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In photovoltaic systems, common-mode voltage (CMV) generates leakage current, which shortens the insulation lifespan and causes safety problems. As for neutral-point-clamped (NPC) inverters, zero CMV PWM (ZCMV PWM) can be implemented to eliminate leakage current, but unable to balance neutral-point (NP) voltage in dc side. This article presents a novel modulation technique for NPC PV inverter. The strategy contains ZCMV PWM and another two modulation modes which keeps CMV constant in one switching period, so as to eliminate leakage current. In order to balance NP voltage, NP current is adjusted by selection of optimal modulation mode and the leakage current is still well eliminated during the process of balancing NP voltage. Compared with conventional ZCMV PWM, the proposed scheme can remove both leakage current and NP voltage deviation. The output performance and limitations of the proposed method are also introduced in the paper. Experiments are provided to verify the validity of the proposed method. [ABSTRACT FROM AUTHOR]

The mechanical parameters, including the Coulomb and viscous friction coefficients together with the moment of inertia, are of great interest for the performance optimization and condition evaluation of surface-mounted permanent magnet synchronous motor (SPMSM) drives. This article contributes a speed-controller-independent mechanical parameter identification scheme using open-loop speed response due to signal injection, which has not been reported in existing works. It is designed based on the premise that the viscous friction torque is disproportional to the rotor speed, which is a new progress toward estimating the mechanical parameters with high precision. Moreover, as a practical full-participation-type solution, this scheme involves a new electrical parameter estimation technique based on differential algebra to assist in identifying the mechanical parameters. This auxiliary technique attains the current-controller-free estimation of the electrical parameters with the inverter nonlinearity eliminated, thereby addressing the current controller self-commissioning and serving to acquire the accurate electromagnetic torque. The experimental evaluations on two prototype SPMSM drive systems validate the feasibility of this suggested scheme. [ABSTRACT FROM AUTHOR]

Due to different solar radiation, temperature, and other reasons of modules in the three-phase cascaded H-bridge (CHB) photovoltaic (PV) inverter, the output power among PV modules will be unequal and lead to unbalanced grid currents, which cannot meet the requirements of grid codes. On the other hand, the second-order voltage ripple is aroused in the dc-link capacitor since each phase-leg of the CHB inverter is made up of a single-phase inverter. Concerning these two issues, this paper proposes an optimized third harmonic injection method based on fundamental frequency zero sequence injection (FFZSI), which is a conventional method to handle the slight power imbalance problem. In this paper, the FFZSI is adopted to redistribute the power among three phases, and then the third harmonic is injected into the three-phase combined reference waveforms according to the two requirements: all modules are free from overmodulation and the dc-link voltage fluctuation is mostly reduced. Therefore, the proposed method can reduce the dc-link voltage fluctuation and extend the power balance range of FFZSI at the same time. The validity and effectiveness of the proposed method are verified by simulation and experimental results. [ABSTRACT FROM AUTHOR]

This paper proposes a family of single-stage converters consisting of three different converters, each of which is able to produce a variety of waveforms through the waveform-subtraction technique. We brand this converter family as waveform-subtraction based single-stage converter (WSSC). By using only a single-stage converter, the proposed waveform generation technique is able to realize dc–dc–ac conversions and suppress input current ripple, meeting the needs of modernized power grid with renewable energy sources. At the same time, converters in this WSSC family are capable of generating a range of shapes, such as triangular, rectangular, and sawtooth waveforms, with simple structures, demonstrating its suitability of being applied in the electrochemical industry, e.g., in the electroplating process. Furthermore, the proposed family converters also realize substantially better input current ripple suppression compared to other conventional converters. In order to prove the validity and efficacy of the proposed WSSC and the theory underpinning it, in this paper, extensive simulations and analyses are conducted to verify the theoretical foundation of the proposed WSSC strategy, and the prototype of a converter from the proposed converter family is built and tested in our laboratory, which validates the functionality of the WSSC. [ABSTRACT FROM AUTHOR]

In order to suppress the torque ripple of an open-end winding permanent-magnet synchronous machine (OEW-PMSM), a q-axis current injection method is typically employed but the disturbances caused by the third flux linkage parameter mismatch can augment the torque ripple. To solve this problem, in this paper, first, a zero-sequence current (ZSC) and zero-sequence back-EMF observer (ZCBO) are proposed based on an adaptive sliding mode control, which is able to simultaneously estimate ZSC and zero-sequence back EMF under varying conditions. Based on this ZCBO, the estimated ZSC and zero-sequence back EMF could compensate for the one-step control delay and suppress the aforementioned disturbances. Second, although central hexagon modulation is employed in the predictive current control (PCC) scheme, zero-sequence voltage (ZSV) still exists in the zero-sequence path due to the dead time of the inverter, affecting the predictive accuracy of the ZCBO. To accurately establish the ZCBO, the ZSV caused by the dead time of the inverter is considered in the ZCBO. Finally, a comparative study of two types of methods is presented, and simulations established by MATLAB software and experimental confirmations are carried out to verify the effectiveness of the proposed PCC scheme in this paper. [ABSTRACT FROM AUTHOR]

