LECTURE 19 WIND POWER SYSTEMS ECE 371 Sustainable Energy Systems 1
GENERATORS Blades convert the wind kinetic energy to a shaft power to spin a generator and produce electricity A generator has two parts Stator stationary part Rotor rotating part Small battery-charging WTGs use dc generators WTG = Wind Turbine Generator. Large grid-connected WTGs use ac generators 2
GENERATORS Maximum efficiency point for rotors should be matched to the windspeed For optimal operation Maximum wind energy capture Since windspeed varies, rotor speed should be changed for maximum efficiency 3
GENERATORS The way to categorize a wind energy system by Rotor rotates at a fixed speed Rotor rotates at a variable speed A fixed-speed WTG is Simple Least-cost approach Inefficient High mechanical stress 4
GENERATORS As far as ac generators are concerned, the choice is limited to: Synchronous Generators Produce all the power in power plants Asynchronous Generators Power production is limited to special situations Wind power generation is an example 5
SYNCHRONOUS GENERATORS Synchronous generators Rotor should turn at a precise speed that is a function of number of poles and frequency Need dc voltage for the rotor AC to DC rectification necessary Exciter Need slip rings and brushes to get the dc voltage to rotor winding Replacing the brushes and cleaning slip rings adds to the maintenance effort 6
ASYNCHRONOUS GENERATORS They are induction generators Most wind turbines use them Induction machines do not turn at a fixed speed The name asynchronous comes from this fact As generators they are not commonly used in conventional power plants As motors they consume 1/3 of the generated power worldwide 7
ASYNCHRONOUS GENERATORS Both modes of operation take place in wind turbines Motor during start-up Generator as wind picks-up Induction machines do not require Exciters Brushes Slip rings 8
ASYNCHRONOUS Induction machines are Less complicated Less expensive GENERATORS Require less maintenance Smaller in size More forgiving in terms of stresses during wind gust conditions The synchronous speed of the magnetic field inside the machine is N s = 120f/p, but the rotor speed is different 9
ASYNCHRONOUS GENERATORS There are two categories of induction machines Wound rotor induction generator (WRIG) Squirrel-Cage induction generators (SCIG) 10
SQUIRREL-CAGE INDUCTION GENERATORS 11
SQUIRREL-CAGE INDUCTION GENERATORS When the shaft is connected to wind turbine blades and stator is provided with three-phase excitation (from the grid or capacitors) The machine will start operation by motoring up toward its synchronous speed When wind speed exceeds the threshold that spins the shaft above synchronous speed, it becomes a three-phase generator 12
SQUIRREL-CAGE INDUCTION GENERATORS The rotor of an induction motor turns at N R which is slower than N S The difference in this speed is called slip speed Slip Speed = N S - N R The slip speed divided by the synchronous speed is called slip, which is dimensionless s = (N S N R )/N S 13
SQUIRREL-CAGE INDUCTION GENERATORS For the grid-connected induction machines, the slip is ± 1% (1782 to 1818 rpm) If the gear ratio is 100:1, then the rotor that is connected to blades turn at about 18 rpm This speed can be changed to about 12 rpm by changing the number of poles from 4 to 6 remotely 14
DOUBLY-FED INDUCTION GENERATORS One of the more advanced WTGs is based on the concept of doubly-fed induction generator (DFIG) It uses a wound rotor induction generator The stator part of the DFIG is conventional The rotor is designed to allow bidirectional power flow from and to the grid 15
DOUBLY-FED INDUCTION GENERATORS 16
DOUBLY-FED INDUCTION GENERATORS When the rotor spins at sub-synchronous speeds, the rotor receives power from the grid When the rotor spins at super-synchronous speeds, the rotor provides power to the grid The usual range of speed control is 40% below to 20% above synchronous speed 17
DOUBLY-FED INDUCTION GENERATORS DFIG uses a small converter that is rated about 30% of the turbine-generator capacity 18
VARIABLE-SPEED SYNCHRONOUS GENERATOR To gain complete control of the speed a fullcapacity converter is needed 19
POWER IN WIND The kinetic energy of a mass (m) moving at a velocity (v) is 20
POWER IN WIND Since power is energy per unit time Where ρ is air density 21
POWER IN WIND Power Density Start Rated 22
POWER IN WIND Therefore, the produced power is a function of Air density, ρ = 1.225 kg/m 3 at 15 o C and 1 atm Swept area of turbine rotor, A= (π/4) D 2 in m 2 Wind velocity, v in m/s Doubling the windspeed increases power by 8 fold Doubling the diameter increases power by a factor of 4 The cost of turbine increases in proportion to rotor diameter Big wind turbines are more economical. 23
POWER IN WIND In wind power calculations, we just cannot use the average wind speed The relationship between power and windspeed is nonlinear MathCAD Example 24
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NREL NREL has a website that wind maps and downloadable wind databases. For wind maps, see https://www.nrel.gov/gis/wind.html 28
30 m US Windmap 29
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Windspeed Data Windspeed from NREL can be downloaded from https://maps.nrel.gov/windprospector/#/?al=rh9ekq%255bv%255d%3 Dt&bL=groad&cE=0&lR=0&mC=40.21244% 2C-91.625976&zL=4 31
Wind Prospector 32
Wind Data for Terre Haute (2012) 33