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In this paper, the characteristics of ripple current of a multimegawatt drive system fed by a pulse-width modulation (PWM) inverter are investigated. Based on the investigation, a novel PWM scheme is devised to reduce the switching loss of the inverter under an identical amount of torque ripple of a motor. The proposed PWM is composed of two parts. The first is minimum torque ripple PWM (MTRPWM), which is optimized in terms of the minimum torque ripple in high modulation index (MI). The second determines the minimum switching frequency while holding the average torque ripple less than the allowable value under various operating conditions. It is termed reduced switching frequency PWM (RSFPWM). Consequently, by simply changing the PWM method from conventional fixed switching frequency continuous PWM to the proposed PWM, MTRPWM and RSFPWM, simulation results show that the switching loss of the inverter driving a 3.5 MW induction motor is reduced to 51.6% while keeping the torque ripple in the allowable boundary, where MI is 1.12. This approach is applied to small-scale experimental sets consisting of an interior permanent magnet synchronous machine (IPMSM) motor-generator set, and an induction machine MG set in an effort to verify the validity and feasibility of the proposed method. Based on these experimental results, the cost indexes of the IPMSM and the induction machine, as defined in this paper, decrease by up to 66% and 77%, respectively, when the proposed method is applied when MI is 1.12 compared with the outcome with the conventional PWM. [ABSTRACT FROM AUTHOR]

This paper discusses and compares phase-shifted pulse-width modulation (PS-PWM) and level-shifted PWM (LS-PWM) in a modular multilevel single-delta bridge-cell (SDBC) inverter for utility-scale grid-tied photovoltaic applications. This three-phase SDBC inverter provides the capability of operating even under power-distribution imbalances not only among three clusters, but also among cascaded bridge cells in each cluster. The main focus of this research lies in analyzing the capability of operation under severe power-distribution imbalances and presenting power-balancing methods based on zero-sequence current injection, which result in an extended power-balancing capability. This paper highlights that LS-PWM is superior to PS-PWM in both power-balancing capability and harmonic performance at the ac side of the SDBC inverter when operating under power-distribution imbalances. Experimental results obtained from a three-phase 10-kW downscaled system show good agreement with those from the theoretical analysis. [ABSTRACT FROM AUTHOR]

This paper introduces a three-phase grid-connected inverter for commercial-scale photovoltaic systems. The circuit topology combines an ac-inductor-less active bridge with a parallel-connected pulsewidth modulation inverter acting as an instantaneous reactive power compensator. The active bridge, called a 120° conduction inverter, operates at a switching frequency equal to the grid frequency and outputs a 120° rectangular current, while the compensator eliminates the harmonic currents included in the rectangular current. The main advantage of this strategy is the reduction in size of the required ac inductor by a factor of 4, as well as the reduction of power losses in the switching devices. The theoretical analysis clearly shows that the compensator can synthesize a sinusoidal output current by controlling only instantaneous reactive power, thus eliminating the need for electrolytic capacitors. A 5-kW experimental setup validates the operating principle and the control method of the proposed inverter and exhibits an inverter efficiency as high as 99.2% with Si superjunction mosfets. [ABSTRACT FROM AUTHOR]

This paper proposes a three-phase four-leg drive for dc-biased sinusoidal current vernier reluctance machine (DC-VRM). By adding an asymmetrical neutral leg to the conventional three-phase inverter, the dc field excitation current can be generated. Compared to the open-winding inverter (conventional drive topology), the three-phase four-leg inverter greatly reduces the number of power devices, and the asymmetrical neutral leg does not have short-circuit straight-through risk. The topology ensures that the stator current can flow in both directions and maximum torque per copper loss can be obtained in DC-VRM. Equivalent carrier-based pulsewidth modulation (PWM) strategy is proposed to produce the dc-biased sinusoidal current and maximize the dc bus utilization. The proposed carrier-based PWM has several different equivalent methods. Compared to the conventional DC-VRM drive, the power loss of the proposed drive with equivalent space vector PWM (SVPWM) can be reduced significantly and can be further reduced by discontinuous PWM minimum (DPWMMIN) in the rated condition. The proposed drive has the characteristics of low cost, high power density, and high efficiency compared to the conventional DC-VRM drive. [ABSTRACT FROM AUTHOR]

A discontinuous pulsewidth modulation (DPWM) strategy for three-level neutral-point-clamped (NPC) traction inverter drive for electric vehicles (EVs) with minimum switching loss for the full power factor (PF) range is developed and presented in this paper. Considering the dynamic loading conditions in an EV propelled by an induction motor (IM), the discontinuous modulation strategy is generalized to obtain minimum switching loss for a wide variations in load PF and modulation depth. The dynamic operating conditions in EV may render frequent unbalances in dc-link capacitor voltages in NPC inverter. It is demonstrated that the addition of an appropriate offset signal with the reference discontinuous signals generates compensating neutral current in the right direction, which can be used to mitigate existing unbalance in two dc-link capacitor voltages. Minimum switching loss at variable PF is attained by positioning the nonswitching clamped regions of the reference auxiliary signals according to the load PF. The effectiveness of the proposed scheme for a wide range of torque/speed variations is studied through simulation by developing a MATLAB/SIMULINK model of an IM traction inverter drive with field-oriented control strategy for closed loop control. Finally, the modulation scheme is validated through experimentation in a small-scale prototype NPC inverter with motor loads. [ABSTRACT FROM AUTHOR]

