---
title: "Consequences of P on f and Q on V"
subject: "Power System Analysis II"
chapter: 1
key: "semester_5/power_system_analysis_ii:pf-qv-consequences-control"
source: https://www.bctengineer.com/q/semester_5/power_system_analysis_ii/pf-qv-consequences-control
site: BCT Engineering
---

# Consequences of P on f and Q on V

Power System Analysis II · Chapter 1: Interconnected Power System

- Asked in: 2082 Bhadra (new course), 3+2 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/power_system_analysis_ii/pf-qv-consequences-control
- Whole chapter: https://www.bctengineer.com/md/semester_5/power_system_analysis_ii/chapter-1

## Question

Explain the consequences of real power on frequency and reactive power on voltage problems in a power system. Also, illustrate the basic control mechanisms in brief to address these are in the real power system.

## Short answer

Real power deficit lowers frequency, risking turbine damage and load shedding; surplus raises frequency. Reactive deficit lowers bus voltage, risking motor overheating and collapse; surplus raises voltage, stressing insulation. Control: P-f via governor and AGC (ACE); Q-V via AVR, shunt compensators, and tap changers.

## Answer

#### Consequences
**Real power on frequency:** if load exceeds generation, the deficit is drawn from the kinetic energy of rotating machines, which slow down, so **frequency falls**; surplus generation makes frequency rise. Frequency is common to the whole interconnection. Large deviations cause under-frequency load shedding, damage to turbine blades, and slow motors.

**Reactive power on voltage:** if reactive demand exceeds local supply, Q flows in over the line reactance and causes a drop $\Delta V \approx QX/V$, so **bus voltage falls**; excess Q (light load, line charging) makes it rise. Voltage is a local quantity. Low voltage overheats motors and can lead to voltage collapse; high voltage stresses insulation.

#### Basic control mechanisms
```text
 P-f loop:  df -> Governor -> Valve -> Turbine -> Pm
                    ^ AGC (integral of ACE)
 Q-V loop:  dV -> AVR -> Exciter -> Field -> Ef -> Q
```
- **P–f:** speed governor (primary, with droop) and Automatic Generation Control acting on $ACE = \Delta P_{tie} + B\Delta f$ (secondary) to bring frequency back to 50 Hz.
- **Q–V:** generator AVR/excitation control, shunt capacitors and reactors, synchronous condensers, SVC/STATCOM and on-load tap changers.

## Check yourself

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

1. If load exceeds generation, frequency:
   A. rises
   B. stays the same
   C. falls as rotors lose kinetic energy
   D. depends only on voltage
   Answer: C. Deficit comes from stored kinetic energy. (Key point: Frequency fall)

2. A consequence of reactive deficit is:
   A. frequency rise
   B. turbine damage only
   C. none
   D. low voltage, motor overheating and possible voltage collapse
   Answer: D. Q shortage lowers voltage. (Key point: Low voltage)

3. Q-V control uses:
   A. AVR, capacitors and reactors, condensers, SVC/STATCOM, OLTC
   B. governor and AGC
   C. ACE only
   D. speed changer
   Answer: A. Governor/AGC are for P-f. (Key point: Controls)

## Related in Power System Analysis II

- [Real power/frequency control in interconnected system](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Areal-power-frequency-control) — Power System Analysis II, chapter 1
- [Effects of real and reactive power mismatch](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Apf-qv-mismatch-effects) — Power System Analysis II, chapter 1, asked 4×
- [Interconnection advantages and real power-frequency balance](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Ainterconnection-advantages-frequency) — Power System Analysis II, chapter 1, asked 3×
- [Reactive power and voltage balance](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Areactive-power-voltage-balance) — Power System Analysis II, chapter 1
- [Generator reactive power at constant terminal voltage](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Agenerator-reactive-power-infinite-bus) — Power System Analysis II, chapter 1
- [Interconnected system load-frequency and Q-V control](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Ainterconnection-control-schematic) — Power System Analysis II, chapter 1

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- [Basic structure of power system](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Abasic-structure-power-system) — Power System Analysis I, chapter 1
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