I could see how they might get confused between hi-pot
something-or-other and capacitance.
But for hi-pot arc-over, you don't really have to have much
capacitance. Just one spot with
too little spacing for it to arc across is enough to fail. i.e. Hi pot
problems unfortunately don't
creep up usually, they just happen when you pretty much least expect it.
oOOOPS. Forgot... In addition, if you somehow do try to measure the
capacitance of the array,
there will probably want to be at least some PV current flowing from
plus to minus when they
are shorted so early morning or evening would work as well as full short
circuit current during
the day I would think.
Some current because the measurement might not get the full array of
capacitance to ground
if all those PV diodes aren't at least forward biased which would happen
when light out.
Don't want any current through the capacitance meter itself of course.
boB
On 3/13/2014 11:29 PM, Jerry Shafer wrote:
I had this issue way back with pvl's unisolar and it was always
written off as hi pot leakage in the conductors and its good to see
that it was put to paper
On Mar 13, 2014 11:20 PM, "b...@midnitesolar.com
<mailto:b...@midnitesolar.com>" <b...@midnitesolar.com
<mailto:b...@midnitesolar.com>> wrote:
Interesting app note. Interesting problem.
How far from this metal roof (grounded I presume) is the bottom of
the modules ?
This app note is mainly talking about the area of the module from
the mounting bracket which should
be quite a bit less than the modules area from a distance above a
grounded metal plate like this roof.
How far above the metal roof are the modules ? If they are real
close, raising them slightly might
be enough to lower that capacitance enough to stop the TL from
tripping.
Another thing you could do, but would of course cost money would
be to split the array up for
more inverters.
This app says that 1400 nano-Farads is an approximate point for
problems. That's for 50 Hz.
It will be around 6/5 worse at 60 Hz.
If you have a capacitance meter (impedance bridge or meter), you
could measure the capacitance
of the array. Disconnect the + and - from the inverter first of
course. Then, tie the plus and
minus of the combined array together and connect that to one
terminal of the capacitance meter.
Connect the other lead of the capacitance meter to the roof and/or
ground and it ~should~ show
the capacitance you are interested in.
This is one of those newer compromises associated with the higher
efficiency TL inverters (anybody's,
not just SMA's of course), and also the newer code requiring TL
(non-isolated) inverters above 300 volts
and everything else new we have to all learn about.
Welcome to the future ! ( from Firesign Theater's "I think
we're all [solar] bozos on this bus" )
boB
On 3/13/2014 7:31 PM, Marco Mangelsdorf wrote:
Friends,
We have run across an odd technical problem at our commercial
install at a laundromat on our island. Attached is the
description of the issue from SMA. We don't know much except
that we are switch out the TL inverters to the old style US
inverters because SMA has no fix for the phantom ground tripping.
The issue is beyond our feeble understanding.
Any observations to share?
Thanks,
marco
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