An Efficient System for Bulk Power Transmission

8/4/2019 An Efficient System for Bulk Power Transmission

http://slidepdf.com/reader/full/an-efficient-system-for-bulk-power-transmission 1/10

An Efficient System for Bulk Power Transmission Balachandar.V (09EER013), Dinesh Kumar.G (09EER027)

[email protected]

[email protected]

III BE ±ELECTRICAL & ELECTRONICS ENGINEERING

Kongu engineering college, Perundurai, Erode - 638 052, India.

Abstract: Beginning with a brief historical

perspective on the development of high voltage direct

current (hvdc) transmission systems, this paperpresents an overview of the status of HVDC systems

in the world today. it then reviews the underlyingtechnology of HVDC systems, and discusses the

HVDC systems from a design, construction, operation

and maintenance points of view. the paper thendiscusses the recent developments in HVDC

technologies. the paper also presents an economic and

financial comparison of HVDC system with those of an ac system; and provides a brief review of reference

installations of HVDC systems. the paper concludeswith a brief set of guidelines for choosing HVDC

systems in today¶s electricity system development.

INTRODUCTION:

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 underwater power cables, HVDC avoids the heavy

currents required by the cable capacitance. For

shorter distances, the higher cost of DC conversion

equipment compared to an AC system may still be

warranted, due to other benefits of direct current

links. HVDC allows power transmission between

unsynchronized AC distribution systems, and can

increase system stability by preventing cascading

failures from propagating from one part of a wider

power transmission grid to another.

THE HVDC TECHNOLOGY:

The fundamental process that occurs in an HVDCsystem is the conversion of electrical current from

AC to DC (rectifier) at the transmitting end and

from DC to AC (inverter) at the receiving end.

There are three ways of achieving conversion

NATURAL COMMUTATED CONVERTERS

(NCC):

Natural commutated converters are most

used in the HVDC systems as of today. The

component that enables this conversion process is

the thyristor, which is a controllable semiconductor

that can carry very high currents (4000 A) and is



able to block very high voltages (up to 10 kV). By

means of connecting the thyristors in series it is

possible to build up a thyristor valve, which is able

to operate at very high voltages (several hundred of

kV).The thyristor valve is operated at net frequency

(50 Hz or 60 Hz) and by means of a control angle itis possible to change the DC voltage level of the

bridge. This ability is the way by which thetransmitted power is controlled rapidly and

efficiently.

CAPACITOR COMMUTATED CONVERTERS

(CCC):

An improvement in the thyristor-based

Commutation, the CCC concept is characterised by

the use of commutation capacitors inserted in series

between the converter transformers and the

thyristors valves. The commutation capacitors

improve the commutation failure performance of

the converters when connected to weak networks.

FORCED COMMUTATED CONVERTERS(FCC):

This type of converters introduces a

spectrum of advantages, e.g. feed of passive

networks (without generation), independent control

of active and reactive power, power quality. The

valves of these converters are built up with

semiconductors with the ability not only to turn-on but also to turn-off. They are known as VSC

(Voltage Source Converters). Two types of

semiconductors are normally used in the voltage

source converters: the GTO (Gate Turn-Off

Thyristor) or the IGBT (Insulated Gate Bipolar

Transistor). Both of them have been in frequent use

in industrial applications since early eighties. The

VSC commutates with high frequency (not with the

net frequency). The operation of the converter

is achieved by Pulse Width Modulation (PWM).

With PWM it is possible to create any phase

Angle and/or amplitude (up to a certain limit) bychanging the PWM pattern, which can be one

almost instantaneously. Thus, PWM offers the

possibility to control both active and reactive

power independently. This makes the PWM

Voltage Source Converter a close to ideal

component in the transmission network. From a

transmission network viewpoint, it acts as a motor

or generator without mass that can control active

and reactive power almost instantaneously.

