规模化光伏电站与电网暂态交互影响定量分析

在电力系统暂态安全定量分析软件平台FASTEST(Fast Analysis of Stability using the Extended equal areacriterion and Simulation Technologies)上以研究规模化光伏电站接入系统后的稳定性分析为目的,建立了一个完整而且适合系统安全稳定分析的并网光伏电站机电暂态模型。基于FASTEST特有的暂态稳定量化分析功能,对并网光伏电站被浮云遮挡和系统侧发生扰动这2种典型情况进行仿真分析,从对系统暂态功角稳定性、暂态电压稳定性和暂态频率稳定性的影响的角度,探讨光伏电站与常规电源在影响电网运行特性方面的区别。结果表明,光伏电站并网后对系统暂态频率稳定性影响最为严重,相比较于带有励磁和调速系统的同步发电机,光伏电站在网侧故障时的暂态响应特性更为剧烈。     

Power Modulation of Photovoltaic Generator for Frequency Control of Power System

This paper describes power modulation of a photovoltaic (PV) generator for frequency regulation. The generator has a small electric double-layer capacitor. The capacitor absorbs rapid fluctuations of PV generation, and allows the generator to change its output at a limited ramp rate. The capacitor voltage is kept at a specified value to maintain adequate energy storage. The generator output is modulated in proportion to the frequency deviation. The responses to step and periodic changes of the frequency are examined. The voltage control is modified so as not to affect the frequency control. It is shown by numerical simulations that the PV generator cooperates well with conventional generators. Some experimental results are demonstrated to validate the proposed method.      

A method for the measurement of the photovoltaic cell or module current-voltage characteristic

For the measurement of the current-voltage characteristic of a photovoltaic (PV) cell or module, an experimental “self testing” method is proposed which uses as “load” one or more PV cells (modules), appropriately connected to the test cell and subjected to a variable irradiance. Depending on the irradiance values towards the test cell (module), the “load” cell will behave as a true load, a reverse voltage generator or a reverse current generator. Therefore, it is possible to obtain, by an automatic data acquisition system, the points of the test cell I = I(U) characteristic in quadrants I, II and IV. On this basis, a simple three quadrant I = I(U) curve tracer, based on a digital or analog storage oscilloscope, is produced. Laboratory and field tests on cells and modules have shown the easy feasibility of implementatitg this method.     

Analytical model of mismatched photovoltaic fields by means of Lambert W-function

A new model of photovoltaic (PV) fields is introduced in this paper. It allows the simulation of a PV generator whose subsections, e.g. cells, groups of cells, panels or group of panels, work under different solar irradiation values and/or at different temperatures. Moreover, different nominal characteristics, rated power, production technology, shape and area can be settled for different subsections. Consequently, the proposed model is able to describe the behaviour of matched as well as mismatched PV fields. It results into a non linear system of equations, which includes bypass and blocking diodes models and is characterized by a sparse Jacobian matrix. The numerical model is reliable and requires a moderate computational burdensome, both in terms of memory use and processor speed. Numeric simulations confirm the accuracy and cheapness of the approach. The proposed model is used to simulate the drawbacks associated to mismatching during maximum power point tracking (MPPT) of the PV generator.     

Simple estimation of PV modules loss resistances for low error modelling

For practical purposes it can be assumed that the power delivered by a photovoltaic generator that is connected to an MPPT is always maximal. When studying the behaviour of a PV generator working in this way, the most interesting aspect is the evolution of the point of maximum power. So, the analysis of the I-V characteristic must be centred in the area of high voltages. In this situation the five parameter model is very appropriate for characterizing the PV generator.On the other hand, in the case of photovoltaic modules the information currently provided by manufacturers is insufficient to do modelling. Thus, to evaluate the loss resistances it is necessary to use any of the different methods that currently exist.The purpose of this paper is to present a new procedure, based on simplified equations, which allows the estimation of the loss resistances of any PV crystalline silicon module. By simulating the modules with the loss resistances calculated in this way highly accurate results are obtained. Especially in the surrounding of the maximum power point the error is always less than 0.5%.     

