Barzam A.B. Automation in Electrical Power Systems (Системная автоматика)

Ch,apter Four

AUTOMATIC

CO}ITNOLS

FOR

MAINTAINING STABILITY

;

i controls which

sense

the

change

in

^r,he

active

power generated

and

'raken

b'y

103

AUTOMATIC

CONTROLS

FOR MAINTAINING

STABILITY

IN

PARALLEL

OPERATION

AI{D

ELIMIN,4.TIOI.{

OF

ASYNCHRONOUS

OPERATION

4-1.

General

The

building

of large

power

grids, the

increased power

output of individual

generating

units

aud

electric

power

generating

stations,

the

long

heaviiy

ioaded

transrnission lines and

joint

operation

of

paralleled

po\t'er

systems

through

n'weak"

Iinks have

imposed

a

series of requirements

upon autoroatic

power

control

systens.

They

are:

(a)

l{aximum

possible

transmission

crpzslllr

must

be

ensured

on

transmis-

sion

lines.

(b)

Disturbances

to the

stabilit,v of tlie

power

systems

operating in

parallel

rsust be

prevented.

(c)

In case of

asynchronous

operation it

sixri:ld

be

rapidliv stopped

and

conditions established

for

normal

po'wer

supply

to

consumers.

These requirements are

met b5'

a

group

of

automa*uic

devices

sometimes

called automatic

mode

controls. The compulsor-v

use

of

these devices

and

the

necessity

of

taking

them

into account

when

caiculating

stabiiit]: are full_v

or'-1oi-orl i- onnr^n-iof o nttl'li^of i^noIi-1.l

9^Plqrueu

r!

qylrrvyrro

uv

lJ

uurrvq

vlvuc'

In recent

years,

to

improve

the

steady-state

stability of

turbogenerators,

besides

the AEC

devices

considered above

automatic

devices to

limit

active

power transfers

(due

to

circulating

currents)

by

acting

upon

the

turbine

gover-

nor system

rirere developed.

In many case-q

dangerous

transmission

]ine overloads

are eliminated b-v

automatically

diseonnecting

some generators

at the

power

suppl-ving

stations.

The

first way

to

improve transient

stabilitf

is

to

guickly

clear

sliort

circuits

and also

just

as

quiekly

reduee

steam

turbine

power

for

a short

time ("strong

power

regulation"),

disconnect

some

of

the generators

or appl5

emergenc]'

brak-

lng.

If

tbe

measures

taken

to prevent.

stability

disturbances

are

not effective

and

asynchronous operation

occurs, it, can

be eliminated

by

using

automatic

sectio-

nalizing

devices. Different

variant-s

of

these devices

can

sectionalize

the

power

system into

roughly balanied loads either instantaneously

or

within

a

certain

period

after

the asynchronous

situation

occurred.

Restoration

of synchronism

in the

power

s-vstem may

be attained by

resynch-

ronization

of

the

paralleled

generators

either spontaneously

after a short-time

or

continuous asynchronous

operation, or

it may

be

eflected

by

the

automatic

individual

parts

of

the

power

system.

It

should be

noted

that

power

s-vstem faults affectiug stability

are

nost

I 1 r1

qangerous

ano

T,ney IIIay ruLe.rrupu

ulle

$uPPry

u(]

u,uuDulut,rb rur

rurrg

Pllt-reus,

For

this

reason, in

addition

to

the

use

of a

number of automatic

delices, special

instructions

stipulate

that

attending

personnel

shall eliminate as1'nchronous

operation

by

sectionalizing

the power

system

at

predetermined points

if

the

duration

of

asynchronous

run exceeds 2

minulssln-2J or if

prolonged

asynehro-

nous

operation

adversely

affects

performance

of

f,he main

equipment with

pos-

sible

damage

to

it.

4-2. P rincipal Relations Determining

Operation

of

Autotnatic

Controls

According

to (1-3)

the

active electric

power

transm,itted

from

a

finite

power

station

over

a

transmission line

to

an infinitel-v

(unlimited)

Iarge

power

system

P:

uEo

,io d

tt2

(4-1)

The

graphical

relationship

(4-1)

is

iliustrated b-v

the

sine

curve in

Fig.

4-1.

Po*'er

*"ransmis"5ioa.

undqr ncrrnal

operating conditiou.s

is carried

out at

the

"iugle

6,,

deteimined b;r

poin'u

a oh

tle abscissa

where

it intersects

sine

curve -I

which is

the

electric"power

characieristic

and

straight

line

3

rn'hich

is

the

turbine

power

characterisiic

correspontiing

to ihe

load

reguired

by

the

consumer at

the

receiving

end of

the

porver

s-vstem.

The

iransmission

stead.y-stafe

stability

limit

is

determined

b-v

the maximum

r r

.

n I t aln J- - 7T-l^- - 1 ^-^- a:--

value oi

lne eiecEllc

poE'el f

.lrla]-

\\'.oeD

o

:

iru

uegTees.

ulJuei uO.frrral

upcrauurg

c.onditions

at

lea-st a

20-per

cent

stcady-state

stabilitl'

margin must

be

provided

^'l^-- flra froncmiccinrr'l inoc

nnnnonf.ino

f-ho nnrrrgr

qtqf.inn

vrith the

ylrtu/cr

distri-

qrv45

vuv

u^sges

b

---

r-

"--

buting

-cYstemln-11'

It-is-

evident

from

(4-t)

tbat

the

steady-state

stabilit-v

may

be

disturbed

either

during

a

smooth

decrease

in

the

generator voltage

(emf)

or

the

voltage

at

the

receiving

end or during a

slow

increase in

the

pos/ei

being

tralsmitted

due

to the

rise

in

the

turbine

power

as

the angle

6 gradually

reaches

90 degrees

:,Li^L, +} ^- rtiofrty}.o fho 1'o1o-na lrafuraan flro trrrhino nnu;or rnd lho olontrin.

}tululf

lugu UIDlulu-

luv

lqrqlvu uvvlf

vvs

elv

vurllJv

rvw

power

of

the

generator.

