Blown Cutout Selective Miscoordination? 1

[Mbrooke]

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?

 

[cranky108]

I suppose you use the typical 2 fast and 2 slow curves on the 23kV recloser?

The recommendation is typically to use a sequence coordination on an electronic recloser.

It dos not work.

The real solution is to use all delayed curves on the 23 kV recloser. But maybe a fast ground curve, if the transformer configuration is delta/wye.

I have had these debates, and proved my point years ago with a different company. But with 35kV high side, and 13.8kV low side transformer.

 

[stevenal]

A one-line would be helpful. Generally you want transformer protection to coordinate with the transformer damage curve, so I would question increasing the fusing there.

 

[Mbrooke]

@Cranky: The 23kv recloser is 1 fast curve two slow curves.

Bank is Wye-Wye.

I'm glad to hear that it does not work instead of spinning wheels about it hopelessly!

This is a safe place to share your findings, I'm all ears.

 

[RRaghunath]

It is always a challenge to select a fuse upstream that coordinates well with the protection curves downstream.
Beyond certain short circuit current level, the coordination is virtually not there.

 

[Mbrooke]

Agree, though in this case the bank is several miles from the sub, so the fault current is low, so it might be of help.

 

[cranky108]

Use the fuses to protect the transformer, use the recloser to protect the line. Set the recloser to all delays on phases. The low side recloser should be set for the low side line protection as normal.

You never said if there were load tap off between the 23kV recloser and the transformer bank. Or if there were assets beyond the transformer bank.

My old company would use platform stations, not the regulator stand-off units.

Problem is if the 23kV recloser has ground curves, they likely will not coordinate with the transformer fuses, and if there was a ground fault in the transformer, the recloser would operate. I've seen this twice in a 25 year time frame.

My present company only uses three phase operating devices on 35kV, so breaker relays are more selective. Then again the load is more compact, and we don't have any transformers that small, except to direct customer voltage.

 

[Mbrooke]

There is load on the 23kv segment- about 500+ customers, as well as load taps on the 13.8kv.


Question- why wouldn't the ground curves on the 23kv recloser coordinate? Curious as to theory behind it is all.

 

[davidbeach]

Or somebody just said “good enough” and moved on to the next problem to be solved.

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

 

[Mbrooke]

@David: That is my theory and my bet at this point. This isn't the first time the bank fuses have blown for a down stream fault.

 

[Mbrooke]

@Stevenal: Sorry for the delay, I was busy, but I hope this helps: