AC COUPLING WITH AN XTENDER INVERTER
AC-COUPLING WITH AN XTENDER INVERTER
A white paper on the use of a grid connected inverter in an islanded system
STUDER Innotec® - April 2010
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The grid connected solar has become a huge market compared to offgrid solar which is the
natural market for solar. For the ongrid market much money was invested and now very good
products exist in term of cost and reliability for solar modules and inverters. Those products are
not suitable for the small offgrid systems
but there is medium size where they can
be used, in the range of a few kW,
typically in hybrid systems and minigrids.
A grid connected solar inverter can be
connected to the AC produced by a bidirectional battery inverter. If the voltage
and frequency are within the accepted
values, then the grid connected inverter
recognizes it like a normal grid, connects
to this island grid and starts to perform
grid-feeding with solar power.
Figure 1: Installed solar power in the world
Source http://www.iea-pvps.org
On the main grid all the available solar power (MPPT) is fed into the main grid to maximize the
return on investment. The situation is different in a stand-alone system; there is the need to
control the power production to match the
demand. If there is more production than
demand, the excessive solar goes to
Isolar
recharge the battery. If the batteries are full,
the power production must absolutely be
reduced, stopped or consumed to avoid
overcharging of the batteries. This can be
50Hz
done with connection/disconnection of
52Hz
sources with switching circuits, dump loads
or other control methods (communication
bus …). The control of the grid connected
inverter can also be done very simply
without any additional component with the
Figure 2 : AC-coupled solar inverter
frequency. This system can be realized with
all the Studer inverters of the Xtender series.
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ON/OFF control with frequency
Grid inverter is programmed to accept frequencies only between limits required by grid
directives, per example for German Renewable Energy Act EEG between 47.5Hz and 50.2Hz. The
battery inverter sets the voltage and frequency of the local grid, then it can increase the
frequency and this will stop the grid connected inverter when out of the limit.
This system works with different scenarios:
1. There is less solar power than user load:  the grid inverter work as MPPT and all the solar
power covers a part of the load need.
2. There is more solar power than user load:
a. The batteries are not full:  the grid inverter work at MPP and the solar power
covers the user load and the excess recharges the batteries.
b. The batteries are full:  the grid inverter is switched ON and OFF with frequency
increase to make the top charge. This looks like a slow PWM.
A SunnyBoy1700 with a standard DE configuration is used below with a Studer XTM2400-24. There
is a constant user load and a constant ‘solar’ production simulated with a DC power supply.
When the grid inverter is on, the batteries are recharged (positive current on graphic below) and
when off, they are discharged. The battery bank is a quite old 250Ah lead acid battery. With a
datalogger function in the Xtender XTM inverter, one point is saved every minute over the day
and the ON/OFF control behavior of the system can be well seen:
Battery current
[Adc]
15
Test of SB1700 with standard DE settings
10
5
0
-5
-10
-15
08:24 09:00 09:36 10:12 10:48 11:24 12:00 12:36 13:12
Battery voltage
[Vdc]
29.5
28.5
27.5
26.5
1hour absorption
25.5
time [hh:mm]
24.5
08:24
09:00
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If the max frequency accepted by the grid inverter can be modified (it is generally the case
because different countries have different regulations and different max frequencies!) and there
is more than one grid inverter, it is interesting to stop one after the other, per example first stops
at 50.2Hz the second at 50.5Hz. Then we can modulate the power production with two steps or
more.
Frequency shift power control like MEDIUM VOLTAGE DIRECTIVE
The 2009 EEG grid code has set new requirement and one is particularly interesting for our
minigrid/AC-coupling application: it is an active power reduction in function of the frequency.
When the frequency increases, the grid connected inverter doesn’t simply stops but reduces its
power linearly between 50.2Hz and 51.5Hz.
By changing a little bit its output frequency, the battery inverter is able to control the solar
production in the island grid to match production–consumption-storage balance. It will be a
more precise control instead of an ON-OFF control.