When the voltage abruptly

drops

due

to

a

short

circuit

or when

the

mutual

resistance

between

the

power

station

and

the

power

system

suddcnly

rises

as

the operating

conditions are

chauged

(during

a

short

circuit or

after

discon-

nection

of s6me

transient

links),

the

stability

may be

disturbed even

after

clearing

the

short circuit.

The

possibility

of

maintaining

stability

depends

.

botb

on

the

duration

and

amount of

the

initial

disturbance

and on

the

operating

conditions

set

up

after

the

fault. These

factors, whicb

cause sudden

changes

in

tbe

operating

conditions,

determine

the transient

(dynarni.c)

stability

of

the

power

station.

CTAPTER

FOUR

With

us€;pc€ted

chsnges

iF elo.rric

loy'Fr

(dre

to disconnection of one of

I tbo turbiDo)

the

trsnsmissiou

tines

oi a

short circ::,t1*j""1:it-""-T:1-":^.:""'.1

li"lY"-f:

i

:+:+ (4-3)

the speed

governor

of

the turbine

has

no

time to

instantaneously

change

the

i

admission

rate

of

water or steam.

A

discrepaacy

arises .between

the

power

ot

or

gaini

speed.

causing

an

increase

in

the angle 6. The

powers

of

the

prime

mover

r@t

ind

geierator

are lalanced

at

point

c.

'The

rotor, howeve!,

coltiDues

to rotate

wbere 0)n

is ths Do*inel

vslu€

of ihe a!8!Iar v€locity.

due

io inertia

and

th€ €lect

c

pou'er

becomes

greater

than the

power

of

the

The"ilertia

colstaDt

L,

ift)

a 3r d; 90

0e

tou

:.r

\tarietioD oI the

relatile angle is

rator,----j.ootl""

case

will be

if

the angle 6 becomes

greater

than the critical

angle

wbe$ Grt

:

momjrt- o{

g:'ratioD,

t

.

mr

O--

""

il"tr;"oiteleg

oI

tbe ch"r"?te"isilc

"uto"?.

Io

tbis eveDt,

the

excesstve

r

:

sDesd of th€ uDit' tDm

! i:.-

(see Section

44).

TsDtetrve

tltD€ ot

errmlirauDs

t,*"

"**

ii"".'itl

ii,i,_

Ii"-b",t.:Ti"

V::

.,y i'-e

i"L"iria*

tiom

the lolloidng.

sr.rpP"i!

lhll {ld9--+:.lt:lf:'it^T.:{il^"i'-g1" ffl-

*^if'1"tcitu-"1ii-

"i

*ii

"ijtatii'"

"oto"

"p€€d

js

pmiottionil ro

the

valtre Art ol

the rorque

oit: Y'unt7;-

'

.j=

t"

(e-

"ppfii,'i-"-n?-ii"*-uiJity-iriporiionot

to

the inertie

i

of the

SeDerrti'g

uDit

(tbe

geDerator

and

TM

I

AUTOMATIC

CONTROI,S

FOR MAINTAINING

STABILITY

iu-"

p.i."

-"""" and

the electric

po*er pr-oduc?4.

by

the

geueraior.

The latter

-

ry:!]P

(44)

:--rr^^ ^.i^r^ a

,rL^

---^.; ^+ +!,- --i'.- h^r'6.

dtz J@.

r,^:#

(4-5)

where P-

is

the nominal

power

of the

generator.

The"

expression

(4-4)

may

be

revfritteD as follos's:

kj/

\/--\-=----'---

+:+ +:

(1-6)

i

''.1-.1

t

'

i

i:

I

r

-

Considering

that

the change

of angular

veiocitf

o,:#:+

+t

G-7)

we obtaiD

-

Aa:

_

1

-Ptoad,

{07,:T

tq-t

(48)

Fic.4-1.

Power cberact€ristic

*_beD

geDelator Fig.

4_2.

Vrrjatio! of

slgle 6 betseeD

eof

i't1r,""i;i"'lfrltlltT"fiiiHl;ffri:";;;;;

""""Li"-'"oo"'ii-o.*"'r.io

svrcblotrism

i'

6"=lly-;7*

(4-9)

The

inertia

constant

!in

of

tbe

units

(generator and

turbine or

{}otor

and

driven

,-99h-

turbine,andthegeueraioIstartsdeceierating.AJtersevera}decaying's.ll.ings"'i"Jii..aiii."iii"j-w-ti6"a"iii"p*".

t["-"o"i"

6 becomis

equal

to the angle

6i. corresponding

to a neq st€ady-skte

"i"arii,i"

;#il;;d iy

ih;

poo'e,

i"t",i""

of

tie

pride

mover aril

the

gene-

t,^.,^,,:2'7if,.rP,?n'

$-""

(4-10)

i6iq*"l""g"JitJ"ig"nandcausesfurtb*1"T]::*i9"""j-'lf-::1::lli'-*1"T::rit"#"i''l."^.i'.l#1i'##i'""iJ*"ililIandtu.hine*-9l9|.!9

105

i""&-iJ"-"i"l'""isiandoperatesasynchronousl.v..Synchro.nism.willbe9re-"oioii,Liia.!ii.-io*a*i".ir,'";;;i.t'l'".rf;;;

served,

if

the

acceleratilg

area

5o6.

does

DOt exc@d

the decelelaullg

area D.dc

tutbiDe

.

Iir.rlrD

ot of

th€ Eolor, Z;ntuor aDd lhe

mecbablltl

jr,E

Dnh'

It ttte'iiiiiiia

constatlls

are redud6d i6 a base

pover,

the tecalculatioD

is

pe

ottaed

rortetive

tiu€.ot

eriei

titrs.a shptl'rc!ii_T..Tj:j_P^l*::.

t""::ji:*:T*t::Y_

rio.so,1,:r,n=\

(4-11)

fi#ifi;;;ii;;io"a

"i,ipiirt::tttrriipbaee

sbor-t

cifcuit)

ind tb€

turbiDe €peed

8o!-

;;;ff;;';id;6"["r"ile;"d;,;;"G;;;iiv'.Tb.;-ii;"i;eturbinepiwerInd-'tisconvedeDttoetPEsstbe!€lative!rI€6r'iDdegrre..Tbeb

uces

an ercessii€

torque

bomeDt

o!

tb6 aoior

stal!

eqDar

to

'

360 I P,-,

Tho,

spproJBatjon

syobol

*is

Dseil bectur€

the.anSlrler,

v€locity o'jslssum

At r frcgueBcI

t:50

Ht

""**;'i:f{'f:'"'."*'",",#Jl:,ll#nl:*#,T;f"T"",ti"T5;n

S:Sfl:ff)or

rhe,orque

01,:',*}

'

P!*u

p

(4-13)

-r-

CIIAPTER FOUN

rTrL

- -L ^

l.Ile

(;ua

in

the

angle

6

from one value

.

