help wanted: overcurrent protection for 2 strings

[Consumerbot3418]

I'm trying to free up an MPPT on my Sunny Boy 7.7 (edit: US-41 model) by paralleling 2 identical strings with an ISC of 9.75A, 19.5A once combined. Each MPPT can only handle 18A, according to SMA's specs. Sunny Design gives the following warning:

Observe the installation guidelines

The short-circuit current of the PV array is too high for the selected inverter. Operation with this combination is only permitted when suitable measures are taken for current limitation (e.g. DC fuses).

Solutions

1. Reduce the number of strings.
2. Adjust the allocation of PV strings to the available inverter inputs.
3. Select a different inverter.
4. Take suitable measures for current limitation.

I'd really like to take suitable measures for current limitation--so what might that look like? A 15 amp fuse between the inverter and the point the strings are combined? Are we trying to protect the MPPT? Because I read a document on this site called "Solar Array Overcurrent Protection", and Appendix B illustrates why 2 parallel strings/panels don't require OCP, since a short will never "see" more than the ISC of one string.

Note that I'm only concerned about protecting the inverter/MPPT. The two strings have their own cables running all the way to the inverter, and I intend to combine them at the inverter, so cable sizing isn't an issue.

 

[Bop]

Strange- the specs on the Sunnyboy 7.7 are far different to that statement... (from HERE)

A 19.5A array is well within the 30A Isc limit of the Sunnyboy per input, (it just can't use all of it in good weather and the MPPT controller will max out at 15A, but you could add a THIRD 9.75A parallel string and the Sunnyboy will still be happy handling it with no issue... (29.5A is still (just!) under that 30A limit!!!)- you would still only get that 15A output in good weather, but your total daily generated power would skyrocket, and your poor weather performance improve by a huge margin...

 

[Consumerbot3418]

Strange- the specs on the Sunnyboy 7.7 are far different to that statement... (from HERE)
View attachment 198177
A 19.5A array is well within the 30A Isc limit of the Sunnyboy per input, (it just can't use all of it in good weather and the MPPT controller will max out at 15A, but you could add a THIRD 9.75A parallel string and the Sunnyboy will still be happy handling it with no issue... (29.5A is still (just!) under that 30A limit!!!)- you would still only get that 15A output in good weather, but your total daily generated power would skyrocket, and your poor weather performance improve by a huge margin...

I guess I should have referenced the specs from SMA. It clearly says 18 amps:


Edit: just realized there's a Sunny Boy 7.7 SE (Storage Edition??). So it looks like I should have specified my actual model, which is the 7.7-US-41

 

[Bop]

Ahha- that explains it... (we use the SE in Australia almost exclusively these days as most home owners either want a hybrid from the get go or the option to have battery storage added later- straight gridties are becoming extremely rare in new installs here)

Sorry- your idea simply won't work re 'current limiting' using fuses etc- that simply isn't how it works...
It would handle it with no issues in normal operations (after all that model is only pulling 7.68kw from the array at best) but should something go wrong, you are over the protection limits of the Sunnyboy and there is no knowing how it will respond if that happens...

Could be as undramatic as a fuse blowing or breaker tripping, up to 'the smoke comes out'
(the manufacturers tend to be conservative and you aren't 'that much over', but still, you are in 'at your own risk' uncharted territory once you exceed their ratings...)
Sounds like whatever internal measures that they have, if you exceed that 18A limit- you have to add your own protection ie fitting the 18A fuse as a 'last resort' should something go wrong

This isn't 'current limiting' however- simply added protection... it current limits by blowing the fuse lol the definition of 'current limiting' is usually 'non destructive' ie a 'constant current limit' (like what is found on many power supplies or indeed what a solar panel is)- where your load will be allowed to draw UP TO but not beyond the set current limit- but doesn't 'do any damage' in the process unlike blowing a fuse or tripping a breaker...

 

[Consumerbot3418]

Sorry- your idea simply won't work re 'current limiting' using fuses etc- that simply isn't how it works...

Well, in fairness, it isn't my idea. The below text is verbatim from SMA's website:

Operation with this combination is only permitted when suitable measures are taken for current limitation (e.g. DC fuses).

I don't expect to find an 18A fuse, which is why I wonder if a 15A would work... I just want to make sure the inverter isn't being abused, or its warranty voided.

 

[Bop]

A 15A HRC fuse will be fine for the job and meet the requirements for the Sunnyboy (you can readily get 18A non HRC fuses- but thats not a good idea in a high voltage DC setup like a solar array)

HRC is a 'sand filled' glass cartridge fuse, designed to quench arcs on a DC supply...
eg
SPF solar fuses Au/Eu/UL listed to 1000v

(btw there is a difference in technical specs between 'current limiting' and 'current limitation'- the first is active capping of a supply maximum current without distruption Ie it will supply up to the limit without cutting off if the demand goes over it, it just 'caps the speed' at that current level so to speak), where current limitation is - 'it goes no higher than this point- ever, don't care if it shuts it down in the process...'