For digital current differential line relays, the sound and simple setting principles utilized for differential relays using current-only as the main operating criteria. This paper presents an implementation of the current differential relay and the directional comparison relay using EMTP MODELS based on closed-loop. The SEL Current Differential/Overcurrent Relay provides protection for any two-input apparatus, such as transformers, motors, generators, and reactors.


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An arrangement of overcurrent relay connected to operate as a differential relay is shown in the figure below. The dotted line represents the element of the system that is to be protected by the differential relay.

Current differential relay system element might be a length of the circuit, a portion of the bus or a winding of a generator or that of a transformer. A pair of current transformers is fitted on the either ends of the section to be protected.

The secondaries of current transformers are connected in series with the help of the current differential relay wires in such a way that they carry the induced current in the same direction.

The operating coil of an overcurrent relay is connected across the current transformer secondary circuit shown in the figure below. When there is no fault current or there is an external fault, then the current in the secondaries of the current transformers are equal, and the relay current differential relay coil, therefore, does not carry any current.

Differential Relay

When the short current differential relay developed anywhere between the two current transformers, then the currents flow to current differential relay fault of both sides, and the sum of the current transformer secondary current will flow through the differential relay.

An arrangement of overcurrent relay connected to operate as a differential relay is shown in the figure below. The arrangement of the overcurrent relay is shown in the figure below. The dotted line shows the section which is used to be protected.

The current transformer is placed at both the ends of the protection zone.

Protective relay

The secondary of the transformers is connected in series with the help of the pilot wire. Thereby, the current induces in the CTs flows in the same direction. The operating coil of the relay is connected on the secondary of the CTs.

current differential relay


For example, a relay including function 51 would be a timed overcurrent protective relay. Overcurrent relay[ edit ] An overcurrent relay is a type of protective relay which operates when the current differential relay current exceeds a pickup value.

  • Protective relay - Wikipedia
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  • Differential Protection Relay
  • Definition of Differential Relay

In a typical application the over current relay is connected to a current transformer and calibrated to operate at or above a specific current level. When the relay operates, one or more contacts will operate and energize to trip open a circuit breaker.

The Definite Time Current differential relay Relay has current differential relay used extensively in the United Kingdom but its inherent issue of operating slower for faults closer to the source led to the development of the IDMT relay.

What is differential relay? - Quora

It is possible to program customised curves in some protective relays [40]: Some numerical relays can be used to provide inverse time overvoltage protection current differential relay Curves derived from BS Relay Characteristic.

If, however, the equipment is a transformer where the turns ratio is not equal to one, the current current differential relay will be different from the current exiting. In this case, the current transformers must be balanced with an equivalent turns ratio differential.

The differential scheme creates a well-defined protection zone, encompassing everything between the two current transformers.

Any fault existing in this protection zone is regarded as an internal fault, while any fault existing outside this protection zone is an external fault. Accurate matching of characteristics of current transformer cannot be achieved hence there may be spill current flowing through the relay in normal operating conditions.

Percentage Differential Relay This is designed to response to the differential current in the term current differential relay its fractional relation to the current current differential relay through the protected section.

In this type of relay, there are restraining coils in addition to the operating coil of the relay.


The restraining coils produce torque opposite to the operating torque. Under normal and through fault conditions, restraining torque is greater than operating torque.

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Thereby relay remains inactive. When internal fault occurs, the operating force exceeds the bias force and hence the relay is operated. This bias force can be adjusted current differential relay varying the number of turns on the restraining coils.

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