Post on 04-Jun-2018
8/13/2019 Transient Angle Stability
1/101
1539pk TS - 1
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
8/13/2019 Transient Angle Stability
2/101
1539pk TS - 2
/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
8/13/2019 Transient Angle Stability
3/101
1539pk TS - 3
$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
8/13/2019 Transient Angle Stability
4/101
1539pk TS - 4
(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
8/13/2019 Transient Angle Stability
5/101
1539pk TS - 5
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
8/13/2019 Transient Angle Stability
6/101
1539pk TS - 6
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
8/13/2019 Transient Angle Stability
7/101
1539pk TS - 7
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
8/13/2019 Transient Angle Stability
8/101
1539pk TS - 8
Response to a Short Circuit
8/13/2019 Transient Angle Stability
9/101
1539pk TS - 9
Stable CaseStable Case
Response to a fault cleared in tcl seconds - stable case
8/13/2019 Transient Angle Stability
10/101
1539pk TS - 10
Stable CaseStable Case cont>dcont>d
Pre-disturbance4
both circuits $BS 4 Pe P
m'
8
operating point a
8/13/2019 Transient Angle Stability
11/101
1539pk TS - 11
+nstable Case+nstable Case
Response to a fault cleared in tc? seconds - unstable case
8/13/2019 Transient Angle Stability
12/101
1539pk TS - 12
+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
8/13/2019 Transient Angle Stability
13/101
1539pk TS - 13
8/13/2019 Transient Angle Stability
14/101
1539pk TS - 14
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'
8/13/2019 Transient Angle Stability
15/101
1539pk TS - 15
?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 ,=
8/13/2019 Transient Angle Stability
16/101
1539pk TS - 16
%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
8/13/2019 Transient Angle Stability
17/101
1539pk TS - 17
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
8/13/2019 Transient Angle Stability
18/101
1539pk TS - 18
%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
8/13/2019 Transient Angle Stability
19/101
1539pk TS - 19
$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 ===
8/13/2019 Transient Angle Stability
20/101
1539pk TS - 20
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
8/13/2019 Transient Angle Stability
21/101
1539pk TS - 21
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
8/13/2019 Transient Angle Stability
22/101
1539pk TS - 22
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
8/13/2019 Transient Angle Stability
23/101
1539pk TS - 23
8/13/2019 Transient Angle Stability
24/101
1539pk TS - 24
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
8/13/2019 Transient Angle Stability
25/101
1539pk TS - 25
,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 =
8/13/2019 Transient Angle Stability
26/101
1539pk TS - 26
,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
=,
8/13/2019 Transient Angle Stability
27/101
1539pk TS - 27
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
8/13/2019 Transient Angle Stability
28/101
1539pk TS - 28
*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
8/13/2019 Transient Angle Stability
29/101
1539pk TS - 29
,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
8/13/2019 Transient Angle Stability
30/101
1539pk TS - 30
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
8/13/2019 Transient Angle Stability
31/101
1539pk TS - 31
8/13/2019 Transient Angle Stability
32/101
1539pk TS - 32
8/13/2019 Transient Angle Stability
33/101
1539pk TS - 33
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
8/13/2019 Transient Angle Stability
34/101
1539pk TS - 34
8/13/2019 Transient Angle Stability
35/101
1539pk TS - 35
*9 Representation of
8/13/2019 Transient Angle Stability
36/101
8/13/2019 Transient Angle Stability
37/101
1539pk TS - 37
Transmission Jine ProtectionTransmission Jine Protection
8/13/2019 Transient Angle Stability
38/101
1539pk TS - 38
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
8/13/2019 Transient Angle Stability
39/101
1539pk TS - 39
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
8/13/2019 Transient Angle Stability
40/101
1539pk TS - 40
8/13/2019 Transient Angle Stability
41/101
1539pk TS - 41
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
8/13/2019 Transient Angle Stability
42/101
1539pk TS - 42
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
8/13/2019 Transient Angle Stability
43/101
1539pk TS - 43
: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
8/13/2019 Transient Angle Stability
44/101
1539pk TS - 44
8/13/2019 Transient Angle Stability
45/101
1539pk TS - 45
8/13/2019 Transient Angle Stability
46/101
8/13/2019 Transient Angle Stability
47/101
8/13/2019 Transient Angle Stability
48/101
1539pk TS - 48
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
8/13/2019 Transient Angle Stability
49/101
1539pk TS - 49
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>
8/13/2019 Transient Angle Stability
50/101
1539pk TS - 50
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
8/13/2019 Transient Angle Stability
51/101
1539pk TS - 51
/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
8/13/2019 Transient Angle Stability
52/101
1539pk TS - 52
8/13/2019 Transient Angle Stability
53/101
1539pk TS - 53
8/13/2019 Transient Angle Stability
54/101
1539pk TS - 54
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
8/13/2019 Transient Angle Stability
55/101
1539pk TS - 55
Tripping can occur
only for impedance
between ,( and ,?'
