620 VDC [RE-wrenches]

[Bill Brooks]

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$500 at ASHRAE. I have not written the public design document yet.

Bill.

-----Original Message-----
From: Joel Davidson [mailto:joel.davidson at sbcglobal.net] 
Sent: Thursday, June 26, 2008 6:03 PM
To: RE-wrenches at topica.com
Subject: Re: 620 VDC [RE-wrenches]


Hi Bill,
Where on the internet is the ASHRAE weather data?
Joel Davidson

----- Original Message ----- 
From: "Bill Brooks" <billbrooks7 at yahoo.com>
To: <RE-wrenches at topica.com>
Sent: Thursday, June 26, 2008 4:06 PM
Subject: RE: 620 VDC [RE-wrenches]


All,

The key is coming up with a defensible number for "lowest expected ambient
temperature." The NEC does not define this number. I have submitted a
proposal to our Technical Review Committee to use ASHRAE weather data for
this number. My recommendation is as follows:


690.7(A)
FPN: One source for statistically valid lowest expected ambient temperature
design data for various locations is the Extreme Annual Mean Minimum Design
Dry Bulb Temperature found in the ASHRAE Handbook - Fundamentals. These
temperature data should be used to calculate maximum voltage using either
manufacturer's temperature coefficients relative to the rating temperature
of 25°C, or Table 690.7.



This information will be produced in a publically-available design document
so that anyone can access the data. The ASHRAE number is significantly
warmer than the record cold. As Drake pointed out, even a 480Vac system has
maximum voltage of about 680Vac. The modules, wire, and disconnects are not
the issue as much as the inverter. Many inverters have been built to handle
voltages far in excess of their maximum (SatCon was actually listed to
660Vdc), but others are not. Most inverters will merely wait while the array
warms up to start rather than spontaneously combusting at 601.

The question is what is the statistically likelihood of cold temperatures
and high irradiance on the system. The code allows for short-term excesses
on various current and voltage parameters. The key is if we are able to
convince the code making panel that we have reduced the likelihood of a
problem sufficiently.

Bill.

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