converter Transformer 22
DC Switchyard and Filtering 6
AC Switchyard and filtering 9.5
Control/Protection/Communication 8
Civil/Mechanical works 14
Auxiliary Power 2.5
Engineering & Administration 17
Total 100
Total cost per KW US$ US$ 170
6.16 Cost ratios for DC and AC Transmission Line construction.
AC equivalent line Cost P.U. HVDC Bipolar line
ratings
Range of costs P.U.
230 KV,Double
Circuit
1.00 +/-260KV,500MW 0.68 to 0.95

HVDC Thesis,2011EEE 079

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