Three-phase Grid-connected Inverter Research Articles

A Simple High-Performance Current Control Strategy for V2G Three-Phase Four-Leg Inverter With LCL Filter

Electric vehicles (EVs) can behave as distributed energy storage devices for providing on-demand smart grid support service, that is, an emerging vehicle-to-grid (V2G) technology. A high-performance and easy-implementation current control strategy for V2G three-phase four-leg inverter with LCL filter is proposed. It consists of a deadbeat (DB) controller and a paralleled repetitive controller (RC). The DB controller is based on the weighted average inductor current (WAIC) scheme, which simplifies the third-order LCL filter to be an equivalent first-order $L$ filter. The stability of the DB-controlled inverter with the unmodeled system time delay is analyzed. The DB controller is of very fast response and easy implementation but is not immune to system time delay and various uncertainties. To overcome the disadvantages, a plug-in RC is added to reinforce the DB controller to remove harmonic distortion from the feed-in current in the presence of parameter uncertainties. A lab prototype of 10-kW grid-connected three-phase four-leg inverter has been built up to validate the proposed current control strategy. The simulations and experiments are provided to demonstrate the validity of the proposed control strategy.

Enhanced Model Predictive Current Control Based on Runge–Kutta Approximation for a Voltage Source Inverter

Abstract This paper proposes a model predictive current control (MPCC) method with duty cycle control based on the Runge–Kutta approximation compared to the Forward Euler Approximation for grid-connected three-phase inverters with output LCL filter. Hence, results of proposed MPCC methods alongside of the conventional MPCC have been investigated to find the best strategy. First, all 7 possible switching states have been checked by the discrete-time system model based on two approximation methods to select a state that minimizes the cost function. However, at this stage only one voltage vector is chosen during one control period, which cannot decrease the current ripples to a minimum value. Hence, for having sufficient performance, the sampling frequency is necessary to be selected high. Then, the idea of duty cycle optimization has been introduced by using two voltage vectors (a non-zero and a zero voltage vector) during one control period. Therefore, the duration of two voltage vectors have been defined according to the principle of current error minimization. Finally, the effectiveness of the proposed MPCC method based on the Runge–Kutta approximation has been verified by MATLAB/Simulink and experimental results exhibit a better steady-state performance with less sampling frequency as compared to the conventional strategy.

A Three-Phase Grid-Connected Inverter Equipped With a Shunt Instantaneous Reactive Power Compensator

This paper introduces a three-phase grid-connected inverter for commercial-scale photovoltaic systems. The circuit topology combines an ac-inductor-less active bridge with a parallel-connected pulsewidth modulation inverter acting as an instantaneous reactive power compensator. The active bridge, called a 120° conduction inverter, operates at a switching frequency equal to the grid frequency and outputs a 120° rectangular current, while the compensator eliminates the harmonic currents included in the rectangular current. The main advantage of this strategy is the reduction in size of the required ac inductor by a factor of 4, as well as the reduction of power losses in the switching devices. The theoretical analysis clearly shows that the compensator can synthesize a sinusoidal output current by controlling only instantaneous reactive power, thus eliminating the need for electrolytic capacitors. A 5-kW experimental setup validates the operating principle and the control method of the proposed inverter and exhibits an inverter efficiency as high as 99.2% with Si superjunction mosfet s.

An SiC MOSFET Based Three-Phase ZVS Inverter Employing Variable Switching Frequency Space Vector PWM Control

In this paper, a variable switching frequency space vector pulsewidth modulation control is proposed. It is used to achieve zero voltage switching (ZVS) for a three-phase grid-connected voltage source inverter with unity power factor. A wide range of ZVS can be realized without any additional sensors, auxiliary circuits, or current zero crossing detection circuits. The switching frequency can be easily calculated by a digital controller. The frequency variation range in a line cycle is only about 1.5 times at any specific load. An LCL filter is used to attenuate the high current ripples at the inverter side, and active damping is adopted to avoid the resonance and reduce the filter power loss. The switching loss can be significantly reduced by using silicon carbide mosfet s so that the conversion efficiency is high. The power density can also be improved due to the high switching frequency and the low inductance value of the filter. The operating principle, design considerations, and loss analyses are discussed in detail. A 3.5-kW simulation and experimental prototype interfacing a 350–400-V dc with a three-phase 110-V ac grid is developed to verify the performance of the proposed control strategy.

