Inverter location, ground loops, and corrosion [RE-wrenches]

[Jim Easton]

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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
>
>
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