High Voltage Direct Current Transmission System Report

SEMINAR REPORT ON

“High Voltage Direct CurrentTransmission System”

Submitted in partial fulfilment for Bachelor of Technology degree at Rajasthan Technical University, Kota

(2014-15)

Submitted To: - Submitted By:-Assit Prof. Anshul Bhati Nadeem KhiljiAssit. Prof. Vikram Rajpurohit B.Tech IV year VIII Sem.

Department of Electrical & Electronics EngineeringVYAS INSTITUTE OF ENGINEERING &

TECHNOLOGY, JODHPUR (RAJ.)

HVDC Transmission System

VYAS INSTITUTE OF ENGINEERING & TECHNOLOGY, JODHPUR (RAJ.)

Department of Electrical & Electronics Engineering

CERTIFICATE

This is to certify that the student NADEEM KHILJI of IV year VIII

Sem EEE Branch, have successfully completed the Seminar report

on titled “High Voltage Direct Current Transmission

System” towards the partial fulfillment of the degree of Bachelor

of Technology (B.TECH). In the Electrical & Electronics Engineering

of the Rajasthan Technical University during academic year 2014-

15.

Guided By Head of the Department

Assist. Prof. Anshul Bhati Prof. Dharmendra

Jain

Assist. Prof. Vikram Rajpurohit


Acknowlegment

I would like to take this opportunity to extend my sincere gratitude to

Prof. Dharmendra Jain, Head of Department, Electrical & Electronics

Engineering, for extending every facility to complete my seminar work

successfully.

I would like to express my sincere indebtedness to Prof. Anshul

Bhati & Prof. Vikram Singh Rajpurohit, Department of Electrical &

Electronics Engineering, for there valuable guidance, wholehearted co-

operation and duly approving the topic as staff in charge.

I also extend my gratitude towards the staffs, students and parents

for their sincere support and motivation.

Nadeem Khilji

11EVEEX032


ABSTRACT

The development of HVDC (High Voltage Direct Current) transmission

system dates back to the 1930s when mercury arc rectifiers were invented.

Since the 1960s, HVDC transmission system is now a mature technology

and has played a vital part in both long distance transmission and in the

interconnection of systems. Transmitting power at high voltage and in DC

form instead of AC is a new technology proven to be economic and simple

in operation which is HVDC transmission. HVDC transmission systems, when

installed, often form the backbone of an electric power system. They

combine high reliability with a long useful life. An HVDC link avoids some of

the disadvantages and limitations of AC transmission. HVDC transmission

refers to that the AC power generated at a power plant is transformed into

DC power before its transmission. At the inverter (receiving side), it is then

transformed back into its original AC power and then supplied to each

household. Such power transmission method makes it possible to transmit

electric power in an economic way.

HVDC Light is the newly developed HVDC

transmission technology, which is based on extruded DC cables and voltage

source converters consisting of Insulated Gate Bipolar Transistors (IGBT’s)

with high switching frequency. It is a high voltage, direct current

transmission Technology i.e., Transmission up to 330MW and for DC voltage

in the ± 150kV range. Under more strict environmental and economical

constraints due to the deregulation, the HVDC Light provides the most

promising solution to power transmission and distribution. The new system

results in many application opportunities and new applications in turn bring

up new issues of concern. One of the most concerned issues from

customers is the contribution of HVDC Light to short circuit currents. The

main reason for being interested in this issue is that the contribution of the

HVDC Light to short circuit currents may have some significant impact on

the ratings for the circuit breakers in the existing AC systems. This paper

presents a comprehensive investigation on one of the concerned issues,

which is the contribution of HVDC Light to short circuit currents.


CONTENTS

Chapter 1 INTRODUCTION 1

Chapter 2 HVDC TECHNOLOGY 2

Chapter 3 HVDC LIGHT TECHNOLOGY 16 Chapter 4 SHORT CIRCUIT CONTRIBUTION OF HVDC LIGHT 22

Chapter 5 CONCLUSION 29

Chapter 6 REFERENCES 30


1. INTRODUCTION

The development of HVDC (High Voltage Direct Current) transmission

system dates back to the 1930s when mercury arc rectifiers were invented.

In 1941, the first HVDC transmission system contract for a commercial

HVDC system was placed: 60MWwere to be supplied to the city of Berlin

through an underground cable of 115 km in length. It was only in 1954 that

the first HVDC (10MW) transmission system was commissioned in Gotland.

Since the 1960s, HVDC transmission system is now a mature technology

and has played a vital part in both long distance transmission and in the

interconnection of systems.HVDC transmission systems, when installed,

often form the backbone of an electric power system. They combine high

reliability with a long useful life. Their core component is the power

converter, which serves as the interface to the AC transmission system. The

conversion from AC to DC, and vice versa, is achieved by controllable

electronic switches (valves) in a 3-phase bridge configuration.

