Three-phase Four-leg Voltage Source Inverter Research Articles

Disturbance Observer-Based Feedback Linearized Controller for Grid-Forming Four-Leg VSI Supplying Unbalanced and Nonlinear Loads

This paper presents the design of a disturbance observer-based controller that regulates the output voltage of three-phase four-leg voltage source inverters (VSIs) deployed for grid-forming operation in Renewable Energy-based Distributed Generation (REDG) Systems. The primary objective of the controller is to provide a symmetric and sinusoidal voltage at the output of the VSI when supplying highly unbalanced and nonlinear loads. The controller employs the feedback linearization (FL) technique and incorporates a disturbance observer (DO) to address a range of disturbances that include oscillations resulting from unbalanced loads, harmonics generated by nonlinear loads, and non-oscillatory disturbances. Notably, the controller adopts a direct control scheme without the need for nested current control loops and does not use any transformation frames. Simulation studies and experimental investigations were conducted to assess the controller’s performance under various load conditions, including both linear and nonlinear types, as well as load transients. The findings demonstrate the controller’s capability to accurately track references while complying with the IEEE power quality standards for the tested conditions.

An individual sequencing control strategy for three-phase four-leg inverter under unbalanced loads

Under the influence of the unbalanced load, the negative and zero sequence components in the output voltage of the three-phase four-leg voltage source inverter(TPFL-VSI) will cause asymmetry voltage. This paper designs an individual sequencing control for the three-phase four-leg inverter to address this problem. The second-order generalized integrator is adopted to extract the current’s positive and negative sequence components. The virtual oscillator control(VOC) controls the positive sequence component, and the negative sequence component is suppressed by designing the virtual impedance and the multiple proportional resonance controller. The proportional resonance control is used to control the zero-sequence voltage of the fourth bridge arm. Considering that voltage deviation is caused by the access of negative sequence virtual impedance and load step, the secondary voltage compensation strategy is further added in the individual sequencing control to improve the droop relationship in the VOC and effectively solve the problem of system voltage instability. Ultimately, a TPFL-VSI model is built in Matlab/Simulink, and simulation results verify the feasibility and effectiveness of the proposed control strategy under different conditions.

Application of Three-Dimensional Unbalanced Coordinate Transformation to Stand-Alone Four-Leg Voltage-Source Inverter

Three-phase four-leg voltage-source inverter has been extensively investigated in recent years for its compactness, small size, and high efficiency, and it has been proved to be the best solution for providing transformer-less neutral connection to three-phase asymmetric loads. The main purpose of this article is to apply 3-D unbalanced coordinate transformation to the four-leg inverter, which supplies stand-alone asymmetric loads, to maintain the balance of the output capacitor voltages, and obtain simpler control structure and better control performance. First, a double closed-loop PI control structure can be designed based on the Clarke and Park transformations. Second, the phases and amplitudes of the capacitor voltages are locked based on the 3-D unbalanced coordinate transformation. Furthermore, the Clarke transformation oriented on the inductor currents is replaced by a 3-D unbalanced coordinate transformation, and the corresponding unbalance indices are continuously adjusted according to the unbalance indices of the capacitor voltages, so as to recover the capacitor voltages to the balanced condition. The simulation and experimental results of the whole control system for the four-leg inverter are provided in the end, so as to verify the feasibility and effectiveness of the 3-D unbalanced coordinate transformation for application to the power electronic systems.

Flatness-Based Grey Wolf Control for Load Voltage Unbalance Mitigation in Three-Phase Four-Leg Voltage Source Inverters

Standalone power-supply systems become a key solution to effectively address the load demand in remote locations, wherein the voltage asymmetry raises as a particular concern in view of a large number of single-phase household and communities loads. Over these loading conditions, the use of the four-leg voltage source inverters (FL-VSIs) appears as a suitable topology to provide clean power with symmetrical voltage waveforms. The reported work employs the differential flatness theory associated with grey wolf optimization in order to guarantee proper operations of the FL-VSIs in different loading conditions. In fact, the differential flatness theory is applied to check the flatness of the FL-VSIs, which allows implementing a reduced control model. Then, a grey wolf algorithm, which acts as a tracking controller [grey wolf controller (GWC)], is developed. The GWC generates an optimal control signal used by the differentially flat model so as to ensure adequate control performance, even under severe load disturbances and model inaccuracies. A comprehensive experimental test is carried out to validate the effectiveness of the proposed flatness-based grey wolf controller.

