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The staircase modulation strategy is the best choice for asymmetric cascaded H-bridge multilevel inverter (ACMI) because it makes the output voltage to present a high number of levels and makes one of the ACMI modules to operate at low frequency. However, the staircase modulation is usually employed without giving its deserved attention. This paper presents a clear understanding of the staircase modulation strategy and its applications in both isolated and grid-connected ACMI.  The main novelty of this paper is the obtaining of the switching function for each module. The principle of operation is carefully described, also indicating how to command the ACMI transistors. The output voltage of the two superior modules is evaluated for different values of iterations. Special attention is given to the power distribution among the ACMI modules when the staircase modulation is applied. It is demonstrated on how the power is processed in each module considering an ACMI with three modules. The analysis is presented showing the staircase modulation within a closed-loop control strategy for isolated and grid-connected modes of operation, showing that the content of this paper is valid for both modes. Experimental results clarify and show the efficacy of the content presented in this paper. [ABSTRACT FROM AUTHOR]

Emulation of an electric machine allows the testing of a drive inverter and controller of an electric drive system, prior to the manufacture of a machine prototype. This paper presents the emulation of a variable flux machine (VFM). The VFM emulator system proposed in this paper uses a detailed look-up table-based machine model for the purpose of emulation. This allows the emulator to mimic all machine magnetic and geometric behaviors such as saturation and torque ripple. In addition, the machine emulator proposed in this paper uses high-performance high-bandwidth linear amplifiers as power amplifiers. This enables a high bandwidth; leading to a high accuracy machine emulation. A detailed control description for the proposed machine emulator system and a machine model look-up table data verification against a prototype machine is initially presented. Experimental results are then presented to validate the utility of the proposed VFM emulator system to emulate various machine transient behaviors at different magnetization levels. Experimental results obtained from the emulator are subsequently compared against the experimental results obtained from a prototype VFM drive to verify the emulation accuracy. [ABSTRACT FROM AUTHOR]

Many research works report the use of LCL filters for grid-interfaced PV systems. Indiscreet selection of passive LCL filter parameters may result in poor filtering and increased power losses. LCL filter parameter selection is critical to get a good performance and at the same time ensure that losses and costs stay low. This paper proposes a novel approach to design the LCL filter for a grid-interfaced PV system. A cost-based minimization approach on a cost per joule basis for each of the filter elements is explored to obtain the optimal filter design. Simulation results for a 10-kVA grid-connected inverter with an LCL filter are presented in order to verify the efficacy of the proposed design. The impact of designed filter parameters on the ripple current is also discussed in this paper. The total inductance of the LCL filter is varied and filter parameter values which give lowest cost is identified while meeting harmonic standard requirements and the fundamental voltage drop across the filter. Finally, the proposed design is also experimentally validated via a hardware prototype. Results validate the design method in terms of the harmonic attenuation and total harmonic distortion. [ABSTRACT FROM AUTHOR]

In this paper, the theoretical analysis of the position estimation error in position-sensorless operation using pulsating square wave signal injection is presented. The purpose of this paper is to analyze the phenomenon that the position estimation performance varies depending on the injection frequency. Mathematical derivation supported by simulation and experimental results shows that, in no-load condition, the voltage distortion induces position estimation error with 3rd-order harmonics if the frequency of the injected square wave is the same to the switching frequency. While, if the frequency is a half of the switching frequency or heavier load condition in either injection frequency, the effect of nonideal characteristics of inverter disappears and there is no 3rd harmonics in the position estimation error. Therefore, only the effect of cross-coupling inductance is reflected in the estimation error. As the result, the position estimation error appears in the form of 6th-order harmonics due to the spatial harmonics of inductances, and the error is relatively small compared with the switching frequency injection case. The theoretical analysis was verified by simulation and experimental results. [ABSTRACT FROM AUTHOR]