and within M
8/13/2019 Transient Angle Stability
56/101
1539pk TS - 56
,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
8/13/2019 Transient Angle Stability
57/101
1539pk TS - 57
8/13/2019 Transient Angle Stability
58/101
1539pk TS - 58
$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
8/13/2019 Transient Angle Stability
59/101
1539pk TS - 59
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
8/13/2019 Transient Angle Stability
60/101
1539pk TS - 60
8/13/2019 Transient Angle Stability
61/101
1539pk TS - 61
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
8/13/2019 Transient Angle Stability
62/101
1539pk TS - 62
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
8/13/2019 Transient Angle Stability
63/101
1539pk TS - 63
Analysis4
Eow does the system come apart as a result of instability2
,ut-of-step protection does not operate on =6
8/13/2019 Transient Angle Stability
64/101
1539pk TS - 64
8/13/2019 Transient Angle Stability
65/101
1539pk TS - 65
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
8/13/2019 Transient Angle Stability
66/101
8/13/2019 Transient Angle Stability
67/101
1539pk TS - 67
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
8/13/2019 Transient Angle Stability
68/101
1539pk TS - 68
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
8/13/2019 Transient Angle Stability
69/101
1539pk TS - 69
Eigh-Speed
8/13/2019 Transient Angle Stability
70/101
1539pk TS - 70
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
8/13/2019 Transient Angle Stability
71/101
1539pk TS - 71
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
8/13/2019 Transient Angle Stability
72/101
1539pk TS - 72
,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
8/13/2019 Transient Angle Stability
73/101
1539pk TS - 73
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
8/13/2019 Transient Angle Stability
74/101
1539pk TS - 74
Regulated Shunt CompensationRegulated Shunt Compensation 0cont>d10cont>d1
8/13/2019 Transient Angle Stability
75/101
1539pk TS - 75
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
8/13/2019 Transient Angle Stability
76/101
1539pk TS - 76
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
8/13/2019 Transient Angle Stability
77/101
1539pk TS - 77
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
8/13/2019 Transient Angle Stability
78/101
1539pk TS - 78
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
8/13/2019 Transient Angle Stability
79/101
1539pk TS - 79
Steam Turbine
8/13/2019 Transient Angle Stability
80/101
1539pk TS - 80
8/13/2019 Transient Angle Stability
81/101
1539pk TS - 81
=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
8/13/2019 Transient Angle Stability
82/101
1539pk TS - 82
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
8/13/2019 Transient Angle Stability
83/101
8/13/2019 Transient Angle Stability
84/101
1539pk TS - 84
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
8/13/2019 Transient Angle Stability
85/101
1539pk TS - 85
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
8/13/2019 Transient Angle Stability
86/101
1539pk TS - 86
8/13/2019 Transient Angle Stability
87/101
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
8/13/2019 Transient Angle Stability
88/101
1539pk TS - 88
: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
8/13/2019 Transient Angle Stability
89/101
1539pk TS - 89
;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
8/13/2019 Transient Angle Stability
90/101
1539pk
%ovember &' (&)* lac#out of%ovember &' (&)* lac#out of
%ortheast +S and ,ntario%ortheast +S and ,ntario
8/13/2019 Transient Angle Stability
91/101
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
8/13/2019 Transient Angle Stability
92/101
1539pk TS - 92
%orth American :astern $nterconnected%orth American :astern $nterconnected
SystemSystem
8/13/2019 Transient Angle Stability
93/101
1539pk TS - 93
: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
8/13/2019 Transient Angle Stability
94/101
8/13/2019 Transient Angle Stability
95/101
8/13/2019 Transient Angle Stability
96/101
1539pk TS - 96
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+
8/13/2019 Transient Angle Stability
97/101
1539pk TS - 97
8/13/2019 Transient Angle Stability
98/101
1539pk TS - 98
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
8/13/2019 Transient Angle Stability
99/101
8/13/2019 Transient Angle Stability
100/101
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
8/13/2019 Transient Angle Stability
101/101
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#