直流模块式光伏发电系统前级DC/DC变换研究

针对传统集中式光伏发电系统结构存在的问题,介绍了直流模块式光伏发电结构,并利用反激变换器作为直流模块式光伏发电系统的前级直流模块,采用变步长的滞环比较法实现光伏电池的最大功率跟踪控制,采用带有输入电压前馈控制的电压闭环控制策略实现直流母线的稳压控制。试验结果表明,该方案可行。     

5 kW DC/DC converter for hydrogen generation from photovoltaic sources

This paper covers the design of a DC–DC power converter aimed for hydrogen production from photovoltaic sources. Power conditioning for such application is usually driven by different constraints: high step-down conversion ratio is required if the input voltage of such equipment has to be compatible with photovoltaic sources that are connected to grid-connected inverters; galvanic isolation; high efficiency and low mass. Taking into account those factors, this work proposes a push–pull DC/DC converter for power levels up to 5 kW. The operation and features of the converter are presented and analyzed. Design guidelines are suggested and experimental validation is also given.     

光伏发电系统的新型DC/DC变换器设计

针对传统全桥DC/DC变换器存在的硬开关损耗和因开关管开关特性不一致使变压器原边磁芯单向饱和问题,设计了一种用于光伏发电系统中的移相ZVS全桥DC/DC变换器,控制电路采用基于芯片UC3875的双环反馈方案,驱动电路由专用驱动集成芯片IR2110S构成.不仅实现了开关管的软开关控制、降低开关损耗、提高系统效率,还可解决偏磁问题,使变压器的磁通密度工作接近于饱和点.实验样机的研制验证了设计的可靠性.     

直流微电网系统的DC/DC变换器控制系统设计

全桥DC/DC变换器由于具有高功率密度、高效率、高变压比及电气隔离的特点,成为直流微电网系统中重要的电力电子接口。而移相全桥DC/DC变换器具有高阶时变非线性的特点,具体应用时较难建立其精确的数学模型,这影响了传统PID控制性能,因此设计了模糊自整定控制器。通过对峰值电流模式下变换器的小信号建模,在传统PI控制的基础上,给出了基于模糊理论的PI参数在线自整定方法。同时,利用Matlab仿真工具给出了模糊控制器的设计方法,并通过DSP对其进行软硬件实现。仿真和试验结果表明,模糊自整定控制与常规PI控制相比,提高了系统的抗扰动能力,改善了系统的动态性能,从而提高了整个微电网系统的可靠性。     

DC/DC converter topologies for electrolyzers: State-of-the-art and remaining key issues

In recent years, the use of electrolyzers to produce cleanly and efficiently hydrogen from renewable energy sources (i.e. wind turbines, photovoltaic) has taken advantage of a growing interest from researchers and industrial. Similarly to fuel cells, DC/DC converters are needed to interface the DC bus with the electrolyzer. Usually, electrolyzers require a low DC voltage to produce hydrogen from water. For this reason, a DC/DC buck converter is generally used for this purpose. However, other DC/DC converter topologies can be used depending on the feature of the electrolyzer and electrical grid as well. The main purpose of this paper is to present the current state-of-the-art of DC/DC converter topologies which can be combined with electrolyzers. The different DC/DC converter topologies are compared in terms of output current ripple reduction, conversion ratio, energy efficiency, and power switch fault-tolerance. Besides, remarks on the state-of-the-art and remaining key issues regarding DC/DC converters are provided.     

Integrated modeling and control of a PEM fuel cell power system with a PWM DC/DC converter

A fuel cell powered system is regarded as a high current and low voltage source. To boost the output voltage of a fuel cell, a DC/DC converter is employed. Since these two systems show different dynamics, they need to be coordinated to meet the demand of a load. This paper proposes models for the two systems with associated controls, which take into account a PEM fuel cell stack with air supply and thermal systems, and a PWM DC/DC converter. The integrated simulation facilitates optimization of the power control strategy, and analyses of interrelated effects between the electric load and the temperature of cell components. In addition, the results show that the proposed power control can coordinate the two sources with improved dynamics and efficiency at a given dynamic load.     

中频隔离型光伏±35kV/500kW直流变流器研究与工程应用

首先提出一种中频隔离型光伏±35 kV/500 kW直流变流器,该直流变流器由三相T型三电平并联中频逆变器模组、中频400 Hz/24脉波移相升压变压器、三相二极管整流桥以及高压滤波电路组成,该变流器具有升压比高、结构简单、成本低的优势;其次,为了实现逆变器并联模组有功、无功功率的实时均分,采用一种瞬时功率均分的功率主从控制方案;接着,提出一种基于中频变压器无功补偿的改进型LCL滤波器参数设计方法以提高变流器的效率。最后,通过Matlab/Simulink仿真以及张北国家大型风电并网系统研发（实验）中心建设的光伏发电中压直流汇集现场示范工程,对所提直流变流器的设计理念和控制方法进行验证,现场并网实测最大效率为96.33%,升压比高达88,输出电压±35 kV,仿真和实验结果证明了所提直流变流器的可行性。     

