Tổng quan hệ thống truyền tải điện một chiều HVDC

ٌٍ‫ثسٌ هللا اىشحَِ اىشح‬

IMPLEMENTATION OF HVDC
TRANSMISSION SYSTEM USING
SIMULATION SOFTWARE

A Report submitted in partial fulfillment of the requirement for
the degree of
B.Sc. (HON) In Electrical and Electronic Engineering
(Power system Engineering)
BY
MOHAMMED MOHAMMED ELHABIB AHMED
MOHAMMED

Under Supervision of
Dr. Kamal Ramadan
To
Department of Electrical and Electronic Engineering
Faculty of Engineering
University of Khartoum
June 2011


Acknowledgment
First of all, I would like to say Alhamdulillah, for giving me the health and strength to
complete this project.
Thanks to my supervisor Dr. Kamal Ramadan for guiding us through this project and
taking by our hand me and my partner. He helped us to refine our thesis and pointed
out the concepts calibrating and organization. Moreover, he taught us patiently until
we knew what to do, he tried and tried with all sustain until we understand what is
supposed to do. Your valuable insight, wisdom and assistance are appreciated.

I would like to thank the crew in the Load Dispatch Center (LDC) in Kilo x station for
feeding us with the required data for the National Distribution Grid, the thanks
involving Dr. A.Karrar for ease our visit to Kilo x station.
And I would like to thank the most important people in my life, my family, and my
friend who had encouraged me throughout this project.
Last but not least, deeply appraising to my partner Awab Abd-Elmoniem for his hard
work, encouragement, team spirit, and the unforgettable times those we spent.

I


Abstract
In times of increasing of the production costs and highly competitive market, it‟s
seems to be so wise to prospect for the most economical method in operating and
production managing. With looking to the future, the fossil fuel will be scarce and
more power will have to be obtained from the sustainable energy sources. The
hydropower schemes which will be the major source of energy are usually planted far
away from the consumption areas. To interconnect the consumption to the production
areas over this long distance between, the transmission systems should be suggested
to be more sufficient and affecting positively in the whole system stability.
This project aims to develop HVDC transmission system that provides more power
saving due to decreasing transmission losses, and provide more reliable transmission
capabilities. ETAP simulation program was chosen for this work, it provides high
simulation capabilities, since it can be run in the real time execution.
The method developed resulted in a system that is reliable, flexible to control and
provide less transmission losses and costs comparing to the prior AC system.

