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CONVENTIONALLY POWER TRANSMISSION IS EFFECTED

THROUGH HVAC SYSTEMS ALL OVER THE WORLD.

HVAC TRANSMISSION IS HAVING SEVER LIMITATIONS LIKE LINE

LENGTH , UNCONTROLLED POWER FLOW, OVER/LOW

VOLTAGES DURING LIGHTLY / OVER LOADED

CONDITIONS,STABILITY PROBLEMS,FAULT ISOLATION ETC

CONSIDERING THE DISADVANTAGES OF HVAC SYSTEM ANDTHE ADVANTAGES OF HVDC TRANSMISSION , POWERGRID HAS

CHOOSEN HVDC TRANSMISSION FOR TRANSFERRING 2000 MW

FROM ER TO SR

COMPARISION OF HVAC & HVDC SYSTEMS

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HVDC: USE less current

Direct current : Rollalong the line ;opposing force friction(electrical resistance )

AC current willstruggle againstinertia in the line(100times/sec)-cuurent inertiainductance-reactivepower

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Better Voltage utilisation rating

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DC has Greater Reach

Distance as well as

amount of POWER

determine the choice

of DC over AC

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DC

The alternating current in a cable leaks current (charging

movements) in the same manner as a pulsating pressure

would be evened out in an elastic tube.

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DIRECT CURRENT CONSERVES FOREST

AND SAVES LAND

Fewer support TOWER, less losses

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CONTROLLING or BEING

CONTROLLED

By raising the level in tank ;controlled water flow

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CONTROLLING or BEING

CONTROLLED

ZERO IF Vr=VI=10V

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HVDC provides increase powerbut does not increase the short

circuit POWER

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ADVANTAGES OF HVDC OVER HVAC TRANSMISSION

CONTROLLED POWER FLOW IS POSSIBLE

VERY PRECISELY

ASYNCHRONOUS OPERATION POSSIBLE

BETWEEN REGIONS HAVING DIFFERENT

ELECTRICAL PARAMETERS

NO RESTRICTION ON LINE LENGTH AS NO

REACTANCE IN DC LINES

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STABILISING HVAC SYSTEMS -DAMPENING OF POWER

SWINGS AND SUB SYNCHRONOUS FREQUENCIES OF

GENERATOR.

FAULTS IN ONE AC SYSTEMS WILL NOT EFFECT THE OTHER

AC SYSTEM.

CABLE TRANSMISSION

.

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CHEAPER THAN HVAC SYSTEM DUE TO LESS TRANSMISSION

LINES & LESS RIGHT OF WAY FOR THE SAME AMOUNT OF

POWER TRANSMISSION

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COST: AC vs DC Transmission

Terminal Cost AC

Terminal Cost DC

Line Cost DC

Line Cost AC

Break Even Distance

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HVDC BIPOLAR TRANSMISSION SYSTEM

2 DOUBLE CIRCUIT HVAC TRANSMISSION SYSTEMS

2000 MW HVDC VIS- A- VIS HVAC SYSTEMS

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AC

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DC

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DC

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HVDC BIPOLAR LINKS IN INDIA

NER

ER

SR

NRNER

ER

SR

NR

RIHAND-DELHI -- 2*750 MW

CHANDRAPUR-PADGE 2* 750 MW

TALCHER-KOLAR 2*1000 MW

ER TO SR

SILERU-BARASORE - 100 MW

EXPERIMENTAL PROJECTER SR

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HVDC IN INDIA

BipolarHVDC LINK CONNECTING

REGION

CAPACITY

(MW)

LINE

LENGTH

Rihand Dadri

North-North 1500 815

Chandrapur -

Padghe

West - West 1500 752

Talcher

Kolar

East South 2500 1367

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ASYNCHRONOUS LINKS IN INDIA

NER

ER

SR

NRNER

ER

SR

NR

VINDYACHAL (N-W) 2*250 MW

CHANDRAPUR (W-S) 2*500 MW

VIZAG (E-S) - 2*500 MW

SASARAM (E-N) - 1*500 MW

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HVDC IN INDIA

Back-to-Back

HVDC LINK CONNECTINGREGION

CAPACITY(MW)

