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Volume V /Issue I /JULY 2015
11-Level Grid-Connected Converter Topology For
Single-Phase Transformerless Pv Systems
K. LAKSHMAIAH1 AND T DEEPTHI PRASANNA2
Department of Electrical& Electronics Engineering, SIR C R Reddy College of Engineering, Eluru
1
Department of Electrical& Electronics Engineering, SIR C R Reddy College of Engineering, Eluru
1
Email: lakshmaiah.kommu@gmail.com, tata.deepti@gmail.com
ABSTRACTThis paper presents a gridconnected single-phase Transformerless photovoltaic
converter based on two cascaded full bridges with
different dc-link voltages. The converter can synthesize
up to eleven voltage levels with a single dc bus, since one
of the full bridges is supplied by a flying capacitor.
Harmonic distortion and electromagnetic interference
(EMI) can be reduced in the multilevel output. The
switching strategy is employed such that to regulate the
flying-capacitor voltage, improves the efficiency for
most devices switch at the frequency and will minimize
the minimum the common-mode leakage cuurent with
the help of a novel dedicated circuit. Simulation results
confirm the feasibility and good performance of the
proposed converter.
Index Terms—pulse width modulation (PWM)
inverters, photovoltaic (PV) systems, leakage current,
multilevel systems.
I INTRODUCTION
GRID connected PV systems provides best
solution for small-scale and domestic applications. .
While classical designs of PV converters feature a
grid frequency transformer, which is a typically
heavy and costly component, at the interface between
the converter and the electrical grid, researchers are
now considering Transformerless architectures in
order to reduce costs and weight and improve
efficiency. With the removal of grid frequency
transformer involves all benefits but worsens the
output power quality and increases ground leakage
current [3], [4]. Though fossil fuel resources are
limited and depleting at an alarming rate, the global
demand for oil has increased significantly in recent
years. Energy consumed and demanded by
transportation sector has risen exponentially due to
increasing number of vehicles [1]. Transportation
accounts for above 20% of the total energy-related
emissions [2]. Today most of the world’s vehicles are
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dependent on conventional energy sources. In this
regard, alternative solutions for “sustainable and
green mobility” are being researched and
implemented by researchers, industries as well as
policy makers. The active parts of PV modules might
be electrically insulated from the ground-connected
mounting frame; a path for ac ground leakage
currents generally exists due to a parasitic
capacitance between the modules and the frame and
to the connection between the neutral wire and the
ground, usually realized at the low-voltage/mediumvoltage (LV/MV) transformer [3]. In addition to
deteriorating power quality, the ground leakage
current increases the generation of electromagnetic
interference and can represent a safety hazard, so that
international regulations pose strict limits to its
magnitude.
This issue must be confronted in all transformerless
PV converters, regardless of architecture. In
particular, in full-bridge-based topologies, the ground
leakage current is mainly due to high frequency
variations of the common-mode voltage at the output
of the power converter [4]. Several solutions can be
found in literature aiming at the reduction of the
common-mode voltage harmonic content [5]–[7].
Once the grid frequency transformer is removed from
a PV converter, the bulkiest wound and reactive
components that remain are those that form the
output filter used to clean the output voltage and
current from high frequency switching components.
Further reduction in cost and weight and
improvement in efficiency can be achieved by
reducing the filter size, and this is the goal of
multilevel converters.
For years multilevel converters have been
investigated [8], but only recently have the results of
such researches found their way to commercial PV
converters. Multilevel converters outperform
conventional two- and three-level converters in terms
of harmonic distortion since they can synthesize the
output voltages using more levels.
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Fig.1 expected 11-level output
Moreover, multilevel converters subdivide the input
voltage among several power devices, allowing for
the use of more efficient devices. Multilevel
converters were initially employed in high-voltage
industrial and power train applications. They were
first introduced in renewable energy converters inside
utility-scale plants, in which they are still largely
employed [9]. Recently, they have found their way to
residential-scale single-phase PV converters;
currently represent a hot research topic [10]–[15].
