Lowest DAYTIME temp records [RE-wrenches]

[Joel Davidson]

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Hello Allan,

I did a project for SMUD over 10 years ago that calculated over 600 Voc
based on historical climate data. SMUD accepted the calculation and the
project. None of the 240 inverters failed due to high Voc.

Data recorded by inverters can be used to void warranties and can also be
shared with system designers so that inverters and systems can be improved.

Joel Davidson

Original Message:
-----------------
From: Allan Sindelar allan at positiveenergysolar.com
Date: Thu, 2 Feb 2006 19:50:24 -0700
To: RE-wrenches at topica.com
Subject: Re: Lowest DAYTIME temp records [RE-wrenches]


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Joel,
I think Bill Brooks has the right idea. You can get record cold ambient
temps, or very close to them, when the sun first hits the array. Voltage is
a hard limit, and most (all?) GT inverters record it internally, so an
overvoltage failure wouldn't be warranted. One overvoltage condition could
fry an inverter. It's open circuit voltage that matters, before the inverter
latches and reduces the voltage to MPPT, so current can be zero; that is, it
wouldn't take any current for the voltage to be excessively high.
Anyway, my two cents worth.  Genuine conservatism, from someone who doesn't
identify as a conservative...
Allan at PosE

----- Original Message ----- 
From: "Joel Davidson" <joeldavidson at earthlink.net>

> Should PV systems be designed using nighttime temperatures when it is
> obvious that voltage is not present in the dark? Should climate data from
a
> weather station be used because it is available even if the system
location
> has a different microclimate? Does this seem reasonable? SMA's string
sizing
> tool has the right idea. They ask you to select the coldest record low
> ambient temperature when sunlight will be on the array.
>
>
> www.weather.com only lists record high and low temperatures. NEC 690.7(A)
> specifies the lowest expected ambient temperature which implies sunlight
on
> the array to cause voltage. Except in rare situations record low
> temperature occurs at night or before sunrise when no voltage is present.
>
> Does anyone know a source for the lowest expected ambient temperature when
> sunlight will be on the solar array?
>
> Joel Davidson
>
>
>

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