Three-level T-type Inverter Research Articles

A New Control for Improving the Power Quality Generated by a Three-Level T-Type Inverter

A new controller based on a fractional-order synergetic controller (FOSC) is proposed for a three-level T-type inverter using a shunt active power filter (SAPF). The SAPF is designed to compensate for the reactive power and eliminate the current harmonics caused by non-linear loads, in cases of distorted or unbalanced source voltages. The proposed FOSC technique with the designed parameters and defined macro-variable is a robust control technique that operates well in both transient and steady-state scenarios, ensuring fast convergence and closed-loop system stability. The FOSC technique utilizes a phase-locked loop (PLL) technique on a self-tuning filter (STF) to enhance the SAPF’s ability to compensate current harmonics and reactive power in all situations involving non-linear loads and source voltage variations according to IEEE Std. 519. The proposed control was implemented and verified using Matlab software, where the obtained results were compared with the results of the conventional control based on proportional-integral (PI) controllers in different operating conditions. The results indicate that the proposed FOSC technique outperformed the traditional control in terms of DC voltage tracking and the minimization of the total harmonic distortion of the current.

Topology and Space Vector Modulation Method for the Reduced Switch Count Quasi-Z-Source Three-Level Inverter

This paper proposes the topology and space vector modulation (SVM) method of a reduced switch count (RSC) quasi-Z-source (QZS) three-level inverter, which can realize voltage buck-boost operation by adopting ten power switches. Compared with the QZS three-level T-type inverter, the presented inverter can further reduce the number of power switches. The SVM method for the proposed topology is designed, which introduces a distribution factor to balance the neutral-point voltage (NPV) actively. In addition, the up-shoot-through (UST) and down-shoot-through (DST) states are inserted within the durations of redundant small vectors to boost the dc input voltage, while the normal ac output voltage is not affected. Detailed comparisons of the RSC QZS three-level inverter and the existing impedance-source-based three-level inverters are conducted, and design guidelines and recommendations for selecting different components are given as well. Simulated tests based on MATLAB software and experimental tests based on a hardware-based test rig are conducted to validate the operating performance of the proposed topology and modulation method, which reveal that single-stage power conversion is achieved. Moreover, the NPV balance is actively controlled by using the proposed method.

The Output Voltage Estimation of Power Transformer Integrated with a Three Phase T-Type Inverter

The issues related to integrating these systems into the grids continue to gain importance with the increasing use and importance of renewable energy sources. Therefore, the importance of power distribution transformers is increasing. Besides, these power distribution transformers are connected to the grid with power electronics circuits and inverters. Considering the modular inverter structures, ease of maintenance, and connection, three-level T-type inverters are chosen for this study. The secondary output voltage of the power transformer is estimated by using circuit parameters such as the dead time of the inverter circuit, PWM switching frequency, and modulation rate. Based on the finite element analysis analysis according to the selected parameters, 810 data are obtained with time-dependent parametric analysis. The adaptive neuro-fuzzy inference system model is constructed by considering the simulation data to estimate the secondary output of the power transformer of these parameters. In the training phase of the model, 648 randomly selected data from 810 data obtained by ANSYS-Electronics/Simplorer are used. The remaining 162 data are used in the testing process to measure system performance. As a result of the analysis made by ANFIS, the Root Mean Square Error (RMSE) error is found as 2.475%. Since the values obtained in the estimation process of the study are very close to the simulation values, the ANFIS method can be used as an estimation method that will give accurate results during the design phase.

