New Conceptual High Efficiency Sinewave PV Power Conditioner with Partially-Tracked Dual Mode Step-up DC-DC Converter

Transcription

1 IEEE PEDS 2015, Sydney, Australia 9 12 June 2015 New Conceptual High Efficiency Sinewave PV Power Conditioner with Partially-Tracked Dual Mode Step-up DC-DC Converter Koki Ogura Kawasaki Heavy Industries, Ltd., Japan Srawouth Chandhaket Walailak University, Thailand Saad Mekhilef and Mutsuo Nakaoka University of Malaya, Malaysia Abstract This paper presents a novel circuit topology of a high efficiency single-phase sinewave PV power conditioner. This power conditioner is composed of time-sharing sinewave absolute pulse width modulated boost chopper with a bypass diode in the first power processing stage and time-sharing sinewave pulse width modulated full-bridge inverter in the second power processing stage operated by time-sharing dual mode pulse pattern control scheme. Its unique operating principle of two power processing stage with time-sharing dual mode sinewave modulation scheme is described with a design example. This paper also proposes a sinewave tracking voltage controlled soft switching PWM boost chopper with a passive auxiliary edge-resonant snubber. The new conceptual operating principle of this novel single-phase sinewave power conditioner related to solar photovoltaic generation system is presented and discussed through the experimental results. Index Terms Time-sharing dual mode sinewave modulation, Boost chopper with auxiliary bypass diode, Full-bridge inverter, PWM, Soft switching, Passive auxiliary edge-resonant snubber, High efficiency power conversion, Solar photovoltaic generation system I. INTRODUCTION The small scale distributed solar photovoltaic generation (PVG) system for residential power applications is becoming more and more popular from the earth environmental point of view. In recent years, the further practical and cost effective requirements as much higher efficiency, smaller on physical size and lighter in weight for small scale stand-alone or utility interactive sinewave power conditioner, and some new conceptual power conditioners based on power electronic circuit controller is developed[1]-[7]. In general, the nonisolated single-phase sinewave PV power conditioner topologies have practical advantages such as lower cost, smaller size, higher power density and higher efficiency power generation systems. Of these, non-isolated sinewave PV power conditioner composed of two cascaded power processing stages; a high frequency PWM boost chopper type DC-DC converter required for boosting a low DC voltage from PV modules in addition to the sinewave modulated single-phase inverter connected to commercial utility AC power grid or stand-alone power utilizations. However, the conventional single-phase sinewave PV power conditioner has some disadvantages such as the poor power conversion efficiency especially in the low output power setting ranges due to switching and conduction losses in boost chopper and single-phase sinewave inverter. Furthermore, the bulky and temperature dependent unreliable electrolytic DC smoothing capacitor bank, which includes a little short life time is actually required constant voltage regulation based on the PWM controlled boost chopper. Moreover, this electrolytic capacitor bank in DC link have sufficiently high capacitance, relatively large volumetric size, heavy weight and high frequency ripple current related power loss due to its equivalent series resistance(esr). Therefore, it is more difficult to implement the cost-effective, compact solar PVG system acceptable for the miniaturization in size and lighter in weight. In this paper, a novel prototype of single-phase sinewave PV power conditioner with time-sharing sinewave absolute modulated boost chopper is presented, which can especially achieve high efficiency power conversion processing for wide range power setting requirements[8]. In addition, its DC link capacitor between the first and the second power conversion processing stages can be extremely reduced. Finally, this paper also proposes a soft switching boost chopper with an auxiliary edge-resonant passive snubber. The operating principle of this novel single-phase PV power conditioner with time-sharing sinewave pulse width modulation is evaluated and verified experimentally with a practical design model in terms of its switching voltage and current waveforms, v-i switching trajectory and actual power conversion efficiency as compared with the conventional type from an experimental point of view. II. CONVENTIONAL