[RE-wrenches] Odd Inverter Behavior

[Darryl Thayer]

I have seen that behavior almost.  if open cirucit is 330 then MPPT should be 
about 264 which is to close to bottom of the window.  Last summer a sunny boy in 
a similar situation of 8 modules in string was droping out on MPPT tracking.  
The inverter had additional module added and started to behave correctly.  
Sometimes the manufacture wants the sale and some inverters might not be in 
spec, so becarful of string sizing calculators.  


Another case the tracking algorithm was defective and the modules were operated 
at to high a voltage above the Vmp voltage, I replaced the inverter and problem 
fixed.     





________________________________
From: August Goers <august at luminalt.com>
To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
Sent: Thu, February 3, 2011 6:01:11 PM
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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