Renewable Energy 30 (2005) 913–929 www.elsevier.com/locate/renene

Data bank

Modelling of a 5-kW wind energy conversion system with induction generator and comparison with experimental results Tolga Su¨rgevil, Eyu¨p Akpınar* Dokuz Eylu¨l University, Department of Electrical and Electronics Engineering, Kaynaklar Kampu¨su¨, 35160 Buca/Izmir, Turkey Received 23 April 2004; accepted 13 September 2004 Available online 19 November 2004

Abstract A 5-kW wind energy conversion system (WECS) having induction generator is designed and implemented. The induction machine is connected to the power system through PWM inverter and PWM rectifier. Two digital PI controllers are used, one of them is for regulating dc link voltage and the other is for speed control of induction machine. The whole system is governed by a single fixed point digital signal processing unit (DSP). A detailed simulation program is prepared by using Matlab facilities in order to predict the performance of the controllers before implementation. q 2004 Elsevier Ltd. All rights reserved. Keywords: Wind energy; Induction generator; PWM converter

1. Introduction The power available from wind changes with the cube of the wind velocity. In order to extract the maximum power at different value of wind speed, it is required to vary the turbine speed over a wide range. This would not be possible if the squirrel cage induction generator were directly connected to the power system at constant voltage and frequency. Because the induction generator with this connection would allow the rotor speed to change in a narrow range between the speed corresponding to maximum torque * Corresponding author. Tel.: C90 232 4531008x1163; fax: C90 232 4534279. E-mail addresses: [email protected] (E. Akpınar), [email protected] (E. Akpınar). 0960-1481/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2004.09.002

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and synchronous speed. Due to the advances in power electronics and digital signal processing, the effective control of power conversion for a wide range of wind speed (from 3 to 20 m/s) has been implemented. Four and five megawatts wind turbines integrated with the power electronic circuits will be used worldwide in near future [1]. The techniques developed on torque and speed control of synchronous and induction machines are also implemented in wind energy conversion systems (WECS) in order to draw the maximum power available from the wind turbine. The selection of the generator type depends on many factors such as application type, machine characteristics, maintenance, price, etc. The well-known advantages of induction generators are their robustness, low cost, and ease of maintenance. In wind energy applications, the wound rotor types of them with the rotor voltage control circuitry are also preferred in the case of direct connection to grid, since they provide some flexibility against fluctuating wind due to asynchronous operation. The major disadvantage is that they draw reactive power from the supply terminals. The converters are usually located between power system and the generators at full power rating of the wind turbine, except the slip energy recovery drive used with wound rotor induction machine [2]. Modern control techniques, such as field oriented and space vector controls, are employed for fast dynamic response to change of wind conditions and power factor regulation. The three-phase PWM rectifiers are used in the WECS in order to obtain unity power factor operation with the minimized current harmonics injected to the power system [3–9]. The system described in this paper differs from the systems that use the same power electronic converter topology given in [8–9] from the viewpoint of control methods. The control of the converters in [8–9] is based on vector control method with fuzzy logic controllers, but the scalar techniques are selected in this work. The dc link voltage regulation and speed control of induction machine are carried out via proportional integral (PI) controllers. A single fixed-point DSP is employed, and the ac currents of the line-side converter and dc link voltage are controlled by hysteresis current control (HCC) [5]. The speed control of the induction machine is achieved by employing the slip regulation technique [10] because it provides inherent current limiting and reduces the amount of measurement devices. These control methods eliminate the necessity of system parameters. The system designed in this work uses a simpler control technique for a low power rated WECS. The schematic diagram and general view of the WECS are shown in Figs. 1 and 2, respectively. The wind turbine is a propeller type, three-bladed turbine and coupled to the induction machine shaft through a gearbox. A detailed dynamic model of the drive system with induction machine is prepared in order to predict the performance.

2. Mathematical model of induction machine The variation of the utility voltage level at the armature terminals of the machine causes a significant change on the torque speed characteristics because the electromagnetic torque is proportional to the square of the voltage. In order to reduce the starting current and increase the starting torque, some of the manufacturers have designed the induction motors

Fig. 1. Variable speed wind generation system.

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Fig. 2. General view of WECS at Dokuz Eylu¨l University Campus.

rated more than 5 kW as double cage. The induction machine used here has been bought as an ordinary motor available in the market. Since its rotor has double cage structure, a model of double cage induction machine given in (1) is considered in abc/qd reference frame [11] by neglecting the effect of magnetic saturation. 2

r1 CL11 p 6 6 2 3 6 va1 6 0 6 6 7 6 6 vb1 7 6 6 7 6 6 7 6 0 6v 7 6 6 c1 7 6 6 7 6 6 7 6 6 v0 7 6 6 q2 7 Z 6 Mp 6 7 6 6 7 6 6 v0 7 6 6 d2 7 6 6 7 6 ur M 6 7 6 6 v0 7 6 6 q3 7 6 4 5 6 6 Mp 6 0 vd3 6 6 4 ur M

