Three-phase Grid-connected Inverter Research Articles

An Advanced Nonlinear Controller for the LCL-Type Three-Phase Grid-Connected Solar Photovoltaic System With a DC–DC Converter

In this article, an advanced nonlinear control scheme is proposed for a three-phase grid-connected inverter and a solar photovoltaic (PV) system connected dc–dc converter. The proposed control scheme is based on a nonlinear adaptive integral backstepping approach. The robustness of the proposed control scheme is achieved through maintaining stability of the overall system, extacting maximum power from the solar PV, and controlling active power. Moreover, the injection of less harmonic components into the grid, the superior performance against different atmospheric conditions, and external disturbances enduring capability are also key features of the controller. The stability of the overall system is achieved through the negative-definite control Lyapunov functions of the proposed control scheme. An adaptive law of the proposed control scheme on external disturbances provides the undergoing capability against the external disturbances. Maximum dc power from the solar PV system is obtained using the proposed control scheme on the dc–dc converter along with the incremental conductance algorithm. The proposed control scheme embedded in the three-phase grid-connected inverter with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> -type filter provides maximum active power in the presence of external disturbances, controls reactive power, and injects less harmonic current into the grid. The simulation results under different atmospheric conditions further demonstrate the robustness of the proposed control scheme. The laboratory scale experiments validate the superiority of the proposed control scheme.

Stability analysis of Three-phase Grid-Connected inverter under the weak grids with asymmetrical grid impedance by LTP theory in time domain

The Grid-connected inverter (GCI) often operates in the weak grid with asymmetrical grid impedance due to the unbalanced and single-phase loads. However, the time-periodic dynamic behavior effect of the Phase-Locked Loop (PLL) on the GCI operating in an asymmetrical system is not investigated in the time domain. This paper explores the influence of the asymmetrical grid impedance on the stability of the weak grid with GCI. Firstly, GCI's complete harmonic state-space (HSS) model considering PLL under the asymmetrical network is established. Secondly, in view of the developed HSS model, the predominant parameters are determined through the eigenvalues of the system state matrix, along with the participation factors. Afterward, with eigenvalue locus analysis, the stability boundaries of the system's critical parameters involving the current reference, current controller, and PLL controller are determined under different grid impedance asymmetrical indexes. It is found that the stability region of system parameters is narrowed with increasing impedance asymmetrical index. Lastly, simulations and experiments are carried out to verify the effectiveness of the proposed HSS model and the theoretical analysis results.Index terms— Grid-connected inverter, asymmetrical grid impedance, Phase Lock Loop, harmonic state-space model, stability boundaries, participation factors.

Optimization of Grid-Connected Photovoltaic Power Generation Technology Based on Nonlinear Back-Stepping Controller

To address the issue of energy scarcity and to use solar photovoltaic energy as a renewable source, a three-phase grid-connected photovoltaic inverter system with uncertain system model parameters is investigated, which converts DC power into AC power, feeds it into the grid, and maintains the grid-connected part’s quality. An enhanced back-stepping approach is proposed to explore the control strategy of three-phase solar grid-connected inverter, which includes adaptive control, dissipative theory, sliding mode control, and rapid terminal sliding mode control, respectively. It is verified that the system is subjected to external interference F = 5 during 0.5 s–0.8 s. As per the simulation results, the improved adaptive terminal sliding mode control method can suppress the interference to a certain extent and has strong robustness to the external interference. When compared with the adaptive sliding mode control approach, the anti-interference ability is better, and the system stability is improved. Then, to improve the above approach, a high speed terminal sliding mode control method is proposed, which can swiftly converge in finite time and the estimated value of uncertain parameters is closer to the actual value, thereby boosting the system’s robustness.