A new transmission and distribution technology, HVDC Light, makes it

economically feasible to connect small scale, renewable power generation

plants to the main AC grid. Vice versa, using the very same technology,

remote locations as islands, mining districts and drilling platforms can be

supplied with power from the main grid, thereby eliminating the need for

inefficient, polluting local generation such as diesel units. The voltage,

frequency, active and reactive power can be controlled precisely and

independently of each other. This technology also relies on a new type of

underground cable which can replace overhead lines at no cost penalty.

Equally important, HVDC Light has

control capabilities that are not present or possible even in the most

sophisticated AC.

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2. HVDC TECHNOLOGY

Electric power transmission was originally developed with direct current.

A high-voltage, direct current (HVDC) electric power transmission system

uses direct current for the bulk transmission of electrical power, in contrast

with the more common alternating current systems. For long-distance

transmission, HVDC systems may be less expensive and suffer lower

electrical losses. For shorter distances, the higher cost of DC conversion

equipment compared to an AC system may be warranted where other

benefits of direct current links are useful.

High voltage is used for electric 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

becoming dominant. 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).

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DC transmission now became practical when long distances were to be

covered or where cables were required. The development of HVDC (High

Voltage Direct Current) transmission system dates back to the 1930s when

mercury arc rectifiers were invented. HVDC transmission systems, when

installed, often form the backbone of an electric power system. They

combine high reliability with a long useful life. Their core component is the

power converter, which serves as the interface to the AC transmission

system. The conversion from AC to DC, and vice versa, is achieved by

controllable electronic switches (valves) in a 3-phase bridge configuration.

An HVDC link avoids some of the disadvantages and limitations of AC

transmission and

has the following advantages:

No technical limit to the length of a submarine cable connection.

No requirement that the linked systems run in synchronism.

No increase to the short circuit capacity imposed on AC switchgear.

Immunity from impedance, phase angle, frequency or voltage

fluctuations.

Preserves independent management of frequency and generator

control.

Improves both the AC system’s stability and, therefore, improves the

internal power carrying

capacity, by modulation of power in response to frequency, power

swing or line rating.

2.1 NEED FOR DC TRANSMISSION

The losses in DC transmission are lower. The level of losses is designed into

a transmission system and is regulated by the size of conductor selected.

DC and ac 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.

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When converters are used for dc transmission in preference to ac

transmission, it is

generally by economic choice driven by one of the following reasons :

1. An overhead dc transmission line with its towers can be designed to be

less costly per unit of length than an equivalent ac. line designed to

transmit the same level of electric power. However the dc converter

stations at each end are more costly than the terminating stations of an

ac line and so there is a breakeven distance above which the total cost

of dc transmission is less than its ac transmission alternative. The dc

transmission line can have a lower visual profile than an equivalent ac

line and so contributes to a lower environmental impact. There are

other environmental advantages to a dc transmission line through the

electric and magnetic fields being dc 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 ac cable systems exceeding 50 km but dc 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 ac 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 ac methods in order to exchange electric

power between them. However, if a dc converter station is located in

each system with an interconnecting dc link between them, it is

possible to transfer the required power flow even though the ac

systems so connected remain asynchronous.

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2.2 ADVANTAGES OF HVDC OVER AC TRANSMISSION:

The advantage of HVDC is 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. High-voltage direct current transmission allows

efficient use of energy sources remote from load centers.

In a number of applications HVDC is more effective than AC transmission.

Examples include:

Undersea cables, where high capacitance causes additional AC losses.

(e.g., 250 km Baltic Cable between Sweden and Germany the

600 km Nor Ned cable between Norway and the Netherlands, and

290 km Bass link between the Australian mainland and Tasmania)

Endpoint-to-endpoint long-haul bulk power transmission without

intermediate 'taps', for example, in remote areas

Increasing the capacity of an existing power grid in situations where

additional wires are difficult or expensive to install

Power transmission and stabilization between unsynchronized AC

distribution systems

Connecting a remote generating plant to the distribution grid, for

example Nelson River Bipole

Stabilizing a predominantly AC power-grid, without

increasing prospective short circuit current

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

Facilitate power transmission between different countries that use AC

at differing voltages and/or frequencies

Synchronize AC produced by renewable energy sources

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Long undersea / underground high voltage cables have a high

electrical capacitance, since the conductors are surrounded by a relatively

thin layer of insulation and a metal sheath while the extensive length of the

cable multiplies the area between the conductors. The geometry is that of a

long co-axial capacitor. Where alternating current is used for cable

transmission, this capacitance appears in parallel with load. Additional

current must flow in the cable to charge the cable capacitance, which

generates additional losses in the conductors of the cable. Additionally,

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. HVDC can carry

more power per conductor because, for a given power rating, the constant

voltage in a DC line is the same as the peak voltage in an AC line. The

power delivered in an AC system is defined by the root mean square (RMS)

of an AC voltage, but RMS is only about 71% of the peak voltage. The peak

voltage of AC determines the actual insulation thickness and conductor

spacing. 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.

