PART IX Modern Systems
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Wind Generation

WTG types 1-4 · DFIG vs full-converter · capacity factor · IEEE 1547 ride-through · collector substation

Wind is the second pillar of NCEES "alternative power generation" alongside PV. Where PV is many small DC sources combined through inverters, wind is fewer large AC machines — squirrel-cage, doubly-fed, or fully-converter-fed — feeding a medium-voltage collector substation. The PE exam asks turbine type, capacity factor, ride-through, and reactive support.

The Big Picture — From Wind to Grid

StageComponentVoltageWhat happens
1. MechanicalRotor, gearbox (or direct drive)—Wind kinetic energy → shaft power
2. GeneratorSCIG / DFIG / PMSG0.69 kV typicalShaft power → electrical power, possibly variable freq
3. Pad-mount TxStep-up transformer at base of tower0.69 kV → 34.5 kVEach turbine has its own MV step-up
4. Collector34.5 kV underground feeders, multiple turbines per string34.5 kVUp to ~ 30 turbines per circuit
5. SubstationCollector → transmission step-up34.5 kV → 138 / 230 / 345 kVProject-level interconnect to BPS
6. POI / PCCPoint of interconnection per IEEE 1547 / NERCTransmissionWhere the wind plant meets the BES

The Power Equation — Why You Can't Capture It All

Power available in moving air through swept area A:

Wind power equation
Pwind = ½ · ρ · A · v³
Pcaptured = Pwind · Cp
ρ ≈ 1.225 kg/m³ at sea level. A = π · r² (rotor swept area, r = blade length). v = wind speed in m/s. Cp = power coefficient — bounded by Betz limit at 16/27 ≈ 0.593.

Real turbines achieve Cp ≈ 0.40–0.50 in their design wind speed range. Cubic dependence on v means doubling wind speed produces 8× the power — and a turbine rated for 12 m/s wind delivers only 1/8 its rated output at 6 m/s.

Wind Turbine Generator (WTG) Types — Type 1 to Type 4

TypeGeneratorSpeed rangeGrid interfaceEra / status
Type 1SCIG (squirrel-cage induction)Fixed (~ 1% slip)Direct to grid via Tx + soft starter1980s–early 2000s. Legacy.
Type 2WRIG (wound-rotor induction) with variable rotor resistance~ 10% rangeDirect to grid; rotor resistance modulates speed1990s–early 2000s. Legacy.
Type 3 (DFIG)Doubly-fed induction generator — stator direct to grid, rotor via slip-ring + back-to-back converter± 30% around synchronousStator direct; rotor through ~ 30% rated converter~ 60% of installed onshore wind 2010-2025. GE 1.5 / 1.7 / 2.x, Vestas V90 / V100.
Type 4Full-converter (PMSG or wound-field SG)Full range; turbine spins at any speed100% of power flows through back-to-back converterDefault for new offshore + most large onshore since 2018. Siemens-Gamesa SG, Vestas V150-class, GE Haliade.
Type 3 · DFIG Stator direct to grid · rotor via ~ 30%-rated back-to-back converter blades gearbox DFIG stator (~70% P) rotor slip-ring B2B converter ~ 30% rated 0.69 / 34.5 kV → collector Converter sized for 30% of nameplate → cheaper than Type 4 but rotor is exposed during faults. Crowbar circuit (resistor on the rotor) absorbs fault energy and protects the converter during grid disturbances. Type 4 · Full-Converter 100% of power through back-to-back converter — turbine fully decoupled from grid blades direct drive (no gearbox) PMSG RECT AC→DC DC link INV DC→AC 0.69 / 34.5 kV → collector Generator electrically isolated from grid — converter decides what voltage and frequency the grid sees. Grid sees only the inverter output: full 4-quadrant Q control, sub-cycle FRT response, no inrush, no synchronization issues. Permanent-magnet synchronous gen needs no field excitation; direct-drive eliminates the gearbox failure mode. Cost: full-rated converter is 2-3× more silicon than DFIG. Offset by simpler nacelle and grid-code compliance margin.

