Transient Angle Stability

8/13/2019 Transient Angle Stability

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Transient Angle StabilityTransient Angle Stability

Description of Transient Stability

An elementary view of TS

Methods of TS analysis Time-domain simulation

Structure of power system model

Representation faults

Performance of protective relaying

Concept of electrical centre!

Case studies

Methods of TS enhancement

Ma"or blac#outs caused by Transient $nstability

%ovember &' (&)* %ortheast +S' ,ntario

blac#out

March ((' (&&& ra.il blac#out

,utline


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/hat is Transient 0Angle1 Stability2/hat is Transient 0Angle1 Stability2

The ability of the power system to maintain

synchronous operation when sub"ected to a severe

transient disturbance

faults on transmission circuits' transformers'

buses

loss of generation

loss of loads

Response involves large e3cursions of generator

rotor angles4 influenced by nonlinear power-angle

relationship

Stability depends on both the initial operating stateof the system and the severity of the disturbance

Post-disturbance steady-state operating conditions

usually differ from pre-disturbance conditions


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$n large power systems' transient instability may notalways occur as 5first swing5 instability

could be as a result of superposition of several

swing modes causing large e3cursions of rotor

angle beyond the first swing

Study period of interest in transient stability studies

is usually limited to 6 to * seconds following thedisturbance7

may e3tend up to about (8 seconds for very large

systems with dominant inter-area swing modes

Power system designed and operated to be stable for

specified set of contingencies referred to as 5normal

design contingencies5

selected on the basis that they have a reasonable

probability of occurrence

$n the future' probabilistic or ris#-based approach

may be used


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(9 An :lementary ;iew of Transient(9 An :lementary ;iew of Transient

StabilityStability

Demonstrate the phenomenon using a very simple

system and simple models

System shown in


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The generator>s electrical power output is

/ith the stator resistance neglected' P e represents the

air-gap power as well as the terminal power


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Power-Angle RelationshipPower-Angle Relationship

oth transmission circuits in-service4 Curve (

operate at point 5a5 0P e = P

m1

,ne circuit out-of-service4 Curve ?

lower P max

operate at point 5b5

higher reactance → higher δ to transmit samepower


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The oscillation of δ is superimposed on thesynchronous speed

8

Speed deviation

the generator speed is practically e@ual to 8' and theper unit 0pu1 air-gap tor@ue may be considered to be

e@ual to the pu air-gap power

tor@ue and power are used interchangeably when

referring to the swing e@uation9

:@uation of Motion or Swing :@uation

where4

P m

mechanical power input 0pu1

P max ma3imum electrical power output 0pm1

H inertia constant 0M/-secBM;A1 rotor angle 0elec9 radians1t time 0secs1

:ffects of Disturbance:ffects of Disturbance

0 r dt d ωω


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Response to a Short Circuit


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Stable CaseStable Case

Response to a fault cleared in tcl seconds - stable case


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Stable CaseStable Case cont>dcont>d

Pre-disturbance4

both circuits $BS 4 Pe P

m'

8

operating point a


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+nstable Case+nstable Case

Response to a fault cleared in tc? seconds - unstable case


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+nstable Case+nstable Case cont>dcont>d

Area A2 above P

m is less than A

1

/hen the operating point reaches e' the #inetic

energy gained during the accelerating period has notyet been completely e3pended

the speed is still greater than ω8 and δ continues to

increase

eyond point e, P e


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Practical Method of TS AnalysisPractical Method of TS Analysis

Practical power systems have comple3 networ#

structures

Accurate analysis of transient stability re@uires detailed

models for4

generating unit and controls

voltage dependent load characteristics

E;DC converters'


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?9 %umerical $ntegration Methods?9 %umerical $ntegration Methods

