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126

HVDc

TAPPI NG STATI ON: POWER TAPPI NG FROMA DC TRANSM SSI ON

LI1 Z

TO

a LOCAL

ACNETWORK

A Ekst r omand P Lamel l

Royal I nsti t ute of Technol ogy, EKC- El ectr i c Power Research Cent er, Sweden

ABSTRACT

The paper descr i bes a new concept f or

t appi ng of f a smal l amount of power f r om a

hi gh vol t age di r ect cur r ent ( HVDC) tr ansm s-

si on l i ne t o a l ocal net work. The pr oposed

new concept f or a ser i es t appi ng i mpl i es

power conversi on i n t wo st eps f r om t he l i ne

t o a l ocal ac network. Thi s w l l make i t

possi bl e to t he use a si ngl e phase t r ans-

f ormer bet ween t he dc l i ne pot ent i al and

ground. The convert er br i dge connect ed i n

seri es i n t he dc t ransmssi on i s of t he cur-

r ent source l i ne- commutat ed t ype, whi l e t he

other t wo conver t ers are of t he vol t age-

source f orced-commutat ed t ype. The basi c

f unct i ons of t he convert er ar e descri bed and

i l l ustr at ed by t he resul t s fr om si mul ati ons

w th t he EMTP program

I NTRODUCTI ON

The HVDC techni que has dur i ng t he 1970t h and

1980t h been w del y accepted as a val uabl e

compl ement and al t ernat i ve t o t he AC tech-

ni que

f or

el ectr i cal power t ransmssi on.

However, t he maj ori t y of t he appl i cati ons

have

so

f ar been t wo-t erm nal or poi nt t o

poi nt t r ansm ssi ons. The need f or and possi -

bi l i ty t o bui l d mul t i t erm nal HVDC t r ansm s-

si on have been di scussed at l east si nce t he

ear l y si xt i es. St udi es have been made r e-

gardi ng the feasi bi l i t i es of seri es connec-

ti on o r paral l el connecti on of t he convert er

st ati ons on t he dc- si de. I t seems to be a

f ai rl y general consensus t hat a paral l el

connect i on

or

const ant dc- vol t age

mul t i t erm nal HVDC scheme i s t o be pre-

f err ed, when t he power r ati ngs of t he di f -

f er ent t erm nal s are of t he same or der of

magni t ude. Two such schemes have been bui l d

or

i s bei ng comm ssi oned, t he Sardi ni c

scheme w t h a t hi r d smal l er term nal at Cor-

si ca and t he Quebec - New Engl and schemes

w t h t hree ter m nal s havi ng al most t he same

power r ati ngs, 2000

-

2200

MW.

There al so seem t o be a need t o t ap- of f a

smal l amount of power at some l ocati ons

al ong t he rout e of t he mai n l i ne f or some

l ong di st ance over head l i ne tr ansm ssi on

syst ems. However , t he conventi onal t echni que

w t h l i ne- commutat ed cur r ent - sour ce convert -

ers i n paral l el on t he dc-si de, i . e. con-

nected between t he l i ne vol t age and gr ound,

does not seemto be ver y att r acti ve f or such

appl i cati ons. The cost per megawatt w l l be

f ai r l y hi gh as t he conver t er br i dge has t o

be desi gned f or t he

f ul l

l i ne- vol t age. The

l ocal ac-network i s al so usual l y very weak

f or such cases requi ri ng i nstal l at i ons of

expensi ve synchr onous compensator . I t i s

al so mostl y not accept ed t hat a m nor di s-

t urbance i n t he connect ed ac- net work can

di st urb t he oper at i on of t he whol e HVDC sys-

t em whi ch i s di f f i cul t t o avoi d as l ong as

l i ne-commutat ed conver t ers connect ed bet ween

l i ne and ground ar e used.

Connect i on of a smal l t appi ng st ati ons i n

seri es on t he dc-s i de seems t o of f er cert ai n

advant age compar ed w t h paral l el connect i on,

bot h w t h regar d t o cost s and per f ormance.

