Balancing Transformer

[YukonDon]

I have 2 Xantrex 5548 connected in 240 VAC via the Trace SWI phasing cable. It works well but I find in need better load balancing. The Trace T240 only supports about 4500 Watts where as my inverters operate to 11Kw if combined to 110 VAC or 240 VAC

Xantrex/Trace used to make a SWI/PAR  load balancing transformer that supported up to 11KW but It is no longer available and I can't seem to find one anywhere.

I have seen posts here on Load Balancing Auto Transformers and I see Outback sell one but it only supports 6KW.

Does anyone know how to get a load balancing transformer (Buy make/model or build) that supports 11KW.  or a used Xantrex SWI/PAR. I have searched the web and  while stores like AltE list the SWI/PAR they are discontinued.

Thanks for your help.
Don

[Ungrounded Lightning Rod]

Do you need an 11 KVA transformer?  Or is the transformer's max load only 5.5 KVA?  Are load balancing transformers rated for the power they handle or the power the system handles?

I assume you have two 5.5 kW inverters at 120V in series and phase-locked to make 240V center-neutral, right?  In that case the transformer's max load is when all the load is on one 120V side and the transformer has to transfer 5.5 kW from the other side's inverter.  Do you have enough load (and fuses) on one side to draw the full 11 kW?

[ChrisOlson]

Ungrounded Lightening Rod makes some good points.  11 kW @ 120 volts is pretty ridiculous.  We have dual Xantrex SW Plus 4024's powering our off-grid home, stacked for 240 split phase.  The heavy loads (electric range, clothes dryer, welder, water heater, well pump) are all 240 volt, which automatically keeps the inverters balanced when any of those loads are running.

We have a Xantrex/Trace T240 balancing transformer that powers our 120 volt loads.  Even with my wife's hair dryer, electric toaster, crockpots, slow cookers, microwave, 'fridge, freezer, and numerous smaller loads like computers, TV and lights - the T240 has never had a problem powering those 120 volt loads.

Before I'd try to find a 90 amp 240->120 step down transformer (what kind of wiring you got in your walls that can even handle that?) I think I'd do it right and rewire motors and heavy loads for split phase.
--
Chris

[boB]

Yeah, it does sound a bit large amount of power for that...

As long as the transformers have the same tuns ratio, (they were both built correctly),
you ~could~ parallel 2 of them if you had to.  For an X240, the idle would go from
around 10 watts up to around 20 watts and you would still need a fan on both of
them (if X240s)

boB

[YukonDon]

Thanks for your replies.
I don't need access to all of the 11KW The problem is that if my wife is using some of the power on a given leg and my Air compressor kicks in I exceed the 5500W on that leg. The SW5548 can't handle the spike or the running load so it dumps the whole grid and the wife gets |@#$$%. 

The SWI/PAR could handle the 11K but I can't find one. I think I'll try and convert the compressor and my water pump the other likely candidate to 220 and see if that helps the grid crashing that Chris here and another friend has suggested.

Thanks again for the support.
Don

[ChrisOlson]

Don, I would switch every load that can be switched to 240 split phase, to it.  It's usually a simple matter of swapping some wires around in the junction box on the motor and re-wiring the unit for split phase from the Main.  That alone will more than likely solve your problem.

If you can do it, and it takes two separate panels to do it, power a split phase panel with the inverter outputs and put in a T240 (or similar) and power a 120 volt panel with a 40 amp two-pole breaker in the split phase panel thru the T240.  This requires pulling all your 120 volt circuits out of the split phase panel and putting them in the 120 volt panel.  This is the way our system is set up.

The upside to the way we have our system wired is that it keeps the inverters perfectly balanced at all times and they run more efficient.  Plus they will handle tremendous loads.  We have SW Plus 4024's, but the 4024's have the same surge capacity as the 5548's.  At 3-4 kW load on our 120 volt stuff each inverter is only carrying 1.5-2 kW load and there's plenty of reserve to even use my Lincoln 225 Spark Welder with that much normal load on the inverters with the way we have ours wired.

