--- title: "Auxiliary systems of a 100 MW Francis plant" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:auxiliary-ancillary-systems" source: https://www.bctengineer.com/q/semester_8/power_plant_design/auxiliary-ancillary-systems site: BCT Engineering --- # Auxiliary systems of a 100 MW Francis plant Power Plant Design · Chapter 4: Electric System Design of a Power Plant - Asked in: 2070 Magh, 8 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/auxiliary-ancillary-systems - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4 ## Question Name the auxiliary and ancillary systems with their function in a hydropower plant with Francis turbine of 100 MW. ## Short answer Auxiliary systems: governor/oil pressure for speed control, cooling water for generator/bearings, excitation/AVR, station AC/DC supply, diesel generator for black start, protection/SCADA. Ancillary systems: fire fighting, EOT crane, HVAC, earthing, lighting, intake gate control. They ensure safe start, operation and shutdown of the 100 MW Francis turbine unit. ## Answer **Auxiliary systems** are the supporting systems that the main generating unit needs to start, run and stop safely. **Ancillary systems** are plant-wide support systems (station services, safety, handling) that are not part of the energy conversion itself. For a 100 MW plant with Francis turbines (assume 2 × 50 MW or 3 units), they are as follows. #### Mechanical auxiliary systems | System | Function | |---|---| | Governor and oil pressure unit | Moves wicket gates to control speed/load; accumulator stores oil pressure | | Main inlet valve (butterfly/spherical) with hydraulic unit | Isolates the turbine from the penstock; emergency closure | | Cooling water system | Cools generator air coolers, bearings, transformer (via heat exchangers); pumps, filters, strainers | | Lubrication / bearing oil system | Lubricates thrust and guide bearings; high-pressure oil lifting during start | | Brake and jacking system | Stops the rotor at low speed; lifts the rotor for inspection | | Drainage and dewatering system | Removes leakage water; empties draft tube and spiral case for maintenance | | Compressed air system | Brakes, governor accumulator air, tools; depresses draft tube water for synchronous condenser mode | | Shaft seal water | Prevents water leakage at the turbine shaft | #### Electrical auxiliary systems | System | Function | |---|---| | Excitation system and AVR | Field current, voltage and var control | | Generator neutral grounding | Limits earth-fault current | | Station service AC (UAT, station transformer, 415 V boards) | Power to all auxiliaries | | Emergency diesel generator | Black start and backup supply | | DC system (battery + chargers) | Protection, control, emergency loads | | Protection, control, SCADA and synchronising | Safe automatic operation | | Metering and communication (PLCC, fibre) | Energy accounting, dispatch link | #### Ancillary systems | System | Function | |---|---| | Fire detection and fighting (CO2 for generator, water spray for transformer, hydrants) | Safety from fire | | Heating, ventilation and air conditioning | Keeps powerhouse and control room within temperature/humidity limits | | EOT crane in powerhouse | Erection and maintenance of heavy parts | | Lighting and emergency lighting | Normal and safe operation | | Earthing and lightning protection | Personnel and equipment safety | | Water supply and sewerage, oil handling and purification | Plant services | | Workshop, stores, CCTV and security | Maintenance and safety | | Intake and gate control, desander flushing, trash rack cleaning | Water conveyance operation | In addition, the plant provides **ancillary services** to the grid: frequency regulation, reactive power/voltage support, spinning reserve and black start. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does the governor and oil pressure unit do in a Francis turbine plant? A. Cools the generator B. Moves wicket gates to control speed and load C. Stops the rotor at low speed D. Removes leakage water Answer: B. The governor positions the wicket gates, with an accumulator storing oil pressure. (Key point: Governor function) 2. What is the function of the main inlet valve? A. Brake the rotor B. Isolate the turbine from the penstock, including emergency closure C. Supply cooling water D. Flush the desander Answer: B. The butterfly or spherical valve isolates the turbine from the penstock. (Key point: Main inlet valve) 3. What does the brake and jacking system do? A. Stops the rotor at low speed and lifts it for inspection B. Provides excitation C. Fills the draft tube D. Controls ventilation Answer: A. Brakes stop the rotor at low speed; jacks lift the rotor for inspection. (Key point: Brake and jacking) 4. How do ancillary systems differ from auxiliary systems? A. Ancillary systems are plant-wide support (fire, HVAC, crane, lighting) not part of energy conversion B. Ancillary systems drive the turbine C. They are identical D. Ancillary systems only include batteries Answer: A. Auxiliary systems help the unit start, run and stop; ancillary systems are general plant support. 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