Abstract: This paper studies the piecewise-affine memory $\mathscr {H}_{\infty }$ filtering problem for nonlinear systems with time-varying delay in a delay-dependent framework. The nonlinear plant is characterized by a continuous-time Takagi–Sugeno fuzzy-affine model with parametric uncertainties. The purpose is to develop a new approach for filter synthesis procedure with less conservatism. Specifically, by constructing a novel Lyapunov–Krasovskii functional, together with a Wirtinger-based integral inequality, reciprocally convex inequality and S-procedure, an improved criterion is first attained for analyzing the $\mathscr {H}_{\infty }$ performance of the filtering error system, and then via some linearization techniques, the piecewise-affine memory filter synthesis is carried out. It is shown that the existence of desired filter gains can be explicitly determined by the solution of a convex optimization problem. Finally, simulation studies are presented to reveal the effectiveness and less conservatism of the developed approaches. It is anticipated that the proposed scheme can be further extended to the analysis and synthesis of continuous-time fuzzy-affine dynamic systems with integrated communication delays in the networked circumstance.

Abstract: Despite many intensive researches regarding frequency-locked loops (FLLs) in recent years, the basic problems of principles, models, and structures of FLL algorithms remained open. Conventional FLL comprises a notch filter and a frequency adaptive loop. The notch filter can be categorized into two types: real coefficient filter (RCF) and complex coefficient filter (CCF). However, the principles, models, and structures of RCF and CCF are generally explained by various theories, and thus, a unified understanding and further developments of FLLs are restricted. This paper proposes a generalized design framework of FLLs based on the Popov adaptive theory. Grounded on the proposed framework, three main contributions can be achieved: 1) the mathematical equivalence of RCF-based FLLs and CCF-based FLLs are revealed; 2) not only the wellknown FLLs (e.g., the DSOGI-FLL and the AVF-FLL) can be derived, but also the other FLLs with novel features can be generated, and thus, the existing case designs of FLLs are developed into a family of designs; 3) the performance differences in terms of stability and convergence speed of the FLLs are quantitatively analyzed, and thereby an enhanced FLL (referred to as EFLL) is recommended. The designs and analyzes have been verified by both simulations and experiments.

Abstract: Voltage source inverter based active power filters are widely used for the compensation of harmonic currents, especially in industrial environments. Loads such as three-phase diode rectifiers, thyristor-based rectifiers or variable-speed drives can cause relevant harmonics up to the 50th order or even higher, depending on the line filter. Usually, the control of the harmonics is either done in multiple rotating frames, where the harmonics appear as dc values, or by resonant controllers in a stationary frame. At this, the calculation effort is rising with the number of harmonics to compensate. In this paper, a predictive current control scheme is presented. It is implemented in addition to common PI current control and used for a three-phase four-wire LCL -filter-based active power filter. Inaccuracies in the model caused by parameter variations are compensated by an additional control structure. Both structures are related only with low computational effort and are not depending on the desired number of the harmonics to compensate. Experimental tests are demonstrating, that both structures individually, and especially the combination of both, are offering a very good behavior in the steady state and in the case of load changes.

Abstract: The modular multilevel converter (MMC) is receiving wide acceptance in both high- and medium-voltage (MV) applications. However, due to the low (fundamental and second-order) frequency ripple powers in the submodule (SM) capacitors, large capacitance is required to smooth the SM voltage. The SM capacitors account for a large portion of volume and weight in the MMC system. Present methods (e.g., circulating current control, power channels linking upper and lower arms) cannot eliminate the fundamental and second-order ripple powers simultaneously. This paper investigates the feasibility of an active power decoupling technique for solving this issue. By adding a buck-type active power filter (APF) circuit (which contains another energy-storage capacitor), the low-frequency ripple powers can be transferred to the APF capacitor. This significantly reduces the SM voltage ripple and therefore the total capacitance of the SM. To enhance voltage ripple suppression, APF capacitor voltage reference is modified in a closed-loop manner, and a proportional-integral plus repetitive controller is proposed. Simulations and experimental results prove the validity of the method. A comparison with the traditional MMC shows that it can significantly reduce system volume and improve power density. The method is best suited for MV applications where power density is given a high priority.

