Market Analysis for Rectifying and Inverter Technologies for HVDC System

Market Analysis for Rectifying and Inverter Systems for HVDC Technology

Haseeb Ahmad

1/2/2012

Submitted to

Dr. Bahri Uzunoglu

Högskolan på Gotland

http://www.google.com.pk/url?sa=t&rct=j&q=h%C3%B6gskolan%20p%C3%A5%20gotland&source=web&cd=1&ved=0CCkQFjAA&url=http%3A%2F%2Fwww.hgo.se%2F&ei=F5oDT_3VKc7c4QSioNWNCA&usg=AFQjCNETFDu1W1xmEeB4xqbnCz_H8uw1pg

Turbine Efficiency and Grid Integration

P r o j e c t S u m m a r y

Project Title Market analysis for rectifying and inverter systems for HVDC technology

Investigator Haseeb Ahmad

The goal of this work is to focus on the emerging transmission technology, the HVDC. In

today’s competitive world, the biggest challenge, for the power companies, is efficiency of

electrical transmission. HVDC is known for low losses in long distance bulk power transmission

however for short distances, losses have become quite negligible in densely populated areas.

HVDC comprises of number of components, among which, converters are the most important

part of an HVDC system. They perform rectification and inversion operation. In this report,

advantages of an HVDC system along with its process, configurations and components are

discussed. In the second half, cost analysis and market players for rectification and inversion

systems are discussed. Moreover, another objective of this document is to provide basic

understanding about HVDC systems.

Key words: High Voltage Direct Current, Rectification, Inversion, Electricity Transmission,


Table of Contents

1. Introduction ........................................................................................................................................... 1

1.1. Advantages and features of HVDC ............................................................................................... 2

1.1.1. Long distance bulk power transmission ................................................................................ 2

1.1.2. Interconnections .................................................................................................................... 3

1.1.3. Multi-Terminal Systems in HVDC ....................................................................................... 4

1.1.4. Support for AC System ......................................................................................................... 4

1.1.5. Limitation of Faults ............................................................................................................... 4

1.1.6. Limitation of Short Circuit Level .......................................................................................... 4

1.1.7. Control of Power Flow .......................................................................................................... 5

1.1.8. Voltage Control ..................................................................................................................... 5

1.1.9. Environmental Benefits ......................................................................................................... 5

1.2. HVDC and Wind Power ............................................................................................................... 6

2. HDVC Process ...................................................................................................................................... 7

2.1. Natural or Line Commutated Converters ...................................................................................... 7

2.2. Capacitor Commutated Converters ............................................................................................... 9

2.3. Forced Commutated Converters ................................................................................................. 10

3. Configurations of HVDC .................................................................................................................... 10

3.1. Mono-polar HVDC System ........................................................................................................ 10

3.2. Bipolar HVDC System ............................................................................................................... 11

3.3. Homo-polar HVDC System ........................................................................................................ 11

3.4. Back-to-back HVDC System ...................................................................................................... 11

3.5. Multi-terminal HVDC System .................................................................................................... 11

4. Components of HVDC System ........................................................................................................... 13

4.1. Classic-HVDC System ................................................................................................................ 13

4.1.1. Converters ........................................................................................................................... 13

4.1.2. Transformers ....................................................................................................................... 14

4.1.3. AC Harmonic Filters ........................................................................................................... 15

4.1.4. DC Filters ............................................................................................................................ 15


4.1.5. HVDC Cables or Overhead Lines ....................................................................................... 15

4.2. VSC-HVDC System ................................................................................................................... 15

4.2.1. Converters ........................................................................................................................... 15

4.2.2. Transformers ....................................................................................................................... 16

4.2.3. Phase Reactors .................................................................................................................... 16

4.2.4. AC Filters ............................................................................................................................ 16

4.2.5. DC Capacitors ..................................................................................................................... 16

4.2.6. DC Cables ........................................................................................................................... 16

5. Cost Analysis of HVDC System ......................................................................................................... 16

6. HVDC Market ..................................................................................................................................... 19

6.1. ABB Ltd. ..................................................................................................................................... 19

6.2. Siemens AG ................................................................................................................................ 20

6.3. Alstom ......................................................................................................................................... 21

7. Conclusion .......................................................................................................................................... 22

Bibliography ................................................................................................................................................ 23


List of Figures

Figure 1. Tower Configuration for AC and DC Transmission (HVDC Power Transmission) ..................... 3

Figure 2. Offshore HVDC Development (Appleyard, David, 2011) ............................................................ 7

Figure 3. Graphical symbols for valves or rectifier (IEC, 2011) .................................................................. 8

Figure 4. Arrangement of Capacitor Commutated Converter (ABB, 2011) ............................................... 10

