TRAVANCORE ENGINEERING COLLEGE

KOLLAM-691516

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

SEMINAR REPORT

ON

HVDC TECHNOLOGY AND SHORT CIRCUIT CONTRIBUTION OF HVDC LIGHT

SUBMITTED BY:-

PREETHA S

YEAR 2010

TRAVANCORE ENGINEERING COLLEGE

KOLLAM-691516

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

CERTIFICATECERTIFICATE

Certified that this report titled “HVDC TECHNOLOGY AND SHORT CIRCUIT CONTRIBUTION OF HVDC LIGHT” is the bonafide record of the work done by

PREETHA S

……………………………………………………………………………………………………

during final year, towards the partial fulfillment of the requirement for the award of the B.Tech Degree in Electrical and Electronics Engineering of the Kerala University.

University Reg.No. DATE:

Staff in charge Head of the Department

ABSTRACT

This paper aims at exploring the HVDC transmission systems and also presents a

comprehensive investigation on one of the concerned issues, which is the contribution of

HVDC Light™ to short circuit currents. Different AC network conditions, load conditions

and fault types are considered under different operation conditions and control modes.

A comprehensive investigation on the issue regarding the contribution of HVDC

Light™ to short circuit current has been performed. The studies lead to the following

conclusions. The HVDC Light™, in contrast to the conventional HVDC that 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.

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. 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-ph fault current at the corresponding fault location and

without HVDC Light™ connected.

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.

CONTENTS

LIST OF FIGURES iii

1.INTRODUCTION 1

2.HVDC SYSTEM OVERVIEW 2

3. MOTIVATIONS OF HVDC TRANSMISSION 3

3.1 TRANSMISSION LINE DELIVERY CAPABILITY 4

3.2 HIGHLIGHTS FROM THE HVDC HISTORY 4

3.3 ARGUMENTS FAVOURING HVDC 5

4. COMPONENTS OF HVDC TRANSMISSION SYSTEM 7

4.1 SUBSTATION CONFIGURATION 8

5.ADVANTAGES AND DISADVANTAGES OF HVDC TRANSMISSION 10

5.1 ADVANTAGES 10

5.2 DISADVANTAGES 12

6 .ECONOMICAL AND ENVIRONMENTAL CONSIDERATIONS 13

6.1 ECONOMIC CONSIDERATIONS 13

6.2 ENVIRONMENTAL CONSIDERATIONS 13

7. HVDC APPLICATIONS 14

8. HVDC LIGHT TECHNOLOGY 16

8.1 HVDC LIGHT TRANSMISSION SYSTEM 16

8.2 ADVANTAGES 17

9 .SHORT CIRCUIT CONTRIBUTION OF HVDC LIGHT 18

9.1 STUDIED AC SYSTEMS 18

9.2 IMPACT OF STRENGTH OF AC NETWORKS 18

9.3 THE IMPACT OF CONTROL MODES 19

9.4 THE IMPACT OF OPERATION POINTS 19

10.CONCLUSION 20

11.REFERENCES 21

LIST OF FIGURES

SL NO: NAME OF FIGURE PAGE NO:

1. SHEMATIC OF GENERAL HVDC 2

2. COST/DISTANCE OF AC vs DC 6

3. HVDC SYSTEM COMPONENTS 7

4. HVDC SUBSTATION CONFIGURATION 9

5. LAYOUT OF HVDC LIGHT SUBSTATION 17

6. IMPACT OF AC NETWORK STRENGTH 18

7. IMPACT OF LOAD LEVELS 19

1. INTRODUCTION

Competition in the electricity power industry, coupled with continued load

growth requires that the existing transmission system assets are utilized more effectively

and some times closer to their technical limits. As the existing AC lines become loaded

closer to their thermal capacity with increasing losses and reduced power quality we face

the risk of declining network stability. One solution would be to simply build new, more

powerful AC lines.

But, it is getting increasingly difficult to obtain permits to build new high

voltage Overhead transmission lines, the right-of-way occupies valuable land. An

overhead line change the landscape, causes public resentment and are often met by

political resistance. People are increasingly concerned about the possible health hazards

of living close to overhead lines.

