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A ZVS Grid-Connected Three-Phase Inverter Driven Renewable Energy Sources
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CHENNAMSETTI PAVANI, 2BEELA RAJESH
M.Tech [Scholar], EEE Dept, VITAM College of Engineering, Visakhapatnam, AP-India
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Assisstant Professor, EEE Dept, VITAM College of Engineering, Visakhapatnam, AP-India
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
Abstract-- A six-switch three-phase inverter is widely
used in a high-power grid-connected system. However,
the anti parallel diodes in the topology operate in the
hard-switching state under the traditional control
method causing severe switch loss and high
electromagnetic interference problems. In order to solve
the problem, this paper proposes a topology of the
traditional six-switch three-phase inverter but with an
additional switch and gave a new space vector
modulation (SVM) scheme. In this paper the simulation
of ZVS three phase inverter, ZVS three phase inverter
with PV cell are produced and a comparative analysis
basing on various reports generated in the matlab tool is
performed.
Index Terms-- ZVS; ZCS; hard switching.
I. INTRODUCTION
IN A high-power grid-connected inverter application, the
six-switch three-phase inverter is a preferred topology with
several advantages such as lower current stress and higher
efficiency. To improve the line current quality, the switching
frequency of the grid-connected inverter is expected to
increase. Higher switching frequency is also helpful for
decreasing the size and the cost of the filter. However,
higher switching frequency leads to higher switching loss
[1]. The soft-switching technique is a choice for a highpower converter to work under higher switching frequency
with lower switching loss and lower EMI noise. In the past
few years, there have been many studies on soft switching
techniques for a three-phase converter. And they can
generally be divided into two configurations according to
the position where the soft-switching function is realized [26] as dc-side and ac-side soft-switching circuits.
The active-clamping ZVS-PWM half-bridge inverter [6-8]
also has lower voltage stress (1.01–1.1 times as high as the
dc-bus voltage). According to [6], in this active-clamping
ZVS-PWM half-bridge inverter, to achieve better softswitching performance, the slow reverse recovery switch
antiparallel diode is the primary choice because the diode
reverse recovery energy is used to obtain the
soft
commutation condition. In the ZVS dc-link single-phase
full-bridge inverter [7], the switch voltage is clamped to the
dc-link voltage. The PWM modulation scheme is modified
to achieve ZVS under different power factor loads. Besides
the dc-side soft-switching technique, there are also some acside soft-switching techniques suitable for higher power
application. The auxiliary resonant commutated pole
(ARCP) converter achieves zero-voltage turn-on for main
switches and zero-current turn-off for an auxiliary switch
[4].
The ARCP converter has excellent performance, but two
lowfrequency capacitors are necessary in the resonant cell
and it is difficult to control the capacitors’ midpoint voltage
without an additional control circuit. A new ZVS-PWM
single-phase fullbridge inverter using a simple ZVS-PWM
commutation cell is proposed in [5]. No auxiliary voltage
source or low-frequency center-tap capacitor is needed in
the cell. The main switches operate at ZVS and the auxiliary
switches operate at ZCS. The inductor-coupled ZVT
inverter achieves the zero-voltage turnon condition for main
switches and the near-zero current turn-off condition for
auxiliary switches [6]–[8]. This topology offers several
advantages over the ARCP. The problems associated with
the split dc capacitor bank are avoided, and the ZVT
operation requires no modification compared to normal
space vector modulation (SVM) schemes. The peak current
stress of the auxiliary switches is half of that of the main
switches.
The major problem of this topology is to use coupled
inductors, which are normally bulky in high-power
applications. An improved ZVS inverter used two coupled
magnetic components in one resonant pole to ensure the
main switches operating under the ZVS condition and the
auxiliary switches operating under the ZCS condition when
the load varies from zero to full. Since an independent
coupled magnetic component structure avoids the unwanted
magnetizing current antiparallel loop, the size of the coupled
inductors can be minimized with lower magnetizing
inductance, and its saturation can be eliminated. The ZVS
timing requirement is also satisfied over the full load range
by using the variable timing control with simple and reliable
ZV detection. The zero-current transition (ZCT) inverter
achieves ZCS in all of the main and auxiliary switches and
their antiparallel diodes. This topology needs six auxiliary
switches and three LC resonant tanks. The simplified threeswitch ZCT inverter [23] needs only three auxiliary switches
to achieve zero-current turn-off in all of the main switches
and auxiliary switches. Compared with the six-switch ZCT
inverter, the resonant tank current stress of the three-switch
ZCT inverter is higher.
