Three-phase Four-leg Inverter Research Articles

Extending 2-D space vector PWM for three-phase four-leg inverters

ABSTRACTThis paper proposes a novel Space Vector Pulse Width Modulation strategy extending conventional 2-D SVPWM strategies for its use in three-phase four-leg inverters. The methodology is based on the segregation of the zero sequence from any initial desired unbalance voltage set-points. In this way, the operation of the fourth leg is reserved to manage the desired homopolar component and to compensate for the inherent homopolar component provided by any 2-D SVPWM technique. This choice not only minimizes the complexity and effort to determine the location of the reference voltage space vector from a 3-D to a 2-D space but also enables to use 75% of the affordable space to synthesize voltages under a 3-D SVPWM technique. Furthermore, the proposal is compared with other four 3-D SVPWM techniques analysing the advantages in terms of execution time, memory program, simplicity and neutral to mid-point voltage quality, resulting into a really good implementing alternative compared with the major 3-D SPVWM techniques analysed. The new modulation strategy is presented in detail, and a complete set of simulations using this particular modulation strategy has been done, and the results of these simulations have been validated on a laboratory set-up using a four-leg inverter.

Three-Phase Four-Leg Drive for DC-Biased Sinusoidal Current Vernier Reluctance Machine

This paper proposes a three-phase four-leg drive for dc-biased sinusoidal current vernier reluctance machine (DC-VRM). By adding an asymmetrical neutral leg to the conventional three-phase inverter, the dc field excitation current can be generated. Compared to the open-winding inverter (conventional drive topology), the three-phase four-leg inverter greatly reduces the number of power devices, and the asymmetrical neutral leg does not have short-circuit straight-through risk. The topology ensures that the stator current can flow in both directions and maximum torque per copper loss can be obtained in DC-VRM. Equivalent carrier-based pulsewidth modulation (PWM) strategy is proposed to produce the dc-biased sinusoidal current and maximize the dc bus utilization. The proposed carrier-based PWM has several different equivalent methods. Compared to the conventional DC-VRM drive, the power loss of the proposed drive with equivalent space vector PWM (SVPWM) can be reduced significantly and can be further reduced by discontinuous PWM minimum (DPWMMIN) in the rated condition. The proposed drive has the characteristics of low cost, high power density, and high efficiency compared to the conventional DC-VRM drive.

A Control Method of Inverter to Implement the Three-Phase Current Imbalance Compensation

Based on three-phase four-leg inverter structure, a control method of inverter is studied to implement the three-phase current imbalance compensation. The control method of three-phase four-leg inverter is studied for micro grid power quality problems caused by three-phase current imbalance, using positive and negative sequence synchronous rotation coordinate conversion and band-stop filter, the positive sequence , negative sequence and zero-sequence current are separated to be controlled, and three-phase inverter output current follows the command, three-phase current imbalance compensation is realized effectively. Based on theoretical analysis, a simulation model and experimental platform are build, and the feasibility and correctness of the system control methods is verified by simulation and experiments.

Lyapunov model predictive control to optimise computational burden, reference tracking and THD of three‐phase four‐leg inverter

Due to the evolution of high processing microprocessors, the model predictive control (MPC) has been widely used in power electronic applications. In spite of simplicity, flexibility and fast dynamic response, the conventional MPC (C-MPC) has a drawback of computational burden. This study focuses on Lyapunov MPC (L-MPC) method, in which Lyapunov control law is employed in the cost function to minimise the error between the desired control variables and the actual control variables of three-phase four-leg inverter to optimise the closed-loop system performance. The proposed control algorithm takes advantage of a predefined Lyapunov control law which minimises the required calculation time by the Lyapunov model equations just once in each control loop to predict the future variables. L-MPC technique improves the digital speed by 23.8% as compared to C-MPC. It reduces current tracking error and total harmonic distortion (THD) in the variation of inverter control parameters. The stability of the system is established through Lyapunov function with the help of backsteping control method. The LabVIEW field programmable gate array (FPGA) rapid prototyping controller is used to validate the proposed control method. The experimental results showed that the proposed system has better performance as compared to C-MPC.

Improved Voltage Unbalance and Harmonics Compensation Control Strategy for an Isolated Microgrid

This paper suggests an enhanced control scheme for a four-leg battery energy storage systems (BESS) under unbalanced and nonlinear load conditions operating in the isolated microgrid. Simplicity, tiny steady-state error, fast transient response, and low total harmonic distortion (THD) are the main advantages of the method. Firstly, a new decoupled per-phase model for the three-phase four-leg inverter is presented. It can eliminate the effect of power stage coupling on control design; thus, the three-phase four-leg power inverter can be viewed as three single input single output (SISO) control systems. Then, using an improved orthogonal signal generation method, the per-phase model of the four-leg inverter in the stationary and synchronous frame is derived. As the second step, a per-phase multi-loop control scheme for the four-leg inverter under unbalanced load conditions is suggested. The proposed control strategy has the ability to provide balanced output voltages under unbalanced load conditions by avoiding the need to deal with the symmetrical components. Finally, a multi-resonant harmonic compensator is used to actively prevent low-order harmonic currents to distort the output voltages of the three-phase four-leg grid-forming power converter. Simulations results are also presented to verify the performance of the suggested control strategy.

