The use of unit protection where differential relays areprotecting transformers. For each of the circuit breaker configurations:

(i) CB A open and CB B closed

(ii) CB A closed and CB B open(all other CBs closed)

(a) To a base of 10 MVA determine initially the fault levels at the 11 kVand 3.3 kV busbars for a fault at the transformer, T1, output asshown.

(b) Determine the current settings and TMSs for the relays shown if theoperating time of the directional overcurrent relays on the 3.3 kV sideof the transformers is 0.5 s.

(Use a 0.4 s discrimination interval.)

State with which of the CB configurations the directional overcurrentrelays (if any) will not operate.

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