A highly efficient PV system using a series connection of DC–DC converter output with a photovoltaic panel

A photovoltaic (PV) power conditioning system (PCS) must have high conversion efficiency and low cost. Generally, a PV PCS uses either a single string converter or a multilevel module integrated converter (MIC). Each of these approaches has both advantages and disadvantages. For a high conversion efficiency and low cost PV module, a series connection of a module integrated DC–DC converter output with a photovoltaic panel was proposed. The output voltage of the PV panel is connected to the output capacitor of the fly-back converter. Thus, the converter output voltage is added to the output voltage of the PV panel. The isolated DC–DC converter generates only the difference voltage between the PV panel voltage and the required total output voltage. This method reduces the power level of the DC–DC converter and enhances energy conversion efficiency compared with a conventional DC–DC converter.     

分布式光伏发电系统改进虚拟同步发电机控制

提出一种计及分布式光伏发电系统源端输出功率波动特征的改进虚拟同步发电机(IVSG)控制策略。对单台虚拟同步发电机功率平衡方程特征值进行分析,明确了光伏电源的基本运行特性,确定了光伏电源稳定运行区域。在传统虚拟同步发电机(VSG)的基础之上进一步采用了直流电压稳定控制技术,提出改进的虚拟同步发电机控制策略。当光伏电源输出功率低于负载需求时起到抑制直流母线电压跌落、维持直流电压稳定的作用,实现按照负荷或并网功率需求进行功率匹配的目的。仿真与实验结果验证了所提控制策略的可行性与有效性。     

Input output feedback linearization control and variable step size MPPT algorithm of a grid-connected photovoltaic inverter

In this paper, the power factor of a grid-connected photovoltaic inverter is controlled using the input output Feedback Linearization Control (FLC) technique. This technique transforms the nonlinear state model of the inverter in the d–q reference frame into two equivalent linear subsystems, and then applies a pole placement linear control loops on this subsystem in order to separately control the grid power factor and the dc link voltage of the inverter. Maximum Power Point Tracker (MPPT) that allows extraction of maximum available power from the photovoltaic (PV) array has been included. This MPPT is based on variable step size incremental conductance method. Compared with conventional fixed step size method, the variable step MPPT improves the speed and the accuracy of the tracking.     

基于滑模变功率跟踪的PV-VSG控制技术

针对含储能装置的传统光伏虚拟同步发电机（PV-VSG）不仅投资成本高且未考虑光伏阵列输出特性的问题,提出一种基于滑模变功率点跟踪（SM-VPT）的PV-VSG控制策略。该方法在滑模控制的基础上引入直流母线电压偏差控制,调整光伏阵列的功率跟踪轨迹,实现光伏出力自适应匹配负载需求,即当光伏容量充足时,只提供与负载相匹配的功率;光伏功率不足时,可实现传统的MPPT控制以减少电力短缺,同时防止直流电压骤降,保证系统稳定运行。该方法使PV-VSG能够按需向负载供电,无需增加额外的储能设备,可实现光伏发电系统直接以VSG形式接入并网,仿真结果验证了所提控制方法的有效性。     

光伏水泵提水系统功率补偿控制策略研究

分析系统等效模型,提出一种采用市电补偿的供电方案和基于直流母线电压的功率补偿控制策略,通过检测直流母线电压判断光伏阵列出力与负载功率之间的关系,为系统不同供电模式的切换提供依据,保证系统供电功率的稳定。经仿真和实验验证控制策略的有效性。     

Management of excess energy in a photovoltaic/grid system by production of clean hydrogen

