Question 3 a b In the example on restricted earth-fault protection in Section 4.3.6, (lead loop resistance = 10) decide the minimum relay operating voltage if the CT secondary winding resistance is 30 and the standard relay operating voltages are 50, 75, 100, 125, 150 and 175V. Using the curve (a) shown below, calculate the primary current to operate the relay if the mean core diameter is 20cms, the knee-point Bm = 1.2 Telsa and the relay operating current is 10mA. (Assume CT knee-point voltage = 2 x relay setting). 2.0 Flux Density - B max. (Tesla) 1.5 in 1.0 0.5 0.5 (b) 1.0 (c) 1.5 Exciting Forss-(AT/smr.ma) (a) 2.0 50 Hz magnetisation curves (a) Hot-rolled non-oriented silicon steel (b) Cold-rolled oriented silicon steel (c) Nickel-iron (80% nickel)

Fig: 1