Iec 61378-3-2015.Pdf

IEC 61 378 3 Edition 2 0 201 5 02 INTERNATIONAL STANDARD NORME INTERNATIONALE Converter transformers – Part 3 Application guide Transformateurs de conversion – Partie 3 Guide d''''application IE C 6 1 3[.]

Conv erter t ransformers –

Part 3: Appl cat ion guide

Transformat eurs de conv ersion –

Partie 3: Guide d'appl cat ion

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Conv erter t ransformers –

Part 3: Appl cation guide

Transformat eurs de conv ersion –

Partie 3: Guide d'appl cation

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CONTENTS

FOREWORD 6

INTRODUCTION 8

0.1 General 8

0.2 Ratin data (Clau e 5) 8

0.3 Win in con g ration (Clau e 6) 8

0.4 Ta pin s an imp dan es (Clau e 7) 8

0.5 In ulation asp cts an dielectric tests (Clau e 8)

9 0.6 L s es (Clau e 9) 9

0.7 Core an sou d asp cts (Clau e 10) 9

0.8 Tran f ormer sp cif i ation (Clau e 1 ) 9

0.9 Short circ it con ideration (Clau e 12) 9

0.10 Comp nents (Clau e 13) 10 0.1 Maintenan e (Clau e 14) 10 0.12 Monitorin an on-site in estigation (Clau e 15) 10 0.13 Ad itional sources of information 10 1 Sco e 1

2 Normative ref eren es 1

3 Terms an def i ition 11 4 Symb ls an a breviation 1

5 Ratin data 12 6 Win in con g ration 13 6.1 General 13 6.2 In u trial a pl cation 15 6.2.1 Rectif ier 15 6.2.2 Voltage reg lation 16 6.2.3 Autotran f ormer desig 17 6.3 HVDC a pl cation 22 6.3.1 Dif ferent typ s of tran f ormer ar an ement 2

6.3.2 Win in ar an ements 2

6.3.3 Con ideration of imp dan e – mutual imp dan e 2

7 Ta pin s an imp dan es – HVDC a pl cation 2

7.1 Value of imp dan e 2

7.2 Varia i ty of imp dan e 2

8 In ulation asp cts an dielectric tests 2

8.1 Hybrid in ulation s stems 26 8.1.1 General 2

8.1.2 Hybrid in ulation for con erter tran formers 2

8.2 Dielectric tests 28 8.2.1 General 2

8.2.2 Dielectric saf ety margin in lon term d.c an p larity reversal testin 3

8.2.3 Comments on the dielectric test proced res 31

9 L s es 3

9.1 General 3

9.1.1 General con ideration 3

9.1.2 L s es an f req en y 3

9.1.3 Cur ent s arin , los es an hot sp t in hig c r ent win in s 3

9.1.4 Cur ent harmonic 3

9.1.5 Tran f ormers with thre or more win in s woun on the same core l mb 3

9.1.6 Dual f req en y testin of HVDC tran formers 41

9.1.7 Tran f ormers with more than one active p rt in the same tan 4

9.1.8 Tran f ormers con ected to a voltage source con erter 4

9.2 Thermal tests 4

9.2.1 General 4

9.2.2 Calc lation of test c r ents an los es f or in u trial tran formers 4

9.2.3 Calc lation of los es an test c r ents for HVDC tran formers 44 9.2.4 Con ideration on the hot spot an lmits of the thermal tests 4

9.2.5 Con ideration on temp rature rise test of in u trial tran formers 4

9.2.6 Tan hot sp ts in in u trial tran f ormers 4

10 Core an sou d asp cts 4

10.1 Core 4

10.1.1 Con tru tional features 4

10.1.2 Ef f ect of harmonic on tran former active p rt cores 4

10.1.3 DC bias voltage ef f ect on main active p rt cores 47 10.1.4 Summary a out cores 4

10.2 Sou d 4

10.2.1 General 4

10.2.2 Fu damental con ideration 4

1 Tran f ormer sp cif i ation 5

1 1 General 5

1 2 Tec nical sp cif i ation vers s fun tional sp cif i ation 51

1 2.1 General 51

1 2.2 Tec nical sp cif i ation 51

1 2.3 Fu ctional sp cif i ation 51

1 3 HVDC tran former sp cifi ation 51

1 4 Notes an comments on sp cif i ation items to b provided in ivid al y by purc aser an s p l er 52 1 5 Inf ormation to b provided by the purc aser or by the s stem desig er 5

1 5.1 General des ription 5

1 5.2 Sy tem data 5

1 5.3 En ironmental data 5

1 5.4 Perf orman e req irements 5

1 5.5 Testin req irements 5

1 5.6 Seq en e of f actory tests 5

1 5.7 Site tests 5

1 5.8 Desig an con tru tion req irements 5

1 5.9 Au i ary eq ipment 5

1 5.10 Avai a i ty an me s res to minimize service downtime 6

1 6 Inf ormation to b provided by the s p l er 6

1 6.1 General 6

1 6.2 General des ription 60 1 6.3 Perf orman e data 6

1 6.4 Sup lementary data 61

1 7 Qual ty as uran e an test program 6

1 8 Avai a i ty an me s res to minimize service downtime 6

12 Short circ it con ideration 6

13 Comp nents 6

13.1 On-lo d ta -c an ers 6

13.1.1 General 6

13.1.2 Con erters for in u trial a plcation 6

13.1.3 Con erter for HVDC a pl cation 6

13.2 Valve-side bu hin s 6

13.2.1 General 6

13.2.2 Station layout 6

13.2.3 Tec nical con ideration 6

13.2.4 Bu hin typ s 6

13.2.5 Testin 6

14 Maintenan e 71

14.1 General 71

14.2 Oi 7

14.2.1 General 7

14.2.2 Oi q al ty an tran f ormer q al ty in icators 7

14.3 In ulation q al ty 7

14.3.1 General 7

14.3.2 Oi sampl n req irements 7

14.3.3 Oi tests 7

14.3.4 Oi han l n , storage an tre tment 7

14.3.5 Oi preservation s stems 7

14.4 Ta -c an ers 7

14.5 Ac es ories an f it in s 79 15 Monitorin 7

15.1 General 7

15.2 Tran f ormer con ition as es ment in service 8

15.3 Typ s of monitor 80 15.3.1 General 8

15.3.2 Oi con ition monitorin 8

15.3.3 Of f-l ne con ition as es ment 81

15.3.4 On an of f -l ne monitors 81

15.3.5 Exp rt s stems 8

15.3.6 Fin erprintin 8

15.4 Monitorin s mmary 84 16 Desig review of converter tran f ormers 8

16.1 General 8

16.2 Sc ed l n an con dential ty 8

16.3 Subject f or desig review 8

16.3.1 General 8

16.3.2 Sy tem inf ormation 8

16.3.3 En ironmental Inf ormation 8

16.3.4 Tran f ormer sp cif i req irements 8

Bibl ogra h 91

Fig re 1 – Brid e con ection f or six-pulse ar an ement 14

Fig re 2 – Brid e con ection f or twelve-pulse ar an ement 14

Fig re 3 – Tran former con ection star delta an star star or alternatively, delta-delta

an delta-star to give a 3 electrical degre se aration b twe n the valve-side

voltages 1

4

Fig re 4 – 15° phase s if tin is prefera ly re l zed by exten ed delta or zig ag

con ection 15

Fig re 5 – Double-star con ection 15

Fig re 6 – Typical con ection of tran d ctors 16

Fig re 7 – Autotran f ormer con e t for o en phases with co rse an f i e reg lation

ste s 17

Fig re 8 – Con entional autotran f ormer con e t f or closed phases with co rse an

f i e reg lation ste s 18

Fig re 9 – Con entional autotran f ormer con e t f or closed phases with multi co rse

reg lation 19

Fig re 10 – Bo ster autotran f ormer con e t f or closed phases with co rse an f i e

reg lation ste s 2

Fig re 1 – Autotran f ormer con e t f or o en phases with co rse an f i e reg lation

