Consumerbot3418
Fitting square pegs into round holes... for fun?
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:
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.
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
- Reduce the number of strings.
- Adjust the allocation of PV strings to the available inverter inputs.
- Select a different inverter.
- 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.
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