---
title: "T/F: HVDC better for longer lines"
subject: "Transmission and Distribution System Design"
chapter: 1
key: "semester_8/transmission_and_distribution_system_design:tf-hvdc-longer-length"
source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tf-hvdc-longer-length
site: BCT Engineering
---

# T/F: HVDC better for longer lines

Transmission and Distribution System Design · Chapter 1: Introduction

- Asked in: 2070 Magh, 1+3 marks
- Video: [If HVDC Is Better, Why Don’t We Use It Everywhere?](https://www.youtube.com/watch?v=_2qB_HGHpIg) — LifeAda English
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tf-hvdc-longer-length
- Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-1

## Question

State whether the following statement is TRUE or FALSE. Justify your answer with a brief explanation: HVDC transmission system is better choice in case of longer length than HVAC transmission system.

## Short answer

True. HVDC avoids line reactance and stability limits, has no charging current, fewer conductors, and lower line losses. Despite costly terminals, beyond 500–800 km overhead, HVDC becomes the more economical choice for long-distance transmission.

## Answer

**TRUE.**

- **No stability limit:** AC power transfer $P = \frac{V_s V_r}{X}\sin\delta$ falls as line reactance $X$ grows with length; HVDC has no reactance in steady state, so it has no length-based stability limit.
- **No charging current and no need for intermediate compensation** on long lines.
- **Lower line cost and loss:** two conductors instead of three, smaller towers, narrower corridor, no skin effect and lower corona loss.
- **Economics:** HVDC terminal stations are costly, but the line saving per km is large. Beyond the break-even distance (about 500–800 km overhead, 30–50 km cable) HVDC has lower total cost.

So for long lengths (above break-even distance) HVDC is the better choice; for short lines HVAC remains cheaper.

## Check yourself

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

1. HVDC is a better choice than HVAC for longer lines. This is:
   A. False
   B. True
   C. True only for under 50 km overhead
   D. Depends on voltage only
   Answer: B. Beyond the break-even distance HVDC has lower total cost and no length-based limit. (Key point: Verdict)

2. Why does AC power capability fall with length while HVDC does not?
   A. X grows with length in P = VsVr sin(delta)/X; DC has no reactance in steady state
   B. DC has higher resistance
   C. AC has no losses
   D. DC lines use more conductors
   Answer: A. AC line reactance increases with length, reducing P; DC has none in steady state. (Key point: Stability limit)

3. Typical break-even distances are:
   A. 5-10 km overhead
   B. 500-800 km overhead, 30-50 km cable
   C. 50-80 km overhead, 5 km cable
   D. 2000 km overhead, 500 km cable
   Answer: B. Above these distances HVDC has lower total cost. (Key point: Break-even)

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