Bell curves [RE-wrenches]

[Bill Brooks]

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William,

Is this 3-phase or single phase? That will make a large difference in
inverter choice and current flows. To answer your question simply, the
nominal power from a PV system is typically delivered at 800 W/m^2 for a
well-designed system. Since current is very proportional with irradiance, a
value of 80% of peak is a reasonable number for average voltage drop. This
is an okay evaluation for the dc side, but should be avoided on the ac side
as other issues (tripping on overvoltage) are more important on the ac side.
Of course you always push the dc voltage to the highest allowable voltage to
reduce current and reduce losses due to high temperature voltage
degradation, module degradation, and shading.

Bill.


-----Original Message-----
From: Jeffery Wolfe, Global Resource Options
[mailto:jeff at globalresourceoptions.com] 
Sent: Wednesday, February 08, 2006 9:33 AM
To: RE-wrenches at topica.com
Subject: RE: Bell curves [RE-wrenches]

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]
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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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