0

0

Mp

0

Mp

0

3

7 72 3 7 i 7 a1 r1 CL11 p 0 76 7 76 7 76 ib1 7 76 7 1 1 76 7 0 r1 CL11 p K Mp K Mp 76 7 2 2 76 ic1 7 76 7 pﬃﬃﬃ pﬃﬃﬃ 76 7 3 3 7 0 7 0 0 0 0 0 0 0 ur Lm Kur Lm r2 CðL22 CL23 Þp Kur ðL22 CL23 Þ ðL23 CMÞp Kur ðMCL23 Þ 76 iq2 7 76 2 2 7 76 7 pﬃﬃﬃ pﬃﬃﬃ 76 0 7 76 id2 3 3 6 0 0 0 0 0 0 0 7 Lm p K Lm p ur ðL22 CL23 Þ r2 CðL22 CL23 Þp ur ðM CL23 Þ ðL23 CMÞp 76 7 7 7 2 2 76 0 7 76 iq3 pﬃﬃﬃ pﬃﬃﬃ 6 74 7 5 3 3 0 0 0 0 0 0 L Kur L ur ðL23 CMÞp Kur ðM CL23 Þ r30 CðL33 CL23 Þp Kur ðL33 CL23 Þ7 7 0 2 m 2 m 7 id3 7 pﬃﬃﬃ pﬃﬃﬃ 5 3 3 0 0 0 0 0 0 Lm p K Lm p ur ðM CL23 Þ ðL23 CMÞp ur ðL33 CL23 Þ r30 CðL33 CL23 Þp 2 2 1 K Mp 2

pﬃﬃﬃ 3 K Mp 2 pﬃﬃﬃ 3 Mp 2

1 K Mp 2

pﬃﬃﬃ 3 K Mp 2 pﬃﬃﬃ 3 Mp 2

ð1Þ

T. Su¨rgevil, E. Akpınar / Renewable Energy 30 (2005) 913–929

The electromagnetic torque developed by the machine is pﬃﬃﬃ 3 P 1 1 0 0 Te Z ði K ic1 Þ ia1 K ib1 K ic1 C iq2 M id2 2 2 2 2 b1 pﬃﬃﬃ 3 1 1 0 0 C id3 ia1 K ib1 K ic1 C iq3 ði K ic1 Þ 2 2 2 b1 The torque balance equation for mechanical motion is 2 TL Z Te K J pur K Bur p

917

(2)

(3)

3. Modelling of drive circuit The mathematical models of both PWM converters will be given in order to predict the steady state and transient response of the drive system using computer simulations. The ideal switch model of the solid-state device is taken into consideration. Therefore, the internal voltage drop during conduction, and switching transients during turning on and off are neglected. The closed-loop dc link voltage and speed control schemes have been also included into the models. The drive circuit is divided into parts; one of them is the rectifier and the other is inverter. 3.1. PWM rectifier This converter is supplied by three-phase utility grid and provides a regulated dc voltage to the inverter circuit with almost sinusoidal ac line currents at unity power factor. For dc link voltage regulation, a digital PI controller is used with hysteresis current control. The mathematical model of the converter is as follows [4]: ! 3 dik udc X Ls C Rs ik Z ek K udc dk K d (4) dt 3 kZ1 k C

3 dudc X Z dk ik K idc dt kZ1

(5)

where kZ1,2,3, and Rs, Ls are the resistance and inductance of the inductor connected between converter and utility grid. C is the dc link capacitor. These series inductors are required for filtering of line current harmonic and boost operation of the PWM rectifier. The balanced three-phase supply voltages (ek), the instantaneous switching states of the upper side IGBTs (ONZ1, OFFZ0) on each leg (dk), and the load current (idc) are the inputs, while the three-phase supply currents (ik) and the dc link voltage (udc) are the states in the model. The open-loop Matlab Simulink model of the rectifier is given in Fig. 3. The closed-loop voltage control of the rectifier is achieved through the ac line current control. The magnitude of the reference ac line currents (Icm) is obtained through

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Fig. 3. Simulink model of the PWM rectifier.

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the digital PI controller. The change of digital controller output (DIcm) in Fig. 1 is calculated from error (E) between reference and measured value of dc link voltage, and change of error (DE) within one sampling period as follows DIcm ½nTsmp Z Kp DE½nTsmp C Ki Tsmp E½nTsmp

(6)

The parameters of digital PI controller are error gain KE Z Ki Tsmp and change of error gain KDE Z Kp . 3.2. PWM inverter The sinusoidal PWM inverter is used to control the speed of the machine The gating signals are generated by comparing a sinusoidal reference signal with a triangular carrier wave. The induction machine stator phase voltages and dc link current are expressed in terms of dc link voltage and switching signals as follows va1 Z udc d10 K

3 udc X d0 3 kZ1 k

(7)

vb1 Z udc d20 K

3 udc X d0 3 kZ1 k

(8)

vc1 Z udc d30 K

3 udc X d0 3 kZ1 k

(9)

idc Z ia1 d10 C ib1 d20 C ic1 d30

(10)

dk0

where is the upper side switching signals of each leg. The open-loop Matlab Simulink model of the inverter is given in Fig. 4. The speed control of induction machine is carried out by adjusting the slip factor, which is based on scalar constant V/Hz control method [10]. The output of the speed PI controller gives the amount of change in slip command ðDuslip Þ and it is added to the rotor speed to generate the command signals of frequency for PWM inverter. The V/f ratio is kept constant while the reference frequency is below the rated value. Above the rated frequency, the V/f ratio is deviated from this constant by limiting the voltage. The currents supplied to the induction machine are indirectly limited by the maximum slip frequency set. The required reference frequency for the operation of inverter is obtained as follows: u ðnTsmp Þ Z Duslip ðnTsmp Þ C ur ðnTsmp Þ

(11)

4. Wind turbine and selection of gear ratio The wind turbine in the system is a horizontal axis, three-bladed, fixed pitch angle turbine with a blade length of 2.75 m. A free yaw mechanism with a tail vane is used to

T. Su¨rgevil, E. Akpınar / Renewable Energy 30 (2005) 913–929

Fig. 4. Simulink model of the PWM inverter.