Differential e@uations to be solved are nonlinear

ordinary differential e@uations with #nown initial

values4

x is the state vector of n dependent variables'

t is the independent variable 0time1

Objectie! solve x as a function of t ' with the initial

values of x and t e@ual to x 0 and t

0 ' respectively9

"ethods! :uler>s Method

Modified :uler>s Method

Runge-Futta 0R-F1 Methods

Trape.oidal Rule

( )t x f

dt

dx ,=


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%umerical stability%umerical stability

Depends on propagation of error

%umerically stable if early errors cause no significant

errors later

%umerically unstable otherwise

$mportant to consider numerical stability in the

application of numerical integration methods


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Stiffness of Differential :@uationsStiffness of Differential :@uations

Ratio of largest to smallest time constants or' more

precisely' eigenvalues

$ncreases with modelling detail

Affects numerical stability

Solution using e3plicit integration methods may

5blow up5 with stiff systems unless very small time

step is used9


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%umerical Stability of :3plicit $ntegration%umerical Stability of :3plicit $ntegration

MethodsMethods

:3plicit Methods

:uler>s' Predictor-Corrector' and R-F methods

Dependent variables 3 at any value of t is computed from

a #nowledge of the values of 3 from the previous timesteps

3nG(

for 0nG(1th step is calculated e3plicitly by

evaluating f03't 1 with #nown 3

:asy to implement for the solution of a comple3 set of

system state e@uations

Disadvantage

%ot numerically A-stable

step si.e limited by small time constants or

eigenvalues


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$mplicit $ntegration Methods$mplicit $ntegration Methods

Consider the differential e@uation

The solution for x at t=t 1=t 0 # t may be e3pressed inthe integral form as

$mplicit methods use interpolation functions for the

e3pression under the integral $nterpolation implies that the functions must pass

through the yet un#nown points at time t (

$ra%e&oidal '(le is simplest method

( ) ττ∫ += d x f x x t

t ,1

001

( ) 00, t t at x x witht x f dt

dx ===


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Trape.oidal RuleTrape.oidal Rule

Simplest implicit method7 uses linear interpolation

$ntegral appro3imated by trape.oids

f(x,t)

f(x0 ,t 0 ) f(x1 ,t 1 )

t 0 t 1t

∆t


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Trape.oidal rule is given by

A general formula giving the value of 3 at t=t n#1

is

InG( appears on both sides of :@uation

implies that the variable 3 is computed as a function

of its value at the previous time step as well as the

current value 0which is un#nown1

an implicit e@uation must be solved

%umerically A-stable 4 stiffness affects accuracy not

stability

Trape.oidal rule is a second order method

Eigher order methods difficult to program and less

robust

]110 0 0 1 t , x f t , x f 2 t x x

]1n1nnnn1n t , x f t , x f 2

t x x


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69 Simulation of Power System Dynamic69 Simulation of Power System Dynamic

ResponseResponse

Structure of the Power System Model4

Components4

Synchronous generators' and the associated e3citationsystems and prime movers

$nterconnecting transmission networ# including static

loads

$nduction and synchronous motor loads

,ther devices such as E;DC converters and S;Cs

Monitored $nformation4

asic stability information

us voltages

Jine flows

Performance of protective relaying' particularly

transmission line protection


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Models used must be appropriate for transient

stability analysis

transmission networ# and machine stator

transients are neglected

dynamics of machine rotors and rotor circuits'

e3citation systems' prime movers and otherdevices such as E;DC converters are represented

:@uations must be organi.ed in a form suitable for

numerical integration

Jarge set of ordinary differential e@uations and large

sparse algebraic e@uations

differential-algebraic initial value problem


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,verall System :@uations,verall System :@uations

:@uations for each dynamic device4

where

3d

state vector of individual device

$d

' and ) components of current in"ection from

the device into the networ#

;d ' and ) components of bus voltage

%etwor# e@uation4

where

L% networ# mode admittance matri3$ node current vector

; node voltage vector

( )