Fig re 15 – Typical imp dan e p t ern 2

Fig re 16 – Comp nents of a typical in u trial con erter tran f ormer in ulatin s stem 2

Fig re 17 – In ulation s stem, eq ivalent R-C circ it 2

Fig re 18 – Voltage distribution b f ore an immediately af ter p larity reversal 3

Fig re 19 – a.c./d.c con ersion – Simpl f ied s etc 3

Fig re 2 – L akage f ield f or a thre -win in tran f ormer with closely coupled valve

win in s 3

Fig re 21 – L akage f ield f or a thre -win in tran f ormer with decoupled valve

win in s 3

Fig re 2 – L akage f ield f or a thre win in tran f ormer with lo sely coupled double

con entric valve windin s 4

Fig re 2 – Examples of a.c d.c an combined electric f ield disp sition adjacent to

HVDC bu hin s an as ociated electric in ulation s stems 71

Ta le 1 – Ar an ements of two win in s 2

Ta le 2 – Data for calc lation of lo d los es with harmonic c r ents 4

Ta le 3 – Monitorin typ s 8

INTERNATIONAL ELECTROTECHNICAL COMMISSION

Part 3: Appl cation guide

1 Th Intern tio al Ele trote h ic l Commis io (IEC) is a worldwid org niz tio for sta d rdiz tio c mprisin

al n tio al ele trote h ic l c mmite s (IEC Natio al Commite s) Th o je t of IEC is to promote

intern tio al c -o eratio o al q e tio s c n ernin sta d rdiz tio in th ele tric l a d ele tro ic f i ld To

this e d a d in a ditio to oth r a tivitie , IEC p bls e Intern tio al Sta d rd , Te h ic l Sp cif i atio s,

Te h ic l Re orts, Pu lcly Av ia le Sp cific tio s (PAS) a d Guid s (h re f ter refere to a “IEC

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a re me t b twe n th two org niz tio s

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th later

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Pu lc tio s

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p te t rig ts IEC s al n t b h ld re p n ible for id ntifyin a y or al s c p te t rig ts

International Stan ard IEC 613 8-3 has b en pre ared by IEC tec nical commite 14: Power

tran formers

This secon edition can els an re laces the first edition publ s ed in 2 0 an con titutes a

tec nical revision

This edition in lu es the f ol owin sig if i ant tec nical c an es with resp ct to the previou

c) ad ition of content f or tran former con ected to voltage source con erters;

d) ref eren e to the work of CIGRE JWG A2/B4.2 on HVDC tran formers

The text of this stan ard is b sed on the fol owin doc ments:

Ful information on the votin for the a proval of this stan ard can b f ou d in the re ort on

votin in icated in the a ove ta le

This publcation has b en draf ted in ac ordan e with the ISO/IEC Directives, Part 2

A l st of al p rts in the IEC 613 8 series, publ s ed u der the general title Co v rter

tra sformers, can b f ou d on the IEC we site

The commit e has decided that the contents of this publcation wi remain u c an ed u ti

the sta i ty date in icated on the IEC we site u der "htp:/we store.iec.c " in the data

related to the sp cifi publ cation At this date, the publ cation wi b

• recon rmed,

• with rawn,

• re laced by a revised edition, or

• amen ed

INTRODUCTION

IEC 613 8 series is writen in thre p rts:

– Part 1 a pl es to tran f ormers as ociated with general "In u trial" con erter u es (Co p r

makin , aluminium smeltin an the electroly is of certain gases)

– Part 2 a pl es to tran f ormers req ired f or HVDC a pl cation

– Part 3 is this a pl cation g ide whic contain the to ic he din s des rib d in 0.2 to 0.13

The s o e of IEC 613 8-1 is l mited to a pl cation of p wer con erters of an p wer ratin

Typical a pl cation are: th ristor rectif iers f or electroly is; diode rectifiers f or electroly is;

th ristor rectif iers f or large drives; th ristor rectif iers f or s ra meltin f urnaces, an diode

rectif iers f eedin in erters f or varia le sp ed drives The stan ard also covers the reg latin

u it uti zed in s c a pl cation as ste down reg latin tran f ormers or autotran f ormers The

valve win in hig est voltage for eq ipment is lmited to 3 kV

IEC 613 8-2 covers con erter tran formers u ed in “HVDC Ap lcation ” There are two typ s

of HVDC p wer tran mis ion s stems k own genericaly as “b c to b c ” an “ ran mis ion”

s hemes The o eration an evaluation of tran formers o eratin within these two s stems

are covered by Part 2 an the present p rt of IEC 613 8

Neither IEC 613 8-1:2 1 nor IEC 613 8-2:2 01, expl citly in lu e in their s o e tran formers

con ected to voltage source con erters (VSC) Becau e VSC a pl cation are b comin more

an more common, some g idan e is provided in this stan ard

0.2 Rating data (Cla s 5)

In b th IEC 613 8-1 an IEC 613 8-2, the method of ratin con erter tran f ormers is dif f erent

to that u ed historical y In the traditional method, the r.m.s value of c r ent was u ed in

defi in the name late ratin of the tran former IEC 613 8 has introd ced a fun amental

c an e in the method of defi in the ratin of tran f ormers The con e t of u in the

f un amental comp nents of voltage an c r ent as the b sis f or the tran f ormer name late

ratin is explained The name late ratin derived from these f un amental comp nents

b comes the b sis for the g arante d imp dan es an los es

0.3 Win ing conf igurations (Cla s 6)

There are a large n mb r of win in con ection an con g ration that are sp cif i to

con erter tran formers for b th in u trial an HVDC a pl cation They have b en develo ed

over man ye rs The o eratin c aracteristic of the variou rectifier con ection are mostly

covered in IEC 6 14 series In the present p rt of IEC 613 8, the con ection are dis u sed

in so f ar as they af fect the con tru tion an some o erational asp cts of the tran f ormer

The u e of reg latin s hemes is common in in u trial a plcation The present p rt of

IEC 613 8 covers the prin iples f or a n mb r of these s hemes

0.4 Tappings a d impe a c s (Cla s 7)

The imp dan e of tran f ormers for HVDC a pl cation req ires sp cial at ention an desig

solution The req irements primari y con ern the l mitation of the imp dan e varia i ty over

the whole ta pin ran e, the l mitation of the imp dan e dif f eren e b twe n tran formers an

in some a pl cation , the imp dan e dif f eren e b twe n star an delta win in s This

stan ard dis u ses these req irements an their practical asp cts

In general, the ta pin ran e in con erter tran f ormers is wider than in con entional

tran f ormers The imp ct of the wider ta pin ran e on the tran f ormer an the ta -c an er is

dis u sed in this stan ard

0.5 Ins lation a pe ts a d diele tric te ts (Cla s 8)

Two asp cts are covered in Clau e 8 First, the in re sin u e of “h brid” in ulation s stems

in tran f ormers for in u trial a pl cation Secon ly, the a i ty of tran f ormer in ulation

stru tures of HVDC tran f ormers when tested with d.c voltage an in service

The b sic prin iples, the method of testin an the test voltage levels u ed f or b th a.c an

d.c testin are dis u sed The safety margin as ociated with the pro osed testin regime

are also reviewed

The present p rt of IEC 613 8 detai s the derivation of the prin iples, testin an calc lation

method u ed that take into ac ou t the ef f ects of non-sin soidal lo d c r ents on con erter

tran formers of al typ s

The prin iples of testin at two freq en ies for HVDC a plcation are detai ed alon with a

worked example of the calc lation The los es derived from these tests an calc lation are

u ed as the b se f or sp cif yin the test los es an c r ents to b u ed in esta ls in the oi

an win in gradients d rin the thermal testin

0.7 Core a d sound a pe ts (Cla s 10)

The ef fects of voltage harmonic an a d.c.-bias circ latin -c r ent on the p rforman e an

con tru tion of the core are dis u sed an s mmarized

The cau es of sou d an the dif feren es that may b exp cted b twe n conventional factory

sou d me s rements an those to b exp cted an exp rien ed at site are reviewed

The latest method of as es ment of sou d as ociated with con erter tran formers are

