Large scale inverter panel configurations

Tuesday, July 20, 2021
Training
by
Veli Markovic

Before looking at large commercial systems, lets take a look at the context with smaller commercial and domestic solar PV systems...

Small commercial and domestic solar systems 

With smaller commercial or domestic solar systems the ability to design the inverter string configurations for the photovoltaic component is fairly straight forward.

Methodically you need to go through all the component details:

  • Inverter specs, size, number of MPPT, number of inputs, max current, etc. 
  • Panel specs, wattage, voltage, current

Then the inverter and panel specifications are matched!

Small commercial: case study example

We have a small to medium commercial system consisting of:

  • Inverters, Fronius, Eco 27 kW x 3
  • 445 watt Canadian Solar panels
  • Goal; to maximize the amount of solar just below 100 kW

So... what do you do with the inverters?


Small commercial: string configuration 1

The process of designing string lengths etc with smaller systems are trial and error. For example:

  • Fronius 1, 18 x 445 watt x 4 strings = 33,040 watts
  • Fronius 2, 18 x 445 watt x 4 strings = 33,040 watts
  • Fronius 3, 18 x 445 watt x 4 strings = 33,040 watts

Total: 96.12 kW. Not quite there yet!

Small commercial: string configuration 2

What about another configuration:

  • Fronius 1, 18 x 445 watt x 4 strings = 33,040 watts
  • Fronius 2, 18 x 445 watt x 4 strings = 33,040 watts
  • Fronius 3, 19 x 445 watt x 4 strings = 33,820 watts

Total: 97.9 kW.... A bit better.

Small commercial: string configuration 3

Here we go again:

  • Fronius 1, 19 x 445 watt x 4 strings = 33,820 watts
  • Fronius 2, 19 x 445 watt x 4 strings = 33,820 watts
  • Fronius 3, 19 x 445 watt x 4 strings = 33,820 watts

Total: 101.46 kW. Hang on, have gone over!!

Small commercial: string configuration 4

Hopefully this is the last one:

  • Fronius 1, 18 x 445 watt x 4 strings = 33,040 watts
  • Fronius 2, 19 x 445 watt x 4 strings = 33,820 watts
  • Fronius 3, 19 x 445 watt x 4 strings = 33,820 watts

Total: 99.68 kW.... Yes, finally!

Trial and error

With small to medium systems you can use a trial and error method of determining the best fit but with larger systems involving multiple inverters and maybe different panel options thrown into the mix the time spent is considerable. So what do you do?


Hello Spreadsheets!

To make the task easier will need to take a spreadsheet approach that takes into account all the factors previously mentioned. 

Through the creative use of a few functions this process can be “automated” to a degree. The important thing is to get all the correct details of the components you are using.

Case study

Looking at larger systems using more than one inverter. In some cases projects may require 2 x PV options as well. 

So, as well as the above factors, you have all the calculations required to optimize system design with the added factor of the physical reality of the roof in which you are installing the system (or ground mount).

The spreadsheet approach...

The spreadsheet approach continued...

Have done calculation to find VocMax of the panel in question by assuming a minimum site temp of -10 degrees C. 

The calculation used the temp coefficient of the panel, the STC and Voc to derive the VocMax. 

The VocMax with buffer allows the designer to add an extra safety margin if required. In this cases the buffer is one


Next on the list

We have established that there will be, possibly, 2 x panel options and the full details have been entered into the spreadsheet. 

Now for the inverters. On many big jobs due to various factors, multiple sized inverters, usually of the same brand can be used. 

In this we are using 100 kW and 30 kW inverters and we now need to input into the spreadsheet the inverter details.

Inverter details

Panels and Inverters

So we have Option 1 and 2 for the solar panels, a 445 and 450 watt option. In most cases one panel model is used on these kinds of projects but we had a recent case where three different panel brands were stipulated!

For the inverters we have a 100 kW unit and a 30.

Inverter details continued

As we are selecting inverter 1 as our first option, column 2 signifies this and acts as a trigger.

Column 4 is the inverter number, in this case one as we are using multiple units

Inverter 1 has 6 x MPPT and 2x inputs per MPPT as indicated in column 5 and 6



Signage is indicated in column three and is automatically generated by a formula that can be dragged down that references:

  • Inverter no.
  • MPPT no.
  • String input number


Panel details

The first column references the panel option which populates the second column.

The fourth and fifth columns do a basic calculation to determine:

  • VocMax of string
  • Wattage of string

"Xlookup"



We now have to determine the Vocmax with the buffer of the panel; and therefore of the string.

I have used a Xlookup function that asks the question if the number ( see arrow) is 1 then search the solar panel option details area, if number is 2 the same thing happens. 


These formulas can be copied to other cells to achieve the same results...


Total wattage of inverter and DC/AC ratio


So for this inverter we can see the  wattage per string and total for the inverter. 

Also we can set up some conditional formatting to warn us if the DC/Ac ratio is exceeded

DC/AC ratio exceeded

In the case above you can see that the 30 kW inverter, number 4, has exceeded the DC/AC ratio considerably so will you need to modify the number of strings, length of strings, panel used or all?

So, to rectify this we have: 

  • Changed the panel, lower VocMax
  • Have reduced the number of strings
  • Have increased the number of panels per string
  • From 60.52 kW to 34.2 kW

See below...

Final results:

So, looking at the outputs above, we have the following:

  • Inverter 1: 100 kW total wattage 128, 160
  • Inverter 2: 100 kW, total wattage, 113,850
  • Inverter 3: 30 kW, total wattage 32400
  • Inverter 4: 30 kW, total wattage 32400

... Which means:

  • Grand total of 308,610 watts
  • Using 2x different inverters
  • Using 2 x different panels
  • Different string configs

Conclusion

The inverters used have a VocMax of 1500 V and a rated input voltage of 1080 V. Triggers can be used with conditional formatting to show cell colour change if certain parameters are exceeded.

We have barely touched on what can be done and in further presentations we will look at more advanced techniques which can help the CEC designer / CEC installer put together these large scale systems more easily using a spreadsheet approach.

If you’d like to see what Greenwood Solutions get up to in the real world of renewable energy, solar, battery storage and grid protection check out our industry and commercial pages:

https://www.greenwoodsolutions.com.au/industry 

https://www.greenwoodsolutions.com.au/commercial

About the author

Veli Markovic

CEC Designer
Veli has nearly two decades of experience in the renewable industry. He is passionate about providing people with valuable education and is highly regarded throughout the industry as an educator and operator.
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