( )d d d d

d d d d

V x g I

V x f x

,

,

=

=

V Y I N =


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,verall system e@uations4

comprises a set of first order differentials

and a set of algebraic e@uations

where

3 state vector of the system

; bus voltage vector

$ current in"ection vector

Time t does not appear e3plicitly in the above

e@uations

Many approaches for solving these e@uations

characteri.ed by4

a1 The manner of interface between the differential and

algebraic e@uations4 partitioned or simultaneous

b1 $ntegration method used

c1 Method used for solving the algebraic e@uations4

- =auss-Seidal method based on admittance matri3

- direct solution using sparsity oriented triangularfactori.ation

- iterative solution using %ewton-Raphson method

( )V x f x ,=

( ) V Y V x I N

=,


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Analy.e transient stability including the effects of

rotor circuit dynamics and e3citation control of the

following power plant with four *** M;A units4

Disturbance4 Three phase fault on circuit ? at


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*enerator %arameters!

The four generators of the plant are represented by an e@uivalent

generator whose parameters in per unit on ???8 M;A base are as

follows4

The above parameters are unsaturated values9 The effect of

saturation is to be represented assuming the d - and +axes have

similar saturation characteristics based on ,CC

Excitation system %arameters!

The generators are e@uipped with thyristor e3citers with A;R and

PSS as shown in


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,b"ective

:3amine the stability of the system with the following

alternative forms of e3citation control4

0i1 Manual control' i9e9' constant E fd

0ii1 A;R with no PSS

0iii1 A;R with PSS

Consider the following alternative fault clearing

times4

a1 898H s

b1 89(8 s


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Computed using the =ill>s version of fourth order R-F

integration method with a time step of 898? s9

/ith constant E fd ' the system is transiently stable

however' the level of damping of oscillations islow

/ith a fast acting A;R and a high e3citer ceiling

voltage' the first rotor angle swing is significantly

reduced

however' the subse@uent swings are negativelydamped

post-fault system small-signal unstable

/ith the PSS' the rotor oscillations are very well

damped without compromising the first swing

stability

Case 0a14 Transient response with the fault clearing

time e@ual to 898H s


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Responses of rotor angle δ with the three alternativeforms of e3citation control are computed

/ith constant :fd' the generator is first swing

unstable

/ith a fast acting e3citer and A;R' the generator

maintains first swing stability' but loses synchronism

during the second swing

The addition of PSS contributes to the damping of

second and subse@uent swings

+se of a fast e3citer having a high ceiling

voltage and e@uipped with a PSS contributes

significantly to the enhancement of the overall

system stabilityO

Case 0b14 Transient response with the fault clearing

time tc e@ual to 89( s


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*9 Representation of


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Transmission Jine ProtectionTransmission Jine Protection


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0a1 ,vercurrent Relaying0a1 ,vercurrent Relaying

Simplest and cheapest form of line protection

Two basic forms4 instantaneous overcurrent relay and

time overcurrent relay

Difficult to apply where coordination' selectivity' and

speed are important changes to their settings are usually re@uired as

system configuration changes

cannot discriminate between load and fault currents7

therefore' when used for phase-fault protection' they

are applicable only when the minimum fault current

e3ceeds the full load current

+sed principally on subtransmission systems' and

radial distribution systems

faults here usually do not affect system stability so

high-speed protection is not re@uired


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0b1 Distance Relaying0b1 Distance Relaying

Responds to a ratio of measured voltage to measured

current

$mpedance is a measure of distance along the line

Relatively better discrimination and selectivity' bylimiting relay operation to a certain range of the

impedance

Types

impedance relay

reactance relay

mho relay

modified mho and impedance relays' and hybrids

Most widely used form for protection of transmission

lines

Triggering characteristics shown conveniently onR-I plane


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Three .one approach4

one ( primary protection for protected line

K8Q reach and instantaneous

one ? primary protection for protected line

(?8Q reach and timed 0896 - 89* s1

one 6 remote bac#up protection for ad"acent line

covers ne3t line and timed 0? s1


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0c1 Pilot Relaying Schemes0c1 Pilot Relaying Schemes

