Modeling and analysis of current harmonic distortion from grid connected PV inverters under different operating conditions

Due to the fast growth of photovoltaic (PV) installations, concerns are rising about the harmonic distortion generated from PV inverters. High current total harmonic distortion (THD) occurs when PV inverters operate under light load conditions due to low solar insolation. A general model modified from the conventional control structure diagram is introduced to analyze the harmonic formation process. Causes of the current harmonics are summarized and its relationship with output power levels is analyzed. The field measurement for current harmonics is carried out at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) microgrid to validate the analysis results. In particular for two-stage inverter, unlike previous papers that assume the DC-link voltage is constant, the DC-link voltage ripple is identified as the source of a series of odd harmonics. A mew periodic time-varying model is proposed by including the DC-link voltage ripple into the conventional current control loop model. This model is able to simulate the characteristics of the harmonic components and show their dependence on the DC-link voltage ripple.     

基于双重约束的光伏逆变器直接前馈阻尼控制研究

针对高比例光伏发电系统网侧变弱潜在的高频谐振问题,提出一种基于输出电流直接前馈的新型阻尼控制方案,并对环路谐振抑制参数整定的全局-局部运行双重约束进行研究。首先,建立光伏多机并网系统数学模型,利用阻抗比判断潜在的高频谐振。其次,对逆变器串电阻环路变换构建等效的输出电流直接前馈控制环路,通过分析不同谐振抑制参数下系统全局高频谐振治理效果及单台逆变器局部运行状态,以全局-局部稳定为双重约束确定了谐振抑制参数可行域,并分析电气参数随运行状态变化下所提方案的抗扰能力。最后,搭建光伏多机并网系统的仿真实验平台验证上述研究的有效性。     

Harmonic distortion assessment for a single-phase grid-connected photovoltaic system

Nowadays, grid-connected photovoltaic (PV) systems constitute an emerging technology. This has given rise to concerns about their contribution to harmonic distortion levels in utility grids. Valuable insight into the electrical behavior of such a system, including its impact on power quality, can be obtained through extensive simulation studies. In this paper, models are developed with a focus on not only accurate but also fast simulation of grid-connected PV systems. First, time-consuming processes are identified and discussed. These are mainly associated with the semiconductor power switching devices and the non-linear characteristics of the system components (isolation transformer, PV generator). Next, models are developed in order to increase simulation speed by avoiding time-consuming procedures. The validity of the approach is ascertained by comparing simulation results with published measurements. A case study is then performed in order to obtain current and voltage waveforms and, subsequently, harmonic distortion levels. Further simulations are carried out using different values for various system parameters in order to make an assessment of their impact in terms of waveform distortion.     

Grid-connected photovoltaic power systems: Technical and potential problems—A review

Traditional electric power systems are designed in large part to utilize large baseload power plants, with limited ability to rapidly ramp output or reduce output below a certain level. The increase in demand variability created by intermittent sources such as photovoltaic (PV) presents new challenges to increase system flexibility. This paper aims to investigate and emphasize the importance of the grid-connected PV system regarding the intermittent nature of renewable generation, and the characterization of PV generation with regard to grid code compliance. The investigation was conducted to critically review the literature on expected potential problems associated with high penetration levels and islanding prevention methods of grid tied PV. According to the survey, PV grid connection inverters have fairly good performance. They have high conversion efficiency and power factor exceeding 90% for wide operating range, while maintaining current harmonics THD less than 5%. Numerous large-scale projects are currently being commissioned, with more planned for the near future. Prices of both PV and balance of system components (BOS) are decreasing which will lead to further increase in use. The technical requirements from the utility power system side need to be satisfied to ensure the safety of the PV installer and the reliability of the utility grid. Identifying the technical requirements for grid interconnection and solving the interconnect problems such as islanding detection, harmonic distortion requirements and electromagnetic interference are therefore very important issues for widespread application of PV systems. The control circuit also provides sufficient control and protection functions like maximum power tracking, inverter current control and power factor control. Reliability, life span and maintenance needs should be certified through the long-term operation of PV system. Further reduction of cost, size and weight is required for more utilization of PV systems. Using PV inverters with a variable power factor at high penetration levels may increase the number of balanced conditions and subsequently increase the probability of islanding. It is strongly recommended that PV inverters should be operated at unity power factor.     