This paper presents an optimization method for selective harmonic elimination in a cascaded multilevel inverter using teaching–learning-based optimization (TLBO). The main objective in selective harmonic elimination (SHE) strategy is to eliminate low-order harmonics by solving nonlinear equations and reaching optimal solution, while the fundamental component is satisfied. In this paper, in one side, the influence of optimal dc sources is investigated to reach the SHE goal, and in another side, comparing optimization methods is considered. In this paper, the TLBO as a recently emerged nature-inspired algorithm is presented to provide better results for the SHE in comparison with genetic algorithm, artificial bee colony, imperialistic competitive algorithm, harmony search, ant colony optimization, particle swarm optimization, and differential evolution. For better comparison of those methods and influence of optimal dc sources, 5-, 9-, and 15-level inverters are chosen and MATLAB software is used for optimization. Simulation results show the superiority of TLBO, higher precision and probability of convergence than other mentioned algorithms. Finally, to validate the influence of optimal dc sources and the accuracy of TLBO results, the experimental setup is conducted for a 5-level cascaded H-bridge inverter with optimal dc sources. [ABSTRACT FROM AUTHOR]

This paper describes the tuning process of the proportional-resonant controller, taking into account the significant computational delay from the digital control system. Different structures of the controller and related contradicting results are discussed. Particular attention is paid to the stability domain and its dependence on different parameters of the proportional-resonant controller. The main outcome of this paper consists in the tuning approach guidelines for selection of controller parameters in case of significant digital system delay. An optimal controller structure and start-up current optimization are proposed. All results are confirmed by simulation and experimental setup. [ABSTRACT FROM AUTHOR]

This paper presents the development of a Class-D full-bridge zero-voltage switching (ZVS) inverter, applicable to wireless power transfer (WPT) systems, operating at 13.56 MHz switching frequency with dynamic dead-time control (DDTC). Resonant-coupled WPT systems are being designed at ultrahigh switching frequencies to reduce the size of the wireless link and the passive components. Maintaining ZVS while controlling the output power delivered to a fixed or variable load is one of the major challenges of designing inverters at multi-MHz switching frequencies. DDTC is the approach deployed in this paper to sustain soft switching of a Class-D full-bridge inverter over the full range of output power while regulating the input dc bus voltage. Simulation results are presented to show that dynamically controlling the dead-time during input dc bus voltage variations reduces switch-node voltage overshoot, prevents large current spikes in the switching devices, and reduces associated high switching loss. Practical results obtained show that DDTC reduces switch-node voltage overshoot, increases the inverter efficiency, and reduces the steady-state temperature of the inverter during output power regulation. [ABSTRACT FROM AUTHOR]

In this paper, a new modular and active snubber cell, easily applicable, and attractive for converters, especially have many switches proposed. This snubber cell is implemented on the single phase of a grid-connected three-phase T-type three-level inverter (T-3LI). In this new converter, the main switches turn-on under zero-voltage transition and turn-off under zero-voltage switching (ZVS). Also, auxiliary switches turn-on under zero-current switching and turn-off under ZVS. All snubber diodes operate under soft switching (SS). There are no additional current and voltage stresses on the main switches. During an SS operation, the switching energies are transferred to a dc voltage source. Then, these energies are processed to input with an independent dc–dc converter. Thus, all switching energies are recycled. In this paper, the principle operation and steady-state analysis of the modular and active snubber cell for a single-phase T-3LI are presented and experimental results, rated 1 kW and 100 kHz, are provided to verify theoretical analysis. [ABSTRACT FROM AUTHOR]

This paper improves the position sensorless drive by sensing actual machine phase voltages. A high-bandwidth phase voltage measurement is developed for pulsewidth modulation (PWM) voltage inverters. On the basis, the actual phase voltage is obtained based on the digital integration of PWM voltage using the capture modulator in existing drive microcontrollers (MCU's). Comparing to existing phase voltage measurement, no separated A/D converter and communication hardware are required because PWM pulses are directly measured using MCUs. However, for standard machines without neutral points, only line-to-line ac PWM voltages can be measured for the phase voltage reconstruction. Since the capture based on transistor-transistor logic (TTL) logics receives only digital signals, a preprocess circuit to convert ac PWM line voltages to equivalent digital signals is proposed. This paper clearly explains the voltage sensing hardware using MCU capture modulator. According to experimental results, a 150-MHz sampling rate for phase voltage measurement is achieved based on the proposed capture-based voltage measurement. Although the physical limitation of back electromotive force estimation still appears, the proposed phase voltage measurement substantially enhances the sensorless drive performance at low speed. [ABSTRACT FROM AUTHOR]

A space vector modulation (SVM) technique of a dual-inverter system for an open-end winding motor drive is described in this paper, where one inverter has a battery power source and the other has an only capacitor across the dc bus. The SVM must be achieved to operate the motor with field-oriented control and simultaneously to control the capacitor voltage at a constant value by using redundant switching states of the dual-inverter system. The control of the capacitor voltage is carried out by selecting a charging or a discharging mode in each redundant switching state, taking the instantaneous motor power factor into account. In addition, it is also required to reduce the error voltage pulses, which are generated in output multilevel voltage waveforms during the dead time. The compensation method of the existing dead-time scheme and the improved SVM sequence to reduce the error voltage vectors are proposed in this paper. The proposed methods are examined through several experimental tests and are confirmed to generate superior output voltage waveforms from the viewpoint of the measured total harmonic distortion and dv/dt. [ABSTRACT FROM AUTHOR]