Power switch failures tolerance and remedial strategies of a 4-leg floating interleaved DC/DC boost converter for photovoltaic/fuel cell applications

In recent years, many researchers have proposed new DC/DC converters in order to meet the fuel cell requirements. The reliability of these DC/DC converters is crucial in order to guarantee the availability of fuel cell systems. In these converters, power switches ranked the most fragile components. In order to enhance the reliability of DC/DC converters, fuel cell systems have to include fault-tolerant topologies. Usually, dynamic redundancy is employed to make a fault-tolerant converter. Despite this kind of converter allows ensuring a continuity of service in case of faults, the use of dynamic redundancy gets back to increase the complexity of the converter. In order to cope with reliability expectations in DC/DC converters, floating interleaved boost converters seem to be the best solution. Indeed, they have much to offer for fuel cells and DC renewable energy sources (i.e. photovoltaic system), including reduced input current ripple and reliability in case of faults. Despite the offered benefits of this topology, operating degraded modes lead up to undesirable effects such as electrical overstress on components and input current ripple increasing. The aim of this paper is to carry out a thorough analysis of these undesirable effects and to propose remedial strategies to minimize them.     

A maximum power point tracking control strategy with variable weather parameters for photovoltaic systems with DC bus

In photovoltaic (PV) system, the most commonly used DC/DC converter is the basic buck or boost circuit to implement the maximum point power tracking (MPPT) due to their simple structure and low cost while there are some MPPT constraint conditions. By contrast, the conventional buck/boost DC/DC converter without MPPT constraint condition is seldom used because of its high cost or poor performance. To keep the advantages of these three DC/DC converters while overcoming their shortcomings, in this paper, the constraint conditions of capturing the maximum power point (MPP) of PV systems with direct-current (DC) bus are found out. Then, on the basis of this work, a MPPT control strategy with variable weather parameters is proposed. In this strategy, a new buck/boost DC/DC converter is proposed, which not only avoids the MPPT constraint conditions of basic buck or boost DC/DC converter but also overcomes the shortcomings of conventional buck/boost DC/DC converter. Finally, lots of simulated experiments verify the accuracy of MPPT constraint conditions, test the feasibility and availability of proposed MPPT control strategy, analyze the MPPT performance of proposed PV system and compare the output transient-state performance with conventional perturb and observe (P&O) method.     

Supercapacitors for power control in a grid-connected photovoltaic system

并网光伏发电系统输出功率的波动性和随机性给并网后系统稳定性,光伏发电消纳以及光伏电站电能质量等方面带来了负面影响,制约了光伏发电的发展.针对这一问题,将超级电容器作为功率调节装置,控制光伏并网系统按指定值平滑,准确地输出功率,使光伏发电具有可调度性.在分析了超级电容特性,系统构成和双向DC/DC变换器状态空间平均小信号模型的基础上,提出功率,电流双闭环反馈滞环电流控制策略,控制超级电容器吸收或补充输出功率的波动成分.在PSCAD/EMTDC 电力系统仿真软件中构建仿真模型,对提出的系统和控制策略进行了仿真分析,良好的仿真结果验证了方法的可行性.     

基于输出端谐振解耦的三端口直流变换器

针对三端口变换器的功率耦合问题,提出基于输出端谐振解耦的三端口变换器,通过在输出端全桥构造谐振腔来解除控制量对光伏端口和储能端口的功率耦合作用,将三端口变换器等效为2个输出并联的二端口变换器,并直接沿用二端口变换器系统的控制策略,使控制过程更加简单平滑,降低三端口变换器的控制环节设计难度。首先介绍变换器的拓扑结构,并分析电路的功率传输特性和变换器的控制稳定性;搭建输出功率500 W的实验样机,对拓扑结构和控制策略的有效性进行验证。     

一种适用于BESS的交错并联双向DC/DC变换器

针对现有电池储能系统(BESS)双向DC/DC变换器(BDC)电压增益低和开关器件电压应力高等特点,提出一种适用于BESS的两相交错并联BDC。该储能系统(ESS)能有效结合Z源网络和交错并联结构的优势特性。详细分析了该ESS的工作原理、Boost和Buck模式,并推导出2种工作模式下的电压变化比。同时对该ESS两相交错并联BDC的带逻辑判断单元的载波移相控制策略进行了详细介绍。在Matlab/Simulink中搭建仿真实验模型,验证了该ESS各工作模式下的主要工作波形。仿真实验结果表明该系统具有电压增益高、开关器件电压应力低和各相电感之间能实现自动均流等优点。     

一种适用于BESS的交错并联双向DC/DC变换器

针对现有电池储能系统(BESS)双向DC/DC变换器(BDC)电压增益低和开关器件电压应力高等特点,提出一种适用于BESS的两相交错并联BDC。该储能系统(ESS)能有效结合Z源网络和交错并联结构的优势特性。详细分析了该ESS的工作原理、Boost和Buck模式,并推导出2种工作模式下的电压变化比。同时对该ESS两相交错并联BDC的带逻辑判断单元的载波移相控制策略进行了详细介绍。在Matlab/Simulink中搭建仿真实验模型,验证了该ESS各工作模式下的主要工作波形。仿真实验结果表明该系统具有电压增益高、开关器件电压应力低和各相电感之间能实现自动均流等优点。     