Vindyachal North West 2 x 250

Chandrapur West South 2 x 500

Vizag I East South 500

Sasaram East North 500

Vizag II East South 500

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BASIC PRINCIPLES

OF

HVDC TRANSMISSION

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AC Transmission Principle

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HVDC Transmission Principle

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6-Pulse Convertor Bridge

3

6

CiLs

4

E1 Ls

Ls

Bi

iA

1

2

I

V'd

5

Vd

IddL

d

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Voltage and Current of an Ideal

Diode 6 Pulse Converter

Alpha = 0

Overlap = 0

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Operation of Converter

Each thyristor conducts for 120

Every 60 one Thyristor from +ve limb and one Thyristorfrom ve limb is triggered

Each thyristor will be triggered when voltage across itbecomes positive

Thyristor commutates the current automatically when the

voltage across it becomes ve. Hence, this process is callednatural commutation and the converters are called LineCommutated converters

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Triggering can be delayed from this point and this is called firing angle

Output voltage of the converter is controlled by controlling the

Rectifier action

If > 90 negative voltage is available across the bridge Inverteraction

Due to finite transformer inductance, current transfer from onethyristor valve to the other cannot take place instantly

This delay is called over lap angle and the reactance calledcommutating reactance. This also causes additional drop in the voltage

Operation of Converter

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Effect of Control Angle

B

A

2

C

1

u u

Vd

u

3

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RECTIFIER VOLTAGE

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INVERTER VOLTAGE

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DC Terminal Voltage

120

RECTIFICATION

0240 180 300 120 60 180

0.866E . 2

LL

E . 2LL

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DC Terminal Voltage

120

INVERSION

0240 180 300 120 60 180

0.866E . 2LLE . 2

LL

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DC Voltage Verses Firing Angle

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0 30 60 90 120 150 180

Vd

alpha

Vd=Vac*1.35 *(cos alpha-uk/2)

Valve Voltage and Valve

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Current

120 180A

u0.866

240120

u

60

FC

D

B E

180A

u

60 60

K

G J L

H

N

M

3000

P

u

S

E . 2LL

60R

Q

RECTIFICATION

=15

+u E . 2LL


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Current

M Q

120 180 R

N

Pu

240 120R

Q

180

u

0

B

F

SA

C

E

DH

60

J

K

G L

INVERSION=15

6060

u u

60

0.866E . 2LL

E . 2LL

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Y

Commonly Used in HVDC systems

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Commonly adopted in all HVDC applications

Two 6 pulse bridges connected in series

30 phase shift between Star and Delta

windings of the converter transformer

Due to this phase shift, 5thand 7thharmonicsare reduced and filtering higher order

harmonics is easier Higher pulse number than 12 is not

economical


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DC VOLTAGE AT = 15

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DC VOLTAGE AT = 90

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DC VOLTAGE AT = 165

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HVDC Link Voltage Profile

I R

DC CABLE or O/H LINE

I Ed rd

RECTIFIER

dio RV

I X2

d c

cos

rI Ed

L I X

2

d c

cos

VdioI

INVERTER

VdR=VdioR cos-Id Xc+Er VdI=VdioI(cos-Id Xc+Er

2 2

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Control of DC Voltage

V 1 V 3 V 5

V 2V 6V 4

Phase A

Ud

Phase B

Phase C

Id

Power FlowAC System DC System

V 1 V 3 V 5

V 2V 6V 4

Phase A

Ud

Phase B

Phase C

Id

AC System DC SystemPower Flow

30 60 90 120 150 180

0

+Ud

-Ud

160

5

Rectifier

Operation

Inverter

Operation

Rectifier Operation Inverter Operation

R l i hi f DC V l Ud d Fi i

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Relationship of DC Voltage Ud and Firing

Angle

30 60 90 120 150 180

0

+Ud

-Ud

160

Limit Inv

5

Limit Rect.

