TCC Curve

[timm33333]

For Coordination study, is it mandatory to plot the TCC's of motor (stator/rotor damage curve, motor stall point, motor starting curve)? Or only plotting the TCC's of motor relay with upstream transformer secondary relay is enough? Thanks

 

[ScottyUK]

Depends whether you know the TCC for the motor definitely protects it. If not then you need to plot the other items.

 

[timm33333]

If we plot the motor TCC, should motor curves (stator/rotor damage curve, motor starting curve, stall point) be on the left and below the TCC of all the relay/fuse curves?

 

[waross]

Which do you want to occur first? Motor damage or relay operation. The relationship of the curves should be such that the relay operates before the motor is damaged.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

If you actually have the motor damage curves, that generally indicates a fairly large and/or important motor so I'd be inclined to plot them. But modern motor protection relays have a complex thermal model that cannot be fully represented in a traditional time-current curve. I would focus on the initial motor starting event and not worry too much about the long-term (after 60 seconds or so) portion of the TCC.

 

[timm33333]

We can ignore motor damage curves; there is also a motor starting curve which shows the FLA in the top part, starting time in the middle part, and the locked rotor current at the bottom part. Should this motor-starting-curve be plotted together with the TCC of the relays?

 

[dpc]

The TCC is a tool to check coordination and determine protective device settings - it is not an end in itself. If any protective device TCC crosses the motor starting curve, this device will trip when the motor starts.

Check the IEEE Red Book or Buff Book for guidance on coordination and TCC development.

 

[jghrist]

The motor starting curve should be plotted against the TCC to ensure that the relay won't trip when the motor starts. If you are sure that the motor relay is set properly to not trip on motor start, then you can dispense with the motor starting curve because if you coordinate the upstream relay with the motor relay, then it won't trip on motor start either.

 

[burntcoil]

Tim its simple. Keep your TCC curve above the motor starting curve and below the damage point.