Fuse and Feeder Settings 2

[Mbrooke]

Single feeder substation, 69kv 5/6.25MVA transformer. Outgoing 13.8kv feeder loaded at 303 amps. Setting the feeder pickup at 200% as done normally results on a grossly oversized primary fuse.

 

[rzgpp]

Is it possible to use IDMT curve...for relay protection of feeder?...

 

[Mbrooke]

IDMT curve? I was planning on an extremely inverse IEEE curve for this application. But I never heard of the pickup being set so close to the max feeder load.

 

[RRaghunath]

For 13.kV feeder, there is no code that stipulates the pick-up shall be so high as 200% of rated current. Typical norm is - the pick-up shall be greater than the FLC + expected short time overloads. Thus, should a pick-up of 125% of rated current not be adequate.
Fuse could be slow blow type in order to be able to keep the rating low.

 

[Mbrooke]

Thanks 125% would be more reasonable.

 

[Mbrooke]

Alright, a quick test:

It works with a 100E fuse, however, that seems grossly over sized being only a 5/6.25MVA trafo. Will it even protect the trafo for a fault between the secondary and station re-closer?

I'm keeping keeping the curves further apart then they need to be (at first glance) due to the delta wye per unit current discrepancy during a phase-phase fault not involving ground.

 

[ElectricRob]

I've always tried to set feeder overcurrent settings based on the rating of the conductor rather than the feeder load current (but I've never had a substation that only has one feeder). If there are normally open ties, you want to be able to pick up load from additional feeders if necessary without worrying about trip setpoints. If you have something like 336 ACSR conductor leaving the station, then a 600 Amp pickup for the feeder would be appropriate.

Overcurrent settings should protect a transformer in the event of a fault, they should not be used in order to protect the transformer from overload. In the event that the transformer loading gets too great, make plans to upgrade the transformer - don't de-energize it and put your customers in the dark. If that transformer was loaded to 200% of its rating for an hour or so, it would have very little effect on its life expectancy (even less if the overload happens during the winter).

My prior utility, a rural cooperative, had installed fuses to 'protect' transformers at several substations. Its a terrible idea, but I inherited the situation. Often, only one fuse blows and 2/3rds of the substation's customers end up with 1/2 of their normal voltage. If you have the feeder breaker set to trip due to undervoltage (on any phase), it would help to protect your customers' equipment for such an event.

If you are forced to use 69 kV fuses, 100E seems appropriate for a 5 MVA transformer. You want some cushion to avoid blowing fuse due to inrush from an upline breaker operation. I've also seen such fuses melt well below their rating because of heating due to a bad connection where the fuse meets the jaws of the holder.

 

[Mbrooke]

Any idea why those primary fuses were blowing? Greater than expected load or did somehow annealing take place of the fuse link?

 

[bwen08]

I've had a mix, some utilize maximum loading capabilities, others utilize lower settings based on historical loading and lack of available fault current on the feeder.

Regardless, for primary fuses, check your breaker coordination taking accounting for preloading and multiple operations (fuse heating/cooling) to make sure you don't end up damaging the fuse during multiple operations of the breaker. I've dealt with "unknown" fuse drop outs due to previous settings being done to a generic standard.

See S&C's bulletin 210-110, Cooling Factor Method on page 34.

https://www.sandc.com/globalassets/...ll-documents/information-bulletin-210-110.pdf

Note: Check the equivalent curve modifier for each operation of the breaker, for 3 operations, it might be operation 2 that damages the fuse. Also check how LG, LL, LLL faults translate through to the high-side.

 

[Mbrooke]

Thanks. I have a theory: Faults at the same time on different feeders might also be behind unexplained fuse drop outs. In essence it would be like having two reclosers in parallel.

 

[ElectricRob]

A few of our blown fuses didn't have an apparent cause (and no corresponding faults past feeder breakers), but several were due to L-G secondary faults within the substation (arrester failure, voltage regulator failure, snake, squirrel, etc.). In some cases the first fuse would blow and then another would blow shortly afterwards - other times, the second fuse never blew.

 

[Mbrooke]

I wonder if the second fuse is weakened in the processes. S&C says unblown fuses do not need to be replaced- but that has always mystified as to how a fuse pushed to its limits can remain undamaged.