The main problems are about voltage and transient stability problems.
The power must be transmitted in a such stable way that there should be more any dangerous case in the overall network. For future information refer to the power system and stability concerning book. I have a book such that
the details are written below:
Author: Prabha Kundur
Name of the book: Power System Stability and COntrol
Edition by EPRI
I think this book or any resembling ones will be very helpful to find your answer..
Comments: In addition to the previous posting:
1. Leakage currents at the overhead transmission lines and insulation quality at the underground transmission lines.
2. HVDC transmission lines need to have a certain minimum length to pay off.
3. Overhead transmission is very vulnerable to the weather conditions, e.g. strong wind, ice, heavy rains, lightnings, direct heat of sun, etc.
4. Overhead transmission is consuming very precious real estate; especially in more populated areas.
5. Transmission line overall efficiency when it comes to energy transmission over long distances.
6. Potential danger to human beings and animals due to stronger electromagnetic radiation, and dropped live conductors lying on the ground.
7. Etc.
