[RE-wrenches] : Strange voltage readings, power quality

[whitley w]

Mac,

First time poster here, sounds like in your original configuration you were loading all inverters on Vb. My suggestion would be to balance the load evenly across all three phases (7.5 - Va-Vb, 7.5 Vb-Vc, 10.0 Va-Vc). It is not perfect, but this may balance out the system when loads are coming on and off and have a more stable voltage.

On Sun, Nov 13, 2011 10:55 AM CST mac Lewis wrote:

>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