Line Voltage Sensorless Control of Grid-Connected Inverters Using Multisampling

Multisampling control provides an attractive way to improve the dynamics and stability of high-power three-phase grid-connected inverters. Beyond the desired performance in current control, more information can be acquired through multisampling. However, such an assumption is rarely discussed in grid-connected inverters. In order to fill this gap, a line voltage sensorless control scheme is proposed for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> -filtered inverters. In this article, a multisampling is performed on the inverter-side current, and the linear regression of the multisampled current during zero vectors of inverter-modulation voltage is employed to estimate the filter capacitor voltage. Consequently, the cost of line voltage sensors is reduced and the failure of line voltage sensors is prevented when using multisampling current control. Furthermore, to address start-up transients, the line voltage can still be estimated by temporarily locking the upper arms of the inverter, and dc-link voltage can be precharged to the target value at the same time. Experimental results validate the effectiveness of the method on a down-scale inverter.

Current-Limiting Virtual Synchronous Control and Stability Analysis Considering DC-Link Dynamics Under Normal and Faulty Grid Conditions

An improved nonlinear virtual synchronous control for three-phase grid-connected inverters, that can maintain a reliable operation under both normal and faulty grid conditions, i.e. balanced grid voltage sags, is proposed. The proposed controller can ensure a desired RMS current limitation at all times, provide virtual inertia and damping via the DC-link voltage and AC system frequency coupling, and realize the desired real and reactive power regulation without requiring accurate knowledge of the system parameters. Opposed to the conventional methods that use saturated PI controllers with or without anti-windup techniques to limit the reference value of the inverter current, the proposed controller includes a nonlinear bounded integrator, which limits the actual value (instead of the reference) of the inverter RMS current and leads to a fast system recovery even after significant grid voltage sags. The closed-loop stability of entire system is rigorously proven using nonlinear singular perturbation theory. Moreover, analytic conditions for the controller parameter selection to guarantee the stability of entire inverter system with the DC-link dynamics are provided. To prove the effectiveness of proposed controller and its superior performance compared to the traditional approaches, extensive Matlab/Simulink-based simulations are performed, followed by Typhoon-HIL hardware-in-the loop implementation using a TI microcontroller.

Reinforcement Q-Learning Based Current Control for Grid-Connected Three-Phase Inverters with Unknown System Dynamics

This paper proposes a Q-learning-based state feedback suboptimal controller to solve the current control problem of three-phase grid-connected LCL coupled inverters with unknown circuit parameters. In practice, the circuit parameters of the inverters will change obviously for reasons such as calculation errors, external environment and operation aging, which makes the dynamics of the inverter system become unknown. With the circuit’s model of the LCL inverters and the reference current dynamic, an augmented system is constructed and a discounted performance function is formulated as the optimal objective of current control which transform it into a H∞ tracking control. Using Q-learning algorithm with model-free characteristics, a current controller is proposed in which an iterative reinforcement learning (RL) algorithm is embedded. Simulations are presented to verify the valid of the proposed control scheme, where especially the results shows that it can keep excellent control performance under the condition of inverter parameter mutation and grid voltage distortion.

Robust Predictive Control with Three-Vector Modulation Connected to the Power Grid

Robust predictive control is presented in this article using a three-vector modulation for a grid-connected three-phase inverter. The aims of this article are proposing a robust controller that is able to deal with some variations of parameters that occur in practical systems and demonstrating the performance of the controller with experimental tests that change the inductance and consequently the parasitic resistance of the plant; the same controller can predictive the changes to obtain the optimal performance. The article also presents mathematical modeling for plants and controllers and a modulation method to solve the variable frequency problem. Experimental tests corroborate the expected results and validate the controller’s efficiency according to the control system analysis requirements and the IEEE 1547.2-2008 standard.

Analysis and design of photovoltaic three-phase grid-connected inverter using passivity-based control

This paper presents photovoltaic three-phase grid-connected inverter with an inductor-capacitor-inductor (LCL)-filter. For robustness against variation of filter parameters and external disturbance, the passivity-based control (PBC) method has been adopted. In this method, there are two interactively coupled feedforward terms and three damping gains in the control loops which are designed to limit the steady state error of grid current. Boost converter with P&amp;amp;O maximum power point tracker (MPPT) is used for each photovoltaic (PV) string to extract maximum power and to raise the PV voltage to a value suitable for the grid-connected inverter. The outputs of all boost converters are connected in parallel and controlled to fixed reference voltage using proportional-integral (PI) controller to make the direct-current (DC) link voltage robust against variations in sun radiation intensity and system parameters change. The suggested system is analyzed, designed and simulated using PSIM program. 1 kW, 2kW, and 3kW PV systems connected to grid of 220V/50Hz are tested and the results show the validity of the suggested grid-connected PV systems and robustness against filter parameters variation.

