Protecting Network Transformers

[Mbrooke]

How are mesh and spot network transformers normally protected? I am very confused.

 

[davidbeach]

Fuses on the high-side, network protectors on the low-side.

 

[Mbrooke]

Where would the high side fuses be placed?

In so far this is the setup: Units have a network protector on the low side, however the high side only comes with a 3 position disconnect mounted to the back of the unit: closed, open and ground. The plans call for having the MV disconnect supply tap directly into EPR feeder cable from the supply substation breaker which also supplies a dozen other transformers around the network in different vaults. The same holds true for other feeders in service.

 

[Mbrooke]

I guess Im asking, is it normal to have unfused units where a transformer fault is cleared via feeder breaker?

 

[davidbeach]

High-side fuses are in the tank.

 

[Mbrooke]

Are they accessible? Or built into the unit during manufacture? Can a publishes trip curve be obtained?

 

[stevenal]

http://www.cooperindustries.com/con...using/transformer_fusing/bay-o-net_fuses.html

 

[Mbrooke]

So these are immersed in the oil, but can not be serviced without unbolting the tank?

 

[dpc]

Yes, if they are bayonet-type.

 

[VTer]

I believe, the way Cooper protects their transformer is that there is a bay-o-net expulsion type fuse in series with a current limiting fuse. The fuses are coordinated such that the bay-o-net acts on through faults and the current limiting for internal faults. Bayonet is easily accessible and replaced without needing access to transformer tank. The current limiting is inside the tank and the only time you need access is if you have a internal fault anyway.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla

 

[Mbrooke]

But there is never a time where a feeder breaker protects one?

 

[dpc]

It's possible the feeder breaker may operate if there is a coordination issue - most likely for a primary ground fault.

 

[Mbrooke]

Looks like I may need external protection then.

 

[davidbeach]

The whole point is that a transformer problem never trip anything other than the transformer, thus the fuses. There should be enough protection on the low side that the only thing that will blow the transformer fuses is a transformer fault. Keep the feeder relay slower than the transformer fuse and all is well.

 

[Mbrooke]

But how will an internal fault to the primary windings be removed?

 

[Mbrooke]

Sorry, to clarify above, If I am not mistaken I believe that the network transformers in question have no primary fusing even at an internal level. Then again I could be wrong...

 

[dpc]

By the fuses if the system is properly coordinated, or by the primary feeder as a backup. The coordination should be pretty easy to achieve since the transformer rating is generally small compared with the feeder rating.