The structure of the ZVS-SVM controlled three-phase PWM
rectifier is similar to the ACRDCL converter. With the
special SVM scheme proposed by the authors, both the main
switches and the auxiliary switch have the same and fixed
switching frequency. The reverse recovery current of the
switch be turned ON under the zero-voltage condition.
Moreover, the voltage stress in both main switches and the
auxiliary switch is only 1.01–1.1 times of the dc-bus voltage.
In this paper, a ZVS three-phase grid-connected inverter is
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proposed. The topology of the inverter is shown in Fig. 2,
which is similar to the rectifier topology proposed in [8]. All
the soft-switching advantages under the rectifier condition
can be achieved in a grid-connected inverter application, and
the voltage stress in both main switches and the auxiliary
switch is the same as the dc-bus voltage. The operation
principle of this SVM scheme is described in detail. The
experimental results of a 30-kW hardware prototype are
presented to verify the theory.
II. SWITCHING TECHNIQUES
A. HARD AND SOFT SWITCHING
In the 1970’s, conventional PWM power converters were
operated in a switched mode operation. Power switches have
to cut off the load current within the turn-on and turn-off
times under the hard switching conditions. Hard switching
refers to the stressful switching behavior of the power
electronic devices. The switching trajectory of a hardswitched power device is shown in Fig.1.
I
Safe Operating Area
On
Hard-switching
snubbered
Soft-switching
Off
V
Fig.1 Typical switching trajectories of power switches.
During the turn-on and turn-off processes, the power device
has to withstand high voltage and current simultaneously,
resulting in high switching losses and stress. Dissipative
passive snubbers are usually added to the power circuits so
that the dv/dt and di/dt of the power devices could be
reduced, and the switching loss and stress be diverted to the
passive snubber circuits. However, the switching loss is
proportional to the switching frequency, thus limiting the
maximum switching frequency of the power converters.
Typical converter switching frequency was limited to a few
tens of kilo-Hertz (typically 20kHz to 50kHz) in early
1980’s. The stray inductive and capacitive components in
the power circuits and power devices still cause considerable
transient effects, which in turn give rise to electromagnetic
interference (EMI) problems. Fig.2 shows ideal switching
waveforms and typical practical waveforms of the switch
voltage. The transient ringing effects are major causes of
EMI.
Fig.2. Typical switching waveforms of (a) hard-switched
and (b) soft-switched devices
In the 1980’s, lots of research efforts were diverted towards
the use of resonant converters. The concept was to
incorporate resonant tanks in the converters to create
oscillatory (usually sinusoidal) voltage and/or current
waveforms so that zero voltage switching (ZVS) or zero
current switching (ZCS) conditions can be created for the
power switches. The reduction of switching loss and the
continual improvement of power switches allow the
switching frequency of the resonant converters to reach
hundreds of kilo-Hertz (typically 100kHz to 500kHz).
Consequently, magnetic sizes can be reduced and the power
density of the converters increased. Various forms of
resonant converters have been proposed and developed.
However, most of the resonant converters suffer several
problems. When compared with the conventional PWM
converters, the resonant current and voltage of resonant
converters have high peak values, leading to higher
conduction loss and higher V and I ratings requirements for
the power devices. Also, many resonant converters require
frequency modulation (FM) for output regulation. Variable
switching frequency operation makes the filter design and
control more complicated.
In late 1980’s and throughout 1990’s, further improvements
have been made in converter technology. New generations
of soft-switched converters that combine the advantages of
conventional PWM converters and resonant converters have
been developed. These soft-switched converters have
switching waveforms similar to those of conventional PWM
converters except that the rising and falling edges of the
waveforms are ‘smoothed’ with no transient spikes. Unlike
the resonant converters, new soft-switched converters
usually utilize the resonance in a controlled manner.
