Wind Turbine Generator Notes

Sustainable Energies: Wind Turbine Generator

Chapter Overview

  • Objectives: This chapter aims to help readers understand the following:
    • Generator principle review.
    • Types of 3-phase generators:
    • Synchronous generators
    • Asynchronous generators
    • Double Feed Induction Generator (DFIG)

Introduction to AC Generators

  • AC generators are generally divided into two main types:
    1. Synchronous Generators:
    • Operates at a fixed speed and frequency.
    • Used in thermal power stations, hydro-power plants, and nuclear plants.
    1. Asynchronous Generators:
    • Operates at variable speeds and frequencies.
    • Commonly used in wind turbines and isolated generation.

Synchronous Generators

Basic Principles
  • The key parameters of synchronous generators include:
    • Frequency (f) and Synchronous Speed (Ns)
    • ( P = ) total number of poles
    • ( N = ) speed of field poles in RPM
    • ( n = ) speed of field poles in RPS
    • ( f = ) frequency of the generated voltage in Hz.
    • The relation is given by: ( f = P \times N / 120 )
Example Calculation
  • Synchronous Speed Calculation: For a generator with a frequency of 60Hz and 2 poles,
    • ( Ns = \frac{120 \times 60}{2} = 3600 ) RPM
    • If the poles are increased to 4:
    • ( Ns = \frac{120 \times 60}{4} = 1800 ) RPM

Asynchronous Generators

General Overview
  • Asynchronous Generator, also known as Induction Generator, can be divided into:
    • Squirrel Cage Induction Generator
    • Wound Rotor Induction Generator (often used in wind turbines)
Operational Characteristics
  • Requires mechanical movement to produce electrical energy.
  • Needs reactive power externally to establish the magnetic field in the rotor.
  • Advantages include low cost, compact size, and rugged design.
Induction Machine Functionality
  • Functions as an induction motor when powered from the grid, and the rotor winding is shorted.
  • Functions as an induction generator when the rotor is fed by DC voltage and operates at speeds above synchronous frequency.
  • Reactive power is critical, approximately half of the machine's KVA at no load.

Wind Turbine Generators (WTG)

Types of Generators Used
  • For small power WTGs, DC generators with permanent magnets are utilized to charge batteries and use inverters to convert DC to AC.
  • For medium power WTGs, AC generators are used; output frequency is low and requires conversion.
  • For large loads, AC generators, specifically Double Feed Induction Generators (DFIG), are preferred. They feature a rotor fed by AC power through converters.
DFIG Principles
  • Widely used in wind turbine operations, with:
    • Stator connected to the grid.
    • Rotor fed from the grid through AC/DC converters and DC/AC inverters.
  • A better alternative exists: Brushless Wound-Rotor Doubly Fed Electric Machine which mitigates efficiency, cost, and size issues associated with slip ring assemblies.

Reactive Power in Induction Generators

Example Calculation
  • Given an induction generator requires reactive power ratings:
    • If rated at 5KW, 400V, 60Hz, and 0.8 power factor:
    • 3-phase reactive power: ( 1Ф reactive power = \frac{3749.4}{3} = 1249.8 Var )
    • Capacitor current required: ( I_{C} = \frac{1249.8}{400} = 3.125 A )
    • Capacitor reactance: ( X_{C} = \frac{400}{3.125} = 128 \Omega )
    • Capacitance required: ( C = \frac{1}{2\pi f X_{C}} = \frac{1}{2 \pi \cdot 60 \cdot 128} = 20.7 \mu F )