International Journal of Automation and Computing
9(2), April 2012, 200-210
DOI: 10.1007/s11633-012-0634-4
A New Switching Scheme for Z-source Inverter to
Minimize Ripples in the Z-source Elements
Sengodan Thangaprakash1
1
Ammasai Krishnan2
School of Electrical Systems Engineering, University Malaysia Perlis, Perlis 02000, Malaysia
2
Dean (Academic), K.S.R. College of Engineering, Tamilnadu 637215, India
Abstract:
This paper presents a modification in pulse width modulation (PWM) scheme with unequal shoot-through distribution
for the Z-source inverter (ZSI) which can minimize ripples in the current through the Z-source inductors as well as the voltage across
the Z-source capacitors. For the same system parameters, the proposed control technique provides better voltage boost across the
Z-source capacitor, DC-link, and also the AC output voltage than the traditional PWM. The ripples in the Z-network elements are
found to be reduced by 75 % in the proposed modulation scheme with optimum harmonic profile in the AC output. Since the Z-network
requirement will be based on the ripple profile of the elements, the Z-network requirements can be greatly reduced. The effectiveness
of the proposed modulation scheme has been simulated in Matlab/Simulink software and the results are validated by the experiment
in the laboratory.
Keywords:
1
Z-source inverter (ZSI), pulse width modulation (PWM), shoot-through, ripples, total harmonic distortion.
Introduction
Z-source inverter (ZSI) has found widespread
applications[1−10] , and attracted the interest of researchers in recent years, since it overcomes the limitations
of the traditional inverters. The schematic of a three phase
ZSI is shown in Fig. 1. ZSI based systems advantageously
utilize the shoot-through states to boost the direct current
(DC) bus voltage by gating on both the upper and lower
switches of the same phase leg. Shoot-through mode
allows simultaneous conduction of devices in same phase
leg and is forbidden in traditional inverter topologies.
Therefore, a ZSI can buck or boost the voltage which
is equal to a desired output voltage that is less/greater
than the DC bus voltage based on the shoot-through time
period and boost factor. Since the shoot-through state has
no harmful effect on the inverter and is advantageously
utilized, the reliability of the inverter is greatly improved.
It also provides a low cost and highly efficient single
stage power conversion structure for reliable operation.
ZSI ensures smooth operation by supplying the desired
voltage to the load even during the supply voltage sags
and fluctuations[11] .
Fig. 1
Three phase ZSI
Manuscript received September 24, 2010; revised March 11, 2011
Inverter bridge with lattice impedance network connected after DC power supply with the feature of
buck-boost capability was first proposed by Peng[11] . In
this research, simple boost control (SBC) method with
constant boost factor was used to control the shootthrough and DC link voltage of the inverter. The range of
shoot-through duty ratio is very limited in SBC in addition
to the high voltage stress across the power insulated gate
bipolar junction transistors (IGBT). Maximum boost
control (MBC) was proposed by Peng et al.[12] to produce
maximum voltage gain (boost) under a given modulation index. MBC turns all traditional zero states into
shoot-through state and voltage stress across the power
IGBT0 s is greatly reduced by fully utilizing the zero states.
Indeed, turning all zero states into shoot-through state
can minimize the voltage stress; however, doing so also
causes a shoot-through duty ratio varying in a line cycle,
which causes inductor current ripple. This will require a
high inductance for low-frequency or variable-frequency
applications. Shen et al.[13] proposed a constant boost
control (CBC), which can obtain maximum voltage gain
at any given modulation index without producing any
low-frequency ripple that is related to the output frequency.
