The inverter to generator ratio is entirely the designer's call and
comes into play for large generators (for residential systems as
mentioned in earlier replies, it's a bad idea).
Interconnecting in spot networks in dense urban areas has a similar
issue in that the distribution system can't handle backfeed. In downtown
Boston a few years ago I believe that the max PV that could be
interconnected was something like 1/10 or 1/15 of the demonstrated
MINIMUM demand for that building over the previous year-- just to make
absolute sure that there wasn't a chance of backfeed. Other solutions
include things like reverse power relays such that if the PV system does
start backfeeding the system, the relay would trip the inverter offline.
Of course any time you're interconnecting on the load side, you of
course take responsibility for anything bad that happens with the
generator. This should lead to an extremely conservative ratio being
used and regular monitoring to verify that the PV never comes close to
backfeeding something it shouldn't. Also, any issues with power quality
(real or perceived) can complicate your project substantially, thinking
of William's recent issues with the UPS and Sunny Islands. I wonder if
this could complicate a generator warranty, too, unless you were somehow
able to get the manufacturer's blessing ahead of time.
Another comment I remember from an earlier Wrench conversation was that
inverter contributions on the load side of a generator won't
proportionally drop fuel consumption-- that is, if your generator's load
is 500kW and your PV is putting out 250kW, you're not going to have a
50% reduction in fuel use. So it takes some equipment-specific analysis
to figure out what exactly your customer's savings would be.
DKC
On 2014/10/28 12:35, Phil Forest wrote:
Interconnecting the inverter output to the utility side of the transfer switch
is not necessarily an absolute rule. A knowledgeable solar EE told me a few
years back, that interconnection can be made on the load side if the inverter
kW output is small compared to the generator kW, that large kW generators can
produce clean AC power allowing the inverter to operate and to operate without
risk of burning out the generator. I don't remember what the inverter to
generator ratio needs to be, or if other criteria needs to be met.
Never tried it, we always connect to the utility side.
Phil Forest
South Mountain Company
On Oct 28, 2014, at 10:48 AM, Corey Shalanski <cshalan...@joule-energy.com>
wrote:
Wrenches,
We frequently encounter residences with backup generators. The question that
arises: how should the PV system be interconnected?
I have searched the archives and found some good information about this subject, but
nothing definitive. I have fallen into the habit of broadly proclaiming that the PV
system *must* be interconnected on the "utility" side of the transfer switch -
in order to prevent backfeeding into the generator and thereby eliminate unintentional
equipment damage. Is this concern justified? If so, should this be an absolute rule or
would it depend on the generator make/model?
Thanks for any insights.
--
Corey Shalanski
Joule Energy
New Orleans, LA
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