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8/6/2019 ATPS - Contingency Selection S8
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Contingency Selection
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Performance Index (
PI)
Measure ± how much a particular outage
might affect the PS
Overload Performance Index :
n
l l
flowl
P P PI 2
max )(§!
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If n is a large number
� PI will be a small number if all flows are within
limits� PI will be a large number if one or more lines
are overloaded
How to use this PI Problem
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Suppose n = 1
A table of PI values, one for each line can
be calculated
Selection Procedure:� Order the PI table from largest value to
least
� Lines corresponding to the top of the list
are then the candidates for the short list ±pick Nc entries
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Problems faced when n=1
� PI does not snap from zero to infinity as the
branch exceeds its limit
� It rises as a quadratic function
� A line just below its limit contributes to PI
almost equal to one that is just over its limit
� PI may be large when many lines are loaded
just below their limit
� Thus PI¶s ability to distinguish / detect bad
cases is limited
± ordering PI values results in a list that is
not at all representative of bad cases at the top
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1P1Q Method
Way to perform an outage case selection
Decoupled power flow is used
Solution procedure is interrupted after oneiteration
- one P- calculation &
one Q-V calculation
PI can use large n value
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1P1Q Contingency Selection
Procedure
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Advantage
a. Sufficient information at the end of
First iteration to give a reasonable PI
a. Voltages can also be included in the
PI
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PI with voltage magnitude
- difference between voltage magnitude (solved
at the end of 1P1Q procedure) & base case
voltage
- Value set by utility engineers, indicating limit of
a bus voltage from changing on one outage
case
m
i
in
l l
flowl
E
E PI
P
P 2
max
2
max)()( §§
(
(!
i E (
max E (
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To complete security analysis PI list is sorted so that largest PI appears at
the top
Security analysis can then start by executing
full power flows with the case at the top of the list
Then solve the second case and so on
This continues until either a fixed number of cases is solved or until
a predetermined number of cases are solvedwhich do not have any alarms
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Concentric Relaxation
Another idea for security analysis
Outage has only a limited geographical
effect even though line is high-voltage
line, heavily loaded
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To realize any benefit from limited
geographical effect of an outage
Power system
Affected part Unaffected part
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Power System Division
Layer zero:
Buses at the end of the outaged line
Layer 1:
Buses that are one transmission line or
transformer from layer zero
Continue this until all the buses in the entire
network are included
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Choose some arbitrary number of layers
All buses included in that layer and
lower-numbered layers are solved aspower flow with the outage in place
Buses in higher-numbered layers arekept as constant voltage & phase angle
(reference bus)
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Layering of outage effects
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Application
1) Solution of layers included becomes Final
Solution of that case and all overloads &
voltage violations are determined from
this power flow
2) Solution is used to form PI for that outage
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Concentric Relaxation
Procedure
Proposed by Zaborsky
Trouble:
Requires more layers for circuits whose
influence is felt further from the outage
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Bounding
Brandwajn ± solves atleast one of the
problems using concentric relaxation
method using an adjustable regionaround the outage ± applied only to linear
(DC) power flow
Extended for AC network analysis
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Analysis in Bounding Technique
Define Three Subsystems of PS:
N1 = subsystem immediately surrounding theoutaged line
N2 = External subsystem that we shall not solvein detail
N3 = Set of boundry buses that separate N1 &N2
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Layers used in bounding analysis
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Fact behind Bounding method
We can make certain
assumptions about phase angle
spread across lines in N2, giveninjections in N1 & maximum phase
angle appearing across any two
buses in N3
(Refer appendix 11A ± line outage modeling
using injections)
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Let
Transmission line in N2 with flow f opq
There is a max. amount that the flow on pq can shift
)](),[( 00max !( pq pq pq pq pq f f f f smaller f
Lo erLimit f
U pperLimit f
pq
pq
p
p
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Translation into max. change in
phase angle difference
)(1
q p
pq
pq x
f UU !
)(1
q p
pq
pq x
f UU ((!(
pq pqq p x f maxmax)( (!(( UU
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From [2] theorem states that
I & j are any pair of buses in N3
i largest in N3
j smallest in N3
jiq p UUUU ((((
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|i ± j| provides an upper limit to
max. change in angular spread across
any circuit in N2
ji pq pq x f UU (((max
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Graphical interpretation of
Bounding process
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I fig. cannot go over limit
II fig. could go over limit
«« line
represents thepoint where circuit pq will go into
overload
Any value right of dotted line representsan overload
pq pq x f
max
(
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Solid line upper limit on
If solid line is left to the dotted line, then
the circuit cannot go into overload
If solid line is right to the dotted line,
the circuit may be shifted in flow due to
outage so as to violate a limit
pq pq x f (
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All circuits in N2 are safe from
overload if the value of is
less than the smallest value of over all pairs pq, where pq
corresponds to the buses at the ends
of circuits in N2
jiUU ((
pq pq x f max(
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If this condition fails, we have to expand
N1, calculate a new in N3
and rerun the test over the newly definedN2 region circuits
When an N2 is found which passes the
test, only region N1 need be studied
jiUU ((
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PROBLEM
Show how bounding technique works sothat not all of the circuits in the system
need be analyzed. Consider the six-bussystem used previously with outage of transmission line 3-6
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From bus To bus R (pu) X (pu) BCAP (pu)1 2 0.10 0.20 0.02
1 4 0.05 0.20 0.02
1 5 0.08 0.30 0.03
2 3 0.05 0.25 0.03
2 4 0.05 0.10 0.01
2 5 0.10 0.30 0.02
2 6 0.07 0.20 0.025
3 5 0.12 0.26 0.025
3 6 0.02 0.10 0.01
4 5 0.20 0.40 0.04
5 6 0.10 0.30 0.03
LINE DAT A
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Bus
No.
