Current quantites during protection curve grading 1

[pavement]

Sorry Guys,
Just looking for some clarification regarding when you undertake cordination calcs, which current quantity do you use i.e. asymmetrical pk, asymmetrical RMS, or symmetrical pk/RMS when your defining your maximum fault levels against the device curves on either log paper or using 'visual' software packages. Im thinking it will be symmetrical RMS but would like a second...or third opinion.

Also i've also heard in the IEEE Red book, that a multiplier of 2.7 can be used to determine the asymmetric peak from the symmetrical equivalant.Has anybody else came across this 2.7 factor to give ballpark quantities?


Many thanks


Pavement

 

[jghrist]

Use symmetrical rms.

The 2.7 factor (applied to symmetrical rms) is to compare closing and latching capacity of breakers preferred after 1987 (ANSI) which are rated by crest current. Closing and latching capability of breakers preferred before 1987 are rated by total asymmetrical rms current; use a 1.6 factor.

 

[veritas]

Hi pavement

Symmetrical (steady state) is by far the most common I;ve seen used. It is also the simplest to calculate and work with. I have rarely seen the need for requring asymmetrical currents for co-ordination purposes though there may be instances they are needed.

Keep in mind also that the majority of modern digital relays react to fundamental current (dc offset and harmonics are filtered out). Also with IDMT grading the asymmetrical component typically lasts <5 cycles. The higher the time multiplier (time dial/time lever) setting the less impact the asymmetrical current on relay timing.

Hope this helps.

Regards.