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Calculation of sequence impedance of an overhead line

 

 

Calculation of sequence impedance of an overhead line

 

The Positive Sequence Impedance of 3-Phase 3-Wire Short Transmission Lines (i.e. Without Overhead Earth Wires)

Figure  below shows a general arrangement of 3-phase 3-wire overhead line  conductors. The insulators, cross arm and the pole supporting the conductors are  not shown

 

General arrangement of a 3 phase 3 wire transmission line


Positive Sequence Resistance

 

For an overhead line it can be assumed that the positive sequence AC resistance of  the conductor is equal to the DC resistance of the conductor. This is similar to the  cable analysis at power frequency (i.e. 50Hz).

 

Positive Sequence Reactance


The positive sequence reactance of the 3 phase 3 wire line can be calculated from the following equation:

 

where:
ƒ - frequency Hz
GMD - Geometric Mean (mm). For an equilateral triangle disposition of the  conductors, GMD = S - or the separation of the conductors. For any other  arrangements of the 3-phase 3-wire line, it can be calculated with the  following equation:

 

 

dab, dbcand dcaare the spacing (mm) between the phase conductors as shown in the  figure  above.  GMR - Geometric Mean Radius of one conductor (mm). It is calculated by  equation 

 

We get the positive sequence impedance of a 3-phase 3-wire short transmission line  combining R1and X1as follows:

 

The Negative Sequence Impedance of a Short 3-Phase 3-Wire Transmission  Line


The negative sequence impedance of the line is equal to the positive sequence  impedance.

 

 

The Zero Sequence Impedance of a Short 3-Phase 3-Wire Transmission Line

 

To calculate the zero sequence impedance of a short 3-Phase 3-Wire transmission line, the 3 conductors must be considered as a group and therefore the equivalent  GMR for the group of conductors must be calculated. The equation to calculate the zero sequence impedance is given below.

 

 

where Rcond  - Resistance of one conductor  Ω/km
ƒ - frequency Hz
ρ- deep layer soil resistivity  Ω-m
GMR1cond  - Geometric Mean radius of one conductor (mm)

 

 

dab, dbc, and dcaare the spacing (mm) between the phase conductors as shown in the  figure  above.

 

Prepared by: zone4info.com

Reference: Fundamental of calculation of earth potential rise in the underground rise by Ashok


Article Tags: #Resistance #overhead line #Zero Sequence #Transmission Line #Negative Sequence #Reactance #Positive Sequence Reactance #sequence impedance #Positive Sequence
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