Protection Scheme of

Transmission Line using

Distance Relaying

BY

ABHIJIT KUVAR SANDESH CHITNIS

001081139 001082255

ECE 592-008 TERM PROJECT

Principle of Distance Relaying:

Distance relays are normally used to protect transmission lines. They respond to the impedance

between the relay location and the fault location. As the impedance per mile of a transmission

line is fairly constant, these relays respond to the distance to a fault on the transmission line.

The R–X diagram is an indispensable tool for describing and analyzing a distance relay

characteristic.

Distance relaying should be considered when overcurrent relaying is too slow or is not

selective. Distance relays are generally used for phase-fault primary and back-up protection on

sub-transmission lines, and on transmission lines where high-speed automatic reclosing is not

necessary to maintain stability and where the short time delay for end-zone faults can be

tolerated.

Type of Distance Relay used:

For this particular project assignment, we have used the impedance relay having the following

characteristics. Impedance relay monitors the impedance between the relay location and the

fault. If the impedance falls within the relay setting, the relay will operate.

Stepped Distance Protection:

Time-stepped distance protection of a transmission line is the distance protection in which the

definitions of over-reach and under-reach need to be addressed.

Figure above shows the stepped distance protection for setting the relay Rab as the time-

stepped distance protection.

It basically employs the three zones for protection.

• Zone 1

It monitors the 80% of Zab. If a fault impedance is lesser to or equal than 80% Zab, then

the relay would trip the circuit breaker to isolate line AB instantaneously. It serves as a

primary protection for line AB.

• Zone 2

It monitors impedance of line AB and 20% of line Zbc. If the fault impedance is lesser

than this value, the relay would trip the circuit breaker to isolate line AB and line BC

with some delay. It serves as backup protection for line BC.

• Zone 3

It monitors value of impedance of line AB and 150% of line Zbc. If the fault impedance is

lesser than this value, the relay would trip the circuit breaker to isolate line AB and line

BC with some delay, this delay being larger than delay for zone 2. It serves as backup

protection for line BC and line next to BC.

Design considerations for Relay:

The relay is to be configured on the lower transmission line having length 200 miles. Given

impedances are,

Z23 = 5.16 + j 112.88

Z34 = 0.63 + j 15.1025

Considering only the magnitudes of the impedances in given zones, the calculations for the

settings of the zones 1,2 and 3 as follows:

Zone 1

Setting: |0.8*Z23| =| 4.128 + j 90.304| = 90.398

Zone 2

Setting: |Z23+ (0.2*Z34)| = |5.286 + j 115.9005| = 116.021

Zone 3:

The transmission line Z23 and Z34 are completely protected in the zone 3 setting of the relay

Setting: |Z23 + Z34| = |5.79 + j 127.9825| = 128.113

The book 'Power System Relaying' by Stanley Horowitz and Arun Phadke states that 'It is a

fundamental principle of distance relaying that, regardless of the type of fault involved, the

voltage and current used to energize the appropriate relay are such that the relay will measure

the positive sequence impedance to the fault. Once this is achieved, the zone settings of all

relays can be based upon the total positive sequence impedance of the line, regardless of the

type of the fault.'

Thus in the Simulink model we have calculated the positive impedance for all types of fault and

compared them with the zone settings.

Implementation in Simulink:

Description of the circuit:

• Generators are modeled using the voltage sources. The +ve, -ve and zero sequence

impedance of the generators is modeled using three-phase pi-section line.

• The transmission lines are also modeled using Three-phase pi-section line block in

Simulink and +ve, -ve and zero sequence impedances are incorporated.

• The circuit breaker is placed on the lower end of longer transmission line. The circuit

breaker is to be tripped using an external command signal ‘com’.

• In order to measure the fault impedance, the 3-phase measurement block is connected

which gives out the values of three phase voltages and currents in Vabc and Iabc

parameters.

• The loads are modeled using R-L Three phase balanced load.

• There are three fault blocks, the first one for simulating fault in zone 1, another one for

simulating fault in zone 2 and next for simulating the fault in zone 3.

• The whole system is implemented as Subsystem 1 with input as ‘com’ and output as

‘Vabc’ and ‘Iabc’.

Logic Circuit Implementation:

Subsystem 2 is implemented as:

Working of Logical Circuit:

• The basis of measuring the fault impedance is the measurement of positive sequence

impedance. The values of Vabc and Iabc are passed the Sequence Analyzers to give

output as +ve sequence voltage and +ve sequence current.

• This value of impedance is passed into subsystem 2 as an input ‘In1’. It is then separated

into magnitude and phase to measure the impedance.

• The value of magnitude of impedance is then passed into function block as input ‘u’ and

following script is written to differentiate between zone 1, zone 2 and zone 3 faults and

send the trip signal to circuit breaker through logical combinations.