RectifierOperation

InverterOperation

tw

o

60=

Ud

o

30=o

0=

o

90= o

120= o

150=

-Ud

tw

Ud

Ud

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NORMAL POWER DIRECTION

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REVERSE POWER OPERATION

Schematic of HVDC

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Schematic of HVDC

M d f O ti

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Modes of Operation

DC OH Line

Converter

Transformer

Thyristor

Valves

400 kVAC Bus

AC Filters,Reactors

Smoothing Reactor

Converter

Transformer

Thyristor

Valves

400 kVAC Bus

AC Filters, shuntcapacitors

Smoothing Reactor

Bipolar

Current

Current

Modes of Operation

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Modes of Operation

DC OH Line

Converter

Transformer

Thyristor

Valves

400 kVAC Bus

AC Filters,Reactors

Smoothing Reactor

Converter

Transformer

Thyristor

Valves

400 kVAC Bus

AC Filters

Smoothing Reactor

Monopolar Ground Return

Current

Modes of Operation

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Modes of Operation

DC OH Line

Converter

Transformer

Thyristor

Valves

400 kVAC Bus

AC Filters,Reactors

Smoothing Reactor

Converter

Transformer

Thyristor

Valves

400 kVAC Bus

AC Filters

Smoothing Reactor

Monopolar Metallic Return

Current

TALCHERTALCHER KOLARSCHEMATIC

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Kolar

Chintamani

Cudappah

HoodyHosur

Salem

Udumalpet

MadrasBlore

+/- 500 KV DC line

1370 KM

Electrode

Station

Electrode

Station

TALCHER

400kv System

220kv system

KOLAR

SCHEMATIC

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Sharing of Talcher Power

Tamil Nadu - 636 MW

A.P. - 499 MW

Karnataka - 466 MW

Kerala - 330 MW

Pondicherry - 69 MW

32%

23%

17% 3%

25%

T.N. A.P.Karnataka Kerala

Pondy

KOLAR SINGLE LINE DIAGRAM

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Project Highlights

FOR TRANSMITTING 2000 MW OF POWER FROM NTPC TALCHER

STPS -II AND FOR SHARING AMOGEST SOUTHERN STATES THE

2000 MW HVDC BIPOLAR TRANSMISSION SYSTEM IS ENVISAGED

AS

EAST SOUTH INTERCONNECTOR II (ESICON II).

THIS IS THE LARGEST TRANSMISSION SYSTEM TAKEN UP IN

THE COUNTRY SO FAR

THE PROJECT SCHEDULE IS QUITE CHALLENGING AGAINST THE 50 MONTHS FOR SUCH PROJECTS, THE

PROJECT SCHEDULE IS ONLY 39 MONTHS

SCHEDULED COMPLETION BY JUNE 2003

TACLHER-KOLAR 500 kV HVDC TRANSMISSION SYTEM

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Project Highlights

KEY DATES

AWARD OF HVDC TERMINAL STATION PKG -

14TH MAR 2000

AWARD OF HVAC PACKAGE -

27TH APR 2000

APPROVED PROJECT COST - RS. 3865.61 CR

THIS IS THE FIRST OF SUCH SYSTEM WHERE THE ENTIRE

GENERATION IN ONE REGION IS EARMARKED TO

ANOTHER REGION.

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Salient Features Rectifier Talcher, Orissa

Inverter Kolar, Karnataka

Distance

1370 km

Rated Power 2000 MW

Operating Voltage 500 kV DC

Reduced Voltage 400 kV DC

Overload

Long time, 40C 1.25 pu per pole

Half an hour 1.3 pu per pole

Five Seconds 1.47 pu per pole

SYSTEM CAPACITIES

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SYSTEM CAPACITIES

BIPOLAR MODE OF OPERATION -- 2000 MW

MONO POLAR WITH GROUND RETURN --- 1000 MW

MONO POLAR WITH METALLIC RETURN MODE --- 1000 MW

DEBLOCKS EACH POLE AT P min 100 MW

POWER DEMAND AT DESIRED LEVEL

POWER RAMP RATE -- 1 300 MW /MIN

POWER REVERSAL IN OFF MODE

SYSTEM CAPACITIES

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SYSTEM CAPACITIES

OVER LOAD CAPACBILITIES

RATED POWER -- 2000 MW

LONG TIME OVER LOAD POWER 8/10 HOURS -- 2500 MW

SHORT TIME OVER LOAD 5 SEC- 3210 MW

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HARMONIC FILTERS

AT TALCHER

TOTAL FILTERS 14

DT 12/24 FILTERS EACH 120 MVAR - 7 NOS


SHUNT REACTORS 138 MVAR- 2 NOS

SHUNT CAPCITORS 138 MVAR- 1 NOSDC FILTERS DT 12/24 & DT 12/36 1 No per pole.

