Question

A 2850 hp, 4 kV induction motor is connected to the supply system through 2.5 MVA transformer, 13.8:4 KV with a reactance of 5.6%. The motor full-load current is 362 A and its locked-rotor current is 1970 A. The supply system short-circuit MVA at the 13.8 kV terminals of the transformer is 431 maxi. mum, 113 minimum, on 100 MVA base. Determine if a phase-instantaneous overcurrent relay can be applied if it is set at half the minimum fault current and twice the locked-rotor current.

Answer 1

The Transmission System d 132 kv. from *as 0 as 4 $200MW, SOMVAY shown in 900MW ; 20% ممكن 132.ku large 132ku power system joor j zor 4 Z 100HN, SOMVAYL @ N values. Sbose Z (P.No :Z (r) x base 20 x 100 i 1922 y (pe? . nj 8.6112 (9.) 100 100 2919 20.5 100 la Poze L 0, os 759 1

القولون sau also ocurg home hvid 13.8kv/uku/ 9.5MVA as we fox to calcubrity for @ motor t min faut current over orthamal Reday Gorrent fas flows to operats when through it a) Given minimum short Girwit MVA on 13.8k. Side E/P terminal 113 m VA Vo v IL 13 MUA Assume Geory thing ij skao Lonneked ) IK : 113803 19138 . 4. 72 x * S3 x 13.8 x 103 Ils & 16 310.14A. IL? 4.72 KA h 727.57A

June The minimum faut cunent on Grimary The minimum fault current ý grater than motor Induction - load corrent . 16310 A > 3C9A. - so current current that adocy can set for minimum fault which į grater than full load But it į always suggested relay set should be as follows. I full load alf) vday < (= fall ) min 6 Blocked rotor current . 1970A Toice the blocked rotor corrent -3940A

Blocked rotor current , full load correndo so rebay will not operate for full load Corrent in Both cases. 1. In this case velay can be set for blocked rotor corrent