Grid-connected Inverter Research Articles

Point-on-Wave Voltage Phase-Angle Jump Sensitivity Analysis of Grid-Connected Single-Phase Inverters

An extensive experimental analysis of the behavior of thirty-one off-the-shelf distributed photovoltaic (DPV) inverters to voltage phase angle jump (VPAJ) disturbance is done in this paper. Classification of undesirable behavior from DPV inverters is carried out based on the responses to voltage phase angle jump and identified the cause of such behavior through simulation and experimental evidence. Grid-connected inverters can respond undesirably to grid disturbance, such as disconnection and power curtailment, which can put <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ 3-5$</tex-math></inline-formula> % of demand at risk and a cumulative <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ 3-4$</tex-math></inline-formula> % grid at risk. This is a much larger contingency than before. Further, statistical analysis demonstrates the impact of such disturbances on DPV inverters in the Australian National Electricity Market. A sensitivity analysis is carried out to identify the vulnerability of inverters for VPAJ that occur at different points-on-wave (POW). This research aims to determine the scale of at-risk generation, understand how they operate, and what had to be performed to stop or reduce the risk. The ultimate impact is how to model this effect properly to guide the responses of DPV inverters, also making a significant study case internationally for systems with high penetration of DPV inverters.

H2/H∞ Robust Observed-State Feedback Control Based on Slack LMI-LQR for LCL-Filtered Inverters

A robust controller for an LCL-filtered grid-connected inverter (GCI) is presented in this paper in the context of the system model uncertainties and non-ideal grid environment. Under the occurrence of exogenous inputs from parameter drifts affecting the controller and observer, the H₂&#x002F;H&#x221E; approach of the linear quadratic regulator (LQR) control is developed to obtain the state-feedback gain and observer gain with the system stability guarantee by means of Lyapunov stability theory. Furthermore, the use of slack-linear matrix inequalities (LMIs) to handle parameter-dependent Lyapunov functions in synthesizing the robust controller can achieve a less conservative condition than the conventional quadratic stability. System robustness and stability are assessed through the closed-loop pole map and the discrete-time frequency analysis approaches. Hardware experiments are conducted for the LCL-filtered GCI to validate the feasibility and effectiveness of the proposed control method. Fair comparisons with the conventional schemes are also performed to highlight the superior performance of the proposed scheme.

A DPWM-Based Quasi-Constant Switching Frequency Control for Full ZVS Range Three-Phase Inverter With Reactive Power Transfer Capability

In this paper, a quasi-constant switching frequency zero voltage switching (ZVS) control strategy is proposed for three-phase grid-connected inverters. Full ZVS range can be achieved for all the switches at any load, modulation index, and power factor using discontinuous pulse width modulation (DPWM). No additional high- frequency sensor or auxiliary circuit is needed. The switching frequency is constant at steady-state. It does not change with the phase angle or the power factor, but it varies with the modulation index, grid RMS voltage and current. The frequency can be easily calculated in a digital controller based on current ripple prediction (CRP) instead of current zero-crossing detection (ZCD). The turn-on loss is eliminated so the inverter can operate at a high switching frequency, especially by using wide band-gap devices. The conversion efficiency, power density, cost, and dynamic response of the inverter can all be improved. A 3.3 kVA experimental prototype using SiC MOSFETs interfacing 400 V DC link with three-phase 110 V AC grid is developed to verify the effectiveness of the proposed control strategy.