A detailed analysis for how the various conventional pulse
width modulation (PWM) strategies can be modified to
switch a ZSI either continuously or discontinuously, while
retaining all the unique harmonic performance features
is discussed in [14]. Modified space vector modulation
(MSVM) algorithm for reducing the switching stress by
capacitor voltage control with good transient response
has been presented in [15]. In this research, the reference
capacitor voltages are derived for minimizing the voltage
stress at any desired alternating current (AC) output
voltage by considering the DC input voltage. In [16],
minimization of voltage stress across switching devices
in the ZSI has been presented by modifying the space vector
S. Thangaprakash and A. Krishnan / A New Switching Scheme for Z-source Inverter to Minimize Ripples in · · ·
algorithm to generate shoot-through states. An integrated control algorithm using modified voltage space vector has been presented by Thangaprakash and Krishnan[17]
to achieve maximum boost in the DC link voltage with
wide range of controllability and the same has been implemented for induction motor drives[18, 19] . The performance of the drive is found to be very smooth when a step
change is applied to the load or during the supply voltage
sags. Development of a ZSI based power conversion system for photovoltaic applications is proposed in [5] and the
same with quasi-Z-source inverters has been discussed in
[20]. Detailed comparison of ZSI with traditional inverters
in terms of total switching device power, passive components requirement and cost of the overall system has been
given in [8]. An indirect controller for the DC link voltage
on the DC side of the ZSI along with AC voltage control
is discussed in [21]. Controller design for specific applications, namely fuel cell and voltage sag compensation are
discussed by Jung and Keyhami[22] . Unified control technique has been presented[23] for ZSI with minimal number
of sensors and PI controllers to achieve good stability of the
overall system. In [24], an improved ZSI has been proposed
with certain modifications in the Z-network connections of
the traditional ZSI topology. Improved ZSI greatly reduces
the inrush current of the Z-network inductors and facilitates the soft start capability. Space vector modulation
(SVM) of nine switches inverter and nine switches Z-source
inverter was proposed in [25] where the switching sequence
is composed of the upper active vectors, the lower active
vectors and the zero vectors. In this research, the upper
and lower active vectors are determined via two space vector diagrams. Comparative evaluation of different PWM
schemes that can be used for ZSI control to achieve buckboost energy conversion with random and reduced common
mode switching has been presented in [26, 27]. Operation of
power inverter fed induction motor drive has been analyzed
in [28, 29].
The same space vector modulation strategies of traditional inverters with proper insertion of shoot-through periods could be applied to three phase ZSI with each having
the same characteristic spectrum as its conventional counterpart. In these methods, the shoot-through period over a
half of the sampling period is divided into three equal intervals and inserted in the switching waveforms without altering the active time periods. Therefore the shoot-through
time period is diminished from the traditional zero vector
time period. By doing so, one can increase the switching frequency and improve the harmonic profile in the AC
output. But this will introduce significant ripples in the
Z-source elements and causes ripples associated with the
output frequency.
In this paper, a new PWM scheme is proposed which can
minimize the ripples in the current through the Z-source inductors and voltage across the Z-source capacitors without
sacrificing the features achieved by the traditional methods
(i.e., DC link voltage boost, voltage gain, switching stress,
and AC output voltage). In addition to the above, proposed method provides better voltage boost in the DC link.
The theoretical and modulation concepts of continuous and
discontinuous switching of ZSI using modified modulation
201
is presented in this paper, and the same have been verified
by the simulation and experimental results.
2
Modulation
(ZSI)
of
Z-source
inverter
Space vector PWM (SVPWM) techniques have been
widely used in PWM inverters due to lower current harmonics and a higher modulation index. The unique features of a ZSI presented in [1] can be accomplished by the
same SVPWM technique with a few modifications to insert
the shoot-through. In addition to the six active and two
null states associated with traditional inverters, the three
phase ZSI has seven shoot-through zero states representing
the short-circuiting of a single phase-leg, two phase-legs or
all three phase-legs. These shoot-through states again boost
the DC link capacitor voltages and can partially supplement
the null states within a fixed switching cycle without altering the normalized volt-sec average. Shoot-through states
short-circuit the corresponding inverter phase leg and produce zero voltage across the output terminals similar to the
traditional zero states. Shoot-through states can therefore
be inserted into the existing PWM state patterns of traditional inverters to derive different modulation strategies for
controlling the ZSI.
