This document discusses the interaction between the Load Frequency Control (LFC) loop and the Automatic Voltage Regulator (AVR) loop in a power plant. It presents a model that combines the LFC and AVR loops to regulate both frequency and voltage. The model is tested on a single-area power system in simulation. Results show the dynamic and steady-state responses of the combined LFC-AVR system with PID controllers minimize frequency overshoot and transient oscillations while maintaining zero steady-state error.

1. Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 3, No 1, February 2014
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MUTUAL EFFECT BETWEEN LFC AND AVR
LOOPS IN POWER PLANT
Siraparapu.Satyanarayana1
, Prof. R. K. Sharma2
, Mukta3
1, 2, 3
Department of Electrical and Electronics Engineering, Lovely Professional
University, Punjab, India.
ABSTRACT
This paper is proposed to show the mutual interaction between of both LFC and the AVR loops. The
coupling effects of these two AVR and LFC loops are studied by extending the linear zed AGC system and
it include the excitation system also. For a complete system model we have to study the oscillation of LFC
and AVR loops with PID CONTROLLERS. This combined model of LFC and AVR loops is tested on
single-area power system. These results are shown in simulation; and will be reachable in dynamic and
steady state responses [1][2].
KEYWORDS
Load frequency control, Automatic voltage regulation, Automatic Generation Control, PID controllers, Excitation
system
1. INTRODUCTION
In a large scale inter connected power system , one of the most significant problem is both
active and reactive power demands are never be steady and they will be continually changes
with rising and falling trend. The active and reactive power can be maintain constant by
adjusting the speed governor parameters in LFC loop and exciter in AVR loop respectively.
However manual regulation is not feasible in interconnected Power System and therefore
frequency regulation and voltage regulation equipment is installed at the generator. The
coupling of both Load frequency control (LFC) and Automatic voltage regulator (AVR) is
generally known as an Automatic Generation Control (AGC). It deals with frequency through
the LFC loop and with voltage through the AVR loop.
Figure 1. Automatic Generation Control with LFC and AVR loops.
The main purposes of these two controlled loops are to maintain a frequency and voltage at a
acceptable values in a power system. The steam input to the turbine must be continuously
regulated in LFC loop because in order to match the active power demand otherwise the

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machine speed will be vary consequent for changes in frequency. Where as in the AVR loop, the
excitation for the generators must be regulated in order to match the reactive power demand
otherwise the voltages at various system may goes to beyond the prescribed limit. The
maximum permissible of change in frequency is about ± 5% Hz and voltage is about is ± 5% if
not there will be a highly undesirable conditions in the power system like frequency and voltage
fluctuations. So it is necessary to keep the frequency and voltage at constant level.
2. NECESSARY TO MAINTAINING FREQUENCY AND VOLTAGE
CONSTANT
2.1. REASONS TO KEEP THE FREQUENCY AT CONSTANT LEVEL
1) Most of the AC motors requires constant frequency supply in order to maintain constant
speed.
2) In industry, it affects the continuous operation of the process.
3) To maintain a synchronous operation of various units in the Power System, it is
necessary to maintain constant frequency
4) Frequency may also responsible and affects the amount of power transmitted through
interconnected lines.
5) Electrical clocks may lose or gain time when they are driven by synchronous motors.
2.2. REASONS TO KEEP THE VOLTAGE AT CONSTANT LEVEL
1) For lighting load like fluorescent and incandescent lamp are acutely sensitive to the
changes in the voltage.
2) For the induction motor loads, variations in the voltage will affect and change the
torque. Generally in motors torque is directly proportional to the square of the terminal
voltage. For example, if the supply voltage is low then the starting torque of the motor
will be too low.
3) If the voltage variation is very large than a prescribed value then the performance of the
equipments will gets affected and the life span of the equipment may reduces.
4) The picture of the television set will starts rolling due to the voltage are below a
specified level because of the fluorescent tube will not glow at low voltages.
3. LOAD FREQUENCY CONTROL (LFC)
LFC is for regulation of system frequency. It is also called a power factor control loop and
influence the active power balances in the power system network. The main principle for the
mechanism of the speed-governor is to adjust itself as per the load variations and the speed of
the rotor also varies of the synchronous machine and hence the system frequency is also
changes. This change in frequency is sensed by the frequency sensor and compared with a
reference frequency value and produces a feedback signal. [5][6]
We know N = 120 f / P
Therefore N α f
Where, N = Speed in rpm.
f = Frequency in Hz.
P = Number of poles.
This feedback signal can makes the variation of generated power by adjusting the opening of the
steam inlet valve to steam turbine. Hence the real power balance between at the generation

