GAINING IMPROVED PERFORMANCES OF AGC IN A MULTIAREA POWER SYSTEM WITH ALL POSSIBLE CONSTRAINTS BY...

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

1Dipayan Guha,

2P.K.Prasad,

3Somalee Mitra,

4Subhankar Mukherjee

1,3

Assistant Professor, Kanksa Academy of Technology & Management-TIG, Durgapur,

West Bengal 2Principal, Abacus Technology and Management, Hooghly, West Bengal

3Final 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,

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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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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

rrrrrrrT

sTsTsT

TTsTKTTKssG



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

KK i

P [for PI-controller]

GC(s) = )3(−++ Di

P sKs

KK [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

[1] Prof. C.S.Indulkar, “Analysis of MW frequency control problem using sampled data

theory”, IEEE, vol.73, June 1992

[2] J.Nanda et. al., “Sampled data Automatic Generation Control of Hydro-Thermal system

considering GRC-a sensitivity analysis”, IEEE, vol.71, June 1990

[3] Ibraheem et. al., “Recent philosophies of Automatic Generation Control strategies in

Power system”, IEEE trans. on Power system, vol.20, no.1, 2005

[4] J.Nanda et. al., “Some New findings on Automatic Generation Control of an

Interconnected Hydro-Thermal System with Conventional controllers”, IEEE Trans. on

Energy Conversion, Vol. 3, No. 1, 2006

[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

system with diverse sources of power system”, Int. Journal of Engg., Science and

Technology, vol.2, no.5, 2010, pp: 51-65



469

[7] Hemin Golpira et. al., “Effect of physical constraints on AGC dynamic behavior in an

interconnected power system”, Int. Journal of Advanced Mechartronics Syst., vol. 3, no.

2, 2011, pp: 79 – 87

[8] Naimul Hasan et. Al.,‘Real Time Simulation of Automatic Generation Control for

Interconnected Power System’, International Journal on Electrical Engineering and

Informatics ‐ Volume 4, Number 1, March 2012, pp: 40 – 51

[9] Elegerd, O.l., “Eletric energy system theory-an Introduction”, second edition, TMH.

[10] Tan Qian Yi, Gowrishankar Kasilingam and Raman Raghuraman, “Optimal-Tuning of

PID Power System Stabilizer in Simulink Environment for a Synchronous Machine”

International Journal of Electrical Engineering & Technology (IJEET), Volume 4,

Issue 1, 2013, pp. 115 - 123, ISSN Print : 0976-6545, ISSN Online: 0976-6553.

[11] Preethi Thekkath and Dr. G. Gurusamy, “Effect of Power Quality on Stand by Power

Systems”, International Journal of Electrical Engineering & Technology (IJEET),

Volume 1, Issue 1, 2010, pp. 118 - 126, ISSN Print : 0976-6545, ISSN Online: 0976-

6553.

[12] Deepika Yadav, R. Naresh and V. Sharma, “Fixed Head Short Term Hydro Thermal

Generation Scheduling using Real Variable Genetic Algorithm”, International Journal

of Electrical Engineering & Technology (IJEET), Volume 3, Issue 2, 2012,

pp. 430 - 443, ISSN Print : 0976-6545, ISSN Online: 0976-6553.

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

GAINING IMPROVED PERFORMANCES OF AGC IN A MULTIAREA POWER SYSTEM WITH ALL POSSIBLE CONSTRAINTS BY...

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