[RE-wrenches] Strange voltage readings, power quality

mac Lewis maclewis1 at gmail.com
Sun Nov 13 08:55:34 PST 2011


Hello wrenches,

I am having an interesting grid-tied issue and would love to get some input.

The installation is 3 Fronius IG Plus inverters all single phase, two 7.5s
and one 10.0.  The service is 120/208.  We have the two 7.5s feeding a
dedicated 3 phase sub and tied back into the main panel after production
meter.  The first 7.5 is tied across Va to Vb, the second is tied in across
Vb to Vc.   The 10.0 feeds directly back to the main panel after production
meter and is tied in across Va to Vb.
It should be noted that this facility has very large capacitive load, about
25 kW of grow lights.  According to an egauge on site, the power factor
ranges from 0.5 to 0.75 on all phases.
The system worked flawlessly for 3 months, suddenly the two 7.5s dropped
out and gave a myriad of grid errors, everything from islanding errors, to
high grid voltage, and low grid voltage etc.

I went over there and found the phase B voltage to neutral, was way out of
wack, anywhere from 120-170.  Oddly, the Line to line voltage was fine,
right around 208 for all phases.  I called the power company and had them
install a monitor at the transformer.  After 5 days they came back and said
that their end was within spec.  I decided to put both of the 7.5s between
Va to Vc.  They would not boot up.  I then went to the main panel and
changed the physical location of the breaker for the subpanels of the 7.5s.
 This seemed to make the difference.  They booted up and are exporting
power.  I tried switching one of the 7.5s back Va to Vb, but it still would
not boot up unless it is connected from Va to Vc.

My working theory right now is that we are getting harmonics traveling back
to the inverters from the ballasts on the grow light across line B.  As far
as explaining why it suddenly happening, my guess is that he has started
turning off inductive loads such as fan motors and compressors as we are
entering the winter.  There may have been a filtering effect going on with
the motors absorbing some of these harmonics and attenuating them.  I am
going to try to verify this with an oscilloscope this week.

Does anyone have any other theories to explain this type of behavior?
 Anyone seen this before?  Should I look for anything else?

Thanks

Mac Lewis


-- 



Mac Lewis

*

"Yo solo sé que no sé nada." -Sócrates
*
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