ISSN 2348–2370
Vol.07,Issue.08,
July-2015,
Pages:1276-1284
www.ijatir.org
A Fuzzy Controlled Dual Unified Power Quality Conditioner for Power
Quality Improvement
M. RAMAKRISHNA1, V. PRAVINYA2
1
HOD, Dept of EEE, LORDS Institute of Engineering & Technology, Himayathsagar, Hyderabad, TS, India,
E-mail: mrkn83@gmail.com
2
PG Scholar, Dept of EEE, LORDS Institute of Engineering & Technology, Himayathsagar, Hyderabad, TS, India,
E-mail: vasthalapravinya@gmail.com.
Abstract: Unified Power Quality Conditioner (UPQC) for
harmonic elimination and simultaneous compensation of
voltage and current, which improves the power quality
offered for other harmonic sensitive loads. UPQC consist
of combined series active power filter that compensates
voltage harmonics of the power supply, and shunt active
power filter that compensates harmonic currents of a nonlinear load. UPQC has the capability of improving power
quality at the point of installation on power distribution
systems In this project a simplified control technique for a
dual three-phase topology of a unified power quality
conditioner iUPQC is presented and it is used in the utility
grid connection. Different from a conventional UPQC, the
iUPQC has the series filter controlled as a sinusoidal
current source and the shunt filter controlled as a sinusoidal
voltage source. Therefore, the pulse width modulation
(PWM) controls of the iUPQC deal with a well-known
frequency spectrum, since it is controlled using voltage and
current sinusoidal references for both series and shunt
active filter controls. A fuzzy controlled dual unified power
quality
conditioner
is
implemented
with
MATLAB/SIMULATION software.
protection devices. Hence, the maintenance and improvement
of electric power quality has become an important scenario
today. Even though the power generation is fairly reliable, the
quality of power is not always so reliable. Fuzzy control is
based on fuzzy logic; it is much closer in spirit to human
thinking and natural language than traditional logical systems
application to many industries and residential usages. Hence
supply of reactive power at the load ends becomes essential
[3-5]. Power Quality (PQ) mainly deals with issues like
maintaining a fixed voltage at the Point of Common Coupling
(PCC) for various distribution voltage levels irrespective of
voltage fluctuations, maintaining near unity power factor
power drawn from the supply, blocking of voltage and current
unbalance from passing upwards from various distribution
levels, reduction of voltage and current harmonics in the
system [6-7]. By using a unified power quality conditioner
(UPQC) [1] it is possible to ensure a regulated voltage for the
loads, balanced and with low harmonic distortion and at the
same time draining undistorted currents from the utility grid,
even if the grid voltage and the load current have harmonic
contents.
The UPQC consists of two active filters, the series active
filter
(SAF) and the shunt or parallel active filter (PAF) [1],
Keywords: Active Filters, Control Design, Fuzzy
[2].
The
PAF is usually controlled as a non sinusoidal current
Controller, Power Line Conditioning, Unified Power
source,
which
is responsible for compensating the harmonic
Quality Conditioner (UPQC).
current of the load, while the SAF is controlled as a non
sinusoidal voltage source, which is responsible for
I. INTRODUCTION
compensating the grid voltage. Both of them have a control
Electric Power quality is a term which has captured
reference with harmonic contents, and usually, these
increasing attention in power engineering in the recent
references might be obtained through complex methods [4],
years. Even though this subject has always been of interest
[5], [14], [17].Some works show a control technique to both
to power engineers; it has assumed considerable interest in
shunt and SAFs which uses sinusoidal references without the
the 2003's. The measure of power quality depends upon the
need of harmonic extraction, in order to decrease the
needs of the equipment that is being supplied. What is
complexity of the reference generation of the UPQC. An
good power quality for an electric motor may not be good
interesting alternative for power quality conditioners was
enough for a personal computer. Usually the term power
proposed and was called line voltage regulator conditioner.
quality refers to maintaining a sinusoidal waveform of bus
This conditioner consists of two single-phase current source
voltages at rated voltage and frequency. Electric power
inverters where the SAF is controlled by a current loop and
quality (EPQ) problems mainly include unbalance voltage
the PAF is controlled by a voltage loop. In this way, both grid
and current, flicker, harmonics, voltage sag, dip, swell, and
current and load voltage are sinusoidal, and therefore, their
power interruption [1], [2]. These power quality problems
references are also sinusoidal. Some authors have applied this
may cause abnormal operations of facilities or even trip
concept, using voltage source inverters in uninterruptable
Copyright @ 2015 IJATIR. All rights reserved.
