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 [email protected]

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 1

• • •

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

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North American Power Grid Load/Generation Contour

Image contours the load (green) and generation (red) 3

Maximum Rotor Efficiency

Rotor efficiency

C P

vs. wind speed ratio λ. Recall λ is the ratio between the downstream wind velocity and the upstream velocity 4

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 Tip-Speed-Ratio (TSR)  Rotor tip speed = Wind speed rpm   D (7.30) 60

v

• • • •

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 5

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 6

Example

• 40-m wind turbine, three-blades, 600 kW, wind speed is 14 m/s, air density is 1.225 kg/m 3 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?

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Example

a. Find the rpm of the rotor if it operates at a TSR of 4.0

Rewriting (7.30), rpm   D 

v

rpm    40m/rev = 26.7 rev/min We can also express this as seconds per revolution: rpm  26.7 rev/min 60 sec/min = 0.445 rev/sec or 2.24 sec/rev 8

Example

b. Tip speed From (7.30): rpm   D Rotor tip speed= 60 sec/min Rotor tip speed = (rev/sec)   D Rotor tip speed = 0.445 rev/sec   40 m/rev = 55.92 m/s c. Gear Ratio Gear Ratio = Generator rpm Rotor rpm = 1800 26.7

= 67.4 9

Example

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

W

 1 2 

v

3 1 2  1.225

    4  40 2   14 3  2112 kW Overall efficiency:   600 kW 2112 kW  28.4% 10

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 11

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. 12

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. 13

Wind Generator Types

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Rotating Magnetic Field

• • • • Imagine coils in the stator of this 3-phase generator Positive current

i A

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) 15

Rotating Magnetic Field

• • Three-phase currents are flowing in the stator At ωt = 0,

i A i B

=

i C

is at the maximum positive value and are both negative Resultant magnetic flux points vertically down 16

Rotating Magnetic Field Demo

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Magnetic Poles

• Synchronous speed depends on the electrical frequency and the number of poles, with

f m

 2

f e P f e P

is the number of poles,

f m

is mechanical frequency Image source :cnx.org/contents/cbb3bd3b-430a-487b-9c53-b17d79e3367c@1/Chapter_5:_Synchronous_Machine 18

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 19

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= d l /dt) Lorenz force on wire with current in magnetic field:

F

I

l B

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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

i R

flows easily through the thick conductor bars 21

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

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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 23

Slip

• • • • • • The difference in speed between the stator and the rotor 

N S



N R N R N S N S s

= rotor slip – positive for a motor, negative for a generator

N S

= no-load synchronous speed (rpm)

f

= frequency (Hz)

p

= number of poles

N S

 120

p f N R

= rotor speed (rpm) 24

The Induction Machine as a Motor

Torque- slip curve for an induction 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 25

•

The Induction Machine as a Generator

The stator requires excitation current – from the grid if it is grid-connected or – by incorporating external capacitors • Single-phase, self-excited, induction generator Wind speed forces generator shaft to exceed synchronous speed 26

• • •

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

N R S

 3636 rpm 27

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 28

Blade Efficiency vs. Windspeed

At lower windspeeds, the best efficiency is achieved at a lower rotational speed 29

Power Delivered vs. Windspeed

Impact of rotational speed adjustment on delivered power, assuming gear and generator efficiency is 70% 30

• • • •

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) 31

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 32

Effect of Rotor Resistance on Induction Machine Power-Speed Curves

Real Pow er Real Pow er

1.6

1.4

1.2

1 0.8

0.6

0.4

0.2

0 -0.2

-0.4

-0.6

-0.8

-1 -1.2

-1.4

-1.6

-0.3

-0.4

-0.5

-0.6

-0.7

-0.8

-0.9

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0 -0.1

-0.2

0.95

1 0 0.05

Slip Real Pow er 0.95

1 -0.05

0.05

Slip Real Pow er 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.

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• • •

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 34