Switched Inductor Z Source Inverters for High Boost

Applications

Kamal V.V
Department of Electrical and Electronics Engineering Akhil Ahammed KE
Kerala Government Polytechnic College ,West Hill Department of Electrical and Electronics Engineering
Kerala, India kerala Government Polytechnic College
vvkamalvv@gmail.com akhilahmedsav@gmail.com

Unnikrishnan P
Department of Electrical and Electronics Engineering
Kerala Government Polytechnic College ,West Hill
Kerala, India
unnisrishylam@gmail.com

Abstract—This paper analyses a different configuration for the work together is called shoot through state. As a solution to all
classical Z-Source Inverters (ZSI) which is named as Switched these problems a new configuration called Z source inverters
Inductor Z-Source Inverters (SL-ZSI) through simulation are proposed which converts the disadvantageous shoot
results. This configuration uses a unique impedance network that
through state into its advantage [2].
helps in the voltage adjustability. Comparing with the ordinary
Z-source inverter, the new configuration provides more voltage
In the sections following, the working and problems
inversion ability and enhances the output power quality of the associated with the classic ZSI will be discussed and in the
main circuit considerably. The various applications of this later sections the new configuration will be introduced. Then
configuration include DC-AC, AC-AC, DC-DC and AC-DC various modes of operation of the SL-ZSI will be discussed,
power conversions. followed by the mathematical analysis of the topology to find
the boost and inversion ability of the new topology and at last
Index Terms—Inverter, voltage source inverter (VSI), Z-source simulation results are used to validate it.
inverter (ZSI), switched inductor Z-source inverter (SI-ZSI)

I. INTRODUCTION
Traditionally the inverters are divided into Voltage Source
Inverters (VSI) and Current Source Inverters (CSI). But they
have some conceptual and theoretical barriers and limitations.
The VSI and CSI are either a boost or a buck converter
and cannot be a buck–boost converter. Their obtainable
output voltage range is limited to either greater or smaller than
the input. Their main circuits cannot be interchangeable. In
other words, neither the VSI’s main circuit can be used for the
CSI’s, nor vice versa. Lastly they are vulnerable to EMI noise
Fig.1. Three phase Voltage source inverter
in terms of reliability. The problem with EMI noise is that it
results in the switching ON of switches in the same leg which II. Z SOURCE INVERTERS
results in dead short circuit across the source. The switching
ON of switches in the same leg of a traditional inverter is a The Z source inverter employs a unique impedance network
forbidden condition in the switching states of the to couple the inverter main circuit to the dc power supply.
inverter.Fig.1 shows a three-phase VSI [3]. This two-port impedance network consist of a split-inductor L1
The switches in the same leg have to work complimentary. and L2 and capacitors C1 and C2 connected in X shape. The Z-
The forbidden state in which the switches in the same leg source inverter utilizes the shoot-through state to boost the DC
bus voltage by gating on both the upper and lower switches of
a phase leg. Fig.2 shows a Z source network [2].

Fig.2 Z source network

Fig.3 Switched Inductor Z source inverter
A. Drawbacks of Z- Source Inverters
From the viewpoint of the switching states of the main
circuit connected with SL impedance network, the operation
The Boost ability of the ZSI is very much controlled
principles of the new impedance network are similar to those
by the shoot through duty ratio (D) that we provide. In order
of the classical Z-source impedance network. Assume that at
to provide a very high boost factor for low voltage DC energy
t=0- sec we had an active state. Now at t= 0 sec our capacitors
source it needs to work with large D. That is the ZSI needs to
are charged .Thus the voltage across the two capacitors will
work in the extreme condition of long shoot through zero
add up to a value greater than the input voltage thus the diode
state. As a result, the modulation index (M) of the circuit is
at the front end is reverse biased as shown in the Fig.4 which
decreased to a very small value. It is important to note that the
is the equivalent circuit during the shoot through state[1]
inversion ability of the inverter very much depends upon the
modulation index. Ultimately the power quality and inversion
ability of the system is very much disturbed by the low M and
high D value.
For fuel cells, batteries and photovoltaic cells we
need an upgraded and enhanced configuration for the
conventional Z source which can provide very high boost with
low D value and high M value [5]. A solution for this problem
is the new switched inductor Z-source inverters.
III. SWITCHED-INDUCTOR Z SOURCE INVERTERS
The switched-inductor Z-source inverter (SL-ZSI) is
considered as a modification of the classical ZSI. This class of
ZSI can provide very high boost with a very minimum value Fig.4 Equivalent circuit during Shoot through
of shoot through and hence the problem of low modulation
index and the related problems of reduced power quality and For the top switched inductor cell, D1 and D2 are ON,
inversion ability can be avoided. This topology is totally and D3 is OFF. L1 and L3 are charged by C1 in parallel. For the
different from any other existing Z-source inverters from the bottom switched inductor cell, D4 and D5 are ON, and D6 is
viewpoint of circuit structures and operation principles. The OFF. L2 and L4 are charged by C2 in parallel. SL cells perform
new configuration has retained the X structure of the classic the same function to absorb the energy stored in the
ZSI and in addition added six more diodes and two extra
capacitors.
inductors.
The new structure is extensible for the further development After the shoot through, the inductors are in charged
using the coupled inductor techniques and other potential condition and the capacitors are in discharged condition. Now
improving techniques. The modulation techniques that can be the non shoot through mode happens. This mode corresponds
applied to the classical ZSI are also applicable to the new to the six active states and two zero states of the main circuit
configuration. and the equivalent circuit is shown in Fig. 5[1]. In this mode
A. Operation principle the diode Din at the front end is forward biased and for the top
Fig. 3 shows a SL- Z-source inverter [1] switched inductor cell, D1 and D3 are OFF, and D5 is ON. L1
and L2 are connected in series, and the stored energy is
transferred to the main circuit. For the bottom switched
inductor cell, D4 and D5 are OFF, and D5 is ON. L3 and L4 are
connected in series, and the stored energy is transferred to the
main circuit. At the same moment, to supplement the Therefore
consumed energy of C1 and C2 during the shoot-through state,
C1 is charged by Vin through the bottom switched inductor cell, (7)
and C2 is charged by Vin via the top switched inductor cell [1].