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Fig. 5. Power coefficient of the wind turbine versus tip-speed ratio.

turn the nacelle to the wind direction. The turbine is coupled to the shaft of induction machine through a gearbox and braking unit. The Cp–l curve given in Fig. 5 has been obtained from the manufacturer data and used to represent the turbine in the Matlab Simulink. The mechanical power developed by the turbine is as follows [12] Pt Z 0:5 Cp ðlÞrpR2 v3 ðWattsÞ

(12)

The gear ratio ngear affects how much electrical power is generated by the system at a value of wind speed. In order to estimate a proper gear ratio to maximize the electrical power available, the simulation program is used. The induction machine is fed by 220 V, 50 Hz power source and driven by the wind turbine having the characteristic given in (12). The electrical output power of the generator is computed at different values of gear ratio and wind speed. The core, friction and windage losses of the machine are also included into the calculation. The no-load test on the induction machine has been carried out in order to determine the core loss and rotational loss of the machine separately [14]. The rotational loss of the mechanical system with the wind turbine is determined by driving the induction motor from a three-phase voltage source while the tail vane is closed (turbine is directed out of wind) and subtracting the core and stator copper losses from the no-load power measured. The results presented in Fig. 6 show that the power available from the generator can be maximized by selecting gear ratio around 10, when the wind speed is vZ10 m/s which is the average wind speed expected. The maximum power conversion at different wind speed can be obtained by changing the frequency and voltage via PWM inverter.

5. DSP Control and system parameters The control panel in Fig. 7 contains the rectifier, inverter, dc link capacitor, inductances at the input of rectifier, power supplies, transducers and DSP. Two 6MBP75RA120 FUJI

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Fig. 6. Electrical power as a function of gear ratio.

IGBT intelligent power modules have been used in rectifier and inverter. Each module contains 6-pack IGBT, which are rated at 1200 V 75 A and provided with internal gate drive and protection circuitry. Two HCPL4504 optocoupler drive interfaces for each module were used to isolate the control signals from the power stages. Control of the complete system has been performed using a single TMS32F240 16-bit fixed-point DSP board. The DSP has 16-channel 10-bit on-chip analog to digital converter, multi-functioning I/O ports, and has the feature of programmable PWM generation with adjustable dead-band, which is especially suitable for digital motor control applications. Also, it has the power drive protection circuitry, which allows the designer to introduce additional protection for power converters in case any fault occurs. The software for controlling the complete system was developed using C compiler provided by Texas Instruments. In order to obtain the feedback signals to the controller, hall-effect voltage and current sensors are used at ac line and dc link. A tacho-generator is also used to

Fig. 7. Control panel of the WECS.

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provide the rotor speed feedback. The PWM inverter frequency is selected to be 2 kHz. The modulation index is linearly changed from 0.128 to 0.96 correspondingly 0 to 50 Hz, respectively, and kept constant at 0.96 above 50 Hz. The flowchart of the control software developed for the DSP is given in Fig. 8. The rest of the system parameters as follows [11,13]: Line-side converter parameters: Ls Z 50 mH;

Rs Z 0:8 ohms;

CZ 1:2 mF

dc link reference voltage, udc Z 500 V Induction machine per phase parameters: r1 Z 3:0 ohms r20 Z 5:05 ohms r30 Z 3:77 ohms 0 x1 Z7:51 ohms x20 Z0:22 ohms x30 Z9:38 ohms xm Z169:4 ohms x23 Z1:39 ohms 2 J Z 0:02 kg:m Controller parameters: KE;dc Z 4;

KDE;dc Z 256;

KE;speed Z 0:125;

KDE;speed Z 0:5;

The moment of inertia of the wind turbine, JtZ151 kg m2 The blade length of the wind turbine, RZ2.75 m Speed-up gear ratio between the turbine and the generator, ngearZ10

6. Simulation and experimental results 6.1. Simulation results First, the three-phase voltage is applied to the input of PWM rectifier and the dc link capacitance is charged to peak value of line to line voltage through the three-phase diode bridge rectifier built in 6MBP75RA120 FUJI IGBT intelligent power modules. Second, the switching signals of the rectifier are applied and the dc link voltage is boosted to its reference value of 500 V by the controller. Since the starting torque of the wind turbine is low, the induction machine is started from standstill to reference speed as a motor. The rotor speed is set to 940 rpm while the tail vane is closed and the turbine is directed out of wind. The simulation results of the electromagnetic torque and rotor speed are given in Fig. 9 for free acceleration of the induction motor from standstill to 940 rpm. In this case, the line-side converter operates as rectifier and draws real power from the utility grid to compensate the no-load losses. Since the total moment of inertia of the rotating mass referred to the generator shaft is approximately JtZ151 kg m2, the starting transients take almost 28 s to settle down. After reaching at steady state operating point at 28 s, the tail vane of turbine is released free and the turbine is directed towards wind. The simulations are carried out at the wind speed of 8 m/s. It is clearly

T. Su¨rgevil, E. Akpınar / Renewable Energy 30 (2005) 913–929

Fig. 8. Flowchart of the DSP control software.

