Single-stage PV Research Articles

Optimization Design and Control of Single-Stage Single-Phase PV Inverters for MPPT Improvement

Due to the inherent double-frequency (2 f 0) ripple in single-stage single-phase photovoltaic grid-connected inverters, the maximum power point tracking (MPPT) will inevitably be affected. To improve the MPPT performances, a passive LC power decoupling circuit with a robust second-order sliding-mode control (SOSMC) is thus proposed in this article. With the passive LC decoupling path, the double-frequency pulsation on the dc link is effectively cancelled out. Thus, the MPPT accuracy is significantly enhanced, and the utilization of a small dc-link capacitor becomes possible. However, resonance between the LC circuit and the main dc-link capacitor may appear, which can be damped through an active damping method. Additionally, the proposed SOSMC ensures good steady-state, dynamic performance (voltage fluctuation and settling time), and the robustness of the dc-link voltage, which is also beneficial to MPPT control in terms of high accuracy and fast dynamics. The systematic design of SOSMC is presented, and a detailed parameter optimization design of LC decoupling circuit is discussed. Experimental tests are performed on a 2.5-kW single-stage single-phase grid-connected inverter, and the results validate the effectiveness of the proposed strategy.

Fast terminal sliding mode control-based direct power control for single-stage single-phase PV system

Today, grid-connected photovoltaic systems have gained widespread penetration among renewable energy systems. For low power applications, a single-phase inverter with less power converter is a good compromise for high efficiency. The control must make it possible to extract the maximum power from the photovoltaic modules, ensure good dynamic performance for active and reactive power injection, ensure power quality, and reject disturbances and parameter mismatch. Besides, the controllers of the grid and PV sides should be coordinated. In this study, a fast terminal sliding mode control combined with Direct Power Control is proposed. Thanks to the two-cascaded control loops, simulations and experimental results with a 1 kW test bench have proven the proposal’s effectiveness in terms of dynamic performances and robustness to irradiance variations. Comparison with deadbeat-Direct Power Control, predictive control, and power hysteresis control shows that our proposal leads to lower Total Harmonic Distortion (3,5%) for the electrical grid’s current and lower time response (one-tenth of half the grid cycle).

Frequency- adaptive control of a three-phase single-stage grid-connected photovoltaic system under grid voltage sags

The low-voltage ride-through service is carried out in this paper according to the voltage profile described by the IEC 61400-21 European normative when short-duration voltage sags happen, and some instantaneous reactive power is delivered to the grid in accordance with the Spanish grid code; the mandatory limitation of the amplitude of the three-phase inverter currents to its nominal value is carried out with a novel control strategy, in which a certain amount of instantaneous constant active power can also be delivered to the grid when small or moderate voltage sags happen. A Multiple second order generalized integrator frequency-locked loop synchronization algorithm is employed in order to estimate the system frequency without harmonic distortions, as well as to output the positive- and the negative- sequence of the αβ quantities of the three-phase grid voltages when balanced and unbalanced voltage sags happen in a frequency- adaptive scheme. The current control is carried out in the stationary reference frame, which guarantees the cancellation of the harmonic distortions in the utility grid currents using a Harmonic compensation structure, and the implementation of a constant active power control in order to protect the DC link capacitor from thermal stresses avoiding the appearance of large harmonic distortions at twice the fundamental frequency in the DC link voltage. A case study of a three-phase single-stage grid-connected PV system with a maximum apparent power about 500 kVA is tested with several simulations using MATLAB/SIMULINK firstly, and secondly, with some experiments using the Controller hardware-in-the-loop (CHIL) simulation technique for several types of voltage sags in order to do the final validation of the control algorithms.

A Novel MPPT Technique for Single Stage Grid-Connected PV Systems: T4S

In this paper, a novel maximum power point tracking (MPPT) technique, which has been named T4S (a technique based on the proper setting of the sign of the slope of the photovoltaic voltage reference signal), is presented and discussed. It is specifically designed with reference to a single-stage grid-connected PV system. Its performance is numerically compared with that of the well-known and widely used perturb and observe (P&O) MPPT technique. The results of the numerical simulations confirm the validity of the proposed MPPT technique which exhibited a slightly better performance, under stationary and also time-varying irradiance conditions. In addition, the T4S technique is characterized by the following features: it does not require explicit power detection or calculation and, moreover, it allows the tracking of the maximum average power injected into the grid rather than the tracking of the maximum instantaneous power extracted by the PV source.

