Paper accqpted for presentation at 2003 IEEE Bologna PowerTech Conference, June 23-26, Bologna, Italy
String and Module Integrated Inverters for
Single-phase Grid Connected Photovoltaic
Systems - A Review
J. M. A. Myrzik, and M. Calais, Member, IEEE
possible future trends.
Index Terms--Inverters, grid connected photovoltaic power
syslems
connected PV inverters available on the European market and
to outline future trends in this rapidly developing technology.
The paper is organized as follows. Section I1 describes PV
system designs and discusses safety aspects concerning
transformerlesstopologies. Then some commercially available
inverter topologies are presented and discussed and an
overview on market shares, price and efficiency developments
is given. Section 111 outlines new developmentsincluding new
system designs such the ‘team’ and ‘Multi-string’ inverter
concept and discusses new transformerlesstopologies. Section
IV summarisesthe main fmdings and developments.
11. COMMERCIALLYAVAILABLE INVERTERTOPOLOGIES
A. Systems Design Aspects
Before discussing inverter topologies we fmt defme three
inverter families, which are related to specific PV system
designs: central inverters, module integrated or module
orientated inverters and string inverters. These inverter
families are highlighted in Fig. 1.
In the middle of the 1980’s the market for PV grid
connected systems developed with central inverters of several
kilowatts in size being the most common. The topologies of
these inverters were based on inverters used in the drive
system industry and were not optimised for the PV
application. Beginning of the 1990’s, results of research
programs, such as the IDDO Roof Program in Germany [1,2],
highlighted technology deficiencies and drawbacks of systems
with central inverters became apparent. These included
Mismatching losses by using Maximum Power
Point (MPP) conaol for a large group of PV
modules;
Losses and risk of electrical arc in DC wumg; and
Poor expandability and adaptability to customers’
requirements, due to little or no design flexibility.
In order to avoid these problems, a modular system
technology was introduced. Additional advantages of a
modular system concept are:
Cost reductions and reliability improvements by
using similar components;
Simpler system design and installation by
combining standard units; and
Simpler faultfinding.
The smallest possible grid connected PV system unit is a
PV module with a module-integrated inverter. In this case,
mismatching losses are minimised, since load matching for the
individual PV module is achieved through its inverter.
Additionally, DC wiring is minimised. However, there are
drawbacks concerning efficiency due to the low power ratings
involved and replacements in case of inverter faults may be
difficult and expensive. Also, cost per watt remains high
unless,mass-productionis possible.
With the pressure to increase system efficiencies while
simultaneously reducing costs, the string orientated or string
inverter was introduced on the market in the mid 1990’s.
String inverters are designed for a system configuration of one
string of PV modules and can be seen as a compromise
between module integrated and central inverter concepts. As
Fig. 1 shows, the string concept bas since remained popular.
Advancements in semiconductor and filter components
have lead to inverter efficiency improvements but with
maximum efficiencies for string inverters ranging today from
94 to above 97 %, there is little room for further improvement.
PV inverter companies therefore seem to be focusing more on
efficiency improvements and cost reduction through new PV
systems concepts such as the team concept and master-slave
concept, which are discussed in more detail in Section 111.
These concepts combine the advantages of the string
technology with the lower cost solution of the central
inverters. Further goals are to increase the overall system
efficiency, to reduce the harmonic distortions and to enhance
monitoring features. [4,5,13,16,19]
B. Are tramformerless inverters a risk f o r customers?
In the past few years the market share for transformerless
inverters has steadily increased. Topologies without
transformer generally have higher efficiencies and may be
cheaper than comparable inverters with transformers. Both are
crucial advantages to make PV systems more competitivewith
traditional generation. The main disadvantage, however, is the
direct connection of the PV array to the grid without galvanic
isolation. Depending on the inverter topology this may cause
fluctuations of the potential between the PV array and ground.
11111IK l Y r n N (I,
Fig 1: PV inverters available in 1994 and 2003.
These fluctuations may have sine or square wave behaviour at
grid frequency or even switching frequency. They generate
two effects:
1.
The surface of the PV array forms a capacitor, with
respect to ground, which is energised by the
fluctuating potential. A person, connected to ground
and touching the PV array, may conduct the
capacitive current to ground, causing an electrical
hazard to the person involved.
2. The voltage fluctuations generate electric and
magnetic fields around the PV array (electromagnetic
interference)
The seriousness of these effects has been an area of
concern and controversial discussion over the past years.