A Variable Switching Frequency Space Vector Modulation Technique for Zero-Voltage Switching in Two Parallel Interleaved Three-Phase Inverters

A variable switching frequency space vector modulation control method is proposed in this paper. It is used to achieve zero-voltage switching (ZVS) for two parallel 180° interleaved three-phase grid-connected voltage-source inverters at unity power factor. A wide range of ZVS can be achieved without any additional sensor, auxiliary circuit, or current zero-crossing detection circuit. The carrier wave frequency is easy to be calculated and updated with digital control. The current ripple is precisely predicted and controlled to minimize unnecessary circulation loss. The high current ripple at the inverter side can be considerably canceled by the parallel interleaved configuration. LCL filter is used to further attenuate the current ripple and improve the power quality at the grid side. The switching loss can be significantly reduced especially by using a wide bandgap device with low turn- off loss. The power density is greatly improved owing to the high switching frequency and low component parameters of the filter. A 4.5-kW simulation and experimental prototype interfacing 400 V dc with a three-phase 110-V ac grid is developed to verify the performance of the proposed control strategy.

Analysis of frequency characteristics of phase-locked loops and effects on stability of three-phase grid-connected inverter

For grid-connected inverters, phase-locked loop (PLL) is an indispensable part for grid currents to track grid voltages. Hence, PLL will have a nonnegligible effect on the stability of the interconnected system involving a grid-connected inverter and a power grid. Therefore, it is necessary to model the frequency characteristic of PLL and analyze its effect on the system stability. In this paper, three kinds of the commonly used PLL circuits are firstly employed to model and compare their frequency characteristics. Second, the impedances of the inverter are modeled with the equivalent impedance model of PLL displayed, and the influences of different PLLs on the impedance characteristics of the inverter are compared. Then, the effects of PLLs on the stability of the interconnected system are studied to discuss the stability boundary conditions of the system. Based on the equivalent impedance model of the PLL, the optimized design method of the PLL is proposed from the perspective of optimizing the inverter impedance characteristics to improve the system stability. Finally, the simulation and experimental results are presented to verify the correctness of the PLL frequency characteristics and the inverter impedance models, and confirm the effectiveness of the optimized design method.

Model-Based Current Control Strategy With Virtual Time Constant for Improved Dynamic Response of Three-Phase Grid-Connected VSI

In this paper, a model-based current control strategy with virtual time constant is proposed for three-phase grid-connected LCL-filtered voltage source inverters. The proposed control strategy is based on controlling the inverter currents in the rotating dq frame by using current-oriented proportional-integral (PI) controllers rather than voltage-oriented PI. The PI controllers determine the inverter current references in the d- and q-axes by regulating the grid current. It is shown that the proposed strategy decouples the inverter current from other variables provided that the inverter-side inductance and its resistance values used in the control variable match the actual values in the system. In addition, the virtual time constant is introduced in the control variables to offer flexibility for adjusting the inverter current dynamics as desired. Moreover, the integral gain of PI controller has the ability to keep the LCL-resonance peak below 0 dB. Unlike the existing methods, the proposed strategy does not require a dedicated active damping. Computer simulations and experimental studies show that the proposed control strategy exhibits a good performance in achieving fast dynamic response and sinusoidal grid current with low THD under balanced, unbalanced, and distorted grid conditions.

Model Predictive Voltage Control with Optimal Duty Cycle for Three-Phase Grid-Connected Inverter

This paper proposes a model predictive voltage control (MPVC) strategy with duty cycle control for grid-connected three-phase inverters with output LCL filter. The model of the system is used to predict the capacitor filter voltage according to the future output current for each possible switching state at each sampling period. Then the cost function for each prediction is determined and the switching state is selected. In the proposed method, two voltage vectors are applied during one sampling interval to achieve better steady-state performance. Finally, the optimal duration of the nonzero voltage vector is defined based on the duty cycle optimization, which is vital to the control system. The proposed strategy offers a better reference tracking error with less THD in linear and nonlinear load situations. The effectiveness of the proposed method has been verified by MATLAB/Simulink and experimental results exhibit a better steady-state performance with less sampling frequency.