A Novel Carrier Scheme Combined with DPWM Technique in a ZVS Grid-Connected Three-Phase Inverter

In this paper, a novel switching scheme using discontinuous pulse-width modulation (DPWM) for a zero-voltage switching (ZVS) grid-connected three-phase inverter is proposed. ZVS in the main and auxiliary switches was achieved. Moreover, the reverse recovery currents of the anti-parallel diodes in the main switches were suppressed. A circuit analysis was performed, and a simulation was carried out. Furthermore, a prototype of the ZVS grid-connected three-phase inverter was constructed to verify the effectiveness of the proposed PWM control scheme. Both the simulation and experimental results verified the validity of the proposed PWM control scheme.

Active Disturbance Rejection Control of Three-Phase LCL Grid-Connected Inverter under Unbalanced Grid Voltage

When the three-phase LCL grid-connected inverter operates under the condition of unbalanced grid voltage, the influence of the negative sequence component in the grid voltage will cause the grid-connected inverter system to produce active and reactive dual-frequency interference. This will cause the grid-side current to have many low-order harmonics, mainly the third harmonic, which does not conform the IEEE-Std. 929–2000 standard. In order to solve this problem, this paper designs a current auto-disturbance-rejection controller, which speeds up the tracking response speed of the system and reduces the harmonic content of the grid-side current under unbalanced conditions. At the same time, optimize the phase-locked loop structure and introduce a notch filter in the modulation link to eliminate the third low-frequency harmonic components and further improve the quality of the grid-side current waveform. Finally, the correctness of the control strategy suggested in this paper is verified by MATLAB/Simulink simulation.

Raspberry Pi Design and Hardware Implementation of Fuzzy-PI Controller for Three-Phase Grid-Connected Inverter

A photovoltaic system is one of the major sources of renewable energy. The grid-connected inverter controllers play an important role in the conversion and transmission of solar energy. Therefore, they must be improved to meet the demands for grid interconnection. This article introduces the design and hardware implementation of the intelligent fuzzy-PI controller of the inverter part of the grid-connected photovoltaic system. First, the paper discusses the design of the three-phase grid-connected fuzzy-PI controller. Next, the paper describes the implementation of a Matlab graphical user interface (GUI) to design any grid-connected inverter and size the photovoltaic systems. The code generation of the fuzzy-PI controller of the system is accomplished by using Matlab Simulink simulation software. The hardware components of the PV system are implemented experimentally. In the hardware implementation, a 70 W prototype is realized to test the functionality of the controller, such that one can develop a realistic controller without taking risks or falling into security concerns in the case of performing experiments on high-power systems. The prototype proves that the controller model can be directly transformed from Simulink to the control device. It also shows that the fuzzy-PI controller is working properly in the 70-watt prototype. The achieved performance parameters of the proposed fuzzy-PI controller are satisfactory. The proposed method to design and implement the fuzzy-PI controller does not require complicated programming, where a Matlab coder is proposed to transform the Simulink controller into C code that can be directly utilized as a software control program loaded in the microcontrollers embedded in the hardware of the controller. The main result is that the fuzzy-PI controller for the three-phase grid-connected systems can be implemented using low-cost reconfigurable microcontrollers.