II

‫المستخلص‬
‫فً صٍبّْب ٕزا ٗاىزي شٖذ رضاٌذًا ٍضطشدًا فً رنبىٍف اإلّزبج ٗاىزشغٍو‪ّ ،‬دذ أُ اىشؤٌخ اإلقزصبدٌخ صبسد‬
‫اىَشاقت اىَقٌٍ إلّشبء اىَشبسٌغ ٗرشغٍيٖب‪ .‬ىزا فَِ اىحنَخ ثَنبُ أُ رْشظ اىجح٘س اىؼيٍَخ ص٘ة ٍشٍى اىزنيفخ‬
‫األقو س٘اء أمبّذ رنيفخ إّشبء أٗ رنبىٍف اىزشغٍو ىٖزٓ اىَششٗػبد‪.‬‬
‫إرا رَذدد سؤٌزْب فً اىَسزقجو ّدذ َّ‬
‫أُ اىؼبىٌ سٍزدٔ إىى ٍصبدس اىطبقخ اىَبئٍخ أٗ اىشَسٍخ أٗ اىٖ٘ائٍخ ّسجخ‬
‫ألسخحٍخ ّض٘ة ٍصبدس اىطبقخ اىحشاسٌخ‪ .‬أي ٍِ ٍصبدس اىطبقخ اىَقزشحخ ٗىْأخز اىطبقخ اىَبئٍخ فً حٍض‬
‫اإلػزجبس قذ رجؼذ ٍْطقخ اإلّزبج ٍئبد اىنٍيٍ٘زشاد ػِ ٍْبطق اإلسزٖالك‪ٗ.‬ىشثظ ٍْبطق اإلسزٖالك ثَصبدس ر٘ىٍذ‬
‫اىطبقخ ٌز٘خت أُ ٌنُ٘ ّظبً اىْقو ػبىً اإلسزقشاسٌخ ٗأمثش حف ً‬
‫ظب ىيطبقخ اىَْق٘ىخ‪.‬‬
‫ٕزا اىَششٗع ٌٖذف إىى رصٌٍَ ّظبً ّقو ىيقذسح اىنٖشثبئٍخ ثإسزخذاً رقٍْخ اىدٖذ اىَجبشش اىؼبىً ‪ٗ HVDC‬اىزي‬
‫ٌسبػذ ػيى حفظ اىقذسح اىَْق٘ىخ ٍِ ٍْبطق اإلّزبج إىى ٍْبطق اإلسزٖالك ػجش رقيٍو فق٘داد اىْقو ػجش اىَسبفبد‬
‫اىطٌ٘يخ ٗمزىل ٌسبػذ ثشنو ٍب ػيى إسزقشاسٌخ ّظبً ر٘صٌغ اىطبقخ اىنٖشثبئٍخ‪ .‬رٌ إسزخذاً ثشّبٍح اىَحبمبح‬
‫‪ٗ ETAP‬اىزي ثٔ ٍٍضح إحز٘ائٔ ػيى إٍنبٍّخ اىزشغٍو اىحً ػيى اىشجنخ اىنٖشثبئٍخ‪.‬‬
‫رٌ اىحص٘ه ػيى ّظبً ٍسزقش ثٔ فق٘داد ّقو أقو ٍَب سبػذ فً ر٘فٍش ٗحفظ اىقذسح اىَْق٘ىخ ثص٘سح خٍذح‪ ،‬اىْزبئح‬
‫اىَسزخيصخ رحَو رشدٍؼًب ػيى رْفٍز ٕزا اىَششٗع ػيى أسض اى٘اقغ‪.‬‬

‫‪III‬‬


Table of Contents
Acknowledgment ........................................................................................................ I
Abstract ..................................................................................................................... II
‫ اىَسزخيص‬................................................................................................................... III
Table of Contents .................................................................................................... IV
List of Figures ........................................................................................................ VII
List of Tables......................................................................................................... VIII
Abbreviations .......................................................................................................... IX
Chapter 1 ................................................................................................................... 1
1.1

OVERVIEW ................................................................................................... 1

1.2 STATEMENT OF THE PROBLEM ........................................................................ 1
1.3 PROJECT OBJECTIVES ...................................................................................... 2
1.4 METHODOLOGY AND TOOLS ........................................................................... 2
1.5 THESIS LAYOUT .............................................................................................. 2
Chapter 2 ................................................................................................................... 3
2.1 INTRODUCTION ............................................................................................... 3
2.2 HISTORY OF HVDC TRANSMISSION .................................................................. 3
2.3 HVDC CONFIGURATION ................................................................................... 6
2.3.1 MONOPOLE AND EARTH RETURN ............................................................ 6
2.3.2 BIPOLAR TRANSMISSION .......................................................................... 7
2.3.3 BACK TO BACK SYSTEM ......................................................................... 8
2.4 SYSTEMS WITH TRANSMISSION LINES ............................................................... 8
2.5 TRIPOLE, CURRENT MODELING CONTROL .......................................................... 9
2.6 ADVANTAGES AND DISADVANTAGES OF HVDC TRANSMISSION ....................... 9
2.6.1 ADVANTAGES ......................................................................................... 9
2.6.2 DISADVANTAGES .................................................................................. 11
2.7 ENVIRONMENTAL CONSIDERATIONS ............................................................... 11
Chapter 3 ................................................................................................................. 14
3.1 TRANSFORMER MODEL .................................................................................. 14
IV


