Not Enabling Instantaneous 50 Pickup 3

[Mbrooke]

I came across this in an IEEE presentation PPT. Is there any truth to this? Is someone really pushing a global mandate?

 

[cranky108]

Pushing what? To 50 elements or not?

There is no current method of setting this as a mandate.

 

[Mbrooke]

Pushing to not have a 50 element enabled. 51 only, for the feeder breaker.

 

[cranky108]

We enable the 50 element for hot line work. Does not coordinate, but it is fast.

On feeders where there is some distance between the sub and the first customer, then the 50 element is a good idea.

 

[Mbrooke]

Well not much I can add other than- excellent advice Cranky108!

I take it your POCO doesn't do fuse saving?

 

[cranky108]

We have tried it. Good for a few things, but not a fix all for everything. Does not work for underground, which is over half our distribution system.

Note some of the Basler over current relays have two 50 elements (older stuff not the microprocessor). I thought was a good idea at the time. Many others could not understand.
Then again, my old company did not use a 50 element for hot line work. The linemen insested on an "A" curve.

 

[Palletjack]

did not use a 50 element for hot line work. The linemen insested on an "A" curve.

??
Having been one before moving to Engineering I still don’t understand why...
we have 351R,651R, and 351S to name most of our relays, and I put the 51 pickups in the HLT logic. Generally the pickups are 200-400 depending on the circuit.
I’ve done some testing with and without the logic in there to show the guys how much faster the HLT logic is with these pickups added.
I generally ramp my test set from 0-2000 amps in 0.010 steps. That’s as fast as my test set will ramp. Still there’s about a 900 amp difference with and without the logic between the 51 pickups and a 105 curve.

And your right, there is no coordination with HLT on.
But then again, I don’t care about it when it’s enabled.

 

[Mbrooke]

Might be (guessing) that 50 might have limited reach and not cover the whole line up to the first recloser. Or the linemen just don't know better.

HLT? Sorry, I'm terrible with acronyms.

 

[davidbeach]

Hot Line Tag.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Mbrooke]

Thanks! Obvious, but like I said, acronyms lose me lol.

 

[bacon4life]

Cranky- I have had a few cases where adding a 50 element as part of a hot linework tag resulted in the substation breaker opening incorrectly on transformer inrush when picking up load with a field disconnect. Future installation will avoid trips by using the new harmonic restraint feature in the most recent hardware upgrade for SEL-451s.

 

[cranky108]

We only use the 50 element (not the built in HLT), for when the line people are working on the line. So we should not be closing transformers in during that time.
Besides closing most switches or fuses can be done with a stick.

We don't use the 451, as it is more processing than we need for most applications. Besides we try to skip some relay models, to reduce our inventory needs and increase our training on the relays we do support.

We also are trying to limit the number of models we support for the 351s, and other relays.

 

[jghrist]

Fuse blowing vs fuse saving is a reliability trade-off. With fuse saving, the 50 instantaneous element is used to trip the substation breaker for faults beyond tap fuses. The breaker is then reclosed and if the fault is still present, the fuse blows before the breaker trips on a 51 inverse-time overcurrent element. If the fault is temporary, the line goes back into service without the fuse blowing, with a momentary outage to the whole feeder.

With fuse blowing, there is no instantaneous and a temporary fault on the tap blows the fuse, causing a long term outage on the tap but no momentary outage on the rest of the feeder.

 

[davidbeach]

We have found that for most of most feeders in our system we can't actually do fuse savings; there's enough fault current available that the fuse blows before the breaker can get open. So there were a lot of cases where a fuse would blow to clear the fault but the breaker would still open, but by the time it opened it was only interrupting load. Where we have relays that can support it, we have been changing to a delayed instantaneous trip (definite time trip). With a 6 cycle delay, the events that are cleared in a cycle or two by a fuse no longer result in a breaker operations, but mainline fault still get a quick first trip. The delay does not affect tripping when the feeder is tagged.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Mbrooke]

@Cranky: what do you use for your primary and back in transmission, distribution and transformers? I like your idea of simplicity.

@jghrist: Yup. Typically I think its best on a case by case or feeder by feeder basis. Fuse blow on commercial, industrial and the like; fuse save on circuits that are predominately resi.

 

[Mbrooke]

@David: what is your fault current and what type of links do you use? Just curious. You're right, beyond a certain current level fuses will not coordinate without other fuses or even breakers. Though a 200T fuse is more likely to coordinate in high fault portions of the circuit than a 100N or K for example.

 

[davidbeach]

T fuses, the biggest is typically a 140T. With fault current in the high 8kA range, we can't save a 140T fuse within a mile and half or so of the substation; a 6T or 10T fuse is almost unsavable over much of the system. Get out in the more rural areas with smaller transformers and longer feeders and fuse savings become more of a possibility.

I think the fuse saving had a time and place, but we've moved beyond it. Back in the day when many of the feeders were 4kV coming from 3-5MVA transformers with a 12kV high-side the fuse sizes would have been larger and the fault currents lower. That combination makes fuse savings much more probable than today's tendency toward lower fuse ratings and higher fault currents.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[stevenal]

David, I know of one utility that uses fuse saving during times of high fire risk. Save the expulsion fuse to prevent ignition by the fuse. Perhaps we went beyond and are circling back. Comments?

 

[davidbeach]

Assuming a cycle for the relay, 3 cycles for the breaker, and a cycle for margin, you can "save" a 15T fuse when you're far enough away from the substation to have your fault current below 430A (minimum melt). If you're willing to push your luck, you have to be below 575A at maximum clear. If you can get to a 140T, those numbers go up to 4500A and 5800A.

In that case I think the change from expulsion to non-expulsion is a more viable approach. Under the highest risk level we block reclosing and if the feeder trips it has to be 100% patrolled prior to reenergization. In that case we want to blow the fuse, if possible, and leave the feeder energized. The non-expulsion fuse is one tool to get there. Another tool is the S&C TripSaver, or equivalent. Those are useful in that we can put them in a one-shot definite time trip and then leave a margin above them and have a faster than normal "fuse blowing" scheme at the feeder breaker. Larger multi-phase taps have problems with both of those approaches, at least at 13kV.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[stevenal]

Non-expulsion as in current limiting? The utility in question (not mine) also employs them. From what I can deduce, current limiting fuses don't coordinate either with themselves or with downline expulsion fuses. Do you accept miscoordination in favor of fire safety?