transfer after trip?

[alehman]

We have a main and alternate source transfer system utilizing two low voltage power circuit breakers. We are having an ongoing discussion as to whether or not it is desirable for the automatic transfer scheme to close the alternate source circuit breaker if the main source breaker trips due to overcurrent.

One side says transferring will improve reliability because the main breaker might have a nuisance trip or the fault may be momentary. The other side says it will reduce reliability because of the possible increased equipment damage and will increase hazard to personnel. This is a "high up-time" facility. The alternate source is a diesel generator.

Does anyone know of any standards or codes pertaining to this question? NEC Article 430 prohibits reclosing faulted motor circuits, but doesn't address other situations.

 

[rbulsara]

alehman:

I beleive it is a matter of safety and sound engineering, if one of the breakers trips on O/C, we normally lock out the bus via 86 device. Make sure the lock out occurs on 'bell alarm' (trip indicating contact)only and not on breaker position contact. There is nothing to gain by closing a breaker on overcurrent situation, only a lot to lose.

You can close the alternate breaker after a delay, if you sense the normal breaker as 'open' only (via aux contact) but do not sense a trip (bell alarm). This safeguards against 'fail to close' breaker situation.

I do not agree with view of 'momentary' fault, if so set up the normal CB to reclose?? why risk a new source. This does not appear to be a utility set-up, so reclosing is out of question, imho.

 

[alehman]

I agree. Do you know if there are any standards that address the question?

Another point I suppose is that a typical automatic transfer switch would attempt to transfer if a fault caused the upstream breaker to trip.

 

[peebee]

There's no code or standard regarding this that I'm aware of.

But if I was designing this critical facility, I'd have it attempt the reclose to alternate source exactly one time. Worst case, the second breaker will trip too. Beyond that, I'd require a manual reclose. In my opinion, multiple automatic attempts to reclose (beyond one attempt to alternate source) should be left to utility companies only.

Keep in mind that an upstream fault might potentially have caused your breaker to trip (on a high magnitude reverse power current flow, or maybe a lightning stroke) -- in that case, there'd be no good reason not to transfer to alternate source.

Similarly, it's possible that any downstream fault might have cleared itself (this is very true if you're using insulated case or molded case breakers, which generally do not coordinate on high-magnitude faults).

Regarding your typical ATS attempting to transfer -- bear in mind that it would have no idea what caused it's normal source outage, it only knows that the normal source is gone. ATS's generally do not include overcurrent sensing.

 

[ppaya]

Alehman:
I don't quite understand what is this second source? If this is source from local distribution utility feeder (second, alternate feeder supply), it is probably not permitted to automaticly switch to another supply. Utility company in my experience is not allowing for automatic scheme. They want to be in charge with feeder configuration at any given moment.

I don't think that A/R is allowed too. Whwn main breaker trips Distribution company send their lineman to operate the switchgear. (Comments are welcome...)

If power supply is critical to this customer I would think they either have to have 2 distribution feeders supply in switchgear or back-up diesel generator that kicks in on loss of voltage (27) after sec.

Could you explain nature of your alternate source?

 

[alehman]

peebee: The proposed system allows for only one shot at closing the generator breaker. If it trips, the system is dead until manually reset. These are power breakers, but are installed in a non-ANSI switchboard and therefore are equipped with instantaneous trip. Your point about possible reverse current is interesting. We have a fair number of motors. Not sure if it's enough to trip the breaker.

ppaya: The alternate souce is a diesel generator. The question is whether or not to close the breaker connecting the generator source to a bus who's normal source breaker has tripped due to overcurrent. The generator and switchgear are owned by the customer. This utility should have no interest in this.

 

[rbulsara]

I am not sure of any 'standards'.closest that I can think of is IEEE orange book but all it says is ATS is desinged to 'withstand' fault untill upstream breaker opens..

I guess, even if you close alternate source, and there is a fault, the breaker is supposed to open. So in theory 'safety' is taken care of.

While I still standby my previous post, only under very special conditions where nuisance trip's are expected by nature of application (say temporary overloads), I may go along with peebee's suggestion but that too assiging all risk to the 'owner', after explaining him the risk (of possibly damageing both breakers..).

 

[Bung]

You would also have to factor in the possibility of subjecting the generator to a close onto a fault, and the effects on the existing generator connected load. Can this load withstand the the voltage dip (at best) or total loss of supply (at worst) if the generator is closed onto a fault?

General utility practice is not to reclose (or auto-transfer) onto cable circuits, as they are generally more likely to be permanent faults. It is acceptable on overhead circuits because air is generally a self-healing insulator.



Bung
Life is non-linear...

 

[alehman]

Thanks for the great info. My position has been that it is generally hazardous (to personnel and equipment) to re-energize a system known to be faulted. The examples being used to argue the other side are:

1. An ATS will tranfer if the upstream primary source breaker trips, re-energizing a fault (this begs the question: Should there be OC protection downstream of the ATS?)

2. Utilities have used auto-reclosing on aerial circuits forever, and with considerable success. Although as Bung said, it doesn't really apply here.

I'm sure the debate will continue.

 

[mc5w]

There is also the problem that your generator may not put out enough fault current to trip its circuit breaker if closed into as fault.

A generator's fault current is ultimately limited by the amount of current that it takes to saturate the steel in the generator. When the steel saturates the output voltage drops to zero. For diesel and natural gas genrators this is usually the primary concern when a fault occurs.

Source: IEEE Orange Book

On the other hand, with large turbines that are attached to a utility grid there are other issues such as slipping poles and possibly bending or breaking shafts and turbine blades. Someone told me that one of the big hydro plants had an arcing fault on the generator bus that snapped a rather large drive shaft and put it through 12 feet of concrete. If an arcing fault occurs between the generator and its circuit breaker you have a BAD problem.

Mike Cole, mc5w@earthlink.net