In this paper, a new family of impedance source inverters is presented. It employs high-frequency electrical isolation between the inverter bridge switches and the load along with voltage clamping across the dc-link voltage. Conventional Z-source inverters (ZSIs) employs an impedance network that consists of inductors and capacitors. It has unique features that realize both step-up/step-down functions and eliminates the need of dead/overlap times. This paper extends this novel concept by using electrical isolation in impedance source inverters. High-frequency isolation has many advantages in terms of immunity and reliability; when applied with impedance source inverters this makes ZSIs a preferable choice for industrial applications. In photo-voltaic systems, the addition of the high-frequency transformer provides safety by avoiding the injection of dc circulating current into the grid, without the need of an external bulky line frequency transformer. The gain of the proposed inverter design can be accurately selected by choosing the turns ratio of the high frequency transformer (HFT) or by adjusting the shoot-through duty cycle (STDC) to the inverter. This allows for greater freedom especially when utilizing a higher modulation index, with the STDC allowing dynamic gain adjusts to be done speedily during operation of the inverter. Additionally, a dc-rail voltage clamping technique for the proposed class of isolated ZSIs is also discussed. This technique provides benefits not only in improving the output voltage quality, but also in reducing voltage stress of the active and passive components by minimizing the voltage spikes across the switching devices. In this paper, several high-frequency isolated ZSIs are presented, and an example isolated improved ΓZSI design is shown and discussed in detail. Simulations are provided for the proposed class of isolated inverters to verify their working. Further experimental investigation has been done for which results for the isolated improved ΓZSI are reported. These empirical results have largely confirmed the expected benefits that were determined through simulation and accurate model-based testing. [ABSTRACT FROM AUTHOR]

This paper proposes an “isolated soft switching current fed LCC-T resonant dc–dc converter for PV/fuel cell applications.” This converter is able to achieve zero voltage switching for front-end inverter switches and zero-current switching for voltage doubler diodes. The majority of dc–dc converters have compact capacitive (C) filter in output, but high harmonic currents through transformer makes them bulky. The proposed converter attempts to improve harmonic content through a transformer under wide load and input variations as well as it can significantly reduce required turns ratio of transformer to achieve required output voltage. For the above reasons, the proposed converter can offer better footprints and therefore it can be a potential candidate for PV/fuel cell applications. This paper presents basic operating principal, detailed analysis, control, and design of the proposed converter with the help of an example. This converter operates in two modes, i.e., constant duty cycle mode and constant frequency mode, depending on load demand, and is explained through operating curves. Finally, experimental results are included for justifying the proposed theory. [ABSTRACT FROM AUTHOR]

Reduced switch active phase converters (APC) are cost-effective solutions for running three-phase loads from single-phase grids. This paper proposes a novel auxiliary-capacitor-based APC (AC-APC) for this application. The combination of the topological location of the auxiliary capacitor and the control of the AC-APC reduces current stress on the semiconductor devices. In this paper, a selection criterion for the optimal auxiliary capacitor value is formulated to reduce the device currents in such a way that the overall efficiency is enhanced. This paper presents a comparative analysis of converter volt-ampere (VA) rating and device power loss for conventional H-bridge active front-end and inverter-based topology, APC, and AC-APC. To evaluate the efficacy of the proposed AC-APC, an experimental study is conducted on the APC and the AC-APC feeding a 3-hp three-phase induction motor. At a full load of $\text{2.5}\,\text{kW}$ and $\text{0.8 } \text{power factor}$ , the measured maximum efficiency of the AC-APC is $2\%$ higher than that of the APC. Analytical evaluation of converter currents and device power loss are in close agreement with experimental observations. [ABSTRACT FROM AUTHOR]

The second harmonic current compensator (SHCC) can be added into the single-phase converters for compensating the second harmonic current (SHC) and thus removing the undesired electrolytic capacitor. In this paper, a virtual impedance, which is in the form of a capacitor and a resistor connected in parallel, is introduced to be in parallel at the bus-side port of the SHCC for effectively compensating the SHC while guaranteeing the system stability. This virtual parallel impedance is realized by feeding forward the bus-side port voltage of the SHCC, and thus the SHCC can be modular designed. The closed-loop parameter design of the SHCC is also presented. A 3.3-kW prototype of a two-stage single-phase power factor correction converter is built and tested in the laboratory, and the experimental results verify the effectiveness and feasibility of the proposed control method. [ABSTRACT FROM AUTHOR]

This paper proposes a modular on-road wireless power transfer system with interoperable power adjustment mechanism. The paper is anticipated to enhance the capability of on-road charging during the movement of automated guided vehicles (AGV), by which the traveling mileage is increased, while the battery volume is decreased. The system design includes an interoperable power adjustment technique based on the detected impedance, and the moving position of AGV can be, hence, comprehended so as to facilitate the adjustment of the output power from each transmission module in a more flexible way. Through the investigation of on-road charging efficiency, all of coil magnetic analysis, misalignment charging evaluation, system stress simulation, and resonant characteristics investigation are performed. Experimental results gained from software simulation and hardware realization are beneficial for AGV charging applications. [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]

Soft-switching techniques of transformerless photovoltaic grid-connected inverters (TLIs) can significantly reduce switching losses, as well as soften switching processes. Conventional dc–ac soft-switching configurations proposed by Dr. Deepakraj M. Divan are invalid in TLIs because of the leakage current problem. In order to develop soft-switching techniques in TLIs, this paper proposes a new soft-switching configuration and a procedure to guide the invention of soft-switching TLIs. First, this paper proposes two basic resonance tanks related to dc bus polarities, and then, uses these basic tanks to elevate four popular full-bridge-type TLIs according to the proposed guideline. As a result, four soft-switching TLIs are gained. Second, this paper picks obtained soft-switching highly efficient and reliable inverter concept (HERIC) as example to analyze its soft-switching operation principle and performance. As a consequence, all active switches of the gained soft-switching HERIC circuit are switched under both of zero-current turn-on and turn-off conditions; the reverse recovery problem of freewheeling diodes is alleviated owing to the zero-current turn-off of diodes; meanwhile, the common-mode voltage at the switching frequency scale is still constant. Finally, some experimental results from a 3-kW universal prototype at 50-kHz switching frequency are provided to verify the effectiveness of the main contributions of this paper. [ABSTRACT FROM AUTHOR]

