Coordination 1

[timm33333]

In doing phase and ground coordination using ETAP, some TCC’s pass through the transformer damage curve and/or cable damage curve. Also some TCC’s mis-coordinate, which means that at one or two points the TCC of the upstream medium voltage fuse touches the TCC of downstream low voltage breaker. It looks that it is not possible to avoid these mis-coordinations. As the fuse upstream of the transformer is sized as per NEC, and also the cables are sized as per NEC, so I would assume that transformers and cables would be protected even if the TCC’s pass through the transformer/cable damage curves.

Is it normal to have some mis-coordination between the upstream devices and downstream devices at one (or two) points on TCC? Also is it normal that the TCC passes through the transformer/cable damage curves?

 

[dpc]

Primary fuses will seldom fully protect the transformer damage curve for delta-wye transformers.

I'd recommend buying a good text on protective device coordination and reviewing that. The Blackburn book on Protective Relaying would be a good start as well as the IEEE Red Book.

 

[timm33333]

I am trying to understand why on TCC the ground fault short circuit current is slightly higher than three phase short circuit current? I would assume that L-G fault current would be similar to single phase current. So if the L-L-L current is 100A, the L-G current should be 100/1.732= 57.7 A. But in reality the L-G current is about 110 A which is even more than the L-L-L current.

 

[timm33333]

I do have Blackburn book and IEEE red book, but could not find this answer over there...

 

[dpc]

For a delta-wye core type transformer, the zero sequence reactance can be around 85% of the positive sequence reactance. This yields fault current at the transformer terminals that is slightly higher than the three-phase value.

 

[unclebob]

Fuses at the primary protect against short-circuit and don't protect well in overload condition. Secondary main breaker will protect against overload. It is curious that the cable damage curve another TCC. Are you drawing the curve for multiple parallel cable, if any? Most of the time, they are at the far right end side of the plot.

 

[timm33333]

Yes these are multiple parallel cables, the cable damage curves remains the same no matter the cables are multiple or single. The cable damage curves are to the far right but they touch the TCC of the transformer primary fuse at one point.

 

[unclebob]

When drawing the cable TCC, one have the choice to draw N-1 cable (that's what i use), this will shift the TCC further to the right. Are these the primary or the secondary conductors?

Is it possible to show us what you have?

 

[timm33333]

Please see attached. The cable curve is touching the fuse curve and main breaker curve, and the transformer curve is touching the fuse curve. I cannot move the fuse curve and main breaker curve further left because then these will touch the curves of the branch breakers.

 

[timm33333]

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[unclebob]

Something's wrong with your breakers TCC. You have a 3000A that touchs a small 150 A. Looks like you drew your feeder breakers at 4.16.
That green cable is wrong, check your cable size.

 

[timm33333]

It looks that if the transformer curve touches the TCC outside of the operating range of the fuse, then there would be no problem. At 4.16 kV: I have 1/2 cycle short circuit current of 19.5 kA and 30 cycle short circuit of 18.8 kV, so the operating range of the fuse is 19.5-18.8 = 0.7 kA, which looks very narrow to me. Is it normal that the operating range of the fuse is so narrow?