Solar (PV) interconnection

[adobejoe]

I see most schematics of systems with multiple strings, and several conductors to the inverter. I would like a single pair of conductors, so I only have one set of wires over 125 ft. The panels are 200 W, 24V DC. I would like to connect all 20 in series, for 480 V DC. Does this make sense?AdobeJoe

[Ungrounded Lightning Rod]

Is there anything that could shadow part of 'em?

[wpowokal]

What do you plan to use to step down to battery voltage?

allan

[richhagen]

It will depend upon the specifications of your inverter, and of course the maximum series voltage rating of the panels.  You will need to ensure that the panels have robust bi-pass diodes in the event of partial shading.  480VDC is a bit scary to me.  A picture posted on this site by Electronbaby a while back comes to mind, it showed an electrical arc of several inches.  I believe that was from somewhere in the 300 volt range.  When you start switching and fusing high voltage DC, the equipment becomes expensive just because of this.  Unlike AC, the current is continuous which makes it harder to break any arcs formed.  Rich  

[RandomJoe]

Depending on the inverter, you may have to go that high anyway.  Pure grid-tie systems (no battery bank) can have quite high voltages coming in from the array.  Some of them won't start working at all until the DC voltage rises to about 250V.

If your system is battery-based or just uses a lower voltage, then you can't (feasibly) go higher than the max the inverter can handle.  The inverters like this that I know of are generally around 150VDC max - and don't exceed it, or poof!

As for multiple conductors to the inverter, you can also just combine them at the array with a combiner box (a bit like a breaker panel, even has breakers for each string) then run the total power back on a single set of wires.  You just have to size the home run properly for the current required.

[watermanhfl]

Hi,

The comments above are good but need to go further.  My 18 panels go to Outback inverter at about 80 volts winter and 65 volts summer, then to a 48volt a batttery bank. But at start up in deep NY winter they can be over 105 volts. I have 6 parallel sets going thru 6 large #2 wires, from two combiner boxs.  Need full size ground these days. over 150 ft to house.  

24 volt panels can go way over 30 volts in cold start up so you need to size for max voltage not nominal.  Start with invertor you plan to use and work backwords to determine best panel, number of panels and how many parallel strings.  All 20 panels in one loop sounds high for any current inverter.  Splitting in 2 strings may be a bit light on voltage.  If you are having 1 or 2 strings then NO shading allowed!

Research some more as big money is involved and you want max performance.  Inverter sites have spring sizing tools.

Regards Ant

[cardamon]

You MUST USE the string sizing chart or software from the inverter manufacturer.  It will ask for the make and model of your modules and local weather data.  That is simply how it is done, dont guess.  The way it often works out, the window for number/size of strings is actually pretty small for year round assurance that: A)  you wont fry the inverter on a cold winter morning, and B) you will have enough voltage for the inverter to start or at least run with MPPT functionality, and C) Utilize the full wattage potential of your panels.

[adobejoe]

Thanks for your input. NO shading will happen. NO batteries, it is grid connect. Thinking of 20 KYO 200 W panels. Most inverters are rated to 600 V DC, so I should be fine at 480 will all in series. I am in Wyoming. Winter will be higher voltages.

AdobeJoe

[cardamon]

That is way to much voltage.  Your panels can and will reach 40 volts each on a winter morning, which is 800 volts all in series.  Then on a hot day the voltage will be HALF of that.  Solectria inverters typically have a 200-550 voltage range, so when you consider that if you have a max temp of 100, and a minimum of -5 then your voltage will double between the two extremes, thats not a lot of play room.  Thus you can see the importance of consulting a string sizing chart.

[adobejoe]

Okay Cardamon, I will be very careful with the inverter spec's, and work it that way. Our temps can swing well below zero, heck we just had minus 20 last week in this storm.

Now, another concern. I already have a 10 KW bergey excel on line, so this would bring the total capacity up to 14 KW (although I have seen the bergey kick it up a notch!).  My concern is I am feeding a 15 KVA pole mount transformer. Is this enough transformer for the job? I think it should be fine, but am a bit concerned about my power company. Thoughts

AdobeJoe

[ghurd]

You sure about that 15KW transformer?

My house (the whole neighborhood) is 100 years old and my house has a 200A service drop.

This house never had a gas stove, and at times had an electric stove.

Not sure a 15KW transformer would power an electric stove and clothes dryer at the same time.

I can not find Electronbaby's pic Rich mentioned above...

But if you saw it, it would scare the poo out of yoo!

DC at 480V is a permanent solution to many problems, like life.

G-

[ghurd]

EDIT:

"This house never had a gas stove, and at times had an electric dryer"

[joestue]

Monitor the line voltage and alert your power company if it get pushed over the +10% mark.

I don't have a number to give you but a 50% overload is still going to take several hours to push the temp up over 80C inside, there is a lot more oil inside those things than it appears from the ground.

[GaryGary]

Hi,

As was mentioned above, you have to use the string inverter sizing software provided by the inverter company -- this is no big deal  -- its free and easy to use.

For example Fronius offers this one:

http://www.fronius.com/cps/rde/xchg/SID-AE3DCD51-29192085/fronius_usa/hs.xsl/2714_8236.htm

For example, if you put in the IG4000 inverter, and Kyocera KC200GT at 200 watts, it will give you all the combinations of this panel you can use.

One of the ones they recommend is 2 strings of 10 for 4003 watts.

For this KC200GT there is no way you could hook 20 in series -- the open circuit voltage for this panel at -10C is 37.3 volts, or 746 volts for 20 in series -- way over the inverter rating.  And, for your area, -10C is still way to warm.

Anyway, you need to run the software for the inverter and PV panel you want.

Be sure to check off the "Coldest Ambient Temp"at -40C before you do the run.

Gary

[cardamon]

Maybe you should talk to your utility and run it by them just to be safe.  15kw is a pretty standard size residential xformer I think.  Interesting how 200 amp services became standard when you consider that you will never get close to that, even intermittently, and that your 15kva xformer can in theory only supply 65 amps continuously.  Thats a fair amount of juice you are feeding back, make sure you are not exceeding your NEC bussbar rating if that the way you are connected.