Inverter nonlinearity and sampling current dc bias can cause harmonic distortion of estimated back electromotive forces (EMFs), and therefore results in position estimation error and degrades control performance of model-based sensorless techniques of permanent-magnet synchronous motors, especially in low speed region. In this paper, the effects of inverter nonlinearity and sampling current dc bias on back EMF estimation are analyzed. A voltage feedforward compensation method based on an iterative learning control (ILC) algorithm is proposed and it is used to eliminate the harmonics of estimated back EMFs and position estimation error, so as to improve the performance of sensorless control. Compensated voltages at different rotor positions are calculated online by the ILC algorithm and stored in a lookup table. The method does not depend on current polarity and inverter model. And it is simple and easy to implement. The effectiveness of the proposed compensation method is verified by experiments. [ABSTRACT FROM AUTHOR]

Dual-inverter configuration has been attracting increasing attention, especially in motor drives, for its advantages in fault tolerance, multilevel modulation effect, dc voltage utilization, etc. However, its switch fault diagnosis is difficult due to the symmetry of the two sub-inverters. A signal processing approach is proposed in this paper to detect the switch open-circuit fault occurrence and further identify the specific faulty switch, benefiting maintenance and fault-tolerant operation. In this scheme, the faulty phase and the faulty switch pair, consisting of the faulty switch and the switch on its diagonal, are identified using the stator phase currents average values. Following that, the bridge of the faulty phase is forced into a unipolar mode by artificially turning off one switch in the healthy pair continuously. Through properly controlling the duty ratios of the other switches in this bridge, the faulty switch is allowed to be further identified from the faulty pair by analyzing the behavior of the induced current in this phase. This approach is independent of either the speed or the voltage information. The effectiveness is validated through experiments on a sensorless controlled open-end winding induction motor drive. [ABSTRACT FROM AUTHOR]

This paper presents a novel voltage regulator design methodology for a three-phase voltage source inverter with uncertain load dynamics. Different from existing literature, the load is treated as a dynamic uncertainty and a robust optimal state-feedback controller is proposed through the integration of optimal output regulation theory and backstepping method. To avoid sensing the inductor current, an output-feedback control scheme is developed as well. Stability of the inverter–load system is rigorously analyzed via the small-gain techniques. It is ensured that the tracking error asymptotically converges to zero through both the proposed state-feedback and output-feedback controllers. A standard proportional resonant controller is also designed for the purpose of comparison. It is shown that the proposed controller has inherent robustness and does not require retuning with different applications. The controller design method is generalized for grid-tie applications, since the grid can be regarded as a stable load system. Simulations and experimental results show the effectiveness of the control approaches. [ABSTRACT FROM AUTHOR]

This paper proposes an effective sliding mode controller (SMC) for a grid-connected 7-level packed U-cell (PUC7) inverter. The aim is to design a simple controller that deals effectively with the complex control problem of the PUC7 inverter (multiobjective control problem). The selection of the control actions is achieved according to the system state error at every sampling time, regardless of the previous values, which makes the control technique model-independent. The control algorithm evaluates online two cost functions (one for each state error), which are derived on the basis of sliding mode theory, and it selects the optimal control input in order to satisfy the reaching conditions of the two cost functions. Compared with the existing solutions, the proposed SMC technique ensures lower average switching frequency by tuning the hysteresis bandwidth of the capacitor-voltage error. The fast implementation, needless of gains tuning, and simple design procedure are the main features of the proposed algorithm. Simulation and experimental results are presented to prove the effectiveness of the proposed technique in controlling the PUC7 inverter with high dynamic performance and robustness against disturbances and parameters mismatch. [ABSTRACT FROM AUTHOR]

Unplanned islanding is intended to be identified due to the adverse effects on personnel and electrical equipment. In this paper, an active islanding identification technique is proposed based on disturbance injection through q-axis controller and an investigation technique, which incorporates measurement of mean of absolute frequency variation of the voltage signal passed through band-pass filter. The proposed technique is proven to be robust, offering decentralized islanding identification and being applicable to dynamic loading scenario, identifies island formation with zero nondetection zone for all the loads of quality factor $ (Q_{f}) \leq$ 2.5, and does not destabilize the system on postislanding. The islanding identification time is $ < $ 180 ms for all the loads of $ Q_{f} \leq$ 2.5, which is applicable to various international standards. The MATLAB2017b/Simulink platform is utilized to validate the efficacy of the proposed methodology. [ABSTRACT FROM AUTHOR]

This paper proposes a new multilevel inverter (MLI) topology that utilizes trinary sequence for the dc sources. It gives maximum output voltage level with minimum dc source and switch count when compared to other sequences, such as symmetric, natural, binary, and quasi-linear. This is due to the fact that the trinary sequence generates of all additive and subtractive combinations of input dc levels in the output voltage waveform. The concept is implemented on a 9-level asymmetric MLI using only four active devices. Multicarrier unipolar pulsewidth modulation technique is adopted to create the switching pulses. Theoretical calculation of total harmonic distortion in both voltage and current waveforms has been performed using asymptotic time domain formula. These values are compared with simulation and experimental values for different modulation indices. Power loss calculation for proposed topology is discussed with appropriate mathematical equations. [ABSTRACT FROM AUTHOR]

