Three-phase Grid-connected Inverter Research Articles

Application of DE Algorithm over Control of Grid-Connected PV Inverter

A novel method for optimum vector PWM (OVPWM) control of three-phase grid-connected photovoltaic inverter was described. The control vector and its corresponding action duration composed of the PWM sequences generated by optimum vector PWM strategy. A Differential Evolution (DE) algorithm was applied to determine the action duration of the output voltage vector based on conventional vector PWM (CVPWM). The Matlab-based model of three-phase grid-connected photovoltaic generation system was established. Comparative analysis proving superiority of the OVPWM technique over the CVPWM, and the results of the simulation confirmed the feasibility and advantage of proposed OVPWM strategy.

Dynamic Response Improvement of the DPC-SVM Three-Phase Grid-Connected Inverter

As the dynamic response of the traditional direct power control using space vector modulation (DPC-SVM) is slow, an improved DPC-SVM for the three-phase grid-connected inverter is proposed. The feed-forward reference powers are used to improve the dynamic response. The operating principles of the traditional and improved DPC-SVMs are illustrated. Experimental results of a 15-kVA three-phase grid-connected inverter with the traditional and improved control methods verify the theoretical analysis.

Performance Improvement of SVPWM-Based Three-Phase Grid-Connected Inverters

An improved control strategy for three-phase grid-connected inverters with space vector pulse width modulation (SVPWM) is proposed. When the grid current contains harmonics, the d-and q-axes grid currents is interacted in the traditional control method, and the waveform quality of the grid current is poor. As the reference output voltage cannot directly reflect the change of the reference grid current with the traditional control strategy, the dynamic response of the grid-connected inverter is slow. In order to solve the aforementioned problems, the d-and q-axes grid currents in the decoupled components of the grid current controller are substituted by the d-and q-axes reference grid currents, respectively. The operating principles of the traditional and proposed control methods are illustrated. Experimental results show that the grid-connected inverter with the improved control strategy has high waveform quality of the grid current and fast dynamic response.

Control Method for Reducing the THD of Grid Current of Three-Phase Grid-Connected Inverters Under Distorted Grid Voltages

This paper proposes a control method for reducing the total harmonic distortion (THD) of the grid current of three-phase grid-connected inverter systems when the grid voltage is distorted. The THD of the grid current caused by grid voltage harmonics is derived by considering the phase delay and magnitude attenuation due to the hardware low-pass filter (LPF). The Cauchy-Schwarz inequality theory is used in order to search more easily for the minimum point of the THD. Both the gain and angle of the compensation voltage at the minimum point of the THD of the grid current are derived with the variation of cut-off frequencies of the hardware LPF. Simulation and experimental results show the validity of the proposed control methods.

Fault Diagnosis and Fault-Tolerant Control of DC-link Voltage Sensor for Two-stage Three-Phase Grid-Connected PV Inverters

This paper proposes a method for fault diagnosis and fault-tolerant control of DC-link voltage sensor for two-stage three-phase grid-connected PV inverters. Generally, the front-end DC-DC boost converter tracks the maximum power point (MPP) of PV array and the rear-end DC-AC inverter is used to generate a sinusoidal output current and keep the DC-link voltage constant. In this system, a sensor is essential for power conversion. A sensor fault is detected when there is an error between the sensed and estimated values, which are obtained from a DC-link voltage sensorless algorithm. Fault- tolerant control is achieved by using the estimated values. A deadbeat current controller is used to meet the dynamic characteristic of the proposed algorithm. The proposed algorithm is validated by simulation and experiment results.

Fuzzy logic-based MPPT controllers for three-phase grid-connected inverters

In photovoltaic (PV) applications, a maximum power point tracking (MPPT) module is necessary to extract the whole energy that the PV module can generate depending on the instantaneous conditions of the PV system. A PV module is obtained by connecting a number of solar cells in series and parallel, which causes voltage and current to increase at module terminations. The present work is based on a three-phase grid-connected inverter designed for a 100 kW PV power plant that uses an MPPT scheme based on fuzzy logic controllers. The whole system presented is simulated in MATLAB. The fuzzy logic-based MPPT controllers show accurate and fast responses and are integrated into the inverter, so that the there is no requirement for a dc–dc converter. The inverter allows full control of reactive power.

