Grid-connected Photovoltaic Generation Research Articles

Research on Topology of the High Step-Up Boost Converter With Coupled Inductor

High step-up boost converters with coupled inductor have attracted much attention in the fuel cell or photovoltaic grid-connected generation system, however, there are few literatures elaborated on the construction ideas and derivation methods of them. Accordingly, in order to obtain a clear roadmap on the derivation and inner connection of these converters, a comprehensive review and analysis are presented in this paper. First, the basic boost converter with coupled inductor is regarded as the basic topology, and its merits and demerits are analyzed in detail. Then, in order to address these demerits, various step-up techniques are introduced, such as the rectifier circuit, the active-clamped circuit, the multi-winding coupled inductor, and the voltage doubler rectifier; and numerous new topologies are continuously proposed by combinations and equivalent simplifications. In addition to a detailed synthesis of each topology, a comparative and quantitative analysis among some important converters is presented, and the optimal one is chosen to build a 250 W prototype. Finally, based on comparisons and analysis, the main characteristics and inner connections of these high step-up boost converters with coupled inductor are identified and clarified.

Nonlinear Stability Analysis for Three-Phase Grid-Connected PV Generators

Numerous emerging oscillatory stability issues have recently arisen as renewable energy sources (RESs) are integrated into the power system. Some of the existing literature attempts to explain these issues through the corresponding simplification, which usually omits the influence of the nonlinear discontinuous elements in the system. Hence, the obtained results are incomplete. To fill this gap, this article proposes a stability analysis method based on the describing function (DF) method to explore the influence of nonlinear discontinuous elements in the system. Without loss of generality, taking the three-phase grid-connected photovoltaic (PV) generator as the research object, the perturbation and observation (P&O)-based power control, which is a typical nonlinear discontinuous element, was carefully studied. First, the complete mathematical model of the three-phase grid-connected PV generator was established, including the outermost P&O-based power control. Then, according to the DF method, the stability of the PV generator was analyzed, and the related influence factors were studied in detail. It was found that the nonlinear discontinuous P&O-based power control has a substantial influence on the stability of the PV generator and can result in the oscillation. At the same time, the DF method and the conventional method were compared, which shows that the proposed DF method can enhance the accuracy of the stability assessment. Especially regarding critical stability, the oscillation magnitude and frequency can be calculated accurately. All the theoretical analyses were verified by the real-time hardware-in-loop (HIL) tests.

Advanced Frequency Support Strategy of Double-Stage Grid-Connected PV Generation

With an increasing penetration of PV generation in the electrical grid, the increasing replacement of large conventional synchronous generators by PV generation will result in deteriorated frequency regulation performance due to the reduced system inertia response. It is urgent for PV generation to take part in frequency regulation. In this paper, three virtual inertial control strategies are proposed for double-stage grid-connected PV generation: virtual inertial control based on the dynamics of phase-locked loop (PLL), virtual inertial control based on the dynamics of low voltage DC capacitor, virtual inertial control based on the dynamics of high voltage DC capacitor. The influence of control parameters on virtual inertial control strategies is also analyzed. Besides, in order to get an even better frequency behavior, a coordinated control strategy between PV generation and conventional synchronous generators (SGs) is also proposed. Finally, the theoretical analysis and control strategies are verified by simulation results.

Study on Shunt Active Power Filter Control Strategies of Three-phase Grid-connected Photovoltaic Systems

This paper deals with the improvement of the energy quality using shunt active power filter. The three-phase grid-connected photovoltaic generator consists in solar panels, a three-phase voltage inverter connected to the grid and a nonlinear load constituted by a diode rectifier bridge supplying a resistive load in series with an inductor. In so doing, three main challenges arise from the application context. First, the harmonic currents and the reactive power must be compensated. The second challenge is the injection of active solar energy into the grid. Third, Maximum Power Point Tracking (MPPT) must be found. This paper proposes a method addressing those challenges. For the first and the second one, direct current and power controls is used. For the third challenge, an algorithm is proposed which take in account the electrical variables and the variation of the solar irradiation. Simulation results of the proposed method are shown. The method is illustrated with two different strategies: Hysteresis Control and Direct Power Control (DPC) for a variable load. Obtained results are presented and compared in this paper to confirm the robustness and the superiority of DPC strategy compared to Hysteresis Control strategy. In the same context, the simulation carried out in this article shows promising results with THD approximates 1.33 %.