This paper presents an improved rotating restart method for a sensorless permanent magnet synchronous motor (PMSM) drive system using repetitive zero voltage vectors. To restart the sensorless PMSM drive system, a method for estimating initial rotor position and speed is required due to the absence of sensors such as encoders and resolvers. The rotor position and speed can be estimated using the short-circuit current vectors generated by applying zero voltage vectors. However, the estimated rotor position and speed can be inaccurate. The inaccurate rotor position that causes the current is distorted by the back-electromotive force of the PMSM, and it is impossible to precisely control the system. Therefore, an improved rotating restart method for the sensorless PMSM drive system using repetitive zero voltage vectors is proposed. The effectiveness of the proposed method is verified through simulation and experimental results using a 5-kW PMSM drive system. [ABSTRACT FROM AUTHOR]

This paper proposes a new robust field-weakening approach for reluctance synchronous motors regulated by direct torque and flux control. Compared to the existing direct torque and flux control based field-weakening methods that cannot achieve maximized DC-link voltage utilization and are parameter dependent, the proposed approach contributes to improve the field-weakening performance of reluctance synchronous motor in two aspects. First, it extends the constant power speed range through autonomous stator flux reference adjustment, which maximizes the dc-link voltage utilization. Smooth transition between the maximum torque per ampere trajectory and field-weakening trajectory is also realized. Second, it enhances the parameter robustness of drives in very high-speed region by employing a torque reference adjustment scheme. This effectively avoids the instability of drives caused by machine parameter variations. The proposed approach is verified experimentally on a laboratory setup. [ABSTRACT FROM AUTHOR]

The control delay weakens the active damping (AD) performance of digitally controlled LCL/LLCL-type grid-connected inverters (GCIs). The equivalent virtual impedance paralleled with the filter branch explains the effect of the capacitor-current-feedback AD. However, the virtual impedance appears capacitive or even negative due to the delay, which significantly weakens the effectiveness of AD. This paper proposes a single-cycle-lag compensator (SCLC) to eliminate the adverse effects caused by the control delay. The method retains effectiveness in applications with high resonant frequency or large control delay. It keeps system a positive equivalent resistance with a high equivalent inductance. The positive equivalent resistance leads to positive damping effect, while the high equivalent inductance causing little resonant frequency deviation can be neglected. Moreover, the effect of the impedance variation that causes the resonant frequency deviation is analyzed and the restrictions of the proposed method are discussed. By optimizing the LCL/LLCL filter parameters, the adaptability of the SCLC to the grid impedance variation can be further improved. With the SCLC, the LCL/LLCL-type GCIs achieve better steady-state and dynamic performances. Finally, experimental results are presented to verify the proposed method. [ABSTRACT FROM AUTHOR]

Laminated busbars connect capacitors with switching power modules, and they are designed to have low stray inductance to minimize electromagnetic interference. Attempts to accurately measure the stray inductance of these busbars have not been successful. The challenge lies with the capacitors, as they excite the busbar producing their individual stray inductances. These individual stray inductances cannot be arithmetically averaged to establish the total stray inductance that applies when all the capacitors excite the busbar at the same time. It is also not possible to measure the stray inductance by simultaneous excitation of each capacitor port using impedance analyzers. This paper presents a solution to the above dilemma. A vector synthesis method is proposed, whereby the individual stray inductance from each capacitor port is measured using an impedance analyzer. Each stray inductance is then mapped into an xyz frame with a distinct direction. This mapping exercise allows the data to be vectored. The total stray inductance is then the sum of all the vectors. The effectiveness of the proposed method is demonstrated on a busbar designed for H-bridge inverters by comparing the simulation and practical results. The absolute error of the total stray inductance between the simulation and the proposed method is 0.48 nH. The proposed method improves the accuracy by 14.9% compared to the conventional technique in measuring stray inductances. [ABSTRACT FROM AUTHOR]

This paper briefly reviews the common-mode voltage (CMV) and neutral point (NP) voltage for neutral point clamped three-level inverter (NPC TLI). Then, the space vector pulsewidth modulation (SVPWM) and traditional virtual SVPWM (VSVPWM) are discussed in terms of these two issues, revealing the drawbacks in reducing CMV or eliminating NP voltage oscillation. A novel virtual space vector modulation, named as RCMV_VSVPWM, is proposed in this paper to reduce CMV and eliminate NP voltage oscillation for NPC TLI. By selecting vectors with lower CMVs, a set of novel virtual voltage vectors are generated. The highlight of the method is zero average NP current in one control cycle and lower CMV. Furthermore, the active NP voltage control suitable for RCMV_VSVPWM is presented and evaluated. The corresponding experimental results are given, which are well-consistent with theoretical analysis. [ABSTRACT FROM AUTHOR]

This paper investigates the use of resistor–inductor (RL) filters for motor overvoltage issues caused by cables connecting inverters to electric machines. Using existing filter design formulae and fixing the desired peak motor voltage, an RL filter is designed for a 15-kW industrial drive. This filter is used with a Silicon IGBT inverter and a matching between the design targets and the experimental results are found. Using the same filter with an SiC MOSFET inverter, a detrimental effect in overvoltage mitigation task is found. From simulations, it is noticed that combination of filter resistor parasitic inductance and high $dv/dt$ causes unpredicted additional high-frequency motor overvoltages. These phenomena require higher filter inductance values with respect to the standard one, thus previous filter design formulae cannot be used. New design formulae, valid when inverters with high $dv/dt$ are used, are proposed and experimentally validated. Interestingly, using the proposed formulae, it is discovered that the best filter resistance is not equal to the characteristic impedance of the cable. A design guideline that permits to optimally design the RL filter without simulation tool, considering the resistor parasitic inductance and the inverter $dv/dt$ is finally proposed. [ABSTRACT FROM AUTHOR]

