Photovoltaic Grid-connected Inverter Research Articles

An AC MPPT with Active/Reactive Power Control Feature for Single-Stage Three-Phase Grid-Connected Photovoltaic VSIs

This article introduces a new AC MPPT (maximum power point tracking) controller for single-stage three-phase grid-connected photovoltaic (PV) voltage source inverters (VSIs) with active/reactive power control capability. Novelty lies in employing the theory of power transfer in short AC transmission lines to achieve MPPT, where two independent control variables exist: load angle and inverter modulation index. Accordingly, the proposed controller maximizes the capabilities of the single-stage VSI and enables it to control either/both active power or/and reactive power, besides achieving MPPT function. Such contribution opens up avenues in MPPTs world in terms of flexibility, fast performance, and low hardware count/cost. AC refers to Alternate Current since the MPPT is managed in AC terms. Theoretical framework of the proposed controller has been addressed. Moreover, the overall system is simulated in MATLAB (The MathWorks, Natick, Massachusetts, USA) environment, and results have been introduced. Design and implementation of proposed algorithms have been carried out using a fixed-point digital signal processor (DSP)TMS320F2812. Furthermore, a breadboard circuit is built up for experimentally investigating the controller's performance using 3-phase VSI that connects PV with the grid. In addition to that, a comprehensive discussion is conducted for demonstrating results. Moreover, a detailed comparison of proposed work with a number of previously published papers in literature is carried out and addressed in this manuscript. Results reveal that introduced controller achieves satisfactory performance.

Design of PR current control with selective harmonic compensators using Matlab

This paper presents a procedure to design a Proportional Resonant (PR) current controller with additional PR selective harmonic compensators for Grid Connected Photovoltaic (PV) Inverters. The design of the PR current control and the harmonic compensators will be carried out using Matlab. Testing was carried out on a 3kW Grid-Connected PV Inverter which was designed and constructed for this research. Both simulation and experimental results will be presented.

Direct power control of a grid-connected photovoltaic system using a fuzzy-logic based controller

In this paper a fuzzy logic-based controller is proposed for direct power control of three-phase grid-connected photovoltaic PV inverters. To demonstrate the effectiveness of the control method, a typical grid-connected PV inverter is implemented and the performance of the proposed technique is compared to a traditional proportional-integral PI controller. Numerical simulation results illustrate that the proposed fuzzy controller performs well in independently regulating the active and reactive output powers. Moreover, comparisons of simulation results between fuzzy and PI controllers show advantages of the fuzzy-based technique in terms of transient response characteristics, such as fast response, small overshoot, acceptable steady-state fluctuation, and high robustness.

New parameter for current-sensorless MPPT in grid-connected photovoltaic VSIs

This paper a new quantity for current-sensorless power-angle-based MPPT for single-stage grid-connected photovoltaic voltage-source inverters (VSIs). The proposed tracker eliminates the use of the current sensor by using only the voltage sensor: The current sensor is substituted with a new quantity, which is the sine value of the power-angle (the phase angle between the inverter output voltage and the power grid voltage). A theoretical proof for this quantity is developed and is provided. Then the controller is designed and implemented using a digital signal processor (DSP). Algorithms are configured on a fixed point DSP TMS320F2812. Moreover, a breadboard has been built-up for testing the use of the proposed tracker with a single-stage grid-connected photovoltaic voltage-source inverter. Both of simulation and experimental results show that the proposed tracker attains a satisfactory performance. On the other hand, it requires a good control tool to accurately achieve the arithmetic calculations. A substantial part of the manufacturing cost and complexity burdens of MPPTs involves the use of current sensors. Considering this investigation saves cost, decreases complexity, and therefore increases the power density of the MPPTs.

