Side-by-side comparison
| Parameter | AC Transmission | HVDC Transmission |
|---|---|---|
| Break-even Distance | Economical below 600–800 km (overhead) | Economical above 600–800 km overhead; 40–80 km for submarine |
| Skin Effect & Reactive Losses | Present; increases effective conductor resistance | Absent; full conductor cross-section used |
| Stability Limit | Power angle δ must stay < 90°; limits loadability | No stability limit; power controlled electronically |
| Reactive Compensation | SVC, STATCOM, capacitor banks needed | Converter itself provides reactive power control (VSC-HVDC) |
| Converter Station Cost | Simple transformers and switchgear | Expensive thyristor/IGBT valves; ±800 kV station ~₹2000 Cr |
| Fault Current | High AC fault current; circuit breakers well-developed | DC fault harder to interrupt; DC CB still maturing |
| Frequency Coupling | Interconnected systems must be same frequency | Asynchronous grids can be linked (back-to-back HVDC) |
| Losses per 1000 km | ~7–10% (reactive + resistive) | ~3–4% (resistive only) + ~0.6% converter losses each end |
| Indian Example | 765 kV Raigarh–Pugalur AC corridor | ±800 kV Bidar–Pugalur (8000 MW, 1900 km) |
| Control Flexibility | Limited; governed by network equations | Full electronic control of P and Q independently |
Key differences
AC transmission develops reactive power losses with distance and hits a stability ceiling where the power angle δ approaches 90°; HVDC has neither constraint. The converter station of a ±800 kV LCC-HVDC scheme costs ₹1500–2000 Cr per terminal, which is why DC is only chosen when line savings over long distance justify that fixed cost. Back-to-back HVDC (zero km DC line) links the Southern and Northern Indian grids asynchronously — impossible with AC. VSC-HVDC (IGBT-based) enables offshore wind integration and multiterminal grids, features LCC cannot offer.
When to use AC Transmission
Use AC transmission for distances under 600 km within a synchronous grid where converter costs cannot be justified. Example: the 765 kV Raigarh–Sipat AC line transmits 6000 MW over ~300 km using standard EHV technology at a fraction of HVDC converter cost.
When to use HVDC Transmission
Use HVDC when distance exceeds 800 km, for submarine cables beyond 40 km, or to link asynchronous grids. Example: the ±500 kV Rihand–Delhi HVDC bipole (1500 MW, 814 km) was India's first HVDC link and remains cheaper per MW·km than any equivalent AC alternative at that distance.
Recommendation
For bulk long-distance transmission above 800 km, choose HVDC — losses, stability, and right-of-way all favour it. For regional meshed networks under 500 km, choose AC; the converter cost savings and simpler protection make AC the default. Never choose HVDC for distances under 400 km unless connecting asynchronous systems.
Exam tip: Examiners ask students to derive the break-even distance formula comparing AC line cost per km plus compensation cost against HVDC line cost plus fixed converter terminal cost — practice this derivation with per-unit costs.
Interview tip: An interviewer at PGCIL or a core power company will ask you to explain why the Southern Regional Grid was connected to the rest of India via back-to-back HVDC before synchronisation — cite frequency asynchronism and the Vindhyachal back-to-back station as your answer.