M. RAMAKRISHNA, V. PRAVINYA
power supplies and in UPQC [10]. In [10], this concept is
PAF, diverging only from the way the series and shunt filters
called “dual topology of unified power quality conditioner”
are controlled. In the iUPQC, the SAF works as a current
(iUPQC), and the control schemes use the p−q theory,
source, which imposes a sinusoidal input current synchronized
requiring determination in real time of the positive
with the grid voltage. The PAF works as a voltage source
sequence components of the voltages and the currents. The
imposing sinusoidal load voltage synchronized with the grid
aim of this project is to propose a simplified control
voltage. In this way, the iUPQC control uses sinusoidal
technique for a dual three-phase topology of a unified
references for both active filters. This is a major point to
power quality conditioner (iUPQC) to be used in the utility
observe related to the classic topology since the only request
grid connection. The proposed control scheme is developed
of sinusoidal reference generation is that it must be
in ABC reference frame and allows the use of classical
synchronized with the grid voltage. The SAF acts as high
control theory without the need for coordinate transformers
impedance for the current harmonics and indirectly
and digital control implementation. The references to both
compensates the harmonics, unbalances, and disturbances of
SAF and PAFs are sinusoidal, dispensing the harmonic
the grid voltage since the connection transformer voltages are
extraction of the grid current and load voltage.
equal to the difference between the grid voltage and the load
voltage. In the same way, the PAF indirectly compensates the
II. DUAL UPQC
unbalances, displacement, and harmonics of the grid current,
The conventional UPQC structure is composed of a
providing a low-impedance path for the harmonic load
SAF and a PAF, as shown in Fig. 1. In this configuration,
current.
the SAF works as a voltage source in order to compensate
the grid distortion, unbalances, and disturbances like sags,
III. OUTPUT PASSIVE FILTER DESIGN
swells, and flicker. Therefore, the voltage compensated by
The iUPQC circuit can be analyzed by a single-phase wiring
the SAF is composed of a fundamental content and the
diagram, as shown in Fig. 4. The utility grid impedance is
harmonics. The PAF works as a current source, and it is
represented by Z s  jLs  Rs , while the coupling transformer
responsible
for
compensating
the
unbalances,
leakage impedance is represented by Z s  jLlg  Rlg and the
displacement, and harmonics of the load current, ensuring a
sinusoidal grid current.
voltage sources
and
represent the equivalent
Fig. 1. Conventional UPQC.
Fig. 2. Dual UPQC (iUPQC).
structures of the series and shunt filters, which generate a
waveform composed of the fundamental component and
harmonics that originated from the commutation of the
switches. These high frequencies must be filtered by the
output passive filters of the iUPQC, ensuring sinusoidal grid
currents and load voltages. Fig.5 shows the equivalent circuit
used for the SAF output impedance analysis, and Fig.6 shows
the equivalent circuit used for the PAF output impedance
analysis. In order to simplify the analysis of the PAF, the
voltage source and the inductance, which are series connected,
were considered as a current source. Observing the equivalent
circuits, we can claim that the PAF output impedance affects
the frequency response of the SAF, while the SAF output
impedance does not affect the frequency response of the PAF.
Therefore, the output passive filter design of the iUPQC
should be started with the PAF design followed by the SAF
design. The high-frequency filter transfer function of the PAF
is derived by analyzing the circuit of Fig.6 and is shown in
(1)
The series filter connection to the utility grid is made
through a transformer, while the shunt filter is usually
The inductor was defined by the power design, so the
connected directly to the load, mainly in low-voltage grid
capacitor will be defined according to the desired cutoff
applications. The conventional UPQC has the following
frequency of the filter. In this design, a 2.9-kHz cutoff
drawbacks: complex harmonic extraction of the grid
frequency was used, resulting in a value of 10Μf for the
voltage and the load involving complex calculations,
voltage and current references with harmonic contents
filter capacitor. Fig.7 shows the PAF frequency response for
requiring a high bandwidth control, and the leakage
the nominal load and no load. The high-frequency filter
inductance of the series connection transformer affecting
transfer function of the SAF is derived by analyzing the
the voltage compensation generated by the series filter. In
circuit of Fig.5 and is shown in
order to minimize these drawbacks, the iUPQC is
investigated in this project, and its scheme is shown in Fig.
