What's in a tower? Electrical insulation

The figure on the right shows schematically a typical high voltage tower with two circuits consisting of each three phases and two shield wires. The shield wires are shown as the small black dots on the left and right in the top of the tower. The phase wires (two for each phase) are drawn as bigger black dots on the bottom of the green lines. These green lines represent the insulator strings.

Because the voltage of each phase is different from each other, and different from ground (defined as zero voltage) all phase wires should be insulated from each other and from ground. The insulation is accomplished by keeping enough distance (air) between the phases and ground. The higher the voltage, the more distance is needed. The phase wires are connected to the tower by an insulator or insulator string which is often made of glass, ceramics or synthetic rubber.
Because of continuous low velocity wind flow perpendicular to the phase or ground wires, those wires can start viabrating (aeolian vibration). This vibration can damage wires, insulators and other connected parts. Therefore often vibration dampers are installed. Mostly the so calles Stockbridge type dampers are used, see photo: red circles.

About phases, circuits and shielding

Electrical energy is normally transmitted through AC (Alternating Current) systems having a frequency of 50 or 60 Hertz. An electrical circuit consists of three phases, mostly called A, B and C. Phase B follows phase A on a distance of one third of a cycle, phase C follows phase B on a distance of one third of a cycle, and the same distance is again between phase C and phase A.

In the top, the tower has on both sides so called shield or ground wires. These wires protect the line from lightning strikes and are usually grounded in every tower.

Below the ground wires on both sides of the tower the bundled phase wires (a bundle of two for each phase) are suspended from the tower by insulator strings. The phases of each circuit are attached on different sides of the tower.

Click on the black or green parts of the tower (on the left or above) to find out more.

About different ways to express voltage

A high voltage line is normally designated by it's system voltage. This is the voltage between two phase wires of one circuit as RMS value. The voltage of a phase wire to ground is lower, and can be calculated here:

In the graph above all voltages are shown with the example of a 275 kV system voltage. This system voltage results in a voltage difference between phase wires ranging from -200 to +200 kV.
For the relation to time in the graph a frequency of 50 Hertz (cycles per second) is chosen.
 

Electrical fields and induced voltage

The same tower shown above is here used to suspend a two circuit 275 kV high voltage transmission line. For this 30 meters high tower the equipotential lines are calculated by the program used at NKF.

The equipotential-lines, shown here on the left, connect points in space with the same induced voltage (= potential) to each other. In the figure the value of the induced voltage is given in kV (RMS value) next to the equipotential-lines. The graphical shape of these lines depends on the shape and configuration of the tower, of course, and on how the phases are distributed in the circuits.
 

Telecommunication on High Voltage Overhead lines