Generator and Motors in TCC

[JensenDrive]

If you have a TCC, first run with motor contribution and generator impedance set at Xd", and the TCC says the trip will take over maybe 0.5 seconds to clear, the TCC seems a bit misleading. Fault current is not as high as the TCC implies by the time the elements operate. Do you rerun the study and tell the software to turn off motor contribution and do you reset generator impedance to Xd' rather than Xd"?

 

[REM76]

Can you attach the TCC? what software are you using?

Thanks

 

[dpc]

The best approach is to stop the individual TCC at approximately the maximum fault current that it will see. So for inverse time curves without instantaneous, you would clip the TCC at the 5 cycle or 30 cycle fault current, as appropriate. For instantaneous elements, you have to look at the 1/2 cycle, momentary current.

The TCC is a good visual tool, but not a complete solution. Not all relays see the same current for the same fault. And not all relays respond to things like asymmetrical current in the same way.

 

[JensenDrive]

The question is not easily explained by viewing a TCC, and I am talking about a system where no instantaneous elements will trip. (side note: I do not follow the logic behind clipping a 51 at 5 or 30 cycle.)

Maybe I can reprase the problem/question:

Assume there is no instantaneous trip. Assume the peak current at the start of the fault is 10kA (motors included and gen impedance = Xd", and ignore DC offset effects). If this current level is maintained, the 51phase trips in 0.4 seconds. However, the current in the relay is falling as the 51 is timing. At 0.4 seconds, motor contribution should be pretty much gone, and the generator is better modeled as Xd'. If you model the system without motors and model the generator as Xd', fault current is maybe 6kA, and the relay clears in maybe 1.0 seconds. This reduced current has effects on coordination margin, esp. if the generator is running in parallel with a utility source (it also has an effect on arc flash calcs, but I am mainly interested in TCC effects at this point).

Do the typical TCCs that others provide make any effort to show the effect of current decay?

 

[rbulsara]

Jensendrive:
Attach a oneline showing the elements you are talking about and the location of fault. You are mixing up short circuit analysis with a coordination study( or I do not understand your question.)

If the 51 relay is at the main breaker and the fault is on bus, the relay never sees the motor contribution! Motor contribution and X"d are important to determine the short circuit rating of the equipmnet such as the main switchboard not necessarily for relay coordination. As dpc inidicated you need to determine acutual fault current (decay or whatever) and check the timing of the relay at that current.

TCCs do not care what the short circuit current is in the system, it only shows the time current curve of the protectiove device at its setting. You need to determine at what current you need to check the timing.

 

[jghrist]

The coordination has to be achieved not only at the maximum fault, but at any fault that can be seen by both relays being coordinated. Usually, the coordination problems occur at higher fault currents, so as the motor contributions fall off and the generator impedance becomes more like x', then coordination intervals are larger.

 

[JensenDrive]

Thanks, I guess I have gleaned enough from the responses.

I have reasonable but tight coordination at peak fault current (motors + gen Xd"), but if I inspect the TCC at what I think are more realistic fault currents, clearing time gets rather long. It concerns me a bit and makes me want to crank down the time dial for even tighter coordination at peak fault current.

 

[dpc]

The motor contributions are only going to last a few cycles. Generally long enough to influence instantaneous trip elements, but not really much impact on inverse time devices.

I know relay engineers who don't even consider motor fault contributions for coordination purposes.