MUTUAL EFFECT BETWEEN LFC AND AVR LOOPS IN POWER PLANT

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. Sharma

2, Mukta

3

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.

MUTUAL EFFECT BETWEEN LFC AND AVR LOOPS IN POWER PLANT

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