multiple microinverter DC inputs sharing multiple panel negatives?

dakleindaklein Registered Users Posts: 14 ✭✭
Hello,  
       new here, and hope this is an entertaining question:

What if two microinverters, each with it's own panel, both share the same panel negative line?    During operation of the microinverter, does the DC negative also float relative to ground, and change during MPPT operation?  Or do they stay at 0 potential?   Anyone already tried and fried microinverters doing this?

The two panels are paralleled to a Midnite Kid + battery bank, for backup power with another small inverter and some DC things.  Each panel has it's own DC breaker on the positive panel line,  and the panel negative's are directly connected to the neg input of the charge controller.   There is a larger 21 panel array grid tied with microinverters (Enphase M190s),  and I would add another 10a breaker to serve the two new microinverters (M210s).

When the power is out, I would turn off the microinverter AC breaker,  and turn on the panel dc breakers.   I would use a moderately intelligent control system to accomplish this switching.

If the microinverters are not happy enough sharing panel negatives,  I'll need to add a DC disconnect on the negative lines.

Thanks!

4.7 kW rooftop grid-tied Enphase M190s (about 1/3 original survivors still, have replaced and re-replaced the rest), 21x SolarWorld 225W
2.6 kW ground mount AC coupled to SMA SI6048 x2 & 1080Ah 48v Exide FLA forklift bank, 10x Talesun 265W (a DC Solar trailer)
3.7 kw  rooftop 12x Astronergy 305W,  9 panels DC coupled w/ Midnite 250-CP CC,  3 panels AC coupled Enphase M210IG
480 W camper system, 2x Panasonic 240W HIT, Midnite Kid CC w/ Whizbang Jr, 2x 6v GC2 FLA 230Ah

Comments

  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    I am not sure--But the micro inverters may be non-isolated devices (no transformer isolation to AC mains). I would not combine the panel negative (or positive) lines into a common bus. I don't know what would happen--But it could range from tripping the ground fault circuit to letting the magic smoke out of the inverters/panels (or, i guess, it could work too).

    People have always been looking at cost effective methods to use micro or central GT inverters and sharing a solar array with DC solar charge controllers. It just does not really make economic sense to try and mix/match these differing components together.

    Too costly and too many things that can go wrong (in my humble opinion).

    A bit easier solution is usually to connect the AC output of the GT inverters via a 240 VAC transfer switch to the input of a (compatible) TSW Offgrid AC inverter... The Off grid inverter sets up your 120/240 VAC "micro grid" (when the AC mains have failed) and the GT inverters can share the AC loads or even "back drive" the AC off grid inverter and recharge the battery bank (more modern off grid inverter can even vary the AC line frequency 60 Hz +/- 1 Hz to "fault the GT inverters" (frequency out of specification) to stop charging the battery bank when full...

    This direct coupling of GT inverter to Off Grid inverter does work and is already supported by some higher end AC inverters--But it is still a bit of a nightmare to setup and operate (OG inverter has to be larger than GT inverter+solar array power rating, and "bang-bang" control of the GT inverter output into the battery bank is not an optimal battery charging technique).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mvasmvas Registered Users Posts: 383 ✭✭✭
    Per Enphase, both DC Inputs from the Solar Panels appear to be floating - an ungrounded system ...
    https://enphase.com/en-us/blog/enphase-installations-simplified-integrated-ground-technology
    http://https//enphase.com/sites/default/files/Equipment_Grounding_in_an_Enphase_IG_System-v2703.pdf

    Send an email to the company that manufactured the micro-inverters, with your drawing.
    They would have the schematics.
    And they would be able to answer your question with authority.
Sign In or Register to comment.