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INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN ELECTRICAL, ELECTRONICS, INSTRUMENTATION AND CONTROL ENGINEERING
Vol. 3, Issue 5, May 2015

Contingency Ranking in Modern Power System

by Exact and Precise Method
Udaykumar1, H.R.Sudarshana Reddy2
P.G. Student, E.E.E Department, UBDT College of Engineering, Davangere, Karnataka, India 1
Professor, E.E.E Department, UBDT College of Engineering, Davangere, Karnataka, India 2

Abstract: One of the important aspects of modern power system security assessment is the consideration of any
contingencies arises due to unplanned or planned line outages leading to system overloads or abnormal system
voltages. Several methods have been developed in the past few years to address this problem but computation time has
been identified as the constraint making the process inefficient. Utilities today are in need of tools, techniques and also
the methods that will enable them to predict the dynamic stability and reliability of the grid in the real-time. A power
system is secure against a given contingency if it operates within tolerable operating limits before and after the
occurrence of the contingency. In practice, it is not possible to secure a power system against all possible contingencies.
Therefore, only the most critical contingencies (imminent disturbances) are considered. Contingency ranking attempts
to estimate the impact of various contingencies without actually solving the power network. Existing methods of
contingency ranking methods suffer from masking effects in approximate methods and slow execution in more accurate
ranking methods. This paper presents an exact and precise method of contingency ranking. The method used here takes
due consideration of both apparent power overloading and voltage violations simultaneously to find indices which in
turn used to rank the contingencies. Here 1P1Q solution is used to find the post contingency voltages and power flows.
The proposed work is simulated on IEEE-14 and IEEE- 30 bus test systems in MATLAB environment. The method
used is based on realistic approach taking practical situations into account. Besides taking real situations into
consideration, this method is fast enough to be considered for on-line security analysis.

Keywords: Power system operation, power system security, contingency analysis, power system stability.

I. INTRODUCTION
Both the historical and present day civilization of mankind  Due to increased competition, existing power systems
are closely related to energy, and there is no reason to are required to provide greater profit or
doubt but that in the future our existence will be more and produce the same service at the lower costs, thereby
more dependent upon the energy. Electrical energy increasing the duration of power systems operating close
occupies the top most position in the energy hierarchy. It to security and stability limits.
finds innumerable uses in homes, industry, agriculture and  Environmental constraints severely limit the expansion
transport. of a transmission network.
Besides its use for domestic, industrial and commercial  Fewer operators are engaged in the supervision and
and industrial purposes it is used for defence and operation of power system.
agricultural production. Electrical power system is a  The transmission capacity for all transactions in the
technical wonder. Electricity and its accessibility are the open excess network needs to be determined.
greatest engineering achievements of the 20th century. Although power generation, transmission and
Power system is made of interconnected components, each distribution are unbundled, there still exists common
designed to play a critical role for smooth operation of the interest for these companies: power system adequacy and
system at all the times. power system security. The adequacy of production and
It is well known that a power system is a complex transmission capacity is maintained in the long-term and is
network consisting of numerous equipments such as related to power system planning.
generators, transformers, circuit breakers, transmission The concept of adequacy is generally considered to be
lines etc. failure of any of these components during its the existence of sufficient facilities within the system to
operation will harm the reliability of the system and hence satisfy the consumer demand. These facilities include
leading to an outages. those necessary to generate sufficient energy and
Thus one of the agenda of the power system planning and associated transmission and distribution networks required
its operation is to study the effects of outages in terms of to transfer the energy to the actual consumer load points.
its severity. Installation of redundant generation capacity Adequacy is therefore considered to be associated with the
or the transmission lines is essential in order to make the static conditions which do not include the system
system run even when any of its components fails. But disturbances.
power system being dynamic doesn‟t guarantee that it will Security on the other hand, is considered to relate to the
be 100% reliable. The following reasons make the ability of the system to respond to the disturbances arising
management of power systems more difficult than earlier. within that system. Security is therefore associated with
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INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN ELECTRICAL, ELECTRONICS, INSTRUMENTATION AND CONTROL ENGINEERING
Vol. 3, Issue 5, May 2015

the response of system to whatever disturbances they are =∑ +∑ (1)

