International Journal of
Applied Sciences, Engineering and Technology
Vol. 03, No. 01, Jan-Dec 2014
Harmonics Analysis of Multilevel Inverter based Single Phase
AC-DC-AC Converter
KR. DHEIVARAYAN, N. PRIYA
Department of Electrical and Electronics Engineering, SASTRA University, Tamil Nadu, India
Email: dheivarayankr@gmail.com, priyalakshanya@gmail.com
Abstract: This paper introduces a new topology for multilevel inverter based single phase AC-DC-AC
converter for different loads. The proposed converter consists of two stages; a controlled rectifier converting AC
supply to DC supply and a multilevel inverter converting DC supply to AC supply cascaded to the rectifier.
Multilevel inverter is preferred for the proposed system, since it increases the number of output voltage level
and hence the Total Harmonic Distortion (THD) is reduced. Thus the different types of loads R, RL and Motor
loads are fed with less harmonic AC voltage which will increase the overall efficiency of the system. Closed
loop control is done for controlling the output voltage of the inverter using PI Controller. Overall system is
simulated using MATLAB/SIMULINK.
Keywords: Multilevel inverter, AC-DC-AC converter, Total Harmonic Distortion
I. Introduction:
Single phase AC-DC-AC converters are employed in
large number of applications such as Motor Drives,
Yaw Drives Traction and Micro Grids. Conventional
AC-DC-AC converters which are widely used for
power utility and drive applications are the indirect
ac-ac converters. It is necessary to concentrate on
adjustable speed drives. The converter connected to
the source is a voltage source rectifier and the load
side converter is a voltage source PWM inverter. The
DC bus is provided between the rectifier and inverter
parts of the drive. The ripple content in the output
voltage of rectifier must be removed before any
transistor switches “on”. If not, this distortion will
show up in the output to the load. The inverter part is
made up of group of transistor and diode
combinations. This converts the DC supply again to
AC. The transistor devices of the inverter are
switched on and off many times for every half cycle,
hence a pseudo sinusoidal current waveform is
generated.
Figure 1.1 Conventional Single Phase AC-DC-AC
Inverter
Normally the induction motor drive with performance
improved AC-DC-AC converter must have the
properties like: In the voltage source Inverter (VSI)
side, mainly it should have significant torque and flux
operation and maximum output torque for given range
of speed operation [1]. In the rectifier fed DC side
there should be Bi-Directional power flow, increased
Total Harmonic Distortion (THD) in input and
Reduction of link capacitor Range.
This method of power conversion has disadvantages
like, it uses an IGBT PWM inverter for DC-AC
conversion, in which the ac line voltage is not pure
sinusoidal, and hence there may be harmonics located
at high frequencies causing more THD. In case if
these inverters are used for Adjustable Speed Drives
(ASD), then the voltage ranges used are low, this
happens due to factors like:
i. The presence of increased dv/dt in the pulse
width modulated ac line voltage is not tolerable
in the average to maximum voltage ranges
ii. Sharing of load power by only four switches of
the inverter.
There are two methods to approximate near-sinusoidal
voltage by utilizing four-switch inverter.
i.Current Source Inverter (CSI) connected to a
capacitive filter.
ii.Voltage Source Inverter (VSI) which includes an
inductive (L) or combination of L&C filter at the
load terminals.
Even though above said topologies have an advantage
of producing near sinusoidal voltage waveforms, but
they have a disadvantage, that load power is shared
only among four power valves for a single phase
inverter. So it is difficult when this converter is
connected to an adjustable speed drive, since it
reduces the motor performance. Hence multilevel
inverter topology is used in the proposed system to
reduce the dv/dt and to share load power among
different switches [4]. Conventional converter also
uses a diode bridge rectifier for ac-dc conversion
hence control over the DC voltage is lost, but it is
converted to firing angle controlled Thyristor Bridge
in proposed model. A LC ripple filter is used to
remove the ripples in dc output of rectifier. Then this
proposed converter is used for a Single phase
Induction Motor provided with an auxiliary winding
for direct phase angle control [7].
IJASET 030104 Copyright © 2014 BASHA RESEARCH CENTRE. All rights reserved.
KR. DHEIVARAYAN, N. PRIYA
Direct phase angle control of Single phase induction
motor involves the Magnitude and Phase angle
control of Auxiliary winding supply of the motor in
order to control the motor torque, which in turn
controls the speed of the motor. This is done by
generating the reference voltage for auxiliary winding
based on the feedback from motor’s speed.
