Design of Power Quality Optimization Scheme for Wind Farms

Design of Power Quality Optimization Scheme for Wind Farms

I. Typical Power Quality Issues in Wind Farms

1. Voltage Fluctuations & Flicker

   • Sudden wind speed changes → Power fluctuations → Bus voltage variations (especially in weak grids)

2. Harmonics & Interharmonics

   • Converter switching frequencies (2-6kHz) generate characteristic harmonics (e.g., 5th, 7th, 11th, 13th)

   • Tower shadow effect causing low-frequency interharmonics (<100Hz)

3. Voltage Sag/Swell

   • Grid faults or large turbine start/stop events

4. Reactive Power Fluctuations

   • Variations in absorbed/delivered reactive power due to control strategies

II. Core Solution Design

Solution 1: Turbine-Level Optimization

• Technical Measures

  •  Full-Scale Converter (Permanent Magnet Direct-Drive)

    • Principle: Decouples turbine and grid via back-to-back converters, suppressing torque-ripple-induced power fluctuations

    • Harmonic Control: LCL filter + PWM optimization (e.g., SVPWM)

  •  Dynamic Reactive Power Support

    • Principle: Reserve 10-15% reactive capacity; response time <20ms (superior to traditional SVG)

Solution 2: Collector Line-Level Mitigation

• Technical Measures

  •  SVG (Static Var Generator) at Bus

    • Principle: Real-time detection of ΔQ at PCC, injects counteractive reactive power via IGBT inverters (response ≤5ms)

    • Capacity Design: 20-30% of total wind capacity (e.g., 20Mvar SVG for 100MW farm)

  •  APF (Active Power Filter)

    • Principle: Sample harmonics → generate compensating currents → cancel harmonics (THD reduced from 8% to <3%)

    • Key Targets: Characteristic harmonics (e.g., 6k±1 orders) & interharmonics

Solution 3: Station-Level System

• Technical Measures

  •  Hybrid STATCOM + Energy Storage

    • Principle:

      • STATCOM provides fast reactive support (±1.5pu capability)

      • Li-ion storage smoothes power fluctuations (time constant: seconds-minutes)

    • Case Study: 5% wind capacity storage (e.g., 50MW/25MWh) reduces voltage fluctuations by 40%

  •  Harmonic Resonance Suppression

    • Principle: Auto-tuned C-type filter banks avoid resonance points (e.g., 11th/13th)

    • Design Criteria:
      $Q=\genfrac{}{}{0ex}{}{1}{R}\genfrac{}{}{0ex}{}{L}{C}$ (Q=30-50 to avoid over-sharpness)
      $f^{res}=\genfrac{}{}{0ex}{}{1}{2\pi \sqrt{LC}}$ (avoid 2.5-3.5kHz range)

Solution 4: Grid Connection Protection

• Technical Measures

  •  DVR (Dynamic Voltage Restorer)

    • Principle: Series-injected compensation voltage stabilizes sensitive equipment (handles sags ≤0.5s)

    • Key Parameters: 60% compensation depth; supercapacitor energy source

  •  Zero-Sequence Harmonic Blocking Transformer

    • Principle: Zig-Zag winding provides high impedance to 3rd harmonics (suppresses zero-sequence components)

III. Key Technical Principles

1. Voltage Fluctuation Suppression
   $\Delta V\%\approx \genfrac{}{}{0ex}{}{\Delta Q\cdot X^{grid}}{V^{nom}}\times 100\%$

   • SVG compensates ΔQ in real-time to minimize grid reactance (X_grid) effects

2. Harmonic Cancellation Model
   APF compensation current generation:
   $i^c(t)=-{\sum }^{h=2}{I}^h\sin (2\pi hft+{\varphi }^h)$

   • FFT decomposes harmonics; IGBT outputs precise phase-inverted currents

3. Interharmonic Mitigation

   • Enhanced PLL (DDSRF-PLL) for accurate fundamental frequency tracking

   • Band-stop filter center frequency:
     $f^{center}=f^{blade}\times N^{blades}$ (blade-passing frequency)

IV. Implementation RoadmapVoltage Fluctuation

V. Expected Performance

VI. Innovative Technologies

1. Wide-Bandgap Devices: SiC converters (>10kHz switching) reduce filter size by 40%

2. Digital Twin System: SCADA-based prediction model adjusts compensation 15s ahead

3. Grid-Forming Converters: Simulate synchronous machine inertia for weak-grid stability