Fig 5 Equivalent circuit during the non shoot-through

Fig.6 Boost ability comparison of the classical Z-source
states impedance network and the proposed SL Z-source impedance
network
From the operation principle, it is quite evident that the
For the comparison of the individual boost ability,
inductors are being charged parallel during the shoot through the curves of the boost factor B versus the duty ratio D for
state by the capacitors. Later during the active states, the classical Z-source impedance network and the new SL Z-
inductors are discharged as in series. Thus the load receives source impedance network, respectively, are shown in Fig.6.
power from the source as well as from the stored energy in the The boost ability of the new SL impedance network is
inductors, thus providing a boosted output at the load end. The significantly increased compared with that of the classical Z-
switching between the parallel and series configuration of the source impendence network.
inductors is one of the unique characteristics of SL-ZSI.
The boost inversion ability of a whole Z-source is
B. Mathematical analysis of SL- Z-source inverter determined by the interactions of Z-source impedance and the
PWM control method applied to the main circuit. The PWM
For mathematical analysis, assume all the inductors
control method called simple boost is been applied here. In
and capacitors to have the same inductance (L) and
simple boost method the obtainable duty ratio of the shoot-
capacitance (C), respectively. In the steady state, we have
through state can be regarded as a constant value, and its
Vc1 = Vc2 = Vc (1)
maximum value is limited to ( 1 − M) as given in [2]. The
The inductor current iL1 increases during switching
voltage conversion ratio of the whole inverter G can be
ON and decreases during switching OFF. During switching
expressed by
ON, the corresponding voltage across L 1 , VL1- ON is equal to VC
. Applying the volt–second balance principle to L 1 , we can get
the corresponding voltage across L 1 during switching OFF,
VL1- OFF , which is expressed by (8)
where vpn is the peak value of the output phase voltage. The
(2) maximum voltage con-version ratio Gmax versus any desired
The inductor current iL3 increases during switching ON and modulation index M can be expressed by
decreases during switching OFF. The corresponding voltages
across L3 are equal to VC 1 and −(VC 2 − Vin + VL1- OFF ). (9)
Applying the volt–second balance principle to L3 ,we have where G0-s is defined as the maximum voltage conversion ratio
DT VC1 =(1 − D)T (VC2 − Vin +VL1- OFF ) (3) of the classical Z-source inverter and its expression depends
or DT Vin =(1 − D)T(VC − Vin − D1 − D VC) (4) on M, given by
Hence

(10)
(5) The shoot-through duty cycle varies in each cycle. The
During switching OFF, C1, L1 ,L3 , and the voltage source Vdc average duty ratio of the shoot-through zero state, D’ is
form a close loop; therefore we have expressed by
VC =Vdc +VL1- OFF + VL3- OFF (6)
 (11)
Substituting (11) in B we get the equivalent boost factor B’
under the condition of variable duty ratios

(12)
Therefore, the maximum voltage conversion ratio Gmax versus
any desired modulation index M approximates to

(13) Fig.8 Simple boost control (SBC)

