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What is the advantage of using the current grading in relay systems?
It improves the frequency stability of the power system.
It reduces the impedance between two sub-stations.
It ensures quick tripping of the faulty circuit.
It overcomes the long time delays occurring in graded time lag systems.
It overcomes the long time delays occurring in graded time lag systems.
Current grading (also known as inverse definite minimum time or current-based relaying) allows relays to operate based on fault current magnitude rather than fixed time intervals. By utilizing the inverse relationship between current and operating time, it eliminates the need for cumulative time delays that occur in simple time-graded systems at the end of the line.
IS 3231: Electrical relays for power system protection
Current grading (also known as inverse definite minimum time or current-based relaying) allows relays to operate based on fault current magnitude rather than fixed time intervals. By utilizing the inverse relationship between current and operating time, it eliminates the need for cumulative time delays that occur in simple time-graded systems at the end of the line.
T=InтИТCKтАЛ тАФ General IDMT relay characteristic formula
IpтАЛ=Pick-up┬аcurrent тАФ Minimum current at which relay starts operation
In standard time grading, each relay must wait for the downstream relay to clear the fault, leading to very long delays for relays near the source. Current grading uses inverse-time characteristics, where the relay operating time T follows the relationship TтИЭIn1тАЛ. As the fault moves closer to the source, the current increases, causing the relay to operate faster, thereby avoiding the excessive time delays required by fixed-time grading.
Reduces the fault clearance time for faults close to the source.
Maintains selectivity through intentional time-current coordination.
Essential for radial feeder protection to prevent system instability due to prolonged fault duration.
Faster fault clearing for heavy fault currents
Improved coordination flexibility
Reduces potential damage to equipment due to prolonged arc exposure
Requires complex coordination studies
Susceptible to transient inrush currents
Requires high accuracy of current transformer saturation characteristics
Radial distribution feeders
Backup protection for transmission lines
Industrial motor protection
Option A is incorrect because frequency stability depends on generation-load balance and turbine governors, not relay grading.
Option B is incorrect as impedance is a property of the transmission line/transformer, which cannot be changed by the relaying method.
Option C is partially true in effect but 'Current Grading' is specifically the technique to solve the time delay accumulation problem inherent in simple time grading.
D is correct тАФ It overcomes the long time delays occurring in graded time lag systems.
Always remember that time-graded systems (using simple overcurrent relays) result in the 'back-up' relay having the longest time delay, which is undesirable for high-magnitude faults near the source.
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