Partitional Collaborative Mitigation Strategy of Distribution Network Harmonics Based on Distributed Model Predictive Control

Dispersed harmonic control makes a critical guarantee for safe and stable operation of distribution networks. However, conventional point-to-point local control method has limitations. In addition, harmonic current injection has volatility and time-variability, and the residual capacity (RC) of multi-functional grid-connected inverter (MFGCI) employed for auxiliary harmonic compensation is uncertain, which challenges mitigation decision-making. In this study, a grid-side distributed mitigation strategy based on distributed model predictive control (DMPC) is proposed. Firstly, the voltage distortion observation node is selected based on node clustering, and sensitivity analysis of harmonic mitigation for observation nodes is conducted to divide the whole network into multiple control areas. Secondly, for global optimal of voltage distortion, the long time scale distributed optimization model for mitigation parameters orienting partitional coordination is established. Finally, a rolling optimization model based on DMPC for correction of conductance parameters is developed from regional autonomy perspective. The results show that: (1) Distributed mitigation scheme can improve the total harmonic distortion of voltage (THDV) of the whole network node; (2) Compared with control references, the corrected parameters can reduce the decision deviation and further lower the THDV by about 2%; (3) In multi-uncertain scenarios, multi-time-scale optimization can alleviate the uncertainty in harmonic control.

Observer-Based Single-Sensor Control Schemes for LCL-Filtered Grid-Following Inverters

In the single-sensor grid-following control of <i>LCL</i>-type grid-connected inverters, extended-observers, which behave as &#x201C;virtual sensors,&#x201D; are typically employed for active damping, disturbance rejection, current tracking, and grid synchronization. However, most of the existing observer-based single-sensor control schemes choose grid current as the system measurement and output, and the performance of other combinations is still unclear. Hence, in this article, with the help of impedance modeling and Nyquist stability criterion, a systematic comparative assessment for selecting different measurements and controlled outputs is introduced. In this comparison, the sub- and super-synchronous oscillation issues induced by grid faults, lower current reference, high phase-locked loop (PLL) bandwidth and large grid inductance, are investigated. In addition, two easily overlooked interval stability corollaries are proved, facilitating the system stability analysis. Finally, the correctness of the theoretical analysis is verified by a series of steady-state and transient experimental tests.

Comparison of FCS-MPC Strategies in a Grid-Connected Single-Phase Quasi-Z Source Inverter

This paper compares two finite-control-set model predictive control (FCS-MPC) strategies in the context of a grid-connected single-phase quasi-Z source inverter (SP-qZSI). Both schemes use discrete-time models of the inductor current and capacitor voltage for the DC side, as well as the output current on the AC side. To enhance the converter’s performance, given the non-minimum phase characteristics of the DC side, a long prediction horizon is implemented for the predictive control. However, a horizon of this nature can be highly demanding in terms of processing load, rendering it inapplicable for some microcontrollers. To address this issue and mitigate the processing load, an alternative control strategy is presented that divides the total number of candidate solutions to be evaluated into smaller segments. The performance of the two control strategies is compared using total harmonic distortion (THD) and simulation times as evaluation metrics. The results indicate that the proposed strategy achieves significantly shorter simulation times than the compared control strategy when increasing the prediction horizon. Additionally, a reduction in the THD was observed in the proposed strategy, reaching an average of 2.8%, which is lower than the compared strategy that exhibited THD close to 3.5%.

An Overview of Multilevel Inverters Lifetime Assessment for Grid-Connected Solar Photovoltaic Applications

Nowadays, due to advancements in power electronic devices as well as the rise in consumer awareness of the need to protect the environment on a global scale, many people are turning to the use of solar photovoltaic (PV) technology in the distributed power generation side. In the field of power electronics, manufacturers need to develop products that have high lifespans. Power electronic device reliability is important for the maintenance of the device and may be scheduled under that information. Rather than preventing failures, reliability can be improved by predicting them. Even though some research has been conducted over the past few years to investigate the reliability of power electronic devices, the reliability is many common circuits has not been investigated and this leads to a big challenge for researchers. In this review paper, an overview of the grid-connected multilevel inverters for PV systems with motivational factors, features, assessment parameters, topologies, modulation schemes of the multilevel inverter, and the selection process for specific applications are presented. In this paper, the findings of a comprehensive reliability analysis of fundamental multilevel inverters are studied. To evaluate the reliability of three basic multilevel inverters, a calculation is made using each component’s mean time before its failure. Two techniques of computation approximate and exact were used to arrive at the final result. To calculate power losses in temperature-sensitive components such as diodes and switches, MATLAB Simulink is employed. In addition, the concept of oversizing photovoltaic (PV) arrays is presented in this study. This concept proposes that energy output may be increased by increasing the size of the PV array under conditions of poor solar irradiation. Finally, the mission-profile-based and Monte Carlo simulation-based methods process flows are discussed for the accurate lifetime prediction and reliability assessments of PV inverters in a real-time scenario, followed by a conclusion with future work.

