Shunt Hybrid Active Power Filter under Non ideal

International Journal of Electronics
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Shunt Hybrid Active Power Filter under Non ideal
Voltage Based on Fuzzy Logic Controller
Papan Dey & Saad Mekhilef
To cite this article: Papan Dey & Saad Mekhilef (2016): Shunt Hybrid Active Power Filter under
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Download by: [University of Malaya]
Date: 21 January 2016, At: 19:50
Publisher: Taylor & Francis
Journal: International Journal of Electronics
DOI: 10.1080/00207217.2016.1138515
Shunt Hybrid Active Power Filter under Non ideal Voltage Based on
Fuzzy Logic Controller
Corresponding Author:
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Papan Dey
Power Electronics &Renewable Energy Research Laboratory (PEARL), Dept. of
Electrical Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia;
E-mail: [email protected]
Contact no. 0060143247702
Saad Mekhilef
Senior member, IEEE, Power Electronics &Renewable Energy Research
Laboratory (PEARL), Dept. of Electrical Engineering, University of Malaya, Kuala
Lumpur 50603, Malaysia;
Shunt Hybrid Active Power Filter under Non ideal Voltage Based on
Fuzzy Logic Controller
In this paper a synchronous reference frame (SRF) method based on a modified
phase lock loop (PLL) circuit is developed for a three phase four wire shunt
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hybrid active power filter (APF). Its performance is analyzed under unbalanced
grid conditions. The dominant lower order harmonics as well as reactive power
can be compensated by the passive elements whereas the active part mitigates the
remaining distortions and improves the power quality. As different control
methods show contradictory performance, Fuzzy logic controller is considered
here for DC-link voltage regulation of the inverter. Extensive simulations of the
proposed technique are carried out in a MATLAB-SIMULINK environment. A
laboratory prototype has been built on dSPACE1104 platform to verify the
feasibility of the suggested SHAPF controller. The simulation and experimental
results validate the effectiveness of the proposed technique.
Keywords: Phase lock loop; Fuzzy logic controller; Harmonic compensation;
Synchronous Reference frame (SRF) method; Total harmonic distortion (THD)
1. Introduction
The presence of current harmonics, unbalanced loading, reactive power and excessive
neutral current play an important role in deciding power quality in a three-phase fourwire distribution system. These harmonics interfere with sensitive electronic equipment
and cause undesired power losses in electrical equipment. In order to solve the power
quality problem and to deliver clean power several methods have been proposed and
developed by the researchers (El-Habrouk, Darwish, & Mehta, 2000).
Passive filters that work as the least impedance path to tune harmonic frequencies, are
simple and less expensive but have several drawbacks including fixed compensation,
they are bulky devices and the resonance problem of the L-C filters; however, APF has
been developed for complete compensation of distortions (Das, 2004). Owing to the
rapid improvement in power semiconductor device technology, active Due to the
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absence of any resonance problem in the system, APF has attracted favorable feedback
compared to tuned passive filters. The APFs use power electronic converters that insert
harmonic components into the electrical network that cancel out nonlinear load
harmonic currents. With the harmonic compensating ability, APFs are also capable of
reactive power reduction and balancing the unbalanced load (Patnaik & Panda, 2013).
Although different types of active filters have been proposed to increase the electric
system quality, pure APF is limited due to its high cost and rating constraints by power
devices. To make the active filter smaller, cheaper and more practicable in industrial
applications, a hybrid configuration of the APF appears very attractive in power
distribution networks. However, series hybrid APF is complex, unreliable and
expensive. Therefore, a shunt hybrid filter topology is characterized by a combination of
a parallel-connected passive filter and a small rated active filter. This has been
considered in this paper for its capability of attenuation and effective operation. Many
harmonic revealing control concepts and approaches for active filters such as fast
Fourier transform (FFT); recursive discrete Fourier transform (RDFT); instantaneous
active and reactive power (p–q); direct testing and calculating (DTC) method;
instantaneous active and reactive current component (id–iq); notch filter, sine
multiplication, generalized integral, synchronous detection algorithm; adaptive filter,
flux based controller; artificial neural network (ANN); adaptive linear neuron
(ADALINE); wavelet transform; particle swarm optimization (PSO); and genetic
algorithm (GA) techniques have been proposed in have been suggested in the literature
(Corasaniti, Barbieri, Arnera, & Valla, 2009). Among these control schemes, SRF is one
of the most conventional and practically applicable method (Mikkili & Panda, 2012).
