Martina Calais Vassilios G. Agelidis
Centre for Renewable Energy Systems Technology Australia (CRESTA)
Curtin University of Technology
GPO Box U1987, Perth 6845, Western Australia
Abstract Multilevel voltage source inverters offer several ad-
vantages compared to their conventional counterparts. By syn-
thesising the AC output terminal voltage from several levels
of voltages, staircase waveforms can be produced, which ap-
proach the sinusoidal waveform with low harmonic distortion,
thus reducing filter requirements. The need of several sources
on the DC side of the converter makes multilevel technology
attractive for photovoltaic applications.
This paper provides an overview on different multilevel
topologies and investigates their suitability for single-phase Fig. 1. Issues regarding grid connected PV systems for the low power
grid connected photovoltaic systems. Several transformer- range.
less photovoltaic systems incorporating multilevel converters
are compared regarding issues such as component count and
stress, system power rating and the influence of the photo- stages of power conversion . Newest trends in this field
voltaic array earth capacitance. are string based units with a power rating around 1 kW ,
 and transformerless concepts , , . For larger
I. INTRODUCTION systems the overall efficiency can be increased through
application of several, small, string inverters replacing a
Grid connected photovoltaic (PV) systems, in particular single unit which avoids losses through module mismatch
low power, mostly single-phase PV “rooftop” systems and and decreases the DC wiring effort. Transformerless
their contribution to clean power generation is recognised concepts (in particular inverters with high input voltages)
more and more worldwide. Grid connected PV rooftop are advantageous regarding their high efficiencies. Their
systems are generally privately owned, single-phase sys- peak efficiencies of up to 97% are equivalent to efficiencies
tems in a power range of up to 10 kW.The main aim of reached in drives applications . Avoiding the transformer
a private operator who owns such a system is to max- has the additional benefits of reducing cost, size, weight
imise its energy yield. Issues such as long life time (20 and complexity of the inverter. However, the removal of
years and longer), high (part-load-) efficiency and good the transformer and hence its isolation capability has to be
environmental conditions (availability of solar radiation) considered carefully.
are hence of importance to the private operator. Other Multilevel converter technology is based on the synthesis
important requirements for these PV systems (see Fig. 1) of the AC voltage from several different voltage levels
are the fulfillment of standards concerning power quality, on the DC bus. As the number of voltage levels on the
electromagnetic compatability, acoustic noise limitations as DC side increases, the synthesised output waveform adds
well as safety and protection requirements. more steps, producing a staircase wave which approaches
The most important issues, however, for PV grid con- the sinusoidal wave with minimum harmonic distortion
nected systems to gain wide acceptance are reliability and . Multilevel converters are particularly interesting for
low cost. Figures from 1995 show that the operating inavail- high power applications such as FACTS since the need
ability of inverters for low power PV systems due to defects of filters is reduced and the efficiency is high because all
is 6 to 7 days per year [l], which compares unfavourably devices switch at fundamental frequency [ 101, [ 111. In low
with household appliances such as refrigerators or washing power applications where switching frequencies are not
machines. And today’s costs for commercially available as restricted as in high power applications various control
low power sine-wave inverters for PV applications range methods such as multicarrier pulse width modulation or
from 0.9 to 3 USSNp where as drive converters in the same multiple hysteresis band control methods can be used to
power range are available for 0.25-0.5 US$/Wp . further reduce harmonics in the stepped waveforms ,
First commercially available grid connected PV inverters [133. Multilevel converter topologies are especially suitable
were line commutated inverters, followed by self commu- for PV applications since due to the modular structure of PV
tated, puls width modulation inverters including either line arrays different DC voltage levels can easily be provided.
or high frequency transformers, often incorporating several This paper provides an overview on various multilevel
0-7803-4756-0/98/$10.00 1998 IEEE 224
topologies which have been suggested or are considered Module Frame
for (transformerless), single-phase grid connected systems.
Each topology is briefly described, listing advantages and
disadvantages regarding issues such as component count
and stress, system power rating and the influence of the
photovoltaic array earth capacitance. Due to quick voltage
and current transitions most power electronic equipment
emits disturbances which propagate either by conduction or
radiation. In transformerless systems additionally leakage
currents due to the photovoltaic array earth capacitance
can occur and increase electromagnetic emissions (both
conducted and radiated). Since the paper focuses on
transformerless systems the issue of leakage currents in
transformerlessphotovoltaic systems will be discussed first. Fig. 2. (a) Maximum and (b) minimum PV module earth capacitance .