Direct Connection of Supercapacitor–Battery Hybrid Storage System to the Grid-Tied Photovoltaic System

Penetration rate of grid-connected photovoltaic (PV) generation to the existing utility grid is rapidly increasing over the years. Since the power generated from PV systems fluctuates according to the weather condition, e.g., cloud passing, this can significantly disturb the stability of a weak utility grid. The integration of energy storage devices and its ramp-rate control technique are required to reduce the impact of PV systems output fluctuations and augment the stability of the utility grid. In this paper, ramp-rate control is applied to the direct connection of energy storage devices in PV generation system configuration. The direct connection of supercapacitors string and battery combination scheme is proposed to reduce the number of power converters so that the efficiency of the system is increased. In this work, the PV system output is controlled by directly controlling the energy storage system (ESS) to limit the changing rate of the PV output to a desired ramp-rate value, hence reducing the battery charge/discharge cycles and extending the expected lifetime of the ESS. The performance of the proposed direct connection scheme of the ESS and its ramp-rate control strategy is verified using a 1-kW PV system prototype.

Modeling, control and simulation of a photovoltaic /hydrogen/ supercapacitor hybrid power generation system for grid-connected applications

Hybrid renewable energy systems (HRES) should be designed appropriately with an adequate combination of different renewable sources and various energy storage methods to overcome the problem of intermittency of renewable energy resources. Focusing on the inevitable impact on the grid caused by strong randomicity and apparent intermittency of photovoltaic (PV) generation system, modeling and control strategy of pure green and grid-friendly hybrid power generation system based on hydrogen energy storage and supercapacitor (SC) is proposed in this paper. Aiming at smoothing grid-connected power fluctuations of PV and meeting load demand, the alkaline electrolyzer (AE) and proton exchange membrane fuel cell (PEMFC) and SC are connected to DC bus of photovoltaic grid-connected generation system. Through coordinated control and power management of PV, AE, PEMFC and SC, hybrid power generation system friendliness and active grid-connection are realized. The validity and correctness of modeling and control strategies referred in this paper are verified through simulation results based on PSCAD/EMTDC software platform.

Single-Phase Two-Stage Seven-Level Power Conditioner for Photovoltaic Power Generation System

This paper presents a new power conditioner with the inherent benefits of boosting, generation of seven-level output voltage with minimum leakage current in a grid-connected photovoltaic (PV) power generation system. The proposed power conditioner is an upgrade of a front-end multioutput dc–dc boost converter and an asymmetrical seven-level inverter. A high-frequency transformer (HFT) employed in front-end converter produces balanced dc-link voltages to generate the seven-level output voltage. The leakage current caused by the parasitic capacitance of the PV panel is minimized by providing a common-mode conducting path to the inverter. This results in a reduction of the leakage current well below the VDE0126-1-1 grid standards. Furthermore, the proposed configuration utilizes a minimum number of devices for every level generation, which reduces the control complexity and also improves the system efficiency. The dynamic performance of the system is tested for intermittent changes in the PV characteristics for grid-connected operation. The proposed power conditioner is simulated using MATLAB software, and a laboratory prototype of 750 W is developed to validate its feasibility. Finally, a comparison is made with other recently proposed seven-level inverters to highlight the benefits of this power conditioner over others.

A New Improved Trans Z Source Inverter based Three Phase UPS

Objective: In applications like Wind Energy Conversion System, electric cell power conversion and Grid-connected PV generation, a small DC source voltage should be boosted to induce fascinating AC output voltage. Methods/Statistical Analysis: Z Structured Impedance Source Inverter (ZSI) is well-liked for its buck to boost operations. The high voltage gain will be obtained from the normal Z Structured Impedance Source Inverter at low modulation index state. On the other side, a little modulation index state produces high voltage stress at the inverter side switches. To defeat this problem, a Trans Z Structured Impedance Source Inverter (TZSI) is developed. In TZSI, transformer winding turns demand is high and the windings get burnt because of high instant current throughout shoot through length. Findings: A new topology of Trans Z Structured Impedance Source Inverter is introduced in this paper to boost the voltage gain fair enough. In this new topology the winding turn’s usage is smaller amount and consequently inverter gain is lift up by lower the modulation index. To validate the ideas planned in theories, experimental results were taken and bestowed in this paper. Application: This inverter is utilized for three faze Uninterruptable Power Supply. The proposed inverter is designed and simulated in MATLAB/Simulink podium for 3 KVA load capacity and it will sustain the specified output voltage with the substantial drop in voltage at battery bank side. Keywords: PWM Inverter, Shoot Through Period, Trans Z Source Inverter (TZSI), Uninterruptable Power Supply (UPS), Z Source Inverter (ZSI)

