Protection of Transmission lines

ByBy

Rohini HaridasAssistant 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

Non- unit type Protection

1. Time graded over current protection

Unit type protection

1. Differential protection

2. Current graded over current protection.

3. Distance protection.

2. 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 lines lines (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 nextsection and the end of first section.

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

2. For the ring mains, parallel feeders ,where power can flow tofault from either direction , a system without directionalcontrol is not suited.

Trip Law for Simple Impedance Relay Using Universal Torque Equation

432

22

1 )cos( kVIkVkIkT +−++= τθ

The universal torque equation is given as :

Thus trip law for simple impedance relay is as follows :

ZsetZseen pIf Then trip, else restrain

Introduction to Distance Protection

Principle of R-X Diagram

Characteristics of impedance relay

** It is non directional. It responds to the faults on both sides of CT,VT locationi.e. forward (I-quadrant )and reverse direction (III-quadrant)

Characteristics of impedance relay on R-X diagram

Implementation of simple impedance relay using balance beam structure

Phasor diagram for directional element

I

Max. Torque line Zero torque line

vφ

Reference quantity

v

1φ

φ

θ

τ

Characteristics of directional impedance relay

Effect of Arc resistance on Reach of SIR (Under-reach)

**The tendency of distance to restrain (not to operate ) at the preset value ofthe impedance or impedances less than preset value is known as under-reach

Trip Law for Simple Reactance Relay Using Universal Torque Equation

The universal torque equation is given as :

432

22

1 )cos( kVIkVkIkT +−++= τθ

Thus ,trip law for simple impedance relay is as follows:

XsetXseen pIf Then trip, else restrain

Characteristics of Reactance Relay

X

Restrain

Line fault characteristics

R

Xset or Xn

TripTrip

Effect of Arc resistance on Reach of Reactance relay

Directional property exhibited by reactance 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

Distance protection of transmission line

Three step Distance protection

Three step Distance protection

Three step Distance protection using mho relay

Three step Distance protection

Overload or overcurrent protection

Combine overcurrent and earth fault protection