Iec tr 60919 3 2016

In Clau e 6, a dis u sion is provided con ernin method of control n active an re ctive p wer of an HVDC l n to improve the ste d -state an /or tran ient sta i ty of the intercon ected a.

IEC TR 6091 9- 3

Edit io 2.1 2 16-0

Per ormance of high- v olt age direct cur ent (HVDC) sy st ems w it h l

ne-commut at ed conv ert ers –

Part 3: Dy namic condit ions

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IEC TR 6091 9- 3

Edit io 2.1 2 16-0

Per ormance of high- v olt age direct cur ent (HVDC) sy st ems w it h l

ne-commut at ed conv ert ers –

Part 3: Dy namic conditions

IEC TR 6091 9- 3

Edit io 2.1 2 16-0

Per ormance of high- v olt age direct cur ent (HVDC) sy st ems w it h l

ne-commut at ed conv ert ers –

Part 3: Dy namic condit ions

CONTENTS

FOREWORD 5

1 Sco e 7

2 Normative ref eren es

7 3 Outl ne of HVDC d namic p rforman e sp cifi ation 8

3.1 Dy amic p rf orman e sp cif i ation 8

3.2 General comments 9

4 AC s stem p wer f low an f req en y control 9

4.1 General 9

4.2 Power flow control 9

4.2.1 Ste d -state p wer control req irements 9

4.2.2 Ste c an e p wer req irement 10 4.3 Freq en y control 12 5 AC d namic voltage control an interaction with re ctive p wer sources 13 5.1 General 13 5.2 Voltage an re ctive p wer c aracteristic of an HVDC s bstation an other re ctive p wer sources 13 5.2.1 General 13 5.2.2 Con erter as active/re ctive p wer source 14 5.2.3 Voltage c aracteristic of a.c network de en in on the p wer lo din at the bu b r of the HVDC s bstation 16 5.2.4 Voltage c aracteristic of a.c f ilters, ca acitor b n s an s u t re ctors f or p wer comp n ation at the HVDC s bstation 18 5.2.5 Voltage c aracteristic of static var comp n ator (SVC) 18 5.2.6 Voltage c aracteristic of s n hronou comp n ator (SC) 19 5.2.7 Voltage c aracteristic of static s n hronou comp n ator (STATCOM) 19 5.3 Voltage deviation on the bu b r of an HVDC s bstation 19 5.4 Voltage an re ctive p wer interaction of the s bstation an other re ctive p wer sources 2

5.4.1 HVDC con erters, switc a le a.c f ilters, ca acitor b n s an s u t re ctors 2

5.4.2 HVDC con erters, switc a le re ctive p wer sources, SVC 21

5.4.3 HVDC con erters, switc a le re ctive p wer sources an s n hronou comp n ators 21

5.4.4 HVDC con erters, switc a le re ctive p wer sources, STATCOM 2

6 AC s stem tran ient an ste d -state sta i ty 2

6.1 General 2

6.2 Characteristic of active an re ctive p wer mod lation 2

6.2.1 General 2

6.2.2 L rge sig al mod lation 2

6.2.3 Smal sig al mod lation 2

6.2.4 Re ctive p wer mod lation 2

6.3 Clas ifi ation of network situation 2

6.4 AC network in p ral el with the HVDC l n 2

6.5 Improvement of the sta i ty within one of the con ected a.c network 3

6.6 Determination of the dampin control c aracteristic 3

6.7 Implementation of the dampin control er an telecommu ication

req irements 31

7 Dy amic of the HVDC s stem at hig er f req en ies 31

7.1 General 31

7.2 Typ s of in ta i ty 3

7.2.1 L o in ta i ty (harmonic in ta i ty) 3

7.2.2 Cur ent lo p in ta i ty 3

7.2.3 Core saturation in ta i ty 3

7.2.4 Harmonic interaction 3

7.3 Inf ormation req ired for design purp ses 3

7.4 Me n avai a le for preventin in ta i ties 3

7.5 Dampin of low order harmonic by control action 3

7.6 Demon tration of satisf actory p rf orman e at hig er freq en ies 3

8 Subs n hronou os i ation 3

8.1 General 3

8.2 Criteria f or s bs n hronou torsional interaction with an HVDC s stem 3

8.3 Scre nin criteria f or identif yin generator u its s s e tible to torsional interaction 3

8.4 Perf orman e con ideration f or uti zin s bs n hronou dampin controls control ers (SSDCs) 3

8.5 Perf orman e testin 3

8.6 Turbine generator protection 3

9 Power plant interaction 3

9.1 General 3

9.2 Sp cif i interaction 3

9.2.1 General 3

9.2.2 Freq en y variation ef f ects 3

9.2.3 Freq en y controls interaction 4

9.2.4 Overvoltage ef f ects 4

9.2.5 Harmonic 4

9.2.6 Subs n hronou an s af t imp ct eff ects 4

9.2.7 Resonan e 41

9.2.8 Overvoltages 41

9.2.9 Stres es in a.c switc in eq ipment 41

9.2.10 Un er f req en y 41

9.2.1 Startin proced re for an HVDC con erter 41

9.3 Sp cial con ideration for a n cle r plant 41

Bibl ogra h 4

Fig re 1 – Elements for re ctive p wer comp n ation at an HVDC s bstation 14 Fig re 2 – P /Q diagram of a con erter 15 Fig re 3 – Re ctive p wer req irements of a we k a.c s stem de en in on the active p wer lo din for variou con tant voltage c aracteristic at the a.c bu of an HVDC s bstation 17 Fig re 4 – Re resentation of the a.c network 17 Fig re 5 – Example of voltage – c r ent c aracteristic s owin p s ible c r ent mod lation ran e in the a sen e of telecommu ication b twe n rectif ier an in erter 2

Fig re 6 – Re ctive p wer mod lation in an HVDC tran mis ion o eratin at minimum extin tion an le γ min 2

Fig re 7 – Re ctive p wer mod lation in an HVDC tran mis ion o eratin at extin tion

an le γ > γ

min 2

Fig re 8 – Sta i ty improvement of an a.c l n or network 2

Fig re 9 – Prin iple ar an ements of a dampin control er 2

INTERNATIONAL ELECTROTECHNICAL COMMISSION

Part 3: Dynamic conditions

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This Consol date v rsion of IEC TR 6 919-3 be rs the e ition n mber 2.1 It con ists

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IEC 6 919-3, whic is a tec nical re ort, has b en pre ared by s bcommite 2 F: Power

electronic f or electrical tran mis ion an distribution s stems, of IEC tec nical commite 2 :

Power electronic s stems an eq ipment

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

edition:

a) this re ort con ern only l ne-commutated con erters;

b) sig if i ant c an es have b en made to the control s stem tec nolog ;

c) some en ironmental con traints, f or example au ible noise lmits, have b en ad ed;

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

A l st of al p rts of the IEC 6 919 series, u der the general title: P erorma c of hig -v lta e

d ire t c r e t (H VDC) s stems with line-c mmutated c n erters, can b fou d on the IEC

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

PERFORMANCE OF HIGH-VOLTAGE DIRECT CURRENT (HVDC)

Part 3: Dynamic conditions

This Tec nical Re ort provides general g idan e on the d namic p rforman e of hig -voltage

direct c r ent (HVDC) s stems Dy amic p rf orman e, as u ed in this sp cif i ation, is me nt

to in lu e those events an phenomena whose c aracteristic freq en ies or time domain

cover the ran e b twe n tran ient con ition an ste d state It is con erned with the

d namic p rf orman e d e to interaction b twe n two-terminal HVDC s stems an related

a.c s stems or their elements s c as p wer plants, a.c l nes an bu es, re ctive p wer

sources, etc at ste d -state or tran ient con ition The two-terminal HVDC s stems are

as umed to uti ze 12-pulse con erter u its comprised of thre -phase brid e (double way)

con ection The con erters are as umed to u e th ristor valves as brid e arms, with ga les

metal oxide ar esters for in ulation co rdination an to have p wer f low ca a i ty in b th

direction Diode valves are not con idered in this sp cif i ation Whi e multi-terminal HVDC

tran mis ion s stems are not expres ly con idered, mu h of the information in this

sp cif i ation is eq al y a plca le to s c s stems

Only l ne-commutated con erters are covered in this re ort, whic in lu es ca acitor

commutated con erter circ it con g ration General req irements for semicon u tor l n

e-commutated con erters are given in IEC 6 14 -1-1, IEC 6 14 -1-2 an IEC 6 14 -1-3

Voltage-sourced con erters are not con idered

This re ort (IEC 6 919-3) whic covers d namic p rforman e, is ac omp nied by publ cation

f or ste d -state (IEC 6 919-1) an tran ient (IEC 6 919-2) p rforman e Al thre asp cts

s ould b con idered when pre arin two-terminal HVDC s stem sp cif i ation

A dif feren e exists b twe n s stem p rforman e sp cifi ation an eq ipment desig

sp cif i ation f or in ivid al comp nents of a s stem Whi e eq ipment sp cif i ation an

testin req irements are not defi ed herein, at ention is drawn to those whic would af fect

p rforman e sp cif i ation f or a s stem There are man p s ible variation b twe n

dif ferent HVDC s stems, therefore these are not con idered in detai This re ort s ould not

b u ed directly as a sp cif i ation for a sp cifi project, but rather to provide the b sis f or an

a pro riate sp cif i ation tai ored to f it actual s stem req irements for a p rtic lar electric

p wer tran mis ion s heme This re ort do s not inten to dis riminate b twe n the

resp n ibi ty of u ers an man f acturers for the work sp cif ied

2 Normative ref erenc s

The folowin referen ed doc ments are in isp n a le f or the a pl cation of this doc ment

For dated ref eren 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 6 14 -1-1, Semic nductor c n erters – Ge eral re uireme ts a d li e c mmutated

c n erters – P art 1-1: Sp cific tio of b sic re uireme ts

IEC/TR 6 14 -1-2, Semico ductor c n ertors – Ge eral re uireme ts a d line c mmutated

c n ertors – P art 1-2: Ap li atio g id e

IEC 6 14 -1-3, Semic ndu tor c n ertors – Ge eral re uireme ts a d ln c mmutated

c n ertors – P art 1-3: Tra sformers a d re ctors

IEC TR 6 919-1:2 0 2 10, P erorma c of hig -v lta e dire t c re t (H VDC) s stems with

line-c mmutated c n erters – P art 1: Ste dy-state c nditio s

IEC TR 6 919-2:2 0 , P erorma c of high-v lta e dire t c re t (H VDC) s stems with li

e-c mmutated c n erters – P art 2: Fa lts a d switc in

3 Outl ne of HVDC dynamic perf orma ce specifications

3.1 Dy amic perf orma c spe if ic tion

A complete d namic p rf orman e sp cif i ation f or an HVDC s stem s ould con ider the

f ol owin clau es:

