THD analysis of SPWM & THPWM Controlled Three phase Voltage Source Inverter

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 04 Issue: 10 | Oct -2017 www.irjet.net p-ISSN: 2395-0072
© 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 391
THD analysis of SPWM & THPWM Controlled Three phase Voltage
Source Inverter
Mohd Esa1 and Mohd Abdul Muqeem Nawaz2
1,2M.E. Student, EED, Muffakham Jah College of Engineering and Technology, Hyderabad, India
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Abstract –The aim of this paper is to determine the Total
harmonic distortion (THD) of three phase voltage source
inverter (VSI) fed R-L load. The modulation Techniques usedis
Sinusoidal pulse width modulation (SPWM) and third
harmonic pulse width modulation (THPWM). The Carrier
frequency is varied to analyze its effect on Current THD and
Voltage THD.This paper also presents the comparison of
SPWM and THPWM controlled Inverter in terms of THD. The
simulation result shows THPWMhasbetter performancewhen
compared to SPWM. The simulation of circuit is done by using
MATLAB/Simulink.
Key Words: SPWM, THPWM, THD, Carrier Frequency.
1. INTRODUCTION
1.1 Stepped & PWM Inverters
Inverter converts input DC voltage into a.c. output voltage.
Three phase inverters are normally used for high power
applications [1].The applications of inverters include
uninterrupted power supply (UPS), a.c. motor speed
controllers etc. Voltage source inverter is capable of
supplying variable frequency variable voltage for speed
control of induction motors.VSI can be operated as stepped
wave inverter or a pulse width modulated (PWM) inverter.
For stepped wave inverter Output voltage can be varied by
varying input DC voltage. When input voltage is DC, variable
DC input voltage is obtained by connecting a chopper
between DC supply and inverter. When input voltage is a.c.,
variable DC input is obtained by connecting a controlled
rectifier between a.c. supply and inverter. The disadvantage
of stepped wave inverterislargeharmonicsoflowfrequency
in output voltage. Due to this low frequency harmonics, the
motor losses are increased at all speeds causing derating of
Motor. Harmonics content in induction motor current
increases at low speeds. The above drawbacks are
eliminated by using Pulse Width Modulated(PWM)inverter.
Harmonics and losses get reduced in PWM inverters.
For PWM inverter output voltage and frequency can be
controlled without external control. When input voltage is
DC, it is directly connected to PWM inverter. When the input
voltage is a.c., DC is obtained by connecting a diode bridge
rectifier and output of rectifier is connected to PWM
inverter.PWM basedinverterisconsideredinthis paper over
stepped wave inverter because of its harmonics reduction
ability.
1.2 Control Strategies
Various PWM control strategies have been developed in the
past two decades [2].To obtain variation of output voltage
and frequency PWM control strategies such as Sinusoidal
pulse width modulation (SPWM),Third harmonic pulse
width modulation (THPWM), Space vector pulse width
modulation(SVPWM) and 60° PWM are most commonly
used for three phase inverters. SPWM is simplest of all the
above PWM techniques. It was introduced by schonung and
stemmler in 1964[3]. The required signals for gates of
inverter are generated by comparing reference sine wave
and triangular carrier signal in SPWM technique. In 1975
Buja developed THPWM technique.THPWM is implemented
in same manner as SPWM the difference is thatreference a.c.
waveform is not sinusoidal but consists of bothfundamental
component and third harmonic component[1],[4]. The
advantages of PWM techniques are that they are easy to
implement and control, reduces lower order harmonics
[5].SPWM and THPWM techniques are analyzed and
compared in terms of harmonics in this paper.
1.3 Total harmonic distortion
Harmonic distortion is caused bynonlineardevicesinpower
system. A nonlinear device is one in which current is not
proportional to applied voltage.IEEE Standard 519-1992
recommends the requirements for harmonic control in
electrical power systems [6]. Thequalityof Outputvoltage of
inverter strongly related to total harmonic distortion
[7].THD is the measure of effective value of harmonic
components of a distorted waveform.
Where h is characteristic harmonic order, is harmonic
voltage and is fundamental voltage.

