---
title: "P-f and Q-V control loops"
subject: "Power Plant Equipment"
chapter: 2
key: "semester_7/power_plant_equipment:pf-qv-control-loops"
source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/pf-qv-control-loops
site: BCT Engineering
---

# P-f and Q-V control loops

Power Plant Equipment · Chapter 2: Power/Frequency Control in Hydro Generator System

- Asked in: 2071 Shrawan, 8 marks; 2079 Baisakh, 8 marks; 2073 Shrawan, 4 marks (asked 3 times)
- Video: [PF and QV Loops | Real power Frequency | Reactive power Voltage control | Power system operation](https://www.youtube.com/watch?v=ubaTYzK-Tk0) — Simple n Easy Learning
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/pf-qv-control-loops
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-2

## Question

Explain with neat diagram, the P-F loop and Q-V loop in hydro generating system.

## Short answer

The P-f loop uses a speed governor and hydraulic amplifier to adjust turbine gates, controlling real power and frequency. The Q-V loop uses an automatic voltage regulator and exciter to adjust field current, controlling reactive power and terminal voltage. Both loops are weakly coupled and analyzed separately.

## Answer

A hydro generating unit has two main control loops. The **P-f loop** (automatic load frequency control, ALFC) controls real power and frequency through the turbine gate. The **Q-V loop** (automatic voltage regulator, AVR) controls reactive power and terminal voltage through the field excitation.

```text
 P-f loop (ALFC)

 f_ref -->(+)--> speed governor --> hydraulic
           ^ -                      amplifier
           |                            |
           |                            v
   frequency sensor               wicket gates
           ^                            |
           |                            v
           +----- speed ------ turbine-generator
```

```text
 Q-V loop (AVR)

 V_ref -->(+)--> amplifier --> exciter
           ^ -                    |
           |                      v
   PT + rectifier          generator field
           ^                      |
           |                      v
           +------ Vt ----- generator terminals
```

#### P-f loop
1. A speed (frequency) sensor measures the shaft speed.
2. The speed governor compares it with the reference and, through the speed droop, gives a command to the hydraulic amplifier (pilot valve and servomotor).
3. The servomotor moves the wicket gates (or needle), changing the water flow and the turbine power $P_m$.
4. Real power balance decides frequency: if $P_m < P_e$ frequency falls. The loop restores balance. Primary control uses droop; secondary control changes the load reference to restore 50 Hz.
5. It is slow (seconds) because of large mechanical and water time constants.

#### Q-V loop
1. A potential transformer measures terminal voltage; it is rectified and filtered.
2. The comparator finds the error $V_{ref} - V_t$.
3. The amplifier boosts it and drives the exciter, which changes the field current of the generator.
4. Field current changes the generated emf and hence the reactive power output and terminal voltage.
5. It is fast (fraction of a second) because electrical time constants are small. Stabilising feedback is used to avoid oscillations.

#### Interaction
Real power depends mainly on rotor angle and frequency, while reactive power depends mainly on voltage magnitude. Also, the AVR loop is much faster than the P-f loop. So the two loops are weakly coupled and are analysed and designed separately (decoupled control).

## Check yourself

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

1. Which pairing is correct?
   A. P-f loop controls reactive power through the field
   B. Q-V loop controls real power through the gates
   C. P-f loop controls real power and frequency via the turbine gate; Q-V loop controls reactive power and voltage via excitation
   D. Both loops act on the gate
   Answer: C. ALFC acts on the gates; the AVR acts on the field excitation. (Key point: Purpose of each loop)

2. How does the Q-V loop correct a voltage error?
   A. Terminal voltage is measured by a PT, compared with V_ref, amplified, and the exciter changes the field current
   B. The gate opening is changed
   C. The governor droop is changed
   D. The frequency sensor adjusts the voltage
   Answer: A. Field current changes the generated emf, hence the terminal voltage and reactive power. (Key point: Q-V loop operation)

3. In the P-f loop, what does secondary control do?
   A. Changes the load reference to restore frequency to 50 Hz
   B. Provides primary droop action
   C. Controls the field current
   D. Closes the main inlet valve
   Answer: A. Primary control uses droop and leaves an error; secondary control shifts the load reference to remove it. (Key point: Primary vs secondary control)

4. Which statement on loop speed and coupling is correct?
   A. P-f loop is faster than Q-V loop
   B. Both loops are equally fast and strongly coupled
   C. Q-V loop is slow because of water time constants
   D. Q-V loop is much faster than P-f loop, so they are weakly coupled and designed separately
   Answer: D. Electrical time constants are small, while the P-f loop has large mechanical and water time constants. (Key point: Speed of the loops)

## Related in Power Plant Equipment

- [Functional block diagram of excitation control](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aexcitation-control-block-diagram) — Power Plant Equipment, chapter 3
- [Operation of pumped storage hydro plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Apump-storage-operation) — Power Plant Equipment, chapter 1, asked 2×
- [Control of water delivery to turbine](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Awater-delivery-control) — Power Plant Equipment, chapter 1
- [Speed control by inertia and load damping](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Ainertia-damping-speed-control) — Power Plant Equipment, chapter 2, asked 6×
- [Function of excitation system and its transfer function](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aexcitation-function-tf) — Power Plant Equipment, chapter 3, asked 2×
- [Fire fighting system in power station](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Afire-fighting-power-station) — Power Plant Equipment, chapter 4, asked 5×

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