Novel random pulse position modulation for three-phase four-leg inverters

Aiming at improving the harmonic spectra spreading effect, two randomized pulse position modulation strategies are proposed for three-phase four-leg inverters. Based on the volt-second balance law and the fixed switching frequency, the first proposed strategy places the center randomly during one carrier cycle with a limited degree of freedom, and the second proposed strategy randomly changes the carrier slope in each carrier cycle. To get better randomization effect of the proposed strategies, Well Equidistributed Long-period Linear algorithm was introduced to generate random numbers. In order to verify the validity of the proposed strategies, the modulation strategies are implemented with dSPACE DS1007-based real-time operating system. The experimental results show that the proposed strategies have dispersed the discrete harmonics to a wider sideband. Compared with the first strategy, a larger degree of freedom can be achieved in the second proposed strategy with lower harmonic spikes.

A new control scheme for three-phase four-leg inverter

A new control scheme for three-phase four-leg inverter

Near State Vector Selection-Based Model Predictive Control with Common Mode Voltage Mitigation for a Three-Phase Four-Leg Inverter

A high computational burden is required in conventional model predictive control, as all of the voltage vectors of a power inverter are used to predict the future behavior of the system. Apart from that, the common mode voltage (CMV) of a three-phase four-leg inverter utilizes up to half of the DC-link voltage due to the use of all of the available voltage vectors. Thus, this paper proposes a near state vector selection-based model predictive control (NSV-MPC) scheme to mitigate the CMV and reduce computational burden. In the proposed technique, only six active voltage vectors are used in the predictive model, and the vectors are selected based on the position of the future reference vector. In every sampling period, the position of the reference current is used to detect the voltage vectors surrounding the reference voltage vector. Besides the six active vectors, one of the zero vectors is also used. The proposed technique is compared with the conventional control scheme in terms of execution time, CMV variation, and load current ripple in both simulation and an experimental setup. The LabVIEW Field programmable gate array rapid prototyping controller is used to validate the proposed control scheme experimentally, and demonstrate that the CMV can be bounded within one-fourth of the DC-link voltage.

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.

Study on the Current-Limiting-Capable Control Strategy for Grid-Connected Three-Phase Four-Leg Inverter in Low-Voltage Network

The three-phase four-leg inverter can produce balanced voltages even with unbalanced loads, yet its controller design is quite complicated. Based on the analysis on time domain equations, a decoupled sequence control strategy for the three-phase four-leg inverter in a low-voltage network is proposed. A negative sequence controller and a zero sequence controller are added to the control strategy besides the positive sequence controller. Furthermore, considering the output limit of the inverter, a current limit design scheme is raised through the analysis on each sequence current in synchronous rotating frame. In the case of asymmetry, the limit value of each sequence current can be adjusted dynamically according to the design scheme. The output currents in each sequence can be controlled and limited for different purposes respectively. Finally, simulation results based on PSCAD/EMTDC V4.5.0 verify the validity of the control strategy.

Modulation With Sinusoidal Third-Harmonic Injection for Active Split DC-Bus Four-Leg Inverters

Three-phase four-leg inverters are commonly employed to supply unbalanced loads avoiding the use of a transformer. A suitable output filter is necessary to improve the harmonic content of the output voltage and current waveforms. To this aim, different kind of filters, presenting both symmetrical and asymmetrical phase-to-phase and phase-to-neutral behavior, can be used. The paper discusses and analyzes the effects of different carrier-based modulation techniques, based on the independent control of the fourth leg from that of the other phases, with reference to the different filter topologies. The comparison, performed through extensive simulations and experimental tests, have demonstrated that in case of the asymmetrical filter utilization, such as the active split dc-bus (also known as split-link ), the conventional continuous and discontinuous space-vector modulations produce a significant harmonic content in the output voltages even at no-load condition. Moreover, the analysis is extended considering the interaction between the modulation strategy and load characteristics as linear, nonlinear, balanced and unbalanced loads. The problem is discussed in detail and a specific design procedure for the active split dc-bus is presented.

Model Predictive Control of Quasi-Z-Source Four-Leg Inverter

This paper presents a model predictive control (MPC) scheme for quasi-Z-source (qZS) three-phase four-leg inverter. In order to cope with the drawbacks of traditional voltage source inverters (VSIs), a qZS three-phase four-leg inverter topology is proposed. 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, temperature, and solar irradiation. 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. Simulation and experimental studies verify the performances of the proposed control strategy under balanced and unbalanced load conditions as well as single-phase open-circuit fault condition.