(2)
2. The scheme of the iUPQC is very similar to the
conventional UPQC, using an association of the SAF and
International Journal of Advanced Technology and Innovative Research
Volume.07, IssueNo.08, July-2015, Pages: 1276-1284
A Fuzzy Controlled Dual Unified Power Quality Conditioner for Power Quality Improvement
voltage imposition on this filter output inductor. The voltage
imposed on these inductors is complementary to the utility
grid voltage harmonics so that it guarantees a sinusoidal
(3)
current through the filter. Different from the conventional
UPQC whose narrow-band frequency control may distort the
load voltage, in the iUPQC, the narrowband frequency control
(4)
may distort the current drained from the utility grid. The usage
of high-power coupling transformers, with low leakage
(5)
inductance, and the design of higher voltage dc link, allowing
As the inductor was defined by the power design, the
the imposition of higher current rate of change on the filter
capacitor will be defined according to the desired cutoff
output inductor, is solutions to change the characteristics of
frequency of the filter. In this design, a 45-Hz cutoff
the filter attenuation in low frequencies
frequency was used, resulting in a value of 1μF for the load
and no load.
Where
Fig.6. Equivalent circuit as viewed by PAF.
Fig.3.Power circuit of the iUPQC.
Fig.7. Control block diagram of the SAF controller.
Fig.4. Single-phase wiring diagram of the dual UPQC.
Fig.5. Equivalent circuit as viewed by SAF.
It can be noted that the filter response has a low cutoff
frequency that can reduce the bandwidth of the SAF,
decreasing its effectiveness under operation with harmonic
contents on the grid voltage. This characteristic of lowfrequency attenuation is undesirable and intrinsic to the
structure due to the leakage impedance of the coupling
transformers. An important contribution of this project and
different from what it was stated in some previous articles,
which deal with the same iUPQC control strategy, is that,
in spite of the SAF operates with sinusoidal reference, the
control of this filter needs to deal with high frequency since
the current imposed by the SAF is obtained through the
IV. PROPOSED CONTROL SCHEME
The proposed iUPQC control structure is an ABC reference
frame based control, where the SAF and PAF are controlled in
an independent way. In the proposed control scheme, the
power calculation and harmonic extraction are not needed
since the harmonics, unbalances, disturbances, and
displacement should be compensated. The SAF has a current
loop in order to ensure a sinusoidal grid current synchronized
with the grid voltage. The PAF has a voltage loop in order to
ensure a balanced regulated load voltage with low harmonic
distortion. These control loops are independent from each
other since they act independently in each active filter. The dc
link voltage control is made in the SAF, where the voltage
loop determines the amplitude reference for the current loop,
in the same mode of the power factor converter control
schemes. The sinusoidal references for both SAF and PAF
controls are generated by a digital signal processor (DSP),
which ensure the grid voltage synchronism using a phase
locked loop.
A. SAF Control
Fig. 7 shows the control block diagrams for the SAF. The
SAF control scheme consists of three identical grid current
loops and two voltage loops. The current loops are responsible
for tracking the reference to each grid input phase in order to
International Journal of Advanced Technology and Innovative Research
Volume.07, IssueNo.08, July-2015, Pages: 1276-1284
M. RAMAKRISHNA, V. PRAVINYA
control the grid currents independently. One voltage loop is
= voltage sensor gain;
responsible for regulating the total dc link voltage, and the
= current sensor gain.
other is responsible for avoiding the unbalances between
The gain is obtained by considering the gain of the multiplier
the dc link capacitors. The total dc voltage control loop has
integrated circuit and the peak of the synchronized sinusoidal
a low-frequency response and determines the reference
signal generated by the DSP.
amplitude for the current loops. Thus, when the load
increases, overcoming the input grid current, the dc link
supplies momentarily the active power consumption,
resulting in a decrease of its voltage. This voltage
controller acts to increase the grid current reference,
aiming to restore the dc link voltage. In the same way,
when the load decreases, the voltage controller decreases
the grid current reference to regulate the dc link voltage.
Considering the three phase input current, sinusoidal and
balanced, the voltage loop transfer function is obtained
Fig. 9. Equivalent circuit of the SAF unbalanced-voltage
through the method of power balance analysis.
loop.
Fig. 8. Equivalent circuit of the SAF voltage loop.
