Differential magnetic-balance protection

 

Magnetic balance differential protection, also known as self-balance differential protection, is the use of magnetic balance principle differential protection to achieve a method of magnetic balance differential protection consists of three self-balancing transformers and three current relays.

This system is necessarily a combination of circulating current and self-balance protection systems. The main advantages of this protection system are increased stability and sensitivity and its application to power transformers provided with load tap-changers. Figure 9.9 illustrates a scheme representing a power transformer having a 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 Iv 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 magnetizing 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 the occurrence of a fault in the protection zone, say at F, excessive current flows through CT1 causing a flow of current in the relay operating coil. Thus relay is energized and the circuit breaker gets tripped.