At transmission line voltages up to 230 kV the insulation level is decided by the requirement of protection against lightning. In EHV, switching surges cause higher over voltages than lightning and therefore mainly responsible for insulation level decision.

Insulation Level of 400 KV EHV Overhead Transmission Line

Insulation level of an EHV overhead transmission line is decided based on various factors. The most important factor for deciding the insulation level of a 400 KV, EHV overhead transmission line is switching overvoltage.

Switching Overvoltage

Switching overvoltage occurs when a circuit breaker or switch is opened to interrupt the power flow in the transmission line. During the interruption, a high voltage surge is generated due to the sudden change in the electric field. This surge can cause insulation breakdown, resulting in flashovers or punctures in the insulation material.

To prevent insulation breakdown, the insulation level of the transmission line must be designed to withstand the switching overvoltage. The insulation level is decided based on the highest switching overvoltage that the line is expected to experience during its lifetime.

Other Factors

Apart from switching overvoltage, other factors that can influence the insulation level of an EHV overhead transmission line are:

- Lightning overvoltage: Lightning strikes can generate very high voltages that can cause insulation breakdown. The insulation level of the line must be designed to withstand lightning overvoltage.
- Corona inception voltage: When the voltage on the transmission line exceeds a certain level, corona discharge can occur. Corona discharge can cause radio and TV interference, as well as insulation breakdown. The insulation level of the line must be designed to prevent corona inception.
- Radio and TV interference: Corona discharge and other electrical disturbances can cause radio and TV interference. The insulation level of the line must be designed to minimize interference.

Conclusion

In summary, the insulation level of a 400 KV, EHV overhead transmission line is decided based on the highest switching overvoltage that the line is expected to experience during its lifetime. Other factors such as lightning overvoltage, corona inception voltage, and radio and TV interference must also be taken into account. The insulation level must be designed to withstand all these factors and ensure safe and reliable operation of the transmission line.

The severity of Switching Surge for voltage 400kv & above is much more than due to Lightning Voltage.