Optimal energy control of a PV-fuel cell hybrid system

This paper deals with a real time implementation of a fuzzy logic-based power management of a small scale generation hybrid power system. The system consists of a photovoltaic array and a fuel cell stack, supported by a single-phase grid that supplies a stand-alone AC load. The proposed supervisory algorithm guaranties the system to switch smart between two operation modes, according to the load demand, the gas level and the PV availability. Obviously, the PV side DC–DC converter is controlled to track permanently the maximum power point by using a fuzzy logic MPPT method; whereas, the fuel cell stack and the grid converters are tuned to cover the remaining power, or alternatively, injecting the exceeding power to the utility. Besides, to feed the AC load with a pure sine wave, a Back stepping algorithm is proposed to control the front-end single-phase inverter. To test the effectiveness of the proposed algorithms, experimental results obtained with a given load profile are presented and commented.     

Combined operation of DC isolated distribution and PV systems for supplying unbalanced AC loads

This paper describes a DC isolated network which is fed by distributed generation (DG) from photovoltaic (PV) renewable sources to supply unbalanced AC loads. The battery energy storage bank has been connected to the DC network via DC/DC converter called storage converter to control the network voltage and optimize the operation of the PV generation units. The PV units are connected to the DC network via its own DC/DC converter called PV converter to ensure the required power flow. The unbalanced AC loads are connected to the DC network via its own DC/AC converter called load converter without transformer. This paper proposes a novel control strategy for storage converter which has a DC voltage droop regulator. Also a novel control system based on Clarke and Park rotating frame has been proposed for load converters. In this paper, the proposed operation method is demonstrated by simulation of power transfer between PV units, unbalanced AC loads and battery units. The simulation results based on PSCAD/EMTDC software show that DC isolated distribution system including PV units can provide the balanced voltages to supply unbalanced AC loads.     

Battery-integrated boost converter utilizing distributed MPPT configuration for photovoltaic systems

In this paper, a battery-integrated boost converter utilizing the distributed maximum power point tracking (DMPPT) configuration for a photovoltaic (PV) system is studied. Each PV module has its own battery and DC/DC converter. Due to the proposed topology and use of battery, the MPPT function is not affected by the load demand and input power from PV. Application of the proposed converter to DMPPT configuration can save the voltage amplification stage and maintain PV voltage during partial shading. Steady-state analysis of the converter to determine the power flow equations is presented. Comparison with the series-connected conventional boost converter is reported in this paper. Simulation and experiment results of a laboratory prototype are presented to verify the effectiveness of the proposed approach. System design considerations are also discussed.     

A grid-connected photovoltaic power conversion system with single-phase multilevel inverter

This paper presents a grid-connected photovoltaic (PV) power conversion system based on a single-phase multilevel inverter. The proposed system fundamentally consists of PV arrays and a single-phase multilevel inverter structure. First, configuration and structural parts of the PV assisted inverter system are introduced in detail. To produce reference output voltage waves, a simple switching strategy based on calculating switching angles is improved. By calculated switching angles, the reference signal is produced as a multilevel shaped output voltage wave. The control algorithm and operational principles of the proposed system are explained. Operating PV arrays in the same load condition is a considerable point; therefore a simulation study is performed to arrange the PV arrays. After determining the number and connection types of the PV arrays, the system is configured through the arrangement of the PV arrays. The validity of the proposed system is verified through simulations and experimental study. The results demonstrate that the system can achieve lower total harmonic distortion (THD) on the output voltage and load current, and it is capable of operating synchronous and transferring power values having different characteristic to the grid. Hence, it is suitable to use the proposed configuration as a PV power conversion system in various applications.     

