Grid-Tie System added to home with generator

fafcosolar · February 2010

Down in Florida we are running into lots of homes with whole-home backup generators with auto transfer switches, which is typically easy enough to work with. But, sometimes a customer wants a PV array on a remote garag that has a subpanel. While it would be ideal to backfeed this subpanel (if appropriately rated), the subpanel, along with all other loads in the home, are served by the generator. As you might imagine, the only way to grid connect the PV array is to run cable all the way back to the house and tie in ahead of the transfer switch so we are not backfeeding the generator when it is running.

I was contemplating different ways to address this situation. An engineer suggested that we could use a contactor to disconnect the PV source when the generator is running. This would still require cable back to the main house, but wire size and cost may be easier to deal with. Any thoughts on this approach? Anyone with experience in this situation?

Solar Guppy · February 2010

Re: Grid-Tie System added to home with generator

Technically, you can just let the GT inverter back feed from the garage, even if its the generator side of the transfer, all that will happen is if the load can't take the energy, the GT inverter will fault with frequency or voltage fault, no big deal and it to code.

fafcosolar · February 2010

Re: Grid-Tie System added to home with generator

I was more concerned with liability associated with potentially damaging the customer's generator. Yes, the inverter would probably fault out, but what about the sometimes sensitive electronics in generator controllers. I would prefer not to risk it. Has anyone actually done this, especially where the PV array is likely to exceed the load significantly?

mike95490 · February 2010

Re: Grid-Tie System added to home with generator

adding to what Guppy said - I doubt the PV/GT will sync and enable, on the genset, if it's not Xtal controlled Freq., the GT will stay offline.

solartek · February 2010

Re: Grid-Tie System added to home with generator

All due respect to Solar Guppy and Mike90045, they're not paying your liability insurance. I would use a relay that trips the GT inverter offline when the transfer switch kicks in the generator. So, yes, you have to run a connection between the garage and the main house, but it can be much smaller cable or wire for the relay system than what would be required to connected the inverter output into the main house service panel.

Scott.

BB. · February 2010

Re: Grid-Tie System added to home with generator

Some GT inverters, if I recall correctly, have an "inhibit" line--you could just wire that up to a transfer switch status bit / mains sense / generator running / etc...

Or, I would see if there is a solid state two pole relay that you could use to disconnect the Inverter. You could even do both--the Inhibit / Relay branch circuit disconnect -- controlled by two independent circuits (primary and spare disconnect) if you are worried about GT + Genset operation (I agree that GT+Gen seems worrying--and one "inhibit/disconnect" would work well--two independent would probably meet code if you are truly concerned about damage/fire to the genset.

Last resort would be a regular two pole AC or DC controlled relay--It would cost you some energy (6 watts or so to stay energized)... Could be normally closed or normally open (for example, if normally closed--generator AC power from before transfer switch would open GT branch circuit--saves energy for the 99% of the time the genset is not running).

In any case--make sure there are enough blinking lights / bypass / test switches so that any faults can be quickly identified a few years down the road. Might be expensive the first time you setup a nice looking panel/controller--but would be nice to offer your next customers down the road and have a standard setup to install and debug as needed.

-Bill

GreenerPower · February 2010

Re: Grid-Tie System added to home with generator

Would connect the GT to the "line side" of the existing transfer switch suffice ? (providing that the existing feed from the main panel meet the current carrying requirement). When the transfer switch throws to connect the sub-panel loads to the generator, the GT would not be connected to the generator/load side. But you would need a separate visible disconnect per code other than the breaker from the main panel.
GP

BB. · February 2010

Re: Grid-Tie System added to home with generator

I think the question is that the garage has 220 VAC and a GT solar array and the transfer switch + genset is somewhere else (probably near the meter)--So how to, cost effectively, disable the GT setup so that it will not attempt to back drive the Genset without having to run a second set of cables from the GT inverter on the detached garage to the line side of the transfer switch.

If yo have to trench for a GT "inhibit" back to the transfer switch anyway--adding a 220 VAC branch back to the main panel may not end up costing that much more money (vs coming up with your own GT inhibit solution and trenching the control line).

-Bill

Solar Guppy · February 2010

Re: Grid-Tie System added to home with generator

By definition the GT inverter only can back feed a connection that can source the energy. If you have a generator, the GT will either not sync at all or for 10 -20 milliseconds sync and cause the line to go out of UL1741 specs, neither would have any effect what so ever on a generator.

GT's are coupled to 100% electronic inverters ( think generator ) all the time which are much more sensitive than rotary generators, if the load can use the energy, fine, it not, a brief over/under voltage measured in milliseconds.

There is no technology issue or concern here, only fear ... This reminds me of all the foot stomping when GT's first came out ... its will kill the utility works or damage equipment ... FUD then and this topic is FUD now

GreenerPower · February 2010

Re: Grid-Tie System added to home with generator

BB. wrote: »

I think the question is that the garage has 220 VAC and a GT solar array and the transfer switch + genset is somewhere else (probably near the meter)

Ah !, somehow I missed this.

