relay setting calculation

Relay setting Calculation

Base MVA=100Fault MVA=1200Source impedance=0.083Cable impedance=.015*4=0.06 ohmTransformer impedance=11 % at 20 MVA base =0.11*100/20=0.55 ohmTotal fault impedance at 6 kV Bus=0.083+.06+.55=0.693 ohm

Fault MVA= 100 MVA/0.693=145 MVA=150 MVA (approx)

3 phase fault current at 6 kV bus=150/1.732*6 =14 KA

Outgoing Feeders setting: 100%, 0.1, CT Ratio=800/5

PSM=14000/800=17.5

Relay opt time based on Std Curve= 0.014/[(PSM)0.02-1] = 2.37

Operating time =2.37*0.1=0.237 s=237 ms

E/F setting: 20%, 0.1

PSM=14000/160=87.5=20(max)

Relay operating time on std formula=0.14/(20)0.02-1= 2.26

Actual operating time= 2.26*0.1=226 ms

Incomer feeder setting: 115%, 0.25, CT Ratio=2000/5

Full load current=1848 amp at 6kv Considering 25% overloading, load current=2310 amp=2300amp (approx)

PSM= 14000/2300=6.086 Relay operating time based on std formula=0.14/(6.086)0.02-1=3.806Actual operating time=3.806*0.25=950 ms


PSM=14000/400=35=20 (max)Relay operating time based on std formula=2.26

Actual operating time=2.26*0.4=906 msBus Section Feeder: 115%, 0.18

PSM=14000/2300=6.086

Actual operating time=3.806*0.18= 685 ms


Actual operating time= 2.26*0.3=678 ms

Stand by E/F: 20%, 0.55

Actual operating time=2.26*0.55=1243 ms

Bus-zone setting: 0.2,200 ohm

Fault current=14000 amp

B/Z CTR=800/1

Rct (Max) =2.3 ohm

Vk (Min) =242 V

Sec Fault current=14000/800=17.5 A

Voltage across CT=17.5*2.3=40.25 V

If current setting =0.2,Rstab=200 ohm

HVREF setting: 0.75A, 35 ohmFault current at 33 kV side = (14000*6/33) =2545 amp

CTR=400/5Rct (max) =0.55 ohmVk (min) =133 voltVk/2=65 volt

Sec current=2545 *5/400=31.8 amp

Voltage across CT=31.8*0.55=17.49 voltIf relay setting =0.75 ampRstab=35 ohmVoltage across CT=23 volt

LVREF setting: 1A, 50 OHM

Fault current at 6kv side=14000 amp

CTR=2000/5

Rct (max) =0.925 ohmVk (min) =293 volt

Sec ct=14000*5/2000=35 amp

Voltage across CT=35*0.925=32volt

If current setting=1 ampRstab=50 ohmVoltage across CT during fault=50*1=50 volt

relay setting calculation

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