Cascaded quasi z-source network on demand

Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30-31, December, 2014, Ernakulam, India
170
CASCADED QUASI-Z-SOURCE NETWORK ON DEMAND
ARYASREE G1
, RESHMILA S2
1,2
Electrical and Electronics Engineering Department, Sree Narayana Gurukulam college of Engineering,
Kadayiruppu, Kolencherry, India
ABSTRACT
This paper presents a cascaded quasi-Z-source converter that is suitable as a power conditioning unit, to
interconnect low dc voltage producing fuel cells or solar panels to residential loads. The proposed converter have reduced
energy loss, reduced shoot through duty cycle and reduced component values as compared to traditional single stage qZS
converter. The comparison of single stage and cascaded stage qZS converter is given in this paper with same input/output
conditions. A closed loop control scheme is also used for the comparison. The entire system is simulated in
Matlab/Simulink. The simulation results were presented and analyzed. Based on the results which showed the
effectiveness of the proposed cascaded quasi-Z-source inverter some applications are discussed.
Keywords: Quasi-Z-Source, Voltage Source Inverter, Z Source Inverter
I. INTRODUCTION
Due to environmental demands, the renewable energy systems are the one that will become wide spread in future.
As a result of dispersed nature of the renewable energy systems, the electric power will have distributed generation.
Distributed power is a concept that covers a wide scheme used for the local electric power generation from renewable and
non-renewable sources of energy in an environmentally responsible way. The schemes are mainly based on wind energy,
solar energy and fuel cells .A Fuel Cell (FC) is the most efficient modern approach to distributed power generation. The
efficiency of conversion could be as high as 65%-70%.The interconnection of FC to residential load demand a special
voltage matching converter [1].
When fully implemented, this can provide a high quality, reliable, cheap electric power. It offers savings in cost
for the reduction of the losses. The renewable sources when compared to hydro and nuclear power plant require a power
conditioning when connected to the domestic loads .Traditional VSI s which is used for power conditioning have the
disadvantage that it always bucks the output voltage and due to this demerit its operation are limited to low voltage . To
prevent this ZSI can be employed which again cause disadvantage of the discontinuous input current .So the qZSI is
proposed which make high voltage operation possible and also makes the current continuous.
Quasi-Z-Source inverter is a LC network which will boost the input voltage. It have two modes of operation that
is the normal mode and the shoot-through mode .In the normal mode the network act as a normal inverter and in the shoot-
through mode it will act as a short circuit which cannot be employed in the VSI . In the shoot-through mode the inductors
will save energy as short circuit carry high current in the circuit .This stored energy is delivered to the output load in the
normal inverter operation [2].
It is advisable to decrease the time for which the shoot-through mode comes into picture as energy loss can be
prevented .Also with the increase in the number of inductor which acts as an energy storage medium the higher energy
can be delivered to load that collected in the short time of the shoot through mode . So the proposal of a cascaded qZS
INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING &
TECHNOLOGY (IJEET)
ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
Volume 5, Issue 12, December (2014), pp. 170-177
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171
converter will be an efficient method for obtaining the above said merit .For that a cascaded converter is designed with
same legal values of the single stage and simulation model of the circuits of single and cascaded is tried with same
input/output values and results obtained are discussed .
II. QUASI-Z-SOURCE DC/DC CONVERTER
The qZS converter consists of a qZS network, an inverter, an isolation transformer and voltage doubler rectifier.
The circuit diagram of the qZS converter is shown in Figure 1.
The qZS converter can be used as a matching converter for interconnecting low voltage producing renewable energy
sources to domestic loads.
Here two modes of operation are there .The normal mode operation and the shoot-through mode operation. The
total time duration can be split into shoot-through and non-shoot through time. So the shoot through duty cycle will come
into picture which is the ratio of shoot through time period and the total time period. The circuit analysis of qZS is
described in the previous papers from which we can find the advantages lower stress or capacitor rating of C2 , continous
input current ,reduction in EMI problems as the circuit have common dc rail between source and load .
The main features of the qZS converter is that it can compensate the input voltage variations by providing the
boost and buck functions in a single stage. In the qZS inverter, the shoot-through states are used to boost the magnetic
energy stored in the dc-side inductors L1 and L2 without short circuiting the dc capacitors C1 and C2. This increase in
magnetic energy, in turn, provides the boost of the voltage seen on the inverter output during traditional operating states.
If the input voltage is high enough, the shoot-through states are eliminated, and the qZS inverter begins to operate as a
traditional VSI [3].
Fig. 1. Quasi-Z-Source DC/DC Converter
III. PROPOSED CASCADED QUASI-Z-SOURCE CONVERTER
The qZS converter circuit can be improved by the introduction of a cascaded qZS network. The cascaded (two-
stage) qZS network is derived by the adding of one diode (D2), one inductor (L3), and two capacitors (C3 and C4) to the
traditional qZS inverter. The circuit diagram of the cascaded qZS inverter is shown in Figure 2.The proposed cascaded
qZS converter also have the same working ways of the traditional qZS converter. But it can reduce the component values
of the capacitor and inductor and also can minimise the shoot-through duty cycle thereby minimising the energy loss in
the shoot-through time. The mathematical equations will be in the same way as that of the single stage qZS converter that
are discussed in previous paper [3] . Due to the decreased shoot-through duty cycle that can obtained in the cascaded
circuit, the values of the inductors and capacitors of the cascaded qZS network could also be decreased. On the other
hand, for the same component ratings and voltage and current stresses, the qZS converter with the proposed cascaded
qZS network will ensure a higher voltage boost factor than with traditional solutions. Because of its higher boost factor
the cascaded qZS inverter can be used for the interconnection of low voltage producing renewable energy sources to the
grid solar water pumping system etc. The proposed converter can be used for for a solar pump system and also for the
interconnection of the renewable energy sources to the grid.