Although it performs an excellent job, it requires a PLL circuit for synchronization.
Different types of PLL have been introduced in the literature; however, the conventional
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types of PLL have low performance in extremely unbalanced and distorted networks. So
this paper is aimed at the competency of the SRF method with the modified PLL. An
additional essential task in active filters development is to maintain a constant DC link
voltage across the capacitor which is attached to the inverter. The energy loss due to the
switching power and conduction losses associated with the IGBTs and diodes of the
inverter in APF must be compensated that reduce the DC capacitor voltage. Normally,
to control the DC link voltage a PI controller is used. However, precise linear
mathematical models are required for the tuning of the PI controller. It is difficult to
obtain and cannot give satisfactory performance under load disturbances and parameter
variations (Ponpandi & Durairaj, 2011).In addition, in this conventional method, the
shaping of source current is realized with a good amount of spikes. Recently, a great
deal of interest has focused on Fuzzy logic controllers regarding their application to
APFs. Fuzzy logic is considered more advantageous than conventional controllers as
there is no requirement of an accurate mathematical model, they are capable of working
with imprecise inputs, handling nonlinearity and are more robust (Suresh, Panda, &
Suresh, 2011).
In this paper, a new technique has been developed on SRF using the PLL algorithm and
its performance is analyzed and demonstrated for unbalanced source conditions. Here, a
Hysteresis current regulator is used to generate switching signals of APF, and for
maintaining the DC link capacitor a voltage fuzzy logic controller is introduced for
robustness and superior performance compared to the conventional PI controller.
The structure of this paper is as followed: The hybrid APF is discussed in section 2.
Section 3 describes the control algorithm of APF. The simulation results and a brief
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analysis are presented in section 4. In section 5 experimental results are shown. Finally,
conclusions are drawn in section 6.
2. Hybrid APF topology
The proposed hybrid filter structure shown in Figure1 comprises a shunt passive filter
and shunt active filter. A 3-leg inverter with split capacitor works as the APF. It is
designed to be associated in parallel with the single phase and three phase loads that are
considered as a non-linear and unbalanced load for a 3-phase 4-wire distribution system
while its complex control circuitry and massive DC link capacitors are necessary for
perfect operation. The inner point of every branch is attached to the power network
through an inductor, which is used to filter the ripples of inverter current. The
considered LC passive filter at the 5th harmonic tuned frequency is connected in shunt to
the power line before the load. This provides a low impedance trap for harmonics to
which the filter is tuned, and, correspondingly, aids in reduction of the active filter
power rating.
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Figure 1. Schematic diagram of 3-phase 4-wire SHAPF
3. Proposed Control algorithm
3.1 Modified PLL Operation
In order to extract the voltage peak and instantaneous phase information the best way is
to convert the three phase unbalanced voltage in a two 2-D stationary frame. If voltage
is perfectly balanced and this transformed voltage vector is drawn on a q-d frame, a
circular vector rotating at speed equal to the frequency is seen. However, for the case of
an unbalanced vector, there are two vectors, one corresponding to positive sequence
rotating in one direction and the other because of the negative vector rotating in the
opposite direction. Thus, if a transformed unbalanced voltage vector in the d-q frame is
drawn, an eclipse is formed as a resultant of both positive and negative sequence
components. Now, suppose the frequency and phase information are already known and
taking an arbitrary frame that is rotating in the same direction as that of a positive
sequence component and at a speed equal to the frequency, then a positive sequence
