Powerline clearances

[KillBill7]

My understanding is that either lightning flashover(BIL) or switching surges(BSL) govern the required phase to ground and phase to phase clearances based on the system voltage levels. Based on some reseach , it seems like at the voltages below 220kV, lightning performance dictates the required insulation and above 220kV its switching surges.

However, when I look at the North American OH line standards like NESC or CSA C22.3 No 1, the required phase to ground and phase to phase clearances are derived from switching surges for the all the voltage levels. Can someone shed some light on this? Do they assume exaggerated switching surge factor that would cover any over-voltage impulse due to lightning phenomena.

 

[waross]

Corona effects?
The maximum voltage for a given spacing on a long transmission line is often limited by corona.
If you Google six phase transmission schemes you will see examples of the relationship between line to line distances and increased allowable voltages due to reduced corona effects.
If a two circuit line, (one circuit vertically on each side of the tower), is converted to a single six phase circuit, the allowable voltage in regards to corona may be increased 173%.
This is not intended as a suggestion that more six phase circuits should be used, but to illustrate a practical real world conditions where line spacing is dependent on corona effects.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[KillBill7]

But Corona losses can be controlled by appropriately sizing the conductors. Corona loss on a conductor is a function of the conductor surface gradient and surface gradient depends on the system voltage, distance between phases and conductor diameter.
By using larger conductor sizes (bigger conductor or bundle configuration) the effect of corona can be reduced and it's probably more economical to rather use bigger conductor than to use bigger structures.

I believe RUS transmission design manual lists minimum conductor sizes for each voltage level based on corona and radio noise considerations.

 

[waross]

Corona loss on a conductor is a function of the conductor surface gradient and surface gradient depends on the system voltage, distance between phases and conductor diameter.

Yes, I agree.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[bacon4life]

Some of the clearance tables have precalculated clearance values for MV/HV circuits whereas for EHV/UHV circuits they just provide formulas for performing the calculations. What indicates to you that the NESC uses switching surges for lower voltage clearances? I had assumed the precalculated clearances were based on lighting rather than switching.