3.1.1 EQUIVALENT

CIRCUIT REPRESENTATION ............................................ 14

3.1.2 REPRESENTATION OF THREE-WINDING TRANSFORMERS ........................ 15

3.2 TRANSMISSION LINE MODEL ......................................................................... 17
3.2.1 ELECTRICAL CHARACTERISTICS ............................................................. 17
3.2.2 POWER FLOW EQUATIONS ..................................................................... 17
3.2.3 EQUIVALENT CIRCUIT OF A TRANSMISSION LINE ................................... 17
3.3 CONVERTOR MODEL ..................................................................................... 19
3.3.1 CONVERTER THEORY AND PERFORMANCE EQUATION ............................ 19
3.3.2 VALVE CHARACTERISTICS ..................................................................... 19
3.3.3 CONVERTER CIRCUITS ........................................................................... 20
3.4 SYNCHRONOUS GENERATOR MODELING ....................................................... 35
3.4.1 INTRODUCTION ..................................................................................... 35
3.4.2 MACHINE VOLTAGE EQUATION ............................................................. 36
Chapter 4 ................................................................................................................. 41
4.1 SOFTWARE USED (ETAP) ................................................................................ 41
4.1.1 INTRODUCTION ..................................................................................... 41
4.1.2 CAPABILITIES ........................................................................................ 42
4.1.3 SPECIFICATIONS .................................................................................... 43
4.2 LOAD FLOW REQUIRED DATA ....................................................................... 45
4.2.1 BUS DATA ............................................................................................ 45
4.2.2 BRANCH DATA ...................................................................................... 45
4.2.3 SYNCHRONOUS GENERATOR DATA ........................................................ 45
4.2.4 STATIC LOAD DATA .............................................................................. 45
4.2.5 CAPACITOR DATA ................................................................................. 46
4.2.6 LUMPED LOAD DATA ............................................................................ 46
4.2.7 HVDC LINK DATA ............................................................................... 46
4.2.8 OTHER DATA ........................................................................................ 47
4.3 CASE STUDY ................................................................................................ 47
4.3.1 STAGE I DATA COLLECTION AND ENTERING .......................................... 47
4.3.2 STAGE II DATA REFINING ...................................................................... 48
4.3.3 STAGE III HVDC IMPLEMENTATION ...................................................... 49

V


Chapter 5 ................................................................................................................. 50
5.1 CONCLUSION .................................................................................................. 50
5.2 DISCUSSION ................................................................................................... 50
5.3 FUTURE WORK ............................................................................................... 51
References .............................................................................................................. 52
Appendix A Simulation Model ............................................................................... 53
Appendix B Lines Data ........................................................................................... 54
Appendix C Transformers Data .............................................................................. 58
Appendix D Generators Data .................................................................................. 62
Appendix E Capacitors and Reactors Data .............................................................. 65
Appendix F Peak Loading Data .............................................................................. 67
Appendix G AC load Flow Report .......................................................................... 69
Appendix H.. DC Load Flow Report ....................................................................... 70

VI


List of Figures
Figure 2-1: Thury HVDC Transmission System......................................................... 4
Figure 2-2: Block Diagram Of A Monopole System With Earth Return ..................... 6
Figure 2-3: Block Diagram Of A Bipolar System That Also Has An Earth Return ..... 7
Figure 3.1: 2-Windings Transformer Equivalent Circuit .......................................... 14
Figure 3.2: Equivalent Circuit Of A Three-Winding Transformer ............................ 15
Figure 3.3: Transmission Line Equivalent Circuit .................................................... 18
Figure 3.4: Major Elements Of The Convertor ......................................................... 19
Figure 3.5: Symbol For Controlled Valve ................................................................ 20
Figure 3.6: Three-Phase, Full-Wave Bridge Circuit ................................................. 20

Figure 3.7: Equiv Circuit For Three-Phase Full-Wave Bridge Converter.................. 22
Figure 3.8: Waveforms Of Voltages And Currents Of Bridge Circuit Of Figure 3.7 . 23
Figure 3.9: Voltage Wave Forms And Valve Currents, With Ignition Delay ............ 25
Figure 3.10: Line Current Waveform ....................................................................... 25
Figure 3.11: Effect Of Overlap Angle On Periods Of Conduction Of Valves ........... 26
Figure 3.12: Periods Of Valve Conduction With Ignition Delay............................... 27
Figure 3.13: Equivalent Circuit During Commutation .............................................. 27
Figure 3.14: Valve Currents Related to Commutation Voltage ................................. 29
Figure 3.15: The Effect Of Overlap During Commutation From V1 To V2 3 ........... 29
Figure 3.16: Bridge Rectifier Equivalent Circuit ...................................................... 31
Figure 3.18: Angles Used In Rectifier And Inverter ................................................. 34
Figure 3.19: Inverter Equivalent Circuits (With

