Feeder protection relay settings

[pwrtran]

Hi

Does any one have some guidance/standards on the criteria of relay settings for distribution feeder protection (27.6kV and below), espacially for those breakers that do not have reclose function and the feeder protection relay does not have both low-set inst. and the hi-set inst.? My concern is how much percent in terms of the total feeder length that the inst. overcurrent pick-up setting should cover, do you want inst. overcurrent to see the end of line fault or you want it only to see the fault at the first tapping pole and the rest will trip on time delayed overcurrent? I think that is a trade-off between fuse-saving and the reduction of SAIDI and SAIFI, but I don't have clear mind on the criteria, so I appreciate any helpful thought!

Thanks in advance?

 

[Electic]

There might not be a general recommendation. For example, if your feeders are long it might not be feasible to set the instantaneous so low as to sense end of line faults. The arcing fault current at the end of a line might be less than load current near the substation.

 

[pwrtran]

Thanks Electric. There is some margin between the end of line fualt and the maximum load current and it is possible to set
200% max load current < the inst. pickup < 50% of the end of line 3Ph fault.
But the problem is most faults will cause the feeder breaker to trip. So any good practice can make trade-off?

 

[davidbeach]

If you've got fused taps and main-line fuses you can coordinate with them, but if all you have is the substation breaker you have to trip for all faults. Add a reclosing relay and let the breaker close in again after the fault clears.

 

[cranky108]

I don't think this a good place to learn this. Maybe classes on distribution protection would be a good idea.

Your orignal post did not specify overhead or underground so reclosing maybe an option if you have overhead.

Coordination is an art, and less of a science.

 

[pwrtran]

It is mostly overhead distribution. Thanks!

 

[cranky108]

Usually the low set over current is used like a fast curve for something like fuse saving. Maybe useful for hot line work, or with reclosing.
The high set over current is useful for fast clearing for close in big faults. Problem is that so often there is a coordination point so close to the substation that it can't be used (A slight delay some times helps with fuse coordination).
For everything else there is time over current, which must be set to see the end of line faults (phase to phase, or phase to ground). There is a little debate on how much lower to go to cover fault impedance.

 

[pwrtran]

Unfortunately, there is no second relay can do the low-set inst., it only has inst. (or the hi-set) and the timed over current and there is no reclose feature on the breaker. So, fuse saving cannot be implemented.

 

[cranky108]

There are several relay manufacturers that can help you with that problem (depending on how big the problem is perceved to be).
And that's the answer: Do the best with what you have, or replace it.

I've seen it many times where when I describe the problem to my boss, and we don't have the money. But a year or two later they find the money to fix the problem because someone better politicly connected complained.

 

[davidbeach]

Good grief, you can put in new relays with multiple settings and reclose capability for a project cost of less that $10k, perhaps considerably less. Live with the existing problems or make a dramatic improvement, your choice. Reclose is a relay function, not a breaker feature.

 

[pwrtran]

Thanks cranky108 and david!

david, I like your correction. I meant the breaker was not wired to have reclose feature.

However, for the existing relays, my thought is to set the hi-set inst. to see about 1/3 of the line instead of the end of line fault, any comments?

 

[davidbeach]

Where are the first main line fuses? How is your time-overcurrent set?

Time-overcurrent should cover the entire line. Instantaneous should cover about 85% of the way to the first set of fuses. If you have no main line fuses, why not run the instantaneous all the way out and let the time-overcurrent back up the tap fuses?

 

[pwrtran]

There is no main line fuse. The farthest first tapped transformer is about 1/4 ~ 1/3 of the total line away from the station.

why not run the instantaneous all the way out

did you mean the inst. setting should see all the way out to the end of line?

 

[davidbeach]

The more of the line you cover with instantaneous, the less through fault damage to your transformer and the shorter the voltage sag those on adjacent circuits are subjected to. Ultimately there is no single "right" answer, just lots of compromises. I'd certainly want to get reclosing.

 

[jghrist]

The farthest first tapped transformer is about 1/4 ~ 1/3 of the total line away from the station.

Farthest? First? Seems like a contradiction of terms. In any case, if you have tap fuses 1/4 - 1/3 of the way out on the line, you aren't going to be able to use instantaneous on the feeder unless you add a reclosing relay (or change the feeder protection relay to one that has a reclosing function).

 

[cranky108]

Sure you can use instantaneous without reclosing. It's the same inconvenience either way. And it will reduce the through fault time on your transformers, which should extent the life of the transformers.

 

[pwrtran]

Thank all of you for the valued posts!