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Sizing a Solar Grid Feeding system (SGI)

Sizing of an SGI is surprisingly simple, the first step is to work out how much money you would like to spend, this will generally determine how many solar modules you can use. After you have determined this follow these simple steps on how to configure the modules.

Any grid feed inverter will have a DC Input voltage range. Use the maximum value and divide it by the maximum open circuit voltage of the solar module, normally around 22volts for 12V nominal modules. This calculates the number of modules allowed to be wired in series. Keep in mind that the open circuit voltage of a module will increase with colder temperatures.

Then ensure that the Maximum Power Point (MPP) voltage of the total modules in series does not exceed the MPP voltage window of the Inverter. Keep in mind that the MPP voltage will decrease with increased temperatures. You must ensure that the total series MPP voltage always remains at least 25% above the minimum MPP voltage of the inverter. If you do not do this it could mean in hotter temperatures the MPP voltage can fall below the operating window of the inverter and no power will be fed to the grid.

 

A Grid Feed Inverter works at a much higher DC input voltage than most battery connected Stand Alone Inverters. The following parameters must not be exceeded.

1. Maximum DC Input Voltage- consider the Open Circuit voltage of the solar module.
2. DC voltage window of Maximum Power Point tracking. (MPP) - consider the MPP voltage of the module multiplied by the no of series modules.
3. Maximum DC input Current- consider current at MPP voltage multiplied by no. of parallel strings.

Solar modules will firstly be connected in series strings and then multiple series strings will be connected in parallel.

EXAMPLE: We have chosen to use an 80watt module and would like to use the 2500watt SEA inverter.
NO OF SERIES SOLAR MODULES The main limiting factor is the maximum input voltage of the inverter.
= 140V /22.1V =6.3(Max input voltage divided by the module open circuit voltage (BP80watt))
=6 series modules x 17.6V = 105.6V (series modules multiplied by MPP voltage of the module). This figure must be within the MPP range of the Inverter. If not then reduce the number of series modules.

MAXIMUM NO OF PARALLEL SOLAR MODULES This is limited by maximum power and the maximum current the inverter can accept. The inverter will limit the power drawn from the modules if the total array power is greater than the inverter maximum rating.

6 MODULES IN SERIES X 80WATTS = 480WATTS per series string
2750watts / 480watts ( Max DC input power of inverter divided by output power of series string)
= 5.7 (say 5) parallel strings
= a total of 30 x 80 watt modules connected to a SEAG-110-2K5.

 
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