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The instability issues of the three-phase grid-tied inverters are major concerns in the distributed generated systems. However, due to the coupling effects in dq frame, the instability mechanisms of the three-phase inverters have not yet been clearly revealed. This paper presents a divided dq small-signal model of the grid-tied inverter in dq frame. The specific expressions of a small-signal output admittance of the inverter are investigated based on the proposed model, which can reveal the influencing mechanism of coupling effects and phase-locked loop (PLL) on the stability of the inverter. Then, a stability analysis method based on the divided dq small-signal model is proposed, by which the procedure of checking the stability of the grid-tied inverters with strong/weak grid is executed step by step, and it could be helpful for the system parameters design. Furthermore, the stability of the multiparalleled inverter system is also studied, and it indicates that as the number of the paralleled inverters increases, the parameters of PLL could be adjusted for a stable operation. The theoretical stability analysis results are verified by simulations and experiments. [ABSTRACT FROM AUTHOR]

Synchronverters are inverters that mimic the behavior of synchronous generators. In this paper, we propose five modifications to the synchronverter algorithm from the paper “Synchronverters: Inverters that mimic synchronous generators,” (IEEE Trans. Ind. Electron., vol. 58, no. 4, pp. 1259–1267, Apr. 2011), to improve its stability and performance. These modifications are implemented in software and do not require any changes in the inverter hardware. The first two modifications concern the control of the virtual field current in the synchronverter so that it is more robust to faults. We prove the stability of the grid-connected synchronverter with this improved field current controller. The third modification is to increase the effective size of the filter inductors virtually. This is motivated using results from the stability analysis of a constant field current synchronous generator connected to an ac power grid and also by practical considerations. Simulations and experiments show that this leads to a much better response to changes in grid frequency, voltage, or to imbalance in the grid. The fourth modification is to change the formula for the (virtual) nominal active mechanical torque to take into account the (virtual) losses in the output impedance of the converter. This makes the tracking of the desired active power much more accurate. The fifth modification is to introduce virtual capacitors in series with the inverter outputs to filter spurious dc components from the current supplied to the grid. [ABSTRACT FROM PUBLISHER]

Droop control strategy is widely applied in ac microgrid to realize the seamless mode transition and achieve average power sharing among different inverters. However, there is a lack of analysis about the operation performance of droop control strategy in grid-connected mode. In this paper, grid fluctuation and control accuracy of reactive power are illustrated to be the main influence factors for power flow control according to the small-signal model. The feedforward of grid frequency and voltage magnitude is proposed to mitigate the impacts of grid fluctuation on power flow control. Since the feedforward items are achieved through the synchronous reference frame phase-locked loop with the input of filter capacitor voltage, extra sensors for grid voltage are avoided. Moreover, voltage magnitude control is introduced to enhance the control accuracy of reactive power flow. In addition, modified droop control loops are also proposed to improve the system stability. With the implementation of the proposed control scheme, improved power flow control and enhanced system stability are achieved with the feature of seamless mode transition maintained. Finally, the major contribution of this paper is verified by a laboratory prototype. [ABSTRACT FROM PUBLISHER]

In this paper, an improved direct torque control (DTC) technique of a two-level inverter-fed five-phase induction motor (FPIM) drive for low-speed operation is presented. The operation of the FPIM with distributed winding requires the elimination of third-harmonic voltage, which, if not taken care of, would generate distorted stator current. For this, the concept of virtual vectors (VVs) is utilized. Before selecting the VVs for the DTC operation, a theoretical analysis is carried out to find the effects of these VVs on the torque and flux response of the FPIM under different speed and loading conditions. It is found that the rotor speed plays a crucial role in the selection of VVs. A particular VV has different effects on the torque change as speed varies. This analysis helps in the selection of VVs, hysteresis torque bandwidth, flux bandwidth, and efficient formation of sectors. Furthermore, in this paper, the performance of the FPIM during low-speed operation is improved by avoiding the demagnetization of the stator flux. To achieve this, a new modified lookup table is proposed. Superiority of the proposed method over the reported approaches is illustrated by experimental results. [ABSTRACT FROM PUBLISHER]

A grid-forming inverter (GFI) is an important component for the operation of an islanded microgrid. Its purpose, similar to a conventional slack bus generator, is to build up a reference voltage for other distributed generating units in the microgrid. Usually, a nested-loop proportional plus integral (PI) control structure is employed to control a GFI in a dq reference frame. However, conventional PI-based nested-loop control method has a deteriorative performance under parameter variations. In this paper, a novel nested-loop control strategy is proposed for control of a GFI system containing an {LC} output filter and loads. The proposed method does not require a precise model for the inverter system and can better deal with uncertainties and {LC}$ filter resonance without using any passive or active damping mechanisms. It utilizes a sliding-mode control in the inner current loop and a mixed {H_2}/{H_\infty } optimal control in the outer voltage loop, which provides the advantages of constant switching frequency, low total harmonic distortion, robustness against parameters variations, and fast transient response. The simulation and hardware experiments presented in this paper demonstrate the proposed controller's improved transient and steady-state performance in various key criteria, over conventional PI-based nested-loop control strategy. [ABSTRACT FROM AUTHOR]

In this paper, a new topology for half-bridge Z-source inverter is proposed. The proposed topology has only one impedance network. Unlike to the conventional half-bridge inverter, the proposed topology can provide zero voltage level at the output. It also increases output voltage level and stabilizes it in the desired value. Capacitor voltage stress in the proposed topology is low, and, therefore, nominal voltage of capacitor and cost decreases. In this paper, the steady-state analysis of the proposed inverter in two new operations which are named synchronous operation of diodes and asynchronous operation of diodes is conducted based on mathematics calculations. A method to obtain high-voltage gains by cascading the Z-network and combining middle inductors is presented that leads to cost, size, and weight reduction. Comparison among the proposed converter with conventional ones shows its excellent performance. The experimental results have good agreement with analytical analysis for the proposed topology. [ABSTRACT FROM PUBLISHER]

