harmonic identification using fft and harmonics mitigation ... · conventional dc motor though have...

International Journal of Electronics Engineering Research.

ISSN 0975-6450 Volume 9, Number 2 (2017) pp. 281-292

© Research India Publications

http://www.ripublication.com

Harmonic Identification Using FFT and Harmonics

Mitigation with Unit Vector Based APF for the

System Connected to BLDC Drive

1G. Devadasu and 2Dr. M. Sushama

1 Department of EEE, CMR College of Engineering & Technology, Hyderabad, TS, India.

2 Department of EEE, JNTUH College of Engineering, Hyderabad, TS, India.

Abstract

The paper presents identification of harmonics in the system with BLDC

drive and also presents the mitigation of harmonics using unit vector based

active power filter. BLDC motor gained importance in recent days due to its

advantages over conventional DC motors with absence of commutator and

brush assembly. The disadvantage with presence of BLDC drive connected

system is injection of harmonics in to the connected system. Harmonics

affects the other sensitive loads connected to power system and needs to be

eliminated. Identification is the initial step in mitigation of harmonics.

Harmonic identification is done using FFT which is a powerful and simple

technique in harmonic identification. The identified harmonics are mitigated

using an active power filter connected to power system. Active power filter

is controlled using unit-vector theory. Proposed concept is simulated using

MATLAB/SIMULINK software and the results are presented for harmonic

identification and mitigation.

Keywords: Harmonics, Mitigation, BLDC, FFT, APF

1. INTRODUCTION

These days power sector is more concentrated on quality of power sent to industries,

commercial loads and domestic loads due to the presence of non-linear type of loads

like, ballast lamps, fluorescent lamps, electric motor drives, arc lamps, switched mode

power supplies and many more. The mentioned types of non-linear loads are

282 G. Devadasu and Dr. M. Sushama

harmonic productive loads and are the main source of production of harmonics in

power system[1]. The produced harmonics affects the other sensitive loads connected

to the same power system where harmonic productive loads are connected. A good

power quality is hard to define since the power quality which is good and suitable for

a simple refrigerator may not be good for other type of sensitive loads. Power quality

is defined as the resemblance of practical power system close to ideal system.

Harmonics, sag, swell, transients, flickers, noise are some of the power quality issues

that are generally present in power system[2]. Harmonics are dominantly occurred as

power system quality issue due to development in electronic sector and evolution of

different types of non-linear loads[3-5]. Injection of harmonics can deteriorate the

source parameters and can heat motor windings eventually leading to system

malfunctioning. Harmonics are mainly induced due to non-linear characteristics of

current or voltage of equipment connected to power system.

Brushless DC (BLDC) motor gained importance in recent days due to advancements

in power electronics. Evolution of power electronics caused to eliminate conventional

brush and commutator assembly giving rise to electronic commutation in many

applications. Conventional DC motor though have good characteristics suffers from

additional losses , wear and tear, excessive heat, sparks that are produced due to the

presence of brush and commutator assembly. Conventional commutator and brush

assembly converts DC type of induced EMF to AC and vice-versa with their

mechanical operation. Power electronic circuit now replaces the mechanical

conventional commutator and brush assembly with electronic circuit pursuing the

same operation as that of a conventional commutator and brush assembly. BLDC

drive is an adjustable speed drive which is a type of non-linear load causing

harmonics to be induced in to the connected power system. Harmonics induced in to

the system deteriorates the power system parameters like source current and voltage

since non-linear type of loads draws non-linear components of currents.

Custom power devices might be a solution to eliminate or reduce power quality

problems caused from many loads. FACTS devices are type of custom power devices

employed to reduce the risk of power quality problems using power electronics

circuits. Active power filter (APF) is a type of FACTS controller placed in parallel to

the power system network to filter out harmonics in the system by injecting

compensating currents[6-9]. This paper presents the harmonic identification using

FFT analysis. Also this paper discusses the harmonic mitigation using unit vector

theory based active power filter. Power switches in active power filter are controlled

from pulses obtained from unit vector control theory. The paper addresses the

harmonic reduction to nominal value of less than 5% when BLDC drive system is

connected as load to power system

Harmonic Identification Using FFT and Harmonics Mitigation with Unit Vector 283

.

Coupled

inductance

Three-phase

source

L

L

L

S1 S3 S5

S2S6S4

Vdc

Diode bridge

rectifier

L L L

RLoad

Fig.1: Power system with APF for harmonics mitigation

Coupled

inductance

Three-phase

source

L

L

L

Diode bridge

rectifier

L L L

S1 S3 S5

S2S6S4

Vdc

S1' S3' S5'

S2'S6'S4'

Vdc

BLDC

Motor

Commutator

Hall

Sensors

Fig.2: Power system with BLDC drive system and APF for harmonics mitigation

2. SYSTEM CONFIGURATION

The system configuration for diode-bridge rectifier as non-linear load connected and

APF for harmonic mitigation is shown in figure 1. The system shown in figure 1

consists of source, diode bridge rectifier as non-linear load and APF. Diode bridge

rectifier is a type of non-linear load possessing the characteristics of non-linearity.

