Showing papers on "Grid-connected photovoltaic power system published in 2006"

Stability of photovoltaic and wind turbine grid-connected inverters for a large set of grid impedance values

Abstract: The aim of this paper is to analyze the stability problems of grid connected inverters used in distributed generation. Complex controllers (e.g., multiple rotating dq-frames or resonant-based) are often required to compensate low frequency grid voltage background distortion and an LCL-filter is usually adopted for the high frequency one. The possible wide range of grid impedance values (distributed generation is suited for remote areas with radial distribution plants) challenge the stability and the effectiveness of the LCL-filter-based current controlled system. It has been found out and it will be demonstrated in this paper that the use of active damping helps to stabilise the system in respect to many different kinds of resonances. The use of active damping results in an easy plug-in feature of the generation system in a vast range of grid conditions and in a more flexible operation of the overall system able to manage sudden grid changes. In the paper, a vast measurement campaign made on a single-phase system and on a three-phase system used as scale prototypes for photovoltaic and wind turbines, respectively, validate the analysis.

Intelligent PV Module for Grid-Connected PV Systems

Abstract: Most issues carried out about building integrated photovoltaic (PV) system performance show average losses of about 20%-25% in electricity production. The causes are varied, e.g., mismatching losses, partial shadows, variations in current-voltage (I-V) characteristics of PV modules due to manufacturing processes, differences in the orientations and inclinations of solar surfaces, and temperature effects. These losses can be decreased by means of suitable electronics. This paper presents the intelligent PV module concept, a low-cost high-efficiency dc-dc converter with maximum power point tracking (MPPT) functions, control, and power line communications (PLC). In addition, this paper analyses the alternatives for the architecture of grid-connected PV systems: centralized, string, and modular topologies. The proposed system, i.e., the intelligent PV module, fits within this last group. Its principles of operation, as well as the topology of boost dc-dc converter, are analyzed. Besides, a comparison of MPPT methods is performed, which shows the best results for the incremental conductance method. Regarding communications, PLC in every PV module and its feasibility for grid-connected PV plants are considered and analyzed in this paper. After developing an intelligent PV module (with dc-dc converter) prototype, its optimal performance has been experimentally confirmed by means of the PV system test platform. This paper describes this powerful tool especially designed to evaluate all kinds of PV systems

Flyback-Type Single-Phase Utility Interactive Inverter With Power Pulsation Decoupling on the DC Input for an AC Photovoltaic Module System

Abstract: In recent years, interest in natural energy has grown in response to increased concern for the environment. Many kinds of inverter circuits and their control schemes for photovoltaic (PV) power generation systems have been studied. A conventional system employs a PV array in which many PV modules are connected in series to obtain sufficient dc input voltage for generating ac utility line voltage from an inverter circuit. However, the total power generated from the PV array is sometimes decreased remarkably when only a few modules are partially covered by shadows, thereby decreasing inherent current generation, and preventing the generation current from attaining its maximum value on the array. To overcome this drawback, an ac module strategy has been proposed. In this system, a low-power dc-ac utility interactive inverter is individually mounted on each PV module and operates so as to generate the maximum power from its corresponding PV module. Especially in the case of a single-phase utility interactive inverter, an electrolytic capacitor of large capacitance has been connected on the dc input bus in order to decouple the power pulsation caused by single-phase power generation to the utility line. However, especially during the summer season, the ac module inverters have to operate under a very high atmospheric temperature, and hence the lifetime of the inverter is shortened, because the electrolytic capacitor has a drastically shortened life when used in a high-temperature environment. Of course, we may be able to use film capacitors instead of the electrolytic capacitors if we can pay for the extreme large volume of the inverter. However, this is not a realistic solution for ac module systems. This paper proposes a novel flyback-type utility interactive inverter circuit topology suitable for ac module systems when its lifetime under high atmospheric temperature is taken into account. A most distinctive feature of the proposed system is that the decoupling of power pulsation is executed by an additional circuit that enables employment of film capacitors with small capacitance not only for the dc input line but also for the decoupling circuit, and hence the additional circuit is expected to extend the lifetime of the inverter. The proposed inverter circuit also enables realization of small volume, lightweight, and stable ac current injection into the utility line. A control method suitable for the proposed inverter is also proposed. The effectiveness of the proposed inverter is verified thorough P-SIM simulation and experiments on a 100-W prototype

