 Open Access
 Total Downloads : 572
 Authors : T.V.L Obula Reddy, I.E.S Naidu, M. Ramu
 Paper ID : IJERTV2IS3070
 Volume & Issue : Volume 02, Issue 03 (March 2013)
 Published (First Online): 08032013
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Application of Active Filters to Nonlinear Loads
T.V.L Obula Reddy I.E.S Naidu M. Ramu

ech (PS&A) Assosiate professor, Department of EEE, Asst Professor,Department of EEE, GITAM University, Visakhapatnam,India GITAM University, Visakhapatnam,India GITAM University, Visakhapatnam,India
Abstract: During the last years, the increasing research in power transmission and the electrical engineering development around the world, has given an incentive to new expectations in transmission systems.
Power harmonics are called electrical pollution which will degrade the quality of the power supply. As a result, filtering process for these harmonics is needed in order to improve the quality of the power supply. Generally passive filters are used to suppress the harmonics. These usually consist of a bank of tuned LC filters to suppress harmonics. It is proposed to use single active filter in place of passive filters with the objective to reduce harmonic currents.
Key words: Power harmonics; active filter; passive filter

INTRODUCTION
Since the rapid development of the semiconductor industry, power electronics devices have gained popularity in converters. Although these power electronics devices have benefited the electrical and electronics industry, these devices are also the main source of power harmonics in the power system. These power harmonics are called electrical pollution which will degrade the quality of the power supply. As a result, filtering process for these harmonics is needed in order to improve the quality of the power supply. Thus, active power filter seems to be a viable alternative for power conditioning to control the harmonics level in the power system nowadays. Power system normally operates at 50 or 60 Hz. However, saturated devices such as transformers, arching loads such as florescent lamp and power electronic devices will produce current and voltage components with higher frequencies into the power line. These higher frequencies of current and voltage components are known as the power harmonics. The harmonics disturbances in the power supply are caused by the nonlinearity characteristic of the loads. Due to the advantages in efficiency and controllability of power electronic devices, their applications can be found in almost all power levels.
CONTROL OF POWER HARMONICS
Passive Filters
Traditional solutions for these problems are passive filters due to their easy design, simple structure, low cost and high efficiency. These usually consist of a bank of tuned LC filters to suppress current harmonics generated by nonlinear loads.
Passive filters have many disadvantages, such as

Resonance

Large size

Fixed compensation character

Possible overload
Those kinds of approaches are usually suitable for low power applications.
Active Filters
To overcome the disadvantages due to Passive Filters, Active Power Filters (APFs) have been presented as a currentharmonic compensator for reducing the total harmonic distortion of the current and correcting the power factor of the input source. The Active Power Filter is connected in parallel with a nonlinear load. The approach is based on the principle of injecting harmonic current into the ac system, of the same amplitude and reverse phase to that of the load current harmonics. This will thus result in sinusoidal line currents and unity power factor in the input power system. In this case, only a small portion of the energy is processed, which may result in overall higher energy efficiency and higher power processing capability. These kinds of approaches are applicable for lowpower to highpower applications. A three phase shunt APF is typically composed of a threephase bridge converter and control circuitry. Most of the previous control approaches need to sense the load current and calculate its harmonics and reactive components in order to generate the reference for controlling the current of a bridge converter. Those control methods require fast and realtime calculation; therefore, a highspeed digital microprocessor and high performance A/D converters are necessary, which yields high cost, complexity, and low stability.


PWM CONTROL OF ACTIVE FILTER
The main aim of an active power filter (APF) is to generate compensating currents into the power system for canceling the current harmonics contained in the nonlinear load current. This will thus result in sinusoidal line currents and unity power factor in the input power system.
Fig.1 shows the configuration of a threephase active power filter. The active power filter is connected in parallel with a nonlinear load. It consists of a power converter, a DC link capacitor (C2) and a filter inductor (L2). To eliminate current harmonic Components generated by nonlinear loads, the active power filter produces equal but opposite harmonic currents to the point of connection with the nonlinear load.
This results in a reduction of the original distortion and correction of the power factor. The inductor L2 is used to perform the voltage boost operation in combination with the DClink capacitor C2 and functions as a low pass filter for the line current of an active power filter.
The exclusive features of this proposed PWM controlled APF are summarized as follows:

The reference frame transformation and a digital low pass filter are used to compute the harmonics of the nonlinear load current.

The voltage decouplers and polezero cancellation method are used in the current controllers of the active power filter to provide fast current harmonic compensation and simplify the control scheme.

