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Abstract—Recently, medium (0.1–5 MW) and large scale (> 5
MW) photovoltaic (PV) power plants have attracted great
attention, where medium voltage grid (typically 6–36 kV)
connection is essential for efficient power transmission and
distribution. A power frequency transformer operated at 50 or 60
Hz is generally used to step-up the traditional inverter’s output
low voltage (usually ≤ 400 V) into medium voltage level, as the
medium voltage PV inverter has not been developed yet. Because
of the heavy weight and large size of the power frequency
transformer, the PV inverter system can be expensive and
complex for installation and maintenance. As an alternative
approach to achieve a compact and lightweight direct grid
connection, a three-phase medium voltage PV inverter system is
proposed in this paper. The 11 kV and 33 kV PV inverter systems
are designed. A scaled down 3-phase 1.2 kV test rig is constructed
to validate the proposed PV inverter. The experimental results
are analyzed and discussed taking into account the switching
schemes and filter circuits. The experimental results demonstrate
the excellent feature of the proposed PV inverter system.
Index Terms—Photovoltaic (PV) power plants, PV inverters,
grid integration, medium-voltage, step-up-transformer-less.
I. INTRODUCTION
INCE
2007 medium (0.1–5 MW) and large scale (> 5
MW) photovoltaic (PV) power plants have attracted great
attention and power plants of more than 10 MW in capacity
have thereby become a reality [1], [2]. More than 200 PV
power plants have already been installed in the world; each of
them generating an output of more than 10 MW. Of these
plants, 34 are located in Spain and 26 in Germany. The
number of PV power plants will continue to rise. More than
250 PV power plants will be installed within the next few
years. Future PV power plants will have higher power
capacity. Indeed, some are to have a capacity in excess of 250
MW. These multimegawatt PV power plants require large
areas of land. Owing to this, they are usually installed in
remote areas, far from cities. The 20 MW PV power plant in
Beneixama, Spain, used about 200 SINVERT 100M inverters
and installed approximately 100,000 PV modules in a land
area of 500,000 m
2
. For power transmission, a step-up-
transformer is usually used in the PV inverter system to feed-
in the solar energy into a medium voltage grid (typically 6–36
M. R. Islam, Y. G. Guo, and J. G. Zhu are with Centre for Electrical
Machines and Power Electronics, University of Technology Sydney, P O Box
123, Broadway, Ultimo, NSW 2007, Australia (e-mail: Md.Islam@uts.edu.au,
Youguang.Guo-1@uts.edu.au, Jianguo.Zhu@uts.edu.au).
kV). ASEA Brown Boveri (ABB) and Siemens developed
inverters for medium scale PV power plants. ABB central
inverters are especially designed for medium scale PV power
plants. The PVS800 version is 3-phase inverters with a power
capacity in the range of 100–500 kW. The PVS800 inverter
topology allows a parallel connection directly on the ac side,
for grid connection through a step-up-transformer. The
transformer steps-up the inverter output voltage from 300 V ac
to grid voltage level. ABB has been delivering worldwide
vacuum cast coil dry-type transformers for PV applications.
Siemens developed SINVERT PVS inverter for medium scale
PV power plants. The ac output voltage and power capacity of
PVS version inverters are in the range of 288–370 V and 500–
630 kW, respectively. The 1–2.52 MW central inverters can
be designed by parallel connection of 2 to 4 PVS inverters
through transformer and switchgear at the grid side. Siemens
developed GEAFOL cast-resin transformers for grid
connection of PV arrays.
Although these special transformers are compact compared
with conventional distribution transformers, they are still large
and heavy for remote area PV applications [3]. The large size
and heavy weight step-up-transformer may increase the
system weight and volume, and can be expensive and complex
for installation and maintenance. The medium-voltage inverter
may be the possible solution to connect the PV power plant to
the medium-voltage grid directly. Moreover, it can be also
possible to ensure electrical isolation through the inverter,
which is important for the connection of PV power plants with
medium-voltage grids. Therefore, medium-voltage inverters
for step-up-transformer-less direct grid connection of PV
systems have attracted great attention since the installation of
large scale PV power plants commercially in 2007.
In 2011, different multilevel inverter topologies were
compared for possible medium-voltage grid connection of PV
power plants [4], [5]. Because of some special features, the
modular multilevel cascaded (MMC) inverter topology is
considered as a possible candidate for medium-voltage
applications. The component numbers of the MMC inverters
scale linearly with the number of levels, and individual
modules are identical and completely modular in constriction,
thereby enabling high-level number attainability. Furthermore,
the MMC inverter does not require any auxiliary diodes and
capacitors. Fig. 1 shows the requirements of auxiliary devices
in different multilevel inverters. However, the MMC inverter
requires multiple-isolated dc sources that must be balanced. In
2011, a high-frequency link was proposed to generate
A Multilevel Medium-Voltage Inverter for Step-
up-Transformer-less Grid Connection of
Photovoltaic Power Plants
Md. Rabiul Islam, Youguang Guo, Senior Member, IEEE, and Jianguo Zhu, Senior Member, IEEE
S
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multiple-imbalanced sources for asymmetrical multilevel
inverters [6]. In the proposed system, only the auxiliary H-
bridges are connected through high-frequency-link. The main
H-bridges are supplied directly from the source, which means
there is no electrical isolation. Therefore, the use of this
inverter is only for isolated winding motor applications.
