Which short-circuit current to use when checking coordination time intervals (CTI)

[recs]

When performing a coordination study on a MV system, we can use many types of short-circuit currents in the TCC curves.

If we use the ANSI C37 standard for calculation, we have many types of short-circuits to show in the TCC. Some of these are:

1. Momentary: Calculated using the first-cycle network

1.1 Asymmetrical (Total) RMS.
1.2 Asymmetrical (Total) Peak.

2. Interrupting: Calculated using the interrupting network

2.1 Symmetrical RMS 2 Cycle.
2.2 Symmetrical RMS 3 Cycle.
2.3 Symmetrical RMS 5 Cycle.
2.4 Symmetrical RMS 8 Cycle.
2.1 Asymmetrical (Total) RMS 2 Cycle.
2.2 Asymmetrical (Total) RMS 3 Cycle.
2.3 Asymmetrical (Total) RMS 5 Cycle.
2.2 Asymmetrical (Total) RMS 8 Cycle.

In the same TCC, you could have several relay-CB combinations that have different types of momentary and interrupting ratings depending on the date of manufacturing.

So two questions arise:
1. Which one of all these short-circuits should be shown in the TCC for coordination purposes?
2. Also, these short-circuit currents will limit the display of the relay curve on the right side of the curve. The coordination time interval (CTI) might be greater or smaller depending on the current we choose (if the relay curves separate or get further apart as current increases).

 

[dpc]

Fault current during a fault tends to decay with time. You need to use engineering judgement to determine which "maximum" fault current applies to each protective device, based on the minimum tripping time of the protective device. Some instantaneous trip elements respond to dc offset current, some do not - this determines if you need to consider asymmetrical or symmetrical.

There is not really a simple answer to your question. If in doubt, use the momentary 1/2 cycle asymmetrical current.

 

[recs]

I agree that the momentary asymmetrical duty (RMS or Crest depending of the rating of the CB's) should the one to use because is higher than the interrupting duties (Symmetrical or Asymmetrical depending on the CB's).

But if you have two relay TCC's growing apart from each other as current increases, the CTI using this larger momentary asymmetrical duties will be larger than the CTI obtained with the interrupting duties calculated at the appropriate cycles(depending of the CB).

So again, which is the proper CTI, the higher one using the larger momentary duties or the smaller CTI using the interrupting duties?

Example, assume that using the higher momentary duty, the CTI is 0.4 and with the lower interrupting duty, the CTI is 0.15. This is because the curves are not parallel and diverge as current increases. One satisfies the suggested IEEE Buff book values and the other doesn't.

I believe we just can't pick the duty that gives as the CTI that meets the IEEE suggestions and neglect the other.

Is there a suggestions from a reputable source that suggests choosing the worst case CTI?

 

[recs]

I want to correct an error with the previous statement. When the relay curves tend to converge in a lower value of time as current values increases, you may have that the momentary duty results in a lower CTI than the CTI obtained by the lower interrupting duty.