SLG bracing and interrupting ratings on 69 kV outdoor sub

[norcalnewb]

We have an existing 230 - 69 kV substation that is rated for 40 kA on the 69 side. We are looking at building a second sub on the south side of the refinery that will operate with a 69 tie between the two stations, keeping them in parallel. Our studies are showing a SLG fault level of 39 kA. How does this relate to the 3 ph rating of the existing station?

Thanks,

Jim

 

[jghrist]

You don't say what is rated 40 kA. Equipment like switches don't care whether the current is 1Ø or 3Ø. Bus bracing is for 3Ø or Ø-Ø faults; 1Ø faults generally don't create much force on buses. Grounding systems don't care about 3Ø faults, only Ø-grd faults.

 

[norcalnewb]

The bus bracing is 40kA and the interrupting rating on the breakers is also 40 kA. My concern is on the breakers. We are hearing the older versions of the ANSI standards allowed for a higher interrupting rating on breakers for SLG faults over 3PH, but we are pulling the newer standard to see if this still applies. I thought I might pose the question here to get some other opinions.

 

[davidbeach]

There are several possibilities out there for doing breaker rating studies, typically in the same software packages used for doing the fault studies.

 

[cuky2000]

The available short circuit of 39 kV is very close to the equipment rating. Check if a series reactor could help to reduce the SC current or consider a 63 kA breakers for your application.