Gaining improved performances of agc in a multi area power system

International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 2, March – April (2013), © IAEME
459
GAINING IMPROVED PERFORMANCES OF AGC IN A MULTI-
AREA POWER SYSTEM WITH ALL POSSIBLE CONSTRAINTS BY
CHANGING THE EFFECT OF TURBINES AND CONTROLLERS
1
Dipayan Guha, 2
P.K.Prasad, 3
Somalee Mitra, 4
Subhankar Mukherjee
1,3
Assistant Professor, Kanksa Academy of Technology & Management-TIG, Durgapur,
West Bengal
2
Principal, Abacus Technology and Management, Hooghly, West Bengal
3
Final Year Student, Kanksa Academy of Technology & Management-TIG, Durgapur, West
Bengal
ABSTRACT
The main objective of the present work to study the effect of small load perturbation
on a multi-area interconnected power system considering all possible constraints in power
system. Three thermal areas of reheat turbine either single-stage or two-stage is considered
with 10% step load perturbation in all the units for study. This paper focused on the
effectiveness of PI and PID controller over conventional I-controller. An attempt is made to
investigate the proper values of sampling time period (T) and speed regulation parameter
(Ri). Further, the effects of variations of reheat gain (Kr) and reheat time constant (Tr) have
been explored. The performance of single-stage and two-stage turbine has been investigated
with PID and PI controllers. The dynamic responses are studied in MATLAB-SIMULINK
environment.
INDEX TERMS: Automatic Generation Control, PI and PID controllers, two stage turbine,
effect of speed regulation parameter, effect of Kr and Tr , SIMULINK.
INTRODUCTION
Everyone expect uninterrupted power supply for their use. However, it is always not
possible for a system to provide continuous power supply to their consumers, since both
active and reactive powers continuously changes with load variations. Any change in the
loads may cause to disrupt the nominal operation of frequency which is highly undesirable,
INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING
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ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
Volume 4, Issue 2, March – April (2013), pp. 459-469
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460
larger change may collapse the system completely. Modern power system networks are
interconnected and exchanging power to the neighbors, the AGC problem is the major
requirement.
The most important part in our daily operation of a power system is the electrical
energy, which must be properly scheduled and controlled. This is the main concern of energy
control center and implemented automatic generation control program as a part of energy
management system. The main purpose of power system operation and control to match the
generations with loads plus losses incurred in the system. The main objective of the Load
Frequency Control (LFC) system is to maintain the frequency and tie-line power to its
nominal values.
Number of researches has been found over the past decades in the field of AGC.
Literature survey shows that most of the paper studied two-area or multi-area either thermal-
thermal or hydro-thermal system using conventional integral controller. Linear perturbation
system is build for the study of dynamic performances. Less attention is paid for study of
same for non-linear type of power system. A realistic study may come if the whole system is
modeled with all possible physical constraints, such as Generation Rate Constraints (GRC),
Dead Band (DB) and time delays in communication channels. [4] has studied a multi-area
system considering GRC of turbine using integral controller only. [7] present two-area
thermal system considering GRC and single-stage turbine. Literature survey shows that less
attention have been paid in multi-area system considering all possible constraints in power
system. The main objectives of the present work are as follows –
• to study the effect of small step load perturbation in multi-area thermal systems
• to compare the performances of single-stage and two-stage turbines with all possible
constraints
• to study the effectiveness of PI and PID controller in lieu of conventional integral
controller with and without constraints
• to study the effect of changes of reheat turbine gain and reheat turbine time constant on
dynamic performances three-area system
• to investigate the maximum permissible values of speed regulation parameter (Ri) and
sampling time period (T) without hampering the dynamic performances
Fig. 1: Block diagram of multi-area system considering all possible constraints

