Inverter used for grid tie?

[farmerfred]

Hi, Thank you for looking at my question.Can an AC inverter (like the kind you use for batteries, modified sine wave) be hooked to the grid? Im not sure , because the ac coming in to the inverter would be much more than coming out. ( 400watt dc genny)Am I wrong or just missing a "crucial" piece to the pie.

Thank you farmerfred

[old55olds]

Am I missing something? If you mean to feed power back into the grid the answer is NO. If you mean to detect power outages to fire up the inverter to feed AC to a completely seperate circuit some do. But the output of the invertor must NEVER be tied into the grid unless it is a grid-tie inverter. If you do, first you will release all that valuable smoke, and without that the inverter will not work. Very hard to put the smoke back in.  If you tie the inverter to the grid when there is a power outage you will likely kill the poor serviceman who is trying to get your power back on. You will feed the 110volts through the transformer and up to 40,000 volts back into the grid. Not a nice thing to grab on to.

Best you tell us what kind of inverter you have and exactly what you want to do.

Ken

[johnlm]

Just a comment here to avert another "if said often enough everyone believes it".

 First let me say the power company demands that you disconnect in the event of a power failure for various good reasons.  I AM NOT advocating keeping your equipment tied to the grid during a power failure.

The statement Im refering to is that if the grid goes down and you have  some kind of inverter tied to it without disconnecting then you WILL likely kill the poor serviceman trying to fix the line.  In my opinion, this may happen in very very rare circumstances such as if you are one of only maybe a few folks on the end of the grid that is isolated from the rest of the grid, or if you have a very very large inverter.  The reality of the situation is that when the grid goes down 10's, 100's even 1000's of houses are sitting there attached to the (isolated section of) grid trying to draw power. (Translation: the grid looks like a dead short as far as your inverter is concerned)  Its pretty unlikely your invertor will be able to hold the voltage high enough to do much damage to the service man with all these houses trying to draw 100's of kilowatts - unless you have a 100 kw inverter.

The statement that you will kill the guy serves the point of scareing people into disconnecting from the grid during a failure, but the more reasonable reason to do so would be to keep your grid tie invertor from going up in smoke trying to power up all your neighbors, or not be syncronized with the grid phase when it comes back up -also resulting in smoke.

Sorry, Im a stickler for accuracy.

Johnlm

[johnlm]

Another clarification.  The reason I ranted on that statement is that Ive read it several times in posts in the past few weeks.  Thus the -If said often enough ...

I wasnt picking on the guy above.

John

[Ungrounded Lightning Rod]

If the whole grid goes down, yes you won't be able to feed it.

If a wire goes down on the little branch on your street, or if they're changing the transformer that feeds your house, you very well MIGHT be able to feed it - and fry the lineman.

It is a VERY real risk.  And both the power company and the linemen are really worried about it.

(When I hotted a circuit in my house after the goose hit the wire up the street and they heard me start the backup genny they dropped what they were doing and came over to ask me to be sure not to backfeed the line.  I had a nice discussion with the linemen and showed them how I was A) aware of the issue and B) had taken steps to avoid zorching them, after which they went back to work.)

A grid-tie inverter senses the grid condition and cuts off its feed on the grid side if there's an outage.  But the model whose documents I examined will keep feeding a separate distribution panel for the loads you want to stay hot during an outage.  You rewire your importand loads to the secondary panel, leave the ones you want off in an outage on the main panel, and it all just works.

(The inverter in question also keeps your batteries charged off the grid and that feeds the protected loads though the inverter when the sun is down and the wind has stopped.  Don't know if it leaves the loads dead or cuts them over to a line connection if the main inverter circuitry fries, the batteries die, etc.)

[srnoth]

Underground,

Which inverter are you refering to? Is it a particular model or are you just talking about grid-tie inverters in general?

Cheers,

Stephen.

[Ungrounded Lightning Rod]

It was a particular model, of the brand that I've seen most highly recommended by posters on this web site.  Unfortunately I don't recall what it is at the moment, which is why I didn't post the name.

It has an add-on grid tie adapter and the manuals for both the inverter and grid-tie adapter are downloadable from the factory web site.

My impression of the adapter is that it contains the relay to tie/untie the inverter output with the grid, along with a voltage, frequency, and phase-difference sensing peripheral for the inverter's computer, which both gives the computer the information needed to synchronize the inverter with the grid before tie-in and detects the frequency errors and voltage droops or humps which indicate the grid has come unhooked from you (or is otherwise screwing up or browning out) and it's time to untie.  (But that's just reverse-engineering guesses from the manual's description of grid-tie operation.)

[Ungrounded Lightning Rod]

Found it.

It was the Xantrex/Trace SW series.

Here's a link to their documentation list.  Part way down you'll see a grid-tie interface document.

I linked the whole list rather than the particular document because some of the (multi-megabyte) inverter docs in this set are also marvelous tutorials, explaining not just what you do but why, in depth.

For instance, one of those manuals is the first place I've ever seen a written discription of the reason for the code requirement for exactly a single connection between your electrical system and your grounding system.  (With diagrams, yet!)

Reason:  To keep ground currents from nearby lightning strikes from taking a shortcut through your house's neutral or protective ground wiring.

[Ungrounded Lightning Rod]

By the way:

Please respond to the above message to let me know you found it.  Thanks.

[srnoth]

Hi Underground,

Ok, I went to the page and it gives me this list to choose from:

Document Library

    * Choose by Document Type

    * Choose by Product Type

    * List Catalogs

    * Archived Documents

Choose Document Type:

    * Application Notes

    * Brochures

    * Catalogs

    * Data Sheets

    * Drivers

    * FAQs

    * Newsletters

    * Other

    * PV Warranty

    * Software Interface

    * Technical Notes

    * User Guides

Which one is it?

I looked around a bit and as you said there is a lot of useful info there. I will definately check back that page when I have some time on my hands. Lots of good reading there it looks like.

Cheers,

Stephen.

[Ungrounded Lightning Rod]

That's odd.  I tried it and got the same thing.

Click:

 -> Choose by Product Type

 -> Inverter/Chargers

 -> SW Inverter/Charger - 120 VAC/60 Hz

 -> PDF emblem on the 2MB file:

    SW Series

    Inverter/Chargers -

    (4.01 Software

    Revision) User Guide

This gets you the owner's manual as a PDF file.

I was intending to post the link to the page just above the PDF download.

[srnoth]

OK! Got it! Thanks.

[Nando]

farmerfred:

Forget it, this type of converters can not me connected to the GRID -- BURN OUT immediately.

For all the GRID tied inverter needs to be looked not like an inverter but LIKE A CONSTANT AC CURRENT SOURCE. that has to be controlled by the GRID voltage.

It takes too long to show what has to be done for an inverter to be connected to the AC GRID power.

Regards

Nando