Evaluation of Circuit Breaker Short Circuit Rating

[Siyou]

I have a 72.5 kV power circuit breaker with a breaking current of 25 kA and a making current of 65kA with a DC component of 44 percent.
What will I used to evaluate the breaking current rating of CB. The symmetrical breaking current or the asymmetrical breaking current?
How can I know the minimum time delay of the power circuit breaker if the manual have details of the opening time, closing time, total breaking time and dead time.
Thank You.

 

[Kiribanda]

To my knowledge you cannot call a 72.5kV circuit breaker as a "POWER CCT BKR (PCB)" because the word PCB is used for
withdrawble type LV heavy power/sc circuit breakers.
You have to consider both sym & asym ratings of the bkr because,all HV circuit breakers are tested for X/R=17 (Time constant=45ms @60Hz)
means, that it can break an asymmetrical current with max. 67% DC component only. Therefore, the asym current capability =2.6 x sym capability (25x2.6=65kA)
But, if the calculated X/R ratio at the bkr location is higher than 17, then either you have to select a bkr with at least one step higher than 25kA (31.5kA)
or specify a breaker manufactured & tested for special case DC time constant (60ms). Then it will break the sc current at the location comfortably without
violating its TRV capabilities.

Also you can delay the relay operating time slightly so that the bkr will pass the 67% point and break the asym sc current at less than 67%.

 

[Siyou]

Thank You Very Much @Kiribanda for the information you have shared.
The Circuit Breaker nameplate used Standard IEC 62271-100,but sadly I don't have that standard.
I don't know how can I get the minimum time delay of that circuit breaker as well as its X/R test ratio.
I used manual calculation of short circuit calculation. And used ETAP software for validating may answer. But ETAP requires the minimum time delay of CB which range from 0.01 sec to 0.3 sec. Upon checking in ETAP and using different minimum time delay its show that the asymmetrical breaking current value decreases as time increases. I dont know what will I used in the minimum time delay so that I can evaluate my short circuit calculation to the rated CB.

Hoping someone who can help me.

Thank You Very Much Everyone for sharing your knowledge and expertise.

 

[Kiribanda]

For IEC breakers,
Minimum Time Delay = Bkr Opening Time + Total Relay trip time including LOR etc etc
In Fact this Miinimum Time Delay in IEC bkrs is equivalent to the contact parting time (CPT) of IEEE C37 bkrs.
If your bkr is made for IEC62271-100, then its name plate should show both opening time & break time of the bkr.
Now from your sc calculations & protection coordination studies you should know the realy operating time.
That means now you can get the minimum time delay (MTD). The more you increase the MTD, the lower the
DC componenet (Asym current) because you are delaying the bkr operation either by increasing the relay operating time
or by specifying a bkr with a longer operating time.I believe that the above answers your question.
By the way could you pl. upload a photo of the bkr name plate?

 

[Siyou]

@Kiribanda, is there any otherway I can know the minimum time delay. The Circuit breaker is a 3 cycle CB and it is equivalent to
2 cycle contact parting time if I used that the calculated asymmetrical breaking current exceeded the rated breaking current of CB as well as the Dc component (Idc). I only did short circuit calculation.
Anyway my system used a 72.5kV SF6 circuit breaker at 69kV line rated in IEC standard with 25kA breaking current and 65kA making current with 44% DC component. The 15kV vacuum CB rated in ANSI standrd is used in the 13.8kV line with 25kA rated short circuit current.
I only used ANSI standard in my short cicuit calculation for the whole system. and I also used ETAP in checking my calculation but it requires the minimum time delay of the 72.5kV since it is rated in IEC.

See attached file for the breaker nameplate.

Thank You.