7 KW UPS - Build system around it?

[(unknown)]

OK, so I have a 7KW UPS available with a 3 phase pure sine wave output.  

I was thinking about using it for a power supply to a cottage I have that has crappy line power (line quality and frequency of outage).

The battery setup that is came with is 4 banks (in parallel) of 16 12V SLA batteries in series.  The batteries are probably shot (stored uncharged for 8 years or so).

I can replace them with 16 100AH deep cycle batteries in series and run with that - a bit apprehensive about dealing with 192V DC.

Also, thoughts on PMA designed for 192V output (love the idea of being able to use "small" guage wire - 12 g)

I'm fairly competent - have not burned the place down yet

Thoughts?

Timbo

[nick1234]

you only have 2.3kw per phase if i am not mistaken

 run that into a 3 phase electrical panel or you will run into trouble

 check the output might be 208v ? per phase

 if its 120v thats fine but you will  have to balance the load

have fun avoid shocking experiences

nick

[Ungrounded Lightning Rod]

A downside to running over 48V is that below 50V you are in the low-voltage part of the code and above 50V you're in the full version.

It's not just an issue for the code enforcement people:  Non-code electrical work voids insurance.

[bob g]

i recently repowered a 6kwatt inverter, 208/120 single phase

with a 192vdc front end buss

i also frequently work with 480vac three phase mains, and i have to say

192vdc scares the living beejeesus out of me!

in my opinion, you would be better off with a 48volt version, even those will bite

you, but not kill you dead.

one bad connection, or bad battery cell in the 192volt string can have horrendous

reprecussions should there be a short or some fault like a shorted cell.

if you are comfortable with enormous amounts of power, then by all means have fun.

as for the NEC thing, placing the batteries and inverter in a separate and non habitated bunker or building and then bringing the 120, or whatever to the house

in a conventional manner will negate most of your worries,

as long as the outbuilding does not burn your house down should it catch on fire

you are ok,, but the insurance company will likely not pay off on the bunker or its contents.

bob g

[scottsAI]

Timbo1080,

Couple pluses and mines to using UPS.

Plus:

Battery charger built in.

Transfer switch built in.

Used are cheap.

Runs motors. (Sine wave type).

Minus:

Uses power all the time. 3kw SmartUPS (sine wave) draws 48w always when connected to power lines and about the same when running as an inverter.

Low loads efficiency is low. OK at higher loads.

The 3kw SmartUPS uses 48v batteries and works on 120v. Used  less than $200.

My home uses several SmartUPS (different sizes).

Some of the bigger UPS convert all power to DC then invert it back to AC. Efficiency sucks below 70%, like adding 40% to electric cost. Check out what you have, may not want to use it.

Have fun,

Scott.

[timbo1080]

The big plus is that it is FREE

Good idea on the outside storage shed/battery house.

Just need to figure out how to do the grid tie and some form of windmill - it is in a class 3 /4 wind zone on the outerbanks of NC - not really worried about efficiency if I'm not paying for electricity.

Also have access to several 2000W UPS (again pure sine wave output) that run on 24V DC - price also FREE

Just like the idea of 7KW...

Timbo

[Ungrounded Lightning Rod]

If you want to backfeed the grid you MUST use an inverter designed and approved for it, inform your utility, and go through the appropriate design approvals and inspections.

Backfeeding risks "islanding" and then electrocuting a lineman who's out trying to restore power after an outage.  Anti-islanding design is a must and homebrew is not acceptable.  (It involves a contactor, accurate frequency and voltage sensors, and some extra computer programming in the controller.  The extra electronics is pricey enough that it isn't installed anywhere it isn't paid for, and the number of units sold is small enough that amortizing the development cost makes them pricier - like by a couple grand.  You won't find it in anything sold as a UPS.)

If you get caught backfeeding with non-approved equipment - or even approved equipment correctly hooked up but without informing the utility and getting their OK and database update - your utility can (and may) cut you off and refuse to provide power to that address in the future.  And the new smartmeters many utilities are rolling out will detect backfeeding as soon as it occurs.  (It looks like one of the "stealing power" frauds they're designed to detect:  Unplugging and inverting the meter for a few days each month.)

[timbo1080]

Was going to use an "approved" grid tie inverter - receive certification from the utility, etc.

At this point I'm trying to figure out the best way to do it - probably add the inverter at the box where the UPS line input joins the line - with the UPS output going to my existing circuit breaker box.

No way to island in that case - if the power company line goes out, the feed to the UPS gets cut and it stops charging the batteries, however output from the UPS continues to feed the house breaker box.  

The grid tie inverter is designed to cut output to the line when it senses that the line voltage goes out (magic to me how it does that - but that's its job).

Thinking out loud here.

Timbo

[Ungrounded Lightning Rod]

My take is to get an inverter with a sell option and a battery charge from-the-line option.  Then it and your house batteries also serve as a UPS.

You put the loads that will only run from line power on one subpanel - probably the original distribution panel.  You move the loads that are to be powered even in an outage on a new subpanel fed by the inverter's AC output.  And you connect the grid side of the inverter either to the grid-only panel or a breaker at the main panel (if the utility requires it).

In "sell" mode the inverter is synced to the grid and has the two panels tied by a contactor.  It uses the grid as a dump load if the batteries are getting overcharged by the R.E. sources, charges them from the grid if they are getting low, and manages their charging rates, equalization, etc.

If the grid flakes out the grid-tie stuff detects the frequency and/or voltage screwups and opens the contactor.  Now your important loads are still running with barely a flicker while your grid-only stuff is off until the grid comes back.

When the grid does come back the inverter verifies it's stable, shifts its phase to match up, and closes the contactor.

So it acts like a UPS of the "always running and synchronized but with a tie relay trading a little flicker on outage for efficiency most of the time" type.