AT KOLAR

TOTAL FILTERS 17DT 12/24 FILTERS EACH 120 MVAR - 8 NOS


SHUNT CAPCITORS 138 MVAR- 5 NOS

DC FILTERS DT 12/24 & DT 12/36 1 each pole

SYSTEM CAPACITIES

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MONOPOLAR GROUND RETURN - 1000 MW POWER CANBE TRANSMITTED THROUGH THIS MODE WHERE THERETURN PATH IS THROUGH THE GROUND WHICH ISFACILITATED THROUGH A EARTH ELECTRODE STATIONSITUATED AT ABOUT 35 KMS FROM THE TERMINALS ANDCONNECTED BY A DOUBLE CIRCUIT TRANSMISSION LINE.

MONOPOLAR METALLIC RETURN - 1000 MW POWER CANBE TRANSMITTED THROUGH THIS MODE WHERE THERETURN PATH IS THE TRANSMISSION LINES OF OTHERPOLE.

BALANCED BIPOLAR MODE 2000 MW CAN BETRANSMITTED THROUGH THIS MODE WHERE WITH ONE+VE AND OTHER VE .

SYSTEM CAPACITIES

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HIGH VOLTAGE DIRECT

CURRENT TRANSMISSION

(HVDC)

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Ad t

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Advantages In a number of applications HVDC is more effective than AC

transmission. Examples include:

Undersea cables, where high capacitance causes additional AClosses. (e.g. 250 km Baltic Cable between Sweden and Germany)

Long power transmission without intermediate taps, for example,in remote areas

Power transmission and stabilization between unsynchronized ACdistribution systems

Connecting a remote generating plant to the distribution grid

Reducing line cost: 1) fewer conductors 2) thinner conductors

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

Di d t

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Disadvantages The disadvantages of HVDC are in conversion,

switching and control. Expensive inverters with limited overload capacity

Higher losses in static inverters at smaller transmissiondistances

The cost of the inverters may not be offset by reductionsin line construction cost and lower line loss.

High voltage DC circuit breakers are difficult to build

because some mechanism must be included in the circuitbreaker to force current to zero, otherwise arcing andcontact wear would be too great to allow reliableswitching.

C t f HVDC Tr mi i

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Cost of HVDC Transmission Costs vary widely depending on power rating, circuit length,

overhead vs. underwater route, land costs, and AC networkimprovements required at either terminal.

For example, for an 8 GW, 40 km link laid under the EnglishChannel, the following are approximate primary equipmentcosts for a 2 GW, 500 kV bipolar conventional HVDC link is:

Converter stations ~$170 M

Subsea cable + installation ~$1.5 M/km

So for an 8 GW capacity between England and France in four links,

little change is left from ~$1.2B for the installed works. Add another$300$450M for the other works depending on additional onshoreworks required

HVDC System Configurations and Components

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HVDC System Configurations and Components

HVDC links can be broadly classified into:

Basic links

Monopolar links

Bipolar links

Homopolar links

Extended links

Back-to-back links Multiterminal links

Monopolar Links

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Monopolar Links

It uses one conductor

The return path is provided by ground or water

Use of this system is mainly due to cost considerations

A metallic return may be used where earth resistivity is too high

This configuration type is the first step towards a bipolar link

Bipolar Links

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Bipolar Links It uses two conductors, one positive and the other negative

Each terminal has two converters of equal rated voltage,connected in series on the DC side

The junctions between the converters is grounded

Currents in the two poles are equal and there is no ground current

If one pole is isolated due to fault, the other pole can operate with

ground and carry half the rated load (or more using overloadcapabilities of its converter line)

Homopolar Links

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Homopolar Links

It has two or more conductors all having the same

polarity, usually negative Since the corona effect in DC transmission lines is

less for negative polarity, homopolar link is usuallyoperated with negative polarity