Figure 5. HVDC configurations (DU, 2007) (Pualinder, 2003) ................................................................. 13

Figure 6. Six Pulse Valve Bridge for HVDC .............................................................................................. 14

Figure 7. Twelve Pulse Valve Converter Bridge with star/delta Arrangement .......................................... 14

Figure 8. Cost structure for converter stations ............................................................................................ 17

Figure 9. Transmission distance and investment costs for AC and DC power transmission lines ............. 18

Table 1. Comparison between HVDC Classic and HVDC Light®…………………………………………………………20

Table 2. Comparison between HVDC Classic and HVDC Plus®………………………………………….……21


Nomenclature and Abbreviations

AC Alternating current

DC Direct Current

HVDC High voltage direct current

MW Mega Watts

HVAC High voltage alternating current

VSC Voltage source converter

IGBT Insulated gate bipolar transistor

TSO Transmission system operator

UHVDC Ultra high voltage direct current

LCC Line commutated converter

CCC Capacitor commutated converter

SCR Silicon controlled rectifier

SCR Short circuit ration

GTO Gate turn-off

PWM Pulse width modulation

CSC Current source converter


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1. Introduction

The High Voltage Direct Current Technology (HVDC) is used for economical power

transmission over very long distances and it’s an adequate way to connect asynchronous grids or

grids having different frequency. In 1954, the first HVDC (10MW) transmission system was

commissioned in Gotland, Sweden. (Hingorani, Apr 1996) Currently, the longest HVDC link

(2071 Km) is between Xiangjiaba and Shanghai the transmission capacity is 6400 MW and it

connects Xiangjiaba dam to Shanghai in China. (Xiangjiaba-Shanghai UHVDC Transmission

project, 2011) In 2012, the longest HVDC link will be between Amazonas and São Paulo

Brazil with length more than 2500 Km. (Rio Madeira, 2011)

HVDC transmission systems, when installed, play a very vital role in electric power

transmission system. The critical feature of HDVC is the high reliability with long useful life.

Power conversion (rectification and inversion) are the most important systems in HVDC

technology. The conversion system is used as an interface to the AC transmission system, and

converts AC to DC and vice versa.

Applications of HVDC

Following are some important applications of HVDC:

HVDC systems can be used for bulk energy transfer through long distance overhead lines

The bulk energy transfer submarine cables can be done by HVDC.

It can be used to link systems having different frequencies by using an asynchronous

Back-to-Back. The HVDC link has no constraints with respect to frequency or phase

angle between the two AC systems.

HVDC helps to create a positive damping of electromagnetic oscillations and enhance the

stability of the network by modulation of the transmission power using a Back-to-Back.

(Chan-Ki Kim et al. 2009) Consequently, it allows the fast and precise control of the flow

of energy.

HVDC systems can be extremely useful from renewable energy source’s perspective

when the consumer and production units are located far away.


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Multi-terminal DC link is helpful to supply power in the regions or countries through

which transmission lines pass.

Since reactive power does not get transmitted over a DC link, two AC systems can be

connected through a HVDC link without increasing the short circuit power, this

technique can be useful in generator connections.

The VSC based HVDC technology is gaining more attention. This new technology has

become possible as a result of important advances in the development of insulated gate

bipolar transistors (IGBT). In this system, pulse-width Modulation can be used for the

VSC as opposed to the thyristor based conventional HVDC. This technology is well

suited for wind power connection to the grid. (Chan-Ki Kim et al. 2009)

1.1. Advantages and features of HVDC

Apart from the main advantage of HVDC system to transmit bulk power over long distances

with low cost and less losses, another significant feature of HVDC is that there is no stability

limit related to the amount of power or transmission distance.

1.1.1. Long distance bulk power transmission

The transmission network is one of the most critical parts of power systems. It is very

important to design it efficiently to transmit power keeping in mind the economic factors,

network safety and redundancy. The transmission network consists mainly of power lines,

cables, circuit breakers switches and transformers. The transmission network is usually

administered on a regional basis by an entity such as transmission system operator (TSO).

HVDC is a good option to transmit power over long distances because the capacitance of an

AC high-voltage cable gives rise to charging current. This charging current effectively reduces

the amount of power a cable is able to transport. The charging current is proportional to the

length of the cable and thus the longer the cable, the lesser power it is able to transport. An AC

high-voltage cable has a maximum length of approximately 60 to 100 kilometer (Fu, 2010).

While for DC, the capacitive reactance of the cable is infinite so no charging currents exist. This

makes the transmission distance virtually unlimited. From the figure below, the difference

between tower configuration of AC and DC is shown. The DC towers are significantly smaller


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than AC tower because they carry two conductors or power lines to transmit DC while three

phase AC current is carried through three conductors or power lines.

