Two identical systems, one 20% down...

[Energetic]

Morning all,

Came across an interesting one yesterday. On a farm, two P1 27.6kw inverters with identical arrays that used to have the same output, since nov2015 one is running about 20% down as i witnessed.

Immediate assumption was that a string was down but string voltages tested out fine, 6 strings max being 723v and min being 695 in changeable conditions.

I tried swapping the arrays and managed to move the low output to the other inverter which indicates to me that it is an array issue but with the strings testing out ok i'm not really sure where to look.

No Fault codes or issues indicated on the inverters.

If anyone has any ideas i'd be greatful to hear them

Thanks, hope your 2016 is going well!

 

[SolarCity]

Are they literally in identical conditions? And have they been through a winter previously?

Just wondering if one of the arrays is suffering from some mild shading which only hits in the winter months?

 

[whinmoor]

Yes, shading is a strong possibility.

Is any monitoring set up? Could you compare the two graphs? You might find the two arrays perform similarly on overcast days but there's a significant difference on a sunny day. The shading may only be apparent at the start or end of the day.

 

[SibertSolar]

Assuming you tested the strings whilst open-circuit, did you monitor the "on-load" voltage and current values reported by the inverters too? Just curious as sometimes panels can behave differently when actually outputting current. Shading is most likely to be the culprit though tbh. Unless perhaps one of the inverters is getting a good bit warmer than the other, and is de-rating at times? Any monitoring data?

 

[Energetic]

Hi all, apologies for being slack.

No shading at all, no monitoring either. Had a call from the farmer the following day to say that it's back to normal. So I still don't know what it was but I seemed to have fixed it by messing around with the mc4 connectors and dc isolators.

Bit of a mystery but he's happy... nice to have one go your way. ..

 

[Gavin A]

Is this a power-one / ABB?

did you check the continuity on the string fuses inside the inverter? those figures could easy be explained by one string fuse being down in the inverter. Other potential is bird crap on one of the strings of panels to such an extent that it's knocking the MPPT off track.

 

[Energetic]

It's not far from his silage and the panels could do with a wash but all of them seem equally dirty, i have advised that they be cleaned.

They are the Power One Trios with the string fuses but ABB told me they are monitored and would alert, that said i didn't put them back in the same inputs so it may be a possibility.

 

[Markos]

Hi, bird poo!! I believe the sat dish guys call it white dielectric material, can cause hot spots on PV cells, looks better on my job card though. However we do have some roof mounted systems that suffer badly from this, also birds nesting under the panels. I agree with earlier post, measure voltage and current under load.