A Novel High-Performance Utility-Interactive Photovoltaic Inverter System

Transcription

1 704 IEEE TRANSACTIONS ON POWER ELECTRONICS, OL. 18, NO. 2, MARCH 2003 A Novel High-Performance Utility-Interactive Photovoltaic Inverter System Toshihisa Shimizu, Senior Member, IEEE, Osamu Hashimoto, and Gunji Kimura, Member, IEEE Abstract This paper presents a novel photovoltaic inverter that can not only synchronize a sinusoidal ac output current with a utility line voltage, but also control the power generation of each photovoltaic module in an array. The proposed inverter system is composed of a half-bridge inverter at the utility interface and a novel generation control circuit which compensates for reductions in the output power of the system that are attributable to variations in the generation conditions of respective photovoltaic modules. The generation control circuit allows each photovoltaic module to operate independently at peak capacity, simply by detecting the output power of the system. Furthermore, the generation control circuit attenuates low-frequency ripple voltage, which is caused by the half-bridge inverter, across the photovoltaic modules. Consequently, the output power of system is increased due to the increase in average power generated by the photovoltaic modules. The effectiveness of the proposed inverter system is confirmed experimentally and by means of simulation. Index Terms Maximum power point tracking control, photovoltaic module, photovoltaic power generation, utility interactive inverters. I. INTRODUCTION NATURAL energy sources are attracting a great deal of interest, because of environmental concerns, and photovoltaic generation systems are currently considered to be one of the most useful natural energy sources. P generation is a flexible power generation method which is applicable to both small and large power generation plants; i.e., plants that generate anywhere from less than 3 ka to more than 100 ka. The main drawbacks of the P generation system are high initial installation cost and low energy conversion efficiency. In an effort to overcome these problems, a great deal of research, such as maximum power point control and high conversion inverter topology, has been conducted over past ten years [1] [4]. In recent years, interest in small P power generation systems designed to be installed and used in private residences has grown [5]. However, especially in urban areas, the following serious problem prevents improvement in the effectiveness and economic feasibility of the P generation system. When a typical urban residence is equipped with a P power generation system, the P modules are normally installed on the roof. Thus, in addition to shadows created by clouds, those created by neighboring buildings, trees, utility and/or telephone poles, and power line cables sometimes partially cover these P modules. In this Manuscript received May 6, 2002; revised December 12, Recommended by Associate Editor G. K. Dubey. The authors are with the Department of Electrical Engineering, Tokyo Metropolitan University, Tokyo , Japan ( shimizu@eei.metro-u.ac.jp; hashimoto@pe.eei.metro-u.ac.jp). Digital Object Identifier /TPEL study, we call this condition unbalanced generation. In order to allow the utility-interactive inverter to obtain sufficient dc voltage, the P modules in small systems are usually connected only in series and have no parallel strings, and therefore a significant reduction in the total output power of the P system is often observed under unbalanced generation. Traditionally, a bypass diode is connected in parallel with each P module, in order to alleviate the power reduction of the P modules under unbalanced generation. However, the shaded P modules cannot generate their inherent power, and hence the problem has no clear solution [6]. In order to solve the problem regarding unbalanced generation, this paper proposes a novel utility-interactive P inverter system which incorporates the additional function of controlling the generation points of each P module by application of a generation control circuit (GCC), comprising a multi-stage buck-boost circuit connected in parallel with each P module [6]. The problems of the conventional method in terms of unbalanced generation are discussed. The circuit configuration and operation characteristics of the proposed inverter system are described, and the effectiveness of the system is verified experimentally and by means of simulation. II. UNBALANCED GENERATION When employing P generation systems in urban areas, we should take into account the influence of partial shading of the system. One characteristic of the P module is that, when illumination intensity decreases, the short-circuit current of the P module decreases, whereas the open-circuit voltage of the P module decreases only slightly. In other words, the generation current of a shaded P module decreases significantly as compared with that of nonshaded modules. Moreover, even if only a very small portion of a P module is shaded, generation current is reduced to a much greater extent than we would expect. As a result, total output power of the series-connected nonshaded and shaded P modules decreases significantly, because the generation current of the shaded P module is less than the optimum current for the other, nonshaded P modules. Fig. 1 shows a conventional method of connecting the bypass diode in parallel with a P module. The method involves an inherent problem that can be explained clearly by reference to two P modules connected in series. In case of unbalanced generation, if we assume that the generation current-voltage characteristic curves of each P module are as shown in Fig. 2(a), the output current-voltage and output power-voltage characteristic curves become as shown in Fig. 2(b). As load current increases from to, the output voltage, which is the sum of the output /03$ IEEE

