International Journal of Electrical and Electronics
Engineering Research (IJEEER)
ISSN 2250-155X
Vol. 3, Issue 4, Oct 2013, 149-160
© TJPRC Pvt. Ltd.
PERFORMANCE OF A CASCADED SHUNT ACTIVE POWER FILTER USING PI
CONTROLLER & FUZZY LOGIC
M. SURYA KALAVATHI1, N. KARTHIK2 & K. SUSMITHA3
1
Department of EEE, JNTU, Hyderabad, Andhra Pradesh, India
2,3
Department of EEE, Bapatla Engineering College, Bapatla, Andhra Pradesh, India
ABSTRACT
The purpose of this work is to study about work of shunt active power filter in reduction of current distortion and
for power quality improvement in electrical systems. In this paper, based on the analysis and modeling of the shunt active
power filter with close-loop control, a feed forward compensation path of load current is proposed to improve the dynamic
performance of the APF. The two H-bridge cascaded inverter is selected for the active power filter. A justification for
topology choosing and corresponding system control method is given. Furthermore, the global framework and operation
principle of the proposed active power filter are presented in detail. Simulation results verify the feasibility of the proposed
active power filter and the high performance of the control strategy during steady-state and dynamic operations with PI
control technique.
KEYWORDS: Active Power Filters (APF), Cascaded Multilevel Inverter, Close-Loop Control, Feed Forward of
Fundamental Load Current, Fuzzy Logic Controller (FLC)
INTRODUCTION
The increasing use of electrical power in place of hydraulic, pneumatic, and mechanical power is demanding more
advanced aircraft power systems. The concept of the all-electric aircraft and the “more electric aircraft” (MEA) have been
introduced to overcome some of the drawbacks found in conventional architectures and bring more attractive advantages,
such as improved fuel consumption and lower maintenance and operation costs. This implies an increase of the electrical
load and power electronic equipment, higher consumption of electrical energy, more demand for generated power, power
quality, and stability problems. Harmonic current compensation by means of active power filter (APF) is a well-known
effective solution for the reduction of current distortion and for power quality improvement in electrical systems. The shunt
compensator behaves as a controlled current source that can draw any chosen current reference which is usually the
harmonic components of the load currents. Meanwhile, more and more APFs are applied not only in harmonic current and
reactive power compensation but also in the neutral line current compensation, harmonic damping application, and power
flow control.
A cascade multilevel inverter has been proposed for both harmonics and static var compensation applications. The
new cascade inverter eliminates the bulky transformers required by Static VAR Compensators (SVC’s) that employ the
multi pulse inverter and can respond much faster. This inverter generates almost sinusoidal staircase voltage with only one
time switching per line cycle. Its superior suitability has been demonstrated for VAR compensation. When the cascade
inverter is applied to line conditioning and active power filtering of a distribution system, it is expected that the initial and
running costs and the EMI will be dramatically reduced below that of the traditional PWM inverter. The new cascaded
multilevel inverter, however, poses challenging problems for both harmonic filtering and reactive power (VAR)
compensation, such as voltage control and balance of each dc capacitor.
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M. Surya Kalavathi, N. Karthik & K. Susmitha
CLOSE-LOOP CONTROL STRATEGY AND ITS FEED FORWARD COMPENSATION
Close-Loop Control Strategy
In the close-loop control, detection and control target is the source current. In the aircraft EPS, the fundamental
frequency is much higher than 50-Hz power system. Furthermore, measure errors, analog to digital conversion time, digital
delay, and other non ideal factors will deteriorate the open-loop compensation effect to a worse degree. As we know,
feedback control has the following merits: It could reduce the transfer function from disturbances to the output, and it
causes the transfer function from the reference input to the output to be insensitive to variations in the gains in the forward
path. Therefore, compared with open-loop control, close-loop control is more suitable for the aeronautical application.
Source Current Direct Control
The source current direct control is proposed in by Wu and Jou. The basic system diagram of the close loop
control scheme is given. This control strategy operates as follows: The dc-link voltage is sent to the voltage regulator, and
the output of the regulator is sent to the multiplier as well as a synchronous sine wave which is detected from the phase
voltage. The output of the multiplier is sent to the current regulator, being the source current reference. The output of the
current regulator will be sent to the modulator to generate the pulse width modulation waveforms.