A Unified Open-Circuit Fault Diagnosis and Condition Monitoring Method for Three-Level T-Type Inverter

Traditional fault diagnosis (FD) and condition monitoring (CM) methods for three-level T-type (3L-T) inverters are studied separately, which is not conducive to guaranteeing the reliability of the 3L-T inverter in an all-around way. This article presents a unified open-circuit FD and CM method for the 3L-T inverter based on the proposed <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> -state voltage measurement circuit (OVMC). The designed OVMC comprises a voltage limiter, an absolute value circuit, and a minimum value circuit, separating all the OVs and actual switching state information with a few devices and terminals. The corresponding sampling method is designed under the space vector pulsewidth modulation mode, and the sampling frequency is only twice the switching frequency. Three exceptional cases are considered during the implementation to achieve accurate OV sampling. The proposed method for fault locating requires only a straightforward lookup table without complicated feature extraction and threshold setting steps. Meanwhile, the FD results would help to correct the OV distribution. Experimental results demonstrate that the proposed unified FD and CM method merits high measurement accuracy, fast response speed, and short FD time.

A Robust Open-Circuit Fault Diagnosis Method for Three-Level T-Type Inverters Based on Phase Voltage Vector Residual Under Modulation Mode Switching

This article proposes a new phase voltage vector residual-based fault diagnosis method to distinguish the similar open-circuit (OC) fault features of different switches in the three-level T-type inverter. The fault feature based on phase voltage residual can be generated with the designed expected phase voltage model considering different modulation strategies and modes. Under the zero-crossing (ZC) and non-ZC conditions of the faulty phase, the voltage vector residual characteristics caused by OC faults can be summarized. An adaptive residual vector amplitude threshold is designed based on uncertainty propagation using parameter error, dead time, and delay time as system error inputs. In addition, a hierarchical fault diagnosis scheme is proposed, which involves two steps. First, the group-level fault can be located according to voltage residual vector amplitude and angle. Next, the three-level modulation mode of all phase legs is switched to the two-level mode to further identify the faulty switch from the faulty group. The proposed method can achieve a reasonable balance between robustness and rapidity. Meanwhile, complexity and adaptability are also considered. Experimental results under various conditions sufficiently verify the robustness and effectiveness of the proposed fault diagnosis method.

Incipient Fault Diagnosis of a Grid-Connected T-Type Multilevel Inverter Using Multilayer Perceptron and Walsh Transform

This article deals with fault detection and the classification of incipient and intermittent open-transistor faults in grid-connected three-level T-type inverters. Normally, open-transistor detection algorithms are developed for permanent faults. Nevertheless, the difficulty to detect incipient and intermittent faults is much greater, and appropriate methods are required. This requirement is due to the fact that over time, its repetition may lead to permanent failures that may lead to irreversible degradation. Therefore, the early detection of these failures is very important to ensure the reliability of the system and avoid unscheduled stops. For diagnosing these incipient and intermittent faults, a novel method based on a Walsh transform combined with a multilayer perceptron (MLP)-based classifier is proposed in this paper. This non-classical approach of using the Walsh transform not only allows accurate detections but is also very fast. This last characteristic is very important in these applications due to their practical implementation. The proposed method includes two main steps. First, the acquired AC currents are used by the control system and processed using the Walsh transform. This results in detailed information used to potentially identify open-transistor faults. Then, such information is processed using the MLP to finally determine whether a fault is present or not. Several experiments are conducted with different types of incipient transistor faults to create a relevant dataset.

A study on performance parameters of three-level T-type inverter based PMSM drives for electric vehicles applications

In this paper, a comparative analysis is obtained to analyse the performances of two-level conventional and three-level diode clamped voltage source inverter-based drives. Carrier-based space vector modulation technique is employed in both three-level & two-level voltage source inverter (2L-VSI)-based drives. Comparative study for 2L-VSI, three-level neutral-point-clamped (3L-NPC) and three-level T-type neutral-point-clamped (3L-TNPC) inverters are performed in view of conduction & switching losses and harmonic profile. Also presented in this work, the dynamic response of field-oriented controlled (FOC) permanent magnet synchronous motor (PMSM) drive under varying load conditions towards EV applications. Rapid changes in speed and loading in Electric Vehicle (EV) traction drives lead to sharp fluctuations in modulation depth and power factor (PF). With variation of modulation index and power factor, power losses of all the three inverters are analysed. Also, in the present work, control logics are simulated in MATLAB to observe various speed-torque combinations of 5.4 kW PMSM drive. Additionally, power losses of inverters are developed in PLECS environment. By using a scaled-down prototype of 5 hp PMSM drive, the results are carried out for hardware validation.