.i

I

t\ ,

rnltlal

)

Lo

a

ronism

occurs after several

s]rnchronous

swings

(Fig.

4-2a)

or aperiodically

(Fig.

4-2b).

maintain

ii during

-uransien'r,

opera'uion

ihe

angie 6

exceeds

i80

degrees

asynchronous

running occur-s and

the

angie 6

periodically

changes

from

0

to

360

degrees.

In

asynchronous

operation

the

electrical

variables

(current,

voltage,

power

car-

ried

by

the

transient

links) alternately

vary

their

quantities

from the

minimum

to the maximum

in

conformit,y

with

the

time variations

of the

angle 6.

When

out

of

synchronism, atrong. n'ith the

a-lteruating

synchronous torque.

in asynchro-

noustorque

appears

s'hich is

dependent

upon the

amounlof

slip,

i.e.,

the

diffe-

rence between

the

frequencies

of

the

system

and the generator,

ind

the

construc-

tion

of

the

out-of-step

machine.

At

certain

relationships

of

these

torques

and

due

to

the

operation

of the turbine

speed

governors,

resynchronization

may

occurl

i.e.,

recover5' of

the

syag[ronous

paral]el

operation.

after

several

"s1'n-

chronous

revolutious-

when

the

speed

of

the

sl.qtion

generators

approximaies

the

synchrouous

value.

As1'ns[psao_us

operation

with subsequent

resynchronization

may

be either

short-time,

onl-v a few

seconds,

v'hen

res-vnchronization

takes

plaie

after

one

to three

asynchronous

revolutions,

or

prolonged.

In

a.number

of instances

re-

s5nchronizatiol

ma-v

not occur

at

all. Resynchronization

occurs,

if the

steady

slip betv'een

the system parts

fallen

out

of synchronism

becomes

less

than

th!

so-calied critical

slip

whose value

depends

upon

the total

reactance

between

these-parts.

The.critical slip

vaiue

is

1.5

to

g.C

iiz

for fairll,-

"rigid" couplings

(sma}}

reactances), and from

0.2

to

0.5 Hz for "weak"

couplings

(rdlative1l:gr*it

reactances) between_

the.sg

palls

9f

tfie

system.

Technigues

for

determinin[

the

critical

siip-are

dealt

with inta-rJ,'*'hich

also covers porver

system

resynch"roni-

zation

problems.

Generaii5, u'ith

compiicaieri

composite power

systems,

ls,"ynchronization

is not aimed at, as the

Ti-ssion

of

pr:Ihng

into

synchronism

is

assigned to

so-ca}-

]ed

aUtOmatiC Seet.ionrliz.inq'

rlo'-'iaos

rvhieh

qoncpn.r^

+L^ -^==,-: +---r

--^ :- a -

sections

at

predeier;ffi^;?iiia.^""'

nrrrvu

rsPardL.' ur'rti

PUwt'r

syrit'urx

'tLo

These

measures

are resorted

to

for

the

foilowing

reasons:

(a) Wiren

one

part.

of the power

system

operatJs

asynchronously

rvith

res-

pect

J,o

another one, the

voltage

variations

at various

points

may

disturb the

stabilit-v

of the

other

par-ts

of

the

system thus

resultinf

in

r

"omp]irated

form

of multi-frequency

as;vnchronous

operation.

(b)

A voltage

decrease

at

the points

near

the

eiectric

centre

of

the

svstem

may distu-rb

the

stabiiity

and

disconnect

substantial

number

of s.v3chr-onous

and asynchronous

motors

used

by

consumers.

-_

(c)

For certain

types

of load

and

apparatus

asynchronous

operation

is

not

aIIowed.

FoI

the

operation analysi-s

s1 automatic

devices

it

is important

to

know the

ype_of

changes occuning

in

the

electrical quantities

due

to

clianges

in

the

angle 6.

Variations

of the

electrical

magnitudes

during

synehroious

swings"

and

lldel

as}'nchronous

running

differ

from their

chang6s

c-aused

by

short

circuits.

The difference

is as

follows.

AUTOMATIC

CONTROLS

FOR

MAINTAII{ING

STABILTTY

107

During_

synchroncus

swings

and

asynbhronous

runnias

the

elec

emf vectors

of

machines

operating

in

p-arallel

and

are-

changed

smoothly

in

time.

With

short-circuit

iaults

ih*.*

ehencoc

qr"a qrrdrlan

o-.1 cfan*,io^

g

qlu

rvE

l/

w.

IDli,

At'heavy

loads,

and

in tle

case

of

swings

and

synchronous

running,

a1l

the

electrical_ quantities

are

symmetric

if there"are

no

asymmetric

transient

coup-

lings

and/or

as1'nmetric

loads.

Asymnnetry

of

etectricat

-q"r"iiti"-s

alu,ays

Fig.

4-3.

Elementari'

system

(o)

net*-ork

diagram;

(b)

circuit

currents

and vol taees whcn chr rrr^Fc ar

iiti"]En

?it"*t*"0'""=""

-'e

paited

A C : f /l ctn ^

eL..

YC

_

*11.(;

BF

:

i.p

sin

g,c:.r1

1 ,

I

nFatl r t:

ec: arcsin-az-

I

apxt

pr

er: arcsinjlS

To

el.ocidate

the

basic

relationships,

let

us

consider

an elemenmry

svstem

(Fig.