A quasi-Z-source modular cascaded converter (qZS-MCC) is proposed for dc integration of high-power photovoltaic (PV) systems. The qZS-MCC comprises series-connected front-end isolated qZS half-bridge (HB) dc–dc converter submodules (SMs). With the front-end isolation, the qZS-MCC achieves high-voltage dc capability, while maintaining modularity and PV panel grounded. The post-stage qZS-HB handles the PV voltage and power flows, dc-link voltage balance, and output-series power integration. Whereas, the front-end isolation converters of all SMs perform a constant duty cycle, lowing the control complexity. There is no double-line-frequency power flowing through the dc-side PV panels, qZS inductors, and qZS capacitors in the qZS-MCC, so small qZS impedance is possible compared to the existing qZS cascaded multilevel inverter. The configuration, operating principle, power loss evaluation, and passive components design of the proposed system are investigated in this part of the paper. The system control, modeling, and corresponding verifications are stated in Part II of this paper. [ABSTRACT FROM PUBLISHER]

Harmonic current sharing is an important aspect of islanded microgrid due to the presence of nonlinear loads. The existing harmonic current sharing strategies suggest a tradeoff between voltage quality and harmonic sharing effectiveness. In this paper, a novel control strategy is proposed to achieve current harmonic sharing by directly controlling the inherent impedance of the inverter. The proposed technique achieves harmonic sharing, in a distributed generation (DG)-based islanded microgrid, without using any communication infrastructure. This paper also proposes a droop technique to distribute the harmonics generated by the nonlinear load as per the kVA ratings of different DG units in the system. The proposed concept of variable inverter impedance and its application in an islanded microgrid is validated using experimental studies. [ABSTRACT FROM PUBLISHER]

Repetitive control strategies have been commonly applied in pulse-width-modulated (PWM) voltage source inverters (VSIs) for many industrial applications. This paper presents a repetitive controller for voltage harmonic mitigation of VSI-based islanded microgrids. The phase delay in the overall control system, e.g., the delay caused by the digital duty cycle calculation, PWM generation, and repetitive controller, has to be compensated by elaborately designed phase lead filters in order to prevent control performance deterioration and system instability. Nevertheless, quantificational analysis and practical design of the time advance unit in such filters are hardly found in existing literature works. In view of this, this paper proposes an explicit analysis of the phase lead filters and a novel design method of the time advance unit in repetitive controllers to ensure system stability. Moreover, with the help of the proposed method, the overall system stability margin is predictable and improved controller performance is achieved as well. The proposed method is implemented experimentally to show the accurate stability margin calculation as well as the excellent steady state and dynamic performances of the repetitive control scheme. [ABSTRACT FROM PUBLISHER]

The limited capacity of a single inverter has encouraged the usage of parallel inverters for the interconnection of renewable energy generation system. The current-sharing control is one of the commonly used operating strategies for parallel inverters; however, the conversion efficiency using conventional current sharing is low at light loads. Therefore, a smart control strategy for conversion efficiency enhancement of parallel inverters is proposed in this paper. The proposed control strategy is based on the conversion efficiency curve of a single inverter, where a particle swarm optimization based algorithm is utilized to find the optimal conversion efficiency of parallel inverters. Integration of the proposed control strategy into a photovoltaic generation system (PVGS) with a rated output power of 20 kW interconnected by ten inverters with rated output powers of 2 kW is simulated in this paper. Simulation results demonstrate that the proposed smart control strategy can effectively enhance the conversion efficiency of parallel inverters at light loads and increase the electrical energy output of PVGSs. A fully digitally controlled single-phase inverter with a rated output voltage of 220 V and a rated output power of 2 kW is also implemented in this paper. Two implemented inverters are used to validate the performance of the proposed smart control strategy for parallel inverters. Experimental results show that the proposed control strategy can enhance the conversion efficiency of parallel inverters at light loads. [ABSTRACT FROM AUTHOR]

This paper proposes a hybrid modulation method for a dual-output five-leg indirect matrix converter topology. The proposed control technique combines a space vector modulation scheme (SVPWM) applied to the rectifier stage so as to control the input currents and a new digital scalar modulation scheme (DSPWM) designed to control the two loads output voltages. A generalized expression of the output voltage references is also developed showing the capability of the proposed method to control more than two independent loads without requiring additional mathematical development. Furthermore, a new modified expression of the output voltages references is proposed to enable the correct operation of the converter under unbalanced grid voltages. Also, this paper proposes a detailed mathematical analysis that allows evaluating the effect of output loads unbalances on the input current harmonic content. Simulation and experimental results are provided to show the effectiveness of the proposed theoretical investigations and confirm the capability of the proposed method to control multiple drive systems as well as ac input currents. [ABSTRACT FROM AUTHOR]

To improve the power factor and reliability for medium/high-power supply, a three-phase single-stage ac–dc converter based on magnetic-combination transformer with power factor correction (PFC) function is presented in this paper. According to the three-phase instantaneous power balance principle and the concept of the magnetic-flux superposition, a novel magnetic-combination transformer is proposed, which can make three-phase symmetrically and sinusoidally fluctuant power add to be constant. Therefore, the induced voltage of the secondary winding is constant. To realize sinusoidally fluctuant power in the input stage, dc-link filter capacitance can be reduced to a several-microfarad level. Single-stage PFC function, no electrolytic capacitor, and constant output voltage can be realized in the converter. The principle and the structure of the novel magnetic-combination transformer are analyzed in detail in this paper. Simulation and experimental results are presented to verify the feasibility of the proposed converter. [ABSTRACT FROM AUTHOR]

Zero-common-mode-voltage (CMV) operation of multilevel inverters results in reduced dc-bus utilization and reduced linear modulation range. In this paper, a method to increase the linear modulation range for zero-CMV operation without increasing the dc-bus voltage using a cascaded multilevel inverter with a single dc supply is presented. Using this method, the peak fundamental output voltage can be increased from 0.499 to 0.637  \textV\text{dc} with zero CMV inside the linear modulation range. Also, various pulse width modulation (PWM) switching sequences are analyzed in this paper, and the PWM sequence that gives minimum current ripple is used for the zero-CMV operation of the inverter. The inverter topology with single dc supply is realized by cascading a two-level inverter with two floating-capacitor-fed full-bridge modules. Simulation and experimental results for steady-state and dynamic operating conditions are presented to validate the proposed method. [ABSTRACT FROM AUTHOR]