The f ast devel opment of new sem conduct or

devi ces havi ng t urn-of f capabi l i ti es w l l

al so make i t i nter esti ng t o use choppers or

f orced- commutat i ng convert ers. Studi es of a

new concept f or a seri es t appi ng st ati on

us-

i ng two f orced- commutated convert ers w t h an

i nt ermedi ate dc- st age w l l be pr esent ed i n

t hi s paper . The general r equi r ement s on

t appi ng stati ons w l l f i rst be t reat ed and

i t w l l be di scussed how t hese coul d be f u l -

f i l l ed w t h paral l el and seri es connected

stati ons of di f f erent t ypes.

GENWAL REOUI REMENTS ON SMALL TAPPI NG

STATI ONS

The tappi ng stati ons consi der ed i n thi s pa-

per are assumed t o have a f ai r l y smal l power

r ati ng of t he or der of 2-5% of t he power

r ati ng of t he mai n HVDC l i ne. I t i s t hought

t hat t he t appi ng st ati ons shal l f eed smal l

communi t i es l ocat ed al ong the l i ne f or whi ch

t he al t ernat i ve power suppl y i s l ocal gen-

er at i on, e.g. t hr ough di esel generators. The

r equi r ements on cost s, mai ntenance and Der -

fo&ance have because' of t hat t o be f avbur-

abl e as compar ed t o what can be of f ered by

al t ernat i ve power suppl i es. AS the l ocal

power syst em- t o be suppl i ed w t h el ect ri cal

power can be an i sol ated syst emw t h no gen-

erati on or connect i on t o other power syst ems

t he t appi ng st ati on m ght be t he onl y power

suppl y. Thi s means t hat no synchronous ma-

chi nes must be connect ed and that t he ac-

f r equency and ac- vol t age m ght be cont r ol l ed

by the t appi ng st at i on. However, cooperat i on

w th local generati on f ac i l i t i es, i f avai l -

abl e, must of course al so be possi bl e.

Short i nt err upt i ons at t he power suppl i es

can pr obabl y be tol erat ed i n conj uncti on

w t h temporary ear t h faul t s on the mai n

overhead

HVDC

l i ne. On t he other hand,

f aul t s i n t he l ocal ac-network must not di s-

t urb t he oper ati on of t he mai n HVDC l i ne.

Fur t her t he cont r ol of t he tappi ng st at i on

shoul d basi cal l y be perf ormed l ocal l y and

not be dependent upon t el ecommuni cat i on

l i nks t o the mai n stati ons.

I t i s al so t o be pref err ed that t he charac-

t eri sti cs of t he f eed i n t o t he l ocal ac-

net work t hrough t he t appi ng st ati on are

si m l ar t o these of f ered by a synchr onous

generator w t h regard t o f aul t current s, as

t he st andard protecti on and eart h faul t l o-

cat i on syst ems t hen can be used i n the ac-

network

Fi nal l y i t i s to be pref err ed i f t he systems

can be ar r anged i n such a way t hat t he power

suppl i ed t o t he l ocal ac- net work can be i n-

dependent upon t he power t ransm t t ed on t he

mai n HVDC l i nk.



127

SERI ES

OR PAPAL

LEL CONNECTI ON OF THE TAPPI NG

3

TAT ONS

As

i l l ustrated i n f i gure 1 a tappi ng stati on

can be connected ei t her i n seri es

r

i n par-

al l el at t he dc-si de. I n the fi rst case t he

t appi ng stati on w l l cause a vol t age dr op

AUd i n t he

HVDC

syst emdeterm ned by

Wher e

P

i s t he power f ed t o the

t appi ng tsMa t r, n. I n t he paral l el case we

w l i get a reducti on i n ci rr ent AI d deter-

m ned by

The change i n di r ect vol t age of mai n HVDC

systemas i l l ust r ated i n equati on 1 i s a ma-

j or

dr awback for t he ser i es al t ernat i ve.

Thi s vol t age dr op has al so t o be speci al l y

l arge when t he t appi ng st ati on i s operati ng

at a power l evel cl ose t o i t s maxi mumval ue

at an i nstant when t he mai n l i ne i n oper at-

i ng w t h a reduced l oad. Cert ai n restr i c-

t i ons w t h regard t o power f eed i n at r e-

duced cur r ent on t he mai n tr ansm ssi on l i ne

have because of t hat t o be appl i ed f or t he

seri es tappi ng al t ernat i ve.

