In: Electrical Engineering
(a) What is a high impedance relay and when is it used?
(b) A motor with differential protection has a full load current of 200 A. When starting, the motor takes 5 × FL current. The motor is remote from the switchgear which houses one set of CTs and the relay. If the CT ratios are 200/1, the secondary winding resistance is 1.0 ohm and the protection loop lead resistance to the motor is 6 ohms, calculate the minimum relay setting voltage. State what the CT knee point voltage should be.