CT Details: CT Ratio : 600-400/1 A CT Class : 5P20 Selected Ta : 400 A Rated Burde : 0.1 VA Relay Type : REF 545 Note: Setting for the Base Angle: Grid Impedance calculations Since, the system is solidly grounded, to have maximum sensitivity X/R ratio of the remote grid = 10 (As per Spec.) Magnitude of grid Impedance = Voltage / Short Circuit Current rating System Nominal Voltage = 33 kV Max. PE S/C current at the remote 33kV Grid = 3.95 kA = 3.95 kA Min. PE S/C Current at the remote 33kV Grid = 2 kA = 2 kA 1During Min. Grid Fault level condition = 33/(sqrt(3) x 2.00) = 9.526 Ω 2 During Max. Grid fault condition = 33/(sqrt(3) x 3.95) = 4.823 Ω 3 Hence, Impedance of the grid during Min. Fault condition, Z gmin = 9.526*(cos(atan(10)) + j sin((atan(10)) = 0.948+9.479i 4 Hence, Impedance of the grid during Max. Fault condition, Z gmax = 4.823*(cos(atan(10))+jsin((atan(10)) = 0.48+4.799i 5 Positive Sequence Impedance of the incoming cable Z c1, Z c2 & Z c3 = 9.8E-002+0.107i Ω/km Zero Sequence Impedance of the incoming cable Z c1, Z c2 & Z c3 = 0.44+7E-002i Ω/km 6Positive Sequence Impedance for a length of 5.4 km of the cable = 0.5292+0.5778i Ω/km Zero Sequence Impedance for a length of 5.4 km of the cable 2.376+0.378i Ω/km Typical Fault resistance value = 0.200 Ohm Impedance seen by directional relay ( direction is tow ards grid) for minimum & maximum fault ANGLE SETTINGS FOR 33kV INCOMING CABLE FOR DIRECTIONAL OVERCURRENT PROTECTION 1) This relay acts as a Back-up to the cable different ial as a primary one, using its 67 IDMT Characteri stic. Page 1 of 3
