Here is a back to basics question. Let me know if this seems irrelevant in which case I will withdraw it. How do you determine the line constants? Do you determine them using your own scripts or use the TLC module built into psse? Is the conductor database in TLC adequate for your needs or do you use your own database?
Planning studies are generally based on calculated line constants. Calculations contain assumptions on soil resistivity, etc. that may not always be correct. Here is an article that discusses the measurement aspect of line constants and its relevance.
[http://www.pacw.org/en/issue/winter2008issue/line_constants.html?0=]
Method to calculate line parameters
Here is a brute force method right out of the text books for calculating the line parameters of a 400KV transmission line.
The various steps in the calculation process are:
First read in the conductor and tower data as follows:
Cond = 'ACSR 54/7/3.53'
Ew = 'GS 7/3.66'
nc = 2. # number of circuits
nb = 4. # number of conductors in bundle
ne = 2. # number of earthwires
f = 50. # frequency in Hertz
V = 400. # base voltage in kV
S = 100. # base MVA
radc = 0.015885 # conductor radius in meters
Rc = 0.0556 # conductor dc resistance at 20 degC in ohms/km
rade = 0.004725 # earthwire radius in meters
Re = 2.5 # earthwire resistance in ohms/km
rho = 169.84 # earth resistivity in ohm-meter
sagc = 13.262 # average sag of conductor
sage = 10.199 # average sag of earthwire
dcs = 0.457 # sub-conductor spacing
kc = 0.81 # stranding factor of conductor
ke = 0.726 # stranding factor of earthwire
The main point to note here is the conductor resistance. I have used a dc resistance value but AC resistance ought to be used in actual practice.
Next read in the tower configuration data:
Tower Configuration Data
xa = -7.545 # Distance between bundle a and tower center line
xb = -6.872 # Distance between bundle b and tower center line
xc = -6.510 # Distance between bundle c and tower center line
xf = 7.545 # Distance between bundle f and tower center line
xg = 6.872 # Distance between bundle g and tower center line
xh = 6.510 # Distance between bundle h and tower center line
xe1 = -3.756 # Distance between ew1 and tower center line
xe2 = 3.756 # Distance between ew2 and tower center line
ye1 = 42.068 # distance between earthwire and ground in meters
ye2 = 42.068 # distance between earthwire and ground in meters
ya = 21.835 # distance between bundle a and ground in meters
yb = 29.835 # distance between bundle b and ground in meters
yc = 37.835 # distance between bundle c and ground in meters
yf = 21.835 # distance between bundle f and ground in meters
yg = 29.835 # distance between bundle g and ground in meters
yh = 37.835 # distance between bundle h and ground in meters
We now have all the input data for our computation.
Next calculate the ...
I spoke with one of my colleagues that worked with distribution design. He said the following:
I’m not sure about how field measurements could help with line parameters, but they do help ascertain ratings. The way we worked out the line parameters at the utility I worked for do it the following ways:
Conductor/cable impedance are obtained from the manufacturer
The impedance is calculated based on the as-builts (i.e.: line designs). The OH line impedance depends on the circuit configuration (on tower). This can be calculated by hand or can be calculated using DIgSILENT which has a OH impedance calculator based on the tower configuration. Normally what we have done in the past is that calculated it by hand and used DigSILENT to verify results. The cable impedance are also based on as-builts, and depends on the way it is buried in the soil (in trefoil or flat configuration for single core cables). There are literatures out there that shows the way to calculate the impedance. Unfortunately, DIgSILENT does not calculate the cable impedance – so no way of verifying it.
With cables however, the harder thing to work out is it’s rating. Normally, in the absence of measurements, you refer to the manufacture ratings which are based on certain conditions. The cable ratings are susceptible to soil resistivity and temperature. So where you have places of high temperature, or soil of insulating properties, you tend to do soil tests. We used to do this to ascertain the cable ratings using CYMCAP software. The ratings are also dependent on the ability to transfer heat in the trench. So you’ll have to calculate a number of things including the way it is laid out in the trench, the number of cables in the trench, the depth, backfill materials etc.
They didn't use PSSE TLC module, but I think in New Zealand they were using DIgSILENT at this utility not PSSE. It sounds like you both have very similar methods. The difference being that you have automated your calculations with a script, while they prefer to calculate by hand.
Thanks for the feedback. Yes, I agree that resistivity estimates have little effect on load flow analysis. But these differences might be important for relay setting calculations. There are cases where estimates of B & C values differ by a factor of 2.
@JervisW thanks once again for taking the pains to dig up information. I see something common here. I use ATP-EMTP sometimes (when I have to calculate the line constants for a new 400KV configuration) to verify my calculations. I can share my script if there is enough interest.
That does sound interesting. I mentioned earlier that I've never been exposed to this level of engineering. I would love the chance to see what designing a 400kV line looks like.
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How we get line constants for studies - written from the perspective of a high level system planner.
Stage one - Planning data When the initial plans are proposed by the system architects, we look at the voltage level of the line and other similar lines in the area to get a default line constants for use in planning studies.
Stage two - design data Consultants will run detailed studies and speak with contract firms that could build this project to project detailed line constants that will be fed back into our system model.
Stage three - as built data Having purchased the conductors and other equipment and with it installed, the as-built line constants are again fed into our system model for future record and later studies.
The application gives the R, X, B (both positive and zero) values of lines in %/km/ckt format. I work at the utility level and therefore am responsible for providing the as-built line constants.
I use a small script to determine the constants - which is essentially a calculation and therefore an approximation. This is by far the easiest and most popular method. But some utilites do take actual measurements. I would like to know how is it done in your part of the world - hence the question.
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I calculated the r0,x0,b0, r1,x1,b1 for Voltage level 33KV, 66KV, 132KV, 220KV, 400kv for 50hz frequency. The conductors considered are DOG. tiger, wolf, Panther , Zebra. You can contact me at Lfplan2@gmail.com
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So if I read correctly, the inputs are: conductor and tower parameters, geometric mean radius and other similar physical constants? Is it true that steps 3 - 9 don't require additional input?
Right, the rest is mere computation.
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My experience is so high level that I cannot answer your question. I am hoping that someone else will be able to chip in with their process. Good question @amaity.