THE COMPONENTS OF AN HVDCTRANSMISSION SYSTEM:

To assist the designers of transmission

systems, the components that comprise the HVDC

system, and the options available in these

components, are presented and discussed. The three

main elements of an HVDC system are:

A. THE CONVERTER STATION:

The converter stations at each end arereplica¶s of each other and therefore consists

of all the needed equipment for going from AC to

DC or vice versa. The main component of a

converter station are:

1. THYRISTOR VALVES:

The thyristor valves can be build-up in different

ways depending on the application andmanufacturer. However, the most common way of

arranging the thyristor valves is in a twelve-pulse

group with three quadruple valves. Each single

thyristor valve consists of a certain amount of series connected thyristors with their auxiliary

circuits. All communication between the control

equipment at earth potential and each thyristor at

high potential, is done with fibre optics.

2. VSC VALVES:

The VSC converter consists of two level

or multilevel converter, phase-reactors and AC

filters. Each single valve in the converter bridge is

built up with a certain number of seriesconnectedIGBTs together with their auxiliary electronics.VSC valves, control equipment and cooling

equipment would be in enclosures (such as

standard shipping containers) which make

transport and installation very easy. All modern

HVDC valves are water-cooled and air insulated.

3. TRANSFORMERS:

The converter transformers adapt the AC

voltage level to the DC voltage level and they



contribute to the commutation reactance. Usually

they are of the single phase three winding type, but

depending on the transportation requirements and

the rated power, they can be arranged in other

ways.

B. FILTERS:

1. AC Filters and Capacitor Banks:On the AC side of a 12-pulse HVDC

converter, current harmonics of the order of 11, 13,

23, 25 and higher are generated. Filters are installed

in order to limit the amount of harmonics to the

level required by the network.. In the conversion

process the converter consumes reactive power

which is compensated in part by the filter banks

and the rest by capacitor banks.

In the case of the CCC the reactive power is

compensated by the series capacitors installed in

series between the converter valves and the

converter transformer. The elimination of switched

reactive power compensation equipment simplify

the AC switchyard and minimise the number of circuit-breakers needed, which will reduce the area

required for an HVDC station built with CCC.

With VSC converters there is no need to

compensate any reactive power consumed by the

converter itself and the current harmonics on the

AC side are related directly to the PWM

frequency. Therefore the amount of filters in thistype of converters is reduced dramatically

compared with natural commutated converters.

DC filters: HVDC converters create harmonics in

all operational modes. Such harmonics can

create disturbances in telecommunication systems.

Therefore, specially designed DC filters are

used in order to reduce the disturbances. Usually no

filters are needed for pure cable transmissions

as well as for the Back-to-Back HVDC stations.

However, it is necessary to install DC filters if an

OH line is used in part or all the transmission

system The filters needed to take care of theharmonics generated on the DC end, are usually

considerably smaller and less expensive than the

filters on the AC side. The modern DC filters are

the Active

2. DC filters:

In these filters the passive part is reduced

to a minimum and modern power electronics is

used to measure, invert and re-inject the harmonics,

thus rendering the filtering very effective.

C. TRANSMISSION MEDIUM:

For bulk power transmission over land, the

most frequent transmission medium used is the

overhead line. This overhead line is normally

bipolar, i.e. two conductors with different polarity.

HVDC cables are normally used for submarine

transmission. The most common types of cables are

the solid and the oil -filled ones. The solid type is in

many cases the most economic one. Its insulation

consists of paper tapes impregnated with a high

viscosity oil. No length limitation exists for this

type and designs are today available for depths of

about 1000 m. The self ±contained oil-filled cable

is completely filled with a low viscosity oil andalways works under pressure. The maximum length

for this cable type seems to be around 60 km.The development of new power cable technologies

has accelerated in recent years and today a new

HVDC cable is available for HVDC underground

or submarine power transmissions. This new

HVDC cable is made of extruded polyethylene, and

is used in VSC based HVDC systems.