Positive) .................................. 35

Figure 3.20: Schematic Representation Of A Three-Phase Synchronous Generator .. 36

VII


List of Tables
Table 4.1: DC links, AC loses, DC loses for each and the percentage of saving.

VIII

49


Abbreviations
HVDC

High Voltage Direct Current

LCC

Line Commutated Converter

CCC

Capacitor Commutative Converters

AVR

Absolute Value Rectifier

LDC

Load Dispatch Center (Kilo x)

IX


Chapter 1

Introduction

Chapter 1
Introduction
1.1 Overview
An electrical power system can be seen as the interconnection of generating sources

and customer loads through a network of transmission lines, transformers, and
ancillary equipment. Its structure has many variations that are the result of a legacy of
economic, political, engineering, and environmental decisions. The first electricity
transmission systems were DC systems. However, the low voltage DC systems could
not be transmitted over long distances, which cause the rise of AC systems.
The development of high voltage valves (mercury arc, thyristors) leads to the High
Voltage DC transmission systems over long distances. HVDC transmission has
become acceptable as an economical and reliable method of power transmission and
interconnection. It offers advantages such as long-distance power transmission,
synchronous interconnection between two ac systems and offers the ability to
precisely control the power flow without inadvertent loop flows in an interconnected
ac system.

1.2 Statement of the problem
The most important generation of sustainable energy comes from the hydropower.
Most of the hydro plants are far away from the consumption area, using the AC
transmission with the reactive power involved and the voltage drop that occurs when
active power and reactive power flow through inductive reactance associated with the
transmission network may usually lead to system instability which may rapidly leads
to system collapse.
Now days the economic visibility leads much of the project planning, such
transmission losses could not be accepted, in order to illustrate a possible future
situation when there only sustainable generation the power could be exported over
long distances with a very efficient transmission network. The backbone of this
network could be HVDC.

1


Chapter 1

Introduction

1.3 Project objective
The objective of this project is to develop HVDC transmission system upon Sudan
national electrical distribution grid using ETAP modeling and simulation software.
Attempt to decrease the transmission losses as much as possible.
Significantly impact on the stability of the associated ac power system, since HVDC
system has the ability to rapidly control the transmitted power.
Instate a basic of HVDC transmission that may be developed at the future to cause
huge decreasing in the power losses and power system collapse.

1.4 Methodology and Tools
A modular approach is used to satisfy the specified objectives. The project is
portioned into major tasks:
Process simulation:

Sudan National Electric Distribution Grid is simulated

using ETAP simulation environment.
HVDC implementation: Some long distance AC transmission lines were replaced
with HVDC systems responding to the generation and losses.

1.5 Thesis Layout
The thesis is organized as follows:
Chapter 2: In this chapter, the historical background and the theoretical background
needed throughout this project is covered.
Chapter 3: In this chapter, the modeling of the backbone equipment needed to
implement the transmission process is discussed.
Chapter 4: The software developed, system design, implementation method and the

operation description, all have been covered in this chapter.
Chapter 5: Conclusion, recommended future work and references are presented here.
Appendix A:

Single line Diagram of the Network.

Appendix B:

Lines Data (R, X, from bus, to bus and length).

Appendix C: Transformer Data (Ratings and Reactance‟s).
Appendix D: Generators data.
Appendix E: Capacitors and reactors data (MVA rating).
Appendix F: 2010 peak loading data on main bus-bars (110,220).
Appendix G: AC Load Flow Losses Summary Report.
Appendix H: DC Load Flow Losses Summary Report.