An adaptive control of DPWM implementable in clamped-three-level inverters with two strings of photovoltaic (PV) panels in cascaded connection is proposed in this paper. The proposed modulation is developed based on the so-called circuit-level decoupling concept. It is able to provide balance line voltage even under unbalanced dc links. The unequal dc voltages appeared due to asymmetric maximum point of power trackings (MPPTs) are controlled by proper injection of zero-sequence voltage to the common mode voltage, while the total dc voltage is regulated based on the voltage commands generated by the MPPT of each string. Thus, both PV strings are able to be operated based on their MPPTs while there is almost no low-order harmonic distortion at the inverter output current. Compared with the previous methods, the high capability of the proposed method is evaluated by simulation study and experimental tests. [ABSTRACT FROM AUTHOR]

This paper investigates a multilevel inverter composed of series/parallel-connected H-bridges. The presented structure can be used for applications in which the use of semiconductor switches with low-voltage and low-current ratings is intended. A comprehensive system model, an overall control strategy to adjust the output voltage with constant magnitude and frequency, and a level-shifted PWM (LS-PWM) strategy based on a voltage vectors unidimensional analysis and a plane analysis are presented. The LS-PWM is capable of mitigating the low-frequency circulating current, thus generating multilevel voltage signals with low harmonic distortion, maximum number of levels, and low dv/dt. In addition, considering a wide range of values of voltages and currents and various power levels, the total converter losses are reduced compared with the conventional multilevel converters. Two multilevel conventional inverters with the same number of semiconductor switches are used for comparison. Simulation and experimental results demonstrate the feasibility of the studied converter. [ABSTRACT FROM AUTHOR]

This paper presents a nine-level transformer-based inverter requiring only eight switches. The envisaged structure consists of two standard H-bridges fed from a single dc source. Besides, a single-phase transformer is employed to aid the process of intermediate voltage level generation. An ad-hoc pulsewidth modulation scheme based on boolean logic form equations is developed to derive the gating pulses. An effortless extension of the proposed inverter to a higher number of voltage levels is also achieved by generalizing the switching functions. Furthermore, the superior performance of the proposed topology is demonstrated through a comprehensive cost-based analysis. Finally, the validation of the proposed topology is accomplished through experiments on a down-scale prototype, and the measurement results are included. [ABSTRACT FROM AUTHOR]

An optimal current control strategy for asymmetrical hybrid multilevel inverters (MLIs) is proposed in this paper enabling their use on motor drives, static synchronous compensators, and photovoltaic and wind generators, where a fast and precise current regulation is required. A key feature of these highly efficient converters is that an ac machine (motor or transformer) operates according to an open-end winding configuration, connected on one side to a main MLI and, on the other side, to an auxiliary two-level inverter (TLI). The first efficiently controls the main power stream operating at a low switching frequency, while the TLI acts as an active power filter, exploiting a conventional high-frequency two-level pulsewidth modulation (PWM) technique. The proposed control scheme optimally exploits the key features of the two inverters by suitably sharing the control task. In fact, a predictive current control is assigned to the MLI, which can be accomplished by low switching frequency operations, while the high switching frequency of the PWM-operated TLI is exploited to accomplish a fast and precise closed-loop current control, processing only a part of the power stream flowing through the system, thus producing low power losses. Simulations and experimental results confirm the consistency of the proposed approach. [ABSTRACT FROM AUTHOR]

This paper illustrates the application of a solving method of linear systems to a multilevel inverter. This study applies a generic pulsewidth modulation (PWM) method to a multilevel topology. This method finally highlights some specific modulation schemes. First, the study describes the mathematical model of the inverter leg. The leg is made of three level T-type structure. The model is extended to the three-phase inverter. The generalized inverse is used to generate the solution set of the conduction rates. Among the modulation strategies possible, three specific modulation schemes are proposed. The modulation schemes are applied to simulate a PWM three-phase inverter. Finally, some experimental results complete the analysis. [ABSTRACT FROM AUTHOR]

This paper presents an improved dc-link voltage balancing algorithm for a three-level neutral-point-clamped inverter by considering phase current direction. Detailed studies on the effects of change in load current direction on the dc-link capacitor voltages are presented. A maximum value of power factor is numerically derived, above which it affects the capacitor voltage balancing capability. Compared with the previously presented research work, the inputs to the space-vector pulsewidth-modulation block are the three phase currents and the difference between the two capacitor voltages. Depending on the states of the two dc-link capacitor voltages and phase current direction, redundant voltage vector sequences are selected. The selected vectors keep the capacitor voltage deviations within 5% of the total dc-link voltage. Two zero switching vectors (i.e., PPP and NNN) are also removed from all subsectors of the earlier proposed strategy, which one used to produce higher common-mode voltages. Detailed simulation and experimental results are presented in this paper for a 6.0-kW surface permanent-magnet synchronous machine. Both the simulation and experimental results show the required performance of the proposed system. [ABSTRACT FROM AUTHOR]

Paralleled inverters are not only a suitable approach to increase the power capacity, but also have the capability of common mode voltage (CMV) reduction, or even elimination. The original zero CMV modulation scheme for paralleled inverters suffers from the current jump problem in phase-leg currents. This current jump phenomenon amplifies the peak values of differential-mode circulating current (DMCC) and zero-sequence circulating current (ZSCC), which is adverse for the design of coupled inductor and common-mode inductor. This paper introduces the modified modulation scheme, which can keep volt-seconds balance in whole fundamental period and mitigate the current jump in phase-leg currents. In addition, the reduction effect of the DMCC and ZSCC can be achieved. To verify the analysis, both the simulation and experimental results are presented, which validates the proposed methods and show that they are beneficial for DMCC and ZSCC reduction. [ABSTRACT FROM AUTHOR]