In a continuous centered SVM state sequence of a traditional inverter, there are three state transitions which
occur in a half of the sampling period and the traditional zero states are placed at the start and end of the
switching cycle. With these three-state transitions (i.e.,
000→100→110→111), three equal-interval (T0 /3) shootthrough states are added immediately adjacent to the active states per switching cycle for modulating a ZSI. In the
three state transitions, the middle shoot-through state is
symmetrically placed about the original switching instant.
The traditional switching pattern of a voltage source inverter (VSI) and ZSI for the sector I has been shown in
Figs. 2 (a) and (b). In this switching pattern, the active
states {100} and {110} are left/right shifted accordingly by
T0 /3 with their time intervals kept constant, and the remaining two shoot-through states are inserted in the end
within the null intervals, immediately adjacent to the left
of the first state transition and to the right of the second
transition[2] . At this switching pattern, the zero state period is reduced from Tz /2 to Tz /2–2T0 /3 and Tz /2 to Tz /2–
T0 /3. As both zero state periods should be greater than 0,
the shoot-through time is less than 0.75 times of Tz at period Tz /2–2T0 /3, and less than 1.33 times of Tz at period
Tz /2 to Tz /2–T0 /3. In this modulation scheme, the DC link
voltage cannot be boosted to the maximum level, since the
shoot-through state is limited to 0.75×Tz , and also it results
ripples in the current/voltage in the Z-source elements.
3
Modified shoot-through distribution
The process of inserting the shoot-through states in
switching waveforms produce ripples in the current through
the Z-source elements. These ripples cause additional heating of the elements and ultimately degrade their lifespan.
The ripple in shoot-through duty ratio results in ripple in
202
Fig. 2
International Journal of Automation and Computing 9(2), April 2012
Switching pattern for sector I of (a) traditional inverter, (b) ZSI in continuous mode, and (c) ZSI in 60 0 discontinuous mode
the current through the inductor, as well as in the voltage across the capacitor. These ripples are influenced by
changes in power factor angle, modulation index, shootthrough time period and amplitude of the load current.
Instead of having equal shoot-through states, this section presents a modified shoot-through state distribution
in SVM scheme for the ZSI which can minimize the ripples
in the current through the Z-source inductors and voltage
across the Z-source capacitors without sacrificing any features derived in the traditional modulation.
The proposed method also results reduction in the AC
output voltage/current harmonics of the ZSI over the traditional switching. State sequence and the placement of
shoot-through for both the continuous and discontinuous
switching of the proposed modulation schemes are presented. Fig. 3 shows the continuous and discontinuous
switching pattern for the MSVM with modified shootthrough states at the space vector angle 0<θ<60 (sector
I). In a continuous switching mode, for a half sampling
period, 16 % of the total shoot-through period is inserted
in the middle state transition (between active states {100}
and {110}), and the rest of the shoot-through periods are
equally inserted in the first and last state transitions (between active and zero states, i.e., {000} to {100} and {110}
to {111}) with each of 42 % of the total shoot-through state.
To realize the features of the proposed method, the shootthrough time is distributed into 5T0 /12, T0 /6 and 5T0 /12
during the half sampling period in the continuous switching
scheme. T0 /6 is inserted in the middle state transition of
the switching waveform and 5T0 /12 is inserted at the first
and third transitions. The active state {100} and {110} are
left/right shifted by T0 /12 and the remaining two shootthrough states with equal interval of 5T0 /12 are inserted at
the zero states. The zero state periods during the start and
end of the switching cycle can be equally reduced from Tz /2
to Tz /2–T0 . This ensures the optimum harmonic profile of
the AC output parameters. The six PWM pulses in the
MSVM should be controlled independently. The simulated
S. Thangaprakash and A. Krishnan / A New Switching Scheme for Z-source Inverter to Minimize Ripples in · · ·
203
Fig. 3 Modified switching pattern for sector I using novel SVM with (a) continuous switching, (b) 60 0 discontinuous switching for
+ve DC rail clamping, (c) 60 0 discontinuous switching for -ve DC rail clamping
PWM switching pattern for the MSVM is shown in Fig. 4.
