620 VDC [RE-wrenches]

[Bill Brooks]

Joel,

There are only a few inverters on the market that can properly run a
UniSolar array and yet a large company (you may be familiar with) routinely
used the wrong inverters on their UniSolar arrays. The problem is less the
temperature difference as it is the wide differential between starting Voc
and "attenuated" Vmp.

Regardless of what SMA or others may say about their products, there is
always the possibility of exceeding 600Vdc even using all-time low. Using
all-time low and Table 690.7 will not "guaranty" never exceeding 600V. It is
simply very conservative.

The number one problem I see in PV design today is not having enough modules
in series. Nearly every array in New Jersey that has an inverter with a
minimum voltage of 330Vdc is improperly designed. These systems routinely
operate off the maximum power point for much of the summer. This problem is
an epidemic and is contributing to poorly operating systems. There are
several reasons for the design mistakes, but one of them is certainly being
too conservative on Max Voltage calculations.

Bill.

-----Original Message-----
From: Joel Davidson [mailto:joel.davidson at sbcglobal.net] 
Sent: Friday, June 27, 2008 8:26 AM
To: RE-wrenches at topica.com
Subject: Re: 620 VDC [RE-wrenches]


Drake,

Uni-Solar and other amorphous silicon solar cells have a flatter IV curve 
than crystalline silicon cells which requires a wider inverter input voltage

window. This means that it is difficult to design a Uni-Solar PV system for 
places like Boston where cell temperatures can range from -25C to +70C. Cell

temperature right before daybreak is almost always the same as ambient 
temperature unless the building leaks a lot of heat.

Joel Davidson

----- Original Message ----- 
From: "Drake Chamberlin" <Drake.Chamberlin at redwoodalliance.org>
To: <RE-wrenches at topica.com>
Sent: Friday, June 27, 2008 8:35 AM
Subject: RE: 620 VDC [RE-wrenches]


>
> Thank you all for your input.  It appears that this is an area that needs 
> work in the Code.  I'm looking forward to Bill's new temperature guidance 
> being implemented.
>
> With the particular system we are considering, we would be using Uni-Solar

> laminates that will adhere directly to a metal roof.  Is there anyway we 
> can get credit for the building heat that will never let the modules get 
> down to the ambient?
>
> Drake
>
>
> At 11:39 PM 6/26/2008, you wrote
>
>>Joel,
>>
>>What does 600-Volts nominal mean to you? As Drake pointed out, 600Vac has 
>>a
>>peak voltage of 850V. Any momentary exceeding of 600V, when using 
>>reasonable
>>statistical design numbers, will be momentary at best and have no affect 
>>on
>>the equipment unless the equipment is faulty to start with.
>>
>>Bill.
>>
>>-----Original Message-----
>>From: Joel Davidson [mailto:joel.davidson at sbcglobal.net]
>>Sent: Thursday, June 26, 2008 6:43 PM
>>To: RE-wrenches at topica.com
>>Subject: Re: 620 VDC [RE-wrenches]
>>The NEC is very specific about 600 volts DC and AC in Chapter 1, paragraph
>>2. The 600-volt Rule is that fundamental. Almost all AWJ and Code Priests
>>interpret Article 690.7 "lowest expected ambient temperature" as the 
>>lowest
>>temperature recorded by their chosen Weather Scribes even if that low
>>temperature only happened once many years ago. From a practical 
>>standpoint,
>>some solar modules can not be used optimally with some inverters. Every 
>>time
>>
>>I get a good module/inverter match, the manufacturers change specs,
>>availability, price, or all three.
>>
>>----- Original Message -----
>>From: "Drake Chamberlin" <Drake.Chamberlin at redwoodalliance.org>
>>To: <RE-wrenches at topica.com>
>>Sent: Thursday, June 26, 2008 12:37 PM
>>Subject: 620 VDC [RE-wrenches]
>>
>
>
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