2


Chapter 2

Literature Review

Chapter 2
Literature Review
2.1

Introduction
High voltage is used for electric long distance power transmission to reduce the

energy lost in the resistance of the wires. For a given quantity of power transmitted,
higher voltage reduces the transmission power loss. The power lost as heat in the
wires is proportional to the square of the current. So if a given power is transmitted at
higher voltage and lower current, power loss in the wires is reduced. Power loss can
also be reduced by reducing resistance, for example by increasing the diameter of the
conductor, but larger conductors are heavier and more expensive.
High voltages cannot easily be used for lighting and motors, and so transmission-level
voltages must be reduced to values compatible with end-use equipment. Transformers
are used to change the voltage level in alternating current (AC) transmission circuits.
The competition between the direct current (DC) of Thomas Edison and the AC of
Nikola Tesla and George Westinghouse was known as the War of Currents, with AC
emerging victorious. Practical manipulation of DC voltages became possible with the
development of high power electronic devices such as mercury arc valves and, more
recently, semiconductor devices such as thyristors, insulated-gate bipolar transistors
(IGBTs), high power MOSFETs and gate turn-off thyristors (GTOs).‎[1]

2.2 History of HVDC Transmission
In 1882 transmission of electric power of 2.5 kW was demonstrated using DC from
Miesbach to Munich which was the first long distance transmission in DC.
One of the first methods of high-voltage DC transmission was illustrated by the Swiss
engineer René Thury [2]
‎ and his method was carried out by Acquedotto De FerrariGalliera Company in 1889 in Italy. This method used motor-generator sets series
connected to build up and increase voltage. Every motor-generator set was nongrounded i.e. (insulated from ground) and driven by prime mover using insulated
shafts from the mover. The line was operated in constant current mode, with up to
5,000 volts on each machine, some machines having double commutators to reduce
the voltage on each commutator. This system transmitted 630 kW at 14 kV DC over a
distance of 120 km‎[3].

3

Chapter 2

Literature Review

Figure 2-1:Thury HVDC transmission system
The Moutiers-Lyon system also used the previous method transmitted 8,600 kW of
hydroelectric power a distance of 124 miles, including 6 miles of underground cable.
The system used eight series-connected generators with dual commutators for a total
voltage of 150,000 volts between the poles, and ran from about 1906 until 1936.
Fifteen Thury systems were in operation by 1913‎[4]. Other Thury systems operating at
up to 100 kV DC operated up to the 1930s, but there were some drawbacks on this
way, that was because the rotating machinery required high maintenance and had high
energy loss.
In 1902 Hewitt developed the first mercury arc rectifier, which was an efficient way
of converting alternating current power to direct current for use in electric railways,
industry, and HVDC power transmission.
One conversion technique attempted for conversion of direct current from a high
transmission voltage to lower utilization voltage was to charge series-connected
batteries, then connect the batteries in parallel to serve distribution loads. While at
least two commercial installations were tried around the turn of the 20th century, the
technique was not generally useful owing to the limited capacity of batteries,
difficulties in switching between series and parallel connections, and the inherent
energy inefficiency of a battery charge-discharge cycle.

4


Chapter 2

Literature Review

By the latter part of the 19th century the AC power transformer had progressed to a
practical reality and when combined with Tesla‟s work on 3-phase systems the
transformer became the catalyst for the development of efficient power transmission.
Because, at that time, static converter equipment did not exist, the DC technology
could not benefit from this breakthrough and its use as a transmission medium lapsed.
But engineers never forgot the advantages of DC and by the 1930s the appearance of
the mercury-arc valve led many to contemplate the return of DC transmission .In the
period 1920 to 1940 The grid controlled mercury arc valve was used for power
transmission .In 1932, General Electric tested mercury-vapor valves and a 12 kV DC
transmission line, which was used to convert 40 Hz generation to serve 60 Hz loads,
at Mechanicville, New York. In 1941, a 60 MW, +/-200 kV, 115 km buried cable link
was designed for the city of Berlin using mercury arc valves (Elbe-Project), but owing
to the collapse of the German government in 1945 the project was never completed ‎[8].
The main reason of executing the project was that buried cables would be less
conspicuous as a bombing target in wars. The equipment was moved to the Soviet
Union and was put into service there.
Introduction of the fully static mercury arc valve to commercial service in 1954
marked the beginning of the modern era of HVDC transmission. A HVDC-connection
was constructed by ASEA between the mainland of Sweden and the island Gotland.
Mercury arc valves were common in systems designed till 1975. It was an undersea
cable, 96 km long, with ratings of 100 kV and 20 MW, but since then, HVDC systems
use only solid-state devices. For the next two decades progress with HVDC was slow.
Steady improvements were made to the control systems and valve technology but the
reliability and maintenance issues of the mercury-arc valves limited the spread of
HVDC.
By the 1970s the emergence of a high power semiconductor device, the thyristor,
transformed the fortunes of HVDC. Converter technology was revolutionized
providing a far more reliable, economic and maintainable product than was possible