Meanwhile different studies [15,21] show that the impact of
PV systems with transformerless inverters on electromagnetic
interference is usually negligible and does not pose a hazard.
However, with respect to capacitive currents certain
recommendations on inverter and PV system design should be
followed to prevent dangerous current levels (above -10 A).
A recent paper from the Fraunhofer Institute for Solar
Energy Systems in Germany [21] discusses the electrical
hazard in case of a person touching the surface of a PV array.
The capacitance between the point of contact and a single PV
module has been calculated to range between 100 - 400 pF.
For the single PV module case, and depending on the
transformerless topology and the applied switching method, a
current of max. 0.2 mA could flow through the human body.
When considering larger PV arrays with ungrounded
structures, the maximum possible current increases as the
surface area of the PV array increases. The capacitance for
larger ungrounded PV arrays has been found to be 50 150nFkW for glass-faced modules and up to IuFkW for
thin-film modules. Currents of several mA can then occur.
However, it is important to note, that by grounding the PV
array structure the respective parasitic capacitance is reduced
and with this the associated hazardous current. Additionally,
the type of transformerlesstopology and the applied switching
scheme influence the magnitude and type of voltage
fluctuation at the PV array with respect to ground. A detailed
discussion of these is given in [21] and some examples on
recommended topologies and switching schemes are presented
in the following. When following the recommendations on
grounding the PV array structure, topology and switching
scheme choice, along with the use of Class I1 equipment,
transformerless PV system do not present an increased hazard
compared to PV systems with transformers.
Inverting element +
Voltage
PV array voltage has to be boosted with a further
1
The
element.
way these three functions are sequenced within an
inverter design detdmines the choice of semiconductor and
Finer
~ l [ ~ l " t -
~
lnverling element +
Current wave shaping+
VoRage adjustment
,
1
-
j=E--p-p%
Flying Inductor
lnveiier
Utility
Grid
Filter
-~
s2
53
?sat
tPlS
,w
....
m,
ZoDd
~
2
m
.
~
nm
~
_
_
_
2md
Yea.
Fig. 6: Flying inductor inverter [24,25]
converter) configuration, it is used for energy storage, which
increases the size and costs of the magnetic component and
reduces the efficiency. The advantage of this topology
however is that the negative of the PV array can he directly
connected to the (grounded) grid neutral. The undesired
voltage fluctuations between PV m a y and ground as
discussed in Subsection B are therefore kept at a minium.
Fig. 7 shows a neutral point diode clamped inverter with a
boost converter at its input. The inverter operates as follows.
For generating the positive half-wave, a PWM control
switches SI and S2 on to produce a positive inverter output
voltage and S2 and S3 on to provide a zero inverter output
voltage. For the negative half-wave S3 and S4 are switched on
to generate a negative inverter output voltage and S2 and S3
are switched on to generate a zero inverter output voltage. In
this way, the topology produces a unipolar or three-level
inverter output waveform. The mid point of the DC bus can he
connected to the grid neutral, which minimises voltage
fluctuation between the PV array and ground. However, each
DC bus capacitor voltages needs to be higher than the grid
voltage amplitude at all times which explains the additional
boost stage at the input of the inverter.
Fig. 8. Average European efficiency development of singlephase PV inverters (1998-2003).
topologies as presented in [26]. From 1994 to 2003 their
average maximum efficiency increased from 93.5% to 96.5%
and from 93.4% to 95.8%, respectively. However, a PV
inverter rarely operates at maximum efficiency due to the
varying intensity of solar radiation. The European efficiency
qeumprovides a means to compare the energy output of
different inverters operating under typical European
irradiation conditions. In consideration of the frequencies of
the different hdiation values, an average qNm valid for
central Europe is given as:
qeuo = 0 . 0 3 ~ ~ + 0 . 0 6 ~ ~ o + 0 . 1 3 ~qio+O.48qso+
~~+0.1
+ 0.2 qlw
(1)
where qqis the efficiency of the inverter at xy% of the rated
power.
Based on the evaluation of different market surveys given
in [3,9,11-13,191, the development of average qeu for
different PV inverter topology types is shown in Fig. 8.
Interesting is the steady increase since 2000/2001. In
comparison, the average maximum efficiencies (not shown in
Fig. 8) have stagnated in the last three years. One reason could
he that the effort to increase maximum efticiencies above
current levels is contrary to reducing costs. It seems that
D. Efjiciencies
companies are rather focusing on improving the efficiency at
The highest maximum efficiencies are achieved with line low power levels of existing topologies or on improving
commutated and self-commutated transformerless inverter system efficiencies. These and other mends are discussed in
Section 111.