Because, HVDC allows power transmission between unsynchronized AC

distribution systems, it can help increase system stability, by

preventing cascading failures from propagating from one part of a wider

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power transmission grid to another. Changes in load that would cause

portions of an AC network to become unsynchronized and separate would

not similarly affect a DC link, and the power flow through the DC link would

tend to stabilize the AC network. The magnitude and direction of power flow

through a DC link can be directly commanded, and changed as needed to

support the AC networks at either end of the DC link. This has caused many

power system operators to contemplate wider use of HVDC technology for

its stability benefits alone.

2.3 DISADVANTAGES:

The disadvantages of HVDC are in conversion, switching, control,

availability and maintenance..HVDC is less reliable and has lower

availability than AC systems, mainly due to the extra conversion equipment.

Single pole systems have availability of about 98.5%, with about a third of

the downtime unscheduled due to faults. Fault redundant bipole systems

provide high availability for 50% of the link capacity, but availability of the

full capacity is about 97% to 98%.

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. Pole 1 of the HVDC scheme between the North

and South Islands of New Zealand still uses mercury arc rectifiers, as does

Pole 1 of the Vancouver Island link in Canada. Both are currently being

replaced – in New Zealand by a new thyristor pole and in Canada by a

three-phase AC link. 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

7


regulated by the inverter control system instead of the inherent impedance

and phase angle properties of the transmission line. Multi-terminal lines are

rare. Another example is the Sardinia-mainland Italy link which was

modified in 1989 to also provide power to the island of Corsica.

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. 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.

2.4 RECTIFYING AND INVERTING:

2.4.1 Components

Most of the HVDC systems in operation today are based on Line-

Commutated Converters. Early static systems used mercury arc rectifiers,

which were unreliable. Two HVDC systems using mercury arc rectifiers are

still in service (As of 2008). The thyristor valve was first used in HVDC

systems in the 1960s. The thyristor is a solid-state semiconductor device

similar to the diode, but with an extra control terminal that is used to switch

the device on at a particular instant during the AC cycle. The insulated-gate

bipolar transistor (IGBT) is now also used, forming a Voltage Sourced

Converter, and offers simpler control, reduced harmonics and reduced valve

cost.

Because the voltages in HVDC systems, up to 800 kV in some cases, exceed

the breakdown voltages of the semiconductor devices, HVDC converters are

built using large numbers of semiconductors in series. The low-voltage

control circuits used to switch the thyristors on and off need to be isolated

from the high voltages present on the transmission lines.

This is usually done optically. In a hybrid control system, the low-voltage

control electronics sends light pulses along optical fibers to the high-

side control electronics. Another system, called direct light triggering,

dispenses with the high-side electronics, instead using light pulses from the

8


control electronics to switch light-triggered thyristors. A complete switching

element is commonly referred to as a valve, irrespective of its construction.

2.4.2 Rectifying & Inverting Systems

Rectification and inversion use essentially the same machinery. Many

substations (Converter Stations) are set up in such a way that they can act

as both rectifiers and inverters. At the AC end a set of transformers, often

three physically separated single-phase transformers, isolate the station

from the AC supply, to provide a local earth, and to ensure the correct

eventual DC voltage. The output of these transformers is then connected to

a bridge rectifier formed by a number of valves. The basic configuration

uses six valves, connecting each of the three phases to each of the two DC

rails. However, with a phase change only every sixty degrees, considerable

harmonics remain on the DC rails.

An enhancement of this configuration uses 12 valves (often known as

a twelve-pulse system). The AC is split into two separate three phase

supplies before transformation. One of the sets of supplies is then

configured to have a star secondary, the other a delta secondary,

establishing a thirty degree phase difference between the two sets of three

phases. With twelve valves connecting each of the two sets of three phases

to the two DC rails, there is a phase change every 30 degrees, and

harmonics are considerably reduced.

In addition to the conversion transformers and valve-sets, various passive

resistive and reactive components help filter harmonics out of the DC rails.

9


2.5 CONFIGURATIONS OF HVDC SYSTEM:

2.5.1 Monopole And 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.

Figure 1: Block diagram of a monopole system with earth return

Therefore it is a type of single wire earth return. The issues surrounding

earth-return current include:

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.

