---
title: "Function of reactor in power system"
subject: "Power Plant Equipment"
chapter: 4
key: "semester_7/power_plant_equipment:reactor-function"
source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/reactor-function
site: BCT Engineering
---

# Function of reactor in power system

Power Plant Equipment · Chapter 4: Substation Equipments

- Asked in: 2076 Chaitra, 2 marks; 2078 Bhadra, 2 marks; 2073 Chaitra, 2 marks (asked 3 times)
- Video: [Why Are Shunt Reactors Used in Power Systems? | TheElectricalGuy](https://www.youtube.com/watch?v=Rdl540lDIAM) — Gaurav J - TheElectricalGuy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/reactor-function
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-4

## Question

What is main function of reactor in power system network?

## Short answer

The main function of a reactor is to limit short-circuit current by adding series inductive reactance, reducing fault current and fault MVA. This localizes the fault, preventing voltage collapse on healthy parts. Shunt reactors also absorb reactive power to control over-voltage on long lightly loaded lines.

## Answer

A **reactor** is a coil of large inductive reactance and very small resistance. Its main function in a power network is to **limit the short-circuit (fault) current** to a value that circuit breakers and equipment can safely handle.

- It raises the reactance between the source and the fault, so $I_f = V/X$ falls and the fault MVA falls.
- It localises the fault so that the voltage on healthy parts of the system does not collapse.
- Shunt reactors also absorb reactive power on long lightly loaded lines to control over-voltage (Ferranti effect).

## Check yourself

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

1. What is the main function of a reactor in a power network?
   A. Limit the short-circuit current to a value breakers can handle
   B. Raise the voltage at the load end
   C. Improve the power factor of loads only
   D. Increase the fault level
   Answer: A. A series reactor adds reactance so fault current and fault MVA fall. (Key point: Main function)

2. Why does adding a reactor reduce the fault current?
   A. It adds resistance that stores the fault energy
   B. It increases the reactance between source and fault, so I_f = V/X falls
   C. It raises the source voltage
   D. It reduces the breaker rating
   Answer: B. Fault current is inversely proportional to the total reactance in the fault path. (Key point: How it limits current)

3. What do shunt reactors do on long lightly loaded lines?
   A. Supply reactive power to raise voltage
   B. Limit short-circuit current at the sending end only
   C. Absorb reactive power to control over-voltage (Ferranti effect)
   D. Reduce the line frequency
   Answer: C. Line charging produces excess VARs at light load; shunt reactors absorb them. (Key point: Shunt reactor)

## Related in Power Plant Equipment

- [Reactor function and ring bus-bar scheme](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aring-bus-reactor-scheme) — Power Plant Equipment, chapter 4
- [Excitation system with stabilizing transformer: transfer function](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aexcitation-stabilizing-transformer-model) — Power Plant Equipment, chapter 3, asked 13×
- [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×
- [Reactor to protect circuit breaker of small station](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Areactor-protect-circuit-breaker) — Power Plant Equipment, chapter 4
- [Feeder reactor: 40 and 30 MVA, 50% reduction](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Afault-40-30mva-50-percent) — Power Plant Equipment, chapter 4, asked 4×
- [Feeder reactor: 40 and 30 MVA, 40% reduction](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Afault-40-30mva-40-percent) — Power Plant Equipment, chapter 4, asked 4×

## Related in other subjects

- [Typical power system network and components](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Atypical-power-system-network-components) — Power System Analysis I, chapter 1
- [Generator-reactor system fault MVA](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Agenerator-reactor-fault-mva) — Power System Analysis II, chapter 3
- [Significance of main() function](https://www.bctengineer.com/question.md?key=semester_1%2Fcomputer_programming%3Asignificance-of-main) — Computer Programming, chapter 6
- [Vertical Rising Main system](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Avertical-rising-main) — Industrial Power Distribution and Illumination, chapter 5