Main features of a transmission line

National Grid uses a variety of tower designs for the support of overhead line conductors which transmit high voltage electricity from generating stations to where it is needed.
The national grid until the late 1950s consisted of a series of overhead lines at a voltage of up to 132kV. As demand grew, a system of 275kV lines was developed to feed the major conurbations. This system was further developed and uprated to 400kV in the 1960s. A 400kV line carries about three times as much power as a 275kV line, and about 18 times that of a 132kV line depending on the precise line designs. Local distribution companies  generally own and operate lines with a voltage of 132kV and below.

The figure shows National Grid’s L2 and L6 double circuit pylons which are those most widely in use for high voltage transmission. The L2, a typical transmission tower from the 1950s, carried steel-reinforced aluminium conductors in pairs from each insulator. When quadruple conductors were introduced in the 1960s, larger and more substantial pylons were needed. The L6 designs were then introduced. The development of lighter all aluminium alloy conductors allowed the smaller L12 design to be brought into use in the 1980s.
The size, height and spacing of pylons are determined by safety, topographical, operational and environmental considerations.

The figure  illustrates the main features of a transmission line.

A typical National Grid overhead line route will involve the use of three main types of tower. They are as follows:-

Suspension pylons: These support the conductor on straight stretches of line. Conductors: are suspended by a vertical insulator string.
Deviation pylons: These occur at points where the route changes direction.
Conductors: are attached by horizontal insulator strings.
Terminal Pylons: These pylons are of greater bulk in order to ensure stability.  They occur at the end of overhead lines where they connect with substations or underground cables.