VOC and ISC test
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firefly
Solar Expert Posts: 66 ✭✭
Can someone explain to me how I can do a VOC test and a ISC test on a Sun Tech 175 watt panel. I have a good clamp on DMM. I think there may be a bad panel in a 7 Panel array wired in parallel, that I have been asked to look at.
Steve
Steve
Comments
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Re: VOC and ISC test
VOC = voltage open circuit - disconnect the string and measure voltage.
Isc = Amps short circuit - disconnect the string, and shut the array down with blankets, tarps, whatever because you do not want to arc weld wires together. Short the - & + feed wires, uncover the array, and read the amps with the clip-on meter. Cover the array up, and remove the short.
re-commission the array or scrap the bad panelPowerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A
solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister , -
Re: VOC and ISC test
Why are the blankets/tarps necessary? Once the array DC disconnect is opened, no current is flowing, so you can disconnect the conductors of the suspect module. You can short and unshort a single module in sunlight without arcing; measure the Isc (shorted) and Voc (open circuit) and make the determination. If a module is bad, open the string OCPD on the string it is in and you can turn the rest of the array back on.VOC = voltage open circuit - disconnect the string and measure voltage.
Isc = Amps short circuit - disconnect the string, and shut the array down with blankets, tarps, whatever because you do not want to arc weld wires together. Short the - & + feed wires, uncover the array, and read the amps with the clip-on meter. Cover the array up, and remove the short.
re-commission the array or scrap the bad panel -
Re: VOC and ISC test
If the voltage is ok on your panel, why would you need to see what the ISC is? why would that matter if your trying to find a bad panel? The voltage alone should be the only test right? I'm just asking this question because based on what i know, finding out the ISC doesn't seem like it'll help if your trying to locate a panels that's throwing the whole array off. -
Re: VOC and ISC testUrbandialect wrote: »If the voltage is ok on your panel, why would you need to see what the ISC is? why would that matter if your trying to find a bad panel? The voltage alone should be the only test right? I'm just asking this question because based on what i know, finding out the ISC doesn't seem like it'll help if your trying to locate a panels that's throwing the whole array off.
A panel can hit its Voc without being able to produce its full current so it is necessary to test both.
A panel producing its full Isc can arc when you pull the wires apart. Keep in mind that for DC a sustainable arc can be made with Voltages over 15, which most panels produce for their Vmp. Couple that with possibly 8 Amp Isc and you do have an arc potential. Mike's testing procedure is safe and sound. -
Re: VOC and ISC test
Most volt meter take very little current... So you could have a burned area in the panel that has enough carbon to read Voc on the panel but only pass a few milliamps.
The Isc test makes sure that you have a solid bond...
However, remember there are bypass diodes too... If you are still having problems, you should connect them to a load/battery that can maintain somewhere around Vmp or a little less. That is the only true measurement that will give 100% pass on a panel. You could have a failed section in the panel (burned high resistance, but not open) and still pass both Voc and Isc testing.
Voc/Isc will probably find 80-90% of the bad panels.
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: VOC and ISC test
Thanks for every ones in put, let me explain my situation a little further. This is my neighbors system, he has asked me for help. There are two 7 panel arrays each consisting of 3 strings of two 175 watt Sun Tech panels in parallel, each string on its own 15 amp breaker and one 175 watt Sun Tech panel on its own 15 amp breaker. These are 24 volt panels. Each array is on a FM80 charge controller. The problem that has arisen is the in the daily Data Log section, information is shown one day, but the next nothing is recorded, even though they are both charging the 12 volt battery bank, every day in full sun. One day recording one day not, this has been going on since installation 22 days ago. I have contacted Outback and their thought is the something in both arrays is causing a short circuit. I don't see how both arrays could have the same problem, unless it is the wiring.
Each array is 1225 watt which is all most the max for the controllers. In the winter one panel is shut down, then only 1075 watts. I hope my explanation makes sense to you.
Does any one have any thoughts?
Steve -
Re: VOC and ISC test
Did I read that right? An array comprised of three parallel strings of two 175 Watt panels in series, then paralleled with one 175 Watt panel?
Short form: it is impossible to have an array with an odd number of panels of the same spec unless all panels are in series.
If these are all the same panels, then the three strings of two will have a Vmp that is 2X that of the single panel. That won't work; might as well take the single panel out.
BTW, one FM80 would probably handle 12 of those panels altogether. Check the array sizing tool at Outback. -
Re: VOC and ISC testCariboocoot wrote: » Did I read that right? An array comprised of three parallel strings of two 175 Watt panels in series, then paralleled with one 175 Watt panel?