A Control Parameters Design Method With Multi-Constrains for an LCL-Filtered Grid-Connected Inverter in a Weak Grid

Under weak grid conditions, the variation of the grid impedance will affect the steady-state and dynamic performance of the LCL-filtered grid-connected inverter and even make the inverter unstable. To ensure the system stability and further improve the dynamic performance in a weak grid, a control parameter design method with multi-constrains considering the system bandwidth for the current controller and active damping is proposed in this paper. First, based on the current controller and active damping with only grid current feedback, the effects of control parameters and grid impedance on the LCL resonant suppression and the performance of the inverter are analyzed. Moreover, the parameter constraints of the controllers are derived considering the grid impedance, including stability, resonance suppression, and margin constraints. Furthermore, as the system bandwidth affects the dynamic performance of the inverter, combined with the obtained multi-constraints, the optimal control parameters are determined by achieving the maximum bandwidth of the system against the impedance variation. Compared with other two methods, when the proposed method is applied, the system can operate with a better dynamic and steady-state performance. Finally, experiments are performed on a 2 kW three-phase grid-connected inverter in the weak grid, which verify the effectiveness of the parameter design method proposed in this paper.

Higher‐order sliding mode current controller for grid‐connected distributed energy resources with LCL$\mathrm{LCL}$ filters under unknown grid voltage conditions

The control of the current injected into the grid with lower harmonics is considered as one of the most important issues for the grid integration of distributed energy resources (DERs). The unbalances and harmonics in the grid voltage usually pollute the current injected into the grid due to the power electronic interfaces, for example, inverters. To address such problems, the present paper proposes a nonlinear higher order sliding mode controller (HOSMC) for grid-connected three-phase inverters with LCL $\mathrm{LCL}$ filters in order to control the current injected into grid and improve the power quality. The proposed current controller injects the desired current into the grid with lower values of total harmonic distortions (THDs) under any grid voltage condition as well as it reduces the harmonics in the grid voltage. Apart from these, the proposed scheme is developed to provide robustness against parametric uncertainties where these uncertainties are modeled using the Taylor series expansion method. Finally, the performance of the system is evaluated using processor-in-loop (PIL) simulations via MATLAB/Simulink platform through the implementation on a system considering the capacity of the DER as 2 kVA per phase and compared with other existing control strategies.

A Novel Frequency Locked Loop With Current Harmonic Elimination Method for the Three-Phase Grid-Connected Inverter

In order to eliminate the harmonics of the grid connected inverters accurate harmonic detection is essential. Though there are many kinds of different harmonic detection methods are available, it can hardly be achieved without the help of grid frequency detection of the fundamental component. In this paper a novel Lock-In Amplifier based Frequency Locked Loop (LIA-FLL) is proposed for the improved harmonic elimination of the grid-connected three phase inverters. Due to the outstanding performance of the LIA-FLL, the harmonics can be detected and compensated accurately under the grid frequency variation. To demonstrate the superiority of the proposed method, it is applied to a 10 kW three phase grid connected inverters. The results obtained by the proposed method are compared with those obtained by the harmonic elimination method with no FLL. The principle of LIA-FLL is discussed and its performance for the harmonic elimination is proved by the simulation and experiments with a 10KW three phase grid connected inverter under the highly polluted grid condition.

Two-Coordinate Decomposed Siso Models-Based Stability Analysis Of Three-Phase Grid-Connected Inverter Systems with Asymmetric Control

Two-Coordinate Decomposed Siso Models-Based Stability Analysis Of Three-Phase Grid-Connected Inverter Systems with Asymmetric Control

Processor in the Loop Verification of Fault Tolerant Control for a Three Phase Inverter in Grid Connected PV System

