**Insulation Design of Modern EHV Lines**

The insulation of modern Extra High Voltage (EHV) lines is designed based on various factors to ensure safe and efficient operation. One of the key considerations in the design process is the switching voltage. Let's understand why the correct answer is option 'D' by exploring the importance of switching voltage in insulation design.

**1. What is Switching Voltage?**
Switching voltage refers to the transient overvoltage that occurs during the operation of power systems, particularly during switching operations. When a circuit breaker or a switch is opened or closed, it can create voltage transients due to the sudden change in electrical conditions. These transients are known as switching voltages.

**2. Significance of Switching Voltage in Insulation Design**
The insulation of EHV lines must be able to withstand the high voltage stresses caused by switching operations. If the insulation is not properly designed to handle these transient voltages, it can lead to insulation breakdown, corona discharge, and other electrical issues. Therefore, considering the switching voltage is crucial in ensuring the reliability and performance of the EHV line.

**3. Insulation Design Criteria**
The insulation design of EHV lines involves selecting appropriate insulation materials and determining the required insulation thickness or voltage withstand capability. This design process is based on several factors, including:

a) **Switching Voltage:** The insulation must be able to withstand the switching voltage without experiencing breakdown or flashovers. This requires selecting appropriate insulation materials with high dielectric strength.

b) **Lightning Voltage:** EHV lines can experience high voltage surges due to lightning strikes. While lightning voltage is an important consideration in insulation design, it is not the primary factor in this case.

c) **Corona:** Corona discharge can occur when the electric field strength in the vicinity of the conductor exceeds a certain threshold. It can cause power losses and damage to the insulation. Although corona is an important consideration in insulation design, it is not the primary factor in this case.

d) **Radio Interference:** Insulation design should also consider minimizing radio interference caused by the EHV lines. However, this is not the primary factor in determining the insulation design.

**4. Conclusion**
In conclusion, the insulation of modern EHV lines is primarily designed based on the switching voltage. This is because the insulation must be capable of withstanding the transient overvoltages that occur during switching operations. By considering the switching voltage, appropriate insulation materials and insulation thickness can be selected to ensure the reliability and performance of the EHV line.