The three-phase four wire converter with neutral point
can be represented by the circuit shown in Fig. 8,
composed of a current source which is in parallel with the
dc link impedance and whose current source represents the
average charge current of the dc link. The resistor
is
absent in the real circuit ( →∞); it just represents
instantaneous active power consumption of the dc link. The
term instantaneous is related to the time of the switching
period, since active power consumption of the dc link is
null for the utility grid voltage frequency. The average
charge current of the dc link is given by
The unbalanced-voltage control loop also has a low
frequency loop and acts on the dc level of the grid current
reference in order to keep the voltage equilibrium in dc link
capacitors. When a voltage unbalance occurs, this loop adds a
dc level to the references of the grid currents, aiming to
equalize both and voltages. The unbalanced-voltage loop
transfer function is obtained through the analysis of the
simplified circuit shown in Fig. 9. The four-wire converter
allows the single-phase analysis, where two current sources
represent the current on the inverter switches. In Fig. 10, the
current i(t) represents the current through the neutral point,
and d(t) represents the duty cycle. Through the mesh analysis
and by applying Laplace, the unbalanced-voltage loop transfer
function is obtained and
(9)
The open-loop transfer function (OLT
) is given by
(10)
(6)
The SAF peak current is considered the same for the
three phases due to balanced current. Through (6), the
voltage loop transfer function is obtained and is
represented by
(7)
Where
Peak of the grid voltage;
Dc link voltage;
Load equivalent resistance;
n
Total dc link equivalent capacitance;
n transformer ratio.
Fig. 10.Single-phase equivalent circuit of SAF.
The open-loop transfer function (OLTFv) is given by
(8)
Where
= multiplier gain;
The current control scheme consists of three identical
current loops, except for the 1200 phase displacements from
the references of each other. The current loops have a fast
response to track the sinusoidal references, allowing the
decoupling analysis in relation to the voltage loop. The current
loop transfer function is obtained through the analysis of the
single-phase equivalent circuit shown in Fig. 10. The voltage
International Journal of Advanced Technology and Innovative Research
Volume.07, IssueNo.08, July-2015, Pages: 1276-1284
A Fuzzy Controlled Dual Unified Power Quality Conditioner for Power Quality Improvement
source represents the voltage on the coupling transformer.
The dynamic model is obtained through the circuit analysis
using average values related to the switching period. Under
these conditions, the voltages (t) and (t) are constants.
Through small signal analysis and by using Laplace, the
current loop transfer function is given by
(11)
Where
(12)
and
Series grid inductance;
Series grid resistance;
Leakage inductance of the coupling transformer;
Series resistance of the coupling transformer.
Fig.12. Single-phase equivalent circuit of PAF.
The voltage loop transfer function is obtained through the
analysis of the single-phase equivalent circuit shown in Fig.
12. The dynamic model is obtained through the circuit
analysis using average values related to the switching period.
Through small signal analysis and by using Laplace, the
voltage loop transfer function is given
(14)
Where
The open-loop transfer function (OLTF) is given by
(15)
Where
shunt filter PWM modulator gain.
Fig. 11. Control block diagram of the PAF voltage loop.
The open-loop transfer function (OLT ) is given by
(13)
Where
series filter pulse width modulation (PWM)
Aiming to track the voltage reference, a proportional
integral derivative (PID)+ additional pole controller was
designed, which ensures a crossover frequency of 4 kHz and a
phase margin of 350. Controller transfer function (HvPF), and
compensated loop transfer function (OLTFvpf + Hvpf) as
shown in Fig.13.
modular gain.
The
gain is equal to the inverse peak value of
the triangular carrier.
Aiming to track the current reference, a PI+pole
controller was designed, which ensures a crossover
frequency of 5 kHz and a phase margin of 70 0. The
frequency response of the current loop is shown in Fig. 11,
including the open-loop transfer function (OLT+),
controller transfer function (OLT+), and compensated loop
transfer function (OLT+).
B. PAF Control
Fig. 12 shows the control block diagram of the shunt
active filter controller. The PAF control scheme is formed
by three identical load voltage feedback loops, except for
the 1200 phase displacements from the references of each
other. The voltage loops are responsible for tracking the
sinusoidal voltage reference for each load output phase in
order to control the load voltages independently.
Fig. 13. Power flow of iUPQC. (a) Vs< V L, (b) Vs >VL .
International Journal of Advanced Technology and Innovative Research
Volume.07, IssueNo.08, July-2015, Pages: 1276-1284
M. RAMAKRISHNA, V. PRAVINYA
Fuzzification Interface: It transforms the crisp input data
V. FUZZY LOGIC CONTROLLER
Fuzzy logic control is deduced from fuzzy set theory;
into fuzzy values that acts as input to fuzzy reasoning process.
which was introduced by Zadeh in 1965. In the fuzzy set
B. Defuzzification
theory concept, the transition is between membership and
The rules of fuzzy logic produce the set of modified
non membership function. Therefore, limits or boundaries
control output in a linguistic variable. The defuzzification
of fuzzy sets are undefined and ambiguous but useful in
module converts these linguistic variables into a crisp value
approximating systems design. In order to implement the
(real number) according to real time applications. The
fuzzy logic control algorithm of an active power line
different methods of defuzzification available are Bisector,
conditioner in a closed loop, the dc-link capacitor voltage
Centroid, Middle of Maximum (MOM), Smallest of
is sensed and compared with the desired reference value.
