Wind Turbine Generators

Wind Turbine Generator - In-Depth Notes

Chapter Overview

This chapter discusses the operation and principles of different types of wind turbine generators, focusing on synchronous and asynchronous generators, including the Double Feed Induction Generator (DFIG).

Objectives

Upon completing this chapter, you will be able to:

  • Understand the principles of generators in the context of wind turbine generators.
  • Identify and differentiate between types of three-phase generators:
    • Synchronous Generators
    • Asynchronous Generators
    • Double Feed Induction Generator (DFIG)

Introduction to Generators

Types of AC Generators

AC generators are typically categorized into two main types:

  1. Synchronous Generators

    • Operate at a fixed speed and frequency.
    • Commonly used in thermal power stations, hydroelectric plants, and nuclear plants.

  2. Asynchronous Generators

    • Operate at variable speeds and frequencies.
    • Frequently used in wind turbines and isolated generation setups.

Synchronous Generators

Characteristics
  • Runs at a fixed speed: The synchronous speed is determined by the formula: Ns=120fPN_s = \frac{120f}{P} where:
    • NsN_s = synchronous speed in r.p.m.
    • ff = frequency of the generated voltage in Hz.
    • PP = total number of poles.
Example Calculation
  • Given a generator frequency of 60 Hz with 2 field poles:
    Ns=120⋅602=3600 r.p.m.N_s = \frac{120 \cdot 60}{2} = 3600 \text{ r.p.m.}
  • If the number of poles increases to 4:
    Ns=120⋅604=1800 r.p.m.N_s = \frac{120 \cdot 60}{4} = 1800 \text{ r.p.m.}
Synchronous Speed Formula

N<em>1=120fPN<em>1 = \frac{120f}{P} where N</em>sN</em>s is the synchronous machine speed.

Asynchronous Generators

Overview
  • Also known as Induction Generators.
  • Two Types:
    1. Squirrel Cage Induction Generator
    2. Wound Rotor Induction Generator (used in wind turbines)
Operating Principles
  • An induction generator requires mechanical movement to produce electrical energy.
  • It needs external reactive power to establish the magnetic field in the rotor.
  • Advantages:
    • Lower cost
    • Smaller size
    • Rugged construction

Induction Generator Functionality

  • Functions both as an induction motor and an induction generator depending on conditions:
    • Induction Motor: Stator fed from the grid, rotor winding shorted.
    • Induction Generator: Rotor fed by DC voltage and rotates faster than synchronous speed.
Reactive Power Requirement
  • An induction generator requires a significant amount of reactive power, approximately half of its KVA rating at no load.
  • It provides no reactive power generation capability.

Wind Turbine Generator (WTG) Characteristics

  1. Low Speed Operation: The prime mover (rotor blades) rotates at low speeds.
  2. Energy Storage Needs: Wind is inconsistent, necessitating energy storage solutions.
  3. Use of DC Generators: Small power WTGs use DC generators with magnets to charge batteries and supply household loads.
  4. Medium Power WTGs: Utilize AC generators with conversions to ensure a stable output frequency.
Large Power WTGs
  • Typically employ induction generators, specifically the Double Feed Induction Generator (DFIG), where:
    • Stator is fed from the grid.
    • Rotor is fed through an AC/DC converter and DC/AC inverter.

DFIG Characteristics

  • Widespread in wind turbine applications.
  • Features a multiphase wound rotor and multiphase slip ring assembly.
  • There are alternatives to slip rings, although they may compromise efficiency, cost, and size.
Capacitor Rating Calculation Example
  • Given Parameters:
    3-phase induction machine ratings of 5KW, 400V, 60Hz at 0.8 pf.
  • Reactice Power Calculation:
    1 Ph Reactive Power=3749.43=1249.8Var1\text{ Ph Reactive Power} = \frac{3749.4}{3} = 1249.8\text{Var}
  • Capacitor Current:
    I=VarV=1249.8400=3.125AI = \frac{\text{Var}}{V} = \frac{1249.8}{400} = 3.125\text{A}
  • Capacitor Reactance:
    Xc=VI=4003.125=128ΩX_c = \frac{V}{I} = \frac{400}{3.125} = 128\Omega
  • Capacitance Calculation:
    C=12πfXc=12π(60)(128)≈20.7μFC = \frac{1}{2 \pi f X_c} = \frac{1}{2 \pi (60) (128)} \approx 20.7 \mu F

Conclusion

  • A robust understanding of wind turbine generators and their contributions to sustainable energy is crucial for future developments in renewable energy technologies.
  • Proficiency in calculations related to generator operations and the unique characteristics of synchronous and asynchronous machines is vital for effective application and innovation in the field.