Iaetsd reactive power compensation using fuzzy

Reactive Power Compensation using Fuzzy
Controlled SVC for Transmission Line
M.Ratan Kumar Dr. G.V. Siva Krishna Rao
Dept.of Electrical Engineering Professor
Andhra University College of Engineering (A) Dept.of Electrical Engineering
Visakhapatnam, Andhra Pradesh India Andhra University College of Engineering (A)
ratan241@gmail.com Visakhapatnam, Andhra Pradesh, India
gvskrishna_rao@yahoo.com
1 Abstract- Flexible AC transmission system (FACTS) is a
technology, which is based on power electronic devices, used to
enhance the existing transmission capabilities in order to make
the transmission system flexible and independent operation. The
FACTS technology is a promising technology to achieve complete
deregulation of Power System i.e. Generation, Transmission and
Distribution as complete individual units. The loading capability
of transmission system can also be enhanced nearer to the
thermal limits without affecting the stability. Complete close-loop
smooth control of reactive power can be achieved using shunt
connected FACTS devices. Static VAR Compensator (SVC) is
one of the shunt connected FACTS device, which can be utilized
for the purpose of reactive power compensation. Intelligent
FACTS devices make them adaptable and hence it is emerging in
the present state of art. This paper attempts to design and
simulate the Fuzzy logic control of firing angle for SVC in order
to achieve better, smooth and adaptive control of reactive power.
The design, modeling and simulations are carried out for 750 k.m
Transmission line and the compensation is placed at the receiving
end (load end).
Index Terms- Fuzzy Logic, FACTS and SVC.
I. INTRODUCTION
he reactive power generation and absorption in power system
is essential since the reactive power is very precious in
keeping the voltage of power system stable. The main
elements for generation and absorption of reactive power are
transmission line, transformers and alternators. The
transmission line distributed parameters through out the line,
on light loads or at no loads become predominant and
consequently the line supplies charging VAR (generates
reactive power). In order to maintain the terminal voltage at
the load bus adequate, reactive reserves are needed. FACTS
devices like SVC can supply or absorb the reactive power at
receiving end bus or at load end bus in transmission system,
which helps in achieving better economy in power transfer.
In this paper Transmission line 750 k.m is simulated using
4π line segments by keeping the sending end voltage constant.
The receiving end voltage fluctuations were observed for
different loads. In order to maintain the receiving end voltage
constant, shunt inductor and capacitor is added for different
loading conditions. SVC is simulated by means of fixed
capacitor and thyristor controlled reactor (FC-TCR) which is
placed at the receiving end. The firing angle control circuit is
designed and the firing angles are varied for various loading
conditions to make the receiving end voltage equal to sending
end voltage. Fuzzy logic controller is designed to achieve the
firing angles for SVC such that it maintains a flat voltage
profile. All the results thus obtained, were verified and were
utilized in framing of fuzzy rule base in order to achieve better
reactive power compensation for the 750 k.m Transmission
line. Based on observed results for load voltage variations for
different values of load resistance, inductance and capacitance
a fuzzy controller is designed which controls the firing angle
of SVC in order to automatically maintain the receiving end
voltage constant.
II. OPERATING PRINCIPLES AND MODELING OF SVC
An elementary single phase thyristor controlled reactor [1]
(TCR) shown in Fig.1 consists of a fixed (usually air core)
reactor of inductance L and a two anti parallel SCRs. The
device brought into conduction by simultaneous application of
gate pulses to SCRs of the same polarity. In addition, it will
automatically block immediately after the ac current crosses
zero, unless the gate signal is reapplied. The current in the
reactor can be controlled from maximum (SCR closed) to zero
(SCR open) by the method of firing delay angle control. That
is, the SCR conduction delayed with respect to the peak of the
applied voltage in each half-cycle, and thus the duration of the
current conduction interval is controlled. This method of
current control is illustrated separately for the positive and
negative current cycles in Fig.2 where the applied voltage V
and the reactor current iL(α) at zero delay angle (switch fully
closed) and at an arbitrary α delay angle are shown. When α
=0, the SCR closes at the crest of the applied voltage and
evidently the resulting current in the reactor will be the same
T
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ISBN:378-26-138420-0292

