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LINE PROTECTION

Objectives of relay protection

Protects apparatus in power system

Protects person & equipment in the surrounding of the powersystem

Separates faulty parts from the rest of the power system to facilitate the operationof the healthy part of the system.

FAULT STATISTICS (generally)

Single phase to ground 80%

Two phase to ground 10%Phase to phase 5%

Three phase fault 5%

The probability of line fault caused by lightening are 2-3 faults /100 km and year.FAULT TYPES

Transient faults

Are common on transmission lines ,approximately 80-85%

Lightning are the most common reason

Can also be caused by bird , trees ,swinging lines etc..

Will disappear after short dead interval.

Persistant fault.

Can be caused broken conductor fallen down.

Can be by tree falling on line

Must be repaired before normal service

MAIN REQUIREMENTS ON LINE PROTECTION

Speed

Sensitivity

Selectivity

Dependability

Security

MEASURING PRINCIPLES

Over current protection

Differential protection

Phase comparison

Distance protection

OVER CURRENT PROTECTION

Are normally used in radial networks with system voltage below 70KV where

relatively long operating time is acceptable. On transmission line directional or non directional relays are used as back up

protection

The reach of overcurrent relay is function of source impedance & fault impedance

which varies considerably making it difficult to get fast & selective tripping.

PILOT WIRE DIFFERENTIAL PROTECTION

Pilot wire can be in soil or on towers

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The resistance in the wire will limit the use on longer lines .the use is mostly

restricted to distance upto 10 km

PHASE COMPARISION

phase comparision relay compare the angle difference between the two currents at

both ends of the line.

The measured time for zero crossing is transmitted to the other endDISTANCE PROTECTION

Power lines have impedance of value 0.3-o.4 ohm/km & normal angles of 60-85

degrees in 50 Hz system

Measurement base on impedance is immune to source impedance & other

problems ,permits fast and selective tripping.

Z=V/I

Switched scheme

This type of schemes consists of start relay to select the measuring loop to the

single measuring relay & range of same measuring unit is extended to Z2 &Z3

after time delay by switching. Non switch scheme

This type of scheme has a measuring element for each measuring loop & for each

zone

Types of characteristics

A) self polarized Mho:-self polarized mho charct.is circle & passes through

origine.it is less prone to power swing, & inherently directional.

B) cross polarized mho:-this charact. is also suitable for short length lines as it

provides enhanced resistive coverage.

C) offset mho:-the offset mho relay encloses origin & provides some coverage

for faults in reverse direction this type of reverse offset is used to for Z3 to give

back protection to busbar.

D) Reactance :- it is represented by line parallel to resistive axis.It is ideally

suitable for short length line. It is non directional hence required to use withdirectional characteristic.

E) Lenticular :-its shape is like lens it provide required coverage of impedance &

restricted coverage of resistance.this type of charact. is suitable for long & heavily

loaded line.

F) Quadrilateral :-its shape is quadrilateral & it is directional both reactive &

resistive reaches are independently adjustable.it is ideally suitable for short length

line.

Basic setting philosophy

Zone 1: 80% of protected line

Zone 2: 100% of protected line+ 50% of adjacent shortest line

Zone 3: 100% of protected line+100% of adjacent longest line

Zone4 reverse: 10% of zone 3 forward to cover bus etc..

Check should be made for reach to transformer adjacent to protected line section

Switch on to fault

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When energizing a power line into a forgotten earthing no measuring voltage

available & if it is at opp end then it will trip in Z2 time in such situation

protection may not operate & in other case operation will be delayed.

A special SOTF function is provided so as relay to operate in Z1 time Different

principles can be used ,from one phase current to unidirectional impedance

measuring Power swing blocking

A power swing can be started by sudden load change due to fault in the network

During power swing 3phase symmetrical V & I variation occurs.

It is either recoverable or non recoverable

Normally tripping is prevented if it enter upto Z2 & remain there for max 2 sec

Tripping is allowed if it enters Z1 locus.

Vt fuse failure

As Z=V/I hence if VT fuse blows Z will be zero and distance relay may trip

A special function VT fuse failure prevent tripping in such condition

Carrier intertrip As Z1 covers 80% of line , various carrier tripping schemes are used to protect

remaining portion of line in almost Z1 time .

Some type of schemes are as under

Permissive underreach transfer trip(PUR) generally this scheme is utilized in our

network.

Permissive overreach transfer trip(POR) generally utilized for short length line

upto 10 kms.

Blocking

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