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Something About Electrical Power Transmission Line Protection

About Electrical Power Transmission Line Protection

The protection relaying of the transmission lines shall be designed to ensure that:

• Faults on any part of the 220\400kV transmission line are detected by at least two high speed independent protection relays that have the capability of independently initiating fault clearance within the required time;

• Protection scheme applied is adaptable and adequate for protection of the entire transmission line and provide backup protection for associated transmission line circuit end equipment and adjacent transmission lines if required;

• Protection scheme is arranged to ensure no part of the primary system is unprotected;

• Protection scheme is capable of independently tripping the circuit breaker(s) selectively to clear the fault within specified fault clearance times;

• Protection for the 400/220 kV transmission lines shall be phase segregated and shall be capable of single pole tripping and Auto reclosing function.

• Protection and control scheme shall consist of two independent protection relays designated the Main-1 protection relay and the Main-2 protection relay. The protection relays must be from different manufacturers or algorithm of operation to achieve redundancy and diversity.

• Protection scheme for 132kV Lines shall have DPS (Distance Protection Scheme) as Main protection with 3-pole tripping only configuration and Numerical Directional Over Current Earth Fault protection shall be used as back up protection.

• DEF with carrier-aided protection shall be utilized for inception and timely isolation of high resistive faults in transmission line;

Distance Protection for 400, 220 KV and 132 KV Lines.

All EHV class transmission lines shall have Distance and\or Differential Protection function in Relays along with other required features

• The distance protection relay shall have at least four zones of protections. Zone-1, Zone-2, Zone-3 shall be kept in forward direction (towards line) with quadrilateral characteristics whereas Zone-4/5 shall be kept in reverse direction (towards bus) than other three zones.

• Quadrilateral Characteristic shall be utilized for ground fault inception and Isolation;

• Quadrilateral/Mho Characteristic shall be utilized for phase fault inception and Isolation;

• For Lines with length smaller than 10KMs having OPGW Communication link availability, shall have Main-1 and Main-2 Line Differential Protection and Distance Protection as backup function in both relays.

• Overvoltage function shall be configured in M1/M2 DPS scheme instead using static stand-alone relays whenever required.

• Other than required protection functions, all other functions shall not be enabled. 

Some notes:

1. CT ratio selected shall be higher then possible Max. loading on line & thermal rating of conductor.
2. The lines with following configuration shall be considered as double circuit lines.
(a) Double circuit lines with point to point connectivity at both ends.
(b) Double circuit lines with partially connected on same tower and partially with other tower however both end points are similar.
(c) Two different transmission lines connected on double circuit tower having different end points. (Partial connectivity on same tower shall also be considered as D/C lines i.e. both lines).
2. If it is single line on towers for total length of line it shall be treated as single circuit.
3. Line length shall be kept similar at both ends in distance protection which shall be duly verified by protection engineer for both ends. The priority for line length consideration at both ends shall be as under.
(a) Line length derived from line signature test.
(b) Line length submitted by respective construction dept. as per their survey.
4. The conductor parameters considered in distance protection settings, shall be similar at both ends which shall be duly verified by protection engineers for both ends 
5. Cable Impedance data can be used that provided by OEM 
6. Conductor Impedance data measurement is recommended and it shall be used for compensation factor calculation when available.
7. For Hybrid Lines (Different overhead conductors, OH + Cable) Impedance measurement is recommended and it shall be used in relay settings.
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