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#272 - 01/06/01 10:38 AM transfer switch problem
Alan McNeil Offline
Member

Registered: 01/05/01
Posts: 18
Loc: Buzzards Bay, MA U.S.A.
when transfer switch goes back to normal source voltage it pops the circuit breaker, I suspect the problem is that the generator output and the utility output are not exactly in phase with each other causing magnetic flux "clashes", that is flux decaying vs. flux buildup because the different voltage sources are out of phase.
We have had this problem in the past and a phase monitoring device was installed by the technician that services some of our transfer switches but the problem has reared its ugly head again.
I am trying to get my boss to replace the one step transfer switch with a dual step transfer switch that would allow the flux in the transformer to decay enough, say a 3 second delay so that when it does make contact with the utility power we do not pop the breaker.
any suggestions, comment, or ideas would be very helpful.

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#273 - 01/06/01 12:27 PM Re: transfer switch problem
Bennie R. Palmer Offline
Member

Registered: 11/09/00
Posts: 72
Loc: Milwaukie, OR 97267 USA
I would contact the technical services engineers of the transfer switch manufacturer. There may be an air core inductor available that would limit inrush current at transfer, for a short time.
Transformer manufacturers can supply information concerning inrush current.
Circuit breaker manufacturers can provide data on time trip curves, for selecting proper breaker.

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#274 - 01/08/01 03:53 AM Re: transfer switch problem
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
It's my understanding that a decaying magnetic flux causes problems , especially if contacts are slow to open. So just how does an air core inductor work?

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#275 - 01/08/01 07:13 AM Re: transfer switch problem
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Sparky,

I might be able to explain a little on theory that might make some sense. If the following is still questionable, let me know.

An air core Inductor works the same as an Iron core or a ferrite core Inductor. There just is not any highly permable material in the core.

The permability of air is 1.0, where Iron is a lot higher.

The reluctance for air is a lot lower than Iron.

What does this all mean, you might ask?? Well, since the Iron has a high permability and reluctance, it will "collaps" slowly when the power is removed, result is a slowly decaying flux that cuts across the conductors [windings], causing the Inductor to create that Inductive "buck", more than an Air core will.

Air core Inductors are quick decaying, but the trade off is that they are larger than their ferrous counter parts, per Milli Henry [mH]. You will typically see Air core Inductors in speaker system crossovers [passive filters]. I have built many Xovers and spent quite a lot of $$$ on those darn Air core perfect layer Inductors, so it's time to wind my own!! [got two methods to calculate the turns and layers- one is a visual basic script, the other is a common calculus formula].

An Air core Inductor can be placed in series with the supply to "Choke" and lower the Inductive surge.


Scott.
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#276 - 01/08/01 04:53 PM Re: transfer switch problem
Alan McNeil Offline
Member

Registered: 01/05/01
Posts: 18
Loc: Buzzards Bay, MA U.S.A.
 Quote:
Originally posted by Scott35:
Sparky,

I might be able to explain a little on theory that might make some sense. If the following is still questionable, let me know.

An air core Inductor works the same as an Iron core or a ferrite core Inductor. There just is not any highly permable material in the core.

The permability of air is 1.0, where Iron is a lot higher.

The reluctance for air is a lot lower than Iron.

What does this all mean, you might ask?? Well, since the Iron has a high permability and reluctance, it will "collaps" slowly when the power is removed, result is a slowly decaying flux that cuts across the conductors [windings], causing the Inductor to create that Inductive "buck", more than an Air core will.

Air core Inductors are quick decaying, but the trade off is that they are larger than their ferrous counter parts, per Milli Henry [mH]. You will typically see Air core Inductors in speaker system crossovers [passive filters]. I have built many Xovers and spent quite a lot of $$$ on those darn Air core perfect layer Inductors, so it's time to wind my own!! [got two methods to calculate the turns and layers- one is a visual basic script, the other is a common calculus formula].

An Air core Inductor can be placed in series with the supply to "Choke" and lower the Inductive surge.


Scott.


The transfer switch in question is an old zenith, one step switching, do you think that a newer model, say a russ electric two step with a 3 second time delay would help, that way the flux in the transformer has time to decay. It is about time to upgrade the transfer switch. the question is do you think this would eliminate the tripping of the circuit breaker.?????

