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Multilevel inverters (MLIs) featuring reactive power ability and common ground (CG) have become much popular in transformerless grid-tied photovoltaic applications. A switched-capacitor (SC) based five-level transformerless inverter configuration is thus proposed in this article, which requires only six switches, one diode, three capacitors, and one input dc voltage supply. Notably, only four power switches are operated in high frequency, thus, the switching losses are reduced. Moreover, the presented topology can effectively tackle the leakage current problem using a CG architecture. A multicarrier phase disposition pulsewidth modulation strategy is employed to achieve reactive power regulation and SC voltage self-balancing. To achieve the neutral point voltage balance of the presented five-level inverter, a simple closed-loop voltage-balancing control method is proposed. The operation principles with modulation strategy and voltage self-balancing of SC are discussed in depth. Comparisons between the proposed and the state-of-the-art MLIs are presented in detail. Finally, experimental tests on a single-phase 1.1-kW prototype validate the appropriate performance of the proposed inverter. [ABSTRACT FROM AUTHOR]

In this article, a nine-level multilevel topology is proposed by stacking of low voltage basic inverter cells and then cascading with H-bridges. The dc-link is split into four equal halves by series-connected capacitors from a common dc-link. Three common coupling points (CCPs) are formed due to the stacking of four capacitors, which are connected across the dc-link. The CCP voltage balancing is ensured by drawing or injecting zero instantaneous currents from each of these points at any instant. A six-phase symmetrical IM is used to generate equal and opposite instantaneous phase currents by applying exactly equal and opposite phase voltages for two opposite phases from the six-phase inverter. Simultaneously, the capacitor voltage balancing of H-bridge inverter cells is also achieved by proper selection of redundancies from the available pole voltage redundancies. The concept is verified with a nine-level inverter laboratory prototype, which consists of a five-level stacked inverter, and followed by two low voltage cascaded H-bridge inverters in each phase. Detailed experimental results of steady-state and transient conditions are included to validate the proposed scheme. [ABSTRACT FROM AUTHOR]

Power filters have been widely used to suppress switching harmonics caused by the modulation of grid-connected inverters. In order to save the total inductance and cost and reduce the volume, different power filters were proposed for the three-phase three-wire grid-connected voltage source inverter. However, in industrial applications, due to the consideration of production cost, the difficulties caused by the deviation of actual parameters have to be addressed. In this article, we propose a new three-phase three-wire power filter with a simple structure, almost the smallest total inductance, and strong resistance to the adverse effects of parameter shifts. The parameters of the proposed filter and the design of the grid-connected system controller are introduced in detail. When studying the influence of parameter offset, a comparison between different types of notch power filters is also given. Simulation and experimental results on a 380 V/50 Hz/6 kW three-phase inverter prototype verify the performance of the proposed power filter. [ABSTRACT FROM AUTHOR]

In this article, a fault-tolerant model-free predictive controller (FT-MFPC) for cascaded H-bridge multilevel inverters (CHMLIs) is proposed. FT-MFPC is based on an association of model-free predictive controller (MFPC) and a proportional--integral (PI) like structure known as current variation controller (CVC). As in MFPC, the dynamic structure (DS) that is used for the prediction of the system output is obtained from past measurements of current variations, but CVC modifies the dynamics before they are stored for prediction. As a consequence, even if the converter operates with bypassed modules, FT-MFPC adapts its DS to represent the real dynamics of the system and enables a closed-loop response with balanced load currents in unbalanced CHMLIs. Simulation and experimental results validate the adaptability of the proposed strategy to operate a seven-level inverter under various configurations of bypassed H-bridge modules. The experimental results show that, for different postfault operations, the maximal current total harmonic distortion and imbalance factor at steady state in the worst case scenario are below 5% and 2%, respectively. [ABSTRACT FROM AUTHOR]

For the grid-connected inverter, grid voltage sag can be a typical phenomenon. From the prospective of safe and robust operation, the inverter should be stable when the grid voltage sag (or large disturbance) occurs. Although the inverter stability in this case has attracted considerable attentions, few studies have given suggestions on the system stability boundary and parameter design. To fill this gap, this article deeply discusses the large-signal stability of the system under different situations and reveals the quantified impact of the phase-locked loop (PLL) on the large-signal stability of the system based on the fundamental current-source model. The large-signal stability criterion and boundaries for grid voltage sags are established, which are instructive to the design of PLL parameters. Besides, a novel design guideline for the PLL parameters is proposed to ensure the robust operation of the system both under the steady-state and grid voltage sags, as well as to achieve good dynamic performance after large disturbance occurs. The theoretical, simulation, and experimental results show that the proposed stability boundary has certain accuracy and the PLL parameters design guideline can ensure the system stability both under steady-state and grid voltage sag conditions. [ABSTRACT FROM AUTHOR]

