620 VDC [RE-wrenches]

[Joe Felker]

I like Bill's statement about the code allowing "short-term excesses".  
The only time we see the maximum voltage is at dawn/dusk when there is 
enough light to get full voltage but not enough to generate usable 
current (and power).  Once there is enough current to kick in the 
inverter's MPPT, the voltage drops nominally to Vmp.


Bill Brooks wrote:
> 
> 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.
> 
> 
> -----Original Message-----
> From: Bob-O Schultze, Electron Connection
> [mailto:bob-o at electronconnection.com] 
> Sent: Thursday, June 26, 2008 1:59 PM
> To: RE-wrenches at topica.com
> Subject: Re: 620 VDC [RE-wrenches]
> 
> 
> Drake,
> I'd have to agree with Walt that you are up against the Code anyway  
> you look at it.
> That said, let's look at reality:
> We're talking Voc here- no current of any kind flowing.
> We're talking Voc at the record lowest temperature recorded.
> Clearly, 600V wire does not break down at 601V
> 
> Does anyone seriously believe this is -or ever could be in the real  
> world- a safety issue?
> Not me.
> Cheers, bob-O
> 
> 
> On Jun 26, 2008, at 12:37 PM, Drake Chamberlin wrote:
> 
> >
> > Hello Wrenches,
> >
> > Some inverters are able to withstand voltages above 600 VDC.  The  
> > Solectria 95KW can take up to 620 VDC without harming the electronics.
> >
> > If we size a system so that the record low temperature is gives a  
> > voltage of under 620 VDC, a viable system can be installed.  There  
> > is a question, however, about the Code compliance of this, since  
> > building wire is rated at 600 volts.  Higher voltage wiring is  
> > available, but will likely not be cheap or convenient to use.   
> > Also, what about 600 V modules and PV cable?
> >
> > Since this is DC, there are not any cycles to create a voltage  
> > peak.  A 600 VAC RMS nominal system can have (if my calculations  
> > are correct) a peak voltage of close to 850 volts.  Also, system  
> > voltages can range up to 10% from the nominal, resulting in  
> > voltages of as high as 660 VAC RMS on 600 volt systems.
> >
> > The question is, "can we have a system with a possible low- 
> > temperature, open circuit voltage of 620 VDC with wiring and  
> > modules rated for 600 volts?  Can the other 20 volts be considered  
> > within the nominal range?
> >
> > Thanks,
> >
> > Drake
> 
> 
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Joe Felker
Senior Electrical Engineer
Pfister Energy
NABCEP Certified Solar PV Installer


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