Performance Comparison of Three-phase Diode Rectifier-Fed Boost Converter and Three-phase PWM Rectifier for Wind Energy Applications Rajat Kumar
Navdeep Singh
Department of Electrical Engineering MMMUT, Gorakhpur India
[email protected]
Department of Electrical Engineering MMMUT, Gorakhpur India
[email protected]
Abstract—A power conditioning device known as power converter is an essential component of a wind turbine generator system (WTGS). It is used for tuning the generator frequency and voltage as required as per the grid. As the power level of wind turbines increase promptly, thereby more powerful generators, power electronics devices and control techniques have to be proposed to make the overall wind power plant more efficient and reliable. This paper represents the modeling and computer simulation of two power electronics converters namely Three-phase Diode Rectifier-Fed Boost converter and Threephase PWM rectifier under variable supply conditions. It also deals with the comparative study amongst both based on performance parameters. Moreover, closed loop controlling for both the converters are also addressed. Index terms—PWM rectifier; diode rectifier fed boost converter; closed loop controlling; wind energy; Total Harmonic Distortion (THD)
I. INTRODUCTION Winds are caused by the differential heating of the earth’s surface by the sun, causing unequal expansion and thereby unequal densities of the air of various regions. This consecutively results in convection currents and movement of large air masses. Renewable, non-polluting, clean, environmental friendly, no greenhouse effect are the positive sides from the wind power. In the present scenario, large wind power-plant is competing with the electric utilities for providing the economical clean power. Due to several improvements in technology, the cost associated with the wind power is nearly equal to the conventional power. So, the wind energy is the most preferable energy amongst all the renewable energy resources [1]-[2]. Fig.1 shows Installed wind power capacity (In India). A general scheme of converter based wind turbine generator system is shown in Fig. 2. Wind turbine basically converts the wind energy into mechanical energy. The turbine rotor is connected to the generator through a gear box that correlates the low speed of turbine to high speed of the generator. Electric generator is one of the necessary component of Wind Energy Conversion System (WECS). In the earlier days, there were fixed speed wind turbine having Squirrel Cage Induction Generator (SCIG) [3].
But for extracting large amount of power, rotor speed must also vary as per the wind speed. It is the involvement of power electronic converters by which varying speed of the machine is possible easily [4]-[5]. Permanent Magnet Synchronous generator (PMSG) and Doubly Fed Induction Generator (DFIG) are the most common electric generators used now a days in WECS [6]. The major problem associated with wind energy sources is its variable nature (in aspects of real power, reactive power, output voltage and frequency). The main function of power converter in WECS is to convert input ac power at given frequency and voltage to output power at different frequency and voltage. Several power converters are currently used for fulfillment of the WECS. To step up the voltage level and to minimize the electrical losses, power frequency power transformer is commonly used. By the help of such briskly establishing technology called power electronics, components can bear large amount of current and voltage ratings, making the reliability upto the great extent [7]. This paper aims to a study of two converters topologies namely three-phase diode rectifier fed boost converter and three-phase pwm rectifier for wind energy conversion system (WECS).
Fig. 1 Installed wind power capacity (In India): Fiscal year end cumulative capacity (in MW)
Here 𝐶1 to 𝐶5 are coefficient of turbine model [9]. III. CONVERTERS DESCRIPTION
Fig. 2 Converter based wind turbine generator system II. WIND TURBINE SYSTEM The wind turbine captures the wind’s kinetic energy in a rotor consisting of two or more blades mechanically coupled to an electrical generator. For enhancing the amount of energy capture, the turbine is mounted on the tall tower. Two different configurations are available for wind turbine design, the horizontal axis configuration and vertical axis configuration. However, most modern wind turbines use horizontal-axis design. Except for the rotor, all other components are the same in both designs, with some difference in their placement. The output mechanical power extracted from the wind turbine can be expressed as [8]:𝑃𝑚 = 𝐶𝑃 (λ,β)
𝜌𝐴𝑣 3
(1)
2
where: ρ: air density A: area swept by the rotor blades v: wind speed Cp:power coefficient (or coefficient of performance) of the wind turbine The mechanical torque on the wind turbine shaft can be expressed as:𝑃𝑚
𝑇𝑚 =
𝜔𝑚
= 𝐶𝑃 (λ,β)
𝜌𝐴𝑣 3
(2)
2𝜔𝑚
A. Three-phase Diode Rectifier fed Boost Converter In the present era, almost all electronic equipments need the dc voltage but all these equipments are basically fed from single phase or three phase ac supply. Usually this ac to dc conversion is done through diode bridge rectifier. All the loads in present time require regulated dc supply to maintain stability and reliability. To accomplishment of this purpose, switching mode regulators are available to convert unregulated to regulated dc supply and this regulation is frequently obtained using Pulse Width Modulation (PWM) technique at a fixed frequency. The switching device is normally BJT, MOSFET, IGBT [10]-[13]. The circuit diagram of three-phase diode rectifier fed boost converter (under variable supply conditions) is shown in Fig. 3.