CIRCUIT CONFIGURATION OF BOOST CHOPPER CASCADED SINGLE-PHASE SINEWAVE PV POWER CONDITIONER The basic system configuration of the single-phase sinewave PV power conditioner is schematically shown in Fig. 1(a). This power circuit consists of the boost chopper type DC-DC converter in addition to the full-bridge singlephase sinewave inverter with low-pass filter and load in parallel. Its operating principle is also depicted in Fig. 1(b). The boost chopper in the first power processing stage is used for boosting the low DC voltage from PV module array up to a constant output voltage (DC V). The active power switch SW C in this boost chopper always operate at high frequency switching modulation to keep a constant /15/$ IEEE 847

2 output voltage in accordance with the fluctuation voltage from PVG source. In general, the boost chopper stage causes switching losses and conduction losses because of the high frequency PWM switching. And the output side of this boost chopper needs the bulky large-volumetric electrolytic DC capacitor. It is actually impossible to implement smaller and lighter in weight as a power conditioner. In addition to these, the bulky electrolytic DC capacitor provides lower reliability such as power loss of ESR based on ripple current, degradation and a little short lifetime. The full-bridge inverter in the second power processing stage is to make utility AC 200V rms under sinewave carrierbased high frequency PWM. In actual, the active power switches (SW 1 -SW 4 ) in the full-bridge inverter occur switching losses and conduction losses because of high frequency switching sinewave carrier-based PWM. As a result, the total system of this single-phase sinewave PV power conditioner is not so high efficiency because of these power losses. III. PROPOSED CIRCUIT CONFIGURATION OF BOOST CHOPPER CASCADED SINGLE-PHASE SINEWAVE PV POWER CONDITIONER WITH TIME SHARING DUAL SWITCH MODES Fig. 2 shows the proposed time-sharing dual mode controlled PWM single-phase sinewave PV power conditioner composed of time-sharing sinewave pulse width modulated boost chopper with a new bypass diode D b assisted boost chopper and complementary sinewave full-bridge inverter. This power circuit includes partially commutated sinewave PWM boost chopper used for converting intermediate input side DC link voltage to quasi sinewave AC absolute PV Module Array C in L b SW C D C C C Boost Chopper + SW 1 SW 2 SW 3 SW 4 Full-Bridge Inverter (a) Circuit configuration L f C f Low-pass Filter v o Load Fig. 2. Circuit configuration of proposed single-phase sinewave PV power conditioner. absolute value in the first power processing stage, and partially controlled sinewave PWM full-bridge inverter with low-pass filter in the second power processing stage with time-sharing dual mode control scheme. The unique operating principle of this single-phase sinewave PV power conditioner with a bypass diode assisted sinewave absolute PWM boost chopper is basically shown in Fig. 3. Observing the operating principle, when the sinewave instantaneous tracking boost chopper operates under a condition of time-sharing sinewave absolute modulation, the single-phase full-bridge inverter is designed for nonoperation. When the full-bridge inverter operates under the condition of sinewave PWM, the boost chopper is also designed for non-operation. As the proposed sinewave PV power conditioner is not necessary to operate both power processing stages like conventional power conditioner in Fig. 1, which consists of the boost chopper type DC-DC converter in addition to the full-bridge sinewave inverter. As the total number of switching operation times in the power circuit can be reduced, the switching losses and conduction losses of two power processing stages can be substantially reduced. Moreover, this time-sharing controlled sinewave PWM boost chopper with a bypass diode is not necessary to keep a constant output voltage, so the large-volumetric electrolytic DC capacitor bank between the first and the second power processing stages is unnecessary in practice. The small film capacitor for high frequency can be employed in place of the electrolytic DC capacitor. The capacitance of reduced scale film capacitor is about 1/1000 as compared with the conventional electrolytic capacitor for DC smoothing. (b) Operating principle Fig. 1. Conventional single-phase sinewave PV power conditioner. Fig. 3. Operating principle of proposed single-phase sinewave power conditioner. 848

