Classification of Secondary Relay Protection

Secondary relay protection can be classified based on function, actuating quantity, connection type, and operating principle.

Classification by Function

Secondary relays are categorized according to their role in the protection scheme :

• Main Relays: Respond directly to changes in electrical quantities such as current, voltage, or power.
• Auxiliary Relays: Operate under the control of other relays to perform auxiliary tasks like time delays, tripping circuit breakers, or signaling alarms.
• Signal Relays: Indicate the operation of other relays through visual or audible signals and can trigger alarms.

Classification by Actuating Quantity

Relays can also be classified based on the electrical quantity they respond to :

• Overrelays: Operate when the quantity exceeds a set value (e.g., overcurrent relays).
• Underrelays: Operate when the quantity falls below a set value (e.g., undervoltage relays).
• Other types: React to power, reactance, impedance, frequency, or directional changes.

Classification by Connection Type

Secondary relays are distinguished by how their sensing elements are connected :

• Primary Relays: Sensing elements are directly connected to the circuit or element being protected.
• Secondary Relays: Sensing elements are connected through current or voltage transformers, which is common in high-voltage systems for safety and measurement accuracy.

Classification by Operating Principle

Relays can also be classified according to how they act on the circuit breaker :

• Direct-acting Relays: Mechanically operate the breaker directly.
• Indirect-acting Relays: Use an auxiliary power source to operate the breaker.

Additional Considerations

Modern secondary relays often follow IEC standards (IEC 60255 for relay design and IEC 61850 for digital communication), ensuring interoperability, reliability, and advanced features like self-diagnostics, remote access, and event logging . They may also be categorized by mechanism (electromagnetic, static, mechanical) or logic (differential, directional, unbalance) for more detailed functional classification . This classification framework helps engineers select appropriate relays for protection, control, and monitoring in secondary distribution systems, ensuring safe and reliable operation of electrical networks.