Non-isolated multiphase boost converter for a fuel cell with battery backup power system

This work describes a step-up non-isolated DC/DC converter aimed for fuel cell stand-alone power systems. The proposed converter has the following features: simple structure based on the basic boost topology that reduces the number of components; it uses the interleaving technique in order to reduce the current ripple at the input and output sides, reduction of the inductors size, higher frequency that reduce the output filter capacitor and easier power losses management. In addition, the use of an inner current control loop in the input side assures power sharing and easy module parallelization. The converter feeds a backup battery that maintains a DC voltage level at the main bus. An outer battery-charging loop controls the converter. Experimental validation is given for a four-phases 1 kW prototype at 100 kHz PWM switching frequency connected to a Nexa Ballard (1.2 kW-46 A) PEM fuel cell.     

Distributed generation from renewable sources in an isolated DC network

This article describes a direct current (DC) isolated network that is infed with distributed generation from renewable sources and cogeneration units. The sources are connected to the network via DC/DC converters to keep the voltage within a defined range and to ensure the required power flow. The consumption is directly connected to the DC network, without any DC/DC converter. The storage is located at a single point in the network. A simulation analysis based on a DC-network model shows that it is possible to operate a DC network with standard elements used for the generation side as well as for the consumption side. The key elements are the DC/DC converters, which control the voltage of the network and optimize the operation of the sources.     

Implementation of photovoltaic water pumping system with MPPT controls

To increase the output efficiency of a photovoltaic (PV) system, it is important to apply an efficient maximum power point tracking (MPPT) technique. This paper describes the analysis, the design and the experimental implementation of the tracking methods for a stand-alone PV system, using two approaches. The first one is the constant voltage (CV) MPPT method based on the optimum voltage, which was deduced experimentally, and considered as a reference value to extract the optimum power. The second one is the increment conductance (Inc-Cond) MPPT method based on the calculation of the power derivative extracted by the installation. The output controller can adjust the duty ratio to the optimum value. This optimum duty ratio is the input of a DC/DC boost converter which feeds a set of Moto-pump via a DC/AC inverter. This paper presents the details of the two approaches implemented, based on the system performance characteristics. Contributions are made in several aspects of the system, including converter design, system simulation, controller programming, and experimental setup. The MPPT control algorithms implemented extract the maximum power point (MPP), with satisfactory performance and without steady-state oscillation. MATLAB/Simulink and dSpace DS1104 are used to conduct studies and implement algorithms. The two proposed methods have been validated by implementing the performance of the PV pumping systems installed on the roof of the research laboratory in INSAT Tunisia. Experimental results verify the feasibility and the improved functionality of the system.     

适用于直流电网的推挽式直流自耦变压器及其控制策略

基于双有源桥拓扑结构的能量传输机理,通过融合直流自耦变压器与模块化多电平换流器的拓扑演绎规律,提出了具有高传输效率和高运行可靠性的推挽式直流自耦变压器.分析了推挽式直流自耦变压器的工作机理、控制策略和系统参数对运行特性的影响.通过采用特定拓扑结构的四绕组变压器并结合推挽式工作模式,推挽式直流自耦变压器内部的特定四绕组变...     

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.     

Modeling and testing of two-stage grid-connected photovoltaic micro-inverters

In this paper, the characteristics, design and control parameters of a 200 W micro-inverter, consisting of two conversion stages are presented; the first one is implemented by a push-pull converter, which provides galvanic insulation and adjusts the DC voltage from the photovoltaic panel to an appropriate voltage with the implementation of a current injected control. The second stage corresponds to a full bridge inverter SPWM with an average current control, which injects energy from the push-pull converter to the grid; it is synchronized with the grid and delivers the maximum power provided by the photovoltaic panel. Power is extracted through the Maximum Power Point Tracking Technique (MPPT). The micro-inverter is simulated and its behavior to irradiance variations is observed. Finally, the transient and stable responses of the implemented micro-inverter are presented. Stable and slightly underdamped output signals (voltage and current) are obtained under current panel variations.     

High voltage step-up integrated double Boost–Sepic DC–DC converter for fuel-cell and photovoltaic applications

In this paper, an integrated double boost SEPIC (IDBS) converter is proposed as a high step-up converter. The proposed converter utilizes a single controlled power switch and two inductors and is able to provide high voltage gain without extreme switch duty-cycle. The two inductors can be coupled into one core for reducing the input current ripple without affecting the basic DC characteristic of the converter. Moreover, the voltage stresses across all the semiconductors are less than half of the output voltage. The reduced voltage stress across the power switch enables the use of a lower voltage and R_DS-ON MOSFET switch, which will further reduce the conduction losses. Whereas, the low voltage stress across the diodes allows the use of Schottky rectifiers for alleviating the reverse-recovery current problem, leading to a further reduction in the switching and conduction losses. A detailed circuit analysis is performed to derive the design equations. A design example for a 100-W/240 V_dc with 24 V_dc input voltage is provided. The feasibility of the converter is confirmed with results obtained from simulation and an experimental prototype.