Breaker Test X/R Ratio 2

[nightfox1925]

I would like to inquire as to what X/R ratio medium voltage circuit breakers are generally tested to sustain their S.C. ratings.

Appreciate any help. Thank you.

 

[dpc]

It's not that simple for ANSI breakers. For peak current, ANSI allows use of 2.7 times symmetrical fault current. For close and latch rating they allow 1.6 times. For interrupting ratings, there are additional factors that must be taken into account.

In addition, the symmetrical current must be calculated per ANSI standards when evaluating adequacy of breaker ratings versus actual duty.

For IEC, it's even worse

 

[Kiribanda]

nightfox1925,

Per ANSIC37 or IEC 60271 (now both are harmonised) all general purpose (not used for capacitor switching) MV/ HV circuit breakers are tested for X/R=17 at 60 Hz (X/R=14 at 50 Hz)

If the calculated X/R ratio is greater than 17 at 60 Hz, like at generating stations, then the manufacturer has to be consulted to purchase CBs tested for a higher X/R ratio so that TRV are with in given limits in both ANSIC37/ IEC 60271.

 

[dpc]

Kiribanda,

I think that is a great oversimplification of the ANSI C37 standards for MV breaker ratings. But I don't have time to do the research now. Low voltage power circuit breakers are rated this way, but I do not believe it is that simple for medium-voltage breakers.

I'd recommend referring to Conrad St. Pierre's book on Short Circuit calculations for a comparison of ANSI and IEC standards. If the medium voltage breaker standards are harmonized, it's news to me and must be recent.

Cheers,

Dave

 

[nightfox1925]

Good news guys. I got hold of the switchgear datasheet and it is indicated that:

Symmterical Current = 40kA
Close and Latch Current = 108kA (64kA) Peak

There goes 40 x 1.6 = 64kA, 40 x 2.7 = 108kA

It seems that the breaker is sized as per IEEE Std C37.010-1999 (on a Symmetrical Current Basis).

 

[dpc]

Yes, but the symmetrical rating is based on certain assumptions that may not apply in all cases. If the maximum fault current is less than 80% of the interrupting rating, you are probably OK. If it is closer than that, ANSI C37 methods must be used.

Electrical analysis software packages such as EasyPower and SKM can do this ANSI duty calculation automatically.

 

[nightfox1925]

Thanks dpc, I will take note of that.

 

[rbulsara]

dpc and kiribanda:
LPSs for you guys for succinct replies.

Rafiq Bulsara

http://www.srengineersct.com