Three optimized pulse-width modulated (PWM) techniques for the overmodulation region of two-level inverter-fed ac drives are introduced and investigated from the harmonic loss minimization point of view. The optimization is elaborated for the lowest loss-factor, which is proportional to the square of the root mean square value of current harmonics. The loss-factors are computed for different switching numbers as the function of the motor fundamental voltage. It is shown that, with respect to the motor heating and torque ripples, the acceptable drive condition can be guaranteed by a relatively low value of an inverter switching frequency up to 96%–97% of maximal possible motor voltage. Furthermore, it is shown that the so-called three-vector methods have considerably better performance in the lower part of the overmodulation region than the so-called two-vector method for the same number of switching. The performance of the techniques is compared with other existing PWM techniques. This paper discusses the implementation details of the proposed optimal PWM techniques. The theoretical results are verified by experimental and simulation tests. [ABSTRACT FROM AUTHOR]

This paper discusses the use of a cascaded multilevel converter for flexible power conditioning in smart-grid applications. The main feature of the proposed scheme is the use of independent dc links with reduced voltages, which makes such a topology an ideal candidate for medium- and high-power applications with increased reliability. The developed control strategy regulates independent dc-link voltages in each H-bridge cell, and allows the selective and flexible compensation of disturbing currents under a variety of voltage conditions without requiring any reference frame transformation. The selective control strategies are based on the decompositions proposed in the conservative power theory, which result in several current-related terms associated with specific load characteristics. These current components are independent of each other and may be used to define different compensation strategies, which can be selective in minimizing particular effects of disturbing loads. Experimental results are provided to validate the possibilities and performance of the proposed control strategies, considering ideal and deteriorated voltage conditions. [ABSTRACT FROM PUBLISHER]

Medium-voltage drives are regularly used in the petrochemical industry. These drives often vary in topology, semiconductor design, and principles of operation. They each have their own merits, not only in terms of structure, design, and reliability, but also with regard to suitability to specific applications. This paper presents a technology overview of commercially available medium-voltage drive topologies with a view on how they match to petrochemical applications. The operating and performance characteristics of each drive topology are reviewed while manufacturer-specific implementation details are excluded. Common industry and application requirements such as harmonics, motor compatibility, regeneration capability, dynamic performance, and robustness to system power disturbances are used as comparison criteria. Application aspects such as commissioning and operation challenges are highlighted from a practical point of view. [ABSTRACT FROM AUTHOR]

In this paper, design and implementation of a flyback photovoltaic (PV) microinverter based on the direct digital synthesis (DDS) technique has been described for both the standalone and the grid-connected operation. The DDS technique adopted provides flexibility in the implementation of various control schemes of the PV microinverter on a simple low-cost digital signal processing type of microprocessor (dsPIC). As compared with the conventional look-up-table method used for generating sinusoidal output voltage waveforms by digital signal processing, a much higher resolution can be obtained in the voltage phase angle and magnitude owing to the adaptive nature of the look-up table implemented within the DDS architecture. The DDS technique is used in the implementation of all control schemes of a PV microinverter, such as maximum power point tracking (MPPT), phase-locked-loop (PLL), anti-islanding, and low-voltage ride-though (LVRT), with an integrated software run on a simple microcontroller. A dedicated computer simulation model is developed, where the PV panel model, the PLL in DQ reference frame, the MPPT algorithm, and the anti-islanding and LVRT features are all taken into account. The experimental results obtained on a 120-W PV flyback microinverter have verified the validity of the proposed technique for both the steady-state and the transient-state operation. The DDS technique is thus found to be quite convenient for application to module integrated converters. [ABSTRACT FROM AUTHOR]

In order to increase the output voltage levels and reduce the isolated dc sources, a symmetrical hybrid nine-level inverter for high-speed motor drive applications is presented in this paper. Each phase of this inverter is composed of a five-level dc/dc converter cell and an H-bridge cell. The dc/dc converter is operated at high frequency with low-voltage devices and the H-bridge is operated at fundamental frequency with high-voltage devices. The three phases are connected to a common dc-link and each drives an isolated winding of an open-winding motor. The operating principles and modulation method are introduced. A detailed analysis of the average currents through the flying capacitor and neutral point of the dc-link is presented and a capacitor voltage balancing method based on phase-shifted pulse-width modulation is proposed. Simulation and experimental results are presented to demonstrate the feasibility of this topology and control method. [ABSTRACT FROM PUBLISHER]

This paper presents an optimized topology for a \text3-\phi three-level inverter with complete elimination of common-mode voltage (CMV). The proposed multilevel inverter (MLI) configuration is realized by modifying a T-structure \text3-\phi inverter. The proposed configuration is an optimized solution with respect to the pulse-width modulation strategy used for CMV elimination. The given three-level inverter structure uses only 16 power semiconductor switches, which is much lower than the existing configurations. A reduced number of power semiconductor devices results in a diminished number of driver circuits, less installation space, and low cost. Further, due to the complete elimination of CMV, the proposed MLI is free from issues such as electromagnetic interference and leakage current with a reduction in filter requirement. The presented topology is also compared with other existing topologies to prove its advantage. It is an optimized solution with respect to the dc bus voltage requirement and the total voltage rating of the devices or the components used in the system. Simulation and experimental results are presented to confirm the capability of the proposed MLI. [ABSTRACT FROM AUTHOR]

