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Small grid-tie inverters? - Page 11

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Posted by Vaughn Simon on June 30, 2008, 12:07 pm

UPS is an overused technology.  Millions of $ are wasted buying them, and
megawatt-hours of power is wasted on the power losses of unnecessary UPS
installations.  Any UPS system represents a potential point of failure.  I have
seen more data closets fail because of UPS failure than any other single item.

And yes, I have seen transfer switches fail too...  One notable example was in
the middle of a hurricane.  In that particular incident, we would have been far
better off without it, since it left a public safety radio system for a medium
size city dead until a suitably qualified human could brave the storm and
manually unjam the switch.

It is simple math.  The more equipment you have, the more equipment failures you
can expect (MTBF).


Posted by Cydrome Leader on July 1, 2008, 3:26 am

I just ran a test similar to the other fellow here.

by ramping up from 0 to 100, 110 and 120 volts with an autotransformer I
measured the VA, current and watts needed to excite modern 10kVA
distribution transformer by back feeding the secondary.

Transfomer was 10kVA, aluminum wound, 3.6% impedance.

current, watt and VA readings were taken off a killawatt meter connected
to the autotransformer. As cheap as these things are, they're pretty

volts fed    excitiation    VA    Watts
to transformer    current
100        0.27        27    27
110        0.33        36    31
120        0.89        94    41

I never noticed this, but the power factor starts to drop like crazy as
the input approaches 120V. The numbers are a bit off, probably because my
line voltage fluctuates enough to make a difference between the time it
takes to hit buttons on the killawatt.

This is about as small as they come these days in distribution
transfomers. Even if you could magically soft start a transformer like
this, with a 250VA source, you aren't going to be able to do anything, and
that assumes nothing else at all is connected but a transformer a few feet

Posted by We Can Do It on July 1, 2008, 6:14 pm
news:g4c85v$d52$> I just ran a test similar to the other
fellow here.

What exactly are these numbers?

With 100 volts backfed into the 120 Volt side one phase to
ground are you saying the Killawatt measured
.27 VA
27 amps
27 watts????????????????????

Was there 10,000 volts on the power line side with the
conductors open.

What happens if the lineman touches the house side of the poll
pig while you are backfeeding it.


Posted by Cydrome Leader on July 1, 2008, 6:32 pm
It's a text table of numbers. If you set your newsreader to correctly use
monospaced fonts, it will be easier to read.

left column is voltage being backfed into the transformer
column 2 is excitation current of the transformer running no load
column 3 is how many VA are being used
column 4 is how many watts the transformer uses idling.

Voltage one the primary of this transformer with secondary at 120 is about
14.4kV, with no load attached.

The point of all this is, a wimpy distrubution transormer can use about
100VA doing nothing. that number will jump once you have anything
connected to it. a 250VA inverter (if it's even accurately rated) isn't
going to be able to run that thing.


Posted by BobG on July 2, 2008, 12:53 am
Well crap. Assuming my little PV system isnt going to make the
neighborhood greener by sharing my 250 watts, will it slow my meter
down on its way up the pole to the transformer?

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