Dynamic behaviour of PV generator trackers under irradiation and temperature changes

Maximum power point trackers (MPPTs) have a decisive role to extract power from the photovoltaic (PV) generators as they have to assume the maximum power output (MPP) whatever are the continuous changes of temperature and irradiation conditions. Therefore, they take a prior place in the global PV system efficiency. These trackers are driven by MPPT algorithms and lot of these MPPT algorithms are proposed in literature. The two most common implemented algorithms for power optimisation are the Perturb and Observe (P&O) and the Incremental of Conductance (IncCond) algorithms, which present a high simplicity of implementation within electronics programmable circuits. With an approach based on realistic parameters such as those found when the generator is integrated in a real photovoltaic installation, the two MPPT techniques are dynamically compared using testing procedures developed with Matlab/Simulink. The study leads us to conclude that both algorithms can be performed for PV exposures in unfavourable but realistic external conditions.     

Optimized photovoltaic generator–water electrolyser coupling through a controlled DC–DC converter

The coupling of a photovoltaic generator and an electrolyser is one of the most promising options for obtaining hydrogen from a renewable energy source. Both are well known technologies, however, since the high variability of the solar radiation, an efficient coupling still presents some challenges. Direct or through a DC–DC converter couplings are the options in isolated applications. In this work, three models, respectively, for a photovoltaic (PV) generator, a controlled DC–DC converter and a complete proton exchange membrane (PEM) electrolyser have been designed by using Matlab/Simulink. A PV-electrolyser specific algorithm to search for the optimum and safe working point for both elements is presented. Simulation results demonstrate that the use of a controlled DC–DC converter with the proposed algorithm shows better adaptability to the variable radiation conditions than the other coupling options. Therefore, it leads to a better compliance between the electrolyser and the sizing of the PV generator.     

Differentiation of multiple maximum power points of partially shaded photovoltaic power generators

Partial shading conditions have a major effect on the electrical characteristics of photovoltaic (PV) power generators. In this paper, the effects of partial shading on maximum power points (MPPs) of a PV power generator have been systematically studied by using Simulink simulation model of a PV power generator composed of 18 series-connected PV modules. It is shown that the local MPPs can be classified into MPPs at low and high voltages based on the MPP operating point of the PV generator. The results also show that based on the MPP current and voltage it is possible to directly know if the MPP at high voltages is a local or a global MPP. The differentiation between local and global MPPs at high voltages is based on the voltage difference between the actual MPP voltage at high voltages and the theoretical MPP voltage under corresponding uniform conditions. This differentiation method was also tested to work correctly by utilizing experimental measurements of the Tampere University of Technology Solar PV Power Station Research Plant. By using this method, it can be identified if the system is operating at a local or a global MPP. This method can further be utilized to develop global MPP tracking algorithms.     

Matching of a DC motor to a photovoltaic generator using a step-upconverter with a current-locked loop

A photovoltaic (PV) generator is a nonlinear device having insolation-dependent volt-ampere characteristics. Because of its relatively high cost, the system designer is interested in optimum matching of the motor and its mechanical load to the PV generator so that maximum power is obtained during the entire operating period. However, since the maximum-power point varies with solar insolation, it is difficult to achieve an optimum matching that is valid for all insolation levels. In this paper it is shown that for maximum power, the generator current must be directly proportional to insolation. This remarkable property is utilized to achieve insolation-independent optimum matching. A shunt DC motor driving a centrifugal water pump is supplied from a PV generator via a step-up converter whose duty ratio is controlled using a current-locked feedback loop     

Power-quality monitoring of a PV generator

It is envisaged that the use of photovoltaic (PV) dispersed generation will become more widespread in the near future due to advances in PV technology which will lead to cost reductions. In order to gain an understanding of the effects that PV dispersed generation may have upon the quality of electric power, this paper presents a power quality study performed on a PV generator. The study includes measured waveforms, trends, a statistical analysis of the measurements and the results of a connection/disconnection test     