subjected. These are considered to be the conditions
causing local and widespread effects and the loss of major
Where : Ranking index of apparent power flow of
generation and transmission facilities.
line
To achieve high degree of reliability and economy,
problem of planning and coordinated operation of a vast
and complex network have to be solved. This is the main ={
intension of power system studies. For planning the
operation, improvement and expansion of power system, a
power system engineer needs the load flow studies. An :
important part of security study therefore, moves around Ranking index of voltage bus
the power systems ability to withstand the effect of
| |
contingencies, a contingency can consist of several
actions: ={
 Simple line outage of a single transmission line.
| |
 Complex line outage of several lines, a number of
generators.
These problems may lead to total imbalance in the
Start
power system, which will bring down the power system to
halt state. Hence preventive measure is essential to avoid
such situations on the power system.
Contingency analysis being very important and Prepare the outage list
sensitive part of power system security, demands
maximum possible accuracy. The number of contingencies
in a large power system can be in more than thousands; Set k = 1
however the time slot available for power system operator
to analyze so many contingencies and take appropriate
action to avoid any post contingency violation is quite
limited. The constraint of time boundation necessitates Read base case system data
screening and ranking of only potential contingencies
followed by detailed analysis and proper control actions
for credible contingencies. All constrains including flow Outage no. k k =k+1
gate limits need to be respected following any credible
contingency.
Contingency analysis, ranking and selection are
Run 1P1Q load flow for
acceptably considered as crucial activities in power Post outage case
security assessment and normally conducted in line with
the voltage stability analysis. Most of contingency analysis
algorithms are meant to perform the contingency selection
in order to identify and filter out worst contingency cases Calculate ERI
for further detailed analysis once the preventive and
corrective measures have been identified. Complex system
mainly caused by the economic and environmental
pressures in continuing interconnections of bulk power Are all outages
systems has caused the system to operate close to its limit are taken ?
of stability. This situation becomes worst when
contingencies occur in the stressed power network.
Contingencies caused by line, generator and transformer
outages are identified as the most common contingencies
that could violate the voltage stability condition of the Arrange ERI list in descending order
entire system.

II. METHODOLOGY
A. Exact ranking technique Display the list
This method aims at finding the exact number of
possible violations following a contingency in power
system. The logic behind this is to have contribution of „1‟
End
by violated line/bus and „0‟ by non-violated line/bus to
ranking index named as exact ranking index (ERI) as Fig 1: Flow chart for exact contingency ranking
given in eqn.
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B. Precise Ranking Technique Step 4: Now pick an outage say k from the outage list and
Exact ranking method has presented fairly acceptable remove that outage from outage list.
and correct results, however it doesn‟t differentiate
between the outages having same severity and hence label Step 5: Now run 1P1Q load flow for the post outage case.
them with same ranking. This technique hereafter called
precise ranking technique addresses the concern of Step 6: Calculate the precise ranking indices.
identical ranking for outages having the same severity and
takes into account in case there is any line or bus reaching Step 7: Repeat the process from step 2 to step 6 until all
near to its limit following a particular contingency. This outages are considered.
ranking is based on new index hereafter called as precise
ranking index (PRI) given in eqn. Step 8: Now arrange the precise ranking indices in the
descending order
=∑ +∑ (2)
Where : Ranking index of apparent power flow
Start
of the line

Prepare the outage list

=
( )
{ Set k = 1

: Ranking index of voltage bus

| | Read base case system data

data
=
| | Outage no. k
( ) | | k =k+1
{

Where is nominal voltage of bus

is real power flow in line l. Run 1P1Q load flow for
Post outage case
maximum loading capacity of line l.
is the difference between voltage
magnitude after 1P1Q solution
an base case voltage magnitude Calculate PRI
at bus i.
is a value set by utility experts
indicating how much maximum
voltage deviation is allowed at any bus. Are all outages
m, n are integers are taken ?
As in the precise ranking method we are considering the
actual value of power flow in the transmission line and
actual value of bus voltages simultaneously for finding the
index values to rank the contingencies according to
severity. This precise ranking method clearly ranks all the Arrange PRI list in descending order
contingencies according to severity. The procedure for this
technique remains same

C. Algorithm for precise ranking indices Display the list

Step 1: Enter the system data; define R, X, sending end

bus and receiving end bus number, line data, bus data,
tolerance limit etc. End

Step 2: Prepare the outage list. The outage list consists of Fig 2: Flow chart for precise contingency ranking
all the line outages.

