Iec tr 62001 3 2016

This res lted in a f urther contribution to the f un amental freq en y voltage on the DC side an cre ted smal direct c r ents in the con erter tran formers, whic ten ed to saturate the t

IEC TR 62001 - 3

Edit io 1.0 2 16-0

THIS PUBLICATION IS COPYRIGHT PROT CTED

Copyr ight © 2 16 IEC, Ge e a, Switzer la d

Al r i hts r es r ve Unle s oth r wis s e ifi d, n p r t of this p blc tio ma b r epr od c d or uti z d in a y for m

or b a y me n ,ele tr onic or me h nic l in lu in p oto o yin a d microfim, with ut p r mis io in wr itin fr om

eith r IEC or IEC's memb r Natio al Commite in th c u tr y of th r eq e ter If y u h v a y q e tio s a o t IEC

c p r i ht or h v a e q iry a o t o tainin a ditio al r i hts to this p blc tio , ple s c nta t th a dr es b low or

y ur lo al IEC memb r Natio al Commite for f ur th r infor matio

Th Inter natio al Ele tr ote h ic l Commis io (IEC) is th le din glo al or ga iz tio th t pr ep r es a d p bls e

Intern tio al Sta d r ds for al ele tr i al ele tr onic a d relate te h olo ie

A bout IEC publc tio s

Th te h ic l c nte t of IEC p blc tio s is k pt u d r c n ta t r eview b th IEC Ple s ma e s r e th t y u h v th

late t e itio , a c r r ig n a or a ame dme t mig t h v b e p bls e

IEC Catalo u - webstore.e ch/cat alo u

Th st an -alo e a plc at io for c on ult in t he e t ire

biblo ra hic al informat io o IEC Int ern tio al St an ard ,

Te h ic al Sp cific at io s, Tec hnic al R ep rt s a d ot her

d c ume t s A v aia le for PC, Ma OS, A ndroid Ta let s a d

iPa

IEC publc t io s s arch - w w w.e ch/s arch ub

Th a v anc ed s arc h e a le t o fin IEC p blc t io s b a

v riety of c riteria (efere c n mb r, text, t ec hnic l

c mmit t ee,…) It als giv es informat io o projec t s, re lac ed

Ele tro edia - www.ele tro edia.org

Th world's le din o ln dic t io ary of elec t ro ic a d

elec t ric l t erms c ont ainin 2 0 0 t erms a d d finit io s in

En ls a d Fre c , wit h e uiv le t t erms in 15 a dit io al

la g a e A ls k nown a t he Int ern t io al Ele t rot ec hnic l

V oc ab lary (IEV ) o ln

IEC Glos ar y - st d.e ch/glos ary

6 0 0 elec t rote h ic l t ermin lo y e t rie in En ls a d

Fre c ex t rac t ed fom t he Terms a d Definit io s cla s of

IEC p blc tio s is u d sin e 2 0 Some e trie h v b e

c lec t ed fom e rler p blc at io s of IEC TC 3 , 7 , 8 a d

CIS R

IEC Cu t omer Servic Centre - webst ore.e ch/cs

If y ou wis t o giv u your fe d ac k o t his p blc t io or

n e furt her a sist an e,ple s c ont ac t t he Cu t omer Servic

Ce t re: c sc@ie c h

IEC TR 62001- 3

Edit io 1.0 2 16-0

CONTENTS

FOREWORD 7

INTRODUCTION 9

1 Sco e 10 2 Normative ref eren es 10 3 Harmonic interaction acros con erters 10 3.1 General 10 3.2 Practical exp rien e of pro lems 1

3.3 In icators of where harmonic interaction is sig if i ant 13 3.4 Interaction phenomena 14 3.5 Imp ct on AC f ilter desig 15 3.5.1 General 15 3.5.2 AC side third harmonic 15 3.5.3 Direct c r ent on the AC side 16 3.5.4 Characteristic harmonic 16 3.6 General overview of model n tec niq es 16 3.6.1 General 16 3.6.2 Time domain AC-DC-AC interaction model 18 3.6.3 Freq en y domain AC-DC-AC interaction model 19 3.6.4 Freq en y domain AC-DC interaction model 19 3.6.5 Freq en y domain c r ent source model 19 3.7 Interaction model n 2

3.7.1 General 2

3.7.2 Coupl n b twe n network 2

3.7.3 Drivin f orces 21

3.7.4 Sy tem harmonic imp dan es 2

3.8 Stu y method 2

3.8.1 Freq en y domain 2

3.8.2 Time domain 2

3.9 Comp site resonan e 23 3.10 Core saturation in ta i ty 23 3.1 Partic lar con ideration f or b c -to-b c con erters 2

3.12 Is ues to b con idered in the desig proces 2

3.12.1 General 2

3.12.2 Fu damental f req en y an lo d is ues 2

3.12.3 Negative phase seq en e 2

3.12.4 Pre-existin harmonic distortion 2

3.12.5 AC network imp dan e 2

3.12.6 Con erter control s stem 2

3.12.7 Combination with "clas ic" harmonic generation 29 3.12.8 Relative mag itu e of p irs of low-order harmonic 2

3.12.9 Sup rp sition of contribution 3

3.13 Paral el AC l nes an con erter tran f ormer saturation 30 3.14 Pos ible cou terme s res 3

3.14.1 AC (an /or DC) f ilters 3

3.14.2 DC control desig 3

3.14.3 Op ratin restriction an desig protection 33

3.15 Recommen ation for tec nical sp cif i ation 3

3.15.1 General 3

3.15.2 Sp cif ied desig data 3

3.15.3 Req irements regardin calc lation tec niq es 3

4 AC network imp dan e model n 3

4.1 General 3

4.2 Implcation of inac urate def i ition of network imp dan e 3

4.3 Con ideration f or network model n 3

4.3.1 General 3

4.3.2 Project l fe exp ctan y an ro u tnes of data 3

4.3.3 Network o eratin con ition 3

4.3.4 Network imp dan es f or p rforman e an ratin calc lation 38 4.3.5 Model n of network comp nents 3

4.3.6 Re resentation of lo d at harmonic f req en ies 4

4.4 Network harmonic imp dan e en elo es 4

4.5 Method of determinin en elo e c aracteristic 43 4.5.1 General 4

4.5.2 L w order harmonic 4

4.5.3 Mid- an e an hig er order harmonic 4

4.5.4 Balan in of is an b nef it 4

4.5.5 Con ideration of toleran es on harmonic b n s 4

4.5.6 Two se arate en elo es f or one harmonic b n 4

4.5.7 Critical en elo e p rameters 4

4.5.8 Imp dan e en elo es for p rf orman e an ratin con ition 49 4.6 Examples of the imp ct of dif f erent network imp dan e re resentation 50 4.6.1 Ef f ect of network en elo e p rameters on res ltant distortion 5

4.6.2 Ef f ect of network minimum resistan e on f ilter ratin 53 4.7 Interharmonic imp dan e as es ment 5

4.8 Me s rement of network harmonic imp dan e 5

4.9 Con lu ion 5

5 Pre-existin harmonic 57 5.1 General 5

5.2 Model n an me s rement of pre-existin harmonic levels 5

5.3 Harmonic p rforman e evaluation, method an dis u sion 6

5.3.1 General 6

5.3.2 "In remental" harmonic p rf orman e evaluation 6

5.3.3 "Ag regate" harmonic p rforman e evaluation 61

5.3.4 Both "in remental" an "ag regate" p rf orman e evaluation 6

5.3.5 "In remental" an "maximum mag ifi ation f actor harmonic p rforman e evaluation 6

5.4 Calc lation of total harmonic p rf orman e in ices 6

5.5 Harmonic ratin evaluation 64 5.6 Dif fic lties with the voltage source/worst network model f or ratin 6

5.6.1 Bac grou d 65 5.6.2 Il u tration of the voltage source/worst network method 66 5.7 Further p s ible calc lation proced res f or ratin evaluation 6

5.7.1 Usin me s red levels of pre-existin distortion 6

5.7.2 Ap lyin comp tibi ty level voltage source at the f ilter bu b r 7

5.7.3 Limitin the f ilter bu harmonic voltage to a maximum level f or f ilter

ratin (ML R) 7

5.7.4 Limitin total source distortion to the def i ed THD 7

5.7.5 Limitin harmonic order of pre-existin distortion 7

5.8 Con lu ion 7

An ex A (informative) L cation of worst-case network imp dan e 76 An ex B (informative) Ac urac of network comp nent model n at harmonic f req en ies 7

B.1 General 7

B.2 L ad 7

B.3 Tran f ormers 8

B.3.1 Tran f ormer re ctan e 8

B.3.2 Tran f ormer resistan e 8

B.4 Tran mis ion l nes 85 B.5 Sy c ronou mac ines 8

B.6 Model n of resistan e in harmonic analy is sof tware 8

An ex C (informative) Further g idan e for the me s rement of harmonic voltage distortion 91

An ex D (informative) Project exp rien e of pre-existin harmonic is ues 93 D.1 General 9

D.2 Third harmonic overlo d of f ilters in a b c -to-b c s stem 93 D.3 Third an f if th harmonic overlo d of filters in a l ne tran mis ion 9

D.4 Overlo d of a DC side 6 th harmonic filter 9

An ex E (informative) Worked examples s owin imp ct of pre-existin distortion 9

E.1 General 9

E.2 Pre-existin distortion 97 E.2.1 Example 1 – Il u tration of mag ifi ation 9

E.2.2 Imp ct of network imp dan e p rameters 101 An ex F (informative) Comp rison of calc lation method 10 F.1 General 10 F.2 Ref eren e case – Con erter generated harmonic only 10 F.3 Method 1 – Source voltages b hin imp dan e sector 10 F.4 Method 2 – Source voltages at f ilter bu (se 5.7.2) 10 F.5 Method 3 – Limitin the f ilter bu harmonic voltage to a maximum level for f ilter ratin (ML R) (se 5.7.3) 10 F.6 Method 4 – Limitin total source distortion to the THD level (se 5.7.4) 10 F.7 Method 5 – Pre-existin harmonic con idered only up to the 10 th , with 10 % margin on con erter generation f or remain er (se 5.7.5) 1

0 Bibl ogra h 1 1 Fig re 1 – Key elements of a complete AC-DC-AC harmonic interaction model 17 Fig re 2 – Eq ivalent circ it for evaluation of harmonic interaction with DC side interaction f req en y gre ter than AC side f un amental freq en y 21

Fig re 3 – DC side 6 th harmonic voltage d e to AC side 5 th harmonic (f i ed an le) an 7 th harmonic (varyin an le) 2

Fig re 4 – Simple circ it u ed to re resent AC network imp dan e at 5 th an 7 th harmonic 2

Fig re 5 – Example of a sin le imp dan e loc s for harmonic orders 2 to 4 41

Fig re 6 – Example of simple circle en elo e en omp s in al s at er p ints f or

harmonic orders 2 to 4 4

Fig re 7 – Example of an imp dan e en elo e for 7

th

to 13th

harmonic with

as ociated s ater plots 4

Fig re 10 – Example of the ne d to exten the b n of harmonic to al ow f or

resonan e ef f ects 4

Fig re 1 – Ap l cation of toleran e ran e in p rcentage of the harmonic n mb r 4

Fig re 12 – Ap l cation of toleran e ran e in p rcentage of the harmonic n mb r,

zo med to s ow 1 th an 13

th

harmonic 4

Fig re 13 – Example s owin two imp dan e en elo es f or a p rtic lar b n 4

Fig re 14 – Example of imp dan e en elo es u der "p rforman e" an " atin "

con ition for harmonic orders 4

th

to 7th

5

Fig re 15 – Example of imp dan e en elo es "p rf orman e" an " atin " con ition

f or harmonic orders 2

th

to 31st

5

Fig re 16 – Dis rete en elo es f or dif f erent groups of harmonic 51

Fig re 17 – Example s owin a distributed generation cau in a out 15 % aten ation

of rip le control sig al at the PCC 5

Fig re 18 – Generic circ it model f or calc lation of harmonic p rforman e or ratin 5

Fig re 19 – Il u tration of b sic voltage q al ty con e ts with time/location statistic

Fig re 2 – Typical voltage mag if i ation factor 6

Fig re 2 – Pre-existin distortion set to me s red levels (plu margin) 68

Fig re 2 – Pre-existin distortion a pl ed directly at the f ilter bu 7

Fig re 2 – Harmonic voltage stres on a s u t ca acitor with IEC plan in levels

a pl ed 7

Fig re A.1 – Eq ivalent circ it model f or demon tration of worst-case resonan e

b twe n AC f ilters an the network 7

Fig re A.2 – Diagram in icatin vectors Z

F, Z

N

H 7

Fig re B.1 – Typical eq ivalent lo d network 8

Fig re B.2 – Relative er or of eq ivalent lo d los resistan e R

n

of u in [2 ]

comp red with Electra 16 [2 ] model 8

Fig re B.3 – Ef f ect of temp rature on tran f ormer lo d los 8

Fig re B.4 – Ratio b twe n harmonic an fun amental freq en y resistan e as

calc lated for b lan ed mode comp nents an calc lated f rom averages of ed ced Z

matrix resistan e values 8

Fig re B.5 – Ratio b twe n harmonic an fun amental freq en y resistan e as

calc lated for b lan ed mode comp nents an calc lated f rom averages of ed ced Z

matrix resistan e values, f or varyin e rth resistivity 8

Fig re B.6 – Comp rison of s n hronou mac ine re ctan e b twe n [4-1]

recommen ation an test me s rements for a sal ent p le h dro generator of 3 0 MVA 8

Fig re B.7 – Comp rison of s n hronou mac ine resistan e b twe n [17]

recommen ation an test me s rements for a sal ent p le h dro generator of 3 0 MVA 8

Fig re B.8 – Comp rison of dif f erent a proximation for resistan e variation 89

Fig re B.9 – Network imp dan e for Araraq ara s bstation 90

Fig re E.1 – Harmonic models for con erter an f or pre-existin distortion 9

Fig re E.2 – Ge metrical vis al sation of selectin worst-case imp dan e f or con erter

harmonic 9

Fig re E.3 – Simple f ilter s heme to i u trate mag if i ation 9

Fig re E.4 – Plots i u tratin mag ifi ation of variou pre-existin harmonic 101

Fig re F.1 – Network imp dan e sector u ed in example 10

Fig re F.2 – As umed f ilter s heme for examples of dif f erent method of calc lation 10

