Experimental Studies on a Single-Phase Improved Switched Inductor ZSource Inverter
Ayman F. Ayad1, Mohamed A. Ismeil1,2, Ralph Kennel1, Mohamed Orabi2
1
TECHNICAL UNIVERSITY OF MUNICH
80333 Munich, Germany
2
SOUTH VALLEY UNIVERSITY
81542 Aswan, Egypt
Tel.: +49 / 89.289.28358
Fax: +49 / 89.289.28336
E-mail: ayman.francees@tum.de; ismeil@ieee.org; Ralph.kennel@tum.de; orabi@ieee.org
URL: http://www.eal.ei.tum.de
Keywords
«Switched inductor», «Z-source inverter», «Boost factor», «Voltage stress», «PWM».
Abstract
This paper presents an experimental work on improved SL Z-source inverter topology. Because of the
limitations of classical Z-source inverter, the SL Z-source inverter has been proposed to increase the
voltage gain and to reduce the voltage stress on the capacitors of the Z-network. The simulation and
experimental results show that the improved SL Z-source inverter increases the boost factor, improves
the voltage stress across the capacitors, and reduces the inrush current.
Introduction
In 2002, the topology of the Z-source inverter was proposed to overcome the limitations of the
traditional inverter such as the boosting mode and the problems of short circuit [1]. The idea of this
topology is based on building an impedance network (Z network) which is used to replace the
traditional DC link as shown in Fig. 1. Therefore; an additional zero state appears following to the leg
shoot- through of one phase, two phases or three phases. This case is not allowed in the control
strategy of the traditional inverter. On the other side, this alternative topology gives more flexibility
and allows boosting the voltage across the DC- Link bus. The boost factor B can be expressed as
follows [2]:
B=
1
1
=
2To 1 − 2 D
1−
T
(1)
where: T0 is the interval of the shoot-through zero state during a switching cycle T. D represents the
duty ratio of the shoot through for each cycle and equals to T0/T. However, this proposed topology of
Z-source inverter has some drawbacks, such as a large voltage stress across the switches and
capacitors, huge inrush current, and small boost factor.
The concept of the switched inductor (SL) techniques has been integrated into the classical Z-source
impedance network [13], consequently a new SL Z-source impedance network topology has been
obtained. This topology is shown in Fig. 2; it is called the SL Z-source inverter.
Fig. 1: Classical Z-Source inverteer
It is clear that the new approaach is totally different from any other existing Z-source inverters
regarding to the circuit structuress and operation principles. The new obtained boost factor is efficiently
increased and expressed as follow
ws:
B=
1+ D
1 − 3D
(2)
Fig. 2: SL Z-source inverter
On the other hand, the voltage stress
s
across the switches and capacitors remainn large and the inrush
current is still within large valuues. Because of this limitation, an improved Z-source
Z
inverter was
proposed [14]-[15]. Indeed, the improved
i
topology has exactly the same componnents as the previous
topology, where the network imppedance is moved to be placed after the inverter as shown in Fig. 3. It
was found that the voltage streess across the capacitors and switches is reduceed, but there was no
improvement for the boost factorr beside the limitation of the range of the index modulation.
m
Fig. 3: Improved Z-source invertter
Improved SL Z-source in
nverter
The topology presented in Fig. 2 is improved in the present work, where the maiin aim is to avoid the
problems of the first topology by
b minimizing the voltage stress across the capacitors and switches,
while the boost factor obtainedd in the previous topology is kept the same. This idea is based on
moving the SL Z-source networkk to be placed after the inverter in series as show
wn in Fig 4. Where, it
is shown clearly that this new toppology is different from that presented in Fig. 3.
Fig. 4: Improved SL Z-source invverter.
Operation principle of im
mproved SL Z-source inverter
The equivalent circuit of the impproved SL Z-source inverter proposed in this papper is shown in Fig 5.
This topology is studied under thhe following assumptions:
1. Improved SL Z-source invverter operates in CCM (continuous conduction mode).
m
2. All components are assum
med to be ideal, where:
L1 = L2 = L3 = L4 = L
, C1 = C2 = C
3. Because of the Symmetryy of the inductors and the capacitors :
VC1 = VC 2 = VC ,VL1 = VL 2 = VL 3 = VL 4 = VL
Fig. 5: Equivalent circuit of the im
mproved SL Z-source inverter.
