Total current vs Symmetrical current method

[KnicksJets]

Can somebody please explain why the asymmetrical current calculated using Total current method is significantly
different to that calculated using Symmetrical current method? I know the X/R multiplier is different for these
methods, but why should a fault current (asymmetrical) calculated using two different methods be so different? Shouldn't
they be essentially same/similar regardless of the method used?
The issue I am having is that a transmission provider calculates the fault current using TOtal current method whose asymmetrical
value is far above the existing breaker's rating (asymmetrical) which I assume is calculated using symmetrical current method
since the breaker is fairly new.

 

[dpc]

There are different multipliers for the impact of local generators in the ANSI standard, depending on how the circuit breaker was tested.

The short circuit current calculated using standard complex impedance reduction is non-conservative when there are multiple sources of current. This is well-documented and is due primarily to the varying X/R ratios of the various sources.

If you use the IEC methods, you will get yet a different answer.





David Castor

http://www.cvoes.com

 

[KnicksJets]

dpc
Thanks for the response.
I understand your point and now I have come to the understanding that different method yields different answer. This makes sense theoretically, but in real life we have to chose one method.
Now, practically speaking, if a vendor tells me that the breaker is lab tested to interrupt a short circuit fault of say 50 kA symmetrical. The Vendor then converts this to asymmetrical by multiplying with a “S” factor of 1.2 per ANSI C37.04-1979 section 5.10.2.2. His asymmetrical current is then 60 kA.
Now, you as a generator owner have to decide whether this breaker is overdutied or not. What method would you chose to calculate the maximum fault available? We used both total current and symmetrical current method to come up with a maximum fault of say 45 kA symmetrical. However, total current method yields 64 kA asymmetrical while symmetrical current method yileds 54 kA asymmetrical. Clearly with the total current method we are overdutied here. An expert advised that I should use symmetrical current method since that’s what the vendors typically use, but my vendor said they use actual test and the “S” factor.

 

[dpc]

What method would you chose to calculate the maximum fault available?

I would use one of the four methods described in the ANSI C37 standard. Any of the four is acceptable.

You would definitely NOT use the total current basis multipliers if the breaker is rated on a symmetrical current basis.

You also need to pay very close attention to the breaker's true contact parting and clearing times. This makes a big difference.



David Castor

http://www.cvoes.com

 

[cosfi]

This may help you for improtance of x/r ratio. First graph is asymmetrical current which is total of second and third graphs, second one is only exponential component and third one is only symmetrical current.Explanations are in Turkish but you can understand equations.There is a also matlab codes in it.