with the old mercury-arc valves. From 1975 to 2000, line-commutated converters
(LCC) using thyristor valves were relied on. Experts predict that for the next 25 years
may well be dominated by force commutated converters, beginning with capacitor
commutative converters (CCC) followed by self commutating converters which have
largely supplanted LCC use. Since use of semiconductor commutators, hundreds of
HVDC sea-cables have been laid and worked with high reliability.
5


Chapter 2

Literature Review

2.3 HVDC Configurations
2.3.1 Monopole and earth return

Figure 2-2: Block diagram of a monopole system with earth return
In a common configuration, called monopole, one of the terminals of the rectifier is
connected to earth ground. The other terminal, at a potential high above or below
ground, is connected to a transmission line. The earthed terminal may be connected to
the corresponding connection at the inverting station by means of a second conductor.
If no metallic conductor is installed, current flows in the earth between the earth
electrodes at the two stations. Therefore it is a type of single wire earth return. The
issues surrounding earth-return current include: [5]
‎


Electrochemical corrosion of long buried metal objects such as
pipelines



Underwater earth-return electrodes in seawater may produce chlorine
or otherwise affect water chemistry.



An unbalanced current path may result in a net magnetic field, which
can affect magnetic navigational compasses for ships passing over an
underwater cable.

These effects can be eliminated with installation of a metallic return conductor
between the two ends of the monopolar transmission line. Since one terminal of the
converters is connected to earth, the return conductor need not be insulated for the full
transmission voltage which makes it less costly than the high-voltage conductor. Use
of a metallic return conductor is decided based on economic, technical and
environmental factors.
Modern monopolar systems for pure overhead lines carry typically 1,500 MW. If
underground or underwater cables are used, the typical value is 600 MW.
Most monopolar systems are designed for future bipolar expansion. Transmission line
6


Chapter 2

Literature Review

towers may be designed to carry two conductors, even if only one is used initially for
the monopole transmission system. The second conductor is either unused or used as
electrode line or connected in parallel with the other (as in case of Baltic-Cable).

2.3.2 Bipolar Transmission
In bipolar transmission a pair of conductors is used, each at a high potential with
respect to ground, in opposite polarity. Since these conductors must be insulated for
the full voltage, transmission line cost is higher than a monopole with a return
conductor.‎[5]

Figure 2-3: Block diagram of a bipolar system that also has an earth return
However, there are a number of advantages to bipolar transmission which can make it
the attractive option.


Under normal load, negligible earth-current flows, as in the case of
monopolar transmission with a metallic earth-return. This reduces
earth return loss and environmental effects.



When a fault develops in a line, with earth return electrodes installed at
each end of the line, approximately half the rated power can continue
to flow using the earth as a return path, operating in monopolar mode.



Since for a given total power rating each conductor of a bipolar line
carries only half the current of monopolar lines, the cost of the second
conductor is reduced compared to a monopolar line of the same rating.



In very adverse terrain, the second conductor may be carried on an
independent set of transmission towers, so that some power may
continue to be transmitted even if one line is damaged.

7


Chapter 2

Literature Review


A bipolar system may also be installed with a metallic earth return
conductor.



Bipolar systems may carry as much as 3,200 MW at voltages of +/600 kV. Submarine cable installations initially commissioned as a
monopole may be upgraded with additional cables and operated as a
bipole.

