IDMT RELAY CO-ORDINATION

By

Y. k. Pandharipande.

IDMT RELAY CO-ORDINATION

IDMT RELAY CO-ORDINATION

Rated Current = 87.5 A (66KV) / 525 A (11KV) S.C. Current for Fault on LV side (assuming infinite source) = Rated Current x 100 / % Zt = 525 x 100 / 10 = 5250 A (11 KV side) = 87.5 x 100 / 10 = 875A (66 KV side) IDMT Relay with Normal Inverse Characteristic (IEC) 0.02 Operating time (t) = (0.14 / ( I - 1) ) x TMS where I = Plug Setting Multiplier (PSM) t @ PSM 10 / TMS 1.0 = 3.0 S t @ PSM 20 / TMS 1.0 = 2.2 S t @ PSM 8.75 / TMS 1.0 = 3.16 S.

IDMT RELAY CO-ORDINATION

11KV O/G FDR (R1)

O/C Setting= 1A (200A), TMS = 0.10, PSM=10500/200 >20Operating time (t) @ PSM>20 / TMS0.10 = 0.22S 11KV B/C (R2)

O/C Setting = 1A (600A), PSM= 5250/600 = 8.75Desired Operating Time (DOT)= 0.22 + 0.30 = 0.52SOperating time @PSM 8.75 / TMS 1.0 = 3.16STMS to achieve DOT of 0.52S = 0.52/3.16 = 0.16 11KV I/C (R3)

O/C Setting = 1A (600A), PSM = 5250 /600 = 8.75DOT = 0.52 + 0.30 = 0.82SOperating time @ PSM8.75 / TMS 1.0 = 3.16STMS to achieve DOT of 0.82S = 0.82/3.16 = 0.26

IDMT RELAY CO-ORDINATION 66 KV SIDE OF TFR (R4)

O/C Setting = 1A (100A), PSM = 875 / 100 = 8.75DOT = 0.82 + 0.30 = 1.12SOperating time @ PSM 8.75 / TMS 1.0 = 3.16STMS to achieve DOT of 1.12S = 1.12 / 3.16 = 0.35

Note :

1. Grading between Relay R3 & R4 is optional since it does not affect down-stream co-ordination. If foregone, identical TMS can be adopted for Relay R4 (TMS 0.26) with similar response time of 0.82S as relay R3. This would reduce the up-stream fault clearance time.

2. Grading margin of 0.3S is used in this example considering EM IDMT relays. With Numerical relays, grading margin can be reduced to 0.2S, in view of reduced timing errors and no over-shoot.