4-34)

including

two

power

stations,

M

and

y'f,

operating

in

parallel

and

I

tt

I

'-

-

CEAPTER

FOuR

D t-i

sI

Fl

:

I

E

I

and

that

the

impedance

between

the

points

to

which

the

emf

is

appiied

is

distributeri

uniformiY'

is constant

in

vaiue

and

direction,

then

il;;;;

the

emf

vectors

(Fig' 4-3b)

as

foliows

hr:

hrtio

The

absolute

value

difference

between

these

emf's

I

Ail

:1h,"-b,

l:

I

E,

(r'u

-1)

l:

aEn,

sin

*

:"sln

j

where

AEra

is

the

maximum

valtte of

the

emf

diJference

(when 6

:

180

degrees)'

Because

generators

of

the

effect

of

the

emf

difference

aE,

current

flou's

between

the

:AEl

IGr:=--:

\

i"',,

:';::sin

$

:

#.t,

+l

where

icruis

the

maximum

val,ue

of

theequalizlng

current

(when 6

:

180

de-

grees)

Here

z1M

N

i,

iU'r

forward

sequence

impedance

of

tbe

line

between

poin^us

M

and

AI-

ln

space

the

vect.,ors

of

currents

in

phas.:

*,

B

and

c

form

a

symmetric

star'

fU."ott*ge

at

a

random

point

P

(Fig' a-3)

wherea,pgistheforwardsegueDceinpedancebetweenpointPand

Iocated

in

the

eleetric

centre,

i.e.,

at

tLe

systeT

poill at

whicb

the

;;;k

,;;,

*hro

the

emf

vecrors

are

at

rhe

angie

6

:

180

degrees.

Thp v(-llrepe at-the

electric

centre

K

of

the

system

(Fig. 4-3b)

Ilr!

rv4wqbv

g

IJ

x:f

*t

]

l,

the

electric

centre

is

the

midpoint

of

edances

on

both

sides

from

point

K

to

pplied.

'point

at

which

the

ang-le.

between

the

fg

phase

is

zero

(point ^L

in

Fig'

t\b.)'

,

-tn"

to"lysis

of

tle

performance

of

the

nductive

reactance

relaYs'

AUTOMATIC

CONTNOLS

FOR

MI'INTAINING

STABILITY

109

108

tages

and

the

P

above-nantioned.

relaiionships

it

folion's

that

the ourreuts,-

vol-

nhase

displacemen-t

between

ihe

current

atrd

Yo

p

reit

for

different

poiltl

wben

tbe

gene-

rator

emf

vectors

of

the

statrons

ope-

rating

in

parallel

are

shifted

relative

to each

other.

4-3. Automatie

Controls

for

Improvement

of SteadY-State'

Stabilih'

(4-141

(4-Is)

I L-4 6\

As

can

be

seen

from

exPression

(4-t), tbe

critical

value

of

the

active

Dower

sent

over

a transmission

line

can

be

increased

if

the

emf

value

Ea

or

the busbar

voltage

U of

tbe

receiv-

ing substation-rises-,

or

if

the mutual

r:eactance

ctr

decreases'

As

meniioned

in

ChaPter

t,

the

inc

ease

of

tiie

value

E

is

controlled

by

tlie

excitation

regulators

rrhich

are

eisential

components

improving

the

stead.v--state

stability

o

fpower

s1'stems'

Otherl

t.r'pes

of automatic

devices

add-

ing

to

the

stead]'-state

stabilit'v

are

Ioid

limiting

devices

on

tie

transmi-

ssion

iines

and

the'devices

which un-

Ioad

the.

power

transmission

system

when

the

amount

oi

power being

t'rans-

I'nentneamouIItUrPUwtrJUt7rIrEt.Idu'--ffi|aonao90|5P

rnirtArl annrnnr.hes ihe criticai

value.

q

Y" .2( zE - | oL.. I

*'";;,:.:'^;:;;"-

ei^, ic Iirni+od hv

.l-IwurYE

lJvwvr

':'""

-J

rffi-. b\2

Up:Ey'

cos"

i+{2ffiiln

TJ

| /, 41\

\.a.r

r/

point

K

voltage

(4-18)

CTIAPTER FOUR

electrical

parameters

dependent

upon

the

value

of angie

6

ma1'

be used

as

detecting elements

of

the

automatic unloading

devices.

The most

simple

wav

is

to

utiiize

a

relay

responding to

current,

actir''e

power

or.imped.ance.

I

-J

--

Unloading

autornaticdcuicesutilizing

current relays.

These

incorporate

relays

with

serie-s-connected

contacts handling

the.4,

B

and

C

cunents

(Fig.

4-r.

rPrprB@

ry,

""'

"t

ll

t-

Fig.

4-5.

Unloading automatic

device

made

up of current

relays

CnA,

CRB

and

C.R" are current

relal'contacts connected into

pirases,4,

3, andfC.

This

type

of connection

prevents

objectionable

functioning

eaused

by

a-sym-

metric short.

circuits

making

ir

possible

to use

the

relal' pick-up

setting

cor-

responding to the

permissible

angle 6 for the

critical

ioad.

IMalfunctions

of

these

automatic devices

are

possibie

if

three-phase

short

circuits,

which

are

very

rare,

oceur

(less

than 0.5

per

cent of the

total

number of faults).

rnom

curreDt

lransformers

sf lJ?p

line

over

which

moi.n

bul* of

In?ter

is tron-

'smilted

!Fig.

4-6.

Unioading

automatic device

made up

of active

power

reiays

{inloadi.ng automatic

dnuices utf,Ilzing

a.ctiue

(real)

power relays.

Active

po'wer

relays are connected

to

handle

either the phase

power,

forward sequence

power,

or

the

total

three-phase

power

(Fig.

4-6). \[7hen

this

device

is used

provision

should

be made

in

its

circuit

to

prevent

malfunctions caused

by a rise

in the

active

power

during

a short circuit

(active

po*rer

losses

in

the

arc and

trans-

mission lines). Often

the eircuit

is

made

r*-ith

a

blocking

relay

responding

to

AU"OMATIC

CONTROLS FOR

MAII\iTAINING

STABTLTTf

current

or voltage

sequence.

unctions

in the

case of

a backrryard

or

g€+s

fpUas+|

6pich-up:0.86.

x70

The operali_ng

setlilgs

of the_real

-power

-relays

Eust

correspond

to

an

angle

:h-up

:

0.86" x 70 degrees

(Fig.