2.3.3

Back to back

A back-to-back station (or B2B for short) is a plant in which both static inverters and
rectifiers are in the same area, usually in the same building. The length of the direct
current line is kept as short as possible. HVDC back-to-back stations are used for:


coupling of electricity mains of different frequency (as in Japan; and
the GCC interconnection between UAE [50 Hz] and Saudi Arabia
[60 Hz] under construction in ±2009-2011)



Coupling two networks of the same nominal frequency but no fixed
phase relationship (as until 1995/96 in Etzenricht, Dürnrohr, Vienna,
and the Vyborg HVDC scheme).



Different frequency and phase number (for example, as a replacement
for traction current converter plants)

The DC voltage in the intermediate circuit can be selected freely at HVDC back-toback stations because of the short conductor length. The DC voltage is as low as
possible, in order to build a small valve hall and to avoid series connections of valves.
For this reason at HVDC back-to-back stations valves with the highest available
current rating are used.‎[6]

2.4 Systems with transmission lines
The most common configuration of an HVDC link is two inverter/rectifier stations
connected by an overhead power line. This is also a configuration commonly used in
connecting unsynchronized grids, in long-haul power transmission, and in undersea
cables.
Multi-terminal HVDC links, connecting more than two points, are rare. The
configuration of multiple terminals can be series, parallel, or hybrid (a mixture of
8

Chapter 2

Literature Review

series and parallel). Parallel configuration tends to be used for large capacity stations,
and series for lower capacity stations. An example is the 2,000 MW Quebec - New
England Transmission system opened in 1992, which is currently the largest multiterminal HVDC system in the world [7]
‎ .

2.5

Tripole: current-modulating control
A scheme patented in 2004 (Current modulation of direct current transmission lines)
is intended for conversion of existing AC transmission lines to HVDC. Two of the
three circuit conductors are operated as a bipole. The third conductor is used as a
parallel monopole, equipped with reversing valves (or parallel valves connected in
reverse polarity). The parallel monopole periodically relieves current from one pole or
the other, switching polarity over a span of several minutes. The bipole conductors
would be loaded to either 1.37 or 0.37 of their thermal limit, with the parallel
monopole always carrying +/- 1 times its thermal limit current. The combined RMS
heating effect is as if each of the conductors is always carrying 1.0 of its rated current.
This allows heavier currents to be carried by the bipole conductors, and full use of the
installed third conductor for energy transmission. High currents can be circulated
through the line conductors even when load demand is low.

2.6 Advantages & disadvantages HVDC
2.6.1 Advantages
1. The ability to transmit large amounts of power over long distances with lower
capital costs and with lower losses than AC. Depending on voltage level and
construction details, losses are quoted as about 3% per 1,000 km.

2. High-voltage direct current transmission allows efficient use of energy sources
remote from load centers.
3. Is more effective than AC in many applications such as:
i. Undersea cables, where high capacitance causes additional AC losses.
ii.

Endpoint-to-endpoint long-haul bulk power transmission without
intermediate 'taps'.

iii.

Increasing the capacity of an existing power grid in situations where

additional wires are difficult or expensive to install.

9


Chapter 2
iv.

Literature Review
Power transmission and stabilization between unsynchronized AC

distribution systems
v.

Connecting a remote generating plant to the distribution grid, for

example Nelson River Bi pole.

vi.

Stabilizing a predominantly AC power-grid, without increasing

prospective short circuit current.
vii.

Reducing line cost. HVDC needs fewer conductors as there is no need

to support multiple phases. Also, thinner conductors can be used since HVDC
does not suffer from the skin effect.
viii.

Facilitate power transmission between different countries that use AC

at differing voltages and/or frequencies
ix.

Synchronize AC produced by renewable energy sources.

Additional losses are generated through the cable due to the Additional current which
must flow in the cable to charge the cable capacitance when using the alternating
current also; there is a dielectric loss component in the material of the cable
insulation, which consumes power.
When, however, direct current is used, the cable capacitance is charged only when the
cable is first energized or when the voltage is changed; there is no steady-state
additional current required. For a long AC undersea cable, the entire current-carrying
capacity of the conductor could be used to supply the charging current alone. The
cable capacitance issue limits the length and power carrying capacity of AC cables.
DC cables have no such limitation, and are essentially bound by only Ohm's Law.

