DIFFERENTIAL PROTECTION DEVICE FOR ELECTRICAL INSTALLATION Russian patent published in 1994 - IPC

FIELD: electrical engineering. SUBSTANCE: differential signal U_d is applied to input of functional adapter. Signal G proportional to level of higher harmonics is separated by means of high frequency filter. After conversion, signal G is compared with U_d by means of threshold element. Positive signal on output of threshold element (which is also output of functional adapter) appears at short circuit outside zone under protection. This signal opens key element and shunts out current relay, thus providing desensitization of protection and reliable tuning out at external short circuits. Under internal short circuit conditions relationship between U_d and converted signal G is other, control signal at functional adapter output is negative, key element is open and protection operates at lower setting which provides higher sensitivity, as in this case setting is chosen to provide desensitizing from normal duty unbalance current, and not from emergency current. EFFECT: enhanced reliability of protection. 2 dwg