2 4 6 8 10 12 14 16 18
0
100
200
300
Number of levels
Number of auxiliary devices
Auxiliary
capacitors in FC
inverter
Auxiliary diodes
in NPC inverter
No auxiliary devices
in MMC inverter
Fig. 1. Auxiliary devices in different multilevel inverters.
PV array
1
Medium-
frequency-link
Medium voltage grid
Module
2
k
1
2
k
1
2
k
Modular multilevel cascaded inverter
Module
Module
Module
Module
Module
Module
Module
Module
Rectifier
(MF)
H-
bridge
Module
MPPT
Inverter
(MF)
MPPT
Inverter
(MF)
MPPT
Inverter
(MF)
1
2
n
Fig. 2. Proposed medium-voltage PV inverter system for step-up-transformer-
less direct grid integration of PV power plants.
In 2011, a medium-frequency-link operated at a few kHz to
MHz was proposed to generate multiple isolated and balanced
dc sources for MMC inverters from a single source [7]. In
order to verify the feasibility of the new technology, a Metglas
amorphous alloy 2605SA1 based medium-frequency link was
developed [8]. Compared with the power frequency
transformers, the medium-frequency link has much smaller
and lighter magnetic cores and windings, thus much lower
costs. The amorphous alloy based medium-frequency link
shows excellent electromagnetic characteristics, such as very
low specific core losses and possibility to generate multiple-
balanced sources [9].
In 2012, by combination of a quasi-Z source inverter into a
MMC converter, a medium-voltage PV inverter was proposed
[10]. The proposed PV inverter does not have isolation
between PV array and medium-voltage grid. Multiple-isolated
dc/dc converter based inverter topologies were proposed in
[11], [12]. In the proposed configuration, the voltage
balancing is the challenging issue, since each H-bridge cell is
connected to a PV array through a dc/dc converter.
Common dc-link may be one of the possible solutions to
minimize the voltage imbalance problem. In 2012, a common
dc-link based PV inverter system was proposed [13], [14].
Although this design may reduce the voltage imbalance
problem in grid side, the generations of common dc-link
voltage from different PV arrays make the inverter operation
complex,and accordingly limit the range of maximum power
point tracker (MPPT) operation.
In this paper, a three-phase medium voltage inverter is
proposed for step-up-transformer-less direct grid connection
of PV power plants. A medium-frequency link (common
magnetic-link) instead of common dc-link is used to generate
all the isolated and balanced dc supplies of MMC inverter
from a single or multiple PV arrays. Accordingly, the link
guarantees electrical isolation between the grid and the PV
arrays. The basic block diagram of the proposed medium-
voltage inverter for medium and large scale PV power plants
is shown in Fig. 2. The 11 kV and 33 kV inverter systems are
designed and analyzed taking into account the specified
system performance, control complexity, cost and market
availability of the semiconductors. To verify the feasibility of
the proposed inverter system, a scaled down 1.2 kV laboratory
prototype test platform is developed with a 5-level MMC
inverter. The design and implementation of the prototyping
test platform, and the experimental results are analyzed and
discussed. The advantages of the proposed PV inverter are: 1)
step-up-transformer-less and line-filter-less medium voltage
grid connection; 2) an inherent minimization of the grid
isolation problem through the magnetic-link; 3) an inherent
dc-link voltage balance due to the common magnetic-link; 4) a
wide range of MPPT operation; and 5) an overall compact and
lightweight system.
Medium voltage grid
Medium-
frequency-
link
n
Cascaded H-bridges
a
b
c
Rectifiers
C
dc
C
dc
C
dc
C
dc
C
dc
C
dc
Q
C
in
PV array
Medium frequency
inverter
MPPT
L
C
dc
H-bridge
Left top
IGBT
Left bottom
IGBT
Right top
IGBT
Right bottom
IGBT
C
Module
Rectifier (medium
frequency)
H-bridge
D
Fig. 3. Detailed power conversion circuit with 3-phase 5-level MMC inverter
(for simplicity single PV array is used).
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II. PROPOSED PHOTOVOLTAIC SYSTEM
In this paper, as an alternative approach to minimize voltage
imbalance problem with wide range of MPPT operation, a
Metglas amorphous alloy 2605SA1 based common magnetic-
link is considered. The boost converter is considered for
MPPT function mainly. The array dc power is converted to a
medium-frequency ac through a medium-frequency inverter.
