| 摘要 |
PURPOSE:To eliminate inadvertent detection of defective VT, even in case of multiple fault spreading to the outside of protective section, by locking a circuit for detecting defective instrument transformer (VT) based on three-phase fault detecting conditions. CONSTITUTION:When a relay is tripped by single-phase fault of system and reclosed upon elapse of a reclose no-voltage time, it is reopened by a sustaining ground fault. A defective VT detecting/locking circuit having a route of an OR logic 9a, a NOT logic 5c and an AND logic 6d locks unnecessary response of a distance relay on condition that single open-phase prevails during no-voltage interval-and zero-phase voltage is 'present' while furthermore zero-phase current is 'not present' for no power flow. Consequently, trip lag due to unnecessary detection of defective VT caused by collapse of coordinated detection timing of zero-phase voltage and zero-phase current can be prevented when a transition takes place from unbalanced fault of internal and external successive faults to a balanced fault. |
