Corona Phenomenon in HV Engineering with Non-Uniform Fields

In High Voltage (HV) engineering, the design and operation of equipment and transmission lines are significantly impacted by electrical phenomena, especially when the electric field is not uniformly distributed. The question asks to identify a crucial phenomenon in HV engineering that is particularly important when non-uniform fields are unavoidable.

Let's consider the given options:

• Skin effect: This phenomenon relates to the tendency of alternating current (AC) to flow through the outer layer of a conductor. It increases the effective resistance of the conductor, especially at higher frequencies. While important in AC systems, it is primarily related to current distribution, not directly to the impact of non-uniform electric fields around the conductor surface which causes breakdown or discharge phenomena.
• Ferranti effect: This effect describes the phenomenon where the receiving end voltage of a long, lightly loaded or no-load transmission line is higher than the sending end voltage. It is caused by the charging current flowing through the line capacitance. While relevant to transmission lines, it is a voltage regulation phenomenon and not directly related to the effects of intense non-uniform electric fields leading to atmospheric breakdown.
• Corona: Corona discharge is a partial electrical discharge that occurs when the electric field intensity around a conductor in air is strong enough to cause ionization of the surrounding air molecules. This typically happens at points of high field concentration, such as sharp edges, corners, or around high-voltage conductors, especially in non-uniform field regions. Non-uniform fields are inherent around conductors, particularly stranded ones or those with surface irregularities, and become significantly intense at high voltages, leading to corona. Corona is important in HV engineering due to associated power loss, radio interference, audible noise, and potential degradation of insulation materials. It is a direct consequence of high electric fields in a non-uniform configuration interacting with the surrounding medium (air).
• Lightning: Lightning is a natural electrical discharge phenomenon between the atmosphere (usually a cumulonimbus cloud) and the ground, or between different clouds. While lightning strikes pose a significant threat to HV systems, leading to surges and potential damage, it is an external atmospheric event rather than an internal phenomenon occurring due to the operational voltage and inherent non-uniform fields within the HV equipment or transmission lines themselves under normal operating conditions.

Considering the context of unavoidable non-uniform fields around conductors and equipment in HV systems under operating conditions, the phenomenon that is a direct consequence of these fields reaching a sufficient intensity to cause discharge in the surrounding medium (air) is Corona.

Therefore, Corona is a critical phenomenon in HV engineering, especially where non-uniform fields are present and high voltages are applied.