Inverter location [RE-wrenches]

[Bill Brooks]

David,

Sorry for taking so long to respond. I just took a closer look at your
design and I'm really wondering why you are only putting 12 modules in
series. You cannot use 310Volts as the operating voltage for that circuit.
It will be more like 250Vdc on a nice day and much lower on a really hot day
(225Vdc before degradation and actual output lower than manufacturer
promised). That is way too low a voltage for that inverter on a hot day.
Since you have such a long run, you should run the array at highest
allowable dc voltage. This will reduce voltage drop and wire size. The
slight penalty in inverter efficiency is absolutely worth it.

If this is a California system, you should be looking at a different number
of modules in series. I would recommend 15 in series, a total of 75 modules,
with 3 sets on one 7kW unit and 2 sets on one 5kW unit. I'm not sure where
this system is located, but you really need more modules in series. Even in
the coldest part of the country, you can get 14 modules in series on that
inverter. Given the long dc run, I would combine at the array and run back
to the inverter at the house.

Since these inverters have ground fault protection, you have to be very
careful about connecting dc negative to ground. With a very long distance,
you can ground the negative at the array and then the manufacturer does it
again in the inverter (not at the dc disconnect since that will blow the GFP
fuse). The equipment grounds on the array would have to be connected to the
same ground rod as the dc negative at the array, and the string combiners
would be the easy place to make all the ground connections (Outback combiner
split in two halves). As you pointed out, doing it this way necessitates
non-metallic conduit and no ground wire in the conduit from the combiner box
back to the inverter (850 feet).

I hope that helps.

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

Ecclesiastes 11:7 Light is sweet, and it pleases the eyes to see the sun.


> -----Original Message-----
> From: David Palumbo, Independent Power & Light [mailto:ipl at sover.net]
> Sent: Wednesday, July 11, 2007 6:33 PM
> To: RE-wrenches at topica.com
> Subject: RE: Inverter location [RE-wrenches]
> 
> 
> 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 


- - - -
Hosted by Home Power magazine

To send a message: RE-wrenches at topica.com

Archive of previous messages: http://lists.topica.com/lists/RE-wrenches/read

List rules & how to change your email address: www.mrsharkey.com/wrenches/etiquette.php

Check out participant bios: www.mrsharkey.com/wrenches/

Moderator: michael.welch at homepower.com
--^----------------------------------------------------------------
This email was sent to: michael_welch at sbcglobal.net

EASY UNSUBSCRIBE click here: http://topica.com/u/?bz8Qcs.bz9JC9.bWljaGFl
Or send an email to: RE-wrenches-unsubscribe at topica.com

For Topica's complete suite of email marketing solutions visit:
http://www.topica.com/?p=TEXFOOTER
--^----------------------------------------------------------------