Dual-mode operation based second-order sliding mode control for grid-connected solar photovoltaic energy system

This paper proposes a second-order sliding mode control technique for the grid-connected solar photovoltaic system. The proposed technique is tested under grid faults to meet the requirements of low-voltage ride through capability. A dual-mode control technique of the converter and inverter is designed to maintain the required dc-link voltage regulation and the active-reactive power during grid fault. The sliding mode control technique is developed from the model of the grid connected to the converter and the dc-link. The proposed dual-mode control technique is validated using real-time simulator OPAL-RT and experimental three-phase grid-connected photovoltaic inverter system. Simulation and experimental results show that the control technique is quite effective in maintaining the dc-link voltage regulation and the power control. As it shows smooth transitions between the two modes of operation (normal and grid fault). Moreover, the sliding mode control approach is robust against unknown disturbances and uncertainties in the system.

Optimal Energy Management Strategy and Novel Control Approach for DPGSs Under Unbalanced Grid Faults

Recently, the use of distributed power generation systems (DPGSs) based on renewable energy resources is increasingly being pursued as a supplement and a reliable alternative to the large traditional energy sources. For it, power-electronic interface technologies and control have also emerged as the most important key elements in the area of energy management and integrating DPGSs. The specification of a power-electronic interface is subject to several requirements that are related not only to the DPGS itself but also to its interactions with the power system especially where the utility grid is subject to events that can potentially lead to large-scale disturbances or even to its collapse if it operates near its capacity without fault margin. This study deals, first, with an optimized energy management strategy and, second, with a newly-conceived control strategy called symmetrical components control algorithm (SCCA) that was proposed for four-leg three-phase grid-connected voltage source inverter (VSI) used for DPGSs with wind–solar–battery sources. A mechanism of negative and zero sequences injection based on the control of ([Formula: see text]) current coordinates has been introduced. The performance of entire control system, to enhance the unbalanced fault ride-through capability of DPGSs, has been evaluated by time domain simulations with MATLAB/Simulink. Advantages of the combined active–reactive control ensuring both current and voltage controls have been achieved compared to the majority of already published strategies. The distinct features of the proposed SCCA strategy prove that it allows to meet the requirements for grid interconnection and the new stricter standards with respect to power quality, safe running, and islanding protection.

Improved Frequency Locked Loop Based Synchronization Method for Three-Phase Grid-Connected Inverter under Unbalanced and Distorted Grid Conditions

To quickly and accurately estimate the parameters of the fundamental positive- and negative-sequence under the unbalanced and distorted grid voltage, a synchronization method is presented in this paper. The proposed method is based on both a harmonic decoupling network consisting of multiple dual second-order generalized integrators (MDSOGIs) and an improved frequency locked loop (IFLL), so it is called the MDSOGI-IFLL. Due to the IFLL, the system has the feature that the dynamic performance of estimating the fundamental frequency is independent of the variation of both the fundamental positive- and negative-sequence voltage. In this paper, a first-order linear frequency adaption model is established for the design of the IFLL. Finally, the good performance of the proposed MDSOGI-IFLL is validated by the simulation and experiment.

