thedual-moduleMPPTcontrolstrategyofstand-alonePVsystem

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 hongwanghitsz.edu.cn, zhangdlhitsz.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 systems 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 PpvPload PpvPload PpvPload Recharge Status 1,2,3 Status 6 X Discharge X Status 5,7 X Dormancy Status 4 Status 8 Status 0 When the power of PV array generation is greater than the load demand, it corresponds to periods of sufficient solar power to satisfy the total demand. The exceeding energy is used to charge the battery. There are four possible modes as follows Mode 1 DC-DC1 operates in the stable voltage mode. DC-DC2 operates in MPPT mode. DC-DC3 does not operate. At this moment PV array operates in MPPT mode, and batteries are recharged with the controlled current. Mode 2 DC-DC1 and DC-DC2 both operate in the stable voltage mode. DC-DC3 does not operate. The capacity PV array powered exceeds the required demand of loads and battery. The output of solar power is limited. The batteries close to full and are recharged with the constant voltage. Mode 3 DC-DC1 operates in the stable voltage mode. DC-DC2 operates in trickle mode. DC-DC3 does not operate. Status of PV array is as same as mode 2. But batteries are recharged in the trickle current mode. Mode 4 DC-DC1 operates in the stable voltage mode. Both DC-DC2 and DC-DC3 do not operate. At this moment the batteries are not operated since the battery is full. The PV array only supplies the power to loads. When the power of PV array generation is less than the load demand, it corresponds to periods of less solar power without enough supply to loads. Battery bank outputs the power as complement for PV array. Another four possible modes are as follows Mode 5 DC-DC1 operates in the MPPT mode, DC-DC2 does not operate, and DC-DC3 operates in the stable voltage mode. PV array operate in MPPT mode, and batteries operate in discharge mode. Both PV array and battery bank supply the power to loads. Mode 6 DC-DC1 and DC-DC3 don ’ t operate. DC-DC2 operates in the MPPT mode. At this moment the sum of power from PV array and power from battery still can not fulfill the required demand of loads. So the PV array operates in MPPT mode and batteries are recharged. Inverter is closed and no power is supplied to loads. Mode 7 Only DC-DC3 operates in the stable voltage mode. It means there is no power from PV array, such like in nights. Batteries discharge the power to loads individually. Mode 8 All of converters do not operate. At this moment PV array and battery bank do not supply any power. The inverter is closed. 94 2010 Chinese Control and Decision ConferenceWhen the power of PV array generation is equal to the load demand, the last mode is as follows Mode 9 Only DC-DC1 operates in the MPPT mode. It is the best operation condition since the load current value is equal to the PV module MPP current value, where the battery doesn ’ t process power. But this status is unstable. The state machine of control strategy for a stand-alone PV generation system is as shown in figure 4. The switch of modes depends on the battery state and the comparison between PV array and load. Fig 4. Stand-alone PV system status machine The stand-alone PV power system needs to be set the initial status. Mode 8 is the sleep mode, and Mode 9 is the start mode. The system could be woken up from sleep mode by voltage of PV array and changed into mode 9. After the system starts, the control system determines the next mode, which is supplying power to the load or charging for batteries according to the match of power from the load, PV array and battery bank. 4 DUAL-MODULE MPPT CONTROL The management strategy of the proposed system is composed of different algorithms, which must control the cooperated dual-module MPPT process mode 1, single MPPT process modes 5, 6, 7 and 9 and the output voltage regulation modes 2, 3 and 4. In order to maintain system stability, the internal energy of the stand-alone PV power system should keep balance. The voltage of DC bus with energy storage capacitor must be stabilized at a certain range. The formulas 1-3 are shown for the energy balance of the system. 21 DCDCPV PPP 1 03311 dtdEPPP CinvDCDCDCDC ηη 2 invinvload PP η 3 In the formulas, PVP is electricity power generated by PV array, DCxP and DCxη are the power and efficiency of each DC-DC conversion module separately, and loadP is the power of load, CE is the energy stored in capacitor at the present time. The PV voltage and current are monitored, and the PV power can be dynamically controlled by two DC-DC converters parallel connecting PV array. Converters must change the impedance of the circuit to ensure the maximum energy generation from the PV array. Thus, MPP operation can be obtained under any operational condition. The control of mode 1 is the most complex. Both two DC modules have an impact on the output of PV array. Load changes can lead fluctuation of DC bus voltage, and the regulator control of DC-DC1 would undermine the MPPT implementation of DC-DC2. Therefore the fast convergence time of the MPPT algorithm is important for this system in order to minimize the transient in the DC bus voltage with the load variation. Shown as figure 5, this paper designs a unified cooperated controller for the two modules. According to two-way current changes, the total PV power is maintained to be constant by cooperatively controlling switches of two topologies. DC-DC1 is to achieve double closed-loop control of voltage and current to make the output voltage stable. DC-DC2 controls the input current by closed-loop to implement MPPT control. The detected input current of DC-DC1 is subtracted by the reference current from the output of MPPT algorithm, and the result is regarded as the input current reference of DC-DC2. MPPT algorithm uses perturbation observation, which utilizes the changes of operating point of PV array caused by the regulators control of DC-DC1 as the system disturbances. PVvPVi*MPPiini1ini1outi1outv1ini2*1outv*1outiFig 5. Dual-module parallel MPPT coordinated control block diagram In the figure 5, PVv and PVi are output voltage and current of PV array; xini is the input current of the Xth module; xoutv and xouti are the output voltage and current of the Xth module. PVi , ini1 , ini2 satisfy the relation about ininPV iii 21 .5 EXPERIMENTAL RESULTS Taking household load as the target, a modular PV power system was developed to verify system configuration optimization and control algorithms. In the experiment, solar simulator E4360A is used to simulate PV panel array. The Figure 6 is the result of MPPT algorithm. The dual-module cooperated MPPT algorithm is realized to control two parallel DC-DC modules harvesting maximum energy from PV array. 2010 Chinese Control and Decision Conference 95Fig 6. Power output of E4360A in MPPT The Figure 7 shows the status of 350V DC bus with payload changed from 250W to 1.5kW. The current ch3 and the voltage ch4 in picture show the response of the system is fast and stable. The Figure 8 shows inverter output with reistance load. The voltage ch1 and current ch2 display its excellent performance. Fig 7. DC bus voltage response with changing load Fig 8. Voltage/Current Output waves of inverter 6 CONCLUSION The stand-alone PV power system with parallel DC-DC circuit and cooperated MPPT control is presented in this paper. The system implemented multifunctional, operating as an MPPT circuit, battery charge, battery regulator, and inverter. The parallel connection of the PV system reduces the negative influence of power converter losses in the overall efficiency during PV power generation. The control algorithm ensures operation at MPP as in the classical system. The main operational aspects of the pa