Figure 1. Tower Configuration for AC and DC Transmission (HVDC Power Transmission)

1.1.2. Interconnections

If we compare the interconnection between two or more independent systems for an AC and

DC, the AC link poses certain issues like security, reliability, frequency control, voltage control,

primary and secondary control of reserve capacity. In most of the cases more than one AC link is

required for reliability and stability of the system. While interconnecting the systems with DC

eliminates any limitation concerning stability problems and control strategies however it does not

eliminate the issues related with primary and secondary control of reserve capacity.

As far as the submarine interconnections are concerned, the voltage variation and instability

increases with power flows due to charging current in AC connections while the installation of

intermediate reactive compensation units become impractical. The maximum practical length of

submarine cable is approximately 80 Kilometers, beyond this length HVDC is the only viable

solution to such problems however slightly higher cost is the only concern. (Zaccone, 2010)


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1.1.3. Multi-Terminal Systems in HVDC

Multi-Terminal system in HVDC provides an opportunity to transmit power to the countries

or regions within one country through that power transmission lines pass. It is economically and

politically very important to offer connections to the potential partners

A multi-terminal HVDC transmission system consists of two or more conversion stations.

This kind of transmission is little complex than point to point transmission. In particular, the

control system is more elaborate and the telecommunication requirements between the stations

become larger.

The first large scale (2000MW) multi-terminal system “Hydro Québec - New England

transmission” was built between 1987 and 1992 by ABB. Now it is the plan to build ±800 kV

UHVDC transmission link in the North East - Agra HVDC in India. The power transmission

system will have the possibility to convert 8,000 MW hence it will be the largest HVDC

transmission ever. (HVDC multi-terminal system, 2011)

1.1.4. Support for AC System

HVDC systems can also be used to provide support and stability to AC systems if the

disturbances are caused by frequency change. The frequency changes can either be caused by the

difference of power generation and demand or by the difference of voltages in different parts of

the network. HVDC can feed (or extract) the active power into the disturbed system

instantaneously. It is due to the damping torque which is an inherent and valuable feature of

HVDC link.

1.1.5. Limitation of Faults

HVDC restricts the effect of certain critical faults on AC system. Faults like voltage

depression on power swing do not transmit across DC barrier. They may appear on the other side

of a DC link as a reduction in power, but voltage will not be affected. (Chan-Ki Kim et al. 2009)

1.1.6. Limitation of Short Circuit Level

The short-circuit level of the system increases with the addition of new AC lines. It is an

unavoidable problem which has to be resolved through expansive modification in switchgear

equipment. In case of DC, no reactive power exist that means active power can be increased

without increasing the short circuit level since impedance and resistance are fixed.


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1.1.7. Control of Power Flow

HVDC link can operate at any condition of voltage and frequency of the two AC systems. An

independent control is therefore available to transmit power, leaving each system’s existing load

frequency control to act normally. This control feature uses reserves to keep the voltage and

frequency within allowable limits.

1.1.8. Voltage Control

In the conversion stations, converters absorb reactive power. This reactive power can be used

to control the voltages. It is important to realize that the normal constant power regime of a DC

link can destabilize an AC network under distress. A normal feature of the DC link is the

voltage-dependent current limit where DC power is limited when voltage drops below the

normal range, so that the reactive power is made available to the AC system. Under disturbed

conditions, it is a good principle to look after the AC voltage first, and then order the power flow

accordingly. There are substantial AC filters at the converter stations, which can be used to

bolster AC voltage if stability is threatened. The DC control drops DC power, so that the

converters absorb less reactive power and the reactive capacity of the filters is available to the

network. Though the loss of power flow is unwelcome, the boost to AC voltage maybe more

valuable. Self-commutated VSCs can provide independent control of active and reactive power.

Reactive power generation or absorption is possible, within converter ratings, at any DC power

transfer rate. (J. Arrillaga et.al 2009)

1.1.9. Environmental Benefits

It is however difficult to compare the environmental benefits of AC and DC but a qualitative

comparison is presented below

The DC line has less visual impact compared with the AC line of the same power. It is an

advantage.

The small right of way width of DC line, compared with AC line, provides suitable routes

in densely populated areas and regions having complex terrain.

All high tension electrical lines generate crackling and humming sound this phenomena is

called corona. (Corona Effect, 2011) It is a power loss and in some cases it can damage

system’s components. DC lines produce fewer coronas compared with AC lines.


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Radio interference is generated by from Corona discharges; it generates high frequency

currents in the conductors producing electromagnetic radiation, in the vicinity of the

lines. Radio noise from a DC line is considerably lower than from AC lines of similar

capacity. (Crane, 2010)

The DC line contains an unchanging electric field that means it doesn’t exert any

magnetic field on the surroundings.