This paper proposes a control of single phase thyristor
bridge rectifier along with a new hybrid Multilevel
Inverter. For the single phase loads, the output voltage
of the multilevel inverter is fed back to the controller
and compared with the reference voltage. Based on
the load voltage requirements the gating signals are
generated for the thyristors.
Figure 1.2 Overall Block Diagram
II. Proposed Hybrid Multilevel Inverter:
The New Hybrid cascaded seven level inverter shown
in Fig. 2.1. is an alternative idea for conventional
cascaded multilevel inverter. The name hybrid
indicates the combination of one H-bridge and two
auxiliary switches with dc voltage source [2] [5]. The
seven level inverter generates a output voltage such
as: 0, +2Vdc/3, +Vdc/3, +Vdc, -Vdc/3, -Vdc/3,-2Vdc.The
circuit configuration of the proposed Inverter consists
of six power switches, one bidirectional switch and
three capacitors to balance the voltage from dc supply
which act as voltage divider.
Table 2.1: Switching States
Output voltage Conducting switches
Zero
Vdc/3
2Vdc/3
Vdc
-Vdc/3
-2Vdc/3
-Vdc
S3-S4(S1-S2)
S4-S6
S4-S5
S1-S4
S2-S5
S2-S6
S2-S3
A. Multiple Reference PWM Technique
The modulation strategy given for the proposed 7level
hybrid inverter is Multiple Reference Modulation
Technique. In this technique, three reference signals
Vref1, Vref2,Vref3 are obtained by taking absolute of
sine wave operating at fundamental frequency, and
the reference signals below zero level is offset with a
value and in phase with each other. All the three
reference signals with carrier frequency (fs) have same
amplitude of Am. It uses one carrier wave which is a
triangular wave with amplitude of Ac operating at
switching frequency (fsw). Fig.10 shows the switching
pattern for the switch 1 to switch 6.
Figure 2.2 Multiple Reference Modulation Technique
for 1kHz
The gate pulses are generated by means of comparing
the 3 reference wave with a one carrier wave as
shown in fig. 2.2. When Vref1 crosses the peak
amplitude of Vcarrier, Vref2 will be compared with Vcarrier
till ref2 crosses the amplitude of carrier signal. From
that, Vref3 will take the responsibility till its amplitude
crosses zero if Vref3 reaches zero, Vref2 will take the
responsibility till its amplitude crosses zero then
Vref1will be compared with carrier signal.
The modulation index for a Multiple Reference
modulation technique is given by:
Figure 2.1 Proposed Hybrid Multilevel Inverter
Table 2.1 describes the operation of new hybrid
multilevel inverter with 7levels and also discussed the
conduction of switches to produce the different levels
of load voltage.
Where,
Am- Amplitude of reference wave
Ac- Amplitude of carrier wave
III.
Design of LC Filter:
This filter consists of inductor L in series with load
and capacitor C across the load. The dominant
harmonics in the output dc voltage of rectifier is
blocked by the inductor. The capacitor which is
connected in parallel to the inductor provides an easy
International Journal of Applied Sciences, Engineering and Technology
Vol. 03, No. 01, Jan-Dec 2014, pp 14-18
Harmonics Analysis of Multilevel Inverter based Single Phase AC-DC-AC Converter
path to the nth harmonic ripple currents. The general
formula for effective filtering is given as [8],
For Voltage Ripple Factor, VRF=0.5, f=50Hz, R L=50
 , LL=10mh, L&C values are calculated as,
10
C=
=50  F,
2
2 R  (2LL ) 2
connected in cascade to Thyristor bridge rectifier. A
230V 1 phase AC input is given to Thyristor Bridge
Rectifier. By controlling firing angle of Thysistor
under closed loop using PI controller, the input
voltage to the load is controlled significantly.
Fig. 5.1 shows the simulation of AC-DC-AC
converter with R, RL and Motor load and closed loop
control of output voltage. Here RMS value of the
output voltage is compared with a reference voltage to
generate an error. This error is synthesized using a PI
controller and then given to the firing angle generator.
This firing angle generator (shown in fig. 5.2.)
generates the required delay angle to thyristors.
 =0.5
1
VRF= 2 

2
3  (2 ) LC 1
IV.
SIMULATION RESULTS
The given AC-DC-AC converter is simulated using
MATLAB/SIMULINK. Proposed 7-Level Inverter is
simulated by generating the gating signals using
Multi-Reference PWM technique. Then inverter is
Discrete,
Ts = 1e-005 s.
