Bias Setting:

What is bias? Why it is required? Why differential protection is called Bias Differential Protection? Bias is applied to transformer differential protection for the same reason as any unit protection scheme – to ensure stability for external faults while allowing sensitive settings to pick up internal faults. Ratio and phase angle correction are done to achieve stability for external and through faults. But power transformers are equipped with on load tap changers (OLTC) generally provided on HV windings. In general the voltage variation achieved through tap changing is in the range of +5% to -10%. Ratio correction of HV & LV CT secondary current is done at normal tap. This equilibrium achieved at normal tap may not hold for other taps during external or through faults reflected on transformer. Hence, the differential protection provided to power transformer will remain stable at normal tap and may mal-operate at other taps under through faults. By selecting minimum bias to be greater than sum of the maximum tap of the transformer and possible CT errors, mal operation of the protection can be avoided in case the transformer is being operated at other taps. How bias setting helps to avoid mal-operation of the differential protection is illustrated through following figures 4.0 (A to D).

At normal tap, the differential current through the relay for external fault is zero as the selected tap of the transformer is normal.

Figure 4.0 B.External fault on transformer with Tap No.13.

Diff. Pick-up =15%(0.15A),Bias =30%(Assumed)  

Modified Diff.Pick-up due to Restraining Effect of Bias

IdiffMod =0.30 x (10+11.1)/2+ 0.15 =3.165 +0.15 =3.315A. 

Actual Diff.Current= 1.1A (IHV – ILV)

As the Actual Diff Current is less than the Modified Diff. current, protection will remain stable under this through fault.