[RE-wrenches] Interconnection with Generator

[Dave Click]

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 <cshalanski at 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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