DISTANCE PROTECTION OF
TRANSMISSION LINE
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Comparison Between Distance Relays
Comparison of distance relays
Comparison of distance relays
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Distance Protection of a Three-phase Line
The problem of providing distance protection to a three-phase line
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Continued…
Presence of sequence components in various faults
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Phase Faults
Phase a sequence network connection for bb-cc phase fault
fault.
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Continued…
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Continued…
Phase a sequence network connection for b-c phase fault.
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Continued…
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Ground Faults
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Continued…
Phase a sequence network connections for a-g fault.
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Continued…
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Continued…
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Continued…
• Complete Protection of a Three‐phase Line
Complete protection of a three-phase line.
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Three-stepped Distance Protection
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Continued…
Three-stepped distance protection simplified representation
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Continued…
Three-stepped distance protection using mho relays.
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Continued…
Loss of selectivity of second zone of RA with second zone of RB .
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Continued…
Philosophy of the three-stepped distance protection
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Trip Contact Configuration for the Three-stepped Distance
Protection
Trip contact circuit of the three-stepped distance scheme.
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Impedance Seen from Relay Side
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Continued…
Impedance seen from the relay side.
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Th
Three-stepped
t
dP
Protection
t ti off D
Doublle-End-Fed
E d F d Lines
Li
Only 60% of the line gets high speed distance protection.
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Review Questions
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1. What
h are the
h drawbacks,
d b k if any, off over-current relays?
l
2. Why are over-current relays not used for primary protection of EHV lines
in spite of the fact that instantaneous over current relays exist?
3 How is
3.
i the
h transmission
i i line
li modelled
d ll d for
f the
h sake
k off distance
di
relaying?
l i ?
4. Arc resistance is a function of spark-over distance, wind velocity and time.
Explain.
5 What
5.
Wh do
d you mean by
b a metallic
lli fault?
f l?
6. How is it different from an arcing fault?
7. What is the effect of fault resistance on the reach of various distance
relays?
l ?
Which relay is the most affected and which the least affected?
8. What do you mean by power swing?
9. Will an over-current relay be affected by power swing?
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Review Questions
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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?
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.
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 threestepped protection of a transmission line section in case of (i) a singly-fed-systems
and (ii) a double-end-fed system?
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
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impedance. Given that the line has a resistance of 1 milliohm per km and a