Research on a novel buck boost converter for wind turbine systems

ICSET 2008 Abstract—For converter-based wind turbine systems in grid-connected applications as distributed generators (DG), variable sources often cause wide changes in the converter input voltage above and below the output ac voltage, thus demanding a buck-boost operation of converters. Many traditional full-bridge buck converters, two-stage converters and single-stage buck-boost converters either have complex structure or have limited range of input dc voltage. The authors have proposed and developed an innovative single-phase, single-stage, flyback-based, buck-boost converter for renewable energy systems, especially for wind turbine systems in grid-connected applications. This paper focuses on the analysis of the working principles, computer simulation of the operation, and design consideration of the converter for grid-connected applications. I. I NTRODUCTION For inverter-based wind turbine systems in grid-connected applications as distributed generators (DG), resources often cause wide variations in the input voltage to inverters above and below the output ac voltage. This is particularly true for PV and wind systems. This then demands the buck-boost (i.e., step-down and step-up) operation of inverters. Traditional full-bridge buck inverters do not have the flexibility of handling a wide range of input dc voltage, and require heavy line-frequency step-up transformers [1]. Although this topology currently has the largest market share of the commercial DG system market due mainly to its simplicity and electrical isolation, it is gradually replaced by advanced topologies using “more silicon and less iron”. This leads to the pursuance of compact designs with wide input voltage ranges and improved efficiency. Two-stage inverters normally accomplish dc voltage boost in the first stage, and achieve buck dc-ac conversion in the second stage, with a typical high-frequency transformer to accomplish the voltage boost [1]. Although they can accommodate a wide range of input voltage, the complicated structure makes them costly, particularly for small wind turbine systems. A single-stage inverter is an inverter with only one stage of conversion for both stepping-up and stepping-down the dc voltage from wind turbine sources and modulating the sinusoidal output current or voltage. Single-stage buck-boost inverters, have a simple circuit topology and low component count, leading to low cost and high efficiency. Previously Bing Hu is with University of New Brunswick, NB, Canada (e-mail: hubing_1977@ hotmail.com) Liuchen Chang is with University of New Brunswick, NB, Canada (e-mail: lchang@ unb.ca). Yaosuo Xue is with University of New Brunswick, NB, Canada (e-mail: y.x@ unb.ca). available single-stage buck-boost inverters either need more than 4 power switching devices or have a limited range of input dc voltage. Most of them have two symmetrical dc-dc converters operating in the opposite phase angle in order to generate a sinusoidal current waveform feeding to a single-phase grid. For small grid-connected wind turbine systems, inverters should be small, inexpensive and reliable. Further efforts have been directed to innovative inverters and controls. II. A NEW BUCK - BOOST INVERTER WITH 4 SWITCHING DEVICES The Authors have firstly proposed and developed an innovative single-phase, single-stage, flyback-based, buck-boost inverter for renewable energy conversion systems, especially for wind turbine systems in both grid-connected and standalone applications. As shown in Figure 1, this buck-boost inverter has 4 switching devices [2]. The operating principles of this buck-boost inverter can be described by three operation modes. Mode 1: Charge mode In this mode, switches T1 and T4 are turned on and switches T2 and T3 are turned off. From an energy point of view, during Mode 1, inductor L1 is charged to store energy and the output current is provided by the discharge of capacitor C. Mode 2: Positive half cycle (PHC) discharge mode Research on a Novel Buck-Boost Converter for Wind Turbine Systems Bing Hu, Liuchen Chang, Yaosuo Xue Fig. 1. A new buck-boost single-stage inverter with 4 switchin g devices. 228 978-1-4244-1888-6/08/$25.00 c  2008 IEEE In this mode, switch T4 is turned off and T3 is turned on, while T1 is keeping on and T2 off.In this mode, the dc source is disconnected temporarily from the output Two current conduction modes can be defined here. If the time of Mode 2 is so short that the inductor current is not decreasing to zero when the next charge cycle Mode 1 starts, the current of energy-storage inductor is continuous, and we define this operation the continuous conduction mode (CCM). On the contrary, if the inductor current drops zero in Mode 2 and probably sustains zero for certain time, the operation is defined the discontinuous conduction mode (DCM). So far, in the PHC of ac output, the energy is transferred from dc source (i.e. Wind Turbine) to ac grid through the alternations of Mode 1 and Mode 2. Mode 3: Negative half cycle (NHC) discharge mode This mode is combined with Mode 1 to provide ac NHC output when switch T1 is tuned off and T2 is turned on. Through a flyback operation, the current of primary side of the coupled inductor L drops to zero suddenly and the current of secondary side reaches to the initial current of primary side, if the inductances and turns of both sides of the coupled inductor are identical and there is no magnetic leakage. The only differences between Mode 3 and Mode 2 are that in