Protective relay apparatus

A protective relay system for providing pilot protection for an alternating current transmission line section having circuit interrupting means at its near and far ends to be protectively controlled, comprising:

• line current deriving means at the near and far ends of the line section, providing near and far composite sequence signals responsive to the magnitudes and phases of the line currents at the near and far ends, respectively,
• communication means communicating the far signal to the near end,
• first means providing an operating quantity in response to the absolute value of the vector combination of said near and far signals, with said vector combination being selected such that the absolute value thereof increases when a fault occurs in the protected section,
• second means providing a restraining quantity in response to the absolute value of at least one of said near and far signals,
• and third means modifying one of said quantities with a predetermined constant such that the difference between the quantities is indicative of a fault in the protected section when said difference bears a predetermined relationship with a predetermined pick-up value, with the magnitude of said modifying constant being determinative of the tripping characteristic of the protective relay system.