HVDC fundamentals — LCC vs VSC, configurations

Reference notes

HVDC transmits power as direct current through specialized converter stations at each end of the line, providing benefits for very long distances, asynchronous grid interconnection, and underground or subsea cable systems. Use Next → to walk through when HVDC wins over HVAC, the two main converter technologies (LCC vs VSC), system configurations, control behavior, and notable installations.

Why HVDC over HVAC?

1. Long overhead lines (> 600–800 km) — HVDC total project cost (including expensive converter stations, ~$300/kW each end) becomes lower than HVAC because of: (a) no line-charging current, (b) no reactive support needs at intermediate substations, (c) no stability-driven substation requirements.
2. Asynchronous interconnection — connecting two AC grids of different frequencies (50 / 60 Hz) or independent frequency control. HVDC is the only practical option.
3. Underground / subsea cable > 80–100 km — AC cable charging current Q ≈ ω·C·V²·length (~1 Mvar/km at 400 kV; scales with ω, the angular frequency) fills the cable's thermal capacity at this length. HVDC has zero charging current, so any length works. NorNed (Norway-Netherlands) is 580 km subsea.

LCC — Line-Commutated Converter (classic)

• Thyristor (SCR) bridges. Thyristors gate ON only — turn off at natural AC zero crossing → "line-commutated".
• Standard topology: 12-pulse bridge (two 6-pulse + 30° phase-shift transformer) → cancels 5th and 7th AC harmonics, reducing filter requirements.
• Mature technology, since 1954. Highest power ratings: ±800 kV (UHV), 6–8 GW per project.
• Absorbs reactive power ~0.6 Mvar/MW → needs large filter banks for reactive compensation.
• Requires a strong AC system at each end (commutation strength).
• Converter losses ~0.7 % per station.

VSC — Voltage-Source Converter (modern)

• IGBT switches — can gate ON and OFF arbitrarily.
• Modern topology: Modular Multi-Level Converter (MMC) — each phase arm = many cascaded sub-modules, each a small IGBT half-bridge with capacitor. Synthesizes near-sinusoidal voltage without external filters.
• Commercialized 1999 by ABB (HVDC Light). Siemens (HVDC Plus), GE (HVDC MaxSine).
• Unique capabilities:
  • Black-start — can form voltage at AC terminals → works into dead grid or passive network.
  • 4-quadrant P/Q control — independent active and reactive power at each end → grid voltage support.
  • Polarity-stable — power reversal by reversing CURRENT (not voltage polarity) — critical for cable systems.
• Higher converter losses (~1.5 % per station) and lower max ratings today (±525 kV / 2 GW+) than LCC, but rapidly closing the gap.

System configurations

• Monopolar — single pole, ground/sea return. Cheap; used for short subsea cables.
• Bipolar — +pole, −pole, neutral. Standard for long overhead lines. If one pole fails, the other carries half capacity. High reliability.
• Back-to-back — both converters in one station, no DC line. Pure async tie. Eagle Pass, Dürnrohr.
• Multi-terminal — 3+ stations on a common DC bus. Quebec-NE, several Chinese systems.
• DC grids — true DC mesh networks. Emerging; Zhoushan (China) connects 5 islands; future North-Sea offshore-wind DC grid in planning.

Control characteristics

• HVDC converters SET the power flow directly. Compare AC: line power follows P_ij = (V_i·V_j / X_ij) · sin(δ_ij) — operators have only indirect control over the flow on any single line via phase-shifting transformers and FACTS.
• Typical ramp rates: 50–200 MW/min following operator dispatch.
• Emergency power modulation: ramp from full power to zero (or reverse) in seconds to arrest frequency excursions.
• AC fault ride-through: VSC can ride through and even supply reactive power during AC voltage dips. LCC trips on commutation failure when AC voltage drops below ~80 %.
• DC fault: harder than AC because no zero crossing. LCC reverses to inverter mode to drive current to zero. VSC needs DC circuit breakers (hybrid mechanical / solid-state, available since ~2015).

Notable installations