Positive and negative sequence impedances for cable, power lines, and transformers

[PowerEng90]

Hi!
I have a question about positive and negative sequence impedances for cable, power lines, and transformers.
Is the positive (and negative?) sequence impedance for cable and power lines the same as rated R and X values?
And is the positive (and negative?) sequence impedance for a transformer equal as the rated leakage impedances?
Many thanks!

 

[cranky108]

R values depend on the frequency.
X values also depend on frequency, and the spacing, as well as the diameter.

 

[HamburgerHelper]

Positive and negative sequence values will be the same for everything that but motors and generators.

Positive sequence will be the impedance under balanced three phase conditions.

X depends on frequency, spacing and diameter like Cranky said. The only thing that is an exception is bundled conductors. In the calculations for that you calculate a geometric mean radius that represents the bundled conductors as a hollow tube with some radius.

R depends on the conductor and the frequency due to the skin effect. Higher the frequency, the thinner the outer portion of the conductor is that actually carries current. No one cares about this unless you are doing transient analyses. Even then I doubt it matter much due to X being much larger than R.

Positive and negative sequence impedance increases as the spacing increases. Zero sequence impedance decreases as spacing increases.

If you want to see something slick, take a look at ABB or Westinghouse's Transmission and Distribution book. They show how to put together negative sequence voltage and current filters with passive components. I didn't think you could filter out the component rotating opposite of positive sequence with just resistors, capacitors, and inductors but you can.



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If you can't explain it to a six year old, you don't understand it yourself.

 

[PowerEng90]

I am aware of how to calculate R and X I was only wondering if they where equal to the negative and positive seq. impedances (for lines, cables and transformers). Thanks for helpful answers!