Inverter Power Ratings [RE-wrenches]

[Kent Sheldon]

I feel I need to respond to this, as I was the engineer quoted in this
thread. Word travels fast. I'm glad people were listening.

I have never understood why designers feel it necessary to push PV
inverters to their limits. You would never buy a motor drive (same
inverter) and run it at full load all the time. Just because your car
red-lines at 8000RPM, do you drive it there every day, all day. A PV
inverter is typically less than 10% of the total PV system cost. Yet
most people try to make sure it will run at 100% of rated power for most
of the day.

I take calls every day from people wanting to put 3200-3600 watts of PV
on the SB2500U, and they want to hang it in the garage, perhaps in a
cupboard. I tell them, sure you can do it, but don’t call me when it
derates when it is hot. There are really two issues here; inverter life
expectancy, and temperature derating. Yes, the SB2500U is rated to run
at 2500 watts continuously, with cool ambient temperature. Yes it is
warranted to do this for 5 years. Will it last 20 years running
continuously at 2500 watts? Who knows. Will it produce 2500 watts at
50-60C? No way. The second issue is temperature derating. When there is
more power available to the inverter than it can process, the PV voltage
will rise as the power tracker is disabled. The higher PV voltage causes
higher losses in the IGBT bride, which translate directly to heat. So,
for example, if your PV array is optimally positioned for summer sun
angle, this will produce max power in the hottest time of the year. The
ambient heat, high power and the high DC voltage will cause the heatsink
to run hotter, which will cause the inverter to reduce AC power output
to regulate the heatsink temperature. So, in effect, you have gained
nothing. BTW, this is true for any inverter, not just Sunny Boys.

Bottom line, a PV inverter is an electronic device. Heat kills
electronics. The cooler they operate, the longer they last and the more
power you can put through them.

____________________________________________
Kent Sheldon
Manager, Industrial Power Systems
SMA America
ksheldon at sma-america.com
925 513 8813 Brentwood Office (Primary)
530 273 4848 Grass Valley Office




-----Original Message-----
From: douglas bath [mailto:douglas.bath at realgoods.com] 
Sent: Thursday, April 18, 2002 1:10 PM
To: RE-wrenches at topica.com
Subject: RE: How would you handle this situation? [RE-wrenches]

Indeed, I pointed this out the engineer that he was suffering from his
marketing department. He would reiterate that his unit could and would
operate at 2500 as specified. They should build the thing to 2800 and
call
it a 2500! Still, the consequences of redlining should be considered,
and
perhaps we need to back off a bit on our equipment for long term
reliability...after all, "it's just a solar electric system".

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