As

t he mai n tr ansm ssi on systembasi cal l y i s

a const ant vol t age system t he addi t i onal

vol t age dr op al ong the l i ne caused by

t appi ng st ati on connect ed i n seri es has t o

be l i m t ed, probabl y to about 10% A too

l arge vol t age dr op w l l i ncrease t he cost

f or t he mai n convert er st at i ons, gi vi ng i n-

cr eased t ap- changer ranges and need f or op-

erati on at l arge del ay angl es i n t he rect i -

f i er or commut ati on margi ns i n the i nver t er.

Because of t hat t he ser i es connect i on i s

mai nl y consi dered t o be an i nt eresti ng al -

t ernat i ve f or cases w t h l ow r ated powers of

t he t appi ng stat i ons. When t hi s i s not t he

case paral l el t appi ng shoul d f i rst be con-

si dered.

One maj or advantage w t h the seri es al t erna-

t i ve compared w t h the paral l el al t ernat i ve

i s that f aul t s and di st ur bance i n t he opera-

t i on of t he convert er equi pment i s supposed

t o mai nl y r esul t i n a br eakdown of t he vol t -

age

AU

For t he paral l el al t ernat i ve on the

other Rand i t al ways exi st s a cer t ai n ri sk

t hat t he cont rol of t he curr ent AI i s l ost

at f aul t or di st urbance i n t he stat?on. Thi s

gi ves t hat general l y t he seri es al t ernat i ve

l ead to l ess ri sk that a di sturbance i n the

operati on of t he tappi ng stati on w l l di s-

t urb t he oper ati on of t he whol e

HVDC

syst em

I n general i t seems al so that f or l ow power

l evel s of t he t appi ng st at i on t he seri es al -

t ernat i ve w l l gi ve cert ai n advant age w th

r egard t o t he cost s f or t he conver t er equi p-

ment . Thi s i s part l y due t o the f act t hat i t

i s easi er t o desi gn a sem conduct or f or hi gh

cur r ent s than f or hi gh vol t ages.

LI NE COMMUTATED AND FORCED COMMUTATED

ONVERTERS

Besi de the quest i on i f t he tappi ng shal l be

connect ed i n seri es or i n paral l el on t he

dc- si de as tr eat ed above, i t has al so t o be

deci ded, whi ch type of conver t er that i s t o

be pref err ed for t he conver si on f r ompower

i n the dc-ci rcui t t o ac-power t o be f ed t o

t he l ocal ac- network. I n the past mai nl y t he

same type of convert ers as used for t he mai n

HVDC t r ansm ssi on have been consi der ed, i . e.

l i ne- commutat ed cur r ent - sour ce conver t ers .

However , dur i ng t he l ast year s a number of

paper s have been publ i shed proposi ng t he use

of f orced- commutat ed convert ers bot h of t he

curr ent - sour ce and vol t age-sour ce t ype. The

di f f erent t ypes are f ur t her descr i bed by

Ekst r om ( 1) I t has al so been pr oposed t o use

chopper s f or t he vol t age tr ansf ormati on on

t he dc-si de.

The maj or advant age w t h the vol t age- source

f orced- commutat ed convert er i s t hat as seen

f r omt he l ocal ac- network t he vol t age-sour ce

f orced- commutat ed conver t er w l l have a be-

havi our f ai rl y si ml ar t o that of a synchro-

nous generator. I t w l l t hen be possi bl e t o

cont r ol bot h t he vol t age and fr equency of

t he ac- network w t hout assi st ance fr om any

r otat i ng machi ne i n the net work. The con-

ver t er i s f urt her not sensi t i ve t o smal l

di st urbance i n the ac- net work i n the same

way as a l i ne- commutated convert er, f or

whi ch a smal l di st ur bance m ght r esul t i n a

commutat i on f ai l ure.

The maj or di sadvantages w t h t he forced-

commutat ed convert ers i s t he costs, whi ch

sti l l are substanti al l y hi gher than for a

l i ne- commutat ed convert ers havi ng t he same

r ated power . I t i s however t o be expect ed

t hat t he cost s f or f orced- commutat ed con-

vert ers w l l decrease as a resul t of t he

f ast devel opment i n t he semconduct or t ech-

nol ogy. The f urt her devel opment of

thyri stors w th turn-off capabi l i ti es and

whi ch ar e sui t abl e f or connect i on i n ser i es

i n order t o obt ai n suf f i ci ent l y hi gh power

r ati ngs of t he conver t er br i dges i s of gr eat

i mpor t ance f or t he r educt i on of t he costs

for

t he conver t ers.

