International Conference on Intelligent and Advanced Systems 2007

Z-Source Inverter for UPS Application

R.Senthilkumar, R.Bharanikumar, Jovitha Jerom
Bannari Amman Institute of Technology PSG College of Technology
Sathyamangalam, Coimbatore
Tamil Nadu, India

Abstract- This project proposes an impedance-source inverter and its (both single phase and single phase voltage source inverter). The
control method for implementing dc-to-ac, ac-to-dc, ac-to-ac, and dc-to-dc switching device can be a conventional MOSFET, thyristor or a
power conversion. The Z-source converter employs a unique impedance
network to couple the converter main circuit to the power source. The Z- power transistor. Voltage source inverter is one in which the dc
source converter overcomes the conceptual and theoretical barriers and source has small or negligible impedance. In other words a
limitations of the traditional voltage-source converter and current-source voltage source has stiff dc source voltage at its input terminals.
converter and provides a novel power conversion concept. The Z-source A current source- fed inverter or current source inverter is fed
concept can be applied to all dc-to-ac, ac-to-dc, ac-to-ac, and dc-to-dc power
conversion. To describe the operating principle and control, this paper with adjustable dc current source. In current source inverter,
focuses on an Uninterrupted Power Supply (UPS) applications output current waves are not affected by the load.

Keywords- ZSI, VSI, CSI, Inverter B. Voltage source inverter [VSI]

The traditional voltage-source inverter input is a dc voltage
I. INTRODUCTION source supported by a relatively large voltage source can be a

T raditionally there are two inverters available. battery, fuel-cell stack, diode rectifier, and/or capacitor. Four
These are voltage source inverters and current source switches are used in the main circuit; each in traditionally
inverters. Each inverter has two switches in the main circuit. bidirectional current flow and unidirectional voltage blocking
These switches are power switches with anti-parallel diodes. capability. The V-source inverter is widely used however; it has
These diodes provide bidirectional current flow and reverse the following conceptual limitations [5].
voltage blocking capability. Traditional inverters have following
limitations .They can operate either as a boost or buck inverter C. Limitations of voltage source inverter
and cannot be a buck-boost inverter. Their output voltage range The V-source inverter is buck (step down) inverter for dc-to-
is limited to either greater or smaller than the input voltage. ac power conversion. For applications where over drive is
Their main circuit is not being interchangeable. In other words desirable and the available dc voltage is limited, an additional
neither the voltage source inverter can be used for the current dc-dc boost (step up) stage is needed to obtain a desired ac
source inverter nor vice versa. They are vulnerable to EMI noise output [1]. The additional power converter stage increases
in terms of reliability. The above limitations can be rectified in system cost and lowers efficiency.
impedance source inverter to get higher efficiency. This concept The upper and lower devices of each phase leg cannot be
can be applied to all AC to DC, AC to AC, DC to DC, DC to AC gated on simultaneously either by purpose or by EMI noise.
power conversions [4]. Otherwise, a shoot-through would occur and destroy the devices.
The shoot-through problem by electromagnetic interference
II. TRADITIONAL SOURCE INVERTERS (EMI) noise’s misgating-on is a major killer to the inverter to the
Traditional source inverters are voltage source inverters and inverter’s reliability. Dead time to block both upper and lower
current inverters. The output of voltage source inverter is a stiff devices has to be provided in the V-source inverter, which
dc voltage supply, which can be a battery or a controlled causes waveform distortion, etc. [1]. An output LC filter is
needed for providing a sinusoidal voltage compared with the
R.Senthilkumar Asst.Professor EEE Department Bannari Amman Institute of current-source inverter, which causes additional power loss and
technology Sathyamangalam.e-mail id: ramsenthil2@gmail.com control complexity.
R.Bharanikumar Asst.Professor EEE DepartmentBannari Amman Institute of
technology Sathyamangalam.e-mail id:bharani_rbk@rediffmail.com
Dr.Jovitha Jerome Professor, C&I Department, PSG college of Technology, D. Current source inverter [CSI]
Coimbatore.e-mail id:jjovitha@yahoo.com The traditional current-source inverter input is a dc current
source feeds by the main converter circuit. The dc current source
can be a relatively large dc inductor fed by a voltage source such
rectifier (both single phase and single phase voltage source
as a battery, fuel-cell stack, diode rectifier, or thyristor
inverter). The switching device can be a conventional MOSFET,
converter. Four switches are used in the main circuit; each is
thyristor or a power transistor.
traditionally composed of a semiconductor switches device with
reverse block capacity such as gate-turn-off thyristor (GTO) and
A. Traditional source inverters
SCR or a power transistor with a series diode to provide
Traditional source inverters are voltage source inverters and
unidirectional current flow and bidirectional voltage blocking.
current inverters. The output of voltage source inverter is a stiff
However, the current -source inverter has the following
dc voltage supply, which can be a battery or a controlled rectifier
conceptual barriers and limitations. [2]

