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 Time Operating 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

 

Electrical Thumb Rules-(Part-8)


Accuracy Class of Metering CT:

Metering Class CT

Class Applications
0.1 To 0.2 Precision measurements
0.5 High grade kilowatt hour meters for commercial grade kilowatt hour meters
3 General industrial measurements
3 OR 5 Approximate measurements

 Accuracy Class Letter of CT:

Metering Class CT

Accuracy Class Applications

B

Metering Purpose

Protection Class CT

C

CT has low leakage flux.

T

 CT can have significant leakage flux.

H

 CT accuracy is applicable within the entire range of secondary currents from 5 to 20 times the nominal CT rating. (Typically wound CTs.)

L

 CT accuracy applies at the maximum rated secondary burden at 20 time rated only. The ratio accuracy can be up to four times greater than the listed value, depending on connected burden and fault current. (Typically window, busing, or bar-type CTs.)

 Accuracy Class of Protection CT:

Class Applications
10P5 Instantaneous over current relays & trip coils: 2.5VA
10P10 Thermal inverse time relays: 7.5VA
10P10 Low consumption Relay: 2.5VA
10P10/5 Inverse definite min. time relays (IDMT) over current
10P10 IDMT Earth fault relays with approximate time grading:15VA
5P10 IDMT Earth fault relays with phase fault stability or accurate time grading: 15VA
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