Emergency backup Generators

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twayne

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We have two Cat gens supporting two different sections of the same facility, during excercise test (under load) they transfer from normal to emergency just fine. When the excercise cycle has ended and transfers back to normal, one of them (350 KVA 480V), feeder breaker (600A frame 400A trip setting) approx 190A load, trips almost everytime. The other gen 200 KW 480V approx 125 amp load acts similar except when it transfers back to normal it causes breakers on the load side to trip. These systems were installed approx a year and a half ago, they were load bank tested and fully functional tested and all worked well. Little if anything has been changed, this problem just surfaced about two weeks ago. The inphase monitors and all other pertinent features have been checked and are functional according to the Cat techs.These systems back up very important equipment in a very important facility. It has been suggested by some engineers that a breaker coordination study be done on this system. My question is if nothing has been changed and the systems were working fine what could happen to cause this problem all of a sudden. Any help finding this grimline will be greatly appreciated.

Thanks,
Wayne
 
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Do the transfer switches have the ability to be programed for a transition delay?

This is basically a slowing of the transfer, the switch will open from the generator delay time starts, delay time ends, then it will close to utility.

The delay time is very short.

From the little I have read this can help if you have a lot of motor loads it gives them a little time to cost down.
 
Gremlin Hunt

You are transferring from a soft source(generator) back to a stiff source(utility feeders) each time these breakers trip.(Note: the utility may have inadvertently increased the fault level to this site after the initial commissioning tests)

This is a common problem and the solution set is site specific each time.

Good Luck

:)
 
Here is some more info..

The Model ZTSD provides an adjustable time delay after the
opening of the closed contacts and before the closing of the
open contacts for transferring large motor and/or
transformer and UPS loads. This delayed transition time
allows for motors to coast down and transformer fields to
decay, thus allowing inductive loads to be re-energized after
transfer with only normal inrush starting currents. The
delayed transition design is an effective method of handling
these applications and can be utilized as an alternative to a
standard transfer switch equipped with an in-phase monitor.
The delayed transition transfer switch is ideally suited for
pumping stations, sewage treatment plants, hospital X-ray
equipment, or wherever the bulk of the load being
controlled consists of large motors and/or transformers.
Major UPS manufacturers strongly recommend the use of
delayed transition type transfer switches to ensure proper
operation of their rectifier circuit and battery system. The
ZTSD Model allows a UPS system sufficient delay to recognize
a power failure and transfer to batteries, acknowledge the
return of power and allow the rectifier to walk-on to the
new source, reducing any transfer anomalies
Click to expand...


Of course this is written by someone trying to sell a product but I think it helps explain the possible need for a delay.

The rest of that can be found HERE
 
Emergency Back up Generators

Emergency Back up Generators

Bob,

Sounds like you have a lot of experience with this type of problem, unfortunately the type of ATS,s we have are "ASCO" and do not have the type of delay that evidently should have been installed according to G E Zenith.

I am currently discussing this problem with several electrical engineers and they seemed to think that cycles of the gen and commercial power were different enough to cause the inadvertent tripping, mainly due to something the power company could have possibly have changed. Well, that theory didn't flush,this morning a "Cat Tech" just happend by to change a faulty jacket water heater on one of the units and while he was here checked the cycles and they are 59.9-60 on the gen and 59.8-59.9 commercial.

Would have any idea whether or not a " time delay" of the proper type can be added to the ASCO ATS's we have or is this an ASCO question?

Been conversing with an Electrical/Testing company and they agree that a breaker coordination study may be necessary but this is very expensive and more importantly the study does not include the price of the remedies.

Really appreaciate your input.

Thanks,
Wayne
 
Wayne,

Just because the freq is the same as the utitily does not mean that the sources are going to be in phase. That link that Bob sent explains it pretty well. Without a delay between transfers I have seen this problem a few times. Not sure of a solution without the delay in transfer time.

Would like to know if you find one though without changing the ATS!

Rob
 
There is little to nothing the utility could have done to cause this situation. I would not waste time and effort on anything except the problem of poor syncronization between your generators and the utility.

Some options:
Add syncronizing control/paralleling equipmnet to your generators and transfer switches.

Change your ATS units to ones that include an imposed neutral position to give your system a chance to "shut down" and then restart on the utility grid.

Add logic control to your large motor loads so they are turned off before the retransfer to utility and then turned back on after the retransfer - this can often be done in existing building managment/SCADA/PLC control programs. If a "pre-retransfer" signal is available from the ATS the logic controller could treat it just like it does a "loss of utility" signal.
 
What kind of circuit breakers do you have in the generator or emergency gear opposed to the normal gear? Does the emergency gear or generator circuit breakers have adjustable trip settings? Could be that the have a different trip curve and while the emergency or generator breaker can handle the surge the normal power circuit breaker cannot. Has there been a change in the load like a motor or transformer added since original construction? These can cause surges.
 
Emergency backup Generators

The gen breakers are Square D and the normal are Cutlerhammer. !st Gen, going back a little when the load testing started,along with the problems the breaker (400A) in the 200KW Cat gen tripped when the load transfered from normal to emergency (Cat Tech increased instant. trip setting), after that during the next test the main breaker (400A Square D) on the load side of the ATS tripped when transfering from emergency back to normal. The instant. trip on the main (load side)was then set up (believe to max). Next test resulted in one of the breakers in the load side panel tripped (225). No further testing has been done on this unit since last week (facility users very upset)!

2nd Gen, 350KW Cat has a Square D breaker and the normal feeder breaker is a Cutlerhammer 600A frame instant trip set to it's max. Todate everytime tis unit it exercised the feeder breaker trips when the ATS transfers back to normal emergency. Occasionaly one of the two computer room air conditioners that this unit services has to manualy restarted after this operation.

One UPS has been added to the second unit since its original construction. Again the Amp load on this 400A system is approx 190A.
 
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