International Journal of Science, Engineering and Technology Research (IJSETR)Volume 1, Issue 1, July 2012

Abstract— This paper presents the boost converter for variable speed small wind turbine. Because of the irregular nature of wind, the wind turbine generator produces variable-voltage and variable frequency. Several different methods were used to transform the variable-voltage, variable-frequency outputs to the reliable constant-voltage, constant-frequency outputs. In variable speed operation, it is necessary to use power electronics converter as an interface between the wind turbine and the grid. The power converter consists of an uncontrolled three-phase diode rectifier, a DC-DC boost converter and a three-phase PWM voltage source inverter. The DC-DC boost converter is modeled by using the blocks of MATLAB SIMULINK. The simulation results are compared with the theoretical results.

Index Terms—Wind turbine, variable voltage and frequency, power converter, DC-DC boost converter, MATLAB SIMULINK

I. INTRODUCTION

The wind turbine technology is one of the most emerging renewable technologies. In wind energy conversion systems (WECS), normally there are two operating modes of wind turbine generators (WTG) system: fixed speed and variable speed operating modes [1]. In order to extract maximum power from the fluctuating wind, variable-speed operation of the wind turbine generator is necessary. This requires a sophisticated control strategy for the generator. Optimum power/torque tracking is a popular control strategy, as it helps to achieve optimum wind-energy utilization. Some of these control strategies use wind velocity to obtain the desired shaft speed to vary the generator speed.

A control strategy for the generator-side converter with output maximization of a (permanent magnet synchronous generator) PMSG-based small-scale wind turbine is developed. The generator-side switch-mode rectifier is controlled to achieve maximum power from the wind. The method requires only one active switching device [insulated gate bipolar transistor (IGBT)], which is used to control the generator torque to extract maximum power. It is simple and a low-cost solution for a small-scale wind turbine [2]-[3].

Most modern WTGs are designed for variable speed operation. Variable-speed wind turbines have many

Yi Lay Nge, Department of Electronic Engineering, Mandalay Technological University ,(e-mail: yeelaynge81@gamil.com), Mandalay, Myanmar.

Zaw Min Nainge, Technological University(Maubin), Maubin, Myanmar, (e-mail; drzaw290615@gmail.com).

Kyaw Soe Lwin, Department of Electronic Engineering, Mandalay Technological University , (e-mail: kyawsoelwin007@gtmail.com).

Maung Maung Latt, Technological University(Taungngu),Taungngu, Myanmar, (e-mail: mgmglatt2020@gmail.com

advantages over fixed-speed generation such as increased energy capture, operation at maximum power point, enhanced efficiency and power quality, and reduced mechanical stresses and audible noise at low wind speed. The reliability of the variable-speed wind turbine can be improved significantly by using a direct-drive permanent-magnet synchronous generator (PMSG) [4]. PMSGs are widely used in the wind energy conversion systems, especially in the small or medium power range, which increases the conversion efficiency and reduces the maintenance cost due to brushless design [5].

This paper focuses on step by step validation of the boost converter. Simulation results are obtained using MATLAB SIMULINK. Although the presented model has been tested on the dc-dc converter theory, it can also be intended in grid-connected application or additional loads by suitable modification of the dc-ac inverter. This paper is organized as follows. The wind turbine topologies are described in Section II, the proposed system is presented in Section III, and the design implementation and simulation results are mentioned in Section IV.

II.WIND TURBINE TOPOLOGIES

The most important technical information for a specific wind turbine is the power curve, which shows the relationship between wind speed and generator electrical output. A somewhat idealized power curve is shown in Fig. 1. According to the power curve, there are three types of wind speeds.Cut-in Wind speed (3-5m/s): The cut-in wind speed VC is the minimum needed to generate net power. Since no power is generated at wind speeds below VC, which portion of the wind’s energy is wasted. Fortunately, there isn’t much energy in those low-speed winds anyway, so usually not much is lost.Rated Wind speed (between11m/s and 16m/s): As velocity increases above the cut-in wind speed, the power delivered by the generator tends to rise as the cube of wind speed. When winds reach the rated wind speed VR, the generator is delivering as much power as it is designed for. Above VR, there must be some way to shed some of the wind’s power or else the generator may be damaged.Cut-out or Furling Wind speed (between 17m/s and 30m/s): At some point the wind is so strong that there is real danger to the wind turbine. At this wind speed VF , called the cut-out wind speed or the furling wind speed (“furling” is the term used in sailing to describe the practice of folding up the sails when winds are too strong), the machine must be shut down. Above VF, output power obviously is zero [6].

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Design Implementation of Boost Converter for Variable Speed Small Wind Turbine

(IJSETR)

Yi Lay Nge#1, Zaw Min Naing*2, Kyaw Soe Lwin#3, Maung Maung Latt*4

International Journal of Science, Engineering and Technology Research (IJSETR)Volume 1, Issue 1, July 2012

Fig.1 Idealized power curveThe overall block diagram of a wind energy conversion system is shown in Fig 2. Wind turbine converts wind power into shaft power to drive an electrical generator. The output power from the WT fed to the Permanent Magnet Synchronous Generator (PMSG). The generated power of continuously varying frequency is fed to local load through suitable power converters, to ensure constant voltage and constant frequency. This paper is focused on the shaded areas of the overall system.