Depending on which leg your biggest 120 volt loads are on, you may still run into a problem running split phase stuff due to one inverter overloading without the T240 (or similar) on there.
--
Chris

[OperaHouse]

The transformer never sees more than half of the total wattage.  There is no reason to even size it half the load.   Think of a much smaller transformer to supply 10-15A.  This should cover 99.9% of the cases of a load that is too much for one inverter.  Isolate the transformer an a seperate dual breaker.  This is the cheaper and more efficient solution.

[ChrisOlson]

The transformer never sees more than half of the total wattage.

I guess I don't understand this.  Our T240 can handle 4.5 kW load, or about 35 amps @ 120 volt.  At 30 amp load on our 120 volt circuits, each inverter supplies 15 amps of it.  But we have 8 kW continuous capacity from the inverters and close to 20 kW surge capacity for up to 10 seconds.

The idea of the transformer is to keep the inverters balanced on a 120/240 split phase system to prevent the type of problems that Don is having.  Your big loads are 240 volt, which automatically balances the load on the inverters.  It's the 120 volt loads that are the problem because you can pull 120 off either leg to neutral on a split phase system.  If that is not balanced it will overload one inverter and cause a shutdown.  The transformer, in step down mode, takes the 240 split phase power and converts it to 120 to run the 120 volt loads.  This keeps the inverters balanced, no matter what.
--
Chris

[OperaHouse]

The transformer doesn't have to be rated more than what one inverter can supply to the other phase.  An inverter may specify a 5kw transformer as being for a 10KW system only adding to the confusion.  The point I am trying to make is that decisions are often made on a if this and if that which are an extreme.  The reality is that loads are generally balanced or they should be.  The condition when all the power of one inverter is balanced over to the other phase is very abnormal.  Having to balance just a couple KW with a transformer is the more common situation.  A smaller transformer can be used as long as it is protected with breakers for a very abnormal case.  Surges are a special case and it depends on how much voltage sag can be tolerated.   In industrial applications transformers can be rated at 3 to 4 times their normal current to prevent voltage sag.  If the most common trip is from a 10A hair dryer, why not just design for that with a one or two KW transformer. 

[ChrisOlson]

I see what you're saying but the balancing transformer for an off-grid stacked dual inverter setup is going to have to be larger than 1 or 2 kW if you're to live any "normal" life around the house.  Just my wife's hair dryer is 1,800 watts.  The microwave is 1,300 watts.  Coffee maker is 900 watts on brew.  One of my wife's crock pots or slow cookers draws 750-800.  If the transformer is only big enough to run one thing at a time, the female of the house is going to be a little upset when she throws dinner in the slow cooker, then goes to dry her hair after taking a shower.

We have gotten along fine with a 4.5 kW T240 on the 120 loads.  Don may need larger (I don't know).  It all depends on what you have to run at peak load times on 120 volt (usually in the evenings).  So the transformer can be rated at the output capacity of both inverters - IF you need that amount of 120 volt power and have the wiring in the house to handle it.

Your heavier loads like well pump, welders, air compressors, water heating (if electric), range (if electric) should be on 240 volt and this eliminates the problem with inverter balance.  And sure the loads should be balanced but they rarely are on split phase.  Your wife is doing something in the kitchen and drawing 3-4 kW and then you have a 120 volt 2 hp air compressor kick in in the shop that happens to be on the same leg of the split phase that your wife is using in the kitchen and you end up with no power because the inverter kicks out.  It's impossible to balance split phase legs when you have big intermittent loads like an air compressor or 120 volt well pump on either leg.

Only proper design of the system is going to fix it, and that includes a decent sized autotransformer that takes power from both sides of the split phase to power the 120 volt stuff.  Already been there, done that, and was in the same situation as Don.  The T240 fixed it, and while it's big enough for us, Outback does make a 6 kW autotransformer that would be better suited to heavier 120 volt loads.
--
Chris

[OperaHouse]

If you have gotten along fine with 4.5KW inverter, a 2KW transformer would give you 6.5KW on that side.  That may be sufficient for many people. If you have a 4KW imbalance I think you need to rethink how the wiring is set up.  It is an option people can take.  My only point is that the transformer does not have to be as large as the inverter to improve thins in most cases. It is easy to think of one side having an extra load.  The assumption that the other side would have a load of zero and could supply 100% is just not realistic. 

