[RE-wrenches] RE-wrenches Digest, Vol 3, Issue 557

[Jason Szumlanski]

It's pretty common to run into this in Florida were there are lots of
whole house generators, and in commercial systems. One solution is to
use a contactor to sense the presence of grid power. Actually, what you
want to do is sense when the generator is running an shut down the PV
system (since absence of grid power does not necessarily mean the ATS is
closed on the utility side). Unfortunately, there is some concern about
how "fail-safe" this strategy is, since contactors can fail.

Whether it is problematic to backfeed a generator, or whether inverters
will even sync to a generator has been discussed here before. Check the
archives.

To answer your questions the best I can:

1. There is nothing in the code that that I know of that specifically
prohibits this scenario, however there are some considerations about
wire and bus sizing that you may run into as you try to backfeed
subpanels that are run in series. That is not specific to ATS
situations, however.
2. The utility won't care as long as they have a means of disconnecting
PV in most cases. We won't get into the need to a utility disconnect
here, but from the utility perspective, this is nothing more than a load
side connected PV system.
3. I haven't heard of an inverter mfr that has made a determination
about this, but I would imagine they would not have a problem. Either
ask now, or don't tell later. :) 
4. You can accomplish what you need to do with a contactor with the
appropriate controls to open or close the circuit based on your logic. I
would not recommend anything manual in a PV system. 


Jason Szumlanski
Fafco Solar

-----Original Message-----
Message: 1
Date: Sun, 26 Sep 2010 12:31:51 -0700
From: "Mark Frye" <markf at berkeleysolar.com>
To: "'RE-wrenches'" <re-wrenches at lists.re-wrenches.org>
Subject: [RE-wrenches] Interconnection downstream of ATS
Message-ID: <D633AB9D228F4318A20CBF22A2808198 at marksdell>
Content-Type: text/plain; charset="iso-8859-1"

Folks,

I am looking at doing a standard grid-dependent PV installation. The
only
catch is that the whole house is backed up by a generator with automatic
tranfer switch.

Due to the layout of the various equipment, it would be a whole lot
easier
to interconnect the PV at a point downstream of the ATS. Because the ATS
takes a minute or so to bring the generator up and transfer, the
inverter
would drop out when the line is lost. If I had a contactor on inverter
output, with the coil held by line voltage and a reset switch, I could
set
it up so that the inverter output would be isolated from the generator
upon
line voltage loss, and manually reset once utility line voltage was
restored.

I am wondering about the following 4 considerations:

1 - Code / AHJ acceptablity: Does the Code address the issue of load
side
connection downstream of an ATS?
2 - Utility (PG&E) acceptability: I don't see how the utility would
care.
3 - Inverter manufacturer acceptability: The manufacturer might not
warrant
this application unless they were confortable that the inverter never
sees
generator voltage.
4 - Functional efficacy: Owner would have to reset inverters manually.

Has anyone worked throught this scenario?
 
Mark Frye
Berkeley Solar Electric Systems
303 Redbud Way
Nevada City,  CA 95959
(530) 401-8024
www.berkeleysolar.com