Model predictive control (MPC) method has been developed as a simple and effective current control technique for voltage-source inverters (VSI). The conventional MPC method applied to VSI adopts all the seven voltage vectors (VVs), including one zero VV and six nonzero VVs, to implement the predictive current control in order to improve the current control performance. However, the common-mode voltage (CMV) is large due to the use of zero VV. Although the MPC strategy only using six nonzero VVs can reduce the CMV, its switching frequency is relatively high due to too many switches between nonadjacent VVs. In this paper, a new MPC-based CMV suppression strategy for VSI is proposed and applied to a permanent-magnet synchronous generator (PMSG)-based control system. Only four nonzero VVs, including three adjacent nonzero VVs and one nonadjacent nonzero VV, are utilized to perform the optimization in every sampling period, resulting in switching frequency and calculation effort reduction with no adverse effect on the current response. Moreover, the influence of the MPC-based CMV reduction algorithm on the speed control of PMSG is tested in this paper. The experimental results confirm the effectiveness of the proposed strategy. [ABSTRACT FROM PUBLISHER]

This paper proposes a sliding-mode control (SMC)-based scheme for the variable-speed direct-driven wind energy conversion systems (WECS) equipped with a permanent magnet synchronous generator connected to the grid. In this paper, diode rectifier, boost converter, neutral point clamped inverter, and L filter are used as the interface between the wind turbine and grid. This topology has abundant features such as simplicity for low- and medium-power wind turbine applications. It is also less costly than back-to-back two-level converters in medium-power applications. The SMC approach demonstrates great performance in complicated nonlinear systems control such as WECS. The proposed control strategy modifies reaching law (RL) of the sliding mode technique to reduce chattering issue and to improve total harmonic distortion property compared to conventional RL SMC. The effectiveness of the proposed control strategy is explored by simulation study on a 4-kW wind turbine, and then verified by experimental tests for a 2-kW setup. [ABSTRACT FROM PUBLISHER]

Model predictive control (MPC) has been widely advocated as a design strategy for many aspects of industrial electronics. The methodology has been strongly promoted by some researchers but has also attracted criticism from others. In this context, the purpose of this paper is twofold. First, we show that many existing and popular control strategies, including finite set MPC and linear controllers [proportional integral, proportional resonant (PR)], can be viewed as special cases of MPC. Second, we show that the predictive control framework allows one to embellish these classical control architectures with novel features and to design new and advanced control architectures to address various challenges posed by power electronics applications. The findings of the paper are supported by a practical example of designing of a novel form of PR controller with superior tracking performance and delay compensation, confirmed via simulation and experiments. [ABSTRACT FROM PUBLISHER]

This paper presents a model predictive control technique applied to a dual-active bridge inverter where one of the bridges is floating. The proposed floating bridge topology eliminates the need for isolation transformer in a dual inverter system and therefore reduces the size, weight, and losses in the system. To achieve multilevel output voltage waveforms, the floating inverter dc-link capacitor is charged to the half of the main dc-link voltage. A finite-set model predictive control technique is used to control the load current of the converter as well as the floating capacitor voltage. Model predictive control does not require any switching sequence design or complex switching time calculations as used for space vector modulation; thus, the technique has some advantages in this application. A detailed analysis of the converter as well as the predictive control strategy is given in this paper. Simulation and experimental results to validate the approach are also presented. [ABSTRACT FROM PUBLISHER]

Typical value of rated phase voltage of pole phase modulated multiphase induction motor (PPMMIM) drives with wider speed range is in the order of few hundreds of kilovolts. This high value of phase voltage for high power density applications results in higher dc-link voltage requirement and switch voltage rating of two-level multiphase inverter. Further, using multiphase space vector pulse width modulation (SVPWM) yields less dc-link utilization. Methods to increase dc-link utilization using SVPWM with offset value of third harmonic order introduces dominant lower order harmonic currents into phase windings. One more major problem in high pole mode of PPMMIMs is higher torque pulsation due to decrease in phase number. To address these problems this paper proposes a dual-inverter-based multilevel voltage excitation scheme for nine-phase PPMMIM with 1:3 speed ratio. In four-pole mode a simple phase grouping technique to eliminate lower order harmonic currents in the phase windings is proposed. In addition each inverter feeding these phase groups is modulated using carrier-based three-phase SVPWM to achieve higher dc-link utilization. This paper also proposes a multilevel voltage generation scheme for 12-pole mode of operation using carrier phase shifted pulse width modulation (PWM) for inherently available equal voltage profile coils with the same dual inverter structure. The torque ripple using phase shifted carriers PWM and single carrier PWM are compared. Finite element method model of nine-phase PPMMIM is developed in Ansys Maxwell two-dimension and is cosimulated with three three-phase dual inverters in Simplorer environment. Experimental validation is done for linear and over modulation case on 9ФIM fed from three three-phase dual inverters controlled using Spartan six field programmable gate array board programmed in VHDL. [ABSTRACT FROM PUBLISHER]

This paper presents an improved five-level bidirectional converter (iFBC) controlled by finite control set model predictive control (FCS-MPC). This control strategy consists in using the discrete time nature of the iFBC to define its state in each sampling interval. Using FCS-MPC the switching frequency is not constant; however, it is suitable to follow the current reference with low total harmonic distortion (THD). The iFBC prototype that was specially developed for obtaining experimental results is described in detail along the paper, as well as its principle of operation, power theory, and current control strategy. The iFBC was experimentally validated connected to the power grid through a second-order LfCf passive filter operating as an active rectifier and as a grid tie inverter. For both operation modes, the experimental results confirm the good performance (in terms of efficiency, low current THD, and controlled output voltage) of the iFBC controlled by FCS-MPC. [ABSTRACT FROM PUBLISHER]