There are many examples today of public agitation against overhead power lines

and the call for them to be buried. Media reports which link living close to power lines

with higher cancer risks and leukaemia in children don’t help the situation. On the other

hand laying an underground cable is an 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 rarely

meet with public opposition. There are technical constraints, which limit the distance of

traditional AC underground cables to around 80km.

And, even though the cost of laying AC cables is rapidly reducing it still costs

more than equivalent over head lines.

Currently there is little incentive for putting high voltage lines underground

particularly when the Network Service provider is predominantly driven by cost to

provide performance-based transmission services at a competitive price. So what is the

solution?

HVDC Light technology has the potential to play an important role in achieving

this solution. It provides improved power quality and power flow control as well as

Introducing extruded DC-cables which have no technical limit to distance which can be

installed, and can provide an alternative to overhead lines particularly when the total

capital and environmental costs are considered.

2. HVDC SYSTEM OVERVIEW

The HVDC technology is used in transmission systems to transmit electric bulk power

over long distances by cable or overhead lines. It is also used to interconnect asynchronous

AC systems having the same or different frequency. Figure 2.1 shows a simplified schematic

picture of an HVDC system, with the basic principle of transferring electric energy from one

AC system or node to another, in any direction. The system consists of three blocks: the two

converter stations and the DC line. Within each station block there are several components

involved in the conversion of AC to DC and vice versa.

Fig 2.1 Schematic of the overall system perspective of a general HVDC system, transferring electric energy

from one AC system or node to the other,in any direction

The traditional HVDC system is built with line commutated current source converters,

based on thyristor valves. The operation of this converter requires a voltage source like

synchronous generators or synchronous condensers in the AC network at both ends. The

current commutated converters can not supply power to an AC system which has no local

generation. The control of this system requires fast communication channels between the two

stations.

3.MOTIVATIONS OF HVDC TRANSMISSION

The question is often asked, “Why use d.c. transmission?” One response is that losses

are lower, but this is not correct. The level of losses is designed into a transmission system

and is regulated by the size of conductor selected. D.c. and a.c. 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 d.c. transmission in preference to a.c. transmission, it is generally by

economic choice driven by one of the following reasons:

1. An overhead d.c. transmission line with its towers can be designed to be less costly

per unit of length than an equivalent a.c. line designed to transmit the same level of electric

power. However the d.c. converter stations at each end are more costly than the terminating

stations of an a.c. line and so there is a breakeven distance above which the total cost of d.c.

transmission is less than its a.c. transmission alternative.The d.c. transmission line can have a

lower visual profile than an equivalent a.c. line and so contributes to a lower environmental

impact. There are other environmental advantages to a d.c. transmission line through the

electric and magnetic fields being d.c. 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 a.c. cable systems

exceeding 50 km but d.c. 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 a.c. 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 a.c. methods in order to exchange electric power

between them. However, if a d.c. converter station is located in each system with an

interconnecting d.c. link between them, it is possible to transfer the required power flow even

though the a.c. systems so connected remain asynchronous.

3.1 TRANSMISSION LINE DELIVERY CAPABILITY

3.1.1 AC line distance effects:

Intermediate switching stations, e.g. every ~250 mi maximum

Lower stability limits (voltage, angle)

Increase stability limits & mitigate parallel flow with FACTS: SVC & SC

Higher reactive demand with load

Higher charging at light load

Parallel flow issues more prevalent

Thermal limit remains the same

3.1.2 DC line distance effects:

No distance effect on stability (voltage, angle)

No need for intermediate stations

No parallel flow issues due to control

Minor change in short circuit levels

No increase in reactive power demand

3.2 HIGHLIGHTS FROM THE HVDC HISTORY

The Transmission and Distribution of Electrical Energy started with direct current.In

1882,a 50 km long 2 KV DC line was built between Miesbach and Munich in Germany.At

that time conversion between reasonable consumer voltages and DC transmission voltages

could only be realised by means of rotating DC machines.

In an AC system voltage conversion is simple.An AC transformer allows high power

levels and high insulation levels within one unit,and has low losses.It is a relatively simple

device,which requires little maintenance.Further, a three-phase synchronous generator is

superior to a DC generator in every respect.For these reasons,AC technology was introduced

at a very early stage in the development of electrical power systems.It was soon accepted as

the only feasible technology for generation,transmission and distribution of electrical energy.