考虑传输线分布电容影响的光伏并网系统谐波谐振抑制策略

以传输线联网的光伏逆变器为研究对象,当传输线中分布电容较大不能忽略时,考虑传输线分布电容的影响下,分析系统的谐振特性以及电网背景谐波在传输线中的传播特性。针对系统的谐波谐振情况,提出基于电容电压的有源阻尼策略有效抑制系统出现的多个谐振尖峰,同时抑制电网中5次背景谐波的传播放大。为更好地抑制7次背景谐波,在上述控制算法的基础上于电容电压反馈通道中引入7次谐振控制器重塑逆变器输出的7次谐波阻抗。最后,在所提方案下,对逆变器输出5次谐波阻抗进行约束来优化控制参数,有效减小背景谐波导致的入网电流畸变。仿真验证所提方案的正确性和有效性。     

Analysis of the maximum power point tracking in the photovoltaic grid inverters of 5 kW

The aim of this article is to describe how closely PV grid-connected inverters (of around 5 kW) operate at the actual maximum power point. These inverters could be installed at any low voltage, PV grid-connected systems. To carry this study out, twelve 50 Hz single-phase inverters were selected from the European market. Each one of them was put into an outdoor grid-connected system installed in Spain. PV power generation with respect to irradiance, ambient temperature and local time was measured under different meteorological conditions. DC voltage and maximum power point tracking efficiency were analyzed. From the results obtained it has been possible to see that the MPPT algorithms used in some inverters do not bring the optimum utilisation of the PV array.     

基于PIR的并网逆变器低频次谐波抑制策略研究

针对电网电压畸变造成并网电流低频次谐波含量较高的问题,提出一种基于比例积分-准谐振（PIR）与电流谐波检测环相结合和控制策略抑制电流谐波。首先分析并网逆变器原理,建立电流内环数学模型。基于内模原理,引入电网电压全前馈消除电网电压对网侧电流和影响,其次分析电流谐波检测与抑制原理,采用电流特定次谐波检测环对逆变器侧电流提取谐波分量,并采用闭环控制抑制电流谐波,利用伯德图对准谐振控制器参数进行设定。最后建立Matlab/Simulink仿真模型并搭建dSPACE-DS1104半实物仿真平台,分析仿真与实验结果来验证所提控制策略和有效性与可行性。     

Advanced control of grid-connected inverters for proton exchange membrane fuel cell system

Voltage support is one of the most important issues for operating grid-connected inverters under grid faults. Many control strategies have been addressed in literature, but most of them focus on the voltage support control without considering issues of power quality due to voltage harmonics. In order to sort out the drawback, a novel advanced control strategy is presented in this paper. It is able to provide the voltage support function. Meanwhile, the power quality is improved by regulating the control coefficients. The time-domain simulation and experimental results are provided to evaluate the conventional and proposed control strategies. The results show that the total harmonic distortion of PCC voltage can be reduced from 9.35% to 2.88% with the proposed solution. Meanwhile, the voltage support is also achieved, which verify the effectiveness of the proposed solution.     

光伏准Z源H桥级联多电平逆变器并网调制技术

该文针对光伏准Z源逆变器的运行特点,基于逆变器两态滞环电流控制工作原理,提出一种能使逆变器直通实现光伏MPPT的三态滞环控制器和单相准Z源级联逆变器三态滞环控制系统,分析三态滞环控制器的工作原理,并用三单元准Z源级联逆变器系统仿真,对比研究三态滞环控制、载波层叠法和载波移相法的控制效果。仿真结果表明,三态滞环控制调制策略表现出更好的稳态和动态响应性能,其并网电流谐波畸变率更小。     

