Calculation of resistor in High Imp. REF protection

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