Author Topic: The Trouble With String Invrters  (Read 4912 times)

LowGear

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The Trouble With String Invrters
« on: January 22, 2018, 06:51:42 PM »
Wow!  My electric bill jumped up above $300 again.  Runing the pool pump too much?  Too many hot showers?  No, a couple of trips out to the SMA inverter and it's obvious that one of two panel strings are not working. 

I've cleaned all the rats nests out but still three KW at noon prevails.  The inverter has two three KW strings so no math majors required.  One of the strings is down.

So I'm sure three KW at about 270 volts is pretty dang dangerous.  You're advice - warnings - heads-ups would be appreciated.

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carlb23

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Re: The Trouble With String Invrters
« Reply #1 on: January 24, 2018, 11:38:11 AM »
Casy,

What sma do you have?  If they have the dc disconnect on the bottom of the inverter you need to check the string fuses in the dc disconnect.   They will be located on the upper left side of the disconnect if i remember and there should be 4 of them.  be careful as the voltage and amps in there can be quite high.   you must open the circuit by turning the know then removing the screw from the know through a little hole in the ring around the knob.  Remove the one screw in the bottom of the disconnect and remove the cover.  you will see the 4 fuse holders in the upper left hand side of the disconnect.   pull each fuse and check for continunity.  if you don't find the problem there then you will have to check the voltage on each string.   Again be careful those strings are probably in excess of 250 vdc at 7 or 8 amps.

Since you have two strings connected you can just steal on of the other 2 unused fuses from the disconnect.  The fuses are only about 1.50 dollars on the mainland.  if you have the installation manual you will clearly see the fuses in the photos in the manual.   Ask me how I know
« Last Edit: January 24, 2018, 11:46:22 AM by carlb23 »

LowGear

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Re: The Trouble With String Invrters
« Reply #2 on: January 24, 2018, 05:34:55 PM »
Yes I will be a better person and check the inverter more often.  At the current time it's every morning and I'm hitting around 3KW - 250X15=3750 - so I'm not doing too bad for 10 year old panels.

It's a SMA 6000 I bought used about 10 years ago.  Don't ever use UPS to ship 150 pound inverters across the ocean.  I must have forgotten the fuses.  I'll check them as that sounds easy.  I did install the unit so I can remember there being just a tease to get the switch handle off to enter the box.  I might get wild and crazy and check the the connections as well. 

Here's an interesting question.  If I throw a tarp over the panels or many of them will that arrest the total strings generation?
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carlb23

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Re: The Trouble With String Invrters
« Reply #3 on: January 25, 2018, 11:31:13 AM »
once you open the disconnect everything past that is dead but  the string connections going to the disconnect are still hot. Don't forgot to open the ac disconnects as well.

« Last Edit: January 25, 2018, 11:48:27 AM by carlb23 »

mike90045

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Re: The Trouble With String Invrters
« Reply #4 on: January 25, 2018, 03:20:22 PM »
Be very careful to not ruin (via DC arc) the gold flash plate on the solar panel MC connectors.

Best practice is to Shut the AC off, which stops most of the power draw from the Array. Then the DC disconnect, which will stop all DC current (voltage is still there)  Now with current at 0, you can disconnect the panels and not draw an arc.  Each panel will still have it's rated voltage, and even 40Vdc can bite you

LowGear

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Re: The Trouble With String Invrters
« Reply #5 on: January 25, 2018, 04:54:05 PM »
Thanks for the input.  I'm pretty careful with DC once it gets past 24 volts and never underestimate the damage arcing DC can do.  I think with enough current they call it welding which can burn.  The string still has the potential to deliver around 300 volts (15 Panels X24 Volts > 300).  Dangerous stuff for a guy that is just recovering from hooking a black wire to a red terminal on a UPS.  I still stop in the garage and talk to myself about that afternoon.

Saturday morning is SMA SB6000 day.  I want you all to be thinking fried gecko and blown fuse.

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LowGear

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Re: The Trouble With String Invrters
« Reply #6 on: January 26, 2018, 07:15:56 PM »
I've got the talking to myself pretty well mastered.  Saturday morning will be a good day.

My neighbor that has the masters in EE recommends extreme caution as Edison once "Fried" a live elephant to show how wrong Tesla was.  Modern day genius but not a good day care provider.  The neighbor also remembered something he needed to do Saturday morning. 