Step 3: Now read the base case system data.

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III. RESULTS AND DISCUSSION Line 10

10 9 1
MATLAB software has been utilized to write the (bus5-bus6)
programs to identify the contingency which is most severe. Line 11
11 3 6
Here exact ranking method and precise ranking method is (bus6-bus11)
used to find the exact ranking indices (ERI) and precise Line 12
12 3 6
ranking indices (PRI) respectively. (bus6-bus12)
The exact ranking indices (ERI) and precise ranking Line 13
13 4 5
indices (PRI) are calculated for all the line outages (bus6-bus13)
separately. Then these indices are used to rank each Line 14
14 1 8
contingency. The indices with the highest value indicate (bus7-bus8)
that particular contingency as most severe and the indices Line 15
15 7 2
with the lowest value indicate that contingency as less (bus7-bus9)
severe. Line 16
16 5 4
This method is applied on the IEEE 14 bus test system (bus9-bus10)
and the results are as follows. Line 17
17 3 6
(bus9-bus14)
Line 18
18 (bus10- 2 7
bus11)
Line 19
19 (bus12- 2 7
bus13)
Line 20
20 (bus13- 1 8
bus14)
TABLE II
PRI BASED CONTINGENCY RANKING
PRI
S. No Outage PRI based
ranking
Line 1
1 25.0948 17
(bus1-bus2)
Line 2
2 27.6835 4
Fig 3: IEEE – 14 bus test system (bus1-bus5)
Line 3
3 27.2959 8
TABLE I (bus2-bus3)
ERI BASED CONTINGENCY RANKING Line 4
4 27.9477 3
(bus2-bus4)
ERI
Line 5
S. No Outage ERI based 5 27.5531 5
(bus2-bus5)
ranking
Line 6
Line 1 6 24.3333 19
1 1 8 (bus3-bus4)
(bus1-bus2)
Line 7
Line 2 7 25.5544 15
2 7 2 (bus4-bus5)
(bus1-bus5)
Line 8
Line 3 8 29.0930 2
3 6 3 (bus4-bus7)
(bus2-bus3)
Line 9
Line 4 9 27.3671 7
4 7 2 (bus4-bus9)
(bus2-bus4)
Line 10
Line 5 10 29.0931 1
5 7 2 (bus5-bus6)
(bus2-bus5)
Line 11
Line 6 11 25.2636 13
6 1 8 (bus6-bus11)
(bus3-bus4)
Line 12
Line 7 12 25.5397 11
7 2 7 (bus6-bus12)
(bus4-bus5)
Line 13
Line 8 13 25.7627 10
8 9 1 (bus6-bus13)
(bus4-bus7)
Line 14
Line 9 14 25.0948 17
9 6 3 (bus7-bus8)
(bus4-bus9)
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Line 15 Line 14
15 27.4930 6 18 8 17
(bus7-bus9) (bus7-bus8)
Line 16 Line 20
16 26.0827 9
(bus9-bus10) 19 (bus13- 8 18
Line 17 bus14)
17 25.3655 12
(bus9-bus14) Line 6
20 8 19
Line 18 (bus3-bus4)
18 (bus10- 24.9831 14
bus11) The results for IEEE – 14 bus test system are obtained.
Line 19 The ranking indices are obtained for all the outages. The
19 (bus12- 25.0195 16 index which is having highest value indicates the
bus13) contingency as most severe and the index with lowest
Line 20 value indicates the contingency as less severe.
20 (bus13- 24.7289 18 It is seen from the results of exact ranking method that,
bus14) the line 8 and line 10 outage both are having same value of
ERI as 9. It implies that these are the contingencies with
TABLE III more severity and whose outages severely affect the
COMPARISON OF ERI BASED CONTINGENCY RANKING AND operation of power system hence these contingencies are
PRI BASED CONTINGENCY RANKING ranked as 1. The line 1, line 6, line 14, line 20 outages are
ERI PRI having same value of ERI as 1. It implies that these are the
S. No Outage based based contingencies with less severity.
ranking ranking Similarly from the precise ranking method it is seen
Line 10 that, the line 10 outage has the PRI as 29.0931 and for the
1 1 1 line 8 has the PRI as 29.0930. Hence the line outage 10 is
(bus5-bus6)
Line 8 ranked as 1 and line 8 as 2. This indicates that in the exact
2 1 2 ranking indices, where the severity of line 10 and line 8
(bus4-bus7)
Line 4 outages is same, the precise ranking indices distinguish
3 2 3 between the two.
(bus2-bus4)
Line 2 Finally the comparison of exact ranking indices (ERI)
4 2 4 and precise ranking indices (PRI) based contingency
(bus1-bus5)
Line 5 ranking is made in the table. It is seen from the table that,
5 2 5 the exact ranking method does not differentiate between
(bus2-bus5)
the outages with same severity hence label them with
Line 15
6 2 6 identical ranking. The precise ranking technique addresses
(bus7-bus9)
the concern of identical ranking for the outages with same
Line 9
7 3 7 severity by taking into account in case there is any line or
(bus4-bus9)
bus reaching near to its limits following a particular
Line 3
8 3 8 contingency. Hence precise ranking method distinguishes
(bus2-bus3) between the outages having same severity.
Line 16 This method is applied on the IEEE 30 bus test system
9 4 9
(bus9-bus10) and the results are as follows.
Line 13
10 5 10
(bus6-bus13)
Line 12
11 6 11
(bus6-bus12)
Line 17
12 6 12
(bus9-bus14)
Line 11
13 6 13
(bus6-bus11)
Line 18
14 (bus10- 7 14
bus11)
Line 7
15 7 15
(bus4-bus5)
Line 19
16 (bus12- 7 16
bus13)
Line 1
17 8 17
(bus1-bus2)