A multifunctional grid-connected inverter (MFGCI) has been a promising solution to mitigate power quality problems in distributed generation system and microgrid applications. However, the capability of a conventional MFGCI is limited when compensating a voltage-based power quality problem. In this paper, a new configuration of MFGCI is proposed to improve both voltage-based and current-based power quality issues and is named shunt-series-switched MFGCI (SSS-MFGCI). The SSS-MFGCI can be connected in series or parallel to the grid by implementing bidirectional switches to provide different modes of operation depending on various grid disturbances. The required compensating current and voltage are analytically derived considering the grid and load conditions for voltage regulation. This paper also presents the compensation strategies, control scheme, and transition scheme between different modes of the proposed SSS-MFGCI. Simulation results to prove the concept are also included for unbalanced sag and swell cases. [ABSTRACT FROM AUTHOR]

Grid-tied inverters and associated control techniques have gained importance in the domain of distributed generation. Different methods of control have been compared in the literature depending upon the ability to meet total harmonic distortion limits, damping offered to resonant oscillations, and stable operation. Multiple-loop methods compared to their single-loop counterparts have proved to be highly effective for meeting the grid regulations. In the literature, while single-loop and two-loop control systems for LCL filter have been assessed, not much study on three-loop control (TLC) has been done, possibly due to the hardware complexity and cost imparted by the increase in the number of sensors. In this paper, a TLC structure for grid connected inverters is analyzed for stability and parametric variations. Robustness of TLC is compared with single-loop grid current feedback control and a two-loop control structure. An approach to estimate the state variables is proposed in this paper, which cuts down the number of sensors and cost of implementation. The performance of the system is analyzed through simulations and results are validated in a hardware setup, under dynamic and steady-state conditions, thereby showing that this is a promising approach. [ABSTRACT FROM AUTHOR]

This paper presents theoretical and experimental discussions on low-voltage-ride-through operation of a modular multilevel single-delta bridge-cell (SDBC) inverter intended for utility-scale photovoltaic (PV) systems. Modern grid codes require grid-tied inverters to provide dynamic grid support during grid-fault events by injecting reactive current. This paper discusses decoupled positive- and negative-sequence reactive-current control, focusing on asymmetric voltage sags with imbalanced magnitude and phase relationships. The main objective is to present a feedforward control method based on calculation of the zero-sequence current required for achieving power balance during normal and grid-fault conditions. Moreover, this paper demonstrates a practical method that minimizes overcurrent stress in the three inverter clusters by adjusting active power drawn from PV arrays. Experimental results on a three-phase 12.6-kVA system prove that the SDBC inverter is capable of seamlessly operating through asymmetric voltage sags. [ABSTRACT FROM AUTHOR]

The pump-back test is widely adopted in the industry for characterizing and qualifying high-power converters. One popular pump-back test configuration is to tie the dc links of two converters together and circulate the current through the inductor connected between the ac terminals. In this configuration, one of the converters operates in the voltage mode to generate the virtual grid and the other converter regulates the circulating current, where hysteresis control could be applied. However, the switching characteristics of hysteresis control are unique due to the pulsewidth modulation type voltage being the virtual grid. This paper contributes to analyzing the switching frequency characteristics of the hysteresis control in this pump-back test configuration. The switching frequency tracking phenomenon is revealed and reported in this paper for the first time. Theoretical analysis, simulation validation, and experimental demonstration are provided to support the major conclusions. [ABSTRACT FROM AUTHOR]

A programmable electronic load (PEL) solution is usually needed for comprehensive testing of power supplies. In this paper, a novel PEL based on a hybrid five-level voltage source inverter (VSI) is introduced to emulate a variety of static load behaviors, such as R, L, C loads, high-frequency harmonic loads, and unbalanced loads. Dynamic load behavior, for example, modulation loads, can also be achieved in this paper. A dedicated control structure is proposed for the emulation of different load behaviors and the detailed compensator design is presented to evaluate the system dynamic performance. The proposed PEL system has isolated active dc links to support multilevel converter topology, and it can either dump the energy to a passive load or recycle the energy back to the utility grid through another VSI. Simulation and experimental results are provided to validate the effectiveness of proposed control methods in terms of successfully achieving a variety of load profile functions. [ABSTRACT FROM AUTHOR]