This rule is set for medium voltage and high
voltage and is not applied for low voltage yet.
Technically this behavior is not very complicated
to implement in the low voltage solar inverters, it
is a simple modification of the control software.
But up to now, the interest from the various
manufacturers was low due to the small size of
the offgrid market and still smaller size of the
minigrid market.
Figure 3 : Frequency dependant power reduction as BDEW
in KACO solar inverter
But already in some inverters it is possible to
activate this control even for inverters connected
to low voltage 230V/50Hz line, per example on
the figure left, the behavior of the Kaco Powador
inverter with the parameter „Activate BDEW‟
(Medium Voltage Directive) is shown.
The power is reduced down to half from 50.2 to 51.5Hz
and stopped over 51.5Hz. It is already a good
improvement to control the solar inverter up to 50%
with the frequency. Per example if there is 1kW load, a
Powador2002 can reduce its power (nominal 1650W)
down to 1kW and match exactly the power
consumption. At that moment, the batteries are full
and the solar produced covers exactly the loads need.
Kaco is the second most sold manufacturer on the
market, and is compatible in ON/OFF or BDEW control.
Figure 4: Test bench with Kaco Powador
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Full frequency shift power control
More optimal than the ON-OFF control cited above or the partial reduction of power as BDEW, is
the complete linear variation of the power injected in dependence of the frequency. It requires
that this control is implemented in the grid connected inverter control, which is not the case for
devices of all manufacturers at the moment. When this will be widespread implemented, it will
be very interesting for the compatibility of elements in offgrid systems. For the moment the only
inverter we know to have this behaviour is the SMA Sunny Boy with the Offgrid settings; SB is the
most sold grid inverter on the market.
Figure 5: frequency shift power reduction as in SunnyBoy OFFGRID mode
Until the reference user frequency +1Hz the grid-feeding is to the maximum and at user
frequency+2Hz the grid-feeding current is zero. Typically in a 50Hz system, the solar production is
at the maximum at 51Hz or below, is half at 51.5Hz and is zero at 52Hz and over.
f<51Hz ok
51Hz< f<52Hz power reduction
f>52Hz stopped
Figure 6: System Schematics of AC-coupling
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Example of a charge curve with power reduction in function of the frequency: no PWM-like
behavior seen. The charge curve of the battery is clean.
Battery current
[Adc]
20
10
0
-10
-20
Test of SB1700 with frequency power control
Battery voltage
[Vdc]
2909:07 09:43 10:19 10:55 11:31 12:07 12:43 13:19 13:55
28
27
26
25
time [hh:mm]
24
09:07 09:43 10:19 10:55 11:31 12:07 12:43 13:19 13:55
When there are many grid inverter stopped at different frequencies, we approach this kind of
behaviour but with steps.
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Threephased configuration
The system can be realised as well with threephased grid inverter, successful experiences were
done with the StecaGrid9000.
Figure 7: Threephased configuration
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MIXING DC AND AC COUPLING: EFFICIENCY AND ROBUSTNESS
It is now well accepted that the hybrid systems offer a suitable solution for the rural
electrification. Standard configurations are now the use of a DC-bus, or an AC-bus, or a mix of
DC and AC bus. Every single system tends to be a unique mix that a project integrator optimizes.
We will focus on the mix, because there are good reasons to make not only DC neither only AC.
That is the ‘Partial AC-coupling’ concept.
Considering the efficiency, AC-coupling and DC-coupling are not similar.
The power profile determines the total efficiency again:
If there is excess solar production during the day and it must be stored into the batteries,
DC-coupling has a better efficiency.
If the solar energy is directly used, there is one conversion less with the AC-coupling.