3 The film capacitor as a non-smoothing DC link can realize small and thin size, low power losses, high reliability and a long lifetime. D = 1 1 vout / V in (2) IV. UNIQUE CONTROL STRATEGY OF PROPOSED PV POWER CONDITIONER A. Circuit Operation The control strategies for the proposed time-sharing dual mode single-phase sinewave PV power conditioner with time-sharing sinewave absolute modulated boost chopper in Fig. 3 are defined and summarized as follows. Period I chp : Partial sinewave absolute value PWM mode Period II chp : Partial auxiliary bypass diode conduction mode Period I inv : Partial sinewave polarity changing mode Period II inv : Partial sinewave zero crossing area PWM mode 1) Operation mode of boost chopper stage Input DC voltage < Desired sinusoidal output voltage When the input DC voltage is less than the absolute of the required sinewave output voltage, the switch SW C in the boost chopper operates at high frequency switching mode for boosting and producing quasi sinusoidal pulse width modulated waveform. On the other hand, the switches SW 1 - SW 4 in the full-bridge inverter are commercial frequencybased synchronous polarity switching. For example, when the positive sinewave of output voltage is required, the switches SW 1 and SW 4 are on-state. And when the negative sinewave of output voltage is required, the switches SW 2 and SW 3 are on-state. 2) Operation mode of full-bridge inverter stage Input DC voltage Desired sinusoidal output voltage When the input DC voltage is greater than or equal the absolute of the required sinewave output voltage, the switch SW C in the boost chopper is always off state in this mode, and the switches SW 1 -SW 4 in the full-bridge inverter operate at high frequency switching sinewave carrier-based PWM switching mode. In this case, the input current from the DC supply does not flow through the boost inductor L b and free wheeling diode D C, but it flows through the bypass diode D b. Therefore, the conduction losses of boost inductor L b and diode D C do not occur in this mode. B. Time-Sharing Sinewave Modulation of Boost Chopper A steady-state voltage conversion characteristic of the boost chopper can be represented by By using (2), the duty ratio D of switch SW C can be specified from the input voltage and absolute value of desired sinusoidal output voltage. Fig. 4 illustrates the steady state boosted voltage ratio ( / ) versus duty ratio characteristics. This operating characteristic is used for experimental setup. Fig. 5 shows the control strategy of time-sharing dual mode sinewave modulation. When <, the boost chopper operates for boosting and producing quasi sinusoidal pulse modulated waveform with duty ratio characteristics of Fig. 4. The full-bridge inverter operates by comparing a triangular carrier signal with a reference signal waveforms, where the modulation index was designed with a value more than 1. The gate voltage pulse sequences of proposed single-phase sinewave PV power conditioner are depicted in Fig. 6. When the full-bridge inverter operates, the boost chopper does not operate. And when the boost chopper operates, the full-bridge inverter operates under the commercial frequency-based synchronous polarity switching. Therefore, the switching cycles of this proposed PV power conditioner can be substantially decreased. C. Unique Features of Proposed Single-phase Sinewave PV Power Conditioner Unique features and excellent advantages of the proposed time-sharing single-phase sinewave PV power conditioner can be summarized in the following points. Firstly, when the full-bridge inverter-side power switches operate, the boost chopper-side power switch does not operate. On the other hand, when the boost chopper-side power switch operates, the full-bridge inverter-side power switches operate only under the condition of commercial frequency-based synchronous polarity switching. However, in the conventional single-phase sinewave PV power conditioner v out Vin = 1 D (1) where D: the duty ratio of boost chopper in switch SW C, : the input voltage, : the output absolute voltage (absolute value of desired sinusoidal output voltage) obtained from the boost chopper. Rearranging (1), the duty cycle of the boost chopper can be obtained by Fig. 4. Boosted voltage ratio vs. duty ratio characteristics of boost chopper. 849