2 SHIMIZU AND HASHIMOTO: UTILITY-INTERACTIE PHOTOOLTAIC INERTER SYSTEM 705 voltages of the respective P modules, and the total generation power of these two P modules change as follows. Operating point A: Each P module generates power, but neither one generates maximum power. Operating point B: The shaded module generates maximum power, but the nonshaded module does not generate maximum power. Operating point C: generates power, but does not generate any power, because the generation current, Io, flows through the bypass diode. Operating point D: generates maximum power, but does not generate any power. Hence, the following defects in the conventional method can be pointed out. (a) Controlling the operating conditions of individual P modules by use of the maximum power point tracking (MPPT) method is difficult, because in this case the output power-voltage characteristic curve of the system has two peaks, as shown in Fig. 2(b). (b) The maximum output power of the system is less than the sum of the maximum generation power of the P modules, because the P modules do not operate at peak power simultaneously if the P modules connected in series differ in optimum current. In order to solve the above-described problems, this paper proposes a novel utility-interactive P inverter system having a generation control circuit. Fig. 1. Series connection of photovoltaic modules. III. CIRCUIT CONFIGURATION AND OPERATIONAL CHARACTERISTICS Fig. 3 shows the proposed utility-interactive P inverter system, composed of a half-bridge inverter and a generation control circuit (GCC). The GCC consists of a multi-stage buck-boost chopper circuit [6]. A. Generation Control Circuit (GCC) In order to grasp the operation principle of the GCC easily, a two-stage chopper system is described. Fig. 4(a) shows the circuit configuration of a GCC consisting of a two-stage buckboost chopper circuit. Fig. 4(b) shows the switching sequence of S and S, and the current waveform at inductor. The relationship between the generation voltages of the P modules and the off-duty ratio of switches is given as D D (1) D D (2) where, D T T,T : OFF time of switch S, T: switching interval. The output current, Io, of the system is given as I I I D I D I (2) Fig. 2. Generation characteristics of series-connected P modules. (a) Output voltage versus current characteristics. (b) Output voltage versus power characteristics. The total output power, P, is obtained as the sum of the output power of the respective P modules, P and P, which can be controlled individually by the off-duty ratio and the total output voltage P P P (3) P I D I (4) P I D I (5)

3 706 IEEE TRANSACTIONS ON POWER ELECTRONICS, OL. 18, NO. 2, MARCH 2003 Fig. 4. Two-staged generation control circuit. (a) Generation control circuit having two P modules. (b) Switching sequence and inductor current waveform. Fig. 3. Circuit configuration of the proposed P generation system. (a) Circuit configuration of the proposed GCC. (b) Switching sequence. Generation current of each P module depends on the currentvoltage characteristics shown in Fig. 2(a), and hence it always satisfies I D I n (6) Hence, in the case that off-duty ratios are kept constant, the second derivative of output power is expressed as P P This result means that for a given off-duty ratio, only one peak point appears on the total output power-voltage characteristic curve, as shown in Fig. 5. The effects of the proposed GCC are summarized as follows. (a) The generation point of each P module, which corresponds to the off-duty ratio of the GCC and total output voltage, can be independently controlled. (b) For a given off-duty ratio, only one peak point is observed in the total output power-voltage characteristic, enabling assured MPPT control on the corresponding off-duty condition. P (7) Fig. 5. Total generation characteristics of the P generation system. (c) Each P module can be operated at its peak power point, by means of controlling the off-duty ratio and the total output voltage of the system. (d) The sum of the maximum output power of all the P modules is output to the load. The GCC is expected to operate at high efficiency, because most of the power generated from the P modules is transferred directly to the output terminal, and only the power difference between respective P modules passes through the GCC.

