Name three main protective-relaying schemes used for transmission lines and feeders.

Name three main protective-relaying schemes used for transmission lines and feeders.

• A. Voltage-regulation, frequency protection, harmonic protection
• B. Overcurrent (inverse/time-overcurrent), distance (impedance-based), differential protection (current balance across protected element)
• C. Ground-fault protection, arc-flash protection, reactive-power protection
• D. Phase-shift protection, tap-changer protection, thermal protection

Answer: (B)

In protecting transmission lines and feeders, you want schemes that can quickly detect faults, know where they are, and do so selectively. The three main protective-relaying schemes used are: Overcurrent protection with inverse/time characteristics. This scheme looks at the current level; when it exceeds a set threshold, the relay trips. The inverse-time feature means faults that produce higher currents clear faster, while coordination with upstream and downstream relays prevents unnecessary trips for normal conditions or minor faults. It’s simple, robust, and effective for a wide range of fault types along a line or feeder. Distance protection (impedance-based protection). This scheme uses the measured impedance from the relay to the fault location. Since impedance on a transmission line changes with distance, the relay can estimate how far the fault is and trip in defined zones. It provides good selectivity along long lines and can distinguish faults on the protected line from faults elsewhere, making it ideal for transmission corridors. Differential protection (current balance across the protected element). This scheme compares the current entering and leaving a protected element, like a line section or a transformer. Under normal conditions the currents match; if an internal fault occurs, current becomes unbalanced, and the relay trips very quickly. It offers fast, highly selective protection for faults inside the protected zone. The other options mix protections that are not the primary line/feeder relay schemes, such as general power quality or equipment safety protections, or protections aimed at transformers or safety rather than the line itself.

In protecting transmission lines and feeders, you want schemes that can quickly detect faults, know where they are, and do so selectively. The three main protective-relaying schemes used are:

Overcurrent protection with inverse/time characteristics. This scheme looks at the current level; when it exceeds a set threshold, the relay trips. The inverse-time feature means faults that produce higher currents clear faster, while coordination with upstream and downstream relays prevents unnecessary trips for normal conditions or minor faults. It’s simple, robust, and effective for a wide range of fault types along a line or feeder.

Distance protection (impedance-based protection). This scheme uses the measured impedance from the relay to the fault location. Since impedance on a transmission line changes with distance, the relay can estimate how far the fault is and trip in defined zones. It provides good selectivity along long lines and can distinguish faults on the protected line from faults elsewhere, making it ideal for transmission corridors.

Differential protection (current balance across the protected element). This scheme compares the current entering and leaving a protected element, like a line section or a transformer. Under normal conditions the currents match; if an internal fault occurs, current becomes unbalanced, and the relay trips very quickly. It offers fast, highly selective protection for faults inside the protected zone.

The other options mix protections that are not the primary line/feeder relay schemes, such as general power quality or equipment safety protections, or protections aimed at transformers or safety rather than the line itself.