Although some DC leakage current continues to flow through the dielectric insulators,
this is very small compared to the cable rating and much less than with AC
transmission cables.
4. For a given power rating, the constant voltage in a DC line is lower than the
peak voltage in an AC line; so HVDC can carry more power per conductor.
5. Because DC operates at a constant maximum voltage, this allows existing
transmission line corridors with equally sized conductors and insulation to
carry more power into an area of high power consumption than AC, which can
lower costs.

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Chapter 2

Literature Review

6. HVDC allows power transmission between unsynchronized AC distribution
systems so it can help increase system stability, by preventing cascading
failures from propagating from one part of a wider power transmission grid to
another.
7. HVDC seems particularly suited to many renewable energy sources such as
Sources of supply (hydro, geothermal, wind, tidal) are often distant from
demand centers, Wind turbines operating at variable speed generate power at
different frequencies, requiring conversions to and from DC and Large hydro
projects, for example, also often supply multiple transmission systems.

2.6.2

Disadvantages:

1. HVDC is less reliable and has lower availability than AC systems, mainly due
to the extra conversion equipment.
2. The required static inverters are expensive and have limited overload capacity.
At smaller transmission distances the losses in the static inverters may be
bigger than in an AC transmission line. The cost of the inverters may not be
offset by reductions in line construction cost and lower line loss. With two
exceptions, all former mercury rectifiers worldwide have been dismantled or
replaced by thyristor units.
3. In contrast to AC systems, realizing multi terminal systems is complex, as is
expanding existing schemes to multi terminal systems. Controlling power flow
in a multi terminal DC system requires good communication between all the
terminals; power flow must be actively regulated by the inverter control
system instead of the inherent impedance and phase angle properties of the
transmission line.
4. High voltage DC circuit breakers are difficult to build because some
mechanism must be included in the circuit breaker to force current to zero,
otherwise arcing and contact wear would be too great to allow reliable
switching.
5. Operating a HVDC scheme requires many spare parts to be kept, often
exclusively for one system as HVDC systems are less standardized than AC
systems and technology changes faster.

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Chapter 2

Literature Review

2.7 Environmental considerations

The electrical environmental effects from HVDC transmission lines can be
characterized by field and ion effects as well as corona effects. 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. ‎[8]‎[9]

2.7.1 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
affect 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.
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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. 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.
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 [8]
‎ .

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Chapter 3

Modeling

Chapter 3
Modeling
3.1 Transformer model ‎[10]
3.1.1 Equivalent π circuit representation
The analysis of 2-winding transformers ends with the equivalent circuit infigure3.1
and this representation is convenient in digital computer analysis of power flow.

Figure3.1: 2-windings transformer equivalent circuit
Where

=

at nominal primary side tap position

=

at nominal secondary side tap position

= primary side nominal number of turns
= secondary side nominal number of turns

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Chapter 3

Modeling

3.1.2 Representation of Three-Winding Transformers
Figure 3.2 shows the single-phase equivalent of a three-winding transformer under
balanced conditions. The effect of the magnetizing reactance has neglected, and the
transformer is represented by three impedances connected to a star. The common star
point is fictitious and unrelated to the system neutral.

Figure 3.2: Equivalent circuit of a three-winding transformer
The three windings of the transformer may have different MVA ratings. However, the
per unit impedances must be expressed on the same MVA base. As in the case of the
two-winding transformer equivalent circuit in the previous section, off-nominal turns
ratios are used to account for the differences between the ratios of actual turns and the
base voltages. The values of the equivalent impedances Zp, Zs and Zt may be
obtained by standard short-circuit tests as follows ‎[10]:
Zps = leakage impedance measured in primary with secondary shorted and tertiary
open
Zpt = leakage impedance measured in primary with tertiary shorted and secondary
open
Zst = leakage impedance measured in secondary with tertiary shorted and primary
open

With the above impedances in ohms referred to the same voltage base, we have

15