Long strings and the 1000V limit

[DomB]

Hi,

We're just installing a 10kWp system made up of 43 panels & an SMA ST-10000 TL inverter.

SunnyDesign recommends two strings made up of 24 & 19 panels. That fits very nicely with how we want to physically lay out those panels.

The panel's Voc is 36.7, Vmax is 28.7 and the system's Vmax is quoted as 1000V (IEC) / 600 (UL). The panels have a tolerance of +/- 3%

I have a couple of Q's:

Firstly Sunnyboy calculates the "Max. PV voltage" for the 24 panel string at dead on 1000V. It defines Max PV voltage as "Open Circuit Voltage of the string at set minimum cell temperature of -10'C". Is there any issue there given its dead on the panel's system Vmax & they have a 3% tolerance?
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Secondly, what is "(IEC)" & "(UL)"!?
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Thanks very much for any thoughts on this.

 

[DomB]

Actually, Sunny Design might be happy but I'm going to split this out into more strings spread across the two trackers and keep well below that 1000V limit

 

[BruceB]

Europe vs US?
Basic knowledge IEC / UL - Rittal - Power Distribution

 

[Guest111]

Would that be wet or dry string? seriously though anyone reccomend any good books on the subject,know nothing about solar and wouldn,t mind reading up on it,although I,m retired I like to keep up to speed.cheers guys

 

[Worcester]

@DomB, you might also get away with 3 strings x 11 and one by 10, I think from memory the STP10000 has four inputs to the 'A' Mppt and one to the 'B' Mppt

You can manually adjust that in Sunny design to get the best for you, and it will advise wether it is Ok for the inverter.

It depends upon the orientation of the panels, if they are all in the same plane, then it should be fine.

That would reduce the voltage, though obviosly delivers a higher current to the inverter.

 

[DomB]

Thanks Worcester.

Yes, I've moved it to 2 strings of 12 panels on MPP A and 1 string of 19 on MPP B.

Interestingly the predicted yield then drops from 8814 to 8717kWh. I guess, as you say, that's due to the increased current going into the inverter?

In which case I'd be keen to stick to two strings. But that 1000V just spooks me.

 

[SolarCity]

Thanks Worcester.

Yes, I've moved it to 2 strings of 12 panels on MPP A and 1 string of 19 on MPP B.

Interestingly the predicted yield then drops from 8814 to 8717kWh. I guess, as you say, that's due to the increased current going into the inverter?

In which case I'd be keen to stick to two strings. But that 1000V just spooks me.

That's quite a drop. Although I would say that the more likely reason would be the drop in voltages that is causing the loss in yield. Strings with higher voltages will generally be more efficient than strings with lower voltages.

Can't you do 23 and 20?

 

[DomB]

Thanks SolarCity.

23 and 20 is definitely better. I'm back to the yield of 8814 then. The Max PV Voltage goes down to 958V

2 Strings is cheaper for me too, less isolators, connectors etc!

Maybe I'll download a trial of PVSol or the like and see what that comes up with. It's all green lights on SunnyDesign (v2.00)

 

[whlphil]

You want to keep the voltages as high as possible, this does two things it keeps the cable losses as low as possible but more importantly you've got a better chance of the nominal being in the sweet spot for the inverter. Look at the Power One brochure against each inverter is a nice colourful diagram that shows where the sweet spot for efficiency is on that particular unit. What your aiming todo is keep the voltage range in that centre spot for longer.

Couple of things to remember key one being that the VOC is being calculated under -10˚C as stated on SBD, if your area of install is known to get colder than this on a regular basis then you need to adjust the minimum figure to what your lowest recored figure is eg -15˚C as this will raise your max VOC.

Remember that over time the system voltage will degrade so the higher you get the initial voltage (with safe limits) the longer over its life you will continue to maintain peak performance by keeping your nominal in the sweet spot. Also the higher your voltage in the winter when max VOC is determined the more likely you will be in the sweet spot in the summer when VOC will be lower.

Its all a juggling act like most things in solar theres trade offs all over.

 

[SibertSolar]

All good advice above. One thing to consider - you need to spec your DC switches according to the DTI Guide, ie: each pole rated to handle 115% of the string's Voc.....