International Conference on Electrical, Electronics, and Optimization Techniques (ICEEOT) - 2016
Power System Contingency Ranking using Fast
Decoupled Load Flow Method
Sandhya rani.Gongada ,Tegala.Srinivasa Rao,P.Mallikarjuna rao, Shaik Salima
srinivasrao_tegala@yahoo.com,sandhya_gongada@rediffmail.com,electricalprofessor@gmail.com, shaiksalima.aiet@gmail.com
Abstract- As power demand is increasing day to day life loading
on present power system is also increasing .As a result voltage
instability occurs with heavily loaded system .So power system
security becomes one of the important aspect where operational
engineers need to estimate the unseen contingency .So contingency
screening and ranking is one of the important procedure for on
line system security assessment .In this paper contingency screening
and ranking is done by fast decoupled load flow by using
performance index i.e., active power (PIp)and voltage performance
index(PIv) for the line outage of the given power system .Ranking is
based on overall performance index (PI)i.e., summation of active
power performance index(PIp)and voltage performance index (PIv)
the highest value of PI ranks first and proceeds in descending order
for further ranking. The proposed work is simulated on IEEE 5
bus system on MATLAB environment.
Keywords: voltage instability, contingency ranking, fast decoupled
method, overall performance index.
I.
INTRODUCTION
It is a known fact that power system is a complex network
consisting of various electrical based equipment .Failure of any
of the equipment disturbs the reliability of the system .Electric
energy demand is rapidly increasing and unending demand so it
was really a challenging task to power engineers, to maintain an
efficient, secure and much reliable power dispatch to the
consumer end without power interruption. In the power system
operation and planning the power system security is important
aspect .One of the main important aspect of power system
planning and operation is to study the effect of line outage in
terms of severity .A detailed study of system security assessment
is necessary to deal with the possible failures in the system ,its
consequences and remedial actions which is known as power
system security assessment .Security analysis revolves around
the power system to withstand the effect of contingency.
There are number of methods for evaluating of contingency
of power system.AC power flow or load flow are important
part of the contingency analysis .Load flow are necessary to
have proper controlling ,scheduling and operation of existing
and operating power system and also need for proper planning in
future expansion. AC load flow methods in outage case is
discussed by the authors in[ 1 ].Decoupled load flow and
compensation method to get post outage voltage and ranking is
based on performance index is discussed by authors in [2]. The
process of identifying the critical contingencies is referred to as
contingency selection which uses the complete AC load flow
program considering outage of each line or generator[3].On this
topic work is carried out which consists of the potential
contingencies cases by using ranking methods or screening
method i.e., bounding methods [4],distribution method[5],expert
and new method for contingency selection [5-10].lot of
advancement in the speed of contingency screening was brought
in the recent development using Artificial Neural Network[11].
II. CONTINGENCY ANALYSIS
Concept of contingency was introduced by Ejebe and
Wollenberg. Contingency is termed as a unpredictable events in
the power system whereas outage is referred as temporary
suspension of power .Contingency therefore defined as possible
circumstance or as future outage which is possible but cannot be
predicted with certainty. Contingency analysis need the
simulation of each contingency for the given power system
model .In order to analyze the contingency analysis easier it
comprises of three important steps.
1) Contingency definition: This is the first step of analysis
where it consists of all possible set of contingencies that may
occur in power system. This step involves the process of
creating contingency list.
2) Contingency selection: This is the second step simply known
of contingency ranking approach where ranking is given
based on the overall performance index which indicates the
severity of contingencies .so in this step we get the
information of list of contingencies that leads to limit
violation in the power flow and voltage magnitude. This step
excludes the lowest severity contingency from the list of
contingencies.
3) Contingency evaluation: This is the third step and also the
important step which involves the necessary control action
and security action in order to mitigate the most effective
contingencies in a given power system.
Power engineers need to concentrate in this analysis for
power security purpose to fulfill the motive of supplying power
without any interruption i.e. continuity of supply.
For contingency analysis need AC power flow program
i.e. FAST DECOUPLED LOAD FLOW[FDLF] is used as it
provides fast solution and it has a advantage of matrix
alternation formula as it is used to simulate the contingency
problem and has transmission line outage without re changing
the system Jacobian matrix for every iterations.
978-1-4673-9939-5/16/$31.00 ©2016 IEEE
4373
International Conference on Electrical, Electronics, and Optimization Techniques (ICEEOT) - 2016
III. APPROACH TO CONTINGENCY RANKINIG
In order to know the severity of contingency in a number of
transmission lines we need to find the severity and need to rank
on the base of some formula .Here for this purpose AC power
flow solution done in MATLAB environment is used to get
active ,reactive power flows in a transmission lines and bus
voltage magnitudes. First we will do it for base case used for
pre contingency analysis and next done to each line outage case
for post contingency analysis. Hence depends upon the severity
of contingency measured in post contingency analysis based on
overall performance index ranking is done. There are two types
of performance index are used widely are
1) Voltage performance index: This index reflects the violation
of bus voltage and given as.
