The maximum through fault current below which the scheme remains stable
Isl= Ukpv/2
2* RL + RCT
where, RL= longest CT connection lead Resistance
Rct= internal CT resistance
= 100/2 = 11.26 A
2* (0.2) + 4.04
Fault Current of 3.3kv Bus = 26 KA (say 16.25 A in secondary terms)
Setting Voltage Vs = IF (2* RL + RCT)/CT RATIO = 26000 (2*0.2 + 4.04)/1600 = 72.15 Volts.
Where, IF = fault current
Resistance of the circuit R = VS _ Burden of Relay
IS (Is)2
= 72.15 _ 0.05
0.1 (0.1)2
= 716.5 ohms
(Ipu = 0.1 assumed)
Power rating of resistance is Pr = (Ukpv)2 = (100)2 = 20 W
R 500
Variable resistor of 0-1 kOhms to be provided.
During maximum primary fault current, if the peak voltage developed across the relay should not exceed 1.5 kV, then metrosil is required to suppress the same.
VFS= IF (2*RL + RCT+ stabilizing resistance)
CT RATIO
=26000 (2*0.2 + 4.04+ 716.5)
1600
= 11715.275 volts
Knee point voltage = 100 volts
Peak voltage VPK = 2*(2 VK(VFS – VK))1/2
= 2*(2*100(11715.275-100))1/2
= 3048.32 volts
= 3.04 kilo volts
Since peak voltage is greater than 1.5 kV, metrosil is required.
Setting voltage is less than 125 v, metrosil type 125v R.M.S., 600A/S1/S256 is