As a result of dead-time, device on-state voltage drop, dc bus voltage measurement error, etc., volt-second errors degrade precise control of torque and flux linkage, particularly at low speeds. This is true for deadbeat-direct torque and flux control, which directly manipulates the volt-second vector sourced by inverters, as well as for indirect field oriented control drives. This paper introduces a real-time sensing scheme to measure the motor terminal volt-second vectors for each switching period with negligible phase lag. Based on the volt-second sensing, a model reference adaptive system-based approach is developed to decouple the volt-second errors from inverter nonlinearity, and dc bus voltage fluctuation and measurement error. By delivering an accurate volt-second vector for each switching period, torque and flux control accuracy, self-sensing performance, and parameter estimation accuracy are significantly enhanced. [ABSTRACT FROM AUTHOR]

This paper proposes and investigates a multilevel ac six-phase motor drive. The system is composed of five isolated three-leg voltage source inverters feeding the open-end windings of an asymmetrical six-phase induction motor (SPIM), which is adequate to generate multilevel voltages for high-power systems with voltage rating restrictions. A simple space vector pulse-width modulation (PWM) based on three similar individual planes and its implementation by means of equivalent level-shifted PWM are presented. A space vector pattern with a high number of voltage vectors redundancies is obtained. These redundancies and the application sequence of the voltage vectors are selected to minimize the amount of changes in the switching states and to decrease the harmonic distortion of the generated voltages. The vector pattern of this optimal modulation is obtained by analyzing only one plane and applied in the same way to the three planes as if they were independent. The developed PWM techniques have low computational complexity and are suitable for low-cost hardware implementations. Simulation results are used to compare the proposed topology with a conventional configuration in terms of harmonic distortion and semiconductor losses. Experimental results demonstrate the feasibility of the proposed drive system. [ABSTRACT FROM AUTHOR]

This paper proposes a hybrid modular multilevel drive system based on a modular multilevel cascade inverter (MMCI) with double-star chopper cells (DSCC) and a two-level (2L) inverter to feed three-phase induction motors with open-end windings (OEWs). The proposed system is an alternative for high-speed motor drives based on a small-scale DSCC topology. A voltage reference technique is presented based on the analysis of the degrees of freedom of the system for generating the output voltage references, considering any voltage ratios of the dc links. The proposed system is evaluated in a symmetric scenario, in which the dc-link voltages are equal. Thus, the 2L inverter is set up to operate at the fundamental frequency, while the DSCC inverter synthesizes the output voltage references from high-quality waveform with a significant number of voltage levels. Experimental results obtained from a downscaled 10-kVA 380-V prototype driving a full loaded 3.7-kW OEW induction motor are provided to verify the viability of the proposed system. [ABSTRACT FROM PUBLISHER]

One of the problems of the pulse-width-modulation (PWM)-controlled ac machines is the acoustic noise that could become unacceptable when used in sensitive environments. Random PWM (RPWM) in industrial and commercial adjustable-speed drives (ASD) results in the partial transfer of power from the discrete spectrum (narrowband noise) of the output voltage to the continuous spectrum (wideband noise), with advantageous effects on reducing the acoustic noise in the motor drive system. In this paper, the theoretical power spectrum analysis as the basis for RPWM is presented. Five state-of-the-art RPWM strategies, their voltage, current, and acoustic noise spectra characteristics are quantitatively evaluated. The PWM schemes and theoretical analysis are validated through a 2.2-kW 380-V 50-Hz ASD induction machine experimental setup. The results provide valuable data for practicing engineering community to choose the best option in real-world applications. [ABSTRACT FROM PUBLISHER]

Common-mode voltage (CMV) in voltage-source inverters (VSIs) has been well defined and is frequently referred to in the literature. While often referred to, there is no real consistency with respect to how the CMV properties of an individual modulation scheme are evaluated, lacking tools to assess the relative merits or disadvantages of a certain topology or modification of a control scheme. The way that it is analyzed is still mostly a matter of the different authors' foci. In this paper, we propose a metric for the analysis of the CMV, aiming to fill the aforementioned gap. Thereby, the CMV in VSIs is quantified by the number of possible levels and the step sizes between different levels, as well as the respective frequencies of occurrence. The first two are given by the number of levels and the number of phases of the inverter. Depending on the modulation (control) schemes, all possibly available levels or a few of these levels are present in the CMV. We illustrate the use of the tool by applying it to thoroughly study the CMV properties intrinsic to the topologies of multilevel multiphase single- as well as double-ended diode-clamped VSI drive systems. As part of this analysis, we uncover new relationships concerning the existence of zero CMV level. Furthermore, we exemplarily apply the developed tool to the assessment of the CMV properties of selected pulse width modulation (PWM) control schemes and translate the findings into an interpretation of the endangerment of a given drive to suffer damage from inverter-induced bearing currents. The theoretical findings are supported by measurements. This paper also reviews the literature on CMV in multilevel multiphase VSI systems. With the proposed metric in mind, the CMV being mostly discussed as an element of different foci becomes even clearer, confirming again the need for the development of such a tool. [ABSTRACT FROM AUTHOR]

This paper identifies the characteristics of a current source inverter operating in discontinuous conduction mode, and proposes a new multiobjective pulse-width modulation (PWM) method to mitigate voltage boosting and harmonic distortion effects. The method takes into account load impedance effects. Controls are also developed to enable transition between the multiobjective PWM method and conventional PWM as the current source inverter operates in discontinuous and continuous modes of operation. The concept is verified experimentally on a 3 kW test bed. [ABSTRACT FROM AUTHOR]