In this article, the authors design a new clean storage device for a photovoltaic system (PV) reinforced by the electrical grid. The photovoltaic system supplies power to a DC load. When the power of the photovoltaic source is insufficient, the electrical grid compensates the energy deficit. On the other hand, if the load is satisfied and the PV source is still able of supplying energy, the energy excess is diverted to an own storage unit materialized by an electrolysis which produces gaseous hydrogen by the process of electrolysis of water. The authors show that the quantity of hydrogen produced is proportional to the photovoltaic energy excess and also to the flow of water injected into the electrolysis. In this case, it is a question of designing an electrolysis with specific characteristics, which takes into account the quantity of energy excess and the flow of water injected into it. The authors abandon the idea of controlling the water flow by means of a pumping-electrovalve system, and initiate the idea of replacing the function of the pump by the action of gravity. The work focuses on the development of an electrolysis optimization approach using the water flow control in its alliance with the PV power excess which is also maximized. For an optimized use of the global system (load and electrolysis), the authors present an architecture based on energy-converting structures (DC/DC and AC/DC). In addition, to increase the reliability and safety of the system, the authors finish by developing a power management strategy (PMS) in the designed system. This power management strategy organizes the energy flow and selects the appropriate path of this flow between the two energy sources (PV and electrical grid) and the two possible energy receivers (load and electrolysis). A complete modeling of the system is developed in the Matlab/Simulink environment. The simulation results show that the hybrid system (PV and electrical grid) is able to permanently supplying the load and potentially storing the excess of the PV energy in the form of hydrogen gas.     

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.     

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.     

The coupling factor: A new metric for determining and controlling the efficiency of solar photovoltaic power utilization

The production of electricity and hydrogen in a renewable fashion, such as using solar energy, can provide a clean and sustainable energy source for electric-powered vehicles, including fuel-cell and battery-electric vehicles. Our research on generating hydrogen and charging batteries using renewable solar photovoltaic (PV) electricity has led to the development of a simple and convenient new metric called the coupling factor that describes the fraction of the maximum PV power transferred to electrical loads. The keystone of the coupling factor concept is a regression model to calculate the maximum PV voltage, current, and power as a function of the instantaneous incident solar irradiance and the photovoltaic module temperature. The coupling factor can range from zero to one, i.e., no transfer of power from the PV system to the load, to complete transfer of the PV power. We describe the derivation of regression models to compute important PV electrical output variables, such as the open circuit voltage, the short circuit current, the maximum power point voltage, the maximum power point current, and the coupling factor as a function of the fundamental measured variables affecting those quantities. The models are derived for PV modules used in our previous research to power an electrolyzer and charge high-voltage batteries. In addition, we develop models for other modules using PV cell technologies different from those used in our PV system. Some of the calculated quantities are compared to measurements for our PV system. The usefulness of these quantities, and especially the coupling factor, in rating the transfer of PV power to electrolyzer and battery loads, is illustrated. Finally, we discuss how the predicted maximum power point voltage can be used for real-time control and efficiency optimization of a dynamic PV-load system.     

一种基于三端口能量路由的单级式光储并网系统及其高压穿越技术

该文提出一种具有高压穿越能力的基于三端口能量路由的单级式光储并网系统（PV-ESS）。光伏最大功率跟踪始终由三相变换器控制完成。多端口能量路由器可根据不同运行工况使储能灵活接入,平抑光伏输出,使得系统具有更高的效率。在电网高压故障下,通过储能装置抬升直流母线电压,从而使系统具有高压穿越的能力,可在电网高压状态下不脱网连续运行,直至故障恢复。     

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.     

一种基于超级电容器的光伏系统电压控制方法研究

根据超级电容器储能特点,建立了用于独立光伏发电的超级电容器储能系统模型,设计了超级电容器电压控制环节,提出了一种带有超级电容器电压模糊反馈的双环控制策略,通过对开环系统bode图的分析,论证了控制系统的稳定性和动态性能。仿真结果表明,在光伏发电系统受到光照及负载扰动的情况下,超级电容器储能系统可以有效地稳定光伏系统的输出电压,提高系统供电的可靠性和电能质量,并且抑制了超级电容器端电压大范围波动对该系统的影响,提高了超级电容器的利用率。     