This paper proposes the enhancement of the control of a grid-connected inverter by a simplified virtual synchronous compensator (S-VSC) model working in parallel with the traditional inverter current control loops. The goal of the integration of this model into the inverter control scheme is to provide grid services, such as virtual inertia behavior, current harmonic compensation, as well as reactive grid support during faults. The S-VSC is only in charge of providing the aforementioned services, working, therefore, always at a low power level. On the other hand, the main power references are sent directly to the inverter control loops. This way, a more stable and damped operation of the inverter is obtained. The proposed structure has been implemented on a 15 kVA grid-connected inverter for experimental validation. [ABSTRACT FROM AUTHOR]

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 proposes an adaptive power management strategy for photovoltaic (PV) generation units in a standalone microgrid. In existing microgrid configurations, the PV units are controlled using droop control technique, which ensures load sharing among the distributed generation (DG) inverters. However, the intermittency in power generation of PV-based DG poses many challenges to the standalone operation of a microgrid, the ineffective volt-ampere (VA) utilization being one among them. This paper proposes an adaptive droop control, which enables the maximum utilization of the VA rating of the interfacing inverter in a PV/battery hybrid system. With the proposed technique, as the generation goes down, the DG inverter is controlled to support the reactive power demand of the load, thereby utilizing the rating of the inverter switches effectively. The applicability of the proposed adaptive control method is analyzed using detailed simulation and real-time hardware-in-loop studies. [ABSTRACT FROM AUTHOR]

This paper proposes a method to suppress the resonant current circulating between integrated half-bridge modules with phase-leg capacitors in two legs. This circuit configuration has the advantage of low stray inductance between power semiconductor devices and dc-link capacitors (phase-leg capacitors). This paper presents an analysis of the equivalent circuit for the circuit configuration that suggests that the dc-link capacitor current will increase depending on the relationship between resonant and switching frequencies. The capacitor current was experimentally evaluated under pulse-width modulation operation at switching frequencies up to 100 kHz using SiC-mosfet s. As a result, the resonant current may occur in the dc side when the resonant frequency between phase legs was close to either the fundamental or the third harmonic of the switching frequency. In order to suppress the circulating resonant current, a graphite bus bar is applied to the phase-connecting bus bar. Using the graphite bus bar, the capacitor current can be reduced by 35% without any influence from the ac-side output current. [ABSTRACT FROM AUTHOR]

In this paper, a generalization for three-phase four-wire inverters is proposed. In this generalization, an arbitrary number of three-phase four-wire dc–ac converters are cascaded by means of injection transformers, in order to compose a resultant inverter with a single dc link. Its converters may have four or three legs composed by a pair of the complementary insulated-gate bipolar transistor. The injection transformers have different turns ratios calculated in order to maximize the number of levels produced in the output voltage. In this way, the converter cells within the inverter are asymmetrical in terms of power rating. In this paper, the proposed generalization is applied to a specific case, where two four-wire converters are cascaded, which generates four different proposed configurations. These configurations are analyzed in detail and compared with the conventional four-leg inverter by means of data acquired from simulation and experimental results. In these results, the proposed and conventional inverters feed an unbalanced three-phase load accordingly to the same reference voltage, and the load current total harmonic distortion, voltage weighted total harmonic distortion, transformers and semiconductors losses, switching frequency, and power rating are analyzed in order to conclude that proposed configurations present better performance, even though they have more semiconductors and demand injection transformers. [ABSTRACT FROM PUBLISHER]

To minimize the ripple on the dc-link current and torque due to carrier signal injections, conventional saliency based self-sensing control schemes using voltage and current carrier signals for interior permanent magnet synchronous machines (IPMSMs) are optimized in this paper. Carrier signal oriented coordinates are introduced to model the high-frequency behavior of IPMSMs and are used for the analysis and signal processing of proposed methods. Moreover, stability issues are discussed in this paper. Experimental results on a commercial high voltage inverter for application in hybrid and electrical vehicles confirm the applicability of the proposed methods. [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]