A phase compensation method combined with power correction for suppressing sub-synchronous resonance of wind generation under weak grid

Since three-phase grid-connected inverters (GCI) play a crucial role as the connection between renewable energy systems and the grid. They are extensively used in wind farms. However, issues related to sub-synchronous resonance (SSR) have been observed in wind farms, which pose a significant threat to the safe and stable operation of large-capacity wind farms and the grid. In this paper, the mechanism and characteristics of the SSR occurring in the GCI system under weak grid are analyzed based on the impedance model. And derives that the origin affecting the stability of the system is the asynchrony of system frames and control frames. Then, a phase compensation method employed in phase-locked loop (PLL) is proposed to suppress SSR, which can introduce a compensation frame to reduce the angle difference with the system frame. The grid strength can be enhanced equivalently. Meanwhile, virtual resistance and virtual reactance will also be introduced through the introduction of reactive power. It will further provide damping to the system, but will limit power transfer capability. Thus, the power needs to be corrected to achieve precise power control by change the value of current reference. By employing the proposed method, the SSR issues of the GCI under weak grid can be effectively solved and achieve precise power control. Finally, the experimental results verify the correctness of the theoretical analysis and the effectiveness of the proposed method

Comparison between a Cascaded H-Bridge and a Conventional H-Bridge for a 5-kW Grid-Tied Solar Inverter

This paper compares the cost and efficiency of two inverter topologies for a 5-kW grid-connected solar inverter application: the Conventional H-Bridge Inverter (CHB) and the Cascaded H-Bridge Multilevel Inverter (CHBMLI). Emphasis is put on power switches and passive elements with a detailed study of the losses. Both designs respect the same constraints (cost, efficiency, and junction temperature of the transistors) to ensure a fair comparison between both topologies. The work highlights the important parameters when choosing the components (MOSFETs, capacitors, and magnetic cores for the inductors). The DC-link voltage ripple and the output AC current ripple are the key parameters for the design of the passive elements (capacitors and inductors). On top of that, the transistors MOSFETs are chosen, in both topologies, to limit the conduction losses (by selecting the Rdson) and the switching losses (by selecting the Qrr and dv/dt). Real components are picked in order to make the comparison as complete as possible. Numerical simulations are performed using the MATLAB platform. All equations and parameters are provided. A CHBMLI prototype was built with eight independent H-Bridges to validate the proposed design with thermal and efficiency measurements.

A New Intrinsic Power Balancing Control Method for Boosting Grid-Connected PV System Power Stability

Photovoltaic sources have recently become economical electrical generation resources with high potential power generation systems. The increasing rate of PV system penetration and grid integration has raised serious concerns about the stability and reliability of energy networks. The grid-connected inverter and its operations are taken into consideration in the architecture to provide grid failure robustness. As a result, the capacity of the Low Voltage Ride-Through photovoltaic grid-connected power production system to inject more reactive power into the grid when the voltage disturbance is large has become the most critical issue correlated to the grid. The proposed IPBC controller circuit automatically adjusts the duty cycle value, a desired constant output voltage value, and the load output. To achieve this objective, a suitable feedback controller is necessary to regulate the output voltage to a reference value by automatically controlling the system’s duty cycle, quick response and output load, minimum overshoot zero steady-state error. The effectiveness and reliability of the proposed IPBC controller-based grid-connected solar power generation are developed and verified MATLAB 2017 b software environment. Several simulation programs are based on the operation and performance under different operating conditions evaluated for each of different parameters like steady-state error, THD, and system efficiency.

