--- title: "Combined phase and earth fault protection of generator" subject: "Switchgear and Protection" chapter: 7 key: "semester_6/switchgear_and_protection:combined-phase-earth-fault-generator" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/combined-phase-earth-fault-generator site: BCT Engineering --- # Combined phase and earth fault protection of generator Switchgear and Protection · Chapter 7: Protective Relays - Asked in: 2071 Magh, 4 marks - Video: [Combined Earth Fault and Phase Fault Protection - Introduction to Protective Relaying](https://www.youtube.com/watch?v=8vMn-jS8PYU) — Ekeeda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/combined-phase-earth-fault-generator - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-7 ## Question Draw the connection diagram of combined phase fault and earth fault protection scheme applied to the generator. ## Short answer Neutral and line-end CTs are star-connected with pilot wires. Phase-fault relays are connected between the pilots and a common star point; an earth-fault relay connects from that point to the CT neutral. Phase faults produce unbalance in phase relays; earth faults cause residual current in the earth relay. ## Answer In the **combined phase and earth fault protection** scheme, a single set of CTs on each side of the generator winding feeds a differential (Merz-Price) scheme for phase faults and, through the residual (neutral) connection of the same pilots, an earth fault relay. This saves CTs and covers both types of fault. #### Connection diagram ```text Neutral-side CTs Stator Line-side CTs (CT)──────────────── R ───────────────(CT) (CT)──────────────── Y ───────────────(CT) (CT)──────────────── B ───────────────(CT) │││ star pt │││ │││ │ │││ │││ R_n (earthing) │││ │││ ⏚ │││ ││└───── pilot B ──────┬───────────────┘││ │└────── pilot Y ──────┼──┬─────────────┘│ └─────── pilot R ──────┼──┼──┬───────────┘ [P][P][P] phase relays │ │ │ (87) └──┴──┴───[E]── neutral earth fault relay (64) ``` #### Arrangement - Identical CTs at the neutral end and the line end of each phase; the two CT groups are each connected in **star**. - Pilot wires join corresponding phases, forming a circulating-current loop. - Two (or three) **phase-fault relays** (P) are connected between the pilots and a common star point. - An **earth-fault relay** (E) is connected between this common point and the star point of the CTs (neutral pilot), so it carries the residual current. #### Operation - **Normal and external faults:** currents at both ends are equal, the secondary currents circulate and no current flows in any relay. - **Phase-to-phase fault inside the winding:** the out-of-balance current returns through the phase relays of the faulted phases, which trip. - **Earth fault inside the winding:** the unbalance current returns through the earth-fault relay (with a lower setting), giving sensitive earth fault protection. - The relays trip the generator breaker, field breaker and prime mover. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the advantage of the combined phase and earth fault scheme? A. It removes the need for CTs B. One set of CTs serves both differential phase-fault and earth-fault relays C. It protects the rotor only D. It gives back-up for the grid Answer: B. The same CTs feed phase relays and, via the residual connection, an earth-fault relay, saving equipment. (Key point: Purpose of combined scheme) 2. Where is the earth-fault relay connected? A. In series with each line B. Across the generator terminals C. Between the common point of the phase relays and the CT star point (residual path) D. In the field circuit Answer: C. It carries the residual current, so it responds to earth faults with a lower setting. (Key point: Earth-fault relay connection) 3. During an external fault, what happens in the relays? A. Phase relays trip B. Earth relay trips C. Secondary currents circulate equally and no relay current flows D. Field breaker only trips Answer: C. Currents at both ends are equal, so they circulate in the pilots and the relays stay idle. (Key point: Behaviour on external fault) 4. On an internal fault the relays trip: A. Only the load breaker B. The generator breaker, field breaker and prime mover C. Nothing until time delay of 5 s D. Only the neutral earthing Answer: B. The generator must be isolated, de-excited and shut down. (Key point: Action on internal fault) ## Related in Switchgear and Protection - [Restricted earth fault for generator](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aref-generator-solidly-grounded) — 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 - [Converting IDMT scheme to differential protection](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aidmt-to-differential-transformer) — Switchgear and Protection, chapter 7 - [Unit protection scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aunit-protection-scheme) — Switchgear and Protection, chapter 7 - [Basic requirements of a protection scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Abasic-requirements-protection-scheme) — Switchgear and Protection, chapter 1, asked 3× - [Earth fault neutralizer reactance](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aearth-fault-neutralizer-reactance) — Switchgear and Protection, chapter 5 ## Related in other subjects - [Restricted earth fault protection](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Arestricted-earth-fault) — Power Plant Design, chapter 4 - [L-G fault and 3-phase fault, 30 MVA generator](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Alg-fault-30mva-voltages-3phase) — Power System Analysis II, chapter 5 - [DLG fault sequence network connection](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Adlg-sequence-network-connection) — Power System Analysis II, chapter 5, asked 3× - [Reactance diagram: 90 MVA generator (15%), two motors](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Areactance-diagram-90mva-generator-15) — Power System Analysis I, chapter 3, asked 2×