4 SHIMIZU AND HASHIMOTO: UTILITY-INTERACTIE PHOTOOLTAIC INERTER SYSTEM 707 These features mean that the GCC alleviates the drawbacks of unbalanced generation. Furthermore, in the case where the number of P modules connected in series increases, as shown in Fig. 3(a), the following relations are obtained by extending the GCC; i.e., increasing the number of buck-boost chopper circuits [6] D D D (8) I D I (9) P I (10) The switching sequence of the switches is expressed as shown in Fig. 3(b). Throughout the switching interval, the switches are turned off alternately, with the other switches being turned on. Therefore, the generation voltages, and, behave similarly to those on the two-stage buck-boost chopper circuit. This phenomenon is very important in considering the ripple voltage of the dc capacitors, which will be discussed in a later section. Fig. 6. Generation control circuit for three P modules. B. MPPT Control Method for GCC Usually, in the case where MPPT control of each P module is required, power detectors must be connected to every P module [4]. However, this inevitably increases production cost, and the increased complexity of the system may reduce the reliability of the system. In response to this problem, we present a novel MPPT control method for the proposed GCC system, whereby each P module can operate at its peak power point by means of detecting only the output power of the system. The proposed MPPT control method is explained by reference to a GCC, which employs a three-stage buck-boost chopper, as shown in Fig. 6. In order to control the generation condition of each P module, not only the voltage ratio of each P module but also the total output voltage of the system must be controlled. For this purpose, voltage ratio control of the P modules and total output voltage control are performed by the GCC and the inverter circuit, respectively. In this study, however, a controlled load is used instead of the inverter circuit. Owing to the GCC, similar to the case of Fig. 5, for a given off-duty ratio only one peak point appears in the total output power-voltage characteristic curve. Then, at the given voltage ratio, the simple hill-climbing method by way of changing power consumption of the controlled load provides the local peak power, P. Here, a restriction of D D D is imposed, so that, as shown in Fig. 7, we obtain the peak power surface as a function of off-duty ratios, D and D,of the GCC. As is obvious, no local minimum point exists, but one maximum power point, which corresponds to the optimum off-duty ratio of the switches, is present. Fig. 8 shows the procedure for searching the optimum off-duty ratio. On a given point of the plane, six kinds of peak power differences corresponding to directions to, are detected, and a new direction in which the peak power increases considerably is determined. By iteration of this process, the off-duty ratio is converged to its optimum value via the shortest Fig. 7. Peak power surface and maximum power point. route; i.e., climbs the steepest slope on the plane. At the optimum off-duty ratio, each P module generates maximum power independently by detecting the total output power of the system. In the case where the circuit configuration of the GCC is extended to the multistage buck-boost chopper shown in Fig. 3, the MPPT control algorithm can also be extended to a function of multi-variables corresponding to the number of off-duty ratios. C. Utility-Interactive Inverter having Generation Control Circuit Fig. 9(a) shows the simplified circuit configuration of the proposed utility-interactive inverter system, and Fig. 9(b) shows the control block diagram. The proposed system is composed of a half bridge inverter circuit, the generation control circuit, and photovoltaic modules. As mentioned above, in this section the GCC is depicted as a two-stage chopper circuit, but operation