New Control Strategy for Three-Phase Grid-Connected LCL Inverters without a Phase-Locked Loop

The three-phase synchronous reference frame phase-locked loop (SRF-PLL) is widely used for synchronization applications in power systems. In this paper, a new control strategy for three-phase grid-connected LCL inverters without a PLL is presented. According to the new strategy, a current reference can be generated by using the instantaneous power control scheme and the proposed positive-sequence voltage detector. Through theoretical analysis, it is indicated that a high-quality grid current can be produced by introducing the new control strategy. In addition, a kind of independent control for reactive power can be achieved under unbalanced and distorted grid conditions. Finally, the excellent performance of the proposed control strategy is validated by means of simulation and experimental results.

Reduction of Current Harmonic Distortion in Three-Phase Grid-Connected Photovoltaic Inverters via Resonant Current Control

The resonant current control has been extensively employed to reduce the current harmonic distortion in a wide range of grid-connected distributed generation applications, including photovoltaic (PV) inverters, wind and water turbines, and fuel-cell inverters. However, the performance of these systems is deteriorated when the utility grid voltage experiences abnormal conditions such as voltage harmonics and imbalances. Several advanced control solutions have been recently introduced to cope with this problem but at the cost of a significant increase in the control computational load. This paper first analyzes the limitations of the standard resonant current control operating under abnormal grid conditions and then introduces a control scheme that improves the current harmonic distortion in such adverse conditions without increasing the computational load of the standard current control. This theoretical contribution is validated by means of selected experimental results from a three-phase PV inverter.

Flexible Voltage Support Control for Three-Phase Distributed Generation Inverters Under Grid Fault

Ancillary services for distributed generation (DG) systems become a challenging issue to smartly integrate renewable-energy sources into the grid. Voltage control is one of these ancillary services which can ride through and support the voltage under grid faults. Grid codes from the transmission system operators describe the behavior of the energy source, regulating voltage limits and reactive power injection to remain connected and support the grid under fault. On the basis that different kinds of voltage sags require different voltage support strategies, a flexible control scheme for three-phase grid-connected inverters is proposed. In three-phase balanced voltage sags, the inverter should inject reactive power in order to raise the voltage in all phases. In one- or two-phase faults, the main concern of the DG inverter is to equalize voltages by reducing the negative symmetric sequence and clear the phase jump. Due to system limitations, a balance between these two extreme policies is mandatory. Thus, over- and undervoltage can be avoided, and the proposed control scheme prevents disconnection while achieving the desired voltage support service. The main contribution of this work is the introduction of a control algorithm for reference current generation that provides flexible voltage support under grid faults. Two different voltage sags have been experimentally tested to illustrate the behavior of the proposed voltage support control scheme.

Research on fast transient and 6 n ±1 harmonics suppressing repetitive control scheme for three‐phase grid‐connected inverters

The repetitive control technique is widely adopted in ac systems, because of its excellent steady-state tracking performance and low total harmonic distortion. The repetitive control method with one-sixth of the system fundamental period T0 as the delay time has been proposed to achieve fast transient response. In this study, an improved control scheme based on the T0/6 repetitive control is proposed for three-phase grid-connected inverters. The proposed scheme adopts T0/6 as the delay time in the positive-rotating and negative-rotating synchronous reference frames to suppress the 6n ± 1 harmonics. Meanwhile, the proportional-integral regulator and the plug-in repetitive controller are combined to reinforce the system performance. A new auxiliary function based on the linear interpolation is proposed to maintain the ideal repetitive control performance when one-sixth of the ratio of the sampling frequency to the grid fundamental frequency is non-integer. The effectiveness of the proposed scheme on improving the T0/6 repetitive control is confirmed by simulation and experimental results finally.