NB
PIv
= ∑ (W/m) {(│Vi │-│ViSP│) / ΔVilim}2m
(1)
i=1
Where,
NB
=Number of buses in the system
W
=Real non negative weighting factor and the value (=1)
M
=Penalty function and the value (=1)
│Vi │ =Voltage magnitude at bus i obtained from FDLF load
Flow solution
│Visp│=Specified voltage magnitude at bus i
Vilim =Deviation of voltage limit (In my case=0.1)
2) Active power performance index: This index reflects the
violation of active power (MW) flow and given as
PIp
NL
=∑ (W/m)(Pl/Plmax)2m
l=1
IV. ALGORITHM AND FLOW CHART OF CONTINGENCY
RANKING
Algorithm for FDLF load flow method involves the
following steps
[1] Read the given system’s bus data and line data
[2] Set the counter to zero
[3] First before considering any line contingency perform the
FDLF load flow for base case i.e. pre contingency analysis.
[4] Simulate a line contingency or line voltage i.e. removing a
line and proceed to further steps.
[5] For this particular outage load flow analysis is done, then
the calculation of active power[MW] flow is done in the
remaining transmission lines and Pmax value is calculated as per
equation 3.
[6] Calculate the active performance index (PI p) as per equation
2 which indicates active power limit violation of the taken
system model.
[7] For the particular line contingency voltages of all load buses
are calculated.
[8]Calculate the voltage performance index (PIv) as per equation
1 which indicates the bus voltage violation of the taken system
model.
[9]Check if all line outages are simulated .if not repeat steps 4 to
8 is computed till the last line outage of transmission line of the
given bus system is complete.
[10]Once the whole above process is completed, contingencies
are ranked based on overall performance index i.e. summation of
PIp and PIv of all transmission line outages.
[11]And do the power flow analysis of the most severe
contingency case and result is printed.
Start
(2)
Read system
bus data and
line data
Set the contingency counter
=0 and perform load flow
for base case
Where,
NL
=Number of transmission lines in the given system
W
=Real non negative weighting factor and the value (=1)
M
=Penalty function
Pl
=Active power (MW) flow in the transmission line
Plmax =Maximum active power (MW) flow in the transmission
line
Plma x =(Vi*Vj)/X
(3)
Where,
Vi =Voltage magnitude of bus i by FDLF load flow
Vj =Voltage magnitude of bus j by FDLF load flow
X =Reactance of the transmission line Connecting bus i and j
There is another performance index come into picture when
reactive power does not lie in the limits .In this present paper the
above mentioned two index are only used.
Simulate the line outage
contingency
1
2
1
Run the FDLF load flow for the
line outage condition
978-1-4673-9939-5/16/$31.00 ©2016 IEEE
4374
International Conference on Electrical, Electronics, and Optimization Techniques (ICEEOT) - 2016
2
Calculate MW flow
and Pmax in all
transmission line
Compute PIp as
per equation 2
and 3
Calculate all bus
voltages using
FDLF
Compute PIv as
per equation 1
Last
contingency
reached?
Calculate
overall
performan
ce index
(PIp+PIv)
Fig 3. Post contingency state of 5 bus system (line 1 outage)
Rank the contingency based
on OPI
Stop
Fig. 1. Flow chart for contingency ranking
V. RESULTS AND DISCUSSION
Entire analysis is done for IEEE standard 5 bus system which
consists of one slack bus and 4 load buses .These 5 buses are
connected with seven transmission lines as shown in figure
2.Respective voltage magnitude and active power flows results
of FDLF of pre contingency case are also shown fig.2 in per
unit representation.
In the fig.3 shows the single line outage of transmission line
1 and its corresponding values of active power (MW) flows and
voltage magnitude of post contingency analysis of outage of line
1 done using FDLF load flow. So by using the MW flows and
voltage performance indexes are calculated using equation 1 and
2.So post contingency analysis is done for every transmission
line outage and corresponding performance indexes. And those
values are tabulated in table III.
Table I. Active power flows in the pre contingency state and
post contingency state (Line -1 outage)
Line
Start
bus
End
bus
Pre contingency
power flow (pu)
Post contingency
power flow (pu)
1
2
3
4
5
6
7
1
1
2
2
2
3
4
2
3
3
4
5
4
5
0.889
0.407
0.247
0.279
0.548
0.189
0.064
0
1.433
0.155
0.039
0.391
0.669
0.223
Table II.
Bus Number
Fig. 2. Pre contingency state of 5 bus system
1
2
3
4
5
Bus voltages in the pre contingency and post
contingency
Pre contingency voltage
(pu)
1.060
1.045
1.021
1.021
1.011
Post contingency voltage(pu)
1.060
0.865
0.865
0.857
0.829
978-1-4673-9939-5/16/$31.00 ©2016 IEEE
4375
International Conference on Electrical, Electronics, and Optimization Techniques (ICEEOT) - 2016
Table III. Performance indices & contingency ranking using
FDLF load flow
Line outage
number
1
2
3
4
PIp
PIv
0.165
0.022
0.023
0.023
8.614
0.1143
0.2174
0.2034
Overall performance
index(OPI)
8.779
0.1366
0.2404
0.2264
5
0.065
1.7482
1.8132
2
6
7
0.018
0.018
0.2473
0.2525
0.2653
0.2705
4
3
Ranking
1
7
5
6
As per table III the overall performance index of transmission
line 1 outage has highest value so it is ranked first.
VI. CONCULUSION
The results can be concluded that the contingency analysis can
be done by using performance index and AC load flows by fast
decoupled method is most suitable. As the power system is a
complex network it is necessary to find the correct contingency
selection as it eliminates number of contingency cases and need
to focus on the most severe contingency case. Highest ranking
of transmission line outage shows that it has got maximum
chances of making the system parameters to operate beyond the
limit. Operational engineers of power system needs the list of
severity of contingencies in the power system used as useful
information to operate the power system with safe, secure
,reliable and continuously.
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