hvdc transmission part 2

7/26/2019 HVDC Transmission Part 2

1/41

HVDC Transmission part 2


2/41

Types of HVDC links:

Monopolar link

Bipolar link

Homopolar link


3/41

Monopolar link:

Having one conductor (-Ve Polarity and ground is used as return

pat!"

#e can operated eit!er in $Ve or %Ve polarity&'ut usuallypreferred -Ve polarity in order to reduce t!e Corona effect"

T!e maor dra)'ack in t!is system is po)er flo) is interrupted

due to eit!er converter failure or DC link"

T!e ground return is o'ectiona'le only )!en 'uried metallic

structures (*uc! as pipes are present and are su'ect to

corrosion )it! DC current flo)"


4/41

Bipolar link:

T!ere are t)o conductors & one is operates at positive and ot!er is

negative" During fault in one pole it )ill operate as monopolar

link" T!is is very popular link in HVDC


5/41

Homopolar link:

+n t!is link& t)o or more conductors !ave same polarity"

,ormally negative polarity are used(to less corona loss and radio

interference"

round is al)ays used as return pat!"

During fault in one pole it )orks as monopolar"


6/41

.pplication of HVDC:

T!e main areas of application 'ased on t!e economics and tec!nical

performances& are

/ong distance 'ulk po)er transmission"

T!e underground of su'marine ca'les"

.sync!ronous connection of .C system )it! different fre0uencies"

Control and sta'ili1e t!e po)er system )it! po)er flo) control"

Based on t!e interconnection& t!ree types of HVDC is possi'le"

Bulk Po)er transmission

Back to 'ack connection

Modulation of .C system


7/41

Purpose of HVDC 'ased on interconnection:

Bulk power transmission

(Transfer t!e po)er from one end to anot!er end )it!out tapping

po)er in 'et)een"2or t!is DC system is t!e 'est option" (3r HVDC

transmission )!ere 'ulk po)er is transmitted from one point to

anot!er point over long distance" Power flow control (Back to Back HVDC)

+f t)o regions are very near'y& )e can monitor t!e po)er flo)

from one region to anot!er to control& emergency support as per our

re0uirement"(3rBack to Back link )!ere rectification and inversion iscarried out in t!e same converter station )it! very small or no DC

lines


8/41

Continues4

To provide stability to AC system

T!is is 'asically used to control t!e po)er and sta'ili1e t!e

system" +t is also used to connect t)o different fre0uencies system"

(Modulation of .C .C system is connected parallel )it! DC system"

(orParallel connection of .C and DC links" #!ere 'ot! .C and DCrun parallel" +t is mainly used to modulate t!e po)er of .C lines"

HVDC is te better option for above cited purposes wile compare

wit its AC system!


9/41

Principle parts of HVDCTransmission:


10/41

Various Parts of HVDC transmission:

Converters

Converter transformers

*moot!ing reactors

Harmonic filters

3ver!ead lines

5eactive po)er source

6art! electrodes


11/41

C3,V65T65*

Converters are t!e main part of HVDC system"

6ac! HVDC lines !as atleast t)o converters& one at eac! end"

*ending end converter )orks as 5ectifier (+t converts .C po)er to

DC po)er" Ho)ever converter as receiving end )orks as +nverter (

it converts DC po)er to .C po)er"

+n case for reversal of operation& 5ectifier can 'e used as inverter or

vice versa" *o generally it is call it as C3,V65T65*"

*everal t!yristors are connector in series and7or in parallel to form a

valve to ac!ieve !ig!er voltage 7 current ratings"

,ote8- Valves (Com'inations of several t!yristors "


12/41

Various T!yristor 5atings:


13/41

Continues4

How to achieve required voltage and

current ratings?

