Ground faults tripping main circuit breaker 3

[Thedroid]

I've seen several issues with our selective coordination of breakers in the plant. Today we had a 480V coil go to ground and it tripped the MCCB feeding it and also the GE magne-blast main circuit breaker in the main substation. There were several circuit breakers in between these two. The magne-blast is located in the main 4160 switchgear and feeds the substation transformer. Then there is a GE AK-2-50 with an AC Pro trip unit in and feeding the MCC is a GE AK-2-25 also with an AC Pro. The ground fault pick up is 320A with a 10ms delay time. These units never pick up a ground fault, the main circuit breaker always trips along with the MCCB. Luckily we don't see a ground fault too often. Where can I find information on setting the units properly. Before I started working in this plant, a short circuit analysis was performed, and reccomendations were made for setting up the AC PRO units. I'm guessing that mistakes must have been made somewhere, and I'd like to look into this deeper.

 

[Laplacian]

I would suggest hiring a consulting engineer to evaluate your ground coordination, then a relay testing contractor to make the settings. There is too much involved to explain over an internet forum because there are many compromises that must be considered depending on your exact system configuration.

What type of transformer is the 4160-480? Wye-wye? If delta-wye, it should not have seen the ground fault. Your main should not have instantaneous ground fault protection in any case to allow for downstream devices to pick up and clear.

 

[Zogzog]

I agree, you seem to have some serious coordination issues, you need to get someone in there who knows what they are doing, then after all the setting are right, password protect them in the AC PRO units so no on changes them without authorization (That could be how you got there in the first place).

Where are you loacted? I probally know someone in your area.

 

[Elmir]

Also have the wiring inspected for proper installation of the AC Pro trip units. All protection devices should of been tested when the protection settings from the coordination study were implemented.

A NETA member company should be able to help you with reviewing the coordination, making recommendations if necessary, and testing the breakers and protective devices.

 

[jimgineer]

Is there a policy on these forums for recommending companies to individuals who have needs for engineering? I don't want to get myself in trouble before doing it.

 

[Thedroid]

The transformer is a 4160/480 delta/wye. With the exception of the mccb's, I have not seen any other downstream devices clear a fault. It's alway the main circuit breaker. I'm relatively new to this field, but have a deep interest in the larger equipment. I do understand that the calculations required are probably over my head, but I'm looking for a basic understanding of whats involved when making these decisions. I can guarantee that hiring anyone outside is totally out of the question, even if that means the equipment is compromised. I'd love to participate,even as an onlooker in such a study.I'm in New Mexico, if anyone has any reccomendations for future reference.

For laughs here is the current configuration of the AC PRO

CT Rating 600
LT Pick Up 600
LT Pick Up delay 10s
ST Pick Up 2400
ST Pick Up delay .30s
ST I^2T Off
Instantaneous P/U 6000
Ground Fault P/U 320
Ground Fault Delay .10s
Ground Fault I^2T On
Unbalance P/U off

This is installed on a GE AK-2-25 600A breaker feeding an MCC. Normaly this MCC draws around 100A

 

[Zogzog]

There are many variables and I still recommend you get a pro in there to help you but seeing how this is feeding a MCC I find it odd that your ST I^2T is off, that could be your problem but I must stress there is not enough info here to be sure.

I know a very good engineer in Phoenix

http://www.cbssw.com/

Ask for Dan


jimgineer, you are allowed to post a web address and contact name, no email addresses or phone numbers as I understand the rules.

 

[Thedroid]

That is where we send our breakers for repair and testing. I believe they performed the install of the AC PRO's.

Where can I find good information on this branch of the field? Protective relays and circuit breakers is something that definately interests me. Any reccomendations on books?

 

[Dumbo2929]

I know one on the east coast if that is closer for you

http://www.PowerEngineersLLC.com

 

[Zogzog]

They may have installed the breakers but your plant has to provide the settings, Dan can figure out your coordination problem. As for books, I dont know too many to list, depends on what exactly you want to learn about. IEEE white papers and EPRI papers are my favorite, very specific on the topic you are looking for.