Online condition monitoring of winding insulation has been proved to be an effective means to improve the reliability of an inverter-fed machine, but little work has been done on the insulation of line-end coil subjected to much higher transient voltage stress with a higher risk of premature insulation break down. In this article, the inverter self-excited high-frequency switching oscillations are utilized for the line-end coil insulation condition monitoring. To improve the condition monitoring sensitivity and its robustness to dynamic operating conditions, a switching oscillation mode decomposition method is proposed. First, the condition-sensitive mode, i.e., high-frequency common-mode (HFCM) current is selected and separated by the variation mode decomposition algorithm among three dominant modes in switching oscillation current. Then, the amplitude-frequency features of HFCM current are extracted for condition monitoring. Finally, the overall insulation condition of three-phase line-end coils and the degree of asymmetric degradation are quantitatively evaluated by the frequency and amplitude features respectively. Experimental work was carried out and the results demonstrate the good performance of high sensitivity, good robustness, and non-contract safety. [ABSTRACT FROM AUTHOR]

In this article, we introduce a model predictive control with optimal switching sequence (OSS-MPC) for a multilevel static synchronous compensator (STATCOM). The studied STATCOM is based on a hybrid multilevel inverter named HC-FNPC formed by the cascaded connection of H-bridge cells with a full-bridge neutral point clamped cell (FNPC). In this article, the optimal switching sequences are derived for HC-FNPC STATCOM by considering the impact of each switching sequence on the voltage of capacitors. Then, the control objectives of hybrid multilevel STATCOM, i.e., ac current control and capacitors’ voltages balancing, are formulated in terms of OSS-MPC for the first time. In contrast to the conventional MPC, this method provides a fixed-switching frequency. Also, by a combination of optimal switching sequences in MPC, the regulation of capacitors’ voltages with unequal values is realized. The control system's performance is verified on a nine-level (9L) 2.4 Mvar/11 kV HC-FNPC STATCOM in MATLAB/Simulink. Also, the experimental results are provided on a 9L 1.25 kvar/220 V scaled-down hardware prototype. [ABSTRACT FROM AUTHOR]

This article introduces a fault-tolerant reconfiguration topology and control strategy for symmetric open-winding multiphase machines. Bidirectional switching devices are proposed to connect the inverter bridge legs on different ends when the open-circuit fault occurs, and the reconfigured circuit can regenerate the missing voltage phasors due to the faults. Furthermore, to prevent overcurrent problems on the reconfigured bridge legs and maximize the torque output capability of the reconstructed system, winding currents are optimized with zero-sequence component injection during fault-tolerant operation. The experiments implemented on the five-phase system validate that the proposed fault-tolerant scheme is effective and has higher torque output capability compared with the traditional fault-tolerant solutions. [ABSTRACT FROM AUTHOR]

Beyond the impacting presence of nonlinear loads, low-voltage microgrids also experience low energy efficiency and resonance phenomena when operating interconnected to a distribution grid that suffers from distorted voltages. This article proposes a model-free control strategy capable of coordinating inverters existing within a dispatchable microgrid, allowing to operate it as a single-controllable entity that behaves like a resistor at selected harmonic frequencies. Such resistive shaping uses a centralized control architecture to steer inverters to distributively compensate reactive and harmonic currents, supporting active current sharing. Consequently, the microgrid point-of-common-coupling operates with a high power factor when the grid imposes distorted voltages. Additionally, if resonant components exist, the strategy supports harmonic resonance damping, which minimizes deterioration of voltage quality. For instance, comparative results show that, for the considered scenario, the proposed resistive shaping damps resonances up to 50% better than a previous approach that compensates harmonics using sinusoidal current synthesis. Simulation results carried out on a three-phase low-voltage microgrid testbench, considering three inverters, demonstrate the abovementioned capabilities of the proposed approach. Experimental results based on a single-phase microgrid prototype comprising two inverters with two linear loads and one nonlinear load validate the applicability of the method to real-life implementations. [ABSTRACT FROM AUTHOR]

