---
title: "Control type of hydropower governor and traffic lights"
subject: "Control Systems"
chapter: 1
key: "semester_3/control_systems:hydro-governor-traffic-type"
source: https://www.bctengineer.com/q/semester_3/control_systems/hydro-governor-traffic-type
site: BCT Engineering
---

# Control type of hydropower governor and traffic lights

Control Systems · Chapter 1: Control System Background

- Asked in: 2073 Shrawan, 4×2 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/control_systems/hydro-governor-traffic-type
- Whole chapter: https://www.bctengineer.com/md/semester_3/control_systems/chapter-1

## Question

What kind of control system could have been in the following? Illustrate with necessary blocks and variables. i) Governor system of Hydropower Station ii) Traffic light system of Kathmandu

## Short answer

The governor system is a closed-loop control that measures turbine speed and adjusts water flow to keep frequency at 50 Hz despite load changes. The traffic light system is open-loop, using preset timers without measuring traffic flow, so no feedback corrects the output.

## Answer

#### i) Governor system of a hydropower station: closed-loop control
The governor keeps the turbine-generator speed (hence frequency, 50 Hz) constant while the electrical load changes. It measures the actual speed and corrects the water flow, so it is a **closed-loop (feedback)** system.

```text
 Nref +   e   +---------+ u +--------+ q +-------+ N  
 ---->(Σ)---->|Governor |-->|Servo + |-->|Turbine|-+->
      ^ -     |(PID)    |   |gate    |   |+ Gen. | |
      |       +---------+   +--------+   +-------+ |
      |                            load ΔPL -->    |
      +------------[Speed sensor / PMG]<-----------+
```

| Variable / block | Meaning |
|---|---|
| $N_{ref}$ | reference speed (synchronous speed, 50 Hz) |
| Speed sensor | measures actual speed $N$ (tachogenerator, PMG, frequency transducer) |
| $e$ | speed error $N_{ref} - N$ |
| Governor | controller (PID / droop) giving signal $u$ |
| Servomotor + gate | actuator; sets wicket gate / needle opening, i.e. water flow $q$ |
| Turbine-generator | plant; mechanical power and speed $N$ |
| $\Delta P_L$ | disturbance: change in electrical load |

When load rises, speed falls, $e$ becomes positive, the gate opens and the turbine power increases until speed returns to normal.

#### ii) Traffic light system of Kathmandu: open-loop control
The lights change according to preset timers; no measurement of traffic is fed back.

```text
 preset   +-----------+ signal +---------+ vehicle flow
 timing ->| Timer /   |------->| Traffic |------------->
          | controller|        | junction|
          +-----------+        +---------+
                                   ^ disturbance:
                                   | traffic volume
```

| Variable / block | Meaning |
|---|---|
| Input | preset green/amber/red durations |
| Controller | timer / sequencer circuit |
| Actuator | signal lamps |
| Plant | road junction and vehicles |
| Output | vehicle flow / queue length |
| Disturbance | varying traffic volume, accidents |

Because the output (queue length) never affects the controller, the system is **open loop**. It can be made closed loop by adding vehicle detectors or cameras whose data adjust the green time.

## Check yourself

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

1. The hydropower governor system is
   A. Open loop
   B. Closed loop
   C. Manual only
   D. Uncontrolled
   Answer: B. It measures actual speed and corrects water flow. (Key point: Governor type)

2. In the governor loop, the actuator is
   A. Speed sensor
   B. Servomotor with wicket gate or needle
   C. Load
   D. Reference
   Answer: B. It sets water flow q. (Key point: Governor actuator)

3. The disturbance in the governor system is
   A. The reference speed
   B. Change in electrical load
   C. The sensor
   D. The gate
   Answer: B. Load change delta PL affects the speed. (Key point: Disturbance)

4. The Kathmandu traffic light is open loop because
   A. Queue length never affects the controller
   B. It has speed sensor
   C. It compares error
   D. It uses PID
   Answer: A. Only preset timing drives the lights. (Key point: Traffic type)

## Related in Control Systems

- [Why Kathmandu traffic lights are open loop](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Atraffic-light-open-loop) — Control Systems, chapter 1
- [Traffic light system of Kathmandu: open or closed loop](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Atraffic-light-justify) — Control Systems, chapter 1
- [General block diagram of a control system](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Ageneral-bd-control-system) — Control Systems, chapter 1

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- [Blocks of a data acquisition system](https://www.bctengineer.com/question.md?key=semester_6%2Fdigital_control_system%3Adata-acquisition-system-blocks) — Digital Control System, chapter 1, asked 2×
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- [Traffic control devices; traffic signal advantages](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Atraffic-control-devices-signal-pros-cons) — Transportation Engineering II, chapter 1