+se communication channels 0pilots1 between the

terminals of the line that they protect

Determine whether the fault is internal or e3ternal to

the protected line' and this information is transmitted


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:ach terminal station of the line has4

?nderreachin@ &one 1 phase and ground directionaldistance relays covering about H*-K8Q of the line

trip local brea#ers instantaneously

Oerreachin@ &one 2 phase and ground directionaldistance relays covering about (?8Q of the impedance ofthe protected line9

send permissive signal to remote end

trip local brea#ers if permissive signal receivedfrom remote end

if apparent remains inside relay characteristicfor fi3ed time 0typically 89N s1' local brea#erstripped without receiving permissive signal


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Relaying uantities During SwingsRelaying uantities During Swings

The performance of protective relaying during electro-

mechanical oscillations and out-out-step conditions

illustrated by considering the following system4

0a1 Schematic diagram

0b1 :@uivalent circuit


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The apparent impedance seen by an impedance relay at C

loo#ing towards the line is given by

$f :A:-(98 pu

0 E E

E A A

) B

) B

A E B

) BE B

A

B A

A$ A

A A> >

∠∠ =

2 cot 2

A

j A 2

A

sin2

cos1 j

2

1A A

sin j 2

sin j cos1A A

10 110 1

10 1A A

10 1

A A A

$

A

$

$ A

$ A

$ A

$ A>


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During a swing' the angle δ changes9 as a function of δ on an ' diagram' when:

A:

-

%ote4 ,rigin is assumed to be at C' where the relay is located9

as a function of δ' with E A=E B


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/hen E A and E

B are e@ual' the locus of

> is seen to be a

straight line which is the perpendicular bisector of the

total system impedance between A and ' i9e9' of the

impedance T the angle formed by lines from A and to any

point on the locus is e@ual to the corresponding

angle δ /hen δ8' the current I is .ero and

> is infinite

/hen δ(K8' the voltage at the electrical centre is .ero the relay at C in effect will see a 6-phase fault at

the electrical centre9 The electrical centre and

impedance centre coincide in this case9

$f E A is not e@ual to E

B' the apparent impedance loci are

circles' with their centres on e3tensions of the

impedance line AB

/hen E AE

B' the electrical centre will be above the

impedance centre7 when E AE

B' the electrical centre will

be below the impedance centre9


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Prevention of Transmission Jine TrippingPrevention of Transmission Jine Tripping

During Transient ConditionsDuring Transient Conditions

Re@uirements for prevention of tripping during swing

conditions fall into two categories4

Prevention of tripping during stable swings' while

allowing tripping for unstable transients9

Prevention of tripping during unstable transients' and

forcing separation at another point9

Prevention of tripping during stable transients

UmhoV distance relay characteristic may be too large

and have regions into which stable swings may enter

$n order to minimi.e the possibility of tripping during

stable swings4

use of ohm units 0blinders1

composite relays

shaped relay 0lens' peanut' etc91


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Tripping can occur

only for impedance

between ,( and ,?'

and within M


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,ut-of-Step loc#ing and Tripping Relays,ut-of-Step loc#ing and Tripping Relays

$n some cases' it may be desirable to prevent tripping of

lines at the natural separation point' and choose the

separation point so that4

a1 load and generation are better balanced on both

sides' or

b1 a critical load is protected' or

c1 the separation is at a corporate boundary9

$n certain instances' it may be desirable to trip faster in

order to prevent voltage declining too far9

Princi%le of o(tofste% relayin@!