11. LEAKAGE CURRENTS IN TRANSFORMERLESS TABLE I
PV MODULE IEARTH CAPACITANCES
Avoiding the transformer in PV inverter topologies results
in a galvanic connection of the grid and the PV array. Due Maue 1
esrd ll00F I 4.2 DF I
to the capacitance between the PV array and earth, potential
differences imposed on the capacitance through switching
actions of the inverter inject a capacitive earth current. The 1.
1 1 MULTILEVEL INVERTER TOPOLOGIES
PV array earth capacitance is then part of a resonant circuit
consisting of the PV array, DC and AC filter elements and A. HalfBridge Diode Clamped
the grid impedance. Due to necessary efficiency optimisa-
tion of PV systems the damping of this resonant circuit can Fig. 3(a) shows a half-bridge diode clamped three-
be very small so that the earth current can reach amplitudes level inverter (HBDC)  as part of a single-phase
well above permissible levels. Also, the resonant frequency transformerless grid connected PV system as suggested in
is not fixed due to the varying, on environmental conditions [ 161. With simultaneously switching on the switches S1and
dependent PV array earth capacitance. Depending on the 5 2 a positive voltage can be created at the inverter output
topology, switch states and environmental conditions the ca- terminal. A zero output voltage is created by switching on
pacitive earth current can cause more or less severe (con- S2 and S3 and a negative voltage is created by switching
ducted and radiated) electromagnetic interference, distortion on S3 and S4 respectively. In order to allow power transfer
of the grid current and additional losses in the system. Mea- into the grid the DC bus voltages VPVAIand VPVAZ
sures to minimise this current are mentioned in  and [ 141 have to be always higher than the grid voltage amplitude
and include e.g. adding passive components to dampen the 6grid. Since currently avatilable PV modules have operating
resonant circuit. voltages around 17 V a large number of modules is required
The magnitude of the PV array earth capacitance depends resulting in a minimum system size of approximately 3 kW.
on weather conditions and physical structure of the array. It An advantage of this system is that the midpoint of the
can be estimated according to the physical dimensions of the PV array is grounded which eliminates capacitive earth
PV array and its grounded frame area. One electrode of the currents and their negative influence on the electromagnetic
capacitance is formed by the photovoltaic cells, the other by compatability of the circuit.
the grounded frame (see Fig. 2(b)). In the worst case the The half-bridge diode clamped inverter can be expanded
complete surface of the PV array is covered by a conducting from three-levels to five:-levels as shown in Fig. 3(b).
layer (e.g. formed through humidity or dirt) increasing the Five switch combinations where always four switches are
area of the grounded electrode of the array (see Fig. 2 (a)). switched simultaneously generate five different voltage
Table I summarises estimations and measurement results levels at the AC output of the inverter, e.g. switching on S1,
of PV module earth capacitances for mono-crystalline mod- S2, S3 and S4 at the same time generates V P V A l + V P V A 2
ules with the following specifications and dimensions: at the AC output, switching on S2, S3, S4 and S5 generates
VPVA2 at the AC output and so forth:In [I71 a three phase
Ppeak 55 w
Cells in series 36 grid connected PV systenn using a diode clamped five-level
V M P P ( 2 5"C) 17V inverter is discussed. By adding more levels on the DC bus
I M P P ( 2 5 "C> 3.23 A the number of levels of the voltage at the inverter output
V O G P" C )
~ 21.2 v terminals are also increased. This allows for reduced distor-
Length x width x depth 1004 mm x 448 mm x 38.5 mm tion of the output waveform. To further reduce harmonics
an extra degree of freedom is given through choosing the
For arrays consisting of several modules the capacitances number of cells in series (and thus the voltages) of the outer
add independent of series or parallel connection. PV sub arrays (1 and 4)differently to those of the middle
PV sub arrays (2 and 3 ). Drawbacks of this topology,
(j!jjsti ,........... .....
IIW.*l DI s2:
Fig. 4. Grid connected PV systems with (a) full-bridge single leg switch
clamped inverter (SLSC) and (b) full-bridge single leg diode clamped
inverter (SLDC)  .
