Bell curves [RE-wrenches]

William Miller wrmiller at charter.net
Tue Feb 7 19:01:11 PST 2006


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Friends:

I'm wrestling with a design for a 30KW grid tie system that has some 
significant distances involved.   The PV is 500 feet from the inverters and 
the inverters are 600 feet from the meter.  Were starting to talk 500 MCM 
to maintain 1-2% voltage drop.

These wire sizes really make me want to evaluate the logic used in 
calculating voltage drop.  I always use the maximum current a system is 
capable of generating. but that value only occurs once a day several times 
per year.  What about all of the rest of the mornings, evenings and low sun 
angle days?  The voltage drop quickly goes down to about zero.  If I'm 
smart, don't I want an AVERAGE voltage drop of 1%?

If this is true, I'm analyzing the bell curve of power production over a 
day.  To quantify the average voltage drop, I need the average power.  Is 
this the RMS value of that curve?  Or is it the average value?  Are they 
the same?

I think PV wire systems may be over designed if a fleeting peak value is 
used in all calculations.


William Miller

PS: The bottom line is to keep the AC and DC voltages at the inverter from 
going too high and causing the inverter to shut down.  After some research 
of the higher math here, it just may turn out that this is the real criteria.

Thoughts?

WM


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