Bundled Conductors | Power Systems - Electrical Engineering (EE) PDF Download

Bundled Conductors Used in Transmission Line

Single stranded conductors are used in transmission system usually up to 220 KV. But it is not possible to use single-stranded conductor for the voltage above 220 KV systems. For very high voltage system, hollow conductor can be used to optimize the flow of current through it. But erection and maintenance of hollow conductors in the ∑HV system are not economical. The problem can be solved by using bundled conductors instead of hollow conductor in the electrical transmission system above 220 KV voltage level.

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.

Advantages of Bundled Conductors

1. Bundling of conductors leads to reduction in line inductance.
   We know that inductance of a line is given by
   
   Where, GMD = Geometric mean distance
   GMR = Geometric mean radius
   For a single conductor of radius r
   GMR = 0.7788r
   For two conductor bundle as shown in figure
   
   
   
   For three conductor bundle
   
   For four conductor bundle
   
   Hence as we increase the number of conductors the GMR increases and hence L decrease. Now, there are many advantages of reduction in inductance of the line, such as-
   • The maximum power transfer capability of the line increases as
     
     Where X = wL …reactance of line 
   • The voltage regulation of the line is also increased as the reactance of the line is reduced.
2. On the similar argument for decrease in inductance of line, we can say that the capacitance of the line increases, as capacitance of line to neutral is given by
   Now since we have L decreased and C increased the net SIL of the line also increases automatically, and hence the power transfer capability too. Hence using bundled conductors is an effective way of increasing SIL, i.e. Surge Impedance Loading.
3. The most important advantage of bundled conductors is its ability to reduce corona discharge. When power is being transferred at very high voltages using a single conductor, the voltage gradient around it is high, and there is a high chance that the corona effect will occur – especially in bad weather conditions. However, using several conductors nearby instead of one conductor, forming a bundled conductor which leads to a reduction of voltage gradient and hence the possibility of corona formation.

   The increase in critical corona voltage depends upon the following-

   • Number of conductors in the group,

   • Clearance between them, and

   • The distance between the groups forming separate phases.
     It has been found out that the optimum spacing between the conductors in a group is of the order of 8-10 times the diameter of each conductor, irrespective of the number of conductors in the bundle.

4. Reduction in the formation of corona discharge leads to less power loss and hence improved transmission efficiency of the line.

5. Reduction in communication line interference due to reduction in corona.

6. The ampacity i.e. the current carrying capacity of bundled conductors is much increased in comparison to single large conductor owing to reduced skin effect.

7. As the bundled conductors have more effective surface area exposed to air, it has better and efficient cooling and hence better performance compared to a single conductor.