What are the advantages and disadvantages of overhead and underground transmission line?

Cost is the major factor. For long transmission system, and higher voltages, overhead are more cost effective. For local distribution, Under Ground Cables System is advantageous for planning , security/safety and many other reasons..

Underground distribution pros: - esthetically preferable - requires smaller right-of-way - right-of-way can be shared - fewer problems with vegetation, automobile accidents, lightning, or vandalism Underground distribution cons: - higher cost of construction - higher repair cost - potential risk from careless digging