电网频率波动时LCL并网逆变器重复控制研究

针对电网频率波动时,并网电流畸变和谐波较大的问题,提出了静止坐标系下LCL并网逆变器的重复控制策略,该策略可有效降低并网电流谐波畸变率;在静止坐标系下,LCL并网逆变器数学模型无耦合现象,省去了复杂的解耦运算,从而该控制策略简单易实现;在确定电路参数的基础上,对重复控制进行了参数设计,并通过仿真试验证明了理论分析的正确性。研究成果对于提高电能质量意义重大。     

弱电网中LLCL型并网逆变器的谐振分析与抑制策略研究

针对弱电网中多个LLCL型并网逆变器并联运行的谐振问题,提出一种混合阻尼控制策略。在建立多LLCL型并网逆变器的数学模型基础上,分析多机并联谐振的产生机理,得出电网阻抗和LLCL滤波器参数变化是产生谐振的关键因素,将影响系统并网运行的稳定性。为此,在不改变LLCL型并网逆变器拓扑结构的前提下,将所提控制策略应用到弱电网多机并联系统中。仿真与实验结果表明,该控制策略能有效地抑制系统谐振,降低并网电流和电压的谐波含量,提高在弱电网条件下多机并联运行的稳定性。     

三电平并网逆变器基于有限集模型预测控制的新型谐波抑制策略

有限集模型预测控制应用于大功率并网逆变器能在控制并网电流的同时降低开关频率,然而随着开关频率的降低,并网电流的谐波显著变大.针对这一问题,提出一种带有新型谐波抑制策略的改进型有限集模型预测控制方法,谐波抑制这一目标被添加到有限集模型预测控制的多目标优化过程中,谐波抑制策略是基于三角函数正交性对谐波幅值进行提取来实现的....     

Interline fuel cell (I-FC) system with dual-functional control capability

In this paper, a new system concept is presented for the grid connection of fuel cells. In conventional grid-connected systems, fuel cells ensure the generated power into a single electrical feeder and control the electrical-line through interfacing elements. In the proposed system, interline fuel cell (I-FC) system shares a common dc-dc converter tied fuel cell at the base of inverters and eliminates the additional fuel cell & dc-dc converter in a multi-feeder system. For this purpose, a fuel cell system is connected to multi-feeders through separate inverters, thereby sharing electrical power into the feeders and attenuating the harmonics at grid-side currents. In this direction, the proposed system presents an economical way for the mitigation of electrical problems for multi-feeders. In order to achieve the functional capabilities of I-FC system, dual-functional control is separately applied in the grid inverters. In the testing stage of I-FC, nonlinear loads in feeder I & feeder II create 31.29% and 27.61% total harmonic distortion (THD) at grid-side currents, respectively. With I-FC, the THD values are reduced to approximately 3% values in both feeders after the harmonic elimination capability. Also, I-FC allocates the active power to both feeders, and reduces the electrical power demand from the utility-grids. The evaluation results verify that I-FC system accomplishes the good performance to control power-sharing and attenuate the current harmonics at grid-side.     

不平衡电网电压下T型三电平光伏并网逆变器的有限集模型预测控制

为实现电网电压不平衡时对T型三电平光伏并网系统输出功率和电流质量的控制,以达到入网功率平稳或电流正弦为控制目标,结合光伏阵列输出功率前馈,在两相静止坐标系下提出一种直流母线电压外环PI控制、并网电流内环有限集模型预测控制的控制策略,并在电压外环中引入2倍频陷波器以获得平滑的入网功率参考值。仿真结果表明:当电网电压不对称时,采用所提控制策略能够实现对入网有功、无功功率2倍频脉动及负序电流的分别抑制或协调控制,且并网电流谐波畸变小、入网电能质量高,同时实现T型三电平逆变器的中点电位平衡。     