In this paper, it is described how the circulating current flows between inverters that have isolated dc link such as cascaded H-bridge (CHB) topology when they are operating in parallel. In this case, zero sequence circulating current, which normally flows through shared dc link, cannot flow between inverters in parallel. However, circulating current is provoked by asynchronous switching instant of devices, difference of dc-link voltages of parallel inverters, and unbalanced impedance. The circulating current should be suppressed by a sharing inductor that is inserted between inverters. Generally, all of the inverters should synthesize the same output voltage reference for load current control to minimize the size of this sharing reactor. However, this conventional method cannot guarantee diminishing circulating current in the transient state and even in the steady state. In this paper, in order to reduce sharing reactance, after deriving circulating current model from general case of n-parallel operation, circulating current control method is devised based on the model. This proposed algorithm is applied to active front end five-level-CHB inverter system for medium voltage drive. Simulation and experimental results are provided to verify the effectiveness of the proposed control scheme. [ABSTRACT FROM AUTHOR]

This paper proposes a new inverter topology based on the active neutral-point-clamped (ANPC) converter with capability to integrate one energy storage system (ESS) per inverter leg with an independent state of charge control, so that each inverter leg has two dc ports and one ac port. The primary dc port is common to all inverter legs and it is connected to the main power source. The secondary dc port is suitable for ESS connection because of its two-quadrant operation, with bidirectional current capability, so that the ESS can be charged and discharged with regulated current. Integration of a secondary dc port with independent control also allows the use of different ESS technologies for each inverter leg to compose a hybrid ESS. Redundant switching states of the proposed structure are used to balance the energy exchanged between the secondary dc port and the dc bus capacitors. Theoretical propositions were experimentally validated through a 1-kW single-phase inverter with a secondary dc port connected to a 230-V ESS and ac port connected to an LC filter feeding a linear load. Low total harmonic distortion (THD) sinusoidal current is generated at the ac port while charging or discharging the ESS with a constant current. [ABSTRACT FROM AUTHOR]

Recently multilevel inverters (MLIs) have received wide attention from industry and academia, as they are changing into a viable technology for diverse applications. To produce high-quality output using less switch count, development of novel reduced switch MLI (RS MLI) topologies has been a focus of current research theme. This paper presents design and control of a switched-diode dual source single switch MLI (SDDS MLI). The generalized SDDS MLI is first designed using an asymmetric basic unit. Proposed SDDS MLI requires less switch count and driver circuit count compared with the few recently developed RS MLI topologies. To improve the voltage quality by eliminating targeted low-order harmonics, a modified version of fish swarm optimization algorithm is examined for computing optimum switching angles required to control the SDDS MLI. Moreover, suitability and superiority of the derived algorithm are established by comparing with traditional selective harmonic elimination techniques. The developed topology is investigated through several MATLAB simulations as well as experimental tests in the laboratory applying the modified control approach. [ABSTRACT FROM AUTHOR]

A gate driving for power devices is a key technology to further improve switching characteristics. With the help of digital gate driver IC, the switching behavior of power devices can be enhanced even under high-speed switching. In this paper, an evaluation platform for determining the optimal switching pattern of an active gate drive control is proposed for an inverter circuit. A high speed optimization system is built up to search for an advantageous switching pattern that reduces total switching loss of two power devices in an inverter circuit and constrains surge voltage simultaneously. The proposed online optimization demonstrates its feasibility for the full-bridge inverter circuit, which is rated at 500 V with digital active gate drive control. Experimental results show that the proposed optimization system is able to obtain optimal switching pattern from $64^{60}$ possible combinations of switching patterns within 15 min, which is 6 times faster than the previous study. Optimizations can also conducted under different load current conditions. Eventually, the obtained optimal pattern yields up to 42% reduction in the total switching loss when it constrains surge voltage to minimum compared with the conventional driving pattern. [ABSTRACT FROM AUTHOR]