Fig re F.3 – IEC plan in levels u ed f or source voltages in the stu y 10

Ta le 1 – Dominant f req en ies in AC–DC harmonic interaction 15

Ta le 2 – Comp rison of calc lated harmonic voltage distortion b twe n two method

of re resentin network harmonic imp dan e 5

Ta le 3 – Comp rison of calc lated harmonic voltage distortion con iderin the

variation of network imp dan e an le 5

Ta le 4 – Comp rison of calc lated f ilter harmonic c r ent con iderin the variation of

network minimum resistan e an filter detu in 5

Ta le 5 – Ampl f i ation factor tanΦ at dif ferent network imp dan e an les 6

Ta le 6 – Variation of calc lated f ilter harmonic c r ent as a f un tion of detu in 71

Ta le B.1 – Con tants f or resistan e adju tment – f i e p rameter eq ation 89

Ta le E.1 – Parameters of elements of a simpl f ied f ilter s heme s own in Fig re E.3 9

Ta le E.2 – Voltage an c r ent distortion f or Z

min

= 1 Ω an varyin Φ 101

Ta le E.3 – Voltage an c r ent distortion f or Φ = ± 5° an varyin Z

min 10

Ta le F.1 – Ta le F.1 – Parameters of comp nents of filters s own in Fig re F.2 10

Ta le F.2 – Comp nent ratin calc lated u in dif f erent calc lation method 10

Ta le F.3 – Ratin calc lation u in Method 3 – f or BP1 13 C1 10

Ta le F.4 – Ratin calc lation u in Method 3 – f or HP2 R1 10

Ta le F.5 – Ratin calc lation u in Method 4 – f or BP1 13 C1 1 0

INTERNATIONAL ELECTROTECHNICAL COMMISSION

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 cific tio 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

Pu lc tio (s)”) Th ir pre aratio is e tru te to te h ic l c mmite s; a y IEC Natio al Commite intere te

in th s bje t d alt with ma p rticip te in this pre aratory work Intern tio al g v rnme tal a d n

n-g v rnme tal org niz tio s laisin with th IEC als p rticip te in this pre aratio IEC c la orate clo ely

with th Intern tio al Org niz tio for Sta d rdiz tio (ISO) in a c rd n e with c n itio s d termin d b

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

2) Th formal d cisio s or a re me ts of IEC o te h ic l maters e pre s, a n arly a p s ible, a intern tio al

c n e s s of o inio o th rele a t s bje ts sin e e c te h ic l c mmite h s re re e tatio f rom al

intere te IEC Natio al Commite s

3) IEC Pu lc tio s h v th form of re omme d tio s for intern tio al u e a d are a c pte b IEC Natio al

Commite s in th t s n e Whie al re s n ble eforts are ma e to e s re th t th te h ic l c nte t of IEC

Pu lc tio s is a c rate, IEC c n ot b h ld re p n ible for th wa in whic th y are u e or f or a y

misinterpretatio b a y e d u er

4) In ord r to promote intern tio al u iformity, IEC Natio al Commite s u d rta e to a ply IEC Pu lc tio s

tra s are tly to th ma imum e te t p s ible in th ir n tio al a d re io al p blc tio s An div rg n e

b twe n a y IEC Pu lc tio a d th c r e p n in n tio al or re io al p blc tio s al b cle rly in ic te in

th later

5) IEC its lf d e n t pro id a y ate tatio of c nformity In e e d nt c rtif i atio b die pro id c nformity

a s s me t s rvic s a d, in s me are s, a c s to IEC mark of c nf ormity IEC is n t re p n ible for a y

s rvic s c rie o t b in e e d nt c rtif i atio b die

6) Al u ers s o ld e s re th t th y h v th late t e itio of this p blc tio

7) No la i ty s al ata h to IEC or its dire tors, emplo e s, s rv nts or a e ts in lu in in ivid al e p rts a d

memb rs of its te h ic l c mmite s a d IEC Natio al Commite s f or a y p rs n l injury, pro erty d ma e or

oth r d ma e of a y n ture wh ts e er, wh th r dire t or in ire t, or f or c sts (in lu in le al fe s) a d

e p n e arisin o t of th p blc tio , u e of, or rela c u o , this IEC Pu lc tio or a y oth r IEC

Pu lc tio s

8) Ate tio is drawn to th Normativ ref ere c s cite in this p blc tio Us of th refere c d p blc tio s is

in is e s ble for th c re t a plc tio of this p blc tio

9) Ate tio is drawn to th p s ibi ty th t s me of th eleme ts of this IEC Pu lc tio ma b th s bje t of

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

The main tas of IEC tec nical commit e s is to pre are International Stan ard However, a

tec nical commite may pro ose the publ cation of a Tec nical Re ort when it has col ected

data of a diff erent kin from that whic is normal y publ s ed as an International Stan ard, f or

example "state of the art"

IEC TR 6 0 1-3, whic is a Tec nical Re ort, has b en pre ared by s bcommit e 2 F:

Power electronic f or electrical tran mis ion an distribution s stems, of IEC tec nical

commit e 2 : Power electronic s stems an eq ipment

This f irst edition of IEC TR 6 0 1-3, together with IEC TR 6 0 1-1, IEC TR 6 0 1-2 an

IEC TR 6 0 1-4, can els an re laces IEC TR 6 0 1 publ s ed in 2 0 This edition

con titutes a tec nical revision

This edition in lu es the f olowin sig if i ant tec nical c an es with resp ct to IEC TR 6 0 1:

a) exp n ed an s p lemented Clau e 6;

b) new Clau e 4;

c) new Clau e 5;

d) new an exes on the location of worst case network imp dan e;

e) ac urac of network comp nent model n at harmonic freq en ies;

f ) f urther g idan e f or the me s rement of harmonic voltage distortion;

g) project exp rien e of pre-existin harmonic is ues;

h) worked examples s owin imp ct of pre-existin distortion;

i) comp rison of calc lation method

The text of this Tec nical Re ort is b sed on the f ol owin doc ments:

En uiry draft Re ort o v tin

Ful information on the votin for the a proval of this doc ment 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 6 0 1 series, publ s ed u der the general title H igh-v ltag d ire t

c re t (H VDC) s stems – Guidan e to the s e ific tion a d de ign e alu tio of AC filters,

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 datein icated on the IEC we site un er "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

A bi n ual version of this publ cation may b is ued at a later date

IMPORTANT – Th 'colour in ide' logo on the cov r pa e of this publ c tion indic te

that it contains colours whic are consid re to be us f ul f or th cor e t

understa din of its conte ts Us rs s ould theref ore print this doc me t usin a

colour printer

INTRODUCTION

The IEC TR 6 0 1 series is stru tured in four p rts:

Part 1 – Overview

This p rt con ern sp cif i ation of AC f ilters f or hig -voltage direct c r ent (HVDC)

s stems with l ne-commutated con erters, p rmis ible distortion l mits, harmonic

generation, filter ar an ements, f ilter p rforman e calc lation, f ilter switc in an re ctive

p wer management an c stomer sp cif ied p rameters an req irements

Part 2 – Performan e

This p rt de ls with c r ent-b sed interf eren e criteria, desig is ues an sp cial

a plcation , f ield me s rements an verif i ation

Part 3 – Model n

This p rt ad res es the harmonic interaction acros con erters, pre-existin harmonic ,

AC network imp dan e model n , simulation of AC filter p rf orman e

Part 4 – Eq ipment

This p rt con ern ste d -state an tran ient ratin s of AC f ilters an their comp nents,

p wer los es, au ible noise, desig is ues an sp cial a pl cation , f ilter protection,

seismic req irements, eq ipment desig an test p rameters

HIGH-VOLTAGE DIRECT CURRENT (HVDC) SYSTEMS –

Part 3: Model ing

This p rt of IEC TR 6 0 1, whic is a Tec nical Re ort, provides g idan e on the harmonic

interaction acros con erters, pre-existin harmonic , AC network imp dan e model n an

simulation of AC filter p rf orman e

The s o e of this doc ment covers AC side f ilterin for the f req en y ran e of interest in

terms of harmonic distortion an au ible freq en y disturb n es It ex lu es filters desig ed

to b ef f ective in the PLC an radio interf eren e sp ctra

This doc ment con ern the "con entional" AC f ilter tec nolog an l ne-commutated hig

-voltage direct c r ent (HVDC) con erters

The folowin doc ments are ref er ed to in the text in s c a way that some or al of their

content con titutes req irements of this doc ment F r dated referen es, only the edition

cited a pl es For u dated referen es, the latest edition of the ref eren ed doc ment (in lu in

an amen ments) a pl es

IEC TR 610 0-3-6:2 0 , Electroma n tic c mp tibility (EMC) – P art 3-6: Limits – As e sme t

of emis io limits for th c n e tio of d istortin installatio s to M V, H V a d EH V p wer

s stems

IEC 610 0-4-3 , Ele troma n tic c mp tib ity (EMC) – P art 4-3 : Te tin a d me s reme t

te h iq e – P ower qu lity me s reme t meth ds

3 Harmonic interaction acros converters

In order to faci tate the analy is of harmonic generation by an HVDC con erter, simpl f yin

as umption are of ten made Typicaly, the HVDC con erter is regarded as a generator of

harmonic c r ents, with an in nite internal imp dan e Su h an as umption is re sona ly

val d f or practical purp ses f or most harmonic , an is the b sis of the calc lation method

des rib d in IEC TR 6 0 1-1

The c stomer s ould b aware, however, that s c a simpl f ied a pro c has lmitation , an

can le d to in or ect analy is an desig in some circ mstan es In practice, the con erter is

a l n b twe n the AC an DC side harmonic s stems, an the AC side harmonic c r ents

may b stron ly in uen ed by the harmonic imp dan e an harmonic c r ent f lows on the DC

side

This is p rtic larly true f or low-order harmonic , an it is stron ly recommen ed that the

analy is of third harmonic distortion an f ilterin req irements s ould take into ac ou t the

AC/DC side harmonic interaction At the 1

also b sig if i ant At hig er f req en ies, althou h interaction oc ur, their practical imp ct

on filter desig an harmonic p rf orman e wi normal y b negl gible

Subclau es 3.2 to 3.15 give an overview of the interaction phenomena, foc sin on practical

impl cation f or AC f ilter desig The tec nical sp cif i ation s ould make it cle r that s c

phenomena have to b taken into ac ou t, an the c stomer s ould b a le to ad res the

s bject in his evaluation of the bid ers’ desig s

The terms "harmonic interaction" an "cros -mod lation" are u ed s non mou ly in this

re ort "Cros -mod lation" is to b u dersto d here as the proces of harmonic tran fer

acros one con erter, not, as it is sometimes u ed in a more sp cif i sen e, as the tran fer of

harmonic from one AC s stem to another via the intervenin HVDC l n

CIGRE Tec nical Broc ure 14 [1]

1

dis u ses in detai the tec nical asp cts related to the

s bject This is a comprehen ive review of the s bject an in lu ed valua le ref eren es to

other publ cation However, it con entrates on the the retical asp cts of calc lation

proced res CIGRE Tec nical Broc ure 5 3 [2] contain more g idan e on the practical

req irements for sp cif yin an evaluatin the tre tment of cros -mod lation d rin a ten er

an s bseq ent desig proces an some asp cts not in lu ed or only brief l covered in [1]

Some of the f un amental con lu ion of [1] [2] an other referen ed b ok an p p rs have

b en s mmarised in this doc ment

Harmonic interaction acros the con erters can b a cau e of pro lems, some examples of

whic are i u trated in 3.2 Pro er con ideration of cros -mod lation d rin the desig

proces can b of b nef it, not only in avoidin s c future pro lems in o eration, but also in

p s ibly simplf yin desig s There are examples of where 3

rd

harmonic f ilterin would have

b en neces ary when u in a simpl f ied clas ic calc lation with a stiff c r ent source, but

s own to b u neces ary when a ful interaction model was a pl ed, takin into ac ou t the

imp dan es on b th sides of the con erter It s ould therefore not alway b as umed that

con ideration of cros -mod lation wi introd ce pro lems or make the desig more dif f ic lt –

it may actual y resolve some dif fic lt is ues

This doc ment do s not recommen pres ribin calc lation proced res an con ition in the

c stomer’s tec nical sp cif i ation In practice, is ues in olvin harmonic interaction have

b en tre ted in very dif f erent way an u in dif ferent stu y method by variou HVDC

contractors in the p st However, c stomers ne d comp ra le bid an want to b in control

of the ris s as ociated with this phenomenon Clau e 3 wi theref ore pinp int the imp rtant

as umption that ne d to b def i ed in a tec nical sp cif i ation an it wi recommen that

contractors s ould ju tify their c osen calc lation proced re an verif y its ac urac

3.2 Pra tic l e perie c of problems

There has b en con idera le exp rien e f rom o erational HVDC s hemes of ad erse

harmonic interaction b twe n AC an DC sides of the con erter Several exp rien es are

des rib d in detai in [1] A brief s mmary of some i u trative is ues is given b low

One of the e rlest in idents of re orted interaction is related to the Kin s orth HVDC l n [3]

The p rtic lar combination of DC re ctors an DC ca le ca acitan e of the Wi es en p le

res lted in a series resonan e con ition at the f un amental freq en y in the DC circ it A

smal 2

n

harmonic present on the AC side theref ore res lted in hig f un amental c r ent on

the DC side, whic in turn gave cau e to u eq al f irin pulse sp cin This res lted in a

f urther contribution to the f un amental freq en y voltage on the DC side an cre ted smal

direct c r ents in the con erter tran formers, whic ten ed to saturate the tran f ormer cores,

generatin further 2

n

harmonic distortion on the AC side An ad itional f l x control lo p in

the HVDC control s stem solved the pro lem This was one of the e rl est examples of what

1

Numb rs in s u re bra k ts refer to th Biblo ra h

is k own as "core saturation in ta i ty" whic has s bseq ently b en o served on several