The inverter also ensures constant output voltage. The inverter
is connected to a primary winding of a multiwinding medium-
frequency link. Each secondary winding works as an isolated
source and is connected to H-bridge cell through a bridge
rectifier. The number of primary windings depends on the
number of PV arrays and the number of secondary windings
depends on number of levels of the inverter. The detailed
power circuit of a 3-phase 5-level PV inverter system is shown
in Fig. 3, which is used to validate the proposed inverter in the
laboratory. In large PV power plants, several PV arrays are
operated in parallel. For this case multi-input and multi-output
magnetic-link can be used, where each PV array is connected
to a primary winding through a booster and medium frequency
inverter as shown in Fig. 2. The magnetic-link provides
electrical isolation between PV array and grid, thus inherently
overcomes common mode and voltage imbalance problems.
0
1
2
3
4
5
6
0
1
2
3
Rated voltage or current (kV or kA)
Per unit price (k AUD)
Price against rated voltage
Price against rated current
Rated current at 0.4 kA
Rated voltage at 1.7 kV)
Fig. 4. Per unit price of IGBT, in kAUD
10 20 30 40 50
2
4
6
8
10
Level number
THD (%)
33 kV system
11 kV system
5% limit (IEEE 1547, IEC61727)
Fig. 5. Calculated THDs at different level number ranging from 7-level to 21-
level for an 11 kV system and 15-level to 55-level for a 33 kV system.
III. D
ESIGN AND ANALYSIS OF THE PROPOSED SYSTEM
If V
ll(rms)
is the grid line to line voltage and L the number of
levels of the inverter, the minimum dc-link voltage of each H-
bridge can be calculated from
)1(
2
)(
(min)
−
=
L
V
V
rmsll
dc
. (1)
To determine the nominal dc-link voltage of each H-bridge
cell, a voltage reserve of 4 % is assumed, i.e.
(min))(
04.1
dcnomdc
VV =
. (2)
If I
p(rms)
is the inverter phase current, the apparent output
power can be calculated from
)()(
3
rmsprmsllc
IVS =
. (3)
The highest voltage rating of commercially available
insulated gate bipolar transistor (IGBT) is 6.5 kV and this is
suitable for 2.5 kV or lower voltage inverter systems with
traditional 2-level inverter topology. Although high-voltage
devices such as 3.3, 4.5 and 6.5 kV IGBTs are available in the
market, they are still costly as shown in Fig. 4. The lower-
voltage devices, such as 0.6, 0.9, 1.2, 1.7 and 2.5 kV IGBTs
are not only mature in technology but also cheap. On the other
hand, the cascaded connection of low-voltage rated
semiconductors can be a cost effective solution for medium
voltage inverter applications. The high-number of levels
means that medium-voltage attainability is possible to connect
the PV array to the medium-voltage ac network directly and
also possible to improve the output power quality. The total
harmonic distortions (THDs) of 11 kV and 33 kV inverter
systems are illustrated in Fig. 5. The component number and
control complexity increase linearly with the increase of level
number. Therefore, optimal selection of number of inverter
levels is important for the best performance/cost ratio of the
PV systems.
Each H-bridge cell communication voltage of a 7-level
topology based 11 kV inverter is 2696 V which may be
supported by the 6.5 kV IGBT. Thus at least 7-level topology
is required to design the 11 kV inverter. The output power
quality of 21-level inverter is good enough to feed into the 11
kV ac grid directly. The cheap 1.7 kV IGBT can be used to
design the 21-level inverter. For a 33 kV system, at least 15-
level topology is required and 55-level topology is sufficient
for the power quality. Therefore, 7-level to 21-level modular
multilevel cascaded inverter topologies are considered for an
11 kV inverter system and 15-level to 55-level topologies are
considered for a 33 kV inverter system. The device voltage
utilization factor (DVUF), ratio of commutation voltage of
respective commutation cells (V
com
) and device commutation
voltage for a device reliability of 100 failures in time (FIT)
due to cosmic radiation (V
com@100FIT
), are summarized in
Tables I and II.