Optimal schedule of grid-connected residential PV generation systems with battery storages under time-of-use and step tariffs

Abstract In recent years, grid-connected residential PV generation systems have been greatly encouraged in China. In a residential power system containing PV, three types of power sources, namely, PV generation, battery storage and the utility grid, need to be properly scheduled. The time-of-use tariff and step tariff have been widely accepted in China. Correspondingly, this paper proposes two optimal energy management strategies for residential power systems to adapt to the two tariffs towards largest benefits. The strategies are formulated as optimization problems, where minimizing the household energy cost is taken as the objective and the dispatching ratio of electricity sold to the grid and used locally is treated as the optimization variable. A genetic algorithm (GA) is then employed to solve the formulated nonlinear optimization problems. A typical residential power system, containing grid-connected PV panels and a battery storage system, is used for verification study. The results show that the proposed energy management strategies, along with a GA based solving method, are effective in scheduling the operation status of the three power sources optimally to achieve maximum economic benefits under the time-of-use tariff and step tariff, respectively.

Current-Source Modular Medium-Voltage Grid-Connected System With High-Frequency Isolation for Photovoltaic Applications

Large-scale grid-connected photovoltaic energy generation systems are progressing remarkably benefiting from the latest developments in solid-state semiconductors technology. In such systems, the photovoltaic arrays can be connected directly to the medium-voltage grid without employing a bulky line-frequency transformer to step up the voltage. Nano-crystalline cores with a small size and a high permeability operating at medium or high frequency can be installed in the power conversion stage. Hence, the necessary isolation as well as voltage boosting features can be provided. However, only a few power converters allow this type of isolation. This paper proposes a new modular converter structure suitable for medium-voltage grid connected systems with high-frequency isolation. The output voltages of the series-connected modules are added in order to provide the necessary voltage boosting. Four different power converter topologies with small input capacitors can be used as submodules for the presented medium-voltage configuration having different advantages and drawbacks. These different topologies are analyzed in terms of power losses, footprint, and functionality. To validate the mathematical analysis and the computer simulations, a scaled-down 5 kVA three-phase, 1 kV prototype is built and tested with four modules for each phase.

Research on Grid-connected PV Power Station Condition Evaluation Technology Based on SCADA and Hierarchical Progression

This paper draw up the conditional evaluation strategy of grid-connected photovoltaic (PV) power generation stations based on the characterization of PV devices, and introduced the conditional evaluation index system includes performance and reliability of PV power generation station. This conditional statement evaluation method is based on both sides of the reliability and performance of PV devices, employ the availability, MRTBF, performance ration and power generation ability as the key evaluation index. In addition, using the operation data recorded by SCADA and recommended threshold values, in order to evaluation the operation conditional statement of grid-connected PV power generation station. Also, this paper proposed the performance degradation analysis method based on the hierarchical progression structure, and achieved the target of fast fixed position of fault PV devices. The firstly established conditional evaluation method is able to comprehensive evaluation the entirely operation statement of PV power generation station. This conditional statement evaluation method promotes the maintenance strategy of grid-connected PV power generation station, and guarantee the safe, stable and efficiency operation of stations. As a result, increase the benefits of operators of PV power generation stations and can be used to guide the PV power engineering.