5 708 IEEE TRANSACTIONS ON POWER ELECTRONICS, OL. 18, NO. 2, MARCH 2003 Fig. 9. Simplified circuit configuration and control block diagram. (a) Simplified circuit configuration of the proposed system. (b) Control block diagram of the proposed system. trary, the GCC holds the voltage of each P module to a constant value that depends on the off-duty ratio control. In this process, the low-frequency ripple current flowing into these capacitors is shared, and hence the ripple voltages in these capacitors are reduced. As a result, each P module can generate power at the desired generation point and overall generation efficiency is improved. Fig. 8. Procedure for scanning the optimum off-duty ratios. (a) Scanning directions. (b) Changing status of off-duty ratios. relating to the following discussion is the same as that with the multi-stage chopper circuit. A high-frequency PWM and instantaneous a.c. current control are utilized in the utility-interactive inverter in order to realize a unity-power-factor and low-harmonic current operation on the utility side. For the first step, the output voltage command,, is generated by the MPPT controller on the basis of a conventional MPPT method, such as the hill-climbing method, and the switching signal for the switches, S and S, on the inverter is generated. In the next step, the MPPT controller adjusts the off-duty ratio of switches, S and S, on the GCC. On the conventional half-bridge inverter, large ripple voltage of frequency double that of the output voltage appears in the dc capacitors, and, because these capacitors are alternately charged and discharged in conjunction with the half-bridge inverter operation. The large ripple voltage causes the operation point of the P- curve to fluctuate, as shown in Fig. 10, and reduces the average output power of the P modules. On the con- I. SIMULATION AND EXPERIMENTAL RESULTS A. The Generation Control Circuit (GCC) In the experiment, 6 P modules are used and hence a six-stage buck-boost chopper circuit is used as the GCC. Furthermore, in this case and are 50% covered by shadow, but the other modules are not covered by shadow. Fig. 11 shows the generation characteristics of the P modules and these operation points, and Table I shows the measured generation power of each P module in the case where the GCC is activated. Fig. 12 shows the total output power characteristics, in the experimental setup with the proposed GCC system and a conventional system, and Table II shows generated maximum power, P, for these two systems. As is obvious from Table II, the GCC recovers 14% more power than does the conventional system. In addition, the output power-voltage characteristic of the conventional method has at least two peaks, making accurate MPPT control difficult. In contrast, the GCC facilitates robust MPPT control, because only one peak point appears on the output power-voltage characteristic.

6 SHIMIZU AND HASHIMOTO: UTILITY-INTERACTIE PHOTOOLTAIC INERTER SYSTEM 709 Fig. 12. Experimental results of total output power characteristics. Fig. 10. Output voltage fluctuation and average power of P module. TABLE II GENERATED MAXIMUM POWER Fig. 11. Output characteristics and operation point of each P module. TABLE I GENERATION POWER OF EACH P MODULE Fig. 13. Simulated waveforms on the unbalanced generation condition. (a) Waveforms of the proposed system. (b) Waveforms of the conventional system. B. Utility-Interactive Photovoltaic Inverter With Generation Control Circuit Fig. 13 shows the simulated waveforms of the inverter output current,, the ripple voltage in the dc capacitors,, and the ripple current flowing through the P modules,. As is clear from the figure, the amplitude of the ripple voltage in the proposed inverter system is reduced to about 22% that in the conventional half-bridge inverter. Fig. 14 shows the experimental waveforms of the inverter for both the proposed system and the conventional system. Output current of the proposed system is about 20% higher than that of the conventional system, because the ripple voltages in the dc capacitors are reduced. Fig. 15 shows the experimental results under highly unbalanced generation. Output current of the proposed inverter retains a sinusoidal waveform, whereas that of the conventional inverter is greatly distorted.. CONCLUSION A serious problem of partial shading of P power generation systems, which results in unbalanced generation, has been solved by employing a novel inverter composed of a utility-interactive half-bridge inverter and a generation control circuit. The advantages of the proposed inverter system were confirmed experimentally and by means of simulation, and can be summarized as follows.