ABSTRACT The techno-economic feasibility of photovoltaic (PV) solar generation systems is greatly dependent on its operating conditions. However, significant penetration of PV sources into alternating current (AC) grids may cause lower system inertia, decreased system damping, and higher frequency fluctuations when grids are subjected to fault conditions. In this paper, a two-stage three-phase grid-connected PV-based inverter system is analyzed under grid fault conditions using a robust fault-tolerant super twisting sliding mode control scheme. Moreover, closed control system is formulated using Lyapunov theorem and its global stability is ensured. The proposed controller is tested in processor in the loop experimentation using a Texas Instrument (TI) Launchpad (TMS 320F28379D). The control performance and resilience of the proposed control system is tested under three-phase faults introduced between the inverter–grid interface buses. Under the above test conditions, a comparative analysis is performed for the proposed adaptive super twisting sliding mode control (ASTSMC) and fixed gain sliding mode control (SMC) control method. Under fault conditions, the inverter power has a settling time of 50 m-s with ASTMC control while with fixed gain STSMC, the settling time is longer and the observed settling time is 115 m-s. Moreover under faulty conditions, a voltage dip of 400 V is recorded with ASTSMC while with fixed gain STSMC controller the observed dip is 600 V. With the proposed ASTSMC controller, THD settles down to under 3% in 50 m-s while the fixed gain STSMC takes 115 m-s.

Magnetic integrated LCL filter design for a 2.5 kW three-phase grid-connected inverter with double closed-loop control

Magnetic integrated LCL filter design for a 2.5 kW three-phase grid-connected inverter with double closed-loop control

A study on the dynamic model of a three-phase grid-connected inverter and an innovative method for its verification

A study on the dynamic model of a three-phase grid-connected inverter and an innovative method for its verification

Modified repetitive control based on comb filters for harmonics control in grid-connected applications

Power quality plays an important role in the operation of grid-connected inverters. The injected current should have a minimum harmonic content in order to comply with the local grid codes. This can be challenging due to the fact that the grid voltage can also be distorted. This paper presents the modified repetitive control method for three-phase grid-connected inverters by means of a digital comb filter application. The proposed method provides multiple harmonics compensation, simple implementation and design. It consists in complementing the Proportional-Resonant (PR) control with an appropriately tuned comb filter, which results in mitigation of the output current harmonics and reduced THD. Two types of comb filters have been proposed as a harmonic compensator, namely feedback and feedforward structures. The compensator included an additional FIR low-pass filter for increased robustness. Regardless of the structure chosen the solution is targeted at simplicity of analysis, design and implementation. Furthermore, the compensator based on the feedforward comb filter is casual, hence its implementation on the digital microprocessors is greatly facilitated. The proposed two-stage design of the control loop ensures the stability even in the case of additional poles being introduced by the feedback comb filter. Investigation on frequency variations impact is also included. The research was conducted by means of the following IT solutions: data workflow management environment, code design and generation (Matlab), and user interface design (ControlDesk). The experimental results have demonstrated the effectiveness of the proposed solution for a grid-connected converter operating in a network with distorted supply voltage.

High performance decoupled active and reactive power control for three-phase grid-tied inverters using model predictive control

Finite control set-model predictive control (FCS-MPC) is employed in this paper to control the operation of a three-phase grid-connected string inverter based on a direct PQ control scheme. The main objective is to achieve high-performance decoupled control of the active and reactive powers injected to the grid from distributed energy resources (DER).The FCS-MPC scheme instantaneously searches for and applies the optimum inverter switching state that can achieve certain goals, such as minimum deviation between reference and actual power; so that both power components (P and Q) are well controlled to their reference values.In addition, an effective method to attenuate undesired cross coupling between the P and Q control loops, which occurs only during transient operation, is investigated. The proposed method is based on the variation of the weight factors of the terms of the FCS-MPC cost function, so a higher weight factor is assigned to the cost function term that is exposed to greater disturbance. Empirical formulae of optimum weight factors as functions of the reference active and reactive power signals are proposed and mathematically derived. The investigated FCS-MPC control scheme is incorporated with the LVRT function to support the grid voltage in fulfilling and accomplishing the up-to-date grid codes. The LVRT algorithm is based on a modification of the references of active and reactive powers as functions of the instantaneous grid voltage such that suitable values of P and Q are injected to the grid during voltage sag.The performance of the elaborated FCS-MPC PQ scheme is studied under various operating scenarios, including steady-state and transient conditions. Results demonstrate the validity and effectiveness of the proposed scheme with regard to the achievement of high-performance operation and quick response of grid-tied inverters during normal and fault modes.