However, high-voltage AC transmission links have disadvantages, which may compel

a change to DC technology:

Inductive and Capacitive elements of overhead lines and cables put limits to the

transmission capacity and the transmission distance of AC transmission links.

This limitaiton is of particular significance for cables.Depending on the required

transmission capacity,the system frequency and loss evaluation,the achievable

transmission distance for an AC cable wil be in the range of 40 to 100 km.It will

mainly be limited by the charging current.

Direct connection between two AC systems with different frequencies is not possible.

Direct connection between two AC systems with the same frequency or a new

connection within a meshed grid may be impossible because of system instability,too

high short-circuit levels or undesirable power flow scenarios.

3.3 ARGUMENTS FAVOURING HVDC:

The most common arguments favouring HVDC are:

1) Investment cost. A HVDC transmission line costs less than an AC line for the same

transmission capacity. However, the terminal stations are more expensive in the HVDC case

due to the fact that they must perform the conversion from AC to DC and vice versa. 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

Above a certain distance, the so called "break-even distance", the HVDC alternative

will always give the lowest cost. The break-even-distance is much smaller for submarine

cables (typically about 50km) than for an overhead line transmission. The distance depends

on several factors, as transmission medium, different local aspects (permits, cost of local

labour etc.) and an analysis must be made for each individual case (Fig. 2.1).

2) Long distance water crossing. In a long AC cable transmission, the reactive power flow

due to the large cable capacitance will limit the maximum transmission distance. With HVDC

there is no such limitation, why, for long cable links, HVDC is the only viable technical

alternative.

3) Lower losses. An optimized HVDC transmission line has lower losses than AC lines for

the same power capacity. The losses in the converter stations have of course to be added, but

since they are only about 0.6% of the transmitted power in each station, the totalHVDC

transmission losses come out lower than the AC losses in practically all cases. HVDC cables

also have lower losses than AC cables.

4) Asynchronous connection. It is sometimes difficult or impossible to connect two AC

networks due to stability reasons. In such cases HVDC is the only way to make an exchange

of power between the two networks possible. There are also HVDC links between networks

with different nominal frequencies (50 and 60 Hz) in Japan and South America.

5) Controllability. One of the fundamental advantages with HVDC is that it is very easy to

control the active power in the link

6) Limit short circuit currents. A HVDC transmission does not contribute to the short circuit

current of the interconnected AC system.

7) Environment. Improved energy transmission possibilities contribute to a more efficient

utilization of existing power plants. The land coverage and the associated right-of-way cost

for a HVDC overhead transmission line is not as high as for an AC line.This reduces the

visual impact. It is also possible to increase the power transmission capacity for existing

rights of way. There are, however, some environmental issues which must be considered for

the converter stations, such as: audible noise, visual impact, electromagnetic compatibility

and use of ground or sea return path in monopolar operation.

Fig 3.1 Total cost/distance of ac&dc transmissions

Engineers were therefore engaged over generations in the development of a technology for

DC transmissions as a supplement to the AC transmissions.

4. COMPONENTS OF HVDC TRANSMISSION SYSTEM

The most relevant components that comprise a HVDC system, are the following:

- The Thyristor or IGBT(Insulated Gate Bipolar Transistor) valves make the

conversion from AC to DC and thus are the main component of any HVDC converter. Each

single valve consists of a certain amount of series connected thyristors (or IGBTs) with their

auxiliary circuits.

Fig 4.1 HVDC system components

- The Converter Transformers transform the voltage level of the AC busbar to the required

entry voltage level of the converter. The main component of a converter station are:

Thyristor valves: The thyristor valves can be build-up in different ways depending on

the application and manufacturer. 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.

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 series-connected IGBTs 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.

- The Smoothing reactor, which main functions are:

• Prevention of the intermittent current

• Limitation of the DC fault currents

• Prevention of resonance in the DC circuits

- The Harmonic Filters, on the AC side of a HVDC converter station, which have two main

duties:

• To absorb harmonic currents generated by the HVDC converter

• To supply reactive power

Also DC filter circuits have to be used. Besides Active Harmonic filters can be a supplement

to passive filters due to their better performance.

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 the harmonics 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 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.