Abstract: An active power filter (APF) plays an important role in compensating the harmonic component of a nonlinear load current. In this paper, deadbeat control (DBC), which has high control performance and fast dynamic response, is used as a current controller for an APF. An improved DBC (Imp_DBC) is introduced to suppress the adverse effect of the current sampling error and to enhance the interference rejection capability of the control system. The characteristics of the conventional DBC and Imp_DBC are analyzed using transfer functions in the discrete-time domain. The robustness of the system is improved with the inductance variation. Specifically, aiming at the dramatic change of the harmonic component of the nonlinear load current, a current-tracking error compensation method is proposed. The phenomena of the upper co-nvex and lower concave in grid current are eliminated. Finally, the experimental setup of three-phase three-line APF is built, and the effectiveness of this method is verified by experiments.

Abstract: The operation of active power filters (APFs) under nonideal grid conditions, such as grid-frequency fluctuation and voltage harmonics, can lead to significant degradation in harmonic compensation performance. This paper proposes an adaptive digital-control scheme for a three-phase APF for use in harmonically distorted and variable-frequency grid conditions. This scheme is comprised of a grid-frequency adaptive resonant current controller and an enhanced synchronous-reference-frame phase-locked loop (SRF-PLL). The PLL uses an inherently stable adaptive-filtering stage to improve grid phase and frequency estimates in the presence of voltage harmonics. The improved PLL frequency estimate is used to update the resonant gains of a PI + vector-proportional-integral current-control scheme, implemented in the SRF. This enables the APF to maintain optimal performance in distorted grid conditions. The performance of the proposed APF control scheme is evaluated in a test microgrid, with a 15-kVA three-phase voltage-source converter configured as the APF, a 90-kVA grid emulator utilized to replicate distorted grid conditions, and a load emulator implemented to draw harmonic currents. The control scheme presented here is shown to demonstrate significant performance improvements under nonideal grid conditions compared with equivalent adaptive and nonadaptive methods.

Abstract: In this paper, the structure, modeling, parameter design, and control method are proposed for a new hybrid structure of a static var compensator in parallel with a hybrid active power filter (SVC//HAPF). The SVC part of the SVC//HAPF is used to dynamically compensate the reactive power. And, the HAPF part is used to provide harmonic power and small amount of reactive power compensation. Due to the large fundamental voltage drop on coupling the LC part, the active inverter part of the SVC//HAPF has low voltage rating. Meanwhile, the parallel-connected SVC acts as a current divider to reduce the active inverter current. Therefore, the proposed SVC//HAPF shows the great promise in compensating harmonic current and wide-range reactive power with a low (both) voltage and current rating active inverter part. To show the advantages of the proposed SVC//HAPF, simulation comparisons among the active power filter (APF), HAPF, SVC in series with HAPF (SVC−HAPF), and the proposed SVC//HAPF are provided. Finally, experimental results based on the laboratory-scaled hardware prototype are given to show the validity of the SVC//HAPF.

Abstract: An active grid impedance cancelator using the concept of series active filter to suppress the effect of the grid disturbance and stabilize the single-phase grid-connected inverters with an inductive–capacitive–inductive filter operating under variable grid condition is presented. Harmonic interaction between the inverter and the grid is thereby avoided owing to the cancelation of equivalent grid impedance. More importantly, the impedance cancelator offers an active damping function to ease the heavy burden of the inverter control, such as power control, phase locked loop, current regulation, impedance shaping, etc. The impedance cancelator is a full-bridge dc–ac converter having no passive inductive–capacitive filter. It is connected in series with the inverter output and is operated as a negative virtual grid impedance. As the volt-ampere rating of the impedance cancelator is low, the efficiency of the entire system is not sacrificed. Starting with the impedance-based analytical modeling method, the basic principle of equivalent grid impedance cancelator is derived and studied. Then, the digital control strategy and the modeling of equivalent grid impedance cancelator are examined. The experimental results of a prototype cancellator for a single-phase inverter are favorably compared with theoretical predictions.

