micro inverter

petertearai

HI I have an off grid system . Thinking of adding an extra small building on site. Could i add a couple of panels on this building with micro inverters , this would add some extra day time assistance to my load , but save the extra cost of lengthy dc cabling and another charge controller . ..Good idea or bad idea ??

BB.

Yes, it can work....

Several caveats:

1. Off Grid inverter should be a true/pure sine wave inverter and 50 Hz +/- 0.2 Hz or so (GT inverter may not sync otherwise)
2. Off Grid inverter wattage rating is >> than GT inverter capabilities (don't over current OG inverter)
3. What would happen if battery bank is full and GT Output > Loads? (How will you prevent battery bank from over charging--safety? Several options available)

To reduce the chance of over charging, the simplest way would be to put a dump controller on the battery bank and set it a few tenths of a volt above your standard charging voltage... Battery gets full and controller turns on dump load. Depending on how much power and your risk/reward feelings, technically you should have two independent methods to ensure the battery bank is not over charged (boiling dry, possible risk of fire).

Not possible to add panels to main installation direct to battery bank?

-Bill

petertearai

Thanks for your reply . On the note about the batterys . I dont see how the batterys would be affected as they are only affected by the fm80 charge contoller . Ie not an inverter charger.
It would be possible to add panels to my present system , but i would have to get a 24 volt inverter , or another charge controller as my pv is at its max for 12 volt system.

scrubjaysnest

petertearai wrote: »

Thanks for your reply . On the note about the batterys . I dont see how the batterys would be affected as they are only affected by the fm80 charge contoller . Ie not an inverter charger.
It would be possible to add panels to my present system , but i would have to get a 24 volt inverter , or another charge controller as my pv is at its max for 12 volt system.

If I understand correctly you plan on running the micro inverters directly from panels mounted to the new building; the new building wouldn't have batteries or charge controller. This will work if the panels supply enough to run the inverters when the sun is out but maybe not all day light hours.

petertearai

scrubjaysnest Yep That is how im thinking .

BB.

Maybe I am a bit confused... Micro GT inverters connected to the AC output of your Off Grid inverter?

In general, GT inverters will not produce AC power--They need to be connected to a "live" AC Power line (230 VAC 50 Hz for NZ?) and "sync up" to the AC mains.

A typical Off Grid Inverter when connected to a GT Inverter+solar array will actually have "reverse power flow" if the GT inverter outputs more power than the AC loads--The Off Grid Inverter will "run backwards" and recharge the DC Battery bank (at least some OG TSW inverters will).

-Bill

petertearai

Hi . My idea is that my present off grid 1800 ps xantrex inverter is essentially my grid . It runs continuously while i am in residence. The new building would have say 2 250 watt panels on it with micro inverters. This would then only be conected via 230volt ac cable . while sun is shining it would then provide all the power my fridge needs and other base loads, this wold mean that my main inverter would provide less to the system and hence batterys would benefit . However thinking on it more i seem to be making my simple system more complicated . Thanks again for reply's.
Regards Peter

Vic

Believe that the "ps 1800 xantrex" is the ProSine 1800, which has NO internal charger -- it is not bi-directional, IIRC. This inverter may behave poorly when there is not enough load to accept all of the microinverters' output. But have never such a setup with any OG inverter.

FWIW, YMMV, so on. VIc

BB.

I am not sure--But I would guess that many Off Grid inverters are actually "bi-directional" capable. It does not have to be an Inverter-Charger to "Back Drive" from the GT back through the OG Inverter to the battery bank. All it takes is higher voltage on the 120 (or 220-240) VAC line to "push backwards" through a standard/typical OG Inverter.

-Bill

Vic

Hi, Bill,

Yea, I do not know the answer, guess it depends upon way the electronics are implemented, but thought that would throw out this wonderment, on my part.
Thanks, Vic

BB.

We tend to think of using diodes as rectifiers (AC to DC)... However, if I understand the inverters correctly, we are "de-rectifing" using an H bridge with FET or similar transistors (replace the "motor" in the article with a transformer) and you have a MSW inverter.

So--I would explain the inverter as really a "synchronous rectifier". Instead of using diodes with there ~0.2 to 1.0 volt drop, you can use FETs and electronics to turn on/off in time with the AC wave form. And you can now have a "more efficient" rectifier for AC to DC conversion (no diode drop, less wasted energy/heat).

That is pretty much what an Inverter does--Except the current flows from higher voltage (inverter) to the loads. When you put an energy source in place (or in parallel) of the AC loads, you can now "force" current "backwards" through the H Bridge (using the electronics to control the bridge FETs) and charge the battery bank.

TSW inverters are similar (but more complex to make a sine wave approximation).

Anyway, that is my guess as to how/why this works (GT inverter "AC output" connected to OG inverter "AC output").

-Bill