– a.c s stem p wer f low an f req en y control (se Clau e 4);

– a.c d namic voltage control an interaction with re ctive p wer sources (se Clau e 5);

– a.c s stem tran ient an ste d -state sta i ty (se Clau e 6);

– d namic of the HVDC s stem at hig er freq en ies (se Clau e 7);

– s bs n hronou os i ation (se Clau e 8);

– p wer plant interaction (se Clau e 9)

Clau e 4 de ls with u in active p wer control of the HVDC s stem to af fect p wer f low

an /or freq en y of related a.c s stems in order to improve the p rforman e of s c a.c

s stems The f ol owin asp cts s ould b con idered at the desig of HVDC active p wer

control modes:

a) to minimize the a.c p wer s stem los es u der ste d -state o eration;

b) to prevent a.c lne overlo d u der ste d -state o eration an u der a disturb n e;

c) to co rdinate with the a.c generator governor control;

d) to s p res a.c s stem f req en y deviation u der ste d -state o eration an u der a

disturb n e

In Clau e 5, the voltage an re ctive p wer c aracteristic of the HVDC s bstation an other

re ctive p wer sources (a.c filters, ca acitor b n s, s u t re ctors, SVC (static var

comp n ator), s n hronou comp n ators) as wel as interaction b twe n them d rin control

of the a.c bu voltage are con idered

In Clau e 6, a dis u sion is provided con ernin method of control n active an re ctive

p wer of an HVDC l n to improve the ste d -state an /or tran ient sta i ty of the

intercon ected a.c s stem by cou teractin electromec anical os i ation

Clau e 7 de ls with d namic p rforman e of an HVDC s stem in the ran e of half

fun amental f req en y an a ove d e to b th c aracteristic an non-c aracteristic harmonic

generated by con erters Me n f or preventin in ta i ties are also dis u sed

In Clau e 8, the phenomenon of ampl f i ation of torsional, mec anical os i ation in turbin

e-generators of a thermal p wer plant at their natural f req en ies, d e to interaction with an

HVDC control s stem (con tant p wer an c r ent reg lation modes), is con idered

Sp cif i ation f or s bs n hronou dampin control are defi ed

The interaction b twe n a p wer plant an an HVDC s stem located electrical y ne r to it is

con idered in Clau e 9, takin into ac ou t some sp cial features of the n cle r p wer plant

an req irements for the rel a i ty of the HVDC s stem

3.2 Ge eral comme ts

An desig req irements f or f uture HVDC s stems b in sp cif ied s ould f al within the

desig l mits covered in publ cation on ste d -state (IEC 6 919-1) an tran ient

(IEC 6 919-2) p rf orman e It is recommen ed that d rin pre aration of the d namic HVDC

s stem p rf orman e sp cif i ation, the pro er HVDC s stem control strateg s ould b

identif ied b sed on detai ed p wer s stem stu ies The priorities of control sig al inputs an

the way they are proces ed s ould b sp cified

4 AC system power f low and f requency control

Active p wer control of an HVDC s stem can b u ed to control the p wer f low an /or

f req en y in related a.c s stems in order to improve the p rforman e of a.c s stems in

ste d -state o eration an u der disturb n e

In this clau e, the HVDC active p wer o eration modes, whic are u ed to improve the a.c

s stem p rforman e for the fol owin purp ses, wi b covered:

– HVDC p wer control to minimize the total p wer s stem los es u der ste d -state

o eration;

– HVDC p wer control f or prevention of a.c lne overlo d u der a disturb n e as wel as

ste d state;

– co rdinated HVDC p wer control with an a.c s stem generator governor control;

– HVDC p wer control for s p res ion of an a.c s stem f req en y deviation u der a

disturb n e as wel as ste d state

HVDC active an /or re ctive p wer modes u ed to improve a.c s stem d namic an tran ient

sta i ty or improve a.c voltage control is dis u sed in Clau es 5 an 6

4.2 Power flow control

4.2.1 Ste dy-state power control re uireme ts

The p wer of an HVDC s stem is sometimes controled to minimize overal p wer s stem

los es, to prevent a.c l ne overlo din , an to co rdinate with the governor control of a.c

s stem generators Su h p wer control req irements dif fer f rom time to time ac ordin to the

role of HVDC s stems in the overal p wer s stem

When an HVDC s stem is u ed to tran mit p wer f rom remote generatin station , the HVDC

tran mis ion p wer control is co rdinated with the governor control of the p wer station

generators In this case, the generator voltage, freq en y or the rotor sp ed may b u ed as

a referen e to the HVDC p wer control s stem

When two a.c p wer s stems are con ected by an HVDC l n , the HVDC p wer is control ed

to a pre-determined pat ern u der normal circ mstan es, but an ad itional fun tion can b

in orp rated to this HVDC p wer control so that the freq en y of either or b th a.c p wer

s stems is control ed When one of the a.c s stems is an isolated s stem, s c as one

s p lyin a se arate islan , freq en y control of this isolated a.c s stem may have to b

re l zed by the HVDC s stem

The a.c s stem f req en y control by an HVDC s stem is dis u sed in 4.3

When two a.c s stems are intercon ected by more than one d.c l n or d.c an a.c l n s, or

when a d.c s stem exists within an a.c s stem, HVDC p wer may b control ed in order to

minimize the total tran mis ion los es of the intercon ected s stems

In some cases of a.c./d.c s stem con g ration des rib d a ove, the HVDC p wer c an e

control can b u ed to prevent overlo din of one or more tran mis ion lnes in the p wer

s stem

In certain sp cial HVDC control s hemes, s c as the one desig ed to improve a.c s stem

p rforman e by in re sin the d.c p wer d ring an af ter a disturb n e, the ste d -state d.c

tran mis ion p wer may have to b set at a restricted value so that the d.c p wer do s not

ex e d the d.c rated p wer, in lu in overlo d ca a i ty, when the control is initiated It is

imp rtant to con ider also the ad itional re ctive p wer s p ly req ired b th by the HVDC

con erters an the a.c s stems in s c a situation

The f ol owin items a) to g) ne d to b con idered in the sp cif i ation of ste d -state control

req irements Note that at the time of pre arin the sp cif i ation, the complete ste d -state

control req irements may not have b en determined or desig ed, but al owan e f or p s ible

f uture inputs is neces ary

a) When a p wer f low control s stem is desig ed to have more than one f un tion, in lu in

the a.c s stem freq en y control, the HVDC control s stem s ould b so desig ed that

priorities are set b twe n e c control fun tion

b) Un er ste d -state con ition , the control f or prevention of a.c l ne overlo din is u ualy

given hig er priority over other p wer flow controls The control f or minimization of power

s stem los es is implemented either by setin the d.c p wer to a p t ern whic has b en

pre-determined by the p wer s stem data, or in resp n e to an on-l ne computation whic

is con u ted in the central lo d dispatc in of fice Us al y, its control resp n e is

relatively slow, b in several secon s or several min tes, even in the later case

c) In isolated s stems or s stems with a relatively large d.c inf eed, freq en y is of ten

maintained by the HVDC p wer In s c a case, HVDC f req en y control could have a

priority over s stem los minimization, but may b l mited by overlo d protection

d) The c an e in re ctive p wer deman ac omp n in the p wer c an es may res lt in

f req ent switc in of re ctive p wer eq ipment In s c a case, it is neces ary to fi ure

out p rtic lar a.c voltage control me s res s c as reactive p wer control by con erter

u its, or to set l mits of the mag itu e of HVDC p wer c an e

e) The ne d f or sp cial p wer order adju tment sig als u iq e to the p wer s stem s ould

b identif ied, stu ied, an sp cified The sig als can ot b p rmited to cau e d.c c r ent

or p wer, or a.c voltage to deviate b yon eq ipment an s stem ratin s an l mits The

priority of two or more input sig als havin simultane u deman on d.c l n p wer

s ould b esta l s ed an co rdinated

f ) Bip lar d.c l n s normal y req ire that d.c p wer an c r ent b ef f ectively s ared

b twe n p les For los of one p le, an overlo d strateg for the remainin p le could b

develo ed to minimize disruption to a.c s stem p wer f low, voltage an f req en y

g) Disruption of the telecommu ication l n b twe n the sen in an receivin s stem of the

d.c l n s ould not cau e disruption to the a.c p wer s stem A minimum sp cif i ation

req irement is that p wer tran mis ion is maintained at the same p wer level whic

existed b fore the telecommu ication f ai ure If ad itional fun tion s c as freq en y

control are req ired d rin temp rary outage of the telecommu ication ln , these s ould

b sp cif ied

4.2.2 Step c a ge power re uireme t

Un er certain p wer s stem con ition , it may b req ired to c an e the HVDC p wer in

ste s in order to improve the p rf orman e of a.c s stems d rin an af ter p wer s stem

disturb n es Un er certain circ mstan es, the ste c an e may in olve d.c p wer reversal

A ste c an e of d.c p wer is re l zed by c an in the set value of d.c p wer order or by

c an in the p wer ran e in resp n e to an input sig al The rate of c an e of p wer an

l mit to the mag itu e of the d.c p wer c an e deman ed by the ste c an e s ould b

adju ta le within sp cif ied l mits ac ordin to a.c s stem req irements F r example,

dif ferent ramp rates may b req ired f or dif ferent events Sp cial con ideration may b

req ired when the ste c an e would in lu e p wer reversal

Power s stem disturb n es to b con idered in sp cif yin d.c p wer ste c an es may

in lu e: a.c l ne trip, los of large p wer s p ly source or large dro in a.c s stem f req en y

an s d en in re se or decre se of p wer s stem lo d with its cor esp n in large freq en y

deviation

In some of the a ove cases of p wer s stem disturb n es, the a.c s stems wi also b

s p orted by the a.c f req en y control provided by the d.c s stem

In sp cif yin an desig in HVDC control f un tion , the eff ects of the ste c an e p wer

f un tion s ould b s rveyed in detai for variou p wer s stem con ition It is b st to

sp cif y l mits an ran es f or p wer c an es an ramp rates rather than sp cif i set in s

Setin adju tment can b made with the d.c s stem in o eration

The sig als f or initiation of HVDC ste p wer c an es in lu e overlo d relay sig als or trip

sig als of p rtic lar tran mis ion l nes whic are tran mited to the HVDC s bstation, or a.c

s stem f req en y whic is detected at the HVDC s bstation or at some p int in a.c s stems

