---
title: "Line as source and sink of reactive power, effects"
subject: "Power System Analysis I"
chapter: 6
key: "semester_4/power_system_analysis_i:line-source-sink-reactive-effects"
source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/line-source-sink-reactive-effects
site: BCT Engineering
---

# Line as source and sink of reactive power, effects

Power System Analysis I · Chapter 6: Performance Analysis

- Asked in: 2082 Kartik (new course), 3+3 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/line-source-sink-reactive-effects
- Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-6

## Question

Why do transmission lines perform as source and sink of reactive power? What are their effects on the power system?

## Short answer

A line's shunt capacitance generates reactive power, series inductance absorbs it. At light load, capacitance dominates, line acts as source, causing voltage rise; at heavy load, inductance dominates, line acts as sink, causing voltage drop and increased losses. Compensation (reactors/capacitors) is needed to maintain voltage.

## Answer

A transmission line has both **series inductance** and **shunt capacitance**, so it both absorbs and generates reactive power. Which one dominates depends on the load.

#### Why a line acts as a source or a sink
- **Shunt capacitance generates** reactive power: $Q_C = \omega CV^2$ per phase (per unit length). It depends on voltage only, so it is almost constant (voltage stays near rated).
- **Series inductance absorbs** reactive power: $Q_L = \omega LI^2$. It depends on the square of load current.

Comparing the two:

| Load condition | Comparison | Line acts as |
|---|---|---|
| Light load ($P < $ SIL) | $\omega CV^2 > \omega LI^2$ | **Source** of Mvar |
| At SIL ($P = $ SIL) | $\omega CV^2 = \omega LI^2$ | Neither (flat voltage) |
| Heavy load ($P > $ SIL) | $\omega CV^2 < \omega LI^2$ | **Sink** of Mvar |

At SIL, $V/I = \sqrt{L/C} = Z_c$, so the two balance exactly.

#### Effects on the power system
**As a source (light load):**
- Receiving-end voltage rises above sending voltage (Ferranti effect); overvoltages stress insulation.
- Generators may have to absorb Mvar (under-excited operation), which reduces their stability margin.
- Shunt reactors are needed at line ends.

**As a sink (heavy load):**
- Voltage falls along the line and at the receiving end; voltage regulation becomes poor.
- Extra reactive current increases losses and reduces the capacity for active power.
- Generators must supply more Mvar; risk of voltage instability or collapse.
- Shunt capacitors, series capacitors, synchronous condensers or SVCs are needed.

So the voltage profile of a line changes with load, and reactive compensation must be adjustable (switched reactors and capacitors, SVC/STATCOM) to keep voltage within limits throughout the day.

## Check yourself

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

1. Which line elements generate and absorb reactive power?
   A. Shunt capacitance generates (wCV^2); series inductance absorbs (wLI^2)
   B. Series inductance generates; shunt capacitance absorbs
   C. Resistance absorbs reactive power
   D. Both generate
   Answer: A. Charging depends on V^2, inductive absorption on I^2. (Key point: Source and sink)

2. At light load (P < SIL), the line acts as
   A. a sink of Mvar because wCV^2 > wLI^2
   B. a source of Mvar because wCV^2 > wLI^2
   C. a sink because current is large
   D. neither source nor sink
   Answer: B. Charging exceeds inductive absorption below SIL. (Key point: Light load)

3. What happens at P = SIL?
   A. Line absorbs maximum Mvar
   B. Voltage collapses
   C. Generated and absorbed Mvar balance, giving flat voltage, with V/I = Zc
   D. Line generates maximum Mvar
   Answer: C. At SIL, V/I = sqrt(L/C) = Zc, so the two are equal. (Key point: At SIL)

4. What is an effect of the line acting as a sink at heavy load?
   A. Voltage rises (Ferranti effect)
   B. Shunt reactors must be added
   C. Generators must absorb Mvar
   D. Voltage falls and regulation is poor; capacitors or SVC support is needed
   Answer: D. Heavy load absorbs Mvar, voltage sags, and capacitive support is required. (Key point: Heavy-load effect)

## Related in Power System Analysis I

- [Line as source and sink of reactive power](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aline-source-sink-reactive) — Power System Analysis I, chapter 6
- [Need and methods of reactive power compensation](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Areactive-compensation-need-methods) — Power System Analysis I, chapter 6, asked 2×
- [Reactive power proportional to voltage drop](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Areactive-power-voltage-drop) — Power System Analysis I, chapter 6

## Related in other subjects

- [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×
- [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×