RATING OF THE TRANSFORMER SECONDARY

[pristava]

Industrial Power System provides five transformers 13.8 / 0.6 kVolts - 2000kVA - 3pcs, 225kVA - 1pce, 150kVA - 1pce,
(a) 2000 kVA transformer has 1926 Amps on secondary side, but 3000 A bus is connected there with 3000 A main breaker.
(b) also, 150 kVA transformer has 144 Amps on secondary side, but 200 A bus is connected there with 200 A main breaker.

I discussed with Senior Electrical Engineer about that - especially about a situation that 2000 kVA transformer gets a load 3000 A on secondary side ( or 150 KVA transformer load 200 A). In both cases transformers are overloaded almost 50 %.
He said that there is NO problem because that situation will be solved by Protective Relaying.
My question - Can that One Pole Diagram be accepted by our Professional Engineer, and stamped by him after that ?
Note : Network is radial.

 

[cuky2000]

The transformer loading depend on the running load connected on the transformer secondary side, not on the bus rating ampacity. If relays are setting properly, the breaker will trip protecting the transformer from excessive overloading.
Oversizing electrical apparatus and component could result in unnecessary additional cost. However, this is not necessarily a bad practice since may help to minimize voltage drop, allow momentarily overload, minimize heating losses, provide allowance for future expansion, etc.
I suggest check the protection one-line diagram and discuss with other colleagues in the project more details about how this will be accomplished.

 

[rbulsara]

Short answer is your senior engineer is right( Most senior engineers are, specailly when you are just starting).

Refer to NEC article 430 completely and 220.

The tansformer is protected by its primary overcurrent protection. Seconday breaker does not noramlly protect the transfomer. There are some excpetion to this but you need to review Articel 430.

At minimium you should have secondary bus rated for not less than 1.25% of the FLA that itself makes it 2400A. So a 3000A bus is not too big, 2500A also would have sufficed. But the main bus should always be generously sized as you do not want it to run too hot. You cetainly do not want to run a main bus at its rated capacity. (Same as you would not run your car to its full rated capacity in normal condiitons).

 

[rbulsara]

Its NEC article 450 not 430 for the Transformers.

Sorry.

 

[a430]

Hi-pristava.Ithink you can acheive correct protection of the secondary of your transformer with the help of protective relays (O/C&EF)since iam sure you will have current transformers incorporated.It is just a matter of cost like rbulsara suggested.