AC side with small DC bus voltages of e.g. 40 V each. High
power applications using cascaded inverters are described
in [lll, and I201.
Fig. 3. Grid connected PV systems with (a) half-bridge diode clamped
three-level inverter (HBDC)  and (b) half-bridge diode clamped D. Step
The step converter  switches PV sub arrays of
different voltages to the AC output. In  a topology using
however, are the high number of semiconductor devices five arrays with nominal voltages of 11 V, 22 V, 44 V, 88 V
required and since the loading of the outer PV sub arrays and 176V is suggested for a grid connected PV system
(1 and 4) is different to that of the middle PV sub arrays as shown in Fig. 6. A first conversion stage generates a
(2 and 3) careful sizing of each PV sub array is neces- rectified AC voltage waveform with 32 different voltage
sary to ensure maximum power transfer from each sub array. levels, a second conversion stage switches the polarity of
every second half-wave generating an AC voltage with 63
B. Full Bridge Single Leg Clamped different voltage levels. The energy delivered from each of
the PV sub arrays increases with increasing voltage. Each
In  and  a full-bridge single leg switch clamped PV sub array has different sizing requirements in order to
inverter (SLSC) is described and suggested for residential ensure maximum power extraction of each individual PV
PV systems. The topology (see Fig. 4(a)) comprises of array during operation.
a conventional full-bridge (switches S a l , Sa29 sbl and
s b 2 ) where a bi-directional switch (realised with Sa3, Sa4, E. Magnetic Coupled
Dal and Da2) is added which controls current flow to
and from the midpoint of the DC bus. When applied in a Fig. 7 shows a single-phase PV system with a mag-
transformerless PV system the minimum system size with netic coupled inverter as described in . The inverter
this topology is approximately 1.5 kW. consists of three full-bridges each with their midpoints
A transformerless PV system with similar characteristics connected to a primary winding of a transformer. The sec-
can be realised with a full-bridge single leg diode clamped ondary windings of the transformers are connected in series.
inverter (SLDC) as shown in Fig. 4(b) . With the single Due to different turns ratios of each of the transformers
leg diode clamped configuration the devices D a 1 , D a 2 , and the ability of each full-bridge to create three different
Sal, Sa2,Sa3 and Sa4 all can be rated for half the blocking voltages across the primary winding (+VPVA, -VPVA
voltage of switches s l and S b 2 , whereas with the single
leg switch clamped configuration this only applies to the
devices D a l , Da2, Sa3 and S a d , not Sal and Sa2. In both ...............
systems both PV sub arrays are symmetrically loaded.
C. Cascaded (CC)
Fig. 5 shows a transformerless grid connected PV
system where a cascaded inverter  is used for DC to
AC power conversion. The topology comprises of two
full-bridges with their AC outputs connected in series. Each
bridge can create three different voltage levels at its AC
output allowing for an overall five-level AC output voltage.
The advantage of this topology is the modular character. In -... :
 the concept is suggested for transformerless PV systems
using more than two full-bridges connected in series on the Fig. 5. Grid connected PV system with a cascaded inverter (CC) [ l I ]
.... .... . ..................
Fig. 7. Grid connected PV system with magnetic coupled inverter.
Fig. 6. Grid connected PV system with a step inverter 1221.
and 0) the voltage at the AC terminals can be comprised
of 27 levels. The advantage of this circuit is the relatively
accurate replica of a sine wave accomplished with low
switching frequencies. A major drawback of the circuit,
Fig. 8. Grid connected PV system with a half-bridge three-level flying
however, is the need for three transformers. capacitor inverter (FC).