I got 14 KW ($4.20) yesterday which isn't bad for a single string of 250X15=3750 * 5 hours = 18KW on 10 year old panels.  28 KW would have been nicer.
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LowGear

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Re: The Trouble With String Invrters
« Reply #7 on: January 29, 2018, 07:27:17 PM »
Carlb23 knows his stuff but most of us already knew this.  Pulled the DC switch cover off the Sunny Boy 6000 US and there were the fuses.  This unit can support 4 strings and each string has it's own fuse.  Pulled the first one and continuity was there.  Pulled the second one and continuity was 0.  Pulled the third fuse and continuity was there so I swapped it out with the open fuse and a couple of minutes later both strings were producing.  Not a bad birthday present.  I haven't checked today so there's excitement in the air.

Another pleasant surprise was the fuses are $3 each on eBay - free delivery.

What confused me was I couldn't get any voltage reading off the strings? 

Thanks again to you all and I've applied to carlb23 Hawaii fan club.

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BruceM

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Re: The Trouble With String Invrters
« Reply #8 on: January 30, 2018, 06:01:35 PM »
Glad you found your fuse problem Casey.  CarlB has impressive grid tie solar capacity and experience, as well as ingeniously simple NG Listeroid conversion/dual use, and DIY solar hot water systems (both flat panel and tracked parabolic) as well.  He has done it all, and very nicely.

I like the MC4 connectors that are now standard; cheap and fairly safe to handle with higher voltages, and easy to add new connectors.  As Glort noted it does require some thought to make sure you get panels in series or parallel properly.  With as many panels as Glort, I'd have to have a map and connection diagram!


BruceM

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Re: The Trouble With String Invrters
« Reply #9 on: January 31, 2018, 05:20:32 PM »
That seems like lots of PV and wiring to me!  I've lived with 875 watts  (5 panels), off grid,  for a decade. Just upgrading by adding 1500 watts more now.  Helps to be in AZ where clear skies are the norm.

Back to stuffing circuit boards and soldering today, I'm updating my (10) battery regulators with the new pcb's with fancy silk screen and solder mask, adjustable bulk, float, equalize voltages, etc.  I got the PCB's for peanuts as an add on when ordering new boards for my neighbor's setup.  My present battery shunt regulators are the original bare prototype boards with plenty of mods. 


carlb23

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Re: The Trouble With String Invrters
« Reply #10 on: February 01, 2018, 10:15:57 AM »
Carlb23 knows his stuff but most of us already knew this.  Pulled the DC switch cover off the Sunny Boy 6000 US and there were the fuses.  This unit can support 4 strings and each string has it's own fuse.  Pulled the first one and continuity was there.  Pulled the second one and continuity was 0.  Pulled the third fuse and continuity was there so I swapped it out with the open fuse and a couple of minutes later both strings were producing.  Not a bad birthday present.  I haven't checked today so there's excitement in the air.

Another pleasant surprise was the fuses are $3 each on eBay - free delivery.

What confused me was I couldn't get any voltage reading off the strings? 

Thanks again to you all and I've applied to carlb23 Hawaii fan club.

Aloha

Thanks for the kind words.  If you are only using 2 strings then you already had two spare fuses.  You wont get any voltage off the strings in the dark  :D.   Did you check for voltage where the strings connect to the disconnect or where they go to the inverter? 

LowGear

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Re: The Trouble With String Invrters
« Reply #11 on: February 02, 2018, 07:59:03 PM »
While carlb23 had the fuse right but there is still something wrong.  I took a tarp up on the roof and covered the new string (it's about 4 years newer than the original string..)  and I was still getting about 2.8 KW.  Covered the original string and the inverter started shutting down - lack of voltage.  So the new string has a problem.  I can't write the words that come to my mouth.  The panels are snug.  Saturday shall be another day of challenge.  I sure wish I knew more about testing panels.
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BruceM

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Re: The Trouble With String Invrters
« Reply #12 on: February 03, 2018, 12:56:13 AM »
Disconnect the new string from the inverter and check the open circuit voltage.  Should be around 44V per nominal 24V panel in series.  If it's  reading no volts, look for a bad connection.  Disconnect each panel's MC4 connectors and confirm open circuit voltage of 44V and short circuit current- I suggest using a meter with fused 20A DC current capability.  The short circuit current should be in the ballpark of your panel's specs or within 75% in full sun. 

That should give you a good idea of where the problem lies.




carlb23

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Re: The Trouble With String Invrters
« Reply #13 on: February 03, 2018, 01:10:35 PM »
Disconnect the new string from the inverter and check the open circuit voltage.  Should be around 44V per nominal 24V panel in series.  If it's  reading no volts, look for a bad connection.  Disconnect each panel's MC4 connectors and confirm open circuit voltage of 44V and short circuit current- I suggest using a meter with fused 20A DC current capability.  The short circuit current should be in the ballpark of your panel's specs or within 75% in full sun. 

That should give you a good idea of where the problem lies.

+1 on what bruce said