High slew rate of the line voltage $(\boldsymbol {dv}/\boldsymbol {dt})$ has been a concern for power inverters based on the emerging wide bandgap switching devices, such as silicon carbide (SiC) mosfets. Particularly, for SiC-based power inverters feeding electric machines interconnected with long cables, there could be more severe insulation stress on the stator windings in electric machines, due to the higher dv/dt output from the SiC inverters. Compared to the conventional lower voltage electric transportation applications (e.g., 270 V dc), higher dc bus voltage of 500–600 V can dramatically reduce cable weight and systematic copper losses, hence improve the power density and efficiency of power converters. However, high dc bus voltage further increases the $\boldsymbol {dv}/\boldsymbol {dt}$ level in the converter ac line voltages. Driven by the necessity of developing SiC inverters with 500 V dc bus in electric transportation applications while attenuating the $\boldsymbol {dv}/\boldsymbol {dt}$ stress to a low level, this paper presents a multi-domain design approach for dv/dt filters that comprehensively considers the constraints in electrical, magnetic, and thermal domains. Experimental results based on a 75 kW SiC inverter are provided to verify the efficacy of this design approach. [ABSTRACT FROM AUTHOR]

This paper presents the bilateral operation of a transformer-less H-bridge inverter as a power flow controller and fault current limiter in low-voltage distribution network. A novel control strategy has been developed to initiate and activate the H-bridge inverter to control the power flow at point of common coupling (PCC). On the other hand, the H-bridge inverter operation like a short-circuit current limiter during grid disturbance event and its smooth reinsertion after fault clearance have also been clarified. Besides, an island mode control strategy incorporating synchronization controller, and proper coordination with flip flops have been implemented in order to synchronize the microgrid with the main grid before reclosing the circuit breaker. The results confirm: The effectiveness and feasibility of the novel control methodology in activating the real power flow; the inverter capability of minimizing the short-circuit current during fault and the successful resynchronization of the microgrid voltage source inverter (VSI) and the H-bridge inverter to the main grid. [ABSTRACT FROM AUTHOR]

Harmonic instability is threatening the operation of renewable energy based power plants where multiple grid connected voltage source inverters (VSIs) are connected in parallel. To analyze and improve the stability of the grid-connected VSIs, the real part of the output admittance of the VSIs is first investigated in this paper. It has been shown that the negative real part of the output admittance will make the system unstable under certain grid condition. Based on the stability analysis, this paper proposes a method to cancel the negative real parts by using VSIs with different LCL filters, sampling frequencies, and control strategies. The proposed method does not require the VSIs to have any active damping function. Three specific cases are studied where the LCL parameters, sampling frequencies, and current control strategies of the VSIs are different. The results show that the combination of different types of grid-connected VSIs can improve the stability of the renewable power plant. [ABSTRACT FROM AUTHOR]

A considerable amount of papers has been published in recent years proposing supervised classifiers to diagnose the health of a machine. The usual procedure with these classifiers is to train them using data acquired through controlled experiments, expecting them to perform well on new data, classifying correctly the condition of a motor. But, obviously, the new motor to be diagnosed cannot be the same that has been used during the training process; it may be a motor with different characteristics and fed from a completely different source. These different conditions between the training process and the testing one can deeply influence the diagnosis. To avoid these drawbacks, in this paper, a new method is proposed, which is based on robust statistical techniques applied in quality control applications. The proposed method is based on the online diagnosis of the operating motor and can detect deviations from the normal operational conditions. A robust approach has been implemented using high-breakdown statistical techniques, which can reliably detect anomalous data that often cause an unexpected overestimation of the data variability, reducing the ability of standard procedures to detect faulty conditions in earlier stages. A case study is presented to prove the validity of the proposed approach. Motors of different characteristics, fed from the power line and several different inverters, are tested. Three different fault conditions are provoked: a broken bar, a faulty bearing, and mixed eccentricity. Experimental results prove that the proposed approach can detect incipient faults. [ABSTRACT FROM PUBLISHER]