- Surge arrester, which main task is to protect the equipment of over-voltages.

- DC Transmission circuit, which include DC Transmission line, cable, high speed DC

switches and earth electrode.

- Control and Protection, mechanism for maintaining entire system security.

4.1 SUBSTATION CONFIGURATION

The central equipment of a d.c. substation are the thyristor converters which are usually

housed inside a valve hall.Figure 4.1 shows an example of the electrical equipment required

for a d.c. substation. In this example, two poles are represented which is the usual case and is

known as the “bipole” configuration. Some d.c. cable systems only have one pole or

“monopole” configuration and may either use the ground as a return path when permitted or

use an additional cable to avoid earth currents.

From Figure 4.1, essential equipment in a d.c. substation in addition to the valve groups

include the converter transformers. Their purpose is to transform the a.c. system voltage to

which the d.c. system is connected so that the correct d.c. voltage is derived by the converter

bridges. For higher rated d.c. substations, converter transformers for 12 pulse operation are

usually comprised of single phase units which is a cost effective way to provide spare units

for increased reliability.

Fig 4.1 HVDC Substation configuration

The secondary or d.c. side windings of the converter transformers are connected to

the converter bridges. The converter transformer is located in the switchyard, and if the

converter bridges are located in the valve hall, the connection has to be made through its wall.

This is accomplished in either of two ways. Firstly, with phase isolated busbars where the

bus conductors are housed within insulated bus ducts with oil or SF6 as the insulating

medium or secondly, with wall bushings. When applied at d.c. voltages at 400 kV or greater,

wall bushings require considerable design and care to avoid external or internal insulation

breakdown.Harmonic filters are required on the a.c. side and usually on the d.c. side. The

characteristic a.c. side current harmonics generated by 6 pulse converters are 6n +/- 1 and 12n

+/- 1 for 12 pulse converters where n equals all positive integers. A.C. filters are typically

tuned to 11th, 13th, 23rd and 25th harmonics for 12 pulse converters. Tuning to the 5th and 7th

harmonics is required if the converters can be configured into 6 pulse operation. A.c. side

harmonic filters may be switched with circuit breakers or circuit switches to accommodate

reactive power requirement strategies since these filters generate reactive power at

fundamental frequency. A parallel resonance is naturally created between the capacitance of

the a.c. filters and the inductive impedance of the a.c.system.

5. ADVANTAGES AND DISADVANTAGES OF HVDC TRANSMISSION

5.1 ADVANTAGES

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:

1. Undersea cables, where high capacitance causes additional AC losses. (e.g., 250 km

Baltic Cable between Sweden and Germany,the 600 km NorNed cable between Norway and

the Netherlands, and 290 km Basslink between the Australian Mainland and Tasmania)

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

example, in remote areas

3. Increasing the capacity of an existing power grid in situations where additional wires are

difficult or expensive to install

4. Power transmission and stabilization between unsynchronised AC distribution systems

5. Connecting a remote generating plant to the distribution grid, for example Nelson River

Bipole

6. Stabilizing a predominantly AC power-grid, without increasing prospective short circuit

current

7. 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

8. Facilitate power transmission between different countries that use AC at differing voltages

and/or frequencies

9. Synchronize AC produced by renewable energy sources

Long undersea high voltage cables have a high electrical capacitance, since the

conductors are surrounded by a relatively thin layer of insulation and a metal sheath. 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. This limits the length of AC

cables. DC cables have no such limitation. Although some DC leakage current continues to

flow through the dielectric, this is very small compared to the cable rating.

HVDC can carry more power per conductor because, for a given power rating, the

constant voltage in a DC line is lower than the peak voltage in an AC line. The power

delivered 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

100% 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.

5.2 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.

In contrast to AC systems, realizing multiterminal systems is complex, as is expanding

existing schemes to multiterminal systems. Controlling power flow in a multiterminal 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. One is in operation at the

Hydro Québec - New England transmission from Radisson to Sandy Pond. 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.

6 ECONOMICAL AND ENVIRONMENTAL CONSIDERATIONS

6.1 ECONOMIC CONSIDERATIONS

A study for Oak Ridge National Laboratory reported on a survey to 3 suppliers of HVDC

equipment for quotations of turnkey costs to supply two bipolar substations for four

representative systems. Each substation requires one d.c. electrode and interfaces to an a.c.

system with a short circuit capacity four times the rating of the HVDC system.