Movement of the apparent impedance under out-of-step

conditions is slow compared to its movement when a line

fault occurs

transient swing condition can be detected using two

relays having vertical or circular characteristics on an

' plane

if time re@uired to cross the two characteristics

0,,S?' ,,S(1 e3ceeds a specified value' the out-of-

step function is initiated


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$n an o(tofste% tri%%in@ scheme' local brea#ers

would be tripped9 such a scheme could be used to

speed up tripping to voltage decline

ensure tripping of a selected line' instead of other

more critical circuits

$n an o(tofste% bloc/in@ scheme'

relays are prevented from initiating tripping of the

line monitored' and transfer trip signals are sent

to open circuits of a remote location

ob"ective is to cause system separation at a more

preferable location


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H9 Case Study - Transient StabilityH9 Case Study - Transient Stability

The ob"ect

demonstrate transient instability and actions of

protective relaying

show methods of maintaining stability

The system

??H& buses' N)H generators' and )*K( branches

the focus is on a plant with K nuclear units' with a

total capacity of H888 M/

all generators and associated controls are modelledin detail

loads are modelled using voltage-dependent static

load model 0P*8Q l G *8Q ' (88Q 1


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The Contingency4

Double line-to-ground 0JJ=1 fault occurs on the *88 #;double circuit line at Wunction I

Time 0ms1 :vent

8 %o disturbance

(88 Apply JJ= fault at Wunction I on circuits ( and ?()N Jocal end clearing4

,pen brea#ers at bus ( for circuit (

,pen brea#ers at bus ? for circuit ?

This occurs )N ms after the fault is applied' and this time is computed asthe sum of fault detection time 0?* ms1' au3iliary relay time 0) ms1' andthe brea#er clearing time 066 ms ? cycle19 At this time' the fault remainsconnected on the ends of circuits ( and ? at Wunction I

(KH Remote end clearing4

,pen brea#ers at bus N for circuit ?

,pen brea#ers at bus 6 for circuit (

Clear fault 0the line is isolated1

This occurs KH ms after the fault is applied' and the time is calculated asthe sum of fault detection time 0?* ms1' au3iliary relay time 0(? ms1'communication time 0(H ms7 microwave1' and brea#er clearing time 066ms ? cycle1

*888 Terminate simulation


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Simulation4

A * second simulation was performed

=6 is seen to lose synchronism and becomes

monotonically unstable

similar behaviour for the other H units of the nuclear

plant

As =( to =K become unstable' the rest of the system

becomes generation deficient absolute angles of all machines in the system drift

slightly


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Analysis4

Eow does the system come apart as a result of instability2

,ut-of-step protection does not operate on =6


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Jine Protection4

Mho distance relays have .one ( coverage of about H*Q of

line length' and .one ? over-reach of about (?*Q of line

length

Apparent impedance enters the .one ? relays at bus ( and

enters .one ( and .one ? relays at bus H

.one ( relay at bus H would trip circuit 6 at bus H and

send a transfer trip signal to brea#ers at bus ( which

would then trip circuit 6 at bus (

true for the companion *88 #; circuit 0N1 which would

be tripped in an identical manner


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Methods of Maintaining Stability4

Reduction of the pre-contingency output of the plant costly to bottle energy in the plant

Tripping of ? generating units 0generation re"ection1

following the disturbance


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K9 Transient Stability :nhancementK9 Transient Stability :nhancement

,b"ectives4

Reduce the disturbing influence by minimi.ing the

fault severity and duration

$ncrease the restoring synchroni.ing forces

Reduce accelerating tor@ue through control of prime-

mover mechanical power

Reduce accelerating tor@ue by applying artificial load


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Eigh-Speed


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Reduction of Transmission SystemReduction of Transmission System

