Differential Magnetic Balance Protection for Transformers:

This differential magnetic balance protection system is necessarily a combination of circulating current and self balance protection systems. The main advantages of this differential magnetic balance protection system are increased stability and sensitivity and its application to power transformers provided with load tap changers. Figure 12.9 illustrates a scheme representing a power transformer having primary (low-voltage) side connected in delta and secondary (high-voltage) side connected in star.

The current transformers used are bus type and are connected for circulating current protection. Their current ratings are so adjusted that they provide equal secondary currents. The relay operating coil instead of being connected to the equipotential point of the pilot wires is fed from another winding on hv side CT (CT₂). Since the core flux of lv side CT (CT₁) is more because of its more ampere-turns, the turns of hv side CT are increased so as to make core flux of CT₂ zero under all load conditions resulting in no current flowing in the relay operating coil under normal operating conditions. The CT₂ is made with high permeability alloy core so as to reduce magnetising current and provide an accurate balance.

For any through fault CT₂ continues to have no flux and so the relay operating coil remains inoperative. On occurrence of fault in protection zone, say at F, excessive current flows through CT₁ causing flow of current in the relay operating coil. Thus relay is energized and the circuit breaker gets tripped.