Abstract: This paper proposes a single-phase onboard battery charger (OBC) for plug-in electric vehicles, where the low-voltage (LV) battery charging circuit is utilized for an active power decoupling function. The OBC is operated in three different modes by sharing the transformer, switches, and capacitors. For a grid-to-vehicle mode or a vehicle-to-grid mode, the LV battery charging circuit serves as an active filter to eliminate the low-frequency power ripple at the DC link. Thus, the small film capacitors can be employed instead of large capacitors at the DC link. For the third operating mode, where the LV battery is charged from the HV battery, the isolation is provided by the dual active bridge (DAB) DC-DC converter. Since some components in the proposed OBC are used in common, the size and cost of the OBC can be reduced significantly. The simulation and experimental results have verified the validity of the proposed system.

Abstract: This paper presents a further significant development to the developed flexible LCC HVDC system with controllable capacitors [1] , which can provide ac voltage/reactive power control [2] . The development involves the installations of fixed parallel capacitors at the valve side of converter transformer, which brings the following significant benefits: First, ac filter banks at the ac side of converter transformer are not needed as a better harmonic filtering performance can be achieved; second, significant reduction of the HVDC station land requirement (compared with traditional LCC HVDC), as the ac filters together with the switchgear can occupy over 50% of the HVDC station footprint; third, up to 50% reduction of the required voltage rating and more than 60% reduction of the capacitance of controllable capacitors for commutation failure elimination can be achieved while similar power system dynamic performance (ac voltage/reactive power control) compared with that in [1] can be demonstrated. Detailed analyses are presented to illustrate the effective commutation process and superb harmonic filtering performance with the fixed parallel capacitors. Selections of the component values are presented. Simulation results in real-time digital simulator are presented to verify the effectiveness of commutation failure elimination, power system dynamic performance, harmonic filtering performance, and show voltage/current stress of the fixed parallel capacitors.

Abstract: The existing control strategies of modular multilevel converter (MMC) balance the capacitor voltage on the premise that the active power of ac side is balanced with that of dc bus. Thus, the symmetrical ac-side current references and the unevenly distributed dc current references in three legs are obtained by coordinate transformation, precise calculation, and numerous filters under unbalanced grid condition. However, by controlling capacitor voltages, the active powers between ac side and dc bus can self-regulate to balance, and this could simplify the obtainment of ac-side current references and dc-bus current references. Based on this idea, this paper proposed a control strategy, which combines the multi-hierarchy control with the arm current control for MMC under unbalanced grid condition. Within the multi-hierarchy control, the symmetrical ac-side current references and the unevenly distributed dc current references in three legs could be obtained easily by three voltage controllers in the abc coordinate, avoiding coordinate transformation, precise calculation, and numerous filters in the existing methods. Besides, the employment of the arm current control removes the need of the three-sequence ac-side current controllers and the three-sequence circulating current suppressing controllers. The proportional regulator with a feedforward steady-state duty cycle is designed for arm current regulator, which can perfectly track its reference and is easy to design, avoiding the complicated design of proportional resonant (PR) controllers. Both system-level simulation results and low-level experiment results verify the feasibility and effectiveness of proposed strategy.

Abstract: LCL -filter is among the best performing filters for grid-connected voltage source inverters Designing of the filter parameters (grid-side and inverter-side inductors and capacitor), takes an iterative approach due to the coherence between the parameters and design requirements such as IEEE-519 Std for harmonic current limitations, reactive power compensation limit, and maximum allowable voltage drop across the filter to limit the switching losses Most of the proposed LCL -filter optimization strategies emphasize on reducing the total inductance and losses of the filter while meeting the design requirements There is less emphasis given on the capacitor selection and optimizing its value Therefore, this paper proposes a method to compute the optimum capacitance requirement of the LCL -filter based on reactive power compensation of the filter rather than calculating it as a percentage of base capacitance of the filter as found in the literature The proposed design methodology compared to the previously proposed designs is capable of reducing filter capacitance by 50% while meeting the harmonic limitation demanded by IEEE-519 Std and also considers the impact of the total inductance on reactive power compensation Based on the proposed methodology an LCL -filter with minimum total inductance and capacitance is realized Functionality of the proposed LCL -filter is verified and validated through simulations and experimental results