ReactanceReactance

Series inductive reactances of transmission networ#s

are primary determinants of stability limits

reduction of reactances of various elements of the

transmission networ# improves transient stability

by increasing post-fault synchroni.ing powertransfers

Most direct way of achieving this is by reducing the

reactances of transmission circuits

voltage rating' line and conductor configurations'

and number of parallel circuits determine the

reactances of transmission lines

Additional methods of reducing the networ#

reactances4

use of transformers with lower lea#age reactances

series capacitor compensation of transmission

lines


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Typically' the per unit transformer lea#age reactance

ranges between 89( and 89(*

for newer transformers' the minimum acceptable

lea#age reactance that can be achieved within the

normal transformer design practices has to be

established in consultation with the manufacturer

May be a significant economic advantage in opting for a

transformer with the lowest possible reactance

Series capacitors directly offset the line series reactance

the ma3imum power transfer capability of a

transmission line may be significantly increased by

the use of series capacitor ban#s

directly translates into enhancement of transientstability' depending on the facilities provided for

bypassing the capacitor during faults and for

reinsertion after fault clearing

speed of reinsertion is an important factor in

maintaining transient stability7 using nonlinear

resistors of .inc o3ide' the reinsertion is practically

instantaneous


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,ne problem with series capacitor compensation is the

possibility of subsynchronous resonance with the

nearby turbo alternators

must be analy.ed carefully and appropriate

preventive measures ta#en

Series capacitors have been used to compensate very

long overhead lines

recently' there has been an increasing recognitionof the advantages of compensating shorter' but

heavily loaded' lines using series capacitors


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Regulated Shunt CompensationRegulated Shunt Compensation

Can improve system stability by increasing the flow

of synchroni.ing power among interconnected

generators 0voltage profile control1

Static ;AR compensators can be used for this

purpose


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Regulated Shunt CompensationRegulated Shunt Compensation 0cont>d10cont>d1


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Dynamic ra#ingDynamic ra#ing

+ses the concept of applying an artificial electrical

load during a transient disturbance to increase the

electrical power output of generators and thereby

reduce rotor acceleration

,ne form of dynamic bra#ing involves switching inshunt resistors for about 89* seconds following a

fault to reduce accelerating power of nearby

generators and remove the #inetic energy gained

during the fault

PA has used such a scheme for enhancing

transient stability for faults in the +S Pacific%orthwest

bra#e consists of a (N88 M/' ?N8 #; resistor

made up of N*'888 ft9 of (B?5 stainless steel wire

strung on 6 towers


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To date' bra#ing resistors have been applied only to

hydraulic generating stations remote from load centres hydraulic units' in comparison to thermal units' are

@uite rugged7 they can' therefore' withstand the

sudden shoc# of switching in resistors without any

adverse effect on the units

$f bra#ing resistors are applied to thermal units' the

effect on shaft fatigue life must be carefully e3amined

$f the switching duty is found unacceptable' the

resistors may have to be switched in three or four steps

spread over one full cycle of the lowest torsional mode

ra#ing resistors used to date are all shunt devices

series resistors may be used to provide the bra#ingeffect

the energy dissipated is proportional to the

generator current rather than voltage

way of inserting the resistors in series is to install a

star-connected three-phase resistor arrangement

with a bypass switch in the neutral of the generator-step-up transformer to reduce resistor insulation and

switch re@uirements

resistor is inserted during a transient disturbance by

opening the bypass switch


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Another form of bra#ing resistor application' which

enhances system stability for only unbalancedground faults' consists of a resistor connected

permanently between ground and the neutral of the L

connected high voltage winding of the generator

step-up transformer

under balanced conditions no current flows

through the neutral resistor

when line-to-ground or double line-to-ground

faults occur' current flows through the neutral

connection and the resistive losses act as a

dynamic bra#e

/ith switched form of bra#ing resistors' theswitching times should be based on detailed

simulations

if the resistors remain connected too long' there is

a possibility of instability on the 5bac#swing5


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Reactor SwitchingReactor Switching

Shunt reactors near generators provide a simple and

convenient means of improving transient stability

Reactor normally remains connected to the networ#

Resulting reactive load increases the generator

internal voltage and reduces internal rotor angle


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Steam Turbine


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=enerator Tripping=enerator Tripping

