AC 2010-498: EMULATION OF A WIND TURBINE SYSTEM

Ruben Otero, Student at University of Puerto Rico - Mayaguez

Apurva Somani, University of Minnesota

Krushna Mohapatra, University of Minnesota

Ned Mohan, University of Minnesota

© American Society for Engineering Education, 2010

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Emulation of a Wind Turbine System

Abstract

Recently there has been an increasing interest in wind power generation systems. Among

renewable sources of energy (excluding hydro power), wind energy offers the lowest cost. It is

therefore imperative that basics of wind power generation be taught in the undergraduate

electrical engineering curriculum. In this paper, an experiment that emulates wind turbine

systems has been developed for this purpose.

It is known that the power that can be drawn from the wind in a wind turbine depends on the

wind speed and the speed at which the shaft of the turbine is rotated. The objective of this project

was to emulate the behavior of such a system using two DC machines. One of the DC machines

was operated under torque control. The torque reference for this machine was generated using

the Power vs. Shaft speed curves for wind turbines. This DC machine emulated the wind turbine

and shaft. The second DC machine was operated under speed control and this machine emulated

the electrical generator.

Simulations were performed to design such a system. The system was implemented in real-time

using Simulink and dSPACE control platform. Two 200W DC machines rated at 40VDC and

4000 rpm were used. The DC machines were controlled using a pulse width modulated (PWM)

power converter. This project was part of an undergraduate research supported by NSF and the

University of Minnesota Research Experiences for Undergraduates (REU) program.

I. Introduction

The objective is to develop a system that emulates a wind turbine. Previous efforts in this

direction have employed separately excited DC machines1,2

with power ratings in the multiple

horsepower range. The intended application of the system described in this paper is for

undergraduate laboratory courses. Thus, a system that works at lower voltages is desired.

Existing laboratory equipment such as DC motors and generators are to be used to describe the

system. Since this experiment was done using two 200W DC machines3 rated at 42VDC and

3600 rpm it is more appropriate for educational purposes.

The kinetic energy from the wind is transferred as rotational mechanical energy to the wind

turbine system. An optional gearbox can be placed depending on the generator specifications to

increase the shaft speed (hence decreasing the torque). This mechanical energy is converted to

electrical energy using a generator. A power electronic interface may be needed to interface the

generator with the supply grid and to provide a control method for the system.

Page 15.458.2

The mechanical power in the wind depends on a few factors and is given by4,5

: �� � 1

2 ��� ��

where ρ is the air density (1.225 kg/m3 at 15

0 C and 1 atm.), A is cross-sectional area of the

blades and Vw is the wind speed. Betz’s Law states that only a fraction of this power can be

captured by the wind turbine [Ref.5]. This fraction of the power in the wind that can be captured by

the wind turbine is called the Power Coefficient (Cp) and is defined as:

�� � �����������

��

The maximum theoretical Cp value is .593 or 59.3%

Assuming that there is no pitch angle control for low variable wind speed, equation (3) describes

the Power Coefficient (Cp) in terms of the Wind Speed and Shaft Speed of the wind:

�� � 12 � �

��� 5.6 !"#.$%&'() �*

where Vw is the Wind Speed in mph and ωm is the Shaft Speed in rad/sec.

Equations (1-3) are used to calculate the shaft power (Pshaft) curves for various wind speeds. The

results are shown in Fig. 1

Note: The power shown in Fig.1 was scaled by a factor of 1/15 and a gear ratio of 50 was used

for the shaft speed. This was done to match the equipment rating used in the experiments. The

diameter of the blades was chosen to be 1m for simplicity.

Fig. 1: Power vs. Shaft Speed curves

0 500 1000 1500 2000 25000

20

40

60

80

100

120

Shaft Speed (rpm)

Psh

aft (

W)

Power transfer to the shaft vs. Shaft speed

20 m/s 15 m/s 10 m/s 5 m/s

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II. Modeling and Emulation of Wind Power Generation System

The modeling and emulation of this wind power generation system are done by controlling the

torque of one DC machines and the speed of another DC machine. The wind turbine is emulated

using a model that generates torque as output depending on wind speed and shaft speed (which

are inputs to the system) as shown in Fig.2. This is implemented using a DC motor which is

operated under Torque-control (Current-control) with torque reference derived from the wind

turbine system Model (Fig.2). The wind turbine system model is either based on an equation or a

look up table.