TABLE I
DVUF
WITH DIFFERENT LEVEL NUMBER OF AN 11 KV SYSTEM
Level
number
V
com
Rated device
voltage kV
V
com@100FIT
DVUF
(%)
7
2696
6.5
3600
75
9
2022
4.5
2250
90
11
1618
3.3
1800
90
13
1348
3.3
1800
75
15
1156
2.5
1200
96
17
1011
2.5
1200
84
19
898
1.7
900
99
21
809
1.7
900
90
Higher DVUF is essential for cost effective design, since
semiconductor cost is the significant figure in medium-voltage
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inverter applications. From Tables I and II, it can be seen that
only a few inverters have high DVUF. In order to ensure cost
effective design, the inverters with level numbers of 9, 11, 15,
19 and 21 for an 11 kV system and 15, 23, 29, 43 and 55 for a
33 kV system were considered for the further analysis. The 11
kV and 33-kV systems with the selected inverter topologies
are designed and analyzed in the MATLAB/Simulink
environment. The number of arithmetic and logic operations
(ALOs) for switching section and cost of semiconductors are
calculated as summarized in Tables III and IV. The number of
ALOs is used to compare the complexity of the inverters. The
THDs are calculated through the MATLAB/Simulink
environment at a switching frequency range of 1–2 kHz. If
P
c_inv
is the conduction loss and P
sw_inv
is the switching loss of
semiconductor devices, the total losses in the inverter section
of the proposed system can be described as
invswinvcinvloss
PPP
___
+=
. (4)
TABLE II
DVUF
WITH DIFFERENT LEVEL NUMBER OF A 33 KV SYSTEM
Level
number
V
com
Rated device
voltage kV
V
com@100FIT
DVUF
(%)
15
3467
6.5
3600
96
17
3034
6.5
3600
85
19
2697
6.5
3600
75
21
2427
6.5
3600
68
23
2206
4.5
2250
98
25
2022
4.5
2250
90
27
1867
4.5
2250
83
29
1734
3.3
1800
96
31
1618
3.3
1800
90
33
1517
3.3
1800
84
35
1428
3.3
1800
79
37
1348
3.3
1800
75
39
1277
3.3
1800
71
41
1214
3.3
1800
65
43
1156
2.5
1200
96
45
1103
2.5
1200
92
47
1055
2.5
1200
88
49
1011
2.5
1200
84
51
971
2.5
1200
81
53
934
2.5
1200
78
55
899
1.7
900
100
TABLE III
I
NVERTER COMPARISON FOR AN 11 KV SYSTEM
Level number 9 11 15 19 21
IGBTs
48
60
84
108
120
THD (%)
9.60
8.20
6.00
4.30
4.25
Cost (AU$)
86400
82159
47066
36670
40744
ALOs
44
55
77
99
110
TABLE IV
I
NVERTER COMPARISON FOR A 33 KV SYSTEM
Level number 15 23 29 43 55
IGBTs
84
132
168
252
324
THD (%)
6.40
4.54
4.12
3.61
3.47
Cost (AU$)
258552
237600
229992
141200
110030
ALOs
77
121
154
231
297
The switching losses in a multilevel inverters was
approximated by [15]
cr
r
invsw
fBI
AI
P )
(
2
_
+=
. (5)
where f
c
is the carrier frequency of multilevel inverter. The
switching loss of an active switch is proportional with the
carrier frequency. The carrier frequency can be reduced
linearly with the increase in number of levels. Although the
number of active switching devices increases linearly with the
number of levels, the reduction of carrier frequency can keep
the total switching loss constant. The conduction losses in a
switch and in an anti-parallel diode were represented by [15]
)
3
8
3
()
4
1
(
2
1
2
_ f
a
CErf
a
trswc
p
m
RIp
m
VIP
p
p
p
+++=
. (6)
and
)
3
8
3
(
)
4
1
(
2
1
2
_ f
a
AK
rf
a
f
rD
c
p
m
RI
p
m
VI
P
p
p
p
−+
−=
. (7)
where m
a
is the amplitude modulation index, p
f
the power
factor of the current, I
r
the device current, and V
t
and V
f
are
the voltage drops at zero current condition, and R
CE
and R
AK
are the forward resistances of IGBT and diode, respectively,
which can be collected from the manufacturer’s data sheets.
The inverter section of the system consists of a series of H-
bridge inverter cells in a cascaded connection. Therefore, the
total conduction losses of an m-level inverter can be
approximated as
))(
1
(6
__
_
Dc
swc
inv
c
PP
m
P +−
=
. (8)
The device communication voltage of m-level inverter is (m-
1) times lower than that of a device in the 2-level inverter. The
on-state voltage drops of an IGBT and forward voltage of a
diode are highly dependent on device voltage ratings. Fig. 6
plots the on-state voltage drops of different rated Mitsubishi
Electric IGBTs. For these reasons, although the number of
devices increases linearly with the number of levels, the total
conduction loss can be constant. Therefore, the efficiency of
the multilevel inverter remains almost constant to the variation
of number of levels, and thereby the efficiency was not
considered for the selection of number of levels.
0 1 2
3 4 5 6
1
2
3
4
5
Device voltage rating
On-state
voltage drops (V)
IGBT on-state
voltage drops
CERtCE
RIVV +=
Fig. 6. On-state voltage drops of 600 A rated IGBTs.
If y is the given value, y
min
the minimum value, and y
max
the
maximum value on a respective row of each inverter system,
the normalized index value can be calculated as