The return path for such a system is through ground

Components of HVDC Transmission Systems

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1. Converters

2. Smoothing reactors

3. Harmonic filters

4. Reactive power supplies

5. Electrodes

6. DC lines

7. AC circuit breakers


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Converters

They perform AC/DC and DC/AC conversion

They consist of valve bridges and transformers Valve bridge consists of high voltage valves connected in a

6-pulse or 12-pulse arrangement

The transformers are ungrounded such that the DC systemwill be able to establish its own reference to ground

Smoothing reactors

They are high reactors with inductance as high as 1 H inseries with each pole

They serve the following: They decrease harmonics in voltages and currents in DC lines

They prevent commutation failures in inverters

Prevent current from being discontinuous for light loads

Harmonic filters

Converters generate harmonics in voltages and currents.These harmonics may cause overheating of capacitors andnearby generators and interference with telecommunicationsystems

Harmonic filters are used to mitigate these harmonics

Components of HVDC Transmission Systems contd.

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p y

Reactive power supplies

Under steady state condition conditions, the reactivepower consumed by the converter is about 50% of the

active power transferred Under transient conditions it could be much higher

Reactive power is, therefore, provided near the converters

For a strong AC power system, this reactive power isprovided by a shunt capacitor

Electrodes

Electrodes are conductors that provide connection to theearth for neutral. They have large surface to minimizecurrent densities and surface voltage gradients

DC lines

They may be overhead lines or cables

DC lines are very similar to AC lines

AC circuit breakers

They used to clear faults in the transformer and for takingthe DC link out of service

They are not used for clearing DC faults

DC faults are cleared by converter control more rapidly

Converter Theory and Performance

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Converter Theory and Performance

Multiple Bridge Converters

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Two or more bridges are connected in seriesto obtain as a high a direct voltage asrequired

These bridges are series on the DC side,parallel on the AC side

A bank of transformers is connectedbetween the AC source and the bridges

The ratio of the transformers are adjustableunder load

Multiple bridge converters are used in evennumbers and arranged in pairs for 12-pulsearrangement


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Two banks of transformers, one connected in Y-

Y and the other Y-are used to supply each pair

of bridges

The three-phase voltage supplied at one bridge is

displaced from the other by 30 degrees

These AC wave shapes for the two bridges addup to produce a wave shape that is more

sinusoidal than the current waves of each of the

6-pulse bridges

This 12-pulse arrangement effectively eliminates

5th and 7th harmonics on the AC side. Thisreduces the cost of harmonic filters

This arrangement also reduces ripple in the DC

voltage

Control of HVDC Systems

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Control of HVDC Systems

Objectives of Control

Efficient and stable operation

Maximum flexibility of power control without

compromising the safety of equipment

Content

Principle of operation of various control systems

Implementation and their performance during normaland abnormal system conditions

Basic principles of control

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Basic principles of control

Direct current from the rectifier to the

inverter

Power at the rectifier terminal

Power at the inverter terminal

Basic means of control

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Basic means of control

Internal voltages, Vdorcosand Vdoicos, can used becontrolled to control the voltages at any point on the line

and the current flow (power)

This can be accomplished by:

Controlling firing angles of the rectifier and inverter (for fast

action)

Changing taps on the transformers on the AC side (slow

response)

Power reversal is obtained by reversal of polarity of

direct voltages at both ends

Basis for selection of control

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Basis for selection of control

Following considerations influence theselection of control characteristics:

Prevention of large fluctuation in DCvoltage/current due to variation In AC side voltage

Maintaining direct voltage near rated value

Power factor at the receiving and sending endsshould be as high as possible

Control implementation

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Power control

To transmit a scheduled power, the correspondingcurrent order is determined by:

Iord

=Po/Vd

Bridge/converter unit control

Determines firing angles and

sets their limits

Pole control It coordinates the conversion

of current order to a firing

angle order, tap changer

control and other protection

sequences

Multiterminal HVDC network

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Multiterminal HVDC network

Successful application of two-terminal DC systems led to the

development of multi-terminal networks

There are two possible connection schemes for MTDC systems:

Constant voltage parallel scheme

Constant current series scheme

Unit 5 HVDC

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Transcript of Unit 5 HVDC