Which of the following is NOT considered a problem produced in the EHV transmission line? 

Explanation:

Surge impedance loading is the maximum power that can be transmitted over a transmission line without causing overvoltage or instability. It is not a problem that is specific to EHV transmission lines. In fact, EHV transmission lines have a higher surge impedance loading than lower voltage transmission lines.

The other three options are all problems that can occur in EHV transmission lines:

Corona loss and radio interference:

Corona is a discharge that occurs around conductors at high voltages. It can cause power loss and interference with radio and television signals.

Heavy supporting structures and erection difficulties: 

EHV transmission lines require heavy supporting structures to support the weight of the conductors. This can make them difficult and expensive to erect.

Insulation requirements: 

EHV transmission lines require high insulation levels to prevent breakdown. This can also make them expensive.