A ZVS-PWM Scheme for Three-Phase Active-Clamping T-Type Inverters

For increasing power density and other performance of T-type three-level inverters, the soft-switching technique is studied. This article proposes a novel zero-voltage switching (ZVS) PWM scheme for active clamping three-phase T-type inverters, which can realize the ZVS operation of all switches and reduce the switching loss. The proposed ZVS PWM scheme is evolved from discontinuous PWM and modified for the requirements of T-type inverter. It enables all switches, including main switches and auxiliary switches, operate at a fixed frequency. However, a larger current on the power semiconductor devices occurs at 60° zone transition. To suppress this current surge, the carriers used in the ZVS PWM scheme are phase shifted. Finally, the novel ZVS PWM scheme is verified on a 10 kW/150 kHz SiC-based ZVS T-type inverter prototype.

Current Sensor Fault Localization and Identification of PMSM Drives Using Difference Operator

The permanent magnet synchronous motor (PMSM) has received increasing attention in traction applications. For those closed-loop control systems, the current sensor is one of the most sensitive components, which determines the control performance and safety operation. Therefore, the fault diagnosis of current sensors is of great importance to realize fault tolerance and further maintenance. In this work, a novel fault localization and identification method of the current sensor is proposed for PMSM drives. The residual between the measured current and estimated currents based on a modified <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha \beta $ </tex-math></inline-formula> reference frame is employed for the fault sensors’ localization. Besides, a second-order difference operator is adopted to identify the fault types. Moreover, a function is defined for preventing misdiagnosis, thus enhancing the algorithm’s robustness. The algorithm for the fault type identification would not be affected by the variation of motor parameters, granting it high flexibility to combine with not only the fault location algorithm in this article but also with other model-based methods. The effectiveness of the proposed approach is experimentally validated on an 11-kW platform of the PMSM fed by a three-level T-type inverter.

Grid Impedance Detection Based On Complex Coefficient Filter and Full-order Capacitor Current Observer for Three-phase Grid-connected Inverters

This article proposes a new grid impedance detection method incorporating the complex coefficient filter (CCF) with full-order capacitor current observer for a T-type three-level grid-connected inverter controlled by the inverter output current feedback. Compared with conventional CCF impedance detection algorithms, the proposed method reduces the number of current sensors and detects the grid impedance accurately. First, based on the sampled inverter output current and grid-connected voltage signals, the grid-connected current is estimated. Then, the CCF method is used to extract harmonics from the grid-connected current and voltage signals to calculate the grid impedance. Finally, the correctness of the full-order capacitor current observer is verified by simulation and the feasibility and effectiveness of the proposed algorithm are verified experimentally based on a laboratory prototype.

A Simple Open-Circuit Fault Diagnosis Method for Grid-Tied T-Type Three-Level Inverters With Various Power Factors Based on Instantaneous Current Distortion

Different open-circuit (OC) faults for grid-tied T-type three-level inverters with various power factors may lead to similar current distortion characteristics, making it challenging to locate the faulty switch. This article proposes an OC fault diagnosis method applied to grid-tied T-type three-level inverters, which hierarchically diagnoses the group-level and device-level faults. Based on the current distortion similarity analysis with various power factors operation, the switches in one phase are divided into two groups. Subsequently, the group-level fault detection is implemented by locating the half-cycle where the zero-section phenomenon occurs. Moreover, the device-level fault location is realized by injecting the specific switching signals. Simulations and experiments are performed to verify the analysis of the current distortion. The result considering various conditions shows the proposed approach’s correctness and robustness. The proposed method is effective in the power factor ranging from 0 lag to 0 lead. The OC fault can be located within 6 ms.