component appears as a stationary and negative sequence component that seems to be
rotating at a speed twice that of the frequency. For safe and consistent operation of an
APF in an unbalanced and distorted grid voltage situation, the frequency and phase
extraction of the positive sequence component of the voltage should be obtained quickly
and accurately. Because of the dynamic behavior, conventional SRF-PLL performance
is unsatisfactory under non-ideal voltage mains (Kesler & Ozdemir, 2011). The
improved PLL developed in this study is shown in Figure 2, which is put forward for
the determination of positive sequence components with stability and rapidity. For nonideal mains voltage:
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V a  V a 
 +
Vsabc = V b  = V b  +
 
V c  V c + 
V a − 
 −
V b  +
 _
V c 
V a 0 
V 
 b0 
V c 0 
= V Sin(θ − 2π / 3) + V Sin(θ + 2π / 3) + Vo
Sin(θ − 2π / 3 
Sin(θ + 2π / 3)
transform, the voltage vectors are:
Vα 
Vαβ =   = [Tαβ ]V abc
V β 
[ T αβ
]= 2 
2 
3 
2 
Vα   V + Sinθ + + V + Sinθ − 
Vαβ =   =  +
Vβ  − V Cosθ + V Cosθ 
Performing d-q transform:
V d 
V dq =   = [T dq ]V αβ
V q 
=  Cos θ
 − Sin θ
Sin θ ^   V + Sin θ + + V + Sin θ − 
Cos θ ^   − V + Cos θ + + V − Cos θ − 
=  V Sin(θ − θ ) + V Sin(θ + θ ) 
 − V Cos(θ − θ ) + V Cos(θ + θ )
The estimated phase angle= θ ^ ; assuming θ ^
≈ ωt .
PLL successfully tracks the phase at
Vd   V − Sin ( 2θ ^ )
V  ≈ 
^ 
 q   − V + V Cos ( 2θ )
is the double frequency to be eliminated. It is the basic concept of the
modified PLL structure. It can provide the positive sequence component by cancellation
of 2ω oscillations (Venkatachalam, Chandrasekaran, & Rama Rao, 2009).The modified
PLL circuit uses Clarke and Park transform for identifying the peak of positive
sequence voltage. The three phase unbalanced voltages are converted to the stationary
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coordinate system. Through the measured phase angle, the voltages in stationary coordination are transformed to the d-q frame. The PI controller forced the d-axis
component Vd to zero so as to align the mains voltage space vector with the q-axis. The
estimated phase angle θ, which is attained by integrating the proportion-integral (PI)
controller output, is, in turn, used for the coordinate transformation process (Sinha &
Sensarma, 2011). Here, a second order resonant filter is used instead of the conventional
low pass filter for suppressing the double fundamental frequency, which is created for
unbalancing and the gain adaptation block shows the convergence of any value of
system voltages and creates normalized templates of fundamental voltage (De &
Ramanarayanan, 2010).The rate limiter block works as an attenuator against the ripple.
The modified PLL can perform satisfactorily as long as the gains of the PI are adjusted
accordingly under highly inaccurate and disturbed system voltages.
Figure 2. PLL circuit block diagram
The transfer function of the resonant filter is given in:
H (s) =
s 2 + 4ω 2
s + 4 ω cut −off + 4 ω 2
and the open loop PLL gain is written as:
G ( S ) = V . .k p .(1 +
). H ( s )
Where T and Kp are the time constant and gain of the PI controller correspondingly.
Now a critically damped system is taken into consideration and the value of the cut-off
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frequency is chosen as it is equal to double the fundamental frequency of the system
(Indu Rani, Aravind, Saravana Ilango, & Nagamani, 2012). For selection of the PI
controller gains, the resonant filter is approximated as a low pass filter and the
parameter design is obtained via the calculation through the optimum symmetrical
T = 4τ
kP =
2V τ
Where τ is the time constant of the low pass filter.
3.2. Reference Current Generation
The three phase load currents are measured using a hall-effect current sensor and
converted into d-q-0 by means of a rotational frame that is synchronous with the system
voltage positive sequence in equation 11.
2π 
 sin(ω s t ) sin(ω s t − 3 ) sin(ω s t + 3 ) 
i d 
 i A 
 i  = 2 cos(ω t ) cos(ω t − 2π ) cos(ω t + 2π ) i 
  3
3  
 i 0 
  
ωst ω is considered here as the transformation-angle of the positive sequence source
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voltage and is delivered by the phase lock loop (Jianben, Qiaofu,Jun,2012).
