Hybrid T-I-D and Fuzzy Logic Based SVC Controller for Transient Stability Enhancement

Poonam Singhal. Int. Journal of Engineering Research and Application www.ijera.com
ISSN : 2248-9622, Vol. 7, Issue 6, ( Part -1) June 2017, pp.31-36
www.ijera.com DOI: 10.9790/9622-0706013136 31 | P a g e
Hybrid T-I-D and Fuzzy Logic Based SVC Controller for
Transient Stability Enhancement
Poonam Singhal*, Sakshi Sharma**
*(Associate Professor, Department of Electrical Engineering, YMCA University of Science & Technology,
Faridabad-121006
** (M.tech Student, Department of Electrical Engineering, YMCA University of Science & Technology,
Faridabad-121006
ABSTRACT
This paper presents a new approach to solve the transient stability problem. The conventional PI based SVC
controller has simple functioning and is economical in operation but has sluggish performance with non-linear
characteristics. so, in order to circumvent this problem, fuzzy based T-I-D controller has been designed to
improve the transient stability of 2 machine 3 bus power system using MATLAB/SIMULINK software.
Keywords: FACTS; PI Controller; SVC; Fuzzy Logic Controller; T-I-D Controller.
I. INTRODUCTION
In modern power systems, system stability
issues are of tremendous significance. In order to
operate a system in a safe, reliable state, it is
imperative to maintain its stability. The stable
parallel operation of AC generators and the power
system stability concepts were proposed by the
experts for the very first time in the early decades of
the 20th century. However, those ideas and concepts
presented in the past are not applicable for the
modern power systems. This has subsequently been
reflected in the definition of Stability, realizing this,
the power system stability definition and
classification from a modern point of view has been
systematically introduced here. Also, there exists a
close relationship between reliability, stability and
security. Maintaining the stability of a system to
ensure its reliability; a secure system is always a
result of its reliable operations. Generally, power
system blackouts begin with the loss of stability.
Experts started to learn from these blackouts that in
order to maintain power systems in a safe, reliable
state, the power system operation should emphasize
on the improvement of the system stability through
various strategies. In summary, the term "stability"
imparts as an important concept for the world's
electric power generation, transmission and
distribution systems. The operation of power systems
cannot be conducted without considering it.
A definition of power system stability can
be proposed: Power system stability refers to the
ability that all the synchronous machines in a power
system have to return to the previous equilibrium
states, or move to new steady states, if there is a
disturbance.
It is known that disturbances play a key role
in triggering destabilization in the systems, and may
cause big problems. Short circuits and sudden loss of
power grid element may happen. The continuous
variation of loads can also affect the operation and
control of generators. Therefore, in practice, the
stability of power systems experiences many
disturbances every time, as the environment keeps
varying. If the system is not properly designed or
operated, a small disturbance may lead to a severe
power outage. In such a scenario, FACTS devices
render themselves as very important elements to the
power system. The recent trends suggest that they are
finding increasing emergence throughout the world.
II. SVC DESCRIPTION
In power transmission systems, the
compensation of reactive power is important,
especially in the cases of heavy loads. As large real
power is transferred from power generation units to
loads, the voltage drops in transmission lines leads to
a poor voltage regulation in the transmission line at
receiving end. Therefore, reactive power is needed in
order to maintain the voltage level in the systems, as
well as to ensure stability.
However, it is common that not enough
reactive power is provided to power systems,
especially at remote areas far from power plants.
Sometimes, even adding the reactive compensation
from the transmission line equivalent shunt
capacitors may not be sufficient. Therefore, extra
reactive power compensations are helpful if a power
system operates in an overloaded conditions , or if
the voltage level needs to be adjusted, SVCs provide
appropriate solutions to these problems. SVCs were
developed in 1960s. They are most commonly used
FACTS devices, and reference defines SVC as a
shunt-connected static VAR importer or exporter
RESEARCH ARTICLE OPEN ACCESS

