Power sharing between mppt's not working

[Niels]

Hello, I currently have a problem with the power sharing function in pvsyst

normally I use full inverters but due to a single string with less modules I need to use the multi-mppt feature

 

Some information about my system

• 12 250kVA inverters, 144 mppt's in total
• 1 string of 24 modules, 610Wp
• 243 strings of 25 modules, 610Wp
• DCAC ratio of 1,24

 

 

PVsyst number 1

I used multi mppt feature but did not select power sharing. the 24 modules string has 1 mppt and the 25 modules 243 strings have 143 mppt's.

This results in a pr 84,74% with  a inverter loss over nominal inv power of 1,3%.

The inverter loss is rather high for such a low DCAC ratio.

 

PVsyst number 2

I used multi mppt feature but did did select the power sharing function. the 24 modules string has 1 mppt and the 25 modules 243 strings have 143 mppt's.

This results in a pr 84,74% with  a inverter loss over nominal inv power of 1,3%.

Attached is an image of the settings of the power sharing. Despite enabling power sharing it did not result in a different outcome.

 

PVsyst number 3

I used the multi mppt feature for the first string of 24 modules, for the strings of 25 modules i disabled the feature, instead of using 143mppt's i used 12 inverters. In theory i now use 1 mppt too much but I did it to test if disabling the multi mppt feature would result in more realistic results.

The result from pvsyst 3 is a pr of 85,64% and a inverter loss over nominal inv power of 0,1%.

As stated, with disabling the multi mppt feature the results of the pvsyst are far more inline with what we see in practise.

 

 

If I missed something, please let me know.

How to fix this issue? for this specific case I can work around it but there are more pvsysts where I need to use the mppt feature a lot.

[André Mermoud]

You have 244 strings on 144 MPPT. I.e. 100 MPPT with 2 strings and 44 with one string. 

This means  244 x 25 modules = 6'100 modules - 1 module.

First option: you neglect the string with 24 modules, and define one only sub-array with 12 inverters with "Use MPPT feature" unchecked.  This will give a quite reasonable result with an error of 1 /6'100 = 0.2 per mille.  The result will be fully reliable in PVsyst.

Second option: the problem is that the PNom sharing within an inverter cannot be used when this inverter PNom is shared with other subarrays. In this case you have to define sub-arrays with a number of strings divisible by the number of MPPT inputs.

Therefore if you want something more sophisticated, you should define 4 sub-arrays:

- SubArray #1 with 11 full inverters and 223 strings  (uncheck the option "Use Multi-MPPT feature).  I.e. you leave 21 strings for the last inverter.

- SubArray #2 with 9 MPPT inputs and 18 strings

- SubArray #3 with 2 MPPT inputs and 2 strings

- SubArray #4 with 1 MPPT inputs and 1 string  of 24 modules.

and you will define an explicit PNomsharing between the subarrays #2, #3 and #4.

Hopefully this should give the same result as the first option, minus 0.02%.

 

[Niels]

Hello Andre, 
Thanks for the answer.

While I agree that the first option is a fairly practical solution it sadly isnt usually applicable due to needing the exact same amount of modules and strings as in a customers contract.

Next time this comes up I will try the second proposed solution. I do have 1 question with this, namely: doesnt using many different subarrays slow down the simulation speed?
Quite often I find myself in the situation where a 40MW subsection of a project already takes multiple hours with only 2-3 subarrays, doing it in this manor would probably lead to many more subarrays.