---
title: "Ferranti effect and skin effect"
subject: "Power System Analysis I"
chapter: 6
key: "semester_4/power_system_analysis_i:ferranti-and-skin-effect"
source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/ferranti-and-skin-effect
site: BCT Engineering
---

# Ferranti effect and skin effect

Power System Analysis I · Chapter 6: Performance Analysis

- Asked in: 2081 Chaitra (new course), 4 marks
- Video: [skin effect, ferranti effect, proximity effect and transposition of transmission line *Bengali*](https://www.youtube.com/watch?v=6pI4VW1wDrY) — Electricaledu
- 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/ferranti-and-skin-effect
- Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-6

## Question

Briefly explain Ferranti effect and skin effect.

## Short answer

Ferranti effect: receiving-end voltage exceeds sending-end voltage under no-load/light-load due to line capacitance charging current flowing through series inductance. Skin effect: AC current concentrates near conductor surface because inner filaments link more flux, raising effective resistance; increases with frequency and conductor size.

## Answer

#### Ferranti effect
The **Ferranti effect** is the rise of receiving-end voltage above the sending-end voltage when a long or medium line is on **no load or very light load**.

- At light load, the line's shunt capacitance draws a **charging current** $I_C$ that leads the voltage by $90^\circ$.
- This current flows through the series inductance and produces a drop $jX_L I_C$, which points **opposite** to $V_R$ (since $jX_L \cdot jI = -X_L I$), so $V_S = V_R - X_L I_C$ is smaller than $V_R$.
- For a nominal-$\pi$ line at no load (neglecting $R$, with $L$ and $C$ the total line values), $V_S = A V_R$ with $A = 1 - \omega^2 LC/2$, so $V_R = V_S/(1-\omega^2LC/2) > V_S$.
- The rise is roughly proportional to the **square of the line length** and is large for EHV lines (about 5–10 % at 400 km).

```text
        jXL*Ic
   Vr --------> Vs  (Vs shorter than Vr)
   ^
   | Ic (leads Vr by 90 deg)
```

Remedy: shunt reactors at the receiving end, or keeping some load connected.

#### Skin effect
The **skin effect** is the tendency of **alternating current** to crowd towards the **outer surface** of a conductor instead of spreading uniformly over its cross-section.

- The inner filaments of a conductor link more flux than the outer ones, so they have higher inductive reactance. Current therefore takes the outer, lower-reactance path.
- The effective area carrying current falls, so the **AC resistance is greater than the DC resistance**.
- It increases with **frequency**, **conductor diameter**, **permeability** and **conductivity**. At 50 Hz it is small for normal conductors (a few percent) but is not negligible for large conductors.
- It is absent for DC.

Reduction: use **stranded conductors (ACSR)** or hollow/tubular conductors; in ACSR the steel core sits at the centre, where little current flows anyway.

## Check yourself

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

1. Why does V_R exceed V_S at light load?
   A. Leading charging current through the series inductance gives a drop that makes V_S = V_R - X_L I_C
   B. Lagging current adds drop in V_S
   C. Resistance causes voltage gain
   D. Corona raises voltage
   Answer: A. At no load, V_S = A V_R with A = 1 - w^2 LC/2 < 1. (Key point: Ferranti cause)

2. What causes skin effect?
   A. Outer filaments have higher reactance
   B. Inner filaments link more flux, have higher reactance, so AC crowds to the outer surface
   C. Voltage gradient at surface
   D. Heating from corona
   Answer: B. Current takes the lower-reactance outer path. (Key point: Skin effect cause)

3. What is the main consequence of skin effect?
   A. AC resistance is lower than DC
   B. Resistance is unchanged
   C. AC resistance is higher than DC resistance, increasing with frequency and diameter
   D. It occurs for DC as well
   Answer: C. Smaller effective area carrying current raises resistance. (Key point: Skin effect consequence)

## Related in Power System Analysis I

- [Ferranti effect and measures to limit it](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aferranti-effect-measures) — Power System Analysis I, chapter 6, asked 2×
- [Skin effect and proximity effect](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Askin-and-proximity-effect) — Power System Analysis I, chapter 4, asked 3×
- [Skin effect with AC and DC current](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Askin-effect-ac-dc) — Power System Analysis I, chapter 4
- [Skin effect in transmission lines](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Askin-effect-brief) — Power System Analysis I, chapter 4

## Related in other subjects

- [Reducing Ferranti effect mathematically](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Aferranti-effect-reduction) — High Voltage Engineering, chapter 3
- [Negative skin friction: causes, effects, remedies](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Anegative-skin-friction-effects) — Foundation Engineering, chapter 8, asked 4×