Analysis and enhancement of PV efficiency with incremental conductance MPPT technique under non-linear loading conditions

This paper presents experimental evaluations for variation in the efficiency of energy extracted from a photovoltaic (PV) module (under non-linear loading) incorporated with an incremental conductance(IC) maximum power point tracking (MPPT) algorithm. The focus is on the evaluation of the PV panel under non-linear loading conditions using the experimental installation of a 100W_p photovoltaic array connected to a DC–DC converter and a KVA inverter feeding a non-linear load. Under the conditions of non-linear loading, both the simulation and experiment show that the MPPT technique fails to attain maximum power point due to the presence of ripples in the current leading eventually to a reduction in efficiency. In this paper, panel current is taken as a function of load impedance in the MPPT algorithm to eradicate power variation, as load impedance varies with supply voltage under non-linear conditions. The system is simulated for different non-linear loads using MATLAB-Simulink. A TMDSSOLAREXPKIT was used for MPPT control. In case 2, the inverter is connected to a single phase grid. When a voltage swell occurs in the grid, PV power drops. This power loss is reduced using the proposed MPPT method. The results of simulations and experimental measurements and cost efficiency calculations are presented.     

Design and implementation of a hybrid regenerative power system combining grid-tie and uninterruptible power supply functions

A hybrid regenerative power system including photovoltaic (PV) and wind powers and combining the functions of the grid?tie system and uninterruptible power supply (UPS) for critical load applications is presented. The proposed system employs six-arm converter topology with three arms for the rectifier? inverter, one arm for battery charging/discharging and two arms for power conversion of the PV module and wind turbine generator. The operation modes include the grid?tie mode and the UPS mode depending on the grid status. A power balance control scheme is presented, which can reduce the grid power and utilise the regenerative power in the most effective way for fulfilling the two requirements of a three-stage charging of the battery and no interruption of the load. Also, the PV and wind powers can be utilised with priority in order to provide the flexibility for adapting to local circumstances. A single-phase 1.2 kW/110 V system is designed and implemented, and the effectiveness of the proposed system and control methodology are verified with some experimental results.     

A single phase parallely connected uninterruptible powersupply/demand side management system

This paper presents the application of a single-phase parallel converter as an uninterruptible power supply and demand side management system. The proposed system consists of a bi-directional inverter that is connected in parallel to the utility system. When the grid system fails, the converter will convert the power from the battery to the AC side (utility side) at the mains voltage and frequency. On the other hand, when the utility is normal the converter will act as a demand side management system. It charges the battery during low load and shaves the transient load at the peak period system. This improves the pattern of the demand variation in the utility side. This paper presents the operational principle of the uninterruptible power supply and demand side management system, laboratory, and simulation results     

Improving a battery charger architecture for electric vehicles with photovoltaic system

In this study, a two-stage battery charger architecture with high-efficiency, multi-input, and output half-bridge LLC (HBLLC) resonance converter that performs a wide load range is proposed. The first input of the HBLLC is provided by the photovoltaic (PV) panel assembly on the vehicle. A high efficiency and fast maximum power point tracking (MPPT) algorithm has been developed for the PV panel to operate at the maximum power point. The other input is supplied by a grid-connected AC-DC bridgeless power factor correction (PFC) converter, which is controlled with the average current mode (ACM) control method. The most important feature that distinguishes the designed topology from previous studies is that it charges the low-voltage battery through the PV panel. In previous studies, the low-voltage battery was being charged via the high-voltage battery. This allowed the high-voltage battery to transfer power to the low-voltage battery even when it was not charged. However, in the proposed architecture, the low-voltage battery is fed by a PV panel. This condition allows the electric vehicle to take more miles with a single charge process. Furthermore, the proposed architecture reduces energy costs in the long term by providing some of the energy demanded from the grid. In addition, the proposed integrated battery charging circuit is intended to reduce the cost of additional cables. The system is designed as 3.1 kW power and operated under no load to full load. As for the performance of the proposed architecture, the peak efficiency of the LLC resonant converter is 95.3%. In addition, peak efficiency of the AC-DC bridgeless PFC converter is 97.3%, while the power factor is higher than 0.99, input current total harmonic distortion (THD) is less than 5%, MPPT method accuracy is higher than 99%, and output voltage ripples (ΔV) is less than 1 V.     