Following computation is done to compare both cases with assumptions:
Grid inverter efficiency:
Battery inverter efficiency:
DC solar charger efficiency (with MPPT):
Battery storage efficiency:
Energy produced by the grid-connected solar inverter must be stored for the night time. When it
is given back to the user, the fraction left of the initial solar energy produced by the solar panel
is:
Energy produced during the day by the grid-connected solar inverter is directly used by user:
Energy produced during the day by the solar charger connected to DC, stored, and used later
by the user:
Energy produced during the day by the solar charger connected to DC and directly given to
user:
Efficiency on
solar energy
Energy stored in
battery
Energy directly
used
DCcoupled
ACcoupled
70.6%
66.4%
88.3%
96%
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There is not a big difference: 3.5%, between using AC or DC
coupling for energy stored in batteries, and a little bit
bigger difference on the direct use during the day: 8% at
the advantage of AC-coupling
that avoids one
conversion.
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Then the ideal case for
efficiency is direct use of ACcoupled solar energy and
storage for night time of the
DC-coupled solar.
But practically, install two
different
types
of
solar
systems, is probably not
interesting. The installer will
prefer a simpler system with
only
one
connection
philosophy even if there are a
few little percents of efficiency
to gain.
Power [Watts]
For DC-coupling, this is true only for a modern solar battery charger with MPPT included. The
values are very different if the solar regulator is a traditional series or shunt. It is claimed by
manufacturers that a MPPT can give up to 30% more energy during a day compared to a direct
connection to a battery (if the battery is never full!). In reality experiences and publications
shows that the real gain is situated between 5% and 15%.
6000
Power profiles
4000
direct use of
AC-coupled solar
storage of
DC-coupled solar
2000
Time [h]
0
0
2
4
6
8 10 12 14 16 18 20 22 24
Solar AC-connected
Solar DC-connected
Figure 8: Split solar between AC and DC-coupling
The mix of AC and DC is interesting on another level: for the robustness of the system. If AC is not
present for any reasons, the solar grid inverter cannot work. That is a weakness in the system: the
solar production depends on the proper operation of the battery inverter that creates the AC;
the battery charging depends on two components instead of one. Per example if the batteries
are empty after a few rainy days, the battery inverter stops in order to protect them. And when
the sun comes back the grid connected inverters don’t start if there is no AC. With AC-coupling
only the system can be blocked in this situation. If there is solar at the DC, it can recharge the
batteries the next sunny day and all the system can restart again. Then we recommend having
always a part of solar to DC when using AC-coupling: partial AC-coupling.
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Generator
All generators have different frequency behaviors. Generally they have a natural power control
with frequency as the frequency is higher at no load. But this cannot be guaranteed in all cases,
depending on the type of speed controller of the genset. The effect of a grid inverter pushing
power at a generator output cannot be predicted for all genset models.
Automatic
start of
genset
Figure 9: Hybrid System
The problem here is the interaction between genset and grid inverter. When the Xtender is
connected to a source at AC-input, the charging of the batteries is always correct.
In case of doubt better install a relay that disconnects the solar inverter when connected to the
generator to avoid problems. The auxiliary relay of the Xtender can perform this function with the
programming on event: 1236 Transfer relay ON (AUX1) or 1344 Transfer relay ON (AUX2)
connected with the C-NC connections of the relay. When the inverter goes to the generator the
relay is activated and goes to C-NO.
Figure 10: Frequency behavior of generators
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Hybrid systems with genset are normally planned to work with some generator running time. It
can be done in two ways:
The genset is used occasionally and only starts up is in function of the battery voltage.
The system is dimensioned for a daily use of the generator. In that case it is best to plan
the start time of the genset and use the automatic start in function of the battery voltage
as a security.
The optimum time to start the genset, is to use it at given time schedules, when there is no sun
but user loads, ideally during the evening. It is better to make a direct used of the energy
provided by the generator than running on batteries and recharging them later with that
energy. With a correct time schedule, cycling of energy in the batteries is avoided, giving a gain
in efficiency and battery lifetime.
The automatic start of a generator can be done with the auxiliary contacts of the Xtender
inverters. Time schedules and conditions on battery status can be combined for activation of the
contacts.