Where h is characteristic harmonic order, is harmonic
current and is fundamental current.
Fast Fourier transform (FFT) is used to do the spectral
analysis of phase voltage and current of inverter output and
used as useful tool for THD calculations. The algorithm
requires a large amount of calculations but with MATLAB
simulation software, calculations are done easily.
2. SINUSOIDAL & THIRD HARMONIC PWM TECHNIQUES
2.1 Sinusoidal pulse width Modulation
Sinusoidal PWM switching scheme is easy to implement in
both analogue and digital circuit. It is most popular in
Industrial applications. A carrier signal of a triangular shape
is compared with three phase sinusoidal reference signal to
generate gating signals for triggering switches of inverter as
shown in figure 2.1.2
Fig-2.1.1: Three phase voltage source inverter
Carrier signal frequency is very high when compared to
reference signal. The modulation index is ratio of amplitude
of reference signal to amplitude of carrier signal.
Where =Amplitude of reference signal, =Amplitude of
Carrier signal
The frequency of reference signal determines the inverter
output frequency & amplitude of reference signal controls
the modulation index and in turn the rms output voltage.
The harmonic distortion of SPWM is higher than other
switching schemes especially at high modulating index.
Switching losses are also high inSPWM. SPWMissimplestto
understand but it is unable to fully utilize DC bus voltage.
2.2 Third harmonic pulse width modulation
In order to improve the inverters performance THPWM
technique was developed. THPWM is improved sinusoidal
PWM technique, which adds a third order harmonic content
into sinusoidal reference signal of fundamental frequency.
When peak of sine+one sixth of the 3rd harmonic signal is
0.866, the amplitude of fundamental equals to unity.
When peak of sine+one sixth of the 3rd harmonic signal is
unity, the amplitude of fundamental equals to 1.155.
.
Fig-2.1.2: Sinusoidal pulse width modulation
Addition of third harmonic to sinusoidal reference leads to
15.5% increase in the utilization rate of the DC voltage. The
comparator output is used for controlling the inverter
switches exactly as in SPWM inverter.
Fig-2.2.1: Third harmonic pulse width modulation

3. SIMULATION & THD ANALYSIS OF SPWM &
THPWM CONTROLLED VSI
3.1 Simulation of SPWM & THPWM controlled VSI
A three phase Voltage source inverter with SPWM and
THPWM controlled techniques is simulated in MATLAB
Simulink.
MATLAB Simulation parameters are
1. Switching Frequency=1 KHz to 3 KHz
2. System Frequency=50 Hz
3. Load resistance(R) =10 Ohm
4. Load Inductance (L) =50e-3 Henry
5. Input D.C voltage=220V
6. Modulating index (M.I) =1(Unity)
Fig-3.1.1: Simulink model for PWM based VSI
Fig-3.1.2: Simulink model for SPWM switching signal
generation
Fig-3.1.3: Simulink model for THPWM switching signal
Generation
Fig 3.1.4, 3.1.5 shows carrier signal, reference signal
comparisons for SPWM and pulses generated by SPWM
strategy for triggering switches of inverter circuit
respectively.
Fig-3.1.4:Comparision of carrier signal and reference for
SPWM generation

Fig-3.1.5:Pulses for triggering switches of 3 phase VSI
using SPWM Statergy
Figure 3.1.6, 3.1.7 shows carrier signal, reference signal
comparisons for THPWM and pulses generated by THPWM
strategy for triggering switches of inverter circuit
respectively.
Fig-3.1.6:Comparision of carrier signal and reference for
THPWM generation
Fig-3.1.7:Pulses for triggering switches 3 phase VSI using
THPWM Statergy
Fig 3.1.8, 3.1.9, 3.1.10 shows phase voltage,line voltage and
phase current waveforms for SPWM statergies and figure
3.1.11, 3.1.12, 3.1.13 shows phase voltage, line voltage and
phase current waveforms for THPWM statergies
respectively.
Fig-3.1.8: Phase voltage waveforms of SPWM controlled 3
phase VSI
Fig-3.1.9: line voltage waveforms of SPWM controlled 3
phase VSI

Fig-3.1.10: Phase current waveforms of SPWM controlled
3 phase VSI
Fig-3.1.11: Phase voltage waveforms of THPWM
controlled 3 phase VSI
Fig-3.1.12: line voltage waveforms of THPWM controlled
3 phase VSI
Fig-3.1.13: Phase current waveforms of THPWM
controlled 3 phase VSI

3.2 THD analysis of phase current and voltage
Fig-3.2.1: THD analysis of SPWM controlled inverter’s
phase voltage at 1000 Hz carrier frequency
phase current at 1000 Hz carrier frequency
Fig-3.2.3: THD analysis of THPWM controlled inverter’s
Fig-3.2.8: THD analysis of TPWM controlled inverter’s
4. RESULT
In this THD analysis of SPWM and THPWM controlled
voltage source inverter the carrier signal frequencyisvaried
from 1000Hz to 3000Hz.Table-4.1showscomparativeTHDV
and THDI values of VSI with SPWM and THPWM control
strategies.
Table -4.1: The variation of current THD and voltage THD
for both SPWM and THPWM controlled 3 phase VSI
Carrier
Frequency
(Hertz)
SPWM THPWM
THDV
(%)
THDI
(%)
THDV
(%)
THDI
(%)
1000 72.97 3.40 62.81 2.80
1500 71.55 2.39 67.80 2.03
2000 65.98 2.06 53.89 1.77
2500 87.69 2.30 76.70 1.64
3000 71.61 2.11 63.49 1.57
The variation of current THD and voltage THD with
reference to carrier frequency in form of plot for SPWM
controlled inverter is shown in chart 4.2 and chart 4.1
respectively.

