[RE-wrenches] SB Power during countdown

[Matt Lafferty]

Drake,

It's normal for some of the SB units to yank on the grid for a second during
the countdown. Don't know about the 2100 specifically. This is part of a
grid integrity check. There's more to grid integrity than just volts and
hertz. Check your gen meter when this happens. Which way does it turn?

I am curious about your reported DC voltage readings, though. If the
inverter guts didn't smell smoky when you pulled the cover, you most likely
have a DC fault outside the inverter. Sometimes these are tough to find and
sometimes they are intermittent. Did the GFP happen to blow after a rain?
Definitely inspect the array for physical damage and potential water in
j-boxes, etc. Thin-films are more prone to leakage current than mono or poly
by nature. What kind of glass is it?

Is there a DC lightning arrestor on this system? If yes, disconnect it and
measure your voltages again. No matter what, use your DC clamp to measure
current before taking any connections apart! If this array has a dedicated
ground rod, check for current on that GEC and whatever bonding jumper you
have.

Chasing ghosts is best done with plenty of sunlight. I won't bother driving
to troubleshoot something like this unless I have two things... Time and
Sunshine. Driving up at 4 o'clock in the afternoon in the middle of November
means driving back another day anyway.

Don't open any connections without checking for current with a clamp meter.
This is for safety as well as diagnostic purposes. Since one of the elements
we are dealing with is static, voltage readings can be somewhat misleading
at times. Current, however, tells a cautionary tale. Now, on with your
case...

If you got the 500V reading with everything in the same state as your "zero"
readings, you must have an open ground path between the point where you're
taking these readings (@ the inverter???) and the module frames/racking.

Did you get a spark when unlanding either of the DC wires?

If the grounding system is intact throughout, and you are actually getting
0V readings between either pole and ground, that means that pole is common
to ground, i.e. shorted. By saying that you are getting 0V between both
poles and ground, but getting 500V between pos and neg.... That doesn't add
up if the equipment grounding is intact. 500mV maybe???

If your equipment ground path is continuous, and you don't have any DC
faults, you should get a floating value when measuring voltage between
positive and ground or negative and ground. Not "zero".

In a monopole DC circuit you have three elements to check.... Positive,
Negative, and Equipment Ground. With all intentional loads and DC circuit
grounds removed:

A)  Measure and Record DC Voltage between Positive and Negative (VOC).
B)  Measure and Record DC Voltage between Positive and Ground.
C)  Measure and Record DC Voltage between Negative and Ground. 

If you have a value close to the expected VOC for A, any value other than
"floating" for B or C indicates either an open equipment ground path or a
ground fault. You cannot actually have the expected VOC between positive and
negative, and have zero volts between each and ground at the same time in a
single circuit with the equipment ground path intact.

I have seen more than a few cases where head-scratcher stuff like this
happened. The following examples remind me of situations you might be
facing.

Multiple inverters and crossed wiring between the array and the inverters.
The installer didn't take the GFP fuses out of the inverters when
installing. Any voltage readings he might have taken appeared to be correct
because the negatives for both systems were common to each other thru the
GFP fuse. The installation happened in the winter and ran fine until it got
a decent shot of sunshine a few weeks later. One system blew the GFP fuse
and they both shut down. Suffice it to say there was a pile of 1A fuses by
the time the guy called me.

Undersized but "legal" equipment grounding conductors are used. If the DC
homerun pipe is plastic and the equipment ground between the inverter and
the array is small (#14 or #12)... You might get those voltage numbers.
Solid wire under that ground screw in the back of a bell box? Look there
first. Basically, look for a broken grounding conductor or any open in the
grounding path.

If the array is Unisolar lams... You can have any number of wild
combinations going on. If the pan isn't bonded to ground really well, you
could have these kinds of numbers and everything might be "fine". If there
are multiple strings tied parallel in a j-box, you might actually have more
than one fault. Especially if that pan isn't bonded really well to that
equipment grounding conductor back at the inverter.

Other clues to consider... How many kWH did the original inverter make? How
many did this one make? When was the system originally installed? How long
did the original inverter last? How about this replacement?

Hope you let us know what your find out. Best of luck and be careful!

Solar Janitor

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Drake
Sent: Saturday, November 06, 2010 8:17 PM
To: RE-wrenches
Subject: [RE-wrenches] SB Power during countdown

Hello Wrenches,

I'm troubleshooting an SWR 2100U.  During the countdown, the inverter begins
producing power.  There is a utility style kWh meter (Hialeah
meter) next to the inverter.  During the countdown the wheel on the meter
spins fast for several seconds, then stops.  The inverter continues to count
down and the green light continues to flash in the waiting mode.  The
display on the inverter never says it is producing power.

The inverter blew its ground fault fuse.  There is no indication of a fault.
I haven't Megger tested yet, but array produces around 500 Voc.  With wires
off the terminals, in free air, there are readings of between 0.0 and 0.1 V
to ground on both the positive and negative wires.  SMA tech support agrees
that this is a very clean reading.

This inverter shows all the signs of being a glitchy unit, for sure.  The
problem is that this is a replacement for the original inverter that had
very similar issues.

What would make an inverter produce power during the countdown?  Is this
part of a larger pattern?  Is there a problem with the system, or did I get
a second, problematic inverter?

Thanks,

Drake