1.2. HVDC and Wind Power

In the past few years, a rapid increase in wind turbine connection to distribution and

transmission networks is observed and the increased penetration makes the power network more

dependent on, and susceptible to, the wind energy production. Large scale wind generation

facilities have become a very obvious component of the interconnected power grid in many

countries especially in Europe. One of the major challenges faced by the electricity industry is

how to effectively integrate significant amount of wind power into the electricity system. For a

successful integration the electricity industry has to deal with challenges arising from market

liberalization, electricity networks renewal and innovation, the limited predictability of wind and

the frequency and voltage capabilities. (European Commission. Directorate-General for

Research., 2006)

AC links are good option if some hundred megawatts are to be transferred through few tenths

of kilometers but if both power and distance increased, DC connections would be more

competitive option. In the present scenario, HVDC with voltage source converters (VSC) seems

to be best suited for such power transmissions. A more detailed introduction of VSC will be

presented later in this report.

There are a couple of projects which have been constructed keeping in view the suitability of

VSC HVDC in conjunction with wind power. One project is at Gotland, Sweden where 60 MW

(80kV) is transmitted through 70 km by DC connection. The other project is in Tjaereborg

Denmark where 7 MW (10kV) is transmitted through 4 km by a DC connection. (Asplund) A

latest and remarkable advancement in the application of HVDC in wind industry is the

acquisition of around $ 1 billion order by ABB from the Dutch-German transmission system

operator Tenne T to supply the Dolwin2 HVDC transmission line connecting offshore North Sea

wind farms to the German mainland grid. By 2015, 900 MW HVDC converter and cable system,

http://www.tennet.org/english/index.aspx


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operated at ±320 kV, is to be installed. This project follows the 800 MW Dolwin1 link scheduled

for 2013 and the 400 MW Borwin1 which was installed in 2009. (Greiner, 2011)

Figure 2. Offshore HVDC Development (Appleyard, David, 2011)

Siemens is also installing multiple transmission links off the German coast, notably the 864

MW SylWin link scheduled for 2014, the 800 MW BorWin2 and the 576 MW HelWin1 links

both scheduled for 2013. The offshore platforms SylWin alpha and DolWin alpha are both being

certified by Det Norske Veritas (DNV). (Greiner, 2011)

2. HDVC Process

The fundamental process in HVDC system is the conversion of AC to DC (rectification) at

the transmitting end and from DC to AC (inversion) at the receiving end. The conversion can be

achieved by following three ways

Natural or line Commutated Converters (LCC)

Capacitor Commutated Converters (CCC)

Forced Commutated Converters

2.1. Natural or Line Commutated Converters

Natural or line commutated converters are most used in HVDC systems. LCC method uses

thyristor, a controllable semiconductor which can carry currents up to 4000 A and block voltages

up to 10 kV, for the conversion process. These thyristors are arranged in series to make a


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thyristor valve which can operate at several hundred voltages. The thyristor valve operates at

certain frequency i.e. 50 or 60 Hz and by means of control angle it is possible to change the DC

level of the bridge. In this way transmitted power is controlled rapidly and efficiently. The

convertor which is used to convert single-phase or three-phase AC voltages into DC voltages is

called rectifier. The rectifier can either be controllable or uncontrollable.

Controllable Rectifier is one which can be forced to turn on by control signals i.e. thyristors are

also called Silicon Controlled Rectifiers (SCRs). The output quantities can be adjusted using

controllable rectifiers. The controlled rectifier can also be used to convert energy from DC

voltage to a single-phase or three-phase ac. It will be the inverting mode of rectifier. (Doncker,

2011) Uncontrollable Rectifier is one which contains all diodes as electric valves. An

uncontrolled converter provides a fixed output voltage for a given ac supply.

Rectification The controlled converter, as well as the uncontrolled converter, consists of diodes.

The uncontrollable converters prevent the output voltage from going negative. Such converters

only allow power flow from the supply to the load. This is called rectification of current, and

such converters are also called unidirectional converters. (Ned Mohan, 2003)

Inversion The controlled converters allow an adjustable output voltage by controlling the phase

angle at which the forward biased thyristors are turned on. The polarity of the load voltage of a

fully controlled converter can reverse, allowing power flow into the supply, it is called inversion

of current, and such converters can also be described as bidirectional converters because they

allow power flow in both directions. (Ned Mohan, 2003)

Figure 3. Graphical symbols for valves or rectifier (IEC, 2011)


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2.2. Capacitor Commutated Converters

The strength of and AC network can be measured by short circuit ratio (SCR) which is

defined as:

SCR = Short Circuit Level of AC Bus/ DC Power

A system is said to be weak if SCR is less than 3. A weak system is more sensitive to voltage

fluctuations which cause problems in the HVDC network and special control methods are

required to partially eliminate this problem. (Mazumder, 2002)

The LCC based HVDC system becomes unreliable when it operates with weak AC systems. The

reason behind the unreliability is commutation failure due to small disturbances in AC system.