Energy Management and Control of Single-Stage Grid-Connected Solar PV and BES System

In general, Solar Photovoltaic (SPV) is integrated to grid through a DC-DC converter and Voltage Source Converter (VSC) for real power injection (called two-stage conversion). In view of efficiency point, the single-stage conversion becomes popular and in which Maximum Power Point Tracking (MPPT) of SPV and real power injection are achieved with VSC alone. But, if Battery Energy Storage (BES) supported by bidirectional DC-DC converter is presented in the single-stage conversion system, then co-ordination between DC-DC converter and VSC is required to achieve simultaneous operation of MPPT and real power injection. In this paper, a co-ordinated control of single-stage grid connected SPV and BES system is proposed along with energy management. In which, the algorithm coordinates VSC and bidirectional DC-DC converter based on the State of Charge (SoC) of the battery such that MPPT and power injection is achieved simultaneously. The proposed method not only injecting real power, but also works to compensate load reactive power and unbalanced neutral current minimization. Further, an active rectification operation during non-SPV hours is discussed for better utilization of VSC capacity. The multi-functional features of the proposed method are explained using simulation studies and are also validated through experimental studies.

An Efficient Single-Sourced Asymmetrical Cascaded Multilevel Inverter With Reduced Leakage Current Suitable for Single-Stage PV Systems

An isolated single sourced multi-output dc/dc converter with a high-frequency link was proposed before to feed different cells of the Asymmetrical Cascaded H-bridge (ACHB) inverter. One of the fundamental advantages of the ACHB is that the main H-bridge is commutated with fundamental frequency, passing the majority of the inverter power. However, the isolated dc/dc converter needs to be connected to the main voltage source with high-voltage/high-frequency switches, which limits the power capability and the efficiency of the converter. In this paper, a switching pattern based on nearest level modulation method is proposed to command all the power to be transferred through the main H-bridge. Therefore, the power circulates between the auxiliary H-bridges with no need for high-voltage switches. As a result, a single-input single-output auxiliary converter is needed with only 5% of the power rating. This solution reduces cost and size while it improves the efficiency of the converter. Furthermore, the architecture makes the inverter more suitable for high power and high-voltage applications. The inverter switching strategy is based on programmable firing angles. The main dc-link of the grid connected inverter is controlled to achieve maximum power point tracking, while the dc/dc isolated converter is controlled to regulate the auxiliary dc-links. The proposed topology is verified by simulation and experimental results using a 1.5 kW hardware prototype.

Low Voltage Ride through Control Capability of a Large Grid Connected PV System Combining DC Chopper and Current Limiting Techniques

This paper presents the development and performance capability of a comprehensive Low voltage ride through (LVRT) control scheme that makes use of both the DC chopper and the current limiting based on the required reactive power during fault time. The study is conducted on an 8.5 MW single stage PV power plant (PVPP) connected to the Rwandan grid. In the event of fault disturbance, this control scheme helps to overcome the problems of excessive DC-link voltage by fast activation of the DC chopper operation. At the same instance, AC current is limited to the maximum rating of the inverter as a function of the injected reactive current. This helps overcome AC-over- current that may possibly lead to damage or disconnection of the inverter. The control scheme also ensures voltage support and power balance through the injection of reactive current as per grid code requirements. Selected simulations using MATLAB are carried out in the events of different kinds of fault caused voltage dips. Results demonstrate the effectiveness of the proposed LVRT control scheme.

A Multiple Improved Notch Filter-Based Control for a Single-Stage PV System Tied to a Weak Grid

In this paper, a control scheme based on improvement in a generalized integrator is implemented on a three-phase single-stage grid-tied solar photovoltaic system with the distribution static compensator capabilities under grid abnormal conditions of voltage distortion and voltage unbalance. The photovoltaic voltage-source converter system compensating for the reactive power consumed by nonlinear load at point of common coupling provides load balancing and mitigates harmonics. The proposed multiple-improved-notch-filter-based quadrature signal generator control approach extracts the load current fundamental component, independent of the grid voltage. This control has better dc offset and harmonics component rejection capability in comparison to a conventional second-order generalized integrator algorithm. The perturb and observe-based maximum power point tracking algorithm is applied for the extraction of maximum power from the photovoltaic array. The system is analyzed under different abnormal conditions of voltage distortions, voltages unbalance, voltage swell, voltage sag, load currents unbalance, and insolation change on a prototype developed in the laboratory. The system performance is found to be satisfactory, within limits as described in an IEEE-519 standard while feeding active power to distribution network and connected loads.

