---
title: "Effect of load rejection"
subject: "High Voltage Engineering"
chapter: 3
key: "semester_8/high_voltage_engineering:load-rejection-effect"
source: https://www.bctengineer.com/q/semester_8/high_voltage_engineering/load-rejection-effect
site: BCT Engineering
---

# Effect of load rejection

High Voltage Engineering · Chapter 3: Overvoltages in Power System

- Asked in: 2070 Bhadra, 4 marks
- Video: [LINE DROPPING AND LOAD REJECTION BY VIMAL](https://www.youtube.com/watch?v=IggukjLHXz8) — Engineering.subjects
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/high_voltage_engineering/load-rejection-effect
- Whole chapter: https://www.bctengineer.com/md/semester_8/high_voltage_engineering/chapter-3

## Question

Explain in brief the effect of load rejection in power system.

## Short answer

Load rejection removes the load current, so the voltage drop across generator and transformer reactances disappears, causing a temporary power-frequency overvoltage. Ferranti effect raises the receiving-end voltage on a long open line, and generator overspeed and possible self-excitation add to it. This sustained overvoltage sets surge-arrester rating.

## Answer

**Load rejection** is the sudden loss of a large load, for example when a breaker at the receiving end trips while the generator stays connected to the line. It causes a **temporary overvoltage** that can last from several cycles up to seconds.

#### Effects

1. **Loss of voltage drop.** Before rejection, the load current caused a drop across the generator and transformer reactances. When it vanishes, the terminal voltage jumps towards the internal emf $E'$, which may be 1.1–1.2 pu.
2. **Ferranti effect.** The line is suddenly at no load. Its charging current raises the receiving-end voltage by the factor $1/\cos\beta l$, which is significant on long EHV lines.
3. **Generator overspeed.** The prime-mover input cannot be cut instantly, so the machine accelerates. Voltage rises in proportion to speed (and frequency), until the governor acts.
4. **Self-excitation.** A generator feeding a long open line sees a capacitive load. Its armature current then *aids* the field flux, and the voltage can build up uncontrolled if the line charging exceeds the machine's capability.
5. The overvoltage is at power frequency and lasts long. Surge arresters cannot absorb it for long, so it sets the **arrester rating** and the insulation design for temporary overvoltage.

```text
  Gen --[X']--+------- long line -------x CB trips
              |                         (load lost)
          V rises: E' + Ferranti + overspeed
```

#### Remedies

- Shunt reactors at the line ends.
- Fast-acting AVR and governor; SVC or STATCOM.
- Generator transformer and line protection that trips the line or generator if the voltage stays high.

## Check yourself

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

1. Load rejection causes what type of overvoltage?
   A. Fast front lightning surge
   B. Temporary power-frequency overvoltage lasting cycles to seconds
   C. Switching surge lasting microseconds
   D. Undervoltage
   Answer: B. It is a temporary overvoltage at power frequency, which can last from several cycles to seconds. (Key point: Nature of overvoltage)

2. Why does the terminal voltage jump when the load is lost?
   A. Frequency falls
   B. Armature resistance increases
   C. The drop across generator and transformer reactance vanishes and the voltage goes towards the internal emf
   D. The line capacitance is removed
   Answer: C. With load current gone, the drop disappears and the terminal voltage approaches E', 1.1-1.2 pu. (Key point: Loss of voltage drop)

3. Which two further effects add to the voltage after load rejection?
   A. Generator overspeed and the Ferranti effect
   B. Corona and skin effect
   C. Back-flashover and current chopping
   D. Ferroresonance and arc extinction
   Answer: A. The prime mover speeds up the generator, raising voltage and frequency, while the unloaded line shows the 1/cos βl rise. (Key point: Overspeed and Ferranti)

## Related in High Voltage Engineering

- [True/false: load rejection raises terminal voltage](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Atf-load-rejection-voltage-rise) — High Voltage Engineering, chapter 3
- [Charging reactive power of line at no load](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Acharging-reactive-power-no-load) — High Voltage Engineering, chapter 3, asked 2×
- [FACTS devices changing the power system](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Afacts-changes-power-system) — High Voltage Engineering, chapter 1
- [Vision of power system 25 years ahead](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Apower-system-vision-25-years) — High Voltage Engineering, chapter 1
- [Issues of EHV-AC power transmission](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Aehv-ac-transmission-issues) — High Voltage Engineering, chapter 1

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- [Pile capacity in sand, group effect, load test](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-capacity-sand-group-load-test) — Foundation Engineering, chapter 8
- [Armature reaction at different load power factors](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aarmature-reaction-power-factor-effect) — Electric Machines II, chapter 1, asked 7×
- [Bunching, field effect and high power oscillator](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Abunching-field-effect-oscillator) — RF and Microwave Engineering, chapter 5