Figure 1: Source Current Control
H-BRIDGE
An H-bridge is an electronic circuit that enables a voltage to be applied across a load in either direction. These
circuits are often used in robotics and other applications to allow DC motors to run forwards and backwards. H bridges are
available as integrated circuits, or can be built from discrete components.
The term H-Bridge is derived from the typical graphical representation of such a circuit. An H bridge is built with
four switches (solid-state or mechanical). When the switches S1 and S4 (according to the first figure) are closed (and S2
and S3 are open) a positive voltage will be applied across the motor. By opening S1 and S4 switches and closing S2 and S3
switches, this voltage is reversed, allowing reverse operation of the motor. The switches S1 and S2 should never be closed
at the same time, as this would cause a short circuit on the input voltage source. The same applies to the switches S3 and
S4. This condition is known as shoot-through.
Figure 2: H-Bridge
Performance of a Cascaded Shunt Active Power Filter Using PI Controller & Fuzzy Logic
151
The H-bridge arrangement is generally used to reverse the polarity of the motor, but can also be used to 'brake' the
motor, where the motor comes to a sudden stop, as the motor's terminals are shorted, or to let the motor 'free run' to a stop,
as the motor is effectively disconnected from the circuit. The following table summarizes operation, with S1-S4
corresponding to the diagram above.
IMPLEMENTATION OF FUZZY LOGIC CONTROLLER
PI controller main disadvantage is it failed to react to abrupt changes in the error signal. It is capable of
determining instantaneous value of error signal only.
Figure 3: Calculation of Signals for Multi Level Converter Switching
To solve the problem with PI controller Fuzzy logic control is proposed as shown.
Figure 4: Basic Representation of FLC
In this the value of output changed according to the error signal(ε) and the rate of error(Δε) .All the variables
fuzzy subsets for error and rate of error are defined using membership functions of inputs as shown.
Figure 5: Membership Functions of Input and Output of Fuzzzy Controller
All the Fuzzy variable subsets for the inputs ε and Δε are defined as (NB,NM,NS,Z,PS,PM,PB).The Fuzzy control
rule is illustrated as shown.
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M. Surya Kalavathi, N. Karthik & K. Susmitha
Table 1
ΔƐ \Ɛ
NB
NS
ZE
PS
PS
PM
PM
NB
NB
NB
NB
NM
NM
NS
NS
NM
NB
NM
NM
NS
NS
ZE
ZE
NS
NB
NS
NS
ZE
ZE
PS
PS
ZE
NB
NS
ZE
PS
PS
PM
PM
PS
NM
ZE
PS
PS
PS
PM
PM
PM
NS
PS
PM
PM
PM
PB
PB
PB
ZE
PM
PB
PB
PB
PB
PB
SIMULINK DIAGRAM
Figure 6
Simulink diagram shown three phase four wire system, discrete block for FFT analysis and different scope
connections for output results.
NON LINEAR LOAD SIMULINK DIAGRAM
Figure 7
Non linear load simulink diagram consists of linear and non linear load. A non linear load can be a thyristor or a
diode. We use synchronized 6 pulse generators to generate pulses.
PI CONTROLLER SIMULINK DIAGRAM
Figure 8
Performance of a Cascaded Shunt Active Power Filter Using PI Controller & Fuzzy Logic
153
In PI controller reference current is compared with the measured current, depending on the error output signal
current is generated.
Figure 9
These GO-TO connections are used to give current signals to the cascaded H Bridge. These cascaded H bridges
are connected in parallel to the line.
CASCADED H-BRIDGES
Figure 10
Cascaded H bridges are connected in parallel to the line through transformer secondaries. Here transformers
primaries are connected in series.
H-BRIDGE
Figure 11
Here 1,2,3,4, are the MOSFET switches. During positive half cycle 1 & 4 are in ON position. During negative
half cycle 2 & 3 are ON.
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M. Surya Kalavathi, N. Karthik & K. Susmitha
Simulation Output without Active Power Filter
Non Linear Load Current
Figure 12
Supply Current
Figure 13
Without active filter load current and supply current waveforms are similar.
FFT Analysis without Filter
Load Side
Figure 14
Source Side
Figure 15
There is no change in total harmonic distortion (THD) without the use of shunt active filter.