Parameter Design of Current Double Closed Loop for T-Type Three-Level Grid-Connected Inverter

Parameter Design of Current Double Closed Loop for T-Type Three-Level Grid-Connected Inverter

PV-Battery Assisted Three-Level T-Type Inverter for AC Residential Nanogrid Realized With Small-Scale HIL Units

PV-Battery Assisted Three-Level T-Type Inverter for AC Residential Nanogrid Realized With Small-Scale HIL Units

Multi-objective model predictive control method for T-type three-level grid-connected

Based on the mathematical model of T-type three-level topology, a T-type three-level inverter model is proposed to predict the multi-objective control method, which mainly includes DC neutral voltage (NP-V), switching state, inverter voltage vector, and multi-objective cost function. Different emphases can be selected according to the application requirements, and coordinated control of different control objectives can be realized by giving different weight values to different control objectives. An extended observer (ESO) is added to observe the error part of the grid-connected inverter system, and the observed results are compensated to the grid-connected reference voltage. The simulation results show that the total harmonic distortion rate of the output current of the inverter is lower than that of the traditional finite control set model predictive control strategy. In addition, the proposed method can realize the balance control of the midpoint potential, and the principle is simple.

Three-Phase Hybrid Converter With Simultaneous DC and AC Outputs

A novel topology of three-phase hybrid converter (TP-HC) which can supply simultaneously both DC and AC loads is presented. This topology is obtained by adding three switches to T-type three-level inverter. Consequently, this converter can achieve one group of DC/DC power transformation and another three-phase three-level AC voltage outputs with adjustable amplitude. The topology and operation modes of the proposed hybrid converter are introduced. Then the working principle and the available switching states of converter are analyzed, including current path, output phase voltage level and blocking voltage of different switching states. The characteristics of three typical hybrid converters are compared. A strategy combining time-sharing modulation and virtual space vector modulation (VSVPWM) is adopted to realize simultaneous DC and AC outputs of the converter. Then the capacitance voltage balance of the DC link is realized. In order to simplify the modulation strategy, a carrier based on pulse width modulation (CBPWM)strategy is proposed. Then, boost ratio of this converter is analyzed in detail. Finally, the feasibility of the proposed topology and the effectiveness of modulation strategy are verified by simulation and experiment.

Modeling and Suppression of Circulating Currents Among Parallel Single-Phase Three-Level Grid-Tied Inverters

T-type three-level inverters have attracted extensive attention for their utility in many applications, especially photovoltaic power generation. For these inverters, parallel operation offers superior performance in capacity, reliability, and scalability. However, the introduced circulating current (CC) may cause overcurrent in line, which seriously threatens the safety and stability of the system. The mechanism of CC lacks detailed analysis in single-phase modular inverters. Thus, an accurate mathematical model is established and a CC suppression method is proposed in this article for parallel-operated single-phase T-type three-level inverters. First, the characteristics of port degradation are analyzed, and an improved equivalent circuit is introduced based on the findings. Then, according to the improved equivalent circuit, a detailed mathematical model is established. Using this model, excitation sources of CC are analyzed and divided into three parts. After that, a CC suppression method involving carrier-based pulsewidth modulation with offset voltage injection is proposed, and a deadbeat controller is designed to calculate the offset voltage. Moreover, the proposed method can balance the neutral-point voltage across dc-link capacitors as well. Simulation and experimental results validate the theoretical analysis and effectiveness of the proposed scheme.