A NEWCONCEPT

FOR

A SERI ES CONNECTED TAPPI NG

STATI ON W TH FORCED

C O W

ATED C O W TER

General

A

new concept f or a ser i es connect ed t appi ng

st ati on based on the use of f orced

commutated conver t s has been st udi ed at t he

Royal i nst i t ut e of Technol ogy i n St ockhol m

The ci rcui t conf i gurati on i s f urt her i l l us-

t rated i n f i g.

2.

The power i s suppl i ed t o

t he l ocal AC- network w t h a vol t age-sour ce

f orced- commut at ed conver t er her e shown as a

si mpl e 6- pul se convert er. I n a real i nstal -

l at i on i t i s f oreseen t hat a hi gher pul se-

number has t o be used to r educe the magni -

t ude of t he harmoni cs. A

s

cal l ed NPC ( neu-

t ral poi nt cl ampi ng)

or

t hree l evel con-

ver t er mght al so be used.

By per f orm ng t hi s power conver si on f r omt he

dc-l i ne t o the l ocal ac-systemi n t wo steps

w t h an i nt ermedi ate dc- st age t he f ol l ow ng

advant ages can be obt ai ned.

A si ngl e phase tr ansf ormer can be used f or

t he conversi on bet ween hi gh dc-potent i al

and gr ound potent i al , whi ch w l l save

cost s f or t he tr ansf ormer, whi ch pr obabl y

i s t he most expensi ve i t emi n t he t appi ng

stati on.

A cur r ent - sour ce l i ne- commutat ed si ngl e

phase bri dge can be used on t he hi gh po-

tent i al .



128

- The frequency of the single phase commuta-

tion voltage can be freely selected tak-

ing into account generation of harmonics,

cost and losses of t he transformer and

the tw o single-phase converter bridges.

-

The intermediate dc-stage will also make

it easy to connect a battery or some

other energy stor age element, which might

be required for starting up and short

time power supplies during disturbances

in either the local ac-system or the main

HVDC system.

One disadvantage of that conversion is per-

formed in two stages is that it gives

slightly increasing losses. This seems to be

of minor importance for this special appli-

cation for which the alternative pow er sup-

ply is local generation.

The function of the tapping station will be

further described based on the fig 2. The

converter

1

on DC line potential is a

current-source line-commutated conver ter and

the converters 2 and 3 voltage-source

forced-commutated converters.

The voltages between the ac-terminals of the

converters

2

and 3 are directly proportional

to t he voltage U across the capacitor con-

nected between tlfe converters. The relation-

ship between the voltage Vv3 and U can be

controlled by PWM (pulse wldth mo&lation)

but this is not considered in th is paper.

The waveshape of the voltage Uv 3 is further

illustrated in fig. 3, assuming completely

smoothed voltage Vc ,ac ross the capacitor C.

The voltage level in the local ac-network

will thus be directly controlled by the con-

trol of the voltage U and the frequency by

the switching

frequent

of the converter 3.

The commutation voltage for converter

1

will

also be directly determined by th e voltage

U and the turn ratio Nl : N2 of the trans-

fgrmer between the converters 1 and 2.

The voltage across the converter bridge

1

will have th e wave shape as illustrated in

figure 4 which also shows the voltage on

the ac-side of converter 2, Uv2. The angle

of overlap and the commutation margin y are

also shown in figure

4.

From this figure we

will also obtain the following relationship.

For th e angle of overlap we obtain

(3)

4 )

Where 012 s the angular frequency for the

converters, 1 and 2 and L is the commuta-

tion reactance for convertelr 1.

Inserting equation 4 in 3 gives

5 )

Inserting equation 3 in 6 gives

N

4 =

(l-

(7)

Regarding th e choice of a suitable value for

the commutation margin

y

at rated operation

it has to be considered that y must include

a sufficiently large control margin if it

requires to feed rated power to the local

ac-network also at a reduced current on the

dc-line.

The control sv st m

A

number of different control strategies

might be possible for the above presented

circuit for a tapping station. Below the

control scheme used at performed simulation

study with the EMTP program will be pre-

sented. The control system is further illus-

trated in figure 5. The frequency of the re-

ceiving ac-network is controlled by the

firing of converter 3. AS no PWM is assumed

for the converter 3 the internal voltage U

is directly determi ned by t he voltag e of tg

capacitor C.

