Correct Answer - Option 1 : Long transmission line
Major factors that needed to be considered for the relay selection in transmission lines are:
- Power swing
- Stability on normal load
- Fault resistance or arc resistance
- Directional property
- Cost
Major relays we use
- Impedance relay
- Reactance relay
- Mho relay
Long transmission lines: Since long lines are highly subjected to power swings (Load shedding cumulative effect adds on and affects the long transmission lines majorly).
Mho relay can protect from power swing better than impedance relay and has the directional property which is essential in transmission lines.
Short transmission lines:
Ground faults are highly subjected here because of the trees falling during the heavy winds. So, fault resistance can affect in estimating the reach of the distance relay.
Reactance relay is immune to the fault resistance but is not directional and also unstable at normal load. So, the mho relay (gets affected with arc resistance) is used in conjunction with the reactance relay here.
Medium transmission lines: Impedance relay with added directional property element is used rather than the mho relay due to the cost factor.