Short form: it is impossible to have an array with an odd number of panels unless all panels are in series.
If these are all the same panels, then the three strings of two will have a Vmp that is 2X that of the single panel. That won't work; might as well take the single panel out.
BTW, one FM80 would probably handle 12 of those panels altogether. Check the array sizing tool at Outback.
Yeah, looking at what he said, it looks like he's going over the VMP rating on the charge controller for a 12 volt system
This is the same thing I did, remember, I fried the Diode in my charge controller doing it too, would of worked had I been trying to charge a 24 volts system, but the rating on my charge controller is MAX VMP is 30 for 12 volts and 50 Vmp for 24 volts.... wired 2 low voltage panels together in series to try to boost the voltage(voc) to 18, it worked but I went over the VMP rating per panel on my charge controller for a 12volt config. -
Re: VOC and ISC test
Be that as it may, I have shorted and disconnected hundreds of single panels of many different makes and models in full sunlight to test ISC and Voc (it is routine to do that when unpacking modules on a job site) and I have never seen one cause an arc.Cariboocoot wrote: »A panel can hit its Voc without being able to produce its full current so it is necessary to test both.
A panel producing its full Isc can arc when you pull the wires apart. Keep in mind that for DC a sustainable arc can be made with Voltages over 15, which most panels produce for their Vmp. Couple that with possibly 8 Amp Isc and you do have an arc potential. Mike's testing procedure is safe and sound. -
Re: VOC and ISC testUrbandialect wrote: »Yeah, looking at what he said, it looks like he's going over the VMP rating on the charge controller for a 12 volt system
This is the same thing I did, remember, I fried the Diode in my charge controller doing it too, would of worked had I been trying to charge a 24 volts system, but the rating on my charge controller is MAX VMP is 30 for 12 volts and 50 Vmp for 24 volts.... wired 2 low voltage panels together in series to try to boost the voltage(voc) to 18, it worked but I went over the VMP rating per panel on my charge controller for a 12volt config.
No worries there: the FM80 will take 150 VDC input.
My concern is: if they wired three sets of two in series against one of the same in parallel, what other stupid wiring did they do? -
Re: VOC and ISC test
The question about stopping current flow before opening the panel connections is that the MCx connectors are not rated to interrupt current... And it is possible to pit the gold connections with even small arcs and cause long term reliability issues.
Will it happen--don't know.
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: VOC and ISC testBe that as it may, I have shorted and disconnected hundreds of single panels of many different makes and models in full sunlight to test ISC and Voc (it is routine to do that when unpacking modules on a job site) and I have never seen one cause an arc.
Be that as it may, that doesn't mean it isn't possible and it's no trouble to make the effort to eliminate the potential altogether.
Some people are luckier than others. -
Re: VOC and ISC test
And, at the risk of beating a dead horse--The Sears $60 (cheap) AC/DC current clamp meter will allow you measure the current in each parallel string while under operation--cannot get much better testing than that.
Make sure the batteries are discharged/under heavy load (heater, pump, etc.) and get the charge controller outputting maximum current... Then current check each string.
If your current is within 10-20%--They are probably OK. If you have >50% difference, you have have found an issue (or need to check your test conditions/measurements).
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: VOC and ISC test
The panels are wired as follows. 2 panels positive to positive, negative to negative then to combiner box and 15 amp breaker. Next 2 the same and next two the same. One by its self to its own breaker. Don,t have proper software on this computer to run Outback sizing tool, but when I did it was ok to run seven in parallel if I understood it correctly, or am I missing something.
Steve -
Re: VOC and ISC test
I think you can just send all the positive wires to the positive bus in the combiner box and all the negative wires to the negative bus in the combiner box, I think the wiring them together outside the combiner in parallel then into the conbiner box is extra wiring..lol, but depending on how the panel connection are on the back of the panel may be the reason why they are wired in parallel outside the conbiner box -
Re: VOC and ISC test
Usually, each positive wire from each panel string should have its own fuse or beaker.
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: VOC and ISC test
Let's call this one "convoluted parallel connections" as per your clarification it seems they are all in parallel, none in series. It's still wrong.
The right way is each of the seven panels has its own fuse sized according to its Isc. After the fuses they all connect together in the combiner box.
However even this misconstrued wiring shouldn't be cause the FM to not log data. Any possibility they accidentally connected the two arrays (+) sides? -
Re: VOC and ISC test
Why can't you parallel two panels on one breaker? -
Re: VOC and ISC test
It depends on the series fuse rating for the panel, the other parallel panels, and the breaker rating...
In general, if all panels are the same Isc rating, and Ifuse~2xIsc, then the "incoming current from the breaker + parallel panel will exceed Ifuse of the "failed" panel.