A Direct Maximum Power Point Tracking Method for Single-Phase Grid-Connected PV Inverters

A direct maximum power point tracking (MPPT) method for PV systems has been proposed in this work. This method solves two of the main drawbacks of the perturb and observe MPPT, namely: 1) the tradeoff between the speed and the oscillations in steady-state and 2) the poor effectiveness in dynamic conditions, especially in low irradiance when the measurement of signals becomes more sensitive to noise. The proposed MPPT is designed for single-phase single-stage grid-connected PV inverters and is based on estimating the ripple of the instantaneous PV power and voltage, using a second-order generalized integrator-based quadrature signal generator. We analyzed the global stability of the closed-loop control system and validated the proposed algorithm through simulation and experiments on an inverter test platform according to the EN 50530 standard. The experimental results confirm the performance of the proposed method in terms of both speed and tracking efficiency.

Microinverter Topology based Single-stage Grid-connected Photovoltaic System: A Review

This paper discussed the topology development of a single-stage microinverter in grid-connected PV system. In general, the microinverter topologies can be categorized into four type of topologies: 1) Flyback inverter, 2) Double-boost inverter, 3) Derived zeta-cuk configuration and 4) Buck-boost inverter. Flyback configuration is widely used for single-stage microinverter which offers protection between solar panel and utility grid. However due to the bulkiness of the transformer, new arrangement circuit employ the Half-Bridge topology with film capacitor and microcontroller provide a good room for research and future developments to obtain greater efficiency and compact design of single-stage microinverter grid-connected PV system. Plus, there are several characteristics need to be taken care for future development of the microinverter technology.

Optimal PI microcontroller-based realization for technical trends of single-stage single-phase grid-tied PV

The paper introduces optimal PI controllers for a single-phase single-stage PV grid-tied inverter. In the proposed model, a time domain objective function based on the integral square error (ISE) of the current and voltage errors is formulated in order to obtain the PI controllers in an offline manner. The performance of the developed controllers is simulated via MATLAB/SIMULINK whereas a prototype was implemented to verify the effectiveness of the developed controllers. The developed controllers enable the developed inverter to provide active power to the utility grid with reactive power compensation. From the simulated and experimental results, the developed controller had better performance over different operating conditions. Besides, it is simple, low cost, and attractive for households’ energy production.

A Single-Stage Stand-Alone Photovoltaic Energy System With High Tracking Efficiency

A single-stage flyback PV inverter with maximum power point tracking (MPPT) high-speed capability is presented in this paper. The proposed stand-alone photovoltaic energy system (SAPES) incorporates the use of hybrid MPPT to ensure peak energy harvesting under all weather conditions. The proposed hybrid MPPT method combines the conventional short current pulse (SCP) MPPT method and the perturb and observe (P&O) methods. Measurement of the offline parameter [short-circuit current of PV module ( ${I}_{\rm sc}$ )] for SCP is made on the basis of the difference between the offline parameter and the instantaneous current of PV module ( ${I}_{\rm pv}$ ). Use of a single power conversion stage is achieved by employing a modified flyback inverter operating in a discontinuous-conduction mode. The control structure of the conventional flyback inverter is also modified to incorporate the hybrid MPPT algorithm. The proposed SAPES method is tested using computer-aided simulations and real-time hardware in the loop experimentation using the dSPACE DS1104 board. The results obtained using the proposed systems are better than those for the conventional algorithms under the environmental conditions tested.

Realization of single-phase single-stage grid-connected PV system

This paper presents a single phase single stage grid-tied PV system. Grid angle detection is introduced to allow operation at any arbitrary power factor but unity power factor is chosen to utilize the full inverter capacity. The system ensures MPPT using the incremental conductance method and it can track the changes in insolation level without oscillations. A PI voltage controller and a dead-beat current controller are used to ensure high quality injected current to the grid. The paper investigates the system structure and performance through numerical simulation using Matlab/Simulink. An experimental setup controlled by the MicrolabBox DSP prototyping platform is utilized to realize the system and study its performance. The precautions for smooth and safe system operation including the startup sequence are fully considered in the implementation.