The time delay in olved in a telecommu ication s stem whic tran mits these initiation

sig als may af fect the a.c or d.c s stem p rf orman e Theref ore, in some cases, a high

sp ed telecommu ication s stem may b req ired When the tran mis ion delay time is large,

this ef f ect s ould b taken into ac ou t

There are some cases in whic sig als are sent to b th HVDC s bstation , or more than one

sig al is received by an HVDC s bstation In these cases, it is neces ary to set priorities of

control fun tion

The mag itu e of d.c p wer ste c an e may b lmited by a.c an d.c s stem con ition ,

an it may b req ired u der certain circ mstan es to detect the c an es in s stem

con ition to update the values of s c l mits

In p rtic lar, when there is a large ste c an e in d.c p wer, the a.c voltage may c an e

s bstantial y For this re son, it may b req ired to stu y the al owa le ran e of a.c voltage

fl ctuation to determine the l mits on ste p wer c an es, or introd ce sp cial a.c voltage

control me s res

The al owa le lmits of a.c voltage deviation can b diff erent f or ste d -state o eration an

tran ient con ition an s ould b sp cif ied

When an HVDC s stem is con ected to a hig imp dan e an /or low inertia a.c s stem, the

ste c an e in d.c p wer may have ad erse ef f ects on the voltage sta i ty, tran ient

sta i ty, an f req en y of the a.c s stem In s c cases, the mag itu e an rate of c an e

of p wer may have to b l mited, or other sp cial me s res may have to b provided, to

prevent deterioration of the a.c s stem d namic p rforman e When an HVDC s stem

intercon ects two a.c s stems, the ef f ect of d.c p wer ste c an e mu t b evaluated in

detai not only f or the a.c s stem in whic a disturb n e oc urs, but also f or the other a.c

s stem in whic a fault do s not oc ur

When the d.c ste c an e of p wer cau es the d.c c r ent to f al b low the minimum

al owa le o erational c r ent of the HVDC s stem, whic is u ual y 5 % to 10 % of the rated

c r ent, the con erter o eration s ould b set to the p sitive minimum c r ent Otherwise the

con erter s ould b bloc ed af ter the al owa le p riod of low c r ent o eration, or b

sp cif ied to o erate down to zero c r ent One p s ible me s re to overcome minimum

al owa le o erational c r ent is to set the p wer f lows of two p les in o p site direction an

let the p wer f low of two p les can el e c other when the HVDC s stem con g ration is

bip lar The dif feren e in the p wer f lows of e c p le is the actual o eratin p wer f low of

the overal HVDC s stem

Becau e of in erter control l mitation an p s ible ris s to a.c s stem o eration, it is not

ad isa le to req est a c r ent order ste c an e larger than the c r ent margin u les sp cial

control action are taken up n los of telecommu ication

Certain con ideration may b req ired when an HVDC s stem is to b started up from a no

lo d stan -by state in resp n e to a ste c an e p wer order (se Clau e 7 of IEC 6 919-1)

4.3 Fre ue c control

The a.c s stem freq en y control by the HVDC s stem can b a pl ed f or the f ol owin

purp ses:

a) f req en y control of the receivin an /or sen in en a.c s stem f or a d.c tran mis ion

f rom remote p wer sources;

b) f req en y control of an a.c s stem in an isolated islan or a smal a.c s stem when it is

intercon ected to a large a.c s stem throu h a d.c s stem;

c) f req en y control of either of the a.c s stems intercon ected by an HVDC s stem, also

takin the freq en y of the other s stem into ac ou t

The a.c s stem freq en y control is exec ted either as a contin ou f un tion of freq en y

u der ste d -state con ition , or when the freq en y deviation of the a.c s stem ex e d

certain l mits It may only b activated u der certain circ mstan es s c as when the local

a.c s stem con ected to the HVDC s bstation is dis on ected (islan ed) f rom the main a.c

s stem Ac ordin ly, the sp cif i ation s ould state the d ties an p rf orman e req irements

of the freq en y control f un tion

If the f req en y at the receivin en is control ed by varyin or mod latin the p wer

tran mited by the d.c ln , there mu t b co rdination of the d.c ln f req en y control with

an governor control on as ociated a.c generators It may b p s ible to u e tran ient

f req en y deviation ca a i ty of an as n hronou sen in en s stem f or s p ort of the

receivin en , provided the a.c generatin eq ipment is desig ed ac ordin ly

When an HVDC s bstation is electrical y far from the centre of the a.c s stem, the phase

an le of a.c voltage at the HVDC s bstation c an es s bstantial y with p wer c an es In

s c circ mstan es, the sp ed of resp n e of the f req en y sig al can b red ced To avoid

this lower sp ed of resp n e, the f req en y sig al can b detected at the centre of the a.c

s stem an tran mit ed to the HVDC s bstation

In f req en y control it may b req ired to provide l mits of p wer c an e an rate of p wer

c an e whic maintain the a.c s stem voltage f l ctuation within an al owa le ran e, or

uti ze sp cial voltage reg lation me s res s c as re ctive p wer control by con erters or

SVC The al owa le l mits of voltage f l ctuation d rin ste d state freq en y control s ould

b sp cif ied

When the d.c contribution to a.c s stem freq en y control is implemented, it is p s ible that

generator freq en y control is degraded u les the controls are pro erly co rdinated When

two dif ferent p wer s stems are intercon ected, it may b req ired to provide a pro riate

de d b n or to select s ita le gain in the freq en y control by the HVDC s stem so that only

large or fast f req en y f l ctuation are comp n ated by the d.c p wer control, an smal or

slow f req en y f l ctuation are control ed by the p wer station b lon in to the in ivid al

a.c s stems

The freq en y control desig ed to cor ect f or severe disturb n es, s c as those cau ed by

the trip in of large generation u its, may b re l zed more ef f ectively if the generator u it

trip sig al is tran mit ed to the HVDC s bstation to initiate the control action

Fast an large mag itu e of d.c p wer c an e f or freq en y control may prod ce

overvoltage or voltage dip in the a.c s stems Su h a situation may b rel eved by lmitin the

rate of p wer c an e or by fast re ctive p wer comp n ation The alowa le overvoltage or

voltage dip, an the al owa le d ration time s ould b sp cif ied

One p s ible me s re for contin ou an smo th o eration of f req en y control is to set the

p wer f lows of two p les in o p site direction an let the p wer f low of two p les can el e c

other when the HVDC s stem con g ration is bip lar This sp cial o eration mode is cal ed

“Freq en y control with zero p wer setin ” However, note that there is ad itional s stem

los an ac omp n in p larity reversals, whic ha p n when cros in the b rder of

minimum c r ent

It is sometimes dif fic lt to set o timal p rameters of freq en y control sin e the p wer s stem

con g ration of ten c an es d e to outages of tran mis ion l nes an /or s bstation f or

maintenan e This could b ac ou ted for by ado tin multi varia le freq en y control

When d.c p wer control is p rf ormed f or the purp se of f req en y control, it is u ualy

neces ary to provide hig sp ed telecommu ication c an els, s c as a microwave c an els

or fib r o tic c an els, b twe n two HVDC s bstation In case of los of telecommu ication

b twe n the two s bstation , f req en y control is u ual y l mited to the network con ected to

the c r ent control n s bstation

When the freq en y detection p int is located far from the HVDC s bstation control terminal,

or when it is inten ed to initiate the f req en y control action by sp cial sig als is ued from

the a.c s stem, telecommu ication c an els are req ired

In an case, the ef fect of telecommu ication time delay s ould b taken into ac ou t

For a dis u sion of telecommu ication c an els, refer to Clau e 13 of IEC 6 919-1

5 AC dynamic voltage control and interaction with reactive power sources

Chan e in re ctive p wer f low d e to lo d c an e, switc in o eration or f aults prod ce

voltage f l ctuation in the a.c network In hig imp dan e a.c s stems, i.e in s stems with

low s ort-circ it ca acity, larger voltage fl ctuation can b exp cted an the ne d f or voltage

control is more pronou ced

Su den voltage c an es in the network s ould b l mited e.g to les than 3 % if oc ur in

f req ently an to les than 10 % if ha p nin seldom Ap ro riate values s ould b sp cif ied

Hig temp rary overvoltages in ex es of the normal o eratin ran e d e to large lo d

c an es an lo d rejection in network with low s ort-circ it ca acity could ris en an erin

station eq ipment Hig temp rary overvoltages can b lmited by trip in of re ctive p wer

sources The ac e ta le l mit an d ration f or temp rary overvoltages s ould b sp cif ied

5.2 Volta e a d re ctiv p wer c ara teristic of a HVDC s bstation a d oth r

re ctiv power sourc s

5.2.1 Ge eral

Dy amic re ctive p wer an voltage control on the a.c bu of an HVDC s bstation can b

o tained by u in dif ferent eq ipment Fig re 1 s ows s hematical y an HVDC s bstation with

elements f or re ctive p wer comp n ation Whic of the de icted elements is u ed de en s

on the a.c network c aracteristic an req irements on the data of the HVDC s bstation

con erned, as wel as on the economic evaluation of the dif ferent p s ible solution

5.2.2 Conv rter a a tiv /re ctiv power sourc

The active/re ctive p wer of an HVDC con erter de en s on the fol owin f actors:

– commutation imp dan e (in lu in commutation ca acitan e, if a pl ca le);

– commutation voltage;

Fig re 1 – Eleme ts for re ctiv power compe s tion at a HVDC s bstation

Con erter o eratin time con tants are the comp site of control s stem, me s rement

s stems, an d.c tran mis ion l ne con tants With typical control s stems, time con tants

are in the ran e of a f ew mi isecon s, an this provides the control of delay an le or

extin tion an le in the ran e of les than 2 ms The resp n e time of the total d.c s stem is

normal y in the ran e of 5 ms to 15 ms

The ta c an er control can b active in ad ition to the con erter f irin control However,

e c ta c an er ste has a time delay of a f ew secon s Therefore, this control is not u ed

f or f ast active/re ctive p wer control, but only f or adju tments to the o timum o eration

con ition at the new o eratin p int

For con ideration of d namic con ition , the active/re ctive p wer diagram of the con erter

s own in Fig re 2 can b u ed (se also Fig re 16 of IEC 6 919-1) The val d ran e of

o eration f or a given maximum d.c c r ent an delay an le b twe n a f ew degre s an 9 °

as def i ed in Fig re 2 could the retical y b u ed

Q

5

10 234

56

78

d

P

U

d,I

d

I

d/ I

However, in re l ty the ran e is l mited ac ordin to the eq ipment desig an o eratin

con ition Examples of the u e of a con erter f or d namic o erating con ition are as

f ol ows:

– o eration with con tant delay an le α or extin tion an le γ In d namic con ition the

re ctive p wer c an es with the active p wer cor esp n in to c rve a in Fig re 2;

– re ctive p wer ke t con tant when active p wer is c an ed (l ne b in Fig re 2) The delay

an le or extin tion an le c an e cor esp n in ly

In this case, al o eratin p ints in ide the are b twe n c rve a an l ne b can b u ed f or

d namic control purp ses, de en in on the a.c network req irements, if the con erter

station is sp cif ied f or the res ltin stres es

With b c -to-b c s hemes, re ctive p wer control of the con erters can b u ed large s ale