Design, Construction, Operation and Maintenance

considerations:

In general, the basic parameters such as

power to be transmitted, distance of transmission,

voltage levels, temporary and continuous overload,

status of the network on the receiving end,environmental requirements etc. are required to

initiate a design of an HVDC system. For tendering

purposes a conceptual design is done following a

technical specification or in close collaboration

between the manufacturer and the customer. The

final design and specifications are in fact the

result of the tendering and negotiations with themanufactures/suppliers. It is recommended that a

turnkey approach be chosen to contract execution,

which is the practice even in developed countries.

In terms of construction, it can take from three

years for thyristor-based large HVDC systems, to

just one year for VSC based HVDC systems to go

from contract date to commissioning. HVDC links

can be operated remotely, in view of the

semiconductor and microprocessor based control

systems included. There are some existing

installations in operation completely unmanned.

Moreover, modern HVDC systems are designed tooperate unmanned. This feature is particularly

important in situations or countries where skilled

people are few, and these few people can operate

several HVDC links from one central location.

Maintenance of HVDC systems is comparable to

those of high voltage AC systems. The high voltage

equipment in converter stations is comparable to

the corresponding equipment in AC substations,

and maintenance can be executed in the same way.

Maintenance will focus on: AC and DC filters,

smoothing reactors, wall bushings, valve-coolingequipment, thyristor valves. In all the above,

adequate training and support is provided by the

supplier during the installation, commissioning and

initial operation period. Normal routine

maintenance is recommended to be one week per

year. The newer systems can even go for two years

before requiring maintenance. In fact in a bipolar

system, one pole at a time is stopped during the



time required for the maintenance, and the other

pole can normally continue to operate and

depending on the in-built overload capacity it can

take a part of the load of the pole under

maintenance. In addition, preventive maintenance

shall be pursued so that the plants and equipmentwill achieve optimally balanced availability with

regard to the costs of maintenance, operatingdisturbances and planned outages. As a guideline

value, the aim shall be to achieve an availability of

98 % according to Cigrè protocol 14-97. While

HVDC systems may only need a few skilled staff

for operation and maintenance, several factors

influence the number of staff needed at a station.

These factors are: local routines and regulations,

working conditions, union requirements, safety

regulations, and other local rules can separately or

together affect the total number of personnel

required for the type of installed equipment.

COST STRUCTURE:

The cost of an HVDC transmission system

depends on many factors, such as power capacity to

be transmitted, type of transmission medium,

environmental conditions and other safety,

regulatory requirements etc. Even when these areavailable, the options available for optimal design

(different commutation techniques, variety of

filters, transformers etc.) render it is difficult to

give a cost figure for an HVDC system.

Nevertheless, a typical cost structure for the

converter stations could be as follows:

As a guidance, an example showing the price

variation for an AC transmission compared with an

HVDC transmission for 2000 MW is presented

below.

The choice of DC transmission voltage level has a

direct impact on the total installation cost. At the

design stage an optimisation is done finding out the

optimum DC voltage from investment and losses

point of

View. The costs of losses are also very important -in the evaluation of losses the energy cost and the

time horizon for utilisation of the transmission have

to be taken into account. Finally the depreciation

period and desired rate of return (or discount rate)

should be considered. Therefore, to estimate the

costs of an HVDC system, it is recommended that

life cycle cost analysis is undertaken. Two different

comparisons are needed to highlight the cost

comparison between high voltage AC and HVDC

systems

1. Between thyristor based HVDC systems

and a high voltage AC transmission

System

2. Between a VSC based HVDC system; anAC system and a local generation source.

THYRISTOR BASED HVDC SYSTEM VERSUS

HIGH VOLTAGE AC SYSTEM:

The investment costs for HVDC converter

stations are higher than for high voltage AC

substations. On the other hand, the costs of transmission medium (overhead lines and cables),

land acquisition/right-of-way costs are lower in the



HVDC case. Moreover, the operation and

maintenance costs are lower in the HVDC case.

