Why power transmission line conductors have no insulation or jacket?

Power Transmission lines are usually very long, EHV Transmission lines used for wheeling of power. They are strung on very high lattice Towers. The transmission lines are of ACSR or AAAC conductors of appropriate current carrying capacities. In case of wheeling of large currents at say 400kV level, bundle conductors are used. They don't need any kind of insulation or jackets etc. as the ambient air serves as Air insulation. Thus maintaining requisite flash over distance vis'a vis, EHV voltages, between the Transmission line conductors and the insulation between the conductors and the transmission towers are maintained with Porcelain or glass discs.

The disadvantage of air insulation is the need for more distance between phases and between phases and ground, for proper isolation. On the other hand, air is a self-restorative isolator, so the transmission line will be able to return to service following a partial discharge.

A partial discharge on a cable would destroy the isolation and would require replacement of the damaged portion of the cable. So, the main advantages of air insulation are:
Cost (essentially free, but requiring larger and taller towers and probably a wider right-of-way, part of the initial investment on the construction of the line)
Weight (which actually offsets some of the additional cost due to the increased distances between phases described above)
Restorative behavior

A secondary difference: due to these increased distances between phases and all, overhead (air-insulated) lines tend to have lower capacitive charging, compared to isolated cables (for the same voltage level and line length). Isolated cables end up with incredibly restrictive problems associated with Ferranti effect and there will be a practical limit on the maximum length of an AC isolated cable. For extra-high-voltage levels (345 kV and above) the limit is probably less than 50 km, perhaps even as little as just 30 km. This is the main reason why under-sea cables (for long crossings) tend to be HVDC links.

In Brazil, for comparison, we have 500kV over-head (air-insulated) transmission lines approaching 300 km in length for a single section (distance between substations).

FAQ , Basics

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