Transformerless inverter in an off-grid stationary RV

Tulumtam Solar Expert Posts: 37 ✭✭
I have a neighbor who has purchased the following equipment and he has asked me to install it in his RV which will be stationary for about 1 year while he is building his house:

Voltronic Power Axpert VM II 3kva 24 volt inverter/charger with 80A MPPT charge controller
4 Trojan L16G batteries
4 330w PV modules

This is in a remote location with no grid power, so completely off grid.

I have installed 80+ off-grid systems, but I have no experience with transformerless inverters in an off-grid situation.  I also have no experience with RVs.  My neighbor assumes that the AC output of the inverter (it is a 120v model) can be wired into the outlet for connecting to mains power at an RV park.  Is this the best approach?  How should grounding be handled?  I have been trying to research transformerless off-grid inverter installations, but there isn't anything on the internet that I can find besides some manuals with little information and some conflicting specs.  My hesitancy is strong, but somewhat vague.  Any warnings, advice and ideas would be greatly appreciated!

I am not sure if I should post this in the off-grid or the RV section.  I am choosing the off-grid section for now.


  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Transformerless inverters have no isolation, so grounding becomes a problem. you will have to read up about the grounding of the inverter and if the PV panels and charge controller must be floating or grounded somehow.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

  • NANOcontrol
    NANOcontrol Registered Users Posts: 263 ✭✭✭
    Never heard them called that. If it runs from 24V there has to be a transformer to boost up the voltage. Look up HF inverters, they don't have a transformer on the output.
  • Tulumtam
    Tulumtam Solar Expert Posts: 37 ✭✭
    The user's manual for the unit says "Insert AC output wires according to polarities indicted on the terminal block and fasten the terminal block screws.  Be sure to connect PE protective conductor (ground) first."  It says the same thing about the AC input wires.  And that is the full extent of the instructions on grounding.

    The lack of galvanic isolation is one of the things that I understand about the unit, but I don't know what to do to make it safe.  I have read that there should be over-current protection on all cables on the DC side, not just the positive.  Maybe I am searching using the wrong terminology and that is why I can't find much information.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
    Transformerless GT inverters are common... And wiring the PV side of the GT inverter (DC solar panel side), with normal 600/1000 volt AC rated wiring, insulation, jboxes, etc. is normal and does not make a huge difference... The DC side of a transformerless GT inverter is a bit less "touch safe". But touching 400 VDC is not very safe anyway.

    Transformerless Off Grid / Hybrid inverters is something to think about. The DC side is normally under 60 VDC (battery bus voltage) and is treated as being (usually) less than fatal if touched.

    What happens to the DC side when you have a transformerless (non isolated) hybrid inverter (connected to utility AC power and operating as a GT inverter)? Does the DC side become "unsafe" (voltages >60 VDC/VAC on battery bus).

    Most systems are designed to have the DC Bus Negative ground bonded for safety. And I guess those inverters you are looking at do that too (various reasons to ground bond DC side for safety). As long as that is still acceptable/required, It should not be any issue.

    One issue that I could see with a transformerless inverter is questioning how the Genset is wired. In North America, the Genset is expected to have its AC Neutral ground bonded for safety too... This only works if there is galvanic isolation at the inverter-charger (as far as I can guess).

    We have a non-isolated common off grid AC inverter.. The MSW inverter (modified square/sine wave inverter). These types of AC inverters do not have DC to AC side isolation. And with most (all?) MSW inverters if you ground bond the DC bus side and ground bond the Neutral/White wire of the AC output, you create a short circuit through the MSW inverter and let out the magic smoke. (because there is no DC to AC galvanic isolation, both AC output leads are "hot" with respect to the battery terminals and therefore one lead cannot be a ground bonded neutral. PSW inverters are transformer isolated, so they can have an output set up as a ground bonded neutral).

    So, a standard MSW does not work in a Utility power / GT Inverter application, at least for North America where ground bonded neutrals are used in our 120/240 VAC split phase residential homes.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Tulumtam
    Tulumtam Solar Expert Posts: 37 ✭✭
    Thanks, Bill, for the very clear response!  I do hope to avoid letting out the magic smoke....  

    The plan is to install this in the RV for the year or so that it will take him to build his house and then move the whole thing to the house when it is done, increasing the size of the array and the battery bank.  He has a small portable genset, though I don't know which one yet.  I will go see it today to see if I can find out if it has the neutral-ground bond.  

    I will also look around a bit more for info on non-isolated inverters instead of transformerless inverters and see if that doesn't bring up more information.  Thanks for helping me with the terminology!