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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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In time graded protection as distance from substation increases, relay operating time will be
Increase
Decrease
Remain same
Zero
Decrease
Quick Summary: In time-graded protection, the relay settings are coordinated such that the relay closest to the fault operates fastest, and the operating time increases for relays located closer to the source. Therefore, as one moves away from the substation (the source) towards the load, the required tripping time for the relay decreases to ensure selectivity.
In time-graded protection, the relay settings are coordinated such that the relay closest to the fault operates fastest, and the operating time increases for relays located closer to the source. Therefore, as one moves away from the substation (the source) towards the load, the required tripping time for the relay decreases to ensure selectivity.
Top=TMS×log10(PSM)+CK — Inverse Definite Minimum Time (IDMT) relay characteristic equation
The principle of time grading relies on assigning a progressive time delay to relays connected in series along a radial feeder. To ensure discrimination, the relay farthest from the source is set with the minimum operating time, while relays towards the source are set with higher time-delay settings. This ensures that the relay nearest to the fault location trips first, isolating the minimum possible section of the network.
Time grading is essential for selectivity in radial distribution systems.
The relay closest to the fault should operate first to maintain system stability.
Discrimination time margin (typically 0.3s to 0.5s) is maintained between consecutive relays.
As we move downstream from the substation, the operating time settings must be lower.
Simple coordination in radial systems
Provides reliable backup protection
Slow operation for faults near the source
Inefficient for complex interconnected loops
Radial distribution feeders
Primary protection in basic substation switchgear
The operating time is inversely proportional to the fault location distance in a graded scheme.
Option A (Increase) describes the inverse of the required logic for protection coordination.
Option C (Remain same) would result in a lack of selectivity, causing total system blackout during a minor fault.
B is correct — In a time-graded protection scheme, relay operating times are staged so that they decrease as you move away from the substation to ensure the relay closest to the fault operates first.
Always remember: 'Closest to the fault = Fastest trip time.' This rule applies to both time-graded and inverse-time overcurrent relays.