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http://en.wikipedia.org/wiki/File:Hvdc_monopolar_schematic.svg


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 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).

2.5.2 Bipolar

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.

Figure 2: 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.

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http://en.wikipedia.org/wiki/File:Hvdc_bipolar_schematic.svg


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.

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.

2.5.3 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).

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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-to-back 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.

2.6 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 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 multi-terminal HVDC system in the world.

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2.7 CORONA DISCHARGE

Corona discharge is the creation of ions in air by the presence of a

strong electric field. Electrons are torn from neutral air, and either the

positive ions or the electrons are attracted to the conductor, while the

charged particles drift. This effect can cause considerable power loss,

create audible and radio-frequency interference, generate toxic compounds

such as oxides of nitrogen and ozone, and bring forth arcing.

Both AC and DC transmission lines can generate coronas, in the former case

in the form of oscillating particles, in the latter a constant wind. Due to

the space charge formed around the conductors, an HVDC system may

have about half the loss per unit length of a high voltage AC system

carrying the same amount of power. With monopolar transmission the

choice of polarity of the energized conductor leads to a degree of control

over the corona discharge.

In particular, the polarity of the ions emitted can be controlled, which may

have an environmental impact on particulate condensation. (particles of

different polarities have a different mean-free path.) Negative coronas

generate considerably more ozone than positive coronas, and generate it

further downwind of the power line, creating the potential for health effects.

The use of a positive voltage will reduce the ozone impacts of monopole

HVDC power lines.

2.8 AREAS FOR DEVELOPMENT IN HVDC CONVERTERS

The thyristor as the key component of a converter bridge continues to be

developed so that its voltage and current rating is increasing.

Gate-turn-off thyristors (GTOs) and insulated gate bipole transistors (IGBTs)

are required for the voltage source converter (VSC) converter bridge

configuration. It is the VSC converter bridge which is being applied in new

developments . Its special properties include the ability to independently

control real and reactive power at the connection bus to the ac system.

Reactive power can be either capacitive or inductive and can be controlled

to quickly change from one to the other.

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A voltage source converter as in inverter does not require an active ac

voltage source to commutate into as does the conventional line

commutated converter. The VSC inverter can generate an ac three phase

voltage and supply electricity to a load as the only source of power. It does

require harmonic filtering, harmonic cancellation or pulse width modulation

to provide an acceptable ac voltage wave shape.

Two applications are now available for the voltage source converter. The

first is for low voltage dc converters applied to dc distribution systems. The

first application of a dc distribution system in 1997 was developed in

Sweden and known as “HVDC Light”. Other applications for a dc distribution

system may be:

1. In a dc feeder to remote or isolated loads, particularly if underwater or

underground cable is necessary.

2. For a collector system of a wind farm where cable delivery and optimum

and individual speed control of the wind turbines is desired for peak turbine

efficiency.

The second immediate application for the VSC converter bridges is in back-

to-back configuration. The back-to-back VSC link is the ultimate

transmission and power flow controller. It can control and reverse power

flow easily, and control reactive power independently on each side. With a

suitable control system, it can control power to enhance and preserve ac

system synchronism, and act as a rapid phase angle power flow regulator

with 360 degree range of control.

There is considerable flexibility in the configuration of the VSC converter

bridges. Another option is to use multilevel converter bridges to provide

harmonic cancellation. Additionally, both two level and multilevel converter

bridges can utilize pulse width modulation to eliminate low order harmonics.

With pulse width modulation, high pass filters may still be required since

PWM adds to the higher order harmonics. As VSC converter bridge

technology develops for higher dc voltage applications, it will be possible to

eliminate converter transformers. This is possible with the low voltage

applications in use today. It is expected the exciting developments in power

electronics will continue to provide exciting new configurations and

applications for HVDC converters.

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3. HVDC LIGHT TECHNOLOGY

A new transmission and distribution technology, HVDC Light, makes it

economically feasible to connect small-scale, renewable power generation

plants to the main AC grid. Vice versa, using the very same technology,

remote locations as islands, mining districts and drilling platforms can be

supplied with power from the main grid, thereby eliminating the need for

inefficient, polluting local generation such as diesel units. The voltage,

frequency, active and reactive power can be controlled precisely and

independently of each other. This technology also relies on a new type of

underground cable which can replace overhead lines at no cost penalty.

Equally important, HVDC Light has control capabilities that are not present

or possible even in the most sophisticated AC systems.

As its name implies, HVDC Light is a dc transmission technology. However,

it is different from the classic HVDC technology used in a large number of

transmission schemes. Classic HVDC technology is mostly used for large

point-to-point transmissions, often over vast distances across land or under

water. It requires fast communications channels between the two stations,

and there must be large rotating units - generators or synchronous

condensers - present in the AC networks at both ends of the transmission.

