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Analysis of an Impedance (Z) Source Inverter Using
Modified PWM Technique with Simple Boost Control
Publication History
Received: 02 September 2015
Accepted: 04 October 2015
Published: 06 November 2015
Jitendra Patil, Vikas Kulkarni. Analysis of an Impedance (Z) Source Inverter Using Modified PWM Technique with Simple Boost Control.
Discovery, 2015, 48(221), 18-23
Analysis of an Impedance (Z) Source Inverter Using
Modified PWM Technique with Simple Boost Control
Jitendra Patil
Vikas Kulkarni
Dept. of Electrical Engg.
Pune, India
[email protected]
Dept. of Electrical Engg.
Pune, India
[email protected]
Abstract—This paper presents an impedance source (also
called Z-source) power converter (ZSC) with its control
technique for converting dc-to-ac voltage, which is source voltage
for a small 3 phase squirrel cage induction motor. Generally, we
need to convert voltage according to the application requirement,
like ac-to-dc, ac-to-ac, dc-to-ac and dc-to-dc conversions
respectively i.e. it can be applied to any type of power conversion.
Traditionally, for dc-to-ac power conversion we use voltagesource inverter (VSI) or current-source inverter (CSI) where
capacitor and inductor are used respectively.
Conventional voltage-source inverters and current-source
inverters are suffering from no. of major limitations that can be
easily overcome with the use of Z-source converter. For many
industrial applications in adjustable speed drives we require
smooth/step less variation in speed that can be achieved by using
modified PWM technique which also has more advantages than
the conventional PWM technique. Simulation results are added
here for analysis purpose.
each phase leg, otherwise a short circuit would take
place (also called as shoot-through state) in the
circuit and it will destroy the device. In other words,
dead-time provision is provided to block upper and
lower devices which produce waveform distortion.
An additional LC filter is required for
smoothing and filtering voltage waveform to get
purely sinusoidal waveform compared to current
source inverter, but it causes more power loss and
control complexity [1]-[3].
Keywords—Voltage source inverter; current source inverter; Z
source inverter; modified PWM technique.
Fig. 1. Traditional VSI and CSI
In VSI we cannot switch ON the gate
simultaneously i.e. the upper and lower devices of
Its ac output voltage always has to be greater
than its dc input rail. It is step-up inverter for dc-to-ac
power conversion and vice-versa [1], [13].
At any time at least one device is gated and
maintained in ON state from upper side and from
lower side. Otherwise an open circuit would occur
which will destroy the device. For safe commutation
we require an overlap time provision which produces
waveform distortion [1].
Current-source inverter works only in close
loop control, so it limits multi-motor applications [2].
Apart from the above barriers and limitations, Voltage
source inverter and current-source inverter have some
common problems, like electromagnetic interference and total
harmonic distortion
We cannot increase its output ac voltage
level than its dc input voltage level. Therefore, it is
step-down inverter for dc-to-ac conversion and viceversa. If over drive is required in any specific
application then additional circuit will be required
which further increases system cost and complexity.
Current-source inverter (CSI) consists of dc source in
series with inductor and 3 phase 6 pulse inverter bridge as
shown in fig. 1[B]. Current source inverter also has some
limitations as explained below –
Previously, there existed only two traditional
converters, which are voltage-source inverter (VSI) and
current-source inverter (CSI) as shown in fig. (1.A) and (1.B)
respectively. To reduce complexity in the circuit, a dc source
is directly used rather than an ac source with rectifier bridge.
Voltage-source inverter (VSI) consist of a dc source with large
capacitor in parallel and 3 phase 6 pulse inverter bridge, in
which 6 power transistors with an antiparallel diodes as shown
in fig. 1[A]. The function of an antiparallel diode is to provide
bidirectional current flow and to block unidirectional voltage
capability [1]-[2], [7]-[9]. In many applications voltage-source
inverters are preferred over the current-source inverters,
however it has following limitations –
Generally, voltage-source inverters and currentsource inverters are used to control motor drive system [15].