whose output VARs are adjusted to maintain bus
voltage of the electrical power system within a
specific range.
Since SVCs have been used in the power
systems for several decades, the technologies have
been improved, and SVCs have brought about many
uses for power systems. As previously mentioned,
SVCs enlarge the transmission capability by
supporting the line voltage Moreover, when the
power systems are in their steady states, SVCs
mainly eliminate the system voltage variations,
smooth the effects of continuous load changes, damp
oscillations, and the reactive compensation function
can also help to correct power factors to improve the
power quality. Furthermore, for the transient process,
if systems suffer from sudden large disturbances
which lead to severe voltage drops, SVCs react
immediately to provide a strong reactive power
support, enhancing the system voltage stability and
transient stability.
In the active control range, the Susceptance
(Bsvc) and, hence, the reactive current is varied
according to the voltage regulation slope
characteristics shown in Fig. 2.
Fig.1 Basic Structure of SVC [4]
The slope is determined by the desired
voltage regulation. Typically, it varies between 1-5%.
The SVC behaves like a shunt capacitor of maximum
value (BCsvc) at the capacitive limit, and as fixed
shunt reactor at minimum value (BLsvc)
corresponding to the inductive limit. These limits are
reached when there are large variations in the bus 15
voltage. The inductive limit is reached when the bus
voltage exceeds the upper limit, whereas the
capacitive limit is reached when it falls below the
lower limit.
Fig.2 Svc Output Characteristics
The SVC can be represented by its shunt
current injection model. The current injection( ISVC
)into the bus, where the SVC is connected, can be
written as,
SVCSVC jBI 
   }2sin2
1


 C
L
LC
LCSVC
X
X
XX
BBB
where,
C
XLX CL


1
, 
and , SVCB , , LC XX , are the reactive power
injected into the bus due to svc can be expressed as,
are the shunt susceptance, firing angle, inductive
reactance and capacitive reactance of the SVC,
respectively. f 2 , where f is the frequency of
the supply. The reactive power injected into the bus
due to SVC can be expressed as,
2
VBQ SVCSVC 
where, V is the voltage magnitude of the bus at which
the SVC is connected.
III. POWER SYSTEM MODEL
A 2 machine 3 bus power system model in
MATLAB/Simulink is used for the purpose of study.
The power rating of machine 1 is 1000 MVA which
is connected to load center with the help of a 700km
transmission line, operating at 500 KV. Load is of
resistive type of 5000MW, supplied by machine 2 of
rating 5000MVA
Fig.3. Single line diagram a 2 machine 3 bus
system
The current and voltage at bus B2 is
measured and the corresponding data is fed as input
to the SVC controller. Now the role of SVC is to
regulate the value of Voltage at the bus. The
difference between the measured value of the voltage
and the desired value of Susceptance (B p.u.)
obtained as output from the voltage regulator.
Subsequently , firing angle delay is given to the
Thyristors of TCRs(Thyristor Controller Reactors)
and TSCs(Thyristor Switched Capacitors).This
mechanism can be illustrated with the help of figure.
4.The firing angle delay decides the nature of current
injected by the SVC into the system.

Fig.5. T-I-D Controller Application in SVC
IV. FUZZY LOGIC CONTROLLER
Fuzzy set theory renders itself as an
excellent means for representing uncertainty in the
any data or unprecedented behavior of a system. It
represents the human control processes and also
employs experimental knowledge in adjusting the
controller parameters.
FIS (Fuzzy Inference System) consists of the
following components:
1. Input variables
2. Fuzzification through membership functions
3. Rule-base (if/then rules)
4. Defuzzification through membership functions
5. Output variables
Fig.6. Fuzzy Logic Controller Block Diagram
i) Input Variables
Typically a fuzzy controller has at least 2
inputs and one output. In this paper, the inputs to the
fuzzy logic controller are the error in the voltage and
change of error, while its output acts as the control
signal.
Input1: )( smeassvcmeasref XVBVVe 
Input2:
dt
XVBVd
dt
de SmeasSVCref )(( 

ii) Fuzzification
Fuzzification is defined as a conversion of real
input to fuzzy set values. For eg.
NS(x) = {0 if X < -1.0 or X >0;}
= (-0.5 + X)/0.1 if X >= -1.0 and X < -0.5;
= (-X + 0.5)/0.1 if X >= -0.5 and X < 0
= 0 if X > 0 }
Fig.7. Input Membership Function
Different types of parameterized membership
functions commonly used are:
Fig.8. Types of membership functions
iii) Rule Base
The Rule Base is described as a “If Then” format
wherein the ‘If’ side represents the condition and the
‘Then’ side represents conclusions.
Rule1:If error in voltage, e is NB and the error
change
dt
de
is NB then the output is NB.
Rule 2: If error in voltage, e is Z and error change
dt
de
is PB, then the output is PS.
Table.1 Fuzzy Inference Rules
NB=Negative Big , NS=Negative Small,
Z=Zero, PS=Positive Small, PB=Positive Big.
iv) Defuzzification
Output
dt
de
e
NB
NB NS Z PS PB
NB NS NB NS Z
NS NS NB Z Z PS
Z NS Z Z PS PS
PS Z Z PS PS PS
PB Z PS PS PS PB

