No street railroad corporation, gas corporation, electrical corporation, telephone corporation or water corporation, shall henceforth begin the construction of a street railroad, or of a line, plant, or system or of any extension of such street railroad, or line, plant, or system, without having first obtained from the commission a certificate that the present or future public convenience and necessity require or will require such construction: provided, that this section shall not be construed to require such corporation to secure such certificate for an extension within any city or county, within which it shall have theretofore lawfully commenced operation, or for an extension into territory whether within or without a city or county, contiguous to its street railroad, or line, plant or system, and not theretofore served by a public utility of like character, or for an extension within or to territory already served by it necessary in the ordinary course of its business: and provided further, that if any public utility in constructing or extending its lines, plant or system, shall interfere or be about to interfere with the operation of the line, plant or system of any other public utility already constructed, or if public convenience and necessity does not require or will require such construction or extension, the commission on complaint of the public utility claiming to be injuriously affected, or on the commission’s own motion, may, after hearing, make such order and prescribe such terms and conditions for the locating or type of the line, plant or system affected as to it may seem just and reasonable: provided, that power companies may, without such certificate, increase the capacity of their existing generating plants.