Application of Four-Switch-Based Three-Phase Grid-Connected Inverter to Connect Renewable Energy Source to a Generalized Unbalanced Microgrid System

In this paper, a four-power-semiconductor-switch-based three-phase inverter is proposed for renewable energy source integration to a generalized microgrid system. The proposed topology <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$b \hbox{-}4$</tex></formula> of three-phase inverter is investigated to make the commercial microgrid system to be cost effective and hardware optimized. A simple sine-pulse-width-modulation-based (SPWM) control strategy is proposed for the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$b\hbox{-}4$</tex></formula> inverter topology instead of the traditional complex four-switch-based space vector techniques. The overall control structure is implemented using the Lyapunov function-based nonlinear controller to track the inverter current directly in the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$a {-} b {-} c$</tex></formula> frame so that a specific amount of active and reactive grid power flow to the grid can be controlled in a decoupled manner along with low total harmonic distortion of grid currents in the presence of nonlinear load at the point of common coupling (PCC). A novel technique of using the spatial repetitive controller (SRC) is also proposed to eliminate the effect of midpoint voltage fluctuation of the dc link even in the case of asymmetrically split dc-link capacitors without any extra voltage or current sensors unlike conventional methods. Detailed experimental results are provided to show the efficacy of the proposed hardware system for grid-connected applications in the microgrid.

Simulation and Implementation of Grid-connected Inverters

Solar, wind and hydro are renewable energy sources that are seen as reliable alternatives to conventional energy sources such as oil or natural gas. However, the efficiency and the performance of renewable energy systems are still under development. Consequently, the control structures of the gridconnected inverter as an important section for energy conversion and transmission should be improved to meet the requirements for grid interconnection. In this paper, a comprehensive simulation and implementation of a threephase grid-connected inverter is presented. The control structure of the grid-side inverter is firstly discussed. Secondly, the space vector modulation SVM is presented. Thirdly, the synchronization for grid-connected inverters is discussed. Finally, the simulation of the grid-connected inverter system using PSIM simulation package and the system implementation are presented to illustrate concepts and compare their results.

Research on Deadbeat Control for Three-Phase Grid-Connected Inverter in Model Based Design

Using model based design(MBD) method, deadbeat control algorithm for three-phase grid-connected inverter was designed in optimization and realized in hardware. According to the advanced idea of MBD and its basic procedure, the software-in-loop(SIL) test, processor-in-loop(PIL) test and hardware-in-loop(HIL) test for deadbeat control algorithm were mainly researched under MATLAB/Simulink environment choosing TMS320F2812 as object hardware board. In such method, embedded codes could be produced automatically and the errors brought in through algorithm model could be tested and corrected earlier. Meanwhile, simply changing the PIL related modules, the self-defined deadbeat algorithm module could be flexibly applied in other hardware platforms. At last, the correctness and efficiency of deadbeat control algorithm were verified in three-phase grid-connected inverter experiment platform.

Design and Implementation of 30kW Three-Phase PV Grid-Connected Inverter

This paper developed a mathematical model of three-phase PV grid-connected inverter, and studied the grid-connected current control method based on PI control in synchronous rotating reference frame. Simulation and experimental results from the prototype of 30kW three-phase PV grid-connected inverter proved the correctness and the feasibility of the control strategy, and this grid-connected inverter can operate at the unity power factor state with a nice dynamic performance, and the output current has high sinusoidal and low harmonic content as well as good symmetry.

Research on Control Strategy for the Three-Phase Grid-Connected Inverter

This paper proposes a control strategy，based on the grid voltage oriented vector control (VOC), which makes three-phase inverter control the active and reactive power of grid-connected inverter under the premise of the direct current control. This paper analyzes the principle of three phase photovoltaic grid connected inverter and describes the control structure of the inverter. The control strategy can overcome the deficiencies of the indirect current control scheme. This paper designs the current closed-loop control system, which not only improve the system dynamic response speed and output current waveform quality, while also reduce its sensitivity to parameter changes to improve the robustness of the system. The simulation results show the validity of control strategy proposed.