dis u sed

0.8 Tra sf ormer spe if ic tion (Cla s 1 )

Tran f ormers for con erters dif f er sig if i antly f rom p wer tran f ormers with resp ct to the

tran former sp cif i ation An outl ne of the detai s req ired in an sp cif i ation is in lu ed as

p rt of the g ide for b th tec nical an f un tional typ s of sp cifi ation

Some g idan e as to what s ould b sp cif ied by the purc aser an what s ould b exp cted

from the man facturer d rin the ten er an order proces is given

0.9 Short circ it con iderations (Cla s 12)

In con entional p wer tran f ormers, the calc lation of the s ort circ it c r ents within the

win in s are de en ent solely on the imp dan e an resistan e comp nents of the

tran former an s p ly to whic it is con ected

In the case of tran f ormers u ed to s p ly con erter a pl cation , there are f ault con ition

within the con erter that ne d to b con idered where the p ak value of the f ault c r ents may

b hig er than those derived f or con entional p wer tran f ormers These con ition are

detai ed in the present p rt of IEC 613 8

0.10 Compon nts (Cla s 13)

In the desig of tran formers for b th in u trial an HVDC con erter a pl cation , the c oice

an o eration of the on-lo d ta -c an er is cru ial The present p rt of IEC 613 8 outl nes

some of the prin iples governin the u e of ta -c an ers in these a pl cation

In the HVDC a plcation, the c oice an integration of the valve-side bu hin s into the overal

desig is of vital imp rtan e

The general req irements an recommen ation f or the con tru tion, integration of the

bu hin s with the tran f ormer an testin are detai ed An IEC stan ard sp cif i al y for HVDC

bu hin is in pre aration an the recommen ation in the present p rt of IEC 613 8 draws

up n the req irements of the new stan ard

0.1 Mainte a c (Cla s 14)

Statistic s g est that the HVDC tran f ormers req ire maintenan e to a hig stan ard Those

items that req ire p rtic lar at ention are on-lo d ta -c an ers an valve-side bu hin s

Atention is drawn in this stan ard to maintenan e req irements

0.12 Monitoring a d on-site inv stigations (Cla s 15)

Tran f ormer monitorin is recommen ed if on-site pro lems are to b minimized an in this

resp ct, con ition monitorin is dis u sed The present p rt of IEC 613 8 also presents

recommen ation for the proced re an practices to b folowed in the event of a f ai ure at

site These recommen ation are made so that vital eviden e an data are not destroyed or

contaminated at the initial stages of the in estigation

The u e of con ition monitorin for this a pl cation is also dis u sed in Clau e 15

0.13 Additional sourc s of inf ormation

The pre aration of the present p rt of IEC 613 8, sp cif i al y f or IEC 613 8-2 HVDC

con erter a pl cation , was sig if i antly in uen ed by the workin p p rs on variou to ic of

CIGRE Joint Workin Group 12/14.10

CONVERTER TRANSFORMERS –

Part 3: Appl cation guide

This p rt of IEC 613 8 provides information to u ers a out sp cif i to ic related to in u trial

an HVDC con erter tran f ormers with desig , con tru tion, testin an o eratin con ition

dif ferin from con entional tran f ormers u ed in p wer s stems In ad ition, it is the aim of

the present p rt of IEC 613 8 to provide man facturers with the tec nical b c grou d that

f orms the b sis for the prin iples u ed within IEC 613 8-1 an IEC 613 8-2

It is inten ed that this p rt of IEC 613 8 is u ed to s p lement an not re lace or s p rsede

the ap l cation g ide for p wer tran formers, IEC 6 0 6-8, sin e man of the general

prin iples contained within it are eq aly a pl ca le to con erter tran f ormers

The f olowin doc ments, in whole or in p rt, are normatively referen ed in this doc ment an

are in isp n a le f or its a pl cation For dated ref eren es, only the edition cited a pl es For

u dated ref eren es, the latest edition of the referen ed doc ment (in lu in an

amen ments) a pl es

IEC 6 0 6-1:2 1 , P ower tra sformers – P art 1: Ge eral

IEC 6 0 6-5:2 0 , P ower tra sformers – P art 5: Ab ity to with ta d s ort circ it

IEC 6 0 6-14:2 13 P ower tra sformers – P art 14: Liquid-immers d p wer tra sformers u in

hig -emp rature in ulation materials

IEC 6 2 6, Fluids for ele trote h ic l a plc tion – U nu ed min ral ins latin o s for

tran formers a d switc g ar

IEC 6 4 2, M in ral in ulatin o s in ele tric l e uipme t – Su ervisio a d mainten n e

g idan e

IEC 613 8-1:2 1 , Co v rter tra sformers – P art 1: Tra sformers for industrial a p c tio s

IEC 613 8-2:2 01, Co v rtor tra sformers – P art 2: Tra sformers for H VDC a plc tio s

3 Terms and definitions

For the purp ses of this doc ment, the terms an defi ition given in IEC 613 8-1 an

Symb l Me ning Units

I

N

r.m.s v lu of th n n-sin s id l in-s rvic lo d c re t in th win in u d r

c n id ratio at rate c n erter lo d

r.m.s v lu of th fu d me tal c mp n nt of th ln -to-ln rate v lta e V

Fre u n y ≥ 15 Hz u e to d termin th distrib tio of e d lo s s (H DC

tra sformer a plc tio s o ly, n t a plc ble for in u trial tra sformer a plc tio s)

NOT “Valv sid ” a d “ln sid ” d f i e th e tern l c n e tio s of th tra sformer win in s Lin sid refers

to th win in c n e te to th a.c n twork a d v lv sid to th win in c n e te to th c n erter

5 Rating data

IEC 613 8-1 an IEC 613 8-2 state that the ratin c aracteristic of the tran f ormer are

expres ed in ste d state sin soidal q antities of c r ent an voltage at the rated

f un amental f req en y The g arante d los es, imp dan es an sou d level s al cor esp n

to these values The rated voltage an c r ent ref er to the f un amental q antities, l ne-to-l ne

voltage an l ne c r ent

Fu damental comp nents are selected to esta l s a common platf orm for g arante d

q antities s c as los es an imp dan es in e en ent of the o eratin con ition an th s

the sp ctrum of harmonic content It s ould b noted that the tests in esta l s in the

o eratin c aracteristic can only b car ied out with sin soidal q antities

When in u trial tran f ormer u its are provided with tran d ctors (satura le re ctors) in their

tan , tran d ctor cores make the test c al en in b cau e of the distortion of c r ent an

voltage wave s a es they cau e An agre ment on the test modal ties ne d to b re c ed

b twe n the s p l er an the purc aser b fore sig in of the contract No method has b en

commonly ac e ted Some are presented in An ex G of IEC 613 8-1:2 1

Guarante d temp rature rise values are relevant f or a sp cif i lo din con ition as def i ed by

agre ment b twe n purc aser an man f acturer As a con erter tran former is exp sed to a

c r ent with a certain harmonic content, the actual los es wi deviate f rom the los es

develo ed with a true sin soidal c r ent In general, harmonic in the c r ent wi give rise to

an en an ement in lo d los es as comp red to o eration with true sin soidal c r ent

From the asp ct of tran f ormer desig , it is imp rtant to distin uis b twe n

– a plcation with es ential y sin soidal voltage acros the tran f ormer l ne win in , an

– a plcation with non-sin soidal voltage where the tran f ormer primary win in is

energized from a con erter circ it f or a.c p wer control or varia le f req en y con ersion

Inf ormation a out the con erter a plcation s ould b s p l ed in the tran former

sp cif i ation

The no-lo d los g arante d value is defi ed by the rated sin soidal voltage

The actual lo d los in con erter o eration is estimated with s f f icient ac urac by the

calc lation proced re presented in IEC 613 8-1:2 1 , 6.2 an in IEC 613 8-2 From this set

of f ormula the cor esp n in test c r ent f or esta l s in the cor esp n in temp rature rise

can b ded ced (se IEC 613 8-1:2 1 , 7.6 or IEC 613 8-2:2 01, 10.5)