Single-phase multilevel converters can be roughly
divided into three categories based on design: neutral
point clamped (NPC), cascaded full bridge (CFB),
and custom. In NPC topologies, the electrical
potential between the PV cells and the ground is
fixed by connecting the neutral wire of the grid to a
constant potential, resulting from a dc-link capacitive
divider [15].
A huge advantage is that single-phase NPC
converters are virtually immune from ground leakage
currents, although the same is not true for three-phase
NPC converters [12], [15]. A recent paper has
proposed an interesting NPC design for exploiting
next-generation devices such as super junction or SiC
MOSFETs [16]. The main drawback of NPC designs
is that they need twice the dc-link voltage, with
respect to full bridge. CFBs make for highly modular
designs. Usually, each full bridge inside a CFB
converter needs an insulated power supply, matching
well with multi-string PV fields [16].
Full bridges are sequentially permuted can be used
to evenly share the power among the parts and to
mitigate the effects of the partial shading [17]. As an
alternative, only one power supply can be used if the
output voltage is obtained through a transformer
[18].CFB can also be used for stand-alone application
which provides many degrees of freedom the control
strategy to the developers. With aforementioned
sequential permutation with phase shifting [19],
artificial neural networks [20] and predictive control
have been proposed to minimize harmonic distortion
and achieve maximum power point tracking (MPPT).
When the supply voltage is the same for each full
bridge CFB can synthesize 2n+1 voltage levels when
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CFB made up of n full bridges (and at least 4n power
switches).
Custom architectures can generally provide more
output levels with a given number of active devices,
and custom converters generally need custom pulsewidth modulation (PWM) and control schemes,
although unified control schemes for different types
of multilevel converters can be implemented. In
addition to using fewer switches, custom
architectures can be devised so that some of the
switches commutate at the grid frequency, thus
improving the efficiency. Reduction in the switchesper-output voltage-level ratio can be achieved in CFB
structures if different supply voltages are chosen for
each full bridge (asymmetrical CFBs). The topology
proposed in this paper consists of two asymmetrical
CFBs, generating eleven output voltage levels.
In the proposed converter, one full bridge is
supplied by dc source whereas a flying capacitor
supplies other one. Different sets of output voltages
can be obtained by suitably controlling the ratio
between the two voltages, the flying capacitor used as
a secondary energy source allows for limited voltage
boosting, as it will result clear in the following
section.
The number of output levels per switch (ten
switches, eleven levels) is comparable to what can be
achieved using custom architectures. In order to
reduce the ground leakage current two additional
very low power switches and a line frequency
switching device [transient circuit (TC)] were
included in the final topology. The custom converter
proposed in [25] generates five levels with six
switches but has no intrinsic boosting capability. In
[24], Rahimet al.used three dc-bus capacitors in
series together with two bidirectional switches (diode
bridge+ unidirectional switch) and an H-bridge to
generate seven output levels; however, they give no
explanations on how they keep the capacitor voltages
balanced. In [26], five switches, four diodes, and two
dc-bus capacitors in series are used to generate five
levels with boosting capability. Again, no mention is
made about how the capacitors are kept balanced.
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Fig. 2. CFB with a flying capacitor.
For PV applications, the output voltage has to be
controlled regardless of the voltage ratio, Even
though the PV field dc voltage is constantly changing
due to variations of solar radiation and to MPPT
algorithm. Measuring the separate full-bridge
voltages and computing online the duty cycles
needed to balance the different voltages, and
analyzing also the power balance between the
separate cells. A similar approach is followed in this
paper. The developed PWM strategy, in addition to
controlling the flying capacitor voltage, with the help
of the specific TC illustrated in Section IV,
minimizes the ground leakage current. Finally, it is
important to put in evidence that the proposed
converter can work at any power factor as reported in
Section III, while not all the alternative proposals can
continuously supply reactive power. The proposed
topology was presented by the authors in a previous
paper [27]. With respect to the previous work, this
paper was rewritten for 11-level output and presents a
better organization and a new set of simulation.
