---
title: "Speed governing of a hydro generator"
subject: "Power Plant Design"
chapter: 4
key: "semester_8/power_plant_design:speed-governing-hydro"
source: https://www.bctengineer.com/q/semester_8/power_plant_design/speed-governing-hydro
site: BCT Engineering
---

# Speed governing of a hydro generator

Power Plant Design · Chapter 4: Electric System Design of a Power Plant

- Asked in: 2072 Asoj, 4 marks
- Video: [The Role of Governors in Hydroelectric Power Plants #governor #generator #turbine #hydropower#energy](https://www.youtube.com/watch?v=lkS-wOKc2MA) — Hydropower
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/power_plant_design/speed-governing-hydro
- Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4

## Question

Describe speed governing system of hydro generator.

## Short answer

The hydro governor senses speed, compares it with a reference, and through an electronic PID controller with permanent speed droop (4–5%) drives an electro-hydraulic servomotor to adjust wicket gates. A transient droop compensates water inertia, and gate closure speed is limited to prevent water hammer.

## Answer

The **speed governing system** of a hydro generator controls the turbine's water flow (wicket gate or needle opening) so that the speed, and hence the frequency, stays constant and load is shared between units running in parallel.

#### Components

- **Speed sensor:** a PMG or toothed wheel with a pick-up giving speed/frequency.
- **Governor controller:** electronic PID; compares the actual speed with the reference and includes **droop** (permanent speed regulation, usually 4–5 %).
- **Electro-hydraulic converter / proportional valve:** turns the electrical signal into oil flow.
- **Pilot and main distributing valves**, **servomotor**: move the wicket gates (Francis/Kaplan) or the needle and deflector (Pelton).
- **Oil pressure unit:** pump, accumulator and sump.
- **Feedback:** gate position transducer.

```text
 f_ref -->( + )--> PID + droop --> E/H valve --> Servomotor
            ^ -                                     |
            |                                 Wicket gates
         Speed sensor <-- Generator <-- Turbine <---+
```

#### Working

- Load increases: the speed falls, the error opens the gates, more water flows and the speed recovers.
- Droop makes each unit pick up load in proportion to its rating.
- Hydro units need **temporary (transient) droop** (dashpot action) for stable control, because the inertia of the water column (water starting time $T_w$) makes the turbine respond slowly and at first in the opposite direction. The gate closing speed is also limited to keep **water hammer** within safe limits. Pelton turbines use a **deflector** for quick load rejection.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does the hydro governor adjust to control speed?
   A. Transformer taps
   B. Grounding resistance
   C. Excitation
   D. Water flow through the wicket gate or needle
   Answer: D. Governor changes gate opening to match power to load, keeping frequency constant. (Key point: Purpose)

2. What happens when load increases?
   A. Deflector operates
   B. Speed falls and the gates open
   C. Gates stay fixed
   D. Speed rises and gates close
   Answer: B. Falling speed gives an error that opens the gates, so more water flows. (Key point: Load increase)

3. What is the purpose of droop (permanent speed regulation)?
   A. Reduce water
   B. Increase speed
   C. Raise voltage
   D. Share load among units in proportion to rating
   Answer: D. Droop (usually 4-5%) lets parallel units share load. (Key point: Droop)

4. Why must gate movement be slow in hydro?
   A. Low oil pressure
   B. Water hammer and water starting time
   C. Poor sensing
   D. High frequency
   Answer: B. Fast closing causes pressure surges, so temporary droop/dashpot slows the response. (Key point: Slow gates)

## Related in Power Plant Design

- [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×
- [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×
- [Solar PV versus hydro and their combination](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Asolar-pv-hydro-combination) — Power Plant Design, chapter 1
- [Generator choice and SCR versus stability](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Agenerator-choice-scr-stability) — Power Plant Design, chapter 4
- [Short circuit ratio and system stability](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Ascr-significance-stability) — Power Plant Design, chapter 4, asked 3×
- [Design: 310 m head plant, HV breaker](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2080-310m-hvcb) — Power Plant Design, chapter 4, asked 3×

## Related in other subjects

- [Shunt generator voltage build-up, critical resistance and speed](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-buildup-critical-define) — Electrical Machines I, chapter 3, asked 9×
- [Long shunt compound generator speed, lap and wave](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Agen-long-shunt-10kw-lap-wave) — Electrical Machines, chapter 3, asked 2×
- [Long shunt compound generator speed, 300 conductors](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Agen-long-shunt-10kw-300-conductors) — Electrical Machines, chapter 3, asked 2×
- [Long shunt compound generator speed](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Along-shunt-compound-300-conductors-speed) — Electrical Machine, chapter 3