High speed bus differential relay

Claim 1

In a high-speed differential protective arrangement for protecting a power circuit from faults internal to a predetermined zone of the power circuit, said power circuit having connected thereto in said zone a lightning arrestor which is adapted to spark over in response to an overvoltage and to thereafter allow follow-through current to flow therethrough typically for times equal to or less than a predetermined maximum period, (a) means for developing a first voltage varying directly with the vector sum of the currents flowing into and out of said zone, (b) a filter energized by said first voltage and developing a second voltage generally free from any aperiodic component of said first voltage, (c) rectifying means energized by said second voltage and having output terminals across which a unidirectional third voltage varying directly in magnitude with said second voltage is developed, (d) a relay having an operating element connected across the output terminals of said rectifying means for energization by said third voltage, (e) a zener diode connected across the output terminals of said rectifying means in parallel with said relay operating element, (f) said relay being operable when the voltage V 0 developed by said third voltage across said operating element is equal to or greater than a predetermined minimum pick-up level V p .u. but remaining inactive at lower values of V 0 , (g) said zener diode having a zener voltage slightly higher than a value of voltage applied thereto that results in a voltage V 0 equal to said predetermined minimum pick-up level being applied to said operating element, (h) said zener voltage being sufficiently low that during lightning arrestor follow-through current the voltage developed across said zener diode substantially equals said zener voltage, and (i) said relay having a sufficiently long pick-up time to prevent relay operation in response to lightning arrestor operations that result in follow-through currents persisting for times equal to said predetermined maximum period.