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station and at the load demand is achieved. In the schematic diagram of LFC loop is consist of
two loops. They are Primary control loop and secondary control loop. Primary control loop is
also called as a speed governor control loop. It consists of generator, speed governor and
turbine. Secondary control loop is consists of primary loop as well as with combination of PID
controllers as shown in Figure.2
Figure2. Load Frequency Loop
4. AUTOMATIC VOLTAGE REULATOR
AVR is for regulation of system voltage magnitude by automatically. It is also called a Reactive
power control loop and influence the reactive power balances in the power system network. The
AVR is achieved by the excitation mechanism. The excitation system can control the field
current of the synchronous machine. Hence the field current is controlled so as to regulating the
terminal voltage of a generator. [5][6]
Figure3. Schematic diagram of AVR (or) Exciter
The voltage of the generator is proportional to the speed and excitation of the generator. If we
maintain a speed at constant level, then the excitation system can control the terminal voltage of
the generator. The voltage control is also called as an excitation control system. For the
generator, the excitation is provided by the exciter. Depending upon the methods in which DC

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supply is given to the field winding of the generator, the exciter is classified as a DC exciter,
AC exciter and Static exciter. The stabilizer is used to improve the dynamic response of the high
gain exciter system. [7]
The generator terminal voltage Vt, is compared with a reference voltage Vref to obtain a error
signal as This signal is applied to the exciter as a voltage function as KA / (1+STA). The
regulator output is feed to the exciter as shown in FIG.5 with transfer function of Ke/(1+STe ).
The exciter output Efd is feed to the field windings of the generator which can capable to adjust
the terminal voltage of the generator. The generator field can represented by a transfer function
KF/(1+STF). Hence the voltage is regulated by the exciter. The total transfer given as
Figure 4. Block diagram of AVR with transfer functions
5. PID CONTROLLERS
PID controllers are the combination of Proportional action, Integral action and Derivative
action. These are feedback loop mechanism and widely used in industrial control system. In
LFC loop these are used to stabilization of the frequency and where as in AVR loop
stabilization of the voltages. [3][4]
Figure5. PID controllers

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The transfer function of output of the PID controller is as follows:
G(s) = Kp +( Ki /S) + Kd S
Kp = Proportional gain. It determines the value of the reaction to the present errors.
Ki = Integral time constant. It determines the value of the reaction based on the sum of the
recent errors.
Kd = Derivative time constant. It is used to determine the rate at which the error has been
changes.
The weighted sum of these three actions is used to adjust the process through the control
elements. These controllers are used to correct the error between the measured variables and the
desired set value or the reference value. And hence it keeps the error minimum.
6. AUTOMATIC GENERATION CONTROL
It is the coupling of both LFC and AVR loops and having PID controllers. The real power and
frequency is regulated by LFC loop, where as the reactive power and voltage is regulated by
AVR loop [6]. Therefore, the active power, reactive power, frequency and voltage can be
regulated in AGC method [7]. The block diagram of AGC with LFC and AVR loops as shown
in Figure.7
Figure6. Automatic Generation Control with LFC and AVR with their transfer functions.