E. Prices
Prices for single-phase PV inverters have significantly
decreased (by around 50%) during 1994 and 2002 and Fig. 9
shows this development. Although the growth rate of the
indushy exceeded 50% per annum in 2002 [13], Fig. 9
indicates that prices stagnated or even slightly increased in
2003. Possibly the introduction of the Euro in 2002 may have
been of influence. It is also interesting to note, that prices for
PV inverters are still up to 50% higher than those for inverters
for drive systems. Fig. 9 also shows 2003 prices for different
topology types ranging from 0.5 Euro/Wattoc for
--i~~~_i--.~~~-i
transformerless topologies and up to 2.6 Euro/Wattm for
module-integrated inverters.
Fig. 7 Neutral point diode clamped inverter [21].
l
.................
..........
......................................................
U.2%
Fig. 10: Market shares of different inverter topologies
based on a market survey of turnkey solar systems [30].
the past years. This has lead to cost reductions in production
and installation of PV systems. String inverter power ratings
have grown alongside the PV module ratings and are now
generally higher than a few years ago.
Growth in the demand for larger string and central inverter
concepts has not stopped module integrated or module
orientated inverter developments and a number of new models
have become available in 2003. Research in this field
concenhlltes on reducing costs and implementing new, mostly
resonant switched topologies [ZO].
The implementation of three-phase inverters in the power
range of several kilowatts has been discussed for quite some
time hut little has eventuated. Although three-phase inverters
have some advantages over single-phase inverters, it seems
that customer demand is too low for this technology in this
power range. The main advantage is that the need for large
electrolytic capacitors in the DC input circuit are significantly
reduced, which limits the lifetime of the inverter and increases
the cost of single-phase inverter.
Generally, cost reduction is still the main goal of the PV
inverter industry. However other desirable features include
simpler installation, better monitoring and faultfinding,
improved reliability and efficiencies and safety.
The
following suh-sections examine new PV system concepts and
inverter topologies addressing these issues.
developments in the PV module industry where size and peak
power rating of PV modules have continuously increased over
A. Team-Concept
This concept, mentioned in [13,31], combines the string
technology with the master-slave concept. A combination of
several string inverters working with the team concept is
shown in Fig. 11. At very low irradiation the complete PV
array is connected to one inverter only. With increasing solar
radiation the PV array is divided into smaller string units until
every string inverter operates close to its rated power. In this
mode every string operates independently with its own MPP
controller. At low solar radiation the inverters are controlled
in a master-slave fashion. Simulation results indicated a
higher energy output of about 4%. Actual measurements on a
123 kW test-field showed that the simulation results can be
exceeded [3 I].
DCBUI
(>650 V I
+ I
I
I
ACCcndon
- Communication
Fig. 11. Theteamconcept [13,31].
B. Multi String Inverters
The Multi String inverter concept [ 5 ] , [32] (see Fig. 12) has
been developed to combine the advantage of higher energy
yield of a string inverter with the lower costs of a central
inverter, Lower power DC/DC converters are connected to
individual PV strings. Each PV string has its own MPP
tracker, which independently optimises the energy output
from each PV string. To expand the system within a certain
power range only a new string with a DCiDC converter has to
be included. All DCDC converters are connected via a DC
bus through a central inverter to the grid. The central inverter
is a PWM inverter based on the well-known and cheap GIFT
technology already used in drive systems and includes all
supervisory and protection functions. Depending on the size
of the string the input voltage ranges between 125 to 750 V.
The inverter has a maximum power rating of 5 kW and
became available in 2002.
C. The HERIC-Concept
In transformerless single-phase inverter bridges, as shown
in Fig. 5(a), different PWM switching patterns can be used.
