String voltage low, do I have a panel or two down?p

[Digon]

I noticed today that my 16 panel system which was installed 14 months ago was showing a lower inverter input voltage than what I remembered.
The system is 2 x 8 Hyundai 245 panels in parallel with a Power one 3600 outdoor inverter.
Today was totally cloudless and the inverter was showing 215 - 217v and about 12amps input for most of the afternoon when I was checking it. The data sheet gives 30.3 v and 8.1A at Pmax which would be about 242V which is what I normally see IIRC.
The panels have no shading at all and look clean from ground level.
Just checked again now sun is starting to fade and its running at 224v and 7.6A which is nearer what I expect to see if I am interpreting the meaning of the data sheet voltage/current at Pmax figures correctly.

Is the system faulty?

 

[Gavin A]

voltage reduces by 0.45% as panel temperature rises, so what you report is to be expected.

As an aside, I wish installers would ensure customers are given at least a basic understanding of how the technology works - then again that'd mean the installers would need to understand it first.

 

[infinity]

Also, when the system is running it will find the MPP range which means the voltage could well be lower if it means a better voltage/ current combination for power output. Which is why when the sun started going in the voltage began to rise closer to the open circuit voltage level

 

[Digon]

Thanks for the replies. I'm glad my assumptions where wrong!

I wonder how much it would cost to get the installer back to rewire the system to split the two strings onto separate mppts. Then it would be easier to see if there was a problem or not.

 

[balbecdaze]

It would cost a lot, the wiring bit would cost tuppence but if your inverter's this one, you would need to get a new inverter with two MPPT inputs.

 

[Gavin A]

wrong inverter. the out-d version has dual MPPT. How much it'd cost to swap it depends on how they;ve wired it - does it have 2 or 4 wires going into the DC inputs?

if 4 then it's a 5 minute job to swap it to dual MPPT, if 2 then they've probably joined it at the roof, so would involve roof work.

 

[Digon]

It has been joined at the roof. It has two dc cables from roof to the dc isolator then four dc cables from there, the short distance, into the inverter.
Do you think it would need scaffolding to alter the connections under the panels. Its a modern detached house with 25 degree concrete tiled roof.