A power allocation strategy of a hybrid energy storage system for a PV grid-connected system

针对光伏并网系统中光伏微电源出力的波动性和间歇性,将蓄电池和超级电容器构成的混合储能系统HESS（hybrid energy storage system）应用到光伏并网系统中可以实现光伏功率平滑、能量平衡以及提高并网电能质量。在同时考虑蓄电池的功率上限和超级电容的荷电状态（SOC）的情况下,对混合储能系统提出了基于超级电容SOC的功率分配策略;该策略以超级电容的SOC和功率分配单元的输出功率作为参考值,对混合储能系统充放电过程进行设计。超级电容和蓄电池以Bi-direction DC/DC变换器与500 V直流母线连接,其中超级电容通过双闭环控制策略对直流母线电压进行控制。仿真结果表明,所提功率分配策略能对混合储能系统功率合理分配,而且实现了单位功率因数并网,稳定了直流母线电压。     

Numerical modeling and analysis of PEMFC integrated with auxiliary power source

A numerical model is developed from a stationary proton exchange membrane fuel cell (PEMFC) system comprising a PEMFC, a DC‐DC buck converter, an auxiliary power supply (a lithium battery and supercapacitor), and a DC‐AC inverter. The transient and steady‐state performance of the PEMFC system is investigated by means of Matlab/Simulink simulations. It is shown that a good agreement exists between the simulated polarization curve of the PEMFC and the experimental results presented in the literature. In addition, it is shown that the DC‐DC buck converter provides an effective means of stabilizing the output voltage of the PEMFC. Finally, the results confirm the effectiveness of the auxiliary power source in enabling the PEMFC to satisfy the peak load demand. Copyright © 2012 John Wiley & Sons, Ltd.     

Optimized power management based on adaptive-PMP algorithm for a stationary PEM fuel cell/battery hybrid system

This research develops an efficient and robust polymer electrolyte membrane (PEM) fuel cell/battery hybrid operating system. The entire system possesses its own rapid dynamic response benefited from hybrid connection and power split characteristics due to DC/DC buck-boost converter. An indispensable energy management system (EMS) plays a significant role in achieving optimal fuel economy and in a promising running stability. EMS as an indispensable part plays a significant role in achieving optimal fuel economy and promising operation stability. This study aims to develop an adaptive supervisory EMS that comprises computer-aided engineering tools to monitor, control, and optimize the performance of the hybrid power system. A stationary fuel cell/battery hybrid operating system is optimized using adaptive-Pontryagin's minimum principle (A-PMP). The proposed algorithm depends on the adaptation of the control parameter (i.e., fuel cell output power) from the state of charge (SOC) and load power feedback. The integrated model simulated in a Matlab/Simulink environment includes the fuel cell, battery, DC/DC converter, and power requirements models by analyzing the three different load profiles. Real-time experiments are performed to verify the effectiveness of EMS after analyzing the simulated operating principle and control scheme.     

Modeling and simulation of three phase multilevel inverter for grid connected photovoltaic systems

This paper presents a control for a three phase five-level neutral clamped inverter (NPC) for grid connected PV system. The maximum power point tracking (MPPT) is capable of extracting maximum power from the PV array connected to each DC link voltage level. The MPPT algorithm is solved by fuzzy logic controller. The fuzzy MPPT is integrated with the inverter so that a DC–DC converter is not needed and the output shows accurate and fast response. A digital PI current control algorithm is used to remain the current injected into the grid sinusoidal and to achieve high dynamic performance with low total harmonic distortion (THD). The validity of the system is verified through MATLAB/Simulink and the results are compared with three phase three-level grid connected NPC inverter in terms of THD.     