New maximum power point tracker for PV arrays using fuzzy controller in close cooperation with fuzzy cognitive networks

Abstract: The studies on the photovoltaic (PV) generation are extensively increasing, since it is considered as an essentially inexhaustible and broadly available energy resource. However, the output power induced in the photovoltaic modules depends on solar radiation and temperature of the solar cells. Therefore, to maximize the efficiency of the renewable energy system, it is necessary to track the maximum power point of the PV array. In this paper, a maximum power point tracker using fuzzy set theory is presented to improve energy conversion efficiency. A new method is proposed, by using a fuzzy cognitive network, which is in close cooperation with the presented fuzzy controller. The new method gives a very good maximum power operation of any PV array under different conditions such as changing insolation and temperature. The simulation studies show the effectiveness of the proposed algorithm

New Maximum Power Point Tracker Using Sliding-Mode Observer for Estimation of Solar Array Current in the Grid-Connected Photovoltaic System

Abstract: A new maximum power point tracker (MPPT) for a grid-connected photovoltaic system without solar array current sensor is proposed. The solar array current information is obtained from the sliding-mode observer and fed into the MPPT to generate the reference voltage. The parameter values such as capacitances can be changed up to 50% from their nominal values, and the linear observer cannot estimate the correct state values under the parameter variations and noisy environments. The structure of a sliding-mode observer is simple, but it shows the robust tracking property against modeling uncertainties and parameter variations. In this paper, the sliding-mode observer for the solar array current has been proposed to compensate for the parameter variations. The mathematical modeling and the experimental results verify the validity of the proposed method

Photovoltaic Power Conditioning System With Line Connection

Abstract: A photovoltaic (PV) power conditioning system (PCS) with line connection is proposed. Using the power slope versus voltage of the PV array, the maximum power point tracking (MPPT) controller that produces a smooth transition to the maximum power point is proposed. The dc current of the PV array is estimated without using a dc current sensor. A current controller is suggested to provide power to the line with an almost-unity power factor that is derived using the feedback linearization concept. The disturbance of the line voltage is detected using a fast sensing technique. All control functions are implemented in software with a single-chip microcontroller. Experimental results obtained on a 2-kW prototype show high performance such as an almost-unity power factor, a power efficiency of 94%, and a total harmonic distortion (THD) of 3.6%

Real-Time Identification of Optimal Operating Points in Photovoltaic Power Systems

Abstract: Photovoltaic power systems are usually integrated with some specific control algorithms to deliver the maximum possible power. Several maximum power point tracking (MPPT) methods that force the operating point to oscillate have been presented in the past few decades. In the MPPT system, the ideal operation is to determine the maximum power point (MPP) of the photovoltaic (PV) array directly rather than to track it by using the active operation of trial and error, which causes undesirable oscillation around the MPP. Since the output features of a PV cell vary with environment changes in irradiance and temperature from time to time, real-time operation is required to trace the variations of local MPPs in PV power systems. The method of real-time estimation proposed in this paper uses polynomials to demonstrate the power-voltage relationship of PV panels and implements the recursive least-squares method and Newton-Raphson method to identify the voltage of the optimal operating point. The effectiveness of the proposed methods is successfully demonstrated by computer simulations and experimental evaluations of two major types of PV panels, namely: 1) crystalline silicon and 2) copper-indium-diselenide thin film

Single-phase single-stage photovoltaic generation system based on a ripple correlation control maximum power point tracking

Abstract: A maximum power point tracking algorithm for single-stage converters connecting photovoltaic (PV) panels to a single-phase grid is presented in this paper. The algorithm is based on the application of the "ripple correlation control" using as perturbation signals the current and voltage low-frequency oscillations introduced in the PV panels by the single-phase utility grid. The proposed control technique allows the generation of sinusoidal grid currents with unity power factor. The algorithm has been developed to allow an array of PV modules to be connected to the grid by using a single-stage converter. This simple structure yields higher efficiency and reliability when compared with standard solutions based on double-stage converter configurations. The proposed maximum power point tracking algorithm has been numerically simulated and experimentally verified by means of a converter prototype connected to a single-phase grid. The results are presented in the paper, showing the effectiveness of the proposed system.

