--- title: "Black start capability of a generating unit" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:black-start-capability" source: https://www.bctengineer.com/q/semester_8/power_plant_design/black-start-capability site: BCT Engineering --- # Black start capability of a generating unit Power Plant Design · Chapter 4: Electric System Design of a Power Plant - Asked in: 2081 Shrawan, 4 marks - Video: [What Is A Black Start Of The Power Grid?](https://www.youtube.com/watch?v=uOSnQM1Zu4w) — Practical Engineering - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/black-start-capability - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4 ## Question What does "black start capability" of a generating unit mean? How would it have been made so? ## Short answer Black start capability is the ability of a unit to start and energise a dead grid without external power. Achieved by independent diesel generator for auxiliaries, DC battery for control and field flashing, excitation from PMG or battery, governor with stored energy, and island-mode controls. Hydro plants are ideal due to quick start. ## Answer **Black start capability** is the ability of a generating unit or plant to **start itself and energise the dead bus and transmission lines without any power from the external grid**, after a total or partial blackout. Such units are used to restore the system. #### How a unit is made black-start capable - **Independent auxiliary supply:** a **diesel generator (DG) set** or a small house/auxiliary hydro unit to power the station auxiliaries (governor oil pumps, cooling water pumps, lubrication, lighting). - **Station battery and DC system** sized to operate breakers, protection, controls and field flashing during the dead period. - **Excitation able to build up from zero:** **field flashing** from the battery (for static excitation), or a **PMG-fed brushless exciter**, which needs no external source. - **Governor and turbine** that can start with stored oil pressure (accumulator) or DG-powered pumps; hydro gates or valves that can open on DC/stored energy. - **Controls and protection settings** for isolated (island) mode: frequency control mode, dead-bus closing logic, synchronising for later reconnection. - The unit must be able to **energise transformers and lines** (charging current, inrush) and carry the initial loads. Hydro plants are ideal black-start sources because they start quickly and need little auxiliary power. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does black start capability mean? A. Ability to start only with grid support B. Ability to start itself and energise the dead bus and lines without external grid power C. Ability to run only at night D. Ability to run at zero load permanently Answer: B. A black-start unit restores the system after a blackout without needing power from the grid. (Key point: Black start meaning) 2. What provides auxiliary power for a black-start hydro unit before it generates? A. The grid B. An independent diesel generator or small house hydro unit C. The unit's own output D. A capacitor bank Answer: B. A DG set or small auxiliary unit powers the governor pumps, cooling water and lighting. (Key point: Auxiliary supply) 3. How can excitation be built up with no external AC supply? A. By field flashing from the battery, or a PMG-fed brushless exciter B. By connecting to the transmission line C. By using a reverse power relay D. By increasing the turbine speed beyond runaway Answer: A. Field flashing from the station battery or a permanent-magnet-generator-fed exciter needs no external source. 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