With lon distan e or ca le tran mis ion, however, con erters can b u ed for re ctive p wer

control only f or a l mited ran e The priority is to ke p con tant the voltage on the l ne or

ca le to tran mit active p wer economical y

When a con erter is to b u ed f or re ctive p wer control, the in uen e on the o eration of

the converter at the other en has to b con idered Althou h the two con erters may b

se arated by a smo thin re ctor in the b c -to-b c s heme or by the smo thin re ctors,

an the d.c l nes or ca les, in lu in d.c f ilters in the case of two-terminal tran mis ion,

d namic c an e of the active/re ctive p wer at one con erter wi in uen e the active/re ctive

p wer of the secon con erter The HVDC control has to b co rdinated at b th en s

For lon distan e or lon ca le tran mis ion, the lon er time resp n e of the d.c

tran mis ion can in uen e the d namic ran e of the active/re ctive p wer Both terminals

can, however, b co rdinated by telecommu ication sig als If the telecommu ication s stem

f ai s, co rdination b twe n the terminals can b b sed on the voltage/c r ent c aracteristic

u ed in the control In this case, however, a slower resp n e res lts

In a b c -to-b c station, co rdination of control action is more e si y ac ieved

5.2.3 Volta e c ara teristic of a.c n twork depe din on the power loa ing

at the busbar of the HVDC s bstation

With d namic voltage control, an a.c network c aracteristic des ribin the de en en e on

the active/re ctive p wer at the a.c bu for dif ferent voltage levels is imp rtant Typical

c rves f or s c ste d -state c aracteristic are s own in Fig re 3 To ke p a certain voltage

level (e.g 1 p.u (p r u it ) con tant, the HVDC s bstation s ould del ver or con ume a

certain q antity of re ctive p wer ac ordin to a relation hip with active p wer that is sp cif i

to e c station

Figure 3 – Re ctiv power re uireme ts of a we k a.c s stem d pe ding on th a tiv

power loa ing for v rious consta t volta e c ara teristic at the a.c bu of a HVDC

s bstation

Fig re 3 can b u ed f or determinin d namic voltage control Curves are normal y calc lated

by lo d flow an sta i ty programs The other p s ibi ty is to red ce a.c network to a

simple Thevenin eq ivalent (se Fig re 4)

E

Z(ϕ)

IEC 19 23/0

Ke

E Eq iv le t ele tromotiv f orc of g n rators of a a.c n twork o tain d b th u e of th Th v nin th orem

Z(ϕ) Eq iv le t imp d n e of a a.c n twork o tain d b th u e of th Th v nin th orem

ϕ Eq iv le t imp d n e a gle of a a.c n twork o tain d b th u e of th Th v nin th orem

Fig re 4 – Repre e tation of th a.c network

In mes ed a.c network with generators electrical y far away f rom the HVDC s bstation bu ,

the drivin voltage E (in Fig re 4) remain a proximately con tant an c an es only if the

a.c network con g ration c an es, e.g trip in of lnes, lo d or generators

When, however, generators are close to the HVDC s bstation, a c an e in active/re ctive

p wer con ition an con eq ently voltage con ition wi in uen e the voltage at the

generators Then generation ex itation control wi act on an in uen e the voltage con ition

at the HVDC s bstation bu

The time con tant f or voltage c an e in this case is a out 10 ms to 5 0 ms The lower value

is val d where the generators are electrical y close to the HVDC s bstation, e.g an isolated

generator HVDC network However, this typ of HVDC infeed ne d more detai ed

con ideration, as the HVDC control an generator voltage control s ould b closely

co rdinated

5.2.4 Volta e c ara teristic of a.c fi ters, c pa itor ba k a d s unt re ctors f or

p wer compe s tion at the HVDC s bstation

To me t re ctive p wer req irements u der ste d -state con ition , a.c f ilters, ca acitor

b n s, commutation ca acitors an s u t re ctors may b in taled As a minimum, the a.c

f ilters req ired to fulf il the harmonic p rforman e s ould b con ected to the HVDC

s bstation The ad itional switc a le re ctive p wer elements can also b u ed for d namic

voltage control an adju tment of re ctive p wer req irements of the s stem

The size of re ctive p wer elements is determined by the a.c network req irements an to

l mit the voltage ste at switc in Tran ient voltage c an e at switc in d rin the o eration

of con erters can b red ced with the aid of con erter control whic damps the c an e of

re ctive p wer Binary switc in , e.g simultane u ly switc in in and out of dif f erent sizes

an typ s of e ctive p wer elements can also b u ed to red ce the re ctive p wer c an e

Con ideration s ould b made in the plan ed de loyment of re ctive p wer comp n ation

a p ratu for the l mitation of a.c closin o eratin times, control s stem proces in times,

an dis harge or d ty c cles of s c eq ipment Sp cial con ideration also ne d to b made

f or p wer circ it-bre ker switc in d ties, in lu in the tran ient recovery voltage (TRV) of

the a p ratu d rin mis-o eration or f ault con ition

5.2.5 Volta e c ara teristic of static v r compe s tor (SVC)

Dy amic voltage control in the a.c network can b maintained by SVCs At the HVDC

s bstation bu , the SVC may b u ed f or voltage control in the event that the HVDC

s bstation is out of o eration or the con erter re ctive p wer control ca a i ty is not

avai a le for other re son There is another o tion f or d namic voltage control: to u e VSC

tec nolog but this tec nolog is not covered by this re ort

The size of SVC con ected to the HVDC s bstation bu s ould b determined ac ordin to

the exp cted voltage variation at the bu an the cor esp n in ne d f or the re ctive p wer

to comp n ate it

The re ctive p wer ratin of the SVC s ould b larger than the size of the largest switc a le

re ctive p wer element b in con ected to the bu , to ena le smo th voltage control

The size of the SVC can also b determined de en in on the req ested overvoltage

l mitation, e.g at lo d rejection For this tas , the overlo d ca a i ty of the SVC can b u ed

When dimen ionin the SVC f or d namic comp n ation, the o eratin p int d rin contin ou

o eration ne d to b taken into ac ou t Startin f rom contin ou o eration, the SVC s ould

have enou h reg lation are to maintain voltage control

An imp rtant asp ct of SVC dimen ionin is the q estion of avai a i ty If d namic

p rforman e of the SVC is also req ired d rin p s ible outages of the SVC, a sp re u it

s ould b con idered or o eratin restriction have to b ac e ted

To me t the re ctive p wer req irements u der ste d -state con ition , a.c filters, ca acitor

b n s, commutation ca acitors an s u t re ctors may b in tal ed

5.2.6 Volta e c ara teristic of s nc ronous compe s tor (SC)

In a.c network with zero or very low inertia, s n hronou comp n ators may b u ed to

in re se the s ort-circ it ratin an the inertia The size of s n hronou comp n ator in this

case de en s on the re ctive p wer an f req en y control req irements In we k a.c

s stems the s n hronou comp n ator may b req ired to ac ieve network imp dan e

decre se ne ded f or sta le o eration of the HVDC s bstation

Fast ex itation s stems with hig cei n voltage are con idered s ita le a pl cation for the

s n hronou comp n ator to s p ort the d namic voltage control The time con tant f or the

s n hronou comp n ator re ction is slower than the resp n e of an SVC However, sin e

the internal voltage of the s n hronou comp n ator b hin its tran ient re ctan e ad s to

the s ort circ it ca acity of the a.c s stem, temp rary voltage variation can b in erently an

in tantly l mited to ac e ta le of fset levels as a con eq en e of desig Action of the

ex itation s stem return the a.c voltage to the normal desired o eratin p int

When s n hronou comp n ators are dimen ioned, the u it rel a i ty has to b taken into

ac ou t Becau e of s hed led maintenan e req irements an p s ible malfun tion , sp re

u its s ould b con idered in some cases

5.3 Volta e de iation on the busbar of a HVDC s bstation

The stren th of the a.c network at the bu of the HVDC s bstation can b expres ed by the

s ort-circ it ratio (SCR), def i ed as the relation b twe n the s ort-circ it level in megavolt

amp res at the HVDC s bstation bu at 1 p.u voltage an the rated d.c p wer in megawats

The ca acitors an a.c f ilters con ected to the a.c bu red ce the s ort-circ it level

sig if i antly The expres ion “ef fective s ort-circ it ratio” (ESCR) is u ed for the ratio

b twe n the s ort-circ it level red ced by the re ctive p wer of the s u t ca acitor b n s

an a.c filters con ected to the a.c bu at 1,0 p.u voltage an the rated d.c p wer

Commutation ca acitors in series with the con erter wi in re se the ESCR

L wer ESCR or SCR me n more pronou ced interaction b twe n the HVDC s bstation an

the a.c network AC network can b clas ified in the f ol owin categories ac ordin to

In the case of hig ESCR s stems, c an es in the active/re ctive p wer of the HVDC

s bstation le d to smal or moderate voltage c an es Therefore, the ad itional tran ient

voltage control at the bu b r is not normal y req ired The re ctive p wer b lan e b twe n

the a.c network an the HVDC s bstation can b ac ieved by switc ed re ctive p wer

elements

In the case of low an very low ESCR s stems, the c an es in the a.c network or in the

HVDC tran mis ion p wer could le d to voltage os i ation an a ne d f or sp cial control

strategies

Dy amic re ctive p wer control of the con erter or an ad itional SVC or s n hronou

comp n ator is theref ore req ired in these s stems The in erter s ould prefera ly o erate in

con tant c r ent mode or con tant d.c voltage mode, desig ed not to lower the p wer factor

nor to in re se the re ctive p wer of the in erter when the a.c voltage dro s an so avoid

voltage in ta i ty

At b c -to-b c station , it is p s ible to u e the HVDC con erters to control the lo d

rejection overvoltage on one side cau ed by los of a.c tran mis ion on the other side (an

vice versa) On the faulted side, valves contin e to con u t direct c r ent throu h a by-p s

p ir On the u f aulted side, valve firin is set to control direct c r ent in this s ort-circ ited

mode in order to draw re ctive p wer, an to control overvoltage in the man er of a th ristor

controled re ctor The p rmis ible d ration of o eration in this mode s ould be s f f icient to

al ow time for re ctive switc in , or it can b as lon as p s ible within eq ipment ratin s to

al ow f or p s ible a.c s stem recovery an therefore ra id restoration of the d.c

tran mis ion

However, this strateg is not avai a le if lo d rejection res lts from a f ault in the HVDC

s bstation In this case, other me s res f or overvoltage red ction may b ne ded

Temp rary overvoltages at lo d rejection of HVDC tran mis ion in re se with the red ction of