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Fig. 9. Free acceleration of induction machine to 940 rpm (top: electromagnetic torque, bottom: rotor speed).

shown in Fig. 9 that the electromagnetic torque is positive until the time when the tail vane is released, then, it is negative which corresponds to generator mode of operation for the induction machine. The steady-state waveforms of the ac line voltage and current are shown in Fig. 10. The rectifier input current is almost sinusoidal and at unity power factor. It is clear that the phase difference between the supply current and voltage is 1808, therefore the line side converter (rectifier) supplies real power to the utility grid. In Fig. 11, the induction machine phase voltage and current waveforms are given for this operating condition. The PWM inverter is operated at 2 kHz, hence, the current is almost sinusoidal. The wind turbine operates at a tip-speed ratio of 3.4 for this operating condition and the corresponding power coefficient is approximately 0.35. The turbine develops a mechanical power of 2.5 kW at this operating point. The net electrical power delivered to the grid is approximately 1.8 kW. 6.2. Experimental results In experimental work, since the starting torque of the wind turbine is low, first of all the induction machine has been run as a motor fed by the drive circuit. The reference rotor speed of the induction machine is set to a fixed value, 940 rpm. The PI speed controller keeps the actual rotational speed almost stable at the reference value. For this operating condition, the wind speed and the dc link current are recorded on-line over a long time range by using TQ-Data Management System connected to PC computer.

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Fig. 10. Supply voltage and current at the input of rectifier.

Fig. 11. Induction machine phase voltage and current.

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Fig. 12. Recorded dc link current at a rotor speed of 940 rpm.

Fig. 12 shows the recorded dc link current. Since the dc link voltage is constant and smoothed at the level of 500 V, the dc link current is proportional to the electrical output power supplied to the utility grid. The sign of the dc link current specifies the direction of power flow. The negative value of the dc link current indicates that the real power flows from generator to the grid and positive value of it shows that the real power flows from grid to generator. Especially, between 97 and 129 s, wind condition is quite stable and approximately 1 kW power is delivered to the grid. The recorded wind speed during this measurement is shown in Fig. 13. The oscillations in the dc link current, and as a consequence the instantaneous electrical power, can be observed in long time recordings due to rapidly changing wind direction and speed. The responses of the converters to these rapid changes are good enough to keep the system stable. The line-side converter (rectifier) quickly responses to the load changes and regulates the dc link voltage at its given reference

Fig. 13. Recorded wind speed at a rotor speed of 940 rpm (3.2 m/s/div).

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Fig. 14. Grid voltage and current on the line side when the rotor speed is fixed at 940 rpm (upper trace: grid voltage—125 V/div, lower trace: grid current—4 A/div).

value. The speed controller also shows a good performance and recovers the deviation of rotational speed from the reference during the disturbing effects of the wind gusts. The recorded waveforms of voltage and current on the utility grid are shown in Fig. 14, when the rotor speed is fixed at 940 rpm. These waveforms are recorded by using digital oscilloscope and a current probe with a frequency bandwidth from dc to 150 kHz, when the power delivered to the grid is measured to be approximately 1 kW. When the current waveform in Fig. 14 is compared to that in Fig. 10, there is a significant difference. The reason is that the wind speed and its direction are changing very fast and affecting the actual current waveform but the simulation result in Fig. 10 is obtained under the constant wind speed of 8 m/s. It is clearly seen from Fig. 14 that the peak of current is almost 4 A during last period of waveform. The change of current peak value is created by varying wind speed and its direction.

7. Conclusions The WECS having induction generator and drive circuit has been modelled in Matlab. The PI controller parameters are selected by using the simulation results. The system has been implemented by using scalar control techniques, which do not require system parameters, such as stator and/or rotor parameters of the induction machine. The fact is that the field oriented control (FOC) of the induction machine is better than scalar controllers for large power rated WECS having yaw control. But the small rated WECS may not have yaw control as it is here, therefore, the stator terminals of the generator are connected to the drive circuit through slip rings and carbon brushes. This connection

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causes the precise estimation of stator and rotor machine parameters not to be possible, and application of the FOC may fail. The Matlab model of the whole system prepared in detail has predicted the steady state and transient behaviour of the WECS during uniform flow of wind. But a rapid variation of the wind direction with respect to the position of turbine should be considered in the model. This consideration will provide a better power flow control in terms of extracting the maximum power available from the wind.

Acknowledgements This work was carried out as a part of research project, ‘wind energy conversion system with induction generator using PWM converters’, sponsored by Turkish Scientific and Research Council under contract 101-E004.

