Electromagnetic Type Distance And Differential RelaySubmitted By: 1.Nivesh Agarwal 2.Himanshu Tiwari 3.Pawan Kumar Yadav

What is Distance Relay ? Used for protection of high and extra high voltage

transmission and sub transmission lines. They are used where over current relay become slow. It provides both primary and backup protection.

Type of Distance Relay: Impedance relay Reactance relay MHO relay

Angle Impedance relay Quadrilateral relay Elliptical and other conic section relay.

Basic Principle and operation of impedance relay: An impedance relay measure the impedance of the line

at the relay location. When a fault occur on the protected line section the measured impedance is the impedance of the line section between the relay location and the point of fault. The relay operates when the measured impedance is less then the given constant. It is non-directional relay. Its characteristic on the R-X diagram is circular with center at origin.

Basic Principle and operation of Reactance relay: A Reactance relay measures the reactance of the line at

the relay location and is not affected by variation in resistance. In case of fault on the protected line the measured reactance is the reactance of the line between the relay location and the fault point. The relay operates when the measured reactance is less then the given constant. It is non-directional relay. Its characteristic on the R-X diagram is straight line parallel to X axis.

Basic Principle and operation of MHO relay: MHO relay measures the component of admittance. It is a directional relay as it detects the fault only in

forward direction. Its characteristic on the R-X diagram is circular passing through the origin.

Advantages of EM type Distance relay: No problem of electrostatic discharge. These type of relays are cheaper as compared to static

relays for low voltage systems. Long lifespan Visual operation Less operating cost

Disadvantages of EM type Distance relay: High power consumption. Losses are more because of moving contact. These are less accurate.

What is Differential relay: Differential protection is a very reliable method of

protecting generators, transformers, buses, and transmission lines from the effects of internal faults.

In a differential protection scheme in the above figure,

currents on both sides of the equipment are compared. 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 secondary's 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.

Type of Differential Relay: Voltage differential relay

Voltage differential relay

Percentage differential relay

Application: Protection of generator Protection of large motors Bus zone Protection

Protection of feeder by pilot wire differential

protection Protection of transmission line by Phase comparison current carrier Protection Protection of transformer.

Conclusion: Hence we had seen that : Distance relay works on the principle of z=v/i. when z

value of the line fall below the set value relay will give trip signal. it is used for transmission line. Differential relay works on the principle of Kirchhoff current law. i.e. incoming current = outgoing current if it is not equal relay will give trip signal. It is used for transformer and generator protection.

References: Wikipedia. WWW.Google.com B.Ram,Power System Protection and Switch Gear, Tata

McGraw-Hill, 1989 S.S.Rao, Switch Gear Protection and Power Systems,

Khanna Publishers

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