This paper presents a modified two-level three-phase inverter for the reduction of the leakage current. With respect to a traditional two-level inverter, the proposed solution reduces the common-mode voltage (CMV), both in amplitude and frequency. Between the dc source and the traditional three-phase bridge, two active dc-decoupling devices and a voltage-clamping network have been added. A dedicated control strategy was developed adopting a modified space vector pulse-width modulation, oriented to the reduction of the CMV. Simulations showing the good performance of the solution are presented. A preliminary prototype was developed and experimental results are presented. [ABSTRACT FROM PUBLISHER]

Inserted between the inverter and the ac motor for the purpose of providing clean sinusoidal voltage at the motor terminals, the pure sine filter (PSF) involves common-mode and differential-mode suppression components. When using PSF in a motor drive and employing advanced pulsewidth modulation (PWM) techniques with discontinuous modulation waveforms, a large common-mode current may circulate through the inverter and the filter resulting in overcurrent stress in the inverter and significant acoustic noise in the filter inductors. This paper first discusses the PSF and advanced PWM techniques, then investigates the performance issues that may arise when combining advanced discontinuous PWM methods with PSF and provides a curement for the overcurrent stress problems by means of a rate of change limiter. The simulations and detailed experimental studies illustrate the inverter overcurrent stress problems and the feasibility of the proposed method to cure the problems. Via experimental performance evaluation of various PWM methods, the paper also suggests, along with PSF, the use of advanced continuous and discontinuous PWM methods with reduced common-mode voltage for the purpose of performance enhancement and filter size reduction. [ABSTRACT FROM AUTHOR]

This paper presents an intensive discussion on conducted electromagnetic interference (EMI) emissions from an inverter-driven air conditioner. Common-mode voltages produced by the pulsewidth-modulated (PWM) inverter may cause a high-frequency leakage current. The use of a spectrum analyzer and a line-impedance stabilization network (LISN) makes it possible to refer the leakage current in the time domain to the conducted-EMI emission levels in the frequency domain, which are prescribed by the EMI regulations. Simulation-based estimation of the conducted-EMI emission levels in advance allows engineers to efficiently design a cost-effective EMI filter intended for meeting the regulations. This paper achieves modeling of the inverter-driven air conditioner with focus on common-mode voltages and leakage-current propagation paths. The simulation circuit developed can be used to predict conducted-EMI emission levels and to design the cost-effective EMI filter. Simulated results agree fairly with experimental ones in both cases of the EMI filter being connected and disconnected. [ABSTRACT FROM AUTHOR]

This paper presents the experimental results of the independent drive characteristics of two three-phase induction motors (IMs) fed by a four-leg inverter (FLI). The FLI is a single inverter that can drive two three-phase ac motors independently. The inverter consists of four legs and two capacitors connected in a series. The U and V phases of both motors are connected in each leg, respectively, whereas the W phase of both motors is connected in the neutral point of two-sprit capacitors. The pulsewidth modulation technique in three-phase voltage source inverter is not directly applicable for the FLI because only two phases must be modulated. Then, this paper also analyzes about the neutral point potential of two-sprit capacitors and inverter output voltage. Next, the modulation technique in the FLI is also shown. The experimental results demonstrate the characteristics of two IM independent drives and the validity of those analytic results. [ABSTRACT FROM AUTHOR]

This paper presents a novel flux-weakening strategy for an induction machine driven by an electrolytic-capacitor-less inverter. In the electrolytic-capacitor-less inverter, the dc-link voltage is fluctuating at six times the frequency of the input three-phase source due to its small dc-link capacitance. Hence, the decoupling of the fluctuation and maximum utilization of the dc-link voltage is a major issue in the electrolytic-capacitor-less inverter. In this paper, the cost function is set to increase the voltage utilization of the inverter for the flux-weakening operation of an induction machine. With the proposed flux-weakening strategy, the operating speed of the induction machine is extended above the base speed without any stability problems. The experimental results show the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

This paper proposes a square-wave control strategy for a single-phase power factor correction with a new boost-up converter which uses the leakage inductance of a motor instead of a boost-up reactor. Since the power supply is connected to the neutral point of the motor, the current distortion in the power supply occurs when the inverter outputs square waveforms. First, this paper describes the characteristic of the proposed circuit and the problems in the square-wave operation. Next, a current control method for the square waveform is proposed to suppress the current distortion. Finally, the validity of the proposed converter and its control strategy are demonstrated by experimental results. [ABSTRACT FROM PUBLISHER]

Many relevant concepts related to series connection and independent control of multiphase machines supplied from a single current-controlled voltage-source inverter (VSI) have recently been demonstrated by several researchers. These previous evaluations give great contribution to further improvements of multimachine series-connected devices and reduced count VSI applications. Following the initial proposition, which includes an evaluation based on the general theory of electrical machines, this paper evaluates pulsewidth modulation schemes for two-motor series-connected motor drives. The goal of this paper is to show how to achieve independent feedforward voltage control and to evaluate the switched voltage profile in the motors' windings and their behavior. Scalar and vector approaches are addressed, and simulation and experimental results are shown to illustrate the details discussed in this paper. [ABSTRACT FROM AUTHOR]