Optimal time for
running genset on a
planned schedule
Power profiles
Power [Watts]
6000
4000
2000
Time [h]
0
0
2
4
6
8 10 12 14 16 18 20 22 24
Solar AC-connected
Solar DC-connected
Figure 11: Best time to start a generator: when there is a load a no sun
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Tests-Experiences
Practical experiments with different manufacturers of the market are shown in this document.
First the stability between a grid connected solar inverter and a Studer battery inverter is tested
to assess the robustness of the system. This system has been used for a quite long time in a system
called Solsafe, which is a backup for a grid feeding system, see www.studer-innotec.com .
Coupling battery inverters and grid connected inverter is not only theoretical but a proven
concept. In the past, one problem observed was that there were often disconnections of the
grid inverter because of the grid tests performed (ENS). This happens because the battery
inverter has higher output impedance than the main grid. It is similar to the ‘end of line’ effect on
the main grid. But now, smarter and more stable tests are performed to detect islanding. The
new methods used allow working with standalone battery inverters as voltage source without
problems.
Various tests were performed with a Studer Xtender inverter and a Solarmax S3000 together. It
worked with the default settings for Germany for the S3000 without any parameter modification.
Similar tests were performed with a standard SMA SunnyBoy1700 and a Kaco Powador 2002. The
stability tests are: load jumps, impedance added between the units, tests of transients that
disturbs the voltage and modify impedance of the line,…
Graphic hereby shows one example
of the behavior of a system where a
S3000 is used with a XTH5000-24 using
40 meters of 10A rated cable to
connect the system together with a
high impedance (approx. 1.2 Ohm). A
load jump of 1kW is done by turning
on a halogen lamp (which has a start
current very high when filament is cold
at start-up). The voltage (yellow) is
distorted due to the high load on the
high impedance line, but the solar
inverter (current in blue) continues its
work almost without noticing it.
Voltage (yellow)
Solarmax current (blue)
Load current (pink)
Figure 12: Big load transition on the system
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Figure 13: Test bench with Solarmax S3000
All inverters don’t have the same behaviour; per example the Solarmax stops 20minutes after a
dozen of overfrequency errors when batteries are full:
Battery voltage [Vdc]
Battery current [Adc]
20
Test of Solarmax3000 with standard DE settings
10
0
-10
-20
-30
29
time [hh:mm]
10:07 10:43 11:19 11:55 12:31 13:07 13:43 14:19
28
27
26
25
time [hh:mm]
24
10:07
10:43
11:19
11:55
12:31
13:07
13:43
14:19
This behavior gives a less effective use of the AC-coupled solar during the top charge of the
battery.
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From experience a summary of the requirements on the elements can be given:
It is feasible to make AC-coupling of Studer battery inverters and grid connected solar
inverters. The presented concepts are not only theoretical but were implemented and
tested on real products available on the market. Many tests have been done to find out
the limits and problems that can occur with the use of grid connected and standalone
inverter together. Many combinations were tested and it was found robust enough to be
used in the field.
The battery inverter must be bidirectional to accept power ‘backward’ at its normal ACoutput. It must be able to control the grid connected inverter with the frequency, relays or
communication. Studer Xtender inverters can perform a frequency control, Studer
Compact inverters must use relays (as in the Solesafe S-Box system).
The battery inverter rated power must be equal or bigger than the grid connected
inverter: if there are no user loads all the produced power goes to battery inverter.
The standard grid test of the grid connected must be more ‘intelligent’ than the old
impedance measurement else there is the risk to connect-disconnect a lot of times (end
of line effect). The old ENS grid impedance test must be deactivated. New tests work
perfectly without deactivations (Solarmax S-series test, Kaco,…). Inverters with an ‘Offgrid’
mode are ok (Sunny Boy).
Optimum design for efficiency is a share of the solar modules between DC-coupling with
a solar charger and AC-coupling with a grid inverter according to the load profile.