Abstract: The single-phase nonlinear loads are going to increase in the future and power quality concerns in the residential distribution grid. Since most of nonlinear loads have a dispersed nature, compensating these phenomena may be complicated. On the other hand, the increased capacity of single-phase grid-connected roof-top PV inverters in residential distribution grid can be an opportunity to engage these systems in the power quality issues as custom power devices. By implementing a proper control for roof-top PV inverters, these systems may in addition to inject the fundamental current, additionally act like a virtual harmonic resistance, and dedicate their additional current capacity to compensate the harmonics of residential distribution grid. In this paper, each roof-top PV system is a grid-harmonic supervisor, where it continually measures the PCC voltage harmonics by the sliding discrete Fourier transform algorithm and then individually compensates the measured harmonics by the proposed adaptive-harmonic compensator. In the proposed approach, a current reference is separately determined for compensating each harmonic component. Then, a specific amount of additional current capacity will be dedicated for each component to compensate by using adaptive gain. Finally, the current reference is controlled by a model predictive current controller. It has been tested in a single-phase transformerless inverter (highly efficient and reliable inverter concept) with an LCL filter at the output. Also, the effectiveness of the control scheme has been verified by MATLAB Simulink and experimental tests.

Abstract: This paper proposes a core loss model for filter inductors of power electronic converters. The model allows a computationally efficient analysis on the core loss of the inductor under the square voltage excitation and the premagnetization condition. First, the core loss of the filter inductor under buck chopper excitation is evaluated with the proposed model and compared with the conventional methods. The comparison shows that the proposed method results in a better core loss prediction under the premagnetized condition than that of conventional alternatives. Then, the core loss of the filter inductor with the pulsewidth modulated inverter excitation is evaluated, which shows that the proposed model not only accurately predicts the core loss but also identifies the hysteresis loss part. These results demonstrate that the approach can further be used for the development of magnetic materials for power electronic applications.

Abstract: This paper proposes a modified one cycle control (OCC) for active power filter (APF) to compensate only harmonic components of load currents. The conventional OCC-based APF cannot differentiate between the harmonic and reactive components of load currents, which leads to the increment in device current rating because of compensation of large reactive currents besides harmonics. The reference signals of the OCC control core have been reconstructed and the associated control equations have been developed, following the control philosophy of simplicity. The modified scheme is easy to implement and only three more sensors are required for measuring load currents compared with the conventional OCC. This modified OCC guarantees the excellent performance of harmonic compensation alone and retains the advantages of the conventional OCC, such as no phase locking loop and the constant switching frequency. The feasibility and performance of the proposed OCC has been validated by experimental studies on a 5 kVA APF hardware platform developed in laboratory.

Abstract: In this work, a novel technique based on adaptive filtering is proposed for the control of a three-phase universal active power filter with a solar photovoltaic array integrated into its dc bus. Two adaptive filters along with a zero crossing detection technique are used to extract the magnitude of a fundamental active component of distorted load currents, which is then used in estimation of a reference signal for the shunt active filter. This technique enables extraction of an active component of all three phases with reduced mathematical computation. The series active filter control is based on synchronous reference frame theory and it regulates load voltage and maintains it in-phase with voltage at point of common coupling under conditions of voltage sag and swell. The performance of the system is evaluated on an experimental prototype in the laboratory under various dynamic conditions such as sag and swell in voltage at point of common coupling, load unbalancing, and change in solar irradiation intensity.