&1).

The

operating settins

of

the

curnent

ig.

tt-I).

The

operating setting

of

the

curnJnt

,

Fie.

4-5

mav

be

oriented

towards

an nnole r\

ooich-up

T Y.o9" -

tu oegrees

(rrE_:

+'L).'L'ne

operatmg settrng

of

the

current

relay used in

the device

shown in

Fig.

4-5

may

ie

oriented

towards

au

angle

d

somewhat

greater

than

that

used

in

the power

relay,

as the

real power

decrdases

7Lt

Fig.

4-7.

Changes

in vallres

o-I r.eal

power

(7),

current

(2)

and voltage

(3\

and

(4t

on iadividual iengths

of

tie

link

of

eiemeniary

system

(3-in

el&tiic

cenrre;

4-at

poilt

P of circuit

as in

Fig.

4-3a

when

ztpxlzlMK

-

0.5i resistance

oJ.

power

tralsmission

line is

neglected,

it being-assurired

that

,t1!f1

:

0)

r*'hen

the

angie

6 exceeds

90 degrees

and the

result

oay

be a failrrre

in the

work

oI

the

Pgyer

rela.v,

while

the

c.r:rreut

grows

until

the angle

0

reaclres 180

degrees

(Fig.

4.7).

A

disadvantage

-when

applying tbe

unloading

automatic

devices,

utilizing

current relal's

or real

po'wer

rela-vs,

is

that

the

operation of

these

relays

depend!

no-t only

on

the

angie

6 but,

also

on voltage.

The

designed

pick-up

setling

6t

the

relay

qus!

correspond

to the

pick-up

angle

6prch-up

ai the

Jowest

voltage

at the

ends of the transmission

s1'stem

under

nofmal

operating

conditions-.

If the'

voltage

and the emf

are

maintained

at

high

values,

then

in

conformit-v

with

(Lr),

the

maximum

power

(current)

corresponding

to

the

maxinum

6-

:

g0o,

as

dictated

by the stead5r-state

stability

requirements.

increases.

Consequently,

the automatic

devices adjusted

to a

current

or

active

power

value

at

the

mini,

mum leve}

of

voltage

or

emf

do not

aliov'

the

ioad-cairying

capacity

of

the

tie

Iine system-

to

be full5r used.

It is better

to

supplement

or replaie the

automatic

devices utilizing

current

or active

power

relays with

relays responding

to impe-

dance values

or

with

directional

inpedance

rela.ys.

-

Unlaad'ing

automatic d.euttes

wtth

directianal

tmpedance

relays.

One

of these

devices is

illustrated

in Fig.

4-8. Current

relays ensure

operationai selectivity

U; I;P

CEAPTER

FOUR

112

ifi.

polroiiJ

tr"rrrformer

circuits

to

which

the

Accord.ing

to

(4-16)

and

(4{7)

the

impedance

i*#;;;;

tEtuy

itsulled

at

point

P of

the

tie line

zp:

z17os

cot

+T/

connected

J,-Y,-?e5

Fig.

4-8.

Unloading

automafic

device

emplo,r'ing

directionai

impedance

reiay

cnA,

cRB""o;;;i*

currealt

rtlay

contactai

zan

is

the

contact

of

impedance

relav

directional

imPedance

relaY

is

across

the

terminals

of

the

,r-

(4-1g)

Fie.

4-9.

Variation

of

t'erminal

im-

-^io-^o ^f irnnpdane,e relatt

versus

'il;i-""o

;;d

fiil"

of

io'i*ll*tioo

in

elementary

system

I

-

in

electric

centre;

I

-.atFoint o-f

emf ipplication:

J

=

at

point

P Dets-eetl

pornif

'aa

and F

of

elementarT

EllsEsm

fiFE

E:'eo-#t

en

z,,

o*-o'5

zrr*'shown

in broken

line

areabeolute

values

ol

zto

aB runction.

S

9ril",:t"l;3;io"f"

t.o-

With

due

consideration

to

t'he

uph

whic.b

iieompensated

up

to the

electric

centre'

The

use

of

a directional

impedance

reiay

with

a

orooJtlru

.tt""iua

setting

in

the-detecting-element.

of

ii,.'"uio*atic

control

i1'stem

in

pJa-ce of

an

active

;G-;;1"";;o-bJ

iubstintiateri

as follo*e'

The

pick-

"i

""ititg

""f

the

power

rela-r

of

tbe

ualoading

automa-

iic contrdl

is fouid

from

the

expression

Ppich-upxa.8P^4:

o'8

ry

*o'8

+

G-za)

Tbe

active

power

across

tbe

power

relay

ter-

minals

(Per

Phase)

P:

[,/o;,f

cos

rp

(4-2f]

The

relay'functions,

if

the

active.power

PTPpich-uP'

above,

tbe

equation

msy

be

wutf,en

-l

f l2-'

rcosQ

:0.8-:--4'3-

zt2

ztnn

R

tl

!/

I

/

V

t'

,60

J60

AUTOMATIC

CONTROLS FOR

MAINTAINING

STABILTTY

ltQ

(4-22J

or

Expression

(4-24)

is

coincides

with

axis

.R.

ffi:#:

l'25''": const

z/cos

g

-

const

characteristic

of

tbe

directional

impedauce

relav

The

diameter

D:

!.25sn

(+_ZS)

Tbe characteristic

_circle

is

tang.eut

to the

axis

of ordinates (Fig.

4-10).

It is seen from

(4-25)

that

the

pick-up

setting

of the

directional ihrp-edance'relay

is cons-

tant and independent

of

the

operating

voltage,i.e.,

the relay

adjusts'itself

to

the voltage

Flg.

!-10,

Char.acteristic Fig.

4-71.

Cbaracteristics

of

detecting

element

Of COSine-type

directiOnal

I

-

cosine..t;ae directional

impedance

retayi 2

-

directional

imDedance

imperiauce reial

reiay witir

internai angle egual

to rmpedairce

angle

ai

a

J

iinir"OiiiG

relaS and directiona_l power

relal.u(series-connected

co'ntacts);

I

_

resi-

value existing at

each

moment.

