Main breaker trips on ATS operation

[oldfieldguy]

I just as well throw this one out there:

I have a facility set up with a 300 kva 3-phase delta 480 system. We have a 400 amp molded-case breaker at the service entrance. It is the normal input to an automatic transfer switch. The emergency feed to the transfer switch is 225 kw diesel generator. The load side of the ATS feeds an MCC with the plant lighting loads and a few motors, the largest of which is an air compressor drive at 50 horsepower.

Here's the problem. We lose the utility frequently. The generator starts up and the ATS trasfers load to the generator without problem. Then, when the utility comes back up, the ATS trasfers the load back to the utility, and the 400-amp breaker trips about half the time.

We have no faults. The circuit breaker has been tested and found serviceable.

The transfer switch is fast. I'm thinking that when the transfer switch operates, the 50-hp motor starts to spin down and presents a reverse and out of phase voltage to the incoming utility breaker. We've been unable to get the breaker to trip on retransfer with that 50-hp motor shut down, but I would really like to nail this problem down. I'm leaning towards a time delay relay to keep that motor offline for a few seconds after voltage loss.

And I'd appreciate your thoughts, ideas, commentary, wisdom, etc.

old field guy

 

[itsmoked]

That would be my thought too.

Yes! Put a time delay on the motor contactor circuit that just delays it's start for 5 seconds. Could be just on transfer back to the util or both. An air comp can usually cycle without the air loads even noticing.

In the refrigeration business that is almost always done. Otherwise on restart after a utility power fail all the compressors at a site are calling for cooling. When the power comes back on it goes right back off as the transformer out front falls flaming to the ground.

Anyway, a bunch of varying delay timers are used to allow each compressor to start at a unique time.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Good advice itsmoked.
There are several possible problems.
1> The motor is acting as you surmised. A time delay will fix the problem.
2> The combined starting currents of all the motor loads exceeds the maximum instantaneous current rating of the circuit breaker. A time delay, and or staggered starting (which is multiple time delays) will fix the problem.
3> The magnetic (or instantaneous) adjustment on the breaker is set too low. Adjust the instantaneous trip setting on the circuit breaker, if possible.
respectfully

 

[jraef]

What they said...

Here's a decent article on the phenomenon itsmoked was referring to. It's written with the help (and accompanying bias) of an engineer from Phase Solutions, a company that makes a monitoring device for the motor starter that decides a little more definitively when and if the contactors should open or not. The concept behind it is sound, I just think a timer will work fine albeit a little less "smart" about it.

http://www.plantservices.com/articles/2005/567.html

The company link is

http://www.phase-solutions.net,

but you must use MSIE to view it. I have Firefox so I can't.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[dpc]

I think your suspicions are probably correct. As an alternative to a time delay on the motor starter, you could also use a voltage relay on the ATS and block transfer until the voltage decays to maybe 20% or less.

 

[Snoball]

The switch back is hot to hot, not restart from blackout. You can do an in phase transfer by approproiate control, or use pre and post transfer contacts to switch off loads, then back on once the switch is made.

 

[oldfieldguy]

Thanks all-

I feel much better about the solution...

old field guy

 

[waross]

Considering snoballs suggestion,
There are fast transfer switches and there are very fast transfer switches. There are ATSs being marketed that are so fast that they include a synchronising circuit. They delay transfer until the line and generator are in sync., similar to the synchronizing gear used for parallel generator operation.
If you have one of the newer very fast transfer switches and it does not have a synchronizing circuit or the synchronizing circuit is inoperative, I would be expecting the breaker on the large motor to be tripping often.
Whatever is causing the breaker trip is possibly also causing excessive mechanical stess on the motor.
Can you post the make and model of the transfer switch and we'll see if we can follow up this angle.
If the switch will transfer fast enough that manually controlled motors that usually need to be restarted when the generator starts, will stay on-line when the switch transfers back to the utility supply, it is a very fast transfer switch.
Thanks for the heads up on this, snoball.
respectfully

 

[mc5w]

Duquesne Light Company in and around Pittsburgh, PA requires industrial and commercial motors to have a magnetically held controller that must be manually turned back on when power is restored or if automatically controlled it must have a power on delay.

This also applies to manually controlled residential motors but is not really enforced on residential customers since that would require refitting table saws and so forth.

They basiscally do not want all of the motors trying to start at the same time.

Also, a lot of single phase and a smaller number of three phase refrigeration compressors operate close to peak motor torque which is higher than starting torque for a design B motor. This is a deliberate overload that is tolerable because the refrigerant cools the motor. These types of compressors must be used with capillary or orifice expansion tube so that back pressure can go back to zero when starting. The more efficient compressors do not do this becaue operation near breakdown torque for a design B motor creates extra heat.

I concurr that you need to slow down the transfer switch and to change the compressor control to either magnetically held or a slow operate relay that keeps the motor from running until say a minute after utility power is restored. If you transfer switch is very fast an on delay relay or magnetically controlled controller will not reset which creates the same problem.

However, come transfer switches have their delay on power resoration rigged such that restransfer occurs very fast and you have no control over how fast that occurs. You would then need a relay that shuts down the compressor and rings an alarm when both generator power and utility power are available. This would then give you a chance to wait for retransfer and then turn the compressor back on.

You might also be able to block the transfer switch from automatic retransfer to utility power and instead ring a bell and light a bulb when utility power is restored.

Mike Cole