cable impedance data in p.u.

[louie]

I'm trying to model a specific type of cable
(1kV, 500 MCM Teck 90 power cable). I've got some sequence impedance values from a table that I looked up that are based on a 1000 foot run and are in ohms. I'm not totally sure what the distance between these cables are, but I'm going to assume 8" (because that's what the data I found assumes). The simulation program that I'm using asks for the p.u. impedance for the entire length of the line. Below list the cable impedance from the table that I have,

positive/negative: 0.0340 + j0.079 (ohms/1000ft)
zero sequence: 0.338 + j0.281 (ohms/1000ft)

This cable lies between 2 buses that are at
600 V. The system MVA that I'm using is 10 MVA.
If I was convert the following to p.u./1000ft would the corrent method be to,

Zbase = (600^2)/(10E6) = 0.036

and then, Zp.u. = Zactual/Zbase, where Zactual is the values from the tables.

If I were then to consider runs that are larger or smaller than 1000 feet, can I just set up a ratio to determine the impedance at the distance I'm trying to find ?

Thanks a lot

 

[bob]

Re: cable impedance data in p.u.

Louie
It been a while but you get the actual impedance for the tables for 1000 ft.

Cable Zpu = (actual Z)(KVA base)/(kv?x 1000)
You can then ratio any length you want.

 

[charlie b]

Re: cable impedance data in p.u.

First, you are free to select any value you wish for ?Base Power? and ?Base Voltage.? Having selected those two, the values of ?Base Impedance? and ?Base Current? can be calculated. But I am concerned about your choices. If you had a real system with 10MVA of power operating at 600 volts, the current would be enormous (9,600 + amps). Are you sure you have made the wisest choices?

Secondly, I am unfamiliar with a power system that operates at 600 volts. A 480 volt system will have insulation systems rated for 600 volts, but I know of no transformers that have 600 volt secondaries. Are you certain about the voltage. Also (KEY POINT!!), is this value ?phase-to-phase? or ?phase-to-ground??

Third, your formula for calculating Base Impedance is incorrect. You have to take care of the ?V? versus ?kV,? the ?KVA? versus ?MVA,? the ?3-phase VA? versus ?1 phase VA,? and the ?voltage phase-to-phase? versus ?voltage phase-to-ground.? Here are a couple of choices:
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• <font size="2" face="Verdana, Helvetica, sans-serif">Base Impedance = (Base Voltage, line to line, in kV)^2 / Base MVA, 3-phase</font>

<font size="2" face="Verdana, Helvetica, sans-serif"></font>

• <font size="2" face="Verdana, Helvetica, sans-serif">Base Impedance = (Base Voltage, line to neutral, in kV)^2 times 1000 / Base KVA, 1-phase</font>

<font size="2" face="Verdana, Helvetica, sans-serif"></font>

• <font size="2" face="Verdana, Helvetica, sans-serif">Base Impedance = (Base Voltage, line to neutral, in kV)^2 / Base MVA, 1-phase</font>

<font size="2" face="Verdana, Helvetica, sans-serif">
Fourth, your formula for conversion of Z-actual to Z-pu is correct, but take care that you multiply the values from the tables times the actual length.

Finally, if you aren?t certain of the geometric spacing of the cables, you need to be aware that the inductive and capacitive reactances (the sources of the ?+j0.079 type of information) will vary with spacing.