## 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

## Voltage Rise in Transformers due to Capacitor Bank:

• The voltage drop and rise on the power line and drop in the transformers. Every transformer will also experience a voltage rise from generating source to the capacitors. This rise is independent of load or power factor and may be determined as follows:
• ### % Voltage Rise in Transformer=(Kvar / Kva)x Z

• Kvar =Applied Kvar
• Kva = Kva of the transformer
• z = Transformer Reactance in %
• Example: 300 Kvar bank given to 1200 KVA transformer with 5.75% reactance.
• % Voltage Rise in Transformer=(300/1200)x 5.75 =1.43%

## Standard Size of Transformer (IEEE/ANSI 57.120):

 Single Phase Transformer Three Phase Transformer 5KVA,10 KVA,15 KVA,25 KVA,37.5 KVA,50 KVA,75 KVA,100 KVA,167 KVA,250 KVA, 333 KVA,500 KVA,833 KVA,1.25 KVA,1.66 KVA,2.5 KVA,3.33 KVA,5.0 KVA,6.6 KVA,8.3 KVA,10.0 KVA,12.5 KVA,16.6 KVA,20.8 KVA,25.0 KVA,33.33 KVA 3 KVA,5 KVA,9 KVA,15 KVA,30 KVA,45 KVA,75 KVA,112.5 KVA,150 KVA,225 KVA,300 KVA,500 KVA,750 KVA,1MVA,1.5 MVA,2 MVA,2.5 MVA,3.7 MVA,5 MVA,7.5MVA, 10MVA ,12MVA,15MVA,20MVA ,25MVA, 30MVA,37.5MVA ,50MVA ,60MVA,75MVA,100MVA

## Standard Size of Transformer:

 Standard Size of Transformer KVA Power Transformer (Urban) 3,6,8,10,16 Power Transformer (Rural) 1,1.6,3.15,5 Distribution Transformer 25,50,63,100,250,315,400,500,630

## Impedance of Transformer (As per IS 2026):

 MVA % Impedance < 1 MVA 5% 1 MVA to 2.5 MVA 6% 2.5 MVA to 5 MVA 7% 5 MVA to 7 MVA 8% 7 MVA to 12 MVA 9% 12 MVA to 30 MVA 10% > 30 MVA 12.5%

## Ref: KSEI Handbook

Rating of T/C (KVA) Primary current (Amp) Secondary Current (Amp) Min. Size of Neutral Earthing Conductor (mm2) Minimum Size of Cable (mm2)
63 3.3 84 25X3 50mm2
100 5.25 133.3 25X3 95mm2 or (2×50 mm2)
160 8.4 213.3 25X3 185mm2 or (2×95 mm2)
200 10.49 266.6 25X3 300mm2 or (2×120 mm2)
250 13.12 333 25X3 2×185 mm2
315 16.53 420 31X3 or 25X4 (2×300 mm2) or (3×185 mm2)
400 21.80 533 38X3 (3×300 mm2) or (2×400 mm2)
500 26.20 666.5 25X6 (3×400 mm2) or (4×240 mm2)
630 33 840 31X6 4×400 mm2
750 39.36 1000 50X4 Bus Bar Trucking (min. Isc 50KA)
1000 52.50 1333 210mm2 Bus Bar Trucking (min. Isc 50KA)
1250 65.50 1667 290mm2 Bus Bar Trucking (min. Isc 50KA)
1600 83.98 2133 380mm2 Bus Bar Trucking (min. Isc 50KA)
2000 105.00 2666 450mm2 Bus Bar Trucking (min. Isc 50KA)

## HT Fuse on Primary Side of Transformer (11KV/433V)

Rating of T/C (KVA) Primary current (Amp) Secondary Current (Amp) HT Fuse
Min (Amp) Max(Amp)
63 3.3 84 10 16
100 5.25 133.3 16 25
160 8.4 213.3 16 40
200 10.49 266.6 25 40
250 13.12 333 32 40
315 16.53 420 40 63
400 21.80 533 40 63
500 26.20 666.5 50 100
630 33 840 63 100
750 39.36 1000 75 160
1000 52.50 1333 100 160
1250 65.50 1667 100 200
1600 83.98 2133 160 250
2000 105.00 2666 200 250

## 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 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 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

## Size of Capacitor for P.F Correction:

 For Motor Size of Capacitor = 1/3 Hp of Motor ( 0.12x KW of Motor) For Transformer < 315 KVA 5% of KVA Rating 315 KVA to 1000 KVA 6% of KVA Rating >1000 KVA 8% of KVA Rating