4 Comparator Multiplier SW C Boost Chopper Calculator / OSC D =1 1 v out/ (, D 0) Comparator v ca r SW 1 SW 2 SW 3 SW 4 Full Bridge Inverter Fig. 5. Control strategy of time-sharing dual mode single-phase sinewave PV power conditioner. Reference waveform SW C SW 1 SW 2 SW 3 - Secondly, the time-sharing sinewave modulated full-bridge inverter operates under the condition of zero or low current value. Therefore, the switching losses and conduction losses of full-bridge inverter stage are kept to be low compared to the conventional type. Thirdly, the smoothing DC link capacitor stage between the boost chopper and the full-bridge inverter is not required for using a large-volumetric electrolytic capacitor. The total of PVG system can achieve downsizing and light in weight in addition to long lifetime and higher reliability operation due to the possibility of the film capacitor usage as the DC link capacitor instead of the unreliable and bulky type electrolytic DC smoothing capacitor bank. Fourth, at the operation mode of the full-bridge inverter, the input current does not flow through the boost inductor L b and diode D C, but it flows through the bypass diode D b. Therefore, the conduction losses of boost chopper can be suppressed. SW 4 V. SOFT SWITCHING BOOST CHOPPER Fig. 6. Gate pulse sequences of proposed single-phase sinewave PV power conditioner. conditioner shown in Fig. 1, it is noted that the boost chopper as well as the full-bridge inverter always operate at high frequency sinewave PWM switching conditions. The switching cycles of the proposed type are substantially decreased as compared to the conventional one. As a result, the newly-proposed single-phase sinewave PV power conditioner can suppress the switching losses and conduction losses at time-sharing partially controlled sinewave PWM switching scheme. A. Circuit Configuration Fig. 7 shows the proposed time-sharing dual mode controlled PWM single-phase sinewave PV power conditioner composed of time-sharing partially controlled sinewave PWM soft switching boost chopper and complementary sinewave PWM full-bridge inverter. This soft switching boost chopper is based on the boost chopper in Fig. 2, which also includes an passive auxiliary resonant snubber circuit composed of a resonant inductor L r, a resonant capacitor C r, a lossless snubber capacitor C S, and auxiliary diodes D 1 -D 3. This soft switching boost chopper operates ZVS under the turn-off transition. 850

5 Intermediate DC Link Film Capacitor PV Module Array D Bypass C C r D 1 SW 1 Diode D b L b D 3 D 2 L r C C SW 3 L f C f v o Load L b D C C r D 3 D 1 D 2 L r V o C in SW SW 2 SW 4 C C s SW C C s Soft Switching Boost Chopper with Bypass Diode Full-Bridge Inverter Low-pass Filter Fig. 7. Proposed time-sharing dual mode single-phase sinewave PV power conditioner with soft switching boost chopper. Mode 0 Mode 3 B. Circuit Operation The mode transitions of this soft switching boost chopper are depicted in Fig. 8. The voltage gate pulse sequence of the active power switch SW C in addition to the operating voltage, current waveforms of each component are indicated in Fig. 9. The operating principle in mode transitions of this boost chopper is explained as follows; Mode 0 The stored energy of the boost inductor L b and input voltage is transferred to the load side. When the power switch SW C is turned on, Mode 0 shifts to Mode 1. Mode 1 When the power switch SW C is turned on at t 0, the current begins to flow in the passive auxiliary resonant snubber circuit. The resonant current flows through the resonant inductor L r, the resonant capacitor C r and the lossless snubber capacitor C S. In this mode, the resonant capacitor C r is charging, the lossless snubber capacitor C S is discharging, respectively. The switch SW C is turned on without soft switching commutation because the voltage and current of switch SW C have rapid dv/dt and di/dt. Mode 2 When the voltage across the snubber capacitor C S becomes zero and the resonant capacitor C r is equal to the output voltage, the auxiliary diode D 2 is automatically turned off. So the resonant current flowing through the inductor L r and the capacitor C r, C S becomes zero at t 1. All the circuit operations are identical to the conduction state of the conventional boost chopper with stored mode of boost inductor L b. Mode 3 When the power switch SW C is turned off at t 2 with ZVS, the current flowing through the boost inductor L b flows through the snubber capacitor C S and the resonant capacitor C r. Therefore, the lossless snubber capacitor C S starts charging, and the voltages across it increases gradually, and the resonant capacitor C r starts discharging at the same time. When the voltage across the lossless snubber capacitor C S is equal to the output voltage V o at t 3 and the voltage across the auxiliary resonant capacitor C r becomes zero, the diode D 1 and D 3 are turned off naturally. At the same time, the diode D C is turned on and Mode 3 shifts to Mode 0. v g(swc) v SWc i SWc i Dc v Dc v Cs i Cs v Cr i Cr v Lr i Lr Mode Mode 1 Mode 2 Fig. 8. Mode transitions of soft switching boost chopper. i Dc i Cs v Lr t 0 t 1 t 2 t 3 (a) Gate voltage pulse sequence i Cr ilr i SWc v Dc v Cr ZVS Turn-off (b) Operating voltage and current waveforms v SWc Fig. 9. Operating modes and operating waveforms of one switching period. v Cs 851