HVDC Light consists of only two elements: a converter station and a pair of

ground cables. The converters are voltage source converters, VSC’s. The

outputs from the VSC’s are determined by the control system, which does

not require any communications links between the different converter

stations. Also, they don’t need to rely on the AC network’s ability to keep

the voltage and frequency stable. These feature make it possible to connect

the converters to the points bests suited for the ac system as a whole.

The converter station is designed for a power range of 1-100 MW and for a

dc voltage in the 10-100 kV range. One such station occupies an area of

less than 250 sq. meters (2 700 sq. ft), and consists of ust a few elements:

two containers for the converters and the control system, three small AC

air-core reactors, a simple harmonics filter and some cooling fans.

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The converters are using a set of six valves, two for each phase, equipped

with high power transistors, IGBT (Insulated Gate Bipolar Transistor). The

valves are controlled by a computerized control system by pulse width

modulation, PWM. Since the IGBTs can be switched on or off at will, the

output voltages and currents on the AC side can be controlled precisely.

The control system automatically adjusts the voltage, frequency and flow of

active and reactive power according to the needs of the AC system. The

PWM technology has been tried and tested for two decades in switched

power supplies for electronic equipment as computers. Due to the new, high

power IGBTs, the PWM technology can now be used for high power

applications as electric power transmission. HVDC Light can be used with

regular overhead transmission lines, but it reaches its full potential when

used with a new kind of dc cable. The new HVDC Light cable is an extruded,

single-pole cable. The easiest way of laying this cable is by plowing.

Handling the cable is easy, despite its large power-carrying capacity. It has

a specific weight of just over 1 kg/m. Contrary to the case with AC

transmission; distance is not the factor that determines the line voltage.

The only limit is the cost of the line losses, which may be lowered by

choosing a cable with a conductor with a larger cross section. Thus, the cost

of a pair of dc cables is linear with distance.

A dc cable connection could be more cost efficient than even a medium

distance AC overhead line, or local generating units such as diesel

generators. The converter stations can be used in different grid

configurations. A single station can connect a dc load or generating unit,

such as a photo-voltaic power plant, with an AC grid.

Two converter stations and a pair of cables make a point-to point dc

transmission with AC connections at each end. Three or more converter

stations make up a dc grid that can be connected to one or more points in

the AC grid or to different AC grids. The dc grids can be radial with multi-

drop converters, meshed or a combination of both. In other words, they can

be configured, changed and expanded in much the same way AC grids are.

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3.1 HVDC LIGHT INSTALLATION

HVDC light system mainly consists of transformers, converter units, phase

reactors and filters.

Figure 4: HVDC Light transmission System

The transformers are used to step-up/step-down voltages and the

converters units converts AC to DC and vice versa. HVDC cables are used to

carry currents and the filters are used for filtering unwanted signals.

3.2 HVDC LIGHT CHARACTERISCTICS

An HVDC Light converter is easy to control. The performance during steady

state and transient operation makes it very attractive for the system

planner as well as for the project developer. The benefits are technical,

economical, environmental as well as operational.

The most advantageous are the following:

• Independent control of active and reactive power

• Feeding of power into passive networks (i.e.

network without any generation)

• Power quality control

• Modular compact design, factory pre-tested

• Short delivery times

• Re-locatable/Leasable

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• Unmanned operation

• Robust against grid alterations

3.1.1 Control Of Active & Reactive Power

The control makes it possible to create any phase angle or amplitude, which

can be done almost instantly. This offers the possibility to control both

active and reactive power independently. As a consequence, no reactive

power compensation equipment is needed at the station, only an AC-filter is

installed. While the transmitted active power is kept constant the reactive

power controller can automatically control the voltage in the AC-network.

Reactive power generation and consumption of an HVDC Light converter

can be used for compensating the needs of the connected network within

the rating of a converter. As the rating of the converters is based on

maximum currents and voltages the reactive power capabilities of a

converter can be traded against the active power capability.

3.1.4 Robust Against Grid Alterations

The fact that a Light converter can feed power into a passive network

makes it very robust and can easily accommodate alterations in the AC-grid

to where it is connected. This is a very valuable property in a deregulated

electricity market where AC-network conditions in the future will change

more frequently than in a regulated market.

3.2 THE CABLE SYSTEM

The HVDC Light extruded cable is the outcome of a comprehensive

development program, where space charge accumulation, resistivity and

electrical breakdown strength were identified as the most important

material properties when selecting the insulation system. The selected

material gives cables with high mechanical strength, high flexibility and low

weight. Extruded HVDC Light cables systems in bipolar configuration have

both technical and environmental advantages. The cables are small yet

robust and can be installed by plowing, making the installation fast and

economical.

