Common Mode Voltage Control in Three Level Diode Clamped Inverter

Shiwani S. Rasekar. Int. Journal of Engineering Research and Application www.ijera.com
ISSN : 2248-9622, Vol. 6, Issue 12, ( Part -2) December 2016, pp.30-36
www.ijera.com 30 | P a g e
Common Mode Voltage Control in Three Level Diode Clamped
Inverter
Shiwani S. Rasekar1
, Heena S Sheikh2
1
(Department of Electrical Engineering, Gondwana University, Gadchicholi, India
2
(Department of Electrical Engineering, Gondwana University, Gadchiroli, India
ABSTRACT
This paper presents simple sinusoidal PWM technique to reduced common mode voltage (CMV) at output
terminal of the inverter. Multilevel inverter (MLI) is more suitable in high & medium power application, CMV
is produced at the time of operation in output terminal of inverter. In this paper, an approach to reduced CMV at
output terminal of MLI by using SPWM technique in three level diode clamped inverter (DCMLI) is proposed.
A good transaction between the quality of the output voltage & the magnitude of CMV is achieved in this paper.
The paper presents phase opposition & phase opposition disposition SPWM technique to reduced CMV in
DCMLI. Simulation & experimental result presented to confirm the effectiveness of the proposed technique to
control CMV.
Keywords: Diode clamped MLI, Common mode voltage, Sinusoidal PWM technique, PO & POD technique.
I. INTRODUCTION
Multilevel inverter (MLI) come to attention
of researchers as soon as it was proposed by Nabae
A in 1981. In the area of high power application
multilevel inverter is a first choice where the
demand is increasing & produced energy does not
cope with the increasing demand. To obtain quality
output voltage & current waveform with minimum
ripple content,two level inverter require high
switching frequency along with various PWM
strategies. Two level inverter may not suit for high
voltage applicationbecause of poor quality
output.MLI have draw tremendous interest in the
power industry application. Output of two level
inverter is modified by increasing number of
voltage level in the inverter without requiring higher
rating an individual device can increase the power
rating. Multilevel inverter consist of number of
thyrister switches depends upon levels used in the
inverter. Now a day's thyrister switches replace by
SCR's, BJT's, MOSFET's & IGBTE's depending
upon respective advantages. By switching action of
the inverter switches staircase output is generated
which is also called as multilevel output. As the
number of voltage level increases the harmonic
content of output waveform decreases significantly.
Output generation by switching action results in
common mode voltage (CMV) which are essentially
zero sequence voltages superimposed with switching
noise which will appear at rectifier, inverter & motor
terminals. If not minimized then they appear on the
stator winding with respect to ground results in
motor life expectancy is shortened. As the number of
voltage level increases the synthesized output
waveform has more steps which produce a staircase
wave that approaches a desire waveform &
harmonic distortion of the output waveform
decreases approaching zero as no of level increases.
Fig 1: General block diagram of AC-DC-AC VSI or CSI fed ASD.
The diode clamped multilevel structure is
more suitable for high & medium voltage drives
which are directly connected to utility power system.
Diode clamped inverter is also known as NPC.
Diode clamped inverter deals with one major
problem that is generation of common mode voltage
(CMV) and it is appear at the output terminal of the
inverter. Generation of common mode voltage
results in large bearing current due to heavy shaft
voltage. This leads to failure of motor bearing.
RESEARCH ARTICLE OPEN ACCESS

Therefore best solution for this problem is only to
eliminate CMV permanently.Variable frequency
,variable voltage require in industrial application ,
medium and high voltage level supply given to three
phase induction motors.multilevel inverter have been
establish better counterpart to conventional two level
inverters. Recently number of techniques are used to
eliminate common mode voltage, but in all the
techniques SPWM techniques is more suitable.In
this paper common mode voltage reduction
techniques for three level diode clamped inverter is
represented by using SPWM technique.
II. BACKGROUND
2.1.Three Level Diode Clamped Inverter:
In high voltage & high power application
diode clamped multi level inverter(DCMLI) become
a research hotspot. Multi level inverter is preferred
in between three to nine level inverter. As the
number of voltage level increases total harmonic
distortion is reduced.Three level inverter is shown in
diagram below.
Fig 2: Schematic diagram of three phase, three level diode-clamped inverter.
Three phase diode clamped MLI have three
legs with common DC bus, this DC voltage is
subdivided into switching capacitors. Each diode has
to block the voltage equal to number of switching
above it times the supplied DC voltage. Basically
one leg of DCMLI consist of 'M-1' capacitors &
'M-2' clamping diodes where 'M' is the total voltage
levels & total number of switches are 2(M-1), in the
three level inverter number of switches used are 12,
for the operation in numbering order of Sa1, Sa2,
Sb1, Sb2, Sc1, Sc2, Sa1', Sa2', Sb1', Sb2', Sc1' &
Sc2'. Dc bus consist of two capacitors C1 & C2, six
diodes are used two for each leg. In DCMLI diode is
used as clamping device to clamp the dc bus voltage
so as to achieve staircase output voltage as shown in
the diagram.
Fig 3: Staircase output of the Three level inverter.