Design and Simulation of Adaptive Controller for Single Phase Grid Connected Photovoltaic Inverter under Distorted Grid Conditions

This paper presents an adaptive controller for single-phase grid-connected photovoltaic inverter under abnormal grid conditions. The main problem associated with the controllers of the grid-connected inverter is that they are tuned for some assumed values of the electrical grid parameters. However, when the parameters, such as voltage and frequency are changed or the grid is subjected to uncertain distortion, these controllers unable to track those variations of the grid parameters and handle the output power within the allowable limit. To overcome such problem, a suitable control strategy is proposed, which is based on frequency adaptive current control and an accurate grid detection. For validity confirmation, a controlled 3KW system, with specific features was designed and simulated. The simulation results confirmed that the strategy is an effective way of control.

Comparative analysis of unity power factor grid-connected PV system with PI and fuzzy-based controllers

Herein, we report the comparative analysis of a proposed fuzzy logic controller with conventional incremental conductance algorithm for maximum power point tracking (MPPT) with proportional integral (PI) controller of a unity power factor grid-connected photovoltaic (PV) inverter. In the conventional controller, incremental conductance MPPT algorithm with PI controller has been implemented. In the fuzzy logic controller, inputs and outputs are chosen for this study, rules relating them are written and the simulation is performed. Simulation model of a 40 W solar panel using single diode model is developed and results are obtained for PI controller and fuzzy logic controller for different irradiation and temperature conditions. Results show the ability of fuzzy logic controller to give a performance nearly similar to that of incremental conductance algorithm with increased robustness. Also it is inferred that when the conditions become low, PI controller shows poor performance when compared to fuzzy logic controller.

Comparative analysis of unity power factor grid-connected PV system with PI and fuzzy-based controllers

Herein, we report the comparative analysis of a proposed fuzzy logic controller with conventional incremental conductance algorithm for maximum power point tracking (MPPT) with proportional integral (PI) controller of a unity power factor grid-connected photovoltaic (PV) inverter. In the conventional controller, incremental conductance MPPT algorithm with PI controller has been implemented. In the fuzzy logic controller, inputs and outputs are chosen for this study, rules relating them are written and the simulation is performed. Simulation model of a 40 W solar panel using single diode model is developed and results are obtained for PI controller and fuzzy logic controller for different irradiation and temperature conditions. Results show the ability of fuzzy logic controller to give a performance nearly similar to that of incremental conductance algorithm with increased robustness. Also it is inferred that when the conditions become low, PI controller shows poor performance when compared to fuzzy logic controller.

Assessment of Grid-Side Filters for Three-Phase Current-Source Inverter PV Systems

In this paper, three grid-side filter topologies for grid-connected three-phase photovoltaic current-source inverters are designed and assessed based on the Total Harmonic Distortion (THD) of the grid currents, on the power factor in the connection to the grid, and on the filter losses. A second-order CL filter with passive damping resistors in parallel with the filter inductors is first sized and evaluated. It is then improved by removing the resistors and using active damping, with the virtual resistor method and feedback of the inductor voltages requiring no additional measurements. The analysis is conducted taking into account the delays introduced in the control circuit and the cross-terms in the state-space equations in the synchronous dq frame. A fourth-order CLCL filter is also sized and considered as an alternative grid-side filter topology to reduce the switching harmonics. Simulation and experimental results obtained with a direct control approach confirm the proper sizing and operation of the filters. Based on the THD of the grid currents and filter losses, the second-order filter with active damping presents the best results with a THD lower than 5 % and nearly negligible losses.

A New Approach to Design Control Method for Grid- Connected WT and PV Inverters in Micro-Grid

Background/Objectives: The objective of this paper is to a simple but very effective method to achieve the predictive direct compensation current control for three phase grid-connected Wind Turbine (WT) and Photo-Voltaic (PV) inverters in micro-grid. Methods/Statistical Analysis: Our proposed grid-connected power inverter for PV consists of a switch mode DC-DC boost converter and Pulse-Width Modulation (PWM) inverter. The proposed control method is comprised of the Advance Synchronous Reference Frame (ASRF) method. Findings: The control method on PV designed to eliminate main harmonics and also is responsible for the injection power to the grid. The novel approach is able to increase the inverter output current around 8 times of conventional one. Applications/Improvements: The simulations for threephase Bridge type inverter have been done in MATLAB/Simulink. To validate the simulation results, a scaled prototype model of the proposed inverter has been built and tested.