Power quality improvement of solar photovoltaic transformer-less grid-connected system with maximum power point tracking control

This paper presents a transformer-less single-stage grid-connected solar photovoltaic (PV) system with active reactive power control. In the absence of active input power, grid-tied voltage source converter (VSC) is operated in the reactive power generation mode, which powers control circuitry and maintains regulated DC voltage. Control scheme has been implemented so that the grid-connected converter continuously serves local load. A data-based maximum power point tracking (MPPT) has been implemented at maximum power which performs power quality control by reducing total harmonic distortion (THD) in grid-injected current under varying environmental conditions. Standards (IEEE-519/1547) stipulates that current with THD greater than 5% cannot be injected into the grid by any distributed generation (DG) source. MPPT tracks actual variable DC link voltage while deriving maximum power from PV array and maintains DC link voltage constant by changing the converter modulation index. Simulation results with the PV model and MPPT technique validations demonstrate effectiveness of the proposed system.     

PV powered smart charging station for PHEVs

A huge inrush of PHEVs is envisioned in the future. There is a growing risk that, this proliferation in the number of PHEVs will trigger extreme surges in demand while charging them during rush hours. To mitigate this impact, a smart charging station is proposed in which the charging of the PHEVs is controlled in such a way that the impact of charging during peak load period is not felt on the grid. The power needed to charge the plug in hybrids comes from grid-connected photovoltaic generation or the utility or both. The three way interaction between the PV, PHEVs and the grid ensures optimal usage of available power, charging time and grid stability. The system designed to achieve the desired objective consists of a photovoltaic system, DC/DC boost converter, DC/AC bi-directional converter and DC/DC buck converter. The output of DC/DC boost converter and input of DC/AC bi-directional converter share a common DC link. A unique control strategy based on DC link voltage sensing is proposed for the above system for efficient transfer of energy.     

Simulation studies of islanded behavior of grid-connectedphotovoltaic systems

Simplified computer models are presented for predicting the run-on time of self-commutated inverters operating in a utility interactive mode. The inverter receives DC power from a photovoltaic (PV) array and delivers AC power to a local load or the utility lines. The models represent the dynamics of a phase-locked loop control circuit that is designed to destabilize the inverter operation and shut down the power-conditioning subsystem when a phase discrepancy between the line and some reference signal is detected. The sustained isolated operation (or islanding) of the PV system poses a possible safety concern to utility personnel and potential damage to utility-connected equipment. Two computer models, the TESLACO model and the APCC model, were implemented on an IBM PC using Pascal. They provide results compatible with experimental evidence and more elaborate computer modeling techniques     

光伏阵列变阵调整输出算法的研究

针对多子系统的光伏发电大系统,提出了光伏阵列变阵思想,并给出了光伏变阵调整输出算法及控制器的设计。当光照强度较好时,各子系统独立的正常工作。随着光照强度降低,变阵控制器根据系统所需求的负载要求,改变大系统的结构,对各子系统进行系统重组。重组后系统将部分子系统的能量资源补充给其他子系统,使部分子系统的能量满足负载的要求,进而延长了发电系统的工作时间。通过一个仿真例子说明了给出方法的有效性及可行性。     

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。     

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.     

Power conditioning system for grid-connected photovoltaic system

This study develops a new control strategy for an expandable grid-connected photovoltaic (PV) system. The proposed system performs dual functions of a PV generation and an active power filter, or either one. To this end, the grid current, instead of output current of the converter, is shaped to be a sinusoidal current in phase (or opposite phase) with the grid voltage. Furthermore, its reference current is generated in multiplying the voltage loop controller output by the grid voltage waveform, and therefore non-active components analysis for harmonics elimination and reactive power compensation in the conventional design is not necessary. As a result, the algorithm is simple and easy to implement, and only one sensor for current detection plus two voltage sensors are required. Experimental results verified the effectiveness of the proposed method.