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#277 - 01/09/01 05:51 AM Re: transfer switch problem
Bennie R. Palmer Offline
Member

Registered: 11/09/00
Posts: 72
Loc: Milwaukie, OR 97267 USA
This problem is based on time and intensity. I have not evaluated the effects of opposing, out of sync, flux. Circuit breaker time /trip coordination is the only way the breaker will remain closed, on a hard start.
In the past, I have froze brakers with freon gas, to enable a transformer to be started.
I was incorrect when I stated the load on the secondary was not a factor. The current flow in the secondary will delay the primary flux build up, extending the time. There could be some effect on the secondary load equipment, concerning out of sync fluxes. I don't know what this effect would be, I'm only mentally examining this possibility.
Transformers, at initial start up are essentially a dead short. There must be overcurrent devices capable of providing service continuity.

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#278 - 01/09/01 03:27 PM Re: transfer switch problem
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
Thanks Scott, i'll keep an eye out
Ben, you actually froze breakers? This is good !, tell me more....

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#279 - 01/09/01 04:27 PM Re: transfer switch problem
Alan McNeil Offline
Member

Registered: 01/05/01
Posts: 18
Loc: Buzzards Bay, MA U.S.A.
I thank everyone for the information, I have much to think about, I don't think I am going to freeze the circuit breaker, but that is certainly a novel idea.
I think what is going to happen is the building will be re-wired from top to bottom because the electrical is in poor condition and has been reworked and added on one too many times. thanks again for all your help and consideration Alan

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#280 - 01/09/01 07:34 PM Re: transfer switch problem
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Bennie covered it well with the hard start situations of a Transformer under load. Glad to see you re-thought the secondary load problem! That's where the problem will fall under.

If you have secondary loads with a long inrush time, the Transformer will need to deliver that via the Primary windings. This, along with the inherit inrush of a transformer will be seen on the Primary as a high and long duration inrush.

Using circuit breakers with adjustable trips could help this situation, unless you can install a C/B frame with time current curves similar to the ones of an FRN/RK5 fuse.

One thought would be to unload any non-essential loads, especially those with high inrush values-like motors- during transfer over to separate systems, just long enough to allow the transfer and stabilize the circuits/system. If you find transfer equipment that has better time lag trips, that is great.

Any Induction motors that are running when the transfer takes place will do the same as if there is a short circuit - they become Transformers. Loaded motors supply the most KVA, which is near the LRA of the motor for a motor loaded to near it's capacity. [kind of interesting, isn't it?? ..]. The transfer acts as if there is a temporary loss of KVA supplying the motor. Since the Induction motor is just a Transformer with a rotating secondary, it will supply KVA through the Inductive Reactance that is present in the primary [windings], along with the XL in the secondaries [rotor]. This continues until either the load stops turning the rotor at a sufficient speed that can create the Transformer action, or the system becomes stable. A locked rotor will not develop KVA outwards, but requires an inrush KVA to get things moving- similar to the way a Transformer needs a high inrush current to get a high flux current flow moving in the core.

After the system becomes energized again, the motors will demand any inrush /LRA according to how much KVA is required to get the loads moving again, up to the speed needed [or slipped below a synchronous speed].

Also, any HID lighting will have the same scenario of starting as it would normally. Some HID ballasts draw a high starting current, which tapers off to the somewhat constant VA load it has while at full operation. CWA [Constant Wattage Autotransformer] Ballasted HID lamps will have the same KVA [or current] load through starting and operating. HPS lamps typically use the CWA ballast, as do most Metal Halides.

Anyhow, hope this was interesting.


***** Bennie:

My father had to do the breaker cooling trick one time. He was the EC on Playboy Mansion West, back in the late 60's/early 70's. When the place was almost finished [everyone knows that situation with clients!!], Mr. Hefner wanted to throw one of his world famous parties. The load was extremely high on one certain subpanel, due to the fact that things were kind of temped here and there for his party!! To keep the breaker from tripping, they used a fire extinguisher to cool the frame [the CO2 type, I guess]. He told me this story about 15 years ago when I was getting serious about the technical side of the trade [you know - the young wanna-be Engineer!!]. Not too sure if He would admit to doing this now

That was the only trouble with that entire project, and everything is still rock solid to this day.

There is a problem with Utility power quality. The location is still fed via 4160 3 phase utility distribution and on a very small branch. Since it's nearly impossible to upgrade for the Utility Co. [DWP], there is a high occurrence of power loss!!

To remedy this, the on-site crew have installed diesel powered backup systems. They have a nice one now, which from what I have heard is really huge! I think it's rated at like 750 KVA, but not too sure.

Need to go back and visit everyone there. I can get some insight to their UPS / backup protocols and all the extra details! It's been over 7 years since I have last been there, so it will be a nice trip.

See everyone in a couple of weeks! I am off for a "Vacation". details to follow.


Scott.
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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