Fig 4: IEEE – 30 bus test system

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TABLE IV Line 28
28 8 9
ERI BASED CONTINGENCY RANKING (bus15-bus23)
Line 29
ERI 29 7 10
(bus16-bus17)
S. No Outage ERI based
Line 30
ranking 30 8 9
(bus18-bus19)
Line 1
1 7 10 Line 31
(bus1-bus2) 31 9 8
(bus19-bus20)
Line 2
2 7 10 Line 32
(bus1-bus3) 32 7 10
(bus21-bus22)
Line 3
3 7 10 Line 33
(bus2-bus4) 33 7 10
(bus22-bus24)
Line 4
4 7 10 Line 34
(bus2-bus5) 34 8 9
(bus23-bus24)
Line 5
5 6 11 Line 35
(bus2-bus6) 35 7 10
(bus24-bus25)
Line 6
6 7 10 Line 36
(bus3-bus4) 36 7 10
(bus25-bus26)
Line 7
7 20 4 Line 37
(bus4-bus6) 37 10 7
(bus25-bus27)
Line 8
8 25 2 Line 38
(bus4-bus12) 38 7 10
(bus27-bus29)
Line 9
9 6 11 Line 39
(bus5-bus7) 39 7 10
(bus27-bus30)
Line 10
10 7 10 Line 40
(bus6-bus7) 40 17 5
(bus28-bus27)
Line 11
11 7 10 Line 41
(bus6-bus8) 41 7 10
(bus29-bus30)
Line 12
12 21 3
(bus6-bus9) TABLE V
Line 13 PRI BASED CONTINGENCY RANKING
13 12 6
(bus6-bus10)
Line 14 PRI
14 7 10 S. No Outage PRI based
(bus6-bus28)
Line 15 ranking
15 7 10 Line 1
(bus8-bus28) 1 14.0577 35
Line 16 (bus1-bus2)
16 9 8 Line 2
(bus9-bus11) 2 14.3998 34
Line 17 (bus1-bus3)
17 7 10 Line 3
(bus9-bus10) 3 14.4636 33
Line 18 (bus2-bus4)
18 10 7 Line 4
(bus10-bus20) 4 15.7486 19
Line 19 (bus2-bus5)
19 7 10 Line 5
(bus10-bus17) 5 13.0393 38
Line 20 (bus2-bus6)
20 7 10 Line 6
(bus10-bus21) 6 14.9873 30
Line 21 (bus3-bus4)
21 7 10 Line 7
(bus10-bus22) 7 27.8616 4
Line 22 (bus4-bus6)
22 31 1 Line 8
(bus12-bus13)
Line 23 8 (bus4- 30.1612 2
23 7 10 bus12)
(bus12-bus14)
Line 24 Line 9
24 12 6 9 13.5283 37
(bus12-bus15) (bus5-bus7)
Line 25 Line 10
25 7 10 10 15.2398 27
(bus12-bus16) (bus6-bus7)
Line 26 Line 11
26 7 10 11 15.7466 20
(bus14-bus15) (bus6-bus8)
Line 27 Line 12
27 10 7 12 28.1640 3
(bus15-bus18) (bus6-bus9)
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Line 13 Line 33
13 (bus6- 20.9154 7 33 (bus22- 15.7198 21
bus10) bus24)
Line 14 Line 34
14 (bus6- 15.0411 29 34 (bus23- 16.7296 14
bus28) bus24)
Line 15 Line 35
15 (bus8- 14.9300 31 35 (bus24- 14.0501 36
bus28) bus25)
Line 16 Line 36
16 (bus9- 18.6769 11 36 (bus25- 15.7701 18
bus11) bus26)
Line 17 Line 37
17 (bus9- 15.4983 25 37 (bus25- 20.2553 9
bus10) bus27)
Line 18 Line 38
18 (bus10- 20.2036 10 38 (bus27- 15.9111 16
bus20) bus29)
Line 19 Line 39
19 (bus10- 14.4636 33 39 (bus27- 15.4611 26
bus17) bus30)
Line 20 Line 40
20 (bus10- 15.1057 28 40 (bus28- 26.5737 5
bus21) bus27)
Line 21 Line 41
21 (bus10- 15.6828 22 41 (bus29- 14.8963 32
bus22) bus30)
Line 22
TABLE VI
22 (bus12- 120.9750 1
COMPARISON OF ERI BASED CONTINGENCY RANKING AND
bus13)
PRI BASED CONTINGENCY RANKING
Line 23
23 (bus12- 15.8882 17 ERI PRI
bus14) S. No Outage based based
Line 24 ranking ranking
24 (bus12- 20.9496 6 Line 22
bus15) 1 (bus12- 1 1
Line 25 bus13)
25 (bus12- 15.5513 23 Line 8
2 2 2
bus16) (bus4-bus12)
Line 26 Line 12
3 3 3
26 (bus14- 14.4636 33 (bus6-bus9)
bus15) Line 7
4 4 4
Line 27 (bus4-bus6)
27 (bus15- 20.6460 8 Line 40
bus18) 5 (bus28- 5 5
Line 28 bus27)
28 (bus15- 17.7011 15 Line 24
bus23) 6 (bus12- 6 6
Line 29 bus15)
29 (bus16- 15.5368 24 Line 13
bus17) 7 6 7
(bus6-bus10)
Line 30 Line 27
30 (bus18- 16.7882 13 8 (bus15- 7 8
bus19) bus18)
Line 31 Line 37
