This document summarizes a research paper that proposes a harmonic mitigation method for a DC-AC converter without using a low pass filter. Specifically, it suggests using sine wave modulation of the converter along with injection of specific harmonics calculated using Fourier analysis to cancel out existing harmonics. A proportional-resonant integral controller is also used to eliminate any DC offset. Simulation results show the total harmonic distortion is reduced to 11.15% using this approach, avoiding the need for an output filter. The proposed method continuously monitors and mitigates harmonics in the output to improve power quality.

1. INTERNATIONAL JOURNAL FOR TRENDS IN ENGINEERING & TECHNOLOGY
VOLUME 5 ISSUE 2 – MAY 2015 - ISSN: 2349 - 9303
230
Harmonic Mitigation Method for the DC-AC
Converter in a Single Phase System
PrabhuPratap Singh.C.S Lakhmanan.M
Anna University,Chennai, EEE Anna University, Chennai, EEE
csppsingh@yahoo.in lakshmanan.muthuramu@gmail.com
Abstract-This project suggest a sine-wave modulation technique is to achieve a low total harmonic distortion of Buck-
Boost converter connected to a changing polarity inverter in a system. The suggested technique improves the harmonic
content of the output. In addition, a proportional-resonant Integral controller is used along with harmonic compensation
techniques for eliminating the DC component in the system. Also, the performance of the Proposed controller is analyzed
when it connecting to the converter. The design of Buck-Boost converter is fed by modulated sine wave Pulse width
modulation technique are proposed to mitigate the low order harmonics and to control the output current. So, that the
output complies within the standard limit without use of low pass filter.
Keywords- DC-AC converter, PWM, Proportional Resonant controller, Injected harmonics, Fourier analysis
I Introduction
The rapid development of renewable energies will
increase the quality of power supplies, efficient and
cheap in generation. Most of the renewable energies
generate DC power, so there is in need of at least one
inverter at the load side. Many commercial inverters are
available in the market. Most of those inverters are use
the variation of sine PWM full bridge inverter, because
of simplicity in design. Additionally low pass filter
isused at the load side to mitigate the harmonics content
of the output. Although the main hurdles of this
application are increased size, cost due to the filter and
losses of the semiconductor switches performing the
inverting operation at the inverter and converter. Several
PWM methods are proposed to reduce the Harmonic
content. Selective Harmonic elimination solves the
transcendental equations characterizing the harmonics,
So that appropriate switching angles are computed.
Theoretically it was prove as satisfying Method. But
quite difficult to do online and the solution these
equations is computationally intensive.Other methods
including the modification carrier signal or reference
sine wave. All of them are open loop schemes and
ignoring the existing harmonic content of the grid
voltage or the distortion caused by the load. They can
able to reduce harmonics in PWM itself and not at the
terminal bus. Also, Sine wave modulated technique is
used in converter cascaded inverter for improving the
harmonic content when the supplied by an ideal constant
DC source.
Fig.1 Converter topology
In this project, sine – wave modulation of the converter
is improved to minimize the remaining output voltage of
the capacitor when the inverter swaps polarity. Specific
harmonics are injected in order to improve theharmonic
content of the output. However, the injected harmonics
are pre-calculated, according to the expected harmonic
in the system created by PWM itself. In this paper,
output harmonic is continuously monitored and
mitigated. Computational power is consumed mostly for
the measurement of the angle and magnitude of output
harmonics rather than creation of cancellation of the
harmonics. The main controller is PRI controller to
mitigate any DC Offset in the control loop will
propagate through the system. An integral controller is