Fig.2 Block diagram of wind energy conversion system

A. Wind TurbineWind turbines can either be fixed speed or variable

speed turbines based upon their operating speed characteristics. The most common conventional wind turbine systems uses multistage gears coupled to a fixed speed squirrel cage induction generator directly connected to the grid. Fixed speed turbines are designed for optimum performance at a fixed rotor rpm and hence its energy yield is low. Doubly fed induction generator topology with converter on the rotor side is the most popular variable speed wind turbine application. To match the low speed turbine rotor and high speed generator, gear box with appropriate gear ratio is used. The modern trend in wind power generation is toward gearless permanent magnet synchronous generator incorporated direct drive system [4]-[6]. Many disadvantages can also be avoided in gearless WTG. The noise caused mainly by a high rotational speed can be reduced and also high overall efficiency and reliability reduced weight and diminished need for maintenance [7]. The wind turbine basic principle is to convert the linear motion of the wind into rotational energy. This rotational energy is used to drive an electrical generator. The captured power of the wind (Pv) for a wind turbine is given by equation (1).

= (1)

The mechanical power (Pm) generated by the wind turbine from captured power of the wind depends on the power coefficient (Cp) of the wind turbine.

. (2)

The Cp is a function of tip-speed ratio λ.

(3)

Therefore, the mechanical power and torque production from wind turbine is given by the following equations:

ρ

(4)

(5)Where: ρ = the air density r = the radius of the wind blade u = the velocity of the wind Cp = the power coefficient CT = the torque coefficient ωm = the rotor speed

When wind speed changes, the rotational speed, ωm has to achieve the best value of Cp.

B. Wind Turbine GeneratorBoth induction and synchronous generators can be used

for wind turbine systems. Induction generators can be used in a fixed-speed system or a variable-speed system. The induction type machine has the advantages of robustness, low cost and maintenance-free operation. However, they have the drawbacks of low power factor and need for an AC excitation source. In the regular wind turbine generator it has an induction generator that it can rotate at a speed of 1000 to 1500 rpm for normal operation and good efficiency.

Permanent magnet generator is chosen so as to eliminate the drawbacks of induction generator. Synchronous generators are normally used in power electronic interfaced variable-speed systems. It provides an optimal solution for varying-speed wind turbines of gearless or single-stage gear configuration. Many disadvantages can also be avoided in gearless WTG. The noise caused mainly by a high rotational speed can be reduced and also high overall efficiency and diminished need for maintenance.

Simplest synchronous generator model consists of a speed controlled voltage source in series with impedance. Because of the ease of handling the rotation dependent parameters, direct and quadrature axis machine quantities in power system study are transferred to arbitrary reference frames. Machine dynamics is represented by differential equations using park transformation with rotor axis as the reference frame [6].

(6)

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International Journal of Science, Engineering and Technology Research (IJSETR)Volume 1, Issue 1, July 2012

(7)

+( ) ]

(8)Where,Ld, Lq d and q axis inductancesR stator winding resistanceid, iq d and q axis currentsvq, vd d and q axis voltageωr angular velocity of rotorλ amplitude of rotor induced fluxp pole pair numberTe electromagnetic torque

C. Power ConverterSince the wind power fluctuates with wind velocity, the

generator output voltage and frequency vary continuously. Therefore, a full power capacity AC-DC-AC power converter is employed to convert the variable frequency variable speed generator output to the fixed frequency fixed voltage.

The varying AC voltage is rectified into DC in a diode bridge and the dc voltage is then regulated to obtain constant voltage by controlling the duty ratio of a DC/DC boost converter. The DC voltage is inverted to get the desired AC voltage and frequency employing a PWM inverter. The duty ratio, δ controls the Boost converter output voltage.

III. DESIGN CONSIDERATION

A. Three-phase RectifierThe three-phase diode bridge rectifier converts the AC

generated output voltage, which will be varying in magnitude and also in frequency, into DC voltage.

The average output voltage of the three phase diode rectifier is obtained as follows:Vdc =(3*Vm )/π (9)

Here, Vm = *Vrms

(10)

And, phase-to-phase rms voltage=VL/ (11)

Here, the output voltage of PMSG is assumed about 400V.

So, Vrms=400/ =230.94V

Vm= *Vrms= *230.94=326.55V

The DC output voltage=Vdc=(3*326.55)/π=311.83V

B. DC-DC Boost Converter

DC-DC converter is a device that accepts a DC input voltage and produces a DC output voltage. Typically the output voltage is at a different voltage level than the input. In addition, DC-DC converters are used to provide noise isolation, power bus regulation, etc. As the name implies, the output voltage is greater than the input voltage.