1-4244-1355-9/07/$25.00 @2007 IEEE ~ 835

 International Conference on Intelligent and Advanced Systems 2007

TABLE 1
Comparison of VSI, CSI and ZSI
E. Limitations of current source inverter
The ac output voltage has to be greater than the original dc
voltage that feeds the dc inductor or the dc voltage produced is Current Source Voltage Source Impedance
Inverter(VSI) Source Inverters (ZSI)
always smaller than the ac input voltage. For applications where Inverter (CSI)
a wide voltage range is desirable, an additional dc-dc boost stage
is needed. The additional power conversion stage increases 1. As inductor is used As capacitor is used As capacitor and inductor
in the d.c link, the in the d.c link, it acts is used in the d.c link, it
system cost and lowers efficiency. [1]At least one of the upper source Impedance is as a low impedance acts
devices and one of the lower devices have to be gated on and high. It acts as a voltage source. as a constant high
maintained on at any time. Otherwise, an open circuit of the dc constant current impedance voltage
inductor would occur and destroy the devices. The open-circuit source. source.
problem by EMI noise’s misgating-off is a major concern of the 2. A current source A VSI is more
converter’s reliability [3].overlap time for safe current inverter is capable of dangerous situation In ZSI mis-firing of the
commutation is needed in the I-source converter, which also withstanding short as the parallel switches are also
causes waveform distortion. The main switches of the I-source circuit across any two Capacitor feeds more acceptable sometimes.
of its output terminals. powering to the fault.
inverter have to block reverse voltage that requires a series diode Hence momentary
to be used in combination with high-speed and high- short circuit on load
performance transistors such as insulated gate bipolar transistors and mis-firing of
(IGBT). This prevents the direct use of low-cost and high- switches are
acceptable.
performance IGBT modules.
3. This is used in only This is also used in a This is used in both buck
F. Limitations in both voltage and current source inverter buck or boost buck or boost and boost operation of
Their obtained output voltage range is limited to either operation of inverter. operation of inverter. Inverter.
greater or smaller than the input voltage [2]. Their main circuit
cannot be interchangeable. In other words, neither the V-source 4. The main circuits The main circuit Here the main circuits are
inverter main circuit can be used for the I-source inverter nor cannot be cannot be Interchange
vice versa. They are vulnerable to EMI noise in terms of interchangeable. Interchange able
able here also.
reliability [5].
5. It is affected by the It is affected by the It is less affected by the
EMI noise. EMI noise EMI noise.

III IMPEDANCE SOURCE INVERTER 6. It has a considerable It has a considerable Harmonics Distortion is
amount of harmonic amount of harmonic low
A. Block diagram of impedance source inverter distortion distortion
To overcome the above limitations of the traditional V-source
and I-source inverter, this thesis deals an impedance-source 7. Power loss should Power loss is high Power loss should
be high because of be low
inverter and its control method for impedance dc-to-ac power filter
conversion. This thesis also deals with how to overcome the
limitations of voltage source inverter and current source inverter 8. Lower efficiency Efficiency should be Higher efficiency
because low because of high because of less
of high power loss power power loss
loss

Single
Phase Rectifier Impedance Inverter Load
AC Unit Network

The proposed impedance source inverter block diagram is

Supply

shown in Fig 1. It is consists of rectifier unit, Impedance

network, single phase inverter and load. AC voltage is rectified
to DC voltage by the rectifier. The rectified output DC voltage is
Figure 1. Block diagram of Impedance Source Inverter
fed to the network

B. Advantages of the impedance source network

The impedance source inverter concept can be applied in all
ac-ac, dc-dc, ac-dc, dc-ac power conversion. The output voltage
range is not limited. The impedance source inverter is used as a
buck-boost inverter. The impedance source inverter does not
affect the electro magnetic interference noise. The impedance
source inverter cost is low. The impedance source inverter

836 ~
 International Conference on Intelligent and Advanced Systems 2007

provides the buck-boost function by two stage power Fig.3, whereas the inverter bridge becomes an equivalent current
conversion. source as shown in Fig 3 when in one of the six active states.
The inverter bridge can be also represented by a current
IV ANALYSIS AND DESIGN OF THE IMPEDANCE
NETWORK source with zero value (i.e., an open circuit) when it is in one of
the two traditional zero states. Therefore, Fig. 3, shows the
A. Equivalent circuit, operating principle, and control equivalent circuit of the Z-source inverter viewed from the dc
The unique feature of the impedance-source inverter is that link when the inverter bridge is in one of the eight nonshoots-
the output ac voltage can be any value between zeros to infinity through switching states. All the traditional pulse width-
regardless of the DC voltage. That is, the impedance-source modulation (PWM) schemes can be used to control the Z-source
inverter is a buck-boost inverter that has a wide range of inverter and their theoretical input–output relationships still hold
obtainable voltage. The traditional V-and I-source inverters [1].
cannot provide such feature. To describe the operating principle
and control of the impedance-source inverter in Fig. 2, let us
briefly examine the impedance-source inverter structure. V CIRCUIT ANALYSIS AND OBTAINABLE OUTPUT
VOLTAGE
D2 L1 From the symmetry and the equivalent circuits, we have
+ + - +
+ + VC1=VC2=VC; VL1=VL2=VL (1)
-
C1 C2