Shoot-through state
The shoot-through actions of thee top and bottom arms of the main circuit and its equivalent circuit are
shown in Fig. 6(a). When the sw
witch S is on, the different states of the diodes Din, D1, D2, D3, D4, D5
and D6 are presented in Table. I. It is obvious that when the two diodes D1 and D2 turn on, the
inductors L1 and L3 charge and the
t diode D3 turns off. On the other side, when thhe two diodes D4 and
D5 turn on, the inductors L2 and L4 charge and the diode D6 turns off. In this case the diode Din remains
off. In this mode, the relationshhip among the input voltage, the capacitor voltaage, and the inductor
voltage can be presented by the following
f
equation:
(3)
Vin + VC − VL = 0
Non-shoot-through state
The equivalent circuit of this mode is shown in Fig. 6(b). When the switch S is off, the different states
d
D1 and D2 turn
of the diodes Din, D1, D2, D3, D4, D5 and D6 are presented in Table I. As the two diodes
off and D3 turns on. This case allows
a
the two inductors L1 and L3 to be conneccted in series. On the
other side when the two diodes D4 and D5 turn off and the diode D6 turns on, the two inductors L2 and
L4 are connected in series. In thiss case, the diode Din remains on.
Table I: The different
d
states of the diode function of S sttate
S
Din
D1
D2
D3
D4
D5
D6
on
o
off
on
on
off
on
On
off
off
o
on
off
off
on
off
Off
on
In this mode, the relationship am
mong the input voltage, the capacitor voltage, the inductor voltage, and
the input DC voltage of the inverrter can be presented by the following equations:
(4)
−VC − 2VL = 0
(5)
Vin − Vdc − VL + VC − VL = 0
wing expressions are
By applying the volt-second ballance principle to the inductor voltage, the follow
obtained:
2D
Vin
1 − 3D
1+ D
=
Vin = BVin
1 − 3D
VC =
(6)
Vdc
(7)
Thus; the boost factor of the SL Z-source
Z
impedance B is expressed as follows:
T
1+ o
1+ D
T
=
B=
1 − 3D 1 − 3To
T
(8)
The resulting gain of the SL Z-soource inverter is then expressed as follows:
Gmax = MB =
(2 − M )
3M − 2
The detailed modeling of the nonn-ideal case was presented in previous work [16].
(a) Shoot-through state
(9)
(b)
Non-shoot-through state
Fig. 6: Equivalent circuit of imprroved SL Z-Source inverter: (a) Shoot-through staate. (b) Non-shootthrough state.
Stress analysis
The voltage stress across the cappacitors in the initial topology of SL Z-source preesented in [5] is given
by:
VC =
1− D
Vin
1 − 3D
(10)
This expression is different froom the one presented in (6) which is deducedd from the improved
topology proposed in this paperr. A comparison between the curves of the volttage stress across the
capacitors versus the duty cycle of the initial topology and the improved topologgy of the SL Z-source
are shown in Fig. 7. It is obvious that the voltage stress across the capacitors of SL Z-source is larger
than the proposed topology.
10
Vc/Vin
8
6
Improved SL Z-source
4
SL Z-source
2
0
0
0.05
0.1
0.15
D
0.2
0.25
0.3
Fig. 7: Voltages stress against duuty cycle.
On the other side; the voltage strress across the switches in the proposed topology Vs, can be expressed
as follows:
(11)
Vs = BVin
This voltage can be rewritten as a function in the gain and the modulation index:
Vs
G
=
Vin M
(12)
Simple boost control metthod for improved SL Z-source inverrter
This control strategy is based onn the use of the two straight lines, a line in the poositive side and a line
in the negative side of the voltagge axis. The value of the positive line has to be kept
k
more or equal to
the peak value of the three phaase reference system voltage which is supposed to be balanced. The
negative line has to be less or eqqual to the negative peak of the reference system
m voltage as shown in
Fig. 8. The carrier signal is the same
s
as in the classical carrier triangular based PWM.
