51 Device Coordination

[Power2all]

So as I understand, the recommended time deference in coordinating electronic relays in series is 0.3 Seconds. I am wondering at what point this time difference should be measure on the TCCs since the delta increases with time due to the log scale. Should it be measure at a particular time?

 

[rcw retired EE]

When is the timing difference most critical?
Wouldn't it be when fault current is causing the relays to operate quickly?

If that is true, then find the highest short circuit current that is seen by both relays and adjust the settings to achieve the required timing margin at that current.

At all lower currents, the margin between the two trip curves should be greater.

 

[dpc]

You need to consider the point where the two curves have the smallest time separation. Don't forget to consider the interval between a downstream instantaneous trip and upstream inverse time.

 

[tinfoil]

...and make sure you consider the actual operating time of the device itself.

If you have a 3-cycle breaker or recloser, that's 50 milliseconds, AFTER the relay decides to trip, that you need to add in.

I have seen that overlooked on many occasions.

 

[krisys]

In the era of numerical relays the discrimination time of 0.3s is little too much. I normally provide 0.15s to 0.2s only.

 

[slavag]

DT or IDMT?
for DT is usually used from 0.18 up to 0.3sec.
depend on the terminal type and breaker.

For IDMT calculation is another.