The non-linear nature of diode bridge rectifier draws only non-linear components of

currents inducing harmonics in to the main power system to which diode bridge

rectifier was connected. Active power filter was connected to main power system as

compensator for harmonics. Active power filter is a type of shunt compensator and so

connected in parallel to main power system. Active power filter compensated

harmonics by injecting compensating currents in to main power system at the point

where it was connected. System configuration with BLDC motor drive connected


injecting harmonics in to the main power system and the APF connected for

harmonics mitigation is shown in figure 2. Figure consists of components with BLDC

drive system, APF. BLDC drive system is fed from DC supply where the main grid

supply of AC type is converted to DC using a diode bridge rectifier. The converted

DC is fed to converter for BLDC where the phases of BLDC are excited or

magnetized for the operation of BLDC through electronic commutation. The position

of BLDC is continuously monitored and fed to electronic

3. FFT ALGORITHM FOR IDENTIFICATION OF HARMONICS

Process to FFT Block

Load RMS Current into

MATLAB file

Test for Harmonics

Test for

Harmonics < 5%

Test for

Harmonics > 5%

Read Load Current

Display Result

Linear Load

Display Result

Non-Linear Load

Fig.3. Flow chart for Harmonic Identification using FFT analysis.

The exactness of custom power device can be assessed with the identification of

harmonics. Many methods are available to identify the harmonics in power system

network but in this paper FFT analysis was used for quick response and also the FFT

analysis is simple in implementation. FFT algorithms are based on fundamental of

discrete Fourier computation. Measuring the harmonic value, harmonic identification

exporting the values to Simulating file is discussed in detail in flow chart.

The algorithm for identification of harmonics using FFT analysis was depicted in

figure 3. Initially the load current is sensed and the read load currents are sent to FFT

block. The RMS component of current is loaded in to the MATLAB file and the

loaded currents are tested for harmonics. If the test result of harmonics measures the

harmonic content less than 5%, the FFT window displays result that the load is linear

load. If the test result of harmonics measures the harmonic content more than 5%, the

FFT window displays result that the load is non-linear load.


Vdcact

2/3(VLa^2+VLb^2+VLc^2) Vmax

+- PI Vdcref

+-

Isabcact

Isabcref

Hysterisis

control

Gate

pulses

VLabc Imag

Sin wt

Sin (wt-120)

Sin (wt-240)

Fig.4: Unit vector control theory for APF

Conversion of time domain to frequency domain is possible with Fourier transforms.

Fourier series is used when the wave is periodical to calculate the magnitudes and

phases of harmonic component and fundamental component. FFT usage can give fast

and quick response which is essential in many applications and also the

implementation of FFT is very simple.

4. APF FOR HARMONIC MITIGATION AND ITS CONTROL

The system configuration with BLDC motor drive injecting harmonics in to the

connected power system and with active power filter (APF) for harmonic mitigation

was shown in figure 2. The control circuit to produce pulses to APF power switches

was shown in figure 4. Initially the line voltage is sent to generate unit vectors by

generating maximum value from line voltage. The obtained maximum value is

divided and originates sin wt from line value. Similarly sinusoidal phases of three

phases were obtained. The DC link actual voltage is compared with reference DC link

voltage and the error is fed to PI controller to obtain current magnitude signal which

is then multiplied to three unit vectors. The obtained current signals are then

compared to reference current signals and the error is fed to hysteresis current

controller to produce pulses for power switches of APF thus producing gate pulses.

Coupled

inductance

Three-phase

source

L

L

L

Diode bridge

rectifier

L L L

S1 S3 S5

S2S6S4

Vdc

S1' S3' S5'

S2'S6'S4'

Vdc

BLDC

Motor

Commutator

Vdcact

2/3(VLa^2+VLb^2+VLc^2) Vmax

+- PI Vdcref

+-

Isabcact

Isabcref

Hysterisis

control

Gate

pulses

VLabc Imag

Sin wt

Sin (wt-120)

Sin (wt-240)

Fig.5: Proposed system configuration with Unit vector control theory for APF and

BLDC drive


Figure 5 shows the proposed system for harmonic mitigation using APF controlled

with unit vector control theory and also BLDC drive system connected to system

injecting harmonics.

5. RESULTS AND DISCUSSIONS

Case 1: Result of FFT analysis Linear Load exists at node A & node B

Fig.6. Simulated Result obtained from Matlab two nodes connected to linear Loads.