The boost converter output voltage is obtained asVinton+(Vin-Vo)toff=0 (12)

Vinton+Vintoff-Votoff=0

Vin(ton+toff)-Votoff=0VinT=Votoff

=

Here, D=

Where, D=duty ratio of the boost converter

(13)

Since the duty ratio D is between 0 and 1 the output voltage must always be higher than the input voltage in magnitude. The variable output voltage is obtained by regulating the ON time of the duty cycle. Here, it is used 0.5 as the duty cycle. From the above calculation, the output voltage of the rectifier is 311.83V. It is the input of the DC-DC converter. The output voltage is

Vo= Vin= *311.83=623.66V

To meet the required boost converter design, the value of L and C can be calculated based on the minimum overshoot level and required voltage range. According to the boost converter design equation from typical design, the proposed converter can be evaluated with high performance. So, the design specification for the proposed boost converter is shown in Table 1.

TABLE 1DESIGN SPECIFICATIONS FOR DC-DC Boost Converter

Input voltage range, Vin 280V ~ 310VOutput voltage, Vout Above 600VOutput current, Iout 4 AmpInput inductor L 1mHOutput resistor R 200ΩOutput capacitors (c,c1) 2000μF

IV. IMPLEMENTATION AND SIMULATION RESULT

The simulation performance is divided into two main parts including three-phase rectifier and dc-dc boost converter. Here, Fig. 3 shows the simulation model of three-phase rectifier part. It is presented for 230V AC voltage with variable frequencies (50~80Hz).

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International Journal of Science, Engineering and Technology Research (IJSETR)Volume 1, Issue 1, July 2012

Fig.3 SIMULINK model of rectifier circuit

Fig.4 illustrates the simulation result of the three-phase rectifier. According to the figure, it can be seen that the theoretical result and simulation result are nearly the same.

Fig.4 The rectifier output voltage

When the frequency changes between 50 Hz and 80 Hz, the various dc output voltages are also changed as shown in Fig.5.

Fig.5The results of variable frequencies

Fig. 6 is the simulation model of DC-DC boost converter.

Fig.6 SIMULINK model of DC-DC converter

According to the simulation, it can be seen that the voltage across in MOSFET as shown in Fig. 7. As the duty cycle is 0.5, ton and toff are the same duration. For this simulation the suitable duty cycle is 50%.

Fig.7 Voltage across switch

Fig.8 DC output voltage of boost converter

Fig.8 presents the output voltage of DC-DC boost converter. At the beginning, the voltage is increased to about 1000V. During 0 to 0.5s, the voltage is continuously decreased to around 600V. After 0.5s, it is stabled over 600V. Therefore, the simulation result is approximately same with the theoretical result. Fig.9 illustrates the corresponding DC output current.

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International Journal of Science, Engineering and Technology Research (IJSETR)Volume 1, Issue 1, July 2012

Fig.9 DC output current

Fig.10 Input vs. Output voltage

Fig.10 shows the relation curve of input and output voltages of boost converter. According to the curve, the boost converter output over 600V can get from the input voltage over 300V at 50 Hz. It can be used as part of a DC-AC inverter. The desired voltage can also be obtained by changing the duty ratio of the boost converter.

V. CONCLUSION

The DC-DC boost converter was considered by using the MATLAB simulation blocks. The target of the system is to get the DC output voltage above 600V. Here, AC 400V is assumed to the PMSG output voltage. The voltage is fed to the three phase diode bridge rectifier and then the output is connected to the DC-DC converter. The input voltage range is (280~310) V with variable frequencies (50~80) Hz. Therefore, the variable output voltage is achieved within the voltage range of 550~610V. In practical, a power MOSFET with parameters of 1500V VDSS, current ID

of 4A and low resistance RDS(on) should be used as the simulation result of output voltage is increased up to about 1000V. Consequently, the boost converter design in this paper has to be found that the system requirements are outfitted to implement the reliable variable speed small wind turbine.

REFERENCES

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[2] Mihet-Popa, L.; Groza, V.; G.; Filip, I.; Szeidert, I.; ‟Variable Speed Wind Turbines Using Cage Rotor Induction Generators Connected to the Grid” Electrical Power Conference, 2007. EPC 2007. IEEE Canada 25-26 Oct. 2007 Page(s):8-13

[3] T. F. Chan and L. L. Lai, “Permanent-Magnet Machines for Distributed Generation: A Review,” in Proc. IEEE Power Eng. Annu. Meeting, 2007, pp. 1-6

[4] R. Datta and V. T. Ranganathan, “A Method of Tracking the Peak Power Points for a Variable Speed Wind Energy Conversion system”, IEEE Trans. Energy Convers., Vol. 18, no. 1 pp. 163-168, Mar.1999.

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[8] Md. Enamul Haque, Michael Negnevitsky, Kashem M. Muttaqi: “A Novel Control Strategy for a Variable-Speed Wind Turbine with a PMSG” IEEE Transactions on Industry Applications, Vol.46, no.1, January-February 2010

[9] Roslinda Zainal, Noriah Bidin and Yaacob Mat Daud; “High Voltage Boost Converter for Capacitor Charging Power Supply” Jabatan Fizik UTM. Vol 3. (2008) 68-73

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[11] Athimulam Kalirasu, Subharensu Sekar Dash; “Simulation of Closed Loop Controlled Boost Converter for Solar Installation:, Serbian Journal of Electrical Engineering, Vol. 7, No. 1, May 2010, 121-130

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