Maximum (SOM) and Largest of Maximum (LOM), etc.,
The error signal (e(v) Vdc-ref Vdc) passes through a
however, the selection of method is a compromise between
Butterworth low pass filter that allows only the
accuracy and computational intensity (that influences
fundamental component. The voltage error signal e(n) and
hardware requirement for real time application). The centroid
change of error signal ce(n) are used as inputs for fuzzy
(or center of gravity) method is used for simplicity and
processing as shown in Fig.14. The output of the fuzzy
accuracy. The linguistic output variable from the rule
logic controller estimates the magnitude of peak reference
evaluator and definition of output membership are used to
current Imax.
calculate the hidden area. Finally, crisp output is obtained by
using output Ai xi / Ai.
Fig.14.Schematic diagram of the fuzzy logic controller.
The fuzzy logic controller is characterized as follows:
 Seven fuzzy sets (NB, NM, NS, ZE, PS, PM, PB) for
each input and output variables.
 Triangular membership function is used for the
implicity.
 Implication using Mamdani-type min-operator,
Defuzzification using the centroid method.
A. Fuzzification
Fuzzy logic uses linguistic variables instead of
numerical variables. In a closed loop control system, the
error signal e(n) , change of error signal ce(n) and output of
peak reference current Imax are considered as membership
functions. It can be labeled as Negative Big (NB), Negative
Medium (NM), Negative Small (NS), Zero (ZE), Positive
Small (PS), Positive Medium (PM), Positive Big (PB) as
shown in Fig.15. Converting numerical variable (real
number) into a linguistic variable (fuzzy number) is the
process of fuzzification.
Defuzzification Interface: It converts the fuzzy sets obtained
from the inference process into a crisp action that constitutes
the global output of the FRBS. Mamdani based fuzzy logic
interfacing rule is adopted for correction of power factor.
Complex power is taken from power measuring block, in
which power angle is taken as input of fuzzy controller.
According to power angle control output (firing angle) is
provided by fuzzy controller. When power angle is large
firing angle is also large. Controlled output is supplied to
variable delay circuit and it is supplied to thyristor. According
to the output of variable time delay circuit firing angle of
thyristor is changed. When power angle is very small then
firing angle is also very small. When power angle is medium
then firing angle is also medium. When power angle is large
then firing angle is also large.
VI. MATLAB RESULTS
Here simulation is carried out in several cases, proposed
UPQC model is evaluated as voltage sag conditions with
respect to sudden load changes and same proposed concept is
applied to intelligent based fuzzy systems to validate the
optimal results and may increase the robustness of the system
as shown in Figs.16 to 24.
Fig.16. shows the Matlab/Simulink model of proposed
iUPQC model with simplified control scheme using
Fig.15. Membership functions (a) the input variables e
Matlab/Simulink platform.
(n), ce (n) and (b) output variable Imax.
International Journal of Advanced Technology and Innovative Research
Volume.07, IssueNo.08, July-2015, Pages: 1276-1284
A Fuzzy Controlled Dual Unified Power Quality Conditioner for Power Quality Improvement
(a)
Fig.19. PAF Currents.
(b)
(c)
Fig.17. Source Voltage & Source Current.
(a)
Fig.20. SAF Currents.
(b)
(c)
(a) THD Analysis of Load Current
Fig.18. Load Voltage & Load Current.
International Journal of Advanced Technology and Innovative Research
Volume.07, IssueNo.08, July-2015, Pages: 1276-1284
M. RAMAKRISHNA, V. PRAVINYA
(c)
(b) THD Analysis of Load Current
Fig.21 THD Analysis of source current & load current,
without compensation source current is equal to load
current then THD value is 11.79%, when compensation
is performed getting 2.04%, with in IEEE-519
standards.
(d)
(a)
(e)
Fig.22. (a) Source voltages and load during a voltage dip in
phase A. (b) Load voltages and source currents (c) Load
voltages and load currents during a load step from 50% to
100%. (d) Load voltages and load currents during a load
step from 100% to 50%. (e) DC link voltages and load
current during a load step from 100% to 50%.
Fig.23. load voltage with Fuzzy based i upqc compensation
scheme.
(b)
International Journal of Advanced Technology and Innovative Research
Volume.07, IssueNo.08, July-2015, Pages: 1276-1284
A Fuzzy Controlled Dual Unified Power Quality Conditioner for Power Quality Improvement
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VII. CONCLUSION
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International Journal of Advanced Technology and Innovative Research
Volume.07, IssueNo.08, July-2015, Pages: 1276-1284