To overcome aforementioned problems regarding VSI and
CSI, this paper presents an impedance source (also called Zsource) inverter as a solution. The Z-source inverter is able to
produce any output voltage range that might be greater than
the ac input voltage by controlling the boost factor [1],[14].
With the help of shoot through state (which is forbidden in
traditional converters) output voltage of the inverter can be
varied. This (shoot through) state is an additional state (ninth
state) in Z-source inverter while a traditional converter has
only eight switching states [1]-[3].
called as self- boost type inverter. Diode (D) is used in Zsource inverter circuit to prevent the discharging of
overcharged capacitor through the source [1]-[2].
The advantage of shoot-through zero state is, it
does not affect pulse width modulation scheme of inverter. It
equivalently produce same zero voltage at the load terminals,
hence, it greatly reduces voltage stress. Available shootthrough period is limited by the zero state periods that can be
determined by modulation index.
There is multiple PWM techniques available like1.Single PWM.
2.Multiple PWM.
3.Sinusoidal PWM.
 Unlike the traditional inverters Z-source inverter does
not requires any dead-time or overlap time provision
which reduces waveform distortion greatly.
 The in-rush current and harmonics in the current can
be reduced due to the inductor.
 It is buck-boost type inverter, so no need to change
circuit configuration during operation.
 Z-source inverter can provide ride-through capability
where voltage sags are major problems.
 Z-source inverter requires small capacitors and
inductors compared to traditional voltage-source
inverters and current-source inverters respectively [1][6].
In Z-source inverter there are basically nine
permissible states and three modes of operation. In first mode
inverter bridge generates one of the six traditional active
vectors. In second mode inverter bridge produces one of the
two traditional zero vector. Mode three is the shoot-through
mode which is forbidden in traditional converters. Depending
on how much voltage boost is required in the circuit, shootthrough interval (T0) and duty cycle (T0/T) is determined and
varied. During this shoot-through state energy is transferred
from capacitors to inductors and as a consequence Z-source
inverter gains the voltage boosting capability and hence it is
Fig. 3. Traditional PWM without shoot-through.
Fig. 4. Modified PWM with shoot-through.
PWM switching is based on sawtooth carrier wave. In this
method at any time, total time period of one switching cycle
will not change the shoot-through zero states are introduced in
Z-source inverter consist of two split inductors L1
and L2 and two split capacitors C1 and C2 which are connected
in X shape to provide an impedance source to main circuit.
IGBT’s or MOSFET’s are used to design main circuit i.e. 3
phase inverter. Z-source network is connected in between dc
side or source and ac side or load. Z-source inverter has no. of
advantages over a traditional converter, they are as follows –
Fig. 2. Z-source network
Most of the time a sinusoidal PWM technique is used
due to its no. of advantages. In PWM techniques there are two
different waves of different frequencies which are compared
in PWM comparator. The traditional pulse width modulation
is used when the dc input voltage is sufficient to produce an ac
output voltage level shown in fig. (3). In other words we don’t
need voltage boosting. In this method shoot-through state is
not distributed in zero states. Fig. (4) shows the modified
PWM technique in which shoot-through state is distributed in
every switching cycle to achieve desired output voltage level
each phase linearly. It should be noted that the active states
remain unchanged.
It should be noted that there are no. of control methods
which includes simple boost control, maximum boost control,
constant boost control and traditional and modified space
vector based pulse width modulation controls. Boost factor is
the key factor in buck-boost operation of Z-source inverter.
The boost factor also depends on the distribution of the shoot
through duty intervals into the non shoot-through states. The
simple boost technique is described in this paper and it is used
widely in applications because of its simplicity and easy
implementations. It uses two straight lines one from upper side
and one from lower side as shown in fig. (4). Straight lines are
compared with saw tooth waves and by varying reference of
straight lines, shoot-through duty interval can be controlled.