Performance of a Cascaded Shunt Active Power Filter Using PI Controller & Fuzzy Logic
Simulation Output with Active Power Filter, PI Control with  =0
Non Linear Load Current
Figure 16
Supply Current
Figure 17
Active Power Filters Current
Figure 18
FFT Analysis with  =0
Load Side
Figure 19
Source Side
Figure 20
155
156
M. Surya Kalavathi, N. Karthik & K. Susmitha
When shunt active power filter is used THD is reduced up to 0.83% at the source side.
Simulation Output with Active Power Filter, PI Control with  =90
Non Linear Load Current
Figure 21
Supply Current
Figure 22
Active Power Filter Current
Figure 23
FFT Analysis with  =90
Load Side
Figure 24
Source Side
Figure 25
157
Performance of a Cascaded Shunt Active Power Filter Using PI Controller & Fuzzy Logic
When shunt active power filter is employed with  =90 THD is reduced below 3%.
Load Side Neutral Current
Figure 26
Source Side Neutral Current
Figure 27
Active Power and Reactive Power
Figure 28
By using shunt active power filter reactive power decreases and active power increases.
Source Current with Fuzzy Controller
150
100
source current(A)
50
0
-50
-100
-150
-200
0. 31
0. 32
0. 33
0. 34
0. 35
t ime(s ec )
0. 36
0. 37
0. 38
0. 39
0. 4
Figure 29
Load Current THD with APF, Fuzzy  =0
Fundamental (50Hz ) =
149.9 ,
THD=
23.53%
100
90
80
Mag (% of Fundamental)
70
60
50
40
30
20
10
0
0
2
4
6
8
Harmonic
10
order
Figure 30
12
14
16
18
20
158
M. Surya Kalavathi, N. Karthik & K. Susmitha
Source Current THD with APF, Fuzzy  =0
Fundamental (50Hz ) = 153.4 , THD= 0.46%
100
90
80
Mag (% of Fundamental)
70
60
50
40
30
20
10
0
0
100
200
300
400
Frequenc y
500
(Hz )
600
700
800
900
800
900
1000
Figure 31
Tabular form Shows Different Results with SAF, PI Controller, Fuzzy Logic
Table 2
Without SAF
23.83
23.9
24.94
0.75
THD at α =0
THD at α =45
THD at α =90
Power factor
PI
0.83
1.26
2.18
0.95
Fuzzy
0.41
0.65
1.08
0.99
Load Current THD with APF, Fuzzy  =90:
Fundamental (50Hz) = 38.3 , THD= 34.64%
100
90
80
Mag (% of Fundamental)
70
60
50
40
30
20
10
0
0
100
200
300
400
500
Frequency (Hz)
600
700
1000
Figure 32
Source Current THD with APF, Fuzzy  =90
Fundamental (50Hz ) = 35.9 , THD= 1.22%
100
90
80
Mag (% of Fundamental)
70
60
50
40
30
20
10
0
0
100
200
300
400
500
Frequenc y (Hz )
600
700
800
900
1000
Figure 33
DC Voltage with PI Controller and Fuzzy Logic
with PI
with Fuz z y
5000
DC voltage (V)
4000
3000
2000
1000
0
-1000
0
0.05
0.1
0.15
0.2
0.25
time(s ec )
Figure 34
0.3
0.35
0.4
0.45
Performance of a Cascaded Shunt Active Power Filter Using PI Controller & Fuzzy Logic
159
CONCLUSIONS
In this work, a load current feed forward compensation method for the source current direct control-based APF
has been proposed. The corresponding system control strategy of the cascaded-inverter based active filter system is shown.
Simulation results are shown to confirm the good compensation behavior for various kinds of nonlinear load condition and
the excellent dynamic performance of the proposed control method. Because of the limitations of the hardware (800-Hz ac
voltage source), experiments of the APF in variable-frequency applications have not been taken.
By using cascaded-inverter based active filter system we reduce the harmonics, neutral current and power flow is
controlled. By using shunt active filter we have observed that the total harmonic distortion (THD) reduced up to 2% at the
source side by PIcontrol. We can reduce harmonic distortion below 2% at the source side by employing fuzzy logic.
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