6 VI. EXPERIMENTAL RESULTS AND EVALUATIONS A. Design Specifications The experimental design specifications and circuit parameter constants of proposed single-phase sinewave PV power conditioner with time-sharing sinewave absolute modulated soft switching boost chopper are listed in Table 1. In addition, Fig. 10 indicates the appearance of experimental power circuit for proposed power conditioner. B. Switching Operating Waveforms Fig. 11 and Fig. 12 illustrate the voltage and current operating waveforms and v-i trajectory in case of turn-off commutation of the power switch SW C in the boost chopper under hard switching and soft switching commutation conditions, respectively. From the voltage and current switching waveforms at turn-off commutation under hard switching switching commutation as depicted in Fig. 11(a), there is an overlapped region of voltage and current switching waveforms. Moreover, observing the v-i trajectory in Fig. 12(a), it spreads out in the first quadrant in the v-i plane, which increases the switching power losses. On the other hand, observing the switching voltage and current operating waveforms shown in Fig. 11(b) under soft switching commutation conditions, except the overlapping period of the switching voltage and falling current during the turn-off period and tail current during the tail period of IGBT, there is no overlapping region. Also, the relevant v-i trajectory illustrated in Fig. 12(b) is nearly moving along the voltage axis and current axis of v-i plane. Therefore, under a soft-switching, the switching power losses of the power switch SW C can be essentially reduced compared with the hard switching conditions. TABLE I DESIGN SPECIFICATIONS AND CIRCUIT CONSTANTS Item Symbol Value Conventional Proposed Current 5A/div DC Input 160V 160V AC Output 200V rms 200V rms Switching Frequency Boost Inductor f S L b 20kHz 1mH 20kHz 1mH 200ns/div Intermediate Capacitor Snubber Capacitor C C C S 3,900 F 2.2 F 18nF (a) Hard switching Resonant Inductor Resonant Capacitor L r C r 7 H 18nF Current 5A/div Filter Capacitor C f 10 F 10 F Filter Inductor L f 1mH 1mH 500ns/div (b) Soft switching Fig. 11. Switching waveforms at turn-off transient of boost chopper switch SW C. Current 5A/div Current 5A/div Fig. 10. Appearance of experimental power circuit for proposed singlephase sinewave PV power conditioner. (a) Hard switching (b) Soft switching Fig. 12. v-i trajectory at turn-off transient of SW C. 852

7 C. Operating Waveforms and Actual Efficiency Fig. 13 shows the time-sharing sinewave tracking current waveform through the boost inductor L b. Fig. 14 depicts the voltage waveform across the intermediate DC link capacitor C C. As shown in Figs. 13 and 14, the boost chopper operates when the input DC voltage is less than the required sinewave output voltage. Fig. 15 illustrates the time-sharing dual mode sinewave modulated voltage waveform of proposed single-phase sinewave PV power conditioner in the input side of the low pass filter stage. Observing this waveform, when the desired sinusoidal AC output voltage is greater than the input DC voltage, the boost chopper operates under a condition of partially controlled sinewave PWM scheme and the fullbridge inverter operates only under a condition of commercial frequency-based synchronous polarity switching. On the other hand, if the desired sinewave output voltage is less than the input DC voltage, the boost chopper does not operate and the full-bridge inverter operates with partially controlled sinewave PWM strategy. The output current and voltage waveforms of proposed PV power conditioner as a resistance load (Output power: 1kW) are shown in Fig. 16. The AC output voltage and current waveforms can produce high quality sinewave. Fig. 17 shows the harmonic orders vs. harmonic contents characteristics of this proposed single-phase sinewave PV power conditioner. The actual maximum harmonic contents is less than 1.5%, and the total harmonic distortion (THD) is 2.82%. They are both meeting the specified value of electric power guidelines in Japan. Fig. 18 depicts the comparative actual power conversion efficiency of