Fig.6. shows result of FFT analysis when linear load is connected at load point. Only

linear load is connected.

Fig.7. Simulated wave form of Source Current at two linear loads connected

nodes A &B.


Figure 7 illustrates the source current of linear loads connected at load point when

only linear type of loads is connected.

Case 2: when linear load connected at node A & Non- Linear Load connected at node

B

Fig.8. Result obtained from Matlab linear load connected at node A & Non-Linear

Load at node B

Figure 8 shows FFT window when linear load is connected at point A and non-linear

load at point B. results illustrates that harmonic load exists at node B and linear load

at A. It shows total harmonic distortion with 9.80%, and from total THD 5th harmonic

component has major contribution with 8.90%

Fig.9. Simulated waveform of linear load connected at node A & Non-Linear Load at

node B

Fig.9.Shows voltage and current is Non-Sinusoidal because the 3-ph ac source is

tethered with Non-Linear Load at node B. Here the harmonics which affects the

source current wave form is presented.


Case 3: when linear load connected at node B & Non- Linear Load connected at

node A

Fig.10. Result obtained from Matlab linear load connected at node B & Non-Linear

Load at node A

Fig.10. Shows that non-linear load affects the voltage and currents with harmonics at

node A. linear load could not affect at node B it is identified from FFT analysis. The

Total Harmonic Distortion = 10.55% at α= 00. The 5th order harmonic component has

major contribution with 9.58%.

Fig.11. Simulated wave form of linear load connected at node B & Non-Linear Load

at node A

Fig.11.Shows voltage and current waveform with Non-Linear Load and linear load is

connected at respective nodes of A and B.


Fig12: Result of Source current, Load current, Injected current, Source voltage of

power system

Figure 12 shows the source current, load current, injected voltage from APF and

source voltage of power system. source current is maintained with normal shape and

very less harmonics. Load current since load is of non-linear nature, draws non-linear

currents as shown in result. Compensating currents are nullifying components to

harmonics injected by load. Source voltage is maintained with normal shape.

Fig. 13 Simulation result of Stator current, Stator back EMF voltage


Figure 13 shows simulation result of stator current and stator back EMF of BLDC

motor. Stator current of BLDC is non-linear in shape and back EMF is trapezoidal in

shape.

Fig. 14 Simulation result of speed and Electromagnetic torque of BLDC

Figure 14 shows the simulation result of speed and torque characteristics of BLDC

motor. BLDC runs at 3000 rpm and torque is at 5Nm shown in results.

Fig. 15 Power factor angle between Source voltage and Source current


Figure 15 shows the power factor angle between source voltage and current. The

power factor angle is almost zero indicating power factor is maintained nearer to

unity.

Fig. 16 Load current THD

Fig. 17 Source current THD

Figure 16 shows the harmonic distortion in load current showing 92.5% of harmonic

distortion in load current. Figure 17 shows harmonic distortion in source current after


compensation showing 5.2% of distortion which is within nominal value. Presence of

APF reduces the harmonic distortion to below nominal value

REFRENCES:

[1] P. Mattavelli, “A closed-loop selective harmonic compensation for active

filters,” IEEE Trans. Ind. Appl.,vol.37, pp. 81-89, Jan./Feb. 2001.

[2] K. Srinivasan, “On separating customer and supply side harmonic

contributions,” IEEE Trans. Power Del., vol.11 pp. 1003–1012, Apr. 1996.

[3] Zainal Salam, Tan Perng Cheng and Awang Jusoh,“Harmonics Mitigation

Using Shunt Active Power Filter: A Technological Review”, ELEKTRIKA,

vol. 8,pp 17-26, 2006.

[4] Lucian Asiminoaei, Frede Blaabjerg, Steffan Hansen,“Evaluation of Harmonic

Detection Methods for Shunt Active Power Filter Applications”, Institute of

Energy Technology, Denmark.

[5] P. Salmerón, R.S. Herrera,” Instantaneous reactive power theory-A general

approach to poly-phase systems”, Electric Power Systems Research ., vol.79,

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[6] S. B. Sakunde , V. D. Bavdhane, “Level Shifted Pulse Width Modulation in

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International Journal of Science and Research (IJSR), Volume 4 Issue 7, pp.

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DSTATCOM with Multiple Feeders in Stand Alone WECS by Mitigating

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[8] S. Rechka, E. Ngandui, J. Xu, and P. Sicard, “A comparative study of

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[9] P. H. Swart, M. J. Case, and J. D. van Wyk,- On techniques for localization of

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Conditions, Stresa, Italy, Sep. 8–10, 1999

http://www.indjst.org/index.php/indjst/article/view/54089



harmonic identification using fft and harmonics mitigation ... · conventional dc motor though have...

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