The delay times of both current response of an active power filter and DClink voltage feedback are considered. This results in decreasing the settling time of the DClink voltage and reducing the high frequency current harmonic components of the power system.
In this block diagram (Fig.1) iaL, ibL, icL are load currents, iaf, ibf, icf are filter compensation currents, Vdc is dc link capacitor voltage of the APF. Here load currents, filter currents and dclink capacitor voltages are taken as feedback components. After that load currents, and filter currents are transformed from abc model to dq model through reference frame transformation.
Figure 1: Control Block Diagram of PWM Controlled Active Power Filter
The idL current is passed through the low pass filter for getting fundamental current i1, then by making idL i1 gets total harmonic currents as reference. Here dc link voltage regulator current also taken into consideration for making direct axis filter reference current idf*. q axis filter reference current iqf* is getting simply by making negative of that iqL.
Now these two currents are compared with the actual filter currents idf, iqf and produce the error .This error is after that passed through the current controllers for getting reference voltages. Again these reference voltages are transformed into abc model. These three reference voltages are compared with the triangular wave, and by the method of bipolar voltage switching produce switching pulses for the APF power converter IGBT switches.
Compensating current calculations:
Consider Fig.1, where ea, eb, ec and Vaf, Vbf, Vcf represent the phase voltages of a power system and the input voltages of a power converter, iaf, ibf, icf and Vdc2 denote the input currents of the active power filter and the DClink voltage, respectively. Neglecting the reactors Ls of the input power system, the differential equations of the threephase active Power filter in Fig. .1 can be described as follows.
Where C2 is the capacitance of the DClink capacitor, R2 and L2 are the resistance and inductance of the active power filter line reactors, respectively, fa, fb, fc are Switching functions, and the possible values . For model analysis and controller design, the threephase voltages, currents and switching functions can be transformed to a dqo rotating frame. This yield,
Where the transformation angle of the rotating frame and x is denotes currents, voltages or switching functions. From equations (1)(5), the state model in the rotating frame can be written as


DESIGN CONSIDERATIONS
Where
is the frequency of the power system and the subscripts d and q are used to denote the components of the d and qaxis in the rotating frame, respectively. Equations (6) – (8) will be used to derive the block diagram of the active power fitter and calculate the input voltage commands of power converter.
Let transformation angle be equal to the angle of phase voltage. Assume that the threephase voltages are balanced. This yields the voltage components:
Where Vm is the peak value of the phase voltage of the input power system.
When the phase voltages of power system are balanced, PL and qL depend only on idL and iqL, respectively. For a fully harmoniccurrent compensated active power filter system, the instantaneous real power Ps and reactive power qs from the power system can be expressed as:
Where the fundamental component of the load current i1 can be obtained from the daxis current idL by means of a low pass filter. The corresponding reference currents, and of the active power filter in the rotating frame are
In this, design of Low Pass Filter for Reference Current calculation, Design of DClink Voltage Regulator, Design of Current Regulators etc. are explained in detail.
Design of Low Pass Filter for Reference Current calculation
When is small, the current harmonics on the source side and the DClink fluctuation voltages will decrease, while the settling time of the DClink voltage and the compensation time of current harmonics will increase. Therefore, the design of the cutoff frequency will strikingly affect the active power filter compensation characteristics. For balanced load.
Introducing the frame transformation of equation (5) to equations (17 to 19) the daxis current for balanced load can be written as.
(20)
Similarly for unbalanced load
Equ. (15) and equ. (16) are obtained from the proposed novel calculation method for reference currents of the active power filter by using the load current feedback, reference frame transformation and a digital low pass filter. It is noted that the reference currents can be obtained simply by subtracting the fundamental component from the measured load currents regardless of whether the load is balanced or not.
It can be seen from equations (20) and (21) that the first are the fundamental component of the load current and both are of DC values. The lowest frequency in the daxis current is the third harmonic for the balanced load and the fundamental frequency for the case of unbalanced load. Thus, to obtain the fundamental component for both the cases of balanced and unbalanced loads, the cutoff frequency must lie between the DC frequency (0 rad/sec) and the fundamental
frequency (377 rad/sec). In this paper,
is chosen. This yields a delay time constant of 10ms.

SIMULATION RESULTS AND ANALYSIS
Active Power Filter components
Input Resistance(R2) = 0.03 , Inductance(L2) = 0.25mH,
DcLink Capacitor Capacitance(C3) = 4800 ÂµF.
The simulation was performed on the
powergui Discrete,
Ts= 1e006 s.
INPUT AC source
iaS ibS icS
iaF
iaL i/p
Nonlinear load
iaL ibL icL iaS i/p ibS i/p icS i /p
Here in Fig. total system is shown as five sub systems, those are three phase ac source, which is supplying for nonlinear load. Active Power Filter (APF) ,that is connected parallel to the load and also Total control circuit which is giving pulses to the APF such that it will inject compensation currents into the power line which are opposite in phase to the harmonic currents introduced by the nonlinear loads. Ultimate goal is to improve source side power factor to unity, and protects the power grid from the harmonic currents, so improving the power quality.
Vaf* i/p1
Vbf * i/p
Tga+
Tgb+
Tgc+
Tga+
Tgb+
Tgc+
ibF
icF
VdC
ibL i/p
icL i /p
iaF i /p
Vaf*
Vbf *

