DISTANCE PROTECTION OF TRANSMISSION LINE

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Comparison Between Distance Relays

Comparison of distance relays

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Comparison of distance relays

Distance Protection of a Three-phase Line

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The problem of providing distance protection to a three-phase line

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Presence of sequence components in various faults

Phase Faults

Phase a sequence network connection for b-c phase fault

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Phase a sequence network connection for b c phase fault.

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Phase a sequence network connection for b-c phase fault.

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Ground FaultsGround Faults

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Phase a sequence network connections for a-g fault.

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Continued…• Complete Protection of a Three‐phase Line

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Complete protection of a three-phase line.

Three-stepped Distance Protection

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Three-stepped distance protection simplified representation

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S.Padmini A.P(Sr.G)/EEE SRM University 16Three-stepped distance protection using mho relays.

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Loss of selectivity of second zone of RA with second zone of RB .

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Philosophy of the three-stepped distance protection

Trip Contact Configuration for the Three-stepped DistanceTrip Contact Configuration for the Three-stepped Distance Protection

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Trip contact circuit of the three-stepped distance scheme.

Impedance Seen from Relay SideImpedance Seen from Relay Side

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Impedance seen from the relay side.

Th t d P t ti f D bl E d F d LiThree-stepped Protection of Double-End-Fed Lines

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Only 60% of the line gets high speed distance protection.

Review Questionsh h d b k if f l• 1. What are the drawbacks, if any, of over-current relays?

• 2. Why are over-current relays not used for primary protection of EHV linesin spite of the fact that instantaneous over current relays exist?3 i h i i li d ll d f h k f di l i ?• 3. How is the transmission line modelled for the sake of distance relaying?

• 4. Arc resistance is a function of spark-over distance, wind velocity and time.Explain.5 Wh d b lli f l ?• 5. What do you mean by a metallic fault?

• 6. How is it different from an arcing fault?• 7. What is the effect of fault resistance on the reach of various distance

l ?relays?• Which relay is the most affected and which the least affected?• 8. What do you mean by power swing?

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• 9. Will an over-current relay be affected by power swing?

Review Questions• 10. How do different distance relays perform with respect to their behaviour on

load,• effect of arc resistance on the reach and response to power swing?effect of arc resistance on the reach and response to power swing?• 11. Prove that a-6 distance measuring unit fed with (Va- Vb) and (Ia - Ib) responds a

correctly to a-b-g faults as well as a-b-c faults. • 12 Why is residual current compensation required in case of ground fault distance• 12. Why is residual current compensation required in case of ground fault distance

measuring unit?• 13. Explain the need for a three-stepped distance protection of a transmission line.• 14. How many distance measuring units will be required for the complete three-• stepped protection of a transmission line section in case of (i) a singly-fed-systems

and (ii) a double-end-fed system?

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• 15. The impedance seen from the relay side in a distance relay is 10 ohms. Given that the CT ratio is 1000 : 1 and the PT ratio is 100,000 : 110, determine the actual impedance. Given that the line has a resistance of 1 milliohm per km and a