BATTERY INVERTER FOR MODULARLY-STRUCTURED PV POWER SUPPLY SYSTEMS
B. Burger, P. Zacharias
Institut für Solare Energieversorgungstechnik (ISET) e.V.
Königstor 59, D-34119 Kassel, Germany, Tel. [phone], Fax: -100
G. Cramer
SMA Regelsysteme GmbH
Hannoversche Straße 1- 5, D-34266 Niestetal, Germany, Tel. [phone], Fax: -100
W. Kleinkauf
Universität Gh Kassel, Institut für Elektrische Energietechnik (IEE)
Wilhelmshöher Allee 71-73, D-34109 Kassel, Germany, Tel. [phone], Fax: -6512
ABSTRACT: The electricity supply in remote areas without public utility is very important worldwide, particularly in
developing and threshold countries. This is an ideal application for isolated or  off grid hybrid power supply
systems. ISET developed a completely new bi-directional battery inverter with a rated power of 3.6 kW for such
systems in cooperation with SMA Regelsysteme GmbH. Power ranges from 3.6 kW to 33 kW can be established
with the parallel connection of inverters in single phase and three phase systems due to the modular design of
the battery inverter. All power producers and consumers are coupled at the AC line in these modular systems.
Keywords: Hybrid - 1: Stand-alone PV Systems - 2: Inverter - 3
inverter must therefore be able to operate it in all four
1. INTRODUCTION
quadrants.
This requires the control of voltage, frequency, active
The supply of small, peripheral consumers in the
power and reactive power of the AC voltage of the
power range from 2 to 30 kW, which cannot be
battery inverter. With appropriate coupling of three
attached to a public grid, is worldwide, in particular in
devices a three phase power supply is possible and
the developing and threshold countries, from large
by the direct parallel connection of several inverters
relevance. This is an almost ideal application for
at a phase an increased power will be achieved (in
isolated or  off grid photovoltaic power supply
development). For simple configurations the battery
systems. The experiences with such systems have
inverter is able to take over the battery management
shown that these systems should be not only very
and load management.
reliably, economical and robust, but above all
The DC voltage is controlled to provide a best
modularly structured and therefore easily
possible battery handling, with respect to temperature
subsequently expandable [1]. Also the connection of
dependent and current dependent voltage limits, the
diesel generator sets and small wind energy systems
execution of regular total charge cycles and the
should be possible in a simple manner. Only a simply
adaptation of the charging algorithms to the battery
structured and flexible system design for these
type and the application conditions. Additionally the
photovoltaic power supply systems will enable a wide
state of charge of the batteries is calculated and
spread application.
displayed.
On the basis of these requirements, the ISET in
The following requirements for battery inverters in
cooperation with SMA and with promotion of the
modular structured island systems can be fulfilled:
German Federal Ministry BMWi developed the
completely new battery inverter  Sunny Island with a " Operating modes: Voltage control - current control
nominal output power of 3.6 kW. The usage of - parallel operation,
advanced microprocessor technology in combination " Modular expandability,
with new power electronic circuit concepts provides a
" Extendibility for 1 and 3 phase island grids,
simply applicable and expandable system
" Intelligent battery management for longest battery
engineering for the power supply of remote areas.
life: Charging and discharging control, regular full
charging
" State of charge display
2. THE BATTERY INVERTER
" Load management for simple basic configurations
of small systems and
The central component of such a modular supply
" High efficiency, also in the partial load range with
system is a battery inverter with the product name
low stand by losses.
