Short Circuit Calculation 1

[Siyou]

Dear Everyone,

The system where I did its short circuit calculation has 72.5kV PCB with 25kA breaking current and 65kA making current with 44% dc component rated in IEC standard which is used in the 69kV line, the 13.8 kV line used 15kV vacuum circuit breaker with rated short circuit current of 25kA, the rating used ANSI standard. Since the system used both ANSI and IEC standards. I'm confused about what standard I will use in my short circuit calculation. I tried using only ANSI standard calculation for the whole system. What will I do in my short circuit calculation? What Standard will I used in the short circuit calculation to evaluate the CB ratings?

Thank You.

 

[waross]

Don't confuse short circuit Amps with Available Short Circuit Current.
North American breakers are often rated in ASCC.
This rating does not include any DC offset.
However the beakers are tested with a specified DC offset, related to a specified X/R ratio.
I believe that the specified X/R ratio is 17 but I am open to correction.
Thus a breaker rated by the ASCC method will be suitable for any source with an X/R ratio below the test X/R ratio (17).
The peak current with a high X/R ratio may approach 2.828 times the ASCC value.
Many users will never encounter a source with an X/R ratio of 17 or greater.
However, there are may be exceptions.
If you are concerned with mechanical forces then you must use the full offset current in your calculations.
This link may help.
Link

[link ]Bill[/url]
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

You need to use the methodology that the breaker testing and rating was based on. If the breaker was designed and tested per IEC, you have to use the IEC method. If the breaker is ANSI rated, you use ANSI C37. If you have both types of breakers, you have to do the calculations twice.

Cheers,

Dave

 

[Siyou]

Do I need to calculate the asymmetrical breaking current and dc current component and use this in the evaluation of the IEC rated CB?
How about if the asymmetrical breaking current calculation is higher as well as the dc current, does it shows that the calculations exceeds the device capability?

Thank You.

Hope to hear you soon.

 

[cuky2000]

Hi Warros,

Below are some reference associated with the X/R ratio on the latest IEEE and IEC marketplaces:

[ul]
[li]X/R = 17 @ 60 Hz[/li]
[li]X/R = 14 @ 50 Hz[/li]
[/ul]
The figure below and the thread238-472233 provide an excerpt from the standard as well some background information that may be relevant for this post.


To bad. The original interrupting rating at X/R=14 (for IEC) needs to be derated and adjusted accordingly. Under any circumstance, the prospective system short circuit current exceeds the short circuit rating of the breaker. A new breaker with a higher rating shall be specified.


>>>

 

[cuky2000]

Waross,


I forgot to mention that the latch and closed multiplication factors became know slightly reduced to 2.6 and 1.5 instead of the traditional values of 2.7 and 1.6 respectively.

 

[Siyou]

Thanks Everyone.

If I dont have the fault duty of the utility source, what can I used?

 

[dpc]

In my opinion, it's pointless to calculate MV breaker duties if you don't know the utility fault data. Ask them what rating breakers you should buy?

 

[Siyou]

The 69kV line uses a CB which has a rating in IEC standard, while on the 13.8 kV it uses ANSI standard.. So meaning I will do twice short circuit calculations ANSI and IEC?

 

[dpc]

Yes.