Selective Miscoordination?

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mbrooke

mbrooke

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A down stream 13.8kv line fault caused A and B phase cutouts to blow tasked with protecting a 23kv-13.8kv auto transformer bank.

A bit of info on the scheme going from back to front: 23kv incoming line, 100K primary fuse links, 3 1254kva 13.2kv-7.9kv step down auto transformers, VT bank for the recloser control, microprocessor controlled recloser as part of a loop scheme, 13.8kv outgoing.

{It appears as though the 23kv upstream recloser's fast curve (not pictured here) can "reach" through the step down bank and clear for a fault on the 13.8kv side, while the upstream's slow curve blows the bank cutouts on A and B phase. The 13.8kv recloser pictured here appears to act merely as a voltage sectionalizer for loss of the incoming 23kv. Not sure if the prior is deliberate or inadvertent}

My question is, could this ever be intended? Should the bank cutouts be changed to 140 or 200K? Is coordiantion with the recloser pictured here practically realistic or worth considering? Shouldn't the 13.8kv recloser do all over current tripping for the 13.8kv side and not the upstream 23kv devices?

https://imgur.com/caxggh9

https://imgur.com/CUsmxmo

https://imgur.com/x881m4K
 
If your question is, "Can/should the primary overcurrent protective device trip for close-in faults on the secondary of a transformer?", then the answer is yes. This is backup protection.

If your other question is, "Should the secondary overcurrent protective device trip for faults on the secondary feeder before the primary overcurrent protective device", then the answer is ideally yes; however, its not always possible to discriminate when the fault magnitudes (as seen by both devices) are very high and similar.
 
If this line is in an area where temporary faults can occur (lightning, tree branches, squirrels), then in theory the "fuse-saving" fast curve of the upstream recloser is appropriate, since it would result in a much shorter outage for a temporary fault. This has been the conventional approach for decades, at least in the parts of the US that get a lot of lightning. Two fast and two slow was the standard. Here in PNW, mostly we see one fast and two slow since there is little lightning west of the Cascades. However "fuse-saving" schemes are falling out of favor for a variety of reasons. Google "fuse-saving vs fuse-clearing" for much more.
 
Xptpcrewx said:
If your question is, "Can/should the primary overcurrent protective device trip for close-in faults on the secondary of a transformer?", then the answer is yes. This is backup protection.

If your other question is, "Should the secondary overcurrent protective device trip for faults on the secondary feeder before the primary overcurrent protective device", then the answer is ideally yes; however, its not always possible to discriminate when the fault magnitudes (as seen by both devices) are very high and similar.
Click to expand...


Suspected as much. Fortunately this is some miles from the substation, so current isn't all that high.
 
David Castor said:
If this line is in an area where temporary faults can occur (lightning, tree branches, squirrels), then in theory the "fuse-saving" fast curve of the upstream recloser is appropriate, since it would result in a much shorter outage for a temporary fault. This has been the conventional approach for decades, at least in the parts of the US that get a lot of lightning. Two fast and two slow was the standard. Here in PNW, mostly we see one fast and two slow since there is little lightning west of the Cascades. However "fuse-saving" schemes are falling out of favor for a variety of reasons. Google "fuse-saving vs fuse-clearing" for much more.
Click to expand...


Can you go in depth on why it is controversial or falling out of favor? I've had folks tell me that fuse saving does not work 100% of the time and that it should just be dropped from protection schemes.
 
mbrooke said:
Right. At some current level the curves encroach and overlap. Where 65, 100 and 140K coordinate, under high current levels all 3 will blow instantly.
Click to expand...

Theoretically all three would blow but practically one or some combination will blow since it’s really a racing condition.
 
For coordination between fuse links, the TCCs do not tell the entire story. Manufacturers publish tables of coordination for different size fuses and the maximum fault current at which they will coordinate (regardless of the what the time-current curves look like). Similar concept holds for current-limiting fuses.
 
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