RESULTS AND DISCUSSIONS
Here the simulation results are shown for different cases i.e, without filter, with passive filter and with PWM controlled
Vcf * i/p
Tga
Tgb
Tgc
Tga
iaS i/p
Tgb ibS i/p Tgc icS i /p
ibF i /p
icF i /p
VdC i /p
Vcf *
APF and the results are compared.
Case 1: Simulation results of uncontrolled rectifier load
PWM controller
Active power filter
Total Controller
without filter:
Fig. Simulink Model of PWM Controlled APF for the Three Phase Power System.
MATLAB/SIMULINK package. Simulink is a software package for modeling, simulating and analyzing dynamic systems. It supports linear and nonlinear systems,
100
80
60
40
20
Current
Current
0
20
40
60
80
100
Uncontrolled rectifier without filter
0 2 4 6 8 10 12
modeled in continuous time, sampled time, or a hybrid of the
Time
4
x 10
two. The simulation models are built by using the MATLAB7.9 (R2009), Simulink toolbox and are simulated using variable step type of solver.
SIMULINK MODELS OF PWM CONTROLLED APF
Detailed simulink models of power system along with the active power filter is shown below.
FigA: Three Phase source currents Iabc with uncontrolled rectifier load without filter
Ias of uncontrolled rectifier without filter
100
80
60
current
current
40
Test system data:
20
TABLE 1 0
ThreePhase Supply
RMS Voltage (Vrms) = 120v , Source inductance(Ls) = 0.4 mH ,
Supply Frequency (f) = 60 HZ.
Nonlinear Load components
Diode Bridge Rectifier with R1 = 8.67 ,
C1 =3300 ÂµF,
Lo= 3.1 mH.
ThreePhase Supply
RMS Voltage (Vrms) = 120v , Source inductance(Ls) = 0.4 mH ,
Supply Frequency (f) = 60 HZ.
Nonlinear Load components
Diode Bridge Rectifier with R1 = 8.67 ,
C1 =3300 ÂµF,
Lo= 3.1 mH.
20
0 2 4 6 8 10 12
Time
4
x 10
Fig B: Phase current Ia of source with uncontrolled rectifier
without filter
without filter
without filter
without filter
180
160
140
120
voltage
voltage
100
80
60
40
20
0
Vdc of uncontrolled rectifier without filter
0 2 4 6 8 10 12
100
80
60
Ia
Ia
40
20
0
20
40
0 2 4 6 8 10 12
Time
4
x 10
Time
4
x 10
Fig C: Vdc of uncontrolled rectifier without filter
Fig.B: Ia of uncontrolled rectifier with passive filter
1
Normalised Harmonic spectrum of ias
Normalised Harmonic spectrum of ias
0.8
0.6
0.4
0.2
200
180
160
140
120
Vdc
Vdc
100
80
60
40
20
0
0
0 0 2 4 6 8 10 12
0 5 10 15 20 25 30 35 40 45
armonic order
Time
4
x 10
Fig D: Harmonic magnitudes of source current without filter
Fig.A shows the three phase source currents and. Fig.B shows the source current in phase A for the non linear load without filter. From fig.A&B it is clear that the source currents are non sinusoidal and results in presence harmonic components in the system. Fig.C shows the output voltage of rectifier .From FFT analysis of source current Harmonic magnitudes in their order are obtained and are shown in Fig.D. it is observed that 5th ,7th ,11th, 13th ,17th and 19th order harmonics are injected into source current. the system performance can be improved by eliminating the lower order harmonics. The THD in this case is 16.06%.
CASE 2: Simulation results of uncontrolled rectifier load with passive filter:
100
80
60
40
Iabc
Iabc
20
0
20
40
60
80
0 2 4 6 8 10 12
Fig.C: Vdc of uncontrolled rectifier with passive filter
1
Normalised Harmonic spectrum of ias
Normalised Harmonic spectrum of ias
0.8
0.6
0.4
0.2
0
0 5 10 15 20 25 30 35 40 45
Harmonic order
Fig.D: Harmonic magnitudes of source current with passive filter
Fig.A shows the three phase source currents and Fig.B shows the source current in phase A for the non linear load with passive filter. From fig.A&B it is clear that the source currents are non sinusoidal but improved compare with previous A&B that is without filter. Fig.C shows the output voltage of rectifier .From FFT analysis the source current Harmonic magnitudes in their order are obtained and are shown in Fig.D. t is observed that 3rd , 5th ,7th ,11th, 13th ,17th and 19th order
Time
4
x 10
Fig.A: Three Phase source currents Iabc of uncontrolled rectifier load with passive filter