SCR In hig SCR s stems, the overvoltage f actor at lo d rejection is b low 1,2 p.u an in

most cases l es u der the critical stres level f or the eq ipment The d ration of the

overvoltage can last f or a lon time u les it is red ced by switc in of f the re ctive p wer

elements This is the normal proced re to red ce the temp rary overvoltages in HVDC

s bstation s p led by stron network

In low an very low SCR network , the overvoltage factor at lo d rejection, if not l mited by

other me n , would re c hig values whic could en an er the a.c an HVDC eq ipment or

in re se the s bstation costs

In this case, re ctive p wer control of the HVDC s bstation is u ual y req ired whic would

l mit the overvoltage at complete an p rtial lo d rejection However, ad itional me s res are

ne ded to red ce the overvoltages in the event that the HVDC s bstation is trip ed b cau e

of f ai ure in the s bstation This can b done with an overvoltage lmiter or ad itional SVC on

the bu b r or f ast trip in of re ctive p wer elements or s n hronou comp n ators

5.4 Volta e a d re ctiv p wer intera tion of the s bstation a d other re ctiv power

sourc s

5.4.1 HVDC conv rters, switc able a.c fi ters, c pa itor ba k a d s u t re ctors

Switc a le re ctive p wer elements without control ed re ctive p wer eq ipment of fers

re ctive p wer c an e in ste s The size of the re ctive p wer elements s ould b desig ed

in s c a way that the voltage c an e at switc in do s not ex e d a certain sp cif ied l mit

The con g ration u in only switc a le re ctive p wer elements is general y u ed f or stron

s stems

However, by makin c an es to the con erter delay an le or extin tion an le by a f ew

degre s, d.c control can o p se the re ctive p wer c an e at switc in To p rmit u e of the

con erter f or ad itional re ctive p wer control within a nar ow ran e, whic may b req ired

in intermediate s stems, only a smal ad itional ratin of the con erters is req ired

At f ul lo d, the c an e of delay an le or extin tion an le by, say, 3

o

, me n a re ctive p wer

c an e at the HVDC s bstation of 10 % of actual active p wer, whic can b ac ommodated

by a 2 % hig er ratin of eq ipment (se Fig re 2) This solution is also s ita le f or lon

distan e HVDC tran mis ion if the co rdination of b th s bstation is avai a le At p rtial

lo d, larger delay an le or extin tion an le is neces ary to ac ieve the same in re se in

re ctive p wer con umption This may res lt in o eration at lower than nominal d.c voltage

an therefore wi af fect the o eration of the other station The lower d.c voltage with its

in re sed c r ent wi res lt in in re sed tran mis ion l ne los es

However, con erters with a larger re ctive p wer ran e are ne ded in we k s stems to

control the voltage The re ctive p wer ran e s ould b at le st in the ran e of the largest

switc a le re ctive p wer element In s c a case, the req ired over atin of the HVDC

s bstation le d to con idera ly hig er costs This solution can b pref era ly u ed in b c

-to-b c HVDC s bstation

The voltage control time con tant of the con erters l es in the ran e of 10 ms to 2 ms Both

sides are control ed simultane u ly: switc in of re ctive p wer elements, de en in on the

req irements of e c a.c side, can in prin iple b done in ste s of 10 ms, thou h a certain

decision time may b req ired in practice If , d e to the dif ferin voltage con ition on e c

side, o timum o eratin con ition can ot b ac ieved by switc in re ctive p wer elements

only, the con ition can b cor ected by ta c an er o eration on the con erter tran formers

This cor ection is q ite slow, in the ran e of a f ew secon s p r ste

5.4.2 HVDC conv rters, switc able re ctiv power sourc s, SVC

If voltage an re ctive p wer control are req ired b cau e the network is we k, but can ot b

made avai a le by the con erter, an ad itional SVC may b in tal ed at the bu of the HVDC

s bstation The ad antage of this solution is that the voltage is control ed even whi e the

HVDC is out of o eration or is b in trip ed One f urther SVC a pl cation arises when one

a.c side has to b voltage controled without an in uen e on the other side of the d.c

tran mis ion In the case of a two-terminal HVDC s stem, voltage control by SVC can ena le

the HVDC to o erate at an economic o eratin p int ir esp ctive of re ctive p wer control

The size of the SVC s ould b desig ed ac ordin to the req ired reg lation ran e, whic

s ould b larger than the largest switc a le re ctive p wer element As a startin p int f or

the reg lation, al p s ible o eratin p ints of the SVC d rin contin ou o eration of the

s heme s ould b con idered In ad ition, atention s ould b given to con ition arisin

when the SVC is out of o eration d e to outages or maintenan e Either two u its s ould b

con idered, or restriction imp sed by op ration without SVC s ould b ac e ted

HVDC s hemes an SVCs e c have their own controls with comp ra le time con tants As

h ntin ef f ects can oc ur b twe n them, caref ul co rdination b twe n the controls s ould b

provided

SVCs are u ed f or cou teraction of temp rary voltage c an es, whi e s u t ca acitors or ta

c an er fol ows af ter the SVC action brin in a new ste d state con ition This co rdination

helps to con g re economical SVC ratin

In te d of SVCs, VSC tec nolog can b u ed for b t er control of voltages an re ctive

p wer comp n ation, but this is not covered by this re ort

5.4.3 HVDC conv rters, switc able re ctiv power sourc s a d s n hronous

compe s tors

For d namic voltage control at the bu of an HVDC s bstation, s n hronou comp n ators

can also b u ed Sy c ronou comp n ators can b uti zed in cases where inertia of the

a.c s stem is low an tran ient lo d c an es or faults would le d to u ac e ta ly large

f req en y deviation Sy c ronou comp n ators can of fer a p rt of the req ired re ctive

p wer comp n ation an voltage control The voltage comp n ation time con tant de en s

on the ex itation s stem of the s n hronou comp n ator an is in the ran e of a out 10 ms

to 2 0 ms, whic is lon er than the d.c control time con tant

A s n hronou comp n ator ad s to the s ort-circ it ca acity of the a.c s stem an is

theref ore u ef ul in preventin voltage in ta i ty in we k network than s to the res ltin

in re se in s ort-circ it ratio

The s n hronou comp n ator reg lation ran e s ould b rated f or more than the largest

switc a le re ctive p wer u it with d e regard to the p s ible o eratin point of the

Becau e of lon er maintenan e times an f orced outages of a s n hronou comp n ator, one

or more sp re u its s ould b con idered, or the restriction imp sed by o eration without a

s n hronou comp n ator in service have to b ac e ted

One major disad antage of a s n hronou comp n ator is its lon maintenan e p riod

Becau e it has rotatin p rts an mas ive mec anical comp nents, its maintenan e req ires

lon er time

6 AC system transie t and ste dy-state stabi ity

The p s ibi ty to control active an /or re ctive p wer of an HVDC l n can b u ed to improve

the tran ient an ste d -state sta i ty of the con ected a.c network if this is req ired to

at ain go d o eratin con ition

An electric p wer s stem is said to b in a con ition of ste d -state sta i ty if , f ol owin an

“smal ” disturb n e, it re c es a con ition of ste d -state o eration whic is identical or close

to its initial con ition

A p wer s stem is said to b in a con ition of tran ient sta i ty with resp ct to a seq en e of

disturb n es (not “smal ” as in the a ove sen e) if , f ol owin this seq en e of disturb n es, it

return to a con ition of ste d -state s n hronou o eration

The phenomena de lt with in this clau e con ern electromec anical os i ation b twe n

dif f erent s n hronou mac ines, groups of mac ines or are s of the p wer s stem fol owin a

disturb n e in the network If s ita le precaution are not taken, s c os i ation could b so

large that the p wer s stem b comes u sta le an generators fal out of s n hronism In

other cases the s stem can b sta le, but with low dampin le din to a prolon ed os i ation

time The electromec anical os i ation res lt in os i ation of active an re ctive p wer in

generators, tran mis ion l nes, etc an of the voltage in the s bstation

The freq en ies of interest are in the ran e of 0,1 Hz to 2 Hz Os i ation b twe n large

network are s normal y f al in the lower p rt of the ran e, whi e os i ation in olvin a low

inertia mac ine, s c as a s n hronou comp n ator, f al in the up er p rt of the ran e

An HVDC ln can sometimes b u ed to improve the sta i ty p rf orman e of the con ected

a.c s stems by automatical y control n the active p wer of the ln to cou teract the

os i ation As an alternative or a complement, the re ctive p wer con umption of the

con erter can, if neces ary, b control ed to improve p rf orman e

One control f eature, whic can b es ential in the as istan e of an HVDC l n to maintain

sta i ty in an a.c s stem is the ste c an e p wer f low req irements dis u sed in Clau e 4

Other asp cts of HVDC l n p rf orman e that have an imp ct on sta i ty but whic are not

tre ted here are p wer tran f er d rin f aults an the recovery f ol owin f ault cle rin These

mat ers are tre ted in 5.3.2 an 5.3.3 of IEC 6 919-2

6.2 Ch ra teristic of a tiv a d re ctiv p wer modulation

6.2.1 Ge eral

Active an re ctive p wer c an es are ac ieved by control of the f irin an les Mod lation

control do s not normal y in olve con erter tran former ta c an er o eration sin e ta

c an ers are to slow to re ct to os i ation over most of the freq en y ran e of interest, or

b cau e the mod lation levels are to low to initiate ta c an er o eration It may also b

p s ible to bloc ta c anger o eration d rin mod lation action

Durin a disturb n e, several os i atory modes may oc ur simultane u ly In s c cases, the

controler may have to resp n to several os i ation f req en ies at the same time In some

cases, however, s stem con ition may b s c that the control er s ould pref era ly not

resp n to certain f req en ies These asp cts can b taken care of by pro er f ilterin of the

input sig als to the controler

The automatic control action can b ef f ected either by large sig al mod lation or smal sig al

mod lation These terms are related to active p wer mod lation L rge sig al mod lation

in olves the simultane u mod lation of the c r ent order in b th rectifier an in erter, whie

smal sig al mod lation is car ied out local y in the c r ent control n station only L rge

sig al mod lation is the most commonly a pl ed method

6.2.2 Larg sign l mod lation

The s ort-term overlo d ca a i ty of the con erter eq ipment can normal y b uti zed for

large sig al active p wer mod lation, an can b an imp rtant f actor in ac ievin an ef f icient

dampin action It s ould therefore b sp cif ied if dampin control action s al determine the

amou t of overlo d req ired, or if the control action s al b lmited to u in the in erently

avai a le overlo d ca a i ty When sp cif yin the overlo d req irements, it s ould b

esta l s ed if mod lation f or dampin purp ses s al b p s ible in combination with s

ort-term or ste d -state overlo d o eration Further asp cts of overlo d req irements can b

