Differential Current Protection of Traction Circuits (Train)

Differential Current Protection of Traction Circuits

There are various schemes for the protection out of which differential current protection scheme for traction circuit is widely used.

This protection is possible with the help of differential relays. These relays take a variety of forms depending on the equipment they protect.
The differential relay is one that operates when the vector difference of two or more similar electrical quantities exceeds a predetermined value.
In general any relay, when connected in a certain way can be made to operate as a differential relay.
In other way it is not the relay construction rather how that relay is connected in a circuit makes it a differential relay.
Most of the differential relay operations are of the current differential type.
The simple arrangement of the differential relay is shown in the Figure (A).

Differential Relay Applications (A)

Two current transformers are connected as shown in the Figure (A) and in between these two CT relay is connected.
The secondaries of the CTs are interconnected and the coil of an overcurrent relay is connected across the CT secondary circuit.
If the two current transformers have the same ratio and are properly connected their secondary currents will not circulates between the two CTs and no current will flow rough the differential relay.
But if the current flows to the short circuit from both sides, the sum of the CT secondary currents will flow through the differential relay and will trip the circuit with the help of circuit breaker.
In other words, the differential – relay current will be proportional to the vector difference between the currents entering and leaving the protected circuit; and if, the differential current exceeds the relay’s pick up value, the relay will operate.
Nowadays, advancement in the conventional methods and new type of relays which can not only provides transformer differential protection without any interposing CTs but can also provide a complete protection and control scheme for the traction transformer.
For example transformer tank protection can also be included in the relay scheme for all types of the traction transformer applications.

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