It s ould also b noted that the actual lo d c r ent in o eration may b hig er than the rated

c r ent, when me s red by an in trument re din r.m.s values of c r ent, sin e the rated

c r ent on the name late ref ers to the f un amental comp nent of the lo d c r ent

6 Winding c nf igurations

This clau e des rib s the dif f erent version of ar an ement of win in s that can exist in b th

in u trial an HVDC a pl cation

In general, the win in s wi b ar an ed to provide a six-pulse con erter brid e f rom a

b lan ed thre -phase s stem The con erter wi car y c r ent twice from e c phase for a

maximum d ration of 12 electrical degre s or one third of a c cle, on e in p sitive an on e

in negative direction Se Fig re 1

Two or more six-pulse brid es may b con ected in series or p ralel When s p lyin the

in ivid al brid es with lne-side, thre -phase voltages displaced in time, the harmonic content

in l ne-side c r ents an valve-side voltages an c r ents wi b red ced With two six-pulse

brid es con ected in a so-cal ed twelve-pulse ar an ement, the two thre -phase s p les

s al b se arated by 3 electrical degre s Sti hig er pulse n mb rs on the con erters

req ire les se aration b twe n the s stems Se Fig res 2 an 3

U

Out put volt ag s a e

IEC

NOT Th s mb l of re tifier me n eith r dio e or th ristor

Figure 1 – Bridge conne tion for six-puls ar a geme t

Out put v olt ag s a e

NOT Th s mb l of re tifier me n eith r dio e or th ristor

Figure 2 – Brid e con e tion for twelv -puls ar a g me t

NOT 1 In re s d p ls n mb r re u e th h rmo ic c nte t in v lta e a d c re ts

A common ar an ement to re c two thre -phase s stems with a 3 -electrical-degre

se aration is two tran formers con ected to a common bu on the l ne side One tran f ormer

has the same win in con ection on the l ne an valve sides, star star or delta-delta The

other tran former has the con ection star delta or delta-star

Figure 3 – Tra sf ormer conne tions star-delta a d star-star or altern tiv ly, delta-d lta

a d d lta-star to giv a 3 ele tric l de re s paration betwe n the v lv -side volta e

Two identical tran f ormers with a win in con ection makin a 15 electrical degre s s if t

b twe n l ne-side an valve-side voltages can b u ed to provide a 3 degre s if t b twe n

the two valve-side voltages One tran f ormer is con ected for p sitive rotation (1U-2U, 1V-2V

an 1W-2W) an the other with negative rotation (1U-2V, 1V-2U an 1W-2W) This

ar an ement wi give +15 electrical degre s an –15 electrical degre s on the valve side as

15°

IEC

15°

Figure 4 – 15° pha e s if ting is preferably re l ze

by e te de delta or zigza conn ction

NOT 2 Th two c n e tio s c n als b u e for oth r s iftin a gle s c a 7,5 ele tric l d gre s for twe

ty-fo rp ls brid e

In u trial u its with tertiary win in s lo ded with f ilter an comp n ation are covered in

IEC 613 8-1:2 1 , 5.7 It s ould b noted that the con ection of the f ilter to the tertiary

win in wi le d to a hig er than rated voltage Theref ore the f un amental c r ent f lowin

throu h the f ilter imp dan e wi b hig er than the rated c r ent as def i ed in the clau e ju t

mentioned So it is imp rtant that the s stem en ine r takes into ac ou t this phenomenon

when sp cif yin maximum tertiary win in fun amental c r ent

6.2 Ind strial appl c tions

6.2.1 Rectifier

In general, con erter tran formers f or in u trial a pl cation wi b desig ed for a fairly hig

c r ent on the valve side The valve win in s s al theref ore have simple con ection s c as

star or delta

For some in u trial a plcation with hig DC c r ents an low DC voltage, it is of ten of

ad antage to u e a double-star ar an ement in te d of brid e con g ration

Figure 5 – Double-star conne tion

The l ne side is either con ected in star or delta an the valve side has two star s stems with

18 ° phase s if t The valve win in s s ould b ph sical y interle ved to avoid DC

mag etization of the core an to minimize mag etic le kage f l xes (ed y los es) related to

even c r ent harmonic An interphase tran former (IPT) b twe n the two star p ints is

ne ded together with a 3 l mbs core to en ure 12 ° valves con u tion an to minimize ef fects

of u b lan e b twe n the 2 neutral c r ents of the valve win in s When these 2 neutral

c r ents are b lan ed, a 5 l mbs core may b u ed in te d of the IPT

The tran former f or a six-pulse brid e con erter is a thre phase u it with the valve win in s

con ected in star or delta

Twelve-pulse re ction can b o tained by u in two six-pulse u its with dif f erent con ection

groups or by u in a tran f ormer with two valve win in s, con ected in star an delta This

last solution req ires a normal thre phase core

More complex cases req ire a dif ferent ar an ement with star an delta on the primary side

an valve win in s con ected in star or delta These p rtic lar ar an ements may also

req ire a sp cial core desig For 12 pulse double-star rectif iers, phase displacement of 3 ⁰

ne d to b ac ieved on the lne side win in s (star – delta, zig-zag, etc.)

Con erter plants with more than twelve-pulses req ire the u e of diff erent s if tin an les

amon the variou u its This is o tained in most cases by u in a zig ag or exten ed delta

con ection or more seldom with p ly on con ection Detai ed des ription an examples can

b f ou d in IEC 613 8-1:2 1 , An ex I

The ph sical ar an ement of the tran f ormer is pref era ly done with the valve win in as the

outer win in to cater f or the simple ar an ement of the valve-side exits The valve-side exit

ar an ement de en s up n the con erter stru ture

If the con erter req ires the presen e of tran d ctors, they are general y located in the same

tan as the tran former an in this case, it is neces ary to provide two le d exits by phase

Tran d ctors, if u ed, may b con ected ac ordin to Fig re 6

IEC

In ic te a tra s u tor

NOT Th s mb l of re tifier me n eith r dio e or th ristor

Figure 6 – Typic l conne tion of tra s uctors

The ph sical layout (b th in ide an outside the tan ) of hig c r ent bu b rs may

sig if i antly af fect the tran former lo d los es, s ort circ it imp dan e an tan hot sp ts

IEC 613 8-1:2 1 , An ex F gives g idan e on how to estimate these ef f ects

6.2.2 Volta e re ulation

In general, there is a ne d f or voltage reg lation b twe n the s p ly an valve voltages an

of ten the reg lation ran e is fairly large f rom a f ew p rcent up to 10 % of the rated valve

win in voltage The purp se of the wide ta pin ran e is to red ce the con umption of

re ctive p wer an harmonic p l ution related to con erter o eration F r practical re son ,

the large reg latin ran e of ten req ires a se arate reg latin tran f ormer b twe n the a.c

terminal an the con erter tran f ormer itself

De en in on the l ne voltage, the reg latin u it can b buit either as an autotran former or

with two se arate win in s to red ce the ris s f rom hig tran ient overvoltages enterin the

con erter tran f ormer

6.2.3 Autotra sformer de ig

6.2.3.1 Ge eral

Variou common con ection are s own in Fig res 7 to 13

6.2.3.2 Star conne tion – ope ph s s

The s hematic con titution of the win in is s own in Fig re 7

Ke

1U – Lin 2U1 – Co v rter tra sformer 2U2 – Co v rter tra sformer

1N – Lin n utral 1V – Lin 2V1 – Co v rter tra sformer 2V2 – Co v rter tra sformer

