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A NOVEL APPROACH OF LOAD
FREQUENCY CONTROL IN MULTI
AREA POWER SYSTEM
*V.SHANMUGA SUNDARAM
LECTURER/EEE, SONA COLLEGE OF TECHNOLOGY,SALEM-5, TAMILNADU INDIA
Dr.T.JAYABARATHI
PROFESSOR/DIRECTOR ,SCHOOL OF ELECTRICAL ENGINEERING ,VIT UNIVERSITY
,VELLORE,TAMILNADU INDIA
Abstract:
The main objective of Load Frequency Control(LFC) is to regulate the power output of the
electric generator within an area in response to changes in system frequency and tie-line loading .Thus
the LFC helps in maintaining the scheduled system frequency and tie-line power interchange with the
other areas within the prescribed limits. Most LFCs are primarily composed of an integral controller.
The integrator gain is set to a level that the compromises between fast transient recovery and low
overshoot in the dynamic response of the overall system. This type of controller is slow and does not allowthe controller designer to take in to account possible changes in operating condition and non-linearities in
the generator unit. Moreover, it lacks in roubustness.Therefore the simple neural networks can alleviate
this difficulty.This paper presents the Artificial Neural Network (ANN) is applied to self tune the
parameters of PID controller. Multi area system, have been considered for simulation of the proposed
self tuning ANN based PID controller .The performance of the PID type controller with fixed gain,
Conventional integral controller, and ANN based PID controller have been compared through MATLAB
Simulation results carried out by 1% system disturbances both single area in multi area power system .
Comparison of performance responses of integral controller & PID controller show that the neural-
network controller has quite satisfactory generalization capability, feasibility and reliability, as well as
accuracy in both single and multi area system. The qualitative and quantitative comparison have been
carried out for Integral, PID and ANN controllers. The superiority of the performance of ANN over
integral and PID controller is highlighted.
Keywords: Power system, Artificial Neural network, Back propagation algorithm, PID Controllers.
I.INTRODUCTION
In electric power generation, system disturbances caused by load fluctuations result in changes to
the desired frequency value. Load Frequency Control (LFC) is a very important issue in power system operation
and control for supplying sufficient and both good quality and reliable power. Power networks consist of anumber of utilities interconnected together and power is exchanged between the utilities over the tie-lines by
which they are connected. The net power flow on tie-lines is scheduled on apriori contract basis. It is therefore
important to have some degree of control over the net power flow on the tie-lines. Load Frequency Control(LFC) allows individual utilities to interchange power to aid in overall security while allowing the power to be
generated most economically.
The variation in Load frequency is an index for normal operation of the power systems. When the
load perturbation takes place, it will affect the frequency of other areas also. In order to control frequency of the
power systems various controllers are used in different areas, but due to the non-linearity in system componentsand alternators, these conventional feedback controllers could not control the frequency quickly and efficiently.
2..PROBLEM FORMULATION
In order to keep the power system in normal operating state, a number of controllers are used in
practice. As the demand deviates from its normal operating value the system state changes. Different types of
controllers based on classical linear control theory have been developed in the past. Because of the inherent non-linearities in system components and synchronous machines, neural network techniques are considered to build
non-linear ANN controller with high degree of performance.
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Most load frequency controllers are primarily composed of an integral controller. The integrator
gain is set to a level that compromise between fast transient recovery and low overshoot in the dynamicresponse of the overall system. This type of controller is slow and does not allow the controller designer to take
into account possible non-linearities in the generator unit.
2.1 Objectives of LFC:
In order to regulate the power output of the electric generator within a prescribed area in response
to changes in system frequency, tie line loading so as to maintain the scheduled system frequency andinterchange with the other areas within the prescribed limits.
3. MODELING OF SINGLE AREA AND MULTI AREA POWER SYSTEMS
3.1Single Area System Modeling
In Single area system, generation and load demand of one domain is dealt. Any load change
within the area has to be met by generators in that area alone through suitable governor action. Thus we can
maintain the constant frequency operation irrespective of load change.
3.2 Generator Model
A single rotating machine is assumed to have a steady speed of and phase angle 0. Due tovarious electrical or mechanical disturbances, the machine will be subjected to differences in mechanical and
electrical torque, causing it to accelerate or decelerate. We are mainly interested in the deviations of speed, ,
& and deviations in phase angle , from nominal.