As the symmetrical shoot-through time periods with dissimilar duration are inserted in the switching waveforms,
the ripples in the Z-source inductor current and the Zsource capacitor voltage can be reduced having retained
all the features such as, voltage gain, voltage stress limitation, AC output controllability, and the optimum harmonic
profile. In addition to the above a better voltage boost
can be achieved in the DC link. During the shoot-through
period, both switches of the phase leg are conducted simultaneously for boosting the DC capacitor voltage. Fig. 4
shows the modified switching with unequal shoot-through
distribution for sector I.
The modified switching can also be realized by carrier
based implementation. For carrier based implementation,
the modified reference signals needed to produce the modified switching pulses to the ZSI in the continuous switching
can mathematically be expressed as follows,
Vmax(sp) = Vmax + Voff + T
T
Vmax(sn) = Vmax + Voff +
4
T
Vmid(sp) = Vmid + Voff +
4
T
Vmid(sn) = Vmid + Voff −
4
T
Vmin(sp) = Vmin + Voff +
4
Vmin(sn) = Vmin + Voff − T
(1)
(2)
(3)
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International Journal of Automation and Computing 9(2), April 2012
where {sp, sn} = {1, 4}, {3, 6}, {5, 2} and T = T0 /3Ts .
The modified reference signals of all the six switching devices are generated for the switching frequency fs =10 kHz,
shoot-through duty ratio D0 = 0.2 and modulation index,
ma = 0.79 using modified modulation with unequal shootthrough state and is compared with high frequency triangular carrier signal to generate the switching pulses to the
gates of the power IGBT0 s of the ZSI.
state transitions. There are only two switching transitions
in a half of the sampling period. 16 % of the shoot-through
period is inserted in between the active states {100} and
{110}, and the remaining 84 % is inserted at the active and
zero states either in between {000} and {100} or {110} and
{111}. The switching pattern of the proposed MSVM with
60◦ discontinuous switching for positive and negative DC
rail clamping is shown in Figs. 3 (b) and (c), respectively.
The existing zero state interval is again reduced to Tz /2–
T0 . Carrier based implementation of proposed modified
modulation with discontinuous switching with positive DC
rail clamping (as shown in Figs. 3 (b)) and negative DC rail
clamping (as shown in Fig. 3 (c)) is possible with the modified reference signals derived using the same procedure as
described in the continuous switching. The reference voltages are given in (7)–(9) for positive DC rail clamping and
(10)–(12) for negative DC rail clamping with 60◦ discontinuous switching mode.
Fig. 4 Switching pattern for the modified shoot-through distribution
In (1)–(3), all the waveforms are altered while inserting
the unequal shoot-through states, by switching the ZSI in
this way ensures a better voltage boost in the DC link along
with added features, however, the voltage stress across
the power devices is similar to the traditional modulation.
These features can also be realized with reduced switching
stress as discussed in the previous section and the switching
pattern for the same is shown in Fig. 5.
For carrier based implementation, the modified reference
signals needed to produce the modified switching pulses
with symmetrical unequal shoot-through states inserted by
only altering four switching waveforms in the continuous
switching can now mathematically be expressed as follows,
5T
Vmax(sp) = Vmax + Voff +
4
Vmax(sn) = Vmax + Voff
(4)
Modified reference voltages for positive DC rail clamping
are as follows:
Vmax(sp) = Vmax + Voff
Vmax(sn) = Vmax + Voff
Vmid(sn) = Vmid + Voff −
T
2
T
2
7T
−
4
(5)
Vmin(sp) = Vmin + Voff − −
(6)
where {sp, sn} = {1, 4}, {3, 6}, {5, 2} and T = T0 /3Ts .
