Long distance DC feeders [RE-wrenches]

[John Berdner]

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Wrenches:
 
Willima called me directly and I respnded off list.
I thohgt others might want to know some of the trade offs of long dc
versus long ac runs.
- = negative, + = positive
 
Long DC Run
 
- Requires yet another "totally unnecessay dc disconnect"
  (one at the array and another at the inverter)
- Longer "antenna" for surges due to lightning. 
  (Install a surge arrestor at the inverter disconnect)
- No available UL Listed 600 Vdc surge arrestors
  (While I can not encourage use of unlisted components I have heard
many people use the Delta LA 600 series)
  (This is not required for code/inspection and, since these are
unlisted, the building inspector does not need to see it)
- Have to pull home runs for each array.
  (Since we ground our arrays here in the USofA the GFDI circuits see
the other GFDI as a ground fault)
  (In the ROW, with floating arrays you can parallel the arrays and use
Sunny Team for higher efficiency and N+1 redendancy)
+ Avoids long AC run
+ No technical issues with higher dc side voltage drop (economic yes,
technical no).
 
Long AC Run
 
- Voltage drop is a killer since the inverter has to trip offline to
meet UL/ IEEE 929
  (you only have 110% of the nominal Vac to work with and the utilities
are allowed to run 5% high)
  (If you design at 3% voltage drop then add in the impedance on the
grid you are cutting it close) 
  (FYI, we have measured impedances of around 0.2 Ohms for a 200 Amp
service (0.4 Ohms @ 100A, 0.1 Ohms @ 100A))
  (Probably means oversized wire at 1.5% drop to make the system work
well)
+ Can parallel ac side in a sub panel at the inverters for single home
run
  (No added cost for the sub panel since you need one anyway)
  (Consider pulling the neutral even if you don't need it for the PV
system now so that later you can get a 120 Vac circuit at the array)
+ Probably little to no added exposure to lightning damage since the
new "antenna" is not much longer than the exisitng one, e.g. the grid.
   (Add a "whole house" arrestor to the sub panel to protect all
inverters at once if needed)
   (set new ground rod to minimize ground impedance and "don't forget
to bond it back to the grounding system")
 
Did I miss any ?
 
Best Regards,
 
John Berdner
    
>>>>>>>>>>>
 
William,
I think that John Berdner will encourage you to set the innerter near
the
array and run AC back, rather than DC, to minimize surge damage. John?
Allan at +E

----- Original Message ----- 
From: "William Miller" < wrmiller at slonet.org >
Sent: Sunday, May 30, 2004 4:13 PM

Have any of you fed DC for Sunny Boy inverters distances on the order
of
800 feet? I suppose one would need a DC disconnect at both ends for
safety
purposes (to service feeder, for example). Any input is appreciated.





 
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