Transmission line costs cannot be so readily defined. Variations depend on the cost of

use of the land, the width of the right-of-way required, labor rates for construction, and the

difficulty of the terrain to be crossed. A simple rule of thumb may be applied in that the cost

of a d.c. transmission line may be 80% to 100% of the cost of an a.c. line whose rated line

voltage is the same as the rated pole-to-ground voltage of the d.c. line. The cost advantage of

d.c. transmission for traversing long distances is that it may be rated at twice the power flow

capacity of an a.c. line of the same voltage.

When electricity must be transmitted by underground or undersea cables, a.c. cables

become impractical due to their capacitive charging current if longer than a critical length

which for undersea applications is less than 50 kM. For distances longer than this critical

length with today’s technology requires d.c. cables. The choice is system specific, and

economic considerations will prevail.

6.2 ENVIRONMENTAL CONDITIONS

The electrical environmental effects from HVDC. transmission lines can be

characterized by field and ion effects as well as corona effects (4), (5). The electric field

arises from both the electrical charge on the conductors and for a HVDC overhead

transmission line, from charges on air ions and aerosols surrounding the conductor. These

give rise to d.c. electric fields due to the ion current density flowing through the air from or to

the conductors as well as due to the ion density in the air. A d.c. magnetic field is produced

by d.c. current flowing through the conductors. Air ions produced by HVDC lines form

clouds which drift away from the line when blown by the wind and may come in contact with

humans, animals and plants outside the transmission line right-of -way or corridor. The

corona effects may produce low levels of radio interference, audible noise and ozone

generation.

6.2.1 FIELD AND CORONA EFFECTS

The field and corona effects of transmission lines largely favor d.c. transmission over

a.c.transmission. The significant considerations are as follows:

1. For a given power transfer requiring extra high voltage transmission, the d.c.

transmission line will have a smaller tower profile than the equivalent a.c. tower carrying the

same level of power. This can also lead to less width of right-of-way for the d.c. transmission

option.

2. The steady and direct magnetic field of a d.c. transmission line near or at the edge

of the transmission right-of-way will be about the same value in magnitude as the earth’s

naturally occurring magnetic field. For this reason alone, it seems unlikely that this small

contribution by HVDC transmission lines to the background geomagnetic field would be a

basis for concern.

3. The static and steady electric field from d.c. transmission at the levels experienced

beneath lines or at the edge of the right-of-way have no known adverse biological effects.

There is no theory or mechanism to explain how a static electric field at the levels produced

by d.c. transmission lines could effect human health. The electric field level beneath a HVDC

transmission line is of similar magnitude as the naturally occurring static field which exists

beneath thunder clouds. Electric fields from a.c.transmission lines have been under more

intense scrutiny than fields generated from d.c. transmission lines.

4. The ion and corona effects of d.c. transmission lines lead to a small contribution of

ozone production to higher naturally occurring background concentrations. Exacting long

term measurements are required to detect such concentrations. The measurements taken at

cross-sections across the Nelson River d.c. lines in Canada failed to distinguish background

from downwind levels. While solar radiation influences the production of ozone even in a

rural environment, thereby maintaining its level, any incremental contribution from a d.c. line

source is subject to breakdown, leading to a resumption of background levels downwind from

the line. Investigations of ozone for indoor conditions indicate that in well mixed air, the half-

life of ozone is 1.5 minutes to 7.9 minutes. Increases in temperature and humidity increase

the rate of decay .

7. HVDC APPLICATIONS

HVDC transmission applications can be broken down into different basic

categories. Although the rationale for selection of HVDC is often economic, there may be

other reasons for its selection. HVDC may be the only feasible way to interconnect two

asynchronous networks, reduce fault currents, utilize long cable circuits, bypass network

congestion, share utility rights-of-way without degradation of reliability and to mitigate

environmental concerns. In all of these applications, HVDC nicely complements the ac

transmission system.The main applications are:

A. Long Distance Bulk Power Transmission:

HVDC transmission systems often provide a more economical alternative to ac

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

electric developments, mine-mouth power plants or large-scale wind farms. Higher power

transfers are possible over longer distances using fewer lines with HVDC transmission than

with ac transmission. Typical HVDC lines utilize a bipolar configuration with two

independent poles. Bipolar HVDC lines are comparable to a double circuit ac line since they

can operate at half power with one pole out of service but require only one-third the insulated

sets of conductors as a double circuit ac line. The controllability of HVDC links offers firm

transmission capacity without limitation due to network congestion or loop flow on parallel

paths.