Chart-4.1: Variation of voltage THD with reference to
carrier frequency for SPWM controlled inverter.
Chart-4.2: Variation of current THD with reference to
carrier frequency for SPWM controlled inverter
The variation of current THD and voltage THD with
reference to carrier frequency in form of plot for THPWM
controlled inverter is shown in Chart 4.4 and Chart 4.3
respectively.
carrier frequency for THPWM controlled inverter.
carrier frequency for THPWM controlled inverter
Minimum current and voltage THD’s for SPWM fed inverter
are 2.06% and 65.98% respectively, are obtained at carrier
frequency of 2000Hz. It is advisable to consider 2000Hz as
carrier frequency for SPWM controlled inverter as current
THD is minimum.
Minimum current and voltage THD’s for THPWM fed
inverter are 1.57% and 53.89% respectively, areobtainedat
carrier frequencies of 3000 Hz and 2000 Hz respectively.
Usually minimum current THD is consider as the best for
selecting the appropriate carrier frequency for a circuit It is
advisable to consider 3000 Hz as carrier frequency for
THPWM controlled inverter as current THD is minimum.
The comparison of current THD’s for SPWM and THPWM
respectively are shown in Chart 4.5 and comparison of
voltage THD’s for SPWM and THPWM respectively are
shown Chart 4.6
Chart-4.5: Comparison of voltage THD’s for SPWM and
THPWM controlled VSI

Chart-4.6: Comparison of current THD’s for SPWM and
THPWM controlled VSI
5. CONCLUSIONS
A three phase VSI has been implemented with SPWM and
THPWM control strategies. Analysis of current THD and
voltage THD is done at carrier frequencies from 1000Hz to
3000Hz.Simulation results of SPWM controlled inverterand
THPWM controlled inverter are compared. From chart 4.5
and chart 4.6 it can be concluded that THPWM provides
better quality of output voltage and current whencompared
to SPWM controlled inverter i.e. both current THD and
voltage THD is lesser in case of THPWM.Although there is
variation in current THD and Voltage THD with variation in
Carrier frequency, it is clear that current THD is well below
5% as specified by IEEE standards in both SPWM & THPWM
inverters.
REFERENCES
[1] Muhammad H. Rashid, “Power Electronics-Circuits,
Devices and Applications” Pearson Education Incorporated,
2005.
[2] R.K. Pongiannan, and N. Yadaiah, “FPGA Based Three
Phase Sinusoidal PWM VVVF Controller,” IEEE ICEES
(International Conference on Electrical Energy Systems), pp.
34-39, 2011.
[3] J.Y. Lee, and Y.Y. Sun, “A New SPWM Inverter with
Minimum Filter Requirement, International Journal of
Electronics, Vol. 64, No. 5, pp. 815-826, 1988.
[4] Berrezzek Farid and Omeiri Amar, “A Study of New
Techniques of Controlled PWM Inverters” European Journal
of Scientific Research, Vol.32, No.1, 2009.
[5] Mahesh A. Patel, Ankit R. Patel, Dhaval R. Vyas and Ketul
M.Patel, “Use of PWM Techniques for Power Quality
Improvement” International Journal of Recent Trends in
Engineering, Vol. 1, No. 4, May 2009.
[6] "IEEE Recommended Practices and Requirements for
Harmonic Control in Electrical Power Systems," IEEE Std
519-1992.
[7] M. Baumann and J. W. Kolar, "Comparative evaluation of
modulation methods for a three-phase/switch buck power
factor corrector concerning the input capacitor voltage
ripple," in Power Electronics Specialists Conference, 2001.
PESC. 2001 IEEE 32nd Annual, 2001, pp. 1327-1332 vol. 3.
BIOGRAPHIES
Mohd Esa is currently pursuing M.E.
(Power Electronics systems) from
Muffakham Jah College of Engineering
and Technology,Hyderabad. HeReceived
his B.E degree from Osmania University,
Hyderabad. He is Member of
International Association of Engineers
(IAENG), Hong Kong. His research of interests includes
Multi level inverters and electric drives.
Mohd Abdul Muqeem Nawaz, was Born
in Hyderabad, India in 1994.He received
his B.E degree from Osmania University;
He is currently pursuing M.E (Power
Electronicssystems)fromMuffakhamJah
College of Engineering and Technology,
Hyderabad. He is Member of
International Association of Engineers (IAENG), Hong
Kong. His research interests include power electronics,
FACTS devices and power electronics applications to
power systems.

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