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used along with the PR controller to ensure that there is
no DC in the output of the inverter side.
2 PWM Techniques and its modification
2.1 Previously proposed techniques:
The duty cycle of the converter is continuously
calculated under constant frequency, so the output DC
voltage equals the rectifier voltage.
D=VDC out/ (VDC out +VDC in)…. (1)
The inverter output containing the higher order
harmonics caused by switching operation of the
switches. The method of modulation is to reduce the
remaining DC voltage at the output of the DC converter
is reduced. When the capacitor has low level of voltage,
then the harmonic contents are improved.
Fig.2. Block diagram of the DC elimination
The idea of reduction of remaining voltage of the
capacitor is accomplished from the block diagram of
Fig. 2. First the ac voltage is sampled in the process.
Then, the Vac, rms and Reference voltage is compared
in the error compensation block .then the duty cycle of
the sine wave of 50HZ is multiplied with error
compensation. Finally the resultant output is compared
with triangular carrier waveform. Initially, the ac
voltage is rectification is proposed for VDC min error
compensation and integrated with comparator for duty
cycle generation.
2.2. Suggested modulation technique
In order to tackle the low order harmonics, cancellation
scheme was proposed with Adaptive harmonic
compensation techniques based on the injection of
mirror harmonics during the construction of the
modulation signal of the DC converter. Harmonic
content of the output is calculated with Fourier analysis.
Then, add low order harmonics to modulation sine wave
of the fundamental frequency, but phase difference of
180 degree to the output harmonics is to be eliminated.
hA(t)=A sin (2*pi*f*t + a)
hB(t)=B sin(2*pi*f*t + b)
hC(t)=C sin(2*pi*f*t + c)……..(2)
Where A,B, C and a, b, c are the phase and magnitudes
and phase angles.
All the three harmonics are at the same frequency, so
convert the function (2) to phasors.
hA(t) to AeJA
hB(t) to Bejb
hC(t) to Cejc
………(3)
The resulting harmonics equals the summation of the
injected and existing harmonics.
Knowing the magnitude and phase of each harmonic in
the output, without harmonic injection, allows us to set
the magnitude of the mirror harmonic injected in this
iteration. Then it will allow for Fourier analysis of the
output to perform again.
Fig.3. creation of pulses for converter

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Fig.4. VDC min elimination
3 Current control scheme with modulation
technique:
The PRI controller was introduced in the converter to
ensure that there is no DC offset in the output side.
Many current controlling scheme was implemented for
current control in the inverter side .In this project,
Current control scheme was implemented in the
Converter side to enhance the modulation technique.
Multi-resonant controller are used with selective
harmonic elimination for improving the harmonic
standards. The main advantage of this method was
simplicity in implementation of the resonant blocks.
But, major drawbacks of this method is affecting the
performance of the controllers due to the variation in the
grid frequency. This will increase the complexity to
control the variation in the grid frequency. Also, the
repetitive controller with harmonic cancellation methods
are sensitive to the frequency variation. This will affect
the stability of the system. The PR controller was used
along with Integral block to overcome those difficulties.
The integral blocks along with the PR to ensure that
there is no DC offset in the output of the system.It was
coupled with Sine wave modulation technique.
Fig.5. block diagram of PRI controller
The PRI controller was derived from the following
equation,
Gi = Ki/s………………(4)
Gpr(s) = Kp + KrS/S2
+Wo2
………………(5)
The plant transfer function is modeled as
Gplant (s)= VDC/Rs + SLs ……….(6)
…………(7)
The above equation shows the PI controller parameters
are then plugged in for the PR controller parameters.
For the PR controller,the expressions obtained are used
for the proportional and resonant gain respectively from
the equations (8) and (9),
……………(8)
………..(9)

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………(10)
Fig. 6. PR Controller transfer function block
Table I
Parameters and their value
Parameter meaning values
VDC DC voltage 65V
Ls Source
inductance
5mH
L Inductance across
converter
3mH
C Capacitance 29Mf
R Load resistance 54ohms
Kp Proportional gain 3.2
Kr Resonant term 594
Ki Integral term 100
4 Simulation results
A DC source of 65v is given to the supply. The buck-
Boost converter operator at a switching frequency of 20
kHz and Inverter changes polarity at 50Hz.
Fig.7. simulation of Buck-boost converter without using filter

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Fig.8. output voltage and current
Fig.9. FFT analysis
The total harmonic distortion of the overall system is
about 11.15%. By further improving the performance of
the PRI controller, the THD of the system will be
reduced within the standard limit. The Fig.9. Shows the
FFT analysis of the system. For the initial state, No
harmonics are to be injected. Then, Fourier analysis of
the output voltage was calculated. By knowing the
harmonic of the output voltage and harmonics are
already injected. Then alter harmonic injection in order
to cancel harmonics at the output was again fed to the
Fourier analysis.
5 Conclusion
The converter is buck-Boost converter in series with
polarity changing inverter. Specific harmonics are
cancelled by using Fourier analysis of the injected
harmonics. PRI controller with the VDC elimination
method are used to improving the harmonic content.
The method makes no distinction of harmonics creation
within the converter or supplied from the sources. The
complete current of PRI controller are also used in Solar
PV application for cancellation of the DC offset to
improving the harmonics standard. The transient
response of the whole system was studied and increase
the overall performance of the system. The PRI
controller and Modulated sine wave method together
improve the quality of the current injected into the load
side.
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Author Profile:
 PrabhuPratap Singh.C.S is pursuing master’s degree in
power electronics and drive in BIT, India.
E-mail – csppsingh@yahoo.in
 Lakhmanan.M is currently working as a Assistant
Professor in electrical and electronics in Anna
University, India.
E-mail-lakshmanan.muthuramu@gmail.com