Inverter location [RE-wrenches]

[David Palumbo, Independent Power & Light]

Bill,

I apologize for the false wrench-mail start earlier.

This is a timely reminder for me to hear right now. I am working on the
design for a 13kw system with a wire run of 850 feet!
An expensive wire run any way we do it.

AC looks simpler at first. Inverters (2 SMA SB 7000's) at the arrays pushing
electrons thru 750 MCM AL conductors in 4" conduit to the service entrance.
But as Bill points out we do not want the inverter(s) to see higher voltages
on it's terminals due to voltage drops. Best to play that safe since any
resistance in that wire run, and connections, not accounted for in your
calculations can shut down the inverters if the utility voltage is on the
high side.

DC wire run involves more conductors but is more of a sure thing.
We may use 72 Evergreen 180's. They are 25.9 vmp and 6.95 amps. With series
strings of 12 modules 310 volts 6.95 amps. I calculate 6 pairs of #4 Cu, or
#2 AL.

I have a question pertaining to paralleling series strings together (3
strings in this case) before the long wire run to the inverter. Is this an
acceptable/workable practice? That would mean that we are still transmitting
310 volts mp but at 20.85 amps (26.06 amps cold temp.) into the SMA 7000. If
we could do this we would only need 2 pairs of DC conductors. I calculate
1/0 CU, or 3/0 AL in this case.

Back to the grounding lesson of a couple of weeks ago Bill. Let me run my
understanding of that by you for this case. This assumes inverters at
service entrance and an 850 foot PV wire run. With a separately derived
system I can ground the pv arrays and bond negative to ground at the dc
combiner box(es). No ground wire in pvc conduit and no ground wire in
trench. At the DC disconnect(s), just before inverter(S), I will bond DC
negative to ground. This is my understanding of separately derived systems.
Your posting on June 22nd is quite clear and the best concise description
that I have seen on pv grounding issues that I have read. I normally do not
bond the negative to equipment ground at the array combiner box. I normally
do run #6 copper in the trench, along side the conduit, to connect the
grounding electrode system of the pv array(s) together with the grounding
electrode system at the house. I have done this with pv arrays that are up
to 250 feet away from the house. I have had very low incidence of lightning
damage over my 20 plus years of installing. Other systems that I have
serviced, that do not have as much copper in the ground, seem to suffer more
damage from lightning induced electrical surges.

Dave

-----Original Message-----
From: Bill Brooks [mailto:bill at brooksolar.com]
Sent: Wednesday, July 11, 2007 12:41 PM
To: RE-wrenches at topica.com
Subject: RE: Inverter location [RE-wrenches]



Kurt,

I have been waiting for the correct response and haven't seen it yet. When
voltage drop issues are essentially equal on the dc and ac sides of an
inverter, as they are with this particular SMA product, it is ALWAYS best to
put the inverter near the service entrance to keep the ac voltage drop to
the lowest possible value. DC voltage drop causes a proportional loss of
power from the array, whereas ac voltage drop can often shut down the
inverter.

SMA recommends no more than 1.5% voltage drop on the ac side and my
recommendation is to keep it below 1%. Having seen numerous inverters
tripping off line because of high voltage at the inverter terminals (due to
3% voltage drop on the ac side), this is a critical operational issue.

Put the inverter at the service entrance. The only time I would consider
putting the inverter at the array is for a 48-Volt array that is more than
75 feet from the service entrance.

Bill.

Bill Brooks, PE
Brooks Engineering
873 Kells Circle
Vacaville, CA 95688
office and mobile phone: 707-332-0761
fax:707-451-7739
email: billbrooks7 at yahoo.com
web: www.brooksolar.com


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