The normal percentage bias setting of differential relay used for transformer protection will be

• A. below 10%
• B. between 50% and 75%
• C. between 10% and 50%
• D. above 75%

Answer: (C)

In bias differential protection for transformer,
There are two basic settings required to setup the percentage biased differential protection.
1. Minimum pickup
2. The slope.

The minimum pick-up covers the transformer magnetizing current typically 1-4% of transformer rating.
A pick up of 0.2 to 0.3 times the TAP is recommended for general cases.

The slope is defined by determining the potential sources of differential current.