[ChrisOlson]

Only problem is, I don't know of a transformer than can sync its output with half the split phase output from one inverter.  I know that the T240 can't.  And I know the Outback FX240 (or whatever it is) can't.  For step-up from 120 to 240 all they do is take the inverter output and make a mirror image of it, 180 degrees out of phase for the other leg of the split phase, so 240 is gotten between the hot on the inverter and the output of the transformer.

For generator leg balancing with a 240 gen on a 120 inverter, or inverter balancing with dual inverters, all it does is step down the split phase output of the gen or inverter to a single sine wave 120 volt amplitude.  It's just a "dumb" transformer with no capability to sync in parallel with another 120 volt source of power.
--
Chris

[OperaHouse]

We have gone from a discussion of transformer balancing to inverter speak mumbo jumbo and I have lost any interest in going further. 

[ChrisOlson]

It's not mumbo jumbo.  I'm familiar with Don's problem because I got basically the same equipment as he does.  There used to be a SWI/PAR load balancing transformer that would parallel the sine waves from two inverters @120 volt, and lots of places still advertise them, but when you call they say that's out of stock.

So there is an alternative method that works very well, and that's method we have our system set up with.  It requires 240 split phase with a balancing transformer on the 120 loads to get full capacity from your inverters.  I just wanted to make sure Don understands this option, as it's much cheaper than the SWI/PAR load balancing transformer used to be, but does take some changes to the loads to use the split phase power where possible.

You just can't do it with a small transformer.  It takes a big one capable of handling ALL your 120 volt loads.

There's one factor with stacked inverters that few people consider, but it's quite important.  And that's efficiency.  If you're constantly loading one inverter to past 50% load the efficiency drops really fast and you're wasting a lot of power.  The idle or parasitic draw of the T240, or similar, is minor compared to the loss in efficiency of the heavily loaded inverter.

Setting up a stacked system with circuits attempting to keep things balanced, but both inverters are not perfectly balanced because you have no transformer, is just a "patch", basically to solve a kick-out problem from leg overload.  The XW's have auto leg balancing built in.  The SW's do not.

This is one of those BTDT things - just passing along what I learned about setting up my system so it performs at peak efficiency.
--
Chris

[boB]

These X240 and T240 auto-formers can do some pretty neat stuff.  When you connect them up like you're talking
about, Chris, it is like placing the two 180 degree out of phase inverters in parallel.  Like you say, the XW basically
does this internally...    It's just a tapped secondary transformer and both legs output windings are on the same
core.

When balancing legs from the 2 inverters onto one of the 120V legs from the T240, remember that due to
winding losses and resistance and such, that it is not perfect.  If you are loading down the L1 side more than
the L2 120V side, the L1 side inverter will still be delivering more power than L2,  but not nearly as much as it would
be without that balancing xformer.

BTW, the Outback X240 transformer is rated for basically 4kW without a fan and 6kW with a fan blowing
air over it.

boB

[ChrisOlson]

BTW, the Outback X240 transformer is rated for basically 4kW without a fan and 6kW with a fan blowing
air over it.

Yeah, I installed one of those Outback transformers on a VFX3624 at a place a few miles south of us where I also installed a wind turbine.  The guy need it to run his air compressor for his well (he has a air powered well).  It works really good on the 3624 and runs that 240 volt air compressor with no problem at all.

I wired it up with a transfer switch so he can also use the autotransformer to step down his 240 volt generator power for the charger in the 3624.  That works good too.

That Outback transformer was less money than we paid for our T240, but it didn't come with an enclosure for it.
--
Chris

[equiluxe]

If the 220 load is the smaller why not put the inverters in parallel and use a step up transformer for the 220v. If the only 220 is a well pump you could even put the switch gear on the low side of the transformer saving power when in the off mode.