as that obtained in steady state with a permanently closed
switch. When the gating of the SCR is delayed by an angle α
(0 ≤ α ≤ π/2) with respect to the crest of the voltage, the
current in the reactor can be expressed [1] as follows
V(t) = V cos ωt. (1)
iL = (1/L) α∫ωt
V(t)dt = (V/ωL)(sin ωt –sin α) (2)
Since the SCR, by definition, opens as the current reaches
zero, is valid for the interval α ≤ ωt ≤ π–α. For subsequent
negative half-cycle intervals, the sign of the terms in equation
(1) becomes opposite.
In the above equation (1) the term (V/ωL) sin α = 0 is offset
which is shifted down for positive and up for negative current
half-cycles obtained at α = 0, as illustrated in Fig.2. Since the
SCRs automatically turns off at the instant of current zero
crossing of SCR this process actually controls the conduction
intervals (or angle) of the SCR. That is, the delay angle α
defines the prevailing conduction angle σ (σ = π-2α). Thus, as
the delay angle α increases, the corresponding increasing
offset results in the reduction of the conduction angle σ of the
SCR, and the consequent reduction of the reactor current. At
the maximum delay of α = π /2, the offset also reaches its
maximum of V/ωL, at which both the conduction angle and
the reactor current becomes zero. The two parameters, delay
angle α and conduction angle σ are equivalent and therefore
TCR can be characterized by either of them; their use is
simply a matter of preference. For this reason, expression
related to the TCR can be found in the literature both in terms
of α and σ [1].
Fig. 1. Basic Thyristor Controlled Reactor
Fig.2. firing delay angle
Fig. 3. Operating waveforms
It is evident that the magnitude of the current in the reactor
varied continuously by delay angle control from maximum
(α=0) to zero (α=π/2) shown in Fig.3, where the reactor
current iL(α) together with its fundamental component iLF(α)
are shown at various delay angles α [1]. However the
adjustment of the current in reactor can take place only once in
each-half cycle, in the zero to π/2 interval [1]. This restriction
result in a delay of the attainable current control. The worst-
case delay, when changing the current from maximum to zero
(or vice versa), is a half-cycle of the applied ac voltage. The
amplitude ILF (α) of the fundamental reactor current iLF(α) can
be expressed as a function of angle α [1].
ILF (α) = V/ωL (1 – (2/π) α – (1/π) sin (2α)) (3)
Where V is the amplitude of the applied voltage, L is the
inductance of the thyristor-controlled reactor and ω is the
angular frequency of the applied voltage. The variation of the
amplitude ILF (α), normalized to the maximum current ILFmax,
(ILFmax= V/ωL), is shown plotted against delay angle α shown
in Fig.4.
Fig.4. Amplitude variation of the fundamental TCR current with the delay
angle (α)
It is clear from Fig.4 the TCR can control the fundamental
current continuously from zero (SCR open) to a maximum
(SCR closed) as if it was a variable reactive admittance. Thus,
an effective reactance admittance, BL(α), for the TCR can be
defined. This admittance, as a function of angle α is obtained
as:
BL(α)=1/ωL(1–(2/π)α–(1/π)sin(2α))
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ISBN:378-26-138420-0293

(4)
Evidently, the admittance BL(α) varies with α in the same
manner as the fundamental current ILF(α).The meaning of
equation (4) is that at each delay angle α an effective
admittance BL(α) can be defined which determines the
magnitude of the fundamental current, ILF(α), in the TCR at a
given applied voltage V. In practice, the maximal magnitude
of the applied voltage and that of the corresponding current
limited by the ratings of the power components (reactor and
SCRs) used. Thus, a practical TCR can be operated anywhere
in a defined V-I area, the boundaries of which are determined
by its maximum attainable admittance, voltage and current
ratings as illustrated in the Fig.5a. The TCR limits are
established by design from actual operating requirements. If
the TCR switching is restricted to a fixed delay angle, usually
α = 0, then it becomes a thyristor switched reactor (TSR). The
TSR provide a fixed inductive admittance and thus, when
connected to the ac system, the reactive current in it will be
proportion to the applied voltage as the V - I plot in the Fig.5b.
A basic VAR generator arrangement using a fixed
capacitor with a thyristor-controlled reactor (FC-TCR) shown
in Fig.6 [1].The current in the reactor is varied by the
previously discussed method of firing delay angle control. A
filter network that has the necessary capacitive impedance at
the fundamental frequency to generate the reactive power
required usually substitutes the fixed capacitor in practice,
fully or partially, but it provides low impedance at selected
frequencies to shunt the dominant harmonics produced by the
TCR.
The fixed capacitor thyristor-controlled reactor type
VAR generator may be considered essentially to consist of a
variable reactor (controlled by a delay angle α) and a fixed
capacitor. With an overall VAR demand versus VAR output
characteristic as shown in Fig.7 in constant capacitive VAR
generator (Qc) of the fixed capacitor is opposed by the variable
VAR absorption (QL) of the thyristor controlled reactor, to
yield the total VAR output (Q) required. At the maximum
capacitive VAR output, the thyristor-controlled reactor is off
(α= 900
). To decrease the capacitive output, decreasing delay
angle α. At zero VAR output increases the current in the
reactor, the capacitive and inductive current becomes equal
and thus the capacitive and inductive VARs cancel out. With a
further decrease of angle α, the inductive current becomes
larger than the capacitive current, resulting in a net inductive
VAR output. At zero delay angle, the thyristor-controlled
reactor conducts current over the full 180o
interval, resulting in
maximum inductive VAR output that is equal to the difference
between the VARs generated by the capacitor and those
absorbed by the fully conducting reactor.
Fig.6. basic FC-TCR type static generator
Fig.8. V-I characteristics of the FC-TCR type VAR Generator
In Fig.8 the [1] voltage defines the V-I operating area of the
FC-TCR VAR generator and current rating of the major power
components. In the dynamic V-I Characteristics of SVC along
with the Load lines showed in the Fig.9[1] the load
characteristics assumed straight lines for Dynamic studies as
easily seen that the voltage improved with compensation when
compared without compensation.
Fig.9. Dynamic V-I Characteristics of SVC with Load lines
VCmax = voltage limit of capacitor
BC = admittance of capacitor
VLmax = voltage limit of TCR
ICmax = capacitive current limit
ILmax = inductive current limit
BLmax = max inductive admittance
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ISBN:378-26-138420-0294