Grid-connected building-integrated photovoltaics: a Hong Kong case study

The first building-integrated photovoltaic system (BIPV) in Hong Kong has been working successfully for three years, as remote system for the first year and grid-connected system in the last two years. A number of issues have been investigated on the experimental system including technical, economical, operation and management topics. This paper presents the findings from this research project funded by the Industrial Support Fund of the Hong Kong SAR Government. Simulation and data monitoring have been completed for energy performance of the BIPV system under Hong Kong weather conditions. The natural ventilation effect of an air gap on PV module’s power output and heat transfer across the PV wall and PV-roof have been investigated. Good agreement between simulation and experimental results was achieved. The system can provide about 41% of electric power for an indoor lighting floor area circuit of 250 m². The harmonics of the power output from the PV system was also measured to check the interference level to the utility grid. Experiments show that the total harmonics current distortion of the grid-connected BIPV system is far lower than that from some conventional equipment, such as personal computers. The total harmonics from this BIPV system is less than 12% for most of the time, even when the incident solar irradiation is very weak.     

基于双序前馈解耦的光伏逆变器并网控制策略

为了消除三相不平衡LCL型光伏并网逆变器在同步旋转坐标下耦合项对电流控制性能的影响,该文提出适用于逆变侧电流反馈的双序前馈解耦的方法。通过双旋转剔除耦合项中负序直流量,使用正序直流量进行前馈解耦,实现电网电压和变换耦合项对控制环路的零干扰。在PI控制的基础上加入重复控制,抑制死区效应,降低并网电流谐波含量。通过仿真和实验,验证所提控制策略的有效性。     

Analysis of the Double Output Induction Generator Using Direct Three-phase Model Part II - Harmonic Analysis

This paper investigates the harmonic content of the rotor current, rotor voltage and the torque of the variable speed constant frequency double output induction generator (VSCF-DOIG). The waveforms of these variables were generated using a direct three-phase model and then analyzed numerically for their harmonic content using a Fourier series subroutine. In addition, this paper investigates the stator distortion currents that are being induced in the stator windings by the rotor current harmonics at frequencies which are not an integral multiple of the stator fundamental frequency. As a check on the analysis the calculated stator current waveform is compared with the experimental oscillogram.     

A Single-Stage Single-Phase Transformer-Less Doubly Grounded Grid-Connected PV Interface

A transformer provides galvanic isolation and grounding of the photovoltaic (PV) array in a PV-fed grid-connected inverter. Inclusion of the transformer, however, may increase the cost and/or bulk of the system. To overcome this drawback, a single-phase, single-stage [no extra converter for voltage boost or maximum power point tracking (MPPT)], doubly grounded, transformer-less PV interface, based on the buck–boost principle, is presented. The configuration is compact and uses lesser components. Only one (undivided) PV source and one buck–boost inductor are used and shared between the two half cycles, which prevents asymmetrical operation and parameter mismatch problems. Total harmonic distortion and dc component of the current supplied to the grid is low, compared to existing topologies and conform to standards like IEEE 1547. A brief review of the existing, transformer-less, grid-connected inverter topologies is also included. It is demonstrated that, as compared to the split PV source topology, the proposed configuration is more effective in MPPT and array utilization. Design and analysis of the inverter in discontinuous conduction mode is carried out. Simulation and experimental results are presented.      

A Single-Stage Three-Phase Grid-Connected Photovoltaic System With Modified MPPT Method and Reactive Power Compensation

Single-stage grid-connected photovoltaic (PV) systems have advantages such as simple topology, high efficiency, etc. However, since all the control objectives such as the maximum power point tracking (MPPT), synchronization with the utility voltage, and harmonics reduction for output current need to be considered simultaneously, the complexity of the control scheme is much increased. This paper presents the implementation of a single-stage three-phase grid-connected PV system. In addition to realize the aforementioned control objectives, the proposed control can also remarkably improve the stability of the MPPT method with a modified incremental conductance MPPT method. The reactive power compensation for local load is also realized, so as to alleviate grid burden. A DSP is employed to implement the proposed MPPT controller and reactive power compensation unit. Simulation and experimental results show the high stability and high efficiency of this single-stage three-phase grid-connected PV system.     

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.