As compared

with the.use

ol'

a real (active)

porxer

rela-t,,

the

detecting

element

utilizing a

directional

impedance

relal'offers

a

gain

in trinsmitted

i,o"[r

which

amounts to

5-10

per

cent

of the

maximum

power

tbat

can be

transmills6

t4-E1.

8-2076

(4-23)

(4-24)

whose

diameter

qEAPTF,R

FOUR

AIJ'TOMAIIC

CONTROLS

FOR IAAINTAINING

STAAILSTT

flow interrupted. The

ZA?,R

reiay sends

a momentary control

pulse

(within

0.3

to

0.5

s)

for

disconnection[1a].

Fig. 4-12. Circuits

responding

to inrusb

of zero-sequenc.e

curyent

(o)

witbotheJ#,i

LII]'ilJ'J#'J u"t,o.l11#3f;t"o'3;.1?f;'"'*iitf.t;f,"#,prevents

mis-

The

circuit shou'n in Fig.

t+-13b

is

more complicated

than

that

in

Fig.

4-13a.

It

produces

an output

signal

when

the

protective

relaying

system

may

be

ino-

perative.

for

exampie, if

the

line is disconnected

manually.

Rapid

remote-trip

dzuice. The rapid response time (Iess

than

0.1

s) and

elimi-

nation

of

interference in remote-control disconnectors

are

obtained

b_v

the

use

of

a freguency-time code.

Under normal

operating

conditions

the

transmitter

of

the remote-trip

device continuously

generates

a canier-frequenc],

(c-f)

signal

8r

TL4

115

Autom.a,ti.c

unloading

tthen

the

tLe

link

u:efuia". From

equation

(41)

it

is

Autornati.c

unlooding

uhen thE

tie

Lmh

ttEtsJtz'ts'

_Io-':gY

"191_\*'/

r:

"

i

deepelqize the outpui

cilcuits of

tbe

deYic€

whep the switch

is disconpest6d

clear

tbai

the

o"u

or

several

tie

Iinks

are

disconnected.

This

is

due

to

an

increasg

ill

tbe

cp

"utur.

To

prevent

disturba*ces

to

the

stabilitl',

the batteries

of

lengthv'ise

capa-

a*onno.nynr\pnqatinn

(if

available) are

forced.

The

result

is a decrease

in

the

:.':::::'^f,,.11l"

-

*\.o

o

-or,o."tnrs e rp disoonnected directiv.

A

remote-cont'rol

CHAPTER

FOUF

Rijrt

is

uickgd

rl? }-T

@esds

4.r:oLore

-

+he 34

€q+i?s€et

i{les

€ejlFrat cgntrgl

siglals:g9ertPd 't:

{'equeaciaa

iJaiii",

'"

-i""tt

in the

c-f channel.

\4hen

thli occurs,

the output

circuits

at

which the

ira-nsnitter oaD

operate

under

the

control ol,

the

startiDg.

d€vices-

ot"i;t7-

i

llf,,Y

i

Power

Engineerins

is

shown

in

Fig.

4-14.

Jn

I

b;tloncl

trqping

lrom

LffiI

{6)

. I

in

tie }ine ,4.8.

116

AUTOMATIC

CONTROLS

TOR

MAINTAINING

STABILITY

iil;";;id;;

automatically

opeu

and a

warning sigral

is_s€lt

to.the,opera-

i

tl" c-.1

tratrsmitters

and receivers

are.,couecte.d to. tl:

transnissi:1,

!iDe:

ii;.:F;;;ffi"]il

oitU"

"t"*iog

"i"!rert

of

tbe rem;te-:trtp

(Rf)

devic€

chaDges

I

via coupling capacitors

and

cotruectioD

filters si&il8r

to those

used

at

the c-f

,,17

.0i,c@aection

i

pI_"1"""_t]ii

"Llt9T:-Tl"

eads of the traNmission

liue

should be

Frovided

with

otsedr.rtors

i

ii'"

.r'"p.'.

Tg.'-plol. i"-t_"I1:l?:: :ylp:::-i:l'^lt-1T'll:^-1'j,'*'^tT:.::d

I

oernrztors

9i{'-.-'

-

I

tbu

input receivei

are connected

to

the carrier cban:rel

through

litre

filters,

l--

--*------T------l

/:\tf I

tU"

genented frequcncy

is

quanz-controlled.

The

diagram ol

a

remote-trip

I l-I ' \l | :--

b-"-'--'.--

-:-l-

i" .,,-

-. .. | , :-J/.*JJ I

u$conoc..L*

|

tltz

| |

,

device developed

by

the AII-UnioD

Scie[tific

Research Instituie

of Electric

l;f#n I

Rrh.rnr-

lAR Lo 3.4.R of

the E? rcc€iver

are connected

to

narrow-band fil-

ff/_ .l

j

ters. Since

the

pilot

frequencl

is

circulatirg

a,t

all

times. the rela-y

7:4?

-is

I

j-

! , . .

'

excited. As

this-bappens.

the

lAR-l a0d

lAR-g cotrlacts are opened,

and tbe

1 ll |

|

z fifr1i i. ,

I

,

.

exctted. As

ttrrs nappens,

l'l]e

tAIT-t aDo

rrl.ri-, Eouracrs al.l' uPEr.Ilru

I

arru LlrE

|

"

I lt

--3?l

"'o::t:*f

"Fl'?g

I

iiji-i *n1J i" cio's"a.'rhe

ciicuit oi

tbe blockiug retay

,/t is closed

(con-

| | loc

sn

I l"

'-

-

-'---

r

tacts 88-1 and. BR-Z

sre closed,

wbile cotrtact .B.A-3

is opeued).

Ifi'hea tbe trans-

hw

pnlulio,

IIbt.+J L-----

.

|

r

rr

r-

r

-'r

i

mitter operates

pilot

lrequency

I

disappears ald

frequenc-v_L

or fregueucy.fg

.lll1-\

lA-

|

:

"pp"urs.'d.p"odiog

on

wiethei

ii"

"t"itiog

re1al'

lS.R

or

ZSR operatis at the

tiansmittine

end.

At

frequency

fi

the relay

Z,AR functions

and makes

the relay

1ARo6 operat.e.

