Momentary Duty and Interrupting Duty

[IndianTechEngg1001]

I have a 2MVA 11kV/400V Transformer, Secondary is connected to a busbar through a MCCB.

ETAP calculations gave following report on duty checking.

1. Momentary Duty--------

Sym KA rms = 52.063
x/R = 2.7
mf = 1.095
Asymmm kA sym= 96.877

2 Interrupting Duty-------

Sym KA rms = 52.063
x/R = 2.7
mf = 1.0
Adj sym kA rms= 52.063

I have selected MCCD Rated Int 65, Test pf =20, 65kA based upon the Interrupting Duty I got above.

My question is what is the purpose of Momentary Duty (Asymmm kA sym= 96.877) and is it taken care of when we select a breaker.

 

[rbulsara]

Momentary Duty (1/2 cycle) is critical for MCCBs as they operate in the first 1/2 cycle. (Power breakers on the other hand takes several cycles to operate, way past the 1/2 cycle peak.)

Asymm current depends on the X/R ratio so it is not the same for every system of a given symmetrical currents. While the breaker are tested to a given ratio per ANSI standards, you still need to check that the calculated asymm (momentary) duty is still within capability of the device. You can also make the adjustments based on the actual X/R ratio.



Rafiq Bulsara

http://www.srengineersct.com

 

[IndianTechEngg1001]

Thanks Rafiq,

So do you think in this case as Asymmm kA sym= 96.877 I must go for ACB?

 

[rbulsara]

If available, try a higher rated MCCB such as 85kA or 100kA rms symmetrical would work. The gear also need to be rated accordingly.

Check the momentary duty report in your software. It should also show rated Momentary duty vs calculated.

Personally, I would prefer a ICCB or ACB for a secondary main of 2MVA substation. Provides better coordination and more rugged.

Rafiq Bulsara

http://www.srengineersct.com

 

[alehman]

I think it's saying your 65kA rated breaker is acceptable.

The momentary duty is accounted for in the 65kA symmetrical rating for X/R up to a certain level (4.9 for MCCB's in the ANSI world). The Interrupting Duty calculation appears to be showing you that no adjustment is needed due to the X/R of your system (2.7).

I'm not an ETAP person and this is my guess as to what it's saying. Read the software manual carefully to be sure.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.