Laboratory Experiment Wind Power Systems (EIEN10) Rev. 1 - M. Collins, 2016

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WIND POWER SYSTEMS – LABORATORY EXPERIMENT OPERATION & CONTROL OF A WIND TURBINE GENERATOR

Objectives The objectives of this laboratory experiment are summarized below:

1. To become familiar with the set-up of a wind turbine generator 2. To understand the characteristics of fixed speed and variable speed wind turbines 3. To understand the advantages of variable speed wind turbines 4. To understand how a variable speed wind turbine is controlled to achieve

maximum power extraction 5. To obtain the plot of the rotor power vs. wind speed as a function of rotor tip speed 6. To obtain the difference in annually electricity production between fixed and

variable speed wind turbines Theory Wind turbines extract power from the wind, converting it to electrical power through an electrical machine. Often, between the wind turbine and the generator, a gearbox is used to connect the low speed wind turbine rotor to the high speed generator rotor. Wind turbine A wind turbine, where the blades sweep an area A, encounters an airflow with a power equal to:

3

21

wAvP ρ=

where ρ is the air density (approximately 1.225 kg/m3) A is the area swept by the turbine rotor blades vw is the wind speed However, not all power is available for extraction:

( ) 3,21

wp vACP βλρ=

w

WT

vRωλ =

Here, Cp is the power coefficient determining how much power may be extracted from the wind. It is a function of tip speed ratio λ as well as pitch angle β, and has a maximum theoretical value, the Betz limit, equal to 59.3%. In reality, wind turbines have a maximum Cp of about 45 %. Cp decreases with increasing pitch angle β, that is as the blades are rotated out from the wind. Below, figure 1 shows the variation of Cp with λ for two different values of β.

Laboratory Experiment Wind Power Systems (EIEN10) Rev. 1 - M. Collins, 2016

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0

0.1

0.2

0.3

0.4

0.5

0 2 4 6 8 10 12 14

Lambda

Cp

Beta = 0 Beta = 4

Figure 1. Example of Cp vs. λ for two different values of pitch angle β

A fixed-speed wind turbine operates always at the same rotor angular speed, which is related to the gearbox ratio, the generator pole pairs number and to the electrical frequency. Clearly, such a wind turbine can operate at optimum Cp only for one wind speed. As the wind speed varies, the tip speed ratio λ will vary thus reducing the power coefficient. In variable speed wind turbines, the rotor angular speed is adjusted depending on the wind speed so that operation at optimal Cp results. The rotor angular speed can be adjusted by properly controlling the electrical system. Operation at maximum Cp is possible only while the wind turbine delivers less than its nominal power. At high wind speed, usually above the range 10 – 12 m/s, the power output must be limited to the nominal value, achieved by increasing the pitch angle, figure 2.

Figure 2. Example of wind turbine power output vs. wind speed

Electrical system There are different concepts of wind turbine setups available on the market. Fixed-speed wind turbines have been widely used in the past and they are usually equipped with an induction generator directly connected to the grid. Here, there are no means to regulate the generator and wind turbine rotor speed. Today, the two most common variable-speed wind turbine concepts adopted are

a) Doubly-Fed Induction Generator (DFIG) b) Permanent Magnet Synchronous Generator (PMSG) or Induction Generator

connected to the grid via a back-to-back Full Scale Converter (FSC) The two concepts are shown in Figure 3. Both offer the possibility to continuously vary the rotor angular speed by controlling the power electronic converters, achieving

Laboratory Experiment Wind Power Systems (EIEN10) Rev. 1 - M. Collins, 2016

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operation at maximum Cp over a wide range of wind speeds. Moreover, these concepts present other advantages over a fixed-speed wind turbine, e.g. the ability to regulate active and reactive power independently.

Figure 3. DFIG and FSC wind turbines

Control scheme for optimal power extraction The variation of the wind turbine angular rotor speed for optimal power extraction at different wind speeds can be achieved with different control strategies. A Maximum Power Point Tracking (MPPT) strategy imposes small variations on the rotor speed and checks if the extracted power increases or decreases. The rotor speed is then controlled in the direction that increases the extracted power. Another approach is to control the power electronic converter to deliver a predefined electric power as a function of the rotor angular speed. One such predefined curve is shown in Figure 4. The right figure, the reference torque, is easily obtained from the left figure.

