Protective relays are generally required for medium voltage (MV) and high voltage (HV) systems, typically above 1 kV, where circuit breakers alone cannot ensure precise and selective protection.

When Relay Protection is Needed

Protective relays are essential in electrical systems where large currents or high voltages are present, and faults could cause significant damage or system instability. While low-voltage circuits often rely solely on circuit breakers, MV (1 kV to 35 kV) and HV (above 35 kV) panels require relays in combination with breakers to detect abnormal conditions such as overcurrent, earth faults, or abnormal voltage . Relays provide fast, selective, and directional protection, which is critical for isolating faults without affecting the entire system .

Role of Current Transformers (CTs)

Relays are powered by secondary currents from current transformers (CTs), which reduce the high primary current to a manageable level (typically 5 A or 1 A) for the relay to process . This ensures that relays can safely detect overcurrent or fault conditions without being exposed to the full line current.

Types of Relays and Applications

• Overcurrent Relays (50/51, 50N/51N): Detect instantaneous or time-delayed overcurrent and earth faults. Used in MV and HV systems to trip breakers quickly during short circuits .
• Voltage Relays: Protect against overvoltage or undervoltage conditions.
• Distance Relays: Used in transmission lines to detect faults based on impedance or distance from the relay location .
• Differential Relays: Protect transformers, generators, and busbars by comparing currents at different points to detect internal faults .

Summary

Relay protection is mandatory for systems where high voltage or high current could cause severe damage, typically in MV and HV networks. Low-voltage systems may not require relays if standard breakers provide sufficient protection. The combination of relays and breakers ensures fast fault detection, selective isolation, and system stability, making them indispensable in modern power systems .