Boost Current Multilevel Inverter and Its Application on Single Phase Grid Connected Photovoltaic System

Abstract: This work presents a novel current multilevel (CML) inverter topology, named boost CML inverter, and its application on energy processing of single-phase grid-connected photovoltaic (PV) systems. The structure allows a high power factor operation of a PV system, injecting a quasi-sinusoidal current into the grid, with virtually no displacement in relation to the line voltage at the point of common coupling among the PV system and the loads. The major appeals of using the CML technique are the balanced current sharing among semiconductor switches and the decrease of the current slope in the circuit devices, with a consequent reduction of conducted and radiated electromagnetic interference (EMI). The CML technique also allows adapting or minimizing current waveforms harmonic content. System description, mathematical approach, and design guidelines are presented, providing an overview of the new topology. In order to validate the proposed concepts, experimental measurements, made in a small-scale laboratory prototype, are also presented. The obtained results evidence the feasibility of the application of this new topology on singlephase grid-connected PV systems.

High-performance solar photovoltaic ( PV) energy conversion system

Abstract: The present invention focuses on the development of a high-performance solar photovoltaic (PV) energy conversion system. The power circuit of the invention is made of a two-stage circuit, connecting a step-up DC-DC converter and a full-bridge inverter in serial. The present invention uses an adaptive perturbation and observation method to increase tracking speed of maximum power position and at the same time reduces energy loss. In addition, the full-bridge inverter's output has to have the same phase with the utility power in order to achieve unit power factor and increase the system efficiency. The present invention uses voltage type current control full-bridge inverter to achieve the goal of merging into utility grid. The present invention provides an active Sun tracking system, by utilizing the character of changing in open circuit output voltage with Sun radiation strength to follow the Sun, and decreases the system cost and increases system effectiveness.

PSIM circuit-oriented simulator model for the nonlinear photovoltaic sources

Abstract: The photovoltaic (PV) generator exhibits a nonlinear i-v characteristic and its operating point depends on the nature of the connected load and solar insolation In this article a simple and accurate PV model suitable for circuit-oriented simulator, PSIM, is developed Necessary theory and mathematical treatment for the solar cell array model is discussed To verify the model, i-v characteristics are generated through simulation for different solar insolations and compared with the experimentally obtained characteristics The developed circuit models are integrated into the PV system and as an example a maximum power (MP) point tracking of PV system with dc-dc converter is considered here Simulation and experimental result are in close agreement with each other This verifies the accuracy and suitability of the developed circuit-oriented models

Modeling and Simulation of Solar PV Arrays under Changing Illumination Conditions

Abstract: This paper proposes a modeling and computing algorithm to simulate and analyze the effect of non-uniform changing shadows (a passing cloud) on the output power of solar PV arrays. Each solar array is composed of a matrix of individual solar cells or solar modules interconnected in series and parallel. Bypass switches and diodes are also modeled. The model is able to determine the power loss in each solar cell and the hot spots of a shaded solar PV array as well as the PV output power. The model is also able to simulate and compute the output power of solar PV arrays for any configuration, with or without bypass diode

System and methods for a self-healing grid using demand side management techniques and energy storage

Abstract: A self-healing power grid control system includes a power grid having a plurality of network islands with a plurality of linear and non-linear loads. A plurality of control sensors communicate with the power grid to monitor the electrical characteristics of the power grid. A plurality of controlled relays are in electrical communication with the plurality of non-linear loads. A battery energy storage system (ESS) is in electrical communication with a main power source and a network island. A first restoration controller is in electrical communication with the control sensors, the controlled relays, and with the battery ESS. The first restoration controller receives control signals from the control sensors, and in response to detecting an irregularity in the power grid, automatically actuates the battery ESS to stabilize power to the linear loads, and disconnects selected controlled relays to disconnect power to a calculated percentage of the non-linear loads.

Methods and systems for intentionally isolating distributed power generation sources

Abstract: A method and system for operating a mini-grid including one or more power generation sources and one or more loads independently from a utility grid is provided, where the mini-grid is disconnected from the utility grid in response to a power disruption over the utility grid. A universal interconnect device forms a connection between the mini-grid and the utility grid. In the disconnected state, the mini-grid operates independently from the utility grid, such that the power generation sources of the mini-grid supply the loads. Frequency and voltage regulation are provided by the universal interconnect device through a monitoring function performed by a controller in conjunction with at least an energy storage device, which can absorb or generate power as needed.