This paper is organized as follows: Section II
presents converter topology with PWM control
strategy chosen in order to maximize the
performance. Section III discusses about CFB
topology for the regulation of flying capacitor used to
supply the second full bridge. Section IV describes
the principle of operation of the additional
components to reduce the ground leakage current.
Section V shows the simulation results, Whereas
Section VI reports the concluding remarks.
II. ELEVEN-LEVEL CONVERTER AND
PWM CONTROL STRATEGY
The proposed converter is composed of two CFBs,
one of which is supplied by a flying capacitor (see
Fig. 2). This basic topology is well known. In this
paper, in order to allow grid-connected operation
with no galvanic isolation (transformerless solution)
different PWM strategy was developed for this basic
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topology. Using PWM strategy alone is not going to
maintain a low ground leakage current, other
components were added which will be discussed in
next section. As it will be described in the following,
the proposed PWM strategy stretches the efficiency
by using, for the two legs where PWM frequency
switching does not occur, devices with extremely low
voltage drop, such as MOSFETs lacking a fast
recovery diode. In fact, the low commutation
frequency of those two legs allows, even in a reverse
conduction state the conduction in the channel
instead of the body diode (i.e., active rectification).
Insulated-gate bipolar transistors (IGBTs) with fast
anti-parallel diodes are required in the legs where
high-frequency hard switching commutations occur.
For grid-connected operation, one full-bridge leg is
directly connected to the grid neutral wire, whereas
the phase wire is connected to the converter through
an LC filter. Flying-capacitor voltage
is kept
lower, at steady state, than dc-link voltage
is
discussed and justified in the following section. The
full bridge supplied by the dc link is called the highvoltage full bridge (HVFB), whereas the one with the
flying capacitor is the low-voltage full-bridge
(LVFB). CFB can also be used for stand-alone
application which provides many degrees of freedom
the control strategy, so that different PWM schemes
can be considered. However, the chosen solution
needs to satisfy the following requirements.
1) To limit the switching losses, most commutations
must take place in the LVFB.
2) To reduce the ground leakage current, the neutralconnected leg of the HVFB needs to switch at grid
frequency
3) To control the flying-capacitor voltage the
redundant states of the converter must be properly
used.
4) The driving signals must be obtained from a single
carrier for a low-cost to be used as a controller.
The switching pattern described in Table I was
developed starting from the above requirements.
Requirement 2), in particular, is due to the
aforementioned parasitic capacitive coupling between
the PV panels and their frames, usually connected to
the earth. Capacitive coupling provides the commonmode current inversely proportional to the switching
frequency of the neutral-connected leg. The converter
can operate in different output voltage zones, where
the output voltage switches between two specific
levels. The operating zone boundaries vary according
to the dc-link and flying-capacitor voltages, and
adjacent zones can overlap (see Fig. 3).
In zones A, the contribution of the flying-capacitor
voltage to the converter output voltage is positive
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(+ve), whereas it is negative (-ve) in B zones.
Constructive cascading of the two full bridges can,
therefore, result in limited output voltage boosting.
Depending on the
/
ratio, one of the (a) or (b)
situations in Fig. 3 can occur afterwards;
nevertheless, the operation of the converter does not
differ much in the two cases. If two overlapping
operating zones can supply the same output voltage,
the regulation of
will determine the operating
zone, will be described in section III. the duty cycles
are calculated on-line by a simple equation, similarly
to the approach presented in [28]. The instantaneous
fundamental component of output voltage ∗ and
the measured values of
and
will define the
switching pattern.
(a).
<0.5
.
(b)
>0.5 .
Fig. 3. Operating zones under different
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regulated depending on the instantaneous output
voltage request. Depending on the operating zone of
the converter (see Fig. 3),
can be added to (A
zones) or subtracted from (B zones) the HVFB output
voltage, charging or discharging the flying capacitor.