These commutation failures eventually initiate other changes such as voltage and frequency

instability in AC networks. Furthermore, LCC stations consume large amount of the rated DC

power which leads to the requirement of adding large capacitor banks and the filters.

(Mazumder, 2002) Such additions increase the cost of the system and may cause operational

problems such as development of low frequency resonance with AC networks.

The chances of commutation failure can be greatly reduced by adding capacitors between the

converter transformers and valve side. This arrangement is called Capacitor Commutated

Converters (CCC)

The (CCC) improve system’s performance and give following advantages (Capacitor

Commutated Converter, 2011)

They are good for reactive power compensation

They give stability to HVDC transmission at low SCR

They provide good control properties at long cable transmission

They are very sensitive to commutation failure.


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Figure 4. Arrangement of Capacitor Commutated Converter (ABB, 2011)

2.3. Forced Commutated Converters

The force commutated converters are also called self-commutated converters utilize the

valves made up of semiconductors with the ability not only to turn-on but also to turn-off.

Another name of such these converters is voltage source converters (VSC). Two types of

semiconductors are normally used in VSC, named Gate Turn off Thyristors (GTO) and Insulated

Gate Bipolar Transistors (IGBT). Both of them have been in use since eighties. The VSC

commutates with high frequency. The operation of the converter is achieved by Pulse Width

Modulation (PWM). With PWM it is possible to create any phase angle and/or amplitude (up to

a certain limit) by changing the PWM pattern, which can be done almost instantaneously. Thus,

PWM offers the possibility to control both active and reactive power independently. This makes

the PWM Voltage Source Converter a close to ideal component in the transmission network.

From a transmission network perspective, it acts as a motor or generator without mass that can

control active and reactive power almost instantaneously. (DU, 2007)

3. Configurations of HVDC

HVDC converter bridges together with lines or cables can be arranged as the following

configurations.

3.1. Mono-polar HVDC System

In the mono-polar system, a single pole line connects the two converters, and positive or

negative DC voltage is used. Only one insulated transmission conductor is installed in mono-

polar systems while ground or sea provides path for the return current. However, a metallic


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conductor may be used as the return path. In 1965, Konti-Skan project and in 1967, Sardinia

Italy project used mono-polar HVDC system. (Arrillaga, 1998)

3.2. Bipolar HVDC System

In the bipolar system, two mono-polar systems are combined; one is run with the positive

polarity voltage and the other with negative polarity. The two poles can be operated

independently if both neutrals are grounded. The bipolar configuration increases the power

transfer capacity. Under normal operation, the currents flowing in both poles are identical and

there is no ground current. In case of failure of one pole, power transmission can continue in the

other pole which increases the reliability. Most overhead line HVDC transmission systems use

the bipolar configuration and it is the most commonly used configuration for HVDC

transmission systems. (Arrillaga, 1998)

3.3. Homo-polar HVDC System

In the homo-polar configuration, two or more conductors have the negative polarity and can

be operated with ground or a metallic return. With two poles operated in parallel, the homo-polar

configuration reduces the insulation costs. However, the large earth return current is the major

disadvantage. (Sood, 2004)

3.4. Back-to-back HVDC System

In the back-to-back HVDC configuration, two converters at the same site are used. They are

connected to each other without any transmission line in between. This option is used when the

aim of the HVDC link is to connect two power systems with different frequencies i.e. 50 and 60

Hz. These configurations are mostly found in Japan and South America. (Biledt et al. 2000)

3.5. Multi-terminal HVDC System

In the multi-terminal configuration, three or more HVDC converter stations are

interconnected through transmission lines and cables, these converters are geographically apart,

and these can either be connected in parallel or in series. In the parallel arrangement, all

converter stations are connected to the same voltage while in series arrangement; one or more

converter stations are connected in series in one or both poles. A hybrid multi-terminal system

contains a combination of both the parallel and series arrangement. Sardinia-Corsica-Italy


12

(SACOI) connection, the Pacific Intertie in USA and the Hydro Quebec - New England Hydro

from Canada to USA employed multi-terminal HVDC system. (Hausler, March 1999)


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Figure 5. HVDC configurations (DU, 2007) (Pualinder, 2003)

4. Components of HVDC System

There are two main components of HVDC system: the convertor station at the transmission

and receiving end and the transmission medium. Rectification and inversion use essentially the

same machinery. However, for better understanding, the components of classic-HVDC and VSC-

HVDC will be discussed separately in the following section.