Photovoltaic (PV) energy conversion has been on the spotlight of scientific research on renewable energy for several years. In recent years, the bulk of the research on PV has focused on transformerless grid-connected inverters, more efficient than traditional line transformer-based ones, but more critical from a power quality point of view, especially in terms of ground leakage current. Neutral-point-clamped (NPC) inverters have recently gained interest due to their intrinsically low ground leakage current and high efficiency, especially for MOSFET-based topologies. This paper presents an active NPC (ANPC) topology equipped with 650-V silicon carbide (SiC) MOSFETs, with a new modulation strategy that allows to reap the benefits of the wide-bandgap devices. An efficiency improvement is obtained due to the parallel operation of two devices during the freewheeling intervals. Simulations and experimental results confirm the effectiveness of the proposed converter. [ABSTRACT FROM PUBLISHER]

Three-level neutral point clamped (NPC) inverters exhibit a natural balancing property that can self-regulate the intermediate neutral point voltage to half the overall dc-link voltage. However, the underlying mechanisms of this process are not well understood, its performance under various load conditions has not been properly explored, and the requirements and benefits of ( $RLC$) balance booster filters to enhance the natural balancing response are not clear. This paper presents a generalized model of an NPC natural balancing process that allows these issues to be explored for various load structures and balance booster configurations (i.e., isolated wye, wye with a grounded star point, etc.). The model accurately predicts the dominant balancing time constant, identifies the harmonics that drive the balancing process, and can quantify the steady-state balance booster losses for a designed target natural balance response. Extensive simulation and experimental tests have been performed to validate the theoretical model. [ABSTRACT FROM PUBLISHER]

Buck-converter-based topologies are used to generate high-frequency sinusoidal outputs. Buck-based inversion circuits such as voltage source inverters or class-D amplifiers have inherent control-to-output linearity in large-signal sense. However, in these topologies, the instantaneous output is always smaller than the dc-input during linear modulation. A differential boost inverter (DBI) is a boost-based topology that is used to generate a sinusoidal output. In DBI, the instantaneous output can be higher or lower than the dc-input voltage. DBI exhibits nonlinear control-to-output behavior in large-signal dynamic sense. Therefore, generating a high-frequency sinusoidal output using this topology is a challenge. The issues associated with a DBI for high-frequency sine wave generation are characterized in this paper. Conventional linear and nonlinear control techniques fail to produce a high-quality sine wave output at higher operating frequency. A nonlinear feedback linearization technique is proposed, which forces the output to be linear with respect to the reference even at higher operating frequency. This leads to a high-frequency high-quality sine wave generation using a DBI. The proposed modulator is verified using a laboratory prototype to generate a sine wave up to 2 kHz. A triangular wave of 100-Hz frequency is also generated by the proposed technique. Superior dynamic responses of a dynamic linearizing modulator controlled DBI for a step change in frequency, load current, input voltage, and reference are also experimentally verified. [ABSTRACT FROM AUTHOR]

In this paper, an online method to calculate the optimum dc bus voltage for motors and inverters with variable dc bus voltages is proposed. The required dc bus voltage in the low-speed region is much lower than that for the rated-speed region. As a result, the machine drive system efficiency can be improved with lower dc bus voltage since the switching losses in the inverter are reduced. To achieve both desired machine output torque and minimized losses simultaneously, an optimum dc bus voltage is required. However, machine parameter estimation errors and spatial harmonics in the machine result in calculation errors for the required dc bus voltage. A voltage disturbance state filter (VDSF), which is formed by a Luenburger-style closed-loop stator current vector observer with voltage command feedforward, is used to mitigate the calculation errors. Voltage errors caused by the parameter deviations and machine spatial harmonics will inherently be estimated by the observer's state feedback controller. Therefore, the calculation voltage error can be corrected by the VDSF's disturbance voltage so that the optimum dc bus voltage can be obtained. [ABSTRACT FROM PUBLISHER]

This paper investigates the effect of voltage stress on the voltage insulation components inside a regenerative active-front-end adjustable speed drive (ASD). It shows that a high potential voltage insulation issue may exist on various components inside the ASD and cause earlier failures. A simplified system model to describe this phenomenon is described, and the voltage stresses of different components inside the ASD under different grounded conditions are analyzed. It is concluded that a much higher voltage stress may exist in different locations of the drive under different grounding systems. Appropriately designing the insulation components is critical to protect the drive. [ABSTRACT FROM AUTHOR]

AC current regulation for low-pulse-ratio inverters is particularly challenging because the controller bandwidth is often comparable to the target fundamental frequency and this constrains the transient response. This paper presents an improved current regulation strategy to address this issue, using state feedback concepts to deterministically compensate for pulsewidth modulation (PWM) transport delay so that the controller gains can be significantly increased. Controller design principles are presented using discrete linear-quadratic-regulator theory, and an elegant strategy is applied to make the system more robust to PWM saturation. Experimental results for both 50-Hz and 400-Hz systems confirm the theoretical concepts and demonstrate the superior transient performance of the proposed strategy. [ABSTRACT FROM AUTHOR]

A new hybrid five-level inverter topology with common-mode voltage (CMV) elimination for induction motor drive is proposed in this paper. This topology has only one dc source, and different voltage levels are generated by using this voltage source along with floating capacitors charged to asymmetrical voltage levels. The pulsewidth modulation (PWM) scheme employed in this topology balances the capacitor voltages at the required levels at any power factor and modulation index while eliminating the CMV. This inverter has good fault-tolerant capability as it can be operated in three- or two-level mode with CMV elimination, in case of any failure in the H-bridges. More voltage levels with CMV elimination can be realized from this topology but only in a limited range of modulation index and power factor. Extensive simulation is done to validate the PWM technique for CMV elimination and balancing of the capacitor voltages. The experimental verification of the proposed inverter-fed induction motor is carried out in the linear modulation and overmodulation regions. The steady-state and transient operations of the drive are verified. The dynamics of the capacitor voltage balancing is also tested. The experimental results demonstrate that the proposed topology can be considered for industrial drive applications. [ABSTRACT FROM AUTHOR]

