Bell curves [RE-wrenches]

Jeffery Wolfe, Global Resource Options jeff at globalresourceoptions.com
Wed Feb 8 09:32:58 PST 2006


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William,

There are two different key criteria, and two different ways to solve
for AC and DC sides.

For the DC, I agree with your wire sizing philosophy, although erring on
the side of big wire. But the key is to design the DC array voltage to
not drop below the DC turn on voltage of the inverter, including wire
loss. (Some would say turn off voltage, but that might leave you off all
afternoon if there is an AC power glitch.) So don't use minimal strings,
and use an inverter with a  wide input window. Which leads me to:

For the AC side, maybe a group of SB6000's would work. Probably cheaper
than a single 30 kW unit. (Although now you are running 5 sets of DC
conductors instead of 1, but all smaller). And then you can set the
SB6000's on 277V operation. At the far end use a transformer to buck
back to your nominal connection voltage of probably 120/240. This
concept should work with either single or 3 phase service. This may add
more complexity than it is worth, or may just replace wire loss with
transformer loss. But at least with a  multi tap transformer you can
probably eliminate worries about under voltage tripping of the
inverters, and you only have one added transformer, not two.

And as you well know, there are some days when big wire may be dumb, but
simplicity rules.

Jeff

Jeffery D. Wolfe, P.E.
Global Resource Options, Inc., Solar Energy Solutions

-----Original Message-----
From: William Miller [mailto:wrmiller at charter.net] 
Sent: Tuesday, February 07, 2006 10:01 PM
To: RE-wrenches at topica.com
Subject: Bell curves [RE-wrenches]
-------------------------------------------------------------------

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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