Abstract: This paper proposes a bidirectional Level 3 (fast) power level Electric Vehicle (EV) charging system where the control strategy consist of two cascaded stages At first stage, the control strategy allows the control structure to utilize a Three- Level (TL) Neutral Point Clamped (NPC) multilevel converter ensuring a unity power factor correction (PFC) by absorbing/injecting a sinusoidal current from/to the grid with low current harmonics At second stage, two dc-side capacitors of the TL NPC on the dc-bus voltage are connected to two Dual Active Bridge (DAB) DC/DC converters paralleled at the battery This stage regulates the current that is delivered to or from the battery according to its State of Charge (SOC) and matches the difference between the dc-link and battery voltages The purpose of the proposed strategy is to deal with higher power levels with reduced stress on components In addition to bidirectional power exchange, active power filter capability is also added to the control strategy for enhancing power quality in presence of reactive, non-linear and/or unbalanced loads at point of common coupling (PCC) based on the Conservative Power Theory (CPT) Simulation results are presented to demonstrate the validity of the proposed charger system offering very flexible, and selective functionalities

Abstract: In this paper, the active differential-mode (DM) electromagnetic interference (EMI) filter topology for ac/dc converters is first briefly investigated. The active DM EMI filter model was developed based on the models of filter components including a current transformer, a high-pass filter, an operational amplifier, and a class AB amplifier. The system model including the active filter, ac/dc converter, and line impedance stabilization networks (LISNs) are explored. With the developed system model, the loop gain and insertion gain of the active filter are derived. Based on the loop gain, the stability of the active filter is investigated and the compensation is applied to achieve stability and good EMI reduction. Both simulations and experiments are conducted to validate the developed technique.

Abstract: A new method for the control of a battery energy storage system and its implementation on a 25 kW solar system to compensate solar power intermittency and improve distribution grid power quality is presented in this paper The novelty of the proposed method is to provide a systematic way to optimize the size of the battery capacity for the desired level of solar power smoothing This goal is achieved by designing a two-stage filter solution The first stage is a fast response digital finite impulse response (FIR) filter that makes a trade-off between smoothing of the solar output and battery capacity This paper proposes an optimal design of a minimum-length, low-group-delay FIR filter by employing convex optimization, discrete signal processing, and polynomial stabilization techniques The new strategy proposed in this paper formulates the design of a length- $N$ low-group-delay FIR filter as a convex second-order cone programming, which guarantees that all the filter zeros are inside the unit circle (minimum-phase) A quasi-convex optimization problem is formulated to minimize the length of the low-group-delay FIR filter The second-stage filter is designed to level the battery charging load The effectiveness and performance of the proposed approach is demonstrated by simulation results and also over a real-case implementation

Abstract: Two types of filter media in groundwater treatment were conducted for a comparative study of surface structure and catalytic performance. Natural filter media was adopted from a conventional aeration-filtration groundwater treatment plant, and active filter media as a novel and promising filter media was also adopted. The physicochemical properties of these two kinds of filter media were characterized using numerous analytical techniques, such as X-Ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS) and Zeta potential. The catalytic activities of these filter media were evaluated for ammonium and manganese oxidation. XRD data showed that both active filter media and natural filter media belonged to birnessite family. A new manganese dioxide (MnO2) phase (PDF#72-1982) was found in the structure of natural filter media. The SEM micrograph of natural filter media showed honeycomb structures and the active filter media presented plate structures and consisted of stacked particle. These natural filter media presented lower level of some trace elements such as calcium and magnesium, lower degree of crystallinity, lower Mn(III) content and lattice oxygen content than that of active filter media, which were associated with its poor ammonium and manganese catalytic activities. In addition, some γ-Fe2O3 and MnCO3 were found in the coating which may hinder the ammonium and manganese catalytic oxidation. This study provides a thorough and comprehensive understanding about the most commonly used filter media in water treatment, which can provide a theoretical guide to practical applications.