Fig. 3 Circuit diagram of three-phase diode rectifier fed boost converter It is clear from the circuit diagram that a variable frequency/amplitude/phase ac supply from the wind turbine generator is firstly converted into dc which is normally unregulated and then is converted into regulated dc supply by boost regulator. Fig. 5 shows the MATLAB/SIMULINK model of the corresponding circuit. Here PI controller is used which provides the control loop feedback machanism. The controller attempts to minimize the error over time by adjustment of a control variable u(t). 𝑡
u(t)=𝐾𝑃 e(t) + 𝐾𝐼 ∫0 𝑒(𝑡) 𝑑𝑡
(6)
The power coefficient 𝐶𝑃 is a nonlinear function of the blade pitch angle β and the tip-speed ratio λ. 𝑅𝜔𝑚
λ={
𝑣
}
(3)
with: R: radius of the turbine blades 𝜔𝑚 : angular speed of the turbine rotor Power coefficient equation with different coefficients of turbine can be written as:𝐶
𝐶𝑃 (λ,β) = 𝐶1 [ 𝜆2 − 𝐶3 𝛽 − 𝐶4 ] [𝑒 𝑖
1 𝜆𝑖
1
0.035
= [𝜆 + .08𝛽] - 𝛽3 +1
𝐶 − 5 𝜆𝑖
]
(4)
(5)
Fig. 4 PI controller where 𝐾𝑃 and 𝐾𝐼 are non negative,coefficient of proportional and integral terms respectively. P accounts for present value of the error and I accounts for past value of the error. In this model, 𝐾𝑃 and 𝐾𝐼 values are taken as 𝐾𝑃 = 5.0 and 𝐾𝐼 = 30 [14]-[16]. Due to low cost and easy in manner, here PWM technique has been proposed to generate switching pulses to
the MOSFET, comparing a constant dc voltage (reference) with a saw tooth wave (carrier). Here the switching frequency has been taken as 50KHz. The diode rectifier based converter system transfers the power from generator to grid i.e. it exhibits unidirectional power handling capability and it also produces large amount of harmonics (i/p side) which affects the performance of the utility side. Still it is easy to implement and having low production cost.
rectifier operates on unity power factor [29]-[30]. Here Sinusoidal Pulse Width Modulation (SPWM) technique has been proposed to generate switching pulses to the MOSFET, comparing sinusoidal signal (reference) with a saw tooth wave (carrier).
Fig. 6 Circuit diagram of three-phase PWM rectifier Fig. 5 MATLAB/SIMULINK model of three-phase diode rectifier fed boost converter B. Three-phase PWM Rectifier The PWM rectifiers are widely used in renewable energy systems, variable speed drives, High Voltage Direct Current (HVDC) Transmission system, controlled power supply and many more applications [17]. It provides high quality dc output voltage with small size of filter capacitor installed at dc side. It also provides reversible power flow between both sides [18]-[23]. The basic circuit diagram of three-phase pwm rectifier is shown in Fig. 6. It is clear from the circuit diagram that a variable frequency/amplitude/phase ac supply from the wind turbine generator is converted into dc. The switching device used here is MOSFET. Fig.7 shows the MATLAB/SIMULINK model of three-phase pwm rectifier. The control strategy used here in dq rotating coordinate system is shown in Fig. 8 [24]. Three-phase stationary coordinate system (a, b, c) can be converted into synchronous rotating (d, q) coordinate system by coordinate transformation. In this rotating coordinate system, d-axis represents the active component and q-axis represents the reactive component [25]. The control strategy involves the voltage outer loop and current inner loop. Due to comparison with the reference dc voltage, the error is minimized using PI controller [26]-[28]. As the dc voltage can be controlled by the control of 𝑖𝑑 , the output value of voltage outer loop of PI regulator is the reference of current inner loop by which the active power of PWM rectifier can be adjusted. As the reference value of reactive current is dependent upon the reference value of reactive power. So when 𝑖𝑞∗ =0, PWM
Fig. 7 MATLAB/SIMULINK model of three-phase PWM rectifier
Fig. 8 Control strategy of three-phase PWM rectifier in dq rotating coordinate system [24]
IV. PARAMETERS USED Three-phase diode rectifier fed boost converter 𝑅𝑆 =1Ω 𝐿𝑆 =1mH L(Boost)=150µH C(Boost)=4800µF R(Boost)=30Ω Three-phase PWM rectifier
𝑅𝑆 =0.005Ω 𝐿𝑆 =0.1mH R(Load)=50Ω C(Load)=4700µF
Fig. 12 o/p current B. Three-phase PWM Rectifier
V. SIMULATION RESULTS A. Three-phase Diode Rectifier Fed Boost Converter
Fig. 13 i/p voltage (showing one phase)
Fig. 9 i/p voltage (showing one phase)
Fig. 14 i/p current (showing one phase)
Fig. 10 i/p current (showing one phase)
Fig. 15 o/p voltage
Fig. 11 o/p voltage
Fig. 16 o/p current
VI. CONCLUSION
Fig. 17 FFT analysis of three-phase diode rectifier fed boost converter
Fig. 18 FFT analysis of three-phase PWM Rectifier
Three-phase diode rectifier fed boost converter
THD=6.93%
Three-phase PWM Rectifier
THD=9.21%
Fig. 19 THD Comparison As seen from the table (Fig. 19) and FFT analysis (Fig. 17,18) of both the converters, Total Harmonic Distortion (THD) value is less for Three-phase diode rectifier fed boost converter. So this paper suggests for the use of Three-phase diode rectifier fed boost converter instead of Three-phase PWM rectifier for wind energy applications. REFERENCES [1] Navdeep Singh and Vineeta Agarwal, Senior Member, IEEE, “Deltamodulated ac–ac converter for PM WECS,” IEEE Transaction on Industrial Informatics,vol. 11, no. 6, December 2015. [2] N. K. Swami Naidu, Member, IEEE, and Bhim Singh, Fellow, IEEE, “Doubly fed induction generator for wind energy conversion systems with integrated active filter capabilities,” IEEE Transaction on Industrial Informatics, vol. 11, no. 4, August 2015. [3] A. A. B. Mohd Zin, H. A. Mahmoud Pesaran, A. B. Khairuddin, L. Jahanshaloo, and O. Shariati, “An overview on doubly fed induction generators controls and contributions to wind based electricity generation,” Renew. Sustain. Energy Rev., vol. 27, pp. 692–708, Nov. 2013.
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