Protective Relay Reference

Q: What ANSI relay device numbers does this reference cover?

This reference covers the most critical ANSI device numbers used in power system protection: 21 (Distance), 27 (Undervoltage), 32 (Reverse Power), 40 (Loss of Field), 46 (Negative Sequence), 49 (Thermal), 50/51 (Instantaneous/Time Overcurrent), 50N/51N (Ground Fault Overcurrent), 50BF (Breaker Failure), 55 (Power Factor), 59 (Overvoltage), 63 (Buchholz), 67 (Directional Overcurrent), 78 (Out of Step), 79 (Recloser), 81 (Frequency), and 87 (Differential). Each includes typical pickup settings, time dial ranges, and application guidelines.

Q: How do I check if a CT will saturate?

Calculate the secondary required voltage: Vs = Is x (Rct + Rw + Rb), where Is is the secondary fault current, Rct is CT internal resistance, Rw is wiring resistance, and Rb is relay burden. If Vs exceeds the knee point voltage (Vk), the CT will saturate. For DC component consideration, use Vs_dc = Is x (1 + X/R) x (Rct + Rw + Rb). The reference explains both formulas with parameter definitions.

Q: What is the coordination time interval (CTI) for digital relays?

For digital relays, CTI = t_relay (0.02s) + t_CB (0.05-0.08s for 5 cycles) + t_margin (0.10s) + t_overshoot (0.05s), totaling 0.30-0.35 seconds. For electromechanical relays, CTI is typically 0.40-0.50 seconds. These values are used to ensure proper time grading between upstream and downstream protection devices.

Q: What IEC 60255 overcurrent curves are included?

Four standard curves are included with their formulas: Standard Inverse (t = 0.14*TDS / ((I/Ip)^0.02 - 1)), Very Inverse (t = 13.5*TDS / ((I/Ip)^1.0 - 1)), Extremely Inverse (t = 80*TDS / ((I/Ip)^2.0 - 1)), and Definite Time (t = TDS constant). These are the industry-standard formulas used for time-overcurrent relay setting calculations.

Q: How does transformer differential protection (87T) work?

Transformer differential relay (87T) compares currents entering and leaving the transformer. Typical settings include operating current at 20-30% of rated, restraint slope of 15-40%, and harmonic restraint at 2nd harmonic 15% and 5th harmonic 25% (to block during inrush and overexcitation). CT ratio correction via TAP settings compensates for transformer turns ratio differences.

Q: What is the 75% rule for fuse coordination?

The 75% rule states that the upstream (source-side) fuse minimum melting time must exceed 75% of the downstream (load-side) fuse total clearing time at any given current level. This ensures the downstream fuse clears the fault before the upstream fuse begins to melt, maintaining selective protection. The reference also covers power fuse (PF), current limiting (CL), and dual element fuse types.

Q: How does ground fault protection differ by grounding method?

In ungrounded systems, 59N/64 detects residual voltage (fault current is very low, only charging current). High-resistance grounded systems use 51N with fault current limited to 5-25A. Solidly grounded systems use 50N/51N with high fault current and fast clearing. Resonant grounded systems (Petersen Coil) use self-extinguishing arcs allowing continuous operation. Each method is detailed with typical relay selections.

Q: What IEC 61850 features does this reference explain?

The reference covers IEC 61850 three-level architecture (Station/Bay/Process), GOOSE messaging for high-speed peer-to-peer trip signals (under 4ms transfer time for direct CB tripping), and Sampled Values (SV) for digitizing CT/PT analog signals over process bus. This provides the foundation for understanding modern digital substation communication-based protection systems.

Free reference guide: Protective Relay Reference

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About Protective Relay Reference

The Protective Relay Reference is a comprehensive cheat sheet for power system protection engineers covering ANSI device numbers, relay settings, CT/PT selection, and protection coordination. It includes 26 entries organized into Protection Relays, CT-PT, and Protection Coordination categories, each with detailed formulas, typical settings, and practical engineering examples.

From overcurrent relay pickup calculations (ANSI 50/51) and differential protection slope settings (87T) to distance relay zone reach (21) and IEC 61850 GOOSE communication, this reference provides the key parameters and equations needed for relay setting studies, TCC curve plotting, and coordination time interval calculations.

Designed for power system engineers, substation designers, and electrical engineering students, this tool runs entirely in the browser with Korean and English support, dark mode, and mobile-responsive design.

Key Features

Complete ANSI device number coverage: OCR (50/51), OCGR (50N/51N), UVR (27), OVR (59), differential (87T/87B), distance (21), directional (67), recloser (79), and breaker failure (50BF)
CT selection guide with 5P20 accuracy class explanation, burden calculation formulas, and saturation check (Vs vs. Vk knee point)
PT/VT specifications including Y-Y, open delta, and GPT connections with residual voltage detection for ungrounded systems
IEC 60255 overcurrent curve formulas: Standard Inverse, Very Inverse, Extremely Inverse, and Definite Time with TDS parameters
Protection coordination tools: TCC plotting order, CTI margin calculation (digital: 0.30-0.35s, electromechanical: 0.40-0.50s), and 75% fuse coordination rule
Equipment-specific protection schemes for motors (49/50/51/46/27/66), transformers (87T/63/51/49), and generators (87G/40/32/46/78/64)
Ground fault protection methods compared across ungrounded, high-resistance grounded, solidly grounded, and resonant grounded systems
IEC 61850 architecture overview with GOOSE trip messaging (< 4ms), Sampled Values, and station/bay/process level structure

Frequently Asked Questions

What ANSI relay device numbers does this reference cover?

This reference covers the most critical ANSI device numbers used in power system protection: 21 (Distance), 27 (Undervoltage), 32 (Reverse Power), 40 (Loss of Field), 46 (Negative Sequence), 49 (Thermal), 50/51 (Instantaneous/Time Overcurrent), 50N/51N (Ground Fault Overcurrent), 50BF (Breaker Failure), 55 (Power Factor), 59 (Overvoltage), 63 (Buchholz), 67 (Directional Overcurrent), 78 (Out of Step), 79 (Recloser), 81 (Frequency), and 87 (Differential). Each includes typical pickup settings, time dial ranges, and application guidelines.

How do I check if a CT will saturate?

Calculate the secondary required voltage: Vs = Is x (Rct + Rw + Rb), where Is is the secondary fault current, Rct is CT internal resistance, Rw is wiring resistance, and Rb is relay burden. If Vs exceeds the knee point voltage (Vk), the CT will saturate. For DC component consideration, use Vs_dc = Is x (1 + X/R) x (Rct + Rw + Rb). The reference explains both formulas with parameter definitions.

What is the coordination time interval (CTI) for digital relays?

For digital relays, CTI = t_relay (0.02s) + t_CB (0.05-0.08s for 5 cycles) + t_margin (0.10s) + t_overshoot (0.05s), totaling 0.30-0.35 seconds. For electromechanical relays, CTI is typically 0.40-0.50 seconds. These values are used to ensure proper time grading between upstream and downstream protection devices.

What IEC 60255 overcurrent curves are included?

Four standard curves are included with their formulas: Standard Inverse (t = 0.14*TDS / ((I/Ip)^0.02 - 1)), Very Inverse (t = 13.5*TDS / ((I/Ip)^1.0 - 1)), Extremely Inverse (t = 80*TDS / ((I/Ip)^2.0 - 1)), and Definite Time (t = TDS constant). These are the industry-standard formulas used for time-overcurrent relay setting calculations.

How does transformer differential protection (87T) work?

Transformer differential relay (87T) compares currents entering and leaving the transformer. Typical settings include operating current at 20-30% of rated, restraint slope of 15-40%, and harmonic restraint at 2nd harmonic 15% and 5th harmonic 25% (to block during inrush and overexcitation). CT ratio correction via TAP settings compensates for transformer turns ratio differences.

What is the 75% rule for fuse coordination?

The 75% rule states that the upstream (source-side) fuse minimum melting time must exceed 75% of the downstream (load-side) fuse total clearing time at any given current level. This ensures the downstream fuse clears the fault before the upstream fuse begins to melt, maintaining selective protection. The reference also covers power fuse (PF), current limiting (CL), and dual element fuse types.

How does ground fault protection differ by grounding method?

In ungrounded systems, 59N/64 detects residual voltage (fault current is very low, only charging current). High-resistance grounded systems use 51N with fault current limited to 5-25A. Solidly grounded systems use 50N/51N with high fault current and fast clearing. Resonant grounded systems (Petersen Coil) use self-extinguishing arcs allowing continuous operation. Each method is detailed with typical relay selections.

What IEC 61850 features does this reference explain?

The reference covers IEC 61850 three-level architecture (Station/Bay/Process), GOOSE messaging for high-speed peer-to-peer trip signals (under 4ms transfer time for direct CB tripping), and Sampled Values (SV) for digitizing CT/PT analog signals over process bus. This provides the foundation for understanding modern digital substation communication-based protection systems.