1W – Lin 2W1 – Co v rter tra sformer 2W2 – Co v rter tra sformer

Fig re 7 – Autotra sformer conc pt f or ope pha e

with coars a d f in re ulation steps

6.2.3.3 Star conne tion – clos d ph s s

Ke

1U – Lin 2U – Co v rter tra sformer

1N – Lin n utral 1V – Lin 2V – Co v rter tra sformer

1W – Lin 2W – Co v rter tra sformer

Figure 8 – Conv ntion l a totra sf ormer conc pt f or clos d pha e

with coars a d fine re ulation steps

1U – Lin 2U – Co v rter tra sformer

1N – Lin n utral 1V – Lin 2V – Co v rter tra sformer

1W – Lin 2W – Co v rter tra sformer

Figure 9 – Conv ntion l a totra sf ormer conc pt f or clos d pha e

with multi coars re ulation 2U

1U – Lin 2U – Co v rter tra sformer

1N – Lin n utral 1V – Lin 2V – Co v rter tra sformer

1W – Lin 2W – Co v rter tra sformer

Figure 10 – Booster a totra sformer conc pt f or clos d pha e

with coars a d f in re ulation steps

6.2.3.4 Delta conne tion – ope pha e

Ke

1U – Lin 2U1 – Co v rter tra sformer 2U2 – Co v rter tra sformer

1V – Lin 2V1 – Co v rter tra sformer 2V2 – Co v rter tra sformer

1W – Lin 2W1 – Co v rter tra sformer 2W2 – Co v rter tra sformer

Figure 1 – Autotra sformer conc pt f or ope pha e

with coars a d f in re ulation steps

2U12U2 2V2 2V12W2 2W1

IEC

6.2.3.5 Two win ing de ig

Ke

1N – Lin n utral 1V – Lin 2N – Se o d ry n utral 2V – Co v rter tra sformer

Fig re 12 – Tra sf ormer conc pt f or clos d pha e

with coars a d fin re ulation steps

6.3 HVDC ap l c tion

6.3.1 Dif f ere t type of tra sf ormer ar a geme t

In general, HVDC tran mis ion are bui t f or s bstantial p wer ratin s with con ection to

hig voltage a.c s stems In order to red ce the harmonic in the a.c an d.c s stems the

brid es are u ual y bui t f or twelve-pulse ar an ements Most of ten the tran formers have the

win in s star con ected on the lne side to take ad antage of non-u if orm in ulation

req irements an a voltage reg lation close to e rth Sy tem ne d an ef f icient o eration of

the con erter of ten req ire fairly large voltage reg lation to b in the ran e of up to a out

4 % of name late voltage

De en in on p wer ratin , voltages an ph sical con traints, the con erter tran formers can

b bui t as sin le phase or thre -phase u its

The folowin solution are p s ible to re l ze HVDC tran formers with twelve-pulse brid es

a) Six sin le phase, two-win in tran f ormers

• Thre con ected in star/delta

• Thre con ected in star/star

b) Thre sin le phase, thre -win in tran f ormers

• Al con ected star/delta-star

c) Two thre -phase tran formers

• One con ected star/delta.

• One con ected star/star

d) One thre -win in tran former

• Con ected star/delta-star

A con erter tran former is general y bui t with two or thre main win in s A tertiary win in is

req ired only in some sp cif i cases, e.g injection of test voltage, con ection to sp cial filter

ar an ements

6.3.2 Win ing ar a g me ts

6.3.2.1 Ge eral

In core typ tran formers, the win in s are c l n er s a ed an ar an ed con entrical y

arou d the core l mb The ta p d p rt of the l ne-side win in general y f orms its own

ph sical y se arated win in , the reg latin win in The in ivid al win in s are almost close

to eq al heig t an have eq al amp re-turn distribution alon the win in heig t

In general, the reg latin win in is located ph sical y close to the main p rt of the l ne

win in but not in b twe n valve an the l ne win in s (se Clau e 7 for ta pin an

imp dan es an Clau e 9 f or los es)

6.3.2.2 Two win ings

In general, there are two way to ar an e the in ivid al win in s in a two-win in tran f ormer

as in icated in Fig re 13 The main an reg latin win in s f orm the l ne-side win in The

valve-side win in con ists of one sin le win in

The main solution is as fol ows.

In sin le-phase tran former desig s, there are general y two wou d l mbs, one f or star/delta

an the secon one f or star/star con ection This me n that the valve win in f or a delta

con ection has 3 times the n mb r of turn f or a star con ected win in but a lo d c r ent

of 1 3 of the c r ent in a star con ected win in

The relative p sition of the in ivid al win in s els are the same as the p sition for a

two-win in tran former (se Ta le 1)

When the ph sical size p rmits for l mited p wer ratin s an l ne voltages, e.g a medium size

b c -to-b c con erter, the tran former may b bui t as a thre -phase, thre -win in

tran f ormer Req irements for the imp dan es to b simi ar b twe n the l ne-side terminal

an the two valve terminals of ten req ire a desig with two win in ar an ements located

axial y on the same l mb – a so cal ed axial spl t The two l ne-side win in s with their

reg latin win in s are con ected in p ral el an one of the valve win in s is con ected for

delta an the other f or star The relative p sition of the in ivid al ph sical y se arated

win in s s al b in ac ordan e with Fig re 13

Ar an ements where the two valve-side win in s e c oc upy two con entric p sition , of ten

on o p site sides of the lne win in , are general y not a via le alternative For a tran f ormer

with voltage reg lation, it wi b dif fic lt to re c the same imp dan e to the two valve-side

win in s for al ta p sition

6.3.3 Consid ration of impe a c – mutual impe a c

The imp dan e p tern b twe n one l ne an two valve terminals can b vis al zed by thre

imp dan es con ected together in one an the same p int with two bran hes toward the two

valve terminals an one toward the l ne terminal, also cal ed the common mutual imp dan e

In general, the common mutual imp dan e s al b as low as p s ible to avoid interaction

b twe n the voltages d rin commutation

In two-win in tran formers, the common mutual imp dan e by the tran f ormer is non

-existent; contribution to the mutual imp dan e can only b ac ieved f rom the common bu

Thre -win in tran f ormers (Fig re 14) where the two valve win in s are located on dif ferent

l mbs wi also have zero mutual imp dan e Thre -win in tran formers u in an axial spl t

wi also ex ibit a low common mutual imp dan e

The selected imp dan e is the res lt of in uen es f rom a n mb r of req irements Althou h it

red ces harmonic c r ent levels, hig imp dan e in re ses the commutatin re ctan e

le din to in re sed re ctive p wer a sorption of the con erter, th s in re sin the costs of

the filter circ it an valve as embly

L w imp dan e has an in uen e on s ort-circ it f orce con ideration an in re ses the size

an weig t of the tran former with con eq ential ef fects on cost an tran p rt

7.2 Variabi ty of impe a c

It is es ential that the actual an me s red tran former imp dan e remain within a given

deviation from a sp cif ied value Deviation may b of s stematic as wel as ran om nature

Sy tematic deviation may have their origin in desig an man facturin con traints Ran om

deviation de en on variation in man facturin an me s rement u certainty Imp dan e

variation as a f un tion of ta p sition can b regarded as s stematic deviation where the

typ of ta ar an ement in uen es the course of deviation, e.g l ne r or reversin ta s

Permis ible imp dan e deviation f rom sp cif ied values may con ern:

– deviation from a sp cif ied imp dan e value at the prin ip l ta ;

– varia i ty b twe n in ivid al phases;

– varia i ty b twe n tran former u its;

– varia i ty within the ta pin ran e;

– deviation b twe n star an delta-con ected win in s

The purc aser may sp cif y l mits f or al f i e pro erties

If not otherwise sp cified, rules for p rmis ible toleran es laid down in IEC 6 0 6-1 wi a ply

together with the req irements b low

The deviation from the imp dan e sp cif ied by the purc aser s al not ex e d 5 % in the

u ual o eratin ran e of ta -c an er Outside this ran e, the deviation may go up to 10 %

(se Fig re 15)

The imp dan e variation b twe n in ivid al phases, tran f ormer u its an star an del

ta-con ected win in s s ould not ex e d 3 % f or the same ta p sition Ef f icient can el n of

harmonic in a twelve-pulse brid e con ection req ires smal imp dan e varia i ty L rge

variation b twe n phases, u its an star an delta-con ected win in s wi in re se the