Improvement of DPC of Grid-Connected Inverter for Wind Power System

This paper analyzes the principle and properties of the traditional direct power control (DPC) system for the three-phase grid-connected voltage source inverter based on its mathematical model. Hysteresis comparators in power loop of DPC control system, leading to the switching frequency not fixed .the fluctuation of power and DC voltage in temporary state. This paper brought an improved program. The active and reactive powers are calculate by estimating the virtual flux. The AC side voltage sensors could be replaced. The SVPWM has been utilized instead of the power hysteresis and switching table of the traditional DPC system.

A ZVS Grid-Connected Three-Phase Inverter

A six-switch three-phase inverter is widely used in a high-power grid-connected system. However, the antiparallel diodes in the topology operate in the hard-switching state under the traditional control method causing severe switch loss and high electromagnetic interference problems. In order to solve the problem, this paper proposes a topology of the traditional six-switch three-phase inverter but with an additional switch and gave a new space vector modulation (SVM) scheme. In this way, the inverter can realize zero-voltage switching (ZVS) operation in all switching devices and suppress the reverse recovery current in all antiparallel diodes very well. And all the switches can operate at a fixed frequency with the new SVM scheme and have the same voltage stress as the dc-link voltage. In grid-connected application, the inverter can achieve ZVS in all the switches under the load with unity power factor or less. The aforementioned theory is verified in a 30-kW inverter prototype.

Exploring Inherent Damping Characteristic of LCL-Filters for Three-Phase Grid-Connected Voltage Source Inverters

This paper investigates the inherent damping characteristic of LCL- filters for three-phase grid-connected voltage source inverters. Specifically, it is found that when the converter-side current is used for implementing the feedback control, there will be an inherent damping term embedded in the control loop, which can neutralize the resonance introduced by LCL- filters. Theoretical analysis is then presented to suggest a general design guideline for choosing values of grid- and converter-side inductors, so that optimum damping can be naturally achieved by solely using converter current control, doing away with active damping, passive damping, or complex state observer. In cases where the design criterion is not fulfilled, the damping information contained in the converter current is extracted by a second-order notch filter, and then processed by a compensation gain to tune the system damping factor. The proposed compensation strategy requires no additional hardware and it will not cause an overmodulation problem due to its free of fundamental component. Both simulation and experimental results are finally provided to validate the theoretical findings developed in this paper.

Reseach on Control Strategy Based on p-q-r Theory for Four-Leg Grid-Connected Inverter

The mathematical model for three-phase four-leg grid-connected inverter is modeled by the simplified p-q-r theory. The control strategy is proposed based on the characteristic of controlled variables. PI algorithm is adopted for variables of p axis and q axis, and PR algorithm is adopted for variable of r axis. The full design and analysis of the proposed control algorithms are given in this paper. Simulation results and experimental result show that quality of four-leg grid-connected inverter output current is higher, and effectiveness of the proposed control strategies is validated by all results.

Robust Predictive Current Controller Based on a Disturbance Estimator in a Three-Phase Grid-Connected Inverter

This paper proposes a robust predictive current controller based on a disturbance estimator for a three-phase grid-connected inverter. The various components that deteriorate the performance of a conventional predictive current controller are regarded as disturbances, and a disturbance estimator is constructed using an inverter output voltage and current values. Furthermore, a grid angle is extracted using a phase-locked loop (PLL) with the estimated reactive component of the disturbance; therefore, sensorless grid voltage control is possible. In order to determine the relevant gains in the disturbance estimator, the frequency components corresponding to the unbalanced grid condition and low-order harmonics, as well as the PLL bandwidth, are considered. Moreover, the stability of the disturbance estimator due to parameter errors in the inductor filter is analyzed. The proposed method has an inherent rapid dynamic response due to the conventional predictive current controller, as well as low-cost implementation and robust control performance with regard to the disturbance and noise interference due to use of the combined estimation algorithm. The feasibility of the proposed predictive current control strategy is verified through the examination of experimental results.