0 500 1000 15000

200

400

600

800

1000

1200

WT speed (rpm)

Ref

eren

ce E

lect

rical

Pow

er (W

)

0 500 1000 15000

0.5

1

1.5

2

2.5

3

3.5

WT speed (rpm)

Ref

eren

ce E

lect

rical

Tor

que

(Nm

)

Figure 4. Optimal electric power (left) and optimal electric torque (right) vs. rotor speed

To understand how the left figure is obtained, remember that at a given wind turbine rotor speed there is only one corresponding wind speed, or tip speed ratio λ, that results in maximum power extraction, i.e. at maximum Cp. The curve on the left figure respesents all of these optimal points vs. the wind turbine rotor speed. If the converter is controlled to deliver an electric power according to the figure, the wind turbine will automatically stabilize at the optimal point at any wind speed. To understand how this stable optimal operation can be achieved, suppose that the wind speed is fixed

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and that the wind turbine is initially operating at the optimal point. Neglecting all losses, the power extracted from the wind is equal to the power delivered to the grid. If the wind speed should now suddenly decrease, the converter will initially continue to set the same electric power reference as before since the rotor speed cannot change instantaneously (recall that the converter sets the electric power reference only based on the rotor speed and does not “know” anything about the wind speed). However, since the wind speed has decreased and because the tip speed ratio is no longer optimal, the wind turbine will deliver a higher electrical power than what is extracted from the wind, slowing down the rotor. Hence, according to figure 4, the converter will decrease the electric power reference until the new optimum point is reached at a lower rotor speed. At this new operating point, the electric power delivered to the grid is equal to the wind turbine mechanical power, i.e. the maximum power that can be extracted from the wind for this new wind condition. Since the mechanical and the electrical power are again equal, the wind turbine rotor speed stabilizes at the new optimal value. Lab set-up The lab set-up consists of a DC motor driving a synchronous generator and of a back-to-back converter, as shown in Figure 5. The DC motor is controlled to emulate the behavior of a wind turbine rated for 1 kW. The control also emulates pitch control of the wind turbine. The synchronous generator can be connected to the grid directly in the fixed speed wind turbine case, Figure 5.a, or via the back-back converter in the case of a variable speed wind turbine, Figure 5.b. A phototograph of the lab set-up is shown in Figure 6.

Figure 5. Schematical representation of laboratory set-up.

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Figure 6. Photograph of laboratory set-up. DC motor The DC motor is controlled through software to behave as a wind turbine, as shown in Figure 7. The power delivered by the DC motor is controlled by changing the armature current Ia of the DC motor via a 4-quadrant DC/DC converter. In the lab, the wind speed can be adjusted in software and, consequently, the DC motor is controlled to generate an output power at the shaft proportional to the cube of the wind speed as for a common wind turbine:

3

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wpvACP ρ=

3Φ VSC

DC/DC Converter

DC Motor (Wind Turbine)

Synchronous Generator

Voltmeter (DC-link voltage)

Variable Transformer (Magnetizing Current)

Variable Transformer (DC-link voltage)

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The area swept by the wind turbine has been chosen to scale the power of the DC motor to 1 kW when the wind speed is 12 m/s. The value of Cp is continuously calculated by the software given the DC motor shaft rotational speed, the wind speed and a predefined table of Cp vs. λ values for a given pitch angle β. One of the main objectives of the lab session is to understand how Cp varies with varying values of λ, and how this influences the wind turbine output. The wind turbine model includes also a pitch controller to limit the power at the shaft for wind speeds higher than 12 m/s. For wind speeds above 12 m/s the wind turbine controller emulates the pitch opening, and as a result Cp will drop, limiting the output power to its maximum value.

Figure 7. DC motor control for wind turbine emulation.

Back-back converter A back-back converter is used to operate the wind turbine at variable speed. The back-to-back converter consists of two independent converters, one on the generator side and one on the grid side. The grid side converter operates at 50 Hz fundamental frequency, controlling the voltage of the DC-link to its nominal value. It also ensures the reactive power delivered to the grid is zero (note that, in the general case, the converter can deliver/absorb reactive power, e.g. for voltage control purposes).

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The generator side operates at the fundamental frequency associated with optimal rotor speed such that maximum power may be extracted, as was discussed in a previous section. Below, an example of typical back-to-back control is schematically depicted.

Figure 8. Example of back-to-back converter control. Experiment A: Fixed speed wind turbine The interface page in LabVIEW to control the wind turbine is shown in figure 9.

Figure 9. Interface page in LabVIEW

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The interface facilitates switching between experimental setups (fixed speed wind turbine mode vs. variable speed wind turbine mode) as well as between manual or wind turbine power mode. In the upper right corner, the wind speed may be varied. During operation, readback of wind power, extracted power (i.e. turbine power), λ, β, Cp, as well as turbine speed is displayed. In this experiment, the synchronous generator is connected directly to the grid. Instructions:

1. Before starting the wind turbine, be sure that the DC motor magnetizing current (Im) is set to 0.8 A and that the DC link voltage (Udc) is set to around 250 V. These are adjusted with the two variable transformers as indicated in figure 6. Check their values on the analog instruments on the instruments panel and on the LabVIEW interface.

2. To begin, select fixed speed wind turbine mode and manual power reference.

Subsequently, press Start. The DC motor is now operated with a constant power reference, driving the synchronous generator connected directly to the grid.