III. Fuzzy Logic Controller
Fuzzy logic is a new control approach with great potential
for real time applications [2] [3]. Fig.10 shows the structure
of the fuzzy logic controller (FIS-Fuzzy inference system) in
MATLAB Fuzzy logic toolbox. [5][6].Load voltage and load
current taken as input to fuzzy system. For a closed loop
control, error input can be selected as current, voltage or
impedance, according to control type [7]. To get the linearity
triangular membership function is taken with 50% overlap.
The output of fuzzy controller taken as the control signal and
the pulse generator provides synchronous firing pulses to
thyristors as shown in fig.11. The Fuzzy Logic is a rule based
controller, where a set of rules represents a control decision
mechanism to correct the effect of certain causes coming from
power system [8] [9]. In fuzzy logic, the five linguistic
variables expressed by fuzzy sets defined on their respective
universes of discourse. Table-I shows the suggested
membership function rules of FC-TCR controller. The rule of
this table can be chosen based on practical experience and
simulation results observed from the behavior of the system
around its stable equilibrium points.
Fig.10. Structure of fuzzy logic controller
Fig.11. Single Phase equivalent circuit and fuzzy logic control structure of
SVC
Table I. Membership function rules
Load voltage
Load
current
NL NM P PM PB
NL PB PB NM NM NL
NM PB PB NM P NL
P P PM NM NM P
PM NM P NM NM PM
PB NL NM NM NL NL
IV. HARDWARE IMPLEMENTATION
An available simple two-bus artificial transmission (λ/8)
line model of 4π line segments with 750 km, distributed
parameters were used in this study. The line inductance 0.1mH
/km, capacitance 0.01µf/km and the line resistance 0.001
were used. Each π section is of 187km, 187km, 188km and
188 km. Supply voltage is 230V - 50 Hz having source
internal resistance of 1 connected to node A. Static load is
connected at receiving end B .The load resistance was varied
to obtain the voltage variations at the receiving end. A shunt
branch consisting of inductor and capacitor is added to
compensate the reactive power of transmission line. With the
change of load and due to Ferranti effect, the variations in
voltages are observed at receiving end B of transmission line
[9] [10]. The practical values of shunt elements are varied for
different loading conditions to get both sending and receiving
end voltages equal. As shown in Table II.
Table II compensated practical values of inductor and capacitor
A. FIRING CIRCUIT DESIGN
IC TCA 785 a 16 pin IC shown in Fig.12 is used in this
study for firing the SCRs. This IC having output current of
250 mA and a fuzzy logic trainer kit with two input variables
and having 5 linguistic sets is used. This can generate 5 X 5
rules. The output of fuzzy logic which varies from DC -10V to
+10V is given to IC 785 controller pin11, which controls the
comparator voltage VC ,and the firing angle α for one cycle
and (180 +α) during negative cycle shown in fig.13
S.NO Load
Resistance
Compensatin
g Inductance
Compensatin
g
Capacitance
(µf)
1. 500 0.8 H 1
2. 400 0.9H 1
3. 300 0.19H 2
4. 200 0.18 H 5
5 150 0.19H 5
6 100 0.22H 8
7 50 0.14 8.5
8 40 0.14 9.0
9 30 0.14 10
10 20 0.14 12
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ISBN:378-26-138420-0295