Note that your two panels into one breaker---Makes the Breaker to be ~2x2xIsc (2xIsc individual fuse rating) times two because you have two panels connected to the breaker.
If, you run the breaker =Ifuse, it will probably false trip under good sun/heavy loads. If you run the breaker >Ifuse, then you are not properly protecting the panel.
When we say 1 or 2 panels in parallel, no fuses needed... We really mean the entire array connected the charge controller.
We do not mean that you need 1 series fuse/breaker per two panels.
There are a few panels that may work with two parallel panels per series fuse (the one I saw were very high voltage thin film panels that had Vmp>100 volts, Imp~2amps, and Ifuse~10-15amps or so); however, that is pretty rare to find panels so rated...
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: VOC and ISC testWhy are the blankets/tarps necessary? Once the array DC disconnect is opened, no current is flowing, so you can disconnect the conductors of the suspect module. You can short and unshort a single module in sunlight without arcing; measure the Isc (shorted) and Voc (open circuit) and make the determination. If a module is bad, open the string OCPD on the string it is in and you can turn the rest of the array back on.
not knowing the configuration, where combiner boxes, # of panels in the string, or even if there are disconnects, I offered the safest way for a neophyte to test.
With out the MC disconnect tool, individual panels can't be disconnected either.Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A
solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister , -
Re: VOC and ISC test
I hope the attached diagram shows properly. This is how it looks like it is wired now. If this is wrong, how can I fix it for him. I found out the only time a breaker tripped was during the winter and it was the breaker between the charge controller and the battery bank, not the breaker at the combiner box. -
Re: VOC and ISC test
Functionally, the setup as drawn should work. Safety wise, it is not "legal" with one breaker per two panels.
Are the two separate arrays each going to their own charge controller solar panel input or are they paralleled in any way (or is there a common ground connection between the two solar arrays ahead of the charge controllers)?
Also, you have a DC clamp on current meter? If so, just check each wire (positive) and see that the current is relatively matched for each panel. If not, you can look for problems there.
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: VOC and ISC test
i don't understand, if you have 2 175 watt 24volt solar panels wired in parallel before sending it into the combiner box, that's 350 watts, that's over 20amps.. then you bring the hot down into the combiner box and put it threw a 15 amp breaker coming off the hott bus?
you said the breaker tripped but it wasn't the one in the conbiner box but the one from the charge controller to battery?
shouldn't 3 of the 15amp breaker trip in the breaker box every day? -
Re: VOC and ISC test
They each have their own charge controller and I have a Extech 730 AC/DC clamp on DMM. The arrays are both grounded and the two FM80's share a common ground. Would replacing the 15 amp breaker with a 30 amp solve the problem? They where originally wired in series to 48 volt, but due to only being a 12 volt battery bank it was suggested it be reduced to 24 volt so the controllers would not work as hard. -
Re: VOC and ISC test
You have a good meter-so you can measure the current from each string/array/connection point in the PV / Battery system.
Don't quite understand, how 7 panels were wired 2 up in series/parallel--but that is OK. In theory, you do need to review the gauge and length of the wiring to ensure that you do not have too much voltage drop (in theory, you need ~6 awg thicker wire for the same percentage voltage drop while running at the lower input voltage).
Won't necessarily cause you problems--but you may have quite a bit of voltage drop/wire losses with the new 7 up parallel connection.
Also, are the two array "Negative" bus connections wired together (should not be--but may not matter to the Outback)--Or are they isolated home runs to each Outback controller (correct connection).
For now, probably don't play with the breakers--they are not your problem regarding the functionality of the Outback charge controllers (unless you start getting nuisance tripping).
Larger breakers only make the safety issue worse. The final fix is to re-wire back to 48 volts or add more breakers (7 per array).
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: VOC and ISC test
Original array was only six panels when wired at 48 volts. When dropped to 24 volt, one panel was added to each array. The negative buss bars are no connected and the wire run is #6, also the ground wire is #6. Outback have now asked for a voltage check on the ground, not sure what the means. Thanks Bill for all your help. -
Re: VOC and ISC test
Not quite sure what they are asking for either...
I would pick a place as "zero volts" (maybe the battery bank negative post), and start measuring voltages (earth ground, battery +, array +/-, charge controller metal case, etc.).
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: VOC and ISC test
I stipulate that covering the modules when shorting and unshorting the connectors will not hurt anything.Cariboocoot wrote: »Be that as it may, that doesn't mean it isn't possible and it's no trouble to make the effort to eliminate the potential altogether.
Some people are luckier than others.
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