f ou d in Clau e 6 of IEC 6 919-1

In b c -to-b c station , it may b p s ible to let the mod lation cau e p wer reversal If

p wer reversal d rin mod lation is req ired in a two-terminal s stem, a hig sp ed tel

e-commu ication s stem may b req ired

In two-terminal HVDC s stem with ca les, p wer reversal imp ses a gre t stres on ca le

in ulation sin e the p larity is c an ed, “con umin ” in ulation l f etime of HVDC ca le It is

ad isa le to l mit the n mb r an sp ed of p wer f low reversal

In a two-terminal HVDC s stem, large sig al mod lation u ual y req ires telecommu ication

f or tran mis ion of c r ent order an sometimes f req en y inf ormation b twe n the HVDC

s bstation

When the telecommu ication is not avai a le, active p wer mod lation can sti b p rformed

in the c r ent control n station but s ould then b restricted in s c a way that the c r ent

margin is not lost This is i u trated in Fig re 5

Figure 5 – Ex mple of volta e – c r e t c ara teristic s owing p s ible c r e t

modulation ra ge in the abs nc of tele ommu ic tion

betwe n re tifier a d inv rter

6.2.3 Smal signal mod lation

In some situation , it may not b practical or p s ible to u e the telecommu ication l n to

tran mit mod lated p wer or c r ent orders This is the case if the telecommu ication delay is

so large that it b comes sig if i ant comp red to the p riod time of the mod lation or if the

telecommu ication has f aied A smal sig al p wer mod lation can then sti b p rformed in

the c r ent control n station The mod lation ampl tu e is normal y l mited to b twe n 3 %

an 5 % of the c r ent margin Su h a smal sig al mod lation can sti provide con idera le

dampin as istan e in cases where os i ation can start growin sp ntane u ly (ste d -state

in ta i ty)

6.2.4 Re ctiv power modulation

In most cases, active p wer mod lation is s f f iciently ef f ective However, mod lation of

re ctive p wer can sometimes b b nef i ial, in p rtic lar f or hig imp dan e a.c s stems, or

b cau e of the location of the d.c infeed It s ould also b p inted out that mod lation of

active HVDC p wer may only le d to u wanted os i ation of the a.c voltage, sin e a c an e

in active p wer is alway ac omp nied by a c an e in re ctive p wer This can sometimes b

c red throu h a combined control of active an re ctive p wer Re ctive p wer mod lation

can for example b p rf ormed to cou teract a.c voltage f l ctuation The input sig al to the

controler is, in this case, the a.c voltage

Re ctive p wer mod lation is ac ieved by alterin the f irin or extin tion an le in the voltage

control n station This is normal y ef f ected on the in erter An in re se of the extin tion

an le in order to in re se the re ctive p wer con umption of the in erter wi res lt in a

red ction of the direct voltage, an the f irin an le in the rectif ier wi theref ore also

automaticaly in re se in order to maintain con tant d.c c r ent or p wer Therefore, re ctive

p wer mod lation at the in erter wi also res lt in variation of the re ctive p wer con umption

of the rectif ier This can sometimes l mit the mod lation ampl tu e

The mec anism des rib d a ove in icates that re ctive p wer mod lation can b made at the

voltage control n station without an sp cial telecommu ication req irements If re ctive

p wer mod lation is ne ded at the c r ent control n station, the control action wi sti have

to b car ied out at the voltage control n station, an the control sig al s ould theref ore b

sent throu h the telecommu ication c an el The res ltin action in the c r ent control n

station wi b ac ieved in irectly throu h the con tant c r ent or p wer control

If the HVDC s stem is o eratin at its minimum extin tion an le (maximum voltage), only an

in re se of the re ctive p wer con umption f rom the ste d -state value is p s ible d rin

mod lation A decre se of the re ctive p wer relative to the ste d -state value can only b

o tained if the HVDC s stem is b in o erated b low the maximum direct voltage an with

ste d -state f iring an extin tion an les a ove the minimum values Fig res 6 an 7 s ow

voltage-c r ent c aracteristic an re ctive p wer variation at in erter o eration f or b th

these situation

It is imp rtant to sp cif y if a decre se in the re ctive p wer con umption d rin mod lation is

p s ible, sin e this wi cau e in re sed cost for valves, con erter tran formers an f ilters

6.3 Cla sific tion of network situ tion

The degre of improvement of a.c s stem sta i ty that can b o tained de en s on the size

of the mod lation ampl tu e relative to the stren th of the network, the c aracteristic of the

network an to what p ints in the network the HVDC l n is con ected One can distin uis

b twe n two con e tual y dif ferent situation One of these is when sta i ty improvement has

to b provided f or an a.c l ne or an a.c network in p ral el with the HVDC l n The other

situation is when the sta i ty in ide a network con ected to one of the HVDC s bstation has

to b improved These two situation are s own in Fig res 8 an 8

6.4 AC network in paral el with the HVDC l nk

This is pro a ly a con g ration in whic the ca a i ty of the HVDC l n to improve sta i ty of

an a.c s stem can b most ef fectively uti zed The normal control action is to mod late the

active p wer tran mited to cou teract variation in the phase an le b twe n A an B (se

Fig re 8 ) The freq en y dif f eren e b twe n A an B or active p wer or c r ent in the

p ral el a.c l n are p s ible inputs to the dampin control er of the HVDC l n Fig re 9

s ows two prin iple ar an ements of a dampin control er The control strateg s ould b

a le to recog ize the situation when the p ral el a.c l n is o ened an network A an B

lose s n hronism The mod lation then b comes me nin les

a d extin tion a gle γ

Figure 7 – Re ctiv power modulation in a HVDC tra smis ion o erating

at e tinction a gle γ > γ

min

Net work B Net work A

IEC 19 30/0IEC 19 29 /0

Figure 8 – AClnk con e ted in paralel

G

2(s)

p wer ord r to p wer c ntrol s stem

Fig re 9 – Prin iple ar a geme ts of a damping control er

The me s rement of f req en y dif feren e is the most commonly u ed method The u e of

p wer in an a.c l ne as input to the control er may sometimes cau e diff ic lties, sin e at hig

phase an le dif feren e acros the l ne, the p wer of the lne may decre se with in re sin

phase an le dif feren e an there y cau e a wron control action by the d.c l n

Subclau e 6.6 des rib s how the tran fer f un tion G(s) can b determined G(s) normal y

have a b n -p s c aracteristic so that ste d -state c an es do not res lt in a non-zero

output f rom the controler Likewise, the tran f er f un tion s ould s p res control er action at

f ast c an es in the input sig als

In case of a p ral el a.c l n , HVDC dampin control can sometimes in re se the amou t of

p wer that can b tran mit ed via the p ral el a.c circ it in a sta le man er The amou t of

improvement de en s on what typ of in ta i ty dictates the p wer l mit In a f irst swin

in ta i ty situation, improvements can only b exp cted if the HVDC l n is large or has a

large s ort-time overlo d ca a i ty Improved dampin can, however, also b o tained in

cases where the mod lation ampltu e is relatively smal Even smal sig al mod lation

p rformed local y in the c r ent control n station has made it p s ible to in re se the

amou t of p wer in p ral el a.c lnes in some cases

6.5 Improv me t of the stabi ity within one of the con e te a.c network

The a i ty of the HVDC l n to improve sta i ty in this case de en s largely on the

con g ration an c aracteristic of the network req irin as istan e an also on the

p s ibi ty to o tain relevant input sig als to the dampin control er In some cases, input

sig als may have to b tran mit ed to the HVDC s bstation f rom other station in the a.c

network Input sig als to the controler can b freq en y, voltage, a.c l ne p wer, l ne c r ent

or a combination of these

When sp cifyin dampin control in a case l ke this, con ideration s ould b given to the

a i ty of the network con ected to the other HVDC s bstation to ac e t the res ltin p wer

swin s

The general con ideration regardin the tran fer f un tion of the control er as given in 6.4 f or

the case of a p ral el a.c network a ply also in this case

6.6 Determination of the d mping control c ara teristic

The ne d f or an ef f ectivenes of HVDC l n dampin control is normal y stu ied u in a

tran ient sta i ty computer program with the a i ty to model HVDC ln s an dif f erent

control er c aracteristic Su h a stu y can reve l p tential pro lems in the interaction at low

f req en ies b twe n the HVDC l n an other p wer s stem elements s c as static var or

s n hronou comp n ators

The control er en ironment in lu es man elements that are, however, model ed inac urately

or not at al Th s model stu ies s ould b au mented by me s rements an tests p rformed

in the field when p s ible to do so

It is imp rtant to stres the ne d for an ac urate re resentation of the HVDC l n an its

control s stem when p rformin tran ient sta i ty stu ies An oversimpl f ied model may give

rise to misle din res lts The d.c l n model in the sta i ty programme s ould theref ore b

val dated The valdation can b car ied out pref era ly again t an HVDC TNA or digital

simulator an af ter con tru tion, again t me s rements p rformed up n the HVDC s stem

Note that the model u ed s ould give con ervative res lts, even if it is model ed as a simple

one

Also, it is very imp rtant to model the PL (phase loc ed lo p) f or HVDC firin control

as precise as p s ible

HVDC simulators an eq ivalent programs can b u ed to val date the implemented tran fer

f un tion of the dampin control er

The re l-time simulator is very u ef ul f or the stu y of val dation of control Closed-lo p test

u in a re l-time simulator ena les a q asi- e l con ition The re l-time simulator can b of

an analog e typ l ke TNA or digital Sin e it is a simulator test, variou kin of disturb n es

can b initiated an help to verif y the control s heme Also, it is recommen ed to car y out

artif i ial fault testin if it is p s ible, esp cial y f or new HVDC s stems

In spite of the f act that a dampin control er is desig ed to act in a freq en y ran e b low

a out 2 Hz, it may have a detrimental ef fect on a control er f or dampin of s bs n hronou

os i ation (se Clau e 8)