Impact of Common Mode Signal Injection on the Leakage Current of a Grid-connected Transformerless Pv Inverter

Leakage current is one of the most relevant problems in grid-connected transformerless photovoltaic (PV) inverters due to parasitic capacitance of the PV modules. This problem is directly related to the common mode voltage generated by inverter modulation. Therefore, this paper presents an analysis of the impact of the common mode signal usually injected in carrier based modulation approaches, aiming the reduction of the leakage current in three-phase grid-connected transformerless PV inverter. The resulting leakage current is analyzed for an inverter topology with a modified LCL filter, which is utilized to attenuate the high frequency components of the voltage on the PV parasitic capacitance. Experimental results are given to evaluate the performance of a 10 kW transformerless PV inverter.

Voltage Ripple-Based Passive Islanding Detection Technique for Grid-Connected Photovoltaic Inverters

One of the main challenges of integrating distributed generation into the power grid is islanding, which occurs when a disconnected power line is adversely energized by a local distributed generation source. If islanding is not quickly detected, it can present serious safety and hazardous conditions. Conventional passive detection techniques used today are entirely dependent on the parameters of the power system, which under certain operating conditions may fail to detect islanding. In this paper, a novel and efficient passive islanding detection technique for grid-connected photovoltaic-based inverters is presented. In this technique, the ripple content of the inverter output voltage at the point of common coupling is monitored for deviations using time-domain spectral analysis. Islanding is then detected whenever the ripple spectral content exceeds a preset threshold level for a certain period of time. The performance of this technique was extensively tested and quantified under a wide range of operating conditions. It was determined that the proposed technique did not exhibit any non-detection zone and was able to detect all types of islanding cases within 300 ms of the allowed delay time. Furthermore, the proposed technique was found to be robust and inherently immune to other degrading factors, since it is relatively independent of system parameters, power system scaling, or the number of distributed generation sources present within the islanding zone.

Design of a Grid-connected Photovoltaic Inverter with Maximum Power Point Tracking using Perturb and Observe Technique

There is no easy way to convert Photovoltaic (PV) energy with high efficiency due to dynamic changes in solar irradiance and temperature. This paper illustrates a control strategy to design and implementation of Maximum Power Point Tracking (MPPT) in a photovoltaic system using Perturb and Observe (P&O) algorithm. The PSIM simulation results confirm proper functioning of the proposed MPPT sub-circuit to achieve a constant 48V DC output from fluctuating voltage of solar panel by varying duty cycle of the MOSFET in the 24V-48V boost converter. The filtered output waveform of the SPWM driven H-bridge inverter via the L-C low pass filter is found to be a pure sine-wave of 48V peak which is then stepped-up 312V peak (220V rms) by using a step up transformer. The frequency of output voltage is found to be 50Hz with a total harmonic distortion (THD) of 0.001 which is much lower than the IEEE 519 standard.

A Robust Continuous-Time MPC of a DC–DC Boost Converter Interfaced With a Grid-Connected Photovoltaic System

The main function of the DC-DC converter in a grid-connected photovoltaic (PV) system is to regulate the terminal voltage of the PV arrays to ensure delivering the maximum power to the grid. The purpose of this paper is to design and practically implement a robust continuous-time model predictive control (CTMPC) for a dc-dc boost converter, feeding a three-phase inverter of a grid-connected PV system to regulate the PV output voltage. In CTMPC, the system behavior is predicted based on Taylor series expansion, raising concerns about the prediction accuracy in the presence of parametric uncertainty and unknown external disturbances. To overcome this drawback, a disturbance observer is designed and combined with CTMPC to enhance the steady-state performance in the presence of model uncertainty and unknown disturbance such as the PV current, which varies nonlinearly with the operating point. An interesting feature is that the composite controller reduces to a conventional PI controller plus a predictive term that allows further improvement of the dynamic performance over the whole operating range. The effectiveness of the proposed controller was tested numerically and validated experimentally with the consideration of the grid-connected PV inverter system and its controller.