In discontinuous PWM, a null state is eliminated either
at the start or end of the switching cycle. In the 60◦ discontinuous switching of ZSI using the proposed SVM two
shoot-through states T0 /6 and 5T0 /6 are inserted at the
(7)
Vmid(sp) = Vmid + Voff
T
6
T
Vmin(sp) = Vmin + Voff −
6
T
Vmin(sn) = Vmin + Voff − .
3
Vmid(sn) = Vmid + Voff −
Vmid(sp) = Vmid + Voff
Vmin(sn) = Vmin + Voff
Fig. 5 Modified switching pattern for sector 1 with unequal
shoot-through distribution
(8)
(9)
Modified reference voltages for negative DC rail clamping
are as follows:
T
Vmax(sp) = Vmax + Voff +
3
T
Vmax(sn) = Vmax + Voff +
(10)
6
T
Vmid(sp) = Vmid + Voff +
6
S. Thangaprakash and A. Krishnan / A New Switching Scheme for Z-source Inverter to Minimize Ripples in · · ·
Vmid(sn) = Vmid + Voff
(11)
Vmin(sp) = Vmin + Voff
Vmin(sn) = Vmin + Voff .
(12)
The proposed modulation can be adopted for all kinds
of applications since it greatly reduces the current/voltage
ripples along with additional boost in the DC link for the
same system parameters and shoot-through duty ratio. The
DC capacitor voltage can be boosted significantly, since the
maximum shoot-through time period in the MSVM is increased to the traditional zero state time period (Tz /2).
4
Current ripple calculation
Current ripples in the DC link can produce ripples in
the output and it would cause ripples in the electromagnetic torque waveform, and ultimately this will lead to
torque pulsation in the rotor during operation. This facilitates an undesirable operation to the ZSI fed induction
motor drive. Expression for the ripple currents through
the DC inductor produced by different modified state sequences can conveniently be formulated by assuming that
the Z-source capacitors and inductors are symmetrical (implying L1 = L2 = L and C1 = C2 = C). Current flowing
through a single Z-source capacitor can then be expressed
as: IC2 = −IL1 in a shoot-through state during time interval T0 and IC2 = IL1 − Iin in a non-shoot-through active
or null state during time interval Ts − T0 . Averaging the
current across a Z-source inductor over a switching period
gives
¶
µ
Ts − T0
× IDC−link .
(13)
IL =
Ts − 2T0
Further the Z-source inductor current in shoot-through
and non shoot-through states can be calculated as
µ
¶
Ts − T0
ILshoot−through =
× IDC−link
(14)
Ts − 2T0
¶
µ
T0
× IDC−link .
(15)
ILnon shoot−through =
Ts − 2T0
To proceed with the formulation of RMS expression for
the inverter input current IDC−link , two assumptions should
preferably be made firstly, in order to simplify the derivations without any degradation of accuracy. These assumptions are respectively the presence of smooth three-phase
sinusoidal currents at the inverter AC output and a sufficiently large inductor in the Z-source network with a constant inductor current. The method followed by Gao et
al.[26] can then be applied to determining the RMS expressions for IDC−link by first expressing the duty ratios of the
active states, k1 and k2 .
√
³
3
π´
cos ωt +
k1 = m a ×
2
6
√
³
3
π´
k2 = m a ×
cos ωt −
.
(16)
2
2
The null duty ratio is then expressed as
µ
¶ µ
¶
T − T0
TZ
T0
k0 =
− k1 − k2 =
−
.
(17)
T
2
2
205
The null state is divided equally at the start and end of
the sampling period. This ensures better harmonic profile
in the output parameters.
ia = I cos (ωt − ϕ)
µ
¶
2π
ib = I cos ωt − ϕ −
3
µ
¶
2π
ic = I cos ωt − ϕ +
3
(18)
where ϕ represents the power flow angle.