HVDC s hemes

At the Chate u uay b c -to-b c con erter station, a simiar phenomenon oc ur ed A 2

n

harmonic resonan e con ition was o served at the AC side of the con erter [4] The initial y

smal pre-existin voltage distortion at the 2

n

harmonic was tran f er ed to the DC side of the

con erter The res ltin f un amental c r ent on the DC side tran fer ed b c to the AC side

harmonic, this con erter c r ent gave rise to a cor esp n in voltage distortion, an the

lo p was closed The pro lem was solved by introd cin an au i ary direct c r ent control er

whic cre ted an external dampin f or the fun amental freq en y comp nent of the direct

c r ent Sh nt f ilters tu ed to the 2

n

harmonic have also b en in tal ed on the AC side

Durin commis ionin of the Gez oub -Shan hai HVDC tran mis ion, non-c aracteristic DC

harmonic c r ents of orders 2, 6 an 18 were o served whic cau ed an u d ly large

eq ivalent disturbin c r ent It was f ou d that pre-existin voltage distortion of order 3, 5

an 7 on b th AC sides cau ed a n mb r of non-c aracteristic voltage distortion on the DC

side These met with ne r resonant con ition at these f req en ies on the DC side res ltin

in the o served c r ent distortion The implementation of ad itional resistive dampin in the

DC f ilters, as wel as c an es to the neutral ca acitor, solved the pro lem

For the same s heme, it was also re orted that the 1

In the desig of the Que ec-New En lan multi-terminal s heme, a 6 Hz comp nent in the

DC circ it cau ed by in u tion from ne rby AC l nes was anticip ted d e to the plan ed DC

l ne route This in u ed f un amental f req en y c r ent would cros the con erter an

generate direct c r ent in the tran former win in , whic could le d to core saturation The

desig of the s heme theref ore in lu ed a series bloc in f ilter tu ed to the fun amental

f req en y in erted in the DC neutral of e c con erter

A secon is ue in the same project was related to the Radis on con erter station [5] The

station is located in an are of the far north of Canada where ge mag etic activity is stron

an the grou d resistivity is hig This combination can cre te hig direct c r ents in

tran former neutrals d rin ge mag etic storms Durin s c an event, 5

th

th

harmonic

distortion on the AC side d e to tran f ormer saturation prod ced ex es ive 6th harmonic on

the DC side res ltin in the fai ure of a DC f ilter ar ester It was f ou d that the con erter

imp dan e as se n from the AC side, whic was he vi y determined by 6

th

harmonic

imp dan e of the DC side, s if ted the resonan e f req en y b twe n the AC network an the

complete con erter station from b low 3

harmonic AC f ilters This re-tu in circ it con ists of a f ilter

re ctor in p ral el with a resistor, in tal ed at the neutral side of the f ilter This circ it, normaly

s ort-circ ited by a byp s in switc , is activated when hig 5

th

or 7th

related to tran former core saturation A solution was re orted whic in olved mod lation of

the con erter f irin an les

At the Kristian an con erter station on the Norwegian side of the Skager ak, HVDC

Tran mis ion s heme tran former core in ta i ties an oc asional sig if i ant harmonic

distortion in the low order ran e have b en o served, whic led to trip in of AC l nes an

tran formers The retical in estigation [6] in icated that the energization of a 4 0kV s u t

re ctor in the Tjele con erter station in Denmark can cau e saturation of the local

tran formers The res ltin AC side 2

n

harmonic voltage distortion cau e a sig if i ant

f un amental c r ent in the DC circ it whic is tran fer ed to the Kristian an con erter station

in Norway The Kristian an con erters tran f orm this f un amental c r ent into a DC c r ent

on their AC side res ltin in u s mmetrical tran f ormer saturation The zero seq en e

c r ents th s generated on the network side can re c values whic are a le to trig er AC

l ne protection The ef f ectivenes of a 5 Hz dampin control simi ar to the one develo ed f or

the Chate u uay project was proven f or the Skager ak s heme

A secon ef f ect, oc asional y o served at Kristian an , was tran f ormer trip in d e to

c r ent overlo d It was fou d that hig negative phase seq en e comp nents of the

f un amental f req en y voltages in Norway, cau ed by he v lo d f low over not f ul y

tran p sed AC l nes, cau e third harmonic c r ent generation by the con erters on the AC

side, an this feed into an AC network whic can exp rien e oc asional resonan e at third

harmonic As these network resonan es can not b avoided, detu in of the imp dan e by

temp rary f ilter trip in , trig ered by a protection related to hig harmonic c r ents, was

defi ed as a general cou ter me s re

The Sasaram 5 0MW b c -to-b c HVDC station intercon ects two as n hronou AC

s stems Durin initial commis ionin testin , on some oc asion the u b lan e in one of the

AC s stems ex e ded the sp cified level of 1 % NPS voltage This res lted in 2

n

harmonic

generation on the DC side an con eq ent 3

rd

harmonic c r ent generation into the AC

s stem This res lted in thermal overlo d of the resistors in the in tal ed 3

rd

harmonic C-typ

f ilters The pro lem did not p rsist, but i u trated the sen itivity of some station eq ipment to

AC-DC harmonic interaction

Durin desig stu ies of a large u derse HVDC intercon ection in Ja an, a v lnera i ty to

core saturation in ta i ty was o served The DC ca le s stem ex ibited series resonan e

ne r the fun amental f req en y, an the AC s stems had an imp dan e resonan e ne r the

secon harmonic The AC s stems were not p rtic larly we k F r purp ses of comp ction,

the s u t ca acitor b n s were implemented u in de d-tan con tru tion at medium voltage,

with tran f ormers con ectin these b n s to the EHV level The tran former ca acitor

combination yielded a series resonan e ne r the third harmonic, introd cin the imp dan e

resonan e with the grid at a lower f req en y ne r secon harmonic Smal sig al analy is

tec niq es, des rib d in [7] were u ed to s re n s stem con ition for this core saturation

in ta i ty phenomenon A modif i ation of the ca acitor b n desig was implemented, whic

has avoided this is ue d rin o eration

3.3 Indic tors of where harmonic intera tion is signif ic nt

The practical impl cation of AC-DC harmonic interaction are mainly related to low order

harmonic Althou h the the ry of cros -mod lation is eq al y val d at al freq en ies, the

in re sin in u tive imp dan e of the con erter tran formers an DC side smo thin re ctors

with risin freq en y ten to l mit hig er order DC side harmonic c r ents an , con eq ently,

their tran f er ed imp ct on the AC sides is cor esp n in ly smal er

Harmonic interaction has to b con idered f or b th harmonic p rforman e an f ilter ratin

typ calc lation as wel as its imp ct on protection an the overal d namic b haviour of the

con erter station an its controls

HVDC s stems con ected to AC nodes with low s ort circ it ratios, i.e implyin hig network

imp dan e at low order harmonic f req en ies, an /or nodes whic ten to exp rien e

sig if i ant negative phase seq en e comp nents of the fun amental voltage, are more lkely

to b at ris of harmonic interaction pro lems

For b c -to-b c s hemes, interaction s ould alway b in estigated, as their DC circ it may

not provide ef f ective smo thin or dampin of harmonic an as they may intercon ect

s stems with as n hronou or even dif ferent nominal f req en ies

, 5th

th

harmonic resonan es on the AC side are esp cial y

sig if i ant in s p ortin harmonic in ta i ties, the DC side s ould not b sen itive to

an DC side network are tu ed to complementary freq en ies, i.e if the AC side has a

p ral el resonan e at freq en y f

In a case where the in u tan e of a DC smo thin re ctor in the DC side is low, 12

th

harmonic c r ent in the con erter on the DC side may also b an is ue, with an imp ct on the

relative levels of AC side 1

harmonic is low,

an , theref ore, the tran f er ed 1

th

an 13th

c r ents wi have a pro ortional y gre ter imp ct

Fu damental f req en y in u tion in DC lnes from ne rby AC lnes also introd ces a ris of

con erter tran f ormer saturation d e to direct c r ent in the valve win in s, an con eq ent

u desira le ef f ects [8] Detaied stu y of the level of fun amental f req en y in u tion an of

harmonic interaction in s c cases is es ential (se 3.13)

3.4 Intera tion phe ome a

The AC voltage an c r ent wavef orms can b con idered to b comp sed of p sitive,

negative an zero seq en e comp nents of the f un amental freq en y alon with p sitive,

negative an zero seq en e components of other f req en ies The DC side waveforms can

simi arly b expres ed as a DC comp nent plu a bro d sp ctrum of other freq en ies The

con ersion proces in olved in the con entional brid e con ected con erter esta l s es a

wel defi ed relation hip b twe n freq en ies on the AC side of the con erter an f req en ies

on the DC side

In general, the relation hip is governed by several simpl f ied rules

• The u grou ded star an delta tran f ormer con ection on the valve side of the

con erter tran f ormers preclu e the tran fer of zero seq en e f req en ies f rom the AC

side of the con erter tran f ormers to the DC side Zero seq en e coupl n is l mited to

secon order ca acitive tran fer ef fects

• An given p sitive seq en e f req en y gre ter than f un amental on the AC side of the

con erter is con erted to a dominant f req en y on the DC side whic is lower in

f req en y than the AC side freq en y by an amou t precisely eq al to the f un amental

f req en y of the AC side of the con erter For AC side p sitive seq en e f req en ies

les than f un amental, the res ltant DC side f req en y is the complement of the AC

side freq en y

• An given negative seq en e freq en y on the AC side of the con erter is con erted

to a dominant freq en y on the DC side whic is gre ter in freq en y than the AC side

f req en y by an amou t precisely eq al to the fun amental f req en y of the AC side

of the con erter

• An given freq en y on the DC side of the con erter is converted to two dominant

f req en ies on the AC side A p sitive seq en e f req en y is cre ted whic is gre ter

than the DC side freq en y by an amou t precisely eq al to the f un amental

f req en y of the AC side of the con erter If the DC side freq en y is gre ter than the

AC side f req en y, a negative seq en e f req en y is also cre ted whic is les than

the DC side f req en y by an amou t precisely eq al to the f un amental f req en y of

the AC s stem If the DC side freq en y is les than the AC side f un amental

f req en y, then in te d of a negative seq en e f req en y, a secon p sitive seq en e

f req en y is generated at a value precisely eq al to the AC side f un amental

f req en y les the DC side f req en y

Ta le 1 provides a s mmary of the dominant freq en ies in olved in an interaction:

Table 1 – Domina t f re ue cie in AC–DC harmonic intera tion

0

D– f

0(n g s q.)

0

0– f

C(p s s q.)

F

D

0(p s s q.)

The a ove rules are not l mited to ju t harmonic f req en ies but in general can b a pled to

al freq en ies Other freq en ies s if ted by multiples of the con erter pulse n mb r times

f un amental f req en y (or their complements) can also b in olved in the con ersion proces

but f or the most p rt, their contribution to an given interaction are of secon order The

relation hips hold for not only ste d -state con ition but can also b o served in q asi

-ste d state con ition as exp rien ed u der prolon ed u b lan e f ault con ition , an even

tran ient con ition (an phenomena lastin gre ter than 1 or 2 c cles)

3.5 Impa t on AC f ilter d sign

Interaction wi in uen e AC filter desig only when the res ltant harmonic are of a

sig if i ant mag itu e so as to af fect either the AC f ilter p rf orman e or the AC f ilter ratin or

b th

Several interaction are k own to in uen e the desig of AC f ilters, an are dis u sed in the

f ol owin s bclau es

3.5.2 AC side third harmonic

One s c set of interaction freq en ies in lu es

• AC side negative seq en e fun amental freq en y,

• DC side secon harmonic, an

• AC side p sitive seq en e third harmonic

The presen e of a s bstantial comp nent of f un amental f req en y negative seq en e

voltage in the commutatin bu voltage (> 1 % to 2 %) wi of ten res lt in large comp nents of

p sitive seq en e third harmonic in the AC side wavef orms an large secon harmonic

comp nents in the DC waveforms Con iderin the two AC network , c r ent flow is l mited by

the series combination of the negative seq en e f un amental freq en y imp dan e an the

third harmonic p sitive seq en e imp dan e With low values of net ef fective imp dan e, an

a large negative seq en e voltage, the negative seq en e c r ent f low an third harmonic

c r ent f low could b large In tal n a low imp dan e s u t con ected third harmonic f ilter to

l mit the third harmonic f low into the AC s stem could actualy exag erate the amou t of

c r ent f low, in re sin the third harmonic c r ent in the f ilter to values a ove the f low without

the AC filter This could b further comp u ded by resonan es b twe n the AC filter an the

AC s stem

The secon harmonic imp dan e of the DC network could also in uen e the desig of the AC

side third harmonic f ilter

3.5.3 Dire t c r e t on th AC side

A secon set of interaction freq en ies af fectin the desig of f ilters in lu es

• AC side secon harmonic p sitive seq en e,

• DC side f un amental freq en y, an

• AC side valve win in DC c r ents

Fu damental f req en y c r ents on the DC side of the con erters can b con erted into

p sitive seq en e secon harmonic an DC c r ents on the AC side of the con erter The

secon harmonic c r ents are tran f ormed an hen e a p ar in the AC network Althou h DC

c r ents are initial y coupled into the AC network, with time, the AC side c r ents decay to

zero and the DC c r ent f low eventual y en s up f lowin throu h the mag etizin p th of the

eq ivalent circ it of the con erter tran f ormer s if tin the tran former saturation

c aracteristic De en in on the mag itu e of the s if t, one-sided tran f ormer saturation

could oc ur res ltin in the generation of secon harmonic p sitive seq en e c r ents d e to

tran f ormer saturation These c r ents could ad to the secon harmonic coupled by the

interaction network exag eratin the c r ent f lows in the thre network

Fu damental freq en y bloc in f ilters on the DC side an secon harmonic s u t con ected

f ilters on the AC side have b en u ed to l mit the interaction ef f ects The desig of the secon

harmonic f ilter s ould take into ac ou t not only the direct interaction ef f ects but also the

p s ible ampl f i ation res ltin f rom interaction with tran former mag etization

3.5.4 Chara teristic harmonic

Interaction can oc ur at c aracteristic f req en ies as wel as low order non-c aracteristic

harmonic is also smal (as can oc ur

on b c -to-b c s hemes with smal or no smo thin re ctors), coupl n can oc ur b twe n

harmonic of b th AC s stem freq en ies could b evident in the resp ctive f ilter wavef orms