By using bipolar PWM (the bridge voltage switches between
+Vpv and -Vpv) the diagonal switch-pairs are switched
alternately. The operation can be described as follows: In
order to generate the positive output sine-wave S1 and S4 are
closed and the inductor current in the output filter increases
The inductor current consists of the average (50Hz sine-wave)
load current and of a triangle ripple (at switching frequency)
current component provoked by the on and off times of the
switches. If S1, S4 are switched off, the switches S2 and S3
are switched on (after a certain blanking time has occurred)
and the inductor current can decrease. In this time the ripple
current of the inductor flows back into the source via the
freewheeling circuit, which includes the switches S2, S3 and
the input electrolytic filter capacitor. This causes additional
losses in the switches and capacitor. These losses become
significant, as the load current is zero or quite low. In this case
the average part of the inductor current is zero but the ripple
part is still high and circulates as a “reactive” current inside
the freewheeling circuit. These losses are mainly responsible
for a low efficiency at low load. In order to reduce the losses
Fig. 12. Multi siring inverter [5,32]
unipolar P W M (bridge voltage switches between +Vpv, Vpv =
0, and -VPV) is commonly used in PV inverters. Due to the
required cosp = 1 a one-phase-chopping can be used. With
this switching pattern only the switches SI and S3 are
switched at high frequency while S2 and S4 are switching at
50Hz. For example at positive grid voltage S4 is permanently
on while SI is switching at e.g. 20 kHz. If SI is closed the
inductor current increases. If S1 opens the inductor current
commutes in its freewheeling circuit consisting of S4, the antiparallel diode of S2, the filter and the load. In this case the
ripple current remains within the load circuit and cannot flow
back into the DC source. The significant disadvantage of this
switching method is that the potential between the PV array
connection points and ground jumps with 50 Hz between
+VPVand zero (or -Vpv and zero respectively) and additional
EMC filtering is required.
A new topology called Highly Efficient Reliable Inverter
Concept ( E R I C ) is proposed in [2122]. Two additional
freewheeling branches as shown in Fig. 13 extends the
common well-known PWM inverter topology. These branches
-
yj#sj
s2
L1
S4
D1
S6
Fig. 13. The HERIC Inverter [21,22].
i
-
topologies are proposed in [17]. These topologies are based on
a parallel-series connection of two buck-boosting converters
as Cuk, D2 or Zeta converters and their versions with high
frequency transformers. Examples are given in Fig. 14.
Common P W M controllers can control the switches. In order
I
to realize a linear control function the input inductance should
I
operate in the discontinuous conduction mode while the
output inductance carries a continuous load current. Due to
zeta mnvener
the higher voltage and current stress of the semiconductors
and input inductor the efficiency of these inverters is approx.
I
1.5% lower than that of string inverters available on the
market. Their important advantagehowever is the reduction of
magnetic material of approx. 40%. This has a significant
impact on the price. These topologies are suitable for
applications where the PV array voltage is lower than the grid
voltage.
I
I
As most other transformerlesstopologies theses topologies
!
cause fluctuations of the PV array potential with respect to
Fig. 14. Topologies derived 60m Cuk, Zeta, and D2ground. In 1231 a new topology is described which achieves a
I Converters [171.
fixed potential between PV array and ground. Instead of
are switching with 50 Hz respectively to the polarity of the
I
connecting two of the same converters (2 Cuk, 2 Zeta or 2 D2
sinusoidal output values. This allows to short circuit the
converters) the new topology is derived from a series-parallel
reactive current and 'to keep it within the load circuit. The part
connection of a Cuk converter with a Zeta-converter as shown
load efficiency in a h g e below 20% of the nominal power is
in Fig. 15. Due to the voltage inverting behaviour of the Cuk3 4 % higher comp+ed to other PWM concepts. This has a
converter a fxed potential can be achieved between PV
strong impact on the overall system efficiency and systems
I
generator and ground. In order to generate the positive sinedesign. Achievable maximum efficiency is approx. 98% for a
wave of the output current the inverter operates as a Zeta3kW inverter. A further advantage of this topology is, that the
converter: S1 is switching at high 6equency and S2, S4 are
bipolar voltage switdhing method causes a sinusoidal potential
permanently closed, D2 operates as free-wheeling diode. To
(at 50 Hz and an amblitude of % Ved) at the connectors of the
generate the negative half-wave of the output current the
PV generator. The Fraunhofer Institute has developed the
inverter operates as Cuk-converter: S2 is switched at high
concept and the first,inverter of 1.4 kW will be entering the at
frequency, S1, S3 are permanently closed, and D1 operates as
the end of 2003.
6eewheeliig diode. In [23] other variations of this topology
D. New topologies using two buck-boosting converters in
are shown, which also enable resonant switching.
'
I.