For a balanced three phase load, the three phase output currents expressed in (18), should satisfy the following
condition,
ia + ib + ic = 0.
(19)
The RMS value of the DC link current can be calculated
by
(IDC−link )RMS =
µ √
TS
9 3
π
× (
× ma × I 2 × sin(2ϕ − ))+ (20)
TS − T0
8π
6
√
¶
TS
3 3
π
(
× I 2 (1 −
sin(2ϕ − ))) .
TS − T0
π
6
By substituting the RMS value of Idc−link into (13), the
amplitude of current ripples can be calculated. The current ripples are significantly reduced while applying modified modulation with unequal symmetrical shoot-through
states and has been shown in the results section.
5
Results and discussion
Simulations have been carried out to verify the effectiveness of the proposed modified shoot-through distribution
over the traditional scheme. A LC filter with 1 kHz cut-off
frequency is placed in between the inverter output and AC
load.
The system parameters considered for simulation are as
given below:
Source voltage: VDC = 250 V.
Z-source inductors: L1 = L2 =1 mH.
Z-source capacitors: C1 = C2 = C =1000 µF.
Load = 5 kW.
Power factor = 0.9 (lagging).
Modulation index: ma = 0.6.
Shoot-through duty ratio: D0 =0.3.
Switching frequency: fs =10 kHz.
Simulation results of the traditional and proposed PWM
for the current and voltage ripples in the Z-source inductor
and capacitor respectively, and DC link voltage, AC output
(out.) voltage/current waveforms are shown in Figs. 6–8.
From these results, it can be observed that, the continuous
switching of the proposed SVPWM offers better DC link
voltage boost for the same shoot-through duty ratio than
the traditional method. The current ripples of the Z-source
inductor (ind.) current in the proposed SVPWM scheme
is reduced by 80 % (from 4 A to 0.8 A), and the voltage
ripples of the Z-source capacitor (cap.) voltage is reduced
by 60 % (from 1 V to 0.4 V). In the proposed scheme, the
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International Journal of Automation and Computing 9(2), April 2012
DC link voltage is boosted to 236 V where in the traditional method it is 220 V, hence for the same shoot-through
duty ratio and modulation index an additional 16 V boost
is offered by the proposed method. After the filter, the
voltage waveform becomes sinusoidal and having the RMS
phase-phase value as 158 V. In traditional scheme the RMS
phase-phase voltage was 148 V. Additional voltage boost is
achieved by turning the maximum time period of the traditional zero states into shoot-through zero states by symmetrical unequal shoot-through distribution. The switching stress across the power devices is directly proportional
to the shoot-through time period and hence the switching
stress is increased while maintaining the shoot-through period for a significant time. It could again be reduced by the
modified modulation scheme which needs only four switches
to be altered while inserting shoot-through state.
traditional SVPWM scheme. Proposed SVPWM provides
same advantages for the discontinuous switching scheme for
positive and negative DC rail clamping as the continuous
switching. The results presented in Fig. 8 show 60◦ discontinuous switching for positive DC rail clamping. Fig. 9
shows the transient response of Z-source capacitor voltage
by traditional modified modulation schemes. It can be seen
that, the proposed modulation scheme provides better DC
voltage boost than the traditional modulation. The output
voltage and current harmonic spectra of the proposed modified modulation scheme have been shown in Figs. 10 (a) and
(b), respectively.