Real-time digital simulation-based modeling of a single-phase single-stage PV system

This paper presents real-time digital simulation-based modeling of a single-phase single-stage PV system. Discrete models of voltage source converter (VSC) controls, including proportional resonant (PR) current control and phase-locked-loop (PLL) are developed in RT-LAB. An improved incremental conductance-based maximum power point tracking (MPPT) method that can mitigate error signal spikes is proposed and modeled in RT-LAB as well. Simulation results demonstrate the faster response of the proposed MPPT and the real-time simulation performance of the developed system model.

The Summary of the Inverter Used in PV System

the inverter is one of the key technologies in PV system. It transforms the DC power output by the PV panel to AC power that the grid needs. The main technologies of the inverter are the circuit topology and the control tactic. The circuit topologies of inverter including the frequency isolated PV grid-connected inverters, the high-frequency isolated PV grid-connected inverter, the single-stage non-isolated PV grid-connect inverter and the multi-level non-isolated PV grid-connected inverter. The control tactics including the voltage oriented control (VOC), the voltage-based direct power control (V-DPC), the virtual-flux oriented control (VFOC) and the virtual-flux based direct power control (VF-DPC).

An Improved Power Electronic Controller with Unity Power Factor for a Single-Stage Grid-Tied PV System

A single-stage PV system using Z-source inverter (ZSI) capable of operating in grid-connected mode or stand-alone mode has been developed for extracting power from PV array. For feeding the extracted power to the grid, an analog current controller adjusting the modulation index of ZSI combined with the shoot-through duty ratio has been designed and developed. The proposed system has the advantage of using less number of power converters and can be built with simple controllers. Experiments have been conducted on a PV array consisting of seven PV panels (each rated for 21.2V, 5.17A) connected in series, feeding a 110V single-phase utility grid through ZSI. A hysteresis current controller for varying the modulation index of ZSI has been built using analog ICs. Using MATLAB/Simulink, the complete system has been simulated and the simulation results closely correlate with experimental ones. The efficacy of the proposed scheme with the power being fed to the grid at unity power factor is indicated by the results. It is also proved that the total harmonic distortion of both voltage and current waveforms fed to the grid terminals is less than 5%.

Power Quality Improvement in a Grid Connected Single Stage PV Generating System by integrating Shunt Active Filter for both connection of PV cells and power Quality Improvement

Power Quality Improvement in a Grid Connected Single Stage PV Generating System by integrating Shunt Active Filter for both connection of PV cells and power Quality Improvement

Research on Single-Stage Grid-Connected Photovoltaic System Based on orCAD/PSpice Analog Behavioral Model

According to mathematical physics model of PV Cell, the simulation model of Si PV array as well as controller model of PV power system is constructed with adoption of orCAD/PSpice10.5 ABM. On this basis, single-stage grid-connected PV system is simulated, and the system which is influenced by intensity of illumination and temperature is analyzed with CAD/PSpice simulation. The simulation results shows that large PV array power system can be simulated to a high approximation degree with CAD/PSpice ABM, and simulation of grid-connected PV power system based on orCAD/PSpice ABM is of practical significance.

Reactive Power Dynamic Assessment of a PV System in a Distribution Grid

Accommodating more and more PV systems in grids has raised new challenges that formerly had not been considered and addressed in standards. According to recently under-discussed standards, each PV unit is allowed to participate in reactive power contributions to the grid to assist voltage control. There are some PV models in the literature however those models mostly assumed unity power factor operation for PV systems owing to the contemporary standards. Therefore, there is a need to develop a PV model considering the reactive power contribution and its dynamic influence on power system. This paper describes non-proprietary modeling of a three-phase, single stage PV system consisting of controller scheme design procedure and coping with the important aspects of three different reactive power regulation strategies and their impact assessment studies. The model is implemented in PSCAD to examine the behavior of the proposed model for recently codified reactive power strategies. Furthermore, this model is integrated in a distribution grid with two PV systems in order to effectively investigate consequences of the different reactive power control strategies on the distribution network.