Research on Fuzzy Active Disturbance Rejection Control of LCL Grid-Connected Inverter Based on Passivity-Based Control in Weak Grid

In response to the problems of impedance variation and rich background harmonics in the weak grid environment of traditional LCL grid-connected inverters, which cause distortion of the grid-connected current and reduce the stability of the entire grid-connected system, this paper proposes a passive fuzzy-active disturbance rejection control strategy. Firstly, passivity-based control (PBC) is introduced into the grid-connected inverter system to ensure the global stability of the entire system. Secondly, in order to solve the problem of difficult selection of inverter-side current, active disturbance rejection control (ADRC) is used to directly track the grid-connected current, avoiding the tracking error of AC signal and suppressing the interference of background harmonics of grid voltage. The combination of fuzzy control and ADRC simplifies the system parameter tuning process, and the FUZZY-ADRC grid-connected current outer loop and PBC inductance current inner loop are combined to form a multi-loop control system. Finally, simulation comparison verifies that the proposed control strategy can effectively suppress the harmonics of grid-connected current, improve the quality of grid-connected current, and enhance the stability of the entire grid-connected system in a weak grid environment.

A Triple Boost Seven-Level Common Ground Transformerless Inverter Topology for Grid-Connected Photovoltaic Applications

This article proposes a single-stage, seven-level (7L), switched-capacitor-based grid-connected inverter architecture with a common ground feature. This topology has the ability to boost the output voltage up to three times the input voltage. The proposed topology can diminish the leakage current in grid-connected photovoltaic (GC-PV) applications, and its capacitor voltages are self-balanced without any additional control strategies. The different operating modes are described in detail with their related mathematical expressions. The design of passive components and a detailed power loss analysis are presented. The merits of the proposed structure are demonstrated using a detailed comparative assessment. The grid-connected operation of the proposed inverter structure is simulated in the MATLAB/Simulink environment, and the results are presented. The laboratory prototype of 935 W is built and analyzed to validate the performance of the proposed structure.

PLL phase margin design and analysis for mitigating sub/super-synchronous oscillation of grid-connected inverter under weak grid

Under weak grid, the grid-connected inverter can easily cause sub/super-synchronous oscillations, which are determined by the oscillation modes of system. Firstly, based on the eigenvalue analysis, the sub/super-synchronous oscillation modes of the grid-connected inverter are revealed with considering the phase-locked loop (PLL) and control delay. With the decline of grid stiffness, the typical roots of sub-synchronous oscillation (SSO) mode gradually shift from the left-half plane to the right-half plane, which means that the grid-connected inverter will oscillate under weak grid. By analyzing the participation factor of SSO mode, it finds that PLL is the dominant factor. Based on the root locus analysis, the stable regions of PLL parameter are obtained. Then, according to the stable regions of PLL parameter, the phase margin of PLL can be calculated, which can make the grid-connected inverter operate stably. It finds that PLL phase margin should be increased for suppressing this SSO. However, when the PLL phase margin is greater than a certain value, the system will enter a supersynchronous oscillation mode, which can cause a supersynchronous oscillation. Thus, to suppress the sub/super-synchronous oscillations of the grid-connected inverter under weak grid, the phase margin of PLL should neither be too small nor too large. Finally, the proposed suppression strategy of the sub/super-synchronous oscillations for the inverter integrated into a weak grid is validated with the simulation and experimental studies.