This paper presents a three-phase open-end winding induction motor drive. The drive consists of a three-phase induction machine with open stator phase windings and dual-bridge inverter supplied from a single dc voltage source. To achieve multilevel output voltage waveforms, a floating capacitor bank is used for the second of the dual bridges. The capacitor voltage is regulated using redundant switching states at half of the main dc-link voltage. This particular voltage ratio (2:1) is used to create a multilevel output voltage waveform with three levels. A modified modulation scheme is used to improve the waveform quality of this dual inverter. This paper also compares the losses in the dual-inverter system in contrast with a single-sided three-level neutral point clamped converter. Finally, detailed simulation and experimental results are presented for the motor drive operating as an open-loop v /f controlled motor drive and as a closed-loop field-oriented motor controller. [ABSTRACT FROM PUBLISHER]

A novel centroid-based diagnostic method of the power switches in five-leg voltage source inverter is proposed in this paper. Using a vectorial multimachine description, a five-phase drive presenting an opened switch or an opened phase faults has typical operating characteristics in comparison to classical three-phase drives. Based on such characteristics, this paper aims to provide a simple and robust diagnostic process for switches fault regardless of the shape of the back electromotive forces (harmonic components) and the transient states due to the load variation. Original theoretical developments are presented. Experimental results are shown to validate the proposed strategy. [ABSTRACT FROM PUBLISHER]

Two three-level inverters driving an open-end winding induction motor can generate equivalent voltage waveforms as a single five-level inverter-based drive. In addition, it can bring in benefits such as reduced dc-link voltage, less number of devices, and fault-tolerant capability, which is favored in high-power motor drive applications. The main challenge with this type of configuration is the appearance of large zero-sequence current circulating between the two inverters, which generates extra losses in the switching devices and the motor as well as affects the normal operation of the machine. Another challenge is to balance the dc-link neutral point voltage of the two three-level inverters. This paper has proposed a simplified decoupled space-vector pulsewidth-modulation (PWM) strategy to eliminate the averaged zero-sequence voltage in each switching cycle by the placement of redundant vectors of each individual inverter using a time shift. Neutral point voltage balance is further achieved by adjusting the time duration of the redundant vectors. In this paper, it is shown that it is possible to operate this kind of drive configuration with a single dc power supply and achieve averaged zero-sequence voltage elimination and neutral-point voltage balancing at the same time. Experimental results of a 5.5-kW dual three-level inverter drive using the proposed PWM strategy are presented, which validates the control method and drive configuration. [ABSTRACT FROM PUBLISHER]

AC cascaded system is one of the basic components of the power conversion system in the modern more-electric aircraft (MEA). However, the ac cascaded system suffers from an instability problem, i.e., even if each subsystem is stable individually, the formed cascaded system may become unstable. Most of the existing stabilization methods are a linear control method, which is based on a small signal model and can only ensure small signal stability near the operation point in theory. Therefore, these existing linear stabilization methods may be ineffective to fix large-signal stability issues. In this paper, a large signal stability constraining dichotomy solution (SCDS)-based model predictive control (MPC) is proposed to improve the stability of the ac cascaded system in the MEA by the Lyapunov criteria. This approach first theoretically presents how to add the system stability item to the MPC cost function in an analytic way instead of using the traditional “experience-based” weight method. Besides, the SCDS-MPC itself is also an improvement to the existing finite control set MPC. It can realize a constant switching frequency and achieve better steady-state/dynamic current control performance. Finally, a 1.5 kW ac cascaded system is fabricated in the laboratory to verify the proposed SCDS-MPC method. [ABSTRACT FROM AUTHOR]

This paper introduces a sliding mode control (SMC) strategy for three-phase dynamic voltage restorers (DVRs) with a 12-switch voltage source inverter. The compensating voltage references needed in the SMC strategy are generated by an adaptive notch filter (ANF), which exhibits excellent performance under grid voltage anomalies such as voltage sags, swells, and unbalanced and distorted grid voltage conditions. The consequence of using the ANF eliminates the use of phase-lock loop or frequency-lock loop and low-pass filter, which makes it distinguishable from the existing reference signal generation solutions. In addition, the use of the SMC strategy with its attractive properties makes the control implementation simple. Theoretical results are supported by simulation results as well as real-time laboratory results over a range of grid voltage anomalies. These results show that the proposed control strategy not only offers an excellent dynamic response independent from the parameter variations and disturbances but also compensates the voltage sags, swells, and harmonics on the load terminals under the defined limits of the IEEE-519 standard. [ABSTRACT FROM AUTHOR]

This paper presents a novel model predictive control (MPC) for the T-type single-phase three-level inverters. The cumbersome procedure in tuning of weighting factors is eliminated so that the implementation of the proposed MPC becomes easy. The constant switching frequency is achieved during the control implementation in order to facilitate the filter design. In order to optimize the distribution of the output current harmonics, multiple voltage vectors are employed in each control cycle with their application times setting inversely proportion to the respective cost function. The redundant small voltage vectors are utilized to balance the neutral point voltage of the inverter by directly regulating the upper and lower dc-link capacitors voltages. Finally, the proposed MPC algorithm is experimentally evaluated and compared with other two unfixed switching frequency MPC algorithms in terms of the steady state, transient performance, and parameter sensitivity. [ABSTRACT FROM AUTHOR]

This paper proposes an observer-based grid-voltage sensorless strategy that allows to implement the synchronization and control of an LCL grid-tied voltage source inverter. By using exclusively the measurement of the current injected to the grid, an observer estimates both fundamental positive- and negative-sequence components of the grid voltage, with a quality (in terms of total harmonic distortion and convergence speed) comparable to that obtained with a classic double second-order generalized integrator-frequency-locked loop (DSOGI-FLL). The unmeasured LCL filter states are also estimated by the observer, and they are used to implement a full-state feedback control strategy suitable for low resonance frequency LCL filters. The effectiveness of the scheme proposed here is demonstrated by comparison with a DSOGI-FLL and with a controller recently proposed in the literature. Experimental and simulation results that demonstrate the validity and effectiveness of the proposal are presented. [ABSTRACT FROM AUTHOR]