Abstract: This paper proposes an improved modulated carrier control with on-time doubler for the single-phase shunt active power filter, which eliminates harmonic and reactive currents drawn by nonlinear loads. This control method directly shapes the line current to be sinusoidal and in phase with the grid voltage by generating a modulated carrier signal with a resettable integrator, comparing the carrier signal to the average line current and making duty ratio doubled. Since the line current compared to the carrier signal is not the peak, but the average value, dc-offset appeared at the conventional control methods based on one-cycle control is effectively addressed. The proposed control technique extirpates the harmonic and reactive currents and solves the dc-offset problem. The operation principle and stability characteristic of the single-phase shunt active power filter with the proposed control method are discussed, and experimental results with laboratory prototype under various load conditions verify its performance.

Abstract: In a variable switching frequency control scheme, it is challenging to design the voltage source inverter (VSI) parameters without knowing the maximum and minimum switching frequencies. The switching frequency variation depends on the system parameters. Therefore, the expression for frequency variation in one inverter topology does not hold in other topologies. The design of hysteresis current controlled four-leg shunt active power filter (APF) requires the switching dynamics analysis to ensure satisfactory performance of the inverter. In this study, a detailed analysis on the switching dynamics of hysteresis current controlled four-leg VSI and design of various inverter components for compensating unbalanced and non-linear loads are presented. The results obtained from these analyses can be used in any applications involving hysteresis current controlled four-leg VSI. These results are supported by detailed simulation studies conducted on a three-phase four-leg shunt APF system using Matlab/Simulink. The theoretical studies and design of passive components are also verified by conducting experiments on a prototype of four-leg shunt APF developed in the laboratory.

Abstract: A proposal of a fractional (1+α)-order low-pass filter is presented in this paper. The proposed filter operates in the current-mode and it is designed using Multi-Output Current Followers (MO-CFs), Dual-Output Current Follower (DO-CF), Dual-Output Adjustable Current Amplifier (DO-ACA) and Adjustable Current Amplifiers (ACAs) as active elements within the presented topology of the filter. The filter possesses ability to electronically control its order and also the pole frequency by changing the current gain of current amplifiers (ACAs) already present in the structure. Three different values of the order and pole frequency of the proposed low-pass filter were tested as an example. Design of the proposed filter is supported by simulation and experimental results. Simulations of the circuit are carried out in PSPICE simulator with behavioral models of used active elements. The experimental laboratory measurements are performed with the help of available devices forming equivalent circuits. Simulations and experimental results of the electronical control of the order and pole frequency are compared in this contribution.

Abstract: Presently, the neutral-point potential offset is an inherent problem for three-level converters on three-phase four-wire active power filters. The conventional three-dimensional space vector pulse width modulation (3-D-SVPWM) strategy is adopted as a normal solution to the problem, but the selection of space vector sectors for four-wire converter is both abstract and complicated in practice. This paper proposes a new PWM strategy accordingly to simplify the selection process and the calculation of duration. Compared to the conventional 3D-SVPWM, the proposed method locates the sector of optimal vector directly with the polarity of three-phase reference voltage. Then, by applying voltage–second balance rule, it can perform a fast calculation of the duration and generate PWM signals to control the switching devices’ on/off. As an approach to correct the imbalance dc-link capacitor voltage, this strategy reconstructs the switching sequence. A portion of state O duration is disassembled into states P and N to suppress the neutral-point potential offset. The proposed strategy is validated with a range of experiments. The results indicate that this simplified PWM strategy can effectively reduce the computational burden, and also the dc-link capacitor voltage is well controlled.

Abstract: Revolutionary design power architectures into electric aircraft system are being actively discussed to cope with the issues of a present state-of-the-art technology. The two demanding objectives in more-electric-aircraft (MEA) concept are power quality enhancement and increased level of redundancy despite of higher on-board power supply requirement throughout aircraft operation. In addressing the open challenges, this study outlines an operational characteristics of a three-phase multilevel shunt active filter (SAF) deployed for distribution power system in emerging MEA. The introduced shunt active filter is based on modular multilevel converter configuration for harmonic-current compensation purposes. On this foundation, a current control strategy for SAF is designed with a finite-control-set model predictive control technique. The developed solution provides several merits into aircraft electrical network as easy implementation on embedded platforms, controllable feature with a wide range of current frequencies, producing exact current harmonic replica onto the supply side over a given prediction horizon, and modular design with redundancy functionality. Dedicated predictive control system helps to better execution-time efficiency together with low sensitivity over the constraints imposed upon the optimization formulation through an integrated perturbation analysis and sequential quadratic programming solver. Simulation and experimental verifications for the three-phase four-level shunt active power filter are discussed, where robust behavior of handling harmonic currents with respect to the supply impedance and fundamental frequency variations is achieved, when the harmonic distortion levels fulfill the IEEE Standard 519 recommendations.