4.1. Classic-HVDC System

The Classic-HVDC consists mainly of converters, converter transformers, AC side harmonic

filters, DC filters and HVDC cables or overhead lines. (R. Rudervall, March 2000)

4.1.1. Converters

Converters are the most critical part of an HVDC system. They perform two functions i.e.

conversion from AC to DC (rectification) at the sending end, and from DC to AC (inversion) at

the receiving end. These converters are connected to the AC network through transformers. The

classic HVDC converters are current source converters (CSCs) with line-commutated thyristor

switches. A six pulse valve bridge, shown in Figure 6 is the basic converter unit of classic

HVDC for both rectification and inversion. (Diodes and Rectifiers, 2011) Similarly a twelve

pulse converter bridge can be made by connecting two six pulse bridges. The bridges are then

connected to the AC system through transformers using star/star or star/delta arrangement. In

figure 7, a twelve pulse valve converter bridge with star/delta arrangement is shown. (Diodes and

Rectifiers, 2011) The valves can be cooled by air, oil, water or Freon but cooling by de-ionized

water is the most efficient way. Air is used for insulation.


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Figure 6. Six Pulse Valve Bridge for HVDC

Figure 7. Twelve Pulse Valve Converter Bridge with star/delta Arrangement

4.1.2. Transformers

As described above, transformers connect the AC network to the valve bridges. Transformers

also adjust the suitable AC voltage level for converters. The transformers can be of different

types depending on the power to be transmitted and possible transport requirements. (R.

Rudervall, March 2000) The HVDC transformers are made by the leading electrical companies

like ABB, Siemens and Alstom. The Tianwei group China, Reinhausen Group and CG Power

Systems are also manufacturing HVDC transformers.


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4.1.3. AC Harmonic Filters

The AC filters restrict the harmonic current from entering into AC network connected to

HVDC system. The harmonics are produced by HVDC converters. The filter banks also provide

reactive power which is usually consumed by the converters during conversion process.

However rest of the power is provided by capacitors banks.

4.1.4. DC Filters

The HVDC converters produce ripple on the DC voltage. Ripple voltage is the undesirable

mixing of AC voltages with DC output. (Diodes and Rectifiers, 2011)The voltage ripple

produces interference in the telephone networks near the DC line. Usually DC filters are neither

required for pure cable transmission nor for back to back HVDC stations. They are only required

if overhead lines are used in the transmission system. The filters may be turned filters or active

DC filters. (Active filters in HVDC applications, 2003)

The companies dealing with harmonic filters are Circutor SA, CG Power Systems, Schaffner,

Zhuhai Wanlida Electric Co., Ltd, etc. Chinese companies are mostly dealing with harmonic

filters.

4.1.5. HVDC Cables or Overhead Lines

The HVDC cables are normally required for submarine transmission while overhead cables

are required for the connections over land. For a back-to-back HVDC system no DC cable or

overhead line is needed. No serious length limitation exists for HVDC cables. Now there is a

growing trend to use cables for land also because of environmental effects of overhead lines.

4.2. VSC-HVDC System

The components of VSC-HVDC system are converters, transformers, phase rectors, AC

filters, DC capacitors and DC cables. (G. Asplund, K. Eriksson, H. Jiang, J. Lindberg, R.

P°alsson, and K. Svensson, 1998)

4.2.1. Converters

The converters are voltage source converters (VSC) and they employ IGBT power

semiconductors one operating as a rectifier and other as an inverter. The two converters can be

connected either back to back or through a DC cable depending on the application.


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4.2.2. Transformers

The role of transformers is the same for both VSC and classic-HVDC system. They adjust

the AC voltage for converters.

4.2.3. Phase Reactors

The phase reactors are used for controlling both the active and the reactive power flow by

regulating current through them. The phase reactors also act as AC filters to reduce the high

frequency harmonic ACs which are caused by the switching operation of the VSCs.

4.2.4. AC Filters

The AC filters are required to reduce the harmonics on the AC side however with VSC, there

is no need to compensate reactive power and current harmonics on the AC side are directly

related to PWM frequency. Therefore the amount of filters is less in this case as compared with

line commutated converters.

4.2.5. DC Capacitors

The objective of the DC capacitors is to provide an energy buffer to keep the power balance

during transients and reduce the voltage ripple on the DC side. There are two capacitors stacks

installed on the DC side, and their size depends on the required DC voltages.