This paper presents a new reconfiguration module for asymmetrical multilevel inverters in which the capacitors are used as the dc links to create the levels for staircase waveforms. This configuration of the multilevel converter makes a reduction in dc sources. On the other hand, it is possible to generate 13 levels with lower dc sources. The proposed module of the multilevel inverter generates 13 levels with two unequal dc sources (2VDC and 1VDC). It also involves two chargeable capacitors and 14 semiconductor switches. The capacitors are self-charging without any extra circuit. The lower number of components makes it desirable to be used in wide range of applications. The module is schematized as two back-to-back T-type inverters and some other switches around it. Also, it can be connected as a cascade modular which leads to a modular topology with more voltage levels at higher voltages. The proposed module makes the inherent creation of the negative voltage levels without any additional circuit (such as H-bridge circuit). Nearest level control switching modulation scheme is applied to achieve high-quality sinusoidal output voltage. Simulations are executed in MATLAB/Simulink and a prototype is implemented in the power electronics laboratory in which the simulation and experimental results show a good performance. [ABSTRACT FROM AUTHOR]

This paper proposes and experimentally assesses two zero-sequence current control strategies that allow continuous operation of the three-phase dual-inverter drive using an open-end winding permanent magnet synchronous machine (PMSM) when freewheel diodes (or semiconductor switches) in either inverters fail open circuit. With an identified failure present, the proposed control strategies inject zero-sequence current to bias the phase current(s) of the faulted phase(s) such that the phase current(s) are unipolar, thereby eliminating current flow in the device(s) that have failed. Two possible injection modes are proposed. The first mode injects the minimum required zero-sequence current to bias the current in the phase that has a semiconductor open-circuit failure(s). The injected zero-sequence current varies over a fundamental cycle. This minimizes the increase in total rms currents in the phases yielding higher torque capability in fault conditions. The second mode injects a zero-sequence current that has a constant value over a fundamental cycle to bias all three-phase currents appropriately. This mode is used if more than one phase has suffered a fault. Additional thermal and current stresses resulting from both techniques were assessed and compared. A detailed electromagnetic torque model was also developed to demonstrate that the zero-sequence current does not affect the average torque output. Experimental results verify the theoretical treatment and are used to examine the advantages of techniques and show the improvement in the maximum available torque under fault conditions. [ABSTRACT FROM AUTHOR]

This paper proposes a single-vector-based model predictive control (MPC) for an induction motor drive supplied by a three-level neutral-point-clamped inverter operating at a low switching frequency. Unlike the torque and flux control in the conventional MPC, the proposed MPC tries to minimize the error between the applied voltage vector and a reference voltage vector obtained based on the principle of deadbeat control, thereby reducing the number of weighting factors in the cost function. To reduce the computational burden and restrict the high jumps in both phase and line voltages, two switching tables are proposed and compared for the preselection of candidate voltage vectors. The neutral point potential fluctuation and switching frequency are also included in the cost function to achieve the balance between the upper and lower dc voltages and a relatively low switching frequency. Furthermore, a speed adaptive stator flux observer with a novel gain matrix is proposed to achieve speed-sensorless operation, which has a higher speed and flux estimation accuracy than conventional fixed gains. Finally, the proposed MPC is extended to the field-weakening operation by adjusting the torque and stator flux references online, which significantly widens the speed range and improves the practical value of the MPC. The effectiveness of the proposed method is confirmed by the experimental results obtained at an average switching frequency of less than 600 Hz. [ABSTRACT FROM AUTHOR]

In this paper, a model-based current control strategy with virtual time constant is proposed for three-phase grid-connected LCL-filtered voltage source inverters. The proposed control strategy is based on controlling the inverter currents in the rotating dq frame by using current-oriented proportional-integral (PI) controllers rather than voltage-oriented PI. The PI controllers determine the inverter current references in the d- and q-axes by regulating the grid current. It is shown that the proposed strategy decouples the inverter current from other variables provided that the inverter-side inductance and its resistance values used in the control variable match the actual values in the system. In addition, the virtual time constant is introduced in the control variables to offer flexibility for adjusting the inverter current dynamics as desired. Moreover, the integral gain of PI controller has the ability to keep the LCL-resonance peak below 0 dB. Unlike the existing methods, the proposed strategy does not require a dedicated active damping. Computer simulations and experimental studies show that the proposed control strategy exhibits a good performance in achieving fast dynamic response and sinusoidal grid current with low THD under balanced, unbalanced, and distorted grid conditions. [ABSTRACT FROM AUTHOR]

Due to the pulsating power at the inverter output terminals, the passive storage elements of the boost stage of the current-fed switched inverter (CFSI) contain a power frequency harmonic component (100 Hz), apart from dc and switching frequency components. In prior designs, the second harmonic component is suppressed by increasing the size of the input filter inductor and capacitor or by using additional circuits. This paper presents a complete characterization of a control-based approach to suppress the power frequency harmonic component in the input inductor current of a CFSI. It also presents a comprehensive characterization of the harmonic power redistribution. Using the proposed approach, it is demonstrated that the inductor value of a CFSI is reduced by 61% for the same peak-to-peak ripple. This leads to an improvement in the conversion ratio, efficiency, and power density of the overall converter. A SiC MOSFET-based CFSI with 75-kHz switching frequency is used to verify the proposed design philosophy. [ABSTRACT FROM AUTHOR]

In this paper, new steady-state analysis is proposed for a series resonant converter (SRC) that provides closed-form expressions for converter waveforms. Using the proposed analysis, explicit equations are obtained to design the converter components. It is shown that an SRC can be modeled through a nonlinear differential equation with discontinuous inputs. A Laplace-based theorem (LBT) is provided to obtain the steady-state analytic solution of the resonant converter differential equations. Using the LBT, a flowchart is proposed to analyze the converter where the nonlinearity of differential equation is removed by defining intermediate variables. The variable duty ratio SRC is analyzed using the proposed flowchart. Accurate, closed-form, and explicit equations for converter waveforms, voltage gain, current phase lag, zero voltage switching, and discontinuous conduction mode boundaries are derived. The proposed analysis is compared with conventional methods and its accuracy is validated through simulations and experimental results. Moreover, using the proposed method, a novel procedure is provided for the optimal design of the converter and is compared with conventional design approaches. [ABSTRACT FROM AUTHOR]