conventional and proposed types, respectively. Observing these results, the actual efficiency of the proposed type is higher than the conventional one for all the required output power ranges. Especially, for the low output power setting condition, proposed single-phase sinewave power conditioner operating at time-sharing dual mode sinewave PWM control scheme can realize higher efficiency characteristics as well as high power density. Moreover, the actual efficiency of the soft switching is higher than that of hard switching for all output power range. Current 5A/div 2ms/div 2ms/div Fig. 13. Current waveform through boost inductor L b. Fig. 15. waveform across full-bridge inverter. =160V Current 5A/div 2ms/div 2ms/div Fig. 14. waveform across intermediate capacitor C C. Fig. 16. Output current and voltage waveforms. 853

8 and verified experimentally. This paper also introduced the soft switching Harmonic contents [%] Actual efficiency [%] Fig. 17. Harmonics orders vs. harmonics contents characteristics. Fig. 19. Output power vs. actual efficiency characteristics of proposed power conditioner. and verified experimentally. This paper also introduced the soft switching boost chopper with a passive configuration auxiliary edge-resonant snubber to achieve further high efficiency power conversion. The effectiveness of the proposed single-phase sinewave PV power conditioner has been proven experimentally and compared with conventional one. Finally, the soft switching with trench-gate IGBT achieved the highest power conversion efficiency because of very low on-state voltage and on-state power loss compared with the planar-gate IGBT. Fig. 18. Output power vs. actual efficiency characteristics. The measurement results of actual power conversion efficiency for the proposed power conditioner under hard / soft switching condition using the planar-gate IGBT and the trench-gate IGBT are shown in Fig.19. The trench-gate IGBT has some excellent features with very low on-state voltage and on-state power loss compared with the planar-gate IGBT. Observing these results in Fig.19, the efficiency of the proposed power conditioner under hard / soft switching with trench-gate IGBT is higher than the one with planar-gate IGBT for all the output power ranges. Moreover, the soft switching with trench-gate IGBT achieves the highest power conversion efficiency for all output power range. VII. CONCLUSIONS In this paper, a novel prototype of time-sharing dual mode single-phase sinewave PV power conditioner controlled by time-sharing sinewave absolute pulse width modulated boost chopper with a bypass diode has been proposed for a small scale PVG system. The unique operating principle of the proposed power conditioner has been described, discussed REFERENCES [1] Y. Nishida, S. Nakamura, N. Aikawa, S. Sumiyoshi, H. Yamashita, H. Omori, A novel type of utility-interactive inverter for photovoltaic system, in Proc. Annual Conf. of IEEE Industrial Electronics Society (IECON), pp , USA, Nov [2] S. Saha, N. Matsui, V.P. Sundarsingh, Design of a low power utility interactive photovoltaic inverter, in Proc. International Conf. on Power Electronic Drives and Energy Systems for Industrial Growth, vol. 1, pp , Australia, Nov [3] S. V. Araújo, P. Zacharias, R. Mallwitz, Highly efficient single-phase transformerless inverters for grid-connected photovoltaic systems, IEEE Trans. Industrial Electron., vol. 57, no. 9, pp , Sept [4] H. Terai, S. Sumiyoshi, T. Kitaizumi, H. Omori, K. Ogura, H. Iyomori, S. Chandhaket, M. Nakaoka, Utility-interactive solar photovoltaic power conditioner with soft switching sinewave modulated inverter for residential applications, in Proc. IEEE Power Electronics Specialist Conf. (PESC), Vol. 3, pp , Australia, June [5] T. LaBella, W. Yu, J.-S. Lai, M. Senesky, D. Anderson, A bidirectional-switch-based wide-input range high-efficiency isolated resonant converter for photovoltaic applications, IEEE Trans. Power Electron., vol. 29, no. 7, pp , July [6] H. Xiao, S. Xie, Y. Chen, R. Huang, An optimized transformerless photovoltaic grid-connected inverter, IEEE Trans. Industrial Electron., vol. 58, no. 5, pp , May [7] R. Gonzalez, J. Lopez, P. Sanchis, L. Marroyo, Transfomerless inverter for single-phase photovoltaic systems, IEEE Trans. Power Electron., vol. 22, no. 2, pp , March [8] K. Ogura, T. Nishida, E. Hiraki, M. Nakaoka, S. Nagai, Time-sharing boost chopper cascaded dual mode single-phase sinewave inverter for solar photovoltaic power generation system, in Proc. IEEE Power Electronics Specialists Conf. (PESC), pp , Germany, June