In accordance, considering a positive value of the
current injected into the grid, the flying capacitor is
discharged in a zone and charged in B zone. Since a
number of redundant switch configurations can be
used to synthesize the same output voltage
waveform, it is possible to control the voltage of the
flying capacitor, forcing the converter to operate
more in A zones when the flying-capacitor voltage is
higher than a reference value or more in B zones
when it is lower than a reference value. Similar
considerations hold in case of a negative injected grid
current. To synthesize the same output voltage
waveform number of redundant switch configurations
is used; it is possible to control the voltage of flying
capacitor, when flying-capacitor voltage in zone A is
more than a reference value or more in B zone when
it is lower than a reference value. Same can be
applied for negative injected grid current. Because of
some adjacent commutations output levels must
inevitably occur, the drawback of a certain increase
in the output ripple current. With simple hysteresis
controller the flying capacitor voltage whether it
should operate in zone A or zone B operation can be
realized.
ranges.
If
=
/4 the converter can synthesize eleven
equally spaced output voltage levels. Fig. 4 refers to
this case and shows the theoretical waveforms, where
one leg of the HVFB operates at grid frequency and
one leg of the LVFB at five times the grid frequency.
Except, zone 2, no high-frequency commutations
occur in the whole HVFB (see Fig. 3).since the
voltage regulation of the flying capacitor takes place
in zone 2, the zone-2 behavior is more articulated and
will be described in detail in the following section.
(a). Flying-capacitor charge
III. REGULATION OF FLYING-CAPACITOR
VOLTAGE
Controlling the voltage of the flying
capacitor voltage
is critical when a gridconnected PV converter is transferring the active
power to the electrical grid. By suitably choosing the
operating zone flying-capacitor voltage
is
(b). Flying-capacitor discharge.
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Fig. 4. Configuration of the circuit for the regulation
of the flying capacitors
If positive grid current
> 0 and
< 0.5
. If
is too low, by switching between the 0 and
−
output level
can be replaced by − ,[
zone 2B, Fig. 5(a)]. Conversely,
−
is
replaced by
if
is too high forcing the
converter to switch between
and
output levels
[zone 2A, Fig. 5(b)].it is shown in Fig 5 that the
devices are switching at low frequency are shortcircuited when on and not shown when off.
For the cases when
> 0.5
similar
regulation strategies can be developed. If
<
0.5
, to minimize the current ripple zone 2 is
chosen when
< ∗ <
−
(zones 3 are
otherwise chosen), Limiting level skipping. Level
skipping always occurs if
> 0.5
; according
to voltage regulation algorithm A or B zone is
chosen. Due to MPPT strategy, the DC-link voltage
can go through sudden variations, it is important that
the converter is able to work in any [ , ]
condition. Through on-line duty cycle computation,
the distortion of the output voltage is minimized. It is
important to estimate the capability of the converter
to regulate the flying-capacitor voltage under
different operating conditions. The ability to control
flying-capacitor voltage through proposed PWM
strategy has been studied in simulation by average
flying-capacitor current under a large span of
and
values. Grid voltage
is maintained
sinusoidal with amplitude of 230√2V in simulations.
Even for different voltages the ratio
/
remains constant. The unity power factor case
results are summarized in Fig 6.
is fully
controllable in the white area and cannot be
controllable in the gray and black regions. In detail,
cannot be decreased in the black region, whereas
in gray region, it cannot be increased. The safe
operating area for the converter is the white region
located between the gray and black ones for stable
operation. If failed in controlling , it can be a
constraint that ensuring flying capacitor cannot be
over charged nor completely discharged inside the
white region. With the variation in the grid current
the results will not going to be affected but power
factor affects the results will going to determine less
going to widen the controllable area. In entire
[ , ] domain the power
is controllable when
converter supplies only reactive power.
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The commutation pattern of Table 1 is that T3 and T4
switch at grid frequency, will commutate at every
zero crossing of
. If negative derivative with
zero crossing is considered.T3 closes and T4 opens,
changing the neutral wire voltage (and thus the
voltage across the parasitic capacitance of the PV
field) from zero to
.because of this reason the
commutation can cause a large surge of leakage
current which will decrease the power quality and
damage the PV modules.