Optimization of Solar Hydrogen Systems Based on Hydrogen Production Cost

Hydrogen is regarded as a potential future energy carrier. It can be produced by the electrolysis of water with the required power supplied by a photovoltaic module. The hydrogen in this study was produced using a hydrogen generator with a solid polymer electrolyte. The required power was supplied by a photovoltaic module rated at 3.4 V, 27.45 A. The experimental system was designed and constructed so that the photovoltaic module was directly coupled to the hydrogen generator. The system characteristics: quantity of hydrogen produced, current/voltage output characteristics of the PV module, PV module and H₂ generator temperatures were measured and analyzed. A method to design a solar hydrogen energy system, providing the most cost effective hydrogen generation, was developed. In this method, the design point is chosen based on the irradiance during system operation under rated capacity. The data supplied by the experimental system clearly showed the importance of considering the ratio of photovoltaic module cost to hydrogen generator cost when designing an optimum solar hydrogen system.     

New control strategy for 2-stage grid-connected photovoltaic power system

This work deals with the performances and responses of a grid-connected photovoltaic (PV) plant in normal and disturbed modes. The system is composed of a solar array, a dc–dc converter and a three-phase inverter connected to the utility grid. On the one hand a suitable control of the dc–dc converter is developed in order to extract the maximum amount of power from the PV generator. On the other hand an active and reactive power control approach (PQ) has been presented for the inverter. This method can provide a current with sinusoidal waveform and ensure a high power factor. Therefore, the grid interface inverter transfers the energy drawn from the PV into the grid by ensuring constant dc link voltage. Modeling and controlling were carried out using the informational graph of causality and the macroscopic energy representation methods. The simulation under MATLAB/SIMULINK and the experimental results show the control performance and dynamic behavior of grid-connected PV system in normal and disturbances modes.     

Effect of flow distribution on the photovoltaic performance of a building integrated photovoltaic/thermal (BIPV/T) collector

The phenomenon of non-uniform flow distribution and its influence on thermal performance within a traditional solar thermal collector is well known. Its effect on the photovoltaic conversion of a hybrid photovoltaic/thermal (PV/T) collector however has received little attention. In this study an investigation has been carried out to determine what effect flow distribution will have on the photovoltaic yield of a BIPV/T collector of various size. A three step numerical analysis was conducted to model flow distribution, temperature variation, and photovoltaic yield for a PV/T collector of various design (manifold sizes), geometric shape (aspect ratio), and operating characteristics (mass flow rate and flow direction in manifolds) in order to vary flow uniformity within the collector. The results revealed that flow distribution within the collector will have a significant influence on the photovoltaic performance of a hybrid PV/T collector. For the scenario where flow distribution was most uniform, photovoltaic performance was improved by over 9% in comparison to a traditional photovoltaic (PV) collector operating under the same conditions. For poor flow however, performance was only improved by approximately 2%. Parameters found to influence flow distribution include the manifold to riser pipe ratio where a ratio of 4:1 was found to be ideal and that increasing to a 6:1 ratio offered negligible improvement. Additionally it was found that array geometry (characterised by its aspect ratio in this study) plays an important role on both flow distribution and photovoltaic yield. This study has identified that the optimal mass flow rate is dependent on the shape or aspect ratio of the array.     

A model for optimal sizing of photovoltaic irrigation water pumping systems

The previous methods for optimal sizing of photovoltaic (PV) irrigation water pumping systems separately considered the demand for hydraulic energy and possibilities of its production from available solar energy with the PV pumping system. Unlike such methods, this work approaches the subject problem systematically, meaning that all relevant system elements and their characteristics have been analyzed: PV water pumping system, local climate, boreholes, soil, crops and method of irrigation; therefore, the objective function has been defined in an entirely new manner. The result of such approach is the new mathematical hybrid simulation optimization model for optimal sizing of PV irrigation water pumping systems, that uses dynamic programming for optimizing, while the constraints were defined by the simulation model. The model was tested on two areas in Croatia, and it has been established that this model successfully takes into consideration all characteristic values and their relations in the integrated system. The optimal nominal electric power of PV generator, obtained in the manner presented, are relatively smaller than when the usual method of sizing is used. The presented method for solving the problem has paved the way towards the general model for optimal sizing of all stand-alone PV systems that have some type of energy storage, as well as optimal sizing of PV power plant that functions together with the storage hydroelectric power plant.     

