WHAT IS BIAS DIFFERENTIAL RELAYS
The term biased-differential or percentage differential is based on the fact that the operating threshold increases with the through current. Because of that, comparison of upstream and downstream currents in the same phases is not convenient for transformer differential protection.
Biased-differential protection is an older design of protection than high impedance circulating current. It is widely applied to two-winding transformers except those connected to the lower distribution voltages.
Until the advent of the high-impedance design, it was also applied to busbar protection, although the last few years have witnessed a
of biased-differential busbar protection schemes.
1. Basic principal
The basic features and principles of operation are illustrated in Figure 1. The biased-differential relay consists of two windings, an operate winding with turns No and a restraints or bias, winding with turns Nb
The relay compares the operating force due to the out of balance current I1-I2 arising from an in-zone fault with the restraint force due to the circulating current through the bias winding, arising from an external fault or load current.
For operation:
.https://electrical-engineering-portal.com/biased-differential-protection