Initial loss levels are higher in the HVDC system,

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

levels increase with distance in a high voltage AC

system. The following picture shows the cost breakdown (shown with and without considering

losses).

The breakeven distance depends on

several factors, as transmission medium (cable or

OH line), different local aspects (permits, cost of

local labour etc.). When comparing high voltage

AC with HVDC transmission, it is important to

compare a bipolar HVDC transmission to a double-

circuit high voltage AC transmission, especially

when availability and reliability is considered.

VSC based HVDC system versus an AC system or

a local generation source: VSC based HVDC

systems cater to the small power applications (up to200MW) and relatively shorter distances (hundred

of km) segment of the power transmission

spectrum. The graph below shows that, the VSC

based HVDC system is the better alternative

economically when compared to either an high

voltage AC system or a generationsource local to

the load centre (e.g., diesel generator).

However, the break-even distance and

power transfer level criteria and the comparative

cost information should be taken in the proper

perspective, because of the following reasons:

01. in the present (and future) industry environment

of liberalised competitive markets and heightened

efforts to conserve the environment. In such an

environment, the alternative for a transmission

system is an in-situ gas-fired combined cycle

power plant, not necessarily an option between an

AC transmission and a HVDC one.

02. Second, the system prices for both AC and

HVDC have varied widely even for a given level

of power transfer. For example, several different

levels of project costs have been incurred for a HVDC system with a power transfer capacity of

600 MW. What this shows therefore is that,

in addition to the criteria mentioned above (power

levels, distance, transmission medium,

environmental conditions etc.), the market

conditions at the time of the project is a critical

Factor, perhaps more so than the numerical

comparisons between the costs of an AC or DC

System.



03. Third, technological developments have tended

to push HVDC system costs downward, while the

environmental considerations have resulted in

pushing up the high voltage AC system costs.

Therefore, for the purposes early stage feasibility

analysis of transmission system type, it is perhaps better to consider HVDC and high voltage AC

systems as equal cost alternatives.

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.[15] 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:

y Undersea cables, where high capacitance

causes additional AC losses. (e.g., 250 km

Baltic Cable between Sweden and

Germany,[16]

the 600 km NorNed cable

between Norway and the Netherlands, and290 km Basslink between the Australian

mainland and Tasmania[17])

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

'taps', for example, in remote areas

y Increasing the capacity of an existing

power grid in situations where additional

wires are difficult or expensive to install

y Power transmission and stabilization

between unsynchronised AC distribution

systems

y Connecting a remote generating plant to

the distribution grid, for example Nelson

River Bipoley Stabilizing a predominantly AC power-

grid, without increasing prospective short

circuit current

y 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

y Facilitate power transmission between

different countries that use AC at differing

voltages and/or frequencies

y Synchronize AC produced by renewable

energy sources

Long undersea / underground high voltage cables

have a high electrical capacitance, since theconductors 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.

However, when 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 ACsystem 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 operatorsto contemplate wider use of HVDC technology for

its stability benefits alone.

DISADVANTAGES:

The disadvantages of HVDC are in conversion,

switching, control, availability and maintenance.

1. 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%. [18]

2. The required static inverters are expensive and

have limited overload capacity. At smaller

transmission distances the losses in the static

inverters may be bigger than in an AC transmission

line. The cost of the inverters may not be offset byreductions in line construction cost and lower line

loss. With two exceptions, all former mercury

rectifiers worldwide have been dismantled or

replaced by thyristor units.

3. 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 toogreat to allow reliable switching.

4. 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.

CORONA DISCHARGE:

Corona discharge is the creation of ions in

a fluid (such as 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 asoxides 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.

