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

The need for corona shields specifically arises in relation to Extra High Voltage (EHV) and Ultra High Voltage (UHV) transmission lines. Corona occurs when the electric field surrounding a conductor exceeds a certain threshold, leading to ionization of air, which can generate ozone and produce radio interference or audible noise.

EHV and UHV lines operate at very high voltages, often exceeding 345 kV and can reach levels above 800 kV, respectively. At these high voltage levels, the electric fields produced are strong enough to cause corona discharge, which can lead to energy loss and damage to the infrastructure over time. Employing corona shields helps to mitigate this issue by creating a more uniform electric field around the conductor and reducing the likelihood of ionization, thereby enhancing the efficiency and longevity of the transmission system.

The other voltage types mentioned, such as High and Medium Voltage, Standard and Extra High Voltage, or Moderate and Low Voltage, generally do not require such protective measures because the voltages and the resultant electric fields are not sufficient to create significant corona discharge effects under normal operating conditions.