Short Answer:

An auxiliary relay in substations is a supporting device that helps perform control, indication, interlocking, and logic functions. It does not directly detect faults but operates when energized by other protection relays to perform actions like tripping a breaker, turning on alarms, or changing circuit status.

Auxiliary relays are essential in coordinating and executing complex protection and control sequences in substations. They ensure the proper working of breakers, switches, and alarms by providing multiple contacts and delay features as needed.

Detailed Explanation:

Auxiliary relay in substations

In electrical substations, the operation of protection relays often needs additional support devices to complete tasks such as sending trip signals, holding circuits, or activating multiple output devices simultaneously. This support is provided by an auxiliary relay, which acts as a helper to the main protection relay.

An auxiliary relay does not sense faults on its own. Instead, it is used to amplify signals, increase the number of contacts, or introduce time delays in the operation of protective equipment. It plays a key role in ensuring the smooth coordination of protection, indication, and automation systems.

Key functions of an auxiliary relay

1. Contact multiplication:
   • Main protection relays may have limited output contacts.
   • Auxiliary relays multiply these contacts to operate multiple devices (breakers, alarms, indicators) at the same time.
2. Interlocking and control logic:
   • Used in interlock circuits to ensure equipment operates only in a predefined safe order.
   • Implements logical sequences required for complex switching operations.
3. Signal amplification:
   • If the protection relay’s output signal is weak, the auxiliary relay strengthens it to operate high-load devices.
4. Time delay functionality:
   • Some auxiliary relays are time-delay relays and introduce intentional delays before triggering operations like tripping or closing.
5. Trip and lockout functions:
   • When a protection relay detects a fault, the auxiliary relay sends the final trip signal to the circuit breaker.
   • In case of critical faults, it can hold the breaker in the lockout position until reset manually.

Types of auxiliary relays

• Instantaneous auxiliary relay: Operates immediately upon energization.
• Time-delay auxiliary relay: Adds a time gap between energization and operation.
• Lockout relay (86 relay): Maintains the trip condition until manually reset.
• Latching relay: Remains in its position after operation until reset.

Importance in substations

• Reliable control: Ensures all control operations work correctly in coordination with protection systems.
• System flexibility: Allows easy implementation of changes in logic or circuit behavior without changing primary relays.
• Safety enhancement: Prevents incorrect switching and improves human safety during operations.

Conclusion:

An auxiliary relay in substations is a supportive device used to execute control, indication, interlocking, and timing functions after receiving signals from main protective relays. Though it does not detect faults itself, it ensures that commands like tripping, alarm activation, or logic sequences are carried out correctly and safely, making it an essential part of substation automation and protection systems.