APPLICATIONS:

01. Three different dynamics - technology

development, deregulation of electricity

industry around the world, and a quantum leap in

efforts to conserve the environment - are

demanding a change in thinking that could make

HVDC systems the preferred alternative to high

voltage AC systems in many other situations aswell. To elaborate:



02. New technologies, such as the VSC based

HVDC systems, and the new ext

uded polyethylene

DC cables, have made it possi ble for HVDC to

become economic at lower power levels (up to

200 M¡

¢ and over a transmission distance of just

60 km.03. Li berali £ ation has brought other demands on

the power infrastructure overall. Transmission isnow a contracted service, and there is very little

room for deviation from contracted technical

and economic norms. HVDC provides much better

control of the power link and is therefore a better

way for providing contractual transmission

services.

04. Li berali £ ation has brought on the phenomenon

of trading to the electr icity sector, which would

mean bi ¤ directional power transfers, depending on

market conditions. HVDC systems enable the bi ¤

directional power f lows, which is not possi ble with

AC systems (two parallel systems would be

required).

05. In the past, when the transmission service was par t of a government owned, ver tically integrated

utility, the land acquisition and obtaining r ights-of-

way was relatively easier, and very of ten was done

under the pr inci ple of ³Eminent Domain´ of the

State. With li berali £ ation, transmission service

provision is by and large in the domain of

corporati £ ed, sometimes pr ivati £ ed, entities. Landacquisition and/or obtaining r ights-of-way is now a

signif icant por tion of the pro ject ¥ s costs. Once

these costs are included in their entirety in the

economical analysis of HVDC versus AC

alternatives, it would be seen that HVDC is much

more economical in this regard, since it requires

much less land/r ight-of-way for a given level

of power.

06. In an environmentally sensitive areas, such as

national parks and protected sanctuar ies, the

lower foot pr int of HVDC transmission systems

becomes the only feasi ble way to build a power link.

07. The functional specif ications could lay down

the power capacity, distance, availability and

reliability requirements; and last but not least, the

environmental conditions.

08. The bidders should be allowed to bid either an

HVDC solution or an AC solution; and the best

option chosen. It is quite conceivable that with

changed circumstances in the electr icity industry,

the technological developments, and environmental

considerations, HVDC would be the preferredalternative in many more transmission pro jects.

CONFIGUR ATIONS:

1. MONOPOLE AND EAR TH R ETUR N:

MONOPOLAR CONVER TER STATION 600MW

± 450 KV DC

In a common conf iguration, calledmonopole, one of the terminals of the rectif ier is

connected to ear th ground. The other terminal, at a

potential high above or below ground, is connected

to a transmission line. The ear thed terminal may be

connected to the corresponding connection at the

inver ting station by means of a second conductor.If no metallic conductor is installed, current f lows

in the ear th between the ear th electrodes at the two

stations. Therefore it is a type of single wire ear th

return. The issues surrounding ear th-return current

include:

y Electrochemical corrosion of long bur ied

metal ob jects such as pi pelines

y Underwater ear th-return electrodes in

seawater may produce chlor ine or

otherwise affect water chemistry.

y An unbalanced current path may result in

a net magnetic f ield, which can affect

magnetic navigational compasses for shi ps 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 conver ters is connected to ear th, 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.[25]

If underground or

underwater cables are used, the typical value is

600 MW.

Most monopolar systems are designed for future bi polar expansion. Transmission line towers

may be designed to carry two conductors, even if

only one is used initially for the monopoletransmission system. The second conductor is

either unused, used as electrode line or connected

in parallel with the other (as in case of Baltic-

Cable).

2. BIPOLAR:

Block diagram of a bi polar system that also has an

ear th return

In bi polar transmission a pair of conductors isused, each at a high potential with respect to

ground, in opposite polar ity. Since these conductors

must be insulated for the full voltage, transmission

line cost is higher than a monopole with a return

conductor. However, there are a number of

advantages to bi polar transmission which can make

it the attractive option.

y Under normal load, negligi ble ear th-

current f lows, as in the case of monopolar

transmission with a metallic ear th-return.