The concept of distribution flexible network PV system

In stand-alone photovoltaic (PV) systems, when the battery is fully charged, the excess generated power is wasted. To solve the problems of wasted excess power, a distributed flexible network photovoltaic (DFNPV) system is studied. It incorporates many PV subsystems each consisting of PV panel, DC/DC converter, and load, and are connected to each other with shared batteries. The excess generated power of the subsystem is transferred between PV subsystems to compensate the lack of power in other subsystems.The control method of transferring power is based on simple voltage control of the subsystems. The output voltage in a given subsystem decreases if a transient excessive load is larger than the generated power; as a result excess power is transferred from another subsystem that has sufficient power and higher voltage output. In this study, this proposed operation method is demonstrated by simulation of power transfer between two subsystems and among four subsystems. Furthermore, to estimate the size of the DFNPV system within an acceptable voltage drop, the relationships between cable length, power loss, and cable types are discussed.     

Photovoltaic field emulation including dynamic and partial shadow conditions

In this paper the development of a new laboratory prototype for the emulation of a photovoltaic (PV) field is presented. The proposed system is based on a DC/DC step-down converter topology and allows to obtain the solar array I–V curves, taking into account the environmental changes in solar irradiance and cell temperature. The DC/DC converter control strategy is deduced by using a comprehensive mathematical model of the PV field whose parameters are obtained from the knowledge of: (a) maximum power point data, measured when the PV plant power converter is running, (b) open circuit voltage and short-circuit current, measured off-line. This approach allows the most accurate representation of the PV source. Computer simulations and experimental results demonstrate that the proposed circuit acts as a highly accurate and efficient laboratory simulator of the photovoltaic array electrical characteristics both in steady state and transient conditions. Partial shading and fluctuating conditions can be reproduced too. Moreover the dynamic behaviour of the proposed laboratory emulator is suitable to its effective connection to power electronic interface to the utility or to load through a DC/DC boost converter.     

A simple photo-voltaic tracking system

This paper describes a simple photo-voltaic (PV) tracking system which has been designed and manufactured using a pyramidal stand as a base. A rotating unit consisting of two pairs of modules fixed at an angle of 170° between them was installed at the upper edge of the stand. The four modules and a DC motor were connected to a bridge circuit making the system sensitive to solar tracking. The PV tracker has a DC–DC (24/2 V) converter, DC–AC (24/220 V) inverter and a battery. The modules are able to provide a maximum power of 100 W, which feeds the converter, inverter and DC motor. Wind resistance of the tracker is quite low. The total area of the four modules is 1.26 m². The inclination angle between the modules and the horizontal plane is adjustable (34°±11°) and the tracking angle is 120°. The system can track solar motion with an error of ±10°. The designed PV tracking system, with modules fixed at an angle of 170° to feed the load as well as the DC motor, exhibited it to be an efficient energy-conversion system. The fabricated system offers low wind resistance. The cost analysis data revealed that the proposed design was very economical and cost effective.     

Improved particle swarm optimization for photovoltaic system connected to the grid with low voltage ride through capability

Grid connected photovoltaic (PV) system encounters different types of abnormalities during grid faults; the grid side inverter is subjected to three serious problems which are excessive DC link voltage, high AC currents and loss of grid-voltage synchronization. This high DC link voltage may damage the inverter. Also, the voltage sags will force the PV system to be disconnected from the grid according to grid code. This paper presents a novel control strategy of the two-stage three-phase PV system to improve the Low-Voltage Ride-Through (LVRT) capability according to the grid connection requirement. The non-linear control technique using Improved Particle Swarm Optimization (IPSO) of a PV system connected to the grid through an isolated high frequency DC–DC full bridge converter and a three-phase three level neutral point clamped DC-AC converter (3LNPC²) with output power control under severe faults of grid voltage. The paper, also discusses the transient behavior and the performance limit for LVRT by using a DC-Chopper circuit. The model has been implemented in MATLAB/SIMULINK. The proposed control succeeded to track MPP, achieved LVRT requirements and improving the quality of DC link voltage. The paper shows superiority of IPSO than Incremental Conductance (IC) method during MPPT mode of PV system.     