Fig re 15 – Typic l impe a c pat ern

8 Insulation aspects and dielectric tests

8.1 Hybrid ins lation s stems

8.1.1 Ge eral

Hybrid in ulation s stems are primari y u ed in in u trial con erter tran f ormers sin e there

are ad antages of workin at elevated temp ratures in s c tran formers

Whenever the term “h brid” is u ed with referen e to an in ulation s stem, it me n that hig

-temp rature-in ulatin materials (s c as aramide p p r or hig temp rature enamel) are

combined with con entional temp rature materials (s c as cel ulose)

In ulatin materials are u ed in several of the dif ferent comp nents that make up a

tran f ormer’s win in : con u tor in ulation, sp cers, c l n ers, an le rin s, etc (se

Fig re 16)

A – An le rin s a d c p C – Static rin s E– Co d ctor in ulatio

Fig re 16 – Compone ts of a typic l industrial conv rter tra sf ormer in ulating s stem

For man ye rs, hybrid in ulation was a pl ed f or oi -co led tran f ormers Hig temp rature

materials are uti zed in p rts in direct contact with the co p r (con u tor in ulation) or close

to it (sp cers), whie al the other in ulatin comp nents con ist of cel ulose Win in s with

h brid in ulation have b en desig ed with an average temp rature rise hig er than the one

al owa le for cel ulose (without red ction of the tran former l f e), th s in re sin the p wer

p r u it of volume

8.1.2 Hybrid ins lation f or conv rter tra sf ormers

Whenever a tran former is u ed in conju ction with a con erter, harmonic c r ents are

injected in its win in s The harmonic c r ents wi in re se the ed y los es of the win in s

by a f actor defi ed in IEC 613 8-1 an IEC 613 8-2 In a con entional tran former, the ed y

los es de en on the radial an axial comp nents of the mag etic le kage f l x an on the

ge metric dimen ion of the con u tor s bjected to this f l x (se IEC 613 8-1) The ed y

los es vary alon the win in heig t They are lower in the central p rt of the win in where

the le kage f l x is mostly p ral el to the thic nes of the con u tor an in re se at the en s

of the win in where the le kage f l x is almost p rp n ic lar to the width of the con u tor

This non-u if orm distribution of los es res lts in a non-u if orm distribution of the temp rature

alon the win ing’s heig t The dif feren e b twe n the hot sp t an average win in

temp rature is therefore in re sed b yon that cau ed by the normal co ln mec anism

In con erter tran formers, when harmonic c r ents are taken into ac ou t, the dif f eren e

b twe n the hot sp t an the average win in temp rature may further in re se b cau e of

the cor esp n in in re se in ed y los es (se IEC 6 0 6-8:19 7, 9.6)

A n mb r of solution to this pro lem are avai a le In agre ment with the purc aser,

ado tion of hig -temp rature-in ulation materials in the hot sp t region is a solution whenever

it is not p s ible to red ce the hot sp t temp rature to l mits in icated in IEC 6 0 6-2:2 1 ,

6.2

In p wer tran formers, h brid in ulation is general y inten ed to in re se the al owa le

average win in temp rature an hen e the p wer ratin Therefore, hig temp rature

in ulation material is u ed ne r to al the win in con u tors In con erter tran formers,

h brid in ulation could also b u ed in the are s ne r to local hig hot sp t temp ratures,

whie average win in rises remain within lmits It also f ol ows that hig -temp ratu

re-in ulation material s ould b a pl ed to the win in con u tors in the hot sp t region

More an more sol d an l q id hig temp rature materials are avai a le in the market This

fact in re ses the n mb r of p s ible solution whenever the average an hot sp t

temp ratures are of con ern The cor ect a pl cation of these materials is covered in

IEC 6 0 6-14

8.2 Diele tric te ts

8.2.1 Ge eral

Dielectric tests an in ulation asp cts primari y refer to HVDC con erter tran formers The

dielectric tests an in ulation asp cts for in u trial con erter tran f ormers are identical to

those of con entional p wer tran f ormers an are covered in IEC 6 0 6-3

The in ulation s stem in a HVDC con erter tran f ormer is exp sed to the stres es cau ed by

the a pl ed voltage an in u ed voltages For the lne win in an its terminals, the voltages

are of the same nature as f or a con entional tran f ormer, that is, ste d state a.c voltages

an tran itory voltages s c as l g tnin an switc in s rges The valve win in s an their

terminals wi in ad ition to the voltages in a con entional tran former also b s bjected to

d.c voltages This wi in turn give rise to a combined a.c an d.c stres

The distribution of al time varia le voltages acros the in ulation s stem is wel def i ed by

the ge metry an p rmitivity of the materials in olved In the f ol owin p ragra h it is

as umed that these materials are mainly tran f ormer oi an celulose The distribution of a

ste d state direct voltage in oi /cel ulose in ulation deviates sig if i antly f rom that of time

varia le voltages (impulse an a.c voltages) The distribution is mainly governed by a p rent

resistivities These resistivities are to a gre t extent de en ent on the mo i ty of sp ce

c arges in the in ulation s stem As a con eq en e, the resistivity is not a con tant value an

varies with a n mb r of p rameters, s c as temp rature, electrical stres , moisture, sp ed of

oi f low, d ration of voltage a pl cation, agein an c emical stru ture These phenomena are

b t er re resented by a sp ce-c arge time model

From a practical p int of view, it is general y ac e ted that an in ulation stru ture can b

model ed as an R-C network to determine its b haviour u der d.c con ition (provided that

ion-drif t ef fect can b neglected) When a d.c voltage is initial y a pl ed, the voltage

distribution is ca acitive, whi e the fi al or ste dy state d.c f ield in a complex stru ture is

determined by the relative resistivities of the in olved materials In most cases, the u e of an

R-C model gives a con ervative an saf e desig margin Resistivity ratios b twe n sol d

in ulation an oi may ran e f rom 10:1 up to 5 0:1 at ro m temp rature As the temp rature is

in re sed, the resistivity ratios typical y decre se up to one order of mag itu e

A – Celulo e B – Oi U – Volta e C – Ca a ita c R – Re ista c

Figure 17 – In ulation s stem, e uiv le t R-C circ it

A model s own in Fig re 17 can b u ed to simulate the in ulation s stem The interf ace

b twe n the oi an the cel ulose b r ier is as umed to b an eq ip tential s rf ace

Becau e the a.c an d.c stres es are predominantly determined by in e en ent p rameters,

it may b as umed that an complex voltage field can b se arated into its a.c an d.c

comp nents The stres d e to e c voltage can b calc lated se arately, an the two

stres es can b combined to give a me s re of the actual stres

Fig re 18 gives an in ication of the stres es at ste d state d.c voltage an immediately af ter

a p larity reversal, whic are the two critical stres p tern The maximum stres es oc ur

when the resistivity ratio of cel ulose/oi is hig

In the ste d state con ition, a resistive distribution of the d.c voltage dominates Normal y,

the resistivity is s c that the major p rt of the total voltage dro wi b acros the cel ulose

Together with the a.c comp nent, the distribution in the comp site s stem wi b l ke the

lower l ne in Fig re 18 Th s, a sig if i ant stres in the sol d in ulation wi develo an this is

mainly d e to the a pl ed d.c voltage

Immediately af ter a p larity reversal ( he up er lne in Fig re 18), the c arge of the cel ulose

in the o p site direction remain , but wi b red ced d e to ca acitive voltage distribution at

the 2U ste (–U to +U) The con eq en e is a sig if i ant in re se of the voltage acros the

oi The value of resistivities an p rmitivities in olved govern this temp rary voltage acros

the oi This voltage is normaly hig er than the voltage acros the oi immediately af ter an

a pl cation of +U or –U, but it wi b red ced to a low value (mainly a.c voltage) in the ste d

state con ition

C – Initial distrib tio after re ers l F – Initial distrib tio b fore re ers l

Figure 18 – Volta e distribution bef ore a d imme iately af ter polarity re ers l