Proper TC must be designed to decrease these surge
currents. The proposed converter topology constitute
two-cell CFB described in Fig 2 with the addition of
TC components. For better understand of TC, the
distributed parasitic capacitance of the PV source was
modeled with equivalent simple capacitance i.e.,
between negative pole and the ground. The TC is
formed from two low-power MOSFETs M1 and M2,
bidirectional switch T9, and resistor . When
converter enters operating zone 1, the HVFB output
voltage must be zero by switching T1 and T3 or T2
and T4 on. Nevertheless, to operate the TC, when
entering zone 1, T1, T2, T3, and T4 are all kept off,
while T9 is on which maintains neutral point floating
and voltage on the parasitic capacitor
stays
constant.
. (a) TC topology.
(a) TC operation
IV. TRANSFORMERLESS PV CONVERTER
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(c) TC waveforms.
Fig. 5. Ground leakage current limitation circuit
topology and behavior.
One of M1 and M2 is turned on (M1 if the slope of
the zero crossing is negative and M2 if positive). At
the same time is charged through
with a firstorder transient, limiting the surge current. Whereas
the TC introduces additional components, they can be
selected with current ratings much lower than the
devices of the CFB considering the power loss in the
additional resistor is negligible.
A dissipation of about 1.2W with
= 200nF
and
= 300V,
=
/T, averaged over a line
period T, is the estimated energy lost while charging
and discharging a capacitor
to
.
Fig. 7. Simulation results with VDC = 300 V
In grid-connected operation, the output voltage is
very close to the grid voltage will not going to affect
the power factor in the operation of TC. For correct
operation of the TC requires the grid voltage
instantaneous angle obtained from a grid voltage
through phase locked loop (PLL) [20].
V. SIMULATION RESULTS
Under MATLAB/Simulink, The simulations
cover entire range of active and reactive power
injected into the grid, Dc-link voltage
= 300V
was used in the simulations if specified. The
sinusoidal grid representation with voltage source at
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50Hz of amplitude
= 230V with a capacitor
=1μF. An inductor
=1.5mH and an additional
inductor
=40μH will represent the total
distributed grid inductance. The PWM frequency was
=20 kHz, and the flying capacitor had a capacitance
of
= 500μF. Surge limiting resistance
was
selected as 1.5 kΩ. With feedforward
through proportional-integral regulator plus feed
forward at
=8.5A rms. The instantaneous
commutations and the injection of both active and
reactive power was simulated the entire performance
will not depend on the power factor. As outputs are
independent of power factor simulations are done for
unity power factor only. As grid voltage angle
information is available. So, PLL is needed. With the
DC voltage ratio the THD of the grid currents
increases, being 2.7%, 3% and 3.3%, respectively.
Fig. 8. TC behavior with a 200 nF parasitic capacitor.
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Fig. 6. Block scheme of the delay-based PLL.
TABLE I
DESCRIPTION OF THE CONVERTER OPERATING
ZONES
Zone
Output
voltage
Zone
3B
Zone
3A
−
−
↔−
Zone
2A
Zone
3B
Zone
1B
Zone
1A
Zone
2A
Zone
2B
Zone
3B
Zone
3A
−
+
↔0
−
↔−
+
−
↔−
−
↔0
0↔
↔
0↔
−
−
↔
↔
+
On
Devices
, ,
, ,
Off
Devices
, ,
,
,
,
,
,
, ,
, ,
, ,
, ,
,
,
,
,
, ,
, ,
, ,
, ,
Switching
Devices
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
For
/
= 0.33,
/
= 0.5
and /
= 0.66. The performance of the TC with
a parasitic capacitance of the PV field of = 200nF
which is shown in Fig.5 with ground leakage current
results in
=30mA rms. By designing the
common-mode filter more accurately the ground
leakage current can be reduced. Without any
overshoot
rapidly (in about 25 ms) rises to the
reference value.
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REFERENCES
Fig. 9. Vfc step variation response.