This paper presents a 5-level inverter topology for open-end induction motor drives by using a single dc source. The open stator windings of the drive are supplied with a 3-level flying capacitor inverter from one end and capacitor-fed 2-level inverter from another end. The voltage ratio of the dc link to the capacitor in 2-level inverter is maintained at 4:1 ratio to generate five-level voltage output. The capacitor in 2-level inverter is balanced by the switching redundant vector combinations from both the inverters while the floating capacitors in the 3-level inverter are balanced by using redundant switching states. The proposed topology gives 5-level operation with less number of floating capacitors and power semiconductor switches compared to other existing topologies. Also, the balancing of the capacitors is independent of load power factor and modulation index. Further, the generalization of the proposed dual inverter scheme for any n-level inverter is also included in this paper. The experimental results and required analysis are also presented to validate the inverter scheme. [ABSTRACT FROM AUTHOR]

This paper presents the current control design procedure of a single-phase grid-tied five-level packed U-cell inverter (PUC5) with an LCL output filter. The PUC5 inverter is used as an interface of renewable energy sources, such as solar applications. The LCL filter is calculated according to the grid-tied operation and converter ratings. An optimal controller, based on a linear quadratic regulator with integral action, is designed to inject a sinusoidal current with low harmonic distortion at unity power factor. For that design, the PUC5 inverter is modelled in the D–Q frame. The sensorless voltage control is incorporated into the switching technique to balance the PUC5 capacitor voltage and generate a five-level waveform at the output. Experimental tests are performed on a laboratory benchmark to confirm the theoretical design. The results prove the efficiency and accuracy of the adopted control strategy in a steady state and under transients of grid current, grid inductance, ac and dc voltage amplitudes. [ABSTRACT FROM AUTHOR]

In complex terrain, semi-airborne transient electromagnetic surveying can effectively solve the detailed exploration problems to replace ground manual operation. This method has the advantages of high efficiency, deep exploration depth, and high signal-to-noise ratio. As the core component of the system, the constant-current transmission converter determines the primary field source of the surveying system. High-accurate and stable-output electromagnetic transmission technology is the key issues of the surveying equipment development. In this paper, the converter circuit topology is designed in detail and a novel model reference adaptive proportional-integral control algorithm is put forward. The rising overshoot of current is eliminated and falling edge is linear. Finally, the 20-kW high-power transmitting instrument has been completed and used for field experiments. A large number of experimental results show that the transmitting instrument can achieve high-power constant-current emission, stable output, and good dynamic performance. And the steady-state accuracy is better than the traditional transmitter. The converter successfully improved the semi-airborne transient electromagnetic system detection capability and achieved satisfactory results in field experiments. [ABSTRACT FROM AUTHOR]

In this paper, many impedance-source inverters with coupled inductors have been investigated to obtain a high step-up boost ratio. However, the leakage inductors in these topologies induce great voltage spikes at the dc link, which will increase the voltage stress of switches. Thus, the power level of inverters is limited. Furthermore, the efficiency of the inverters can be degraded by the losses associated with leakage inductors. To address abovementioned issues, this paper proposes a family of low-spike high-efficiency Y-source inverters. The proposed inverters have the ability of eliminating voltage spikes at the dc link and recycling the leakage energy via the additional diode and capacitor. In order to show the excellent characteristics of the proposed topologies, this paper compares the performances of the proposed inverters and the improved-Y-source inverter in many aspects. Simulation and experimental results have verified the abilities of the proposed inverters. [ABSTRACT FROM AUTHOR]

This paper proposes an explicit predictive current control scheme implemented with a low carrier frequency pulsewidth modulation (PWM) on an induction machine fed by a three-level neutral point clamped inverter. The PWM carrier and the main current sampling frequency are both set to 1 kHz, resulting in a 500 Hz average switching frequency per device, which is very suitable for large drive applications. The explicit predictive control is introduced to optimize the available bandwidth provided by such a low sampling frequency, maximizing the dynamic performance. The strategy has been tested in a 2.2-kW induction motor experimental prototype. [ABSTRACT FROM AUTHOR]

Three-phase pulsewidth modulation inverter fed induction motor drive system is widely applied in high power drive applications. Sensor faults are very common in the drive system, which, once occur, might result in degraded system performance or even system shutdown. In order to rapidly and accurately diagnose the sensor faults, this paper proposes an intelligent time-adaptive data-driven method to identify the fault location and fault type of sensors in the drive system. An emerging machine learning technology named extreme learning machine (ELM) is applied to learn the sensor fault dataset; an ensemble ELM classifier is then designed to improve diagnostic accuracy, based on which a time-adaptive fault diagnosis process is proposed to achieve a high and balanced diagnostic accuracy and speed. As a data-driven method, the proposed method only employs the phase current, dc-link voltage, and speed signals as the inputs to the ensemble ELM classifiers and requires no additional sensors and other hardware. Simulated and experimental tests show that the proposed method can rapidly and accurately detect the fault sensor location and identify offset fault, stuck fault, and noise faults with an average diagnostic accuracy of 98% and the average decision time of 10 ms after the fault occurs. Moreover, such diagnosis method is robust to the fluctuation of catenary voltage and dc-link voltage, fault severity, and variation of model parameters, speed, and load. [ABSTRACT FROM AUTHOR]