Quasi-Z-Source Four-Leg Inverter with PV by using Model Predictive Control Scheme

The Implementation of Quasi-Z-Source Four-Leg Inverter with PV by using Model Predictive Control Scheme is proposed in this paper. In order to reduce the drawbacks of traditional three phase voltage source inverter (VSI). Photovoltaic (PV) is a term which converts the light into electricity. This topology features a wide range of voltage gain which is suitable for applications in renewable energy-based power systems, where the output of the renewable energy sources varies widely with operating conditions such as wind speed, solar irradiation and temperature. To improve the capability of the controller, an MPC scheme is used which implements a discrete-time model of the system. The controller handles each phase current independently, which adds flexibility to the system. The performance of quasi z source three-phase four-leg VSI with PV by using model predictive control (MPC) was simulated using MATLAB Simulink under balanced and unbalanced load conditions as well as single-phase open-circuit fault condition.

Internet of Things Platform for Energy Management in Multi-Microgrid System to Improve Neutral Current Compensation

In this paper, an Internet of Things (IoT) platform is proposed for Multi-Microgrid (MMG) system to improve unbalance compensation functionality employing three-phase four-leg (3P-4L) voltage source inverters (VSIs). The two level communication system connects the MMG system, implemented in Power System Computer Aided Design (PSCAD), to the cloud server. The local communication level utilizes Modbus Transmission Control Protocol/Internet Protocol (TCP/IP) and Message Queuing Telemetry Transport (MQTT) is used as the protocol for global communication level. A communication operation algorithm is developed to manage the communication operation under various communication failure scenarios. To test the communication system, it is implemented on an experimental testbed to investigate its functionality for MMG neutral current compensation (NCC). To compensate the neutral current in MMG, a dynamic NCC algorithm is proposed, which enables the MGs to further improve the NCC by sharing their data using the IoT platform. The performance of the control and communication system using dynamic NCC is compared with the fixed capacity NCC for unbalance compensation under different communication failure conditions. The impact of the communication system performance on the NCC sharing is the focus of this research. The results show that the proposed system provides better neutral current compensation and phase balancing in case of MMG operation by sharing the data effectively even if the communication system is failing partially.

A three-dimensional active zero state PWM for common-mode voltage reduction of a three-phase four-leg voltage-source inverter

PurposeThis paper aims to investigate the common-mode voltage (CMV) issue of a three-phase four-leg voltage-source inverter. A new space vector modulation method, named as three-dimensional active zero state Pulse-width modulation (PWM) (3-D AZSPWM), is proposed to reduce the CMV level.Design/methodology/approachPWM is a general method to generate the switching signals of the power converters in order to obtain high-quality output voltages. However, the CMV produced by PWM methods has become a serious problem. 3-D AZSPWM is proposed to solve this issue. In 3-D AZSPWM, instead of using zero voltage vectors with high CMV level, appropriate complementary non-zero vectors are introduced to synthesize reference vector. The proposed method is classified into four types of AZSPWM1(a), AZSPWM1(b), AZSPWM2(a) and AZSPWM2(b) based on different complementary vectors chosen for each type. An extend software simulation using MATLAB/Simulink is performed to verify the superior performance of the proposed methods.FindingsCompared to other reduced CMV methods, the proposed method not only reduces the CMV but also retains the positive characteristics of the three-dimensional classical space vector PWM (3-D CSVPWM).Originality/valueThe proposed method does not suffer from linear modulation region limitation and also does not impose additional switching loss. Furthermore, calculated output voltage harmonic distortion factor illuminates acceptable quality of output voltage produced by the proposed method.

