power line vs transmission line

power line and transmission line both are nouns.

Word	Noun	Adjective	Verb	Adverb
power line	Yes	No	No	No
transmission line	Yes	No	No	No

As nouns, transmission line is a hypernym of power line; that is, transmission line is a word with a broader meaning than power line:

power line: cable used to distribute electricity
transmission line: a conductor for transmitting electrical or optical signals or electric power

Other hypernyms of power line include cable, line.

power line (noun)	transmission line (noun)
cable used to distribute electricity	a conductor for transmitting electrical or optical signals or electric power