ECN Forum Forums General Topics of the Trade Alternative Energy Grid tie inverter question

If I had a grid tie inverter and the power was out, could I isolate the line side with the appropriate transfer equipment and switch in a small battery operated inverter to tickle the array back into operation and then run offline (using appropriate load management)?

Once it was going, the load on the battery inverter should be nil because the grid tie would be putting out a slightly higher voltage. You could then use some of the developed power to trickle charge the battery.

For that matter the "battery" could just be a 12v solar charger.

I know this sounds like a perpetual motion machine but the power is coming from the sun. The only question is how you handle sundown.

The solar charger would deal with that.

With a battery you would need some kind of diode isolation and a push to start button.
You could certainly make the controller smarter but I was just wondering if the idea would work.

In my jurisdiction such a set-up would disqualify you from all California and Federal tax subsidies.

The regulations, as interpreted, INSIST on PV systems going absolutely dead during system power interruptions.

Attaining power during outages is limited to stand-by generators.

I suppose that some NEMA company could devise a scheme that would met the utility's safety concerns -- but it's not any kind of priority.

The current PV schemes are boondoggles. They entail trivial, retail, installations that can't approach the economy efficiencies of large power systems.

PV arrays should be sited where natural conditions are absolutely prime: the high deserts of New Mexico. These have low rainfall and consequently no farming nor much grazing. It's sterile land -- typically over 7,000 feet up. Such land is absolutely ideal for massive PV arrays.

In contrast, the solar boiler schemes are now understood to kill / fry migrating birds. They are that powerful.

The other idea that stinks is wind turbines. They're killing off everything from eagles to migratory birds. They're uneconomic: wind energy is so variable that to make the mechanism last it has to be wildly over engineered. That ruins the economics.

All over California there are broken wind turbines. The engineers simply never imagined that peak wind energy would be so hard to shed. So the gearboxes burn up.

PV is the only solar powered scheme that looks like it can hold up: it's solid state. It scales well.

There are schemes afoot to have next generation cells able to split water at efficiencies of 40%. With that, finally PV would come into its own. Right now the technology is just too primitive, expensive.

It's only looking good because the government is massively cross-subsidizing it. That comes at the expense of all the other electrical goods that we, as professionals, love to install.

For every dollar going into solar, multiple dollars that would've gone into plain old electrical wiring don't get sold -- or installed.

Spain took this to the limit -- and broke their national budget. They so over subsidized PV systems that a national mania took hold. At the end of the day, the factories and contractors imploded. The two biggest German producers shut down entirely -- writing off billions!

And Spain is left with power bills that are crippling every consumer of electric power... large and small. Staggering rates like $0.40/ kWHr are just too much -- when French power sells for $ 0.13/ kWHr -- right over the border.

PV is elegant... but can't be so exaulted that rates are taken to wholely uneconomic levels.

Do they not allow battery back-up systems like the ones available from Outback Power Technologies?

Outback offers systems for Grid-Tie Solar Voltaic with battery back-up.

I'm much in favor of Solar Energy versus Wind Energy.

The answer to managing power from the Sun will come in the form of storage...battery storage. One of the top companies in the country working on development of such a battery is Ambri in Massachusetts, started by MIT professor Donald Sadoway. He is working on a Liquid Metal Battery for Grid Storage and it nears production.

Although this battery may not prove to be the answer to storing the Suns power...eventually someone will come up with one that will.

IMO, harnessing the Suns power is the answer to our energy needs here on Earth...and for our Planets sake the sooner the better.

A transfer switch would solve the safety problem and if this was done after you cashed the rebate check it would solve the California problem.

My real question is whether it would work.
How much power does it take to tickle the inverter into turning on? The way I understand it they only use the line to clock the inverter if solar output is greater than load.
I would be very frustrated if the power was out, I had $50,000 worth of solar PV on my roof and I was still running a dollar a KWH generator. That was be the real deal breaker for me when I was looking at it ... before the end of the Florida rebates.

It will depend on your system, it's configuration, local ordinances, and local utility rules regarding grid tie systems. Most inverters I worked with can be either a grid tie or off grid, not both. It maybe just more then a transfer switch. Plus start tinkering with a grid tie system will make the loco poco nervous. In a sense you are tinkering with their grid. An off grid configuration would do what you want but you won't be selling power to the grid.

Once you flip the transfer switch, it is not on the grid, any more than a generator would be. The PoCo is out of it.

Greg...if your asking if you can use a generator for the power source to activate the Grid Tied Inverter...that's a bad idea because where will the excess power go if the loads are less than Solar production? Right back into the generator and that's not OK. A grid tied inverter is not designed to work with a gen-set and will also be a problem and both would void warranty's.

That's why they have systems like the Outback with battery storage.

I think SMA just came out with a grid tied inverter with a single circuit 120volt receptacle that's still works when the grid is down...but it only works when the sun is up.

