Short Cable Length -Fault Contribution

[PTCruiser85]

Hi All,

This is my first time and have seen some similar posts like my question. My feeder is 1.5 km from utility and the fault level at that point and my feeder is approx. same and some how my fuse cannot coordinate with their instantenous overcurrent protection.

Could you advise me how a situation like this can be resolved. Is their some kind of methodology to discriminate between same fault level at different location between fuse and instantenous electromechanical relay.

Thanks

 

[jghrist]

You'll have to get the utility to use an inverse time overcurrent relay. Maybe they can use the instantaneous along with reclosing and an inverse relay after the first trip.

 

[racobb]

If the utility instantaneous attachment "sees" the fault at your equipment, there will be no coordination unless they do as jghrist says or something similiar.

You could ask them to raise their inst setting, but good luck with that.

Alan

 

[davidbeach]

It may also be that their first trip is an instantaneous trip which is then blocked for subsequent trips. Very rarely will the utility have an instantaneous trip on a line and lock out following the instantaneous trip. There will almost always be a time trip before lock out.

 

[rbulsara]

Speak to your utility contact and ask to discuss your finding with their protection person. Send them the TCC curves. I have found most utility companies accommodating and helpful. After all coordination is one of the important things that they do well. Only time I had some difficulty when dealing with a less experienced person at their end.

You may want to make sure that what you are suggesting is also reviewed by some experienced engineer at your end.



Rafiq Bulsara

http://www.srengineersct.com

 

[PTCruiser85]

Thanks Gentleman. I am glad to seek advise from people who know it the best. But I have still have some confusion and need your advise.

Utility is supplying us with a cable feeder and they dont use reclosing on U/G cable feeds. As I recall they have have seprate old individual Inst O/C and TOC Very inverse relay.

As per the suggestion in the post, if they disable their instanteous for my case, it will take their relay atleast .5 secs(relay operating time)+ breaker cycle(air breaker 5-8 cycles) to clear a 12000 A fault on 13.8 kV.

All this time, rest of the customers on the same feeders will see a voltage drop for atleast .5 sec.

1 ) What is your take on this perspective.
2) What does a fault flowing for .5 sec means.

An unrelated question in the same post:

A PILC lead cable or XLPE unarmoured cable; Which one of these cable has a better fauly handling capaciy.

If I look at a TCC, How do I quantify damage on a cable for the fault amplitude and duration. I would appreciate if there is some kind of literaure avaible to know this in detail.

Thanks

 

[davidbeach]

Is it really an underground feeder, or are you just fed from an underground portion of a feeder with lots of overhead? On our underground feeders we only use instantaneous tripping when the circuit is tagged. An instantaneous trip on an underground feeder doesn't generally allow enough damage to be able to find the fault to allow it to be repaired.

 

[PTCruiser85]

Thanks for your reply davidbeach.

It is an actual underground cable feeder fed right from the substation breaker and this feeder have other customers as I have been told. Could you kindly reply other part of my (1,2) questions in the above post as well.

Thanks

 

[dpc]

I'd recommend getting a copy of the IEEE Red Book to give you some background on short circuit calculations and coordination issues.

There is no single correct answer as to the best approach for coordination philosophy, especially for distribution feeders. The best you can hope for is a compromise that doesn't leave anyone too unhappy.



David Castor

http://www.cvoes.com

 

[PTCruiser85]

Thanks dpc. I appreciate the recommendation.

Could you still advise the practical(historical & statical ) impact of fault current flow for .5 sec and the cable fault capability question in my post.

 

[jghrist]

All this time, rest of the customers on the same feeders will see a voltage drop for atleast .5 sec.

If the utility protection does not have enough delay for your protection to coordinate, then the rest of the customers will see a long voltage drop to zero volts when the utility breaker trips unnecessarily for a fault on your system.

 

[PTCruiser85]

Thanks jghrist. My objective of asking this question to get an introspection in to repercussions of fault current flowing for a certain period of time

Should I presume that a voltage drop of .5 sec can take down computers motors and lot of other nuisance stuff. As soon as power will be restored after -5 secs, motors will regain inrush current and breakers can trip.

Thanks for your input.

 

[steelerfan28655]

What type of fuse are you using? Are you using the fastest fuse but it still will not coordinate with the utility?
I agree that you should work with the utility coordination engineer to solve your problem. Have they supplied you with their coordination curves?

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