Current Differential Relay | Principle for Line Protection

Line differential

An alternative principle for line protection that’s quickly turning into the norm is differential protection. Differential protection is predicated on Kirchhoff’s laws, stating that each one current into a network node shall add up to zero in a perfect system. What this much suggests that for a line protection application is that the present that’s measured to flow into the road ought to even be measured to come back out.

The differential relay is one that operates once there’s a distinction between 2 or a lot of similar electrical quantities exceeds a planned worth. In differential relay theme circuit, there square measure 2 currents come back from 2 elements of associate degree power circuit. These 2 currents meet at a junction purpose wherever a relay coil is connected. in step with physicist Current Law, the resultant current flowing through the relay coil is nothing however summation of 2 currents, coming back from 2 totally different elements of the power circuit.

current differential relay

A relay that senses and operates the part distinction between this moving into the electrical system and therefore the current deed the electrical system is named a current differential relay. an appointment of overcurrent relay connected to work as a differential relay is shown within the figure below


The arrangement of the over current relay is shown within the figure below. The line shows the section that is employed to be protected. this electrical device is placed at each the ends of the protection zone. The secondary of the transformers is connected nonparallel with the assistance of the pilot wire. Thereby, this induces within the CTs flows within the same direction. The operative coil of the relay is connected secondary CTs.

In the traditional in operation condition, the magnitude of current within the secondary of the CTs remains same. The zero current flows through the in operation coil. On the incidence of the fault, the magnitude of the present on the secondary of CTs becomes unequal thanks to that the relay starts in operation.


Biased or Percentage Differential Coil

This is the foremost used variety of differential relay. Their arrangement is same as that of this differential relay; the sole distinction is that this technique consists a further restraining coil connected within the pilot wires as shown within the figure below.

The operative coil connects within the centre of the restraining coil. The quantitative relation of current within the current electrical device becomes unbalance attributable to the fault current. This drawback is resolved by the employment of the restraining coil.


It is clear from the on top of clarification, larger the present flowing through the restraining coils, higher the worth of the present needed for operative coil to be operated. The relay is termed share relay as a result of the operative current needed to trip are often expressed as a share of through current

Induction Type Biased Differential Relay

This induction kind relay consists a disc that freely rotates between the electromagnets. The every of the magnet consists the copper shading ring. The ring can move in or out of the magnet. The disc experiences a force as a result of the restraining and therefore the operative part.

The resultant force on the shaded ring becomes zero if the position of the ring is balanced for each the component. however if ring moves towards the iron core then the unequal torques engaged on the ring thanks to the operational and restraining coil.


Voltage Balance Differential Relay

The current differential relay isn’t appropriate for the protection of the feeders. For the protection of the feeders, the voltage balance differential relays area unit used. The voltage differential relay uses 2 similar current electrical device places across the protecting zone with the assistance of pilot wire.

The relays square measure connected serial with the secondary of the present electrical device. The relays square measure connected in such some way that no current flows through it within the traditional operative condition. The voltage balance differential relay uses the air core CTs within which the voltages induces

When the fault happens within the protection zone, the present within the CTs become unbalance because of that the voltage within the secondary of the CTs disturbs. the present starts flowing through the in operation coil. Thus, the relay starts in operation and offers the command to the breaker to operates.

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