Sunny Island [2],[3]. The AC output of the inverter
To create a flexible applicable device, closed loop
must provide constant voltage and frequency for the
control and local management system (operational
consumers. A lead acid battery is used as energy
control) are taken over by its own processor. This
buffer. Intelligent management and control algorithms
allows the integration of a fast closed loop control
integrated in the device enable it to supply not only
and a complex management into the new battery
different consumers but also to connect different
inverter. The fast control allows the required
generators, e.g. string inverters, small wind energy
operation modes and a parallel connection of
converters or diesel generator sets. The battery
inverters. The management system takes over the
battery management, enables a limited load and reaches already after approximately 1 ms again
management and makes communication interfaces its desired value. A further improvement of the
available for optional management devices. Figure 1 dynamics would be possible only with a higher DC-
shows a block diagram of the battery inverter. The link voltage, since the inverter here already operates
battery and the AC output are connected via circuit as control in the delimitation. This would increase
breakers. There are 8 relay contacts available for the however the losses of the power electronic
following tasks: components, so that the efficiency would be reduced.
" Starting a generator and connecting it to the
Ch1: Vbat
island grid
Ch2: Vlink
" Switching of wind energy, consumers, utility and
Ch3: Vgrid
dump load
Ch4: Igrid
" Automatic control of the fan for the battery room
and an optional electrolyte circulation pump.
63 A DC 16 A AC
battery island grid
e.g. 60 V
230 V, 50/60 Hz
generator start
closed loop
fan generator connect
control
wind energy
circulation
consumers
management
battery temp.
utility
system
generator voltage
Figure 4: Load branch with 2 kW
dump load
generator current
RS485
communi-
3-phase
RS485 display
If the output of the inverter is short circuit, an
synchroni- cation
Powerline
zation
additional closed loop control for the choke current
limits the output current of the inverter to the
Figure 1: Block diagram of the battery inverter
maximum current, which the power semiconductors
can process. This current is higher than 60 A
2.1 Circuit of the static inverter
resulting in the fact that the inverter is able to trip
Figure 2 shows the power electronic circuit of the
normal circuit breakers of Class A with a rated
battery inverter. A bi-directional Cuk converter
current of 16 A. Figure 5 shows a measured short
changes the battery voltage, which depending upon
circuit. During the short circuit the voltage V is
grid
number of cells and charge can be between 40 V and
nearly zero and the current I rises to the current
grid
80 V, into a regulated DC-link voltage of 380 V. The
limit. After approximately 15 ms the circuit breaker
HF transformer provides a electric separation
trips and the voltage rises again on its desired
between battery and grid, so that the battery is
sinusoidal value. Thus a selective protection is
potential free. By the high frequency of 16.6 kHz the
possible by circuit breakers in the island system just
transformer is substantially lighter and smaller than a
as in the public grid.
comparable transformer for 50 Hz. To the DC-link a
single phase inverter with L-C-L filter is connected,
Ch1: Vgrid
which generates the sinusoidal voltage for the island
Ch2: Igrid
grid. Since both, the Cuk converter and the inverter
operate bi-directional, the static inverter can charge
and discharge the batteries.
L
=
N
~ PE
battery bidirectional inverter single phase
DC-link
60 V DC-DC converter 3.6 kW / 4 kVA island grid
380 V
C1 C2
L1 L2
L5
L3
S3 S5
Tr
S1 S2
C3
C4
S4 S6
L4
Figure 2: Circuit of the static inverter Figure 5: Short circuit over a 16 A circuit breaker
2.2 Closed loop control 2.3 Management system
For the control of the Cuk converter a state The management system is responsible for all
control with overlaid closed loop PI control is used for functions, which do not have to be processed faster
DC-link voltage control. The digital closed loop than in one second. These are above all
control operates at the half clock frequency of the communication over the serial interfaces and the user
hardware, i.e. with 8.3 kHz. interface via keyboard as well as the graphic display.