This reduces ear th return loss and

environmental effects.

y When a fault develops in a line, with ear threturn electrodes installed at each end of

the line, approximately half the rated

power can continue to f low using the ear thas a return path, operating in monopolar

mode.

y Since for a given total power rating each

conductor of a bi polar line carr ies only

half the current of monopolar lines, the

cost of the second conductor is reduced

compared to a monopolar line of the same

rating.

y In very adverse terrain, the secondconductor may be carr ied on an

independent set of transmission towers, so

that some power may continue to be

transmitted even if one line is damaged.

A bi polar system may also be installed with a

metallic ear th return conductor. Bi polar systems

may carry as much as 3,200 MW at voltages of +/-

600 kV. Submar ine cable installations initiallycommissioned as a monopole may be upgraded

with additional cables and operated as a bi pole.

A block diagram of a bi polar HVDC

transmission system, between two stations

designated A and B. AC ± represents an alternating

current network CON ± represents a conver ter

valve, either rectif ier or inver ter, TR represents a

power transformer, DCTL is the direct-current

transmission line conductor, DCL is a direct-

current f ilter inductor, BP represents a bypass

switch, and PM represent power factor correction

and harmonic f ilter networks required at both ends

of the link. The DC transmission line may be veryshor t in a back-to-back link, or extend hundreds of

miles (km) overhead, underground or underwater.One conductor of the DC line may be replaced by

connections to ear th ground.

A bi polar scheme can be implemented sothat the polar ity of one or both poles can be

changed. This allows the operation as two parallel

monopoles. If one conductor fails, transmission can

still continue at reduced capacity. Losses may

increase if ground electrodes and lines are not

designed for the extra current in this mode. To

reduce losses in this case, intermediate switching

stations may be installed, at which line segments

can be switched off or paralleli ¦ ed. This was done

at Inga±Shaba HVDC.

3. BACK TO BACK :

A back-to-back station (or B2B for shor t) is a

plant in which both static inver ters and rectif iers

are in the same area, usually in the same building.

The length of the direct current line is kept as shor t

as possi ble. HVDC back-to-back stations are used

for



y 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)

y coupling two networks of the samenominal frequency but no fixed phase

relationship (as until 1995/96 inEtzenricht, Dürnrohr, Vienna, and the

Vyborg HVDC scheme).

y 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.

4. SYSTEMS WITH TRANSMISSION

LINES:

The most common configuration of an HVDC link

is two inverter/rectifier stations connected by an

overhead power line. This is also a configuration

commonly used in connecting unsynchronised

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 largecapacity 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. [26]

5. TRIPOLE: CURRENT-MODULATING

CONTROL:

A scheme patented in 2004 (Current modulation of

direct current transmission lines) is intended for

conversion of existing AC transmission lines to

HVDC. Two of the three circuit conductors areoperated as a bipole. The third conductor is used as

a parallel monopole, equipped with reversing

valves (or parallel valves connected in reverse

polarity). The parallel monopole periodically

relieves current from one pole or the other,

switching polarity over a span of several minutes.

The bipole conductors would be loaded to either

1.37 or 0.37 of their thermal limit, with the parallel

monopole always carrying +/- 1 times its thermal

limit current. The combined RMS heating effect isas if each of the conductors is always carrying 1.0

of its rated current. This allows heavier currents to be carried by the bipole conductors, and full use of

the installed third conductor for energy

transmission. High currents can be circulated

through the line conductors even when load

demand is low, for removal of ice.

As of 2005, no tri-pole conversions are in

operation, although a transmission line in India has

been converted to bipole HVDC. Cross-Skagerrak

consists of 3 poles, from which 2 are switched in

parallel and the third uses an opposite polarity with

a higher transmission voltage. A similar

arrangement is HVDC Inter-Island, but it consists

of 2 parallel-switched inverters feeding in the same pole and a third one with opposite polarity and

higher operation voltage.

CONCLUSION:

Thus the various advantages of the HVDC system

over the normal transmission lines , its

temperament and characteristics were discussed in

this paper. The proper implementation of this

system ensures lesser loss of energy during

transmission.

Documents

An Efficient System for Bulk Power Transmission