461
SYSTEM INVESTIGATED
The AGC system investigated comprises an interconnection of three areas of equal
size. All areas comprising reheat type thermal unit, turbines are either single-stage or two-
stage. Appropriate values of physical constraints are considered for study. The nominal
values of all parameter with physical constraints are given in appendix-1. Fig. 1 shows the
basic block diagram of three-area system with constraints present in power system.
TWO-STAGE REHEAT TURBINE
The approximated Transfer Function (T.F) of two-stage reheat turbine is modeled in
[2]. This has been used to demonstrate the effect of small load perturbation in AGC system.
Fig. 2(a) and 2(b) shows the schematic and T.F representations of two-stage reheat turbine,
respectively. It comprises four main cylinders as very high pressure (VHP), high pressure
(HP), intermediate pressure (IP) and low pressure (LP). MW rating of each cylinder is α, β, γ,
and δ so that( )1=+++ δγβα .
Fig. 2(a): Schematic diagram of two-stage turbine
Fig. 2(b): T.F model of two-stage turbine
Linear approximated T.F of two-stage turbine is defined by eqn
: [1]
{ } ]1[
)1)(1)(1(
)(1
)(
21
21
2
22211
−
+++
++++
=
rrt
rrrrrrr
T
sTsTsT
TTsTKTTKs
sG

462
CONTROLLERS
MW frequency control or AGC problem is that sudden small load perturbation may
continuously disturb the system operation. To maintain system stability the system is
operated with different types of controllers, hence selection of proper controller is very
important so far system stability is concerned. This paper is focused on the effectiveness of
Proportional (P), Integral (I), Proportional-Integral (PI), Proportional-Integral-Derivative
(PID) controller on system dynamics. The transfer function model of PI and PID controller
are as follows –
GC(s) = )2(−−−+
s
K
K i
P [for PI-controller]
GC(s) = )3(−++ D
i
P sK
s
K
K [for PID-controller]
The optimum values of KP, KI & KD are considered as 0.8036, 0.6356 and 0.1832
respectively [4].
SIMULINK RESULTS
Fig. 3: Frequency error in single-stage 3-area system with different controllers
Fig. 4: Frequency error in single-stage 3-area system for different Ri with PID-controllers

463
Fig. 5: Frequency error in single-stage 3-area system with constraints for different controllers
Fig. 6: Frequency error of single stage system with constraints with PID-controllers
Fig. 7: Frequency error of two-stage system without constraints using different controllers

464
Fig. 8: Frequency error of two-stage 3-area system for different values of Ri using PID
controller
Fig. 9: Frequency error in 3-area system without constraints using PID-controllers
Fig. 10: Frequency error in 3-area system with constraints using PID-controllers

465
Fig. 11: Frequency error in two-stage 3-area system with constraints using different
controllers
Fig. 12: Frequency error in two-stage system with constraints for different values of Ri
Fig. 13: Frequency error in single-stage 3-area system with constraints for different values of
Kr

466
Fig. 14: Frequency error in single-stage 3-area system with constraints for different values of
Tr
Fig. 15: Frequency error in two-stage 3-area system with constraints for different values of
Kr
Fig. 16: Frequency error in two-stage 3-area system with constraints for different values of Tr

