8/23/03
The Sunny Boy family of inverters is probably the most
popular grid-tie inverter on the market today. With the large number
of systems using these high voltage series-string inverters, questions
often arise about how many PV modules to use with these inverters along
with other design and installation questions. This article will present
guidelines for proper sizing and installation of these inverters
Basic Design Criteria
Voc, Vmp and Imp
The most common question asked is how many modules to use with these
inverters. There are three basic design criteria that need to be met.
First, each series-string must have
an open circuit voltage (Voc) that is less than the maximum allowable
input voltage of the inverter; 600 VDC for the SB2500 and 400 VDC for
the SB1800. Since the Voc of PV modules increases as the module becomes
colder, you need to design the Voc at the coldest temperature the module
will experience during daylight. The next factor is to make sure the
volatage at maximum power (Vmp) of the series-string is equal to or
greater than the grid voltage. This will insure that the inverter can
maximum power point track (MPPT) the PV modules. The Vmp of PV modules
decreases as the temperature of the module increases so you need to
confirm that the array output current (Imp) is less than the inverter's
DC input current limit; 13 amps for the SB2500 and 12 amps for the SB1800.
The Imp of PV modules varies only slightly with temperature changes.
If these three basic design criteria are met, the PV array should operate
the Sunny Boy with reasonable reliability.
String-Sizing Program
If you are not comfortable with or you do
not want to do the necessary calculations to determine the correct values
for Voc, Vmp and Imp of the PV series-string, SMA America (Sunny Boy)
has a string-sizing program on their web site (www.sma-america.com)
that works very well. When using their program you select teh specific
inverter and module model. You then enter the minimum and maximum ambient
temperatures that the modules will experience during daylight hours.
The program then displays the full range of series-string combinations
that will work witht the PV modules you have chosen. You can easily
change one factor, like the module size, and rerun the sizing. By playing
with the various input factors, you can determine the optimal inverter
and module configuration for a given application. Some module sizes,
like the Sharp 185, sometime match up better with one inverter than
another, like the SB1800 rather than the SB2500, so definitely play
with the varous combinations. This program is highly suggested to help
with your design - Its quick and easy and it works!
Amorphous Modules
There is a special consideration when using amorphous silicon
modules such as Uni-Solar modules. For the initial 8 to 10 weeks of
operation the Voc of these modules is about 11% higher than the published
value. You will need to factor this additional voltage into the design
calculations to avoid exceeding the 600 VDC maximum DC input voltage.
Be advised the SMA sring-sizing program does
not consider this initial higher Voc, so you
may need to limit the maximum number of modules per series-string to
avoid an over-voltage condition.
Other Considerations
PV Array Size
There are other factors that effect
how well the sytem operates. If these factors are not addressed then
net power production could be reduced and/or nuisance errors may occur.
First SMA strongly recommends limiting the maximum size (in peak watts
@ STC) of the PV array powering an inverter to between 112% to 120%
of the inverters rated output. The maximum recommended array size for
the SB2500 is between 2800 Wp and 3000Wp and for the SB1800 between
about 2000 Wp adn 2100 Wp. Although a larger array could be connected
to these inverters without exceeding the maximum input voltage or current
limits, the inverter will limit the output power to 2,500 watts and
then has to dissipate additional power as waste heat. This additional
heating causes the inverter to thermally de-rate itself to prevent over
heating. When it de-rates, the maximum output power is progressivly
reduced from 2,500 watts until the thermal condition is corrected. So,
installing too large of a solar array will actually reduce the net AC
power produced and also increases the cost of the system.
Cooling
For installations where the Sunny Boy may not have adequate airflow
for proper cooling of the heat sink (like many indoor installations),
SMA makes a small fan kit called the Sunny Breeze. It attaches directly
to the heat sink and is thermostatically controlled to turn ON whenever
the heat sink temperature reaches about 65° C.
DC Input Voltage
Another design factor that effects net AC power production is the actual
DC input voltage powering the sytems. The lower the DC input voltage,
the greater the conversion efficiency to the AC output. There is about
1.5% increase in conversion efficiency operating the SB2500 at 300 VDC
compared to operating it at 500 VDC. The inverter will operate correctly
at either voltage, but more power will be delivered by operating it
at the lower end of its voltage range. Because the cost of the inverter
is typically a minor fraction of the overall sytem cost, you won't save
money operating the inverter at the maximum ratings. It is better to
design the system to operate the inverter at 80% to 90% of its rating.
AC Output Wiring
Proper sizing of AC output wiring is critical to trouble-free system
operation. There needs to be less than a 1% to 1.5% voltage-drop in
the wireing from the AC terminal of the inverter to the utility meter.
UL standards require the inverter to disconnect from the grid if the
grid voltage, measured at the inverter, exceeds a definded range (-12%
to +10% of nominal line voltage). If there is a significant voltage-drop
across the AC output wiring, the inverter has to increase its output
voltage to compensate for this drop in voltage. In some locations, the
utility power voltage tends to be on the higher end of the allowed voltage
window, if the inverter has to increase its output voltage even higher
to compensate for large line-losses, its output voltage can excede the
UL limit. This can lead to nuisance disconnects, loss of power production
and customer dissatisfaction.
SMA Web Site
In closing, we suggest you visit the SMA web site (www.sma-america.com).
Aside from the string-sizing program there is also a good section on
technical articles, a FAQ section, on-line manuals and software downloads.
In particular, check out the technical updatee on the SquareD HU361
DC Disconnect switch. There is UL approval for using up to 11.5 amps
(Isc) per pole on this 3-pole switch. Its wire lugs are approved for
two #10 wires per lug, so this disconnect could also act as a combiner
box. This can help simplify installation and reduce costs.