8.2.2 Diele tric s f ety margins in long term d.c a d polarity re ers l te ting

Sig if i ant variation in resistivity f or oi an celulose, an hen e resistivity ratios, wi oc ur

d rin service This raises the q estion whether the f actory testin re resents con ition that

demon trate that there is a s f ficient dielectric safety margin for the desig

The aim of the lon term d.c se arate source test is to demon trate that there is a s f f icient

saf ety margin over the o eratin stres es in service acros the cel ulose in ulation The test

is made at ro m temp rature, hen e the resistivity ratio b twe n cel ulose an oi is

comp ratively hig This wi ac ieve stres es that are es entialy a ove the most extreme in

service Testin at hig temp rature wi res lt in a lower saf ety margin

The aim of the p larity reversal test is to demon trate that there is a s f f icient safety margin

over the o eratin stres es in service in the oi d cts However, the d ration of the p larity

reversal stres u der testin con ition is q estioned It has b en s own that resistivity ratios

close to 1:1 in service may prod ce stres es in the oi d cts that are not adeq ately verif ied

by the p larity reversal test For this re son, a new 1 h se arate source a.c test with p rtial

dis harge me s rement has b en introd ced This test cre tes a lon -term oi d ct stres

s f f icient to demon trate an adeq ate saf ety margin u der the worst con ition o tained

u der p larity reversal in service

The direct voltage testin s al b car ied out at a temp rature of 2 °C ± 10 °C.(se 10.4.3.1

of IEC 613 8-2:2 01) If the tests are car ied out immediately af ter the heat ru test, the time

con tants f or c argin an rec argin of the in ulation s stem wi b s orter d e to the hig

temp rature This may have the ef f ect that the dielectric margin are not demon trated in a

pro er way

Ref eren e is made to 8.2.3 an CIGRE JWG 12/14.10 p p r

The recommen ation f rom the CIGRE p p r have b en in orp rated into the dielectric

testin req irements of IEC 613 8-2 In s mmary, IEC 613 8-2 in orp rates thre dielectric

tests to val date the d.c in ulation stru ture of the valve win in s:

– lon d ration d.c se arate source voltage with tan test;

– the p larity reversal test;

– a.c se arate source voltage with tand test

CIGRE JWG A2/B4.2 has f urther in estigated the ef fectivenes of PR an DC tests in their

present f orm (se CIGRE Tec nical Broc ures 4 6 an 4 7) The key is ue is k owled e

a out oi resistivity

Su h k owled e is vital b cau e:

– ste d state stres distribution in the HVDC tran f ormer in ulation s stems is a fun tion of

material con u tivities;

– oi resistivity in uen es the time ne ded by the in ulation s stem to re c ste d state

an , therefore, the ef fectivenes of present tests Tests d ration ne d to b lon er than

the time to re c ste d state

CIGRE JWG A2/B4.2 also determined that oi con u tivity is not me s red in an u if orm an

con istent way by HVDC tran former man f acturers an u ers (se CIGRE Broc ure 4 6)

The mat er of oi con u tivity has b en taken up by CIGRE JWG A2/D1.41 HVDC tran former

in ulation Oi con u tivity an its res lts wi b ref lected in edition 2 of IEC 613 8-2

8.2.3 Comme ts on the diele tric te t proc d re

Figure 19 – a.c./d.c conv rsion – Simpl f ie s etc

8.2.3.2 DC s parate sourc volta e withsta d te t

multipl cation f actor to o tain this p ak value from the a.c phase-to-phase r.m.s voltage

In l ne with the same proced re u ed to esta l s the normal a.c lon -term testin values, the

multipl er 1,5 is a pl ed to o tain the test voltage

8.2.3.3 Polarity re ers l te t

The test level U

pr

is expres ed in the eq ation in IEC 613 8-2:2 01, 8.2.4

The same prin iples as mentioned in 8.2.3.2 a ply However, when d.c with s p rimp sed

a.c is con idered, the d.c comp nent is doubled for the p larity reversal an the a.c

comp nent is ad ed but not doubled Th s only half of the a.c comp nent is ad ed, whic

gives the factor 0,3 The f i al res lt is multipled by 1,2 to give a safety margin

The p larity reversal is made f rom p sitive to negative p larity an vic -v rs The re son is

that the in ulation stru ture is not p rfectly s mmetrical an the imp ct of the tran ient

stres es d rin the reversal wi dif fer de en in on p larity

8.2.3.4 AC s parate sourc volta e te t

The test level U

ac

is expres ed in the eq ation given in IEC 613 8-2:2 01, 8.2.5

The a.c voltage test level cor esp n s to the s m of the d.c voltage level of the valve

win in an the p ak value of the a.c phase-to-e rth voltage an division by 2 gives the

r.m.s value In l ne with the proced re of IEC 6 0 6-3 u ed to esta l s the normal a.c lon

term test value, the multipl er 1,5 is a pl ed to o tain the test voltage

It s ould b noted that this test is not a s bstitute to the p larity reversal test It is only a

s p lementary test to b car ied out f or the re son des rib d a ove

9.1.1 Ge eral con iderations

Whenever an alternatin c r ent f lows throu h the win in of a tran former, in ad ition to the

I R los es, it prod ces:

– ed y los es in win in s;

– ed y los es in hig c r ent bu b rs, when present;

– stray los es in stru tural ste l p rts

The external s ort circ it con ection, to b set up f or the me s rement of the lo d los es,

in uen es the me s rin res lts An agre ment s ould b re c ed b twe n the s p l er an

purc aser b f ore startin the test on how to cor ect the me s rin res lts owin to los es an

imp dan es of the test circ it

When u its provided with tran d ctors are s bject to lo d los es tests, the electrical

q antities mig t not b sin soidal An agre ment s ould b re c ed b twe n the s p l er an

purc aser b f ore startin the test on how to cor ect the me s rin res lts

These los es originate as fol ows

An a.c c r ent f lowin throu h a win in prod ces an alternatin mag etic flu This

alternatin fl x in u es electromotive f orces in al con u tive materials that it en ou ters

These electromotive f orces prod ce ed y c r ents in al con u tive materials an hen e they

generate ed y an stray los es

Electromotive f orces are pro ortional to the derivative of the mag etic f l x If we con ider a

sin soidal state, then electromotive forces are pro ortional to:

f× B

where

f is the f req en y; an

B is the mag etic f l x den ity

The mag itu e of the in u ed ed y c r ents is pro ortional to:

(f× B) R

where R is the resistan e en ou tered by the ed y c r ents

Con iderin that B is general y pro ortional to the win in ’s c r ent I, then the ed y los es

are pro ortional to:

(f2

× B2

Ed y c r ents, in u ed in a con u tive material by an alternatin mag etic f ield, distribute

themselves u evenly in the con u tor section an con entrate ne r the con u tor s rf ace

This phenomenon is k own as “s in ef f ect

The p netration de th of ed y c r ents de en s on the material, electrical an mag etic

pro erties together with the freq en y of the alternatin mag etic f ield This relation hip can

b expres ed as f ol ows:

σπ

=

f1

σ is the material electrical con u tivity

The s in ef f ect then in uen es the value of the con u tor resistan e at a given f req en y

Th s Eq ation (1) may b rewriten as:

)(

c on t ant

22

e d

fR

If

P

×

×

The con u tor’s resistan e remain con tant up to a certain f req en y (cal ed tran ition

f req en y), if the f req en y is in re sed b yon this, the resistan e in re ses in a man er

des rib d by the f ormula given in An ex A of IEC 613 8-1:2 1

Ed y los es in a given con u tor wi in re se with the s uare of the f req en y up to the

tran ition freq en y F r hig er freq en ies, the relation hip is:

where x is 0,5 < x ≤ 2 an con tantly decre ses with an in re se in f req en y

The value of tran ition f req en y of a sp cif i con u tor de en s sig if i antly on the

ge metric dimen ion of the con u tor itself , hen e the larger the dimen ion , the lower the

tran ition f req en y (se IEC 613 8-1:2 1 , An ex A)

The dif ferent con u tive materials in ide the tran f ormer may b divided into two groups:

– win in con u tors, the dimen ion are u ual y

1

comp ra le with the s in de th at

f un amental f req en y;

