ECE 333 Renewable Energy Systems Lecture 9: Wind Power Systems Prof. Tom Overbye Dept. of Electrical and Computer Engineering University of Illinois at Urbana-Champaign overbye@illinois.edu

Announcements Read Chapter 7 HW 4 is 7.1, 7.2, 7.4, 7.5; it will be covered by an in-class quiz on Thursday Feb 20

In the News: On Feb 12 AWEA Released Report on Wind Reliablity Report addressed issue of how much wind energy could be integrated into the US grid Finding is wind could provide more than 40% of our total electric energy In 2013 Iowa and South Dakota got 25% of their electricity from wind, and for ERCOT it as 10.6% Key to integrating large amounts of wind is that the wind plant outputs are not correlated across large areas Changes in the wind tend to cancel out Report: awea.files.cms-plus.com/AWEA%20Reliability%20White%20Paper%20-%202-12-15.pdf

North American Power Grid Load/Generation Contour Image contours the load (green) and generation (red)

Maximum Rotor Efficiency Rotor efficiency CP vs. wind speed ratio λ. Recall λ is the ratio between the downstream wind velocity and the upstream velocity

Tip-Speed Ratio (TSR) Efficiency is a function of how fast the rotor turns Tip-Speed Ratio (TSR) is the speed of the outer tip of the blade divided by wind speed D = rotor diameter (m) v = upwind undisturbed wind speed (m/s) rpm = rotor speed, (revolutions/min) One meter per second = 2.24 miles per hour

Tip-Speed Ratio (TSR) TSR for various rotor types If blade turns too slow then wind passes through without hitting blade; too fast results in turbulence Rotors with fewer blades reach their maximum efficiency at higher tip-speed ratios Figure 7.18 A higher TSR is needed when there are fewer blades

Example 40-m wind turbine, three-blades, 600 kW, wind speed is 14 m/s, air density is 1.225 kg/m3 a. Find the rpm of the rotor if it operates at a TSR of 4.0 b. Find the tip speed of the rotor c. What gear ratio is needed to match the rotor speed to the generator speed if the generator must turn at 1800 rpm? d. What is the efficiency of the wind turbine under these conditions?

Example a. Find the rpm of the rotor if it operates at a TSR of 4.0 Rewriting (7.30), We can also express this as seconds per revolution:

Example b. Tip speed From (7.30): c. Gear Ratio

Example d. Efficiency of the complete wind turbine (blades, gear box, generator) under these conditions From (7.7): Overall efficiency:

Converting Wind into Electric Energy Design challenge is to convert rotating mechanical energy into electrical energy This is, of course, commonly done in most power plants. But the added challenges with wind turbines are 1) the shaft is often rotating a variable speed [because of changes in the wind speed], and 2) the rate of rotation is relatively slow (dozens of rpm) Early wind turbines used a near fixed speed design, which allowed use of simple and well proven induction generators, but gave up aerodynamic efficiency. Modern turbines tend to use a variable speed design to keep tip-to-speed ratio near optimal

Electric Machines Electric machines can usually function as either a motor or as a generator Three main types of electric machines DC machines: Advantage is they can directly operate at variable speed. For grid application the disadvantage is they produce a dc output. Used for small wind turbines. AC synchronous machines Operate at fixed speed. Used extensively for traditional power generation. The fixed speed had been a disadvantage for wind. AC induction machines Very rugged and allow some speed variation but usually not a lot for efficient operation.

Types of Wind Turbines by Machine From an electric point of view there are four main types of large-scale wind turbines (IEEE naming convention) Type 1: Induction generator with fixed rotor resistance Type 2: Induction generators with variable rotor resistance Type 3: Doubly-fed induction generators Type 4: Full converter generators which main use either a synchronous generator or an induction generator Most new wind turbines are either Type 3 or Type 4 In Europe these are sometimes called Types A, B, C, D respectively.