B. Cable Transmission:

Unlike the case for ac cables, there is no physical restriction limiting the distance or

power level for HVDC underground or submarine cables. Underground cables can be used on

shared ROW with other utilities without impacting reliability concerns over use of common

corridors. For underground or submarine cable systems there is considerable savings in

installed cable costs and cost of losses when using HVDC transmission. Furthermore, there is

a drop-off in cable capacity with ac transmission over a distance due to its reactive

component of charging current since cables have higher capacitances and lower inductances

than ac overhead lines. This can be compensated by intermediate shunt compensation for

underground cables at increased expense, it is not practical to do so for submarine cables.

With a cable system, the need to balance unequal loadings or the risk of post-contingency

overloads often necessitates use of a series-connected reactors or phase shifting transformers.

These potential problems do not exist with a controlled HVDC cable system.

C. Asynchronous Ties :

With HVDC transmission systems, interconnections can be made between

asynchronous networks for more economic or reliable system operation. The asynchronous

interconnection allows interconnections of mutual benefit while providing a buffer between

the two systems. Often these interconnections use back-to-back converters with no

transmission line. Asynchronous HVDC links act as an effective “firewall” against

propagation of cascading outages in one network from passing to another network.

D. Offshore Transmission :

Self-commutation, dynamic voltage control and black-start capability allow compact

VSC HVDC transmission to serve isolated loads on islands or offshore production platforms

over long distance submarine cables. This capability can eliminate the need for running

expensive local generation or provide an outlet for offshore generation such as that from

wind. The VSC converters can operate at variable frequency to more efficiently drive large

compressor or pumping loads using high voltage motors.

E. Power Delivery to Large Urban Areas :

Power supply for large cities depends on local generation and power import capability.

Local generation is often older and less efficient than newer units located remotely. Often,

however, the older, less-efficient units located near the city center must be dispatched out-of-

merit because they must be run for voltage support or reliability due to inadequate

transmission. Air quality regulations may limit the availability of these units. New

transmission into large cities is difficult to site due to right of way limitations and land use

constraints. Compact VSC-based underground transmission circuits can be placed on

existing dual-use rights-of-way to bring in power as well as to provide voltage support

allowing a more economical power supply without compromising reliability. The receiving

terminal acts like a virtual generator delivering power and supplying voltage regulation and

dynamic reactive power reserve. Stations are compact and housed mainly indoors making

siting in urban areas somewhat easier. Furthermore, the dynamic voltage support offered by

the VSC can often increase the capability of the adjacent ac transmission.

8. HVDC LIGHT TECHNOLOGY

HVDC Light represents electric power transmission by HVDC based on voltage

source converters. This newly developed technology has various interesting characteristics

that make it a very promising tool for transmission of electric power to distant loads, where

no other transmission is possible or economic. The technology is briefly presented here

together with its application to a pilot transmission. Emphasis is on the characteristics that are

of importance for feeding of networks or loads without own generation. This refers

specifically to the generation by internal control of the phase voltages in the inverter, that

could serve the loads in the connected AC network.

New DC power cables based on a modified triple extrusion technology and a specially

designed DC material have been developed. DC power cables with ratings 30 MW at 100 kV

can be accomplished weighting only 1 kg/m. Such cables can be installed at low cost by e.g.

ploughing techniques.Voltage source converters together with these cables constitute an

excellent tool for providing power to any distant location. Thereby the advantages of a large

network can be brought to basically any place. For the moment the technology considers

designs that work within the power range of 1-60 MVA and with direct voltages up to around

+/-100 kV. For the future both powers and voltages will increase and extension to pure DC

networks will be possible.

8.1 HVDC LIGHT TRANSMISSION SYSTEM

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, as shown in Fig. 81. 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. Regarding the reactive power control, the feedback control loop can be formulized

such that it either tracks thepredetermined reactive power order, or tracks the given AC

voltage reference.