When the carrier wave (saw tooth) is greater than upper
straight line or lower than bottom straight line at that time only
the circuit turns into shoot-through mode, otherwise it operates
just like traditional PWM control as shown in fig. (4). The
method is very simple ; however the resulting voltage stress
across device is more because some zero vectors are not
utilized properly [1],[4]-[6].
Before going through the simulation results, theoretical
calculations are added which is very important for comparing
If TS is the total switching period, T0 is the zero
state time period and D0 is the shoot-through duty ratio which
is given by,
Dc supply
= =
= =
Switching frequency
Modulation index,
Shoot through duty ratio,
Initial capacitor voltage,
and (2)
. B .
voltage and
is the ac output voltage ,
is the input dc
is the modulation index which is given by,
= (1-
index. In other words for high voltage gains small
modulation index should be used [1], [3].
150 V
1000 μ
10 kHz
335 V
With the help of duty ratio, the boost factor B is
Calculated as follows,
In simple boost control as shown in equation (5) shoot
through duty ratio is inversely proportional to the modulation
×( .
= 3.52
. B .
= (0.642) x (3.52) x (150/2) = 169.5 V
Traditional voltage-source inverters and current-source
inverters use capacitors and inductors to store energy and
The equation (8) shows peak phase voltage which in turn
helps to calculate the line-line peak voltage which is
0.18 kW
400 V
50 Hz
0.119 Ω
0.0997 Ω
0.07142 H
0.07142 H
0.762 H
0.001 kgm2
and (G) is the voltage gain of inverter which is given by,
Output power
RMS line voltage
Input frequency
No. of poles
Stator resistance,
Rotor resistance,
Stator inductance,
Rotor inductance,
Mutual inductance,
Moment of inertia, J
Boost factor is given by,
filtering element to suppress voltage and current ripples
respectively. Here Z-source inverter is used, which uses both
two split capacitors and inductors to store energy which boosts
voltage and acts as a second order filter. , ,
same values because of circuit symmetry [1]-[2]. Table I and
II shows the parameters used for 0.18 kW induction motor and
Z-source inverter respectively. Also the simulation graphs are
given for analysis purpose. All reference values of Z-source
network are tabulated below [1].
calculated by multiplying that phase voltage by √3 and which
is equal to
x √3
= 169.5 x√3
= 293.58 V
By using equation (11), the rms voltage is calculated
which is,
Fig. 7. 3 phase IM current
= 207.59 V
Equation (11) and (14) shows that the output ac voltage
is boosted up to 293.58 V (peak) or 207.59 V (rms).
Simulation results are also shown below to confirm above
calculations. Fig. (5) and (6) shows the performed simulation
results of line-to-line and phase voltage respectively for 150 V
dc input voltage with 0.642 modulation index and 0.358 shootthrough duty ratio. Waveform shows that the output ac voltage
of inverter is nearly about 298 V for 0.642 modulation index.
Fig. 8. Total harmonic distortion in voltage profile
Fig. (7) shows three phase motor currents individually
for Phase A, Phase B and Phase C. Initially during the
transient period distortions are visible. Phase currents attains
stability during steady state operation. Fig. (8) shows Total
Harmonic Distortion in phase voltages, carried out using FFT
Fig. 6. Phase voltage of 3 phase induction motor
Fang Zheng Peng, “Z-source inverter”,IEEE transaction on industry
applications., vol.39,no.2, march/april, 2003
The Z-source inverter can produce any desired output
voltage which is required for application which may be greater
than the input voltage level with the help of modulation index
and shoot-through duty ratio. This feature is forbidden in
traditional voltage-source inverters and current source
inverters, hence now a day’s researchers are moving towards
this technology. In this work theoretical analysis and
simulation results are described. The results obtained by
simulations have close agreement with results obtained by
theoretical calculations which proves Z-source inverter
concept. Modified PWM technique with simple boost control
is used because of its no. of advantages over the traditional
PWM control.
Fig. 5. Simulation waveform of 3 phase line-to-line voltage
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