T!e current rating of converter stations can 'e increased 'y

putting

Valves in parallel

T!yristors in parallel Bridges in parallel

*ome com'inations of a'ove"

T!e voltage ratings of converter stations can 'e increased 'y

putting Valves in series

Bridges in series

Com'ination of a'ove"

Bridge converters are normally used in HVDC systems"


14/41

Main re0uirement of t!e Valves are:

To allo) current flo) )it! lo) voltage drop across it during

t!e conduction p!ase and to offer !ig! resistance for non

conducting p!ase"

To )it!stand !ig! peak inverse voltage during non conducting

p!ase"

To allo) reasona'ly s!ort commutation angle during inverter

operation"

*moot! control of conducting and non conducting p!ases"


15/41

Continues4

T)o versions of s)itc!ing converters are feasi'le depending

on )!et!er DC storage device utili1ed is"

.n inductor-Current source converter

. Capacitor-Voltage source converter" C*C is prefera'le for HVDC system

V*C is prefera'le for 2.CT* like *T.TC3M&*VC&etc


16/41

Comparison of CSC andVSC:

CSC VSCInductor is used in DC side Capacitor is used in DC side

Constant current Constant voltage

Higher losses More ecient

Fast accurate control Slo control

!arger and more e"pensive Smaller and less e"pensive

More fault tolerant and morerelia#le

!ess fault tolerant and lessrelia#le

Simpler control Comple" control

$asil% e"panda#le for in series $asil% e"panded in parallel forincreased rating


17/41

C&'V$(T$(T()'SF&(M$(S:

2or si9 pulse converter& a conventional t!ree p!ase or t!ree

single p!ase transformer is used"

Ho)ever for ; pulse configuration& follo)ing transformer are

used"

*i9 single -p!ase t)o )indings

T!ree single- p!ase t!ree )indings

T)o t!ree- p!ase t)o )indings

+n converter transformer it is not possi'le to use )inding close toyoke since potential of its )inding connection is determined 'y

conducting valves"

Here entire )inding are completely insulated"


18/41

Continues4

.s leakage flu9 of a converter transformer contains very !ig!

!armonic contents& it produces greater eddy current loss and !ot spot

in t!e transformer tank"

+n case of ;-Pulse configuration& if t)o t!ree p!ase transformers

are used& one )ill !ave star-star connection& and anot!er )ill !avestar delta connection to give p!ase s!ift of "

*ince fault current due to fault across valve is predominantly

controlled 'y transformer impedance& t!e leakage impedance of

converter transformer is !ig!er t!an t!e conventional transformer"

3n-line tap c!anging is used to control t!e voltage and reactive

po)er demand"


19/41

*M33TH+, 56.CT35*:

.s its name& t!ese reactors are used for smoot!ing t!e direct

current output in t!e DC line"

+t also limits t!e rate of rise of t!e fault current in t!e case of

DC line s!ort circuit"

,ormally Partial or total air cored magnetically s!ielded

reactor are used"

Disc coil type )indings are used and 'raced to )it!stand t!e

s!ort circuit current"

T!e saturation inductance s!ould not 'e too lo)"


20/41

Harmonic filters

Harmonics generated 'y converters are of t!e order of np?in .C side and

np is t!e DC side" #!ere p is num'er of pulses and n is integer"

2ilter are used to provide lo) impedance pat! to t!e ground for t!e

!armonics current"

T!ey are connected to t!e converter terminals so t!at !armonics s!ould notenter to .C system"

Ho)ever& it is not possi'le to protect all !armonics from entering into .C

system"

Magnitudes of some !armonics are !ig! and filters are used for t!em only"

T!ese filters are used to provide some reactive po)er compensation at t!eterminals"


21/41

3ver!ead lines:

.s monopolar transmission sc!eme is most economical and t!e first

consideration is to use ground as return pat! for DC current"

But use of ground as conductor is not permitted for longer use and a

'ipolar arrangement is used )it! e0ual and opposite current in 'ot! poles"

+n case of failure in any poles& ground is used as a return pat! temporarily" T!e 'asic principle of design of DC over!ead lines is almost same as .C

lines design suc! as configurations&to)ers&insulators etc"

T!e num'er of insulators and clearances are determined 'ased on DC

voltage"

T!e c!oice of conductors depends mainly on corona and field effectconsiderations"