A Common-Mode Voltage Elimination Method With Active Neutral Point Voltage Balancing Control for Three-Level T-Type Inverter

Nowadays, the three-level T-type inverters (3LT2Is) are widely employed in applications with high-performance requirements. The 3LT2Is commonly suffer from neutral point voltage (NPV) unbalance and common-mode voltage (CMV) variation. The medium vector pulse width modulation (MVPWM), which generates the reference voltage using medium and V0[OOO] zero voltage vectors, eliminates the CMV variation in 3LT2Is. However, it cannot keep the NPV balance, negatively affecting power quality. This article proposes a hybrid method to simultaneously eliminate CMV variation and guarantee the NPV balance without topological change or extra hardware. The proposed method uses MVPWM when NPV is balanced, eliminating CMV variation. If NPV goes unbalanced, it utilizes novel high/low triangle PWM (H/LPWM) and MVPWM in a hybrid manner to balance the NPV. The H/LPWM, similar to MVPWM, synthesizes voltage using medium and V0[OOO] voltage vectors to maintain the CMV elimination advantage during the NPV balancing process. However, the difference between H/LPWMs and MVPWM switching patterns adjusts the average neutral point current to balance the NPV. Noteworthy that the proposed method provides NPV balancing ability for the entire power factor range. As a result, it is usable for various applications of 3LT2I, such as photovoltaic inverters and motor drives. The proposed method is carrier-based, which reduces the execution time and simplifies the practical implementation. The simulation study and experimental evaluation have been accomplished, and their results demonstrate the effectiveness of the proposed method.

ANN Optimization of Weighting Factors Using Genetic Algorithm for Model Predictive Control of PMSM Drives

Model predictive control (MPC) has become one of the most attractive control techniques due to its outstanding dynamic performance for permanent magnet synchronous motor (PMSM) drives. However, the tuning of weighting factors for the cost function of MPC is a time-consuming procedure and weighting factors can significantly affect torque ripples of PMSM drives. For the purpose of optimizing the weighting factors in cost functions of MPC, this article proposes an artificial neural network (ANN) based method, which applies genetic algorithm as the back propagation algorithm. Since this method is trained offline, it does not increase any computation complexity of MPC. Consequently, any optimization targets that combine the performance metrics of MPC are defined and the optimal weighting factors for the given targets can be fast and precisely found through the proposed method. Besides, this method is robust to the variations of motor parameters like stator resistance and permanent magnet flux. The superiority of the proposed method over conventional ANN is evaluated by comparative simulations, in terms of current total harmonic distortion, tracking performance, and neutral-point potential balance. Finally, the feasibility and robustness of the proposed method are verified on the platform of PMSM drives fed by three-level T-type inverter.

A Novel Model-Free Predictive Control for T-Type Three-Level Grid-Tied Inverters

The model-free predictive control (MFPC) scheme is an effective scheme to enhance the parameter robustness of model predictive control. However, the MFPC scheme can be affected by the current gradient updating frequency. This paper proposes an improved MFPC scheme for a T-type three-level inverter. First, a novel current gradient updating method is designed to estimate all current gradients per control period, which uses the current gradient relationship between different voltage vectors and eliminates the effect of current gradients updating stagnation. Moreover, a sector judgment method based on the current gradient is proposed. Redundant small vectors are accurately judged and the computational burden is greatly reduced. Finally, simulation and experimental comparisons on a T-type three-level inverter verify the effectiveness of the proposed MFPC scheme.

Model Predictive Control With Constant Switching Frequency for Three-Level T-Type Inverter-Fed PMSM Drives

Finite control set model predictive control (FCS-MPC) shows superior dynamic performance with simple structure for motor drives. However, conventional FCS-MPC method presents unfixed switching frequency, making it intractable for the design of heat sinks and output filters. To address this issue, this article is devoted to developing a new FCS-MPC strategy with constant switching frequency for permanent magnet synchronous motor (PMSM) drives fed by three-level T-type inverter. The switching frequency regulation is realized by extending the period control technique, where the periods are measured between each rising edge and falling edge of the gate signals. The proposed algorithm can be embedded in the iterative process of the FCS-MPC, which can easily achieve the fixed switching frequency with balanced neutral point potential. The performance of this control strategy is numerically evaluated and experimentally verified by an 11-kW PMSM drive platform.