| 摘要 |
PROBLEM TO BE SOLVED: To prevent erroneous operation of a current differential relay due to CT saturation completely by discriminating overcurrent by means of an overcurrent detection element, and locking the current differential relay when it is judged that proportion is lower than a threshold value by means of a percentage detection means. SOLUTION: An overcurrent detection element 1a discriminates generation of overcurrent if it is in excess of a threshold value. Threshold value is set to a larger value than load current running at the time of ordinary operation. A proportion detection element 1b judges it as external failure if proportion is in excess of a threshold value and it as internal failure if the proportion is less than the threshold value. External failure is judged by using the proportion detection element 1b which detects the proportion of a sum of the magnitude of differential current and those of respective currents for the current running through a system in addition to the overcurrent detection element 1a; therefore, it can exactly be judged whether or not differential current flows regardless of the magnitude of current. By locking a current differential relay based on the exact judgment, it is possible to prevent erroneous operation of the current differential relay. |