[ChrisOlson]

Because you need that Sine Wave Paralleling Kit to parallel the sine waves from the two inverters.  You can't get that kit anymore.  When the inverters are stacked with a stacking cable they put out their sine waves 180 degrees out of phase with one another for split phase.
--
Chris

[OperaHouse]

I was just using my LEM power analyzer to see how much my plane consumed.  I used an ACME 1KW 24V stepdown transfprmer to power it.  From my childhood watching Road Runner cartoons I love anything ACME.  As mentioned before these transformers have losses even when no power is consumed.  This one measured about 15W.  It would be a nice UNO microprocessor project to sense inverter balance and only connect the balancing transformer when ever there was a signifigant inbalance  between the inverters or over a certain current level.  Enough smarts could be included for a seamless transition. 

[bergmanj]

Folks,

I bought an 8KW 120/124V autotransformer from this outfit for about $250.00 a couple of years ago; it's worth looking-at: "http://tortran.com/single-phase-dry-type-low-voltage-distribution-transformers.html?utm_source=May+2012+Tortran+email&utm_campaign=UA-27766632-2&utm_medium=email".

Regards,   JLB

[ChrisOlson]

As mentioned before these transformers have losses even when no power is consumed.  This one measured about 15W.

Typically, systems that have dual inverters with a balancing transformer will have a quite large battery bank and the generating capacity of the system is probably also quite large.  15-20 watts idle power consumption from a balancing transformer is usually negligible for these systems as the balancing transformer is rarely, if ever, idle.

If you have a system that generates maybe 2-3 kWh per day then these small things become more important.  We generate, and use, 25-30 kWh/day from wind and solar, and the "overhead" power consumption of our equipment (controllers, inverters, transformer) probably would consume all the power generated by a small system on a daily basis.  But for us it is not even noticeable.  You just bolt an extra solar panel on the rack and forget about it.
--
Chris

[Ungrounded Lightning Rod]

While a balancing transformer doesn't need to have power/current handling capability higher than the output of one side of the dual-inverter system, the one you have is less than that.

Like a motor, a balancing transformer can handle large overcurrents for a short time (say, 15 seconds) because the failure mode is overheating and the wires have significant thermal mass.  But long-term overload can result in insulation damage, turn shorting, and fire.  So if it were hooked up to the main it might work just fine for years - then (when you had a major long-term imbalance or a one-side inverter failure) suddenly fail.  And because it was out of code your fire insurance would be void.

If I were in this situation I'd:

 - Switch every load I could to 240V split-phase.
 - Put the remaining 120V loads (or the bulk of them, including any that are especially overvoltage-sensitive) on a subpanel with the transformer,  with a subpanel main breaker (either on the subpanel or in the main panel where the subpanel is fed) that is slow-blow and rated no larger than the current rating of the transformer.

That way the subpanel main breaker will protect the balancing transformer from overcurrent - at the cost of dropping the 120V loads if the current goes too high.  It has the disadvantage that balanced loads count against the transformer's protection breaker.  But it is a safe way to run with a transformer that is rated below the available power.  (Note that the transformer will also try to power any unbalanced loads on the upstream side by backfeeding power through the breaker, though it will still be protected.  So restricting the amount of 120V load on the subpanel doesn't necessarily keep the transformer from overloading and tripping out - though it does reduce the amount of BALANCED load from 120V loads on both sides that is mistakenly counted against the transformer's limit by the breaker.)

A simpler alternative is to just connect the transformer to the panel through a slow-blow breaker.  That very slightly reduces the effectiveness of the balancing (due to the impedance of the breaker) and means that if the transformer overloads the load is thrown back to the inverters (which will most likely immediately shut down the one on the heavy-load side).  But you're protected from transformer overload despite having an undersized transformer and the transformer can run up to the breaker's rating.  This might be adequate for your present situation and existing equipment.

If you try any of these suggestions please check the codes to be sure the configuration is legit.

[ChrisOlson]

[quote author=Ungrounded Lightning Rod link=topic=146768.msg1007554#msg1007554
That way the subpanel main breaker will protect the balancing transformer from overcurrent - at the cost of dropping the 120V loads if the current goes too high.
[/quote]

Some of the more expensive autotransformers, like our Xantrex T240, have overload protection built-in.
--
Chris