Wind turbine Types 1-4 — SCIG / WRIG / DFIG / Full-converter PMSG

Reference notes

Wind turbines are classified into Types 1-4 by their generator + power-converter topology. Each successive type increases controllability and grid integration but adds power-electronic hardware. Type 3 (DFIG) dominated 2005-2020 onshore; Type 4 (full-converter PMSG) dominates offshore and all new installations since 2018.

Wind power fundamentals

• ρ = air density ≈ 1.225 kg/m³ at sea level (decreases ~10% per 1000 m altitude).
• A = rotor swept area = π·R² for a single rotor.
• v = wind speed.
• Cube law on wind speed: doubling wind speed → 8× the power. Drives site selection.
• Modern utility turbines achieve C_p ≈ 0.45-0.50 at rated wind speed.
• Tip-speed ratio λ = ω·R / v; optimal λ ≈ 6-9 for 3-blade designs.

Wind power curve

• Cut-in speed (3-4 m/s): below this, no net power.
• Cube-law region (cut-in to rated): P ∝ v³; MPPT via blade pitch and rotor speed.
• Rated speed (12-14 m/s): turbine reaches nameplate output.
• Flat (rated) region (rated to cut-out): blades pitch to spill excess wind; output held constant.
• Cut-out speed (25 m/s): turbine shuts down to protect blades; blades fully feather, rotor parks.

Type 1 — Squirrel-Cage Induction Generator (SCIG), fixed speed

• SCIG directly connected to grid; no power-electronic converter.
• Operates near synchronous speed (~1-2% slip).
• Capacitor bank supplies reactive power (~25-30% of rated MVAR).
• Soft-starter ramps voltage during startup; bypassed at full speed.
• Pros: simple, rugged, low cost.
• Cons: fixed speed (no MPPT, 5-15% lower energy capture); voltage flicker on local grid; cannot provide reactive support; limited voltage-sag ride-through.
• Common 1990s-2000s; rare in new installations after ~2005.
• Examples: Vestas V47, Bonus, Nordtank.

Type 2 — Wound-Rotor Induction Generator (WRIG), variable rotor R

• WRIG with external rotor resistance varied by thyristor-controlled chopper.
• Variable rotor R changes slope of torque-speed curve → limited variable speed (typical ±10% slip range above synchronous).
• Pros over Type 1: limited variable-speed for higher energy capture, reduced drive-train torque oscillations.
• Cons: slip power dissipated as heat in rotor resistors (efficiency penalty).
• Niche 1990s-2000s; Vestas OptiSlip notable example.

Type 3 — Doubly-Fed Induction Generator (DFIG), partial converter

• Wound-rotor induction machine. STATOR directly to grid; ROTOR via back-to-back IGBT converter through slip rings.
• Partial converter ~25-30% of rated MVA — back-to-back IGBT with shared DC link capacitor bank; controllable power factor at the grid-side converter. Much smaller and cheaper than Type 4's full converter.
• Variable speed ±30% around synchronous. Super-synchronous (rotor exports) or sub-synchronous (rotor imports).
• Stator always generates at grid frequency.
• Pros: MPPT capability; reactive-power control; smaller cheaper converter.
• Cons: slip rings + brushes (maintenance); limited voltage-sag ride-through (needs crowbar to protect rotor converter); SSCI risk with series-compensated lines.
• DOMINANT onshore architecture 2005-2020 (~80% of installed capacity in that era).
• Examples: GE 1.5/2 MW, Vestas V90/V100, Siemens SWT, Gamesa G80-G87, Suzlon.

Type 4 — Full-converter (PMSG / SCIG with full converter)

• Full back-to-back AC-DC-AC converter at 100% of generator MVA.
• Generator options:
  • PMSG — Permanent-Magnet Synchronous Generator: dominant choice. 90-95% efficiency. Direct-drive feasible (no gearbox). Multi-pole, slow-speed (10-20 RPM) for direct-drive.
  • SCIG with full converter: older niche use.
  • EE-IM (electrically-excited): uncommon.
• Direct-drive: eliminates gearbox failure mode (critical for offshore where repair is expensive).
• Pros: total decoupling from grid; any rotor speed; full reactive/V/F support; suitable for grid-forming; less SSCI risk; required for offshore.
• Cons: full-rated converter is expensive (1.5-2× DFIG cost); generator-side harmonics need filtering; higher converter losses (twice).
• Dominant in all offshore + most new onshore since 2018.
• Examples: Vestas EnVentus / V162-7.0, Siemens-Gamesa SG 14-222 DD, GE Cypress / Haliade-X 14 MW, MingYang 18 MW.

Type comparison summary

Standards and grid-code compliance

• IEEE 2800-2022 — Interconnection of Inverter-Based Resources to bulk power system. V/F ride-through, reactive support, fault current, harmonics, anti-islanding.
• FERC Order 661-A — wind reactive-power capability (typical 95% lag to 95% lead) and ride-through.
• FERC Order 901 (2024) — IBR reliability standards after Odessa Texas 2021 cascading-trip event. Improved ride-through, accurate dynamic modeling, faster recovery.
• IEEE 1547-2018 — DER interconnection. Applies to distributed wind under 20 MW.
• IEEE P2800.2 (in development) — grid-forming-specific requirements.

Grid-forming wind — the frontier

• Type 4 full-converter turbines controlled as VOLTAGE SOURCES (not current sources).
• Provides synthetic inertia (programmable H_eff = 1-3 s) and damping.
• Required for high-IBR grids approaching 100% renewable (Hawaii, Ireland, AEMO).
• Pilots: AEMO South Australia, Eirgrid SOEF, Hawaiian Electric, ERCOT.
• Solves stability issues that plague grid-following on weak grids (SCR < 3) — see L101 small-signal-stability-pss.

Offshore wind

• Almost entirely Type 4 direct-drive PMSG (gearbox failure offshore is hugely expensive).
• Modern turbine sizes: 8 MW (2020) → 14-15 MW (2024) → 18+ MW expected by 2027.
• Notable projects: Hornsea (UK), Vineyard Wind (US), Dogger Bank (UK), Empire Wind (US).
• Capacity factor 35-50% (better wind resource than onshore).
• Floating offshore wind emerging for deep-water sites (Norway, California, Atlantic).

Capacity factor comparison

Repowering and future trends

• Replacing aging Type 1/2/3 turbines with new Type 4 designs is a major investment trend.
• Floating offshore wind for deep-water sites.
• Hybrid wind-solar-storage projects.
• Hydrogen production from wind via electrolysis.
• Multi-rotor turbines (research stage).