DC
Vd - Vi i1
Given that the inverter bridge is in the shoot-through zero state
for an interval ofT0, during a switching cycle, T and from the
- - L2 + equivalent circuit, Fig. 3 one has
Figure 2. Equivalent Circuit of Impedance Source Inverter VL=VC; Vd=2VC; Vi=0 (2)
Now consider that the inverter bridge is in one of the eight
The single -phase Z-source Inverter Bridge has six nonshoots- through states for an interval of T, during the
permissible switching states unlike the traditional single-phase switching cycle, from the equivalent circuit,
V-source inverter that has five. The traditional single-phase V-
source inverter has five active vectors when the dc voltage is Vl=V0-VC: Vd=V0; Vi=VC=VL=2VC-V0 (3)
impressed across the load and one zero vector when the load
terminals are shorted through either the lower or upper single Where VO is the dc source voltage and T=T0+T1.
devices, respectively. However, the single-phase impedance- The average voltage of the inductors over one
source inverter bridge has one extra zero state. switching period (T) should be zero in steady state, from
When the load terminals are shoot-through both the upper and equation (2) and equation (3), we have
lower devices of any one phase leg. This shoot-through zero
state is forbidden in the traditional V-source inverter, because it VL=Vl =T0.VC+ (T1 (V0 - VC))/T = 0 (4)
would cause a shoot-through. We call this third zero state the
shoot-through zero state, which can be generated by seven VC/V0=T1/(T1-T0) (5)
different ways: shoot-through via any one phase leg, Similarly, the average dc-link voltage across the inverter bridge
combinations of any two phase legs, and all single phase legs. can be found as follows:
The impedance source network makes the shoot-through zero
state possible. Vl=Vi1=T0+T1 (2VC-V0))/T=
(T1/ (T1-T0)) V0=VC (6)

For the traditional V-source PWM inverter, we have the well

known relationship:
VS=M.BV0/2 (7)
Equation shows that the output voltage can be stepped up and
down by choosing an appropriate buck-boost factor,

Bb=M*B (0 to Į) (8)
From (1),(6) and (7), the capacitor voltage can expressed as
Figure 3. Equivalent circuit of the impedance source inverter viewed form the
dc link VC1=VC2= ((1-(T0/T))/ (1-2(T0/T)) V0 (9)
The inverter bridge is equivalent to a short circuit when the
inverter bridge is in the shoot-through zero state, as shown in

~ 837
 International Conference on Intelligent and Advanced Systems 2007

The buck–boost factor is determined by the modulation index

and boost factor. The boost factor can be controlled by duty
cycle (i.e., interval ratio) of the shoot-through zero state over the
nonshoots-through states of the inverter PWM. Note that the
shoot-through zero state does not affect the PWM control of the
inverter, because it equivalently produces the same zero voltage
to the load terminal.

The available shoot through period is limited by the zero-state

period that is determined by the modulation index. The
impedance source network should require less capacitance and
smaller size compared with the traditional V-source inverter.
Similarly, when the two capacitors are small and approach zero
the impedance source network reduces to two inductors in series
and becomes a traditional I-source. Therefore, a traditional I-
source inverter’s inductor requirements and physical size is the
worst case requirement for the impedance source network. Figure 5. Input and Output voltage waveform
Considering additional filtering and energy storage by the
capacitors, the impedance source network should require less The simulation proved the impedance source inverter concept.
inductance and smaller size compared with the traditional I- The waveforms are consistent with the simulation results.
source inverter [1].

