Bell curves [RE-wrenches]

[Ray Walters]

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Hi William;

You could play with my wire sizing spread sheet if you like. Its 
available at http://www.solarray.com/TechGuides/TGmain_T.php  . GO to 
the design section to the far right it is labeled "advanced wire sizer". 
The whole idea is to compare watts lost on the line to the cost of the 
wire. Your case is exactly why I cooked this up: the wire costs are 
going to be outrageous vs. the power being lost. The spread sheet lets 
you put in wire costs, installed PV per watt cost etc, and compare 
different wire choices. It also figures whether you're using MPPT, and 
it considers for how many hours a day you're running at full amperage. 
(Wire should be sized larger for loads that are continuous 24 hrs/ day 
vs. a 30 minute a day load.) I also tried to add in sizing for NEC 
compliance (you can input both ambient temp and wire temp), PV array NEC 
compliance, and then threw in normal wire sizes used in the automotive 
field (NEC would demand 4/0 cable and a 300 lb starte for instance)
The whole spread sheet is still just version 1, so your feed back would 
be greatly appreciated.

Ray Walters

William Miller wrote:

> 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?



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