The advancement in power electronics has made the use of any number of phases in ac machines possible since with the inverter, any number of phase voltages can be realized as long as the corresponding inverter exists. To supply multiphase drives requires multiphase voltage source inverters (VSIs) whose output voltages depend very much on the methods of generating the pulses to turn on and turn off the inverter devices appropriately. This paper presents an analytical technique for the determination of the expressions for the modulation signals used in the carrier-based sinusoidal and generalized discontinuous pulse width modulation schemes for two-level, five-phase voltage source converters. These expressions can be used for both balanced and unbalanced phase voltages generated by any two-level multiphase inverter, in generating the required reference signals appropriately. An example of a five-phase VSI is presented to illustrate the strategy. Simulation and experimental results are presented in which a close examination shows that both results are similar for the balanced and unbalanced load voltage cases. [ABSTRACT FROM AUTHOR]

This paper proposes a smooth transition control strategy for voltage-source inverters between standalone (SA) and grid-connected (GC) modes of operation. In the GC mode, the amount of power exchanged with the utility grid is controlled by regulating the phase currents. In the SA mode, the load voltage is regulated by the inverter with its phase dictated by the inverter control. The transition between SA and GC operations that will ensure continuous power delivery to the load requires continuation in the phase of the system voltage. The proportional–integral, trapezoidal, sinusoidal, and staircase frequency variation techniques have been analyzed to find the best approach for minimizing the total harmonic distortion (THD). A smooth frequency variation technique has been developed, which provides lower THD on the voltage waveforms compared with the other techniques. The new algorithm has been implemented on a 5-kW single-phase utility interactive inverter having the SA operation capability. The simulation and experimental results show that the proposed technique provides seamless transition between the inverter modes of operation with minimal distortion at the inverter output voltage. [ABSTRACT FROM PUBLISHER]

Operating a dual three-level inverter system from a single dc link circuit is an economic solution for generating five-level output voltage waveforms and to double the output power compared to a single three-level inverter. Such an inverter system is used for feeding a drive motor with open stator windings. A mechanism is then required to reduce the common-mode voltage components of the motor voltages. This paper describes an offline optimization method that minimizes both the harmonic distortion of the motor currents and the common-mode voltage components. The optimization permits reduction of the switching frequency to a very low value of 200 Hz without compromising the harmonic distortion. High performance operation of the drive system at low switching frequency is experimentally demonstrated. [ABSTRACT FROM PUBLISHER]

The inherent low switching frequency in medium-voltage alternating-current industrial drives presents power-quality and filter-design challenges. In this paper, four multilevel pulsewidth-modulation methods; phase disposition (PD), switching-loss minimization (SLM), and selective harmonic elimination (SHE) up to the 17th and 29th harmonics, respectively, are considered. The characteristics of long-cable effects on common-mode voltage (CMV) and differential-mode voltage (DMV), inverter losses and efficiency, induction machine voltage, and current harmonics are analyzed. Very little has been published in these quantitative comparisons. It is shown that the SHE method has reduced CMV as compared with the PD and SLM algorithms. Elimination of up to the 29th harmonic achieves the best harmonic performance without needing an output filter, at the expense that the losses are higher with a lower efficiency. Analytical and simulation results using the Piecewise Linear Electrical Circuit Simulation for the power-electronic circuits and MATLAB/Simulink for control systems are experimentally verified with a 1000-hp 4160-V neutral-point-clamped adjustable-speed-drive system that includes a 24-pulse front-end voltage source converter. [ABSTRACT FROM AUTHOR]

This paper demonstrates improved performance of a three-phase voltage source inverter (VSI) when digital multisampled space vector modulation is used. The modulator and the inverter controller are implemented entirely using a field-programmable gate array platform, thus achieving increased bandwidth with respect to a typical digital signal processor or microprocessor-based controller. Increased controller bandwidth results in lower output voltage harmonic distortion in the frequency range above the fundamental and below the switching frequency. Experimental validation is presented together with the analysis carried out using a state space model of a VSI with an output LC filter. [ABSTRACT FROM AUTHOR]

In large-drive applications, load-commutated inverters (LCIs) are one of the most used technologies mainly because of their excellent reliability records. However, LCIs are known to generate interharmonics. They can interact with the mechanical system at torsional natural frequencies of the rotating train, on both the inverter and rectifier sides in weakly connected power systems, such as offshore oil and gas platforms. These interactions can lead to accelerated shaft fatigue, lifetime reduction, gear damage, and system blackouts. On the other hand, pulsewidth-modulated (PWM) voltage-source inverters (VSIs) are known to produce less torque ripple compared to LCIs. Consequently, VSIs are supposed to be less subject to exciting torsional resonances in mechanical shaft systems. Even with reduced torque ripple, mechanical failures have been consistently reported due to motor air-gap torques supplied by PWM drives. This paper is focused on solving torsional vibration issues. Due to system uncertainties, these issues cannot be excluded in the design phase of LCIs and VSIs for high-power applications. With regard to LCIs, the dc-link inductance is used as an integrated energy-storage unit. Motor and generator interactions are therefore decoupled. Excited eigenmodes on the grid side are damped with the rectifier; the inverter is driving the variable-speed motor. This approach is also successfully applied to damping resonance modes in the motor side. Simulation and selected experimental results on a 30-MW LCI system are provided to validate the proposed design approach. As for VSIs, a more general approach of damping or controlling excited shafts is proposed. This method is successfully applied using the dc-link capacitor as an integrated energy-storage source. This approach is used to optimize the design of new systems; otherwise, it is used to improve the performance of existing systems with minor modifications. [ABSTRACT FROM AUTHOR]

