Smartsolar MPPT's and polarized DC MCB's

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Triton asked Apr 18, '23

Smartsolar MPPT's and polarized DC MCB's

Hi All

This might be an odd question but I have recently seen several videos of what happens when polarized DC circuit breakers are not wired correctly as far as polarity goes i.e. up in flames when tripped and trying to extinguish a reverse DC arc.

This made me wonder, even if wired correctly and there is reverse current flow due to system design and fault condition these MCB's can still be a fire hazard.

So now my question is, what type of protection does a Victron SmartSolar MPPT have to ensure a reverse current situation at night when there is no PV power can't occur from the battery. Is it highly unlikely for reverse current flow from "MPPT battery out" to "PV in" under fault condition if everything is wired correctly? And is it safe to use polarized DC breakers if wired correctly with PV as the source? I am having a really hard time finding stock of non-polarized DC breakers so pretty much forced to use polarity sensitive.

I noticed this thread where others had issues with measured PV voltages at night and it has me a bit concerned...

https://community.victronenergy.com/questions/3209/power-non-return-to-zero-at-night-on-a-mppt-25060.html

MPPT SmartSolar

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Triton commented · Apr 18, 2023 at 08:50 PM

Sorry I forgot to mention, my question relates to a single string so no risk of reverse current from paralleled strings joined in a combiner box. Just trying to explore the potential scenario of MPPT reverse current failure under shorted string fault condition or at night when no power on "PV in" of MPPT.

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Answer 1 · 2023-04-19T04:12:51Z

Alexandra answered · Apr 19, 2023 at 04:12 AM

@Triton

The faliure is usually due to the arc being higher than what the breaker can quench. And sometimes combined with not rechecking the torque on the screw clamps on the mcb heat and arc....

Sometimes for higher voltages you have to break only one side and loop through the breaker, NOARK is one of them.

They all come with install instructions, so follow that.

So most often it is not installed correctly (or it cannot do what it says on the box - so quality). Buy a good or known brand, don't combine strings on it. The string combo is particularly dangerous as when victrons mppts are preventing overcharge they short the input, alot of breakers can't take that if there are several in there.

Most importantly recheck connections once in a while.

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Answer 2 · 2023-04-19T11:24:25Z

Triton answered · Apr 19, 2023 at 11:24 AM

@Alexandra

@Guy Stewart (Victron Community Manager)

Thank you for your advice. As far as I know the breaker I have is a quality brand but it is not specifically for PV applications, rather general DC applications with a source and a load and polarity must be taken into account for correct wiring as per pictures attached. Bi-directional current flow is not permitted. Bi-directional MCB's are very hard to come by where I live. I will only be connecting one string so no string combinations risk.

My concern is that, even with correct wiring,

if the load suddenly becomes the source (a reverse current flow fault condition from battery through the MPPT as per https://community.victronenergy.com/questions/3209/power-non-return-to-zero-at-night-on-a-mppt-25060.html )

AND

the source becomes the load (PV string negative and positive shorted fault or panels generating no power at night, and MPPT failing as per above)

you would have a reverse current flow condition and if the breaker is manually or automatically tripped the arc will not be pulled toward the quenching device inside the MCB but toward the internals of the MCB and cause a fire as many Youtube videos show.

The way I see it the breaker will never even operate in forward current as an automatic current breaker since a shorted PV string only puts out around 11A so it is pretty much only intended to be a disconnecting device and protection against large reverse currents. The question is can this breaker also disconnect a reverse fault current and the answer seems to be no. My hope was that the MPPT has thorough protection against reversed current flow so that this does not have to be accounted for but after reading above thread I am not so sure anymore.

Or am I missing something?

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Alexandra ♦ commented · Apr 19, 2023 at 07:28 PM

I imagine you have watched a few you tube videos on this.

If you have questions about electrical installation, or are unsure, you need to find an experienced professional.

Install it correctly for normal polarity running conditions.

Add fusing too if you want extra protection. Fuses blow faster than breakers trip anyway and they don't care about polarity.

What voltage string are you using?

And don't use it on the battery side....

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Answer 3 · 2023-04-21T19:26:49Z

Hi @Alexandra, thanks for your message. I have double checked the datasheets and also confirmed wiring with the supplier. Indeed using fuses for each string negative and positive in series with the breaker along with a good quality SPD both rated for 1000Vdc operating Voltage. Probably redundant seeing as I have a relatively small installation with only 2 strings and 131.57Voc per string at coldest temperature extreme (2 x Smartsolar 150/35's used). Strings each to their own MPPT because of drastically different azimuths on roof (no optimal azimuth roof slope available without significant shading).

The fuses are rated at 16A FF speed vs the breaker at 20A. The fuses don't really serve a purpose seeing as Isc is only 11.52A and I am quite literally only using them to try and catch the highly unlikely reverse current situation (such as major problem in the MPPT along with a short between the corresponding string's plus and neg) before the breaker can trip under reverse current and be hazardous.

I am currently actually considering sending the Smartsolars back (still boxed) and upgrading to a SmartSolar RS 450/100 and then running all 6 in one string to comfortably exceed the start up voltage of the RS. The RS has galvanic isolation between PV in and Batttery out plus built in Ground Fault detection which seems like a better option plus I can add another PV string at a later stage on its 2nd MPPT.

GFP remains a headache with the Smartsolars/BlueSolars and the lack of galvanic isolation is not ideal as far as potential ground currents go. DC voltages on ground make me nervous seeing as I have a fixed installation and equipment grounds are all connected to the main utility AC earth which is connected to the AC neutral at the grid provider's supply DB. Potential fault current paths are too complex for me to account for/comprehend but atleast with built in GFP I will know when isolation between my PV panel frames and pos/neg is compromised many years from now and know when to act.

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