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。     

Techno-economic analysis of a stand-alone hybrid photovoltaic-diesel–battery-fuel cell power system

The main objective of this work is to model a renewable energy system that meets a known electric load with the combination of a photovoltaic (PV) array, a diesel generator and batteries. The replacement of conventional technologies with hydrogen technologies is examined. The analysis utilizes the power load data from an electric machinery laboratory located in Kavala town, Greece. The modeling, optimization and simulation of the proposed system were performed using HOMER software. Different combinations of PV, generators, and batteries sizes were selected in order to determine the optimal combination of the system on the basis of the Net Present Cost (NPC) method.     

A method for the identification of configurations of PV/wind hybrid systems for the reliable supply of small loads

Autonomous systems on the basis of PV and wind generators in combination with a battery storage are an option for the supply of small electrical loads at remote locations. The systems may be supplied by a wind generator or a PV generator alone, or by a combination of both. To decide on the system configuration that should be applied in a certain case it is necessary to identify all possible configurations that satisfy the desired level of reliability. From these configurations, an optimal one may be extracted by the application of economic measures for example. For European sites we present a new approach to the determination of the combinations of generator and storage sizes that will guarantee a high level of system reliability. A set of equations has been identified that link the sizes of the system components (PV generator, wind generator, battery) directly with parameters that characterize the meteorological conditions at the respective site. The meteorological parameters applied are all gained from data sets given in the European Solar Radiation Atlas and the European Wind Atlas.     

Dynamic modeling of a photovoltaic hydrogen fuel cell hybrid system

The objective of this paper is to mathematically model a stand-alone renewable power system, referred to as “Photovoltaic–Fuel Cell (PVFC) hybrid system”, which maximizes the use of a renewable energy source. It comprises a photovoltaic generator (PV), a water electrolyzer, a hydrogen tank, and a proton exchange membrane (PEM) fuel cell generator. A multi-domain simulation platform Simplorer is employed to model the PVFC hybrid systems. Electrical power from the PV generator meets the user loads when there is sufficient solar radiation. The excess power from the PV generator is then used for water electrolysis to produce hydrogen. The fuel cell generator works as a backup generator to supplement the load demands when the PV energy is deficient during a period of low solar radiation, which keeps the system's reliability at the same level as for the conventional system. Case studies using the present model have shown that the present hybrid system has successfully tracked the daily power consumption in a typical family. It also verifies the effectiveness of the proposed management approach for operation of a stand-alone hybrid system, which is essential for determining a control strategy to ensure efficient and reliable operation of each part of the hybrid system. The present model scheme can be helpful in the design and performance analysis of a complex hybrid-power system prior to practical realization.     

Simulation of solar hydrogen energy systems

Solar hydrogen is a promising long-term global energy option for the post-fossil fuel era. On the other hand, solar hydrogen may have already found an early commercial application in the form of seasonal energy storage for remote stand-alone photovoltaic (PV) applications. In a stand-alone solar hydrogen energy system, the photovoltaic array is coupled with an electrolyser to produce H₂ which is stored to be later converted back to electricity in a fuel cell. The system setup comprises several subsystems which have to be controlled in an optimal way. Numerical simulations are used to get a closer insight into the transient response behavior of these elegant, but rather complicated systems during variable insolation conditions and to estimate the overall system performance accurately over extensive periods of time. The simulations are performed with the H2PHOTO program which has been successfully used for the design of a solar hydrogen pilot plant. It has also shown good accuracy against experimental data.     

Steady-state performance of DC motors supplied from photovoltaicgenerators with step-up converter

A simple step-up converter circuit consisting of a single power transistor and an inductor is used as an interface between a PV (photovoltaic) generator and a shunt DC motor driving a centrifugal water pump. The step-up converter allows maximum power output from the PV generator to the motor at all insolation levels. Steady-state performance of the motor is vastly improved as its input voltage and current are stabilized by the regenerative action of the converter. The PV generator operates at maximum power regardless of insolation variations. The converter duty ratio can be set at a fixed optimal value which is valid for all insolation levels. This remarkable property makes this device economically attractive since it is easy to build and does not require any insolation-dependent control as compared to other peak-power tracking devices