Generator calculations questions

[Davids1964]

I am working on a 14 yr old generator that keeps tripping at the shunt trip mechanism. The 2000amp breaker has been replaced along with a new trip unit. It still keeps shutting down with only have the capcity load (750amps). The generator is a 15KW 480V with 5 sets of 500mcm wire to the switch gear. Switch gear only 5 ft away.The wire has been megged. An engineener is saying the wiring is under rated. I would like to know if he is correct on that issue. We also had a representaive from the manufacture of the breaker come out to look at the breaker. Yes, we have addressed vibrations also. Has anyone ever ran across a situtaion as this. The generator is a Steward Stevenson. Any help would be appreicated.

 

[Lxnxjxhx]

Can you post a simplified schematic for this setup?

 

[mdshunk]

How is a 15kw genny running 750 amps of load?
What all controls have the capability of energizing the shunt trip?

 

[mivey]

He must have meant a 15 hundred kW Stewart & Stevenson

 

[boboelectric]

How are you "working"on this generator?

 

[Davids1964]

Commericail building emergency power. There's only 750 amp total load to this generator before it trips. The breaker is rated at 2000 amps. Durnig a load test of approximatley 1400 amps it tripped after 40 min. into the test. However we have had similar trips with only the building load after only 20 min. It's never consistent

 

[LarryFine]

David, you might be able to tempororily disconnect the shunt-trip wiring so you can determine whether the breaker is tripping due to the load or the control circuitry.

As with most troubleshooting, use a process of elimination of the possible cuases of tripping, one at a time: actual over-current, breaker too sensitive, shunt triggering, etc.

 

[Davids1964]

Disconnecting the shunt trip was performed durning a test and the breaker still tripped. This breaker has been tested on the bench with no problem.

 

[chaterpilar]

David,

Can you check,

a) The current flowing in neutral conductor.

b) Is the Load balanced ? ( 750 amps) in each phase and within 5% tolerance?

c) Is there any Earth fault protection in the breaker? if yes can you reduce the sensitivity.

d) Last, but not least what is the temperature of Busbars at the input and output terminals of breaker, many a times loose connections/underrated busbar can increase the breaker temperature and induce a trip.
(You may use Infra red temperature scanner for this)

Cheers and good luck.

 

[coulter]

I am working on a 14 yr old generator that keeps tripping at the shunt trip mechanism. ...

...Disconnecting the shunt trip was performed durning a test and the breaker still tripped. ...

Tell me why you think it is the shunt trip mechanism that is causing the problem.

This breaker has been tested on the bench with no problem

Then you know it is not the CB. Was the trip uit also tested?

The model of the gen doesn't matter much. What is the CB make and model? What is the trip unit m&m? Where are the current sensors? Were these tested as well? Is there any readout on the trip unit that tells you why it is tripping.

The information so far points to a mis-set trip unit, or a mis-tapped sensor.

One thing - be careful of jumping out trip elements, something might be wrong and the CB/tripunit is doing it's job.

carl

 

[mayanees]

An engineer is saying the wiring is under rated. I would like to know if he is correct on that issue.

David,

To answer your question about ampacity, the engineer may be looking at the typical 500kCM THHW/THWN 75 deg C rating of 380 * 5 = 1900.
Without inspection of the system I would agree that the wiring is underrated for 2000 amps.
But frankly that's the least of your worries!
To put the installation in complaince with NEC - if there's an adjustable trip unit on the 2000 A breaker for the phase pickup - just adjust it to 0.9.

And if it's an 80% rated breaker, it may be limited to 1600 amps by design - in which case I wouldn't do anything.

Post the breaker part number if you have it.

JM