VII. CONCLUSION
This paper has proposed a novel elevenlevel grid connected transformerless PV converter
with full bridges with CFB topology, one of which is
supplied by a floating capacitor, in order to improve
the efficiency a suitable PWM strategy was
developed (at low frequency most devices
commutate) and with the help of specific TC, the
ground leakage current is minimized. The proposed
PWM strategy can regulate the voltage across the
flying capacitor. Simulations were performed to
assess the ability to regulate the flying capacitor
voltage in wide range of operating conditions.
Extensive simulations and experiments confirm the
results of the theoretical analysis and show the good
performance of the converter as far as
Harmonic distortion and ground leakage current are
concerned. The proposed converter can continuously
operate at arbitrary power factors, has limited
boosting capability and can produce eleven output
voltage levels.
[1] G. Buticchi, L. Consolini, and E. Lorenzani,
“Active filter for the removal of the dc current
component for single-phase power lines,”IEEE
Trans. Ind. Electron., vol. 60, no. 10, pp. 4403–4414,
Oct. 2013.
[2] G. Buticchi and E. Lorenzani, “Detection method
of the dc bias in distribution power transformers,”
IEEE Trans. Ind. Electron., vol. 60, no. 8, pp. 3539–
3549, Aug. 2013.
[3] H. Xiao and S. Xie, “Leakage current analytical
model and application in single-phase transformerless
photovoltaic grid-connected inverter,”IEEE Trans.
Electromagn. Compat., vol. 52, no. 4, pp. 902–913,
Nov. 2010.
[4] O. Lopez, F. Freijedo, A. Yepes, P. Fernandez
Comesaa, J. Malvar, R. Teodorescu, and J. DovalGandoy, “Eliminating ground current in a
transformerless photovoltaic application,” IEEE
Trans. Energy Convers., vol. 25, no. 1, pp. 140–147,
Mar. 2010.
[5] S. Araujo, P. Zacharias, and R. Mallwitz, “Highly
efficient single-phase transformerless inverters for
grid-connected photovoltaic systems,”IEEE Trans.
Ind. Electron., vol. 57, no. 9, pp. 3118–3128, Sep.
2010.
[6] D. Barater, G. Buticchi, A. Crinto, G.
Franceschini, and E. Lorenzani, “Unipolar PWM
strategy for transformerless PV grid-connected
converters,”IEEE Trans. Energy Convers., vol. 27,
no. 4, pp. 835–843, Dec. 2012.
[7] T. Kerekes, R. Teodorescu, P. Rodridguez, G.
Vazquez, and E. Aldabas, “A new high-efficiency
single-phase transformerless PV inverter topology,”
IEEE Trans. Ind. Electron., vol. 58, no. 1, pp. 184
191, Jan. 2011.
[8] S. Kouro, M. Malinowski, K. Gopakumar, J. Pou,
L. Franquelo, B. Wu, J. Rodriguez, M. P. Andrez,
and J. Leon, “Recent advances and industrial
applications of multilevel converters,”IEEE Trans.
Ind. Electron., vol. 57, no. 8, pp. 2553–2580, Aug.
2010.
[9] Y. Xue, B. Ge, and F. Z. Peng, “Reliability,
efficiency, and cost comparisons of mw-scale
photovoltaic inverters,” inProc. IEEE ECCE,
Raleigh, NC, USA, Sep. 2012, pp. 1627–1634.
[10] C. Townsend, T. Summers, and R. Betz,
“Control and modulation scheme for a cascaded Hbridge multi-level converter in large scale
photovoltaic systems,” inProc. IEEE ECCE, Raleigh,
NC, USA, Sep. 2012, pp. 3707–3714.
IJPRES
168
INTERNATIONAL JOURNAL
OF PROFESSIONAL ENGINEERING STUDIES
[11] S. Essakiappan, H. Krishnamoorthy, P. Enjeti,
R. Balog, and S. Ahmed, “Independent control of
series connected utility scale multilevel photovoltaic
inverters,” inProc. IEEE ECCE, Raleigh, NC, USA,
Sep. 2012, pp. 1760–1766.
