Protection Engineering Basics and Schemes Protection Engineering | Page 116

PSM here is 1.1; CT ratio is 400 / 1, so tripping time will be 0.05 t = 0.14 ∗

1.1 0. 02 − 1 t = 3.66872s Tripping time is decreasing as PSM increasing, TMS is Fixed once in relay, it cannot be change like PSM during fault condition, PSM is changing because it defines the ratio of actual current to secondary current and actual current will raise during abnormal condition, but we can change TMS at any time except at fault time as per requirement of relay coordination or time grading.

Note: Time grading is actually the methodology by which we scaled Protection pick up timing of our protection schemes, time grading help us to prevent from unnecessary load isolation, wrong fault location and wide area fault trace inspection. Time grading also help us to limit fault at the faulty equipment / loop else fault current