Fig.12. Firing Scheme with TCA 785 IC
Fig.13.Generation of wave forms of TCA 785 IC
TEST RESULTS
The transmission line without any compensation was not
satisfying the essential condition of maintaining the voltage
within the reasonable limits. The effect of increasing load was
to reduce the voltage level at the load end. At light loads, the
load voltage is greater than the sending end voltage as the
reactive power generated is greater than absorbed. At higher
loads the load voltage drops, as the reactive power absorbed is
greater than generated, as shown in Table III. Fig.14 and
Fig.15 indicates unequal voltage profiles. Fig.16 clearly
showsthe firing angle and inductor current control.
Table III Load voltage before and after compensation
Tr Line
Parameters
for
Lt=.10mh/k
m
Ct
=0.1µf/km
R.= 0.001
Before
compensation
For Vs= 230V
(p-p)
After
Compensation
For L= 0.19H
C= 8µ f
For Vs= 230 (P-P)
R VS
(rms)
Volts
VR
(rms)
Volts
IR
rms
Amp
VR
(rms)
Volts
IR
(rms)
Amp
α.
500 162.6 270.8
0
0.54 162.1 2.032 90
400 162.6 268.1
0
0.67 162.4 2.036 100
300 162.6 268.0
0
0.89 162. 2.099 102
200 162.6 261.1
0
1.30 162.7 2.182 103
180 162.6 258.1
0
1.43 162.4 2.198 105
160 162.6 256.1
0
1.59 162.3 2.232 106
140 162.6 250.3
0
1.78 162.8 2.299 108
120 162.6 243.8
0
2.03 161.8 2.357 109
100 162.6 234.2
0
2.34 162.4 2.459 112
80 162.6 219.5
0
2.74 163.3 2.651 117
60 162.6 195.8
0
3.26 162.3 3.071 128
50 162.6 156.5
0
3.91 162.5 4.124 158
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ISBN:378-26-138420-0296

Fig.14. Uncompensated voltages for heavy loads
Fig.15. Uncompensated voltage for light load
FIG.16. INDUCTOR CURRENT FOR FIRING ANGLE 165
CONCLUSION
This paper presents an “online Fuzzy control scheme for
SVC”and it can be concluded that the use of fuzzy controlled
SVC (FC-TCR) compensating device with the firing angle
control is continuous, effective and it is a simplest way of
controlling the reactive power of transmission line. It is
observed that SVC device was able to compensate both over
and under voltages. Compensating voltages are shown in
Fig.17 and Fig.18. The use of fuzzy logic has facilitated the
closed loop control of system, by designing a set of rules,
which decides the firing angle given to SVC to attain the
required voltage. With MATLAB simulations [4] [5] and
actual testing it is observed that SVC (FC-TCR) provides an
effective reactive power control irrespective of load variations.
Fig.17. Compensated VR =VS (RMS voltage) for R=200
Fig.18. Compensated VR=VS (instantaneous voltage) For R=200
REFERENCES
[1] Narain. G. Hingorani, “Understanding FACTS, Concepts and
Technology Of flexible AC Transmission Systems”, by IEEE Press
USA.
[2] Bart Kosko, “Neural Networks and Fuzzy Systems A Dynamical
Systems Approach to Machine Intelligence”, Prentice-Hall of India New
Delhi, June 1994.
[3] Timothy J Ross, “Fuzzy Logic with Engineering Applications”,
McGraw-Hill, Inc, New York, 1997.
[4] Laboratory Manual for Transmission line and fuzzy Trainer Kit Of
Electrical Engineering Department NIT Warangal
[5] SIM Power System User Guide Version 4 MATLAB Manual
Periodicals and Conference Proceedings:
[6] S.M.Sadeghzadeh M. Ehsan “ Improvement of Transient Stability Limit
in Power System Transmission Lines Using Fuzzy Control of FACTS
Devices ,IEEE Transactions on Power System Vol.13 No.3 ,August
1998
[7] Chuen Chien Lee “Fuzzy Logic in Control Systems: Fuzzy Logic
Controller”. Part I and Part II. IEEE R. IEEE transactions on system,
man ,and cybernetics ,vol.20 March/April11990
[8] A.M. Kulkarni, “Design of power system stabilizer for single-machine
system using robust periodic output feedback controller”, IEE
Proceedings Part – C, Vol. 150, No. 2, pp. 211 – 216, March 2003.
Technical Reports: Papers from Conference Proceedings unpublished):
[9] U.Yolac,T.Talcinoz Dept. of Electronic Eng.Nigde 51200,Turkey
“Comparison Compariiison of Fuzzy Logic and PID Controls For TCSC
Using MATLAB”
[10] Jaun Dixon ,Luis Moran, Jose Rodrfguz ,Ricardo Domke “Reactive
power compensation technology state- of- art- review”(invited paper)
Electrical Engineering Dept Pontifica Universidad Catolica De CHILE.
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Iaetsd reactive power compensation using fuzzy

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