seminar- hvdc technology and short circuit contribution of hvdc light

Seminar report on

HVDC TECHNOLOGY AND SHORT CIRCUIT CONTRIBUTION OF HVDC LIGHT

ByJijo Francis

DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING

FEDERAL INSTITUTE OF SCIENCE AND TECHNOLOGY (FISAT)

MOOKKANNOOR P O,

ANGAMALY-683577,

Affiliated to

MAHATMA GANDHI UNIVERSITY

2011

FEDERAL INSTITUTE OF SCIENCE AND TECHNOLOGY (FISAT)

Mookkannoor P O, Angamaly-683577.

Affiliated to

MAHATMA GANDHI UNIVERSITY, Kottayam- 686560

DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING

CERTIFICATE

This is to certify that this report entitled “HVDC TECHNOLOGY AND SHORT

CIRCUIT CONTRIBUTION OF HVDC LIGHT” is a bonafide report of the seminar

presented during 7th semester by JIJO FRANCIS (57164) in partial fulfillment of the

requirements for the award of the degree of Bachelor of Technology (B.Tech) in Electrical

& Electronics Engineering during the academic year 2010-2011.

Head of the Department

Date:

Place: Mookkannoor

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 17

Chapter 4 SHORT CIRCUIT CONTRIBUTION OF HVDC LIGHT 24

Chapter 5 CONCLUSION 31

Chapter 6 REFERENCES 32


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.

Electrical & Electronics Engineering, FISAT 1


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).



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.



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.



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



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 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 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 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.



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.


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


These effects can be eliminated with installation of a metallic return conductor between the

two ends of the monopolar transmission line. Since one terminal of the converters is

connected to earth, the return conductor need not be insulated for the full transmission

voltage which makes it less costly than the high-voltage conductor.

Use of a metallic return conductor is decided based on economic, technical and

environmental factors. Modern monopolar systems for pure overhead lines carry typically

1,500 MW. If underground or underwater cables are used, the typical value is 600 MW.

Most monopolar systems are designed for future bipolar expansion. Transmission line

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.


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


However, there are a number of advantages to bipolar transmission which can make it the

attractive option.

Under normal load, negligible earth-current flows, as in the case of monopolar

transmission with a metallic earth-return. This reduces earth return loss and

environmental effects.

When a fault develops in a line, with earth return electrodes installed at each end of

the line, approximately half the rated power can continue to flow using the earth as

a return path, operating in monopolar mode.

Since for a given total power rating each conductor of a bipolar line carries only

half the current of monopolar lines, the cost of the second conductor is reduced

compared to a monopolar line of the same rating.

In very adverse terrain, the second conductor may be carried on an independent set

of transmission towers, so that some power may continue to be transmitted even if

one line is damaged.

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).

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.

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.

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. Many

two level converter bridges can be assembled with appropriate harmonic cancellation

properties in order to generate acceptable ac system voltage wave shapes. 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.



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.

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.

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

• 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.



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 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 both reduced pollution levels and increased fuel economy. The power exchange between the networks is also very easy to measure accurately.



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 DCvoltage 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.

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 voltagereduction 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

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 changerin transformer, the AC network will observe a different current although the maximum current of theconverter 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 alsoshown 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, theshort 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 tofaults. 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 reduced. Therefore, with Qctrl control mode the contribution of VSC to the short circuit current is almost neglectableindependent 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.



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 faultcurrent 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 isunbalanced 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 faultoccurs 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. Inother 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.




Figure 8: Different fault currents in per unit of the corresponding 3-phase fault current without HVDC Light.

(a): 1-ph close fault current.(b): 3-ph close fault current.(c): 1-ph distant fault current.(d): 3-ph distant fault current.

Figure 9: AC voltage and fault current with different control strategies.

(a): AC voltage measured at CCP.(b): Case 1 – with Uac ctrl and no change on current order limit.(c): Case 2 – with Uac ctrl and current order limit depending on voltage.(d): Case 3 – with Qctrl and current order limit depending on voltage.


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 faultedand 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 soonas 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 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.



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; TheHVDC 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 circuitcurrent 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 iscomparable 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 willbe 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.



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.) Wikipedia & other web resources.


seminar- hvdc technology and short circuit contribution of hvdc light

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