References [1] Ackermann T, So¨der L. Wind energy technology and current status: a review. Renew Sustain Energy Rev 2000;4:315–74. [2] Akpınar E, Pillay P. Modeling and performance of slip energy recovery induction motor drives. IEEE Trans Energy Convers 1990;5:203–10. [3] Wu R, Dewan SB, Slemon GR. Analysis of a PWM ac to dc voltage source converter under the predicted current control with a fixed switching frequency. IEEE Trans Ind Appl 1991;27:756–64. [4] Blasko V, Kaura V. A new mathematical model and control of a three-phase AC–DC voltage source converter. IEEE Trans Power Electron 1997;12:116–23. [5] Ooi BT, Salmon JC, Dixon JW, Kulkarni AB. A three-phase controlled-current PWM converter with leading power factor. IEEE Trans Ind Appl 1987;IA-23:78–84. [6] Dixon JW, Ooi BT. Indirect current control of a unity power factor sinusoidal current boost type three-phase rectifier. IEEE Trans Ind Electron 1988;35:508–15. [7] Wu R, Dewan SB, Slemon GR. A PWM AC-to-DC converter with fixed switching frequency. IEEE Trans Ind Appl 1990;26:880–5. [8] Simoes MG, Bose BK, Spiegel RJ. Design and performance evaluation of a fuzzy-logic-based variablespeed wind generation system. IEEE Trans Ind Appl 1997;33:956–65. [9] Pena R, Cardenas R, Blasco R, Asher G, Clare J. A cage induction generator using back to back PWM converters for variable speed grid connected wind energy system. In: Proceedings of the IEEE industrial electronics conference; 2001. p. 1376–81. [10] Dubey GK. Power semiconductor controlled drives. New York: Prentice-Hall; 1989. [11] Su¨rgevil T, Akpinar E. abc/qd, qd/abc model of a double-cage induction machine and determination of parameters using genetic algorithm. J Electr Power Components Syst 2003;31:1115–31. [12] Buehring, IK, Freris, LL. Control policies for wind-energy conversion systems. IEE proceedings 1981 128. pt. C p. 253–61 [13] Su¨rgevil T. Modeling and simulation of wind energy conversion system using PWM converters. PhD Thesis, Graduate School of Natural and Applied Sciences, Dokuz Eylu¨l University; , 2004. [14] IEEE standard no. 122-1996. Test procedure for polyphase induction motors and generators, New York; 1997.

Data bank

Modelling of a 5-kW wind energy conversion system with induction generator and comparison with experimental results Tolga Su¨rgevil, Eyu¨p Akpınar* Dokuz Eylu¨l University, Department of Electrical and Electronics Engineering, Kaynaklar Kampu¨su¨, 35160 Buca/Izmir, Turkey Received 23 April 2004; accepted 13 September 2004 Available online 19 November 2004

Abstract A 5-kW wind energy conversion system (WECS) having induction generator is designed and implemented. The induction machine is connected to the power system through PWM inverter and PWM rectifier. Two digital PI controllers are used, one of them is for regulating dc link voltage and the other is for speed control of induction machine. The whole system is governed by a single fixed point digital signal processing unit (DSP). A detailed simulation program is prepared by using Matlab facilities in order to predict the performance of the controllers before implementation. q 2004 Elsevier Ltd. All rights reserved. Keywords: Wind energy; Induction generator; PWM converter

1. Introduction The power available from wind changes with the cube of the wind velocity. In order to extract the maximum power at different value of wind speed, it is required to vary the turbine speed over a wide range. This would not be possible if the squirrel cage induction generator were directly connected to the power system at constant voltage and frequency. Because the induction generator with this connection would allow the rotor speed to change in a narrow range between the speed corresponding to maximum torque * Corresponding author. Tel.: C90 232 4531008x1163; fax: C90 232 4534279. E-mail addresses: [email protected] (E. Akpınar), [email protected] (E. Akpınar). 0960-1481/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2004.09.002

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and synchronous speed. Due to the advances in power electronics and digital signal processing, the effective control of power conversion for a wide range of wind speed (from 3 to 20 m/s) has been implemented. Four and five megawatts wind turbines integrated with the power electronic circuits will be used worldwide in near future [1]. The techniques developed on torque and speed control of synchronous and induction machines are also implemented in wind energy conversion systems (WECS) in order to draw the maximum power available from the wind turbine. The selection of the generator type depends on many factors such as application type, machine characteristics, maintenance, price, etc. The well-known advantages of induction generators are their robustness, low cost, and ease of maintenance. In wind energy applications, the wound rotor types of them with the rotor voltage control circuitry are also preferred in the case of direct connection to grid, since they provide some flexibility against fluctuating wind due to asynchronous operation. The major disadvantage is that they draw reactive power from the supply terminals. The converters are usually located between power system and the generators at full power rating of the wind turbine, except the slip energy recovery drive used with wound rotor induction machine [2]. Modern control techniques, such as field oriented and space vector controls, are employed for fast dynamic response to change of wind conditions and power factor regulation. The three-phase PWM rectifiers are used in the WECS in order to obtain unity power factor operation with the minimized current harmonics injected to the power system [3–9]. The system described in this paper differs from the systems that use the same power electronic converter topology given in [8–9] from the viewpoint of control methods. The control of the converters in [8–9] is based on vector control method with fuzzy logic controllers, but the scalar techniques are selected in this work. The dc link voltage regulation and speed control of induction machine are carried out via proportional integral (PI) controllers. A single fixed-point DSP is employed, and the ac currents of the line-side converter and dc link voltage are controlled by hysteresis current control (HCC) [5]. The speed control of the induction machine is achieved by employing the slip regulation technique [10] because it provides inherent current limiting and reduces the amount of measurement devices. These control methods eliminate the necessity of system parameters. The system designed in this work uses a simpler control technique for a low power rated WECS. The schematic diagram and general view of the WECS are shown in Figs. 1 and 2, respectively. The wind turbine is a propeller type, three-bladed turbine and coupled to the induction machine shaft through a gearbox. A detailed dynamic model of the drive system with induction machine is prepared in order to predict the performance.

2. Mathematical model of induction machine The variation of the utility voltage level at the armature terminals of the machine causes a significant change on the torque speed characteristics because the electromagnetic torque is proportional to the square of the voltage. In order to reduce the starting current and increase the starting torque, some of the manufacturers have designed the induction motors

Fig. 1. Variable speed wind generation system.