Improved one-cycle control algorithm for diagnosis and optimization in low-voltage SVG device

Nowadays low voltage SVG (Static Var Generator) has got a certain amount of practical applications. Current PWM control is a key unit of SVG control process. Taking the three-phase four-leg inverter for research object, this paper designs the current controller base on one-cycle control algorithm and derivates the control equation in detail. Aimed at the problem of control shocks existing in traditional one-cycle control, the improved control unit is given out with the fundamental positive sequence active component of load current as control objectives. This improved control unit could realize reactive power, harmonics, and negative sequence imbalance current comprehensive compensation function, and duty cycle stability are improved. Control performance and response time of the control unit were significantly optimized. The proposed improved one-cycle control unit was validated in terms of simulation and experiment result, and has high practical value of low-voltage SVG application.

A Fault-Tolerant Control Strategy for Six-Phase Transverse Flux Tubular PMLM Based on Synthetic Vector Method

The six-phase transverse flux tubular permanent magnet linear motor (PMLM) has many advantages compared with conventional linear motors. However, the reliability of its drive system can still be influenced by power converter or machine faults. This paper presents a fault-tolerant control strategy for six-phase transverse flux tubular PMLM based on synthetic vector method when the machine has a single-phase open-circuit fault. The operation of six-phase transverse flux tubular PMLM is described as well as its basic mathematical model. Then, the principles of synthetic vector method are introduced. Based on these theories, the fault-tolerant control strategy is derived in detail under the single-phase open-fault condition. In addition, a fault-tolerant dual three-phase four-leg inverter's drive topology that can be applied under this faulted condition is proposed. Moreover, to implement the control algorithms proposed, a modified space vector modulation is presented. Both the finite-element simulation and MATLAB/Simulink simulation results illustrate the validity of the proposed fault-tolerant control as well as the associated control topology and space vector modulation.

A novel three-phase four-leg inverter based load unbalance compensator for stand-alone microgrid

• This paper proposes a novel load unbalance compensator (LUC) for the stand-alone microgrid using three-phase four-leg VSI. • The model of a three-phase four-leg VSI for the LUC and the microgrid are simulated using PSCAD/EMTDC. • This demonstrates that the proposed LUC increases the stability of stand-alone microgrid under unbalance load conditions. This paper proposes a three-phase four-leg voltage sourced inverter (VSI) based load unbalance compensator (LUC) including its control algorithm, which is a component of a microgrid. The purpose of proposed three-phase four-leg VSI based LUC is to improve power quality of the standalone microgrid. Power quality of the microgrid which was installed in Mara-island, Korea is analyzed using a real operational data. In this work, the microgrid in Mara-island which includes a photovoltaic power generation system, a diesel generator, a battery energy storage system, and a power management system is modeled in PSCAD/EMTDC, and proposed three-phase four-leg VSI based LUC is also modeled and applies to the modeled microgrid. Power flow and stability of the modeled microgrid with the LUC is analyzed under variable irradiance and unbalance loads. The results show that the proposed LUC helps to improve stability of the stand-alone microgrid. The proposed three-phase four-leg VSI based LUC and its control algorithm can be effectively utilized to the stand alone microgrid which has large unbalance loads.

Sliding Mode Control of Three-Phase Four-Leg Inverters via State Feedback

To optimize controller design and improve static and dynamic performances of three-phase four-leg inverter systems, a compound control method that combines state feedback and quasi-sliding mode variable structure control is proposed. The linear coordinate change matrix and the state variable feedback equations are derived based on the mathematical model of three-phase four-leg inverters. Based on system relative degrees, sliding surfaces and quasi-sliding mode controllers are designed for converted linear systems. This control method exhibits the advantages of both state feedback and sliding mode control. The proposed controllers provide flexible dynamic control response and excellent stable control performance with chattering suppression. The feasibility of the proposed strategy is verified by conducting simulations and experiments.

Implementation of digital FCS-MP controller for a three-phase inverter

In this paper, a digital current control of a three-phase four-leg inverter based on predictive cost function is studied. Inverters are important for power electronics applications in three-phase four-wire systems, such as active power filters and uninterruptible power supplies. There are many methods for control of inverters, such as pulse-width modulation and three-dimensional space vector modulation. These methods are complex. In this paper, a simple way for current control of a three-phase four-leg inverter is presented. Here, by using a predictive cost function and fixed-point computations, the optimal switching state to be applied in the next sampling is selected. Finally, to evaluate the accuracy of the fixed-point computations, several cases for balanced and unbalanced loading conditions are verified. In each case, the parameter that measured to confirm computation accuracy is total harmonic distortion.

The Control Technology of the Z-Source Three-Phase Four-Leg Inverter Based on Computer Simulation

Z-source inverter can boost the voltage of the DC-side, allow the two switches of the same leg conducting at the same time and it has some other advantages. The zero-sequence current flows through the fourth leg of the three-phase four-leg inverter so the three-phase four-leg inverter can work with unbalanced load. This paper presents a Z-source three-phase four-leg inverter which combines a Z-source network with three-phase four-leg inverter. The circuit uses simple SPWM modulation technique and the fourth leg uses fully compensated control method. The inverter can maintain a symmetrical output voltage when the proposed scheme under the unbalanced load.

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.