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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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Which of the following is non unit type of protection in transmission line?
Pilot wire differential protection
Phase comparison
Carrier current protection
Current graded over current protection
Current graded over current protection
Quick Summary: A non-unit type of protection is one where the protection zone is not restricted to a specific section of the transmission line and does not compare inputs at both ends of the protected zone. In 'Current Graded Overcurrent Protection', the relay coordination depends on time settings or current magnitude settings which cover overlapping segments of the system, rather than isolating only one specific zone.
A non-unit type of protection is one where the protection zone is not restricted to a specific section of the transmission line and does not compare inputs at both ends of the protected zone. In 'Current Graded Overcurrent Protection', the relay coordination depends on time settings or current magnitude settings which cover overlapping segments of the system, rather than isolating only one specific zone.
Ttrip=Trelay+Tcb — Total time taken for fault clearance
Ipickup>Iload(max) — Fundamental condition for overcurrent protection
The principle relies on time-grading or current-grading. By setting different relays with different pickup times or current thresholds, the relay closest to the fault is designed to trip first. Since these zones overlap into adjacent sections, they are classified as non-unit systems because they do not exclusively respond to faults within a fixed boundary.
Unit protection responds only to faults within the protected zone.
Non-unit protection has no fixed zone boundary and provides back-up for adjacent sections.
Pilot wire, phase comparison, and carrier current are all types of unit protection.
Graded protection requires coordination studies to prevent maloperation.
Simple construction
Economical and reliable for primary protection
Provides inherent backup protection for downstream zones
Coordination becomes difficult in complex networks
Slower fault clearance compared to unit protection
Sensitivity decreases as number of cascaded sections increases
Radial distribution feeders
Back-up protection for transmission lines
Distribution network protection
| Feature | Unit Protection | Non-Unit Protection |
|---|---|---|
Zone Definition | Strictly defined (Unit) | Undefined/Overlapping (Non-Unit) |
Unit schemes like Differential Protection use the principle of Kirchoff's Current Law (Iin−Iout=Ifault), making them highly selective.
Carrier current protection uses high-frequency signals over the power line to compare conditions at both ends, ensuring unit-style selectivity.
D is correct — Current graded overcurrent protection is a non-unit type of protection as it relies on time and current settings that overlap into adjacent system sections.
Always remember: 'Unit' = 'Selective/Differential' (comparing boundaries), 'Non-Unit' = 'Graded' (relying on time/current magnitude).