Figure 3. SRF based control block diagram
The system under observation is a three phase-four wire in which the active component
id and oscillating part iq are reflected. After the load currents id and iq are found, they are
allowed to pass over a low pass filter to separate the AC and DC part through which the
active and reactive fundamental current components (id-DC, iq-AC) are obtained. Both
the currents (id-DC, iq-AC) AC parts are related with the responsibility for active and
non-active harmonic components. The filters used in the circuit are the 2nd order
Butterworth type and their cut off frequency is identical to one half of the fundamental
frequency (Mikkili & Panda, 2011). In consideration of the reactive current, the passive
filter provides its DC value while the VSI delivers an AC voltage to sink the harmonics.
The filtered active and non-active currents from equation 12 are used for generation of
the accurate references to the modulator:
 i d   i dDC 
 i dAC 
 =   − i
 qAC 
 i q   qDC 
As well as providing harmonic currents, the DC voltage of the PWM VSI should be
maintained for accurate operation (Saad , Tarek & Nasruddin, 2011). The voltage of the
capacitor is controlled by regulating the reactive current (Salmeron & Litrán, 2010) as
shown in Figure 3. Then the a-b-c frame reference currents are:
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cos(ω s t ) 
 iCA   sin(ω s t )
i  = sin(ω t − 2π ) cos(ω t − 2π ) i d *
 
 CB  
3  i q *
i CC  
sin(ω t + 2π ) cos(ω t + 2π )
3 
3.3. DC Link Capacitor Voltage Control
3.3.1. Fuzzy controller
Figure 4. DC voltage optimization with Fuzzy approach
A Fuzzy logic controller is used to regulate the DC link voltage and added to the active
part of the fundamental load current. The obtained voltage of the capacitor is matched
with a preset reference value (Mikkili & Panda, 2011). The loss component is then
handled by a Fuzzy logic controller, which contributes to the zero steady error in
tracking the reference current signal, as shown in Figure 4. A low pass filter and a rate
limiter block have been used here for optimum performance. The supply current peak
value is calculated using the output component of the fuzzy logic controller
(Park,Jeong,Kwon,2001),(Younus, Rahim& Mekhilef,2009).
A linguistic control approach is converted into an automatic control strategy, and with a
knowledge database, Fuzzy rules are constructed by proficient experience. To begin
with, ‘µ(E)’, the input error, and ‘µ(∆E)’, the change in error, have been used as the
input variables of the fuzzy logic controller (Singh, Singh, & Mitra, 2007). Then the
control current ‘Imax’ represents the output variable. Seven Fuzzy sets are chosen here
to adapt these numerical variables into linguistic variables: PB (positive big), PM
(positive medium), PS (positive small), ZE (zero), NS (negative small), NM (negative
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medium) and NB (negative big), as can be seen in Figure 5. The intransient state
requires coarse input/output variables for controlling based on large errors and fine
input/output variables are essential for controlling the small errors properly with the
development of Fuzzy logic rules in the steady state (Saad & Zellouma, 2009). With this
perception, the rule table elements and view of the surface is obtained, as shown in Fig.
6, where ‘µ (E)’ and ‘µ (∆E)’ are used as inputs.
Figure 5. Normalized membership functions for the input and output variables
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Figure 6. (a) Fuzzy control rule (b) Surface Viewer (E vs ∆E)
3.4. Harmonic current generator
The active filter switching patterns are decided by the current controller. A hysteresis
comparator is used here for a quick response and appropriate sinusoidal current tracking
capability (W.u,2011). The desired current, Ia (t), and the injected inverter current,
Ia*(t) are compared with one another. The logic of switching is as follows:
If Ia< (Ia*-hb) upper switch S1 is OFF & lower switch S4 is ON in leg of ‘a’ of APF. If
Ia> (Ia*-hb) upper switch S1 is ON & lower switch S4 is OFF in leg of ‘a’ of APF.
Correspondingly, in the legs ‘b’ & ‘c’ the switches are initiated. Here ‘hb’ is the
hysteresis band and determines the variation of load current harmonics and switching
frequency of the devices.