The AC f ilters l mit the imp ct of the interaction to the con erters themselves an as a res lt

the interaction is not normal y evident in the AC s stem voltage an c r ent wavef orms The

desig of the AC f ilters however s ould recog ize the p tential for interaction with s ita le

al owan e in the filter p rforman e an ratin

3.6 Ge eral ov rview of mod l in te hniq e

A the retical y comprehen ive in estigation into the harmonic b haviour of an HVDC s stem

is only ful y p s ible if a complete AC-DC-AC interaction model is u ed This is feasible on e

al the AC an DC filter desig is complete an the filter comp nent p rameters are k own

Also, ful AC-DC-AC interaction models but u in a proximate re resentation of the f ilters,

p rha s simply as AC ca acitors of the estimated Mvar ca acity or DC f ilters with the

predicted ca acitor size, can b u ed to investigate low order resonan e con ition d rin an

e rly desig stage when smo thin re ctors an tran f ormer imp dan es are selected, as

wel as the p s ible ne d f or low order f ilterin on either or b th sides

However, it is general y not practica le to u e a complete AC-DC-AC interaction model f or the

detai ed f ilter desig itself , as it impl es that the desig of the in ivid al f ilter s hemes on b th

AC sides an the DC side would have to b done simultane u ly Al s c f ilter desig

calc lation req ire variation of an exten ive n mb r of s stem p rameters an f actors

af f ectin detu in , an to do this for al f ilters in the complete HVDC s stem simultane u ly

would b an extremely compl cated iterative proces This practical dif f ic lty has led to the

u e of red ced models whic at empt to de-couple the variou elements an al ow the desig

of the variou groups of filters to b made in e en ently

One of the most common a pro c es is to p rform the f ilter desig at one con erter station

with a relatively simple f req en y domain model in whic con erters are model ed as a

harmonic c r ent source Cur ent source values f or this eq ivalent are derived from simpl f ied

calc lation The DC filter desig calc lation can also b p rformed u in a simi ar model

with the con erters as harmonic voltage sources However, there are sig if i ant in erent

inac uracies in an s c red ced model

If it can b s own that there is very l t le harmonic coupl n b twe n the two con erter

station , for example d e to a lon HVDC ca le, then it may b p s ible to tre t the two

station in e en ently It may also b u eful to tre t the two station se arately in a p rtial

analy is to in estigate some asp ct of the harmonic tran fer at one station alone

Variou f ul an red ced model n tec niq es are dis u sed in the s bseq ent s bclau es

Al refer to some or al of the s stem comp nents re resented in Fig re 1

More detai s, p rtic larly on the mathematical b c grou d, are given in [1] an [9]

Figure 1 – Ke eleme ts of a complete AC-DC-AC harmonic intera tion model

The circ it s own in Fig re 1 s ows the key elements of an model u ed to stu y AC-DC-AC

harmonic interaction The detai s of these elements are as f ol ows

• AC s stem voltage (U

o): Re resents the p sitive an negative seq en e f un amental

f req en y s stem voltage, plu an pre-existin harmonic distortion, in lu in its

phase seq en e

• AC network harmonic imp dan e (Z

s): This models the ran e of p s ible harmonic

imp dan es (R

s

± j X

s) of the AC network

• AC harmonic f ilters (Z

fn): In lu es the n mb r of con ected f ilters an the typ of f ilter,

f or example tu ed, hig p s , double f req en y Detu in d e to variou re son has

to b re resented if it aff ects the p rtic lar harmonic(s) u der stu y Sh nt ca acitors

are also re resented as their presen e af f ects the overal imp dan e at low harmonic

orders

• Con erter tran former: This re resents the imp dan e of the tran f ormer, also

con iderin an imb lan e b twe n star/star an star/delta con ection an

imb lan es b twe n phases an turn ratios The tran f ormer saturation

c aracteristic may also b re resented, u in a model whic resp n s to direct

c r ent in the valve-side win in s The ran e of the ta -c an er s ould b con idered,

as this has a sig if i ant ef f ect on the tran fer ed imp dan es

• HVDC con erter: Models the f irin controls of the con erter, in lu in an toleran e

ef f ects, plu the hig er level control s stem fun tion s c as con tant c r ent control

an an sp cial fun tion whic af fect harmonic interaction

• DC smo thin re ctor (Z

rn): This is the series in u tan e an resistan e of the re ctor

• DC filter (Z

d): Con iders the n mb r of con ected DC f ilters an the typ of f ilter, for

example tu ed, hig p s , double tu ed Fi ter outages an de-tunin s d e to variou

re son have to b taken into ac ou t The neutral bu ca acitors (not s own) are

also re resented

• Tran mis ion imp dan e: This in lu es the imp dan e of the DC s stem (overhe d

wire or s bmarine/u dergrou d ca le) re resented by the exact tran mis ion l ne

eq ation In the case of a b c -to-b c station, this imp dan e is zero In case of

overhe d l nes, it mig t b neces ary to con ider ad itional voltages/c r ents in u ed

by p ral el AC overhe d l nes L n electrode l nes or neutral con u tors s ould also

b expl citly model ed

For a ful , closed-lo p model, al the a ove AC an DC side s stems an con erters ne d to

b re resented

3.6.2 Time domain AC-DC-AC intera tion model

The most comprehen ive model is a f ul re resentation in the time domain of the AC an DC

side circ its an the con erters, as this in lu es al asp cts of " e l l f e" interaction The

stu ies can b p rf ormed u in electromag etic tran ient programs, in lu in f ul

re resentation of the con erters an their control s stems, s c as PSCAD/EMTDC or EMT

-RV A f ul thre -phase detai ed switc in model of the con erter is in lu ed, in lu in its

control s stem Caref ul at ention s ould b given to c o sin an a pro riate time ste to

ac urately re resent harmonic generation (some programs u e time-ste interp lation to

rel eve some of these is ues) When coupled to models of the AC side an DC side s stems,

this gives a p werf ul to l to analy e the AC-DC-AC interaction acros the con erters The

model can b u ed to calc late the voltage an c r ent wavef orm at an location of interest A

Fourier analy is of the waveform wi determine the harmonic comp nents an the phase

seq en e of these comp nents Exp rien e with F urier analy is to ls is vital an care

s ould b taken to avoid misle din res lts

Althou h a p werf ul tec niq e, s c time domain stu ies req ire a con idera le exp rtise

an ef fort to p rf orm, esp cial y con iderin the lon ru time f or s c complex models an

the man cases whic ne d to b con idered f or f ilter desig Hen e, a complete in estigation

of al relevant f ilter desig cases (in lu in variation of AC network imp dan e an

comp nent toleran es as wel as al lo d an con g ration cases) is not f easible with this

model More ver, sp cif i AC network models are u ed, whic ef f ectively removes the

ca a i ty to in e en ently model network imp dan es at the variou harmonic f req en ies,

s c as s ould b done when the AC network imp dan es are def i ed by en elo es in the

R-X plane Hen e, this tec niq e is normal y u ed as a valdation of the f req en y domain

stu ies for certain sp cifi comp rison cases, esp cial y for low order harmonic res lts

Another p s ible u e for time-domain models is to determine l ne rised harmonic coupl n

p rameters, to re resent the con erters in a s bseq ent f ul closed-lo p f req en y domain

analy is (se 3.6.3) Eff ectively, this a pro c makes the closed lo p freq en y domain

analy is actual y a h brid b twe n freq en y- an time-domain a pro c es

3.6.3 Fre ue c domain AC-DC-AC intera tion model

A model of simi ar extent, but implemented in the f req en y domain, can also in lu e al key

elements to calc late interaction The b nefit of this model is that a very large n mb r of

desig cases can b calc lated very ef f iciently Model n of AC network imp dan e en elo e

in the R-X plane as wel as variation of toleran es can b e si y re lsed

One of the most imp rtant f eatures of a f req en y domain model is the way it con iders the

con erter imp dan e on the AC an DC sides resp ctively Where s simpler models of ten

only u e the commutatin re ctan e of the con erter tran formers, more so histicated models

u e imp dan e tran f ormation fun tion to ac ou t f or al imp dan es An example of s c

f un tion has b en given in [10] The a p rent con erter imp dan e at a given freq en y on

the DC side is in uen ed by AC side imp dan es at variou other freq en ies – an vice

versa A common a pro c is to l mit the n mb r of con idered freq en ies to the most

sig if i ant ones

However, the determination of the case sen itive con erter tran f ormation fun tion (i.e

voltage, c r ent an imp dan e) is dif f ic lt A f un tion takin into ac ou t the ef f ects of

closed-lo p controls can b determined by calc lation u in a cor esp n in time domain

model This mat er has b en ela orated in [10] Interaction can also b stu ied u in an

a pro riate smal-sig al model n to l, s c as des rib d in [9] In either case, f i al

verif i ation of analytic res lts req ires u e of time-domain simulation

In a les so histicated o en-lo p freq en y-domain model n of AC-DC-AC harmonic

interaction, the imp ct of con erter controls is neglected (i.e con tant f irin an les ac ordin

to the selected o eratin p int are u ed) Un er this prereq isite, the tran fer f un tion of

voltages, c r ents an imp dan es can b calc lated analytical y

More detai s on this s bject can b f ou d in [1]

3.6.4 Fre ue c domain AC-DC intera tion mod l

This is a red ced freq en y domain model whic neglects the imp ct of harmonic generated

at the remote station an the tran f er ed imp dan e of that station The DC circ it within s c

a model is simply modeled by a p s ive imp dan e Obviou ly, the b nef it of this

simpl f i ation is that calc lation can b p rf ormed se arately f or e c con erter station an

the n mb r of desig cases is red ced sig if i antly This model mig t b a pl ca le if 2

n

harmonic s u t DC f ilter were in tal ed at the remote station, th s ef f ectively s ort-circ itin

the con erter an remote AC s stem as se n from the DC side

The disad antage of doin so is that the DC imp dan e u ed is les ac urate as it do s not

take into ac ou t the AC network and AC filter imp dan e of the remote station More ver,

the cros -mod lated harmonic contribution of the remote station, whic can b sig if i ant,

wi not b model ed

3.6.5 Fre ue c domain c r e t sourc model

In this simple freq en y domain model n , it is as umed that at al harmonic the con erter

b haves on the AC side as a source of harmonic c r ent This model provides a very ef f icient

way to in estigate the AC f ilter desig f or one con erter station an has b en u ed in [1 ]

when dis u sin other is ues of AC filter desig Harmonic c r ent source values have to b

determined u in an AC-DC interaction model as des rib d a ove, but with only generic or

even no con ideration of AC network an AC filter imp dan e

However, this as umption may introd ce sig if i ant er ors f or low order harmonic For

example, in cases where there may b a natural resonan e b twe n the AC filters an the AC

network close to 3

rd

harmonic, the con erter may b have more lke a harmonic voltage source

than a harmonic c r ent source In s c cases, the said resonan e wi res lt in a hig

imp dan e f or the 3

rd

harmonic as se n f rom the con erter This hig imp dan e wi b

tran fer ed to the DC side an se n mainly as a 2

harmonic c r ents on the AC side, these c r ents would theref ore b de en ent up n the

network an f ilter imp dan e, rather than b havin as a con tant c r ent source

This me n that a time con umin iterative desig proces s ould b u ed whic

re-calc lates c r ent source values u in the anticip ted AC f ilter desig an the AC network

imp dan e Fol owin this, the AC f ilter desig has to b re-con rmed (or c an ed) u in the

new source values Nevertheles , s c a proced re may b q ic er than the u e of a

complete AC-DC-AC interaction model

3.7 Intera tion model ng

For an given set of AC an DC side o eratin con ition (AC voltage, DC voltage an

c r ent an f irin an le), the DC con erter can es ential y b tre ted as a lne r p s ive

device simi ar to a thre win in tran former but whic tran f orms voltages an c r ents

b twe n the thre network (at thre dif f erent f req en ies) in olved in an interaction as

i u trated in Fig re 2 The f i ure de icts the con ition where the DC side interaction

f req en y is gre ter than the fun amental f req en y of AC s stem Re resentation of the

other con ition is simi ar with the seq en e an f req en y of e c of the network

esta l s ed from the ta le of freq en ies s own a ove

The mag itu e of voltages an c r ents whic a p ar in an given interaction are a f un tion

of the coupl n b twe n the network , imp dan es of the AC an DC network at the

resp ctive f req en ies as wel as the mag itu e of the drivin force (or f orces) whic

esta l s es the f req en ies in the f irst place

3.7.2 Coupl ng betwe n n twork

The amou t of coupl n f rom network to network is a direct f un tion of the DC o eratin

con ition an can b q antified in terms of the f irin an le an overla angle If overla is

not con idered in the analy is, the coupl n network b haves as an ide l thre win in

tran former where the "eq ivalent turn ratios" are de en ent on the f irin an le The

harmonic c r ent f low into e c of the thre network is a fun tion of the "eq ivalent turn

ratio" an as s c ef f ectively con ects e c of the two AC network an the DC network in

series In lu ion of overla an le into the analy is is eq ivalent to ad in le kage an

mag etizin re ctan es to the ide l tran former

1 A p sitiv s q e c n twork at fe u n y f

C+ f

0

2 A n g tiv s q e c n twork at fe u n y f

C– f

0

3 Tra sformatio matrix tra sformin v lta e a d c re ts at th h rmo ic fe u n ie

4 D n twork at f re u n y f

C

Figure 2 – Eq iv le t circ it f or e alu tion of harmonic intera tion with DC side

intera tion f re u nc gre ter tha AC side fun ame tal fre u nc

3.7.3 Driving f orc s

Drivin f orces could originate in the AC s stems The drivin f orce could b harmonic in

nature res ltin f rom the presen e of harmonic generatin devices s c as other DC station ,

static var comp n ators (SVCs), tran former saturation, non-l ne r lo d Also, the drivin

f orce could simply b negative seq en e fun amental freq en y voltage res ltin from

u b lan ed lo d in the vicinity of the DC con erter, he vi y lo ded u tran p sed

tran mis ion l nes feedin the con erter or as mmetrical faults an /or 2 phase o eration

d rin sin le p le reclos re

The drivin f orce could originate in the DC con erter itself , with harmonic c r ents and/or

voltages driven by f irin an le j t er, u b lan e in commutatin re ctan es or p s ibly

u b lan e in con erter tran f ormer turn ratios

The third source of drivin forces in lu es voltages an c r ents of the DC tran mis ion

network either coupled electromag etical y or electrostatical y f rom ne rby AC tran mis ion or

directly coupled as a res lt of some AC/DC interaction phenomena at the remote con erter

