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PostPosted: Tue Mar 11, 2014 3:15 pm 

Joined: Fri Feb 28, 2014 11:50 am
Posts: 4
Hi,

how does PVsyst (version 6.20) handle the fact that not all inverters are able to distinct between local and global MPPs?
For exampel: OptiTrac Global peak is a MPPT/algorith implemented in new inverters from SMA. Older ones, like Sunny Mini Central 5000A, does not have this function, instead it uses a conventional MPPT which cannot "see" whether a MPP is the true (global) MPP or a local one?
Operation at a local MPP will decrease the power outout from the potential output. But I am not sure how likely it is that an inverter actually will operate the system at a local MPP. It seems to me that PVsyst don't consider the issue - am I right?

Best regards,
Siri


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PostPosted: Wed Mar 12, 2014 10:20 pm 
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Joined: Mon Apr 16, 2012 7:29 pm
Posts: 1680
In the "Module Layout" calculation, PVsyst always takes the best MPP point indeed.
I don't see how to do differently: how to decide which maximum is really taken by the inverter ?

By the way this represents a very low energy uncertainty:
- the shading situations with 2 MPPt are not very significant: they only concerns the situation when the diffuse component is higher than the beam component, due to particular shading dispositions.
- for usual systems, the full electrical shadings loss (module layout calculation) is less than a very few percents during the year. The additional loss would be a fraction of this loss.
- the power difference between both points is not necessarily very high.


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PostPosted: Mon Apr 20, 2015 6:08 pm 

Joined: Thu Aug 02, 2012 3:38 am
Posts: 105
One item not on your list that is probably more influential to the array power output is the actuation of the shaded model's bypass diodes. In an array with multiple strings in parallel the first MPPT point will be the point where all the strings except the one with the shaded module are at MPP and the string with the shaded module is not contributing any power because the bypass diodes are clamped open by the high string voltage.

If the string voltage is reduced the power output will go down until the point where the bypass diodes actuate on the shaded module. At that point the string will contribute power to the array and the second MPPT point will occur. The addition of the power from the shaded string is usually more than the power lost from the rest of the array being off MPP so the overall result is a higher MPPT point than then first one.

This of course would require a high granularity of the array model to perform a simulation and a way to identify inverters that can perform the MPPT function. Not all inverters use an MPPT algorithm that looks for the second MPPT point, one reason being that it reduces MPPT efficiency so when using unshaded arrays it will reduce the system performance.


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