The

relar'3/.R

functions

and

the relay

2ARo,r1

closes at fre-

i

ft'ei*r

/_,

1,

,^,

r

.

qu"ni-v

1..

The time

take! by

tbe coDtacts o{

tbe relal's zr4fi

aDd JAfl 10 cbalge

L-----J!----!=----

"'----)

over

ii insuJficiert

for tbe i?.R

relay

to drop out and break

tbe coDtact 8rg-2.

The suppl-r:

to

the c.oil

of

reial;

BR_is

recovered

t,hiotgh

tire e,jicuit

2AR-3*

IAR-?

or 3AR-3-1AR:3.

"

Onloading

th.e ti.e

llnks

by

power

statton

seperatton with

wbsequent

two-brarrch

operation.

Tlijs metliod

is clear

from Fig.

4-15. Under

normal_

operuting

condi-

t,ions

power station

,4

supplies

power to

power

s,vstems

B

and

C.

The tie lines

A-B arc

loaded

close

to

their maxirnum

capacit-\'.

Any

disturbance

to the norma]

operatitrg

corrditions

(for

exaTple, sponta-

neous

tiisconneciion

of

t,he

generai.ing unit in

power

s]'siem

.8, d.iseonneeiion

of one

of

the

parallel

Lines

at

some

section

of

the

AB

tie

iine

performed

b,r' swit-

fmn

cnI

|

-'

{-'-^'-'---i:-

-'-::.

-':;-

----; -i--.---

r...irpr

I

cles

l througi 8. and

disconoection

of one

ol the liDes

ot

tbe

AC

tie

liDL by

ilrrl-61 |

-

|

switch.l0

ttiiough

l3) causes

a tie

liDe overload betweeD

pow-er station

.d

and

lU

poa'er

sysrem EI

Tbe disturbance

can

be

preveuted

by

guickll'-

unloading

this

I

i.i. fine.'The

required

uoloading

is accomplisbed

br: s6ctionilizi.ng

sr-itch.9

ff'npnt"ii'h

i

r:lxlt'"T:H3Ttf,3;:::";ff"5XX"'ffifJ'iiiif":ti"l?'tgTiFl,:;

i

statioE .4

lrom

shich the

two

parallel

lines

are !uD.

Fis.

4-rs.

Automatic

uDroadi's

devrce

i - 9ry--.1c,:h-1^::"t-i111lJ:l-1c^,tY::*J:]",*o,,1:10,'^*.RfJ"T"1,'*;":,o,"""f:

Fig.

4-13.

Automatic

uDloailiDg

devrce

i

Qp€oing

ihe ssctionaliuing

svrtch

(y)

a$o

nelps 10

preveDr

ols[ulDaDcEs

(o}!niI(D)Ya!t&t!otdetecdnael6neni.;(c)!td!Ag€ol;r€c€dtDeoD€r'Engcondltlon'|totni'11a13ia1i-3'+1l1I."J-:::TPf:i-'i:"".jg:*"h:l:"Ij:":i:1l

I

example,

ir the case

of load

tlrow-off in

power

system B

or aD open circuit

rhe frequency

band

generated.

by

thg.jllls.xai!1:It

ll *f:_b:-* :S::::::X I

-

iil;d;;t

ot

seDerator.s_to.

be

disconuected simultaneously

with

tl:

:gp::

l"i'u,i.iitii.'a-fiu'oei,i."]'""tit'!::l"^"l9|]::t}ic*1."**3:?:"::1""1|*''r"iiitr"l]'"i-iFa'aacdepeldsupo,nthepowercarried-

;'ffi^;fiffit;d

tbe output

iircuits

91!!g l-.lT-gil.*l'"T *:*-i:i:; I

goiter

lh€

pir-{au+

conditioas aui

.tle

load on

eicb

gererator. rh"-

1}1!"1

y

ilii'p#al'6-"ir-g

io"Jto

tui

4'ry1jrygr'::-."1,

the FceiYer

at

tbe r€ceiYing

I

d";;;99 l"i;1"ur"a iD

the

siabilily *lgll"Jlo11

:1_9:,,0:.y"1_i:Pl-f-{

;|:fi;

id;ff"ti";

and

produo&

8!

ottput

sigBal.

|

-"t be

€Ifected by

a contrcl

compuier

ione

of

the tirst installations

is in.uso at

ATIIODIATIC

CONTROI,S FOR

MAtrITTAINING

SIABIT,MA-

119

Suislat ton

bus

Tronsmilling

end

C=l

commuritcotton channe/

Receiwno

end

F-{

Autpu!

circuii

of ET

recelYer

:b:-".9:!':::---

Ft ^-; -- f ; *.. ^l-^l *-, ^

vLu)Ltlv LLtrl(

c(c.(t

utt.t

relay"and blocking

urcutr

FLg.4-14'Schematicdiagramofrernote-tripc-fdevice

(a).bloc.k

ilia€iraEl

I

-

c-f master

oscillator:

2

-power_amp_lijr_e5; ,l

-

line

filter; 4

-

inout filter:

5

_

beterod-T;;:;-:

tr{qu.enc."*

conveiter:

7

--aroplifier-limiter:

E

-

nar-

roiv-band

til'ersl

9

-

amplifiefii

lAR

.n

ae-n--arixiiialf

relays;

(b)

-

connection

of

---output

circuits

of c'f n?

rec€iver

Fig.

4-i5.

Combined,operation

of.porv'er.

s]'stems

B

and

C

through

busbars

of

rarge-ratrng

h1'droelectric

station,4

r€lays

.I

and 2

operate

simultaneousiy

and a

greater

number

of

machines

are

disconnected.

Foi

example.

tbe

sectionalizinf

switch

aod

o".

o, two

more

e

in

which

the

number

of

disconnected

alue

in

.pre--f_qult

operation

being

con_

iays

with

different

operaring

set:tings.

u.i"ng

portion

gf the

power

sysiern.

