Line Differential Relay [87L]:

The principle of operation of line differential protection is same as transformer differential protection. The only difference is transformer differential protection uses only one differential relay whereas line differential protection relay uses two or more differential relays at each end of the line. Communication is established among these relays.

These relays require a high speed and secure communication medium such as fibre-optics in order to transfer all of the collected analog data among all of the relays so each relay can perform its own calculations and provide high-speed tripping.

Line Current Differential Relay [87L]:

Differential protection for universal use with power lines and cables on all voltage levels with phase-segregated measurement (87L). Single pole tripping is possible with phase-segregated measurement.

Line current differential relays operate on a difference in current into the line compared to the current out of the line. This is called current differential method.

The differential current can be measured in different ways:
· Magnitude comparison
· Phase comparison
· Phasor or Directional comparison (magnitude and angle)
· Charge comparison
· Combinations of the above

Relays Communication:

Communications is an integral piece of a line differential relay, as the CT Secondary currents from one line terminal must be sent to relays at other line terminals to perform the differential calculation. This requires the use of a digital communications channel, which is commonly a multiplexed channel where channel switching may occur.

The most common methods currently in use are twisted pair cable, coaxial cable, fibre optic cable, power line carrier and wireless communication (Microwave).

A fibre optic pair available for exclusive use by the relays provides optimal performance for digital communications. Dedicated Protection data interface port gives a fast and error-free point-to-point connection.

Data communication for differential current measurement is effected via fiber-optic cables or digital communication and telephone networks, so that the line ends can be quite far apart. All the relays needed to time-synchronized for making the operational and fault event messages identical.

Working of Line Current Differential Relay:

Line differential relays basically operate on a difference in current into the line, compared to the current out of the line.

For an internal fault, the current will flow into the line from both line terminals, with the polarity of the current transformers as shown in Figure.

The above figure shows this for a line with two ends. Each device measures the local current and sends the information of measured currents and phase relation to the opposite end.

Digital data communication for differential current measurement is effected via fiber-optic cables, digital communication networks or pilot wires, so that the line ends can be quite far apart.

I_Operate= I_{Loca l} – I_Remote

I_Restraint = (I_{Loca l} + I_Remote)/2

%Slope = (I_Operate/ I_Restraint)*100

Operation of Line Current Differential Relay:

The relay shown in figure is used for cable differential purpose. Two relays at each end of the cable separated by some distance with the communication path between the two relays so that they exchange information together.

• These relays operate on a current differential method.
• These relays connected using fiber to transit digital information.
• The digital information contains the current magnitudes and other diagnostic parameters and is transmitted continuously between connected stations.
• Tripping is initiated when differential relay exceed the relays restraint   characteristic.
• Failure of the fiber communication path will automatically block the scheme and initiate an alarm.
• There is a chance to break the communication medium due to any reason, so distance relay function used as back up to the line current differential relays.