2.2.Common Mode Voltage:
Common mode voltage is defined as the
voltage between neutral point of the load & system
ground. It may also defined as the voltage between
neutral point of the load & dc midpoint given
diagram shows generalized drive system , where
Vag, Vbg & Vcg are the voltages between ground to
phase & Vng is the voltage between neutral point of
the motor and system ground. i. e. CMV & it is
given as:
CMV = Vng = (Two level inverter)
CMV = Vng = (Three level
inverter)
CMV= Vng = (Five level
inverter).
Output of the inverter is not purely
sinusoidal but square wave (discrete) , hence sum of
the instantaneous value is not zero & develops
common mode voltage. Meanwhile in purely
sinusoidal three phase system sum of the
instantaneous voltage is zero hence CMV is zero.
Neutral point voltage variation results in generation
of common mode voltage three times the
fundamental frequency. If common mode voltages
are not reduces they results in bearing failure of the
motor connected as load in output terminal.
Fig 4: Three phase VSI inverter with Induction motor connected as a load.
III. CMV CONTROL STRATEGIES
Control strategies for common mode voltage can be
classified as given below:
1. Using some extra hardware circuitry such as
isolation transformer, active passive filter,
common mode choke &dual bridge inverter etc.
2. Using modification in control strategy such as
space vector PWM technique (SVPWM) &
sinusoidal PWM technique (SPWM).
In the first method common mode voltage is
minimized by selecting proper rating of the chock,
but the main issue is that the total harmonic
distortion (THD) increases with controlling CMV.
Fig 5: Sinusoidal pulse width modulation technique

High frequency triangular carrier signals
superimposed with three sinusoidal reference signal
known as the modulating signals to generates the
gating signal for the inverter switches this process
known as sinusoidal PWM technique. SPWM
technique provides effective switching frequency of
load voltage is 3 times the switching frequency of
each cell as determine by its carrier signal & require
less complex algorithm with ease to implement.
Instead of maintaining width of all pulses same as in
case of multiple pulse width modulation, the width
of each pulse is varied in proportion to the amplitude
of sine wave evaluate at the center at the same pulse.
The distortion & lower order harmonics are reduced
significantly. The gating signal generated by
comparing sinusoidal reference signal with
triangular carrier wave. When instantaneous value of
the sine reference is greater than the triangular
carrier wave then the output is at +Vdc & when the
reference signal is less than the carrier signal then
the output is at -Vdc.
Different SPWM technique can be classified as:
a. Phase disposition method
b. Phase shifted method
c. Phase opposition disposition method.
3.1 Phase disposition method:
Phase disposition method based on the
comparison of sinusoidal reference waveform with
triangular carrier waveform, all the carrier waveform
have same frequency and amplitude & all the carrier
waves are in phase with each other. If 'M' is the
number of voltage level in inverter then 'M-1' carrier
signals are used to generate voltage levels.
Fig 6: Phase disposition PWM technique.
3.2 Phase opposition disposition:
In this method sinusoidal reference wave
superimposed on the carrier triangular wave (which
are phase above & below the zero reference) like in
phase opposition method but difference is that,
triangular wave above & below the reference zero
axis have equal frequency and amplitude but they
are phase shifted by 180 degree with each other. The
reference signal is compared with all carrier waves
thus gate pulses are generated & are related into
each switching device.
Fig 7: Phase opposition disposition SPWM technique

IV. EXPERIMENTAL RESULTS
Figure shows the simulation diagram of the
proposed inverter. The circuit is designed in
Matlab/simulink and the generation of pulses has
been made by comparing every carrier wave with the
sine wave and the resultant pulses has been given to
the appropriate switches to produce the three level
staircase waveform
.
Fig 8: Simulation diagram for proposed 3 level MLI
Diagram below shows experimental results
of three level diode clamped inverter common mode
voltage reduction by PD-SPWM technique with &
without filter. Fig 9 shows output of inverter which
is staircase wave. Sinusoidal waveform obtain after
filtering inverter output.
Fig 9: Output waveform of proposed MLI using PD technique.
Fig 10: Filtered output waveform of proposed MLI

Fig11: FFT analysis of proposed technology using PD technique.
The total harmonic distortion (THD) of any
waveform is define as the closeness to the
fundamental wave shape. FFT analysis is done to
determine the THD of the output waveform.
Fig 12: Output waveform of proposed MLI using POD technique.
Fig 13:Filtered output waveform of MLI.
Fig 14: FFT analysis of proposed technology using POD technique.

Phase opposition disposition method uses
reference wave & carrier wave, carrier waves are
180 degree phase opposite to each other. Comparing
reference signal with carrier wave results in gating
pulses shown in diagram. Fig. 13 shows filtered
output of the MLI.
Table 1: Comparison between PD & POD technique.
SPWM Technique THD CMV
Without Filter With Filter
PD 45.13 0.77 Vdc/2
POD 85.36 18.58 Vdc/2
V. CONCLUSION
The proposed technique reduces CMV at
the output terminal of the DCMLI. Experimental
results & simulation shows the effectiveness of the
proposed techniques giving reduction in magnitude
of common mode voltage. Results prove that PO-
SPWM technique is superior to reduced Total
harmonic distortion & CMV.
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