For the lat er, harmonic of the remote AC s stem freq en y would b in olved res ltin in

non-harmonic interaction f req en ies at the local station

IEC 1

4

Z

ac p(f

C+ f

0)

e

sp(f

C+ f

0)

i

aC(f + f

0)

e

C(f

C+ f

0)

i

D(f

C)

Z

D(f

C)

e

D(f

C)

i

ac n(f

C– f

0)

e

c n(f

C– f

0)

e

s(f

C– f

0)

Z

ac n(f

C+ f

0)

3.7.4 Sy tem harmonic impe a c s

The imp dan e of the thre network at their resp ctive interaction f req en ies play an

imp rtant role in the mag itu e of the voltages an c r ents whic can a p ar at the

con erters Series an p ral el resonan es can oc ur within e c leg of the eq ivalent

network, but also b twe n the thre leg For example an ef f ective series resonan e could

a p ar b twe n the p sitive seq en e AC network an the DC network res ltin in large

c r ent f low at the resp ctive freq en ies b twe n the two network To the negative

seq en e network, the resonan e con ition could a p ar as a p ral el resonan e (with a hig

imp dan e) In f act, the c r ent f low in the p sitive seq en e an DC network could b

ex ited by a smal voltage in the negative seq en e network

3.8 Study methods

3.8.1 Fre ue c domain

Both the p rforman e an ratin calc lation f or f ilters have b en car ied out traditional y

u in freq en y domain analy is an desig to ls A network solution of voltages an

c r ents are calc lated for e c selected harmonic an the weig ted voltages an c r ents for

e c freq en y are combined mathematical y to esta l s some overal p rf orman e or ratin

in ex Stu y of interaction ef f ects req ires the exp n ion of the sin le f req en y network

model into a multi-f req en y model The model could b the simple thre freq en y model

des rib d a ove or could b exp n ed to in lu e a bro d sp ctrum of freq en ies

With freq en y domain analy is, it is p s ible to f oc s on the exact nature of a sp cifi

interaction The AC s stem an DC side harmonic imp dan es can b re di y varied within a

k own spre d of values to esta l s if interaction is l kely to oc ur In the event that

interaction can oc ur, the same proced re can b u ed to esta l s lmitin con ition for the

desig in lu in DC control p rameters an comp nent ratin s It can also b u ed to trade

of f DC desig with p s ible AC an DC s stem o eratin restriction

Freq en y domain analy is is f or the most p rt lmited to "smal sig al" analy is For harmonic

interaction analy is, this is normal y val d as harmonic comp nents are typical y several

orders of mag itu e les than the AC side f un amental freq en y an DC side DC

comp nents of their resp ctive waveforms

The main c alen e in olved in freq en y domain analy is is the derivation of the coupl n

co f ficients whic mathematical y couple the AC an DC network These can b derived

n merical y or analytical y an can b set-up to in lu e the in uen e of the DC controls

In sin le freq en y analy is tec niq es, HVDC con erters are of ten tre ted as ide l harmonic

c r ent (AC side) an voltage (DC side) sources with mag itu es of non-c aracteristic

harmonic u ed in the calc lation b sed on exp rien e f rom me s rements on simulators or

other DC s hemes When con iderin harmonic interaction, this tre tment may not b

completely val d For example, the third harmonic generated by the con erter is for the most

p rt dictated by the mag itu e of negative seq en e voltage at the con erter bu , an hen e

an ide l third harmonic voltage source would provide a more ac urate tre tment in the

analy is than the con entional y u ed c r ent source

3.8.2 Time domain

With the avai a i ty of digital simulation tec niq es a pro c in (or ac ievin ) re l time

ca a i ty, time domain analy is is an ef f ective to l when coupled with Fourier series or

Fourier tran form analy is of the voltage an c r ent waveforms The a pro c in olves the

simulation of a set of sp cif i AC an DC o eratin con ition On e a ste d state con ition

is ac ieved, the voltage an c r ent waveforms are recorded an analy ed f or their f req en y

content The waveform comp nents are then n merical y combined to o tain the traditional

filter p rf orman e an ratin in ices This analy is could b car ied out on a contin ou b sis

providin "on-lne" monitorin of the p rforman e an ratin in ices

The major ad antage of this method is that the simulation is in f act car yin out the harmonic

lo d f low, hen e there is no req irement to compute the coupl n co ff icients A sig if i ant

secon b nef it is the a i ty to o serve s stained interaction whic may b trig ered by some

disturb n e to the network

The major disad antage of the time domain solution is the l mit imp sed on the extent of the

AC network whic can b practical y re resented in an given simulation Without a detai ed

model, AC s stem o eratin con ition , whic may res lt in an interaction, may not b

simulated an hen e the in uen e of the p tential interaction would not b in lu ed in the

filter desig

3.9 Composite re ona c

This is a term whic des rib s a resonan e in olvin the circ its on b th AC an DC sides of

a con erter, in lu in the harmonic tran fers acros the con erter an the action of the

con erter control [1] [14] It is imp rtant to recog ize that the critical resonan e con ition of

the s stem may not b determined by analy is of ju t the AC side or ju t the DC side circ its,

but s ould con ider the whole interl n ed s stem, as s own in [13] This ref eren e presents a

freq en y s an method u in a time domain simulation to l to o tain a more ac urate

freq en y c aracteristic f or identif yin harmonic in ta i ty in HVDC s stems It s ows that

dif ferent imp dan e c aracteristic are o tained if AC an DC imp dan es are determined

in e en ently than if the interln ed s stem is con idered

3.10 Core s turation instabi ty

This phenomenon may b regarded as a sp cial case of comp site resonan e, with the

introd ction of an ad itional “freq en y s if t” with AC side 2

n

harmonic b in generated d e

to one-sided saturation of the con erter tran f ormer core by direct c r ent cau ed by DC side

f un amental f req en y c r ent [1] [3]

3.1 Partic lar considerations f or ba k- o-ba k conv rters

In a b c -to-b c HVDC s heme, there is virtual y no DC tran mis ion imp dan e, there are

no DC f ilters, an smo thin re ctors may b smal or non-existent, d e to the a sen e of DC

s ort-circ its an the lac of interferen e from DC side harmonic Due to the close coupl n

of the rectif ier an in erter, there wi b a sig if i ant harmonic contribution at the rectifier AC

side f rom the in erter an at the in erter AC side from the rectif ier The mag itu e of these

cros -mod lated harmonic can b typical y 10 % to 2 % of the local contribution an is

de en ent up n the imp dan e within the circ it, whic can b se n as the AC side

imp dan es (i.e tran formers, AC f ilter an AC network), tran fer ed to the DC side by

con erter o eration at b th terminals, plu the DC smo thin re ctor, if in tal ed

Althou h a DC smo thin re ctor has an in uen e on the mag itu e of these cros

-mod lated harmonic comp nents, the presen e of s c a re ctor do s not f ul y el minate

them In some desig s, no DC side smo thin re ctors are u ed, res ltin in con idera ly

lower smo thin an hen e in hig er tran f er ed harmonic When smo thin re ctors are

u ed, they normal y have low in u tan e values, as it is impractica le to man facture air core

re ctors with hig in u tan e for very hig direct c r ent ratin s

NOT Th smo thin efe t of a low in u ta c re ctor in th low v lta e, hig c re t circ it c n b a hig or

hig er th n a mu h larg r in u ta c re ctor in a hig v lta e ln tra smis io

If the nominal f un amental f req en ies of the two AC s stems are dif f erent, then d e to

cros -mod lation, harmonic of one f un amental f req en y wi a p ar in the AC s stem of

the other side of the b c -to-b c l n as interharmonic f req en ies, to whic more strin ent

l mits may a ply Be tin of the dif ferent freq en ies wi res lt in s b-harmonic freq en ies

whic may cau e l g t f lic er or torsional ef fects on mac ines

Therefore, in the analy is of b c -to-b c HVDC l n s, it is not ad isa le to u e red ced

interaction models whic con ider only the imp ct of one con erter station

3.12 Is u s to be con idere in the de ign proc s

3.12.1 Ge eral

As dis u sed in the precedin s bclau es, d e to the complex nature of harmonic interaction,

complex an time con umin calc lation are req ired to take al asp cts into ac ou t For

practical re son , simpl f i ation are therefore ne ded, esp cial y d rin the tender stage

when only lmited time is avai a le 3.12 dis u ses as umption an proced res commonly

u ed

Firstly, a distin tion s ould b made b twe n the actual desig proces an evaluation of a

desig , where the later is made simply to demon trate the adeq ac of the desig The

actual desig of the f ilters is of ten made with simpl f ied as umption , for example model n

the con erter as a c r ent source, takin harmonic interaction into ac ou t throu h simpler

model n as dis u sed in previou s bclau es, or by simple man al calc lation A major

re son for this simpl f i ation l es in the nature of desig in a HVDC s heme, where typical y

man asp cts of the overal desig of ten have to b made in p ral el That is, main circ it

calc lation, AC f ilter desig , DC f ilter desig an other stu ies start more or les

simultane u ly an ru in p ral el This is more the case f or a s ort-d ration ten er, but to

some extent it wi sti hold true d rin exec tion of a project when station layout, civi

drawin s, etc await the outcome of tec nical s stem stu ies an s bseq ent eq ipment

sp cif i ation

Therefore, in man cases f ilter desig s have b en completed, an wi contin e to b in the

future, without detai ed con ideration of harmonic interaction In s c situation it is the

desig er’s resp n ibi ty to en ure that there is an in erent margin in the desig s c that it

could demon tra ly me t req irements of ratin an p rforman e, if harmonic interaction

were to b con idered expl citly

Su h a demon tration is of ten made u in time domain to ls, s c as PSCAD/EMTDC, EMTP

or simiar, thou h other method are u ed A restricted n mb r of simulation are u ual y

s f f icient in order to demon trate that a desig is ac e ta le The contractor ne d to b

ca a le of explainin wh s c restricted cases are governin or s f ficient f or desig

Whic model an sof tware to u e is b st determined by the contractor ac ordin to his

ca a i ties an normal practice However, the contractor s ould b pre ared to ju tif y an

verif y the model u ed, throu h the retical an analytical re sonin or an other me n

ac e ta le to the c stomer Esp cial y for newer con erter to ologies, f or whic there is les

practical exp rien e, s c ju tifi ation s ould pref era ly b verif ied throu h practical

me s rements on an actual con erter in service

Further, if a time domain model is u ed an harmonic are evaluated u in Fourier analy is,

some al owan e s ould b made f or er ors introdu ed throu h the l mitation of the model

The ac urac wi b a trade of f b twe n simulation time (how close the simulation comes to

ste d state con ition ) an time-ste in simulation (a to large time ste can res lt in

u re lstical y hig calc lated harmonic levels)

In the dis u sion b low, the foc s of harmonic interaction is on low order harmonic , as

the ry an practical exp rien e in icate that harmonic interaction is mainly an is ue in this

ran e

3.12.2 Fun ame tal fre ue c a d loa is ue

The f un amental freq en y asp cts that can af fect the harmonic interaction are the negative

phase seq en e comp nent of AC bu voltage, the freq en y variation (inasmu h as this

af f ects filter detu in ) an , to some extent, the lo d level of the con erter

It is common practice in f ilter desig to con ider a defi ed o erational ran e f or s stem

f req en y an p sitive seq en e voltage of the intercon ected AC s stems It is also

es ential to def i e a maximum value f or the negative seq en es voltage to b con idered.

Se arate sets of p rameters f or p rforman e an ratin typ calc lation are of ten u ed

When u in s c p sitive seq en e values f or the drivin source U

o

in a model ac ordin to

Fig re 1 alon with a given fun amental AC network impedan e, the res ltin f un amental

f req en y voltage oc ur in at the con erter bu b r may b u re l stic an ex e d normal

o erational l mits Su h a proced re do s not take into ac ou t the f act that in re l AC

s stems, active an re ctive lo d disp tc wi avoid s c ex e tional bu b r con ition

Therefore, the ran e of f un amental voltage is of ten con idered to b def i ed at the con erter

bu rather than b hin the network imp dan e Alternatively, the mag itu e of the source

voltage could b adju ted s c that f un amental f req en y voltage at the con erter bu

cor esp n s to the actual o erational ran e If several dif f erent fi ter con g ration are

stu ied, then the source levels may have to b adju ted f or dif f erent simulation

The con erter lo d levels s ould b selected s c that the variou f ilter con g ration that

can af fect the res lt are al con idered (se IEEE Std 1 2 -2 0 , 5.4.1 [1 ])

If harmonic interaction of c aracteristic harmonic is to b stu ied, then minimum lo d s ould

If the level of negative phase seq en e (NPS) voltage to b con idered is def i ed by the

tec nical sp cif i ation to oc ur at the con erter bu , then when u in the model in Fig re 1

the negative seq en e source voltage is also adju ted s c that the desired mag itu e at the

con erter bu is re c ed However, the situation for the negative phase seq en e

f un amental comp nent is sl g tly diff erent f rom that f or p sitive seq en e The o eration of

the con erter may in itself generate negative seq en e c r ent, whic when f lowin into the