Analytical and Experimental Validation of Parasitic Components Influence in SiC MOSFET Three-Phase Grid-connected Inverter

Analytical and Experimental Validation of Parasitic Components Influence in SiC MOSFET Three-Phase Grid-connected Inverter

Performance comparison of Si IGBT and SiC MOSFET power devices based LCL three‐phase inverter with double closed‐loop control

Grid-connected inverters are essential equipment for DC-AC energy conversion between renewable energy generation and power grids, and their performance directly affects the stability of the operation of the power grid. Compared to the traditional L or LC filter, LCL filter is widely used in the grid-connected inverter due to its harmonic attenuation performance and system stability. Because of the requirements for higher switching frequency to increase the power density and reduce the cost of the inverter. Compared to the traditional silicon (Si) insulated gate bipolar transistor (IGBT) power device, the silicon carbide (SiC) metal-oxide-semiconductor field-effect transistor (MOSFET) has shown apparent advantages in high-power density inverters with a high switching frequency. This study first analyses and compares the suppression effects of passive damping methods on the resonance peak of the LCL filter; then, a double-current closed-loop control strategy as the active damping control is applied in the LCL filter three-phase grid-connected inverter; finally, the experimental platforms of IGBT inverter and SiC MOSFET inverter are built, and the filtering performances of the two inverters are compared. The experimental results show that the double-closed-loop control method significantly reduces the system harmonics and confirms the feasibility of the control strategy.

A Novel Low Voltage Ride-Through Technique of Three-Phase Grid-Connected Inverters Based on a Nonlinear Phase-Locked Loop

In this paper, a novel low voltage ride-through (LVRT) technique for three-phase grid-connected inverters is proposed. The proposed technique consists of two parts: a nonlinear phase locked loop based on complex-coefficient filters (NLCCF-PLL) and an LVRT control scheme. Generally, the synchronization process of three-phase grid-connected inverters is performed via PLL with a relatively low bandwidth, which delays the detection of voltage sag and recovery during the LVRT process. To accelerate the synchronization process, the NLCCF-PLL with adaptive controller gains is proposed to improve both the filtering capability and dynamic performance of PLL at the same time. The stability of the NLCCF-PLL is validated by the second method of Lyapunov in the nonlinear model, and the superiority of its operating performance is verified. The proposed LVRT control scheme consists of a reference current calculation block to effectively suppress the power ripples and an inner loop controller with strong robustness as well as fast dynamic response. By comparing the proposed LVRT technique with the existing LVRT technique on the basis of experimental results, the superiority of the proposed LVRT technique is confirmed.

Control Method for Three-Phase Grid-Connected Inverter PV System Employing Unity Power Factor (UPF) Strategy in Microgrid

Microgrids (MGs) are developing owing to the rapidly growing distributed power generation systems. The MG controls the flexibility of the network to ensure the requirements of reliability and power quality are satisfied. A typical MG normally consists of dispersed generation resources, which are connected by power electronic inverters, storages, and non-linear loads. This study deals with a compensation control method of a photovoltaic grid-connected inverter using unity power factor (UPF) strategy in MG. In this case, the proposed control method can provide output currents without distortion and with the UPF. Further, it is able to increase the inverter output current to approximately 19 times of the value obtained conventionally. The proposed control method can be applied to three-phase grid interfaced converters such as DG inverters and can also be easily integrated into the conventional control scheme without installation of extra hardware. A theoretical analysis is presented and the performance of the proposed control method for a grid-connected inverter in a MG is evaluated through simulation results.

An intelligent dc current minimization method for transformerless grid-connected photovoltaic inverters

Due to the scaling and zero-drift of current sensor errors, unbalanced grid voltages, tolerance of power switching devices, and asymmetry of PWM gate driving pulses, transformerless grid-connected inverters usually have certain amount of dc components injected to the ac grid. Therefore, power quality of the grid is degraded. Many efforts, such as using blocking capacitors, a dc current feedback control method, and a voltage dc component feedback control method, etc., have been introduced to minimize the dc injection. This paper proposes an intelligent control strategy of dc current injection suppression to the grid by utilizing adaptive-back-propagation (ABP) neural network PID controller. The performance of the proposed scheme is evaluated and compared with the traditional and existing method. Finally, the control scheme is verified on a 2-kW three-phase grid-connected inverter in the laboratory.

