Protection Engineering Basics and Schemes Protection Engineering | Page 136

I short circuit secondary = I SCL 1000

= 10.52632 ∗

1.732 ∗ 0.433

I SCL = 14035. 9A

CT Ratios:

We can take CT with ratio ranging minimum from 50 / 1. Accuracy limit factor ranges minimum from 10 I took 200 / 1 CT with class 5P20

Time Over current Relay setting( 51);

We selected 120 % of Primary short circuit current as our pick up value.

I PFL = 26.24A Pick up phase = 120 % of 26.24 Pick up phase = 31.488A

CT ratio is 200 / 1, pick up at relay will be 31. 488 I >= Pick up phase = = 0. 15744 ≅ 0. 16A 200 since I PSC = 552.5A

I PSC

PSM =

= 552. 5 = 17. 5A

120 % of I PFL 31.488 If our time grading is 0.3sec, and our expected tripping time is 0.3sec also( which means our t = 0.6sec), our

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