---
title: "T/F: 765 kV stability limit vs thermal limit"
subject: "Transmission and Distribution System Design"
chapter: 2
key: "semester_8/transmission_and_distribution_system_design:tf-765kv-stability-vs-thermal"
source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tf-765kv-stability-vs-thermal
site: BCT Engineering
---

# T/F: 765 kV stability limit vs thermal limit

Transmission and Distribution System Design · Chapter 2: Transmission Voltage Level and Number of Circuits

- Asked in: 2072 Magh, 3 marks
- 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-765kv-stability-vs-thermal
- Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-2

## Question

State whether the following statement is TRUE or FALSE. Justify your answer with a brief explanation: For a 765 kV line transient stability limit is more than the thermal limit.

## Short answer

FALSE. For a 765 kV line the transient stability limit is less than the thermal limit because EHV lines are long and stability-limited; thermal capacity is very high (4000–6000 MW) while stability limit is only about 1–1.5 × SIL (around 2250–3375 MW).

## Answer

**FALSE.** For a 765 kV line the transient stability limit is **less** than the thermal limit (for practical line lengths).

- Thermal limit depends on conductor area; a 765 kV line uses 4–6 sub-conductor bundles, so its thermal capacity is very large (about 4000–6000 MW, i.e. 2–3 × SIL).
- The SIL of a 765 kV line is about $765^2/260 \approx 2250$ MW. Because EHV lines are long (300–600 km), the power limited by stability/voltage drop is only about 1–1.5 × SIL.
- Hence in EHV/UHV lines power transfer is **stability-limited**, not thermally limited; only short lines (< 80 km) are thermally limited.

That is why series compensation and FACTS are used on such lines: they raise the stability limit toward the thermal limit.

## Check yourself

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

1. For a 765 kV line, transient stability limit exceeds thermal limit:
   A. True
   B. False, it is less
   C. Equal
   D. True for short lines
   Answer: B. EHV lines are stability-limited. (Key point: Verdict)

2. SIL of 765 kV line is about:
   A. 500 MW
   B. 2250 MW
   C. 10000 MW
   D. 100 MW
   Answer: B. 765^2/260 = about 2250 MW. (Key point: Typical numbers)

3. To bring stability limit towards thermal limit, use:
   A. Shunt reactors only
   B. Series compensation and FACTS
   C. Smaller conductors
   D. Lower voltage
   Answer: B. They raise stability limit on long lines. (Key point: Remedy)

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