Why are bundled conductors used in transmission lines?

What is Bundled Conductor?

      We call bundled conductor to those conductors which form from two or more stranded conductors, bundled together to get more current carrying capacity.

      Here, we use two or more stranded conductors per phase. Also, to increase the current carrying capacity of the system, a bundle conductor also contributes various facilities to the electrical transmission system. A bundled conductor reduces the reactance of the electric transmission line. It also reduces voltage gradient, corona loss, radio interference, surge impedance of the transmission lines.

      By making bundle conductor, the geometric mean radius (GMR) of the conductor increased. As the self GMR of the conductor increases, the inductance of the conductor decreases. Theoretically, there is an optimum sub-conductor spacing in bundle conductor that will give minimum voltage gradient on the surface of bundle conductor. The optimum spacing between sub-conductors for reducing voltage gradient is eight to ten times of the diameter of the conductor.

      Since the voltage gradient gets reduced, the radio interference also gets reduced.
As the inductance of the bundled conductor is reduced, the surge impedance of the line is reduced became the formula of surge impedance is √L/C.

      Where L is the inductance per phase per unit length, and C is the capacitance per phase per unit length of the transmission line. As the surge impedance gets reduced due to bundling the conductor, the surge impedance loading, of the conductor gets increased. Increased surge impedance loading makes the transmission capacity of the system more.

      In bundled conductors two or more stranded conductors are used per phase.

Now, why they are used :

• It reduces the reactance of the electric transmission line
• It also reduces corona loss, which improves the transmission efficiency as loss due to corona effet is countered (Corona effect can be observed when the air medium present between the phases charged up and start to ionize and acts as a conducting medium)
• Bundled conductor lines do have the higher capacitance to neutral, when compared to single lines. This results in higher charging currents which helps in improving the power factor
• In bundled conductor, as the GMR (geometric mean radius) of the conductor increases, the inductance per phase of the conductor decreases.
• The optimum spacing (8 to 10 times of the diameter of conductor) between sub conductors in bundled conductors gives a minimum voltage gradient on the surface of bundle conductor
• As the voltage gradient is reduced, the radio interference is also reduced or minimized
• With decrease in inductance of bundled conductors, the surge impedance of the line is also reduced
• Decrease in surge impedance due to bundling the conductor, surge impedance loading (SIL) of the conductor increases. This makes the transmission capacity of the system more.

There are many advantages of using bundled conductors in transmission lines which are as follows.

• Reduced Inductance:

L=(u0/2π)* ln⁡(Mutual GMD)/(Self GMD)) H/m/phase

      Due to bundling of conductors the self GMD of the conductors is increased (as radius increased), so Inductance of conductors decreases which leads to reduced voltage drop in line, so voltage regulation improves(decreases). Due to reduced value of Inductance, value of reactance decreases which maximize the maximum power flow Pmax (see the power flow equation), so stability of system increases.

• Reduced corona loss:

       There is a optimum spacing between sub conductors of a bundled conductor which give minimum gradient on the surface of sub conductors and hence highest critical disruptive voltage and hence reduced corona loss (see formula of corona loss in terms of critical disruptive voltage). Other explanation of reduced corona loss can be, by using bundle conductor, the value of current per conductor reduced which leads to reduced current density. As Current density is directly proportional to electric field intensity, electric field intensity reduces, so corona loss also reduces.

• Increased Capacitance: 

      Due to increase in self GMD, the capacitance per phase increases (formula for capacitance having logarithmic term in denominator) which leads to improved power factor of the system.

• Reduced surge impedance: 

      Surge impedance of a lossless line is given as square root of (L/C) as inductance is reduced and capacitance is increased (due to self GMD), so surge impedance reduces. Due to this surge impedance loading (which is the ideal loading) capability(V*V/Zc) of line increases.

• Radio interference:

      Radio interference is reduced due to reduced current per conductor or we can say due to increase in self GMD the inductance reduces and thus the flux linkage with telephone lines also reduces.