Studies show that the power cycling mean time to failure (MTTF) of the insulated-gate bipolar transistor (IGBT) bond wire in an adjustable speed drive may be very short under some very common conditions. This paper proposes a switching frequency reduction method based on the junction temperature variation (\DeltaTj) of an inverter IGBT. It has the following advantages. First, it reduces the pulsewidth-modulation frequency under low-speed higher torque condition. Second, the inverter is started more smoothly and safely under higher command switching frequency and high-torque condition. Third, the overall MTTF of the inverter IGBT can be improved. The theory analysis, simulation, and experimental result are provided to show these advantages. [ABSTRACT FROM PUBLISHER]

In this paper, a novel grid integration system for a variable-speed wind turbine using an interior permanent-magnet synchronous generator (IPMSG) is developed. The power conditioner system (PCS) consists of a series-type 12-pulse uncontrolled diode rectifier powered by a phase-shifting transformer and then cascaded to a pulsewidth-modulated (PWM) voltage source inverter. The active current of the grid-side PWM inverter is only controlled to follow the optimal active current reference which is determined by using a simple maximum power point tracking (MPPT) control strategy. The MPPT algorithm requires only three sensors in order to track the maximum power of the wind turbine. The most significant advantage of the proposed system is that the passive filter together with a series-type 12-pulse rectifier provides high efficiency by compensating the power factor angle of the IPMSG and suppresses distortions presented in the IPMSG voltages and currents. The laboratory results indicate that the proposed construction and scheme are simple, cheap, and efficient. [ABSTRACT FROM PUBLISHER]

This paper proposes a new dc-voltage-balancing circuit for a five-level diode-clamped inverter intended for a medium-voltage motor drive with a three-phase diode rectifier used as the front end. This circuit consists of two unidirectional choppers and a single coupled inductor with two galvanically isolated windings. The inductor produces no net dc magnetic flux because the individual dc magnetic fluxes generated by the two windings are canceled out with each other. This makes the inductor compact by a factor of six, compared with the balancing circuit including two noncoupled inductors. Moreover, introducing phase-shift control to the new balancing circuit makes it possible to adjust the midpoint voltage. As a result, the dc mean voltages of all the four split dc capacitors can be balanced, independent of inverter control. Experimental results obtained from a 200-V 5.5-kW downscaled model verify the effectiveness of the new balancing circuit. [ABSTRACT FROM PUBLISHER]

This paper analyzes the insulated gate bipolar transistor (IGBT) power cycling capability for a dual-bridge matrix converter (DBMC) used as a motor drive and compares the result with the traditional dc/ac inverter and the conventional matrix converter (CMC). Two pulsewidth modulation (PWM) control methods for the DBMC are investigated. One is called zero-current PWM (ZCPWM), which allows zero current commutation of the rectifier side switches. The other is named zero-voltage PWM (ZVPWM), which generates zero-voltage switching in the inverter IGBT. It is found that the DBMC under the ZVPWM method shows much higher power cycling capabilities in the IGBT than that of the ZCPWM and the other two topologies. Moreover, by appropriately selecting the sequence of the ZVPWM and ZCPWM methods, the DBMC shows overall advantages on the sizing of the IGBT and the balancing of the thermal system. It is also shown that the IGBT applied in the DBMC inverter is easier to operate under higher switching frequency conditions. [ABSTRACT FROM PUBLISHER]

Unexpected voltage sags or outages often interrupt the operation of manufacturing processes or even lead to equipment damage. Numerous critical loads rely on the uninterruptible power supply system to uphold the power during these events and maintain the normal operation. As disturbances occur in the utility, the uninterruptible power supply (UPS) should take over the load within 1-5 ms to prevent any interruptions. However, the startup process of the UPS can cause the significant inrush current phenomenon on the load transformer due to magnetic saturation. The inrush current could result in reduced line voltages and even trigger the UPS's overcurrent protection. To prevent the inrush current, this paper proposes a flux offset compensation technique based on the synchronous reference frame (SRF) to regulate the flux of the load transformer. The proposed technique can be integrated with the existing voltage control and the pulse-width modulation (PWM) of the UPS inverter without any additional sensors. The proposed technique can provide a very smooth and timely transition without the risk of inrush current for critical loads under power outages and voltage sags. [ABSTRACT FROM AUTHOR]

A voltage-source inverter without dc-link components is an ac-to-ac converter having dual bridges of a pulsewidth-modulated (PWM) rectifier and a PWM inverter that can be controlled independently. While conventional matrix converters have disadvantages, such as a complicated commutation scheme and necessity of a large-sized clamp circuit, commutation, and protection of our inverter can be implemented easily. In order to control the PWM rectifier and the PWM inverter independently, snubber circuits for the PWM rectifier are required to assure the path of load current during dead time. In this paper, analytical method of a snubber circuit and operating characteristics of a snubber circuit are described when a O.75-kW induction motor is driven by our inverter. [ABSTRACT FROM AUTHOR]