Mode 3, the grid is in the negative half cycle and the discharging current has an opposite direction. Then similar arguments regarding energy exchange and transfer in Mode 2 can be also applied to Mode 3. As a result, in the NHC of ac output, the energy is transferred from dc source to ac grid through L1, L2 and C by the alternations of Mode 1 and Mode 3. In summary, during each switching interval, the energy-storage inductor is charged from a dc source (i.e. Wind Turbine) and discharged to a grid through a low pass filter. The inductor current can be discontinuous as shown in Figure 2, and continuous as shown in Figure 3. The simulation waveforms for the buck-boost inverter subject to a variable dc voltage sources, controlled by an open loop feedforward compensation [3], are shown in Figure 4. The current total harmonic distortion (THD) of the 120V grid side is 2% for a switching frequency of 9.6 kHz. The simulation waveforms for the buck-boost inverter subject to a variable dc voltage sources, controlled by a closed-loop sinusoidal PWM modulation [3], are shown in Figure 5. The current total harmonic distortion (THD) of the 120V grid side is 3.4% for a switching frequency of 9.6 kHz. It is noted that the grid voltage has been assumed containing significant harmonic contents. Fig. 2. Buck-boost inverter operation in the discontinuous current mode. Fig. 3. Buck-boost inverter operation in the continuous current mode. Fig. 4. Simulated waveform of the buck-boost inverter under the open loop feedforward compensation control. 229 III. A NEW BUCK - BOOST INVERTER WITH 3 SWITCHING DEVICES Based on the buck-boost inverter with 4 switching devices as developed by the Authors for small distributed generators, further improvements have been proposed, which leads to a new buck-boost inverter with 3 switching devices [4]. This inverter is shown in Figure 6. The simple circuit topology of this invention provides the possibility for a low cost and high efficiency dc-ac converter appropriate for small wind turbine applications. The inverter has a low component count with only 3 power semiconductor switches to accomplish dc-ac conversion. The inverters can accommodate a wide range of input dc voltage for an improved energy output from variable wind turbine resources. The input source and the output grid are separated based on flyback operation principles. As compared to traditional buck inverters with line-frequency transformers, two-stage buck-boost inverters, and other single-stage buck-boost inverters, both the component count, cost and size of the newly proposed buck-boost inverter are reduced, thereby presenting a more reliable and economical design for wind turbine systems and other distributed generators. The two coupled inductors L 1 and L 2 have the same inductance L. Since only one switch is turned on in each operation mode and an inductor is always connected in the charge/discharge circuit, the dead time for preventing two switches from shoot-through can be eliminated. The inverter operation can be divided into charge and discharge operation working in the positive half cycle and in negative half cycle, similar to the buck-boost inverter with 4 switching devices, as presented in the previous section. Mode 1: Charge mode During Mode 1, switches 2 Q and 3 Q are turned off, and switch 1 Q is turned on to charge inductor 1 L from the dc source through diode 1 D . Capacitor C provides the continuous current for the grid in Mode 1. The governing equations are the same as in Mode 1 of the 4-switch buck-boost inverter. Mode 2: Positive half cycle (PHC) discharge mode Mode 2 is the discharge mode in positive half cycle. During Mode 2, switches 1 Q and 3 Q are turned off, and switch 2 Q is turned on to discharge the energy, which was stored in inductor 1 L , to the grid through diode 2 D . Figure 7 is the operation waveforms in a positive half cycle. I(Q1) I(Q2) Iout1 Ich+ Idisch+ Ich+ Idisch+ Idisch+ Q2 Q1 D1 D2 ON ON ON ON ON ON ON ON t1t0 I(L1) Mode 3: Negative half cycle (NHC) discharge mode Mode 3 follows Mode 1 in a negative half cycle of the grid voltage. During Mode 3, switches 1 Q and 2 Q are turned off, and switch 3 Q is turned on. The energy which is stored in the coupled inductor 1 L will transfer to the coupled inductor 2 L and then discharges to the load through switch 3 Q and diode 3 D . Figure 8 is the operation waveforms in a negative half cycle. Assume that the resistance of the switches, diodes, and coupled inductors are negligible; two coupled inductors are perfectly coupled; the inverter works in discontinue current mode (DCM); the averaged current of Mode 2 is the average output current of the inverter and can be expressed as, Fig. 5. Simulated waveform of the buck-boost inverter under the closed loop sinusoidal PWM control. Fig. 6. Newly proposed buck-boost inverter with 3 switching devices. Fig. 7. Operation waveforms in a positive half cycle. 