comparative study between shunt active filter and hybrid
TRANSCRIPT
مجلة الجامعة الأسمرية للعلوم الأساسية والتطبيقية
م7132، الجزء الثاني ، ديسمبر (13)العدد
65
Comparative Study between Shunt Active Filter and Hybrid Filter
MOHAMED Muftah Saleem
Department of Electrical and Computer Engineering, Almergib University, Alkhoms, Libya
Abstract—This paper presents an comparative study between shunt active filter and shunt
hybrid filter based on configuration system and harmonic isolation. The harmonic isolator is
based on Self Tuning Filters (STF) (it can be tuned at any frequency). The study of the active
filter control is divided in two parts. The first part deals with the harmonic isolator which
generates the harmonic reference currents. The second part focuses on the generation of the
switching pattern of the IGBTs of the inverter by the modulated hysteresis current controller.
The studied hybrid active filter consists of an active filter and three phase tuned LC filters for
7th
harmonic frequency. The nonlinear load is a diode rectifier feeding a (R, C) parallel load.
The use of STF's simplifies the control scheme by reducing the number of extraction filters.
The use of STFs instead of classical extraction filters allows extracting directly the voltage
and current fundamental components. The effectiveness of this study has been verified by
computer simulation and comparing the results.
Index Terms—shunt active filter, hybrid active filter, harmonics, self tuning filter (SFT).
I. Introduction
The power system must be free from harmonics and which will lead to numbers of
benefits. A clean network has less strain on appliances and their lifespans are lengthened.
Nowadays with the advancement of technology, the demand for electric power is increasing at
an exponential rate. The performance of the end user equipment is heavily dependent on the
quality of power supplied to it. But the quality of power delivered to the end user is affected
مجلة الجامعة الأسمرية للعلوم الأساسية والتطبيقية
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by various external and internal factors. They are like voltage and frequency variations, faults,
outages etc.
The main affect caused by these problems is the presence of harmonics. This leads to the
overheating of the equipment, insulation failure and over speeding of induction motors etc
[1,2]. The solution to overcome these problems is to filter out these harmonics.
There are many filters topologies present in the literature. Filters can be classified into three
types (passive, active, hybrid) [3,4]. This study presents and compare two types of filters and
the control strategy to control the filter in such a way that the harmonics are reduced.
The hybrid filter which is a combination of active filter and shunt passive filter is studied.
While the passive filter mitigates load produced harmonics the active filter helps to enhance
filtering properties of passive filter. This ensures a great diminution of the rating of the active
filter guiding to an economical practical system.
II. Active filter and control strategy
A. System configuration
The harmonic currents are the source of adverse effects for many types of equipment's
such as heating in distribution transformer, perturbation of sensitive control equipment's and
resonances with the grid. Many solutions have been studied in the literature to mitigate the
harmonic problems, such as filtering (passive, active, and hybrid) with various topologies
(shunt, series or both). These solutions have been proposed to improve the power quality of
the AC mains.
The passive filter which is the simple way to eliminate the harmonic currents, has many
drawbacks such as the series or parallel resonance with the system impedance and it cannot
completely eliminate all of the harmonic currents [5].
The active filter which is developed and widely used to overcome to the drawbacks of the
passive filters and improve power quality. As known, the performances of the active filter
مجلة الجامعة الأسمرية للعلوم الأساسية والتطبيقية
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system mostly depend on the accuracy of the harmonic isolation and on the current control
technique used to generate the switching patterns for the inverter [6]. Hysteresis and PMW
techniques are usually used.
The hysteresis strategy allows faster dynamic response and better robustness according to the
variation of the non-linear load. Nevertheless, with this strategy, the switching frequency is
not constant. In this paper, to fix the switching frequency, we studied a three-phase non-linear
current controller resulting from the hybridization between a PWM current controller and a
hysteresis current controller, so called “modulated hysteresis controller”. More, a focus is
made on the improved harmonic isolator based on Self Tuning Filter (STF) [5,7].
This filter (STF) is used instead of classical harmonics extraction based on High Pass filters or
Low Pass Filters. The effectiveness of the proposed method is verified by computer
simulation.
Fig.1. Active filter system.
