This document discusses using Active Disturbance Rejection Control (ADRC) to improve the control of an Automatic Voltage Regulator (AVR) system for a generator's excitation system. The AVR system aims to maintain the generator's voltage within limits by adjusting the excitation. Initially, the AVR system shows highly oscillatory behavior with large overshoot and long settling time. By adding rate feedback compensation and then decoupling the system with ADRC control, the response is improved with faster rise time, smaller overshoot, and quicker settling. Simulation results show the ADRC approach provides satisfactory stability and zero steady state error under changing loads.

1. Abstract— In this paper, An Active Disturbance Rejection
Control (ADRC) is decupling with the system of Automatic
Voltage Regulator (AVR) of generator excitation system. The
role of the generator excitation system is to maintain
generator voltage and to control the reactive power flow.
When we change the real power that affects mainly on
the frequency, but if we get any change of reactive
power that will affect on the voltage magnitude. The
sources of reactive power are generators, capacitors, and
reactors. The generator reactive powers are controlled
by field excitation. The primary means of generator
reactive power control is the generator excitation control
using automatic voltage regulator (AVR). The role of an
(AVR) is to hold the terminal voltage magnitude of a
synchronous generator at specified level. The simulation
results demonstrate the effectiveness of the designed
system in terms of reduced settling time, overshoot and
oscillations.
I.INTRODUCTION
HE- The objective of the control strategy is to generate
and deliver power in an interconnected system as
economically and reliably as possible while maintaining the
voltage and frequency within permissible limits. The
function of excitation control is to regulate generator
voltage and reactive power output. Figure1. Illustrates the
schematic diagram of LFC and AVR of a synchronous
generator. Reactive power is a concept used to describe the
background energy movement in an Alternating Current
(AC) system arising from the production of electric and
magnitude fields. Power flows, both actual and potential,
must be carefully controlled for a power system to operate
within acceptable voltage limits. Reactive power flows can
give rise to substantial voltage changes across the system,
which means that it is necessary to maintain reactive power
balances between sources of generation and points of
demand. Changes in real power affect mainly the system
frequency, while reactive power is less sensitive to changes
in frequency and is mainly dependent on changes in
voltages magnitude. An increase in the reactive power load
of the generator is accompanied by a drop in the terminal
voltage magnitude. The voltage magnitude is sensed
through a potential transformer on one phase. This voltage
is rectified and compared to a dc set point signal. The
amplified error signal controls the exciter field and
increases the exciter terminal voltage. Thus, the generator
field current is increased, which results in an increase in the
generated emf. The reactive power generation is increased
to a new equilibrium, raising the terminal voltage to the
desired value. Many investigations AVR of an isolated
power system have been reported and a number of control
T
1
Department of Electrical and Computer Engineering, Gannon University,
Erie, PA 16451, USA.
schemes like Proportional and Integral (PI), Proportional,
Integral and Derivative (PID) and optimal control have
been proposed to achieve improved performance I will look
briefly at the simplified models of the component involved
in the AVR system[1-3].
In this paper I am going to use ADRC system to examine
the open loop output and to see the effect of the ADRC
system. ADRC offers a solution where the necessary
modeling information needed for the feedback control
system to function well is obtained through the input-output
data of the plant in real time. Consequently, the control
system can react promptly to the changes either in the
internal dynamics of the plant, or its external disturbances.
As first shown in [5] .
Figure1. Schematic diagram of LFC and AVR of a
synchronous generator.
II.CASE STUDIES
A. A Nonlinear System
The aim of this control is to maintain the system voltage
between limits by adjusting the excitation of the machines.
The automatic voltage regulator senses the difference
between a rectified voltage derived from the stator voltage
and a reference voltage. This error signal is amplified and
fed to the excitation circuit. The change of excitation
maintains the AVR balance in the network.[4]. The
arrangement of AVR is shown in Figure.2.
Figure2. AVR Arrangement.
Reactive Power and Voltage Control of
Generator Excitation system
Ahmed Al Hashim1