This can be expressed in Laplace transform operator notation as
Pmech - Pelec = M s (1.11)
Equation (1.11) can be represented as shown in figure
Figure 1.Relationship between mechanical and electrical power and speed change
3.3 Load Model
The load on a power system comprises of a variety of electrical devices. Some of them are purely
resistive. Some are motor loads with variable power frequency characteristics, and others exhibit quite different
characteristics. Since motor loads are a dominant part of the electrical load, there is a need to model the effect of
a change in frequency on the net load drawn by the system. The relationship between the change in load due to
the change in frequency is given by
PL (freq) = D (or)
D = PL (freq) / (1.12)
1 / M sPmech
Pelec
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The net change in Pelec in figure ( 1.1 ) is
Pelec = PL + D
No frequency frequency
Sensitive load sensitive load
Change change
Incorporating this in the figure2,
Figure 2.Block diagram of rotating mass an load as seen by prime- mover output
3.4 Prime-Mover Model
The prime mover driving a generator unit may be a steam turbine or a hydro turbine. The models
for the prime mover must take account of the steam supply and boiler control system characteristics in the case
of a steam turbine, or the penstock characteristics for a hydro turbine. Here only the simplest prime-mover
model, the non reheat turbine, is considered. The model for a non reheat turbine shown in figure 1.3, relates the
position of the valve that controls emission of steam into the turbine to the power output of the turbine.
Figure3. Prime mover model
1 / M sPmech
PL
D
1 / (M s + D)
PL
Pmech
1/(1+ s T CH )
Pvalve Pmech
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Figure 4. Prime mover generator load model
3.5 Governor Model
Suppose a generating unit is operated with fixed mechanical power output from the turbine, the
result of any load change would be a speed change sufficient to cause the frequency-sensitive load to exactly
compensate for the load change. This condition would allow system frequency to drift far outside acceptablelimits. This is overcome by adding mechanism that senses the machine speed, and adjusts the input valve to
change the mechanical power output to compensate for load changes and to restore frequency to nominal value.
The earliest such mechanism used rotating fly balls to sense speed and to provide mechanical motion in
response to speed changes. Modern governors use electronic means to sense speed changes and often use a
combination of electronic, mechanic and hydraulic means to effect the required valve position changes.
The simplest governor, is synchronous governor, adjusts the input valve to a point that brings
frequency back to nominal value. If we simply connect the output of the speed-sensing mechanism to the valve
through a direct linkage, it would never bring the frequency to nominal.
To force the frequency error to zero, one must provide reset action. Reset action is accomplished
by integrating the frequency (or speed) error, which is the difference between actual speed and desired or
reference speed. Speed-governing mechanism with diagram shown in figure 1.6.1.5 The speed-measurement
devices output, , is compared with a reference, ref, to produce an error signal, .
The error, , is negated and then amplified by a gain KG and integrated to produce a control
signal, Pvalve , which cause main steam supply valve to (Pvalve position) when is negative. If, for
example, the machine is running at reference speed and the electrical load increases, will fall below ref and
will be negative. The action of the gain and integrator will be to open the steam valve, causing the turbine to
increase its mechanical output, thereby increasing the electrical output of the generator and increasing the speed
. When exactly equals ref, the steam valve the new position (further opened) to allow the turbine generator
to meet the increased electrical load.The synchronous (constant speed) governor cannot be used if two or more
generators are electrically connected to the same system since each generator would have to have precisely the
same speed setting or they would fight each other, each trying to pull the systems speed (or frequency) to its
own setting. To run two or more generating units in parallel, the speed governors are provided with a feedback
signal that causes the speed error to go to zero at different values of generator output.
1/(1+ s T CH )
Pvalve Pmech
PL
1 / (M s +D)
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The result of adding the feedback loop with gain R is a governor characteristic as shown in
figure5. The value of R determines the slope of the characteristic. That is, R determines the change on the units
output for a given change in frequency.
The basic control input to a generating unit as far as generation control is concerned is the load reference
set point. By adjusting this set point on each unit a desired unit dispatch can be maintained while holding system
frequency close to the desired nominal value.