Analysis and implementation of an adaptative PV based battery floating charger

In a system composed of a photovoltaic (PV) cell, a converter and a resistive load, the Maximum Power Point Tracking (MPPT) techniques are applied at the output of the PV panel and not at the level of the load. In this study, the considered load is a battery at different States Of Charge (SOC) that is charged by the PV panel. The power consumed by the battery is related to its SOC. Consequently, an empty battery consumes more current than a charged one. At full state of charge, the battery does not call for more energy and thus it is not rewarding to extract more power from the PV panel.Besides, in a stand-alone photovoltaic system, the size of the PV panel and the battery should be respected. Thus, the PV current at different irradiances should be compatible with the charging current required to charge the battery at different SOC. A critical situation occurs at high irradiance when the PV panel delivers a high current at Maximum Power Point (MPP) that exceeds the tolerated charging current. The current reaches the top limit when the battery is totally empty, caused by the big difference in potential between the converter output and the battery voltages. In this case, the battery starts to gas when attempts are made to charge it faster than it can absorb the energy. On the other hand, in a fully charged battery, the difference in potential between the converter and the battery is zero. In this case, there is no need to track the MPP.In this study, we will focus on the load type and suggest new methods to reach the MPP depending on the load state. In the proposed designs, the components of the stand-alone system are protected even if they are not well sized. In addition, we will focus on the development of the PV array mathematical model. The results achieved with the system, as well as the experimental results of a laboratory prototype, will be given.     

An effective and safe charging algorithm for lead‐acid batteries in PV systems

In this paper a new charging algorithm is proposed to charge lead‐acid batteries in photovoltaic (PV) systems. This algorithm can return discharged lead‐acid batteries to their 100% state of charge (SOC) quickly and at the same time can avoid the associated problems of the excessive gassing phenomenon at overcharge. The proposed algorithm can be applied in the PV systems by using a DC‐DC converter, which differs from the traditional on/off regulators in that it cannot only be used to charge the battery and protects it from overcharging, but it can also be used to quickly and safely charge the battery to 100% SOC through better exploitation of the available PV energy. The simulation results verify that, using the proposed algorithm, the discharged battery can always restore its 100% SOC compared with the conventional charging algorithms. Copyright © 2010 John Wiley & Sons, Ltd.     

Fuzzy logic based MPPT controller for high conversion ratio quadratic boost converter

In this study, a maximum power point tracking DC–DC quadratic boost converter for high conversion ratio required applications is proposed. The proposed system consists of a quadratic boost converter with high step-up ratio and fuzzy logic based maximum power point tracking controller. The fuzzy logic based maximum power point tracking algorithm is used to generate the converter reference signal, and the change in PV power and the change in PV voltage are selected as fuzzy variables. Determined membership functions and fuzzy rules which are design to track the maximum power point of the PV system generates the output signal of the fuzzy logic controller's output. It is seen from MATLAB/Simulink simulation and experimental results that the quadratic boost converter provides high step-up function with robustness and stability. In addition, this process is achieved with low duty cycle ratio when compared to the traditional boost converter. Furthermore, simulation and experimental results have validated that the proposed system has fast response, and it is suitable for rapidly changing atmospheric conditions. The steady state maximum power point tracking efficiency of the proposed system is obtained as 99.10%. Besides, the output power oscillation of the converter, which is a major problem of the maximum power point trackers, is also reduced.     

Optimisation strategy for an operational planning of a large photovoltaic system with enhanced electrical vehicles

Photovoltaic (PV) systems are rapidly gaining acceptance as some of the best alternative energy sources. In the installation area of a large PV system, appropriate operation is required, for example, of a method considering power output fluctuation and solar radiation forecast errors. In addition, it is better to minimise the size of the battery and its capital cost. This paper proposes an optimisation approach to determine the operational planning of the power output for a large PV system. This approach includes the method of determination of the charge/discharge amount for the battery of an electric vehicle as a demand response. The method aims to obtain a more beneficial deal with the sale of electrical power. The optimisation method applies the genetic algorithm to smoothen the fluctuating power output due to the PV system, and also to determine the initial state of charge of the battery. The validity of the proposed method is confirmed by the simulation results.