SG, what if the generator is inverter type like the Honda EU series . Not sure what happens when GT exports more than load and backfed the EU . Likely it would cause the EU to go out of UL1741 spec either voltage or Hz and the GT would drop. .. Then the GT would sync and exports again later ... whether this could cause any harm to the EU gen/inverter . And during the brief period when the EU goes out of spec and GT dropped, whether this would cause any issue with sensitive electronic loads.

GP

halfcrazy · February 2010

Re: Grid-Tie System added to home with generator

I have a few systems out there wired this way they work fine as SG say's one I have watched simply never try's to sync as the generator is to out of spec any ways. And the grid is a big generator also right?

Solar Guppy · February 2010

Re: Grid-Tie System added to home with generator

GT inverter when they connect don't just slam on all the available energy, they ramp up over the period of 10's of seconds. If the "grid" , regardless of the source can not source the GT energy, the line will rise or the frequency will change to the load change and the GT will stop selling.

It will work just fine on the generator output side , most likely will never sync as generators don't have the ability to maintain the required UL1741 tolerances ( basically 60hz +- 0.5hz. And this is checked every 16 milliseconds, off for more than 3 cycle, offline it goes for 5 minutes and when waiting to go online has to be perfect for 5 minutes, which will probably never happen

I'm not sure how to explain this better than what I have written, I know first hand the design of two generations of Xantrex GT inverters so this isn't some guess, it fundamental to how GT inverters have to work.

As for liability, you can be sued for any reason ... you can go thru all the effort of a new line, put it ahead of the transfer switch and if something fails you can still get sued.

halfcrazy · February 2010

Re: Grid-Tie System added to home with generator

Excellent description SolarGuppy. Like I said I have several of these systems and I have never worried about it.

BB. · February 2010

Re: Grid-Tie System added to home with generator

Thank you SG,

The fact that the GT inverters ramp slowly up over 10's of seconds in something that I was not aware of (one of many things :roll:)--that does make the generator issue sound like it should not be a problem.

Have you ever run a GT inverter with an "Inverter/Generator"?

-Bill

Solar Guppy · February 2010

Re: Grid-Tie System added to home with generator

Yes, back when I did the Suntie work I tried syncing to a diesel generator, never could get a sync as the frequency would not be stable to meet the UL1741 checks

GreenerPower · February 2010

Re: Grid-Tie System added to home with generator

SG, it's not clear, is your "yes" to use the GT against an inverter type generator ? your explaination followed was with a diesel generator.
Over OB forum, I recalled someone successfully "sync'ed" (AC coupled) a GX inverter to an EU Honda inverter generator AC output and have the GX inverter "support" the generator AC loads. This tells me that the EU AC output is stable within the UL1741 spec. I would guest that any battery-less gridtie would be able to sync and "connect" to an inverter-type generator, at least the Honda EU series.
The unknown factor is whether the inverter design in the generator is "bi-directional" or not. If not, when the GT export exceeds AC loads, the AC voltage quickly goes out of spec and the "resync" would happen every ~ 5 minutes. If the inverter is "bi-directional", the excess energy would be dumped into the DC input side of the gen's inverter i.e. the DC output of the motor/alternator. That's probably a 3-phase retified with a cap, so probably no harm done to the motor/alternator but the DC would jump up and the gen inverter probably shutdown due to the DC input "out of range". This would cause a "brown-out" on the AC output, just my guess. OB has an application note on AC coupling to detect this scenario i.e. "battery" voltage goes up passed some limit, to disconnect other "AC coupling" source from the GX AC output. Another scenario is when the GT export just enough to support all the AC loads (assumed it would sync to the Honda EU AC output), if suddenly, say a 1000W AC load drops out, would this extra 1000W back-feed the gen for ~ 16ms at worst ?

GP

tallgirl · February 2010

Re: Grid-Tie System added to home with generator

Greener,

AC Coupling, as mentioned on the OB forum and in their application note, is backfeeding a V-series inverter with a grid-tied (battery-less, though a battery-fully G-series inverter can also be used, based on my conversations with OB engineering) inverter. In that scenario, grid-tied inverters are fed into the AC outputs of the V-series inverters. Excess power is then sent to the batteries.

There are instances where people have used inverter-generators to feed G-series inverters on their AC inputs. Based on the technical specs for the G-series inverters, you can't feed them on their AC outputs or a backfeed error occurs.

GreenerPower · February 2010

Re: Grid-Tie System added to home with generator

Julie, my point is the Honda EU can be "sync'ed" and "support sell" by the GX i.e. the EU AC output is within UL1741 spec. It would be "sync" and "support sell" by inverters like the GT . In is case, the GX would "AC couple" and backfeed the EU's inveter (i.e. the other way around). True that I should say "to disconnect other AC coupling source from the GX AC input" if you are nit-picking. Of course support sell of the GX is done via its AC input. The explanation on the EU's inverter behavior whether it's "bi-directional" or not is pretty obvious, isn't it ?
GP

Grid-Tie System added to home with generator

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