4. Simulation results and analysis
The performance of a 3-legs 4-wire Hybrid APF for filtering current distortions,
compensation of neutral current, load current balancing and reactive power mitigation
has been examined under unbalanced source-balanced load. The goal of this case study
is to demonstrate the validity and evaluation of the proposed approach, where the source
is greatly unbalanced.
The results are specified in the following simulation studies for before compensation,
compensation using only passive filter and after the operation of the hybrid filter. Three
and single phase diode rectifier nonlinear loads are coupled in the power system for
producing unbalanced, reactive current and harmonics in the load current as well as a
neutral current. The widespread simulation outcomes are discussed below.
4.1 Unbalanced source condition
Voltage unbalance occurs when the relative magnitudes and phase angle of the three
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phases are not equal. The source voltage magnitude of phase-b is 20% smaller than the
source voltage of phase-a, and the phase-c voltage is 50% lower than phase-a. This is
done by using a three phase programmable voltage source block in Simulink.
The three phase unbalanced voltages are considered as: Vsa= 100 sin ωt, Vsb= 80 sin
(ωt-120) and Vsc= 50 sin (ωt+120). In unbalance source voltages, the fundamental
components are first derived using PLL to make the unit templates for the balanced and
sinusoidal voltages. Then the synchronizing phase angle is extracted and simulation is
performed through it. The source voltage is shown in Figure 7 (a). The load current and
its FFT graph without compensation are given in Figure 7 (b) and (c), which is highly
non-linear in nature. After the application of the passive filter lower order harmonics are
reduced but non-linearity and distortion still exists, which is presented in Figure 7 (d).
The THD of the source current is reduced from 21.07% to 1.33% in the case of the
fuzzy controller, which is below the IEEE standard as shown in Figure 7 (g). In Figure 7
(l) the extracted transformation angle from PLL is also shown. The values of source
current and THD levels are listed in table 1.Although, the source currents are not purely
balanced but the results are quite satisfactory.
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Figure 7. Simulation results for operational performance of SHAPF a) Source voltage b)
load current c) THD before compensation d) passive filtering source current e)THD
after passive filtering f) source current g) THD after SHAPF h) injecting current i) DC
link voltage J) Real power (p) k) Reactive power (Q) l)Transformation angle (wt)
Table 1.System performance for unbalanced source condition
Source current
Before filter
After Passive
After Hybrid
active filter
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THD (%)
Fuzzy logic controller performance evaluation
DC voltage
settling time
with Fuzzy
Real & Reactive
Active filter
Real & reactive power with
Active filter
910w, 370var
Table 3.Simulation parameters
Source impedance
Load impedance
R = 0.01Ω, L = 2 mH
3-phase diode rectifier R = 80 Ω, L = 60 mH
1-phase diode rectifier R= 0.1Ω, L=1 mH
Passive Filter
Shunt APF
5th harmonic tuned, R=0.2Ω, L=5mH, C=60μF
R=0.001Ω, L=2mH,
Two DC link capacitor = 2100μF,Vdc=220 V
100 V (r.m.s), 50 HZ
Source voltage
It has been observed that the shunt hybrid APF performance with fuzzy logic controller
shows outstanding characteristics compared to the conventional controller under
unbalance voltage condition. The calculation of real (p) and reactive (Q) power for
unbalance case are arranged in Table 2. It also shows the effectiveness of the fuzzy
logic controller. The parameters used in this study for SHAPF system are listed in Table
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Table 4. THD comparison between recommended topology and conventional controller
Before SHAPF
P-Q theory
Modified P-Q
PI controller
with SRFPLL(corasanti)
A comparative analysis between the proposed control method, conventional method
(Corasaniti et al., 2009), P-Q theory (Mikkili & Panda, 2012) and modified P-Q theory
(Patnaik & Panda, 2013) is shown in Table 4 in terms of THD % and reactive power.
The P-Q theory and modified P-Q theory are simulated with the used parameters. From
the table it is observed that they are capable of mitigating harmonics but fail to reduce
the THD % below 5% under a non-ideal voltage source. The proposed method allows
2% THD values of source current and compensates reactive power, which is satisfactory
when compared with the conventional control technique.