22/41

5eactive po)er source

.s suc! converter does not consume reactive po)er 'ut due to

p!ase displacement of current dra)n 'y converter and t!e voltage in

.C system& reactive po)er re0uirement at t!e converter station is

a'out @=-A= of real po)er transfer& )!ic! is supplied 'y

filters&capacitors&and sync!ronous condensers" *ync!ronous condensers are not only supplying reactive po)er 'ut

also provide .C voltages for natural commutation of t!e inverter"

Due to !armonics and transient& special designed mac!ines is used"


23/41

6art! electrodes:

T!e eart! resistivity of at upper layer is !ig!er (=== o!m-m and

electrodes cannot 'e kept directly on t!e eart! surface"

T!e electrode are 'uried into t!e eart! )!ere t!e resistivity is around (


24/41

Constitution of 6HV .C and DC links:

6HV transmission links& superposed on a lo)er voltage .C

net)orks& or interconnecting t)o suc! net)orks& or connecting

distant generating plants to an ac system& are compared as to t!eir

principle components and arrangements t!ereof& according to

)!et!er t!e line operates on .C or DC" Belo) single line diagram& is single circuit t!ree p!ase .C line" +n

suc! system re0uires transformer at 'ot! ends-step up transformers

at t!e sending end and step do)n transformer at t!e receiving end"

Most long over!ead .C lines re0uire series compensation of part of

t!e inductive reactance"(one 'ank of series capacitor


25/41

Continues*

T!e t!ree p!ase .C lines cannot 'e operated& e9cept for a very s!ort

time(less t!an sec )it! one or more conductors are open& 'ecause suc!

operation causes un'alanced voltages in t!e .C system and interference in

p!one telep!one lines"

T!erefore t!ree-pole s)itc!ing is al)ays used to clear t!e permanent

faults& alt!oug! suc! fault may involve in any one conductor" T!is 'eing

so& t)o parallel t!ree p!ase circuits re0uired for relia'le transmission"


26/41

Continues*

T!e line itself usually !as t)o conductors& alt!oug! some lines !ave

only one& t!e return pat! 'eing in t!e eart! or sea )ater or 'ot!"

.t 'ot! end of t!e lines are converters& t!e components of )!ic! are

transformers and group of mercury arc valves"

T!e converter at t!e sending end- 5ectifier" T!e converter at t!e receiving end-+nverter"

6it!er converter can function as rectifier or inverter& permitting

po)er to 'e transmitted in eit!er direction" 3f course it is preferred

for .C line& also !as t!is reversi'ility" T!e circuit 'reaker are installed only on t!e .C side of t!e

converters" T!ese 'reakers are not used for clearing faults on t!e dc

line or misoperations of t!e valves& for t!ese faults


27/41

Continues*

Can 'e cleared more rapidly 'y grid control of t!e valves" Ho)ever

'reaker is also re0uired for clearing t!e faults in transformers or taking

t!e )!ole DC link out of service"

Harmonic filters and s!unt capacitors for supplying reactive po)er

to t!e converters are connected to .C sides of t!e converter" /arge inductance called dc smoot!ing reactors are connected in

series )it! eac! pole of t!e DC line"


28/41

Continues*

+f !ig!er relia'ility is re0uired of a DC line t!an t!at provided 'y

t)o conductors& t!ree or four conductors may 'e provided"

Here one pole of four conductor line is s!o)n )it! t)o converters

per terminal"

T!e 'us-tie s)itc!es are normally open" +f a permanent faultoccurred on t!e lo)er conductor& t!e converters connected to it

)ould 'e controlled so as to 'ring t!e voltage and current on it to

1ero" T!en s)itc!es < )ould 'e opened& isolating t!e faulted line"


29/41

Continues*

,e9t t!e converter voltages )ould 'e raised to e0uality )it! t!ose

of t!e respective adacent converters& after )!ic! s)itc! )ould 'e

closed"

T!e capa'ility of all converter )ould 'e usa'le& and t!e po)er

normally carried 'y t)o conductors )ould t!en 'e carried 'y one"


30/41


31/41


32/41


33/41


34/41


35/41


36/41


37/41


38/41


39/41


40/41


41/41

hvdc transmission part 2

Documents