35kV Cable Bus Application 1

[rockman7892]

Anyone have any experience with using cable bus for 35kV applications with short distances?

I'm looking at a preliminary design which uses (6) sets of 750MCM cables installed underground to connect 75MVA transformer 34.5kV secondary to secondary switchgear. Cable distance is approximately 80' between transformer secondary and switchgear located in prefabricated control building.

We're currently in the process of revising substation layout and have an option to move control building closer to transformer. With the control building located in closer proximity to transformer I was looking into feasibility of using overhead cable bus between transformer and switchgear.

To me overhead cable bus vs underground cable presents an option for lower cost installation and easier maintenance and reliability.

I've used non-seg busduct successfully at 5kV & 15kV but from what I understand this becomes a bit more bulky and prohibitive at 35kV.

Interested to hear any pros/cons experience from others with cable bus at MV applications. Is there a maximum distance for cable bus beyond which makes its applicable not practical?

On a similar note is it typical for a yard having multiple transformers to have each transformer's secondary switchgear in a separate control building vs combined into a single control building?

 

[7anoter4]

If the transformer ratings are 75 MVA at 34.5 kV the rated current will be 75/sqrt(3)/34.5=1.2551 kA. For 6 cables per phase 1255.1/6=209.2 A, but for 3 cables per phase will be 419 A per one cable.
If the single core cables of 35 kV rated shield will be grounded on one end only then, according to NEC Table 310.60(C)(77) detail 2, you need only 3 sets of 750 mcm copper.
According to NEC Art. 394 on a cable tray of 24" width 3 sets of 350 mm copper 35 kV single core cables in trefoil arrangement and spaced 2.15 times the outside diameter of one cable, will do the job.
A cable bus duct keeps the single core cables far one from the another so the cooling is better and so higher ampacity. Also, the phase arrangement it is so calculated then the unbalance between parallel cable currents is minimum.
You need then a cable bus duct for 45 kV and 1600 A as per MPHusky - for instance.

 

[rockman7892]

7anoter4

Thanks for the feedback.

For the cable bus option is there a maximum distance for which overhead cable bus my not make sense or be practical?

For example with 75MVA transformer oil closest the control building can be to transformer is 50ft without a blast wall per FM standards. Does 50' typically exceed the length for cable bus or busduct which i have traditionally seen on shorter runs between transformer and equipment?

How much flexibility is there with cable bus on the terminations to equipment. Does it connect to Equpment with a throat or flange to which the individual conductors are then broken out and terminate in a similar fashion as single conductor cables? Also trying to evaluate connections as they relate to ease of removal from equipment for testing or equipment replacement.

Thanks

 

[FacEngrPE]

Likely best to talk with Husky about a concept. The literature talks about single pull cable length limit of 1000 ft. so 50 ft should be well within reasonable limits.

https://www.mphusky.com/literature/brochures

 

[7anoter4]

I agree with FacEngrPE.It will be better to contact MPHusky.