T. Su¨rgevil, E. Akpınar / Renewable Energy 30 (2005) 913–929

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Fig. 2. General view of WECS at Dokuz Eylu¨l University Campus.

rated more than 5 kW as double cage. The induction machine used here has been bought as an ordinary motor available in the market. Since its rotor has double cage structure, a model of double cage induction machine given in (1) is considered in abc/qd reference frame [11] by neglecting the effect of magnetic saturation. 2

r1 CL11 p 6 6 2 3 6 va1 6 0 6 6 7 6 6 vb1 7 6 6 7 6 6 7 6 0 6v 7 6 6 c1 7 6 6 7 6 6 7 6 6 v0 7 6 6 q2 7 Z 6 Mp 6 7 6 6 7 6 6 v0 7 6 6 d2 7 6 6 7 6 ur M 6 7 6 6 v0 7 6 6 q3 7 6 4 5 6 6 Mp 6 0 vd3 6 6 4 ur M

0

0

Mp

0

Mp

0

3

7 72 3 7 i 7 a1 r1 CL11 p 0 76 7 76 7 76 ib1 7 76 7 1 1 76 7 0 r1 CL11 p K Mp K Mp 76 7 2 2 76 ic1 7 76 7 pﬃﬃﬃ pﬃﬃﬃ 76 7 3 3 7 0 7 0 0 0 0 0 0 0 ur Lm Kur Lm r2 CðL22 CL23 Þp Kur ðL22 CL23 Þ ðL23 CMÞp Kur ðMCL23 Þ 76 iq2 7 76 2 2 7 76 7 pﬃﬃﬃ pﬃﬃﬃ 76 0 7 76 id2 3 3 6 0 0 0 0 0 0 0 7 Lm p K Lm p ur ðL22 CL23 Þ r2 CðL22 CL23 Þp ur ðM CL23 Þ ðL23 CMÞp 76 7 7 7 2 2 76 0 7 76 iq3 pﬃﬃﬃ pﬃﬃﬃ 6 74 7 5 3 3 0 0 0 0 0 0 L Kur L ur ðL23 CMÞp Kur ðM CL23 Þ r30 CðL33 CL23 Þp Kur ðL33 CL23 Þ7 7 0 2 m 2 m 7 id3 7 pﬃﬃﬃ pﬃﬃﬃ 5 3 3 0 0 0 0 0 0 Lm p K Lm p ur ðM CL23 Þ ðL23 CMÞp ur ðL33 CL23 Þ r30 CðL33 CL23 Þp 2 2 1 K Mp 2

pﬃﬃﬃ 3 K Mp 2 pﬃﬃﬃ 3 Mp 2

1 K Mp 2

pﬃﬃﬃ 3 K Mp 2 pﬃﬃﬃ 3 Mp 2

ð1Þ

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The electromagnetic torque developed by the machine is pﬃﬃﬃ 3 P 1 1 0 0 Te Z ði K ic1 Þ ia1 K ib1 K ic1 C iq2 M id2 2 2 2 2 b1 pﬃﬃﬃ 3 1 1 0 0 C id3 ia1 K ib1 K ic1 C iq3 ði K ic1 Þ 2 2 2 b1 The torque balance equation for mechanical motion is 2 TL Z Te K J pur K Bur p

917

(2)

(3)

3. Modelling of drive circuit The mathematical models of both PWM converters will be given in order to predict the steady state and transient response of the drive system using computer simulations. The ideal switch model of the solid-state device is taken into consideration. Therefore, the internal voltage drop during conduction, and switching transients during turning on and off are neglected. The closed-loop dc link voltage and speed control schemes have been also included into the models. The drive circuit is divided into parts; one of them is the rectifier and the other is inverter. 3.1. PWM rectifier This converter is supplied by three-phase utility grid and provides a regulated dc voltage to the inverter circuit with almost sinusoidal ac line currents at unity power factor. For dc link voltage regulation, a digital PI controller is used with hysteresis current control. The mathematical model of the converter is as follows [4]: ! 3 dik udc X Ls C Rs ik Z ek K udc dk K d (4) dt 3 kZ1 k C

3 dudc X Z dk ik K idc dt kZ1

(5)

where kZ1,2,3, and Rs, Ls are the resistance and inductance of the inductor connected between converter and utility grid. C is the dc link capacitor. These series inductors are required for filtering of line current harmonic and boost operation of the PWM rectifier. The balanced three-phase supply voltages (ek), the instantaneous switching states of the upper side IGBTs (ONZ1, OFFZ0) on each leg (dk), and the load current (idc) are the inputs, while the three-phase supply currents (ik) and the dc link voltage (udc) are the states in the model. The open-loop Matlab Simulink model of the rectifier is given in Fig. 3. The closed-loop voltage control of the rectifier is achieved through the ac line current control. The magnitude of the reference ac line currents (Icm) is obtained through

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Fig. 3. Simulink model of the PWM rectifier.