Resonance is allowed to occur just before and during the
turn-on and turn-off processes so as to create ZVS and ZCS
conditions. Other than that, they behave just like
conventional PWM converters. With simple modifications,
many customized control integrated control (IC) circuits
designed for conventional converters can be employed for
soft-switched converters. Because the switching loss and
stress have been reduced, soft-switched converter can be
operated at the very high frequency (typically 500kHz to a
few Mega-Hertz). Soft-switching converters also provide an
effective solution to suppress EMI and have been applied to
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DC-DC, AC-DC and DC-AC converters. This chapter
covers the basic technology of resonant and soft-switching
converters. Various forms of soft-switching techniques such
as ZVS, ZCS, voltage clamping, zero transition methods etc.
are addressed. The emphasis is placed on the basic operating
principle and practicality of the converters without using
much mathematical analysis.
B. Resonant Switch
Prior to the availability of fully controllable power
switches, thyristors were the major power devices used in
power electronic circuits. Each thyristor requires a
commutation circuit, which usually consists of a LC resonant
circuit, for forcing the current to zero in the turn-off process.
This mechanism is in fact a type of zero-current turn-off
process. With the recent advancement in semiconductor
technology, the voltage and current handling capability, and
the switching speed of fully controllable switches have
significantly been improved. In many high power
applications, controllable switches such as GTOs and IGBTs
have replaced thyristors. However, the use of resonant
circuit for achieving zero-current-switching (ZCS) and/or
zero-voltage-switching (ZVS) has also emerged as a new
technology for power converters. The concept of resonant
switch that replaces conventional power switch is introduced
in this section.
A resonant switch is a sub-circuit comprising a
semiconductor switch S and resonant elements, Lr and Cr.
The switch S can be implemented by a unidirectional or
bidirectional switch, which determines the operation mode
of the resonant switch. Two types of resonant switches,
including zero-current (ZC) resonant switch and zerovoltage (ZV) resonant switches, are shown in Fig.3 and
Fig.4, respectively.
switch S is turned on, it carries the output current Io. The
supply voltage Vi reverse-biases the diode Df. When the
switch is zero-voltage (ZV) turned off, the output current
starts to flow through the resonant capacitor Cr. When the
resonant capacitor voltage VCr is equal to Vi, Df turns on.
This starts the resonant stage. When VCr equals zero, the
anti-parallel diode turns on. The resonant capacitor is
shorted and the source voltage is applied to the resonant
inductor Lr. The resonant inductor current ILr increases
linearly until it reaches Io. Then Df turns off. In order to
achieve ZVS, S should be triggered during the time when the
anti-parallel diode conducts. It can be seen from the
waveforms that the peak amplitude of the resonant capacitor
voltage should be greater or equal to the input voltage (i.e.,
Io Zr > Vin). From Fig.5(a), it can be seen that the voltage
conversion ratio is load-sensitive. In order to regulate the
output voltage for different loads r, the switching frequency
should also be changed accordingly.
1 cycle
ILr
IO
t2
0
t0
t1
t1'
t1"
t
t 2'
t3
t4
t3
t4
vc
ZrIO
vi
0
t
t0
t1
t1 '
t1"
t2
t 2'
(a) Circuit waveforms.
1
0.9
0.9
0.8
0.8
0.7
M
0.6
Lr
Lr
0.5
0.5
0.4
0.3
0.2
0.2
Cr
S
0.1
0.1
S
Cr
0
0
(a)
(b)
Fig.3 Zero-current (ZC) resonant switch.
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
g
(b) Relationship between M and g.
Fig.5 Half-wave, quasi-resonant buck converter with ZVS.
Lr
Lr
Cr
S
S
(a)
Cr
(b)
Fig.4 Zero-voltage (ZV) resonant switch.
