Purpose of backup protection is

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Examoogle · Accepted Answer

to guard against failure of primary

Explanation: Backup protection is a secondary protection scheme designed to trip the circuit breakers in the event that the primary protection fails to detect or clear a fault within the specified time. Its primary purpose is to ensure system stability by isolating the faulty section when the primary system is non-responsive.

Working Principle: The backup protection relay operates with a intentional time delay, which is greater than the operating time of the primary relay plus the breaker clearing time. If the primary relay clears the fault, the backup relay resets without triggering. If the primary relay fails (e.g., due to CT saturation, battery failure, or mechanical sticking), the backup relay completes its timing cycle and triggers the backup breaker to isolate the circuit.

Key Formulas: $t_{backup} = t_{primary} + t_{breaker} + \Delta t_{margin}$ — represents the discrimination time interval required for relay coordination

Key Points: Backup protection must be independent of the primary protection to avoid common-mode failures.; It typically involves a time-delayed trip setting to ensure selectivity.; Relay coordination is critical to prevent unnecessary tripping of healthy sections of the network.; Common types include remote backup and local backup protection.

Advantages: Ensures system reliability against relay hardware failure.; Prevents catastrophic damage to equipment if primary protection fails.

Disadvantages: Increases the total fault clearing time during backup operation.; Can lead to larger sections of the network being de-energized.

Applications: EHV transmission lines; Power transformers; Industrial busbar systems

Additional Info: The discrimination time margin ($\Delta t$) is typically set between $0.2s$ to $0.5s$ depending on breaker performance.. Option A is incorrect as backup protection actually adds delay; Option B refers to the 'zone of reach' which is a configuration setting, not the 'purpose'; Option C is a result of effective design but not the defining purpose.

Answer: D is correct — The purpose of backup protection is to act as a fail-safe mechanism when the primary protection fails to operate during a fault condition.

Exam Tip: Always remember: Primary protection prioritizes speed, while backup protection prioritizes reliability and security through intentional time delays.

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