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the digital PI controller. The change of digital controller output (DIcm) in Fig. 1 is calculated from error (E) between reference and measured value of dc link voltage, and change of error (DE) within one sampling period as follows DIcm ½nTsmp Z Kp DE½nTsmp C Ki Tsmp E½nTsmp

(6)

The parameters of digital PI controller are error gain KE Z Ki Tsmp and change of error gain KDE Z Kp . 3.2. PWM inverter The sinusoidal PWM inverter is used to control the speed of the machine The gating signals are generated by comparing a sinusoidal reference signal with a triangular carrier wave. The induction machine stator phase voltages and dc link current are expressed in terms of dc link voltage and switching signals as follows va1 Z udc d10 K

3 udc X d0 3 kZ1 k

(7)

vb1 Z udc d20 K

3 udc X d0 3 kZ1 k

(8)

vc1 Z udc d30 K

3 udc X d0 3 kZ1 k

(9)

idc Z ia1 d10 C ib1 d20 C ic1 d30

(10)

dk0

where is the upper side switching signals of each leg. The open-loop Matlab Simulink model of the inverter is given in Fig. 4. The speed control of induction machine is carried out by adjusting the slip factor, which is based on scalar constant V/Hz control method [10]. The output of the speed PI controller gives the amount of change in slip command ðDuslip Þ and it is added to the rotor speed to generate the command signals of frequency for PWM inverter. The V/f ratio is kept constant while the reference frequency is below the rated value. Above the rated frequency, the V/f ratio is deviated from this constant by limiting the voltage. The currents supplied to the induction machine are indirectly limited by the maximum slip frequency set. The required reference frequency for the operation of inverter is obtained as follows: u ðnTsmp Þ Z Duslip ðnTsmp Þ C ur ðnTsmp Þ

(11)

4. Wind turbine and selection of gear ratio The wind turbine in the system is a horizontal axis, three-bladed, fixed pitch angle turbine with a blade length of 2.75 m. A free yaw mechanism with a tail vane is used to

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Fig. 4. Simulink model of the PWM inverter.

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Fig. 5. Power coefficient of the wind turbine versus tip-speed ratio.

turn the nacelle to the wind direction. The turbine is coupled to the shaft of induction machine through a gearbox and braking unit. The Cp–l curve given in Fig. 5 has been obtained from the manufacturer data and used to represent the turbine in the Matlab Simulink. The mechanical power developed by the turbine is as follows [12] Pt Z 0:5 Cp ðlÞrpR2 v3 ðWattsÞ

(12)

The gear ratio ngear affects how much electrical power is generated by the system at a value of wind speed. In order to estimate a proper gear ratio to maximize the electrical power available, the simulation program is used. The induction machine is fed by 220 V, 50 Hz power source and driven by the wind turbine having the characteristic given in (12). The electrical output power of the generator is computed at different values of gear ratio and wind speed. The core, friction and windage losses of the machine are also included into the calculation. The no-load test on the induction machine has been carried out in order to determine the core loss and rotational loss of the machine separately [14]. The rotational loss of the mechanical system with the wind turbine is determined by driving the induction motor from a three-phase voltage source while the tail vane is closed (turbine is directed out of wind) and subtracting the core and stator copper losses from the no-load power measured. The results presented in Fig. 6 show that the power available from the generator can be maximized by selecting gear ratio around 10, when the wind speed is vZ10 m/s which is the average wind speed expected. The maximum power conversion at different wind speed can be obtained by changing the frequency and voltage via PWM inverter.

5. DSP Control and system parameters The control panel in Fig. 7 contains the rectifier, inverter, dc link capacitor, inductances at the input of rectifier, power supplies, transducers and DSP. Two 6MBP75RA120 FUJI

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Fig. 6. Electrical power as a function of gear ratio.

IGBT intelligent power modules have been used in rectifier and inverter. Each module contains 6-pack IGBT, which are rated at 1200 V 75 A and provided with internal gate drive and protection circuitry. Two HCPL4504 optocoupler drive interfaces for each module were used to isolate the control signals from the power stages. Control of the complete system has been performed using a single TMS32F240 16-bit fixed-point DSP board. The DSP has 16-channel 10-bit on-chip analog to digital converter, multi-functioning I/O ports, and has the feature of programmable PWM generation with adjustable dead-band, which is especially suitable for digital motor control applications. Also, it has the power drive protection circuitry, which allows the designer to introduce additional protection for power converters in case any fault occurs. The software for controlling the complete system was developed using C compiler provided by Texas Instruments. In order to obtain the feedback signals to the controller, hall-effect voltage and current sensors are used at ac line and dc link. A tacho-generator is also used to

Fig. 7. Control panel of the WECS.

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provide the rotor speed feedback. The PWM inverter frequency is selected to be 2 kHz. The modulation index is linearly changed from 0.128 to 0.96 correspondingly 0 to 50 Hz, respectively, and kept constant at 0.96 above 50 Hz. The flowchart of the control software developed for the DSP is given in Fig. 8. The rest of the system parameters as follows [11,13]: Line-side converter parameters: Ls Z 50 mH;

Rs Z 0:8 ohms;

CZ 1:2 mF

dc link reference voltage, udc Z 500 V Induction machine per phase parameters: r1 Z 3:0 ohms r20 Z 5:05 ohms r30 Z 3:77 ohms 0 x1 Z7:51 ohms x20 Z0:22 ohms x30 Z9:38 ohms xm Z169:4 ohms x23 Z1:39 ohms 2 J Z 0:02 kg:m Controller parameters: KE;dc Z 4;

KDE;dc Z 256;

KE;speed Z 0:125;

KDE;speed Z 0:5;

The moment of inertia of the wind turbine, JtZ151 kg m2 The blade length of the wind turbine, RZ2.75 m Speed-up gear ratio between the turbine and the generator, ngearZ10