This paper presents a high-efficiency inverter with the aid of the soft-switching technology, which is accomplished by the resonance of the in-series inductance with the snubber capacitance. The equivalent circuit of the quarter-phase ultrasonic motor is initially deduced, based on which the topology along with the operations of the novel inverter is subsequently presented. The configuration of the new quarter-phase inverter shows its uniqueness in terms of the shared arms and the especially designed snubber capacitance in comparison with the existing inverters. The circuit analysis infers that the dead time of the inverter is the key factor to arrive at the zero-voltage-switching or the zero-current-switching operations. Experimental measurements have demonstrated that the efficiency of the whole drive increases by a factor of 1.25 after replacing the traditional inverter with the proposed one. It should also be noted that the proposed inverter in association with the design method of the dead time not only can work for the quarter-phase ultrasonic motor, but also can be applicable for other kinds of quarter-phase devices. The soft-switching operations are achieved in the quarter-phase inverter for the first time, whose amplitudes of the output voltage and switching frequency are not required to be constant any more. [ABSTRACT FROM AUTHOR]

Two parallel interleaved two-level (2L) three-phase voltage source inverters (VSIs) can be analyzed as a single three-level (3L) VSI. A generalized carrier-based method is proposed in this paper for implementing different 3L pulsewidth modulation (PWM) techniques using two parallel interleaved 2L VSIs. The proposed method modifies the given three-phase reference signals of 3L VSI for implementation of the PWM technique using two parallel interleaved 2L VSIs. The influence of nonidealities on the equivalent 3L PWM switching sequences is also discussed. To overcome the effect of nonidealities, a new modified discontinuous PWM technique (MDPWM1) is proposed. The analysis of different performance indices is carried out for different 3L PWM techniques, and the key advantages of different PWM techniques are identified. Simulation and experimental results are also presented. [ABSTRACT FROM AUTHOR]

The improved LCL filter is used in transformerless three-level photovoltaic inverter system for leakage current suppression. However, the common-mode (CM) resonance circulating current (CMRCC) is generated, which causes inverter-side current oscillation, leakage current increment, and system instability. To overcome these issues, an improved CM circulating current model with detailed analyses of CMRCC is proposed in this paper. Based on above theories, the CMRCC is influenced by filter parameters, zero-sequence duty ratio, and number of switching transitions. To suppress CMRCC, a hybrid control strategy composed of proportional integral (PI) and feedforward control is presented. It is realized by real-time adjusting zero-sequence duty ratio in space vector modulation technique. Compared with the conventional damping resistor techniques, the CMRCC and leakage current are effectively suppressed with the proposed method. Thus, the performance of inverter-side currents and leakage current suppression is greatly improved. Besides, the capacitor-voltage-feedforward control is adopted to eliminate resonance peak in the differential-mode circuit. Simulation and experimental results are given to demonstrate the effectiveness of theoretical analyses and the proposed method. [ABSTRACT FROM AUTHOR]

The nine papers in this special section focus on photovoltaic module and sub-module level power electronics. Grid connected photovoltaic energy systems have experienced an explosive growth over the last decade, with a cumulative installed capacity surpassing the 400 GW milestone as of 2017. Among PV system configurations, distributed module-level converter architectures can lead to a higher energy yield by mitigating partial shading, persistent shading (soiling, snow, bird droppings, and fallen leaves), mismatch, and aging, through a higher maximum power point tracking (MPPT) efficiency. Also, distributed electronics might be the key for implementing diagnostic and prognostic actions at a module level. Among these configurations, microinverters (also known as ac-module inverter), which connect a single PV module to the grid, and PV power optimizers, which are dc–dc converters performing the MPPT function at a module level, have attracted the academic and industrial interest in the last decade. So much so, that both microinverters and dc–dc power optimizers are commercialized by tens of companies around the world, with a great variety of circuit topologies, which comprise combinations of one or more power stages, interleaved converters, resonant converters, topologies with and without isolation, etc. [ABSTRACT FROM AUTHOR]

This paper explores a power-conversion system for single-phase transformerless ac module application. A nonisolated high-gain dc–dc stage is coupled with a doubly grounded dynamic dc-link inverter to implement the microinverter topology. The front-end boost stage steps up the single-module voltage to a higher dc voltage, typically to 200 or 400 V as required for the following inverter stage. It is a hybrid of an interleaved boost and switched capacitor concept capable of achieving high gain while simultaneously maintaining reduced voltage and current stress, and thus lowering switching and conduction loss for most of the switches. By directly connecting the grid neutral to the photovoltaic (PV) negative, the doubly grounded voltage swing inverter has the advantage of zero capacitive-coupled common-mode ground currents, critical for transformerless PV inverters. The two dc links of the inverter stage share the double-line frequency power decoupling with a combination of higher value of mean voltage and relatively large 120-Hz ripple component–the dynamic dc-link approach. Through an optimization algorithm, the decoupling capacitance is minimized enabling an all-film capacitor implementation while satisfying the converter's operating constraints over a wide range of power factors. The simulation as well as experimental results from a GaN-based hardware prototype validate the converter's performance. [ABSTRACT FROM AUTHOR]

The development of a photovoltaic ac module composed of a high static gain nonisolated dc–dc converter with current source output characteristic followed by an unfolding low-frequency inverter is presented in this paper. The operation of the dc–dc converter with high static gain and with reduced switch voltage is obtained using the Cuk converter integrated with the switched-inductor technique. The nonisolated structure allows the elimination of the volume and losses of the power transformer and leakage inductance, and an output filter configuration is proposed to reduce the high-frequency ground leakage current. The grid-tied inverter operating in a discontinuous conduction mode (DCM) allows a sinusoidal output current with low harmonic distortion (THD = 3.72%) to be obtained without current control loop and without the use of a current sensor at the output. The maximum power point tracking operation is obtained without the use of the current sensor. The DCM operation increases the current stress, but the proposed converter presents two switches and two input inductors sharing the input current in these components. The theoretical analysis, design procedure, and experimental results are presented for a prototype with a dc input voltage Vi = 30 V, an ac output voltage Vo = 127 V, and output power Po = 180 W. [ABSTRACT FROM AUTHOR]

