Install a grid-tie inverter behind a UPS

jaapkrijger

Hi,
we have quite a few black-outs here, am wondering if I could install a grid-tie inverter on the UPS-supported circuit to push the support-time of the UPS?

waynefromnscanada

Re: Install a grid-tie inverter behind a UPS

jaapkrijger wrote: »

Hi,
we have quite a few black-outs here, am wondering if I could install a grid-tie inverter on the UPS-supported circuit to push the support-time of the UPS?

So what do you propose as a power source to feed the grid-tie inverter? They are designed to take DC power, such as from a solar installation, and feed that energy out into the grid.

jaapkrijger

Re: Install a grid-tie inverter behind a UPS

utility feeds ups
PV feed GTI

both UPS and GTI outputs are connected to UPS supported circuit(S)

my main worries are:
where will excess power from GTI go when running UPS on battery?
Will GTI adjust to cycles of UPS when switching over to battery?

rg,
Jaap

GreenerPower

Re: Install a grid-tie inverter behind a UPS

There are 2 potential issues with this:
1 - Most UPS are mod-sine, grid-tie inverters might not sync to "sell" to the UPS AC output. True-sine UPS is OK.
2 - When grid is down, the grid-tie would "back-feed" the UPS AC output (if it can sync). If the grid-tie "sell" power is more than loads, the UPS AC output might go poof.

You would be better off using PVs and CC to charge directly the UPS batteries.

GP

jaapkrijger

Re: Install a grid-tie inverter behind a UPS

Thanks for this info, all understood, wouldn't the charging of the batteries limit the power from the solar to be only usefull when operating the UPS on battery, thus during black-outs? I'd like to maximize the power available from the PVs that's why I wanted to use a GTI

jaapkrijger

Re: Install a grid-tie inverter behind a UPS

the UPS I have is an APC, as per the manual is't pure-sine so that question is answered OK, as for the feeding back on the UPS.... now that's one of my worries, could a diode help me here?

GreenerPower

Re: Install a grid-tie inverter behind a UPS

jaapkrijger wrote: »

Thanks for this info, all understood, wouldn't the charging of the batteries limit the power from the solar to be only usefull when operating the UPS on battery, thus during black-outs? I'd like to maximize the power available from the PVs that's why I wanted to use a GTI

If you could find a grid-tie inverter that uses the same battery voltage as the UPS, you could run the inverter from the same UPS battery and "sell" to the AC input of the UPS and set the "sell battery voltage" slightly above float voltage. Well, you might as well replace the UPS with a grid-tie inverter with battery backup system. Or were you thinking of battery-less grid-tie ?

jaapkrijger wrote:

...as for the feeding back on the UPS.... now that's one of my worries, could a diode help me here?

It's AC, so diode won't help. Depending of the DC-AC design of the UPS, if it's "bi-directional", the extra power would be "dumping" unregulated to battery - still no good.

GP

jaapkrijger

Re: Install a grid-tie inverter behind a UPS

from the diode reply, I see you have understood what I want to do, let me ponder over the feeding back by the GTI.
the idea is to run battery-free on the solar system. in the original plan the GTI sits right at the meter, am just looking at how to maximize the use of the solar system, especially when dealing with a black-out... the outback inverters are extremely expensive... my GTI (250W) is only USD 150, the nearest outback is 10 times that amount...

GreenerPower

Re: Install a grid-tie inverter behind a UPS

jaapkrijger wrote: »

...the idea is to run battery-free on the solar system. in the original plan the GTI sits right at the meter, am just looking at how to maximize the use of the solar system, especially when dealing with a black-out... the outback inverters are extremely expensive... my GTI (250W) is only USD 150, the nearest outback is 10 times that amount...

So, you are thinking about the Enphase et al. inverters. You still can connect the AC output of the inverter to the UPS AC output but need to put in a "safe-guard" circuit to disconnect the inverter AC output when "AC in down" AND "load less than AC export" . A NC relay would do and the relay is activated on those conditions.

