Differential protection is a very reliable method of protecting generators, transformers, buses, and transmission lines from the effects of internal faults.
Figure: Differential Protection of a Generator
In a differential protection scheme in the above figure, currents on both sides of the equipment are compared. The figure shows the connection only for one phase, but a similar connection is usually used in each phase of the protected equipment. Under normal conditions, or for a fault outside of the protected zone, current I1 is equal to current I2 . Therefore the currents in the current transformers secondaries are also equal, i.e. i1 = i2 and no current flows through the current relay.
If a fault develops inside of the protected zone, currents I1 and I2 are no longer equal, therefore i1 and i2 are not equal and there is a current flowing through the current relay.
Differential Protection of a Station Bus
The principle of the differential protection of a station bus is the same as for generators.
The sum of all currents entering and leaving the bus must be equal to zero under normal conditions or if the fault is outside of the protected zone. If there is a fault on the bus, there will be a net flow of current to the bus and the differential relay will operate.
Figure: Single Line Diagram of Bus Differential Protection
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