The figure shows a distribution substation with a 25MVA (max), 69/12.47kV, Dyn1 transformer whose short-circuit impedance is 9% with an X/R=15. The three-phase short-circuit current at the 69kV is known to be 7kA and the equivalent impedance of the transmission system can be assumed to have X/R=5. The phase and ground relay element settings have been partially calculated, as shown in the table on the attachment. All relays have a U3 type curve (handout in the attachment). You can use CI=0.3 sec.
You are requested to perform the following tasks:
1. Calculate the current for a three-phase and single-phase-to-ground fault at a point close to one of the feeders’ breakers.
2. Determine the time dial and instantaneous setting of the ground relay element associated with the main circuit breaker.
3. Determine the time dial and instantaneous setting of the phase relay elements associated with the 69kV circuit breaker.
4. Assuming that the transformer damage curve given in class can be applied, plot the transformer damage curve and the phase relay elements you calculated in part 3. The current axis must be in amps referred to 69kV. Indicate –graphically– all significant elements (times, currents) used to determine your settings in part 3