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Small grid-tie inverters? - Page 9

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Posted by Solar Flare on July 14, 2008, 2:08 am
 
The water would have to be mighty pure for that one.

They do it live for the voltages up to 27kV, I have watched, here.




Posted by Bruce in alaska on June 28, 2008, 8:12 pm
 


Just a Note here, When we are considering Islanding and Backfeeding,
one thing to consider IS, that if some Yahoo does "accidently" Plug
his 2Kw Genset into a house with the Main Breaker Still on during
an outage, it isn't the 2Kw of power that is going to travel back up
the Distribution System, but the Voltage Spike that that 2k Genset
is going to cause, untill either it's internal Breaker Trips, or the
Magic Smoke pours out of the Genend.  It will be the spike that
zaps the folks up the line.  Yes, ALL Linemen, know what Grounding Staps
are for, and use them, as well as Hot Sticks, Hv Gloves, and all the
other protective gear associated with Power Transmission Systems,
BUT, the NEC REQUIRES Transfer Switches and Anti-Islanding Devices
for a reason, and it IS really, Safety Related.

--
Bruce in alaska
add <path> after <fast> to reply

Posted by Neon John on June 28, 2008, 9:57 pm
 

Well, there's safety and there is blind dumbshit idiocy that ignores common
sense masquerading as safety.  Much of the NEC is in that category.  

Before a 2kW generator can do anything to that lineman down the road who is
careless enough to be handling a conductor without his gloves and without
grounds, it first has to power up the house, any other houses connected to the
pig, the magnetizing current of the pig, the capacitance loading of the
primary, the magnetizing current of any other pig on the "island, any other
houses on any other pig, etc, excess...

Ain't gonna happen.  What the generator sees is essentially a dead short. With
most small generator designs, the field de-excites and the generator does
nothing.  The only time that islanding is even a remotely credible hazard is
if someone is at the end of a long primary line in a rural setting and the
primary line is downed near the pig feeding the facility.  A large farm might
fit that description.  A large farm is likely to also have a PTO-driven or
stationary 20kW or larger generator.  Even if he did manage to backfeed and
energize the primary, when the first lineman on the scene does what every
lineman is trained to do automatically, secure grounds, the generator is
dead-shorted.

That's really not what this thread is about since we were discussing a 250
WATT inverter and not a 2kW one.  There has been so much yammering that I
decided to do an experiment.  I went down to my shop and rolled out a 20kVA,
14.4kV pole pig (doesn't every shop have one or two?)  I don't have a 250 watt
inverter but I do have several 350 watt ones.

The experiment consists of hooking the inverter to a pair of 100 amp-hour 12
volt deep cycle batteries in parallel to make sure there is enough inrush
available on the 12 volt side.  A 120 volt Jesus cord runs directly from the
inverter to the neutral and one hot leg of the pig's secondary.  The primary
is unconnected except for a short jumper leading over to an electrostatic
kilovoltmeter.  An electrostatic meter has infinite impedance and presents no
load.

I did 10 shots.  That is, with everything wired up, I flipped the inverter
switch to "on".  Ten out of ten times the inverter tripped instantly.  The
voltmeter didn't move a bit.  I guess that I could have hooked a scope and HV
probe to the primary but it wouldn't have told me much more.  Obviously the
inverter's protective circuitry trips it off long before it can complete even
a single cycle.

I tried the same experiment with a 1.5kW inverter that I had handy.  Same
result except that I could see just a blip on the electrostatic voltmeter.
Maybe a half-cycle of output completed before this inverter shut down.

This pole pig is extremely efficient.  I once measured the standby losses.  I
don't recall the exact number but it was something tiny, on the order of 10
watts or so.  HOWEVER!  It needs several kVA of magnetizing "wattless" power.
That's the demand the inverter sees and can't supply.

This experiment on a pole pig with NOTHING attached to it shows just how silly
it is to be discussing back-feeding from a 250 watt inverter.

I know from other experience that it is very difficult, almost impossible, to
energize this pig even from a 7kVA generator by closing the breaker once the
generator is up to speed.  That makes it kinda difficult to feed a large
Jacob's ladder from the generator.  The only way that I can make it work is to
close the breaker before starting the generator.  Then, most of the time, they
come up together.

John
--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com  <-- best little blog on the net!
Tellico Plains, Occupied TN
I like you ... you remind me of me when I was young and stupid.