• The actual settings that are used in the function are calculated by observing fault

impedance values using Simulink and then writing a MATLAB function block in the

Simulink.

The code that has been implemented in the MATLAB function block in subsystem 1 is as

follows:

Code/Relay implementation:

function [y,z,x,a] = fcn(u) %#codegen a=1; if u<=110 y=0; z=0; x=0; else y=1; x=1; end if (u>110 && u<130) z=0; y=0; x=1; else z=1; x=0; end if u>135 && u<150 a=0; end

We take four outputs from the function, 'y' for zone 1, 'z' for zone 2, and 'a' for zone 3. The output 'x'

is used for resolving the ambiguity between outputs for zone 1 and zone 2. The values of the outputs

vary between values of '0' and '1' according to the conditions on the input value of the magnitude of

impedance given in the code. This is for the permanent faults.

For temporary faults, the code in the MATLAB function block in the main circuit is used. It checks if

the voltage is within limits and thus interprets healthy condition, resuming closed CB configuration while

if the voltage is outside limits, outputs value '0', meaning opening the CB.

Temporary fault code:

function y = fcn(u) %#codegen y=0; if u<4.3e5 && u>3.8e5 y=1; end

• A series of logical operators i.e. AND,NOT and OR gate, is used to send the proper tripping signal from

the MATLAB function. The appropriate delays are also added i.e. 0.3 s for zone 2 and 1 s for zone 3.

Faults in zone 1 are tripped instantaneously.

• Sample-and-hold circuits are used for holding the values of the output to the zero value of tripping

signal which otherwise would have been varying due to the varying nature of the 'Z' waveform.

• The timer of 0.1 s is used to ignore the initial part of the simulation, i.e. the energizing current

variations, so that the CB does not trip inappropriately. This is done by giving the timer signal to the

'enable' input of the subsystem1.

• L-L , L-L-G, and L-G faults in the respective zones do not trip due to the low value of impedance of the

line. This is because the fault impedance comes out to be higher than zone settings.

The simulation diagrams i.e. the three phase current graphs and the impedance angle of the faults for

temporary and permanent L-L-L-G and L-L-L faults in zones 1,2, and 3 are shown below.

Permanent L-L-L-G fault in zone 1 : Fault at t=1.2 s

Fault angle seen by the relay:

Temporary L-L-L-G fault in zone 1: Fault at t=1.2 s and cleared at t=1.8 s

Angle of fault impedance of zone 1:

Permanent L-L-L fault in zone 1: Fault at t=1.2 s

Angle of impedance seen by relay:

Temporary L-L-L fault in zone 1: Fault at t=1.2 s and cleared at t=1.8 s

Angle of impedance seen by relay:

Permanent L-L-L-G fault in zone 2: Fault at t=1.2 s. Breaker opened after delay of 0.3 s at t=1.5 s.

Angle of impedance seen by relay:

Temporary L-L-L-G fault in zone 2: Fault at t=1.2 s and cleared at t=1.8 s. Breaker opened at t=1.5 s after

delay of 0.3 s.

Angle of fault impedance seen by relay:

Permanent L-L-L fault in zone 2: Fault at t=1.2 s and cleared after delay of 0.3 s.

Angle of fault impedance seen by relay:

Temporary L-L-L fault in zone 2: Fault at t=1.2s and cleared at t=1.8s. Breaker opens at t=1.5s after delay

of 0.3s.

Angle of fault impedance seen by relay:

Permanent L-L-L-G fault in zone 3: Fault at t=1.2s and breaker opens at t= 2.2 s after delay of 1s.

Angle of fault impedance seen by relay:

Temporary L-L-L-G fault in zone 3: Fault at t=1.2s and cleared at t=2.4s and breaker opens at t=2.2s.

Angle of fault impedance seen by relay:

Permanent L-L-L fault in zone 3: Fault at t=1.2s and breaker opens at t= 2.2 s after delay of 1s.

Angle of fault impedance seen by relay:

Temporary L-L-L fault in zone 3: Fault at t=1.2s and cleared at t=2.4s and breaker opens at t=2.2s.

Angle of fault impedance seen by relay:

CONCLUSION:

• As seen in all the above graphs, for the specified faults, the fault impedance angles lie

between 50 to 90 degrees thus concluding that, these angles are in the first quadrant of

the R-X Diagram. The directional property of the relay is conclusively utilized.

• As seen here, the zone 2 settings of the relay here does not provide a complete backup

protection for line Z34, thus we have also used the zone 3 settings in the relay to which

basically extends to the complete protection of line Z34 and also backup for the

generator if required.

Thus 'Distance Protection' scheme is implemented using a directional impedance relay in

'Simulink'.