Generally, 230kV or higher system overhead transmission line uses single-stranded conductors; above this voltage system
multi-stranded like two, three, or four strands with bundle conductor spacers is
required.
As per the nature of current flowing on conductive
materials, maximum electrons tend to flow through the surface of the conductor
rather than the inner. Considering this property hollow conductor is more suitable to
optimize the current flow for higher capacity power transmission in an extra-high-voltage (EHV) system.
Theoretically, the hollow conductor may be viable, but
practical conductors are not economical for maintenance and erection. So, the concept
of a bundle conductor is the best solution to instate a hollow conductor.
Advantages of Using bundle conductors in EHV overhead
transmission system
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A bundle conductor in a transmission line is used to get
the following advantages:
1. Bundle conductor of
transmission line reduces the reactance of the electric transmission line;
2. Bundle conductor in power
system also reduces corona loss, which improves the transmission efficiency as
loss due to corona effect;
3. Bundle conductor in EHV
transmission system provide the higher capacitance to neutral when compared to
single lines. This results in higher charging currents which helps in improving
the power factor;
4. In bundled conductor lines,
as the effective
GMR (geometric mean radius) of the bundled
conductor increases the conductor diameter, virtually acts like a larger diameter hollow
conductor, and the inductance per phase
of the conductor decreases;
5. The bundle conductor spacers
give optimum spacing (8 to 10 times the diameter of the conductor) between sub
conductors in bundled conductors that gives a minimum voltage gradient on the
surface of bundle conductors;
6. As the advantage of bundled
conductors in transmission lines by reducing voltage gradient, the radio
interference is also reduced or minimized;
7. With the decrease in inductance
of bundled conductors, the surge impedance of the line is also reduced, L=(u0/2π)*
ln(Mutual GMD)/(Self GMD)) H/m/phase;
8. Due to bundling the
conductor uses the
surge impedance reduction, so more SIL-surge impedance loading (Z=(L/C)^1/2)) of
the conductor increases which increases the transmission capacity of the system.
The remarkable phenomenon of the overhead transmission
system is the “Corona effect” that can be
observed when the air medium present between the phases is charged up and start to
ionize and acts as a conducting medium.
Primarily bundle conductor concept is employed to
reduce the corona loss and radio interference of transmission lines. Especially
EHV at around 400 kV or higher corona losses are very high that can reduce
using bundled conductors.
To know more details about Corona Effect and Radio interference,
read-
Bundle Conductor Formation
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Usually following three types of bundle conductor
formation used in power transmission systems:
1.
Duplex formation;
2.
Triplex formation;&
3.
Quadruplex.
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| Bundle Conductor in Overhead Line |
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| Usually Used Bundle Conductor Formation |
The geometric mean radius of a bundled conductor for
a two-conductor (duplex), a three-conductor (triplex), and a four-conductor
(quadruplex) arrangement is shown in Figs.
In conclusion, we can
say bundle conductors are used for transmission purpose as it helps in
obtaining better voltage regulation and efficiency by reducing the inductance
and skin effect that are present in the power lines. 