Development of an Intelligent Control Strategy for a Hybrid Energy System Integrated with the HEV Drive

Efficient and accurate performance of a control system plays a major role in developing an energy-efficient intelligent control system (ICS) for the Grid-connected Hybrid Electric Vehicle (HEV) applications. The ICS constitutes the control of hybrid energy split among the renewable sources (PV and Fuel Cell), HEV drive control is achieved by controlling the grid-connected inverter and bi-directional DC–DC converter. MPPT is designed in ICS to extract the maximum amount of power from solar PV. RESS control provides the maximum amount of extracted energy from PV and FC. In this proposed system, a bi-directional DCDC converter along with the storage system is designed to transfer energy in forward and reverse directions from source to storage element and storage element to load end in the process of charging and discharging of the battery which constitutes the bidirectional energy transfer. In addition to these control operations, ICS provides HEV drive control by controlling the grid-connected inverter. The operation of the developed ICS is compared with the conventional controllers and the results show that the proposed system with ICS is highly efficient than conventional controllers. The proposed system is tested with a three-phase load and HEV drive, both speed and torque controls are performed for the grid-connected HEV drive, and the system is tested with the different standard drive cycles.

Design and Implementation of an Integrated Control Scheme for GaN-Based Multiple Power Converters

In response to the rapid changes in the international energy environment, developing renewable energy (RE)-based distributed generation (DG) and various smart micro-grid systems is crucial for creating a robust electric power grid and new energy industries. In this aspect, there is an urgent need to develop hybrid power systems suitable for coexistent AC and DC power grids, integrated by high-performance wide ban gap (WBG) semiconductor-based power conversion interfaces and advanced operating and control strategies. Due to the intrinsic feature of variation in RE-based power generation, the design and integration of energy storage devices, real-time regulation of power flow, and intelligent energy control schemes are key technologies for further promoting DG systems and micro-grids. This paper investigates an integrated control scheme for multiple GaN-based power converters in a small- to medium-capacity, grid-connected, and RE-based power system. This is the first time that a complete design case demonstrating three GaN-based power converters with different control functions integrated with a single digital signal processor (DSP) chip to achieve a reliable, flexible, cost effective, and multifunctional power interface for renewable power generation systems is presented. The system studied includes a photovoltaic (PV) generation unit, a battery energy storage unit, a grid-connected single-phase inverter, and a power grid. Based on system operation condition and the state of charge (SOC) of the energy storage unit, two typical operating modes and advanced power control functions are developed with a fully digital and coordinated control scheme. Hardware of the GaN-based power converters and digital controllers are designed and implemented. The feasibility and effectiveness of the designed controllers and overall performance of the proposed control scheme are verified with results from simulation and experimental tests on a 1-kVA small-scale hardware system.

Optimal control for a photovoltaic integrated grid system using PSO and modified whale optimization to enhance power quality

The paper proposes a modified & optimized controller for PV-integrated grid-connected PV systems. As the solar power output depends on the environmental conditions, i.e., irradiation and atmospheric temperature, the maximum power from the photovoltaic (PV) module is extracted using perturb and observe (P & O) algorithm. With a variety of controllers, such as conventional proportional and integral (PI) controllers, particle swarm optimization (PSO), and the modified Whale optimization algorithm (MWOA) method based on optimum control, the power as compared to extracted from PV panels using MWOA-based controllers. A complete simulation and implementation of a single-phase grid-connected inverter are shown to verify the proposed PV controller. The power flow control and grid synchronization are also presented when the above controllers act as MPPT controllers in the proposed system. The performance of proposed system is evaluated and compared among conventional and optimal controllers under a MATLAB-Simulink environment.

New Modeling and Improved Current Control Strategy to Eliminate the Impact of Synchronization Method and Parks Transformation for Grid-connected Four-leg PWM Inverter

The synchronization method’s ability is one of the fundamental guarantees for the stability and accuracy of injected current control for grid-connected converter-based distribution resources. These synchronization methods, such as phase locked loop (PLL), are generally based on Park transformation and grid voltage regulation, which may affect an unstable phenomenon under distortion and unbalanced grid voltage conditions and result in more computational complexity. In this present paper, analogous to the traditional voltage-oriented control strategy (VOC) in the synchronous rotating frame (dq0-frame) based on PLL and Parks transformation, an improved voltage-oriented control strategy (IVOC) without synchronization methods and Parks transformation is proposed for grid-connected four-leg inverters (GC-FLVSI) to achieve accurate current control with high-quality performance in the dq0-frame. This proposed strategy is not only used for controlling the GC-FLVSI but also to provide the module of GC-FLVSI in the dq0-frame based on the instantaneous active and reactive powers theory (DPC). The proposed IVOC strategy has the same properties and identical performance as the traditional VOC when the grid phase angle is correctly detected by any synchronization method, with the advantages of both traditional DPC and VOC at the same time. In order to validate the superiority and excellent dynamic and steady-state performances of the proposed IVOC strategy in comparison with the traditional VOC strategy, some simulation scenarios using MATLAB/Simulink under different operations and grid conditions have been performed and presented.