230 ³ === − 2 0 grids 2 1 grids 2 1 2 dc 11 22 1 T s VT LI VLT TV dti T i (1) where, grid 1 2 V LI T = , and T S is the switching period. It is assumed that the utility line voltage V grid is expressed as a sinusoidal waveform: )sin(2 grid tVV ω = (2) One of the algorithms for sinusoidal PWM is to control the turn-on time of switch Q 1 in proportion to the utility voltage V grid . )sin( s1 tkTT ω = (3) where k is the coefficient factor. Substituting (2) and (3) into (1), the ac output current i ac is expressed as, )sin( 22 2 s 2 dc ac t L kTV i ω = (4) In practical implementation of an inverter control, a sinusoidal reference wave, serving as the modulating signal, is compared with a triangular wave, serving as the carrier signal. The intersection points determine the switching angles and pulse widths as in Figure 9 [4]. The current ratings of the power semiconductor switches of the 3-switch buck-boost inverter are the same as those of the 4-switch buck-boost inverter presented in the previous section. The voltage stresses of the power semiconductor devices in the charging control circuits are the same for the 4-switch buck-boost inverter (T1 and T4) and for the 3-switch buck-boost inverter (Q1), and are equal to the V dc +v c , where v c is the capacitor voltage and is in the same order as the grid voltage. The voltage stresses of the power semiconductor devices in the discharging circuits of the 3-switch buck-boost inverter and the 4-switch buck-boost inverter are somewhat different. The blocking diodes in the discharging circuits of the two inverters have the same reverse voltage of V dc +v c . However, the switching devices (IGBTs or MOSFETs) in the discharging circuits of the 3-switch inverter have a reverse voltage of 2v c , which is twice the reverse voltage of the 4-switch inverter of v c . For a 120V/60Hz single-phase grid, the peak value of v c is in the level of 200V. In summary, the voltage stress of some switching devices of the 3-switch buck-boost inverter is twice that of the 4-swich inverter, but still in the range readily available in commercial IGBT or MOSFET devices. IV. S IMULATION RESULTS OF THE 3-S WITCH BUCK - BOOST INVERTER In designing the parameters of the inverter, a consideration for overall inverter operation under various input voltage is required. Through simulation studies, the required modulation index and operation region under variable dc input voltages are presented in Figure 10. Fig. 8. Operation waveforms on a negative half cycle. Fig. 10. Control-to-output curves of SPWM control. (L = 150 × 10-6H, V = 120V) 231 Based on SPWM strategy of Figure 9, the inverter gating signals are shown in Figure 11. The operation of the inverter is simulated for different dc source voltages from 50V to 300V, as if it were from a wind turbine source. The inverter is designed for a rated power of 1 kW. The grid voltage is fixed at 120V/60Hz. The switching frequency is set at 5 kHz, considering a compromise between reducing switching losses and ensuring output current quality. Figures 12-15 present the simulated output current waveforms. Table I summarizes the simulation parameters and output current performance. From the figures, it has been seen that the newly proposed flyback single-stage single-phase buck-boost inverter can accomplish both buck and boost operation, feeding power to a grid with a reasonable power quality from a widely variable dc source. Fig. 11. Gating signals of the 3-device buck-boost inverter. TABLE I SUMMARY OF SIMULATION RESULTS Fig. 12. Output current waveform when the dc voltage is 50V. Fig. 13. Output current waveform when the dc voltage is 100V. Fig. 14. Output current waveform when the dc voltage is 200V. Fig.15. Output current waveform when the dc voltage is 300V. The implementation of the 3-switch buck-boost inverter is still yet to be done. The output current waveforms are to be improved, possibly using a close-loop sinusoidal PWM as presented V. C ONCLUSION The Authors have proposed an innovative single-phase, single-stage, flyback-based, buck-boost inverter for renewable energy conversion systems, based on a previously developed 4-switch buck-boost inverter. The simple circuit topology of this inverter provides the possibility for a low cost and high efficiency dc-ac converter. The inverters have a low component count with only 3 power semiconductor switches to accomplish dc-ac conversion with a high output power quality. The inverter can accommodate a wide range of input dc voltage for an improved energy output from variable PV resources. The inverter separates the input source from the output grid through a flyback operation. As compared to traditional buck inverters with line-frequency transformers, two-stage buck-boost inverters, and previous single-stage buck-boost inverters, both the cost and size of the newly proposed inverter are reduced, thereby presenting a more reliable and economical design for wind turbine systems. The DC voltage Power factor Output current Output power THD (%) 50 V 0.30 1.24 A 44.6W 2.05 100 V 0.50 1.52 A 91.2W 2.00 200 V 0.95 5.66 A 645.2W 4.50 300 V 0.98 8.50 A 999.6W 4.90 Fig. 9. Sinusoidal pulse-width modulation. 232 analysis of the working principles, and computer simulation of the operation for this inverter have proved its feasibility for dc-ac conversion in wind turbine applications. A CKNOWLEDGMENT The authors wish to thank Natural Sciences and Engineering Research Council of Canada (NSERC) for the financial support to this research project. R EFERENCES [1] Xue, Y., Chang, L., Baekhj Kjaer, S., Bordonau, J. and Shimizu, T., “Topologies of single-phase inverters for small distributed power generators: an overview,” IEEE Trans. Power Electronics, vol. 19, pp. 1305-1314, Sept. 2004. [2] Liu, Z., Study Of Single-Phase Single-Stage Buck-Boost Inverters, University of New Brunswick M.Sc. Thesis, Aug. 2004. [3] Xue, Y., Study Of Single-Phase Single-Stage Buck-Boost Inverters, University of New Brunswick M.Sc. Thesis, Jan. 2004. [4] Xue, Y., Chang, L., "Closed-Loop SPWM Control for Grid-Connected Buck-Boost Inverters,” IEEE Power Electronics Specialists Conference 2004, Aachen, Germany, Vol. 5, pp.3366-3371, June 2004. 233