AC network imp dan e wi generate a negative seq en e voltage at the con erter bu The

an le of this comp nent relative to the original source NPS wi de en on the AC an DC

s stem imp dan es, an may b ad itive In that case, the level of NPS at the con erter bu

wi b hig er than the sp cif ied pre-existin source level This wi not b se n by, or s bject

to, an s stem voltage reg lation s c as a pl es to p sitive seq en e voltage, an therefore

s ould b taken into ac ou t as a re l p s ibi ty

Therefore, althou h it is f airly common practice f or tec nical sp cifi ation to def i e NPS

either at the con erter bu or in a general way without sp cif yin location, it would b more

cor ect to def i e it in the same way as a pre-existin harmonic, that is, b hin the relevant AC

network imp dan e Simulation s ould also take into ac ou t the relative phase s if t of the

negative seq en e comp red to the p sitive seq en e, as this may interact with con erter

o eration, aff ectin the DC side secon harmonic voltage an con eq ently b th the negative

seq en e fun amental an p sitive seq en e 3

rd

harmonic AC side c r ents f rom the

con erter

The sp cif i ation of NPS an the analy is of the imp ct of NPS fun amental freq en y

voltages at a b c -to-b c con erter con ectin as n hronou s stems of the same nominal

f req en y can b p rtic larly c al en in The fun amental f req en y NPS in e c s stem

introd ces a secon harmonic comp nent into the DC c r ent whic can b at in mag itu e as

the phase of the two s stems slowly sl p again t one another d e to sl g t dif feren es in

f req en y b twe n the two s stems The secon harmonic comp nent is then injected into

the AC s stems on e c side as a 3

rd

harmonic p sitive seq en e an negative seq en e

f un amental whic ten s to mod late the pre-existin NPS voltage In the extreme case, the

harmonic filters) d e to the reinforcement of the NPS comp nents from e c s stem

The fun amental f req en y NPS in e c s stem wi also b mod lated by the con erter, an

the variation in fun amental f req en y phase voltage can b come large enou h to af f ect

ta c an er controls The ef f ect can b p rtic larly ac te when the con erter is in tal ed at the

en of a sin le lon u tran p sed AC tran mis ion l ne whic is then lo ded to multiple times

the s rge imp dan e lo din In this case, an previou ly me s red values of NPS in the AC

s stem would gre tly u derstate the mag itude of NPS voltage that would b exp rien ed at

the con erter station when o eratin at ful lo d, an th s the c stomer s ould b aware that

a “hig er than me s red” value of NPS is sp cified as a plyin at the con erter station

3.12.4 Pre-e istin harmonic distortion

Distortion of the AC s stem voltage at the converter bu also in uen es the res ltin

con erter harmonic These voltage distortion are cau ed by harmonic c r ents generated by

the con erters as wel as by pre-existin distortion If an interaction model is u ed whic can

con ider b th ef f ects simultane u ly, it is s f f icient to u e actual values f or pre-existin

distortion (with an alowan e f or f uture growth) as the drivin voltage U

o

in a model

ac ordin to Fig re 1 Se arate sp ctra f or p rf orman e an ratin typ calc lation may b

u ed

For the harmonic interaction to b model ed cor ectly, not only the mag itu e but also the

phase seq en e of the pre-existin harmonic is ne ded, as this af fects the order of harmonic

tran fer acros the con erter In case information is lac in on seq en e, then a general

as umption of 2

n

, 5th

an 8th harmonic b in mainly negative seq en e, an 4

th

, 7th

an

10th harmonic b in p sitive seq en e may b re sona ly vald An 3

rd

, 6th

th

harmonic generated by saturation of tran formers an mac ines wi be zero seq en e (in

whic case they can b ig ored as they do not cros the con erter), but electronic con erters

(b th domestic an in u trial) an an other ne rby HVDC con erters wi generate 3

rd

harmonic of p sitive seq en e, an so it s ould not b as umed that al pre-existin 3

rd

harmonic is of zero seq en e

General y, the practical imp ct on f ilter desig of hig er order harmonic is normal y

negl gible

The relative phase an le (with resp ct to f un amental f req en y voltage) of pre-existin

harmonic is also an imp rtant p rameter in the AC-DC-AC tran f er proces Fig re 3

i u trates this by s owin the DC side 6th harmonic voltage as generated by 5

In most cases, it is virtual y imp s ible to predict the phase an les, as they

are of a ran om nature Theref ore, in order to as es the pro a le imp ct of harmonic

interaction with some con den e, a statistical a pro c would b ne ded u in a large

n mb r of simulation with phase an les as stoc astic varia les For a worst-case

as es ment however, s c as for ratin purp ses, then lne r ad ition s ould b u ed

From a practical p int of view, the typical situation is that desig s are demon trated to b

val d by simulatin harmonic interaction f or one or a f ew selected harmonic , f or example the

harmonic interaction b twe n 5

s c case, it is f airly e s to predict resp n e, simply by defi in the source voltage b hin

network imp dan e in mag itu e an letin the phase an le of one harmonic vary

3.12.5 AC network impe a c

Harmonic AC network imp dan es are normal y sp cif ied as an are in the complex

imp dan e plane As there is pro f (se An ex A) that worst-case imp dan es f or resonan e

with the con erter station imp dan e wi alway b located on the b u daries of s c are s,

there is no ne d to s an the complete en elo e when con iderin ju t local worst-case

resonan e, but only to s an the b u dary to f i d the decisive imp dan e f or e c harmonic

However, when we con ider the doubly-tran f er ed ef f ect of the remote AC network

imp dan e on the local s stem, it is neces ary to con ider al p s ible p ints within that

remote AC network imp dan e en elo e, as what is imp rtant is the a p rent AC side

imp dan e of the local con erter

Furthermore, as in the complete interaction model, this situation has to b reversed to stu y

the harmonic distortion at the remote terminal, it b comes cle r that the f ul are s of b th

network imp dan e en elo es have to b s an ed, in estigatin the ef fect of every

combination of al sampled p ints within e c en elo e

Furthermore, f or a sin le freq en y in the DC circ it, the combination of at le st f our

imp dan e are s ( wo freq en ies at e c AC side) have to b s an ed This tas theref ore

req ires a con idera le computin resource an intel gent selection of the n mb r of

sampled p ints to b stu ied within e c network imp dan e en elo e

This complexity of calc lation would only b ju tif ia le in case there was a stron con ection

b twe n the resp ctive sides of a DC tran mis ion, f or example in a b c -to-b c s heme

with l t le or no smo thin re ctan e For man tran mis ion s stems, s c as those with lon

l nes or ca le tran mis ion, this may not b the case, or only the case for low order

harmonic , an the model can b sig if i antly simplf ied by s an in only the AC imp dan e

b u daries at e c con erter station in ivid al y to maximise the harmonic contribution of that

con erter an ad in contribution ac ordin to the method of s p rp sition as ela orated

b low

For stu ies made u in time domain simulation , the network imp dan e wi have to b

re resented by a circ it eq ivalent In man cases, a model as u ed in d namic p rforman e

stu ies is adeq ate, as s c an eq ivalent is of ten tu ed s c that it wi b re sona ly

re resentative for lower order harmonic In some cases, it can b s f ficient to u e an even

simpler eq ivalent In the example b low (Fig re 4), the aim is to re resent the network f or 5

th

th

harmonic , with the s ort circ it re ctan e tu ed to give desired phase an le at the

me n of the two, i.e at the 6th harmonic The series resistan e, RS, is selected to give

f un amental f req en y X to R ratio an R

p

calc lated s c as to give the desired phase an le

at 6

th

harmonic ac ordin to Eq ation (1) This model would however only b cor ect f or that

p rtic lar harmonic an may theref ore give misle din res lts for other harmonic in the

Fourier analy is of the simulation res lts

Figure 4 – Simple circ it u e to repre e t AC network impe a c

at 5th

a d 7th

+

−

φRXX

R

22

s6

s2

2

66

s66

p

t an4

t an4

t an

t an

t an2

1

(1)

3.12.6 Conv rter control s stem

The imp ct of con erter controls on the harmonic tran f er throu h the con erter has lon

b en evident in practical situation Some pro lems whic oc ur ed in very e rly HVDC

s hemes d e to f irin an le mod lation were red ced or removed with the introd ction of

phase loc ed os i ators an eq idistant f irin Other is ues have b en resolved by the

introd ction of ad itional lo ps within the con tant c r ent control, aimed to af fect the

interaction with the control with one or more low order harmonic An example of s c

ad itional control con ists of an alpha b lan in circ it, whic mod lates the f i al p int on

wave of f irin to ac ieve b lan ed (eq al) f irin an les b twe n the two valves in e c l mb of

e c six-pulse con erter It prod ces a fun amental freq en y mod lation of valve firin an

acts to prevent mag if i ation of secon harmonic in the AC s stem

When model n harmonic interaction, it is recommen ed to in lu e the imp ct of the c r ent

control, if the b n width of the control is s c as to act on f un amental f req en y an low

order harmonic an the DC side imp dan e do s not s f ficiently aten ate these

f req en ies Con ersely, if the control b n width is s c that it do s not resp n sig if i antly

at these freq en ies, or the DC side imp dan e provides stron fun amental freq en y an

low order harmonic dampin , then the imp ct of the c r ent control may b neglected An

initial DC resonan e stu y could b p rformed in the time domain at an e rly stage of the

calc lation to test this, an there y p s ibly al ow f or simpl fi ation of the f uture harmonic

interaction stu ies

For most cases, a simpl f ied generic control model is exp cted to b s f f icient to evaluate if

there are harmonic interaction is ues The f i al setin s of a control s stem are in an case

generaly not u ual y avaia le u ti late in the project, af ter d namic p rforman e stu ies

have b en completed

3.12.7 Combination with "cla sic" h rmonic ge eration

Un er the simpl f yin "clas ic" as umption f or calc lation of con erter AC side harmonic

generation, the direct c r ent is as umed to b completely smo th, with no harmonic content

an con eq ently no con ideration of cros -mod lation Un er these con ition the

c aracteristic harmonic are generated, plu non-c aracteristic harmonic d e to variou

f actors, amon whic are the pre-existin distortion an u b lan e of the AC voltage source

Therefore, for example, there is a 3

rd

harmonic c r ent generation cau ed by a negative

phase seq en e voltage u b lan in the s mmetry of the commutation p riod with resp ct

to the dif ferent phases It is imp rtant to recog ize that this is distin t from the 3

harmonic c r ent are imp s ible to distin uis in re l l f e or in

an time domain simulation, an in e d dif f ic lt to tre t analytical y In a simplfied view, they

may b con idered as in e en ent sources, with a p s ible phase an le displacement whic

de en s on the phase an le of the 2

n

harmonic c r ent

It is imp rtant to u derstan that in a f ul time domain simulation of the harmonic interaction,

b th ef fects wi b natural y model ed an cor ectly simulated In a f req en y domain stu y,

however, they wi have to b se arately calc lated an combined, with some u certainty as

to the cor ect vector relation hip to u e f or s mmation

3.12.8 Relativ ma nitud of pairs of low-ord r h rmonic

Variou f actors have b en o served to have a stron in uen e on the relative levels of the

p irs of low order harmonic tran fer ed acros the con erter from DC to AC sides For

example, where s ac ordin to mod lation the ry a 2

n

harmonic DC side c r ent s ould

res lt in eq al values of AC side NPS f un amental an 3

rd

harmonic, time domain simulation

of typical HVDC s stems ten to s ow a stron u b lan e, with NPS generaly (but not

alway ) b in hig er an 3

rd

harmonic b in lower than predicted The dif feren es in some

cases may b so hig that the NPS c r ent is several times the mag itu e of the 3

harmonic d e to commutation p riod u b lan e as

des rib d in 3.12.7 a ove Another is the action of the control s stem, whic can have a

stron in uen e an acts with dif f erent c aracteristic at the rectif ier an the in erter The

relative phase an les of the AC side negative seq en e voltage an the DC side 2

n

harmonic c r ent are also sig if i ant

It is extremely dif f ic lt, even with time domain simulation , to se arate an explain the

dif ferent in uen es of variou factors, whic in re l ty have interde en ent action This

b haviour is u l kely to b o served in simpl f ied models whic de en on tran fer f actors

calc lated ac ordin to mod lation the ry

For p irs of hig er order harmonic , this u b lan e ef fect is les o viou , p rtly b cau e of

the red ced in uen e of the control s stem

The l terature has l t le or no mention of this phenomenon, but it is one whic s ould b

o served in an re l stic simulation an one whic s ould b taken into ac ou t in the desig

proces

3.12.9 Superp sition of contributions

Freq ently, harmonic interaction stu ies may b made by imp sin harmonic voltage sources

on one side of a HVDC s stem in ste d state con ition an evaluatin the outcome in

harmonic lo d f low, then re e tin the same proced re f or the other side This is a

simpl f i ation, but in general it is ad antage u as it provides b t er u derstan in of the

b haviour of the circ it an as the er or introd ced typical y is ac e ta le The o viou

q estion is how to ad contribution f rom the dif ferent origin

Where b th sides of the HVDC l n o erate at the same nominal f un amental f req en y, the

harmonic freq en ies can b ad ed up u in either an RSS or arithmetic b sis Of ten, RSS is

u ed f or p rforman e evaluation, where s an arithmetic s m is u ed when esta l s in

maximum p s ible eq ipment stres es, in p rtic lar if there is a stron harmonic con ection

b twe n b th sides of the HVDC ln

In the case where b th sides o erate at dif f erent f un amental f req en ies, the cros

-mod lated contribution wi b at dis rete f req en ies an s ould b evaluated ac ordin ly

That is, p rf orman e evaluation s ould b made con iderin inter harmonic p rforman e

criteria In esta l s in eq ipment stres es, interharmonic comp nents s ould b treated as

in ivid al f req en ies, as this b st re resents their ph sical ef f ects on comp nents

3.13 Paral el AC l ne a d con erter tra sf ormer s turation

In some situation , DC tran mis ion l nes ru in p ralel with AC tran mis ion lnes over p rt

of their l ne route This may oc ur d e to ge gra hical features, wayle ve restriction , desire

to ke p al tran mis ion in a cor idor, or may b ph sical y u avoida le in the a pro c to a

con erter station

The retical y, b th ca acitive an in u tive coupl n b twe n the l nes wi oc ur, but for

practical purp ses at typical l ne se aration the ca acitive element is negl gible In u tive

coupl n can however b of gre t imp rtan e, with fun amental freq en y c r ents b in

driven in the DC tran mis ion circ it

In a bip lar tran mis ion, the in u ed c r ent in the two HVDC con u tors wi b in the same

sen e, an theref ore b mainly of grou d mode with a return p th throu h the neutrals or

electrodes of the con erter station However, d e to the dif f erent distan es b twe n e c

DC con u tor an the AC l ne, the in u tion in e c wi b of dif ferent mag itu e, an so an

u b lan ed or p le mode c r ent wi also f low

Typical levels of in u ed f un amental freq en y c r ent are u ualy not hig enou h to make

an sig if i ant imp ct on either the DC side harmonic p rf orman e or the ratin of DC side

f ilters an other eq ipment Their main sig ifi an e is in the ef f ect whic they can have,

throu h cros -mod lation, on saturation of the con erter tran formers Fu damental

freq en y c r ent in the DC circ it is cros -mod lated to a p ar mainly as direct c r ent an

p sitive seq en e 2

n

harmonic in the valve side tran f ormer win in s The direct c r ent is

divided amon the thre phases of the valve-side win in in pro ortion de en in on the

relative phase an le of the in u ed c r ent to the a pl ed AC source voltage an the firin

an le:

παI

π

11

d1

d1t

2

c os3

c os3

ϕϕ

The mag itu e of the DC comp nent wi theref ore b b twe n 0 an

1d3

I

π

de en in on φ

1

an α As the phase an le b twe n the in u ed f un amental an the con erter source AC

voltage is an u k own q antity, it is as umed that the direct c r ent in an one phase may

re c its maximum p s ible value, whic is

1d3

I

π The s m of the thre phases s ould b

zero as there is no neutral grou d p th on the valve side win in s

There are other p s ible sources of direct c r ent in the tran f ormer whic s ould also b

taken into ac ou t when as es in the ris of saturation These are: f urther DC side

f un amental f req en y c r ent res ltin b th from AC side 2

n

harmonic voltage an from

p s ible f irin an le u b lan e, stray direct c r ent f rom ne rby electrodes, an , p s ibly,

ge -mag etical y in u ed c r ent [12] These later two f low throu h the l ne side win in s via

the neutral p int grou din of the Y-win in , an there y also contribute to core saturation

The relative mag etizin ef f ect of direct c r ents in the valve an in the l ne side win in s wi

de en on the n mb r of turn in e c In order to derive a total ef fect ref er ed to say the l ne

side win in , a direct c r ent in the valve win in can b a proximately re resented by a

direct c r ent in the l ne win in , if s ita ly multipl ed by the tran formation ratio Of course,

the direct c r ent can ot in re l ty cros the tran former to the l ne side win in s, an care

s ould b taken when u in certain wel k own time-domain programs whose tran f ormer

models do implement s c u re l stic tran formation of direct c r ents an voltages The

extent of saturation wi also de en on the zero seq en e resistan e, as s own in [13]

The res ltant s if t toward sin le-sided saturation of the core res lts in the generation of a

bro d sp ctrum of harmonic in the mag etizin c r ent on the AC side of the con erter

tran former The au ible noise prod ced by the tran former wi in re se gre tly an there is

a ris of localzed overhe tin an gas in Tran former protection may o erate with

p tential trip in of the HVDC l n The ad itional harmonic generation could res lt in

overlo d an trip of harmonic f ilters u les adeq ately con idered in their desig an ratin

Final y, if the secon harmonic c r ent prod ced by saturation se s a hig network/f ilter

imp dan e, the res ltin in re sed 2

n

harmonic voltage may res lt in f urther DC side

f un amental voltage an c r ent, closin a lo p whic would res lt in core saturation

in ta i ty

It is therefore cle r that in u tion f rom p ral el AC l nes can have extremely seriou

con eq en es an s ould b careful y stu ied The AC l nes can e si y b re resented in

detai within a typical f req en y domain or time domain model of the DC l nes an con erter

station Imp rtant f actors are:

• len th of exp s re(s) – the in u ed c r ent is pro ortional to the exp s re len th;

• location of exp s re(s) – this maters b cau e of the stan in wave p tern alon the

DC l ne; the f un amental c r ent at the con erters wi not b the same as that at a

remote location of in u tion;

• ge metric layout of the AC an DC lnes, in lu in grou d wires;

• sep ration distan e – s ould b cle r whether this is centre l ne to centre l ne or

b twe n ne rest con u tors;

• grou d resistivity – hig er resistivity in re ses the in u ed c r ent;

• o eration mode of the HVDC tran mis ion (bip lar, mono olar grou d or metal c

return);

• maximum c r ent levels in the AC l ne, in lu in p rcentages of negative an , very

imp rtantly, the zero seq en e comp nents;

• tran p sition of the AC l ne an p s ibly the DC lne

If calc lation s ow a hig ris that in u tion levels wi b s c that the con erter

tran formers wi b saturated, then there are thre p s ible mitigation me s res

1) Tran p sition of the AC lnes alon the len th of the exp s re This wi can el the

in u tion d e to p sitive an negative seq en e comp nents, whose eff ects de en

on the varyin distan e to the DC l ne of the thre AC phases It wi however have no

imp ct on in u tion d e to an zero seq en e comp nent of AC c r ent

NOT 1 Tra s o itio of th DC ln c n u tors in te d wo ld b relativ ly in ff ectiv a it wo ld h v n ef fe t

o th in u tio of gro n mo e c re t, whic is ofte th d min nt c mp n nt

2) Implementation of a c r ent control s stem action whic wi ten to damp the f low of

f un amental f req en y c r ent in the DC circ it This is by f ar the e siest an le st

costly solution to implement The negative con eq en e of s c mod lation may

however b the generation of a ran e of harmonic on b th the AC an DC sides,

whic cre tes dif ferent pro lems

3) Introd ction of either series bloc in f ilters in the neutral side of the con erter circ it,

or s u t f ilters, tu ed to f un amental f req en y Of these, series f ilters have alway

b en the pref er ed o tion Sh nt filters may act as a byp s f or fun amental c r ent

in u ed in the DC l ne, but they also provide a very low imp dan e p th for

f un amental f req en y c r ent generated by the con erter (f or example d e to AC side

2n

harmonic), res ltin in in re sed direct c r ent in the tran former

Series bloc in f ilters have b en s own to b a very ef f ective solution an have b en u ed on

a n mb r of HVDC projects However, this solution has a hig cost as the f ilters con u t the

f ul direct c r ent an theref ore the re ctors are simi ar to smo thin re ctors – in f act

sometimes the same desig is u ed

NOT 2 Howe er, a re ctor of th blo kin fiter d e n t c ntrib te to th smo thin eff ect for dire t c re t, a

it is b p s e b a p ralel c p citor

Bloc in f ilters have to b s arply tu ed an may b come largely inef fective at wide

f req en y variation d e to s stem disturban es Durin s c large f req en y variation ,

saturation of the tran formers may oc ur d e to the detu in of the bloc in f ilters The ris of

thermal pro lems an con eq ent protection action would have to b careful y evaluated in

relation to the anticip ted d ration of the wide freq en y deviation

3.14 Pos ible counterme s re

3.14.1 AC (a d/or DC) f ilters

Sh nt con ected AC filters can b u ed to l mit the imp ct of interaction on the AC s stem by

providin a low imp dan e p th for interaction c r ent to f low A low imp dan e at the

interaction freq en y res lts in a smal cor esp n in voltage at the con erter bu In some

in tan es, it may b more ad antage u to desig the filter to introd ce dampin into the

network In re sed dampin at the interaction freq en ies red ces the ampl f i ation of

voltages an c r ents whic may res lt from the interaction

DC side bloc in f ilters can b u ed to avoid interaction The f ilters typical y con ist of p ral el

ca acitor re ctor resistor comp nents con ected in series with the DC con erter The filter

restricts the c r ent f low at the tu ed freq en y, th s decoupl n the DC network at the

interaction freq en y Often, this is al that is req ired to el minate the interaction

Interaction can of ten b avoided by selectin an a pro riate value f or the in u tan e of the

smo thin re ctor(s) an s ita le selection of DC f ilter p rameters to avoid series or p ral el

resonan es at critical DC interaction f req en ies Smo thin re ctors are an ef fective me n

of l mitin interaction d e to cros mod lation ef f ects

3.14.2 DC control de ign

DC controls are an extremely cost ef f ective way to cou ter harmonic interaction They are

most ef fective inl mitin interaction in u ed by drivin forces external to the con erter at low

harmonic interaction freq en ies Typical control desig in olves the implementation of a

circ it whic resp n s to voltage or c r ent of one of the interaction network at the

cor esp n in interaction f req en y The circ it ad s a smal cor ection to the f irin an le of

e c valve in s c a way as to red ce the mag itu e of the me s red q antity Chan in

control p rameters ef f ectively alter the gain an phase of the mathematical co f f icients whic

couple the network at the interaction f req en ies

3.14.3 Operating re trictions a d de ign prote tions

Althou h u desira le, the most cost ef f ective solution to an interaction pro lem may b to

avoid the o eratin con ition whic res lts in the interaction If the lkel ho d that s c an

o eratin con ition could oc ur is extremely remote, imp sin an u desira le s stem

o eratin restriction may b more at ractive than the exp n e (an p s ible in on enien e)

as ociated with a large ca acity low order AC harmonic f ilter If s c a strateg is ado ted, it

would b pru ent to en ure that f ilter an s stem protection detect an resp n to the

interaction (s ould it oc ur) an smo thly brin the AC-DC o eratin con ition to a safe

situation

3.15 Re omme d tion for te hnic l spe if ic tion

3.15.1 Ge eral

A tec nical sp cifi ation s ould b a solutely cle r a out the p rf orman e an ratin

req irements with resp ct to harmonic interaction is ues an the con ition u der whic they

are met To s p ort a comprehen ive desig , the sp cif i ation s ould in lu e al s stem data

whic could in uen e the p rf orman e of the HVDC plant in this resp ct 3.15 s ould serve as

a g idel ne for writin a complete an detai ed sp cif i ation

From exp rien e, it is l kely that the c stomer wi b presented with rather dif ferent method

of calc lation f rom dif ferent bid ers, an in e d sig if i antly dif f erent calc lation res lts f or

harmonic whic de en on cros -mod lation ef fects Furthermore, d rin the l mited time of

a bid in proces , s p l ers may ten to simpl f y their desig a pro c an estimate the

res ltin ris f or p s ible in re sed costs when the f i al detaied desig is made d rin the

contract stage

The bid ers may b as ed to explain their resp ctive methodologies an ju tif y their

calc lation res lts, but it may b dif f ic lt f or a c stomer with l mited sp cial zed tec nical

exp rien e in this are an no in e en ent calc lation to ls to as es whic , if an , of the

presented stu y res lts is ac urate, an to comp re the res ltin bid The b st protection f or

the c stomer is to state in the tec nical sp cif i ation that the contractor is ultimately

resp n ible f or f ulf ilment of al related req irements, whic s ould b verif ied by test

me s rements d rin an af ter commis ionin The on s is on the c stomer to en ure that

s c testin an me s rements are later car ied out The tec nical sp cif i ation could also

req ire that the contractors prove their res lts by time domain simulation of critical selected

cases u in a detai ed HVDC model with a pro riate re resentation of AC network

3.15.2 Spe if ie de ign d ta

The fol owin inf ormation is req ired by contractors

• Fu damental f req en y

This s al in lu e inf ormation on the ran e of p sitive an negative phase seq en e

voltages The def i ition of the pre-existin negative seq en e voltage s al make cle r

whether this value is defi ed at the con erter bu or b hin a network imp dan e

(3.12.2).The ran e(s) of f req en y deviation s ould also b defi ed The c stomer s ould

note that in some cases there could b an in re se in NPS d e to in re sed lo din in

u tran p sed tran mis ion l nes when the con erter is o eratin

• Pre-existin harmonic

The mag itu es of pre-existin harmonic s ould b provided, an whether these values

a ply to the con erter bu or b hin a network imp dan e Ide l y, inf ormation on phase

an les relative to the f un amental as wel as the cor esp n in phase seq en e of the

harmonic s ould b in lu ed, but in most cases this information is not k own to the

c stomer an it may vary with time The req ired rule f or s p rp sition of harmonic

originatin at the two terminals of the s stem (3.12.9) s ould b stated Dif ferent

pre-existin harmonic sp ctra may b sp cif ied (e.g f or f ilter p rforman e an ratin ) an the

sp cif i ation s ould in lu e g idan e on the sp cif i calc lation the cor esp n in

sp ctrum has to b u ed f or

• AC network imp dan e

Imp dan es have to b determined an sp cified fol owin the g idel nes as given in

Clau e 4 Detai ed information on al owa le simpl f i ation of imp dan e model n is

req ired If several imp dan e are s are sp cif ied (e.g for p rforman e an ratin ), the

sp cif i ation has to in lu e g idan e on the sp cif i calc lation the cor esp n in

imp dan e has to b u ed for In case of dif feren es b twe n p sitive an negative phase

seq en e imp dan e of the AC network, these dif f eren es s al b stated Su h may b

the case if the plan ed con erter station is located close to a generator station

• DC side imp dan e

If the DC circ it, or p rt of it, is outside the contractor’s s o e of s p ly, the sp cif i ation

in lu es the relevant model n data (l ne len th, tower con g ration, con u tors)

• Paral el AC l nes

Pos ible p ral el or ne r p ralel exp s res of the DC l ne to AC overhe d l nes have to b

sp cif ied The f actors l sted in 3.13 are defi ed

• Req ired l mits

It s ould b stated in the tec nical sp cif i ation that harmonic interaction acros the

con erters (or cros -mod lation) s ould b ful y taken into ac ou t in f ilter p rf orman e

an ratin calc lation If there are other l mits on the external imp ct of harmonic l kely

to b af f ected by cros -mod lation, s c as the f low of 5

th

th

harmonic c r ent in

ne rby generators, then these s ould b def i ed With regard to in u tion f rom p ral el

AC l nes, it s ould b stated that this s ould not res lt in saturation of con erter

tran formers even d rin wide freq en y variation , an ad erse ef fect on the control

s stem, or o eration of an protection

3.15.3 Re uireme ts re arding c lc lation te h ique

A bid er wi ne d the latitu e to select the proced re u ed to determine the imp ct of

harmonic interaction d rin the ten er stage as this decision is of ten in uen ed by non

-tec nical con traints s c as resources or time The bid ers s al b req ired to ela orate on

the calc lation proced re u ed in terms of the f ol owin items

• Model n tec niq es

As ela orated in 3.6, a wide ran e of model n tec niq es are in u e The bid er s ould

b as ed to des rib the calc lation method(s) to b u ed an to provide verif i ation of its

s ita i ty an ac urac

• Variation an toleran es

Eac element within an interaction model is s bject to variation an toleran es The

bid er s ould detai proced res for sen itivity c ec s More ver, the tre tment of f ilter an

s u t ca acitor outages an red n an y s al b clarif ied

• Summation laws/s p rp sition

De en in on the u ed model n tec niq es, the bid er in lu es inf ormation on the

s mmation of in ivid al contribution to an in ivid al harmonic distortion If a pl ca le,

the tec niq es to combine clas ical models (i.e the con erter as a c r ent source on the