R is equal to pu change in frequency, divided by pu change in unit output. That is,
upP
R .
The combined block diagram of single area system with, governor prime mover - rotating
mass/load model is shown in figure7.
1/(1+s Tg )
1/R
0 0.5 1.0
per unit output
Frequency
2 3
Pvalve
Load reference
ref
1. Load reference setting fornominal speed at no load
2. Nominal speed at 0.5 pu output
3. Nominal speed for full output
Figure.5 Block diagram of governor with droop
Figure.6. Governor characteristics
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Figure7.Block diagram of single area system
Suppose that this generator experience a step increase in load ,
s
PsP LL
)(
The transfer function relating the load change PL, to the frequency change is
DMssTsTR
DMssPs
CHG
L1
1
1
1
111
1
)()((1.13)
The steady state value of(s) may be found by
steady state = lim [ s (s)]
s 0
steady state
DR
PL
1 (1.14)
3.6 Multi Area System Modeling
In multi area system load change in one area will affect the generation in all other interconnected
areas. Tie line power flow should also be taken into account other than change in frequency. We will discuss
two area and three area systems in the following session.
3.7 Two Area System
In two-area system, two single area systems are interconnected via the tie line. Interconnection
established increases the overall system reliability. Even if some generating units in one area fail, the generating
units in the other area can compensate to meet the load demand.
GsT1
1
CHsT1
1
DMs
1
1 / R
Load
reference
set point Governor prime mover PL rotating mass
& load
Pvalve
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3.8 Tie Line Model
The power flowing across a transmission line can be modeled using the DC load flow method as
)(1
21 tie
tieflowX
P (1.15)
This tie flow is a steady-state quantity. For purposes of analysis here, we will perturb the above
equation to obtain deviations from nominal flow as a function of deviations in phase angle from nominal.
)(1
)(1
)(1
)]()[(1
21
2121
2211
tie
tieflow
tietie
tie
tieflowtieflow
XP
then
XX
XPP
(1.16)
where 1 and2 are equivalent to 1 and2 .
Then equation (1.16) can be expressed as,
)( 21 s
TP
tieflow
where T is tie-line stiffness coefficient and ,
T = (23.1450) 1 / Xtie (for a 50-Hz system).
Suppose now that we have an interconnected power system broken into two areas each having one
generator. The areas are connected by a single transmission line. The power flow over the transmission line will
appear as a positive load to one area and an equal but negative load to the other, or vice versa, depending on the
direction of flow. The direction of flow will be dictated by the relative phase angle between the areas, which is
determined by the relative speed -deviations in the areas.
A block diagram representing this interconnection can be drawn as in figure 8.
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It should be noted that the tie power flow was defined as going from area 1 to area 2; therefore,
the flow appears as a load to area 1 and a power source (negative load) to area 2. If one assumes that mechanical
powers are constant, the rotating masses and tie line exhibit damped oscillatory characteristics are known as
synchronizing oscillations.
It is quite important to analyze the steady-state frequency deviation, tie-flow deviation and
generator outputs for an interconnected area after a load change occurs. Let there be a load change PL in area 1.
In the steady state after all synchronizing oscillations have damped out, the frequency will be constant and equal
to the same value on both areas.
Then
1 =2 = and
0)()( 21
dt
d
dt
d (1.17)
Finally we have,
21
21
1
11DD
RR
PL
(1.18)
11
1
GsT 11
1
CHsT 11
1
DsM
1 / R1
Load
reference
Pvalve
1
21
1
GsT 21
1
CHsT 22
1
DsM
Loadreference
1 / R2
T / s
2
PL1
PL2
Figure 8. Block diagram of interconnected areas
Ptie
Pvalve
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21
21
2
2
1
11
1
DDRR
DR
P
P
L
tie
(1.19)
The new tie flow is determined by the net change in load and generation in each area. We do not
need to know the tie stiffness to determine this new tie flow, although the tie stiffness will determine how much
difference in phase angle across the tie will result from the new tie flow.
4. BACK PROPAGATION ALGORITHM
Almost any function can be approximated using the multilayer network if we have sufficient number
of neurons in the hidden layer. Infact it has been shown that two layer networks, with sigmoid transfer functions
in the hidden layer and linear transfer functions in the output layer can approximate virtually any function of
interest to any degree of accuracy , provided sufficiently many hidden units are available.