3. Flip the lower switch, selecting wind turbine mode. The DC motor is now

controlled as a wind turbine as outlined in figure 7. Now, the wind speed may be varied from the interface as desired. Observe the readback.

4. In this experiment, the output power delivered by the fixed speed wind turbine to

the grid as a function of wind speed will be measured. Along with generated power, the power coefficient Cp, the tip speed ratio λ, and the pitch angle β will also be measured. Please fill out the below table.

vwind (m/s) λ Cp β (degrees) Pel (W) 5 6 7 8 9

10 11 12 13 14 15 16 17

Note The wind turbine may now be disconnected from the grid. Do this by gradually decreasing the wind speed to 5 m/s, flipping the lower switch back to manual power reference, and pressing Stop.

Laboratory Experiment Wind Power Systems (EIEN10) Rev. 1 - M. Collins, 2016

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Plot each of the above variables as a function of wind speed in the following

figures. What is the minimum wind speed at which the fixed-speed wind turbine is able to

generate power? ……………………………………………………………………………………...

What are the corresponding λ, and Cp?

……………………………………………………………………………………...

Is the wind turbine operating close to its optimum point for low wind speeds? ……………………………………………………………………………………...

For high wind speed range? ……………………………………………………………………………………...

What is the value of λ that corresponds to maximum Cp, and hence maximum

possible power extraction at a given wind speed? ……………………………………………………………………………………...

Laboratory Experiment Wind Power Systems (EIEN10) Rev. 1 - M. Collins, 2016

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Laboratory Experiment Wind Power Systems (EIEN10) Rev. 1 - M. Collins, 2016

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Experiment B: Variable speed wind turbine lab Instructions: In this second experiment, the synchronous generator is connected to the grid via the back-to-back converter.

1. Before starting the wind turbine, be sure that the DC motor magnetizing current Im is set to 0.8 A and that the DC link voltage (Udc) is around 250 V. These are adjusted with the two variable transformers as indicated in figure 6. Check their values on the analog instruments on the instruments panel and on the interface.

2. To begin, select variable speed wind turbine mode and manual power reference.

Subsequently, press Start. The DC motor is now operated with a constant power

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reference, driving the synchronous generated connected to the grid via the back-to-back converter.

3. Flip the lower switch, selecting wind turbine mode. The DC motor is now

controlled as a wind turbine as outlined in figure 7. Now, the wind speed may be varied from the interface as desired. Observe the readback.

4. As in the previous experiment, the output power delivered by the fixed speed

wind turbine to the grid as a function of wind speed will be measured. Along with generated power, the power coefficient Cp, the tip speed ratio λ, and the pitch angle β will also be measured. Please fill out the below table.

vwind (m/s) λ Cp Β (degrees) Pel (W) 5 6 7 8 9

10 11 12 13 14 15 16 17

Note The wind turbine may now be disconnected from the grid. Do this by gradually decreasing the wind speed to 5 m/s, flipping the lower switch back to manual power reference, and pressing Stop. Plot the results obtained for this case in the above plots along with the results

obtained for fixed speed wind turbine mode. At which minimum wind speed is the wind turbine now able to deliver power?

……………………………………………………………………………………...

What is the difference compared to the fixed-speed wind turbine case?

……………………………………………………………………………………...

Laboratory Experiment Wind Power Systems (EIEN10) Rev. 1 - M. Collins, 2016

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At which wind speeds is the wind turbine operating in optimal condition of maximum power extraction?

……………………………………………………………………………………...

Do you see any advantage in having a variable speed wind turbine for high wind

speeds, when it comes to power extraction? Why?

……………………………………………………………………………………... What are the values of λ and Cp at optimal power extraction?

……………………………………………………………………………………...

Estimate the total annual energy production in using fixed speed and variable

speed wind turbines, assuming the following probabilities of operating at a given wind speed.

vwind (m/s) Probability N. hours PWT fix ω PWT var ω EWT fix ω EWT var ω ΔE

0 0 0.0 1 0.0326 285.6 2 0.0619 542.2 3 0.0852 746.4 4 0.1008 883.0 5 0.108 946.1 6 0.1073 939.9 7 0.1002 877.8 8 0.0885 775.3 9 0.0744 651.7 10 0.0597 523.0 11 0.0458 401.2 12 0.0337 295.2 13 0.0238 208.5 14 0.0161 141.0 15 0.0105 92.0 16 0.0066 57.8 17 0.004 35.0 18 0.0023 20.1 19 0.0013 11.4 20 0.0007 6.1 21 0.0004 3.5

Sum the ΔE column to attain the total energy difference ΣΔE.

……………………………………………………………………………………...

Multiply ΣΔE by 2000 to estimate the energy difference for a 2 MW wind turbine.

……………………………………………………………………………………...

Multiply this new number by 100 to estimate the energy difference for a wind

power plant with 100 wind turbines rated for 2 MW.

……………………………………………………………………………………...