Fig.1 presents the schematic diagram of the three-phase active power filter and the
associated control strategy for harmonic cancellation. The filter is connected in parallel with
Rd
Ld
Modulated
hysteresis
Current controller
Active Filter A/D
D/A
Harmonic
Isolator
DSPACE
(DS1104)
i*
fa, i*fb, i
*fc
ifa, ifb
Vdc
iLa iLb
6 control signals
Si SL
FL Diode Rectifier
Fi
Vd
c
vs
Li
vsb vsa
مجلة الجامعة الأسمرية للعلوم الأساسية والتطبيقية
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the ac three-phase three-wire system by three inductors Lf. The switching patterns of each
inverter leg are complementary to each other to avoid any short circuit.
The non-linear load is a three-phase diode rectifier feeding a RL load. This load will generate
harmonics currents in the supply system.
The control method is divided in two parts. The first one consists in the harmonic isolator
which generates the harmonic reference currents and is implemented into a DSPACE DS1104
development board. The second part is the generation of the switching pattern for the inverter
by using an analogue modulated hysteresis current controller.Simulation results
Fig.2 shows the simulation results for the system depicted in the Fig.1 under sinusoidal
voltage conditions. The simulation parameters are defined in the Table I.
The total harmonic distortion (THD) of the load current is 28.08%. The THD of the supply
currents is reduced to 2.3% after compensation. A difference can be noticed in the figure 2
between the fundamental components values of the load current and the supply current.
It is justified by the fundamental component of the filter current in phase with the supply
voltage to regulate the dc bus voltage. The harmonic isolator is implemented by using the
DSPACE system associated with a DS1104 development board. It generates the harmonic
current references.
TABLE I. System parameters
System frequency 50 Hz
System voltage 130 Vmax
Inductor : LF 3 mH
Inductor: LC 0.8 mH
DC bus voltage 400 V
Capacitor: Cd 1100 µF
Resistor: Rd 48.6 Ω
Inductor: Ld 40 mH
مجلة الجامعة الأسمرية للعلوم الأساسية والتطبيقية
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0.1 0.12 0.14 0.16 0.18 0.2
-5
0
5
Time (s)
i L (A)
0 5 10 15 20 25 30 35 400
1
2
3
4
5
6
Harmonic order
Mag
(A)
THD = 28.08 %
-a-
0.1 0.12 0.14 0.16 0.18 0.2
-5
0
5
Time (s)
i L (A)
0 5 10 15 20 25 30 35 400
1
2
3
4
5
6
Harmonic order
Mag
(A)
THD = 2.3 %
-b- 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10
100
200
300
400
500
600
Time (s)
Vdc (
V)
-c-
مجلة الجامعة الأسمرية للعلوم الأساسية والتطبيقية
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Fig. 2. Simulation results for: (a) load current, (b) supply current after compensation, (c) DC-
Bus voltage Vdc (v).
III. HYBRID FILTER
B. System configuration
The cost of active filters in industrial applications could be very high. The power rating of
power converter for active filters is very large. For these reasons, the application of active
filters for power system is cost limited [8]. In the last few years, many different topologies of
hybrid active filters with various control strategies have been proposed in the literature as
lower cost alternatives to active filtering for harmonic compensation. Nowadays, hybrid active
filters are considered as one of the best solutions for improving power quality [9]. Fig.3 shows
the hybrid active filter topology studied in this paper.
The hybrid filter consists in a three-phase LC filter tuned for the 7th harmonic frequency,
connected in series with an active filter without any transformer. The passive filter absorbs
harmonic currents generated by the load whereas the active filter improves filtering
performances of the passive filter. The associated control scheme combines a feedback and
feedforwad loop [10,11].
Fig. 3. Parallel hybrid filter configuration.
SL
Diode Rectifier
Active Filter
FL
FC
Fi
Si Li
dcC
Vvdc 105
dC
dR
مجلة الجامعة الأسمرية للعلوم الأساسية والتطبيقية
م7132، الجزء الثاني ، ديسمبر (13)العدد
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Fig. 4. Control scheme of the hybrid active filter.
IV. SIMULATION RESULTS
The efficiency of the proposed control scheme has been examined by computer simulation
using MATLAB and associated toolboxes “Simulink” and “Power System Blockset”. The
parameters of the system are given in Table II.
The simulated hybrid filter consists in a three-phase LC filter tuned at the 7th
harmonic
frequency, connected in series with a three-phase active filter based on MOSFETs power
semiconductors. The nonlinear load is a diode rectifier feeding a RC parallel load. Table III
shows THD values for different choices of the gain value K for the feedback loop of the active
filter. The optimal value is equal to 20 which provides better filtering characteristics.