Selective tripping of generating units for severe

transmission system contingencies has been used as a

method of improving system stability for many years

Re"ection of generation at an appropriate location in the

system reduces power to be transferred over the critical

transmission interfaces

+nits can be tripped rapidly so this is a very effective means

of improving transient stability

Eistorically' the application confined to hydro plants7 now

used on fossil and nuclear plants

Many utilities design thermal units so that' after tripping'

they continue to run' supplying unit au3iliaries7 permits theunits to re resynchroni.ed to the system and restored to full

load in about (* to 68 minutes

Ma"or turbine-generator concerns4

the overspeed resulting from tripping the generator

thermal stresses due to the rapid load changes

high levels of shaft tor@ues due to successive

disturbances


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Controlled System Separation and JoadControlled System Separation and Joad

SheddingShedding

May be used to prevent a ma"or disturbance in one part of

an interconnected system from propagating into the rest of

the system and causing a severe system brea#up

Severe disturbance usually characteri.ed by sudden

changes in tie line power

if detected in time and the information is used toinitiate corrective actions' severe system upsets can

be averted

$mpending instability detected by monitoring one or more of

the following4 sudden change in power flow through

specific transmission circuits' change of bus voltage angle'

rate of power change' and circuit brea#er au3iliary contacts

+pon detection of the impeding instability' controlled

system separation is initiated by opening the appropriate tie

lines before cascading outages can occur

$n some instances it may be necessary to shed selected

loads to balance generation and load in the separated

systems

:3amples4 P B θ relay on the tie lines between ,ntarioEydro and Manitoba Eydro


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Discontinuous :3citation ControlDiscontinuous :3citation Control

Properly applied PSS provides damping to both local and inter-

area modes of oscillations

+nder large signal or transient conditions' the stabili.er

generally contributes positively to first swing stability

$n the presence of both local and inter-area swing modes'

however' the normal stabili.er response can allow thee3citation to be reduced after the pea# of the first local-mode

swing and before the highest composite pea# of the swing is

reached

Additional improvements in transient stability can be reali.ed

by #eeping the e3citation at ceiling' within terminal voltage

constraints' until the highest point of the swing is reached

Discontinuous e3citation control scheme referred to asTransient Stability :3citation Control 0TS:C1 has been

developed by ,ntario Eydro to achieve the above

improves transient stability by controlling the generator

e3citation so that the terminal voltage is maintained near

the ma3imum permissible value of about (9(? to (9(* pu

over the entire positive swing of the rotor angle


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uses a signal proportional to change in angle of

the generator rotor' in addition to the terminal

voltage and rotor speed signals

angle signal is used only during the transient

period of about ? seconds following a severe

disturbance' since it results in oscillatory

instability if used continuously

angle signal prevents premature reversal of field

voltage and hence maintains the terminal voltageat a high level during the positive swing of the

rotor angle

e3cessive terminal voltage is prevented by the

terminal voltage limiter

/hen TS:C used on several generating stations in an

area7

system voltage level in the entire area is raised

increases power consumed by loads in the entire

area' contributing to further improvement in TS


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1539pk TS - 87

$ntegrating E;DC Parallel Jin#s$ntegrating E;DC Parallel Jin#s

E;DC lin#s are highly controllable9 Possible to ta#e

advantage of this uni@ue characteristic of the E;DC lin#

to augment the transient stability of the ac system

Parallel application with ac transmission can be

effectively used to bypass ac networ# congestion ,ften' provides the best option for using limited right of

way

Provides a firewall against cascading outages during

ma"or system disturbances


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:3amples of E;DC Parallel Jin#s:3amples of E;DC Parallel Jin#s

Pacific E;DC $nter-tie in the +S west

(N88 #m long NN8 #; bipolar E;DC overhead line from

Columbia River in ,regon to Jos Angeles' California

uilt in the early (&H8s' with a capacity of ('NN8 M/7

upgraded over the years to 6'(88 M/

Eas operated successfully for over 68 years in parallel

with *88 #; AC transmission

$taipu E;DC Jin# in ra.il

K88 #m long )88 #; bipolar E;DC overhead line

from


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;SC-ased E;DC Technology;SC-ased E;DC Technology

E;DC transmission systems built over the years use

converter bridge circuits that rely on natural voltage

of the ac system for commutation4 line-commutated

converter technology!