467
Fig. 17: Frequency error in 3-area system with different values of T using PID controllers
EFFECT OF SPEED REGULATION CONSTANT
This paper demonstrates the effect of speed regulation parameter on dynamic
performances of a multi-area system without deteriorating the system performances. 10%
load perturbation in all area considered for this study. Initially, frequency error ( fi) is
obtained considering Ri of 4% (2Hz/p.u MW) for all areas. The system has been investigated
in two stages-(i) without constraints and (ii) with constraints. Fig.3 (for single-stage) & Fig.7
(for two-stage) and Fig.5 (for single-stage) & Fig.11 (for two-stage) show the responses at
nominal value of Ri for without and with constraints, respectively. Then, value of Ri is
increased gradually step by step from 4% to 8% and responses are shown in Fig.4 (for single-
stage), Fig.8 (for two-stage) and Fig.6 (for single-stage), Fig.12 (for two-stage), respectively.
It is observed form the foregoing results that in both the cases the oscillations are deteriorated
and system approaches to instability.
EFFECT OF CHANGE OF KR
The effects of change of gain of reheat turbine on system dynamics
have been observed. To study the effectiveness of Kr, it is being changed by ±25% of
nominal value (i.e., 0.5) and responses are shown in Fig.13 and 15. It is clearly seen from the
results that the settling time as well as steady state error gets minimized at a faster rate for
higher value of Kr.
EFFECT OF CHANGE OF TR
The effects of change of time constant of reheat turbine on system
dynamics have been observed. To study the effectiveness of Tr, it is being changed by ±25%
of nominal value (i.e., 10 sec) and responses are shown in Fig.14 and 16. It is clearly seen
from the results that the settling time as well as steady state error gets minimized at a faster
rate for lower value of Tr.

468
OBSERVATION
Following points are observed from the SIMULINK results –
1. Fig. 3, 5 and 7 reveals that undershoots and settling time both are reducing using PID
controllers
2. Fig. 4 show with increasing of Ri, system undershoots increases while the settling time is
almost constant
3. Fig. 6 and 12 reveals that system responses exhibits fast oscillations with increases of Ri
and system gradually moves towards instability
4. Dynamic responses get improved when system operating with two-stage turbine compare
to single-stage turbine, Fig.9.
5. The performances of two-stage turbine with all possible constraints is quit better when it
works with PI-controller in lieu of other two controllers, Fig. 10 and 11.
6. The settling time and overshoots both are minimized with increase of reheat turbine gain
(Kr), Fig. 13 and 15.
7. The settling time and overshoots both are minimized with decrease of reheat turbine time
constant (Tr), Fig. 14 and 16.
CONCLUSION
This paper made an attempt to study the effect of system constraints on dynamics of
multi-area interconnected power system. The system gets non-linear with addition of system
constraints. It is seen that dynamics of power system is improved with PID-controller in lieu
of PI or I controllers. It is also concluded from the preceding discussion that system
undershoots increases with physical constraints, especially due to the effect of transportation
lag in communication channels. Neglecting the constraints such as GRC, DB and TD
decreases the efficiency of the controller, hence for getting improved responses these must be
considered. The performance with two-stage turbine is much better than single-stage turbine
working with PID controllers.
REFERENCES
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theory”, IEEE, vol.73, June 1992
[2] J.Nanda et. al., “Sampled data Automatic Generation Control of Hydro-Thermal system
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[3] Ibraheem et. al., “Recent philosophies of Automatic Generation Control strategies in
Power system”, IEEE trans. on Power system, vol.20, no.1, 2005
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[5] J.Nanda et. al, “Automatic Generation Control of a multi-area system with conventional
integral controllers”
[6] K.S.S Ramakrishnan et. al., “Automatic Generation Control of interconnected power
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469
[7] Hemin Golpira et. al., “Effect of physical constraints on AGC dynamic behavior in an
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APPENDIX – 1
Pr = rated power in two areas = 2000MW
f = nominal frequency of operation = 50Hz
T12 = T23 = T31 = synchronizing time constant = 0.086 sec
Bi = biasing factor = 0.425
Kps = gain of power system = 120
Tps = time constant of power system = 20 sec
Tt = turbine time constant = 0.3 sec
Tsg = speed governor time constant = 0.08 sec
Ksg = gain of speed governor = 1
Kr1 = Kr2 = steam turbine reheat constant = 0.5
Tr1 = Tr2 = steam turbine reheat time constant = 10 sec
Ri = speed regulation constant =2 Hz/p.u MW
KI = gain of Integral controller = 0.6356
KP = gain of proportional controller = 0.8036
KD = gain of derivative controller = 0.1832
GRC = Generation rate constraint = 5.0±
DB = Dead band = 0.036 sec
Time delay (TD) in communication channel = 1 sec

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