P
In this control
strategy, the boost factor is consttant and the maximum gain is presented in (9).
The shoot through state can be occurred when the triangular wave is greater/leess than the reference
signal and the positive/negative straight line. In the other situation, the control sttrategy is the same as
the classical triangular PWM.
Fig. 8: Waveforms and switchingg strategies of simple boost control
Simulation results
To validate the theoretical advvantages of the proposed topology of the SL
L Z-source inverter,
simulation results under PSIM arre presented with simple boost control method. The
T parameters of the
SL Z- source are chosen as follow
ws:
1. SL Z-source impedance network:
n
L1 = L2 = L3 = L4 = 220mH , C1 = C2 = 100μF
2. the output filter :
L f = 1mH , C f = 25μF
3. the switching frequency: f S =
1
= 20 kHz
T
4. resistive load: R = 680Ω
5. all components are assum
med to be ideal.
According to equations (7), (8) and (9) and at M=0.8, the following values can bee calculated:
D=0.2, B=3, G=2.4, VDC= 45V, VC1=VC2=15V, Vac =36V.
In the simple boost control method, the duty cycle of the shoot-though state is constant, therefore; it is
easy to implement the control cirrcuit by an analog PWM circuit. The simulationss results are presented
in the following figures, where the
t output voltage and capacitor voltage are recoorded, respectively in
Fig.9.
Fig. 9: Output voltage and capacitor voltage
Fig. 10 shows the DC bus voltage and it is clear that the discontinuous mode makes some increases in
the DC bus.
Fig.10: DC bus voltage.
As a comparison between switched inductors and improved switched inductor Z-source inverter. Fig.
11 shows that, in the proposed topology, the inrush current is strongly decreased from 98A to 32A.
The steady-state performance in the simulation is identically matching with the theoretical analysis.
a)
b)
Fig. 11: Input current a) improved switched inductor b) switched inductor.
Experimental results
The single-phase improved switched inductor Z-source inverter is built at the laboratory, where the
inverter experimental test was performed to identify the operating characteristics of the single-phase
Improved SL ZSI. Fig.12 (a), (b) and (c) show the experimental results with 15 V input voltage and
680Ω resistive load. From the experimental results, the inverter operates in boost mode with shootthrough D=0.2. The PN voltage across the device is boosted to around 40-V, thus increasing the output
voltage to 82V P-P. The modulation index of the PWM conditions is 0.8. It was successfully
demonstrated that the Z-source inverter can greatly boost the output voltage as desired where the main
advantage of this topology is to decrease the capacitor voltage stress as shown in Fig.12 c
a)
b)
c)
Fig. 12: Experimental Results for Improved Switched Inductors: A) Output Voltage B) Dc Link
Voltage C) Capacitor Voltage
Fig.13 (a) ,(b)and (c) show the experimental results with 15 V for the previous topology (switched
inductor Z-Source) and as it shown the boost factor and output voltage are obtain . The main
difference is that the proposed topology decreases the capacitor voltage stress. Where with the same
boost factor the capacitor voltage decreases from 42 V to 23 V.
a) Output voltage
b) DC link voltage
c) Capacitor voltage
Fig. 13: Experimental results for switched inductor: A) Output voltage B) DC Link voltage C)
Capacitor voltage
It seems that both topologies have the same problems with DC link voltage and the noise at the start of
non-shoot through state. But these problems will disappear in case of heavy loads.
Conclusion
This paper deals with the presentation of a new and improved topology of the SL Z-source inverter,
where the main goal is to improve the behaviors quality of the Z-source inverter. Also to solve the
main drawbacks of previous topologies, such as voltage stress across the capacitors, voltage stress
across the inverter switches, and the inrush current. In the same time the quality of the boost factor has
to be kept improved. To fulfill these requirements, the proposed topology is studied under simple
boost control strategy. The simulation and the experimental results are presented; these results validate
the theoretical study. On the other side, the advantages of the new topology have been achieved such
as increasing of the boost factor, improving the voltage stress across the capacitors, reducing the
inrush current of the inverter.
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