– hig c r ent bu b rs an stru tural ste l p rts where at le st one dimen ion is

sig if i antly gre ter than the s in de th at fun amental freq en y

The ge metric dif feren es b twe n the two groups are ref lected in a dif ferent relation hip

b twe n ed y los an f req en y Ed y los es in win in con u tors vary as f

The u e of ju t two exp nents is a simpl fi ation in resp ct to what is des rib d in

IEC 613 8-1:2 1 , An ex A However, several me s rements an simulation have s own

that these exp nents are b th safe an re sona le

9.1.3 Cur e t s aring, los e a d hot spot in high c r e t windings

Valve win in s in con erter tran formers f or in u trial a pl cation are of ten c aracterized by

a smal n mb r of turn an large rated c r ents (several kA an more) Of ten the

con eq en e of this f act is that the valve win in has to b made of several coi s con ected

in p ral el

Whenever a win in is made up of coi s con ected in p ral el, the s arin of the total win in

c r ent amon the coi s wi b in uen ed by the self an mutual re ctan e of e c coi an

by the lo din of other win in s In general, coi s exp sed to radial le kage f l x wi car y a

hig er c r ent than the coi s exp sed to axial le kage f l x only

Cur ent values f or coi s placed at the en s of a valve win in may b typical y 1,2 to 1,7 times

the value of c r ent cor esp n in to a p rf ectly even s arin amon coi s In ad ition, it s al

b noted that also the c r ent s arin amon the stran s formin the turn of these hig er

lo ded coi s is u eq al u les even c r ent s arin amon the stran s is ac ieved by p rfect

tran p sition or by u in contin ou ly tran p sed ca le

This me n that these win in s wi ex ibit a dif feren e b twe n hot sp t an average

temp rature rise whic can b sig if i antly hig er than that of other win in s

Therefore a simple hot sp t f actor can ot b as umed an lo d los es ne d to b computed

ac urately In p rtic lar the man f acturer s ould compute:

a) I R los d e to u even c r ent s arin amon the coi s makin up the hig c r ent

win in ;

b) I R los d e to u even c r ent s arin amon the stran s makin up the turn of e c

coi ;

c) ed y los in e c stran makin up the turn of e c coi

These calc lation can b car ied out by me n of mag etic f ield simulation whic take into

ac ou t b th the con ection amon the dif ferent coi s an win in s in the tran former an

sin soidal variation of c r ents vers s time

There is a ne d for one simulation for e c le kage field p tern (se s bclau e 9.1.5)

9.1.4 Cur e t harmonic

The harmonic c r ent wi in re se the tran f ormer ed y an stray los es Su h in re ses can

re c levels that may le d to tran f ormer fai ure

_ _ _ _ _ _

1

Foi win in s are a e c ptio to this (s e IEC 613 8- :2 1 , An e A Eq atio (A.4)

Con erters, to whic tran f ormers are con ected, act as sources of harmonic c r ents The

c aracteristic of these harmonic de en on:

– the con erter’s p wer circ it con g ration;

– the con erter’s control tec niq e;

– interaction b twe n the con erter an the s stem to whic it is a pled;

– the c aracteristic of the p wer electronic comp nents

The tran f ormer man facturer do s not have the neces ary inf ormation an k owled e to

predict the c r ent harmonic generated by the con erter Ac ordin ly, IEC 613 8-1 an

IEC 613 8-2 req ire that a harmonic sp ctrum b in lu ed in the sp cifi ation in terms of

ampl tu e an phase The purc aser has the resp n ibi ty of sp cif yin the harmonic to

whic the tran former wi b s bjected, whi st the tran former man f acturer has the

resp n ibi ty of desig in the tran former takin into ac ou t the sp cif ied harmonic

IEC 6 14 series identif ies dif f erent con erter p wer circ it con g ration For e c of these

con g ration , a pulse n mb r p was as ig ed so that it would b p s ible to predict the

harmonic that, the retical y, s ould b present

Cur ent develo ments in electronic make p s ible the a plcation of new re l-time control

tec niq es that sig if i antly alter the b haviour of the con erter The res lt is that a cle r

relation hip b twe n the con erter p wer circ it con g ration an its n mb r of pulses, an

hen e the value of the c r ent harmonic , is u certain

9.1.5 Tra sf ormers with thre or more windings woun on th s me core l mb

Win in ar an ements an le kage f ield are des rib d by u e of an example of core-form

tran formers The same con ideration can b a pl ed to s el -form tran formers

In a two-win in tran former, amp re-turn are b lan ed if the mag etizin c r ent is

neglected The harmonic c r ents flowin in the valve win in are b lan ed by harmonic

(with the same p.u mag itu e) in the l ne win in ; therefore the ed y los en an ement

factor is the same for b th the l ne an valve win in s

In a tran former with thre win in s, it is k own that the s m of al win in s amp re-turn

ad s up to zero an so it is neces ary to con ider in detai how to calc late e c win in

ed y-los en an ement f actor

It is p s ible to identif y the f ol owin con g ration f or thre win in core-typ tran f ormers

c aracterized by the coupl n b tween valve win in s:

a) close coupl n – two-valve win in s interwoven an one l ne win in ;

b) no coupl n – two p irs of valve-l ne win in s se arated by an intermediate yoke;

Af ter me s rin c r ent harmonic at al thre tran f ormer terminals, it is p s ible to o serve

that, whi e some harmonic injected into the valve win in s a pe r on the lne with an

identical p.u value, other harmonic are not present on the l ne side

Therefore, it is p s ible to divide the harmonic injected into the valve win in s into two

groups:

1) harmonic in phase – there is no phase displacement b twe n these harmonic f lowin in

the valve win in s; they s m an a p ar on the l ne side;

2) harmonic in phase o p sition – there is a 18 ° phase displacement b twe n these

harmonic f lowin in the valve win in s; they can el e c other out an do not a p ar on

the lne side

2

Harmonic c r ents in phase alway contribute to the total ed y los value

The ef fect of harmonic c r ents in phase o p sition is related to the valve win in coupl n

(se IEC 613 8-1:2 1 , 6.4):

a) close coupl n – harmonic in o p sition are b lan ed b twe n the interwoven valve

win in s; they prod ce a negl gible le kage f l x so that they only prod ce I R los es in

the valve win in s (se Fig re 2 );

b) no coupl n – the intermediate yoke se arates the mag etic circ its of the two p irs of

valve-l ne win in s; harmonic in o p sition are b lan ed b twe n e c valve-l ne

win in s couple so that they prod ce b th I R an ed y los es in b th l ne an valve

win in s (se Fig re 21);

c) lo se coupl n o tained with double con entric valve win in s – harmonic in o p sition

do not f low in the l ne win in as they are b lan ed b twe n the valve win in s where

they prod ce b th I R an ed y los es (se Fig re 2 );

d) lo se coupln o tained with two l ne win in s in p ral el axial y-displaced – harmonic in

o p sition are almost completely b lan ed b twe n e c valve-lne win in s couple so

that the same con ideration made for no coupl n a ply (se Fig re 2 ) However, local

los distribution may dif f er sig ifi antly, se p ragra h b low

When rectif iers cau e harmonic c r ents of o p site direction in lo sely con ected valve

win in s, the res ltin mag etic le kage f ield have sig ifi ant radial comp nents at the

win in en s These radial comp nents generate local ed y los es in the up ermost an the

lowest p rts of e c valve win in The win in ar an ement s own in Fig re 2 req ires

sp cial atention b cau e of the hig con entration of harmonic c r ent of certain orders in

the lowest p rt of the up er valve win in an in the hig est p rt of the lower valve win in

Su h service con ition can ot b prod ced in a normal temp rature rise test where the valves

are not con ected to the tran former Theref ore, if req ired by the purc aser, the ef f ects of

the harmonic in o p sition s al b stu ied by me n of the a pro riate mag etic f ield

simulation to ls to val date the desig solution Simi ar con ideration , b sed on the mutual

coupl n of the win in s, a ply when more than thre win in s or s el typ tran f ormers are

Ke