The generator controls the shaft speed in modern wind turbines. The generator is usually a three-

phase induction motor or a permanent magnet ac motor. However, in this paper the generator is

emulated using a DC motor. This DC motor (DC-M1) is operated under speed-control as shown

in Fig.3. The shaft speed of this DC motor (DC-M1) along with the wind speed set the torque

reference for the other DC motor (DC-M2) as shown in Fig.2.

The power in the shaft can be represented by the torque generated in the shaft depending on the

shaft speed. The torque vs. shaft speed curves for different wind speeds are shown in Fig.4.

Fig. 3: Emulation of wind turbine and generator using two DC motors

Torque Control Speed Control

Wind Turbine

DC-M2

Generator

DC-M1

Fig. 2: Wind Turbine System Model

��

+�����

� Wind Turbine

Model

Page 15.458.4

,-./01 � 2-./013-./01

�4

Torque control of the DC motor is done by controlling the machine current. The torque reference

is set using equations (1-4). The PI controller used for the torque control6 is shown in Fig.5.

The speed control of the DC generator is similar to the torque control because it uses a PI current

controller. The reference input to the current PI controller is the output of a speed PI controller

that has a reference shaft speed as input. The controller used for speed control of the DC

generator is shown in Fig.6.

Fig. 5: PI current controller

Kp_i

Ki_i 15

Integrator

IL_ref

IL

VL

Fig. 4: Torque vs. Shaft Speed curve

0 500 1000 1500 2000 2500 3000 0

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16S

haf

t T

orq

ue

(N.m

) Torque vs Shaft Speed

20 mph

15 mph10 mph

5 mph

Shaft Speed (rpm)

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III. Hardware Setup and Results

The experiment setup consisted of the following equipment:

1. Two permanent magnet DC machines rated at 200W, 3600rpm and 40V

2. Power Electronics converter board7

3. 40V DC power supply

4. dSPACE DS1104 board

5. Computer with MATLAB/Simulink for real-time interface

A photograph of the hardware setup is shown in Fig.7.

Fig. 7: Hardware setup

Fig. 6: Speed controller

PI

Speed

Controller PI

Current

Controller

ωm_ref

ωm

Ia

Ia_ref

V0

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The model of the turbine and the

in real time by dSPACE. The PW

A block diagram of the motor con

Fig. 9.

The model is executed in dSPAC

speed inputs, namely 5, 10 and 15

measured. The results are shown

Fig. 9: Simulink model of the syste

Fig. 8: Block diagram of motor co

U

DC Power Source

Input Command

(Speed)

Control In

he generator are implemented in Simulink. This m

WM8 signals for the power converter are generate

control system is shown in Fig.8. The Simulink mo

CE and results are obtained. Results are taken for

15 mph. The rotor speed is varied and the shaft to

n in Fig. 10.

ystem

control system

Power

Processing

Unit (PPU)

Controller Speed Feedback

Adjustable Form

Input

Motor

Senso

model is executed

ated by dSPACE.

model is shown in

for various wind

t torque is

sor

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IV. Conclusion

A wind turbine model has been developed using permanent magnet DC machines. Simulations

are performed to design the system and hardware verification is done. This model can be

utilized in an undergraduate electric drives laboratory to introduce wind power generation.

Further work may include incorporating an induction machine to emulate the generator.

Bibliography

1. Md. Arifujjaman, M.T. Iqbal, John E. Quaicoe, “Emulation of a small wind turbine system with a separately-

excited DC machine,” Journal of electrical & electronics engineering, Istanbul University, vol 8 no 1 yr 2008

2. Ovando, R.I.; Aguayo, J.; Cotorogea, M.; , "Emulation of a Low Power Wind Turbine with a DC motor in

Matlab/Simulink," Power Electronics Specialists Conference, 2007. PESC 2007. IEEE , vol., no., pp.859-864,

17-21 June 2007

3. Motorsolver Dyno Kit Specification, http://www.motorsolver.com/files/Download/DYNO-MOTOR-SPECS.pdf

4. “Simulation Study of Wind Energy Conversion Systems,” M.S. Thesis, Rohit Tirumala, University of

Minnesota, 2000

5. Renewable and Efficient Electric Power Systems by Gilbert M. Masters, 2004 edition by John Wiley & Sons,

Inc., Hoboken, New Jersey.

6. Electric Drives: An Integrative Approach by Ned Mohan, 2003 Edition, http://www.mnpere.com

7. HiRel Systems Inverter Board, http://www.hirelsystems.com/shop/Inverter-Board.html

8. First Course on Power Electronics by Ned Mohan, 2003 Edition, http://www.mnpere.com

Fig. 10: Experimental results for torque-speed curves

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