Grid Connected Energy System four leg Inverter for DC Voltage and Power Improvement

The first power systems were DG systems designed to meet the needs of local areas.Full load DG applications showed greater benefits in terms of power and performance as well as reducing transmission losses. GDs are very suitable for a specific location and for specific applications because they require a short construction time and require little investment. It is defined on the basis of the size of the plant, which can vary from a few KW to MW (10-50 MW). GD options can be classified as renewable or non-renewable sources from fuel sources. This study deals with a newly-conceived voltage control method for three-phase four-leg voltage source inverters (VSIs) which are being required in autonomous power generating units devoted to supply both three-phase and single-phase electrical loads

Design and Evaluation of an Efficient Three-Phase Four-Leg Voltage Source Inverter with Reduced IGBTs

This paper presents a new three-phase four-leg voltage source inverter (VSI), which achieves a high cost effectiveness for mega-watt level system applications. The proposed four-leg inverter adopts the integrated topology with thyristors and insulated-gate bipolar transistors (IGBTs), which aims to reduce the number of IGBTs. In order to handle the zero sequence current, a neutral leg via incorporating IGBTs is artfully integrated with the regular phase legs. Furthermore, the modelling principles are elaborated and analyzed, which emphasizes switching states and voltage vectors in six segments based on the states of thyristors. Finally, by using the carrier-based pulse width modulation (PWM) method, the closed-loop current control of the proposed inverter is verified by both simulation and experimentation.

Design of a Hybrid Controller for the Three-phase Four-leg Voltage-source Inverter with Unbalanced Load

The three-phase four-leg voltage-source inverter topology is an interesting option for the three-phase four-wire system. With an additional leg, this topology can achieve superior performance under unbalanced and nonlinear load conditions. However, because of the low bandwidth of conventional controllers in high-power inverter applications, the system cannot guarantee a balanced output voltage under the unbalanced load condition. Most of the methods proposed to solve this problem mainly use the multiple synchronous frame method, which requires several controllers and a large amount of computation because of frame transformation. This study proposes a simple hybrid controller that combines proportional–integral (PI) and resonant controllers in the synchronous frame synchronized with the positive-sequence component of the output voltage of the three-phase four-leg inverter. The design procedure for the controller and the theoretical analysis are presented. The performance of the proposed method is verified by the experimental results and compared with that of the conventional PI controller.

Simplified Model Predictive Control Method for Three-Phase Four-Leg Voltage Source Inverters

A simplified model predictive control method is presented in this paper. This method is based on a future reference voltage vector for a three-phase four-leg voltage source inverter (VSI). Compared with the three-leg VSIs, the four-leg VSI increases the possible switching states from 8 to 16 owing to a fourth leg. Among the possible states, this should be considered in the model predictive control method for selecting an optimal state. The increased number of candidate switching states and the corresponding voltage vectors increase the calculation burden. The proposed technique can preselect 5 among the 16 possible voltage vectors produced by the three-phase four-leg voltage source inverters, based on the position of the future reference voltage vector. The discrete-time model of the future reference voltage vector is built to predict the future movement of the load currents, and its position is used to choose five preselected vectors at every sampling period. As a result, the proposed method can reduce calculation load by decreasing the candidate voltage vectors used in the cost function for the four-leg VSIs, while exhibiting the same performance as the conventional method. The effectiveness of the proposed method is demonstrated with simulation and experiment results.

An Improved Control Strategy for a Four-Leg Grid-Forming Power Converter under Unbalanced Load Conditions

This paper proposes an improved multiloop control strategy for a three-phase four-leg voltage source inverter (VSI) operating with highly unbalanced loads in an autonomous distribution network. The main objective is to balance the output voltages of the four-leg inverter under unbalanced load conditions. The proposed control strategy consists of a proportional-integral (PI) voltage controller and a proportional current loop in each phase. The voltage controller and the current control loop are, respectively, used to regulate the instantaneous output voltage and generate the pulse width modulation (PWM) voltage command with zero steady-state tracking error and fast transient response. A voltage decoupling feedforward path is also used to enhance the system robustness. Since the outer voltage loop operates in the synchronous reference frame, tuning and stability analysis of the PI controller is far from being straightforward. In order to cope with this challenge, the stationary reference frame equivalent of the voltage controller in the rotating frame is derived. Subsequently, a systematic design based on a frequency response approach is provided. Simulation results are also carried out using the DIgSILENT PowerFactory software to verify the effectiveness of the suggested control strategy.