The sp cifi ation of the sta i ty improvement feature of an HVDC l n can b determined in

two con e tualy dif f erent way The dif f eren e b twe n the two way l es in who determines

the tran f er f un tion an other c aracteristic of the dampin control er, the uti ty purc asin

the l n , or the s p l er of the HVDC s bstation eq ipment

In the f ormer case, the control- elated asp cts of the sp cif i ation s ould sp cif y the

c aracteristic of the control er in terms of input q antities, tran f er fun tion(s), ac es ory

logic an output q antities The control eq ipment can then b desig ed to cor ectly

implement the sp cif ied c aracteristic , an then verif ied by tests

In the secon case, the situation b comes more complex The s p l er s ould b provided

with a f ul set of network an generator data to al ow him to p rform tran ient sta i ty an

related stu ies f or an adequately bro d ran e of o eratin con ition In this case, the

p rf orman e req irements are complex; it may b dif f ic lt to formulate cle r p rf orman e

criteria an negotiation may b req ired If this method is selected, sta i ty stu ies s ould

also b p rformed prior to is uin the sp cifi ation in order to esta l s the level of sta i ty

an /or dampin that can b exp cted to b ac ieved by d.c l n control action

In either case, the ca a i ty of the s bstation eq ipment, in p rtic lar the valves, s ould b

con idered in the determination of the gain an ampl tu e l mitation of the control er Expl cit

provision s ould b made for en ine rin review of controler p rf orman e d rin

commis ionin tests an p riodical y there fter

6.7 Impleme tation of the dampin control er a d tele ommunic tion re uireme ts

The dampin control er can e si y b integrated into the HVDC control eq ipment It is u ualy

al ocated to the HVDC s bstation control level (se Clau e 12 of IEC 6 919-1)

In the case of large sig al mod lation, con ideration s ould b given to the ca acity of the

telecommu ication l n b twe n the two HVDC s bstation , whic s ould b a le to tran mit

the mod lation sig als an in man cases also input sig als (e.g f req en y) to the control er

without cau in to large phase displacements

Where the mod lation control req ires telecommu ication, the rel a i ty of the commu ication

l n is of primary imp rtan e The telecommu ication s stem s ould b desig ed in s c a

way that “out of phase mod lation” can ot res lt f rom commu ication faiures, i.e if there is

an inter uption in the telecommu ication c an el, the s stem s ould b desig ed so that

mod lation is res med cor ectly when the c an el res mes o eration

7 Dy amics of the HVDC system at higher frequencies

This clau e ad res es the d namic performan e of an HVDC s stem in the ran e of half

f un amental f req en y an a ove HVDC con erters generate an re ct to os i ation at

b th integer an non-integer multiples of fun amental freq en y, an th s the con erter

p rforman e is de en ent on the imp dan e of the a.c an d.c s stems In some

circ mstan es, in ta i ty can oc ur, so that the con erter p rf orman e is u ac e ta le For

present purp ses, the distin tion b twe n sta i ty an in ta i ty is that in a sta le s stem,

ef f ect (e.g non-c aracteristic harmonic ) is pro ortional to cau e (e.g u b lan e in an a.c

s stem), where s in ta i ty u ual y in olves the generation of os i ation at non-integer

multiples of f un amental f req en y an can “grow from nothin ” even in a p rf ectly b lan ed

s stem A detai ed des ription of these s bjects is covered in ref eren es l sted in the bibl

o-gra h an in p rtic lar in CIGRE TF14-0 /IEEE, Part 1 [1]

This tec nical re ort wi theref ore only provide brief des ription , but it wi outl ne the

relevant req ired information an p rtic lar asp cts of the sp cif i ation of HVDC s hemes

7.2 Type of instabi ity

7.2.1 Loop instabi ity (harmonic instabi ity)

This in ta i ty was f ou d on some e rly HVDC s hemes, an was of ten refer ed to as

harmonic in ta i ty It can oc ur at b th integer an non-integer multiples of the f un amental

f req en y an it in olves the main p wer circ it an control circ it (in lu in me s rements)

lo p It is stron ly related to the control an me s rin s stem p rameters In ta i ty can

oc ur, even if the a.c s stem is p rf ectly b lan ed an without distortion, if u s ita le control

c aracteristic are u ed an can b se n on b th the a.c an the d.c side It of ten starts

f rom a ne r integer multiple of fun amental freq en y, ne rly alway close to the main circ it

resonan e It may then loc into the ne rest harmonic freq en y as the in ta i ty ampl tu e

grows

In ta i ty rarely oc urs with present day “eq idistant f irin ” typ of control

7.2.2 Cur e t loop instabi ity

The sp ed of the resp n e of an HVDC s stem is l mited primari y by the ca acitan es an

in u tan es of the a.c an d.c s stems The resp n e of the con erter itself is generaly

con idera ly f aster than that of an a.c s stem or mac ine However, ex es ive f orcin of the

resp n e by the u e of hig gain in the c r ent control lo p can le d to in ta i ty of the

HVDC s stem, p rtic larly if the resp n e of the d.c s stem with its me s rin an control

devices is comp ra le to the resp n e time of the a.c s stem The ne d to provide a s stem

whic is sta le overal may, in some cases, me n that the HVDC control resp n e has to b

slowed down

7.2.3 Core s turation instabi ty

This in ta i ty in olves p rtial saturation of the con erter tran formers A f un amental

f req en y comp nent in the d.c c r ent cau es secon harmonic an d.c in the valve

win in c r ent If the d.c comp nent re c es a level comp ra le to say 5 % of the

tran former mag etizin c r ent, the p rtial saturation of the tran former may res lt in

sig if i ant levels of ad itional harmonic (in lu in secon harmonic) in the mag etizin

c r ent The variou secon harmonic c r ent contribution en an e the secon harmonic in

the a.c voltage, an u der some circ mstan es a f ul in ta i ty can res lt

A hig imp dan e resonan e ne r to the secon harmonic on the a.c side, an /or a l

ow-imp dan e resonan e ne r to f un amental freq en y on the d.c side makes the in ta i ty

more l kely In some cases, it is practica le to desig the main circ it p rameters (e.g d.c

re ctor or a.c f ilters) s c that these resonan es are avoided In some cases however, the

l ne imp dan e is so dominatin that resonan e at f un amental or harmonic f req en ies

can ot b avoided by selection of re l stic p rameters f or HVDC s bstation eq ipment In

cases where s c resonan es can ot b avoided, the in ta i ty can b ”c red” either by

modif yin the control con tants or by the provision of sp cial f eedb c lo ps in the controls

However, in s c cases sp cial care s ould b taken to control tran ients, e.g overvoltages,

p rtic larly if the resonan e is lg tly damp d

7.2.4 Harmonic intera tions

Harmonic in the a.c voltage wi res lt in the generation of harmonic in the direct voltage at

the two side an f req en ies, an th s in the f low of c r ent on the d.c side at these

freq en ies Simi arly, harmonic in the direct c r ent wi res lt in the generation of harmonic

a.c c r ents in the tran f ormer valve win in s an a.c s stems at the two side an

freq en ies, an th s in the cre tion of harmonic voltage distortion on the a.c side at these

freq en ies Un er resonan e con ition this may res lt in u ac e ta le harmonic distortion

on either the a.c side or the d.c side, or b th

Unl ke the in ta i ties des rib d in 7.2.1 to 7.2.2, this phenomenon oc urs only if there is an

internal drivin source in either the a.c or the d.c s stem One example of this phenomenon

is the s p rimp sition of a fun amental freq en y comp nent on the direct c r ent, as may

oc ur if an overhe d d.c l ne ru s p ral el to an a.c l ne over a sig if i ant distan e The most

severe case is that a.c l ne an d.c l ne are on the same tower The fun amental f req en y

comp nent wi then cau e direct c r ent an secon harmonic in the con erter valve

win in s It is not feasible to red ce the secon res ltant harmonic comp nent injected into

the a.c s stem at the same time Th s, it may b neces ary to take action s c as

tran p sin the a.c l ne or by a plyin harmonic f ilterin on either the d.c or the a.c side of

the con erter

Other drivin sources are u b lan es in the a.c s stem (negative seq en e fun amental

f req en y, or imp dan e u b lan e) or ineq al ty of le kage re ctan es of con erter

tran f ormers; al of these cau e secon harmonic on the d.c side, whic re cts b c on the

a.c side One side an of the lat er is at third harmonic, an can sometimes req ire the

ad ition of third harmonic f ilters to prevent ex es ive a.c voltage distortion

Becau e the mec anism f or the generation of non-c aracteristic harmonic de en s up n

circ lation of harmonic on the d.c side of the con erters, there may b an interaction

b twe n them Unles the d.c l ne or ca le an the d.c filters have a very low s u t

imp dan e comp red to the imp dan e of the other con erter, the two con erters are coupled

at these f req en ies Theref ore, the tre tment of non-c aracteristic harmonic can ot b

car ied out in e en ently f or the two HVDC s bstation , sin e the coupl n b twe n them

me n that e c contributes to the non-c aracteristic harmonic b haviour of the other There

is even the p s ibi ty that a stan in wave may b set up on the d.c l ne, mag if yin the

ef f ect at b th HVDC s bstation Only an analy is in whic the circ it p rameters are

del b rately distorted to exp se this p s ibi ty is l kely to b u eful

7.3 Inf ormation re uire for de ig purpos s

In order to en ure that the HVDC s stem gives satisf actory d namic p rforman e at hig er

f req en ies, the close interaction b twe n the dif ferent p rts of the overal s stem s ould b

taken into ac ou t in the desig The overal s stem in lu es the a.c s stems at b th HVDC

s bstation , the tran mis ion l nes / ca les, the HVDC s bstation main circ it, an the

con erter control s stem It is u l kely that me nin f ul stu ies can b p rf ormed at the

sp cif i ation stage to def i e sp cifi req irements f or the HVDC s bstation comp nents

Therefore, the sp cif i ation s ould b fun tional, req irin that the man f acturer desig s the

HVDC s bstation so that satisfactory p rforman e is ac ieved at hig er f req en ies f or al

practical combination of s stem con ition A dis los re of the proced res by whic the

man facturer inten s to demon trate satisfactory d namic harmonic p rf orman e at hig er

f req en ies can b req ested

The con ition external to the HVDC s bstation whic ne d to b taken into ac ou t when

desig in satisfactory d namic p rforman e of the HVDC s stem at hig er freq en ies are as

f ol ows:

– the imp dan e an phase an le of the a.c s stem an the k own d.c s stem as se n

f rom e c con erter Both we k an stron a.c s stem re resentation s ould b

provided, an if p s ible, network con g ration with pro a le outage contin en ies;

– resonan es on the a.c or d.c side, in p rtic lar whether complementary resonan es can

0

is the f un amental freq en y an ƒ

re ,aan

ƒ

re ,dare a.c an d.c side resonant f req en ies resp ctively);

– u b lan es in the a.c s stem imp dan e or voltage;

– the presen e of other harmonic sources with cor esp n in source imp dan es an non

-l ne r lo d electrical y close to the HVDC s bstation a.c an /or d.c terminals The

presen e of a f un amental freq en y source on the d.c side is a sp cial case of this;

– p s ible a.c l nes coupl n to the d.c l ne at f un amental f req en y This req ires

ge metrical con g ration of a.c an d.c l nes when ru nin in p ral el, a.c l ne maximum

c r ent an voltage, p ints of tran p sition, etc

– imp dan e vers s feq en y c aracteristic , in lu in variation of s u t ca acitor

n mb rs;