Output feedback nonlinear control of three-phase grid-connected PV generator

<span lang="EN-GB">This paper addresses the problem of controlling three-phase grid connected PV system involving a PV arrays, a voltage source inverter, a grid filter and an electric grid. This paper presents three main control objectives: i) ensuring the Maximum power point tracking (MPPT) in the side of PV panels, ii) guaranteeing a power factor unit in the side of the grid, iii) and ensuring the asymptotic stability of the closed loop system. Interestingly, the present study features the achievement of the above energetic goal without resorting to sensors of currents of the grid. To this end, an output-feedback control strategy combining a state observer and a nonlinear control laws is developed. The proposed output-feedback control strategy is backed by a formal analysis showing that all control objectives are actually achieved</span>

Evaluating the Effect of Financing Costs on PV Grid Parity by Applying a Probabilistic Methodology

This paper presents a study that analyses the effect of financing costs on grid parity in photovoltaic (PV) installations by applying a probabilistic methodology. Three different case studies, located in Spain, have been considered, with 500 kW, 50 kW and 5 kW grid-connected PV generators. The technical and economic calculations were performed, considering the interest rate, yield across the Spanish geography, and PV module cost as parameters. The Monte Carlo method was applied to consider the full probabilistic range of values given to the different variables. The goal of this study was to determine, for the studied cases, the levelised cost of energy (LCOE) and the internal rate of return by considering realistic values of the variables. A success rate parameter was calculated, which determined the likelihood of the number of times that the LCOE was below the retail cost of electricity. All the cases were evaluated by applying 10,000 iterations, considering the standard deviations and means defined.

Five-Level Transformerless Inverter for Single-Phase Solar Photovoltaic Applications

Transformerless inverters are extensively employed in grid-connected photovoltaic (PV) generation systems due to its advantages of achieving low cost and high efficiency. However, the common-mode voltage issues have been motivated the proposition of new topologies, control, and modulation schemes. In common ground PV inverters, the grid neutral line is directly connected to the negative pole of the dc bus. Therefore, the parasitic capacitances are bypassed and the leakage current can be eliminated. In this paper, a five-level common ground transformerless inverter with reduced output harmonic content for PV systems is proposed. In addition, the proposed inverter can process reactive power and it presents a maximum dc-voltage utilization in opposition to half-bridge-based topologies. The operation modes of the proposed inverter, a simple modulation strategy, as well as the design guidelines are analyzed in detail. Finally, experimental results demonstrate the feasibility and good performance of the proposed inverter.

Three-Phase Three-Level Flying Capacitor PV Generation System with an Embedded Ripple Correlation Control MPPT Algorithm

This paper presents the implementation of a maximum power point tracking (MPPT) algorithm in a multilevel three-phase photovoltaic (PV) system using the ripple correlation control (RCC) method. Basically, RCC adopts the inherent oscillations of PV current and voltage as perturbation, and it has been predominantly used for single-phase configurations, where the oscillations correspond to the 2nd order harmonics. The implementation of an RCC-MPPT algorithm in a three-phase system has not been presented yet in the literature. In this paper, the considered three-phase three-level converter is a three-level flying capacitor (FC) inverter. The proffered RCC method uses the 3rd harmonic components of PV current and voltage for the estimation of the voltage derivative of the power dPpv/dVpv (or current, dIpv/dVpv), compelling the PV array to operate at or very close to the maximum power point. The analysis and calculation of the low-frequency PV current and voltage ripple harmonic components in the three-phase flying capacitor inverter is presented first, with reference to centered carrier-based three-level PWM. The whole grid-connected PV generation scheme has been implemented by MATLAB/Simulink, and detailed numerical simulations verified the effectiveness of the control method in both steady-state and dynamic conditions, emulating different sun irradiance transients.

Novel adapted de‐loading control strategy for PV generation participating in grid frequency regulation

Existence PV generation mostly operates under the Maximum Power Point Tracking (MPPT) mode and has no ability to increase or decrease active power, accordingly, PV generation does not participate in grid frequency regulation. Due to an increasing penetration of grid-connected PV generation, when a frequency contingency event occurs, grid may have neither enough inertia response nor governor support and frequency deviation may have a risk of exceeding the limit. Therefore, a novel adapted de-loading control strategy without MPPT for grid-connected inverters is proposed here. This control strategy provides a reserve power in PV system to meet the frequency control demand and an online output Active Power-Voltage Matching (APVM) technique, which is generated as a control flowchart, is implemented in this control strategy to make it possible to adjust PV output voltage performance matching different active power demand with different operation modes. The proposed control strategy of PVs is simulated using PSCAD/EMTDC Software. Results verify the correctness and effectiveness of the proposed adapted de-loading control strategy under several operating conditions.