The ac-voltage of the receiving ac-network

will be controlled by adjusting the refer-

ence value for the regulator controlling the

voltage of the capacitor C.

The difference between the current I and

Id3.wil l be equal to the current in t% ca-

pacitor. This gives

If we assume that both the ac-load and th us

I

and the

D C

current I dl are constant

ti% time derivativ e of th e capacit or voltage

is directly determined by an adjustment in

the commutation margin Ay The equation 7

and 8 give

The control of the capacitor voltage by con-

trol of y at constant values of I dl and

I

corresponds to the two upper control blocftla

in fig. 5. The value of yin steady state op-

eration with

Uc=

U can be calculated

from equation

7

whigegfves

Inserting the expression for u according to

equation 4 gives

The four lower blocks in fig. 5 correspond

to the calculation of y according to equa-

tion

11.

By having this direct control of y

from the measured direct current Idl in the

dc-system and the measured load current in

converter 3, I d3 ,, it is possible to avoid

that fast changes in the currents I

or

Id3

will give large tran sient chang es og'the ca-

pacitor voltage

Uc.

As

we here have neglected th e losses we will

for steady state operation obtain



129

EMTP si mul at i ons

The f uncti on of t he tappi ng stati on bui l t up

of a ci r cui t accor di ng t o f i gure

2

has been

st udi ed by t he use of a di gi t al comput er

programcal l ed EMTP.

The f ol l ow ng i ni t i al val ue f or curr ent s and

vol t ages and data f or t he el ectr i cal ci rcui t

have been used.

P= 50

[MW]

I dl =

2000

[A]

L1( l - phase) = 1.64 [ m ] (ek 0.2

Vol t age rati o ( l - phase) N1 :N2=

1 . 6 2 : 1.0

c= 1000

[ .IF]

UCr ef = 25 [kVl

L3( 3-phase)= 12.35 [ml (ek=0.14)

Vol t age r at i o(3-phase)=N3:N4= 1.0:2.14

I n t he l ocal ac network ther e i s a 5t h, 7t h

and HP-f i l t er i nstal l ed t o r educe t he har-

moni cs. Each fi l t er i s generati ng reacti ve

power of

0 1 S

(S=

57.7

MVA).

Zac net work=

41.57

+

j.24.0

[ l

Cur rent st eD i n the dc- l i ne cur r ent . The re-

sponse t o a s t ep change w t h

? 2 0 ?

200 A)

i n the dc- l i ne curr ent has been studi ed.

Fi gur e 6 shows t he recordi ngs f r om a st ep

i ncrease of t he curr ent at t he t i me i nst ant

20 ms. The i ncreased cur r ent w l l cause a

i ncrease i n commutat i on margi n and i n the

angl e of overl ap and a decr eased di r ect

vol t age component i n Udl .

For t he mathemat i -

cal equat i on t her e have been assumed that

t he vol t age

Uc fs

s t i f f w thout r i ppl e, thi s

w l l gi ves a di f f er ence bet ween the cal cu-

l ated val ue and t he r esul t f r om t he EMTP

si mul at i on.

The vol t age Uc w l l be rather unaf f ect ed ex-

cept f or a i ncrease i n t he ri ppl e cont ent .

Thi s w l l cause a sl i ght decrease i n t he ac-

vol t age and cur r ent due t o a mnor def i -

ci ency i n t he cont rol . I n a real stati on

t here w l l be a 6t h harmoni c fi l ter at t he

i ntermedi ate dc- st age

A si mul ati on w t h a step change of 20

i s

gi vi ng a si ml ar resul t.

Steu l oad chancre i n t he l ocal ac- net work.

The network i mpedance i s di vi ded i nto t hree

di f f erent par t s t o make i t possi bl e to make

a l oad change of

f 2 O

. The data f or the i m

pedances

Z1= 249.5 + j.144.0

[R]

Z =

49.9 j.28.8 [Q]

Z3= 166.4 j.96.0

[R]

The i ni ti al val ue of t he i mpedance i s ob-

t ai ned by paral l el connect i on of

Z

and

Z ,

Z

//Z2=

41.6

+ j.24.0 [a]. The l oa& ri se sf

t ? O

i s obt ai ned by connect i on of t he t hree

i mpedance

Z

/ / Z / / Z

-

33.3

j.19.2

[Q] and

20

i s oktai sed % Z .