E Flying Capacitor (FC)
the systems, Pr,min, and respectively the number of re-
In Fig. 8 a half-bridge three-level flying capacitor quired PV modules is biased on a maximum grid voltage
inverter is suggested for a transformerless grid connected amplitude of Ggrid,max = 1.1 4 * 240 V. All listed
PV system. Flying capacitor converters (which are also topologies have step-down characteristics. Therefore,
referred to as floating capacitor or imbricated cell multilevel for the “half-bridge” topologies (HBDC, FC, HB) the
converters) are described in  and [lo]. The features of DC bus voltages VPVA~L VpVAZ have to be always
this topology are similar to the diode clamped topology. higher than ijgrid,max. For the “full-bridge” topologies
Important for the operation of this converter is a stable V P v A i = VPVA2 > Cgrid,max/2 applies. Since the
voltage ratio of V P V A l / V C 2 = V P V A 2 / V C 2 = 1. There- operating voltage of silicon cells reduces with increasing
fore control methods are required which ensure that the temperature, the DC bus voltage is lowest on hot summer
average current flowing in the capacitor C2 is zero. This days. This lowest operating voltage determines the mini-
complicates the control of the inverter and excludes solution mum number of cells which have to be connected in series
with varying duty-cycles (e.g. hysteresis control). to ensure energy transfer from the PV array to the grid at
all times. Then, based on the minimum number of cells
connected in series, the highest possible voltage, the open
circuit voltage on the coldest day has to be calculated since
IV. DISCUSSION it determines the voltage rating of the DC bus capacitors
as well as those of the semiconductor devices. For silicon
The following system comparison does not include all solar cells the temperature behaviour and hence the voltage
described topologies. It excludes the magnetic coupled variations can be estimated according to . The system
topology since it focuses on transformerless systems. sizes listed in Table I1 have been calculated for environ-
Considered are also only those topologies, where the mental conditions for Perth, Western Australia (maximum
amounts of energy extracted from each PV sub array are ambient temperature in summer: 45” C, minimum ambient
equal, which simplifies the design of the systems, hence the temperature in winter: OOC) and for typical, available
step and the half-bridge diode clamped five-level topology PV modules as specified in section 11. Major drawbacks
are not included in the comparison. Table I1 compares regarding the minimum size of all discussed systems is the
the remaining topologies regarding minimum rated power, lack of flexibility and the relatively high number of modules
P r,mi n, number of DC bus capacitors, number of semicon- required (for half-bridge f.opology systems twice as many as
ductor devices and their ratings, possible levels of the AC for full-bridge topology systems). By adding additional step
voltage at the inverter output terminals and the negative up conversion stages, sizing flexibility can be enhanced,
influence of the PV array earth capacitance. Additionally, however, overall system e:fficiencies will decrease. Modules
transformerless systems incorporating full-bridge (FB)and with higher operating voltages are favourable for the
half-bridge (HB) topologies are included. discussed systems since installation costs can be reduced.
The determination of the minimum rated power of Today’s availability of high voltage modules with operating
promising topologies. However, with the CC topology
(when applied in a transformerless system) measures are
I I FB I SLSC I SLDCl CC I FC I HBDC I H B ] necessary to decrease the capacitive earth currents which
are caused by potential differences imposed on the PV ar-
ray earth capacitance. Also further research is required in
modules order to evaluate, whether the advantages of multilevel con-
No. of Ca- version justify higher cost due to higher component count.
The step-down nature of all topologies requires system
sizes of 1.5 kW upwards in transformerless applications due
to the relatively low operating voltage of most currently
available PV modules. Availability of PV modules with
Vblock,max 0.7 0.7 0.7 0.35 0.7 0 7
. 1.4 higher operating voltages is desirable since this would re-
/kV or or duce system cost. An additional step-up conversion stage
between PV array and inverter can increases the flexibility
Imax/A 9 9 9 9 18 18 18
regarding the system size, but will reduce the systems’ over-
AC volt- 5 5 5 2
3 3 3
I aee levels
1 1 1 1 1 I 1 1
I all efficiency.
NUMBER OF SWITCHES The authors wish to acknowledge the valuable discussions
with Mr Michael Dymond and Mr Andrew Ruscoe, Power
Ilblock,max = 700 v Vblock,max = 350 v
T O ~ O ~ O ~ Y Imax = 9 A =
Imax 9 A Search Ltd, Perth, Western Australia, with Dr. Mike Mein-
SLSC 4 2 hardt from PEI Technologies, National Microelectronics Re-
SLDC 2 4 search Centre, Cork, Ireland and with Mrs Johanna Myrzik,
cc 0 8 ISET, University of Kassel, Germany.
voltages above 30 V, however, is still limited.
Advantageous in respect to minimising leakage currents
are t o p ~ ~ ~ gwhere the PV array can be grounded. Here
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