Figure 4 shows the behavior of the closed loop Additionally the management determines the
control during a load branch with 2 kW. The voltage operating mode. The following operating modes are
V (channel 3) has only a short drop after switching implemented:
grid
2.3.1 Voltage controlled operation battery temperature as well as the calculated state of
In the voltage controlled operation mode the charge and desired value of the charging voltage
output voltage of the inverter is regulated to its RMS over one week. The desired value of the charging
value. The output frequency can be defined with a voltage was not achieved here, since the battery was
resolution of 10 mHz, the RMS value of the voltage charged only with solar energy.
with a resolution of 100 mV. A change from 50 Hz to
2,6 100
60 Hz can be made easily by software. 2,5 charging voltage 90
cell voltage
2,4 80
state of charge
2,3 70
battery temperature
2.3.2 Current controlled operation
2,2 60
In the current controlled operation mode the
2,1 50
inverter synchronizes to an external voltage supply. 2 40
1,9 30
This can be a public grid or a generator. Depending
1,8 20
on the given direction of current, the battery can be
1,7 10
charged in this operating mode or the grid can be
1,6 0
10
supported.
5
0
charging current
-5
2.3.3 Three phase operation
discharging current
-10
In the three phase operation mode three inverters
operate with 120° offset, so that a three phase
current supply system is established. Synchronization
Figure 6: Charge of the battery in the process of one
is done with a digital interface.
week
2.3.4 Parallel operation
In the parallel operation mode several inverters
3. SINGLE PHASE APPLICATIONS
operate synchronized on one phase, so that the
available output power is increased to a multiple of
A simple single phase island grid can be
3.6 kW.
established with one battery inverter and a lead acid
battery. The closed loop control will enable the
2.4 Battery management
increase of output power by parallel connection of up
The battery management is responsible for the
to three battery inverters at one phase. In order to
charging and discharging control of the battery. It
feed solar electricity into the island grid, conventional
calculates the desired value for the charging voltage
PV inverters e.g. string inverters from the Sunny Boy
and is able to start an additional generator over relay
series from SMA can be used. Furthermore the
outputs, in order to charge the batteries additionally
integration of wind or hydroelectric power plants is
to the PV power. When the battery has a low state of
possible with static inverters or single phase
charge, a low priority load can be disconnected from
generators. For the increase of security of supply
the grid by another relay. In the case of a fully
usually still a backup generator (e.g. Diesel
charged battery, a dump load will be connected or
generator) is used. Often these generators are
the PV power will be disconnected or short circuited.
already installed and can be integrated into the hybrid
Additional relays are available for controlling a fan for
system. If a public grid is available from time to time
the battery room and for the pump of an electrolyte
as in many developing countries, then it can be
circulation system.
attached also to the inverter. It operates then like an
For the calculation of the state of charge of the
UPS and supplies the consumers in the case of
battery an algorithm is integrated, which uses a
power failures. Figure 7 shows the structure of a
balance of ampere-hours combined with a calculation
single phase island grid with the battery inverter
of losses and multilevel full charge recognition. Also
Sunny Island and with different generators and
an adapting current-voltage model of the battery is
consumers.
used to recalibrate the state of charge when the
battery is not fully charged [4]. For the current-voltage
model the linear correlation between the open-circuit
wind- or
voltage and the state of charge is used. The
combustion
hydro power
engine
plant
correlation is determined in phases after a full charge
photovoltaic
by means of the then well known ampere-hours
asynchronous generator
modules
G GS
generator set
balance. This is important, since the correlation
between open-circuit voltage and state of charge for PV-string-
=
inverter
different battery types can be very different. Thus it ~
(e.g. Sunny Boy)
1~ / 230 V
becomes possible in most PV systems, the ampere-
50 Hz / 60 Hz
hour balance not only to recalibrate after full charges
but practically each night, when the battery is
battery inverter
~
Sunny Island
discharged only with small currents. By the definition
=
electric
of a lower open-circuit voltage for the end of
consumers
M
batteries
discharging additionally the ampere-hours capacity of
60 V
a battery can be measured. For the determination of
the battery capacity therefore no capacity test is
Figure 7: Example of the structure of a single phase
necessary. Figure 6 shows the measured values of
modular island grid
charge and discharge current, cell voltage and
cell voltage
state of charge, temperature
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current
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4. THREE PHASE APPLICATIONS
The smallest three-phase system is a 11 kW
power supply consisting of three Sunny Island each
connected to a different phase. The three phases are
synchronized via RS485 while the operating data is
additionally sent over this link. Three phase systems
make it much more easy to connect diesel or wind
generators as these mostly are only available in three
phase versions. Larger island systems consist of 6 or
9 inverters with two or three connected to each
phase, all in all resulting in a total output power of
33 kW. The Sunny Islands can be freely connected to
any battery set, i.e. several Sunny Island can use one
or several sets of batteries. Although it is
recommended to establish one single battery set for
Figure 9: Prototype of a three-phase hybrid system
three phase systems. Figure 8 shows the structure of
in the DeMoTec centre of ISET
a three phase island system in principle and Figure 9
the prototype of a three phase hybrid system in the
The ISET and SMA thank the German Federal
DeMoTec centre of ISET. First demonstration plants
Ministry BMWi for the promotion of the project
will be built on the Greek island Kythnos [5].