DC/AC conversion efficiency of grid-connected photovoltaic inverters in central Mexico

The DC/AC conversion efficiency of grid-connected photovoltaic inverters depends on climatic characteristics, technical characteristics of the inverters and PV modules, array orientation, ratio of array peak power to inverter nominal power and DC input voltage fed to the inverter. The influence of the DC input voltage in DC/AC conversion efficiency was investigated by Zhu et al. (2011) for a high-latitude maritime climate. In this paper, all the aforementioned factors, including the DC input voltage, are analyzed for the case of the low-latitude semi-desert climate of Aguascalientes, central México. Solar irradiance and temperature measurements of 10years are used to generate a year of typical characteristics which is the basis of the research. Two commercial grid-connected photovoltaic inverters with different efficiency behavior with respect to DC input voltage and PV technologies based on crystalline silicon and Cadmium Telluride are characterized. The research reveals that maximum losses in annual inverter efficiency due to a sub-optimal string configuration are 1.3% for the analyzed technologies. Rules for sizing the array voltage and array power which keep high annual inverter efficiency while reducing the balance-of-systems costs are recommended. For optimally oriented systems, the recommended array-to-inverter power sizing ratio is 1.05 for c-Si and 0.95 for CdTe independently of the chosen inverter at central México. The methodology can be replicated to other locations, inverters and photovoltaic technologies.

Compensation techniques for non-linearities in H-bridge inverters

This paper presents compensation techniques for component non-linearities in H-bridge inverters as those used in grid-connected photovoltaic (PV) inverters. Novel compensation techniques depending on the switching device current were formulated to compensate for the non-linearities in inverter circuits caused by the voltage drops on the switching devices. Both simulation and experimental results will be presented. Testing was carried out on a PV inverter which was designed and constructed for this research. Very satisfactory results were obtained from all the compensation techniques presented, however the exact compensation method was the most effective, providing the highest reduction in harmonics.

A ripple-based maximum power point tracking method for three-phase grid-connected photovoltaic inverter

As a prevailing solar energy utilization equipment, the three-phase grid-connected photovoltaic (PV) inverter is widely operated in partially shaded conditions and thus tends to generate multiple local maximum power points on its power-to-voltage and current-to-voltage characteristic curves. In order to identify the global maximum power point (GMPP) quickly and precisely, this paper proposes a ripple-based maximum power point tracking method. It aims to perform the optimization of tracking using the segmented scanning of DC-side voltage. An improved adaptive perturb and observe (AP&O) method is introduced to maximize the solar conversion and to increase working stability. This method applies a hybrid model of fixed and variable step-size perturbation to restrain the fluctuation of PV-side voltage. It belongs to a two-stage GMPP tracking method. That is, when environmental factors such as irradiance and temperature change quickly PV power fluctuates sharply. Correspondingly, the AP&O method tracks the latest maximum power point (MPP) with a large fixed-step voltage reference command. When the PV power fluctuates smoothly under a slow environmental change rate, the algorithm applies multiple small and variable step-size voltage perturbations to vibrate round the location of GMPP. Simulation and experimental results show that this method improves the efficiency of the PV inverter tracking performance. In addition, the stability of DC bus voltage is guaranteed, and the operational stability of the photovoltaic power generation system is improved.