I n f i gure 7 t he

case w t h a st ep change % t he i mpedance of

t o 8 0 of t he i ni t i al val ue i s shown.

As

shown i n f i gur e 7d t he step change i n t he

i mpedance gi ves a corr espondi ng st ep change

i n the ac- cur r ent as expect ed.

Si nal e-uhase t o eart h f aul t . The resul t f rom

t he si mul ati on of a si ngl e phase to ear t h

f aul t i s present ed in f i gure 8. The f aul t

i mpedance i s 48

[ Q ] .

The f aul t does not di s-

t urb t he operati on of t he conver t ers. I t

w l l gener ate a smal l second har moni c compo-

nent i n t he vol t age

Uc

The second har moni cs

component can al so be seen i n t he vari at i on

i n the commutat i on margi n y as t he cont r ol

system vi a t he cont r ol , accor di ng t o equa-

ti on 10, tr i es t o avoi d that f l uctuati on i n

the curr ent I d3 w l l af f ect t he vol tage Uc

CONCLUSION

As presented i n t he paper t appi ng st at i ons

connected i n ser i es i n t he dc-ci r cui t w t h a

vol t age-sour ce convert er f eedi ng t he l ocal

ac-network w l l of f er an i nt eresti ng sol u-

t i on when t he r ated power of t he t appi ng

st ati on i s onl y a f ew percent of t he rated

power of t he HVDC syst em However, t he cost

f or such st at i ons i s st i l l expected t o be

f ai r l y hi gh and comparabl e t o t he cost s f or

l ocal gener ati on of power. The most expen-

si ve i ndi vi dual i t em i s expected t o be the

cost s f or t he t r ansf ormer f or t he conversi on

of power f r om hi gh pot ent i al t o gr ound. A

sol ut i on w t h conver si on i n two st eps i s

pr esent ed, whi ch w l l make i t possi bl e to

use onl y one si ngl e phase t r ansf ormer.

EMTP si mul at i ons have shown t hat t he pr o-

posed ci r cui t can be expected t o of f er good

dynam c pr opert i es.

REF

E E

1. Ekstr om A . , Forced commutat ed convert er

fo r

HVDC

appl i cat i ons , EECPS, Capr i , May

89.

ACKNOWLEDGEMENTS

The present ed st udy i s part of a research

proj ect suppor t ed by Vatt enf al l ( Swedi sh

State Power Board) and

ABB

Power Syst ems

AB,

Sweden, whi ch i s grat ef ul l y acknow edged.

n P . ; . n e r a t o r poad q, Generator

Seri es

tapping

aral e

tapping

Fi gure 1 Ser i es and par al l el t appi ng



130

Stat ion B

Station A

Figure 2 DC transmission line with a series tapping.

Figure

3

The waveshape of the voltage Uvg.

T

J

1

1

Figure

5

Control system for the tapping station.

Figure 4 The waveshape of the voltages Uv2

and Udl.



131

I /

I I I I I I , , ,

I I I I I , , I I

0.0

I

I

I I

-.Om

.._e-

l l 0 0 ..PO

..eo .o.om

OO

_ _ _ - _ _ _ _ - _ _ _ _ _ _ _ _ _ - _ _ _ _ _ _ _ _ _ _ _ _

*. I I XeL.LLI..OD D.

Fi gur e

6

Cur r ent st ep i n t he mai n dc- l i ne

cur r ent

a. Dc-l i ne curr ent I dl

b. Vol t age across seri es connected

c. I nt ermedi ate vol t age Uc

d. Phase vol t age i n the l ocal

e. I nstant aneous t hree- phase power

conver t er Udl

ac- net work

5 . 0 0

a.

*.--

1.011

I'

I

i v a o _ m

1.00

- _ D o

Fi gur e

7

Step l oad i ncr ease

a. Vol t age acr oss seri es connect ed

b.

I ntermedi ate vol t age

U

c. Phase vol t age i n t he &a1

d. Li ne curr ent i n t he l ocal

e. I nstant aneous t hree- phase power

conver t er Udl

ac- net work

ac- net work

a.

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