When an external fault occurs near the breaker-and-a-half busbar of UHV transmission line

two current transformers may become saturated. This can bring about great difficulty to the current phasor differential protection

which adopts an input current to be the sum of current of two current transformers. This paper proposes a countermeasure to the problem of mentioned current transformer saturation. The proposed scheme can identify the internal and the external faults near the breaker-and-a-half busbar which cause the problem of current transformers saturation by low voltage start-up. Because usually there is no current transformers saturation in the first short data window

it is not difficult to use a threshold to separate the internal faults differential current from the external faults capacitive current near the breaker-and-a-half busbar. Then the external fault mal-operation can be blocked which is caused by current transformer saturation. The proposed scheme has been applied to WXH-803 digital transmission line protection tester and passed EPRI dynamic experiments for 750 kV transmission line protection successfully.