6. Simulation and experimental results 6.1. Simulation results First, the three-phase voltage is applied to the input of PWM rectifier and the dc link capacitance is charged to peak value of line to line voltage through the three-phase diode bridge rectifier built in 6MBP75RA120 FUJI IGBT intelligent power modules. Second, the switching signals of the rectifier are applied and the dc link voltage is boosted to its reference value of 500 V by the controller. Since the starting torque of the wind turbine is low, the induction machine is started from standstill to reference speed as a motor. The rotor speed is set to 940 rpm while the tail vane is closed and the turbine is directed out of wind. The simulation results of the electromagnetic torque and rotor speed are given in Fig. 9 for free acceleration of the induction motor from standstill to 940 rpm. In this case, the line-side converter operates as rectifier and draws real power from the utility grid to compensate the no-load losses. Since the total moment of inertia of the rotating mass referred to the generator shaft is approximately JtZ151 kg m2, the starting transients take almost 28 s to settle down. After reaching at steady state operating point at 28 s, the tail vane of turbine is released free and the turbine is directed towards wind. The simulations are carried out at the wind speed of 8 m/s. It is clearly

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Fig. 8. Flowchart of the DSP control software.

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Fig. 9. Free acceleration of induction machine to 940 rpm (top: electromagnetic torque, bottom: rotor speed).

shown in Fig. 9 that the electromagnetic torque is positive until the time when the tail vane is released, then, it is negative which corresponds to generator mode of operation for the induction machine. The steady-state waveforms of the ac line voltage and current are shown in Fig. 10. The rectifier input current is almost sinusoidal and at unity power factor. It is clear that the phase difference between the supply current and voltage is 1808, therefore the line side converter (rectifier) supplies real power to the utility grid. In Fig. 11, the induction machine phase voltage and current waveforms are given for this operating condition. The PWM inverter is operated at 2 kHz, hence, the current is almost sinusoidal. The wind turbine operates at a tip-speed ratio of 3.4 for this operating condition and the corresponding power coefficient is approximately 0.35. The turbine develops a mechanical power of 2.5 kW at this operating point. The net electrical power delivered to the grid is approximately 1.8 kW. 6.2. Experimental results In experimental work, since the starting torque of the wind turbine is low, first of all the induction machine has been run as a motor fed by the drive circuit. The reference rotor speed of the induction machine is set to a fixed value, 940 rpm. The PI speed controller keeps the actual rotational speed almost stable at the reference value. For this operating condition, the wind speed and the dc link current are recorded on-line over a long time range by using TQ-Data Management System connected to PC computer.

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Fig. 10. Supply voltage and current at the input of rectifier.

Fig. 11. Induction machine phase voltage and current.

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Fig. 12. Recorded dc link current at a rotor speed of 940 rpm.

Fig. 12 shows the recorded dc link current. Since the dc link voltage is constant and smoothed at the level of 500 V, the dc link current is proportional to the electrical output power supplied to the utility grid. The sign of the dc link current specifies the direction of power flow. The negative value of the dc link current indicates that the real power flows from generator to the grid and positive value of it shows that the real power flows from grid to generator. Especially, between 97 and 129 s, wind condition is quite stable and approximately 1 kW power is delivered to the grid. The recorded wind speed during this measurement is shown in Fig. 13. The oscillations in the dc link current, and as a consequence the instantaneous electrical power, can be observed in long time recordings due to rapidly changing wind direction and speed. The responses of the converters to these rapid changes are good enough to keep the system stable. The line-side converter (rectifier) quickly responses to the load changes and regulates the dc link voltage at its given reference

Fig. 13. Recorded wind speed at a rotor speed of 940 rpm (3.2 m/s/div).

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Fig. 14. Grid voltage and current on the line side when the rotor speed is fixed at 940 rpm (upper trace: grid voltage—125 V/div, lower trace: grid current—4 A/div).

value. The speed controller also shows a good performance and recovers the deviation of rotational speed from the reference during the disturbing effects of the wind gusts. The recorded waveforms of voltage and current on the utility grid are shown in Fig. 14, when the rotor speed is fixed at 940 rpm. These waveforms are recorded by using digital oscilloscope and a current probe with a frequency bandwidth from dc to 150 kHz, when the power delivered to the grid is measured to be approximately 1 kW. When the current waveform in Fig. 14 is compared to that in Fig. 10, there is a significant difference. The reason is that the wind speed and its direction are changing very fast and affecting the actual current waveform but the simulation result in Fig. 10 is obtained under the constant wind speed of 8 m/s. It is clearly seen from Fig. 14 that the peak of current is almost 4 A during last period of waveform. The change of current peak value is created by varying wind speed and its direction.

7. Conclusions The WECS having induction generator and drive circuit has been modelled in Matlab. The PI controller parameters are selected by using the simulation results. The system has been implemented by using scalar control techniques, which do not require system parameters, such as stator and/or rotor parameters of the induction machine. The fact is that the field oriented control (FOC) of the induction machine is better than scalar controllers for large power rated WECS having yaw control. But the small rated WECS may not have yaw control as it is here, therefore, the stator terminals of the generator are connected to the drive circuit through slip rings and carbon brushes. This connection

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causes the precise estimation of stator and rotor machine parameters not to be possible, and application of the FOC may fail. The Matlab model of the whole system prepared in detail has predicted the steady state and transient behaviour of the WECS during uniform flow of wind. But a rapid variation of the wind direction with respect to the position of turbine should be considered in the model. This consideration will provide a better power flow control in terms of extracting the maximum power available from the wind.

Acknowledgements This work was carried out as a part of research project, ‘wind energy conversion system with induction generator using PWM converters’, sponsored by Turkish Scientific and Research Council under contract 101-E004.

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