"modular battery inverter: development of a battery
for the modular system technology in PV systems"
and the European Commission for the promotion of
wind generator
the projects  PV-MODE ,  MORE an  HYBRIX .
turbine set
photovoltaic
modules
G GS
REFERENCES
PV-string-
= = =
inverter
~ ~ ~
(e.g. Sunny Boy)
consumers
[1] W. Kleinkauf, J.Sachau: Components for
Modular Expandable and Adaptable PV
3~ / 400 V
battery inverter Systems, 12th European PV Solar Energy
~ ~ ~ ~
Sunny Island
= = = =
Conference, Amsterdam, April 1994
[2] B. Burger, G. Cramer, A. Engler, B. Kansteiner,
batteries
60 V
P. Zacharias: Battery Inverter for Modularly-
Structured PV Power Supply Systems, 2nd
Figure 8: Example of the structure of a three phase
World Conference and Exhibition on
modular island network
Photovoltaic Solar Energy Conversion, Hofburg
Congress Center, Vienna, Austria, July 1998
[3] B. Burger, P. Zacharias, G. Cramer, W.
5. PERSPECTIVES
Kleinkauf: Hybrid Systems  Easy in
Configuration and Application, 16th European
Due to the modular conception of the battery
Photovoltaic Solar Energy Conference and
inverter, power ranges from 3.6 kW to 33 kW can be
Exhibition, Glasgow, United Kingdom, May 2000
achieved by the parallel connection of inverters in
[4] M. Rothert, B. Willer: Möglichkeiten und
single phase and three phase systems. Additionally it
Grenzen der Ladezstandsbestimmung von
is possible to extend existing systems after the unit
Bleibatterien in PV-Anlagen, 13. Symposium
construction or to extend single phase systems to
Photovoltaische Solarenergie, Kloster
three phase systems. The application of this modular
Banz/Staffelstein, 1998
battery inverter will reduce planning and system
[5] P. Strauss, D. Mayer, C. Trousseau, S.
costs for hybrid island grids.
Tselepis, P. Romanos, F. Raptis, J. Reekers, M.
The use of stand alone hybrid grid systems on the
Ibrahim, R.-P. Wurtz, F. Perez-Spiess, M.
basis of the battery inverters "Sunny Island" will also
Bächler: Stand-Alone AC PV Systems and
enable a power supply in remote areas without mains
Micro Grids with New Standard Power
connection for the first time and therefore will reduce
Components, 16th European Photovoltaic Solar
the consumption of resources for the electric energy
Energy Conference and Exhibition, Glasgow,
production. So even lower social classes have
United Kingdom, May 2000
access to electricity for lighting, household and for
small workshops.
The consistent modularity of this new system
oriented concept allows a commercial use and self
supporting retail structures (leasing of the systems or
sale of the produced electricity), since the
components (battery inverter, batteries, diesel sets...)
do not have to be adapted for individual applications,
but are universally applicable.