T/F: 765 kV stability limit vs thermal limit

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.