Posted by Duane C. Johnson on June 28, 2008, 11:18 pm
 Neon John wrote:

 >
 >
 >>
 >>
 >>>this sounds pretty accurate. The inrush current to even power on a small
 >>>distribution transformer is extremely high and would make any small
 >>>inverter just shut down thinking it was shorted out.
 >>
 >>Just a Note here, When we are considering Islanding and Backfeeding,
 >>one thing to consider IS, that if some Yahoo does "accidentally" Plug
 >>his 2Kw Genset into a house with the Main Breaker Still on during
 >>an outage, it isn't the 2Kw of power that is going to travel back up
 >>the Distribution System, but the Voltage Spike that that 2k Genset
 >>is going to cause, until either it's internal Breaker Trips, or the
 >>Magic Smoke pours out of the Genend.  It will be the spike that
 >>zaps the folks up the line.  Yes, ALL Linemen, know what Grounding Straps
 >>are for, and use them, as well as Hot Sticks, Hv Gloves, and all the
 >>other protective gear associated with Power Transmission Systems,
 >>BUT, the NEC REQUIRES Transfer Switches and Anti-Islanding Devices
 >>for a reason, and it IS really, Safety Related.
 >
 >
 > Well, there's safety and there is blind dumbshit idiocy that ignores common
 > sense masquerading as safety.  Much of the NEC is in that category.
 >
 > Before a 2kW generator can do anything to that lineman down the road who is
 > careless enough to be handling a conductor without his gloves and without
 > grounds, it first has to power up the house, any other houses connected to the
 > pig, the magnetizing current of the pig, the capacitance loading of the
 > primary, the magnetizing current of any other pig on the "island, any other
 > houses on any other pig, etc, excess...
 >
 > Ain't gonna happen.  What the generator sees is essentially a dead short. With
 > most small generator designs, the field de-excites and the generator does
 > nothing.  The only time that islanding is even a remotely credible hazard is
 > if someone is at the end of a long primary line in a rural setting and the
 > primary line is downed near the pig feeding the facility.  A large farm might
 > fit that description.  A large farm is likely to also have a PTO-driven or
 > stationary 20kW or larger generator.  Even if he did manage to backfeed and
 > energize the primary, when the first lineman on the scene does what every
 > lineman is trained to do automatically, secure grounds, the generator is
 > dead-shorted.
 >
 > That's really not what this thread is about since we were discussing a 250
 > WATT inverter and not a 2kW one.  There has been so much yammering that I
 > decided to do an experiment.  I went down to my shop and rolled out a 20kVA,
 > 14.4kV pole pig (doesn't every shop have one or two?)  I don't have a 250 watt
 > inverter but I do have several 350 watt ones.
 >
 > The experiment consists of hooking the inverter to a pair of 100 amp-hour 12
 > volt deep cycle batteries in parallel to make sure there is enough inrush
 > available on the 12 volt side.  A 120 volt Jesus cord runs directly from the
 > inverter to the neutral and one hot leg of the pig's secondary.  The primary
 > is unconnected except for a short jumper leading over to an electrostatic
 > kilovoltmeter.  An electrostatic meter has infinite impedance and presents no
 > load.
 >
 > I did 10 shots.  That is, with everything wired up, I flipped the inverter
 > switch to "on".  Ten out of ten times the inverter tripped instantly.  The
 > voltmeter didn't move a bit.  I guess that I could have hooked a scope and HV
 > probe to the primary but it wouldn't have told me much more.  Obviously the
 > inverter's protective circuitry trips it off long before it can complete even
 > a single cycle.
 >
 > I tried the same experiment with a 1.5kW inverter that I had handy.  Same
 > result except that I could see just a blip on the electrostatic voltmeter.
 > Maybe a half-cycle of output completed before this inverter shut down.
 >
 > This pole pig is extremely efficient.  I once measured the standby losses.  I
 > don't recall the exact number but it was something tiny, on the order of 10
 > watts or so.  HOWEVER!  It needs several kVA of magnetizing "wattless" power.
 > That's the demand the inverter sees and can't supply.
 >
 > This experiment on a pole pig with NOTHING attached to it shows just how silly
 > it is to be discussing back-feeding from a 250 watt inverter.
 >
 > I know from other experience that it is very difficult, almost impossible, to
 > energize this pig even from a 7kVA generator by closing the breaker once the
 > generator is up to speed.  That makes it kinda difficult to feed a large
 > Jacob's ladder from the generator.  The only way that I can make it work is to
 > close the breaker before starting the generator.  Then, most of the time, they
 > come up together.
 >
 > John
 > --
 > John De Armond
 > See my website for my current email address
 > http://www.neon-john.com
 > http://www.johndearmond.com  <-- best little blog on the net!
 > Tellico Plains, Occupied TN
 > I like you ... you remind me of me when I was young and stupid.

Hi John;

I like your ingenuity in performing this experiment.
Thanks!

But, (there is always but).

I was thinking that your test into what amounts to be
a dead short might not be the case that UL-1741 is
testing for.

The grid tie inverter should be assumed to be driving
the pig in an already energized condition. I would
guess that the inverter could keep up with the small
losses once running.

Is there a way you could try to do your energizing test
again. Except this time try to get the transformer
humming before you directly connect the inverter.

You might do this by initially placing a resister in
series to get it going. To do the test with your
350W inverter I would try a 100W incandescent lamp
for the resister.

It would be nice to see if the transformer currents
could be maintained after energization.

Thanks!
Duane

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Posted by Vaughn Simon on June 28, 2008, 11:56 pm
 

   3-100W lamps would be better, but I wonder how valid the test would be unless
you are using a sine wave inverter.

   This is turning into an interesting thread!  I also thank John for performing
the experiment.

Vaughn



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