RM S
Vline
powergui
firing angle
frequency
pulse width
Van
50
frequency
rms signal
PID Controller
alpha_deg
pulse
PI(s)
ref
Step
4 Pulse Generator
FIRING ANGLE
GENERATOR
+ v
-
5
pulse width
g
7-level
INVERTER
L1
+
+
A
-
B
A
B
+ -i
-
Scope1
+ v
-
C
Thyristor Converter
Figure 5.1 Simulation of AC-DC-AC Converter (Thyristor Bridge)-Closed Loop
RM S1
Trigonometric
Function1 Divide1
Gain2
1
alpha_deg
-K-
acos
Fcn
2
ref
(2*1.4144/pi)*u(1)
rms
signal
Figure 5.2 Firing Angle Generator
Vab
Amplitude (V)
400
200
0
-200
-400
0.1
0.105
0.11
0.115
0.12
0.125
0.13
0.135
0.14
0.145
0.15
0.13
0.135
0.14
0.145
0.15
Iab
Amplitude (A)
4
2
0
-2
-4
0.1
0.105
0.11
0.115
0.12
0.125
Time
Figure 5.3 Output of Multilevel Inverter
International Journal of Applied Sciences, Engineering and Technology
Vol. 03, No. 01, Jan-Dec 2014, pp 14-18
1
Vline
KR. DHEIVARAYAN, N. PRIYA
For R load, R=100ohm, L=10mH
Vab
Amplitude (V)
500
0
-500
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.6
0.7
0.8
0.9
1
Iab
Amplitude (A)
5
0
-5
0
0.1
0.2
0.3
0.4
0.5
Time
FFT Analysis of output current of Converter:
Figure 5.4 Simulation Output of AC-DC-AC Converter for R-Load (Thyristor Bridge)-Closed Loop
For RL load, R=100ohm, L=10mH
Vab
Amplitude (V)
500
0
-500
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.6
0.7
0.8
0.9
1
Iab
Amplitude (A)
5
0
-5
0
0.1
0.2
0.3
0.4
0.5
Time
FT Analysis of output current of Converter
Figure 5.4 Simulation Output of AC-DC-AC Converter for RL-Load (Thyristor Bridge)-Closed Loop
For Single Phase Induction Motor load
Vab
Amplitude (V)
400
200
0
-200
-400
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
1.6
1.7
1.8
1.9
2
Iab
20
10
0
-10
-20
1
1.1
1.2
1.3
1.4
1.5
Time
FFT Analysis of output current of Converter:
Figure 5.4 Simulation Output of AC-DC-AC Converter for Motor-Load (Thyristor Bridge)-Closed Loop
International Journal of Applied Sciences, Engineering and Technology
Vol. 03, No. 01, Jan-Dec 2014, pp 14-18
Harmonics Analysis of Multilevel Inverter based Single Phase AC-DC-AC Converter
Figure 5.3 shows that for a change in reference
voltage from 200V to 250V, there is change in output
voltage of the converter. FFT analysis shows that the
THD of the output current of the converter is given in
table 5.1.
Table 5.1 THD Comparison
LOAD
THD in %
R
16.32
RL
3.79
Motor
8.32
V. Conclusion:
Thus, a new configuration of AC-DC-AC converter is
designed. The proposed converter requires less
number of power switches compared to conventional
model, it also achieves less total harmonic distortion
and the obtained output voltage is approximately
sinusoidal wave. RL and Single phase motor type of
loads are discussed. Calculations of LC filter to
remove ripples in output of rectifier are done. FFT
analysis shows that THD for output current of R, RL
and motor load is shown. Also a closed loop control
to maintain required output voltage at the load is
provided. This shows that the converter proposed in
the paper reduces the harmonics in output voltage
effectively and reduce THD in AC-DC-AC converter
by utilizing Multilevel Inverter and responses are
shown for different types of Load. So the proposed
converter is well suited for single phase adjustable
speed drives, like Yaw drives employing single phase
motors and controls the axle of wind blade according
to wind direction also for Traction Drives and UPS
Systems.
References:
[1] Euzeli Cipriano dos Santos, Jr., Member, IEEE,
October 2011, Cursino Brandão Jacobina,
Senior Member, IEEE “Component Minimized
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[2] Rodriguez, J. Lai and FZ. Peng, Aug. 2002,
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controls,
and
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IEEE
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of
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[4] B.-R.Lin and H.-H.Lu, July 1999, “Multilevel
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International Journal of Applied Sciences, Engineering and Technology
Vol. 03, No. 01, Jan-Dec 2014, pp 14-18