Partial AC-coupling is better in term of robustness; it is more reliable to have at least a part
of the solar production connected directly to DC, or even only DC coupling.
The possibility to use standard grid connected solar/wind inverters within offgrid systems to
interface the solar panels or wind turbine to an island AC minigrid can facilitate the system
design. Per example it is possible to place solar panels far from the batteries and at different
places. DC cable must be short and very thick because the standard battery voltage is low (1224-48Vdc) and it cannot be used over long distances.
The advantages of the AC-coupling configuration are:
• Price/availability of the grid connected solar modules.
• Longer distance from the solar roof to the batteries is possible.
• Very good efficiency on the direct use of solar energy during the day.
AC-coupling has become very popular recently, but it is just a technical possibility not necessary
the best scheme to implement in an offgrid system, as disadvantages we can mention:
• Less efficiency if energy must be stored in the batteries for the night (double conversion).
• Multi-locations of installation.
• Price/KW of a grid inverter is much higher compared to a MPPT charger. Now MPPTs with
a larger input range make possible the use of grid connected modules as well.
• Dependence of the proper operation of the battery inverter to maintain the batteries 
problem of robustness.
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Xtender Programming
To program the frequency control with the Xtender, there are, at Expert level, parameters to
control AC output frequency in function of the battery voltage:
{1549} “Inverter frequency increase in function of battery voltage”
Yes
The maximum increase of the frequency is given with:
{1546} “Max frequency increase”
4Hz by default
Those parameters are in the “Inverter” menu of the Xtender settings.
The output frequency is changed in function of the battery voltage with the following
relationship:
Output
Frequency [Hz]
Battery cycle
voltage reference
{1546}
50Hz {1112}
Battery voltage
[V]
0.5V for 12V 1V
for 24V
2V for 48V
Figure 14: AC-output frequency behavior in function of the battery voltage for Xtender with param
1549
The normal user frequency given by parameter {1112} is used. But when the battery voltage
comes close to the desired reference voltage of the current battery cycle state (absorption,
equalisation, floating or reduced floating voltage), then the inverter increases its output
frequency up to a maximal value ({1112}+{1546}). The range over which the frequency increases
linearly is centered on reference voltage and is 0.5V in 12V system, 1V for 24V and 2V for 48V
system.
This works the same with 60Hz, setting {1112} to 60Hz. Xtender 120V/60Hz are available.
It is important to have the two frequencies accorded: if the grid inverter stops at a
given frequency, the Xtender parameter must be programmed OVER this value,
per example set 50.4Hz if the grid inverter stops at 50.2Hz.
For frequency shift control set the 1546 parameter “Max frequency increase” at
two times the limit of the grid inverter, in order to have the power production
completely stopped exactly at the wanted battery voltage. Per example the
SunnyBoy stops at 52Hz, then parameter 1546 must be 4Hz.
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ON/OFF Control
The ON/OFF control of a grid inverter can be done with a simple frequency step with the
parameter:
{1536} “Inverter frequency increase when battery full”
Yes
Instead of a frequency ramp, a step is done which instantly stops the grid inverter when above
the targeted battery voltage. When the battery voltage comes under the floating voltage minus
a given value (0.5V in 12V system, 1V for 24V and 2V for 48V system) then the frequency goes
back to the initial value.
Output
Frequency [Hz]
Voltage
51 Hz
{1112}+{1546}
50 Hz {1112}
Battery
voltage [V]
0.5V for 12V 1V
for 24V
2V for 48V
Floating
voltage
reference
Battery cycle
voltage reference
Figure 15: AC-output frequency behavior with a step when the battery is full
for Xtender with param 1536
The parameters {1549} and {1536} should not be activated both at the same time, choose the
most appropriate for your case.
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Examples of systems
Ecosun company has good experience installing this type of standalone systems in Greece. Here
are a few examples from their news (www.ecosun.gr):
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