Which types of voltage on transmission lines require a corona shield?

The requirement for a corona shield on transmission lines is primarily associated with Extra and Ultra high voltages. At these voltage levels, the electric field intensity near the conductors can become significant enough to ionize the surrounding air, leading to a phenomenon known as corona discharge. This discharge not only results in energy loss but also causes radio interference and can lead to the deterioration of the line insulation and hardware components over time.

By utilizing corona shields, which are typically metallic structures placed around the conductors, the electric field can be mitigated. These shields essentially help in redistributing the electric field, lowering its intensity and thereby reducing the likelihood of corona discharge at these high voltage levels.

While Low, Medium, High, and Very High voltages do have their own considerations regarding insulation and safety, they generally do not exhibit the same levels of corona effect that necessitate the use of shields like Extra and Ultra high voltages do. Therefore, the answer focusing on Extra and Ultra high voltages accurately reflects the specific conditions under which corona shields become essential in transmission line design and management.