[RE-wrenches] Odd Inverter Behavior

[August Goers]

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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