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Calculate IDMT over Current Relay Setting (50/51)


  • Calculate setting of  IDMT over Current Relay for following Feeder and CT Detail
  • Feeder Detail: Feeder Load Current 384 Amp, Feeder Fault current Min11KA and Max 22KA.
  • CT Detail:  CT installed on feeder is 600/1 Amp. Relay Error 7.5%, CT Error 10.0%, CT over shoot 0.05 Sec, CT interrupting Time is 0.17 Sec and Safety is 0.33 Sec.
  • IDMT Relay Detail:
  • IDMT Relay Low Current setting: Over Load Current setting is 125%, Plug setting of Relay is 0.8 Amp and Time Delay (TMS) is 0.125 Sec, Relay Curve is selected as Normal Inverse Type.
  • IDMT Relay High Current setting :Plug setting of Relay is 2.5 Amp and Time Delay (TMS) is 0.100 Sec, Relay Curve is selected as Normal Inverse Type

Calculation of Over Current Relay Setting:

(1)  Low over Current Setting: (I>)

  • Over Load Current (In) = Feeder Load Current X Relay setting = 384 X 125% =480 Amp
  • Required Over Load Relay Plug Setting= Over Load Current (In) / CT Primary Current
  • Required Over Load Relay Plug Setting = 480 / 600 = 0.8
  • Pick up Setting of Over Current Relay (PMS) (I>)= CT Secondary Current X Relay Plug Setting
  • Pick up Setting of Over Current Relay (PMS) (I>)= 1 X 0.8 = 0.8 Amp
  • Plug Setting Multiplier (PSM) = Min. Feeder Fault Current / (PMS X (CT Pri. Current / CT Sec. Current))
  • Plug Setting Multiplier (PSM) = 11000 / (0.8 X (600 / 1)) = 22.92
  • Operation Time of Relay as per it’s Curve
  • Operating Time of Relay for Very Inverse Curve (t) =13.5 / ((PSM)-1).
  • Operating Time of Relay for Extreme Inverse Curve (t) =80/ ((PSM)2 -1).
  • Operating Time of Relay for Long Time Inverse Curve (t) =120 / ((PSM) -1).
  • Operating Time of Relay for Normal Inverse Curve (t) =0.14 / ((PSM) 0.02 -1).
  • Operating Time of Relay for Normal Inverse Curve (t)=0.14 / ( (22.92)0.02-1) = 2.17 Amp
  • Here Time Delay of Relay (TMS) is 0.125 Sec so
  • Actual operating Time of Relay (t>) = Operating Time of Relay X TMS =2.17 X 0.125 =0.271 Sec
  • Grading Time of Relay = [((2XRelay Error)+CT Error)XTMS]+ Over shoot+ CB Interrupting Time+ Safety
  •  Total Grading Time of Relay=[((2X7.5)+10)X0.125]+0.05+0.17+0.33 = 0.58 Sec
  • Operating Time of Previous upstream Relay = Actual operating Time of Relay+ Total Grading TimeOperating Time of Previous up Stream Relay = 0.271 + 0.58 = 0.85 Sec

(2)  High over Current Setting: (I>>)

  • Pick up Setting of Over Current Relay (PMS) (I>>)= CT Secondary Current X Relay Plug Setting
  • Pick up Setting of Over Current Relay (PMS) (I>)= 1 X 2.5 = 2.5 Amp
  • Plug Setting Multiplier (PSM) = Min. Feeder Fault Current / (PMS X (CT Pri. Current / CT Sec. Current))
  • Plug Setting Multiplier (PSM) = 11000 / (2.5 X (600 / 1)) = 7.33
  • Operation Time of Relay as per it’s Curve
  • Operating Time of Relay for Very Inverse Curve (t) =13.5 / ((PSM)-1).
  • Operating Time of Relay for Extreme Inverse Curve (t) =80/ ((PSM)2 -1).
  • Operating Time of Relay for Long Time Inverse Curve (t) =120 / ((PSM) -1).
  • Operating Time of Relay for Normal Inverse Curve (t) =0.14 / ((PSM) 0.02 -1).
  • Operating Time of Relay for Normal Inverse Curve (t)=0.14 / ( (7.33)0.02-1) = 3.44 Amp
  • Here Time Delay of Relay (TMS) is 0.100 Sec so
  • Actual operating Time of Relay (t>) = Operating Time of Relay X TMS =3.44 X 0.100 =0.34 Sec
  • Grading Time of Relay = [((2XRelay Error)+CT Error)XTMS]+ Over shoot+ CB Interrupting Time+ Safety
  • Total Grading Time of Relay=[((2X7.5)+10)X0.100]+0.05+0.17+0.33 = 0.58 Sec
  • Operating Time of Previous upstream Relay = Actual operating Time of Relay+ Total Grading Time.
  •  Operating Time of Previous up Stream Relay = 0.34 + 0.58 = 0.85 Sec

Conclusion of Calculation:

  • Pickup Setting of over current Relay (PMS) (I>) should be satisfied following Two Condition.
  • (1) Pickup Setting of over current Relay (PMS)(I>) >= Over Load Current (In) / CT Primary Current
  • (2) TMS <= Minimum Fault Current / CT Primary Current
  • For Condition (1) 0.8 > =(480/600) = 0.8 >= 0.8, Which found  OK
  • For Condition (2) 0.125 <=  11000/600 = 0.125 <= 18.33,  Which found  OK
  • Here Condition (1) and (2) are satisfied so
  • Pickup Setting of Over Current Relay = OK
  • Low Over Current Relay Setting: (I>) = 0.8A X In Amp
  • Actual operating Time of Relay (t>) = 0.271 Sec
  • High  Over Current Relay Setting: (I>>) = 2.5A X In Amp
  • Actual operating Time of Relay (t>>) = 0.34 Sec
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