IV. RESULTS
The results pertaining to the schematic discussed in the
previous section are presented here in this section. When the
ZVS converters can be operated in full-wave mode. The
circuit schematic is shown in Fig.6(a). The circuit
waveforms in steady state are shown in Fig.8(b). The
operation is similar to half-wave mode of operation, except
that VCr can swing between positive and negative voltages.
The relationships between M and g at different r are shown
in Fig.8(c).
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ILr
Cr
Dr
Io
Lr
Lf
+
v oi
-
+ vc -
Df
+
Vo
-
Cf
(a) Schematic diagram.
Fig.8: Output Voltage & Current of Proposed Three Phase
ZVS based Grid Connected Inverter
1 cycle
ILr
IO
t2
0
t0
t1
t1'
t1"
t
t 2'
t3
t4
t3
t4
vc
ZrIO
The corresponding output waveforms of the proposed
model presented in the circuit given as ZVS three phase
inverter is given in the Fig.8. Similarly Fig.10 depicts the
simulation results of the Fig.9 circuit.
ZVS using PV cell
vi
0
t
t0
t1
t1 '
t1"
t2
t 2'
(b) Circuit waveforms.
1
0.9
0.9
0.8
0.8
0.7
M
0.6
0.5
0.5
0.4
Fig.9: simulation diagram of a ZVS using PV cell
0.3
0.2
0.2
1
0.1
0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
g
(c) Relationship between M and g.
Fig.6 Full-wave, quasi-resonant buck converter with ZVS.
Comparing Fig.5(b) with Fig.6(c), it can be seen that M is
load-insensitive in full-wave mode. This is a desirable
feature.
A thorough description of the proposed ZVS three phase
inverter model with and without PV cell are given in this
section. There are presented in the Fig.7 through Fig.10.
ZVS 3 phase inverter
Fig 7 Simulation diagram of ZVS three phase inverter
Fig.10: Output waveforms of the proposed ZVS 3 phase
inverter
VII. CONCLUSION
Through study and analysis, of the ZVS schematic and its
results are made with a motive to understand its
characteristics. In practice, ZVS-QRCs are usually operated
in half-wave mode rather than full-wave mode. By replacing
the ZV resonant switch in the conventional converters,
various ZVS-QRCs can be derived. A comparative analysis
in the previous section paved a path towards vital
conclusions in its applications.
V. REFERENCES
[1] N. Mohan, T. Undeland, and W. Robbins, Power
Electronics: Converters, Applications and Design. New
York: Wiley, 2003, pp. 524–545.
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[2] M. D. Bellar, T. S. Wu, A. Tchamdjou, J. Mahdavi, and
M. Ehsani, “A review of soft-switched DC–AC converters,”
IEEE Trans. Ind. Appl., vol. 34, no. 4, pp. 847–860,
Jul./Aug. 1998.
[3] D. M. Divan, “Static power conversion method and
apparatus having essentially zero switching losses and
clamped voltage levels,” U.S. Patent 48 64 483, Sep. 5,
1989.
[4] M. Nakaok, H. Yonemori, and K. Yurugi, “Zero-voltage
soft-switched PDMthreephaseAC–DC active power
converter operating at unity power factor and sinewave line
current,” in Proc. IEEE Power Electronics Spec. Conf.,
1993, pp. 787–794.
[5] H. Yonemori, H. Fukuda, and M. Nakaoka, “Advanced
three-phase ZVS- PWM active power rectifier with new
resonant DC link and its digital control scheme,” in Proc.
IEE Power Electron. Variable Speed Drives, 1994, pp. 608–
613.
[6] G. Venkataramanan, D. M. Divan, and T. Jahns,
“Discrete pulse modulation strategies for high frequency
inverter system,” IEEE Trans. Power Electron., vol. 8, no. 3,
pp. 279–287, Jul. 1993.
[7] G. Venkataramanan and D. M. Divan, “Pulse width
modulation with resonant dc link converters,”
inProc.Conf.RecordIEEEInd.Appl.Soc.Annu.
Meeting,
1990, pp. 984–990.
[8] Y. Chen, “A new quasi-parallel resonant dc link for softswitching PWM inverters,” IEEE Trans. Power Electron.,
vol.13,no.3,pp.427–435,May 1998.