– p s ibi ty of islan in , remainin with ne rby generators, isolated f om rest of the a.c

network

The a ove information s ould b in lu ed in the sp cification or alternatively, s f ficient data

des ribin the a.c an d.c s stem to ena le this inf ormation to b devised s ould b

provided It can b se n that the a ove-l sted information largely cor esp n s to that req ired

also for the ste d -state desig of the HVDC s bstation (se IEC 6 919-1) However, it

s ould b noted that in some a pl cation , the fu damental f req en y ƒ

0can vary over a

large ran e (e.g for an isolated generatin station), an that this may exp se resonan es not

normal y en ou tered

7.4 Me ns a ai able f or pre e ting instabi tie

Most in ta i ties at hig er f req en ies can b avoided or cor ected by alteration to the

control s stem This may simply in olve the selection of a pro riate tu in In other cases, it

may b neces ary to introd ce ad itional f eedb c lo ps, e.g f om the a.c or d.c voltage or

the a.c valve win in c r ents or the d.c c r ent The solution ado ted may difer b twe n

man f acturers an in e d fom s heme to s heme

Control tec niq es are alway in estigated first, b cau e they normal y of fer a relatively

c e p solution to in ta i ty pro lems However, there wi b oc asion in whic the control

s stem actin alone is in uff i ient to ac ompl s al of the desired activity On e this stage

has b en re c ed, either a control target has to b compromised, or other me n f ou d

Bro db n damp d f ilters can sig ificantly improve the p rf orman e of the overal s stem by

removin s arp resonan es However, the harmonic s p res ion at in ivid al f req en ies is

mu h lower p r kVAr for a damp d fi ter than for a tu ed fi ter Therefore, this method of

preventin in ta i ty can b very costly, p rtic larly if the in ta i ty oc urs at a relatively low

f req en y However, in the case of sig ificant external harmonic sources, s c as an

u tran p sed a.c l ne b in ru in p ral el to an overhe d d.c l ne over a gre t distan e, it

may not b p s ible to o tain satisfactory p rorman e without the a pl cation of exp n ive

f un amental f req en y bloc in f ilters on the d.c side or secon harmonic s u t f ilters on the

a.c side

7.5 Damping of low order harmonic by control a tion

Modern (eq idistant firin ) control s stems are a le to actively damp low order non

-c aracteristic harmonic , (e.g f rom the a.c network) This is ac ieved by s ita le mod lation

of the delay an le (α) This can b b neficial u der sp cial circ mstan es, s c as the

a pl cation of HVDC to we k a.c network an /or to a.c network whose phase con u tors

are not tran p sed, or even d rin recovery f rom a.c network faults when the wavef orm is

temp rari y distorted Althou h it is often p s ible to adju t the control s stem to exert a

restrainin influen e on the low u c aracteristic harmonic , only a sin le varia le (α) is

avai a le to ac ommodate al s c activity Th s, the same tec niq e may b in a a le of

restrainin simultane u ly the secon harmonic on the d.c network an the third harmonic on

the a.c network

7.6 Demonstration of s tisfa tory performa c at hig er fre ue cie

Traditional y, HVDC simulators have b en u ed to demon trate d rin the desig stage that

an HVDC s heme is f re f rom u desira le in ta i ties Simulators are a con enient to l f or

s c analy is, ena l n a multipl city of s stem con ition an config ration to b

in estigated relatively ra idly The in estigation are general y car ied out as ste resp n e

stu ies, with ra id dampin of the resp n e s owin go d resistan e to in ta i ty However,

the ac urac of s c in estigation is s bject to simulator comp nent lmitation

For example, many of the normal harmonic to b o served are so smal that they ten to b

buried by spuriou harmonic cau ed by model comp nent u b lan es Model con erter

tran f ormers have re c ed a hig degre of p rfection in simulatin re l tran former B-H

c aracteristic , but most models are not adeq ate to c ec for core saturation in ta i ty

Computer simulation has b come a via le alternative to the HVDC TNA simulator for

demon tration of a sen e of in ta i ty A computer simulation is lkely to b more con enient

than one u dertaken by me n of the HVDC TNA simulator, an the analy is can b

p tential y more ac urate, sin e the actual s stem an comp nent los es can b re resented

Unles the computer simulator o erates in re l time it wi not b practical to u e the actual

an complete control hardware It is imp rtant that the simulation is val dated by comp rison

of res lts with actual me s rements p rformed on a re l s stem

Usef ulnes of the re l-time simulator s ould b mentioned here A closed-lo p test can b

p rformed b f ore implementation of the control er, u in re l-time simulator Variety of test

cases can b demon trated

Sin e the con erter interacts with its harmonic en ironment (harmonic imp dan es on the a.c

an d.c sides as wel as harmonic injection f rom other sources), it is imp rtant to

demon trate that the con erter do s not mag if y low order non-c aracteristic harmonic to an

u ac e ta le extent an that large ampltu es of s c harmonic wi not cau e in ta i ty

Stu ies can b u dertaken by model n the con erter an its controls in a s stem with low

order harmonic complementary resonan es b twe n the as ociated a.c an d.c network

By a pro riate tests, it is of ten p s ible to s ow that the b haviour of the con erter an its

controls are not u d ly disturb d by harmonic , even when the harmonic ex ibit s bstantial

ampl tu e

Durin commis ionin it s ould b ar an ed (if a pl ca le) for the a.c s stem an /or the d.c

s stem to b in worst resonan e with the con erter station, an the adeq ac of the in ta i ty

cou ter me s res s ould b demon trated by me s rin the s stem resp n e to ste

c an es an /or smal sig al mod lation at the a pro riate f req en ies of the c r ent order

8 Subsynchronous osci lations

Only turbine-generator u its located ne r a rectifier s bstation an havin we k con ection to

the a.c network are v lnera le to s bs n hronou os i ation Commonly, torsional natural

f req en ies l e in the ran e of 15 Hz to 4 Hz F r large n cle r turbine-generators, the

lowest torsional f req en y may b as low as 5 Hz, with other torsional modes at hig er

f req en ies

Torsional vibration in a turbine-generator rotor may b stimulated by man common electrical

disturb n es in the tran mis ion s stem Furthermore, u der certain con ition , torsional

vibration may b ampl f ied, or reinf orced, by a ne r resonant interaction b twe n the turbin

e-generator rotor an a series comp n ated tran mis ion l ne, or by interaction with an HVDC

control s stem

Sin e turbine-generator torsional modes have very l tle in erent dampin , s af t os i ation

p rsist for a con idera le time af ter they are stimulated Re e ted stimulation of ex es ive

mag itu e can le d to los of s af t l f e, an in the extreme, fatig e f ai ure

In an HVDC s stem, the con tant p wer an con tant c r ent reg lation modes may have a

desta i zin in uen e on rotor torsionals Within the typical b n width of an HVDC c r ent

control reg lator, two or thre s bs n hronou torsional modes of os i ation may b present

To cle rly identif y the diff eren e b twe n SSR an s bs n hronou torsional interaction, the

lat er is cal ed an SSTI whic is an interaction b twe n HVDC control an turbine-generator

Subs n hronou resonan e (SSR) in olvin series comp n ated a.c tran mis ion l nes has

some fun amental dif f eren es with the HVDC torsional in ta i ty phenomenon SSR

as ociated with series comp n ation af f ects primari y the hig er ran e of torsional

f req en ies whi e torsional interaction with HVDC primari y af fects the lower f req en y

torsional modes SSR as ociated with series comp n ation can b mu h more severe than

HVDC related torsional interaction in the mag itu e of torsional desta i zation

8.2 Criteria f or s bs nc ronous torsional intera tion with a HVDC s stem

Negative dampin to turbine-generator torsional modes of os i ation is in erent to the

o jective of control n c r ent in an HVDC s stem Some degre of negative dampin wi

exist within the b n width (BW) of the c r ent control reg lation lo p

Only turbine-generator u its located ne r a rectifier s bstation an havin a we k con ection

to the a.c network are v lnera le to torsional interaction Units ne r an in erter do not

exp rien e mu h desta i zation sin e in erters re ct dif ferently to phase an le variation

than do rectif iers

Turbine-generator rotor motion cau es variation in b th mag itu e an phase an le of the

a.c voltage s p lyin the con erter The ef fect on con erter an le of delay, an the closed

lo p control re ction to the a p rent f irin an le s if t res lt in c an es in direct voltage an

c r ent, there y, d.c p wer tran fer The ultimate ef f ect of c an e in HVDC p wer is c an e

in generator electrical torq e If the generator ac umulated phase lag b twe n the c an e in

the generator s af t sp ed, an the res ltin c an e in electrical torq e on the generator rotor

ex e d 9 °, the torsional os i ation may b come u sta le

A con tant p wer lo d presents a negative c aracteristic to an deviation of generator s af t

sp ed

With the in erter control n c r ent, the d.c voltage wi f ol ow the rectif ier When the

generator rotor sp ed in re ses, so wi a.c voltage If the rectif ier is in an alpha control

mode, d.c voltage wi also in re se an le d to a p wer in re se Th s p sitive dampin

res lts within the c r ent control BW an a pro c es the in erent p sitive dampin

c aracteristic as ociated with no control action

Turbine generator u its ne r the in erter of an HVDC s stem wi alway have a p ral el a.c

intercon ection to the lo d an hen e exp rien e les in uen e on the HVDC control than

mig t a u it ne r a rectif ier Also, when s af t sp ed in re ses on a u it ne r the in erter, the

voltage mag itu e at the in erter in re ses, whic cau es a decre se in d.c c r ent

mag itu e (i.e le din to an o p site dampin ef f ect comp red to the rectif ier)

A ste d -state f irin an le of o eration of the con erter has sig if i ant imp ct on the

interaction This arises from the in erent non-l ne rity cosine relation hip b twe n f irin an le

an d.c voltage Gain l ne rization f or the c r ent reg lator f irin an le wi red ce but not

el minate the interaction

Op ration at large an les of delay can sig if i antly red ce torsional sta i ty Theref ore,

con ideration s ould b given in the HVDC s stem sp cifi ation when o eration is plan ed

at red ced voltage for voltage control purp ses, or b cau e of red ced in ulation stren th of

the tran mis ion s stem

Torsional interaction do s not oc ur on h dro generator u its, only on thermal generator

u its For very low an medium sp ed (low he d) u its, the very large h dro generator inertia

with resp ct to turbine inertia red ces interaction with the electrical s stem an practicaly

el minates the p s ibi ty of torsional interaction pro lems b th with the series comp n ated

a.c tran mis ion an HVDC For a pl cation uti zin hig sp ed (hig he d) u its, the

inertia ratio of h dro generator an turbines may not b large, in re sin the p s ibi ty of

torsional interaction The sp cif i ation s ould req ire clarif i ation on whether an SSTI

pro lem can oc ur in the given con g ration An example of the simpl f ied method f or