172
Fig. 2. Cascaded Quasi-Z-Source coverter
The proposed converter can use the logic that derived from the control principle of single phase qZS converter
The switching states means the one in which one switch in the leg conduct and in the shoot through mode the two switch
in the same leg conduct .To generate this shoot through state two reference signals Up and Un can be used .If the triangular
waveform is greater than Up or lower than Un the shoot through state will be in the picture .So for this logic we have to
develop a control scheme using gates . The active states are controlled by two pulse generator phase shifted by 180o
. NOT
gates are used in between to prevent the simultaneous conduction of the two switch in same leg .The shoot through states
are produced by triangular wave generator ,two comparators and reference signal .The operating graph is given in Figure
3 . The control circuit based on the operating graph for obtaining the normal mode and shoor-through mode is given in
Figure 4.
The PWM1 and PWM2 are the two phase generators that are phase shifted by 180o
. NOT gate is provided to
prevent the simultaneous conduction. These input are given to the four switches T1, T2, T3, T4. Thus a normal inverter
mode can be made by means of these logic .The shoot through mode can be obtained by the comparison of the triangular
wave G with the positive and reference signal Up and Un . Whenever the triangular wave go above the positive reference
signal Up and go below the negative reference signal Un it will enter the shoot through mode. Entering the shoot through
mode means it have to develop a short circuit. It means the two switches in same leg conduct simultaneously .For that the
triangular wave generator , reference signals Up and Un and comparators are used and by means an OR gate the pulse signal
are given to the four switches .
Fig. 3. Operating graph of the proposed converter

173
Fig. 4. Control circuit for the proposed inverter
IV. APPLICATIONS
The simulation results will prove that proposed converter is the one that have a better reliability. Based on the
above concept two applications are suggested .One is a solar water pumping system and other is the connection of PV
renewable energy connection to a micro grid .The schematic figure of solar water pumping system using cascaded qZS
converter is given in Figure 5 .The control circuit for grid connection is given in Figure 6.
Fig. 5. Schematic figure of solar water pumping model using cascaded qZSI
Fig. 6. Grid connected qZS converter

