620 VDC [RE-wrenches]

Fri Jun 27 09:39:30 PDT 2008

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Bill,

I agree. Solar cells and inverter power components have well-known voltage 
characteristics, but module manufacturers make whatever suits their internal 
needs, not what customers want or what works well with inverters. In 
Germany, a country known for rational engineering design (most of the time) 
and a climate somewhat similar to New Jersey, voltage is not an issue 
because designers can work up to 1000 volts. "over 600 volts" first appeared 
as Section B in the 1975 NEC edition and was institutionalized in the 
electrical industry by the time Article 690 was adopted. The 600-volt 
distinction is not sacrosanct. It was arbitrary and provincial from the 
start, is a barrier to PV system design, and should be changed to 1000 
volts.

Joel Davidson

----- Original Message ----- 
From: "Bill Brooks" <billbrooks7 at yahoo.com>
To: <RE-wrenches at topica.com>
Sent: Friday, June 27, 2008 8:49 AM
Subject: RE: 620 VDC [RE-wrenches]

>
> 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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>
>
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