Introduction:
A negative sequence relay is a protective device commonly used in electrical power systems to detect and protect against negative sequence faults. These faults are characterized by an imbalance in the three-phase currents, with the negative sequence component being present. This relay is primarily used to protect alternators from damage caused by negative sequence faults.

Explanation:
- Alternator Protection: An alternator is a device that converts mechanical energy into electrical energy. It is an essential component of power generation systems. The negative sequence relay is used to protect the alternator from negative sequence faults.
- Negative Sequence Faults: Negative sequence faults occur when there is an imbalance in the three-phase currents, with the negative sequence component being present. These faults can be caused by various factors such as unbalanced loads, faulty equipment, or system faults.
- Impact on Alternators: Negative sequence faults can have detrimental effects on alternators. They can cause overheating, mechanical stress, and unbalanced magnetic fields, leading to insulation breakdown, rotor overheating, and mechanical damage. These faults can significantly reduce the lifespan of the alternator and result in costly repairs or replacements.
- Role of Negative Sequence Relay: The negative sequence relay is designed to detect the presence of negative sequence currents and initiate protective actions to prevent damage to the alternator. It continuously monitors the three-phase currents and compares their magnitudes and phases. If an imbalance is detected, indicating the presence of negative sequence currents, the relay operates to isolate the faulty section of the system.
- Operation of Negative Sequence Relay: The negative sequence relay operates based on the principle of symmetrical components. It uses mathematical calculations to determine the presence of negative sequence currents. When an imbalance is detected, the relay sends a trip signal to the circuit breaker, isolating the faulty section from the rest of the system.
- Other Applications: While the primary application of the negative sequence relay is to protect alternators, it can also be used to protect other components such as transformers, transmission lines, and bus bars. However, in this context, the correct answer is option 'A' because the question specifically asks for the most common application, which is alternator protection.

Conclusion:
The negative sequence relay is commonly used to protect alternators from the damaging effects of negative sequence faults. It continuously monitors the three-phase currents and detects any imbalances caused by negative sequence components. When an imbalance is detected, the relay initiates protective actions to isolate the faulty section and prevent damage to the alternator. While it can be used to protect other components as well, its primary application is alternator protection.