Reciprocating steam engines have been used for mechanical power sources since the 18th Century, with notable improvements being made by James Watt.  The very first commercial central electrical generating stations in New York and London,  in 1882,  also used reciprocating steam engines. As generator sizes increased, eventually turbines took over due to higher efficiency and lower cost of construction.  By the 1920s any central station larger than a few thousand kilowatts would use a turbine prime mover.