Regulations on Relay Protection Setting Numbers

Relay protection setting numbers are standardized by ANSI/IEEE C37.2 and IEC standards to ensure consistent identification and coordination of protective devices in power systems.

ANSI/IEEE Device Numbering

In North America, protective relays and devices are identified using ANSI/IEEE standard device numbers, as defined in IEEE C37.2 . Each number corresponds to a specific function:

• 50 – Instantaneous overcurrent relay
• 51 – Time overcurrent relay
• 59 – Overvoltage relay
• 86 – Lockout relay
• 87 – Differential relay (87T for transformer, 87G for generator) Suffix letters are used to indicate specific applications or connections:
• G – Ground (e.g., 51G for ground overcurrent)
• N – Neutral (e.g., 59N for neutral displacement)
• T – Transformer (e.g., 86T for transformer lockout) These numbers are used in single-line diagrams, schematics, and relay documentation to clearly identify the function of each device .

IEC Standards

The International Electrotechnical Commission (IEC) provides alternative symbols and terminology for protective devices under IEC 60617 and IEC 617 . While IEC numbers differ from ANSI, the principle of standardized identification remains the same, ensuring interoperability and clarity in international projects.

Practical Relay Setting Considerations

Relay settings are determined based on system parameters, coordination, and protection zones :

• Zone 1: Typically set to 80–90% of the line impedance for instantaneous operation, with no intentional time delay.
• Zone 2: Covers the protected section plus 50% of the adjacent line, with a time delay coordinated with Zone 1.
• Zone 3: Extends to the protected section plus 120% of the longest adjacent line, with time delays set to coordinate with Zone 2. For transformer differential relays (e.g., SEL-787), TAP settings are used to convert secondary currents to per-unit values, ensuring proper scaling and directional reference. The ratio of TAPmax/TAPmin is typically limited to ≤7.5 to maintain relay accuracy .

Key Principles

• Reliability: Relays must operate correctly under fault conditions and remain stable under normal conditions.
• Selectivity: Only the faulty section should be isolated, avoiding unnecessary tripping.
• Speed: Relays must respond quickly to faults to minimize system disruption.
• Sensitivity: Relays must detect faults under actual operating conditions, including low currents or abnormal voltages . By following ANSI/IEEE or IEC standards and applying proper relay setting calculations, engineers ensure safe, coordinated, and effective protection of generators, transformers, lines, and other power system components.