Why Type 4 Won

Type 3 dominated 2005-2018 because the converter was small (cheap). Type 4 took over because:

DriverType 3 limitType 4 advantage
Grid code complianceFRT performance limited by partial-converter design; crowbar trips disconnect generatorInverter inherently rides through; meets all major grid codes (FERC 661/827, NERC PRC-024)
Low-voltage ride-through (LVRT)Without crowbar — converter overcurrent. With crowbar — loses dynamic Q control during the faultContinues 4-quadrant Q support throughout the fault — improves grid stability
Reactive power range± 0.95 PF typical at full P± 0.85 PF or wider; even Q-only operation when P = 0
Gearbox failure3-stage gearbox is the #1 failure mode — typical replacement cost $250 k–500 kDirect-drive PMSG eliminates the gearbox entirely
OffshoreGearbox maintenance offshore is brutally expensiveDirect-drive + sealed nacelle wins offshore
Grid-forming roadmapDFIG cannot easily run grid-formingFull-converter platform supports GFM firmware (already standard on new fleet)

Capacity Factor — The Most-Quoted Number

Capacity factor (CF) is the ratio of actual energy produced over a period to the theoretical maximum if the turbine ran at nameplate the whole time. Wind CF is a function of site wind regime, turbine class (IEC 61400-1 Class I/II/III/IV), and availability.

Capacity factor
CF = (Annual energy MWh) / (Nameplate MW × 8,760 h)
Typical: onshore Class I wind ~ 35–45%. Class III ~ 30%. Offshore ≥ 50%. Solar PV is 18–28% for context.

IEEE 1547 + NERC — Fault Ride-Through

Wind plants must remain connected and supporting the grid during voltage and frequency disturbances of the magnitudes and durations defined by IEEE 1547-2018 (interconnection ≤ 60 kV) and NERC PRC-024-3 (BES generator performance, ≥ 100 kV). Both define an envelope: voltage vs time, frequency vs time. Inside the envelope you must stay connected; outside it you may trip.

DisturbanceIEEE 1547-2018 Cat IIINERC PRC-024-3
0.0 pu (zero) voltageRide through 0.16 s (10 cycles)Ride through 0.15 s
0.5 pu voltageRide through 0.32 sPer FERC Order 661
0.7 pu voltageRide through 2.0 sSustained envelope
0.9 pu voltageContinuous operationContinuous
1.2 pu voltageRide through 0.16 sRide through 0.5 s
57.0 Hz (or below)Trip after 0.16 sPer regional reliability standard
61.5 Hz (or above)Trip after 0.16 sPer regional

Reactive Power Support — Voltage Control at the POI

Wind plants are required to provide reactive power within at least ± 0.95 PF at the POI under FERC Order 827 (2016). Modern Type 4 plants ship with ± 0.85 PF or wider. The interconnect study sets the actual range. Reactive support comes from three places: WTG converters themselves (most economical), STATCOM at the substation (added when WTG range alone is insufficient), and switched capacitor banks (the cheapest, but discrete steps).

Worked Example 1 — Capacity Factor for a Sample Site

Example 01 · CF math100 MW wind farm in West Texas. 50 turbines × 2.0 MW. Annual energy production: 380,000 MWh.
  1. Theoretical max:
    100 MW × 8,760 h/yr = 876,000 MWh/yr
  2. Capacity factor:
    CF = 380,000 / 876,000 = 43.4%
  3. Equivalent full-load hours (FLH):
    FLH = CF × 8,760 = 3,802 hours/yr
  4. What this means physically: Plant produced its rated power equivalent for 3,802 hours; rest of the year it produced less or zero. In Texas this maps to good year-round wind, particularly nighttime and shoulder seasons.
  5. Compare to peers: Onshore Texas / Iowa / Kansas Class II–III sites typically 40–45% CF. Offshore N. Atlantic ≥ 50% (steadier wind). California Altamont ~ 25% (poorer wind class). Solar PV in the same Texas site would be ~ 24%.
  6. Revenue calculation: At $35/MWh PPA, plant revenue = 380,000 × $35 = $13.3 M/year. 100 MW × $1.5 M/MW capex ≈ $150 M project cost → simple payback ~ 11 years before tax credits, ~ 6 years with PTC.
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Why CF dominates wind project economics
Wind capex is roughly fixed per MW; revenue scales linearly with CF. A 5 percentage-point CF improvement (from 35% to 40%) is a 14% revenue lift on a multi-decade asset. This is why developers spend years on met-tower data before committing to a site, and why repowering existing sites with taller towers + larger rotors (extracting better wind aloft) is increasingly common.