Control Strategies for Distributed Power Generation Systems Operating on Faulty Grid

Abstract: The distributed power generation based on renewable energy sources such as wind and sun experiences a high penetration in the power systems around the world, having in some countries a large contribution to the total energy production. In order to protect the distribution and transmission systems, the grid operators are more and more talking about the ability of the distribution systems (mainly wind turbines WT systems) to behave as a conventional power plant. This paper discusses the possibilities of the distributed power generation systems (DPGS) to deliver power when grid disturbances are present in the utility network. The focus is set more on the control strategies for active power generation, and mainly on the creation of the reference currents which fulfill the demanded output power. Considerations about reactive power are also stated. Experimental results are presented in order to validate the theory behind the proposed control strategies on faulty grid.

Power management of hybrid photovoltaic - fuel cell power systems

Abstract: Photovoltaic (PV) solar energy systems are widely used as an important alternative energy source. To overcome the problem of intermittent power generation, PV power systems may be integrated with other power sources. Fuel cells are an attractive option because of high efficiency, modularity and fuel flexibility; however, one main week point is their slow dynamics. On the other hand, current technology batteries by themselves are usually insufficient to provide the long-term energy that the increasing loads require. Hybrid systems composed of fuel cells and batteries can be integrated with PV power systems to provide uninterrupted high-quality power. The goal of this study is to design an effective power management system for a PV/fuel cell/battery hybrid power system so that the combination can be used as a reliable power source. In this paper, the structure of the hybrid power system is described, and control strategies for power management of the hybrid power system are discussed. The proposed hybrid power system is then verified by numerical simulation.

Survey of the Power Conditioning System for PV Power Generation

Abstract: This paper is a survey on photovoltaic (PV) power conditioning systems (PCS) and their control strategies. Different inverter structures used in the PCSs are summarized. A novel Z-Source inverter based PCS is presented. The requirements and standards of PV PCS are also introduced.

Effects of PV Grid-Connected System Location on a Distribution System

Abstract: The grid-connected photovoltaic (PV) power generation system is expected to be more widespread in distribution systems due to increasing fossil-fuel cost. Recent advances in PV technology and great demand has led to cost reductions. This paper presents the effects of PV system installation location on the power quality of the distribution system. A measurement setup is prepared for a PV grid-contected system. The system consists of a PV array and a grid-connected inverter. The measured data are used to investigate the effects of a PV system. A feeder line with PV system is simulated in Matlab. This study focuses on both radial and loop operated distribution system. The distribution system's power quality, in terms of voltage, active power, total losses, and power factor of the network are presented. The simulation results show that implementation of the PV grid-connected system could improve the power quality of a distribution feeder.

System and method for array and string level monitoring of a grid-connected photovoltaic power system

Abstract: A grid-connected photovoltaic electrical power system with both array level and string level remote monitoring and production and efficiency analysis capabilities. The inventive system includes an array level monitoring component, software for recording and analyzing data obtained through the array level monitoring component, and a string level monitoring component.

Comparison of z-source and boost-buck inverter topologies as a single phase transformer-less photovoltaic grid-connected power conditioner

Abstract: In PV grid connected power conditioners; usually is required to step up the PV array voltage, and to step down the dc bus voltage for injection of a sinusoidal current to the grid. In this paper, the design procedure of the traditional double stage boost-buck inverter and single stage Z-source converter, as a single phase PV grid connected transformer-less power conditioner is presented. The reduction of the component cost is a research goal in PV grid connected systems. The omission of the isolation transformer, and the usage of a single-stage Z-source inverter, helps to develop a cost-effective PV-PCU. In addition, the optimum modulation method for each converter is proposed. The performances of both topologies are investigated. The energy storage elements and the efficiency of the converters are compared.

Modeling and Control of a Grid-connected Wind/PV Hybrid Generation System

Abstract: This paper deals with power control of a wind and solar hybrid generation system for interconnection operation with electric distribution system. The proposed system consists of a variable-speed direct-drive wind generator, wind-side converter, solar array, dc-dc converter and grid interface inverter. Power control strategy is to extract the maximum energy available from varying condition of wind speed and solar irradiance while maintaining power quality at a satisfactory level. In order to capture the maximum power, variable speed control is employed for wind turbine and maximum power point tracking is applied for photovoltaic system. The grid interface inverter transfers the energy drawn from the wind turbine and PV array into the grid by keeping common dc voltage constant. Modeling and simulation study on the entire control scheme is carried out using a power system transient analysis tool, PSCAD/EMTDC. The simulation results show the control performance and dynamic behavior of the wind/PV system