However the line characteristic impedance Z=SQRT[(R+j*w*L)/(G+j*w*C)]
where R,L,C,G=specific resistance, inductance, capacitance and conductance[ohm/km for instance]
These parameters depend on overhead line constructive details-height, distance between conductors, number of circuits on the same tower, conductor material and diameter, earth resistivity and other parameters, and w=2*pi()*freq.
See for instance:
See page 15 of this paper on transmission line characteristic impedance calculations. This is a simplified equation and it works well for this application. In former times, when we made line tuners, this is the formula we gave to customers.
Thanks for the replies! That was a very helpful article scottf.
cranky108, how did you get that 300 ohms? I was told to assume the same value by someone else.
Also the information I have is the positive and zero sequence impedances. How do I calculate the characteristic line impedance using the formula in the article mentioned by scottf??
Yeah I did that and ended up getting 581 ohms. I just want to see if I can get the same value using Z = sqrt(L/C). Not sure how to deduce L and C values..
Work backward from your per unit reactance and admittance values to the ohmic values and then convert from ohms to Henrys and Farads. I'll try a sample later.
The 300 ohms comes from a PLC class I took years ago that was by an ex-GE employee.
Usually the tuner has a range it can be set at, and if it has the correct range, you can start at some point and do a series of reflected power testing at the center of the frequencies you are intending to operate at.
By adjusting the tuning you should find a dip in the reflected power at the sweet spot.
If you calculate the tuning value, you are almost always off by some, and the tech's will be expecting you to be spot on. You will never hear the end of it.
I'm not sure what I did with the books I had from that class, but likely in my basement somewhere.
That is correct, the value is just for use as a starting point for the impedance matching transformer. If it can't be tuned with that setting, yo just take it to the next setting up or down depending on tuning results. It's not an exact science for sure. But if you give the factory your best guesstimate of the primary line characteristic impedance, it saves your field techs some time in tuning, as the line tuner is already close to tuned when received.
Most utilities that stock line tuners ask for the IMT to be set at 300 ohms on the primary side.
