The conductor system, by means of which electrical energy is conveyed from bulk power source of sources (generating stations or major substations supplied over transmission line) to the consumers is known as high-voltage (primary) distribution and lowvoltage (secondary) distribution.
Primary Distribution system. From generation stations the electrical power is usually transmitted to various substations through extra high tension transmission lines at voltage from 33 to 765 kV. At these substations this voltage is stepped down to 11 or 6.6 or 3.3 kV (most usually to 11 kV) and power at this voltage is conveyed to different substations for distribution and to the bulk supply consumers. Such a system is known as high-voltage or primary distribution system. The voltage employed for primary distribution system depends upon the amount of power to be conveyed and the distances of the substations required to be fed.
Secondary distribution system. At distribution substations the voltage is stepped down to 415 volts.
From these substations various low voltage (415 volts between phases and 240 volts between phase and neutral) distributors radiate out and feed the consumers. this system of distribution power is known as low voltage or secondary distribution system.
Bundle Conductors A bundle conductor is a conductor made up of two or more conductors called the subconductors, per phase in close proximity compared with the spacing between phases. The basic difference between a composite conductor and bundle condutor is that the subconductors of a bundle conductor are seperated from each other by a constant distance varying from 0.2 m to 0.6 m depending upon designed voltage and surrounding condition throughout the length of the line with the help of spacers whereas the wires of a composite conductor touch each other.
Although the bundle conductors are used on EHV transmission lines primarily to reduce corona loss and radio interference, but they have several other advantages over single conductors such as reduced losses thereby giving higher transmission efficiency, higher charging currents owing to higher capacitance of bundle conductors to neutral resulting in pf improvement, lesser inductance per phase, comparatively low surge impedance owing to lesser inductance and higher capacitance resulting in increase in the miximum power transfer capability.