Introduction:
Differential protection is a commonly used method for protecting busbars in electrical power systems. It involves the use of current transformers (CTs) to measure the current flowing through the busbars and detect any fault conditions. The CT ratio plays a crucial role in the proper functioning of differential protection.

Explanation:

CT Ratio:
The CT ratio refers to the ratio between the primary current input to the secondary current output of the CT. It determines the transformation ratio of the CT and is an essential parameter for accurate current measurement.

Primary Current:
The primary current refers to the current flowing through the busbars or feeders that are being protected by the differential protection scheme. Each feeder has its own primary current, which may vary depending on the load and configuration of the system.

Secondary Current:
The secondary current refers to the current output of the CT, which is proportional to the primary current. It is the current that is measured and used for differential protection.

Explanation of Options:
a) Based on total expected fault current:
This option suggests that the CT ratio is determined based on the total expected fault current. However, this is not correct as the CT ratio is not directly related to the fault current.

b) Equal and based on primary current of lowest current carrying feeder:
This option suggests that all the CT ratios are equal and based on the primary current of the feeder with the lowest current. This is not accurate as CT ratios should be determined based on the actual primary current of each feeder, not just the lowest one.

c) Based on their respective feeder current:
This option suggests that the CT ratio for each feeder is based on its respective current. This is partially correct, as the CT ratio should indeed be determined based on the primary current of each feeder.

d) Based on primary current input to secondary current output at full load:
This option is the correct answer. The CT ratio should be determined based on the primary current input to the secondary current output at full load. This means that the CT should be able to accurately measure the primary current and provide a proportional secondary current output under normal operating conditions.

Conclusion:
In conclusion, the CT ratio in the differential protection of busbars is based on the primary current input to the secondary current output at full load. This ensures accurate measurement of the primary current and proper functioning of the differential protection scheme.