AC side an a voltage source on the DC side) with an interaction model have to b

ad res ed

• Con erter imp dan e

The bid er s al ela orate on how his calc lation proced re determines the con erter

imp dan e If a simpl f ied a pro c is c osen, the bid er s al q al f y his method in more

detai

Man asp cts of harmonic calc lation an practical f ilter desig are, for relative simpl city,

b sed on an AC side c r ent source model, as umin smo th direct c r ent an DC side

voltage source model as umin purely fun amental f req en y AC side drivin voltage

Harmonic cros -mod lation is the f actor whic u dermines the val dity of s c simpl f i ation

an h gely complcates the calc lation proces

For normal f ilter desig purp ses, harmonic interaction ten s to b of sig if i an e only up to

u ual y has l tle practical imp ct at hig er harmonic

Even with the computin p wer now re di y avai a le, the complexity of a complete f ilter

desig for b th of the AC sides an the DC side of a typical HVDC l n , u in a ful an

ac urate re resentation of cros -mod lation, is general y to gre t to b practical within the

time an resource l mitation of a ten er p riod or contract desig p riod Simplf i ation

theref ore have to b made, an p rts of the desig de-coupled f rom other p rts This

introd ces some degre of ris , whic s ould b covered by the ad ition of s ita le margin

to the calc lated res lts

Cu tomers s ould b aware of the complexities in olved in sp cif i ation of p rf orman e an

ratin when a precia le cros -mod lation is l kely, an se k tec nical ad ice if req ired The

c stomer s ould also b aware of sig if i ant dif f eren es in calc lation tec niq es typical y

u ed by dif ferent HVDC s p l ers an b pre ared to q estion the res ltin dif f eren es in

calc lation res lts an filter desig b th d rin the bid evaluation an also in the project

desig phase an to req est verif i ation of the valdity of the tec niq es an simpl fyin

as umption a pl ed

4 AC network impeda ce model ing

IEC 6 0 1-1 an IEC 6 0 1-4 dis u s the imp rtant in uen e of network harmonic

imp dan e on b th the p rforman e and ratin asp cts of the AC f ilter desig F r a

c stomer, it is one of the most dif f ic lt asp cts to sp cif y, esp cial y if the c stomer is not the

owner of the network an has l tle direct k owled e of its comp sition an p s ible f uture

develo ment The purp se of Clau e 4 is to ampl f y the re son wh the cor ect sp cif i ation

of network harmonic imp dan e is cru ial to an o timal desig of AC f ilters an also to

provide f urther detai ed g idan e as to its as es ment

IEC 6 0 1-1 dis u ses that normal y the c stomer def i es the ran e of network imp dan e to

b u ed for f ilter desig but that in some cases the c stomer le ves the prosp ctive

contractors to p rf orm this as es ment

Clau e 4 reinf orces a recommen ation that in the prod ction of the tec nical sp cif i ation by

the c stomer for an HVDC s stem, the c stomer rather than the prosp ctive contractors is

generaly b st s ited to, an s ould b resp n ible for, the def i ition of the AC network

imp dan e c aracteristic This me n that the stu y is done only on e an avoid al

prosp ctive contractors havin to make their own in ivid al as es ment of the provided data,

s c as s stem sin le l ne diagrams an as ociated relevant data, detai s of normal an

a normal o eratin con ition an lo din , an the ef fects of f uture network exp n ion (s c

data are of ten only k own to c stomer or uti ty) This would then have the inevita le ris that

e c prosp ctive contractor would as es the network imp dan e in a dif ferent man er with

dif f erin res lts, le vin the c stomer to determine whic is cor ect or whether an of them

are adeq ate The c stomer s ould therefore take resp n ibi ty for these stu ies, either

directly or throu h a con ultant He can take ad antage of the lon er p riod that is generaly

avai a le b fore the is ue of the tec nical sp cif i ation to pre are this information, rather than

req irin the prosp ctive contractors to in ivid aly make the as es ment d rin the s orter

ten er stage

It has b come common practice that the sp cif i ation an desig of the AC harmonic filters

esta l s ed at the ten er stage f orm p rt of the later contract If the c stomer decides to

p stp ne the detai ed network imp dan e stu y u ti exec tion of the contract, he wi ne d to

b aware of the fol owin disad antages an manage the ris s

• Received bid may not b en b sed on the same as umption , hen e may not b

comp ra le

• Cost an sp ce req irements of the AC f ilter s heme determined d rin the ten er

stage may not b s f f icient

• The contractor may have to claim c an e/variation orders

• The time ne ded f or the f i al desig stage may b prolon ed

There are some in tan es in whic the method f or determinin the network harmonic

imp dan e des rib d in [1 ] or in this doc ment may b ina pro riate or req ire sp cial

con ideration Su h situation in lu e the fol owin

• Where a pro osed HVDC s heme is to b con ected in p ral el with an existin

s heme whic is o eratin with adeq ately desig ed AC harmonic f ilters an there is a

preferen e for the f ilters to b as ociated with the new s heme to have identical

c aracteristic as the existin u its, at le st for the con erter c aracteristic harmonic

In this case, an c an e in the defi ition of network harmonic imp dan e f rom that

u ed for desig of the original s heme wi req ire caref ul con ideration of the

contin in via i ty of the existin f ilters an of the combined o eration of the original

an new filter desig s

• Where a pro osed HVDC s heme wi b con ected to an AC network that is only

o erated in an "islan ed" mode; that is a smal an wel defi ed network f or whic it

may b pref era le to model the tran mis ion l nes, ca les, tran formers an

generators etc expl citly rather than to employ imp dan e en elo es

4.2 Impl c tions of ina c rate definition of network impe a c

Due to the dif f ic lties in ac urately as es in the network harmonic imp dan e, it can b

at ractive f or a c stomer to b se his sp cif i ation on a simpl f ied network def i ition with fairly

arbitrary p rameters, pro a ly biased toward con ervative values However, a to

con ervative as es ment of network imp dan e (e.g an imp dan e havin ex es ively hig

dampin an les an /orex es ive ran e) can have several sig if i ant disad antages in resp ct

of AC f ilter desig :

• an in re sed n mb r an /or dif f erent typ s of f ilters may b req ired to cater for

network imp dan e con ition that in practice may not oc ur;

• an in re se in switc yard sp ce would b ne ded to cater f or red n an y

req irements as a res lt of the provision of a larger n mb r of dif f erent f ilter typ s;

• the req irement f or a gre ter n mb r of s arply tu ed f ilters, the a pl cation of whic

can in ur ex es ive harmonic ratin s esp cial y when con iderin the ef f ects of

pre-existin harmonic distortion;

• the ne d, esp cial y at low tran mit ed p wer levels, for AC f ilters with a total re ctive

p wer in ex es of that whic can b ac e ted by the AC network an therefore the

req irement f or the con erters to o erate at either in re sed control an les or of ten

the u e of hig ca ital cost s u t re ctors, b th of whic give rise to in re sed los es;

• hig er initial an project l f etime o eratin costs

Con ersely, however, a desig whic is b sed on to nar ow an as es ment of network

imp dan e may fai to me t the req ired harmonic p rforman e criteria an /or sometimes

can ot remain in service d e to comp nent overlo din b cau e of resonan es b twe n the

AC f ilters an the network whic were not predicted In s c cases, the economic

con eq en e of s c s ortcomin s could b more seriou than those l sted a ove d e to an

over con ervative desig

It is theref ore evident that ef forts s ould b made to ac ieve as ac urate as p s ible an

as es ment of the network harmonic imp dan e

4.3 Consid rations for network model ng

IEC 6 0 1-1 gives ref eren es [14] [15] to variou method of derivin network harmonic

imp dan e

In atemptin to p stulate criteria to b con idered in determinin network harmonic

imp dan e, there are very few generic rules that can b a pl ed u iversal y for al network

worldwide; theref ore e c network s ould b tre ted on a case by case b sis

The extent of the network to b modeled is also s stem de en ent an no general rules can

b def i ed One a pro c is to start by model n a relatively smal are of the s stem, but

retainin s ff icient to in orp rate al of the contin en ies req ired to b stu ied The analy is

is then re e ted a n mb r of times with more of the AC network retained e c time, u ti

there is no sig if i ant c an e in the harmonic imp dan e c aracteristic

4.3.2 Proje t l f e e pe ta c a d robustne s of data

The sp cif ied l f e for an HVDC project can typical y vary b twe n 2 ye rs an 4 ye rs The

cost of AC harmonic f ilters f orms a s bstantial p rt of the overal con erter eq ipment costs,

as they are inevita ly c stom-made items with u iq e layouts an comp nent sizes Also they

are very diff ic lt to alter sig if i antly on e con tru ted It is o viou ly desira le that their

desig , in terms of compl an e with p rforman e req irements an their ratin , is s ff iciently

ro u t s c that a redesig or recon g ration with at en ant len th outages is not req ired

p rt way throu h their service lfe For mature an stron ly intercon ected network s c as

those in the UK an continental Euro e, it s ould b e sier to predict network develo ments

than it is for a ra idly develo in cou try However, man so-cal ed mature network are now

also s bject to sig if i ant infrastru ture develo ments to ac ommodate the req irements of

renewa le energ sources Su h develo ments were not f orese n u ti recently

4.3.3 Network operating conditions

In derivin the variation of network imp dan e the fol owin ef f ects s ould b con idered as a

minimum to en ure that al practical y f easible an l kely o eratin s enarios are ca tured:

• Sy tem lo d/generation variation f or a maximum deman day

• Sy tem lo d/generation variation f or a minimum deman day

• Sy tem lo d/generation variation f or intermediate deman day(s)

• Dif ferent AC s stem generation con ection con ition , f or example dif ferin mixes an

location of h dro, n cle r, thermal, win , other HVDC l n s Where ne rby generation

exists, it is general y recommen ed that the lowest practical levels of s c generation

are u ed f or the variou s enarios, in order to model the we kest s stem whic f or low

orders generaly gives the largest imp dan e en elo es

• Statu of re ctive comp n ation plant, b th d namic an f i ed (e.g mec anical y

switc ed ca acitors an re ctors) typ s In this resp ct, al p s ible combination of

s u t re ctive comp n ation at or close to the con erter station AC bu b r are

con idered b cau e where more than one s c device is con ected, these are l kely to

interact, there y formin dif f erin resonan e con ition

• Simi arly, where there is another HVDC l n electrical y close enou h to have a

sig if i ant imp ct on the network imp dan e, it is modeled expl citly, rather than b in

in lu ed as a lump d element within the network, with its as ociated AC f ilters b in

s bjected to the ef f ects of their detu in (d e to c an es in s stem freq en y, ambient

temp rature, ca acitor element f ai ures, etc.) together with the variation in the n mb r

an typ s of f ilters whic may b con ected with varyin lo d

• AC network tran mis ion outages (contin en y an plan ed) The contin en y

outages, i.e sin le or double circ it etc that ne d to b con idered are a f un tion of

the man er in whic the uti ty o erates the network (i.e n-1, n-2 etc criteria ac ordin

to its sec rity stan ard) an for whic it either req ires harmonic p rf orman e l mits to

b met, or req ires AC harmonic filters to b rated whi e not neces ari y ac ievin

p rforman e l mits The clas if i ation of these contin en y an plan ed (e.g

maintenan e) outages s ould b defi ed by the uti ty De en in on the complexity of

the network u der con ideration, it would b u ual that at le st 5 sig ifi antly

dif ferent network con ition would ne d to b stu ied for e c lo din con ition to

give a s ita le an rel a le ran e of p s ible imp dan es

However, an network con ition that are u re l stic p rtic larly in terms of generation an

lo d s enarios (i.e those con ition whic imply imp s ible o eratin s enarios or whic

mig t f ai to provide a con ergent f un amental f req en y lo d f low) s ould not b in lu ed If

the sof tware do s not al ow f or lo d-f low calc lation , it s ould at le st b verified that the

f un amental f req en y s ort circ it imp dan e, calc lated f or the same cases as the

harmonic impedan e, is within the anticip ted ran e

It ne d to b recog ised that the aim is to develo a network imp dan e c aracteristic whic

is val d f or al re sona ly p s ible s stem develo ments over the exp cted l f e of a project

Even mature network do not u ualy have plan b yon the next 2 ye rs to 2 ye rs The

network imp dan e def i ition theref ore may have to cover a p riod up to twice as lon as the

plan in horizon There is theref ore some dif f ic lty in how to cater f or the ye rs af ter the

plan in horizon an the res ltant u certainties Some g idan e is given in 4.5.4 an 4.5.5 on

this to ic

4.3.4 Network impe a c s f or perf orma c a d rating c lc lations

General y, it is neces ary to determine the network harmonic imp dan e c aracteristic for

b th AC harmonic f ilter "p rforman e" an " atin " con ition Generation an lo d s enarios

an contin en y con ition f or these two req irements can of ten dif f er sig if i antly as

dis u sed b low

IEC TR 610 0-3-6, other simi ar standard an national grid codes relatin to the as es ment

of harmonic emis ion l mits dis u s plan in levels f or harmonic voltage distortion b sed on

con ition that cover typical y 9 % of the time an ual y b sed on a statistical average, an

dis u s "normal" o eratin con ition of the network "Normal" general y in lu es al

generation variation , lo d variation an re ctive comp n ation states, plan ed outages an

ar an ements d rin maintenan e an con tru tion work, non ide l o eratin con ition an

normal contin en ies u der whic the con idered network an the disturbin in tal ation (e.g

the HVDC con erter) have b en desig ed to o erate

However, "normal" network o eratin con ition typical y ex lu e those con ition whic arise

as a res lt of a f ault or a combination of f aults b yon those plan ed f or u der the network’s

sec rity stan ard These in lu e ex e tional situation an u avoida le circ mstan es (e.g

f orce majeure, ex e tional we ther con ition an other natural disasters, acts by publ c

authorities, in u trial action ), cases where other network u ers may sig ifi antly ex e d their

emis ion (p rf orman e) l mits or do not comply with the con ection req irements, an

temp rary generation or s p ly ar an ements ado ted to maintain s p ly to c stomers d rin

maintenan e or con tru tion work, where otherwise s p ly would b inter upted Su h

s enarios typical y form the b sis of " atin " con ition , in ad ition to those des rib d a ove

relatin to p rf orman e con ition

The res ltant dif f eren es in the variation of network harmonic imp dan e when comp rin

"p rf orman e" an " atin " con ition can b sig if i ant, esp cial y at hig er order harmonic

This is dis u sed in 4.5.8