4.2.6. DC Cables

The cable used in the VSC-HVDC applications is a new developed type where the insulation

is made of an extruded polymer that is particularly resistant to DC voltage. Polymeric cables are

the preferred choice for HVDC mainly because of their mechanical strength, flexibility and low

weight. (Weimers, December 2000)

5. Cost Analysis of HVDC System

The cost of HVDC systems depends on various factors such as power capacity to be

transmitted, transmission medium, environmental conditions, and other safety, regulatory

requirements etc. HVDC transmission systems often provide a more economical alternative to ac

transmission for long-distance, bulk-power delivery from remote resources such as large scale

wind farms. The concept of “break-even distance” always arises with long distance power

transmission. It comes when the savings in line costs and lower capitalized cost of losses offsets


17

the higher converter station costs. The break-even distance depends on several factors such as

transmission medium (cable or overhead lines) and different local aspects (permits, cost of local

labor etc).

A typical cost structure for the converter stations is shown in the figure 8 below. (D.M.

Larruskain)

Figure 8. Cost structure for converter stations

The cost of the traditional HVDC system is high because it requires for filters, capacitors

and other auxiliary equipment. The traditional HVDC system is designed for the transmission of

large amounts of energy measured in hundred of megawatts. This system is not economical less

for than 20 MW loads. (D.M. Larruskain)

In the following figure, the bipolar HVDC transmission is compared with a double-circuit high

voltage AC transmission. (Chan-Ki Kim et al. 2009)

(1) Represents the initial cost of HVAC power transmission

(2) Represents the initial cost of HVDC power transmission which is higher than HVAC due

to higher valve cost included in HVDC.

(3) Represents the cost of construction for HVAC power transmission

(4) Represent the cost of shunt capacitor installed in HVAC transmission


18

(5) Represents the cost of construction for HVDC power transmission which seems less than

that of HVAC because shunt capacitors must be installed after every 100 or 200 Km in

HVAC systems to maintain the electrostatic stability.

(6) Represents the losses in HVAC transmission

(7) Represents the losses in HVDC transmission, Initial loss levels are higher in the HVDC

system, but they do not vary with distance. In contrast, loss levels increase with distance

in HVAC system.

(8) Represents the total AC cost

(9) Represents the total DC cost

Figure 9. Transmission distance and investment costs for AC and DC power transmission lines


19

6. HVDC Market

Each component, in the HVDC system, plays an important role for the success of the project.

One of the key components in HVDC system is the converter station which performs two vital

processes: rectification and inversion. In this section, companies offering HVDC technology will

be discussed. ABB and Siemens, the world’s leading companies in electrical power transmission,

are racing to overcome the biggest challenge of transmitting power more efficiently. According

to Peter Leupp, ABB’s head of power transmission, “The market potential for HVDC is $10

billion a year in the next few years” and “Circuit breakers could add a lot to that”. (Simonian,

2011).

ABB and Siemens are the two rivals in HVDC technology; they used to leapfrog each other in

terms of higher voltages, power capacities and distances. These two account for 80 percent of the

market shares. (Simonian, 2011)

6.1. ABB Ltd.

ABB is a leader in power and automation technologies that enable utility and industry

customers to improve performance while lowering environmental impact. The ABB Group of

companies operates in around 100 countries and employs about 130,000 people. (Our businesses,

ABB, 2011).

ABB offers HVDC Classic and HVDC Light®

; these are highly efficient alternatives for

transmitting bulk power and for special purpose applications. The HVDC Classic is a traditional

technology which is based on thyristors valves. It is used to transmit electricity by overhead lines

and submarine cables over long distances. Another attribute of the technology is to interconnect

separate power systems where traditional AC systems do not work. (The Classic HVDC

Transmission, 2011)

HVDC Light® is relatively new, IGBT based technology. It was developed in 1997 to

transmit power underground and under water over long distances. It gives enormous benefits like

invisible power lines, neutral electromagnetic fields, oil-free cables and compact converter

stations. HVDC Light® uses extruded polymer insulated cables which make this technology

economical and environment friendly. As far as the costs are concerned, the direct cost of HVDC

Light® including converters, cables and their installation, for a case having power capacity 1700


20

MW and distance 400 Km, is approximately $ 275 to $ 420 million. This wide range is due to

differences in installation costs and local market conditions. The direct investment cost for

HVDC Light® is 0.6 to 3.2 times the cost for overhead lines which is much improved than

formerly anticipated figures of 5 to 15 times. (Dag Ravemark, Bo Normark, 2005)The difference

between these technologies is presented: (Dr. Le Tang, Feb 9, 2010)

Features HVDC Classic HVDC Light®

Converter Technology Thyristor Valve IGBT

Connection valve - AC grid Converter transformer Series reactor (+ transformer)

Max. Convertor rating at present 6400 MW, ±800 kV (OH line) 1200 MW, ±320 kV (cable)

2400 MW, ±320 kV (overhead)

Relative Size 4 1

Typical delivery time 36 months 24 months

Reactive power demand Reactive power demand = 50%

power transfer

No reactive power demand

Reactive power compensation &

control

Discontinuous control (Switched

shunt banks)

Continuous control (PWM built

in converter control)

Independent control of active

& reactive power

No YES

Scheduled maintenance Typically < 1% Typically < 0.5%

Typical system losses 2.5 - 4.5 % 4 - 6 %

Multi-terminal configuration Complex, limited to 3 terminals Simple, no limitations

Table 1. Comparison between HVDC Classic and HVDC Light®

6.2. Siemens AG

Siemens AG, a leading company, besides ABB which is dealing with HVDC systems.