Wh-eu

riug

system

the

above_desciibed

teohni_

ID

IAR-

2 TD

LD

olD- ) laD

-,

t

LnAllatt\L*

CEI.APTER

FOIIR

AUTOMATIC

CONTROIJ

POR

MAII{TAINING

STABILITY

lues

lor-rmdoading+h@g

urits below 90

pe!

gept

of

its

rat:d

:allei

A

voltage

drop i3

another

lactor

cbaracte-

at

tbe

supplylug

st"tion

-rv

-r"frii-i-ffi6d'd"qu"ucv'

TEe

tie

lin

s'

also

be

unloaded

by raprdly

drsconDecti-og

lart

of

tbe

loads

tbrough

a renote-

n-""f,i

lii

iJ i"a"Jtn"i

th"

"t"r

dy-"t"t6

ititirit:'

can

be

effectively

iuproved

;;;;#-;;-a;'reuiiurn'g

"#;;;.]-

;ii""q"i"t

""t'-"ti.-ii"i"'""t'"'

llri**:,rui:::ili"*,#-i*lTl,f..SH.":Tj,;'j:r:'Tiy.iTff#,"'?:"j:

121

120

rverload. This

method

is

the

better

one,

lrruptions

in

power

co-nsumption

(some

the

production

is

of

less

importalce'

IoadJ

ma]'

be

remotely

accomplished

if

of

the

remote-trip

command

is

dependent

upon

the voltage

value

at

the

consumer

and

is

allowed

if

the

voltage

drops

as

well.

4-4. Lutomatic

Controls

for

Improvement

of

Transient

Stabiliby

As

mentioned

previousi-v,

rapid

isolation.

of

short

circuits

is

the

principal

o,r1, of

improving

iransient"(dp'amic)

stabilitl'.

It

is seen

from

equ-ation-(4-13)

thal

the

critical

ii*.

"i

isolating

a

short

circult,

without

stabilit.l'

dist.urbance,

depends

on

the

criti"al

angle

6"rlwithin

wiiich

the

emf

vectors

mal'

depart

under

tbe

transient

conditions.

th.

ftss

the

short

circuit

time.

the

}ess

the

departure

angie

is iikely

to reach

the

value of

6"".

"A,

".o

beieen

from

Fig.4-1,

the

aft.erfault

value of-P,oil

is

of

importance'

This

vaiue

is dependent

on"the

magnitude

of

cp undet

the

same

condition5la-10)'

Some

automatic

devices

consid'ered

above-is

a means

of

improving

steadt'-

:ransient

stabilitf

if

they are

of

quick

apidiv

unload

short-circuited

t,ie links

re

dticonnected.

and

also devices

that

tant

to

transient

stability

are

emergencl

g"o"mtoir.

The

brakiog

"utt

be accJmpiislied

b;v

oiher

methods.

The

first loost

simile

method

consist-s

in

the

aut,omat,ic

isolat

ion

of some

^n*nra*nra ^r *ha condina cfnfinn or thp su'itchinE over of

part

of

the

porvgr

BEnEIAUUIJ

q!

u^!v

!'vrrurlb

- -

---

- -

o

!yrt.*

includinj

the accJlerated

gene.rators

lo.

q

certain

load'

To

this

end the

;;;;^tiai_"

r.Jponding

devices,-as

those

q'hich

prevent

steadl--state

stabilitl'

disturbances

are

used

(see

Section

4-3).

A

more

complicat.ed

method

of

decelerat,ing

generators

is. by

short-tir.ne

connectioo

of

re'sisiances

into

the stator

circuii

of

the

generator

or into

the

l*n"tr

of

the tie

line.

The

resistances

are

cut

in

once or

repeatedl5'

in

case

of

a short

circuit

on

the

tie

line,

depending

upon

tbe

drop

in

tire

acti\re

power

oI

*n-r

tire angle

6

reaches

its'preissigne?

r'alue.. There

is

a

proposal

to

cut in

braking

resis-tances

s'hen

the

eiectric-power^derivative

reverses,

rn'hich

corres-

;;;a;

;"

the angle

6

of

90

degrees.

The

.electrical

braking

methods,

however.

are

iol*ia"f-o-

*pp[i*a-aithough"experimental

instailations

have

been

tested. Per-

haps,

oooioeiti"

srn'itches

i'itnout

movable

pa-rts

(thyristor

type'

for

instance)

v'iil

allos'

this

method

to

be

used more

effectively'

--ifa.rn"nicai

braking

of

speeded-up

generatori

is

even

more

difficult

ait-

houjtr

experimenir

ir"-ur

shown

that'suih

braking

is

feasible.

Hoq'ever,

due

to i[s

awkwardness

this method

is

not applied'

Dtsconnectian

of

Znd

orouP

ol

maclinei

-*

\

Iq

b1R

It

1F

4-16.

\

.;\

\!

R

L,r

S*'

at

, i{-t

I

i

agri

first

rg c]

(o)

!-

\B

'\\

l-\r\

itR

evlc

rrH-

)of

(b)

{

rc);

(

FS

*t

ds

EN

N\

Dev.

t

!RT

I

)up

o

);

(b)

ca8e):

(c)

na]

nl

1s

I

.'her

d)c

portl 0r

u:net

0.5

to 1

'srn-z

ower

E'.

case;

(d

$r

s(

S$

\tt\

ru

TE

o-t

rdi

.lo

,u

41

rd

loa

rfi

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ut1

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eln

\

s

\

I

Unli

po\+'er

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rn

the

i

change

po

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enl

Ir s:

s.:s

s;s

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<\S E

dbt

dir

fi

ng

Fio

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the

swl:

am;

gro

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ne{

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lce

It-3

rct

i

(cj

Ior

prol

s

about 0

tchines);

rnge

ln

pc

another

c

for

1

s

abo

.chint

nge

l:

.3

is

abou

machines

:hange

rn

c)

anothe

llng

o\\'

than

tot

lhan

to5

on

pow

NSInII

De

of

he

tim

line

r

tra

tir

nt

lon

WCT

tJ

pplng

r

ler

than

ilsmlsslc

Ldden

il

power

trippin

'eater

th

transmr!

sudden

I

(grea

the

tra

er at

sl

ofp

rtses

rthel

r

e

(8ro

ttre

tr

eratl

ssloD

the b

reofr

rlses

I

ol

lo

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sys

brtake

f

maxtn

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smoo'

load

tssl

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of

num

thly

Barzam A.B. Automation in Electrical Power Systems (Системная автоматика)

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