Breaker Coordination Curves

[mikeybmdb32]

Are the "main" and "tie" breakers in the attached coordination curve coordinated?
I'm specifically looking at the vertical sections of the purple and blue curves where they pretty much overlap 100%. If I'm reading this curve right, considering a fault of about 950 amps, the purple breaker will start to operate at about 0.25seconds and will not clear until about 9 seconds. Meanwhile, the blue breaker will start to operate at about 0.45 seconds, so that the two are not coordinated.

Also, is the clearing time really almost 9 seconds in this region? Seems like a long clearing time.

 

[rcwilson]

Faults less than 8,000 A and above 12,000 A will be cleared by the tie breaker, as long as other loads on the Main are less than about 1000A. (Tie doesn't see those loads).

A fault in the 8 -11 kA range might trip both breakers.

Many times, system constraints cause compromises in Main & Tie coordination. Raising the Main's ST pickup may create overlap with the fuse. Lowering the tie might mis-coordinate with downstream loads.

Since operating with a Tie closed is an uncommon occurrence, and the odds of having a fault in that range is low, it is not a high risk compromise for most applications.

 

[kingpb]

Without a one-line or system operation explanation it is difficult to ascertain. You are showing fuses for a Xfmr, are these on LV side? Where is transformer damage curve?

Your main is 1600A, but the tie is also 1600A, typically the tie is going to be closer to half the current capability of the main, since the main has to carry the whole bus, but the tie, only around half; depending on how loads are split.

For a fault on the side of the bus fed by through tie breaker, you want the tie to trip first otherwise you risk taking out the whole bus. Maybe this is acceptable for your operation.

When developing coordination curves, certain time intervals should be maintained between the curves of various protective devices. This interval is measured between the relay characteristic curves at the lesser value of either the instantaneous pickup setting of the downstream device or the maximum fault current that can be experienced simultaneously by both relays. The minimum coordination time interval is based upon the circuit breaker opening time and the relay tolerance and setting error. Often a coordination interval of 0.3 second is used between tripping characteristics.

The tie breaker should trip first in all parts of the curve with some margin to avoid taking out whole bus, and very often the main breaker instantaneous is disabled. As shown, looks like the fuses, which are on XFMR, will take out everything above 27kA before the relays can respond.

 

[mikeybmdb32]

The fuses are on the HV side. The damage curves and inrush points for the transformers aren't shown either, unfortunately. This is more of a conceptual exercise as I read up a lot on curves, but wondered about overlap in the vertical region of the curves.

I really appreciate the input from everyone!