Bus
type
V
(pu)
Pgen
(pu MW)
Pload
(pu MW)
Qload
(pu MVAR)
1 Swing 1.05
2 Gen 1.05 0.50 0.0 0.0
3 Gen 1.07 0.60 0.0 0.0
4 Load 0.0 0.7 0.7
5 Load 0.0 0.7 0.7
6 Load 0.0 0.7 0.7
BUS DAT A
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X matrix for six-bus system
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Base case DC power flow
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MW limits on transmission lines
Line MW limit
1-2 30
1-4 50
1-5 40
2-3 202-4 40
2-5 20
2-6 30
3-5 203-6 60
4-5 20
5-6 20
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Line MW limit
(pu)
1-2 0.30 0.253 0.047 0.20 0.0094
1-4 0.50 0.416 0.084 0.20 0.0168
1-5 0.40 0.331 0.069 0.30 0.02072-3 0.20 0.018 0.182 0.25 0.0455
2-4 0.40 0.325 0.075 0.10 0.0075
2-5 0.20 0.162 0.038 0.30 0.0114
2-6 0.30 0.248 0.052 0.20 0.01043-5 0.20 0.169 0.031 0.26 0.00806
3-6 0.60 0.449 - - -
4-5 0.20 0.041 0.159 0.40 0.0636
5-6 0.20 0.003 0.197 0.30 0.0591
pq pq x f max(max pq f ( pq x
)( pu
f pqQ
100 MVA base
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Analysis
Analyze system as if N1 & N3 regions
consist of only line 3-6
If bounding criteria is met, no other
analysis need be done as it
establish that no overloads exist
anywhere in the system
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If bounding criteria fails, expand the
bounded region ie, it includes buses
2,3,5 & 6
ie, boundary of region N3 consists of buses 2 & 5
Check bounding criteria, if it satisfies,
stop the analysis
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)2(
)(,
nmmmnnk
k iminnmi
X X X x
x X X
!H
nm
i
nmi P
UH
(!,
- sensitivity factor
Pnm - original power flow over line nm (k) before it was dropped
± phase angle
xk - reactance of line k
X - element of X matrix of the system
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nnk
k innmi
X x
x X
!,H
mmk
k im
nmi x
x X
!,H
for m = ref. bus
for n = ref. bus
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12865.0
)2(
)(
36663336
36363336,3 !
!
X X X x
x X X H
11953.0)2(
)(
36663336
36666336,6 !
! X X X x
x X X H
111437.063 !(( UU
Smallest value of 0075.042 !((!(( UUUU q p
Criteria fails, Consider new region
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Calculate
Bounding criteria is satisfied
If d factors are used, all line flows are to
be found out
003564.052!((
UU
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Line outage distribution factor
lineK=1
(1-2)
K=2
(1-4)
K=3
(1-5)
K=4
(2-3)
K=5
(2-4)
K=6
(2-5)
K=7
(2-6)
K=8
(3-5)
K=9
(3-6)
K=10
(4-5)
K=11
(5-6)
l=1
(1-2)0.64 0.54 -0.11 -0.5 -0.21 -0.12 -0.14 0.01 0.01 0.13
l=2
(1-4)0.59 0.46 -0.03 0.61 -0.06 -0.04 -0.04 0 -0.33 0.04
l=3
(1-5)0.41 0.36 0.15 -0.11 0.27 0.16 0.18 -0.02 0.32 -0.17
l=4(2-3)
-0.1 -0.03 0.18 0.12 0.23 0.47 -0.4 -0.53 0.17 0.13
l=5
(2-4)-0.59 0.76 -0.17 0.16 0.3 0.17 0.19 -0.02 -0.67 -0.19
l=6
(2-5)-0.19 -0.06 0.33 0.22 0.23 0.24 0.27 -0.03 0.31 -0.26
l=7
(2-6)-0.12 -0.04 0.21 0.51 0.15 0.27 -0.20 0.58 0.20 0.44
l=8
(3-5)-0.12 -0.04 0.2 -0.38 0.14 0.27 -0.17 0.47 0.19 0.42
l=9
(3-6)0.01 0 -0.03 -0.62 -0.02 -0.03 0.64 0.6 -0.02 0.56
l=10
(4-5)0.01 -0.24 0.29 0.13 -0.39 0.24 0.14 0.15 -0.02 -0.15
l=11
(5-6)
0.11 0.03 -0.18 0.12 -0.13 -0.23 0.36 -0.4 0.42 -0.18
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Line MW limit
(pu)
1-2 0.30 0.253 0.257
1-4 0.50 0.416 0.416
1-5 0.40 0.331 0.3222-3 0.20 0.018 -0.220
2-4 0.40 0.325 0.316
2-5 0.20 0.162 0.148
2-6 0.30 0.2480.508
3-5 0.20 0.169 0.380
3-6 0.60 0.449 -
4-5 0.20 0.041 0.032
5-6 0.20 0.003 0.191
Power flow using d factors
)(
63
pu
f out
pq
)( pu
f pqQ
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Overloads exist on lines 2-3, 2-6, 3-5
These lines are within bounded region
N1 + N3
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Reference
1. Allen J.Wood and Bruce F.Woolenberg, ³Power
generation, operation and control´, John Wiley
& sons Inc.
2. Brandwajn, ³Efficient Bounding method for
Linear Contingency Analysis,´ IEEE
Transactions on Power Systems, Vol. 3, No. 1,February 1988, pp.38-43.