2 Strings on 1 MPPT
I have 6x 550W panels on west roof connected to MPPT 1 and 6x 550W panels on east roof connected to MPPT 2. (No north facing roof) 8Wk Inverter.
My west roof is multifaceted with a 18 Degree slope and a 30 degree slope. I want to add another 6 panels to the west roof on 30 degree slope and parallel them with the existing 6 (18 degree) into MPP 1 input B. I am expecting more than a 10% voltage difference as the sun sets between the 30 degree string and the 18 degree strings. The large voltage difference will probably only last for about an hour or so before sunset.
Will this be acceptable or should I have blocking diodes to isolate the strings? (Is this a good idea anyway if a string malfunctions?)
The SunSynk article below says that the voltage difference of the 2 strings on the same MPPT should not exceed 10%
I agree. The voltage remains at a constant high as possible value for most of the day and the MPPT draws what current it can without loading the voltage too much because voltage is squared in the power calculation.
I only asked because of the SunSynk document posted in my initial post. In the last screen grab that I posted you will see 222v on one string vs 86v volts on the other for the last 25min of sunset. This is 2.5x and much more than the safe 10% voltage difference mentioned by Sunsynk. Granted, this is a worse case using the current east / west example.
I just wondered if one string is at a voltage level higher than the other string then reverse current will flow into the string that has the lower voltage. I know that this is how they test for bad solar cells using electroluminescence by feeding limited power into a string and measuring emitted IR light. I don't think the inverter will have an issue. I was more concerned about damage to the panels.
I just watching the old video on electroluminescence again, and realise that panels are a quite resilient to reverse power. (Fyi: https://www.youtube.com/watch?v=8huUsh9eu8U )