For multilayer networks output of one layer becomes input to the following layer.
Step-1
Propagation of input forward through the network
m
mmmmm
aa
MmforbaWfa
pa
,1,...,2,0)(
,
1111
0
Step -2
Propagation of sensitivities backward through the network
.1,2,...,1,)(*)('
,)(*)('2
11
MformsWnFs
atnFs
mTmmmm
MMM
Step -3
Updation of weights and biases using approximate steepest descent rule
.)()1(
,)()()1(1
mmm
Tmmmm
skbkb
askWkW
Basic steps of using MATLAB toolbox, nntool .
In this thesis training is carried out using the two methods. However the updation of weights and
biases from the initial set values, sensitivities, trained data, coordination between the target and trained datacant be explicitly in the second method (using nntool). NNTOOL method provides the facility to train through
one of the methods Say conjugate gradient method, Levenberg-Marquardt method for back propagation. It is
superior to approximate steepest descent method. Hence at first training is carried out using the nntool method.
In the neural network we have employed TANSIG as transfer function in the hidden layer and PURELIN in theoutput layer. Then the obtained weights and biases are chosen as the initial weights and biases. Now the training
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is carried out using the first method (using approximate steepest descent method). All results such as the
updating of weights and biases from the initial set values, sensitivities, trained data, coordination between thetarget and trained data can be obtained.
A. Design of ANN Controller:
The range over which error signal is in transient state, is observed. Responding values of theproportional, integral and derivative constants are set. This set is kept as target. Range of error signal is taken as
the input.
This input target pair is fed and new neural network is formed using nntool in the MATLABSimulink software. Weights and Biases obtained are fed to back propagation algorithm using approx. steepest
descent method. Thus the neural network is well trained. Updated weights and biases are given to a fresh neural
network. Now the neural network is ready for operation.In ANN controller is trained with the set of data to determine the PID controller parameters:
Proportional constant, KP
Integral constant, KI
Derivative constant, KD.
Normal PID controller has fixed constants for proportional, integral and derivative term .Butduring transient state controller effect can be made to perform better by variation of the constants. This is
accomplished by ANN controller.
5.SIMULATION STUDY:
5.1 Single Area LFC:
Figure.9. Comparison of PI, PID And ANN Controllers for single area system
5.2 Multi Area LFC: Area 1
Figure10. Comparison of PI, PID And ANN Controllers for multi area 1system
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5.3Multi Area LFC: Area 2
Figure11. Comparison of PI, PID And ANN Controllers for multi area 2 system
Inference: From the above responses of Figure1,2&3 shows that comparison of PI , PID And ANN Controller
which gives the ANN Controller less undershoot and settling time to other controllers.
PERFORMANCE INDICES
CONTROLLER IAE ISE ITAE
INTEGRAL 0.1495 0.0065 0.3496
PID 0.1000 0.0018 0.3409
ANN 0.0951 0.0016 0.3159
Table 1. Single Area LFC
MULT1 AREA LFC
PERFORMANCE INDICESCHANGE IN
FREQUENCY IN Hz
IAE ISE ITAE
INTEGRAL
f1 0.1793 0.0072 0.5572
f2 0.2932 0.0142 1.2304
PID
f1 0.0443 8.2026e-004 0.0602
f2 0.0513 0.0014 0.0649
ANN f1 0.0487 8.0973e-004 0.0799
f2 0.0577 0.0014 0.0885
Table 2. Multi Area LFC
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MULTI AREA LFC: TIE-LINE FLOW
PERFORMANCE INDICES
CHANGE IN
FREQUENCY IN Hz
IAE ISE ITAE
INTEGRAL
tie 0.3306 0.0132 1.9921
PID
tie 0.0789 8.8451e-004 0.3469
ANN
tie 0.0784 8.9196e-004 0.3339
Table 3. Multi Area LFC Tie-Line floe
With the reference to the results obtained as shown in Table1. we can conclude that theperformance indices IAE/ISE/ITAE are minimum both Area 1 and Area 2 .When ANN controller compared to
that of Integral and PID controllers.