Siemens provide HVDC Classic, Ultra HVDC and HVDC Plus®

for economical power

transmission over very long distances and also a trusted method to connect asynchronous grids or

grids of different frequencies. HVDC classic for both Siemens and ABB are the same where

thyristors are used for commutation.


21

HVDC Plus® employs IGBT technology like HVDC Light

®. It uses new concept of modular

multilevel voltage-sourced converters, HVDC Plus®

is the preferred solution where shortage of

space is a criterion. It is ideal for connection of remote offshore platforms and wind farms to the

onshore grid as well as for power supply high-density areas such as mega cities. (HVDC PLUS,

2011) The HVDC Plus®

technology is used in “Trans Bay Cable Project” where 53 miles long

HVDC cable and converters are installed between two substations in California USA. The rating

is 400 MW and 200 kV are the transmission voltages. HVDC Plus provided a benefit of

approximately $40 billion which had to be consumed in reactive power compensation. (Trans

Bay Cable Project, 2007) In table 2, a comparison between HVDC classic and HVDC Plus® is

shown (Trans Bay Cable Project, 2007)

Another remarkable achievement in the field of HVDC is Ultra HVDC by Siemens, by this

technology voltage level for power transmission reached up to 800 kV and power capacity up to

7 gigawatts. This is technically and economically feasible for the first time. China Southern

Power Grid Co. in Guangzhou is scheduled to commence commercial service by mid-2010 using

this technology. (Ultra HVDC Transmission System, 2011). It employs two 400 kV systems

which are connected in series. A converter station links the DC transmission line at each end to

the AC grids. For UHVDC application; innovative solutions have been implemented to fully

meet the extended requirements for ultra-high voltage bulk power transmission.

Table 2. Comparison between HVDC Classic and HVDC Plus®

6.3. Alstom

Alstom, the world leader in transport infrastructure, power generation and transmission

provides HVDC technology with the name of HVDC MaxSine®. The company has its setup in

100 countries and has 92700 employees. In the 1990s, Alstom successfully developed a voltage

FACTOR HVDC Classic HVDC Plus®

Height of converter station

building

65 feet

35 feet

Noise Along Illinois St. in San

Francisco

72 dB 48 dB

Lightning Arrestor Posts 85 feet 65 feet

Footprint ~5 acres ~3 acres

AC Filters Required Not required

Transformers --- Smaller than Classic


22

source converter for reactive power applications (STATCOM technology) using GTOs. In this

technology IGBTs are used as switching devices.

Alstom Grid has been awarded a contract worth approximately €240 million by the Swedish

utility Svenska Kraftnät for the 1440 MW South-West Link. The project will connect Barkeryd

in central Sweden to Hurva in southern Sweden, using High Voltage Direct Current (HVDC)

technology. The contract is registered in the third quarter of Alstom’s fiscal year 2011/2012. This

transmission project will use Alstom Grid’s HVDC MaxSine® Voltage Source Converter (VSC)

technology. Under the terms of the contract, Alstom will supply HVDC converter stations at both

ends, as well as control and protection, converter transformers, switchyard equipment,

construction and project management. The project will be completed by the end of 2014.

(Alstom Grid , 2012)

7. Conclusion

It is quite conceivable that with changed circumstances in the electricity industry, the

technological developments, and environmental considerations, HVDC would be the preferred

alternative in many more transmission projects. To implement the grid that is required for the

future, collaborative planning is needed using a long term, system perspective. We know where

the wind is, and we know where the loads are. Jointly, we can identify strategic broad-based

interstate system plans to harvest renewable resources before the individual projects develop.

Rather than project by project, piecemeal solutions, we must develop and justify an integrated

system. In order to capture the full scale of benefits that high capacity technologies such as

HVDC provide, the system must be examined on an interregional scale that matches the reach of

those benefits. As concluded from the discussion above, the HVDC transmission has high market

potential. The DC transmission fell into disuse decades ago compared with the more

conventional alternating current. But the thirst for power from fast growing economies, such as

China, India and Brazil, and the vast distances over which electricity often has to be transmitted

there, have sparked a huge revival of HVDC.


23

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Market Analysis for Rectifying and Inverter Technologies for HVDC System

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