SMA long distance Grid tie [RE-wrenches]

[John Berdner]

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Wrenches:
 
As you can see there are several ways to skin the cat.
For long distance runs there are basically 3 solutions that we
suggest.
Each has pro's (+) and con's (-) and we try to walk people through all
of them.
If I missed something please jump in for the good of all.
 
1) Step up / Step Down transformers.
This is not normally our first suggestion for many of the reasons
others pointed out in previous posts.
+ The only way to do it if the wire is already "in the ground" and
replacing it with larger wire would be cost or logistically prohibitive
- Added complexity - more things to design and/or break.
- Harder to explain to the average inspector.
- Higher losses - losses from one transformer is bad enough!

2) Bigger wire on the DC side
+ No issues with AC voltage drop pushing the inverter out of the IEEE
929 window (88% to 110% of Vnom)
+ Can go to 5% voltage and normally only have impact on system output,
i.e. 5% less output
+ Current on DC side is usually a little bit lower for slightly smaller
system losses.
- Have to pull multiple pairs of wires over a long distance
- May need dc disconnects at arrays AND at inverters to keep the
inspector happy
- Increased susceptibility to lightning damage due to longer "antenna"
on dc side
(I can't recommend the use of unlisted equipment but I have heard some
installers have installed DELTA LA600 series surge arrestors in the DC
disconnects near the inverters after the inspector has done their job) 

3) Bigger wire on the AC side
+ Simplest to design and install.
+ Looks routine to inspectors.
- May need big wire to hold voltage drop to < 1.5% to avoid over
voltage trips in areas with high grid voltage.
- Inspector may require ac disconnect or OC protection at inverter
location (not a bad idea anyway)
- Inspector may require additional ground rods at array and bonding
wire to exiting grounding system. 
- Increased susceptibility to lightning damage due to longer "antenna"
on ac side 
(We suggest AC surge arrestors and ground rod(s) in sub panel / pull
box at array location)
 
As a side note: keeping the ground wire runs as short as possible (low
inductance) helps surge arrestors (AC or DC) do their job. 
 
Best REgards,
 
John Berdner 



 

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