VI. SIMULATION CIRCUIT AND RESULTS OF THE

IMPEDANCE SOURCE INVERTER
Simulations have been performed to confirm the above
analysis. Fig shows the main circuit configuration of impedance
source inverter for UPS application. The impedance network
parameters are L1=L2=160µH and C1=C2=C=1000µF. The
Figure 6. Modified gating pulse
purpose of the system is to produce single phase 208V rms
power from the DC source whose voltage changes 150-240V dc
depending on load current. VII. EXPERIMENTAL RESULTS
L1
The z-source inverter is practically implemented and the
160u hardware results obtained satisfy the specifications.
M1
M3
The Figure. 7, shows the PWM pulses with a phase shift of
1800 each other and is applied to the MOSFETs of single
V2
D1 D3 V1 IRFP450
IRFP450
SD51 SD51

V5 C1 C2
R1
phase inverter. Here there is no delay time between the pulses
but there is a shoot through in between the pluses. The pulses
100u 100u
5k

D4

are generated at a voltage of magnitude 14 volts

D2 M2
SD51 M4
SD51
V3 IRFP450 V4
IRFP450

L2

160u

Figure 4. Circuit Diagrams of impedance source inverter

Figure. 5, shows the input voltage and output voltage of the

z-source inverter. Input voltage is 100V AC supply. The output
voltage 100V DC is given by the rectifier unit. The output
voltage of impedance source inverter is shown above.

Figure 7. Pulses before the Driver Circuit

838 ~
 International Conference on Intelligent and Advanced Systems 2007

The voltage waveform is obtained after the impedance improved reliability, strong EMI immunity and low EMI. The
source terminals. This is a sine waveform which is fed to the impedance source technology can be applied to the entire
inverter. There is no need of output filter. Impedance source acts spectrum of power conversion.
as a second order filter.

REFERENCES

[1] F. Z. Peng, “Z-Source inverter,” IEEE Trans. Ind Applicat., vol. 39, pp.504–
510, Mar. /Apr. 2003.
[2] F. Z. Peng, X. Yuan, X. Fang, and Z. Qian, “Z-source inverter for adjustable
speed drives,” IEEE Power Electron. Lett., vol. 1, no. 2, pp. 33–35, Jun.
2003.
[3] F. Z. Peng, M. Shen, and Z. Qian, “Maximum boost control of the z-source
inverter,” in Proc. 39th IEEE Industry Applications Conf., vol. 1, Oct. 2004.
[4] M. Shen, J.Wang, A. Joseph, F. Z. Peng, L. M. Tolbert, and D. J. Adams,
“Maximum constant boost control of the Z-source inverter,” presented at the
IEEE Industry Applications Soc. Annu. Meeting, 2004.
[5] Theory on single phase inverters are presented by M.H.RASHID in power
electronics circuit device and applications, 2nd edition, Englewood cliffs,
Figure 8. Input of Inverter Circuit (Z-Source Output Voltage) N.J.,prentice-hall, 1993.
[6] Design of the impedance network is presented by COMPTUN.K.T in
electrics handbook, 6th edition, London, 1947.
This is a sine output voltage waveform of the inverter
circuit across the load terminals and has the amplitude of
30Volts and frequency of 50Hz BIOGRAPHY

Senthil Kumar.R was born in Tamilnadu, India, on

November 2, 1966. He received the B.E degree in Electrical
and Electronics Engineering from Madurai Kamaraj
University, in 1989. He received his M.E (Power systems)
from Annamalai University, in 1991. He has 15 yrs of
teaching experience. Currently he is working as Asst.
Professor in EEE department, Bannari Amman Institute of
Technology, Sathyamanglam. Currently he is doing research
in the field of power converters for UPS Applications.

Bharanikumar R. was born in Tamilnadu,

India, on may 30, 1977. He received the B.E degree in
Electrical and Electronics Engineering from Bharathiyar
University, in 1998. He received his M.E Power Electronics
and Drives from college of Engineering Guindy Anna
University in 2002.He has 8 yrs of teaching experience.
Currently he is working as Asst. Professor in EEE
department, Bannari Amman Institute of Technology,
Figure 9. Output Voltages across the Load
Sathyamanglam. Currently he is doing research in the field of
power converter for special machines; vector controlled based
synchronous machine drives, converters for wind energy
Variable inputs and load conditions are tabulated below. The conversion systems.
input AC voltage ranges from 100V to 160V and the load to be
fed to the switching equipment also varies correspondingly. For
any such variation in the input side as well as the load, the Dr. Jovitha Jerome was born in Tamilnadu, India, on June
output AC voltage changes according to the input voltage. 2, 1957. She received the B.E. degree in Electrical and
Finally a pure constant AC voltage is obtained and it is fed to Electronics Engineering and M.E. degree in Power Systems
the switching equipment of the UPS. This voltage is utilized to from College of Engineering, Guindy, Chennai. She did her
DEng in Power Systems. Presently she is working as
track the route to provide efficient UPS Application. Hardware Professor and Head in Instrumentation and Control
results also ensure it. Engineering Department of PSG College of Technology,
Coimbatore

VIII. CONCLUSION
A new type of inverter for UPS application has been proposed
and corresponding simulated waveforms are verified. The
Impedance source inverter is specially suited for above
applications. Unique features like single stage power conversion,

~ 839