Two-stage isolated battery-driven grid interface (IBDGI) with front-end three-phase dual active bridge (DAB) dc–dc converter is promising to be applied in the residential battery energy storage system for supporting photovoltaic (PV) integration, load shifting, and backup power purposes. However, the second harmonic current (SHC) caused by the pulsating power of the downstream single-phase inverter will increase the battery's degradation and the component stress of the front-end converter. In this paper, a reduction method is proposed for the IBDGI with three-phase DAB converter using a load current feedforward control. The proposed idea is to incorporate virtual impedance to the output impedance of the front-end converter. The easy-to-be-implemented method does not need to modify the original voltage closed-loop controller but suppress SHC while keeping good dynamic performance. As the first step, the small-signal model of the three-phase DAB converter is provided. The parameter sensitivity analysis is proposed to demonstrate that the SHC can be well-suppressed within $\pm \text{20}\%$ parameter error of the feedforward control. Finally, a 1 kW experiment setup is built to verify the performance of the proposed SHC reduction method. [ABSTRACT FROM AUTHOR]

In this paper, a new Lyapunov-based model predictive controller is proposed for a 7-level packed U-cell (PUC7) grid-connected inverter. A cost function of the proposed model predictive controller is designed from the system stability point of view, inspired by the Lyapunov control theory. The proposed controller eliminates the need for gains that are associated with cost function coefficients, as seen in classical model predictive control based controllers. Therefore, the control design problem is significantly simplified. In addition, the average switching frequency is also reduced, as shown in both simulation and experimental results, leading to a reduction in switching losses. Simulation and experimental tests, on a PUC7 lab prototype, demonstrate the excellent performance of the proposed control system, in terms of high disturbance rejection, robustness to parameter mismatches, and fast dynamic response. Such features qualify the proposed control strategy as a good candidate for grid-tied applications. [ABSTRACT FROM AUTHOR]

One promising solution for future large wind turbine power conversion is to use medium-voltage power conversion systems for high efficiency and high power-density, e.g., by using a cascaded structure presented in this paper. However, a medium-frequency (MF) oscillation issue was observed while testing the experimental prototype of the above system. Given the complex multivariable coupling models of the multiwinding transformer and multiple inverters, conventional single-variable theories are not suitable. Therefore, this paper presents a new decoupling analysis method. Using a proposed linear transformation matrix, the output currents of multiple inverters can be transformed to a summation (grid) current and difference currents, where the former is the summation of inverter currents and the latter are the differences between currents of any two inverters. This transformation has enabled the use of single-variable theories, e.g., Bode diagram to analyze the summation current and difference current separately. It was found that the MF oscillation issue is caused by the equivalent impedance of the summation current being much larger than that of the difference current. Specifically, using small controller parameters cannot ensure the grid current quality and using large parameters may cause oscillation in the difference current. The proposed method can provide guidelines to design the parameters of current regulators and filters in multiple-inverter systems. Experimental results of a downscaled wind energy conversion system are presented to verify the proposed method. [ABSTRACT FROM AUTHOR]

Motors and inverters are increasingly being used in vehicles. As more vehicle components are being adopted, requirement for safety and installation space are increasing. Growing attention is being paid to the use of six-phase permanent magnet synchronous motors (PMSMs) and their parallel control by two inverters, because of their reduced size and enhanced safety. This paper proposes a pulsewidth modulation (PWM) control algorithm for a six-phase PMSM. In low-voltage vehicle components such as electric power steering, the current is usually acquired with resistors placed under the low-side switching devices. In the proposed control algorithm, the neutral voltage of the two inverters is shifted to either a lower or higher voltage within a certain range to allow for the signal conditioning time delay after the switching device turns on or off, and the time taken for the phase current to rise or settle. The paper proposes a novel PWM control algorithm that ensures that the on-time of the low-side switching devices satisfies the restriction on current sensing with resistors and reduces the current in the capacitor at the same time. The work contributes to reducing the current in a practical configuration involving low-voltage vehicle components. [ABSTRACT FROM AUTHOR]

A major challenge for practical wireless power transfer (WPT) applications is to attain stable power transfer with high and constant transfer efficiency under a dynamic change of coupling condition. In order to address the issue, this paper proposes a novel nonlinear parity-time (PT) symmetric model, wherein the nonlinear saturable gain is provided by a self-oscillating controlled inverter. In this paper, the transfer performance and stability criterion of the nonlinear PT-based WPT system are analyzed based on the coupled-mode theory. The theoretical analysis shows that the proposed system automatically achieves constant output power with constant transfer efficiency against the variation of coupling coefficient. Moreover, based on the gain saturation mechanism, the control strategy for the inverter needs to detect only the current in the transmitter, which eliminates auxiliary circuits of wireless communication for feedback control from the receiver. As a case study of dynamic charging, a drone-in-flight wireless charging platform is improved by applying the nonlinear PT-symmetric model. Experimental results show that when the flying drone hovers in a confined three-dimensional volume of space above the WPT platform, a stable output power is maintained with approximately constant transfer efficiency of 93.6%. [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]

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]

