Protection of transmission lines (distance)

Protection of Transmission lines (Distance Protection)

By,Rohini Haridas

Assistant Professor,Dept of Electrical Engineering,

SSGM College of Engineering,Shegaon

� As the length of electrical transmission line isgenerally long enough and it runs through openatmosphere, the probability of occurring faultin electrical power transmission line is much higherthan that of transformers and alternators .

�That is why a transmission line requires much moreprotective schemes than a transformer and analternator.

Features of protection of transmission line

1. During fault, the only circuit breaker closest to the fault pointshould be tripped.

1. If the circuit breaker closest the faulty point, fails to trip1. If the circuit breaker closest the faulty point, fails to tripthe circuit breaker just next to this breaker will trip as back up.

2. The operating time of relay associated with protection of lineshould be as minimum as possible in order to preventunnecessary tripping of circuit breakers associated with otherhealthy parts of power system.

The main methods of transmission line protection

1. Time graded over current protection 4.Differential protection

2. Current graded over current protection.

3. Distance protection.

4.Differential protection

5.Carrier current protection

Protection of Radial Feeder

� In radial feeder, the power flows in one direction only, that is from source to load.

�This type of feeders can easily protected by using either definite time relays or inverse time relays.

Line Protection by Definite Time Relay

ADVANTAGE

� simplicity

DISADVANTAGE

� If the number of sections inthe line is quite large, the timesetting of relay nearest to the

� during fault, onlynearest CB towards thesource from fault pointwill operate to isolatethe specific position ofthe line.

setting of relay nearest to thesource, would be very long. Soduring any fault nearer to thesource will take much time tobe isolated. This may causesevere destructive effect on thesystem.

Over Current Line Protection by Inverse Relay

Over Current Protection of Parallel Feeders

Protection of Ring main system

The two lines leaving the generating stations should be equippedwith non-directional over current relays ( in this case relay 1 and 8)

At each bus directional relay should be placed in bothincoming and outgoing line (2,3,4,5,6,7)Direction of tripping should be away from the bus.

**If the direction of flow of power is same as that of the direction of relaythen only relay trips

There should be relative time setting of the relay. Goinground the loop E-A-B-C-D-A-E, the outgoing relaysare set with decreasing time limits (relays 1,3,5,7)

Similarly Going round the loop in opposite direction E-A-D-C-B-A-E, the outgoing relays are set withdecreasing time limits (relays 8,6,4,2)Direction of tripping should be away from the bus

Protection of Ring main system

Current graded protection

**The short ckt current along the length of protected ckt decreases withincrease in distance between supply end and fault point

Difficulties in current graded protection

1. The relay can not discriminate between the fault in the next section andthe end of first section.

**Hence for discrimination the relays are set to protect only part of the line,usually 80%

2. fault currents are different for different types of fault so difficulty2. fault currents are different for different types of fault so difficultyexperienced in relay setting

3. For the ring mains, parallel feeders ,where power can flow to fault fromeither direction , a system without directional control is not suited.

For this reason current grading alone can notbe employed

Distance protection of transmission line

Three step Distance protection (FIRST STEP)

Three step Distance protection (FIRST STEP)

Three step Distance protection



Step Reach Operating Time Remarks

FirstStep

80 to 90% of lineSection

Instantaneous i.e. Nointentional time delay(T)

1. Provides primary protection to lineupto 80 to 90%

SecondStep

100 % of line sectionunder consideration+50% of adjoining line

T + Selective timeinterval =T1

1. Provides primary protection to thepart of line left out of first step (20to 10%.

2. Full Backup protection to linesection under consideration

3. Backup protection to next line upto50%

ThirdStep

100 % of line sectionunder consideration+100% of adjoining line+ 10 to 20% Extra

T1+ Selective timeinterval =T2

1. Full Backup protection to linesection under consideration

1. Full Backup protection to next line


Three step Distance protection using mho relay

Comparison between Distance RelayFactors Simple

impedance relay

Reactance relay Mho relay

Operating quantity Current Current

Directional element

Restrainingvoltage

Directional Voltage Restrainingquantity

voltageDirectional

element Voltage

Directional property

No No Yes

Effect of fault resistance

Under reaches Reach unaffected

Under reaches

Area occupied on R-X diagram

Moderate Very large Smallest

Overload or overcurrent protection

Combine overcurrent and earth fault protection