Iso-phase bus capacitance

[VLFit]

I now there are many different geometries and sizes to iso phase bus installations, but I wondered if there is any kind of guide to the capacitance of commonly used iso bus in the US, typically used at generation plants. A pF or uF rating per 100' or some kind of rule of thumb would be great. New installs have the data but not old installations. It would be nice to have some idea of the AC hipot kVA size needed when quoting a job.

Thank all

 

[ScottyUK]

IPB is quite amenable to using the basic principles from your high school physics class to calculate the capacitance of two concentric cylinders. It doesn't account for elbows, tee's, and the leakage at bushings and post insulators but you can get a fair estimate. I remember doing this about 12 years ago on a fairly large 16kV IPB following some major repairs and the error wasn't enormous when we got the test set connected up and got real data.

 

[VLFit]

Thanks ScottyUK. You are right about that but if I can simplify the process for the guys in the field so they can do it without being terrified by seeing an ln, it would help. Often its the customers that call needing the test and they don't have much info. Thanks again.

 

[ScottyUK]

What about a simple spreadhseet with a couple of cells to enter the OD of the shield (which is close enough to the ID for this level of accuracy) and the OD of the busbar, and a third for the sum of the lengths of straight sections? The rest can be hidden in locked cells so it doesn't look like big scary equations. ;-)

Thinking about this a bit more, you might need to have do two sets of calcs and sum the capacitances because on machines with unit aux transformers and/or exciter transformers tee'd off the main bus it is quite common to have a reduced size IPB serving the auxiliaries which will have a different capacitance per unit length. I wouldn't be surprised to find that the auxiliary bus has a greater capacitance p.u. than the main bus given the different geometry.

 

[VLFit]

I will do that and your right about the auxiliary bus needing testing as well. I can do the spreadsheet but I still need to gain more data about the typical capacitance of a handful of commonly used bus voltage sizes and arrangements so I can estimate what's needed with some degree of accuracy. I'll let you know how I make out.

 

[electricpete]

Just to elaborate Scotty's suggestion
First link from google - see last page:

http://faculty.polytechnic.org/phys...._capacitors/2._pdf's/capac_of_coax_cable.pdf

C/L = 2*pi*Epsilon0 / ln(Router/Rinner)
Epsilon0 = 8.854E-12 F/m, which provides the expected units for C/L
If you can estimate length,Router, Rinner, you're in business

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(2B)+(2B)' ?