[12] G. Konstantinou, S. Pulikanti, M. Ciobotaru, V.
Agelidis, and K. Muttaqi, “The seven-level flying
capacitor based ANPC converter for grid integration
of utility-scale PV systems,” in Proc. IEEE PEDG,
Aalborg, Denmark, Jun. 2012, pp. 592–597.
[13] G. Brando, A. Dannier, A. Del Pizzo, and R.
Rizzo, “A high performance control technique of
power electronic transformers in medium voltage
grid-connected PV plants,” inProc. ICEM, Rome,
Italy, Sep. 2010, vol. 2, pp. 1–6.
[14] G. Buticchi, E. Lorenzani, and G. Franceschini,
“A five-level single-phase grid-connected converter
for renewable distributed systems,”IEEE Trans. Ind.
Electron., vol. 60, no. 3, pp. 906–918, Mar. 2013.
[15] Y. Kashihara and J. Itoh, “The performance of
the multilevel converter topologies for PV inverter,”
in Proc. CIPS, Beijing, China, Mar. 2012, pp. 1–6.
[16] Y. Noge and J. Itoh, “Multi-level inverter with
H-bridge clamp circuit for single-phase three-wire
grid connection suitable for super-junction–SiC
MOSFET,” inProc. IPEMC, Harbin, China, Jun.
2012, vol. 2, pp. 88–93.
[17] C. Cecati, F. Ciancetta, and P. Siano, “A
multilevel inverter for photovoltaic systems with
fuzzy logic control,”IEEE Trans. Ind. Electron., vol.
57, no. 12, pp. 4115–4125, Dec. 2010.
[18] I. Abdalla, J. Corda, and L. Zhang, “Multilevel
dc-link inverter and control algorithm to overcome
the PV partial shading,” IEEE Trans. Power
Electron., vol. 28, no. 1, pp. 14–18, Jan. 2013.
[19] J. Chavarria, D. Biel, F. Guinjoan, C. Meza, and
J. Negroni, “Energybalance control of PV cascaded
multilevel grid-connected inverters under levelshifted and phase-shifted PWMS,”IEEE Trans. Ind.
Electron., vol. 60, no. 1, pp. 98–111, Jan. 2013.
[20] A. Bidram, A. Davoudi, and R. Balog, “Control
and circuit techniques to mitigate partial shading
effects in photovoltaic arrays,”IEEE J. Photovoltaics,
vol. 2, no. 4, pp. 532–546, Oct. 2012.
[21] G. Grandi, C. Rossi, D. Ostojic, and D. Casadei,
“A new multilevel conversion structure for gridconnected PV applications,”IEEE Trans. Ind.
Electron., vol. 56, no. 11, pp. 4416–4426, Nov. 2009.
[22] S. Daher, J. Schmid, and F. Antunes, “Multilevel
inverter topologies for stand-alone PV systems,”
IEEE Trans. Ind. Electron., vol. 55, no. 7, pp. 2703–
2712, Jul. 2008.
[23] F. Filho, L. Tolbert, Y. Cao, and B. Ozpineci,
“Real-time selective harmonic minimization for
Volume V /Issue I /JULY 2015
multilevel inverters connected to solar panels using
artificial neural network angle generation,”IEEE
Trans. Ind. Appl., vol. 47, no. 5, pp. 2117–2124,
Sep./Oct. 2011.
[24] P. Cortes, S. Kouro, F. Barrios, and J.
Rodriguez, “Predictive control of a single-phase
cascaded H-bridge photovoltaic energy conversion
system,” inProc. IPEMC, Harbin, China, Jun. 2012,
vol. 2, pp. 1423–1428.
[25] N. Rahim, K. Chaniago, and J. Selvaraj, “Singlephase seven-level gridconnected inverter for
photovoltaic system,”IEEE Trans. Ind. Electron., vol.
58, no. 6, pp. 2435–2443, Jun. 2011
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