How the Bias characteristic in a Differential protection relay can be tested?

If you just need to verify that it operates, I've tested Differential protection on generators by putting the unit online at low load, installing shorting screws in one of the CT terminal blocks to short out one phase to CO. Then the differential elements in the protective relay are seeing different values for that phase.

It shouldn't trip at very low load. As you increase load, it should eventually trip. If you need exact values, you might need to get hold of one of the testing services that's equipped to simulate the conditions you require.

This is a great technical paper:

http://www.rflelect.com/pdf_files/Current%20Differ...

Biased Differential Protection

You need to read the manufacturer's instruction book for the relay you are dealing with. Differential relays are not all identically the same, and different manufacturers may design their relays in different ways that affect how the relay can be tested. Without knowing specifically which relay you are working with, it is impossible to answer your question.