A Novel DC Current Injection Suppression Method for Three-Phase Grid-Connected Inverter Without the Isolation Transformer

Due to current sensor errors, tolerance of power switching devices, and asymmetry of PWM gating driving pulses, grid-connected inverters without isolation transformer usually have certain amount of dc components injected to the ac grid. Many efforts, such as using a blocking capacitor, a current dc component feedback control, and a voltage dc component feedback control have been introduced to try to suppress the dc injection to the grid. This paper proposes a novel control strategy of suppressing dc current injection to the grid for a three-phase inverter by accurately sensing the dc component of line voltages of three-phase inverter and adding a dc component control loop. A dc suppression control scheme is presented, and the suppression performance of the proposed dc rejection method is evaluated and compared with the traditional method. Finally, the control scheme is verified on a 20-kW three-phase grid-connected inverter.

Operating principles and practical design aspects of all SiC DC/AC/DC converter for MPPT in grid-connected PV supplies

A 20 kW, 20 kHz high frequency (HF) link maximum power point tracking (MPPT) converter for a grid-connected PV supply, based on all silicon carbide (SiC) power semiconductors, is presented. In the developed converter, SiC power MOSFETs are used in the low-voltage PV panel side and SiC Schottky diodes on the high-voltage DC output, in order to maximize the power conversion efficiency and the power density. Operating principles of the resulting dual H-bridge MPPT converter and the practical aspects of the converter design and its circuit layout, are described in detail. The implemented converter performance is compared with that of a classical Si-IGBT and hybrid-IGBT based MPPT converter in terms of efficiency. This configuration can compete with the non-isolated MPPT converter topologies, such as the boost converter commonly used in grid-connected PV systems, since it allows the possibility of using a conventional two-level, three-phase grid-connected inverter. This is due to the enhanced common-mode EMI performance as compared to non-isolated MPPT topologies, resulting in a competitive high efficiency PV converter design with galvanic isolation. It has been shown that the converter size can be shrinked up to a power density of 1.6 kW/lt, with a DC-DC converter full-load efficiency of 98%. The resulting compact and highly efficient SiC power MOSFET based HF link MPPT converter is suggested to be a part of grid-connected, multi-string PV supplies with simple inverter topologies in the future.

A Harmonic Compensation Strategy in a Grid-Connected Photovoltaic System Using Zero-Sequence Control

Mitigation of harmonics for a grid-connected inverter is an important element to stabilize the control and the quality of current injected into the grid. This paper deals with the control method of a three-phase Grid-Connected Inverter (GCI) Photovoltaic (PV) system, which is based on the zero-sequence current adjuster. The proposed method is capable of removing the harmonic current and voltage without using any active and passive filters and without the knowledge of the microgrid topology and also impedances of distribution bands and loading conditions. This concept is adopted for the control of a Distributed Generator (DG) in the form of grid-connected inverter. The proposed control can be applied to the grid connected inverter of the PV. The fast dynamic response, simple design, stability, and fast transient response are the new main features of the proposed design. This paper also analyzes the circuit configuration effects on the grid connected inverter capability. The proposed control is used to demonstrate the improved stability and performance.

An Improved Feedforward Control Method Considering PLL Dynamics to Improve Weak Grid Stability of Grid-Connected Inverters

Phase-locked loop (PLL) is commonly used for three-phase grid-connected inverters to obtain the information of grid synchronization, and PLL dynamics are the key factors for stable operation of the inverters. Under weak grid conditions, the coupling between PLL and grid impedance can result in harmonic resonance, or even instability in the system. In this paper, the effect of PLL dynamics and grid impedance on the stability of three-phase grid-connected inverters is studied with the d – q impedance model. Besides, the variable transfer relationship is used to analyze the influence of PLL perturbations on output current under weak grid conditions. To suppress low-order harmonics of the network current caused by PLL perturbations under weak grid conditions, a novel feedforward control method is proposed to compensate PLL perturbations and revise the output impedance, where the operation of the inverter and PLL dynamics have been taken into account in the design process. By analyzing the impedance characteristic of the system, it can be demonstrated that the proposed method can enhance the stability of grid-connected inverters under weak grid conditions and reduce the impact of PLL perturbations on grid-connecting current. The experimental results indicate that the low-order harmonics of the network current can be suppressed effectively, which verifies the analysis.