6.CONCLUSION
From the simulation results obtained for load disturbances for ANN controller, PID controller,Conventional integral controller we can conclude that ANN controller is faster than the other, Peak undershoot
is reduced, Settling time is reduced. The superiority of ANN controller is established in the cases of two area
systems. From the Qualitative and Quantitative comparison of the results we can conclude that the ANNcontroller yields better results. ANN controller gives minimum IAE/ISE/ITAE compared to the conventional
integral and PID controllers. The simulation studies were also done for change in the operating conditions like
change in governor time and turbine time constants. The Qualitative comparisons of all the controllers show thatthe ANN results in robust performance. Hence ANN controller has large potential to be used as a control
strategy for the Load Frequency Control.
V.REFERENCES
[1] Aanstad, O. J. and Lokay, H. E. (1970) Fast valve control can improve turbine generator response to transient disturbances.Westinghouse Engineer, July, pp. 114-119.
[2] Dr.Chaturvedi D.K Prof.Satsangi P.S & Prof.kalra P.K, Application of Generalized Neural Network to Load Frequency ControlProblem.
[3] Djukanovic, M., Sobajic, D. J. and Pao, Y. H. (1992) Neural net based determination of generator-shedding requirements in electricpower systems.IEEE Proceedings--C139, 5 427-436.
[4] Elgerd OI. Electric energy systems theory: An introduction. McGraw-Hill;[5] Kundur.P (1994) Power System Stability and Control New York McGraw-Hill[6] Hadi Saadat Power System Analysis Tata Mcgraw-Hill Publishing Company Limited New Delhi.[7] Hertz, J., Krogh, A. and Palmer, R. G. (1991) Introduction to the Theory of Neural Computation. Addison-Wesley, Reading, MA.[8] Ha QP. A fuzzy sliding mode controller for power system load-frequency control. In: IEEE Second Conf -Knowledge-Based
Intelligent Systems, 1998
[9] Jaleeli N, VanSlyck LS, Ewart DN, Fink LH,Hoffmann AG. Understanding automatic generation control. IEEE Trans Power Systems1992;7(3):110612.
[10] Kanniah J, Tripathy SC, Malik OP, Hope GS. Microprocessor-based adaptive load-frequency control. IEE Proc C1984; 131(4):1218.[11] Pan CT, Liaw CM. An adaptive controller for power system load-frequency control. IEEE Trans Power Systems 1989;4(1):1228.[12] Reddoch P, Julich TT, Tacker E. Model and performance functional for load frequency control in interconnected power systems. In:
IEEE Conf Decision and Control, 1971.
[13] Talaq J, Al-Basri F. Adaptive fuzzy gain scheduling for load frequency control. IEEE Trans Power Systems 1999;14(1):14550.[14] Vajk I. Adaptive load-frequency control of the Hungarian power system. Automatica 1985; 21(2):12937.[15] Yamashita K, Miyagi H. Multivariable self-tuning regulator for load frequency control system with interaction of voltage on load
demand. IEEE Proc D 1991; 138(2).
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APPENDIX:
Parameters Area 1 Area 2
Turbine time constant 0.5 s 0.6s
Generator Time constant 0.2s 0.3s
Generator Angular
Momentum
10 MJrad/s 8 MJrad/s
Governor Speed Regulation 0.05 pu 0.065 pu
Load change for Frequency
change of 1% D = p / f
0.6%
0.6
0.9%
0.9
Rated output 250 MW 250 MW
Sudden Load Variation 250 MW
250/250=1p.u
250 MW
250/250=1p
BIOGRAPHY:
V.Shanmuga Sundaram. received the M.E. degree in Power Systems Engineering from Periyar University,
Salem Tamilnadu, India, in 2005. He is a research Scholar of VIT University, Vellore ,Tamilnadu India.Currently, he is an Lecturer in Sona College of Technollogy Salem, His interests are in power system controldesign and Deregulated power systems.
Dr. T.Jayabharathi received the Ph.D., degree in Power Systems Engineering from Anna University , Chennai
in 2003. She has published more than 30 journals of both national and international. Currently she is an Director
and Senior professor of School of Electrical Engineering in VIT University, Vellore. His research interests arePower system optimization, Soft computing ,Economic dispatch control problems and Deregulated power
systems.
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