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3.3 APPLICATIONS

3.3.1 Overhead Lines

In general, it is getting increasingly difficult to build overhead lines.

Overhead lines change the landscape, and the construction of new lines is

often met by public resentment and political resistance. People are often

concerned about the possible health hazards of living close to overhead

lines. In addition, a right-of-way for a high voltage line occupants valuable

land. The process of obtaining permissions for building new overhead lines

is also becoming time-consuming and expensive. Laying an underground

cable is a much easier process than building an overhead line. A cable

doesn’t change the landscape and it doesn’t need a wide right-of-way.

Cables are rarely met with any public opposition, and the electromagnetic

field from a dc cable pair is very low, and also a static field. Usually, the

process of obtaining the rights for laying an underground cable is much

easier, quicker and cheaper than for an overhead line. A pair of HVDC Light

cables can be plowed into the ground. Despite their large power capacity,

they can be put in place with the same equipment as ordinary, AC high

voltage distribution cables. Thus, HVDC Light is ideally suited for feeding

power into growing metropolitan areas from a suburban substation.

3.3.2 Replacing Local Generation

Remote locations often need local generation if they are situated far away

from an AC grid. The distance to the grid makes it technically or

economically unfeasible to connect the area to the main grid. Such remote

locations may be islands, mining areas, gas and oil fields or drilling

platforms. Sometimes the local generators use gas turbines, but diesel

generators are much more common. An HVDC Light cable connection could

be a better choice than building a local power plant based on fossil fuels.

The environmental gains would be substantial, since the power supplied via

the dc cables will be transmitted from efficient power plants in the main AC

grid. Also, the pollution and noise produced when the diesel fuel is

transported will be completely eliminated by an HVDC line, as the need for

frequent maintenance of the diesels. Since the cost of building an HVDC

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Light line is a linear function of the distance, a break-even might be reached

for as short distances as 50-60 kilometers.

3.3.3 Connecting Remote Power Grids

Renewable power sources are often built from scratch, beginning on a small

scale and gradually expanded. Wind turbine farm is the typical case, but

this is also true for photovoltaic power generation. These power sources are

usually located where the conditions are particularly favorable, often far

away from the main AC network. At the beginning, such a slowly expanding

energy resource cannot supply a remote community with enough power. An

HVDC Light link could be an ideal solution in such cases. First, the link could

supply the community with power from the main AC grid, eliminating the

need for local generation. The HVDC Light link could also supply the wind

turbine farm with reactive power for the generators, and keeping the power

frequency stable.

When the power output from the wind generators grows as more units are

added, they may supply the community with a substantial share of its

power needs. When the output exceeds the needs of the community, the

power flow on the HVDC Light link is reversed automatically, and the

surplus power is transmitted to the main AC grid.

3.3.4 Asynchronous Links

Two AC grids, adjacent to each other but running asynchronously with

respect to each other, cannot exchange any power between each other. If

there is a surplus of generating capacity in one of the grids it cannot be

utilized in the other grid. Each of the networks must have its own capacity

of peak power generation, usually in the form of older, inefficient fuel fossil

plants, or diesel or gas turbine units. Thus, peak power generation is often a

source of substantial pollution, and their fuel economy is frequently bad. A

DC link, connecting two such networks, can be used for combining the

generation capacities of both networks. Cheap surplus power from one

network can replace peak power generation in the other. This will result in

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both reduced pollution levels and increased fuel economy. The power

exchange between the networks is also very easy to measure accurately.

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4. SHORT CIRCUIT CONTRIBUTION OF HVDC LIGHT

The HVDC Light transmission system mainly consists of two cables and two

converter stations. Each converter station is composed of a voltage source

converter (VSC) built up with IGBTs, phase reactors, ac filters and

transformer. By using pulse width modulation (PWM), the amplitude and

phase angle (even the frequency) of the converter AC output voltage can be

adjusted simultaneously.

Since the AC side voltage holds two degrees of control freedom,

independent active and reactive power control can be realized. Regarding

the active power control, the feedback control loop can be formulized such

that either tracks the predetermined active power order, or tracks the given

DC voltage reference. This gives two different control modes, i.e., active

power control mode (Pctrl) and DC voltage control mode (Udc ctrl). If one

station is selected to control the power, namely, in Pctrl mode, the other

station should set to control the DC voltage, namely, in Udc ctrl mode.