Wind Generator Types

Rotating Magnetic Field Imagine coils in the stator of this 3-phase generator Positive current iA flows from A to A’ Magnetic fields from positive currents are shown by the bold arrows Magnetic flux is proportional to current, with direction given by the right-hand rule (from Ampere's circuit law)

Rotating Magnetic Field Three-phase currents are flowing in the stator At ωt = 0, iA is at the maximum positive value and iB=iC are both negative Resultant magnetic flux points vertically down

Rotating Magnetic Field Demo

Magnetic Poles Synchronous speed depends on the electrical frequency and the number of poles, with Image source :cnx.org/contents/cbb3bd3b-430a-487b-9c53-b17d79e3367c@1/Chapter_5:_Synchronous_Machine

Synchronous Machines Spin at a rotational speed determined by the number of poles and by the frequency (3600 rpm at 60Hz, 2 pole) The magnetic field is created on their rotors Create the magnetic field by running DC through windings around the core A permanent magnet can also be used A gear box if often needed between the blades and the generator Some newer machines are designed without a gear box Slip rings are needed to get a dc current on the rotor

Asynchronous Induction Machines Do not turn at a fixed speed Acts as a motor during start up; can act as a generator when spun faster then synchronous speed Do not require exciter, brushes, and slip rings Less expensive, require less maintenance The magnetic field is created on the stator not the rotor Current is induced in the rotor (Faraday's law: v= dl/dt) Lorenz force on wire with current in magnetic field:

Squirrel Cage Rotor The rotor of many induction generators has copper or aluminum bars shorted together at the ends, looks like a cage Can be thought of as a pair of magnets spinning around a cage Rotor current iR flows easily through the thick conductor bars

Squirrel Cage Rotor Instead of thinking of a rotating stator field, you can think of a stationary stator field and the rotor moving counterclockwise The conductor experiences a clockwise force Figure 6.16

The Inductance Machine as a Motor The rotating magnetic field in the stator causes the rotor to spin in the same direction As rotor approaches synchronous speed of the rotating magnetic field, the relative motion becomes less and less If the rotor could move at synchronous speed, there would be no relative motion, no current, and no force to keep the rotor going Thus, an induction machine as a motor always spins somewhat slower than synchronous speed

Slip The difference in speed between the stator and the rotor s = rotor slip – positive for a motor, negative for a generator NS = no-load synchronous speed (rpm) f = frequency (Hz) p = number of poles NR = rotor speed (rpm)

The Induction Machine as a Motor As load on motor increases, rotor slows down When rotor slows down, slip increases “Breakdown torque” increasing slip no longer satisfies the load and rotor stops Braking- rotor is forced to operate in the opposite direction to the stator field Torque- slip curve for an induction motor

The Induction Machine as a Generator The stator requires excitation current from the grid if it is grid-connected or by incorporating external capacitors Wind speed forces generator shaft to exceed synchronous speed Single-phase, self-excited, induction generator

The Induction Machine as a Generator Slip is negative because the rotor spins faster than synchronous speed Slip is normally less than 1% for grid-connected generator Typical rotor speed

Speed Control Necessary to be able to shed wind in high-speed winds Rotor efficiency changes for different Tip-Speed Ratios (TSR), and TSR is a function of windspeed To maintain a constant TSR, blade speed should change as wind speed changes A challenge is to design machines that can accommodate variable rotor speed and fixed generator speed

Blade Efficiency vs. Windspeed At lower windspeeds, the best efficiency is achieved at a lower rotational speed

Power Delivered vs. Windspeed Impact of rotational speed adjustment on delivered power, assuming gear and generator efficiency is 70%

Pole-Changing Induction Generators Being able to change the number of poles allows you to change operating speeds A 2 pole, 60 Hz, 3600 rpm generator can switch to 4 poles and 1800 rpm Can do this by switching external connections to the stator and no change is needed in the rotor Common approach for 2-3 speed appliance motors like those in washing machines and exhaust fans Increasingly this approach is being replaced by machine drives that convert ac at grid frequency to ac at a varying frequency (covered in ECE 464)

Variable-Slip Induction Generators Purposely add variable resistance to the rotor External adjustable resistors - this can mean using a wound rotor with slip rings and brushes which requires more maintenance Mount resistors and control electronics on the rotor and use an optical fiber link to send the rotor a signal for how much resistance to provide

Effect of Rotor Resistance on Induction Machine Power-Speed Curves Left plot shows the torque-power curve from slip of -1 to 1 with external resistance = 0.05; right plot is with external resistance set to 0.99 pu.

Variable Slip Example: Vestas V80 1.8 MW The Vestas V80 1.8 MW turbine is an example in which an induction generator is operated with variable rotor resistance (opti-slip). Adjusting the rotor resistance changes the torque-speed curve Operates between 9 and 19 rpm Source: Vestas V80 brochure