Fig. 7 Simulation results of novel modified SVM (continuous
switching) mode SVM for D0 = 0.3 and ma = 0.6
Fig. 6 Simulation results of traditional modified SVM for shootthrough = 0.3 and modulation index = 0.6
The total harmonic distortion (THD) of the output voltage/current waveforms is also found to be superior to the
traditional SVPWM. In the proposed SVPWM, the THD
for the output voltage is reduced to 0.53 % from 2.33 %
and THD for the output current is reduced to 0.63 % from
2.43 %. Hence the output voltage and current waveforms
are also found to be improved proportionally and holds
the superior harmonics profile when compared with the
Even though the ripples in the Z-source elements are reduced significantly than the traditional modulation, still
some ripples are seen in the current through the Z-source
inductors as well as the voltage across the Z-source capacitors. This is because of increasing the shoot-through duty
ratio since shoot-through duty ratio has inverse relationship
with ripples in the Z-source elements. The graph between
voltage gain and modulation index has been depicted in
Fig. 11 and from which, one could see that the possible operation region is extended with the increase of modulation
index. The AC voltage gain is high for a lower range of
modulation indices and less for higher range of modulation
S. Thangaprakash and A. Krishnan / A New Switching Scheme for Z-source Inverter to Minimize Ripples in · · ·
207
indices. Comparatively, the proposed modified modulation
provides better voltage gain than the traditional modulation technique. Modified control technique maximizes the
shoot-through period without effecting the active states by
turning maximum time period of the traditional zero states
into the shoot-through zero state, thus maximum output
voltage could be obtained for a given modulation index.
Fig. 10 Harmonics profile of the proposed modulation scheme
(a) Output voltage; (b) Output current
Fig. 8 Simulation results of novel modified SVM (discontinuous switching mode for +ve DC rail clamping) SVM for shootthrough = 0.3 and modulation index = 0.6
Fig. 11
Fig. 9 Transient response of Z-source capacitor voltage by traditional SVM and modified SVM
Graph between voltage gain and modulation index
The results shown in simulation are verified by the experiment in the laboratory. The control system is implemented
by LM3S611 processor for the voltage control and modified
space vector modulation with unequal shoot-through distribution. The AC output voltage and current are sensed by
isolation devices, amplifiers, and a 12 bit analog-to-digital
converter within the processor board. The gate driver
circuit is placed together with the power circuit board.
The PWM signals coming from the control circuit board
(LM3S611) are isolated through an optocoupler (6N135)
for separating the control and power grounds and UC3705
is used as the IGBT driver. The power circuit components
are selected to minimize parasitic effects. The front end
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International Journal of Automation and Computing 9(2), April 2012
diode rectifier supply is realized using low voltage drop
40CPQ080 Schottky rectifier and low on state resistance
IGBT (N745AB), respectively. The PWM pulses embedded
with shoot-through pulses were then sent out through six independent PWM channels to gate the six switches through
the isolation and driver circuit. The Z-source network is
constructed with L1 = L2 = L=1 mH/10 A inductors and
C1 = C2 = C=1000 µF/600 V capacitors.
Figs. 12 and 13 show the steady state Z-source inductor current (IL ) waveform in response to the shoot-through
pulses using traditional and modified modulation respectively. When the bridge is under shoot-through state, the
Z-source capacitors charge the inductors and IL increases.
When the bridge is under non-shoot-through state, the energy stored in the inductors discharges over the load decreasing IL . Simulation results for inductor current shown
in Figs. 6 and 7 are given for the duration 0.1 s–0.2 s whereas
in the experimental results they are shown for very small
duration to demonstrate with clarity. From Figs. 12 and 13,
one could note that, the Z-source inductor current waveform has reduced ripples while applying proposed modified
modulation. And also these figures show the shoot-through
time period (T0 ) over the total switching cycle (TS ). Fig. 14
and 15 show the experimental results of the traditional and
modified modulation of the ZSI, respectively. The DC link
voltage, voltage across the Z-source capacitors and the output AC voltage waveforms after a LC filter with cut-off frequency 1 kHz (ma =0.6, D0 =0.3, VDC =150 V, and switching frequency=5 kHz) are shown.
ratio. The voltage across the Z-source capacitor is maintained constantly in desired level during the supply voltage
sag/fluctuations. It can be observed from the results that
the experimental results have a close agreement with the
simulation results of the ZSI circuit. This validates that the
ZSI operates as expected in the theoretical analysis given
in this paper.