5. Experimental Results
The system parameters selected for the experimental verification: Vs=50V (peak),
f=50HZ, Ls=5mH, Lc=5mH, RL=100Ω, Ll=20mH, Cdc=2000μF, Vdc =70V. Parameters
of passive filter, Lf =3mH, Cf =60μH. A prototype model of three-phase- four wire APF
is developed in the laboratory for experimentation shown in Figure 8.
For the generation of unbalancing in the voltage, two different values of resistance are
connected in series with the two phases of supply, and, just after that, a constant 3-phase
load is fixed. Another phase is kept as usual. The flow of current through the series
impedance will cause a different voltage drop in each phase, so that unbalanced voltages
occur in the system. In this case, the equivalent supply will be after the series
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impedance and constant 3-phase load.
There are two single-phase bridge inverters connected in series in each phase. Suitably
designed snubber circuit is connected across each device. DC link capacitor is
connected at DC side of the inverter. Three coupling inductors are connected in series
with the three lines on AC-side of the inverter. Three-phase diode rectifier with R-L
elements on DC side is used as a nonlinear load which draws nonlinear current. The
AC and DC voltages are sensed using the isolation amplifier (AD202JN). The AC
source currents are sensed using the PCB-mounted Hall-effect current sensors (LEM25-NP). The IGBT driver circuits are used for pulse amplification and isolation
purposes. The driver circuit has special features like the fault protection and protection
against the under-voltage lockout. The load current, the source voltage and injected
filter currents, ifabc, are measured. The sensed signals are used for processing in the
designed control algorithm. Because real-time simulation is an important aspect for
control engineering, the same is true for the automatic generation of real-time code,
which can be implemented on the hardware.
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Figure 8. Experimental setup for shunt hybrid APF
5.1 Unbalance source condition
For an unbalanced source, 14% amplitude unbalance is introduced in the ‘b’ and ‘c’
phases so that the phase voltages are 50V in phase ‘a’, 43 V in phase ‘b’, and 38 V in
phase ‘c’. The unbalance is also reflected in the load currents in the three phases.
However, the source currents are balanced with the proposed APF. Figure 9 shows the
total response of the system for this condition. From the results it is observed that the
proposed control method gives satisfactory results under unbalanced source. The load
current waveform of phase ‘a’ is distorted before filtering shown in Figure 9(b). FFT
result is also listed. After the operation of hybrid APF controlled by fuzzy controller
sinusoidal source current waveform is obtained instantaneously after the insertion of
active filter as illustrated in Figure 9 (f). In figure it is also shown that the experimental
results of the dc-side capacitor voltage which is nearly constant with small ripple. The
current THD is reduced from 28% to 4.8% as shown in the frequency current spectrum.
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Figure 9. Experimental results for unbalance source a) Supply voltage (25v/div) b) Load
current before compensation (0.8A/div) c) %THD before compensation d) Source
current after passive filtering e) %THD after passive filter f) Source current after
compensation g) %THD after SHAPF compensation h) DC link voltage
6. Conclusion
A modified PLL with SRF theory is developed for computation of the reference current
and employed effectively for grid voltage synchronization under unbalanced mains
conditions. It is seen from the simulated figures that the Fuzzy logic controller shows
better dynamic performance to mitigating current harmonics and compensating reactive
power. The figures illustrate that the feedback currents of the filter approximately agree
with the generated reference currents. The proposed method has been evaluated with
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simulations under non-ideal mains voltage conditions where it is shown that percentage
of THD is reduced from 21.07% to 1.33% for unbalance source. Moreover, 53%
amount of reactive power has been compensated after using the shunt hybrid APF. The
impact of source current unbalance has also been minimized. A laboratory prototype is
developed and it has been found that value of THD has been reduced from 28% to 4.8%
for unbalance voltage which is in the mark of 5% specified in the IEEE-519 standard.
The value of power factor has been increased to 0.632 to 0.963. Therefore, with the
combination of fuzzy logic and modified SRF theory approach, SHAPF can be
considered as a reliable harmonic isolator for its quick response and good quality of
The authors wish to acknowledge the financial support from university of Malaya HIRMOHE project UM.C/HIR/MOHE/ENG/24.
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