Fig. 5 show simulation results that present the capability of compensating the fifth harmonic
frequency successfully.
sin (1)& cos(1 ) abcv
PLL
Shi K
i~
i~ di
~
qi~
*
dcv
dcv
*
AFv Self
tuning
filter
*
1qi
PI
Sabci
Feedforward Loop
Feedback Loop Loop
dq
dq abc
abc
i
i
*
5v 5i
5i *
5v
Labci Self
tuning
filter
abc
abc
Calculation
of
V*
α5 &V*β5
مجلة الجامعة الأسمرية للعلوم الأساسية والتطبيقية
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Fig. 5. Simulation results for the : Source current iS (A), Load current iL (A) and DC-Bus
voltage Vdc (v).
The major aims of this paper were to compare the efficiency of the shunt active filter and
hybrid filter. As presented in Table III, the THD of the non-linear load Li is equal to 27.8%
Si
)(st 0.7 0.705 0.71 0.715 0.72 0.725 0.73 0.735 0.74 0.745 0.75
-100
-80
-60
-40
-20
0
20
40
60
80
100
Li
)(st 0.7 0.705 0.71 0.715 0.72 0.725 0.73 0.735 0.74 0.745 0.75
-100
-80
-60
-40
-20
0
20
40
60
80
100
dcv
)(st 0.7 0.705 0.71 0.715 0.72 0.725 0.73 0.735 0.74 0.745 0.750
50
100
150
مجلة الجامعة الأسمرية للعلوم الأساسية والتطبيقية
م7132، الجزء الثاني ، ديسمبر (13)العدد
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because of the large amount of the 5th
harmonic current while it is equal to 2.9% for the source
current Si . The results we obtained demonstrated an improvement of the THD values. The LC
filter is tuned at the 7th
-harmonic frequency and absorbs the voltage of the network at the
fundamental frequency. Consequently, the dc voltage of the inverter dcv can be reduced as low
as 105V. This enables the hybrid filter to use low-voltage MOSFETs which are less
expensive.
Table III presents load current and source current THD as the harmonic-to fundamental
current ratio in (%). It demonstrates the effectiveness of the control scheme by using STFs in
the feedback and feedforward loops.
TABLE II. Simulation parameters
Capacitor : CF 57.6 F
Inductor : LF 2.5 mH
Inductor: LS 0.15 mH
Quality factor: Q 22
DC bus voltage 105 V
Capacitor: Cd 1500 F
Resistor: Rd 21
Capacitor: Cdc 1500 F
System
frequency 50 Hz
System voltage 480 V
TABLE III. Load current and source current THD and harmonic-to fundamental current ratio
(%)
مجلة الجامعة الأسمرية للعلوم الأساسية والتطبيقية
م7132، الجزء الثاني ، ديسمبر (13)العدد
74
5th
7th
11th
13th
17th
19th
THD
Li
26
7.5
4.7
3.2
1.5
1.4
27.8
Si
1
0.4
1.6
1.3
0.9
0.9
2.9
V. CONCLUSION
This paper has discussed the configuration system, control and performances of a shunt
active power filter and hybrid filter. The control of the active filter was divided in two parts, a
digital one realized by the DSPACE system to generate the reference currents, , the modulated
hysteresis current controller used for the switching pattern generation. STF used instead of
classical extraction filters for both grid voltages and load currents. The proposed hybrid filter,
which is composed of an active filter and three phase tuned LC filters. The simulation results
have demonstrated and conforted the major advantages of using HSFs in the filter control.
This paper describes a comparative study between shunt active power filter and shunt
hybrid power filter. Simulation results proved that performance of the hybrid filter is much
better than the active filter. The DC link voltage of hybrid filter is twice more than that of
active filter. The combined system of passive and an active filter has some features as (Source
impedance no longer governs the filtering characteristics, the active filter has the ability to
dump the parallel and series resonance between the source and the passive filter, the required
rating of hybrid filter is much less than a conventional active filter used alone). Table IV
summarize some major advantages of hybrid filter over the active filter.
TABLE IV. Comparison between shunt active filter and hybrid filter.
Active filter
Hybrid filter
Type of switch
IGBT MOSFET
مجلة الجامعة الأسمرية للعلوم الأساسية والتطبيقية
م7132، الجزء الثاني ، ديسمبر (13)العدد
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Voltage (Vdc) at the terminals of the
capacitor
400 V 105 V
Capacity value (Cdc) DC side
8 mF 1,5 mF
Energy stored in the storage element 1,96×103 J 8,26 J
Total cost of the system high Reduced
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مجلة الجامعة الأسمرية للعلوم الأساسية والتطبيقية
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