Reclose Attempts; Conductor Damage Curve

[distributionPlanner]

Are there any distribution protection engineers who consider the conductor damage curve when deciding on the number of reclose attempts?
It seems fairly common to compare the timed-overcurrent curves with the conductor damage curve.

However, if there are reclosing attempts, the conductor will heat up during the first operation, trip and cool down during the open interval, then heat up again on the reclose onto a fault.

The timed overcurrent curve may be below the conductor damage curve for 1 individual operation, but with successive operations, the *combined* exposure to fault current and duration could be above the conductor damage curve.

(In other words, multiply the timed overcurrent curve with the # of operations to lockout and compare this curve against the conductor damage curve)

Does anyone consider this?

 

[krisys]

Technically it makes sense to add up the I²t of each duration. On a conservative side, the heating may be considered as adiabatic heating. (i.e. no cooling shall be considered for the short period of recloser open).

 

[Marmite]

If you ignore the fact that some of the recloser trips are generally instantaneous, not IDMT, and assume that there is no recloser dead time for the cooling to take place, which obviously there is, then you will probably conclude that parts of your overhead network are going to melt onto the ground at the first sign of a fault.
Regards
Marmite

 

[stevenal]

Would the heating/cooling effect of a conductor be much different from a fuse element? Cooper publishes K factors for use when coordinating reclosers with upstream fuses. As far as number of reclosures, we generally set each open interval longer than the last so that the effect of prior operations becomes less.

 

[distributionPlanner]

I can appreciate that some cooling will occur during the open intervals.

However, our typical open intervals are on the order of seconds, and I doubt that the conductor would be able to cool to pre-fault temperature during the open interval.

It seems to me that there must be some kind of reasonable approximation between: assume no cooling during open interval, and assume conductor cools back to pre-fault temperature.

stevenal, do you consider the conductor damage curve when setting open intervals? Or, are you increasing the open interval time on subsequent operations to allow the fault to clear itself (bird to fall off, tree to swing away).

 

[stevenal]

There's generally a large enough margin between the curves that I don't adjust the interval for that purpose. And yes, the increasing open interval is mainly for increasing the chance of a successful subsequent reclose.