--- title: "Protection for alternator and step-up transformer" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:mega-project-protection" source: https://www.bctengineer.com/q/semester_8/power_plant_design/mega-project-protection site: BCT Engineering --- # Protection for alternator and step-up transformer Power Plant Design · Chapter 4: Electric System Design of a Power Plant - Asked in: 2080 Chaitra, 6 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/mega-project-protection - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4 ## Question List out the most essential protection schemes required for Alternator and Generator Step Up transformer in a mega size power project. ## Short answer Essential alternator protections: differential (87G), stator earth fault 95% and 100% (59N/64G, 27TN/64S), rotor earth fault (64R), loss of excitation (40), negative sequence (46). Essential GSU protections: transformer differential (87T), restricted earth fault (64REF), Buchholz (63), overfluxing (24). Overall unit differential (87GT) and master trip (86) cover both. ## Answer In a mega (large) hydro or thermal project, the alternator and its **generator step-up (GSU) transformer** are protected by a set of main and backup relays, usually grouped as a **unit protection** scheme with an overall differential zone. #### Generator protections (ANSI number) | Protection | ANSI | Purpose | |---|---|---| | Generator differential | 87G | Stator phase-to-phase faults | | Stator earth fault 95 % | 59N / 64G | Earth faults on 95 % of winding | | 100 % stator earth fault | 27TN / 64S | Faults near neutral (3rd harmonic or injection) | | Rotor earth fault | 64R / 64F | Field winding earth fault | | Loss of excitation | 40 | Field failure, under-excitation | | Negative sequence | 46 | Unbalanced load, rotor heating | | Reverse power | 32 | Motoring of the unit | | Over/under frequency | 81 | Abnormal speed | | Over/under voltage | 59 / 27 | Load rejection, AVR failure | | Overfluxing (V/Hz) | 24 | Core over-excitation | | Voltage-restrained overcurrent / impedance | 51V / 21 | Backup for external faults | | Pole slipping | 78 | Loss of synchronism | | Stator thermal / RTD | 49 | Overload, cooling failure | | Inadvertent energisation | 50/27 | Breaker closed on a stopped unit | #### GSU transformer protections | Protection | ANSI | Purpose | |---|---|---| | Transformer differential | 87T | Internal phase faults | | Restricted earth fault (HV) | 64REF / 87N | Earth faults near HV neutral | | Buchholz relay | 63 | Incipient internal faults, gas | | Oil and winding temperature | 26 / 49 | Overheating | | Pressure relief device | 63PR | Sudden internal pressure | | HV overcurrent and earth fault | 50/51, 50N/51N | Backup | | Overfluxing | 24 | Over-excitation of core | | Oil level, cooling failure | 71 | Alarm | #### Overall scheme An **overall (unit) differential 87GT** covers the generator, GSU and unit auxiliary transformer together. Relays are duplicated as **Main-1 and Main-2** groups with separate CTs and DC supplies for very large units. Trips go through a **master trip relay (86)** to the GCB, HV breaker, field breaker and turbine shutdown. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which protection detects stator phase-to-phase faults of the alternator? A. Reverse power (32) B. Generator differential (87G) C. Loss of excitation (40) D. Negative sequence (46) Answer: B. 87G compares currents at both ends of the stator winding and trips for internal phase faults. (Key point: Generator differential) 2. Why is a 100% stator earth fault scheme (27TN/64S) added to the 95% scheme? A. To detect faults near the neutral end that the 95% scheme misses B. To detect rotor earth faults C. To detect overfluxing D. To protect against motoring Answer: A. The 95% scheme leaves the last part of the winding near the neutral uncovered, so 3rd harmonic or injection schemes cover it. (Key point: 100% stator earth fault) 3. What does negative sequence protection (46) guard against? A. Motoring of the unit B. Unbalanced load causing rotor heating C. Core over-excitation D. Field winding earth fault Answer: B. Unbalanced currents produce negative sequence currents that induce double-frequency currents and overheat the rotor. (Key point: Negative sequence protection) 4. What does the overall differential 87GT cover, and how are trips issued? A. Only the GSU; trip directly from each relay B. Generator, GSU and unit auxiliary transformer together; trips via master trip relay 86 C. Only the busbar; trip via Buchholz D. Only the generator; trip via 51V Answer: B. 87GT forms one zone over the unit, and tripping goes through the master trip relay (86) to the GCB, HV breaker, field breaker and turbine shutdown. (Key point: Overall unit differential) ## Related in Power Plant Design - [Protection of a medium size generator](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Amedium-generator-protection) — Power Plant Design, chapter 4, asked 3× - [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× - [Transformer protection, selection and vector group](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Atransformer-protection-vector-group) — Power Plant Design, chapter 4 - [Quotation for a power transformer](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Apower-transformer-quotation) — Power Plant Design, chapter 4 - [Limits of the alternator capability curve](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Acapability-curve-limits) — Power Plant Design, chapter 4 - [Specification for procuring a power transformer](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Apower-transformer-specification) — Power Plant Design, chapter 4, asked 4× ## Related in other subjects - [Step-up chopper and controllable power transfer](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Astep-up-chopper-power-transfer) — Power Electronics, chapter 4 - [Differential protection of power transformer](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Atransformer-differential-protection) — Switchgear and Protection, chapter 7 - [Earth fault protection: 20 MVA alternator](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aearth-fault-20mva-5-ohm) — Switchgear and Protection, chapter 7 - [Percentage differential protection of alternator](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aalternator-percentage-differential) — Switchgear and Protection, chapter 7, asked 2×