Regarding the reactive power control, the feedback control loop can be

formulized such that it either tracks the predetermined reactive power

order, or tracks the given AC voltage reference. This also gives two control

modes, i.e., reactive power control mode (Qctrl) and AC voltage control

mode (Uac ctrl). The two control modes can be chosen freely as desired in

each station.

Under the normal operation condition, the VSC can be seen as a voltage

source. However, under abnormal operation conditions, for instance, during

an ac short-circuit fault, the VSC may be seen as a current source, as the

current capacity of the VSC is limited and controllable.

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4.1 INVESTIGATION OF SHORT CIRCUIT CURRENTS

4.1.1 Studied AC System

The studied AC system has a mixture structure in radial and mesh

connection. It includes high, medium and low voltage buses. The AC

transmission lines are modeled with p-link. The loads are constant current

loads. Three types of fault, namely, the close-in fault; the near-by fault and

the distant fault, are applied at bus A, B and C, respectively. A 3-ph close-in

fault results in a voltage reduction of almost 100%, whereas a 3-ph near-by

fault and distant fault result in voltage reduction on CCP bus of about 80%

and 20%, respectively. In the following discussion, the short circuit ratio

(SCR) is defined as the short circuit capacity of the AC system observed at

CCP divided with the power rating of the converter.

Figure 5: SLD of studied AC system

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4.1.2 The Impact of Strength of AC Networks

The possible maximum relative short circuit current increment (∆Imax) is

determined by the short circuit ratio (SCR). Supposing that the ∆Imax is

defined as (1), it is found that the ∆Imax is inversely in proportional to the

SCR as the solid curve shown in Figure 6.

Figure 6: Characteristic showing the impact of AC network strength.

where, Isc is the short-circuit current of the original AC system alone at a 3-

ph fault and I SC_HVDC_L , is the short-circuit current of the AC system with

converter station connected and in operation at the same fault. It should be

noticed that the solid curve in Figure 6 is valid if there is no tap-changer, or

the tap-change is at the position corresponding to the nominal winding

ratio. If there is a tap changer

in transformer, the AC network will observe a different current although the

maximum current of the

converter is a fixed value. Therefore, the maximum possible short circuit

current increment is in the boundary defined by the two dashed curves. AC

networks with SCR equal to 1.85, 3.14 and 12 have been simulated and the

results are also

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shown in figure 6 with black dots.

Different control modes and different operation points may change the

short circuit current contribution from the VSC. However, it will not be

higher than the ∆Imax. For instance, the

short circuit current contribution from the VSC will not exceed 12% if the

SCR is 10 and voltage tap-change range is ± 20%.

4.1.3. The Impact of Control Modes

The current is mainly limited by the impedances of transmission lines and

transformers when a short circuit occurs. Since the impedance of lines and

transformers is dominated by the inductive impedance, the short circuit

current is mainly consisted of reactive current.

Because of that, the choice of different control modes in respect of the

active power control does not give any impact to the short circuit current.

Therefore, the following discussion will focus on the choice between the

control modes Qctrl and Uac ctrl.

It is important to notice that the change of short circuit current and the

variation of bus voltages usually go hand in hand. The increase of short

circuit current, namely, the increase of short circuit capacity, will improve

the voltage stability and minimize the reduction of bus voltage due to faults.

Inversely, the reduction of short circuit current may leads to voltage

instability and voltage collapse during faults, in particular in weak AC

systems. With Uac ctrl control mode, the reactive current generation will be

automatically increased when the AC voltage decreases. Therefore, the

Uacctrl control mode provides the possibility of improving the voltage

stability and minimizing the reduction of bus voltage due to faults. On

contrast, with Qctrl control mode it has the potential risk of getting voltage

instability or voltage collapse during faults if the AC system is weak and no

control protection action is taken. One way to avoid this potential risk is that

the control is automatically switched to Uac ctrl if the AC voltage is detected

out of the specified range (Umin~Umax, for instance, 0.9~1.1 per-unit). The

other way is that the maximum value for the current order should be

decreased with the AC voltage decreasing during faults. If the current from

the VSC is reduced, its contribution to the short circuit current will also be

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reduced. Therefore, with Qctrl control mode the contribution of VSC to the

short circuit current is almost neglectable independent of operation points,

or load level. It will then be only interesting to discuss the Uac ctrl control

mode in respect of different operation points.

4.1.4 The Impact of Operation Points

As it has been discussed, the maximum possible short circuit increment

(∆Imax) due to HVDC Light is determined by the SCR. It will occur if the VSC

is operating at zero active power, namely, it is operating as an SVC or

STATCOM. Figure 7 shows the characteristic of short circuit current

contribution versus the load level. The two dashed curves are the result by

taking into account the transformer winding ratio variation due to the tap-

changer.AC networks with SCR equal to 3.14 has been simulated. For

different load levels the observed short circuit currents, during a 3-phase

close fault, are marked with black dots in Figure 7. It should be noted that

the short circuit current would be also reduced if the current order is also

limited with the Uac ctrl. The black dot with a circle in Fig. 4 shows the

result when the current order is limited to 35% of the rated current during

the AC fault.