The objective of Defuzzification is to find
one single crisp value which summarizes the fuzzy
set. There are several mathematical techniques that
are used to perform this.
a. Centroid Method,
b. Centre of Sums,
c. Weighted Average Method
d. Modified Center of Area
e. Mean of Maximum
f. Centre of Gravity
Centroid method is employed where the output will
be calculated as



i
i
i
ib
O

 )(
5
1
Where μi symbolizes the membership function and bi
symbolizes the membership of member i of output
fuzzy set.
V. SIMULATION RESULTS
A three phase fault is created at bus 1 for a
period of 0.1s. from 5 to 5.1 seconds. The designed
controller has a precise control to make the system
stable after the fault is cleared. During the fault when
there is a dip in the bus voltage (high current), svc
injects the reactive var into the transmission line
where it is connected to regulate the voltage. The
MATLAB/Simulink is given below: The Tilt Integral
Derivative controller is a newly designed controller
which has good performance and gives reliable
results over PI based controller. Moreover, TID
compensator has a good transient rejection ratio,
simpler tuning, and a few effect on the closed loop
system response. The first overshoot is reduced using
T-I-D Controller. The deviations in Bus terminal
voltage and speed of generator are also reduced.
Fig.9. MATLAB/Simulink Model for system
Fig.10. Hybrid TID-Fuzzy Logic Controller
Following Simulation Results are obtained using
MATLAB/Simulink Software with the help of
Power System ToolBox.
a) Terminal Voltage at bus 2: Time response of
machine 1 Terminal Voltage at bus 2

Fig.11. Terminal Voltage at bus 2(With Fuzzy Logic
based T-I-D SVC Controller PI Controller ,and
Conventional PI SVC Controller)
b) Speed Deviation between Generator1 and
Generator2: Below shows the Time Response
simulation result of Rotor Speed Oscillations between
Generator1 and Generator 2.
Fig.12. Speed deviation (w2 – w1) (With Fuzzy Logic
based T-I-D SVC Controller PI Controller ,and
Conventional PI SVC Controller)
c)Rotor Angle deviation between Generator1 and
Generator 2:
Time Response of Rotor Angle Oscillations
Fig.13. Rotor Angle deviation between Gen1 and Gen
2 (With Fuzzy Logic based T-I-D SVC Controller PI
Controller ,and Conventional PI SVC Controller)
VI. CONCLUSION
i) Terminal Voltage at bus 2
The first undershoot in Bus Voltage at Bus 2
using TID Controller is 0.945 p.u and that with PID
and Conventional SVC Controller is 0.95p.u and
0.874 p.u. respectively. T-I-D SVC Controller is able
to deal with non-linearity faster than PID Controller.
ii) Speed Deviation between Generator1 and
Generator2
The speed deviation between Generator1
and Generator2 .i.e (w2- w1) radian/seconds are
reduced from 115 radian/seconds to 68
radians/seconds using TID SVC Controller.
iii) Rotor Angle Difference between Generator1
and Generator2
The rotor angle difference for PID
Controller is not as desired and keeps on increasing.
Out of PI Controller and TID Controller the latter has
slightly better response.
VII. FUTURE SCOPE
In this paper, hybrid T-I-D and Fuzzy Logic
Based SVC Controller is tested on a 2 machine 3 bus
SIMULINK model in MATLAB. It can be concluded
that this controller improves the transient stability of
the system by reducing the overshoot in voltage and
by giving a faster response and smaller settling time
than PI and PID Controllers. The performance of the
designed controller is reliable and is quite stable.
Further work will be done on improving the transient
stability of SVC using Genetic Algorithm Technique
to optimize the Gain Constants of T-I-D Controller.
REFERENCES

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Hybrid T-I-D and Fuzzy Logic Based SVC Controller for Transient Stability Enhancement

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Hybrid T-I-D and Fuzzy Logic Based SVC Controller for Transient Stability Enhancement