Fig 8.1 Typical Layout Of HVDC Light SubStation

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.

8.2 ADVANTAGES

Reduced environmental impact, an underground cable has no visual impact on

the landscape. Once it's installed the cable route can be replanted with Native

vegetation.

Faster and easier issue of permits using DC underground cables. Underground

cables rarely meet with public opposition and often receive political support.

The system reliability is enhanced with reduced risk of damage from natural

causes such as storms, wind, earthquakes and fire. You simply bury it and

forget it.

Operation and maintenance costs of the transmission easement are virtually

eliminated as there is no need for long term contracts to maintain the easement

with suitable access roads, thermographic checks of conductors joints,

insulator replacements, constant trimming and removal of regrowth vegetation

and public safety and security.

The width of the corridor to install the underground cable can be as narrow as 4

meters, which will give greater flexibility with the selection of a transmission

route.

9 SHORT CIRCUIT CONTRIBUTION OF HVDC LIGHT

9.1 STUDIED AC SYSTEMS

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

shown in Fig. 2. It includes high, medium and low voltage buses. The AC transmission lines

are modeled with π-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.

9.2 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 it is found

that the ∆Imax is inversely in proportional to the SCR as the solid curve shown in Fig. 9.1.

where, ISC is the short-circuit current of the original AC system alone at a 3-ph fault and

ISC_HVDC_L is the short-circuit current of the AC system with converter station connected

and in operation at the same fault.

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 shown in figure 9.1 with black dots.

Fig 9.1 Characteristics showing the impact of ac network strength

9.3 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.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 Uacctrl control mode, the reactive current

generation will be automatically increased when the AC voltage decreases.

9.4 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. Fig.9.2 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-ph close fault, are marked with black dots in Fig. 9.2.

Fig 9.2:Characteristics showing impact of load levels

10 CONCLUSION

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 distribution, HVDC systems

are 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.HVDC systems remain the best

economical and environmentally friendly option for the above conventional

applications. However, around the world, 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 situations.

HVDC Light is a new technology that has been specifically developed to match the

requirements of the new competitive electricity markets. It provides the ability to

connect renewable generation to the AC grid. It allows us to supply power to remote

locations and islands replacing local diesel generation. The technical merits are that

by virtue of their standardised prefabricated modular constructions which lead to short

delivery times, it is relocatable and can be expanded to meet growing demand.

Moreover, a key advantage is that it provides accurate control of the transmitted

active power and independent control of the reactive power in the connected AC

networks. A pair of lightweight DC cables can be laid direct in the ground in a cost-

effective way which is comparable to or less than a corresponding total life cycle cost

of AC overhead line. For these reasons HVDC Light provides an important role as a

business concept and opens up new opportunities for both investors and

environmentalist.

11 REFERENCES

1. http://en.wikipedia.org/wiki/High-voltage_direct_current

2. G. Asplund, “Application of HVDC Light to Power System Enhancement”,

presented at IEEE/PES Winter Meeting, Singapore,January 2000.

3. http://hvdcusersconference.com/wiki/

4. M. P. Bahrman, B. K. Johnson, “The ABCs of HVDC transmission technologies,”

IEEE Power & Energy, vol. 5, pp.32-44, Apr. 2007.

5. J. Zhu, H. Chao, R. Mukerji, D. Wang and L. Brown, “Economic assessment for

transmission upgrades in a deregulated market,” 2006 Session, CIGRE C1-115.

6. M. P. Bahrman, B. K. Johnson, “HVDC Transmission overview”,IEEE

7. SIEMENS, “High voltage direct current transmission - proven technology for power

exchange,” Mars 2007, brochure from SIEMENS, Source http://www.siemens.com.

8. G. Asplund, “Application of HVDC Light to Power System Enhancement”,

presented at IEEE/PES Winter Meeting, Singapore,January 2000.

9. U. Axelsson, A. Holm, C. Liljegren, M. Åberg, K. Eriksson and O.Tollerz, “The

Gotland HVDC LIGHT Project – Experiences from Trial Commercial Operation”

Presented at CIRED Conference, Amsterdam,The Netherlands, June 18-21, 2001.