Outback X-240 Autotransformer [RE-wrenches]

[Drake Chamberlin]

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Hi Christopher,

The applications that I'm trying to find the efficiency for, is for 
AC coupled systems.  We have some SMA systems with 240 V Sunny Boys 
connected to the grid through the transfer switches of 120 volt Sunny 
Island inverters, via Outback X-240s.  Some installers use a relay to 
bypass the X-240 to connect directly to the grid for sell back.  In 
this case, is the relay necessary, due to transformer losses, or are 
the losses so low that the relay can be omitted?

In a related issue, I'm considering a system design where off grid 
VFX inverters will be used to charge batteries through AC coupling 
with an on grid inverter, when the grid is down.  The on grid 
inverter (SMA or PV Powered) will connect to the grid directly via a 
relay when the grid is up.  When the grid goes down, it will sync 
with the AC out of the VFX (through the X-240, when using one inverter).

The reason for this design, is to allow generator backup as well as 
grid sell back.  We would need to use a dump load, of course.  Do you 
see any problems or conflicts with this approach?

We AC coupled with the Grid Tie VFX inverters in a Windy Boy 
system.  Will this arrangement work with the off grid VFX inverters 
for battery charging?

Thanks,

Drake


At 05:04 PM 1/7/2008, you wrote:

>Darryl, Wrenches -
>
>The efficiency of a transformer is a bit complex and takes some special
>equipment to measure accurately - here is more info.
>
>There are two types of losses involved with any transformer - the first
>is the "idle" or magnetization loss and the second is the "conduction"
>losses which relates to the amount of power the transformer is carrying.
>  This can be easily measured by a watt meter with the application of
>120VAC to one of the windings.
>
>The idle power loss of the X-240 is about 10 watts.  This is consumed 24
>hours a day unless the transformer is disconnected. This can be easily
>measured by a watt meter with the application of 120VAC to one of the
>windings.
>
>The conduction loss of the X-240 is about 97% efficient when shifting
>2.5kW from one leg to the other - which is about 75 watts of losses.
>This can be measured by the voltage drop through the transformer when
>carrying the stated load level.
>
>So - for a system using two inverters providing 120/240 VAC - the idle
>loss of 10 watts is made up for by the system turning the second
>inverter (slave) to standby - reducing its idle power consumption but
>still keeping the 120/240VAC available via the transformer.
>
>So - for a two inverter system there is a small advantage using the
>X-240 autotransformer.  For a quad system is has even more advantage.
>
>The real benefit in my opinion of including the X-240 is a split phase
>system (120/240VAC) is that it gives higher surge power on each of the
>AC legs - and therefore better voltage regulation and stability.  Its
>nice to be able to overload one leg above that leg's inverter ability
>for short term conditions - such as when a hair dryer and a waffle iron
>just happen to be plugged in on the same leg of 120VAC power.  Without
>an X-240 you would either need bigger inverters on each AC output leg or
>would need to be very careful to not have this occur.
>
>So - the X-240 is not a bit source of losses in a system - especially if
>the AC loads are reasonably well balanced with imbalances occurring
>intermittently.
>
>Christopher


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