Method for realizing metal longitudinal differential protection of parallel multi-terminal direct-current power transmission system

The invention discloses a method for realizing metal longitudinal differential protection of a parallel multi-terminal direct-current power transmission system, and relates to the technical field of direct-current power transmission systems. A first convertor station, a second convertor station and a third convertor station are respectively provided with a metal loop longitudinal differential protection device, and the third convertor station is also provided with a metal loop fault line selection device; and the third converter station also has a confluence bus area and is provided with a rapid isolation switch. The method comprises the steps that a metal loop longitudinal differential protection device of the converter station triggers a corresponding protection action according to the current of an antipode direct-current line of the converter station and the current of an antipode direct-current line of an opposite station; and the metal loop fault line selection device judges thefault type of the metal l