Optimal digital control of a three-phase four-leg voltage source inverter

This paper presents a three-dimensiona (3-D) space vector pulse width modulation (SVPWM) technique via a 4 � 4 orthonormal transformation matrix that has been used as a new approach in controlling a three-phase four-leg voltage source inverter (VSI). A fully optimal digital control scheme for the closed-loop regulation of the three-phase four-leg VSI has been used to synthesize a sinusoidal waveform with the help of the proposed method. Discrete-time modelling of each phase has been obtained independently in abc reference frame, and its optimal controller has been designed using a predened performance index. A DSP-based controlled three-phase four-leg prototype VSI has been designed to verify the proposed discrete time modelling and the control scheme. The simulations and the real-time experiments show that satisfactory results have been obtained for the modelling of the three-phase four-leg VSI in abc reference frame and the 3-D SVPWM technique via 4 � 4 orthonormal transformation matrix at 20 kHz switching

Model-reference sliding mode control of a three-phase four-leg voltage source inverter for stand-alone distributed generation systems

This paper proposes a discrete-time model-reference sliding mode control of three-phase four-leg voltage source inverters for stand-alone distributed generation systems. Three-phase four-leg voltage source inverters (VSIs) are used in a broad area. A discrete-time model-reference sliding mode controller, having superior advantages such as insensitivity to external disturbances, system parameter variations, and modeling errors, is applied to control the output voltage of three-phase four-leg VSIs. As a new study, the discrete-time model-reference sliding mode control with a supplemented output feedback integral controller is proposed so that the controlled output voltages of the VSI tracks the input reference signal. The controller is designed using the augmented discrete-time error dynamics presented in this paper and is applied to control the output voltage control of a three-phase four-leg based uninterruptable power supply. It is shown that the output voltage of the VSI controlled by the proposed method is accurately regulated for various load conditions such as linear balanced/unbalanced, nonlinear balanced/unbalanced, and instantaneous load changes. The robustness of the proposed method against variations by 20{\%}--50{\%} in R, L, and C filter parameters and 5 different cases are showed. \vs{1mm}

A Near-State Three-Dimensional Space Vector Modulation for a Three-Phase Four-Leg Voltage Source Inverter

A near-state three-dimensional space-vector modulation (NS 3-D SVM) switching scheme, which aims to reduce the common-mode noise in a three-phase four-leg voltage source inverter, is proposed. The impact of common-mode noise, which is related to electromagnetic interference issues for a high-voltage level four-leg system, is investigated first. Identification of the section in a 3-D space, selection of the near-state switching vectors, and sequence of the selected switching vectors are then introduced in steps to describe the proposed switching scheme. The proposed switching scheme is based on classical 3-D SVM, producing higher dc-link utilization, less harmonic content, and reduced switching loss compared to sinusoidal PWM. Theory, simulation, and experimental results show that the near-state 3-D SVM can work under both balanced and unbalanced load conditions.

Digital current control of a three‐phase four‐leg voltage source inverter by using p – q – r theory

In this study, balanced/unbalanced linear/non-linear current control in three-phase four-leg voltage source inverter (VSI) is carried out by using modified p - q - r method instead of the classical p - q - r method, digital proportional integral derivative (PID) control rule and three-dimensional (3D) space vector pulse width modulation (3D SVPWM). For this purpose, three-phase four-leg power inverter and PWM signal generation circuits are designed and modelled separately. Even if the power source voltage is unsymmetrical, the proposed modified p - q - r method leads to measurement and to generating the instantaneous harmonic and reactive components of load current accurately. The performance of the classical p - q - r theory under non-ideal mains voltages is improved and the accuracy of the proposed modified p - q - r theory is demonstrated both through simulation and real-time application. Each phase of the three-phase four-leg VSI modelled directly in abc reference frame is decoupled from each other This simplifies the digital PID controller design significantly. Modelling of the whole system, modified p - q - r measurement and digital PID design steps are given in detail. Real-time experimental results for the designed three-phase four-leg VSI in stand-alone mode are compared with the simulation results.