31 (bus19- 18.3579 12 9 (bus25- 7 9
bus20) bus27)
Line 32 Line 18
32 (bus21- 15.5513 23 10 (bus10- 7 10
bus22) bus20)
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Line 16 (bus29-
11 8 11
(bus9-bus11) bus30)
Line 31 Line 26
12 (bus19- 8 12 34 (bus14- 10 33
bus20) bus15)
Line 30 Line 19
13 (bus18- 9 13 35 (bus10- 10 33
bus19) bus17)
Line 34 Line 3
36 10 33
14 (bus23- 9 14 (bus2-bus4)
bus24) Line 2
37 10 34
Line 28 (bus1-bus3)
15 (bus15- 9 15 Line 1
38 10 35
bus23) (bus1-bus2)
Line 38 Line 35
16 (bus27- 10 16 39 (bus24- 10 36
bus29) bus25)
Line 23 Line 9
40 11 37
17 (bus12- 10 17 (bus5-bus7)
bus14) Line 5
41 11 38
Line 36 (bus2-bus6)
18 (bus25- 10 18
bus26) The results for IEEE – 30 bus test system are obtained.
Line 4 The ranking indices are obtained for all the outages. The
19 10 19
(bus2-bus5) index which is having highest value indicates the
Line 11 contingency as most severe and the index with lowest
20 10 20
(bus6-bus8) value indicates the contingency as less severe.
Line 33 The results for IEEE – 30 bus test system are obtained. It
21 (bus22- 10 21 is seen from the results of exact ranking method that, the
bus24) line 22 outage has value of ERI as 31. It implies that this is
Line 21 the contingency with more severity and whose outage
22 (bus10- 10 22 severely affects the operation of power system hence this
bus22) contingency is ranked as 1. Similarly it is seen from the
Line 32 table that, for the line 13 and 24 outages the ERI obtained
23 (bus21- 10 23 is 12 which indicates that these are the transmission lines
bus22) having same severity hence these outages are ranked with
Line 25 identical ranking and so on.
24 (bus12- 10 23 Similarly from the precise ranking method it is seen that,
bus16) the line 22 outage has a value of PRI as 120.9750. Hence
Line 29 the line outage 22 is most severe so it is ranked as 1 in the
25 (bus16- 10 24 ranking list. Similarly it is seen from the table that, for the
bus17) line 13 and 24 outages the PRI value is 20.9154 and
` Line 17 20.9496 respectively therefore the line outage 13 is ranked
26 10 25 as 7 and line 24 as 6. This indicates that in the exact
(bus9-bus10)
Line 39 ranking indices, where the severity of line 13 and line 24
27 (bus27- 10 26 outages is same, the precise ranking indices distinguish
bus30) between the two.
Line 10 Finally the comparison of exact ranking indices (ERI) and
28 10 27 precise ranking indices (PRI) based contingency ranking is
(bus6-bus7)
Line 20 made in the table. It is seen from the table that, the exact
29 (bus10- 10 28 ranking method does not differentiate between the outages
bus21) with same severity hence label them with identical
ranking.
Line 14
30 10 29 The precise ranking technique addresses the concern of
(bus6-bus28)
identical ranking for the outages with same severity by
Line 6
31 10 30 taking into account in case there is any line or bus
(bus3-bus4)
reaching near to its limits following a particular
Line 15
32 10 31 contingency.
(bus8-bus28)
Hence precise ranking method distinguishes between the
33 Line 41 10 32 outages having same severity.
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IV. CONCLUSION International Journal of Resent Trends in Electrical and