Results in generation of lower-order harmonics

and consumption of reactive power' which in turn

call for counter measures

$n recent years' self-commutated voltage-sourced

converter 0;SC1 technology! has been developed and

advanced for E;DC transmission application with the

following technical benefits4

Active and reactive power can be controlledindependently

:3cellent dynamic response

Can be connected to very wea# ac networ#

Earmonic filter re@uirements are significantly less

=ood blac#-start! capability

Jower overall footprint! re@uirements

;SC-based E;DC converters are more e3pensive and

have higher losses

Depending on the nature of the application' these

may not be significant issues


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1539pk

%ovember &' (&)* lac#out of%ovember &' (&)* lac#out of

%ortheast +S and ,ntario%ortheast +S and ,ntario


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1539pk TS - 91

%ovember &' (&)* - lac#out of%ovember &' (&)* - lac#out of

%ortheast +S and Canada%ortheast +S and Canada

Clear day with mild weather7

Joad levels in the regional normal

Problem began at *4() p9m9

/ithin a few minutes' there was a complete shut

down of electric service to

virtually all of the states of %ew Lor#'

Connecticut' Rhode $sland' Massachusetts'

;ermont

parts of %ew Eampshire' %ew Wersey and

Pennsylvania

most of ,ntario' Canada

%early 68 million people were without power for

about (6 hours

President Wohnson ordered Chairman of


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%orth American :astern $nterconnected%orth American :astern $nterconnected

SystemSystem


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:vents that Caused the (&)* lac#out:vents that Caused the (&)* lac#out

The initial event was the operation of a bac#up relay

0one 61 at ec# =S in ,ntario near %iagara


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Special Protections $mplemented after theSpecial Protections $mplemented after the

(&)* lac#out(&)* lac#out

P Relays on %iagara Ties trip %iagara ties to %L7

cross-trip St9 Jawrence ties to %L

in place until mid (&K8s

+nderfre@uency load shedding 0+


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1539pk TS - 97


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Reliability :nhancement after the (&)*Reliability :nhancement after the (&)*

lac#outlac#out

All utilities in %orth America began to review

reliability related policies' practices and procedures

Coordination of activities and information e3change

between neighbouring utilities became a priority

:ach Regional Council established detailed Reliability

criteria and guidelines for member systems

Power system stability studies became an important

part of operating studies

led to the development of improved Transient

Stability programs

e3change of data between utilities

Many of these developments has had an influence on

utility practices worldwide


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1539pk TS - 100

March ((' (&&& ra.il lac#outMarch ((' (&&& ra.il lac#out

Time4 ??4()488h' System Joad4 6N'?88 M/

Description of the event4

J-= fault at auru substation as a result of lightning

causing a bus insulator flashover

The bus arrangement at auru such that the fault is

cleared by opening five NN8 #; lines

The power system survived the initial event' but

resulted in instability when a short heavily loaded

NN8 #; line was tripped by .one 6 relay

Cascading outages of several power plants in Sao

Paulo area' followed by loss of E;DC and H*8 #; AClin#s from $taipu

Complete system brea# up4 ?N'H88 M/ load loss7

several islands remained in operation with a total

load of about (8'888 M/

Restoration of different regions varied from 68

minutes to N hours Complete blac#out of Sao Paulo and Rio de Waneiro

areas for about N hours


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March ((' (&&& ra.il lac#outMarch ((' (&&& ra.il lac#out 0cont>d10cont>d1

Measures to improve system security4

Woint /or#ing =roup comprising :J:CTR,RAS'

C:P:J and ,%S staff formed

,rgani.ed activities into K Tas#

Transient Angle Stability

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