Step-Up Step-Down transformer Protection and Coordination

[rockman7892]

I'm reviewing an application in which 480V paralleling switchgear feeds a circuit which steps voltage up to 4.16kV for transmission over a long distance before it is stepped back down to 480V for use in the 480V emergency system.

The feeder breaker in the paralleling switchgear is a 3000A breaker which feeds a 2000kVA 480V - 4.16kV step up transformer with a 450A fused switch coupled on transformer secondary. From this transformer the cables travel some distance before hitting new substation which has a 750kVA 4.16k-480V step down transformer with a 150A primary switch and a 1600A 480V breaker in the downstream switchboard.

Are there any special consideration that need to be given to the protection & coordination of such a circuit aside from the considerations given to a normal MV single transformer step-down circuit?

What kind of inrush would the paralleling switchgear feeder see from the two transformers being energized in series? Would it still only see the inrush from the first initial step-up transformer and not really see the inrush of the downstream step-down transformer.

NEC Table 450.3(A) does not provide requirements for primary breaker protection with a breaker 600V or less (only for 600V or less with secondary protection). Are there requirement that govern the 480V feeder breaker on the LV side of the step-up transformer.

 

[ScottyUK]

Not commenting on the NEC aspects, but the earthing arrangement of the 4160V section is a major factor in how you would protect it. Are you using back-to-back Dyn/Dyn distribution transformers?

 

[rockman7892]

ScottyUK

Yes back-t-back Dyn/Dyn transforemrs are being used so that both the 4.16kV and downstream 480V systems are solidly grounded through wye point.

 

[ScottyUK]

Dyn both ends would leave the HV section ungrounded. So at the source you have a YNd (LV delta, HV star) transformer? That should make things simpler.

 

[rockman7892]

Yes my apologies the source has a YNd transformer. I confused the terminology.

The problem that I'm having is that the 450E fuse on the secondary of the 2000kVA step-up transformer cuts through the middle of the damage curve and therefore does not protect damage curve in the long time region. In order to protect damage curve in the long time region I need to lower the long time delay of the 3000A feeder breaker below the transformer thermal damage curve (shift curve for L-G faults). This leads to an issue with the secondary fuse and primary breaker not coordinating because tapped off the line side of the fuse (4.16kV) there is another 450E fuse that feeds a different circuit.

Am I correct in bringing the breakers LTD down below the transformer thermal damage curve since the secondary fuse cuts through the damage curve and does not provide complete protection for low level faults? In this case you have to sacrifice coordination for the sake of transformer protection?

For most step-down transformers the secondary breaker usually sits well below the damage curve and then primary fuse protect mechanical damage point and cuts through damage curve however I haven't come across an application where the secondary device does not completely sit under damage curve.

 

[rockman7892]

I've attached a TCC to help better explain the situation.

You can see that the secondary 4.16kV fuse (HV-1) cuts through the transformer damage curve and thus does not protect it in the thermal overload region to the left of the red line (where the fuse intersects damage curve)

In order to protect the complete overload region of the thermal damage curve my thoughts were that the primary 480v HV-1 FDR breaker has to sit completely below the transformer damage curve (shifted curve to reflect primary) in order to protect the area of the damage curve that is not protected by the fuse.

It would appear that you can raise the long delay setting of the HV-1 FDR breaker to better coordinate with the fuse but by doing so you will not adequately protecting the damage curve? The issue there is trying to coordinate with fuse while protecting transformer which may not be possible.

Am I on the right track here?