2. A. Abbreviations and Acronyms
LFC Load Frequency
Control
AVR Automatic Voltage Regulator
AC Alternating Current
Amplifier Gain
Exciter Gain
Generator Gain
Sensor Gain
Generator Terminal Voltage
Reference Voltage
B. Equations and Calculations
The transfer function of the amplifier model is
(1)
The transfer function of a modern exciter is
(2)
The transfer function relating the generator terminal
voltage to its field voltage can be represented by a gain
and a time constant , and the transfer function is
(3)
The voltage is sensed through a potential transformer and,
in one form, it is rectified through a bridge rectifier. The
sensor is modeled by a simple first order transfer function,
given by
(4)
The open-loop transfer function of the block diagram is
(5)
The closed loop transfer function relating the generator
terminal voltage to the reference voltage is
(6)
Or
(7)
For step input , using the final value theorem,
the steady-state response is
(8)
The PID controller transfer function is
(9)
The open-loop transfer function of the AVR system shown
in Figure 3. is
Figure3. A simplified automatic voltage regulator block
diagram.
When I applied the AVR system in MATLAB, I got the
result as shown in Figure4.
The time-domain performance specifications are
peak time= 0.791sec, Rise time = 0.247 sec, Settling time =
19.04 sec, and Percent overshoot = 82.46%.
From the result, I see that for an amplifier gain =10, the
response is highly oscillatory, with a very large overshoot
and a long settling time. Furthermore, the steady-state error
is over 9 percent and I cannot have a small steady-state
error and a satisfactory transient response at the same time.

3. 0 2 4 6 8 10 12 14 16 18 20
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
t,sec
terminal VoltageStepResponse
Figure4. The output response of AVR system.
B.Linearized Model of the system
The system has only one input and one output as
shown in Figure5.
Figure5. Input and output of the system.
To improve the relative stability, we should introduce a
controller, which would add a zero to the AVR open-loop
transfer function.[4]. One way to do this is to add a rate
feedback to the system as shown in Figure6.
Figure6. Block Diagram of the compensated AVR system.
0 2 4 6 8 10 12 14 16 18 20
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Terminal Voltage Step Response
t,sec
Figure7. The output response of the sysrem.
The step response is shown in Figure7. The time-domian
performance specifications are:
Peak time = 6.08sec, Rise time = 2.95sec, Settling time =
8.08sec, and Percent overshoot = 4.13%.
The result show a very satisfactory trainsent response.
Because this system has single input and single output, (SI-
SO),I can applying the ADRC system by decoupling with
the system that I have. Figure8. Illustrates the decoupling of
the system with ADRC system.
y
f
d y/dt
yo u t
T o W orksp a ce
In1 O ut1
S u b syste m
S te p
x' = A x+B u
y = C x+ D u
S ta te -S p ace
S co p e1
B a n d -Lim ite d
W h ite N oise
Figure8. Decoupling with ADRC system.
0 1 2 3 4 5 6 7 8 9 10
0
2
4
6
8
10
12
t, sec
Terminal Voltage Step Response
Figure9. The output response of the voltage of the system.

4. y
f
dy/dt
yout
T o Workspace
In1 Out1
Subsyste m
S tep
x' = A x+Bu
y = Cx+Du
S tate-S pace
S co pe1
wc^2
Gain2
1/40
Gain1
wc^2
G ain
Band -Lim ited
White Noise
Figure10. Decoupling of the system with ADRC control.
0 1 2 3 4 5 6 7 8 9 10
0
0.2
0.4
0.6
0.8
1
1.2
1.4
t,sec
Terminal Voltage Step Response
Figure11. The ADRC control output of the voltage
response.
III. CONCLUDING REMARKS
The quality of the power supply is determined by the
constancy of frequency and voltage. Minimum frequency
deviation and good terminal voltage response are the
characteristics of a reliable power supply. The conventional
controllers used for this problem have large settling time,
overshoot and oscillations. Hence, when evolutionary
algorithms are applied to control system problems, their
typical characteristics show a faster and smoother response.
An intelligent technique has been proposed for combined
voltage and frequency control in an isolated power system.
The proposed ADRC control approach provides a
satisfactory stability between frequency overshoot and
transient oscillations with zero steady state error. The
simulation results demonstrate the effectiveness of the
proposed controller under changing loads and regulations.
APPENDIX: THE DESCRIPTION OF VARIABLES FOR AVR [1]
The AVR system Parameters
Gain Time Constant
Amplifier = 0.1
Exciter = 1 = 0.4
Generator = 1 = 1.0
Sensor = 1 = 0.005
Acknowledgment
I would thank Dr.Zeng who encourages me to do
this project.
REFERENCES
[1] H.D. Mathur and S.Ghosh, “A comprehensive analysis
of intelligent control for load frequency control”, IEEE
Power India conference, 2006.
[2] D.M.Vinod Kumar, “Intelligent Controllers for
Automatic Generation
Control”, Proc. of IEEE region 10 International
conference on global connectivity in Energy,
Computer, Communication and Control, 1998, pp557-
574.
[3] P.Kundar, “Power System Stability and Control”, Tata
Mcgraw
Hill,Newyork, 1994.
[4] Hadi Saadat, Power system analysis, McGraw
Hill,2004.
[5] Y. Huang, K. Xu, J. Han, and J. Lam, “Flight
Control Design Using Extended State Observer and
Non- Smooth Feedback,” Proceedings of the 40th
IEEE Conference on Decision and Control, pp. 223-
228, 2001.