[RE-wrenches] Odd Inverter Behavior

[vince at solarenergydesign.com]

Hi August
I find it unlikely that you have only one problem with the design and
construction of this system. A quick estimate of string sizing shows that
for most climates 8 modules to a series string is inadequate. But this
time of year low voltage would not be the problem. After you add the NEC
required equipment grounding conductor, a good place to start would be
checking individual series string conductors for both voltage and amperage
while the system is operational. If you don't have a good DC clamp meter I
would recommend getting one ASAP. My guess is that you have some kind of
power output loop from the array where  one series string is feeding back
in to another series string or is shorted to ground somewhere. One of my
least favorite jobs is trying to fix another contractors work because it
makes you have to guess how much the other contractor knew. In this case I
think you may as well start from zero.

> Hi All -
>
>
>
> Thanks for the ideas. It definitely sounds like should trying wiring in at
> least another module in series to see how it acts.
>
>
>
> Best, August
>
>
>
>
>
>
>
> *From:* re-wrenches-bounces at lists.re-wrenches.org [mailto:
> re-wrenches-bounces at lists.re-wrenches.org] *On Behalf Of *Warren Lauzon
> *Sent:* Friday, February 04, 2011 5:55 AM
> *To:* RE-wrenches
> *Subject:* Re: [RE-wrenches] Odd Inverter Behavior
>
>
>
> That 284 volt number is if the panel is at 25 degrees C. If the panels are
> in full sun and with little wind, they can be higher temperature than that
> even in cold weather.
>
>
>
> An easy way to get a rough check on that is to disconnect the panels and
> temporarily haywire an extra panel in series from another string. I
> suspect
> the array might have to be rewired for 2 x 12.
>
>
>
>
>
> *From:* August Goers <august at luminalt.com>
>
> *Sent:* Thursday, February 03, 2011 5:01 PM
>
> *To:* RE-wrenches <re-wrenches at lists.re-wrenches.org>
>
> *Subject:* [RE-wrenches] Odd Inverter Behavior
>
>
>
> Hi Wrenches -
>
>
>
> We were called out today to check out an under performing system installed
> by another contractor. This is an odd case.
>
>
>
> The system has 24 Suntech 180s in three strings of 8 and a SMA Sunnyboy
> 4000us. Open circuit voltage for each string was about 330 Volts and we
> verified that everything appeared to be wired in correctly. There is no DC
> side grounding at all. Just a positive and negative coming down off the
> roof
> for each string.
>
>
>
> With all modules in direct light the inverter is only pumping out about
> 250
> Watts steady state. We noticed that when we turn it off and on again that
> the power went up to about 1500 Watts as it started tracking the max power
> point. Then it slowly veers off course and settles out at about 250 Watts
> and is running very close to the open circuit voltage of about 330 volts.
> I
> would expect the MPP voltage to be closer to ~280 Volts. The inverter MPP
> tracking range goes down to 250 Volts so I don't think that's the problem.
>
>
>
> Anyone ever seen anything like this before? What might be causing the
> inverter to get confused? The only thing I can think of is that something
> fishy is going on with the lack of a DC side ground and that certainly
> needs
> to be fixed anyway.
>
>
>
> All thoughts welcome.
>
>
>
> Best,
>
>
>
> August
>
>
>
>
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