I’m looking at adding another string to my DC-coupled 24V system and I’m mulling the idea of having the new string primary-connected to a new 1kW GTIL inverter and secondary-connected to my existing SCC, both connections being made through forward-biased diodes.
The new string has slightly higher Voc and Vmp than the older string and the SCC needs a minimum of Vbat + 2V to draw any power (while the GTIL can operate all he way down to Vbat -2V).
So by connecting the new string through diodes to both MPPTs, I’m hoping the GTL inverter will have first dibs and can draw as much of the new strings power as it wants (by dipping below Vbat+2V where the SCC cannot follow) and then once the GTIL says ‘no mas’ and has more power than it can use, it can raise MPPT voltage above the voltage of the SCC to choke off input power and redirect it to the SCC.
If/when the SCC also has more input power than it can use (battery nearing full) and raises string voltage above Vmp, the MPPT of the GTIL inverter will need to track and once both SCC and GTIL reach Voc, input power from the new PV string will be completely choked off.
Either GTIL or SCC can drop voltage back under Voc once it has need of more power.
Placing a diode in the path means that I’ll be losing ~0.7V / Vmp = ~1.75% of my string power (or less if the diode drop is only 0.5V), but I believe it’s going to allow the GTIL inverter to draw as much power as it can use before redirecting any excess over to the SCC.
Because the MPPT of the SCC is also serviced directly by the existing string, it will never be able the ‘take’ any power from the new string and will be limited to ‘accepting’ whatever power the GTIL decides it cannot use.
If the SCC tries to hog power from the shared string by dropping voltage to it’s minimum of Vbat +2V, the GTIL inverter will drop below that to get all of the current/power from the new string. Once it’s only getting current/power from the old string at Vbat +2V, the SCC will increase input power by increasing votage to Vmp, at which point the GTIL inverter can also increase input power by increasing voltage to just under Vmp (meaning the string voltage of the new string will increase to just under Vmp + Vdiode).
This situation will be stable and will mean all current/power from the new string is going to the GTIL inverter while all current/power from the old string is going to the SCC.
This situation will continue until the GTIL wants to taper down input current/power by increasing voltage all the way to Vmp+Vdiode a which point excess string current/power is redirected to the SCC (including redirecting all current/power to the SCC by increasing voltage sufficiently above Vmp + Vdiode).
Would appreciate any comments on this logic as especially elements I may be missing. And of course, if anyone has tried anything similar, I’m all eyes…
The new string has slightly higher Voc and Vmp than the older string and the SCC needs a minimum of Vbat + 2V to draw any power (while the GTIL can operate all he way down to Vbat -2V).
So by connecting the new string through diodes to both MPPTs, I’m hoping the GTL inverter will have first dibs and can draw as much of the new strings power as it wants (by dipping below Vbat+2V where the SCC cannot follow) and then once the GTIL says ‘no mas’ and has more power than it can use, it can raise MPPT voltage above the voltage of the SCC to choke off input power and redirect it to the SCC.
If/when the SCC also has more input power than it can use (battery nearing full) and raises string voltage above Vmp, the MPPT of the GTIL inverter will need to track and once both SCC and GTIL reach Voc, input power from the new PV string will be completely choked off.
Either GTIL or SCC can drop voltage back under Voc once it has need of more power.
Placing a diode in the path means that I’ll be losing ~0.7V / Vmp = ~1.75% of my string power (or less if the diode drop is only 0.5V), but I believe it’s going to allow the GTIL inverter to draw as much power as it can use before redirecting any excess over to the SCC.
Because the MPPT of the SCC is also serviced directly by the existing string, it will never be able the ‘take’ any power from the new string and will be limited to ‘accepting’ whatever power the GTIL decides it cannot use.
If the SCC tries to hog power from the shared string by dropping voltage to it’s minimum of Vbat +2V, the GTIL inverter will drop below that to get all of the current/power from the new string. Once it’s only getting current/power from the old string at Vbat +2V, the SCC will increase input power by increasing votage to Vmp, at which point the GTIL inverter can also increase input power by increasing voltage to just under Vmp (meaning the string voltage of the new string will increase to just under Vmp + Vdiode).
This situation will be stable and will mean all current/power from the new string is going to the GTIL inverter while all current/power from the old string is going to the SCC.
This situation will continue until the GTIL wants to taper down input current/power by increasing voltage all the way to Vmp+Vdiode a which point excess string current/power is redirected to the SCC (including redirecting all current/power to the SCC by increasing voltage sufficiently above Vmp + Vdiode).
Would appreciate any comments on this logic as especially elements I may be missing. And of course, if anyone has tried anything similar, I’m all eyes…