Automatic Transfer Switch

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naplespete57

naplespete57

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Eastern Kentucky
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Project Manager in Conveyor Industry
Took a 20+ year hiatus from industrial electrical and what appears? Automatic Transfer Switches! Now that I am back in the saddle I need to school myself quickly on this technology. Anybody have any ideas as to how I might get up to speed quickly? If there is anyone knowledgeable in this area that would entertain questions I would love it. Have been exposed mostly to Cummins brand
 
mhulbert said:
If you have specific questions, just ask them here, there are plenty of people that have experience with ATS's

mike
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OK, fantastic. An ATS has three sets of lugs UTILITY, GENERATOR, & LOAD.

If three phase power is lost at the utility the generator will automatically start and feed the load.

Is the loss of Utility power sensed electronically or by means of a latch and unlatch relay?

What is the failsafe that keeps the generator or utility from backfeeding onto one another? I am told it is impossible for backfeed to happen and I just want to know how it works.

Thanks alot!

Pete
 
Break before Make

Break before Make

naplespete57 said:
... What is the failsafe that keeps the generator or utility from backfeeding onto one another? I am told it is impossible for backfeed to happen and I just want to know how it works.
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The actual physical switch is a "break before make" type, with the "load" on the moving contacts and the "gen" and "utility" on opposing stationary contacts.

The moving contacts are either on the "gen" contacts or the "utility" contacts. (or neither during the actual changeover)

They can't physically be on both at the same time.

It's just a multi-pole, double-throw switch which MUST go thru "off" when switching.
 
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Pete, picture a 3-way switch, where one terminal, the common, can connect to one traveler terminal or the other, but not to both at the same time; plus, the two traveler terminals can never contact each other.

Now, picture a 3-way with one traveler terminal fed from the POCO, the other traveler terminal fed from the generator, and the common terminal supplying your panel. Now, picture three of these switches operating together.

The utility power is sensed by an electronic circuit that is usually adjusted to switch the ATS over after the voltage is below a set level for a set amount of time, and will switch back after the voltage is above a set level for a set time.
 
All very fine explanations. How do you descibe an ATS with a closed transition back to utility power. Are both sources connected for a fraction of a second or are the perfectly sychronized and then the switch is thown in a fraction of a cycle? Also, don't they have ATS's with overlapping neutral pole?
 
sceepe said:
All very fine explanations. How do you descibe an ATS with a closed transition back to utility power. Are both sources connected for a fraction of a second or are the perfectly sychronized and then the switch is thown in a fraction of a cycle? Also, don't they have ATS's with overlapping neutral pole?
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I take it by "closed transition" you mean zero interruption of power on transfer back to "utility".

The smaller/residential ATSs I've worked with simply do a VERY FAST changeover (using some fairly substantial solenoids to rapidly move the contacts from "gen" -- thru "off" -- to "util") and transfer the entire load at-once. This happens fast enough that most residential type loads (including HVAC, computers, clocks, and TVs) keep on running. I suppose you could implement a gen sync device -- or even auto-paralleling w/ a smooth load transfer. It's all about what the customer needs and how much he's willing to pay.

In the larger (multi-meg) multi generator setups I've delt with, while we had fully automatic paralleling and load transfer within the plant ... we could parallel to "utility" only under operator control and then, w/ procedures limiting parallel ops to only long enough to smoothly transfer the load (i.e. a few seconds). I don't know that this return-to-utility process isn't automated elsewhere (I'm almost sure that it IS given the number of generators backing-up remote unmanned sites -- and it's not technically difficult) ... I've just not personally worked with any of them.

Obviously the POCO gets a "vote" in any implementation involving parallel ops w/ the utility.

Re: your neutral question ... some ATSs do switch the neutral (making the genset a separately-derived system) while others implement an unswitched neutral.
 
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An overlapping neutral really isn't an issue (as far as having the utility and generator connected together) since both are grounded anyway.

Steve
 
steve66 said:
An overlapping neutral really isn't an issue (as far as having the utility and generator connected together) since both are grounded anyway
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Absolutely right.

In a non-SDS genset install, the genset's neutral bond IS the main bonding jumper at the utility service. (i.e. the neutral is ground-isolated at the actual genset -- just as it is in a sub-panel)

However, an ATS which does switch the neutral -- thereby making the genset a SDS [defined at 250.20(D) FPN 1] -- does invoke different grounding requirements; specifically 250.30.
 
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NoVA Comms Power said:
In a non-SDS genset install, the genset's neutral bond IS the main bonding jumper at the utility service. (i.e. the neutral is ground-isolated at the actual genset -- just as it is in a sub-panel)

However, an ATS which does switch the neutral -- thereby making the genset a SDS [defined at 250.20(D) FPN 1] -- does invoke different grounding requirements; specifically 250.30.
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Absolutely right again.

In case anyone is wondering why switching the neutral is an option, and why you'd choose one over the other:

If the generator's neutral and ground are irreversibly bonded, the neutral should be switched by the ATS.

In other words, the generator is the determining factor. When you have the option, I suggest the non-SDS method.
 
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