No I was talking about using a small inverter to feed the big inverter a clock signal and jump start it.

I believe APS now offers UPS that will start up off the internal battery without being connected to the wall outlet (or grid). That might get the inverter started. The Outback inverter that I used in a mobile application would take the shore power input and charged the batteries with whatever current was left over from driving the connected load.

How about utilizing a transfer switch in a different way?
Connect the output of the grid tie inverter to the LOAD terminals.
Connect the Grid terminals to the grid.
Connect the house loads to the Generator terminals.

When the grid is present, the GTI feeds into the grid. When the grid goes away, manually transfer the GTI to power up house loads. The transfer switch only has to be sized to match the inverter output or the house loads whichever is larger.

There has to be a second contactor or transfer switch to isolate the powered up house loads from the grid.

Or just use a large central battery bank to feed a bunch of local battery less UPS's and let each UPS take care of its local load. The UPS's do not have to synched to each other. Use a central charge controller to keep the batteries in shape.

The main advantage of grid tie inverters is you do away with all of that battery stuff.

Hmmm...not a bad idea...maybe an auxiliary load bank for when there is more PV power than needed?

Greg, is your inverter currently in place and selling power back to the POCO? If not, an off grid inverter is your answer. It still connects to the power, just doesn't back feed the grid

I have not bought anything yet. I was interested when we still had the rebate. Without it I am trying to come up with a reason to do it.

"where will the excess power go if the loads are less than Solar production?" Is that an actual problem? I am under the impression that you can open circuit PV panels. The inverter will only draw the power that it can use. I agree that backfeeding a small generator is not a good idea, especially the new inverter style.

Even if you could put a generator on the line side, running a generator is inefficient. You'd be better off just putting the generator on the load side of the inverter via a transfer switch but then you run into the gambit of dealing with transfer switch sizing, generator panel, etc. Trying it tie I'm a generator with a grid tie system can be more trouble then it's worth

I can't speak for the rest of the nation...

But out my way, if the AHJ ever find out that you've gotten tricky with field engineering...

Your tax subsidies are TOAST.

Here's the problem. Trust.

The Poco has already decided that it doesn't trust contractors to engineer failure proof systems.

As for economics...

If the grid is actually down... who wants their power to be backed up ONLY during daylight hours? Around my house, power NEED (as againt use -- we'll bear up under a hot day) is at a maximum after dusk... unless it's an insanely hot day.

True back-up power has to be available right through the night. In which case, the cost per kWHr is trivial. The money is all expended by the capital budget. (the shear cost of installation)

The ding to one's wallet from diesel/ natural gas/ gasoline use is utterly trivial. It's only ever going to happen on a very occasional basis.

No.

The Poco has it right: the sole economic value of PV arrays is to displace retail demand during ordinary operations. There is no merit to messing around with even the slightest mistakes -- backfeeding the grid -- from an errant installation. The threat to their line crews is way to high vs the trivial -- and imagined -- gains from using PV to power the home during outages. They're just too rare, too brief. You'll NEVER recover the expense of the complications -- in a century of use.

It flat out doesn't pencil out.

It was more of a theoretical question but in any serious emergency, "feeding the monster" (fuel) is far from a trivial task.
Proper transfer equipment is going to be needed for the generator anyway so any "field engineering" will not be attached to the grid. This would be used in lieu of the generator during the day.

I have this response from others...

The generator is a demand source, i.e., a voltage source, that keeps the AC bus at a constant voltage by delivering whatever current is necessary to do that. A GT inverter, on the other hand, cannot regulate its current output to maintain line voltage - it behaves as a current source. If you have a generator running on an AC bus alongside a GT inverter and the demand from the loads on the bus falls below the output of the GT inverter, the only way that the generator can regulate the line voltage is to sync rather than source current, which most generators cannot do without sustaining damage.

Here is some more...

The GTI will try to drive the generator as a motor. Depending on the design of the generator this might drive the voltage higher or increase the speed of the generator. If you are fortunate this will just cause the generator voltage or frequency to go out of range and shut the GTI down.
In the worst case the reverse current will interact with the speed and voltage regulation of the generator to cause damage.
A normal GTI's anti islanding circuitry will simply never see the generator as a low enough network impedance and will not run in the first place.

shortcircuit

Is there an inverter available that can operate in grid tie OR island mode? That would solve the issue. Even if you had to turn the inverter off and then back on to switch between modes.

The Outback Radian...

http://www.outbackpower.com/outback-products/inverters-chargers/item/radian-series-gs8048

Ill have to check but I do believe Outback FX series does. I have only used them on off grid systems but the menu options are supposedly there

The Outback GFX series is grid/off grid enable as well. Haven't work with the Radian system yet.

http://outbackpower.com/downloads/d...b_022013_gtfxgvfxseries_specsheet_lr.pdf