The presence of a neutral current is quite common in three-phase (3P) four-wire (4W) distribution systems due to an unequal distribution of linear and nonlinear single-phase (1P) loads and small distributed generators. However, a high neutral current can overload the neutral conductor and distribution transformer, which can cause electrical safety concerns and even fire. Among several existing neutral current compensators, the 3P four-leg (4L) voltage-source inverter (VSI) provides better control flexibility and more efficient performance than the passive compensators but requires a higher VSI capacity for the fourth-leg operation. To provide a solution to the aforementioned problem, this paper presents a novel control method to utilize the available capacity of a 3P-4L VSI after active and reactive power regulation to enhance the neutral-current compensation. A smart VSI (SVSI) is designed to operate with a solar photovoltaic unit, regulate the ac side voltage, and minimize the neutral current. Case studies are conducted with actual load data from a commercial building in the PSCAD/EMTDC software environment. The designed system with the proposed control method can provide a significant improvement in the neutral-current compensation, phase balancing, and unbalance factor compared to a fixed-capacity 3P-4L SVSI. Experimental results using a TMS320F28335 digital signal processor microcontroller and modified Semiteach 3P-4L inverter are presented to verify the robustness of the designed controller and the enhancement to the neutral-current compensation using the proposed dynamic capacity-control method. [ABSTRACT FROM AUTHOR]

To reduce the common-mode voltage (CMV) and eliminate the CMV spikes for two-level voltage source inverters (2L-VSIs), an improved model predictive control (MPC) based CMV reduction method is proposed in this paper considering dead-time effects. First, an improved voltage vector (VV) preselection strategy is proposed to reduce the CMV spikes caused by the dead time. Second, a new hybrid VV preselection strategy based on a delay compensation strategy and a modified current sector division strategy is proposed to completely eliminate the CMV spikes. As the proposed hybrid VV preselection based MPC strategy makes full use of the six nonzero VVs of the 2L-VSI, the current total harmonic distortions and ripples are reduced. The simulation and experiments are carried out to verify the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

In this paper, a fixed switching frequency model predictive current controller is designed and implemented on a sensorless five-level packed U-cell (PUC5) inverter interfacing the utility. The predictive control methodology is based on a voltage vector generation technique for the cost function optimization by nullifying its derivative. The aim of the controller is to allow a fixed-frequency operation with no iterations while operating as a utility line current regulator. A pulsewidth modulator based on employing PUC5 redundant switching states fixes the PUC5 auxiliary dc bus capacitor voltage to its desired level with no necessity of a regulator; the overall technique reduces system complexity with less sensors and no loops in the predictive algorithm making it suitable for industrial applications. System discrete model is developed and experimental results verify the performance of the designed controller in making a fast response in regulating the grid current and balancing the capacitor voltage. [ABSTRACT FROM AUTHOR]

To solve the problem of poor robustness due to the high dependence of model predictive control (MPC) on the three-phase inverter model, the influence mechanism of the mismatch of circuit parameters R and L, the model discretization error, and sampling errors on MPC prediction error are analyzed in this paper. A feedback self-correction is proposed, which the next current errors are predicted and used to modify the model, based on the previous prediction errors. This proposed method involves simple structure and a small amount of calculation. Steady-state effects of the proposed method are quantized to prove a good inhibitory ability on multifactorial prediction errors. Finally, experimental results in a three-phase inverter are presented to support the theoretical analysis and demonstrate the feasibility of the proposed method. [ABSTRACT FROM AUTHOR]

Multilevel inverters are now-a-days being applied in electric vehicles (EV). Considering the dynamic operating conditions of an EV propelled by an induction motor, this paper proposes a flexible discontinuous modulation strategy for three-level neutral-point-clamped (NPC) inverter that can shift modulation pattern seamlessly from sinusoidal pulsewidth modulation (SPWM) to discontinuous PWM (DPWM). The modulation scheme is generalized at each degree of discontinuity to ensure minimum switching losses for variable load power factors. Additionally, a hybrid voltage balancing strategy, which comprises two sets of compensating offset signals with complementary features, has been introduced to mitigate voltage deviation problem at neutral point of the inverter. The unique feature of the proposed hybrid balancing approach is that it generates uniform compensating neutral current regardless of the level of discontinuity. This results in identical and predictable voltage balancing dynamics for the entire modulation transition covering SPWM and DPWM. It has been shown that the proposed hybrid balancing strategy does not lose its unbalance compensation strength even during transition from one modulation pattern to another. The effectiveness of the proposed scheme for wide variations in operating conditions is studied through extensive simulation, and validated in experimentation using a prototype NPC inverter with induction motor loads. [ABSTRACT FROM AUTHOR]

Resonant inductive power transfer (RIPT) is gaining in popularity for wireless charging applications of future electric transportation. A fundamental impediment to the efficient operation of an RIPT system is the existence of bifurcation phenomenon in a doubly tuned circuit. Bifurcation can be avoided by either selecting complicated control strategies or by proper design strategies. Many control strategies have been presented in the literature for avoiding bifurcation. However, systematic design strategies for avoiding bifurcation are still unknown. This paper presents a simplified, and easy to follow set of design guidelines for series–series compensated RIPT (SS-RIPT) systems. The design guidelines avoid bifurcation by calculating the parameters for a given load profile in a systematic and easy to follow approach. Following the design guidelines, a 3.6-kW setup has been fabricated as an example and verified using a finite element analysis as well as experimental testing. Selection of a resonant frequency and output voltage for 3.6 kW was influenced by the availability of testing equipment available in the lab. However, the design guidelines are generalized, and by following it, an SS-RIPT system of any frequency and output voltage rating, applicable for EV charging, can be designed. [ABSTRACT FROM AUTHOR]