I
parallel-series connection
I
Costs reduction is one of the main goals in the PV inverter
industry, especially lat small string and module orientated or
integrated inverters ?here the price per watt is still high. Their
efficiencies and costs suffer either 60m the second converter
in cascade conneciion or from the transformer which is
required to boost the mostly low PV array voltage to grid
voltage level. In orher to avoid this kind of connection new
I
PV
Anay
-
1
- -
IV. SUhlhlARY
This review has focussed on small (<6 kW) grid connected
single-phase PV inverters. The recent market developments
and the variety of inverter topologies available have been
presented. It was found that the prices of these inverters have
fallen by about 50% over the last decade and that relatively
cheap and efficient transformerless inverters now have a
market share of about 55%. Their topologies and apparent
risks are discussed. Due to limits being reached in individual
inverter electrical and cost efficiencies, Multi String and team
concepts have been introduced to achieve improved system
performance and reduce costs. There are also several
transformerless inverter topologies under development, such
as the HERIC, which can provide benefits such as improved
efficiency and good electromagnetic compatibility.
1201 S. B. Kjaer,J.K. Pedenen, F. Blaabjerg, Power Inverter Topologies for
Photovoltaic Modules - A Review, IEE-IAS Annual Meeting 2002,
Pittsburgh, PA, USA.
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Symposiums Photovoltaische Solorenergie, Ostbayrisches TechnalogieJohanna M.A. Mynik was barn in Darmstadt
Transfer-Institut e.V. (OTn), Regensburg, Germany, pp. 163.168, 1997.
Germany in 1966. She received her MSc. in
191 P. Welter, Inverter Market Survey, (Marktilbersicht Wechselrichter, in
Electrical Engineering from the Darmstadt
German), PHOTON, no. 3, pp, 60-65, May-June 1998.
University of Technology, Germany in 1992. From
[lo] F. Greizer, Inverters for grid connected photovoltaic systems (Gestaltung
1993 to 1995 she worked as a researcher at the
von Stroamrichtern fU netzgekoppelte Photovoltaikanlagen. In Proceedings of
Institute for Solar Energy Supply Technology
rhe 4. Kasseler Symposium, Energie Syslemlechnik Kossel, G e r m y ,
(ISET e.V.) in Kassel, Germany. In 1995 Johanna
November 1999.
joined to the Kassel University, where she finished
[I I ] P.Welter, Power up. prices down, grid wMected inverter market survey
her PhD thesis in the field of solar inverter
(Leistung rauf, Preise runter, Marktubersicht netzgekoppelter Wechselrichter,
topologies in 2000. Since 2000, Johanna is with the
in German), PHOTON, no. 3, pp. 48-57, May-June 1999.
Eindhoven University of Technology, the
[ 121 A. K r e u u m m , Inverter market survey, (Marktubenicht Wechselrichter,
Netherlands. In 2002, Johanna became an assistant professor in the field of
in German), PHOTON3no. 3, pp. 46-55, March 2001.
1131 I. KrampiIz and A. Kreumnann, From masterpiece to team work, inverter distributed generation. Her fields of interests are: power elecmnics,
market survey: SMA‘s new circnit wncept promises higher yields (Vom renewable energy, distrilbuted generation, electrical power supply.
MeistentUck LW TeamarbeiC MarMLlbersicht Wechselrichter: Ein neues
Martioa Calais was born in Munich, Germany, in
Schalkomept Yon SMA verspricht hahere Enage, in German), PHOTON,
1969. She received her Electrical Engineering
no. 3, pp. 56-67, March 2002.
degree from the Darmstadt University of
1141 Fraunhofer Gesellschat? Institut for Solare Energiesysteme, Course book
Technology, Germany in 1996. In 1996 Martina
far the Seminar Photovoltaic Systems, prepared as part of the EU Comen
worked as a research engineer on automotive power
Project “Sunrise”, 1995.
Supplies in the Electrical h i v e s Department at
[IS] G. Bopp, lnwieweit bagen PV-Anlagen zum Elekaosmog bei?, I4
Daimler Benz Research and Technology, Frankfurt,
Symposium Pholovoltaische Sofinenenergre,Staffelstein, Germany, 1999.
Germany. Currently Martina is shldying towards a
[16] G. Cramer and K.-H. Toenges, Modular system technology (string
PhD with the Centre for Renewable Energy and
inverters) for grid connected PV sytems in the 100 kW - 1 MW power range
Sustainable Technologies (CRESTA) at Curtin
(Einsatz der modularen Systemtechnik (String-WR) zur Netzkopplung von
University of Technology, Perth, Australia In 2000
PV-Anlagen im Leistungsbereich von IOOkW-IMW, in German), 12
Martinajoined the School of Engineering Science at
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