cable impedance

[tonyflair]

I'm trying to compare some impedance values between a table that I've found with values that are modeled through a computer program. The cable that I'm considering is 350 MCM 69 kV (Teck90) 3/C. The software that I'm using models it with the following parameters (this is in p.u., based on a 1000 foot run, Base MVA = 10 MVA, and the systems voltage is 600 V),

Software: R = 2.2929 , X = 0.40317 , B = 1.74E-6,
Ro = 2.2314, Xo = 0.2552, Bo = 1.74E-6

From the table that I've acquired, this specific cable has an impedance of (in ohms - based on 1000 ft run),

Positive/Negative = 0.040+j0.047
Zero Sequence = 0.316+j0.170

If I convert the table value to a p.u. value (base KV = 0.6, base MVA = 10 MVA) my values aren't very close at all.

Why do you suggest the difference is so great ?

Thanks in advance

 

[jwerthman]

Why would you use a 600 volt base to calculate per unit for 69kV cable? Is the cable actually connected to a 600 volt system? If not, I think you are using the wrong base voltage for that cable. Remember that the base MVA stays the same, but base voltages change across transformers.

 

[cuky2000]

The zero sequence impedance and the associated values for resistance and reactance should be greater than the correspondent values for positive resistance and reactance respectively. Therefore, the values provided by the software should be revised.
On the other hand, the referred values from the table, [(Ro = 0.316 > R = 0.040) & (Xo = 0.170 > X = 0.047)] Ohm /1000 ft, appear to be more in line with the expected results for positive and zero sequence.

The enclose site will provide an estimate to support the above statement. For exact results, refer to a 600 V table from the cable manufacturer or perform a calculation using realistic data.

http://www.kerite.com/catalog/catalogfiles/impedance_data_power.htm

 

[wareagle]

Check this site for more information.

http://www.kerite.com/catalog/catalogfiles/impedance_data_power.htm