A new inverter control approach, called enhanced virtual synchronous machine (eVSM), is proposed based on the VSM concept. Unlike existing VSM approaches, the eVSM does not emulate the rotating inertia based on an assumption of the unlimited energy storage, but it deploys the physically existing inertia of the dc-link element. The eVSM adopts an innovative way of enlarging the inertia utilization range, which obviates the need for having a large dc-link element or a dedicated battery storage system, while still providing the same inertia response of an equivalent synchronous machine. Theoretical developments and numerical results presented in this paper confirm that the proposed eVSM can present a stabilizing support to the grid similar to a synchronous machine despite the small size of its dc-link element. Moreover, its transient responses can be improved beyond those of conventional synchronous machines thanks to the possibility of more flexibly adjusting damping and governor functions. [ABSTRACT FROM AUTHOR]

In this paper, the concept of an Optiverter is proposed as a novel class of photovoltaic (PV) module level power electronics systems. Functionally, the Optiverter is a hybrid technology that combines the ultrawide maximum power point tracking (MPPT) voltage window of the PV power optimizers with the direct ac connectivity and inherent safety of the PV microinverters. Thanks to the advanced multimode control with variable dc-link and the shade-tolerant MPPT algorithm, the proposed Optiverter ensures efficient energy harvest from the PV module in different shading scenarios. To justify the superiority of the concept, the performance of a 300 W prototype of the PV Optiverter is experimentally compared to that of the industrial microinverters in different operation conditions, including an extreme case with opaque shading of two out of three substrings of the PV module. [ABSTRACT FROM AUTHOR]

Variable frequency asymmetrical pulsewidth modulation (VFAPWM) is widely used in wireless power transfer (WPT) systems to control the amount of power transferred. This control scheme is also adopted in Qi, one of the world's most well-known wireless charging standards. Conventionally, the analysis of WPT systems is based on first harmonic approximation, which only considers the fundamental harmonic signals. This could be inaccurate for variable frequency asymmetrical pulsewidth-modulated WPT systems because VFAPWM inherently produces multiple harmonics. In this paper, a new analytical technique, called multiple harmonics analysis (MHA), is developed to analyze series–series-compensated WPT systems. The MHA technique is a comprehensive design foundation and provides a set of closed-form solutions to predict quantities, such as zero-voltage switching (ZVS) and voltage gain. New phenomena in ZVS and voltage gain are discovered. The accuracy of MHA is also confirmed by both simulation and experimental measurements. [ABSTRACT FROM AUTHOR]

Wireless power transfer with a self-tuning LCC inverter using pulsewidth modulation (PWM) controlled switched capacitor is proposed in this paper in order to compensate for the variations of Tx inductance. Usually, receivers contain magnetic and conductive materials for shielding and mechanical support. The inductance of the Tx coil is sensitive to the variation of Tx–Rx alignment because the interaction between the Tx coil and the Rx magnetic–conductive materials is also varied. It is shown that the detuning of the Tx inductance value exacerbates the losses in the LCC inverter. In the proposed method, the PWM duty ratio of a single capacitor adjusts the effective output impedance of the LCC inverter. The PWM feedback loop is designed such that the power factor and the real-part impedance of the LCC inverter are maximized. The PWM switch is turned on at zero voltage and turned off with low dv/dt, minimizing switching losses. Such a soft-switching tunable capacitor can handle higher power compared to the traditional hard-switching capacitor, p-n junction capacitor, or an analog IC capacitor. Another advantage of the proposed technique is the continuity in achievable capacitance value using only one capacitor. The feedback can be fully implemented by analog components, obviating the necessity of digital samplings, ADC/DACs, and microcontrollers. The power of 54 W is transferred at 76.6% and 60.7% with and without the proposed method, respectively. [ABSTRACT FROM AUTHOR]

In this paper, the wear-out performance of an impedance-source photovoltaic (PV) microinverter (MI) is evaluated and improved based on two different mission profiles. The operating principle and hardware implementation of the MI are first described. With the experimental measurements on a 300-W MI prototype and system-level finite-element method simulations, the electrothermal models are built for the most reliability-critical components, i.e., power semiconductor devices and capacitors. The dependence of the power loss on the junction/hotspot temperature is considered, the enclosure temperature is taken into account, and the thermal cross-coupling effect between components is modeled. Then, the long-term junction/hotspot temperature profiles are derived and further translated into components’ annual damages with the lifetime and damage accumulation models. After that, the Monte Carlo simulation and Weibull analysis are conducted to obtain the system wear-out failure probability over time. It reveals that both the mission profile and the thermal cross-coupling effect have a significant impact on the prediction of system wear-out failure, and the dc-link electrolytic capacitor is the bottleneck of long-term reliability. Finally, the multimode control with a variable dc-link voltage is proposed, and a more reliable dc-link electrolytic capacitor is employed, which results in a remarkable reliability improvement for the studied PV MI. [ABSTRACT FROM AUTHOR]

To acquire high efficiency and reduce electromagnetic interference in wireless high-power charging for electric vehicles (EVs), it is of great importance to realize accurate zero voltage switching angle (ZVSA) control of the inverter. However, the traditional zero-crossing detection cannot be applied in high power because of considerable noise and poor accuracy in wide power range applications. In this paper, to suppress the noise in high power, a current hysteresis comparator is adopted, and an enhanced phase detection methodology is proposed to measure the phase of resonant current by using a reference signal produced by a processor. Meanwhile, to improve the accuracy of ZVSA, a uniform time delay compensation method (UTDCM) by considering the whole time delays synthetically is proposed, and a uniform time delay equation can be obtained to guarantee the high accuracy of phase detection, especially in the wide power range. Finally, a novel control strategy with the ZVSA loop based on UTDCM is proposed for battery charging in wireless high-power transfer system. A 500-W wireless power transfer prototype is built to verify the accuracy of ZVSA based on UTDCM and the system efficiency can achieve 94.17% with $k$ = 0.22. [ABSTRACT FROM AUTHOR]