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7. SIMULATION RESULTS
Figure7.Simulation block diagram for Automatic Generation Control with LFC and AVR loops.
Table 1. . LFC parameters in per unit value.
KI R TG TT KP TP
1 1.7 0.06 0.32 102 20

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Figure8. Only LFC loop response with a simulation time constant T= 20.0.
Table 2. AVR parameters in per unit value
Figure9. Only AVR loop response with a simulation time constant T= 20.0.

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Figure10 Combined LFC and AVR loop (AGC) response with simulation time constant T=20.
8. CONCLUSION
The quality of power supply is determined by constant of frequency and voltage. The reliable
power supply has the characteristics of minimum frequency deviation and good terminal voltage
response. The terminal voltage and frequency responses of AVR and LFC loops are inter act
with different proportional gains were analyzed. The LFC is used to maintain a zero steady state
error, while the AVR loop is to maintain the machine output voltage with- in a specified time. It
can be concluding that PID controllers are used to minimize the frequency over shoot and
transient oscillations with in a zero steady state error were obtained.
REFERENCES
[1] V. Shanmuga Sundaram: A Fuzzy Approach of Autonomous Power Generating Systems,
Proceeding of 38th SciVerse Science Direct volume 38, 2012, pp. 753-762.
[2] Elyas Rakhshani: A New Combined Model for Simulation of Mutual Effects between LFC and
AVR Loops, Proceeding on Asia-Pacific Power and Energy Engineering Conference, 2009
(APPEEC 2009), Wuhan, China.
[3] Dr.B.U.Musa: Modeling and Simulation of LFC and AVR with PID Controller, Proceeding on
International Journal of Engineering Science Invention ISSN (Online): 2319 – 6734, ISSN (Print):
2319 – 6726, Volume 2, Issue 7, July. 2013, PP.54-57.
[4] M. Deysi: A New Method to Damping of Low Frequency Oscillations, Proceeding on Australian
Journal of Basic and Applied Sciences, pp. 1231-1238, 2011,ISSN 1991-8178.
[5] T R Shyama: Design of FGSPI Controller Based Combined LFC and AVR of Two Area
Interconnected Power Generating System, Proceeding on International Journal of Engineering and
Advanced Technology (IJEAT) ISSN: 2249 – 8958, Volume-1, Issue-4, April 2012.
[6] A.Soundarrajan, Member: Particle Swarm Optimization Based LFC and AVR of Autonomous
Power Generating System, Proceeding on IAENG International Journal of Computer Science, 37:1,
IJCS-37-1-10.
[7] J. Raja, Improved Power System Dynamic Performance Using SMES For Frequency Excursion,
Proceeding on Journal Electrical Systems 7-2 (2011): 193-205.

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Authors’ information
Siraparapu. Satyanarayana was born at, Andhra Pradesh, in India on 1990.He
received his B.Tech in Electrical and Electronics Engineering from Jawaharlal
Nehru Technological University Kakinada, India in 2012.He is currently pursuing
the M.Tech degree in Electrical and Electronics Engineering at the Lovely
Professional University, Punjab, India. His specialisation includes in Power System.
Prof.R.K.Sharma who was the Head of the Department of Electrical and Electronics
and Engineering since from 2002 to till date in Lovely Professional University
(LPU), Punjab, India. His specialisation includes Power System and Power
Electronics
Mukta, Assistant Professor in Lovely Professional University, Phagwara, Jalandhar
from July 2013 in Electrical Engineering. She has done Masters of Engineering
(M.E.) in Power Systems from PEC University of Technology, Chandigarh
“formerly Punjab Engineering College” in 2011-2013.She has done B.E. in electrical
engineering, from CRSCE, Murthal (Haryana) in 2010. She has published 5 papers
in total in various international journals and conferences(2 in International
conferences conducted by IRD, 2 in online International journals-IJSCE,IJEAT, 1 in
national conference conducted by GGGOI, Shahabad). She has got best paper award
for one of her research paper in an International Conference by IRD on 25th
Nov, 2012.