1 INTRODUCTIONWith the environment pollution and energy consumption worsen, the renewable power generation technologies have been actively researched and developed in many countries. Stand-alone PV generation system can provide electric power to remote areas, islands, the equipments in no-man land and vehicles. It can be widely applied in military, disaster response or other emergency occasions. But the high cost of PV systems and the low conversion efficiency of PV panel array are the major obstacles to using the renewable solar energy on a large scale[1,2] . Stand-alone PV generation system is normally composed of PV panel array, battery and power source. Due to technical limitation the cost and efficiency of PV panel array or battery can not be improved at the moment. So the high efficiency of power source has received considerable attention. Since the cost of battery bank plays a fundamental role in the overall system cost, it is critical to operate the battery bank carefully, with the objective of extending its operative life[3,4] . The maximum power of PV array can output depends on climatic conditions. There is only one value of current and voltage for each maximum power point (MPP). The PV current changes with the solar irradiation level, whereas the PV output voltage changes with the temperature of the PV module. In order to harvest the maximum power of the PV panel array, the maximum power point tracking (MPPT) is generally implemented by DC-DC converter between PV panel array and battery. A step-up inverter is necessary to provide alternative power to AC loads. Considering that in the series connection, the DC-DC converters always process all power generated, the total efficiency are reduced by several conversions. As an alternative to this configuration, the parallel connection of the MPPT circuit was introduced in many papers[5-8] . This paper presents a stand-alone PV generation system with parallel battery charger. The MPPT is coordinately implemented by two DC-DC converters. The total efficiency of the system is raised since sections of DC-DC conversions lessen. The operation principle, theoretical analysis, design methodology, and experimental results of laboratory prototype of the PV generation system are presented in this paper. 2 PARALLEL SYSTEM CONFIGURATION The stand-alone PV generation system is often the series connection of a DC-DC converter between PV array and battery. For AC loads, the step-up inverter is used to convert DC power of battery to AC power. The series ways of PV power source are mainly as following[9-11] . 1? Interactive series: There are two DC-DC converters and one inverter in the interactive PV system. One DC bus is used to connect all modules. One DC-DC converter performs the MPPT control to harvest maximum solar energy, the other dual-direction DC-DC converter manages the charging and discharging of battery. The dual-direction DC-DC converter has to be implemented. The system efficiency is also reduced since the serial Two-level conversion of system is used for battery charging. Fig 1. PV interactive conversion 2) On-line series: Batteries are directly connected to DC bus. One DC-DC converter (PV controller) presents both functions of MPPT control and battery charger. For AC loads, the other step-up DC-DC converter is necessary to The Dual-module MPPT Control Strategy of Stand-along PV System Hong Wang, Donglai Zhang Harbin Institute of Technology Shenzhen Graduate School, Shenzhen 518055, China E-mail: hongwang@hitsz.edu.cn, zhangdl@hitsz.edu.cnAbstract: The key to expand PV application is cost cutting and performance enhancing for the extensive application of stand-alone PV generation system. In this paper, the improvement of general efficiency is considered. The new configuration with several DC-DC modules is designed. The conversion steps are decreased and system conversion efficiency is increased. The dual-module MPPT coordinate control is designed to increase PV utilizing efficiency. The battery management is optimized for better performance and longer life time. The performance and total efficiency of PV generating system can be improved for stand-alone load. The experimental results of the prototype verify the effectiveness of proposed protocol and strategy.Key Words: Stand-alone PV system, Dual-module MPPT, DC-DC convertors93978-1-4244-5182-1/10/ $26.00 c 2010 IEEEincrease the battery bank voltage to the required DC bus level required by inverter. The drawback of the architecture is that batteries are involved in energy conversion, affected by load disturbance directly, which could affect battery life-time and utilization. Fig 2. PV on-line conversion According to above analysis, there are several converters that are in series in the PV system. The total efficiency is the product of all conversion efficiency. Its performance greatly depends on the efficiency of each converter. Therefore, the efficiency of each stage has to be optimized in order to avoid an excessive decrease in efficiency, which would increase the system cost and the number of PV modules. Fig 3. PV parallel coordinated conversion To improve the efficiency of battery charging and optimize the battery management, this paper puts forward an improved proposal of PV system. As shown in figure 4, the stand-alone PV generation system is implemented based on parallel battery charger. The power source, battery bank and PV cells are connected as parallel circuits by a boost circuit DC-DC1 and buck circuit DC-DC2. DC-DC1 converts electricity to DC bus which used by inverter. DC-DC2 charges the battery bank. The two circuits can carry out MPPT control individually or simultaneously. DC-DC3 is the battery discharging circuit. DC-AC inverter provides the AC electric power to AC loads. The main advantage of this configuration is that only one DC-DC converter is used for charging battery or DC bus, and each converter processes only a part of the generated power. Compared with the series configuration, the total system efficiency is improved. At the same time, the time of batteries powered is shortened. The management of battery charging and discharging is individually optimized. The battery overcharge or excessive discharge is avoided. So the proper control of power source can prolong the life-time of PV panel array and battery. 3 PV SYSTEM GENERATION STRATEGY The capability of the PV generation system to satisfy the power demand depends on the atmospheric conditions. Such conditions and the status of battery bank (recharging or discharging) will define different operation modes of the system. According to the principle of energy balance, the system s energy should be balanced between the total generation and the total demand. The total demand includes the payload and the required power to recharge (plus) or discharge (minus) the battery bank. The comprehensive control algorithm is essential to efficiently manage the operation of the power source according to those modes. Considering climatic conditions and load variation, the nine possible modes of generation are presented, as shown in table 1. Table1. Stand-alone PV system status determination Status of Battery Bank The status of PV array Ppv>Pload Ppv
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