power grid vs power system

power grid and power system both are nouns.

Word	Noun	Adjective	Verb	Adverb
power grid	Yes	No	No	No
power system	Yes	No	No	No

As nouns, power grid and power system are synonyms defined as:

power grid and power system: a system of high tension cables by which electrical power is distributed throughout a region

Other synonyms of power grid include grid.

power grid (noun)	power system (noun)
a system of high tension cables by which electrical power is distributed throughout a region	a system of high tension cables by which electrical power is distributed throughout a region