MV transformer protection / transformer damage curve 4

[rockman7892]

When looking at MV transformer protection I curious what is the correct method for determining weather or not the transformer damage curve is protected.

I know the NEC allows the primary of a transformer to be protected by a fuse up to 300% of the transformer rating, but by going by this 300% alone does this always guarentee adequate protection of the transformer. Must we look at the transformer damage curve on a TCC in oder to determine if the transformer is adequately protected on the primary? If so must the damage curve lie completely above the primary fuse on the TCC?

When we are looking at the damage curve are we only looking at it from a fault perspective or is it used for an overload perspective as well. For example from an overload perspective if the damage curve falls below the primary fuse on the TCC but is still above the secondary protective device curve is the transformer considered adquately protected from an overload standpoint? In this case how would we confirm weather or not it was protected from primary faults or through faults?

 

[dpc]

Overloads are not faults. You seem to keep worrying about overloads. The NEC overcurrent protection requirements provide very minimal protection against extreme overloading conditions. The intent of the overcurrent protection is mainly to prevent fires, not to eliminate all transformer overloading. Utilities routinely overload their transformers up to 200% for short periods of time because it saves them money.

 

[rockman7892]

dpc

I see your point about confusing overloads with fault. I agree they are two seperate issues.

Is it safe to say that typically the primary device does not provide the overload protection for the transformer so this usually fall on the secondary device?

Are there more stringent overload guidelines to follow besides those in the NEC which are minimum as you mentioned when providing overload protection of transformer? In other words the NEC allows secondary protection up to 300% on MV transformer secondaries. I assume we should strive to protect lower than this.

It sounds like setting for overload and through-fault protection are not related at all since one occurs in the long time region and the other occurs in the short time region. So therefore selecting protection based on one should have no effect on the other correct?

 

[dpc]

Is it safe to say that typically the primary device does not provide the overload protection for the transformer so this usually fall on the secondary device?

No, because sometimes the primary protection is all there is.

I would say that for long-term overloads, the overcurrent relays will provide some protection for serious overload, but that it may or may not protect the transformer from all damage. Most relay curves don't even start until 1.5x or 1.25x pickup. And the TCC stops at 1000 seconds. So we are looking at a limited range of events when we coordinate using TCCs. Also, transformers can carry more load when the ambient is lower, and the overcurrent relay (usually) knows nothing about that.

The best protection against long-term overloads would be internal winding temperature monitoring.