--- title: "Protection of a medium size generator" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:medium-generator-protection" source: https://www.bctengineer.com/q/semester_8/power_plant_design/medium-generator-protection site: BCT Engineering --- # Protection of a medium size generator Power Plant Design · Chapter 4: Electric System Design of a Power Plant - Asked in: 2082 Shrawan, 6 marks; 2079 Chaitra, 4 marks; 2077 Chaitra, 4 marks (asked 3 times) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/medium-generator-protection - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4 ## Question List out the necessary protection schemes to be employed for a medium size generator in a power plant. ## Short answer Core electrical protections: differential (87G), stator earth fault (64G), rotor earth fault (64R), loss of excitation (40), negative sequence (46), reverse power (32), voltage-restrained overcurrent (51V) or impedance (21), thermal overload (49), over/under voltage/frequency, overfluxing (24), out-of-step (78). Mechanical: overspeed, bearing/vibration. ## Answer A medium size generator (roughly 5–100 MVA) is protected against internal faults, abnormal operating conditions and system disturbances. Typical schemes, with ANSI device numbers: | ANSI | Protection | Purpose | |---|---|---| | 87G | Generator differential | Fast clearing of stator phase-to-phase faults | | 87GT / 87T | Overall gen-transformer / transformer differential | Faults in GSU and connections | | 64G (59N + 27TN) | Stator earth fault (95% neutral overvoltage + 100% third harmonic) | Earth faults in stator winding (high-resistance grounded via NGT) | | 64R | Rotor (field) earth fault | First earth fault in field winding | | 40 | Loss of excitation (field failure) | Prevents running as induction generator and drawing VArs | | 46 | Negative phase sequence | Unbalanced loads/open phase; rotor heating | | 32 | Reverse power | Motoring when turbine power is lost | | 51V / 21 | Voltage-restrained overcurrent / impedance | Backup for external faults | | 49 | Stator thermal overload (RTDs) | Overheating | | 59 / 27 | Over/under voltage | Load rejection, AVR failure | | 81O / 81U | Over/under frequency | Speed and grid frequency deviations | | 24 | Overfluxing (V/Hz) | Protects generator and GSU core | | 78 | Pole slipping (out of step) | Loss of synchronism | | 50/27 | Inadvertent energisation | Breaker closed at standstill | | 50BF | Breaker failure | Backup tripping of adjacent breakers | #### Mechanical/hydraulic protections Overspeed, bearing and oil temperature, vibration, cooling water failure, low oil pressure, shear pin failure. #### Tripping logic Faults like 87G, 64G, 64R (second stage) trip the GCB, field breaker and turbine (emergency shutdown); abnormal conditions like 46, 49 or 81 first alarm, then trip after a time delay. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which protection clears stator phase-to-phase faults quickly? A. 32 Reverse power B. 87G Generator differential C. 46 Negative sequence D. 81 Frequency Answer: B. 87G compares currents at both ends of the stator winding. (Key point: Stator phase faults) 2. Device 40 (loss of excitation) protects against: A. Motoring by lost turbine power B. Unbalanced loads C. Running as an induction generator and drawing VArs D. Stator overheating Answer: C. Without field the machine absorbs reactive power from the grid. (Key point: Loss of excitation) 3. Negative phase sequence protection (46) guards against: A. Rotor heating from unbalanced loads or open phase B. Overfluxing C. Breaker failure D. Reverse power Answer: A. Negative sequence current induces double-frequency currents that heat the rotor. (Key point: Negative sequence) 4. How are faults and abnormal conditions treated? A. All only alarm B. Faults like 87G/64G trip the GCB, field breaker and turbine; abnormal conditions alarm then trip after delay C. All trip instantly D. Abnormal conditions trip, faults alarm Answer: B. Internal faults need immediate tripping; abnormal conditions are alarmed first, then tripped with a time delay. (Key point: Tripping logic) ## Related in Power Plant Design - [Sketch of a medium head PROR plant](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Apror-medium-head-sketch) — Power Plant Design, chapter 3, asked 2× - [Components of a medium head high discharge plant](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Amedium-head-high-discharge-components) — Power Plant Design, chapter 3, asked 2× - [Protection for alternator and step-up transformer](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Amega-project-protection) — Power Plant Design, chapter 4 - [Unit generator-transformer schemes](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aunit-generator-transformer-schemes) — Power Plant Design, chapter 4, asked 3× - [Design: 245 m head plant, standby generator](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2073-245m-standby) — Power Plant Design, chapter 4, asked 14× - [Design: 260 m head plant, generator lead](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2078-260m-generator-lead) — Power Plant Design, chapter 4, asked 5× ## Related in other subjects - [Differential protection: 11 kV, 100 MVA generator](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Agenerator-differential-11kv-6-ohm) — Switchgear and Protection, chapter 7 - [Lightning protection schemes](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Alightning-protection-schemes) — Switchgear and Protection, chapter 5 - [Skin depth and power in dissipative medium](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Askin-depth-time-average-power-density) — Electromagnetism, chapter 4 - [Combined phase and earth fault protection of generator](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acombined-phase-earth-fault-generator) — Switchgear and Protection, chapter 7