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4.1.5 The Impact of Fault Type and Location

If the fault current is evaluated in per unit with the base value equal to the

3-phase fault current at the corresponding fault location and without HVDC

Light connected, it turns out that the impact of the fault location seems to

be insignificant. Under the same load and operation condition, the 1-ph fault

current is usually smaller than the 3-ph fault current. This is because the

average voltage reduction is smaller for 1-phase fault, thereby the required

reactive power generation is smaller during a 1-phase fault. In addition, the

VSC only generates balanced 3-phase currents, even if the AC bus voltage is

unbalanced due to 1-phase faults. As an example, Figure 8 shows 1-phase

and 3-phase fault currents at different locations (bus B and bus C in Figure

5) under the same operation condition (SCR=3.14, P=-0.8 and Uac ctrl).

Currents in plot (a) and (b) have one base value, and currents in plot (c) and

(d) have another base value. Plot (b) shows that the peak value is slightly

higher than 1, which means the short circuit current with HVDC Light is

slightly higher than that without the HVDC Light for the same fault. It should

be noticed that when a close-in short-circuit fault occurs the connected

converter station will only feed the fault current. This implies that the

current during the fault in the rest AC lines will be the same as the original

AC network alone. In other words, the close-in fault isolates the HVDC Light

terminal from the AC network. If it is the circuit breakers in the AC network

to be mainly concerned, this type of fault will be less significant. This is why

that the performed studies do not focus on this type of faults.

4.1.6 Line Current during Faults

It is seen that the contribution from the HVDC Light makes the difference

between the current of health lines and faulted lines larger, which may have

a positive impact in distinguishing the faulted

and health line. When a short circuit occur in the AC network, the sudden

AC bus voltage variation may result in over current to the converter due to

the measurement and control delay. As soon

as the over current in the converter is detected, the protection will trigger a

temporary blocking of converter.. It is obvious that the transient and steady

state current contribution from the HVDC Light is different. Nevertheless, it 30


should be noted that usually the circuit breakers do not react to the over

current spontaneously, and it often has a delay time of about 60 ~100 ms.

Therefore, it is the steady state current during the fault that should be

considered.

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5. CONCLUSION

From detailed analysis it is seen that HVDC system is used for long distance

transmission and its more reliable and best method for power transmission

when compared to ac power transmission.

A comprehensive investigation on the issue regarding the contribution of

HVDC Light to short circuit current has also been performed. The studies

lead to the following conclusions; The HVDC Light, in contrast to the

conventional HVDC which does not contribute any short circuit current, may

contribute some short circuit current. The possible maximum short circuit

current contribution is determined by the SCR. It is inversely in proportional

to the SCR and it occurs when the transmission system is operating at zero

active power. Hence, it is comparable to the STATCOM as long as the

maximum short circuit current contribution is concerned. The amount of

contribution depends on control modes, operation points and control

strategies. With the reactive power control mode, the short circuit current

contribution will be limited due to the current order limit decreasing with

the voltage.

With the AC voltage control mode, the short circuit current contribution will

be increased with the decreasing of active power, if the current order limit

is not changed. If the current order limit is decreasing with voltage, the

short circuit current contribution will be small even if the load level is low.

The contribution to the short circuit current is irrelevant to the fault location

if the fault current is evaluated in per unit with the base value equal to the

3-phase fault current at the corresponding fault location and without HVDC

Light connected. Under the same load and operation condition, the 1-phase

fault current is usually smaller than the 3-phase fault current. Finally, it

should be noticed that in associated with higher short-circuit current the

voltage stability and performance is likely to be improved. If the HVDC Light

contributes a higher short-circuit current, the voltage dip due to distant

fault is possibly reduced and thereby the connected electricity consumers

may suffer less from disturbances.

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6. REFERENCES

1.) DC Transmission based on voltage source converters, Gunnar Asplund, Kjell Eriksson and Kjell Svesson,1997.

2.) The ABCs of HVDC transmission technologies, IEEE Power and Energy Magazine, 2006.

3.) A Course in Electrical Power, J.B. Gupta.

4.) On the Short Circuit Current Contribution of HVDC Light, IEEE , Y. Jiang-Hafner, M. Hyttinen, and B. Paajarvi.

5.) www.wikipedia.org

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High Voltage Direct Current Transmission System Report

Engineering

Transcript of High Voltage Direct Current Transmission System Report