Microgrids (MGs) are often unbalanced due to the integration of single-phase generators, unbalanced loads, and asymmetrical faults. To better analyze such MGs, this paper presents an approach to analyze the transient response for an inverter-based MG under the unbalanced condition. The dynamic phasor (DP) concept is used for the MG modeling under the stationaryabcreference frame. First, the DP model of the inverter-based distributed generation (DG) is developed. The influence of unbalanced conditions on the inverter including oscillations on the dc side is considered in this paper. Then, the model of the network and loads is developed. Finally, all the submodules are combined on a time-variable system frequency to obtain the complete DP model of the unbalanced MG. To validate the proposed approach, the DP method is applied to an MG test system with three-phase and single-phase DGs. Small-signal analysis is carried out to derive the dominant modes and their influence on the system response. Simulation results from the DP model are compared against the detailed model built in MATLAB/SimPowerSystem. The results from load disturbances and asymmetrical faults are used to verify the DP model. [ABSTRACT FROM AUTHOR]

In grounded photovoltaic (PV) systems, dc leakage to earth is formed by the contribution of the PV modules, inverters, and dc circuitry. Ground faults in such systems occur when there is an unintentional connection between any dc-carrying conductor with a grounded surface or earth. On the occurrence of ground faults, dc leakage will be intensified and will flow through the earth or other conductive paths, before returning back to the source. Thus, it is highly probable that some portion of the dc leakage will attempt to return to the source through any low-resistance paths that exist in the nearby vicinity, for example, through nearby buried pipelines. Therefore, unattended dc leakage may induce stray current corrosion concerns on these pipelines. To this extent, this paper presents a detailed dc interference modeling for assessing the dc leakage activity in PV systems—particularly under undetected fault conditions. First, the modeling carried out embraces the topologically accurate consideration of all sources of dc leakage that are associated with the grounded PV systems. A second feature presented in the paper is the modeling of blind-spot dc faults and their impact on third-party infrastructure. [ABSTRACT FROM AUTHOR]

In a sensorless vector control method, at low speeds and self-commissioning for AC motors, it is necessary to model dead time, rise/fall time, parasitic capacitors, and semiconductors’ voltage drop accurately. In this paper, voltage drop in each phase due to the nonlinear behavior of the inverter from the control system's viewpoint is precisely modeled by using two lookup tables. These lookup tables make the proposed model robust against the dc-link voltage and carrier frequency variations in the entire current range. In conventional methods at standstill, the measured voltage in the dc current test contains the nonlinearity of two phases. Therefore, to obtain the nonlinear voltage of each inverter phase, it is assumed that the values of inverter nonlinearity voltage at both high current and low current are equal. This simplification leads to distortion in measured voltage in the low currents. However, in this paper, the voltage drop equation of each phase is solved by numerical method that is realizable in conventional processors. The results obtained in actual voltage measurement show the superiority of the proposed method over traditional methods in the low currents. The proposed method is verified by using a voltage source inverter feeding a 2.2-kW three-phase induction motor. [ABSTRACT FROM AUTHOR]

Islanded grids operate isolated from the main grid and they allow the use of ac with a different frequency from the main grid. In this paper, it is proposed to use more than one frequency at the same time in such autonomous systems. The grid voltage amplitude at the fundamental frequency can be increased while the voltage peak is constant by adding odd harmonics to the voltage waveform. The grid currents and the power converter currents are decreased for the same power level, which leads to reduced losses in the system. The currents can be minimized if waveform shaping is additionally applied to the current waveforms. The approach also offers the possibility of mitigating the influence of low power factor in single-phase systems. This paper presents the impact of multifrequency power transfer on single-phase converters and grid components such as power cables and protection, and verifies the results with analysis, simulations, and experiments. [ABSTRACT FROM AUTHOR]

A direct speed control strategy based on finite control set model predictive control (FCS–MPC) with a voltage smoother is presented herein. In the proposed concept, a finite set of smoothed voltage vectors characterized by an adjustable amplitude and a movable origin is introduced as voltage candidates in the FCS–MPC scheme. The controller predicts the future current and speed and outputs the optimal smoothed voltage using pulse-width modulation. Owing to this control scheme, an abrupt change in the output voltage, which causes a large current ripple, is avoided without additional computational costs. Simulation and experimental results obtained using a permanent magnet synchronous motor fed by a two-level three-phase inverter show that the proposed method effectively reduces the current ripple while achieving a high dynamic drive compared with the conventional FCS–MPC scheme. [ABSTRACT FROM AUTHOR]

In islanded ac microgrids, the mismatched impedances of the interfacing feeders between the inverters and the load bus cause poor reactive power sharing when the conventional frequency and voltage droop control technique is employed. Such operation endangers the whole microgrid reliability as it may lead to overloading certain inverters and, consequently, triggering protection relays and causing cascaded failure. Thus, this article proposes an accurate reactive power sharing strategy that considers the mismatched feeder impedances in islanded ac microgrids. It is based on the optimal tuning of the virtual complex impedance for each inverter. The proposed strategy has several advantages. First, it has a physical meaning as it establishes an explicit relationship between the mismatched values of the actual resistive–inductive feeders and the assigned values for the proposed virtual complex impedance to each inverter. Hence, further degradations in the microgrid voltages are prevented. Second, there is no need for prior knowledge about the actual feeder impedances in the design stage as they are estimated online from the available measurements. Finally, the proposed control is reliable and fault-tolerant as it copes with unexpected failures such as failure of (or sudden switching off) some inverters and communication disruptions/delays. It ensures accurate power sharing even under the primary controllers after losing communication links with the secondary controller. The simulation and experimental verification results are presented to validate the performance of the proposed technique. [ABSTRACT FROM AUTHOR]

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