Electronics Engineering 2010.
In this paper two methods for the contingency ranking i.e. [13] M.A. Kamarposhti, H.Lesani, “Contingency Analysis for
exact ranking method and precise ranking method were Voltage Stability Analysis using Continuation Power Flow
described. Both of these methods correctly identify all the Method”, International Journal of Resent Trends in Electrical
and Electronics Engineering ISSSN-1392-1215, 2010.
unstable contingencies. Further precise ranking method Shobha Shankar, T.Ananthapadmanabha, “Fuzzy Approach to
[14]
provides more distinction and information which can Critical Bus Ranking Under Normal and Line Outage
enable the power system operator to have more effective Contingencies”, International Journal on Soft Computing Vol.2,
control action. No.1, pp 59-69, Feb 2011.
[15] Ahamadi Kamarposhti, Barak Mozafari “Study the Effects of
The result shows that the calculation of indices gives a Power Plant Outages on Maximum Loading in Power System”,
measure of severity of possible line outage contingencies Journal of Basic and Applied Scientific Research , pp.2410-
in the system. The highest value of indices provides the 2416, 2011.
severity of outage and has maximum possibility of making [16] Veenavati Jagadishprasad Mishra, Manisha D. Khardenvis,
“Contingency Analysis of Power System”, International
the system parameters to go beyond the operating limits. Conference on Emerging Frontiers in Technology for Rural
The effectiveness of this method is demonstrated on Area, 2012.
IEEE – 14 and IEEE – 30 bus test system respectively. [17] J.B.Gupta, “A Course in Power Systems”, Xth Edition.
The results clearly have shown that both the methods are [18] D.P.Kothari, I.J.Nagrath, “Modern Power System Analysis”,
effective in ranking the contingencies according to the IVth Edition.
[19] K.Uma Rao, “Computer Techniques And Models in Power
severity. Also both the methods are compared to know the Systems”, I.K.International Ist Edition.
relative degree of preciseness in ranking the contingencies. [20] Allen J. Wood, Bruce F. Wollenberg, “Power Generation
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Publication, First edition.
which is more realistic approach. In addition to being
accurate and precise, this method is fast enough to be
considered for online security analysis in present and
future complex mixed power system.

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