174
V. SIMULATION AND RESULTS
The simulation of single stage qZS converter and the cascaded qZS converter are carried out in Matlab/Simulink
environment with same input/output conditions. The results obtained were compared.
The single stage converter consists of a single stage qZS network, a single phase inverter, transformer and voltage
doubler rectifier. The simulink model of single stage qZS converter is shown in Figure 7 .The control circuit is used same
for both single and cascaded and is shown in Figure 8. The system specifications are as follows: L1, L2 = 50 µH, C1,C2 =
360 µF, C3,C4 = 100 µF and Vin = 44 V, Vout = 600 V,Transformation ratio=3.75 . The simulation model of the proposed
qZS inverter was developed using Matlab/Simulink 7.14.
Fig. 7. Simulink Model of Single Stage Qzs Converter
Fig. 8. Simulink Model of the Control Circuit

175
The shoot through pulses of the single stage qZS converter are shown in Figure 9. The simulation result shows
that the single stage qZS inverter requires a shoot-through duty cycle of 3.39% to accomplish an output voltage of
600Volts.
The Simulink model of the proposed inverter is shown in Figure 10. The proposed inverter consists of a cascaded
qZS network, a single phase inverter, transformer, voltage doubler rectifier and on load side we have inverter, LC filter and
R load. The filter circuit is added to the output of the inverter in order to comply with the harmonic standards. The system
specifications are as follows: L1, L2 = 33.5 µH, C1,C2, C3,C4 = 180 µF, C5,C6, C7 = 100 µF and Vin = 44 V, Vout = 600 V
,Transformation ratio=3.75 . The simulation model of the proposed qZS inverter was developed using Matlab/Simulink
7.14.
The shoot through pulses for the proposed cascaded qZS inverter are shown in Figure 11. The simulation result
shows that the cascaded qZS inverter requires a shoot-through duty cycle of 1.83% to accomplish an output voltage of
600Volts.
Fig. 9 Shoot Through Pulses of Single Stage Qzsi
Fig. 10. Simulink Model of the Proposed Converter

176
Fig. 11. Shoot Through Pulses of Cascaded Qzsi
The comparison between single stage and cascaded qZS converter is given in the table below.
From (1) it is clear that, for the same boost factor, the two stage qZS inverter requires 33.33% less shoot-
through, when compared with the single stage qZS inverter. The component values of the cascaded convereter have
reduction .The inductor value reduce by 33.5% and capacitor value reduce by 50% compared to single stage qZS
network.
VI. CONCLUSION
A cascaded quasi-z-source network based inverter is proposed in this paper. The proposed inverter shows a
reduction in shoot-through duty cycle when compared with the traditional single stage quasi-z-source inverter.
Simulation results shows that the proposed cascaded qZS inverters shoot-through duty cycle was reduced by 46%, when
compared with the traditional single stage qZS inverter. The proposed converter can be used in solar water pumping
system and in grid connected PV cell circuit.
VII. ACKNOWLEDGEMENT
This is the most satisfying, yet the most difficult part of the report, to present gratifying words, because most
often we fail to convey the real influence, others have had on one’s life or work.
First and foremost, I stand grateful to Prof.V.M.Vijayan, Head of the department of Electrical & Electronics, for his
valuable advice and motivation. Also I express my heartful thanks to my group tutor Asst.Prof. Ajith K.A and other
supporting faculties from the department for their valuable input.
I express my effective gratitude to my internal guide Mrs. Reshmila S for her helpful feedback and timely assistance.
I also express my sincere thanks to all other faculty members for their help and encouragement. I thank all my Friends
who have helped me with their inspirations and co-operation.
Lastly and most importantly I thank Almighty who gave me the inner strength, resource and ability to complete my work
successfully, without which all my efforts would have been in vain.
Once again I convey my gratitude to all those persons who had directly or indirectly influence on my work.
TABLE 1 Comparison Of Qzs Converters
Parameter Single stage qZS converter Cascaded qZS converter
Voltage boost factor 1/(1 – 2Ds) 1/(1 – 3Ds)
Source current Continuous Continuous
Shoot-through duty cycle 3.39% 1.83%
Component count 4 7

177
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