GP

mike95490

Re: Install a grid-tie inverter behind a UPS

Get a "hybrid" inverter GT & Battery (Xantrex XW series) and that is your solution.

It is UPS when grid fails, and backfeeds when the sun shines.

Oh, you need a MPPT battery charger for it. Go with 48V system, 8, 6V batteries in series.

jaapkrijger

Re: Install a grid-tie inverter behind a UPS

The xantrex option is running into big figures, I'm going to have a look at the earlier option you gave me, thanks for the support

snuffy

Re: Install a grid-tie inverter behind a UPS

jaapkrijger wrote: »

Hi,
we have quite a few black-outs here, am wondering if I could install a grid-tie inverter on the UPS-supported circuit to push the support-time of the UPS?

Maybe I misunderstand but it seems to me an inverter/charger and battery bank would do the trick. The inverter/charger keeps the batteries charged from grid power then switches to battery power and outputs 120 volts ac to your UPS when the grid goes down. I don't see where you need a grid tied inverter to solve this problem.

BB.

Re: Install a grid-tie inverter behind a UPS

I believe that jaapkrijger is out of South Africa--So he may have limitations on the hardware available (at a good price) too.

It sounds like he wants to optimize by using the panels to supply AC power when the power is good (save money) and keep the UPS's running when the power is down (save generator fuel).

With a steady load (computers exceed GT Solar power on UPS)--it could be done--although it is a bit dicey--it would be possible that the UPS could become unstable if the GT solar exceeds the computer loads (something shut down)--would the UPS turn off, over charge the battery bank, or what?

So, the easiest would be to set up a voltage controlled switch on the UPS battery bank... When the Battery Bank is over 14.5 volts (or whatever), it could turn of the the GT to AC power connection. When battery voltage falls (say 12.7 or so), the AC switch is turned back on (and possible 5 minute timeout on the GT inverter for reconnect).

More or less, that is what some others have done (attempted) commercially (from what little I have seen).

-Bill

snuffy

Re: Install a grid-tie inverter behind a UPS

I didn't realize he had panels. I thought he was just looking for options to extend the run time of his UPS. Every APC UPS I've ever seen had a built in battery charger that run off 120 or 240 AC. An inverter charger and battery bank would give him the 120 AC the UPS needs and extend his run time on his UPS. Not sure whether his UPS needs pure sine wave to run it's charger but I doubt it.

BB.

Re: Install a grid-tie inverter behind a UPS

It is possible that he could "back feed" the TSW UPS that he is using and have both Grid Tied step (used most of the time to reduce power bills) and when power is down, to support/recharge the battery bank with the panels (with the caution that we don't know his UPS is capable of being back fed, and he would probably need to put a battery charging control of some sort in place to prevent battery damage or worse).

Could be done for much less money than installing a new Hybrid Inverter and stuff. Also, could be handy for the smaller installation where a 4+ kW hybrid inverter may be overkill.

-Bill

System2

Re: Install a grid-tie inverter behind a UPS

GreenerPower wrote: »

So, you are thinking about the Enphase et al. inverters. You still can connect the AC output of the inverter to the UPS AC output but need to put in a "safe-guard" circuit to disconnect the inverter AC output when "AC in down" AND "load less than AC export" . A NC relay would do and the relay is activated on those conditions.

GP

Hi, got intrigued by your relay idea. Any further lead as to where to find such thing?

jemsol

Please help me
I am off-grid no electrical power
I have 250kw solar panels With a Siemens 2*160kw VFD to drive 2*120kw water pumps coupled with a 300kw diesel generator. VFD has two inputs
1. DC Port for solar PV
2. AC Port for generator input
Currently we bought 5*50kw GTI no grid!!!!
How could I use these GTI without grid voltage?
Could I connect some strings  to a GTI then to the AC input of the VFD and Some string to a DC Port of VFD? Please help me!!!!!!!!!

BB.

Jemsol, please keep your posts on this project here: https://forum.solar-electric.com/discussion/357459/grid-tie-inverter-connection-to-vfd-ac-input-without-grid-voltage#latest -Bill Moderator B.