MItigation plan on potential higher Voc

[denmek]

Hello guys,

during the planing we did not sonsidered Voc @ very low temperatures.Assuming that might happen the Max MPPT votlage will be reached or even exceed by 30V.
Unfortunatlly to change the inverter setup to accomodate higher voltage version is a bit tricky (need to change from LV to HV battery system). Is there a reasonable way to mitigate the potential high Voc event ?(actually I udnerstood that should be empty battery, ultra low temp) Maybe a TVS diodes , or something elve ?

My actual VoC on the PV string is around: 540V @ 0°C , Inverter Max is around 580V. Theoretical calculation seems to result in voltage ~630 under worst case condition.
I would like to avoid to re-do the PV panel wiring, so the idea about the TVS to surpress the potential Voc going above 580V.

any ideas ?

 

[Bluedog225]

Everything I’ve heard said you need to redo your system. If TVS is transient voltage spike, I’m not sure a freezing sunning morning will be transient.

 

[codfish]

Without seeing your setup it is difficult, but…
it seems like your could lower the VOC by half just by splitting the array into two parallel strings at the middle. Just a couple wires, not exactly a do over. Of course that would double the amps from the array but many SCC can self limit when over paneled whereas they are fried by over voltage.

edit: the switch from series to parallel and back could be seasonal, just replug a couple MC4 connectors ( after dark, and array disconnecited)

 

[Quattrohead]

If you are in a hot state and the wethermans go into hysteria mode about a cold night, then just disconnect the panels until it warms up.
Or how about a DC voltage monitor system that disconnects the panels at a set voltage below VOC of the inverter.

 

[MisterSandals]

My actual VoC on the PV string is around: 540V @ 0°C , Inverter Max is around 580V. Theoretical calculation seems to result in voltage ~630 under worst case condition.

630V seems like an unusually high number given the you've calculated 540V @ 0C.

What temps are you expecting and how are you calculating the additional 90V from temp drop?
What is the panel listed VoC (at 25C)?

 

[denmek]

yeah, its not a single event, and injecting the current should reduce the VOc a bit. BUt basically I like the idea about the voltage monitor more.
The condition of ultra cold temperatures and sun ist almost never there betweeen nov and feb

thanks

 

[denmek]

630V seems like an unusually high number given the you've calculated 540V @ 0C.

What temps are you expecting and how are you calculating the additional 90V from temp drop?
What is the panel listed VoC (at 25C)?

Hmm, I guess you are right, there was a mistake in the calc:

Panel Voc is 41.9V and coefficient is at 0.3%/K, calculating at -10°C will end as following:
Vadj = Voc × {100% + [(Tmod - 25°C) × TC Voc]}
Vadj = 41.9*0,003*35 = 0,439V
Voc max = 41.9+0.439 = 42.34V
String of 14 modules, 42.34*14 = 592V , still a bit too high, Vmax of inverter is at 580V

 

[denmek]

update: we did measured VoC on the string: endup in around 38.5V @ around 0°C. According to spec it should be ~ 42-1V.
for the complete string it is a delta of 50V ! Panels are brand new. The second string showed similar value ~38.6V@0°C.
Its even much lower then the spec @ 25°C ~41.9V. Is this normal behavior ? Anyone experienced such differences?
Modules are from Trina 380W model:

https://solar-pac.de/mediafiles/Datasheet/Datenblatt_TrinaSolar_TSM-DE08M08_mono_375Wp.pdf

rgds, DAniel

 

[chrisski]

I only have 100 watt and 175 watt panels to compare that to, but my VOC is within +/- 2 volts, more than can be explained by temp, so yes I’ve seen that. Seems more to do with an at noon summertime sun high in the sky make higher voltages than the actual temperature. I wish I‘d wrote the volts down along with temp and sky conditions.

 

[codfish]

update: we did measured VoC on the string: endup in around 38.5V @ around 0°C. According to spec it should be ~ 42-1V.
for the complete string it is a delta of 50V ! Panels are brand new. The second string showed similar value ~38.6V@0°C.
Its even much lower then the spec @ 25°C ~41.9V. Is this normal behavior ? Anyone experienced such differences?
Modules are from Trina 380W model:

https://solar-pac.de/mediafiles/Datasheet/Datenblatt_TrinaSolar_TSM-DE08M08_mono_375Wp.pdf

rgds, DAni

you used the STC value for VOC. STC is a cell temperature of 25C, but the cells get quite warm with even a small amount of sun light.
The NMOT values might be a better starting point, 20C ambient and 800 W/m2, for a 39.5v VOC. If the illumination was was somewhat less than 800 ( pretty sunny, and orthogonal to array), then you might get closer to your measured value.
And the measurement was completely open circuit? Even a tiny current will pull it back down the I-V curve.