7 710 IEEE TRANSACTIONS ON POWER ELECTRONICS, OL. 18, NO. 2, MARCH 2003 Fig. 14. Experimental results of output waveforms under unbalanced generation. (a) Output waveforms of the proposed system. (v : 10 /div, i : 0.5 A/div). (b) Output waveforms of the conventional system. (v : 10 /div, i : 0.5 A/div). Fig. 15. Experimental results of output current under unbalanced generation. (a) AC current of the proposed system. (i : 1.0 A/div). (b) AC current of the conventional system. (i : 1.0 A/div). a) The MPPT control algorithm for the proposed inverter system allows each P module to generate peak power simply by detecting the output power of the system. The proposed MPPT control algorithm requires fewer power detectors than are employed in conventional systems. b) The GCC allows each P module to generate its inherent power without being affected by the generation condition of the other modules. Hence, the proposed inverter system supplies to a utility line the sum of the maximum output power of all the P modules. REFERENCES [1] C. Hua, J. Lin, and C. Shen, Implementation of a DSP-controlled photovoltaic system with peak power tracking, IEEE Trans. Ind. Electron., vol. 45, pp , Feb

8 SHIMIZU AND HASHIMOTO: UTILITY-INTERACTIE PHOTOOLTAIC INERTER SYSTEM 711 [2] E. Koutroulis, K. Kalaitzakis, and N. C. oulgaris, Development of a microcontroller-based, photovoltaic maximum power point tracking control system, IEEE Trans. Power Electron., vol. 16, pp , Jan [3] M. Matsui, T. Kitano, D.-H. Xu, and Z.-Q. Yang, New MPPT control scheme utilizing power balance at dc link instead of array power detection, in Proc. IPEC-Tokyo 00, vol. 1, 2000, pp [4] T. Noguchi, S. Togashi, and R. Nakamoto, Short-current pulse based adaptive maximum-power-point tracking for photovoltaic power generation system, Proc. IEEE, vol. 1, pp , [5] H. Fujimoto, T. Kagotani, and H. Kidoguchi, Photovoltaic inverter with a novel cycloconverter for interconnection to a utility line, Proc. IEEE IAS 95 Conf., pp , [6] T. Shimizu, M. Hirakata, T. Kamezawa, and H. Watanabe, Generation control circuit for photovoltaic modules, IEEE Trans. Power Electron., vol. 16, pp , May Osamu Hashimoto was born in Tokyo, Japan, in He received the B.S. and M.S. degrees from Tokyo Metropolitan University, Tokyo, in 1999 and 2001, respectively, both in electrical engineering. His research interests includes power converters and photovoltaic power generations. In 2001, he joined SONY corporation, Tokyo, Japan, where he is currently a Research Engineer. Mr. Hashimoto is a member of the Institute of Electrical Engineers of Japan. Toshihisa Shimizu (SM 02) was born in Tokyo, Japan, in He received the B.E., M.E., and Dr.Eng. degrees in electrical engineering from Tokyo Metropolitan University, Tokyo, in 1978, 1980, and 1991, respectively. In 1998, he was a isiting Professor at PEC, irginia Polytechnic Institute and State University, Blacksburg, for four months. He joined Fuji Electric Corporate Research and Development, Ltd., in In 1994, he joined the Department of Electrical Engineering, Tokyo Metropolitan University, as an Associate Professor. His research interests include power converters, high frequency inverters, photovoltaic power generations, UPSs, and EMI problems, etc. Dr. Shimizu has received the TRANSACTIONS Paper Award from the Institute of Electrical Engineers of Japan in He is a member of the Institute of Electrical Engineers of Japan (IEEJ) and the Japan Society of Power Electronics. Gunji Kimura (M 90) was born in Tokyo, Japan, in He received the B.E. and Dr.Eng. degrees from Tokyo Metropolitan University, in 1978 and 1996, respectively. He has been with Tokyo Metropolitan University, where he is currently a Professor. Dr. Kimura is a member of the Institute of Electrical Engineers of Japan (IEEJ).