In SBC, there are five modulation curves: two shoot
where G0-m is defined as the maximum voltage conversion through envelop signal Vp and Vn and three modulating
ratio of the classical Z-source inverter and its expression reference sinusoidal signal Va , Vb and Vc. The amplitude of
depends on M, given by shoot through (ST) envelop signal should be greater than or
equal to peak value of modulating sinusoidal reference signal.
By comparing DC signal with the high frequency triangular
(14)
carrier, shoot through switching pulses are generated. The
Fig.7 shows the maximum voltage conversion ratios
three-phase modulating reference signals are compared with
of the proposed inverter under the simple boost control
high frequency triangular signal to produce the switching
condition where curve 1 and curve 2 correspond to the
pulses. These two signals are compared by a comparator.
proposed SL Z-source inverter and the classical Z-source
Therefore when triangular signal is greater than upper
inverter, respectively. It is shown that the voltage boost ability
envelope Vp or less than lower envelope Vn, the circuit enters
is unavailable at M = 1. However, if M<1, with the decreasing
into shoot-through (ST) state. The waveforms for simple boost
of M, the voltage boost inversion ability of the proposed
control are given in Fig.8 by logical OR gate shoot-through
inverter becomes much stronger than that of the classical Z-
states are inserted into switching waveform. These pulses are
source inverter. It means that for a given voltage conversion
sent to the gate of the switching devices in the inverter. For
ratio, a higher modulation index can be used in the proposed
SBC the modulation index (M) increases with the decrease in
inverter to improve the inverter output performance.
the shoot through duty ratio (Do).

IV. SIMULATION RESULTS

To verify the theoretical results, a simulation example
for the voltage inversion from DC 36 V to AC 198 V (rms) is
performed in the MATLAB/Simulink software environment.
The main circuit parameters are chosen as follows:
1) SL Z-source impedance network: L1 = L2 = L3 = L4 =1 mH,
C1 = C2 = 800 µF
2) Three-phase output filter: Lf = 1 mH, Cf = 22 µF;
3) Switching frequency fs =1/T = 10 kHz;
4) Three-phase balanced RL load, R = 10 Ω, L= 15 mH;
5) All components are assumed ideal.
A. Three-phase SL Z-Source Inverter simulation results
For the three-phase SL ZSI, with an input DC voltage
Vin=36 V, shoot-through duty ratio D=0.3 and M=0.7, then the
expected boost ratio B is 13 and the overall inverter boost G is
Fig.7 Maximum voltage conversion ratios of the proposed 9. It means the output must be nine times the input voltage.
inverter under the simple boost control condition Fig.9 shows the output voltage waveform. From the
C. Modulation technique for SL Z source inverter simulation results, it is seen that the output line-to-line voltage
In this project simple boost sine PWM is applied for the is around 280V (peak) and the RMS voltage is found out as
switching of the proposed inverter. Fig 6 shows the control 198V.
scheme [3].
 Fig.13 Capacitor Voltage (C1)
Fig.9 Output Voltage
The output current is found out to be having a peak value of
14 A. Fig 10 shows the output current waveform.

Fig.13 Capacitor Voltage (C2)

The simulation result in Fig. 14 shows the inductor
current.

Fig.10 Output Current

With input DC voltage of Vin=36V, the DC link
voltage Vdc is expected to be equal to 468V which is 13 times
the input DC voltage. The simulation result is shown in
Fig.11.

Fig.14 Inductor current

B. Single-phase SL Z-Source Inverter simulation results

For a single-phase SL ZSI with an input DC voltage of
3.5V, the simulation result shows an output AC voltage of 15V
Fig.11 Dc link voltage (peak). This means that in single-phase inverter, the overall
inverter boost G has reduced to 4.2 from 9 in the three-phase
The capacitor voltages C1 and C2 are supposed to be inverter. Fig.15 shows the output voltage waveform.
of equal value and they should together contribute for the DC
link voltage which means each capacitor should have a
voltage equal to 240V. Fig.12 and Fig.13 represent the
capacitor voltages of C1 and C2 respectively
 Fig.18 capacitor voltage (C1)

Fig. 15 output voltage waveform

Fig.16 shows the simulation result for the output Fig.19 capacitor voltage (C2)
current. It can be seen that the output current is 0.7 A(peak).
It is seen that the capacitor voltage waveforms are identical in
nature.

V.CONCLUSION
This paper analyses an improvised configuration for
classical Z-source inverters through simulation results. This
inverter uses a unique SL Z-source impedance network to
couple the low dc voltage energy source to the main. The
ability of the inductor configuration to switch between parallel
and series connection during the shoot through and active state
allows this topology to provide very high boost as compared
Fig.16 Output current
with the classical Z-source inverter. The high boost and
inversion ability of this topology was analyzed through
For the single-phase SL ZSI, the DC link voltage is shown
simulation results in MATLAB/Simulink for three-phase and
in Fig.17. The DC link voltage is found out to be 15 V. In a
single-phase SL Z source inverters. Single-phase SL Z-source
three-phase inverter, the theoretical boost for the DC link is
inverter is not found to be effective as three-phase SL S-
thirteen times the input voltage. But, for a single phase
source inverter. In single phase SL ZSI, the theoretical boost
inverter, it is seen that the boost factor B is only 4.2.
and inversion abilities is almost reduced by half. Hence, it is
concluded that the new SL-Z source inverter is more effective
in three-phase circuits as compared to the single phase
circuits.
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