 

[denmek]

you used the STC value for VOC. STC is a cell temperature of 25C, but the cells get quite warm with even a small amount of sun light.
The NMOT values might be a better starting point, 20C ambient and 800 W/m2, for a 39.5v VOC. If the illumination was was somewhat less than 800 ( pretty sunny, and orthogonal to array), then you might get closer to your measured value.
And the measurement was completely open circuit? Even a tiny current will pull it back down the I-V curve.

yeah completelly open, just wires. In the final setup I have also the lightning surge protection in .
BTW in dead my values are close to NMOT. WIll monitor it more closer. THanks for the feedback so far.

 

[codfish]

I don’t know where you are, but attached is my solar radiation for yesterday. Perfectly crystal clear at mid day. 41.72N. and ~ 550W/ m2. But of course that doesn’t account for orientation and tilt. Throw in the off axis effects of not pointed at solar noon, and not tilted up enough to get the winter sun, then my panels report far fewer watts than the 600 watts per meter would otherwise suggest. ( peak, about 54% of rated capacity at that time for the “best” single panel)

 

[denmek]

thanks, we even do not remember how the sun looks like , since weeks only 50 shades of gray here .

 

[codfish]

thanks, we even do not remember how the sun looks like , since weeks only 50 shades of gray here .

Yes, so I suspect your measurement was taken at substantially less than the 800 W/m2 of the NMOT conditions. Even if you did the measurements at solar noon, and with the panels pointed directly at the “sun” such as it was. It is going to be hard to get a VOC measurement that directly relates to the spec sheets.
Cloudy, drizzling day today, only 91 W/m2 at noon. That knocks some volts off the VOC based on the charts in the spec sheets.

Edit: but as Hedges says, use temperature corrected STC for planning. it only takes a moment of over voltage to fry the SCC

 

[Hedges]

TVS wouldn't do it, they would just fry. Tiny little thing, maybe the size of a quarter. Data sheet quotes a massive number of amps it can take briefly, but "Joules" (watt-seconds) isn't much, can't dissipate continuous power. Only meant for millisecond spike.

A circuit to protect SCC either has to open-circuit the PV connection (and withstand the voltage), or short-circuit (and withstand the current.) While short circuiting it also has to withstand the surge of discharging SCC capacitors, unless it has an anti-backfeed diode.

You should start with Voc spec at STC, and adjust according to temperature coefficient of Voc and record cold temperature for your location.
Measurement is good for sanity check (make sure panel voltage isn't higher than documented), but don't use it prove lower voltage.

Just make sure your series/parallel design absolutely never can exceed SCC max input voltage. You don't want to destroy hardware.

 

[efficientPV]

It takes very little current (as opposed to what the array can supply) to drop the VOC considerably. Some have built shunt controllers to heat water or even a space heater, considering this is a winter situation. A simpler solution may be to tap off the lower panel and switch it off with a voltage relay. The on and off voltages can be considerably different to prevent fast switching. This would be switching fairly low voltage.

 

[Hedges]

You could use a relay or SCR/triac to short out a single panel of series string.
Hysteresis in control of relay to prevent toggling.
SCR/Triac would latch and keep that panel shorted until reset by zero current at night.

 

[efficientPV]

VOC only happens when a charge controller has no load. It can be as simple as turning on an electric heater when there is high panel voltage.

 

[Hedges]

VOC only happens when a charge controller has no load. It can be as simple as turning on an electric heater when there is high panel voltage.

Yes - but that is an active circuit to prevent damage. It isn't failsafe, in that an open circuit in the heating load allows over-voltage and damage to SCC.
Same is true for the clamp across a single circuit board that I suggested.
Having PV panels feed through a limiter circuit, rather than branching off to one, reduces (but doesn't eliminate) the chances of protection being disconnected.

Best to consider panel and SCC specs, design array before purchasing anything. A different set of panels would make a good array for a given SCC.
I've got a 145V max SCC (Sunny Island Charger) and SunPower 327W panels. 1s isn't enough voltage. 2s is OK for my mild weather location but not for one with extreme cold. A different PV panel would work for all locations - SunPower 427W works for 1s. A lower voltage panel would be OK 2s or 3s.

 

[efficientPV]

An easy fix, for a bad design, that he can't possibly make, for a problem that probably won't happen anyway.