REF RELAY SETTING CALCULATION

Transformer Data :

  8 MVA , 6.6KV/ 415 ,  Impedance = 0.0835 p.u., Tap + 10% ( 440Kv – 360KV

CTR = 800 /1

Relay  : CAG 14 / P120

 

                                Fault MVA = MVA  / Transformer Impedance

                                        =  8 / 0.0835 = 95.8084

Fault Current  =    Fault MVA  x 1000 / ( 1.732 x  Voltage rating )

                      =   95.8084 x 1000 ( 1.732 x 6.6 )

                      =    8281.06  Amp primary

                          = 8281.06 / 800  = 10.4763  Amp Secondary

Fault Voltage developed across the relay   =  fault current  x loop resistance

 at the time of Maximum fault conditions

                                    V s       =    If ( Rct + 2 RL)     RL  = CT Lead Resistance

                                                 =   10.4763  x (  Rct + 2 RL)

                                                =    10.4763  x ( 4 + 2)  ( assumed RL = 1 ohms)

                                                =     62.8579 volts

The relay has to be set at   Is = 0.1 In for maximum sensitivity

The stabilizing resistor shall be set at value of resistance during fault minus the relay resistance

                                                =   62.85    -    1 VA

0.1                            (0.1) Square       

                                            = 628.5      -  100

                                                    R     =     528.5 ohms