Help please .Transmission line parameters in per unit

In summary, according to the conversation, there are formulas that can be used to calculate parameters for a transmission line in per unit system. The formulas come from knowing the macroscopic parameters, such as the per unit length and resistive losses, as well as the line's geometry. There are also well established formulas that can be used if the parameters are known. If the parameters are not known, or if a more accurate model is needed, then a model of the transmission line in point form may be necessary.
  • #1
Serena_hm
18
2
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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  • #2


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.
 
  • #4


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.
 
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  • #5


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 !?
 
  • #6


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.
 
  • #7


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 !
 
  • #8


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.
 
  • #9


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 !
 

Related to Help please .Transmission line parameters in per unit

1. What are transmission line parameters in per unit?

Transmission line parameters in per unit are a set of normalized values that are used to represent the impedance, admittance, and other characteristics of a transmission line. These values are expressed as a ratio compared to a base value, typically the nominal voltage or current of the line. This allows for easier comparison and analysis of different transmission lines.

2. Why are per unit values used for transmission line parameters?

Per unit values are used for transmission line parameters because they simplify calculations and analysis. By normalizing the values, it eliminates the need to constantly convert between different units, making it easier to compare and analyze different transmission lines. It also allows for the use of standard equations and formulas, regardless of the specific values of the transmission line.

3. How do per unit values affect the performance of a transmission line?

Per unit values have no direct effect on the performance of a transmission line. They are simply a way to represent the characteristics of the line in a normalized manner. However, by using per unit values, it becomes easier to identify potential issues or differences between different transmission lines, which can ultimately impact performance.

4. Can per unit values be used for all types of transmission lines?

Yes, per unit values can be used for all types of transmission lines, including overhead lines, underground cables, and even high-voltage direct current (HVDC) lines. As long as the values are normalized and consistent, per unit representation can be applied to any type of transmission line.

5. How are per unit values determined for a transmission line?

Per unit values are determined by dividing the actual value of a transmission line parameter (such as impedance or admittance) by a base value. The base value is typically the nominal voltage or current of the line. This ratio is then expressed as a percentage or decimal value, and it represents the per unit value for that parameter. The base value can vary depending on the specific analysis or application, but it is usually chosen to be a convenient and easy-to-use value.

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