A Generic Multicell Network Control for Three-Phase Grid-Connected Inverters

The inherent resonance of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> filters threatens the inverter stability seriously. Moreover, the strong <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$dq$</tex-math></inline-formula> -axis cross-coupling of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> -filtered grid-connected inverters in a synchronous frame goes against the independent regulation of active ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$P$</tex-math></inline-formula> ) and reactive ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$Q$</tex-math></inline-formula> ) power that is required in various applications. Besides, the imbalance and harmonics of the grid voltage will deteriorate the injected grid current, so the disturbance suppression performance of grid-connected inverters is important. Furthermore, excellent steady-state and dynamic performance is essential for grid-connected inverters. To solve the above four issues simultaneously, a complex-domain model of grid-connected inverters is established, and a generic multicell network control (GMNC) structure is proposed in this article. Based on the proposed GMNC structure, a number of control schemes are obtained, and the quantitative relationship between inverter performance and control parameters can be given directly and precisely for each scheme. Therefore, it is convenient and simple for engineers to select proper control schemes depending on the practical conditions and to design all the control parameters for an expected steady-state, dynamic, and antidisturbance performance without the resonance and the coupling issues. Finally, the effectiveness of the proposed control is verified by the experiments.

Wideband Series Harmonic Voltage Compensator Using Look-Ahead State Trajectory Prediction for Network Stability Enhancement and Condition Monitoring

A wideband series harmonic voltage compensator (WSHVC) with an operating bandwidth of 10 kHz for microgrids with a plurality of grid-connected inverters (GCIs) is proposed. It generates a voltage to compensate for high-frequency harmonic voltage at the point of common coupling (PCC), resulting in a virtually zero high-frequency impedance at the aggregate output of the GCIs and, thus improving system stability. The generation of the harmonic voltage is based on predicting the state trajectory of the WSHVC after a switching period per half-switching cycle to determine the moment at which the switches in the WSHVC change their states. The PCC impedance and the equivalent impedance of the GCI network are extracted by analyzing the system response after injecting the entire system with particle-swarm-optimization (PSO) optimized multisine power disturbances. Therefore, system stability is predicted. The condition of passive components in the WSHVC is also monitored by estimating their values with the sampled state variables. A 500 VA, 1 MHz, FPGA-controlled, GaN-based prototype is evaluated on a 6.5 kVA testbed that consists of three commercial GCIs, nonlinear load, and adjustable grid impedance. A comparative study of the system performance with and without the WSHVC is conducted. Experimental results are favorably compared with theoretical predictions.

A Single-Phase Transformerless Common-Ground Type PV Inverter With Active Power Decoupling

To solve the problems of existing non-isolated PV grid-connected inverters such as leakage current, power pulsation and harmonic current, a single-phase common-ground-type non-isolated grid-connected inverter with power decoupling capability is proposed in this paper. The integration of the traditional boost converter and the switched capacitor technology obtains the structure that the neutral point of the grid is directly connected with the negative terminal of PV array, and the leakage current is eliminated. Due to the time-sharing multiplexing of circuit components, which can achieve power decoupling and high step-up, fewer semiconductor devices are required in proposed inverter. At the same time, the improved control strategy ensures that the inverter can adapt to the voltage ripple of large decoupling capacitor. This paper introduces the working principle, mode, performance and control strategy of the novel topology in detail. Finally, the experimental prototype with rated output power of 320W verifies the superior performance of the inverter.