Integral‐resonant control for stand‐alone voltage source inverters

This study deals with a newly-conceived voltage control method for three-phase four-leg voltage source inverters (VSIs) which are being required in autonomous power generating units devoted to supply both three-phase and single-phase electrical loads in four-wire electric power distribution systems. To deal with VSI operating conditions providing three-phase voltage supply of both unbalanced loads and non-linear loads, an integral + resonant (I + R) voltage control structure is discussed and experimentally verified by means of a 25 kVA rated power converter prototype. It is shown that the proposed voltage control allows simplifying the overall control configuration, achieving zero-overhead power factor regulation and fast compensation of load unbalances. Further to that, a complete current limiting strategy is described and tested under different load conditions.

Dynamic Hysteresis Current Control to Minimize Switching for Three-Phase Four-Leg VSI Topology to Compensate Nonlinear Load

This letter presents a dynamic hysteresis current control algorithm for a three-phase four-leg voltage-source inverter (VSI) topology. In order to improve the performance and life of the power switches of VSI, we require an optimized control technique that reduces its switching frequency. Proposed hysteresis current control method is a generalized control method that minimizes switching operations to compensate nonlinear load under both balanced and unbalanced supply voltages. It is shown that this control scheme is able to reduce the switching frequency by more than 50% compared to conventional hysteresis current control method and retain the quality of load compensation. The effectiveness of the proposed control scheme is illustrated through simulations using Power Systems Computer-Aided Design (PSCAD) 4.2.1.

A Minimally Switched Control Algorithm forThree-Phase Four-Leg VSI Topology toCompensate Unbalanced and Nonlinear Load

In this paper, a minimally switched control algorithm for a three-phase four-leg voltage source inverter topology is proposed for compensation of unbalanced and nonlinear loads. An optimized control technique is necessary for the three-phase four-leg topology, otherwise it requires higher switching operations of the switches in the voltage source inverter for tracking three-phase and neutral currents through conventional hysteresis band control. In the paper, a control technique has been formulated and its effect of reducing the switching frequency over a period of time has been studied and verified. A simulation study of three-phase, four-wire compensated system is carried out using Matlab/Simulink to validate the proposed method. Detailed experimental results presented further confirm the efficacy of the proposed control.

Novos algoritmos de limitação para inversores de tensão PWM a quatro braços utilizando modulação space vector

Este artigo propõe dois novos algoritmos de limitação para inversores de tensão trifásicos a quatro braços. Estes baseiam-se na limitação do vetor de comando dentro da região de operação linear do inversor. O primeiro algoritmo utiliza um elipsóide inscrito no dodecaedro definido pelas extremidades dos possíveis vetores de comutação do inversor, e o segundo é baseado nos planos limites desse poliedro. Uma descrição detalhada, abordando os principais pontos requeridos para a implementação digital, da modulação space vector e dos algoritmos de limitação são apresentados. Ainda, as tensões de saída e as correntes nos indutores são dinamicamente reguladas, por meio de servo controladores MIMO em eixos síncronos dq0. Estes controladores são projetados usando a técnica do regulador linear quadrático discreto de regime permanente, o qual assegura estabilidade para o sistema em toda a faixa de operação. Além disso, para prover transições suaves entre os diferentes modos de operação do inversor, são propostos algoritmos MIMO não lineares para limitar a sobrecarga da ação integral, quando o vetor de comando for limitado, e atualizar dinamicamente as variáveis dos servo controladores. Finalmente, são apresentados resultados experimentais obtidos de um protótipo de 15 kVA, totalmente controlado por um DSP TMS320F241, para validar os algoritmos propostos e demostrar o desempenho do sistema como um todo.