Help please .Transmission line parameters in per unit

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Discussion Overview

The discussion revolves around converting transmission line parameters (resistance R, inductance L, and capacitance C) into a per unit system using MATLAB. Participants explore various methods to obtain these parameters, including measurements, catalog properties, and geometric calculations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant seeks assistance with formulas to convert transmission line parameters into a per unit system.
  • Another suggests obtaining parameters from measurements, catalog properties, or line geometry, mentioning specific relationships between parameters.
  • Links to external resources are provided for further information on transmission line types and calculations.
  • A participant expresses skepticism about the accuracy of using per unit length parameters for high-frequency simulations, emphasizing the need for a detailed model.
  • Clarification is requested regarding the use of base voltage (Vbase) and base power (Sbase) in the calculations.
  • One participant shares their experience with a specific simulator (Powerworld) and mentions issues with accessing a required feature for calculations.
  • Another participant acknowledges a misunderstanding regarding the type of transmission lines being discussed, indicating a shift in focus from RF to power distribution lines.
  • A final participant summarizes the formulas they found for converting resistance, reactance, and susceptance into the per unit system.

Areas of Agreement / Disagreement

Participants express differing views on the best methods to obtain transmission line parameters and whether per unit length parameters are suitable for accurate simulations. The discussion remains unresolved regarding the optimal approach for the specific context of the original poster's needs.

Contextual Notes

Some participants mention the importance of frequency in determining resistance and the potential limitations of using simplified models for complex transmission line scenarios.

Who May Find This Useful

This discussion may be useful for individuals working on electrical engineering projects involving transmission lines, particularly those interested in the per unit system and simulation software.

Serena_hm
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Help please...Transmission line parameters in per unit

hello there
I'm trying to work on a program that converts electrical quantities of transmission line ( R , L , and C ) into per unit system using Matlab
the thing is that I'm finding difficulties with the formulas , how I can obtain them , each one individually !

Does anybody have clue about it or maybe any other way can be suggested instead !?

I'll be grateful
sincere regards
 
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Obtain them from measurements on the transmission line, or from its catalogue properties, or compute them from the line's geometry?

From macroscopic parameters, v-2=L*C and Z2=L/C is enough, where L and C are per unit length. Resistive losses per length link R and L*2piF, where R is per unit length at the measurement frequency - very important, since skin effect let's it differ completely from DC resistance. Something like exp(-x*R/L2piF) but please check for the constants.

From a line's geometry, many simple form do have an algebraic solution... Coaxial, bifilar, wire over ground, two wires over ground. But not when a dielectric fills a part of the space, for instance in a printed circuit line; experimental formulas then.

Again, from Z you get C and L then, feeding the permittivity in as often needed.
 


I have not tried this, but I am not sure you can use R', L' and C' ( per unit length) to simulate a transmission line that accurate. To get accurate representation, you have to have a model that accommodates very high frequency. Remember, if you read over the explanation of this RLC model, you have to take the infinitesimal length to form the model, and it is almost like in differential or point form. You don't just use 1 meter or some particular length to start simulating.

There are well established formulas, why don't you want to use those.
 
Last edited:


hello there !
thanks for your reply ...well I'm talking about per unit system where we use Vbase & Sbase in the circuit , I'm afraid that I got a little bit lost ...did you get what I exactly mean !?

the program is supposed to calculate the parameters with knowing Vbase , Sbase , length and also the conductor type
Are there any formulas can be used !?
 


Serena_hm said:
hello there !
thanks for your reply ...well I'm talking about per unit system where we use Vbase & Sbase in the circuit , I'm afraid that I got a little bit lost ...did you get what I exactly mean !?

the program is supposed to calculate the parameters with knowing Vbase , Sbase , length and also the conductor type
Are there any formulas can be used !?

I don't know your program. All I am saying is using R', L' and C' might not be the best way as I described above. I have to look around later, I have to go out for a few hours. Time for the gym.
 


well , thanks for your reply all and I really do appreciate your help
actually I'm working on Powerworld simulator and in that program I'm supposed to enter the transmission line parameters in per unit system " p.u " for ( R , L , and C ) I'm going to work on practical grid that has transmission line parameters in (ohm/km ...etc )
I do know that there is an option in the program to make these calculations ( as separate file called TransLineCalc ) but sadly it didn't work with me cause I don't have the full version of the program & I've contacted the help center in order to provide me with the full version but they replied that they only deal with companies or universities...

you may need to check the link below so you can really understand what I'm really looking for

http://www.powerworld.com/products/translinecalc/overview

sincere regards !
 


You are talking about power line, power distribution! I was talking about RF transmission lines. Forget what I said, forget it all. I don't know your transmission lines at all! Sorry.
 


no worries , I finally got what I'm really looking for
R = R'/Zbase...(p.u)
x = X'/Zbase...(p.u)
B = B'/Ybase...(p.u)

where R' : Total series resistance in Ohms
X' : Total series inductive reactance in Ohms
B' : total series susceptance in Siemens

Thanks to you all for your time & attention !
 

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