harmonics are injected into source current. The magnitudes of 5th , 7th ,11th and 13th harmonics are reduced compare to the fig.D. The THD in this case is 8.81%.
CASE 3: Simulation results of uncontrolled rectifier load with Active Power Filter (APF):
70
60
50
40
Ial
Ial
30
20
10
0
10
1
Normalised Harmonic spectrum of ias
Normalised Harmonic spectrum of ias
0.8
0.6
0.4
0.2
0
0 5 10 15 20 25 30 35 40 45
Harmonic order
20
0 2 4 6 8 10 12
Fig.E: Harmonic magnitudes of source current with APF
300
250
200
Current
Current
150
100
50
0
Time
Fig.A: Load current of phase A with APF
Ias uncontrolled
4
x 10
Fig.A shows the load of phase A current, Fig.B shows the source current of phase A and Fig.C shows the filter current for the non linear load with active filter. From fig.B it is clear that the source currents are almost sinusoidal. Fig.D shows the output voltage of rectifier .From FFT analysis the source current Harmonic magnitudes in their order are obtained and are shown in Fig.E. It is observed that 3rd and 5th order
harmonics are injected into source current. The magnitudes of 5th and 7th harmonics are very much reduced compare to the previous two cases. The THD in this case is 6.31% only.
50
0 2 4 6 8 10 12
TABLE 5.2 comparison of THD for different cases
250
200
150
100
Current
Current
50
0
Time
Fig.B: Source current of phase A with APF
Iaf Uncontrolled
4
x 10
Alfa(deg ree)
THD
with APF (%)
THD
without filter (%)
THD with passive filter (%)
Uncontro lled case
4.53
13.23
4.98
0
4.54
10.14
5.54
30
5.1
15.12
5.78
60
7.49
18.84
8.27
Alfa(deg ree)
THD
with APF (%)
THD
without filter (%)
THD with passive filter (%)
Uncontro lled case
4.53
13.23
4.98
0
4.54
10.14
5.54
30
5.1
15.12
5.78
60
7.49
18.84
8.27
50
100
150
0 2 4 6 8 10 12
Time
Fig.C: Filter current of phase A
4
x 10
180
160
140
120
Voltage
Voltage
100
80
Vdc uncontrolled
Table5.3 Harmonic values in percentage of fundamental value for = 0
Harmonic
3rd
5th
7th
11th
13th
17th
19th
Without filter
0
14.2
6.41
2.28
1.55
0.93
0.784
With passive filter
.877
7.28
3.55
2.5
1.77
0.79
0.738
With active filter
4.87
1.79
0.4
0
0
0
0
Harmonic
3rd
5th
7th
11th
13th
17th
19th
Without filter
0
14.2
6.41
2.28
1.55
0.93
0.784
With passive filter
.877
7.28
3.55
2.5
1.77
0.79
0.738
With active filter
4.87
1.79
0.4
0
0
0
0
60
40
20
0
0 2 4 6 8 10 12
Time
Fig.D: Vdc of uncontrolled rectifier load with APF
4
x 10
Table5.4 Harmonic values in percentage of fundamental value for = 30
Harmonic
3rd
5th
7th
11th
13th
17th
19th
Without filter
0
36.4
5.62
8.39
1.41
3.85
1.31
With passive filter
0.45
12.2
3.88
5.06
0.66
2.42
0.53
With active filter
5.88
0.93
0.4
0
0
0
0
The Magnitudes of harmonic components in percentage of fundamental frequency components for = 0 is shown in Table5.3. It is observed that passive filter is able to reduce the magnitudes of harmonic components but is not able to eliminated them completely. But Active filter is able to eliminate harmonics of 11th and above. It is able to reduce the magnitude of 5th and 7th orders.
The Magnitudes of harmonic components in percentage of fundamental frequency components for = 30 is shown in Table5.3. It is observed that passive filter is able to reduce the magnitudes ofharmonic components but is not able to eliminated them completely. But Active filter is able to eliminate harmonics of 11th and above. It is able to reduce the magnitude of 5th and 7th order harmonics.

CONCLUSION

In this project, a threephase shunt APF with PWM control has been proposed. The proposed control approach senses only the mains current and the zero crossing of grid voltage. Furthermore, there is no need to calculate the reference for APF inductor current so that the intensive digital computation is eliminated. The proposed approach employs constant switching frequency modulation that is desirable for industrial applications. The simulation results have demonstrated that the proposed approach effectively cancels the harmonic component of the load, so that the all threephase line current are near sinusoidal.
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