Equivalent Modeling of Photovoltaic Power Station Based on Canopy-FCM Clustering Algorithm

Grid-connected photovoltaic power generation has become an effective way to utilize solar energy. In order to accurately analyze the dynamic characteristics of a grid-connected photovoltaic power station, an equivalent modeling method based on the Canopy-FCM clustering algorithm is proposed. Considering the lack of theoretical basis for the selection of the traditional clustering index, this paper deduces the transfer function of the grid-connected inverter control system based on the detailed model and proposes a new clustering index, which takes the zero-pole expression of the closed-loop transfer function as the prototype. In order to improve the fuzzy C-means clustering method, which is sensitive to the initial clustering centers and the outliers, a Canopy algorithm with lower computation cost is introduced. Before fuzzy C-means clustering analysis, the Canopy algorithm is used to determine the initial clustering centers and the number of clusters to improve the accuracy and efficiency of the clustering algorithm. The effectiveness of the proposed method is verified by simulation examples.

Influence of tilt angle on energy yields and performance ratios of grid connected photovoltaic generators in Southeast Asia

This study aims to analyse the influence of tilt angle on energy yields and performance ratio of photovoltaic (PV) systems with three PV technologies over the selected cities in Southeast Asia. 10 kWp grid connected PV generator is proposed with net-metering option in Jakarta, Jayapura, Kuala Lumpur, Singapore and Bangkok. Simulation is carried for three different PV technologies positioned at tilt angle (0° to 90° with 15° step variations). It is observed that, PV generators performance in Southeast Asia is in order of Si-amorph, Si-mono and Si-poly. Energy yield is moderate in PV generators at 0° (horizontal) and at zenith, i.e., 90° the output is little. PV generators positioned at 15° tilt angle in Jakarta, Jayapura, Kuala Lumpur and Bangkok shows better energy yields and performance ratio (PR). In similar, PV generator positioned at close to 0° tilt at Kuala Lumpur and Singapore City shows better energy yields and PR.

An Advanced Two-Stage Grid Connected PV System: A Fractional-Order Controller

A fractional-order (FO) based controller for a grid-connected PV system is presented in this paper. A single phase two-stage grid-connected photovoltaic generator (PVG) is used to test the performance of the FO controller. The main objectives of the proposed controller are: (1) To regulate the output voltage of PVG at a point where the maximum power is drawn. (2) Constant DC-link voltage control. (3) Power factor control (PFC) at the inverter output with low total harmonic distortion (THD). To solve the first problem, a non-linear control method known as fractional-order back-stepping control (FOBSC) is used to regulate the output voltage of PVG. A maximum power point tracking (MPPT) technique known as perturb and observe (P&O) is used to generate a reference voltage which is suitable for tracking the maximum power generation of PVG. The generated reference is used to regulate the output voltage of PVG using FOBSC. The DC-link voltage fluctuation problem is tackled using FO based PI controller. The last objective is achieved through utilizing FOBSC to obtain maximum power factor of the grid. Lyapunov candidate function is used to verify the stability of the system. To test the performance of the proposed controller, it is compared to conventionally known Integer-order (IO) controller, where the results have shown a significant improvement in THD and efficiency of the system. The proposed controller offers 0.94%, 1.43% and 1.86% lower THD in comparison with IO controller at 100%, 80% and 70% of the power generation capacity of PVG, respectively . The overall efficiency of the system for 100%, 80%, and 70% of the dynamic powers of the system is noticed to be better in case of FO controller.

Optimal placement of grid-connected photovoltaic generators in a power system for voltage stability enhancement

The high penetration of photovoltaic (PV) generation can cause many technical issues such as power quality and impact on the power system voltage stability. To improve voltage stability and reducing power loss in a power system with PV generators, appropriate planning of PV generators is considered by optimal placement of PV. Thus, an effective heuristic optimization technique such as the Wind Driven Optimization (WDO) technique is applied for determining optimal location of PV generators in a power system. For determining the optimal location of PV, the objective function considers maximizing the Improved Voltage Stability Index. The proposed method for optimal location of PV generators is implemented on the IEEE 118 and 30 bus transmission systems and the 69-radial distribution system. The optimization results show that integrating PV into the test systems improves voltage stability in the system. Comparing the performance of the WDO with the particle swarm optimization technique, it is shown that the WDO technique gives faster convergence.