Fig. 14 Experimental results for traditional switching (a) DC
link voltage (before filter), (b) Z-source capacitor voltage, (c)
Output voltage
Fig. 12 Inductor current waveform using traditional modulation
with respect to shoot-through pulses
Fig. 13 Inductor current waveform using modified modulation
with respect to shoot-through pulses
In addition to that mentioned earlier, the following observations are noted when the unequal shoot-through duration
based modified modulation is applied to the three phase Zsource inverter. The DC link voltage has reduced ripples
and the voltage across the Z-source capacitor is boosted
well, and hence the AC output voltage. The Z-source capacitor voltage and inductor current waveforms during 25 %
supply voltage sag have been improved well as discussed
above by the proposed modulation as discussed above. The
DC boost is found good for the same shoot-through duty
Fig. 15 Experimental results for modified switching (a) DC link
voltage (before filter), (b) Z-source capacitor voltage, (c) Output
voltage
S. Thangaprakash and A. Krishnan / A New Switching Scheme for Z-source Inverter to Minimize Ripples in · · ·
6
Conclusions
A new modulation scheme with modified shoot-through
distribution has been presented in this paper for ZSI. Presented scheme has several advantages over the traditional
modulation scheme:
1) Reduces currant and voltage ripples in the Z-source
elements wile retaining all the unique features offered by
the traditional methods.
2) Offers additional voltage boost across the DC link.
3) Provides optimum harmonic spectrum while modifying the shoot-through states.
The theoretical and modulation concepts of continuous
and discontinuous switching modes of the proposed modulation have been presented.
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impedance source inverter. Iranian Journal of Electrical &
College of Engineering and the Sri Shakthi Institute of Engineering and Technology, Tamilnadu, India. Currently he is working
as a senior lecturer in the School of Electrical Systems Engineering, University Malaysia Perlis (UniMAP), Malaysia. He
has authored more than twenty papers in international journals
and conferences. He is a member of IEEE, IEEE-Power Electronics Society, IEEE-Communications Society and a life member of the Indian Society for Technical Education (ISTE). He
was an organizing Chair for the IEEE sponsored second International Conference on Computer Communication and Informatics
(ICCCI 2012) held at Coimbatore, India during 10–12, January
2012. He is an editorial board member for the International
Journal of Engineering, Science and Technology, Nigeria and a
reviewer for the European Transactions on Electrical Power and
technical program committee member for various IEEE international/national conferences.
His research interests include power electronics circuits, renewable power conversion systems and solid state control of electrical drives.
E-mail: thangaprakash@unimap.edu.my (Corresponding author)
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Sengodan Thangaprakash received his
B. Eng. degree in electrical and electronics
engineering and his M. Eng. degree in power
electronics and drives from Bharathiar
University and Anna University Chennai,
Tamilnadu, India in 2002 and 2004, respectively. He then received his Ph. D. degree
in electrical engineering from Anna University Chennai, India in 2011. From 2004 to
mid-2011, he was working with the KSR
Ammsasi Krishnan received the B. Sc.
degree in electrical engineering and the
M. Sc. degree in control systems from
Madras University, India in 1966 and 1974,
respectively. Then he received the Ph. D.
degree in electrical engineering (control,
computers) from Indian Institute of Technology, Kanpur in 1979. He has been in
the field of technical teaching and research
for more than four decades at Government
College of Technology and Coimbatore Institute of Technology,
Tamilnadu, India. From 1994 to 1997, he was an associate professor in electrical engineering at University Pertanian Malaysia
(UPM), Malaysia. Currently he is a Dean with K.S.R. College
of Engineering, Tamil Nadu, India. He has published more than
200 papers in International Journals and Conferences. He is a
senior member of IEEE, life fellow Institution of Engineers of
India, IETE of India and Computer Society of India.
His research interests include control systems, power electronics and electrical machines.
E-mail: a krishnan26@hotmail.com