corrosion [RE-wrenches]
blair may
tump at hughes.net
Wed Jul 18 12:17:30 PDT 2007
<x-flowed>
Jim,& other esteemed wrenches re/ corrosion; I posted a request re/
this issue the other day & I guess it was lost in topica land.
Issue is; S5 Clamps & standing metal roofing. Anyone have any
thoughts regarding some sort of plastic,rubber between the clamps &
roofing to prevent galvanic action between the dissimilar metals?(
Alu S 5 clamps & Steel roofing)I have NOT seen anything from the
Manufactures or any else for that matter.
>Here in San Diego we have had some quite costly galvanic corrosion
>problems associated with grounding and DC currents.
>
>DC is more corrosive than AC because one electrode is continuously
>attacked as opposed to the effect of AC where, both grounded
>electrodes share the burden and, to some extent, each half-cycle
>reverses the effect of the preceeding half-cycle.
>
>When an AC neutral is bonded to separate grounding electrodes, there
>may very well be a DC current flowing because of differences in
>galvanic potential, however there should not be DC forced by the
>load potential drop across the neutral conductor joining the two
>grounding electrodes.
>
>The lightning chokes I have seen are field fabricated using
>intentional slack in the normal current carrying conductors.
>
>Because of the potential for destruction of equipment safety
>grounds, I am very wary of ground loops associated with DC
>power. This equipment is supposed to last decades with minimal
>maintenance, so it does not take rapid deterioration to ultimately
>create a serious hazard. And in this case, there seems to be a
>quite reasonable alternative.
>
>With best regards,
>
>Jim Easton, PE
>(E 11974)
>4364 Bonita Road, No. 166
>Bonita, CA 91902-1421
>Tel: 858-527-0240
>Fax: 619-470-8616
>----- Original Message ----- From: "Bill Brooks" <bill at brooksolar.com>
>To: "'Jim Easton'" <jeaston at ucsd.edu>; <RE-wrenches at topica.com>
>Cc: <ipl at sover.net>
>Sent: Tuesday, July 17, 2007 2:14 PM
>Subject: RE: Inverter location, ground loops, and corrosion
>
>
>Jim,
>
>You bring up some interesting food for thought. I'm trying to see how
>galvanic corrosion could possibly be an issue with grounding systems
>separated by 850 feet. I also don't see how ac or dc could make much of a
>difference in the galvanic corrosion. The difference in potential at each
>end of ac neutral with current flow is a dc voltage is it not? It may have
>an ac component, but the driver is the dc voltage difference--correct?
>
>You make a good point about lightning protection, and I think that lightning
>protection is a matter of a subjective nature rather than being something to
>stake a reputation on. You suggest surge arrestors at the array and no
>connection to ground at the array. I concur for distances of a 100 feet or
>so or less, but if we are talking about 850 feet, I want to ground
>everything at the array, install surge arrestors at the grounding system at
>the array, and run nothing but current carrying conductors from the array to
>the inverter. Chokes might help, but I'm not familiar with suppliers or
>listed devices (any info on this would be interesting--probably could make
>your own by wrapping wire around a non-metallic cylinder).
>
>Once the current-carrying conductors get to the inverter, these conductors
>should have a second set of surge arrestors to dissipate any voltage that is
>induced on the wiring from the array to the inverters. The SMA inverters are
>internally grounded in the inverter for the GFP function (essentially
>disabled in this case for lack of a real current path). This reduces the
>copper requirements and conductive paths. Since the distance is so great,
>the GFP (not GFCI--just to keep from confusing people about ac devices) will
>have no current flow because the ground resistance is too great and voltage
>difference too low to get current to move.
>
>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: Jim Easton [mailto:jeaston at ucsd.edu]
>>Sent: Tuesday, July 17, 2007 7:27 AM
>>To: RE-wrenches at topica.com
>>Cc: bill at brooksolar.com; ipl at sover.net; boillbrooks7 at yahoo.com
>>Subject: Re: Inverter location, ground loops, and corrosion
>>
>>A caveat about multiple grounds (Ground Loops) in DC systems.
>>
>>If a DC neutral is grounded at multiple locations, even if the ground
>>current is too small to trip tha GFCI (Assuming one is installed), it will
>>still cause corrosion. The amount will depend on the total neutral
>>current
>>and the ratio of resistances between the neutral conductor and the
>>ground-to-ground path.
>>
>>DC is much more aggressive than AC in causing electrolytic corrosion. This
>>may, over time, cause a failure of the equipment ground with serious
>>safety
>>implications.
>>
>>When wishing to provide lightning protection for a remotely located PV
>>array, rather than bonding the neutral to the array ground, I would
>>suggest
>>instead installing a lightning arrestor at the array, and possibly putting
>>in lightning chokes (Coils having negligible DC resistance but impedance
>>at
>>high frequencies) between the arrestor and the wire run from the array to
>>the inverter(s).
>>
>>Unfortunately, I do not know of any UL/NRTL Listed arrestors for this
>>purpose, however our inspectors here are usually sympathetic to using UL
>>Recognized arrestors for this purpose.
>>
>>Jim Easton, PE
>> (E 11974)
>>4364 Bonita Road, No. 166
>>Bonita, CA 91902-1421
>>Tel: 858-527-0240
>>Fax: 619-470-8616
>>
>>----- Original Message -----
>>From: "Bill Brooks" <bill at brooksolar.com>
>>To: <RE-wrenches at topica.com>
>>Sent: Monday, July 16, 2007 1:02 PM
>>Subject: RE: Inverter location [RE-wrenches]
>>
>>
>> >
>> > 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
>> >
>> >
>> >
>> >
>
>
>- - - -
>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
>
>
>
>
>
Tump at hughes.net
www.SWNL.net
Serving Mid Coast Maine & Northern California
Me.# 207-832-7574 Cl.# 610-517-8401
******** MAINE'S CHARTER ********
NABCEP "Certified PV Designer & Installer"
******** MAINE'S CHARTER ********
Trace Xantrex "Certified" Dealer / Installer"
--
[Non-text portions of this message have been removed]
- - - -
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
--^----------------------------------------------------------------
</x-flowed>
More information about the RE-wrenches
mailing list