Question

When using differential relaying, why are the CTs connected differently in a 3-phase Y-Delta transformer?

A. To account for the phase shift between the primary and secondary currents.
B. To provide proper current balancing between the CTs.
C. To improve the accuracy of the differential protection scheme.
D. To eliminate the need for differential relaying in Y-Delta transformers.

Answer 1

In a 3-phase Y-Delta transformer, the CTs are connected differently to account for the phase shift between the primary and secondary currents, ensuring accurate differential protection.

Step-by-step explanation:

In a 3-phase Y-Delta transformer, the current transformers (CTs) are connected differently to account for the phase shift between the primary and secondary currents. The Y configuration on the primary side causes a phase shift of 30 degrees between the primary and secondary currents. The Delta configuration on the secondary side helps to negate this phase shift.

By connecting the CTs differently, the phase relationship between the primary and secondary currents is properly accounted for, allowing for accurate differential protection scheme. This ensures that any fault current imbalances between the primary and secondary sides are detected and the transformer is protected.