Abstract: The output currents of high-power voltage source inverters (VSIs) are distorted by the switching harmonics and the background harmonics in the grid voltage. This paper presents an active harmonic filtering scheme for high-power, low-frequency switching VSIs with an additional auxiliary VSI. In the approach, the auxiliary low-power, high-frequency switching VSI is used to compensate both the switching harmonics of the high-power VSI and the low-order harmonics introduced by the distorted grid voltage. Impedance modeling and analysis are implemented for the hybrid VSI system, where the impacts of the grid impedance are analyzed. Different control targets are then formulated based on the analysis, and the corresponding control strategies are proposed. Last, simulations and experimental results are provided to validate the theoretical analysis and the performance of the proposed control strategy.

Abstract: Modular multilevel cascaded converter (MMCC) is a promising technique for medium/high-voltage high-power photovoltaic systems due to its modularity, scalability, and capability of distributed maximum power point tracking (MPPT) etc. However, distributed MPPT under module-mismatch might polarize the distribution of ac output voltages as well as the dc-link voltages among the modules, distort grid currents, and even cause system instability. For the better acceptance in practical applications, such issues need to be well addressed. Based on mismatch degree that is defined to consider both active power distribution and maximum modulation index, this paper presents an efficient modulation strategy for a cascaded-H-bridge-based MMCC under module mismatch. It can operate in loss-reducing mode or range-extending mode. By properly switching between the two modes, performance indices such as system efficiency, grid current quality, and balance of dc voltages, can be well coordinated. In this way, the MMCC system can maintain high-performance over a wide range of operating conditions. Effectiveness of the proposed modulation strategy is proved with experiments.

Abstract: Uninterruptible power system, photovoltaics inverters, home appliances, motor drives, and automotive systems require highly efficient, reliable, and compact single-phase inverters. This paper presents different industrial design approach and experimental performance of high power density (100 W/in3) single-phase inverter with power output of 2 kVA that was presented for Google Little Box challenge. Multiple technical challenges were addressed, different methodologies described to achieve high power density and high efficiency reliable design. High power density was achieved by using the combination of optimized forced air cooled heatsinks, fully digital control, optimized component packaging, and by the introduction of a series active filter that enabled to reduce the size of dc bus capacitors. Low-loss high flux density magnetic materials such as Amoflux powder iron for power inductors and nanocrystalline magnetic materials for electromagnetic interference filter were also used. To minimize inverter switching losses, SiC devices operating at low switching frequency were adopted. Also, an innovative ground current control strategy is devised to minimize the ground leakage currents.

Abstract: For railway traction drives, the active front end usually adopts a single-phase rectifier. However, the dc-link voltage of this single-phase rectifier contains a second-order fluctuating component due to the fluctuation of the instantaneous power at both the ac and dc sides. Fed by the fluctuating dc-link voltage, the traction motor suffers from severe torque and current pulsation. The hardware solution with an additional LC resonant filter is simple, but it will reduce the power density of the system. An alternative solution is to eliminate the beat component in the stator voltage/current through modulation ratio or frequency compensation. However, it is difficult to achieve high performance for the conventional feedforward method. In this paper, an improved closed-loop torque and current pulsation suppression method is proposed, which can eliminate the q -axis current pulsation component in the field-oriented control system through frequency compensation. The torque pulsation suppression is also achieved automatically. Simulation and experimental results show that the proposed scheme can effectively reduce the torque and current pulsation in various operation modes compared with the conventional feedforward method.