Worked Example 2 — Sizing the Collector Substation

Example 02 · Collector + POI100 MW wind farm. 34.5 kV collector → 138 kV transmission interconnect.
  1. Plant rating + reactive range. 100 MW at the POI, ± 0.95 PF range. Apparent power required: S = 100 / 0.95 = 105.3 MVA. Round up to 120 MVA for design margin and future flex.
  2. Substation transformer: 1 × 120 MVA, 34.5 / 138 kV, ONAN/ONAF/OFAF cooling, %Z = 9.5% (typical for power transformers). Δ on HV / Y-grounded on LV (or vice versa per interconnect requirement).
  3. Collector feeder count. Industry rule of thumb: 25–35 MW per 34.5 kV feeder, typically 5–10 turbines per feeder. For 100 MW: 4 collector feeders × 25 MW each.
  4. Collector feeder current at full output. I = 25,000,000 / (√3 × 34,500 × 0.95) ≈ 440 A per feeder. Use 1000 kcmil Al underground cable with ampacity ≥ 500 A (NEC 310.60(C)(83)).
  5. Bus configuration. 100 MW is small enough for a single-bus or sectionalized-bus arrangement (per §22). Larger plants (≥ 250 MW) typically use ring bus or breaker-and-a-half.
  6. Reactive support assessment. WTG reactive range from 50 turbines ≈ ± 50 MVAR. POI requirement (FERC 827, ± 0.95 PF at 100 MW) = ± 32.9 MVAR. Adequate from WTGs alone — no STATCOM needed at the substation. Add later if FRT studies show transient droops below limits.
  7. Protection. 87T (Tx differential), 87B (bus differential), 50/51 backup, 21 (line distance toward POI), 67 (directional), 27/59 (bus voltage), 81 (frequency). All consistent with substation protection in §22.
What you can do after this section
  1. Identify Type 1 / 2 / 3 / 4 WTGs from a one-line and explain the converter implications.
  2. Apply the wind power equation P = ½ρAv³ and compute capacity factor from annual energy.
  3. Distinguish IEEE 1547 ride-through requirements from NERC PRC-024 and explain when each applies.
  4. Size a wind plant collector substation transformer including reactive headroom for FERC 827 PF range.
  5. Explain why most new fleet uses full-converter direct-drive PMSG.

Drill — Quick Self-Check

Drill 1 · Betz limit

Maximum theoretical Cp is ___?

Drill 2 · Type 3 vs Type 4

In a Type 3 DFIG, what fraction of total power flows through the converter?

Drill 3 · Cube law

If wind speed doubles, available wind power increases by what factor?

Drill 4 · Capacity factor

Plant produced 175,000 MWh from 50 MW nameplate. CF?

Drill 5 · Collector voltage

Standard wind farm collector voltage in North America?

If You See THIS, Think THAT

If you see…Think / use…
"Type 3" or "DFIG"Doubly-fed induction; ~ 30% partial-rated converter; legacy onshore standard.
"Type 4" or "PMSG"Full-converter; default for new offshore and most new onshore.
"Betz limit"Theoretical max Cp = 16/27 ≈ 0.593.
"Capacity factor"Annual MWh / (MW × 8,760). Typical 30–50% wind.
"34.5 kV collector"Standard MV collector network voltage in NA wind plants.
"FRT" / "LVRT"Fault ride-through — must stay connected per IEEE 1547 / NERC PRC-024 envelope.
"FERC 661" / "FERC 827"Wind interconnection rules: tech standards (661), reactive PF (827).
"Crowbar"DFIG rotor protection — short-circuits the rotor through resistors during faults.
"PMSG"Permanent-magnet synchronous gen — no field excitation, common in direct-drive Type 4.
"Direct drive"No gearbox; mechanical shaft from rotor straight to generator. Standard offshore.
"IEC 61400-1"Wind turbine class standard: Class I (highest wind) to Class IV (lowest wind).
"Pitch control"Blade angle adjusts to limit power above rated wind speed (~ 12–13 m/s typical).
"Cut-in / cut-out"Wind speeds at which the turbine starts (~ 3–4 m/s) and stops (~ 25 m/s) generating.
"POI"Point of interconnection — where the wind plant meets the transmission grid.
"PCC"Point of common coupling — where harmonics and voltage are measured per IEEE 519.
Also see
§25Solar
PV & Energy Storage
PV is the other half of "alternative power generation" — interconnection and IBR concepts shared.
§40PowerEl
Power Electronics Fundamentals
DFIG converter and Type 4 back-to-back inverter physics live here.
§22Subs
Substations & Switchyards
Wind plant POI substation: bus configurations, IEEE 80 grounding, protection schemes.
§15PQ
Power Quality
STATCOM reactive support, FRT, IEEE 519 harmonics from wind plant inverters.
§17Relays
Protection & Relaying
87T / 87B / 21 / 67 protection on wind collector substations.