This article presents a ground breaking traction drive for fuel cell-powered light rail vehicles based on a multilevel cascade converter with H-bridge cells. The converter provides dc–ac power conversion with unique buck–boost capability to compensate for the variation of fuel cell terminal voltage with the load power. In comparison to a conventional boost inverter, based on the cascade connection of a boost dc–dc converter and a voltage source inverter, the proposed converter has the significant benefits of much smaller inductor size, lower dc side current ripple, and multilevel voltage waveforms at the ac output. This article presents the converter modeling and design of the boost inductor and submodule capacitors, taking into account the full speed range of the motor. The concept has been fully validated on a bespoke experimental prototype driving an induction motor, showing the suitability of the proposed converter as a traction drive for hydrogen-powered rail vehicles. [ABSTRACT FROM AUTHOR]

This article proposes a modular approachto the power sharing control of permanent magnet synchronous bearingless machine. The selected machine topology features a winding layout with phases distributed into nonoverlapping three phase groups, a solution whose twofold aim is to increase the fault tolerance and to allow for the radial force generation. The three phase subwindings are supplied by standard three-phase inverter, leading to a modular system architecture. A throughout explanation of the methodology used to develop the control algorithm is presented considering the torque and force control in combination with the power sharing management of the machine. Special emphasis is placed on validating the modeling hypotheses based on a finite element characterization of the machine electro-mechanical behavior. The proposed control strategy is also extended to cater the possibility of one or more inverters failure, thus validating the intrinsic advantage of the redundancy obtained by the modularity of the system. An extensive experimental test campaign is finally carried out on a prototyped multi three-phase permanent magnet synchronous drive. The obtained results validate the bearingless power sharing operation in healthy and faulty scenarios, both at steady state and under extreme transient condition. [ABSTRACT FROM AUTHOR]

A voltage source inverter generating a polygonal voltage space vector structure (SVS) having 117- concentric 30-sided polygons of different radii are proposed. Modulation using a 30-sided SVS eliminates lower order harmonics till 25th order from motor phase voltage. The proposed scheme combines the advantages of a 30-sided SVS with that of a multilevel inverter. The topology consists of two cascaded five-level inverters feeding from either end of the open-end induction motor stator windings. One of the cascaded five-level inverter is connected to an active dc source and delivers the required active power for the scheme, while the other is capacitor fed and acts as a switched capacitive filter. In this article, nonaligned 30-sided polygons, i.e., regular 30-sided polygons that are not symmetric with respect to $\alpha$ and $\beta$ axis are also considered for inverter operation. The harmonic analysis of such nonaligned 30-sided polygons is investigated in this article. Nearest level switching is employed throughout the modulation range, enabling further reduction of switching loss in the system. Extensive hardware experimentations are carried out to validate the proposed scheme and the results are included in the article. [ABSTRACT FROM AUTHOR]

The single-phase five-level transformerless inverters have been gradually applied in photovoltaic grid-connected systems due to low voltage stress and excellent waveform quality. However, they also suffer from serious leakage current as conventional three-level inverters. In order to reduce the leakage current, a single-phase five-level transformerless inverter is proposed in this article. The proposed inverter guarantees that the common-mode (CM) voltage is clamped to a half of the dc-link voltage when outputting ±Vdc/2 and zero voltage levels. Consequently, the high-frequency CM voltage is avoided and the leakage current is suppressed. The voltage stresses of eight switches and two flying capacitors are only Vdc/4, thus the system investment cost and total losses are reduced. The conduction and switching losses are analyzed in detail and a comparison with some existing topologies is also made. Finally, simulations and experiment are carried out based on a 1-kW prototype. The results show that the proposed single-phase five-level inverter can obtain both excellent differential-mode and CM performance. [ABSTRACT FROM AUTHOR]

In this article, a new high step-up soft-switched double-input micro-inverter is introduced, which is proper for low-power applications. This inverter is composed of a forward resonant inverter and two series boost converters that are coupled electrically and apply five switches. The input powers are controlled separately using the duty-cycle of input switches, and if one of the inputs is omitted, the operation of the inverter is not interrupted. The energy is transmitted to the output and provides soft-switching conditions without any auxiliary switch. Secondary side switches are controlled at low frequency to control the direction of current in each half-cycle of the output voltage. The proposed inverter is analyzed, and design considerations and soft-switching range are discussed. Also, the schematic of the control circuit is presented. The proposed inverter is implemented to supply a 100-W load, and experimental results are presented to justify the theoretical analysis. [ABSTRACT FROM AUTHOR]