inverter shoot-out at high noon [RE-wrenches]

Tom Lane, Energy Conservation Services tom at ecs-solar.com
Fri Mar 26 10:55:15 PST 2004


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as per grid connected inverters I have a $100 dollar magical device that
solves all arguments  --- put a  new electric meter on the  output of all
inverters    gator tom ----- Original Message ----- 
From: "John Berdner" <jberdner at sma-america.com>
To: <RE-wrenches at topica.com>
Sent: Friday, March 26, 2004 1:48 PM
Subject: RE: RE: inverter shoot-out at high noon [RE-wrenches]


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**** Bounces like rubber! Shatters like ceramic! ****
Discover Crazy Aaron's Thinking Putty in grown up
handfuls. It's the creativity unleashing, mood enhancing
desk toy!
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Wrenches:

Wow, I once again find myself wondering if I was just publically
insulted by Bill Brooks ;^)
Well maybe, so I'll respond.
I have personally been asking the CEC for independent performance
verification of inverter performance for over three years.  We have been
told this is an extremely complex problem that requires lots of study.
Reminds me of an old Firesign Theater line: "It's very electrical and
dangerous".  Fortunately it sounds like the idea it has spent enough
time in the aging drawer to re emerge as a good idea again.  I just
looked at the Sandia Endecon testing protocol and it looks like a
reasonable start.  Still needs to be verified against real world
conditions to see if it actually means anything but it is a step in the
right direction.

Back to Bill's comments:

2) SMA does take SYSTEM performance very seriously and we do think it
matters to you the system designers.

1) I am forced to agree with this first point In a very limited way due
to a logic failure in the statement.
The only way we are unaware of a problem is if it is discovered and no
one lets us know.
Given that I have had zero feedback from Bill on any "problems" he has
discovered I can only assume there is no problem.  But since Bill has
not given us any feedback then I guess we do fall into category 1.
This is, of course, not to say we choose to remain blissfully ignorant
as implied by Bill's statement.
We want to hear about performance issues with systems so we can work to
understand them and take corrective action.  This is true even if it has
little to due with our inverters.

One recent example with a happy ending comes to mind:

A customer in Pennsylvania had (2) 1800's that were having frequent
over current faults.  We sent out replacement product with no success.
We then had our senior engineers both here in the US and in Germany do
extensive troubleshooting by email and over the telephone, again with no
success.   We then sent the customer a digital storage scope and
instructions on how to capture some waveforms.  By everything we could
see it looked like the ac neutral was shifting causing the inverters to
trip offline.  Unfortunately we still could not figure out what loads in
his house were causing shift and so the customer was still frustrated by
the problem.  We were preparing to send one of our senior engineers to
Pennsylvania when the customer by chance noticed a strange correlation.
He was outside measuring the current in the ac ground wire running from
the ground rod.  All of sudden he saw several Amps flowing in the ground
wire.  Sure enough he heard the inverters trip offline and, at the same
time, he noticed the sound of the trolley car going by his house.
Turns out they have a DC trolley system in his town and the inverters
were tripping offline every time the trolley cars went by.  Further
investigation by the customer discovered that lots of folks in the town
were having mysterious power quality problems.  We now believe the
transit district has a bad rectifier at one of the sub-stations feeding
dc to the track.  This would explain the large ground current spikes and
neutral shifts every time the trolley goes by.  A new 120/120 to 120/120
isolation transformer nicely solved the problem.

What we learned from this installation is that is always an inverter
performance problem - no matter what.

REgards,

John Berdner


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>>> billb at endecon.com 3/26/2004 09:16:40 AM >>>

Chris,

Although I certainly support side-by-side testing--and am currently
doing
just that on 12 systems for the CEC, the information that is gleaned
from
this testing process is incredibly valuable. I would love to see
both--let's
get everyone together and propose it to someone who can fund it. That
being
said, I still want to challenge you to participate in the development
and
refinement of the test procedure--you're viewpoint is very valuable.

I have learned things about the inverters we have tested so far that
would
never have been learned from the manufacturer because they either 1.)
didn't
know there was a problem; or, 2.) didn't think it mattered to a system
designer. We make huge assumptions about how an inverter is supposed to
do
its job, and then are shocked when it doesn't meet our expectations.
If
Outback has not performed this test on their inverters, I am sure there
are
some things about your inverter's performance that you are unaware of.

Our industry has been starving for a metric to provide a baseline for
the
performance of an inverter. Your message below supports that notion.
The
inverter development efforts at Sandia are a totally separate issue,
and I'm
not sure that is the best program either. But the need for a
performance
test is unquestionable. Don't throw the baby out with the bathwater.

My hope is that all these new inverters, including Outback, will
succeed as
high-quality, efficient new options for system designers and
integrators. I,
for one, wouldn't put one on a customer's facility until I learned a
lot
more about the unit. This test procedure is just one of several hoops
that
any sane installer would force a prospective product to jump through
before
being willing to install it at a customer's facility. I think every PV
installer in California that has been working for more than 3 years
can
personally attest to that truth.

Bill.









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