A Voltage Controlled PV Inverter for a Certs Microgrid Applications

The Photovoltaic (PV) sources are more compatible for Microgrids because of their ability to internally aggregate and balance with other renewable sources. The conventional grid connected PV inverters are basically current source controlled so unable to control ac voltage or frequency. The PV inverter using the Consortium for Electric Reliability Technology Solutions (CERTS) concepts can control ac voltage and frequency but have a major problem with load transients. The over loaded PV micro source causes dc bus voltage collapse and finally resulting in an ac voltage drop during the load transients. A photovoltaic inverter is capable of operating in both island and grid-connected mode by means of a reconfigurable control scheme. This paper presents an efficient control strategy which causes the PV inverter to act as voltage source to maintain stable dc bus voltage under load transients. With this PV inverter control configuration, it is shown that the PV micro source can operate as a voltage source in the CERTS microgrid. Evolutionary changes in the regulatory and operational climate of traditional electric utilities and the emergence of smaller generating systems such as micro sources have opened new opportunities for on-site power generation by electricity users. In this context, distributed energy resources (DER) - small power generators typically located at users' sites where the energy they generate is used - have emerged as a promising option to meet growing customer needs for electric power with an emphasis on reliability and power quality. The portfolio of DER includes generators, energy storage, load control, and, for certain classes of systems, advanced power electronic interfaces between the generators and the bulk power provider. This paper proposes that the significant potential of smaller DER to meet customers' and utilities' needs can be best captured by organizing these resources into Micro Grids. The Consortium for Electric Reliability Technology Solutions (CERTS) MicroGrid concept assumes an aggregation of loads and micro sources operating as a single system providing both power and heat. The majority of the micro sources must be power electronic based to provide the required flexibility to insure operation as a single aggregated system. This control flexibility allows the CERTS MicroGrid to present itself to the bulk power system as a single controlled unit that meets local needs for reliability and security. The CERTS Micro Grid represents an entirely new approach to integrating DER. Traditional approaches for integrating DER focus on the impacts on grid performance of

A Unified Current Loop Tuning Approach for Grid-Connected Photovoltaic Inverters

High level penetration of renewable energy sources has reshaped modern electrical grids. For the future grid, distributed renewable power generation plants can be integrated in a larger scale. Control of grid-connected converters is required to achieve fast power reference tracking and further to present grid-supporting and fault ride-through performance. Among all of the aspects for converter control, the inner current loop for grid-connected converters characterizes the system performance considerably. This paper proposes a unified current loop tuning approach for grid-connected converters that is generally applicable in different cases. A direct discrete-time domain tuning procedure is used, and particularly, the selection of the phase margin and crossover frequency is analyzed, which acts as the main difference compared with the existing studies. As a general method, the approximation in the modeling of the controller and grid filter is avoided. The effectiveness of the tuning approach is validated in both simulation and experimental results with respect to power reference tracking, frequency and voltage supporting.

계통연계형 태양광 인버터 병렬운전 상황에서의 단독운전 검출

The present invention relates to a method for detecting a stand-alone operation of a grid-connected photovoltaic parallel inverter. More specifically, the method prevents application of external disturbances in a parallel operation of the inverter to prevent a negative effect on the power quality of the inverter while enhancing the stand-alone operation detection performance. Accordingly, the method includes the steps of: detecting an angular frequency () of each output voltage from a phase locked loop (PLL) unit while executing a D-Q conversion operation on outputs of the inverter connected to a solar power generation system; measuring a frequency error () in a specific frequency band after removing a noise component in each frequency in a band pass filter (BPF) unit; and increasing the frequency error () to reflect an invalid current increase amount (gain(Kpf)) on an invalid current command value (id-ref) to increase an invalid current when the frequency error () exceeds a preset zero-band range.

Study on model current predictive control method of PV grid- connected inverters systems with voltage sag

According to the limitations of the LVRT technology of traditional photovoltaic inverter existed, this paper proposes a low voltage ride through (LVRT) control method based on model current predictive control (MCPC). This method can effectively improve the photovoltaic inverter output characteristics and response speed. The MCPC method of photovoltaic grid-connected inverter designed, the sum of the absolute value of the predictive current and the given current error is adopted as the cost function with the model predictive control method. According to the MCPC, the optimal space voltage vector is selected. Photovoltaic inverter has achieved automatically switches of priority active or reactive power control of two control modes according to the different operating states, which effectively improve the inverter capability of LVRT. The simulation and experimental results proves that the proposed method is correct and effective.