Three-phase voltage-source inverters typically feature a significant amount of dc energy storage to maintain a stiff dc bus. In these cases, the dc-link capacitor is sized to store enough energy to maintain several tens of cycles of ac output at the rated power. In this paper, we propose to reduce the size of the dc-link capacitor dramatically to store enough energy to provide just one high-frequency switching cycle of the ac output power. The dc bus is no longer stiff, and hence, classical sinusoidal pulse-width modulation cannot be used. However, the stored energy modulation (SEM) proposed herein synthesizes high-quality sinusoidal output voltage waveforms, even with such tiny dc-link capacitors. In SEM, the switching intervals of the interconnecting switches are carefully determined nonlinear functions of various operating parameters such as reactive component sizing, switching frequency, load levels, etc. This paper presents the analytical and detailed design development of the SEM approach along with circuit simulation, experimental results, and comparative evaluation of a dc-to-three-phase system example. [ABSTRACT FROM PUBLISHER]

One of the main challenges of the solid-state transformer (SST) lies in the implementation of the dc–dc stage. In this paper, a quadruple-active-bridge (QAB) dc–dc converter is investigated to be used as a basic module of a modular three-stage SST. Besides the feature of high power density and soft-switching operation (also found in others converters), the QAB converter provides a solution with reduced number of high-frequency transformers, since more bridges are connected to the same multiwinding transformer. To ensure soft switching for the entire operation range of the QAB converter, the triangular current-mode modulation strategy, previously adopted for the dual-active-bridge converter, is extended to the QAB converter. The theoretical analysis is developed considering balanced (equal power processed by the medium-voltage (MV) cells) and unbalanced (unequal power processed by the MV cells) conditions. In order to validate the theoretical analysis developed in the paper, a 2-kW prototype is built and experimented. [ABSTRACT FROM PUBLISHER]

The paper shows the reduction of core losses by using a three-level inverter over a two-level inverter for the same dc-bus voltage and switching frequency. A synchronous reluctance machine stator-core toroid was used in the analysis. Hence, the analysis is realistic as it accounts for mechanical effects through the stator core, and the distorted supply of the line-to-line inverter voltage supplies. The paper also shows the limitations of using finite element-derived excitation; hence, the toroid was directly supplied by the inverter. The reduction of core losses by use of a three-level inverter is significant at very high flux densities and frequencies; approximately 60% lower core losses. Therefore, it can reduce the cooling burden especially in the hard to cool teeth, increase the service life, and allow increased output. The latter is because the output of the three-level fundamental voltage is higher for the same dc-bus and switching frequency at lower machine losses. [ABSTRACT FROM PUBLISHER]

Paralleled inverters have the merits of high power rating, improved reliability, and convenient maintenance. However, zero-sequence circulating current (ZSCC) will occur and lead to current distortion and system loss. The ZSCC control is more complicated when three-level topology is utilized and paralleled number is increased. Therefore, this paper proposes a simplified ZSCC suppression scheme for multi-paralleled three-level T-type inverters (3LT2Is). A novel equivalent model is developed and the ZSCC controller based on the new model is easier to implement. Thus, the number of paralleled inverters can be increased without complicating the control scheme and the inverters could be switched on or off optionally. Besides, Feed-Forward (FF) strategy is adopted to eliminate the ZSCC spikes caused by symmetrical three-level space vector modulation. The effectiveness of the proposed control scheme is verified by both simulation and experimental results. [ABSTRACT FROM AUTHOR]

This paper presents an improved designed induction motor, used for photovoltaic (PV) array fed water pumping system by modifying the rotor and stator slots. The overall system is designed without a mechanical sensor to reduce both cost and complexity with the simultaneous assurance of optimum power utilization of a PV array. The proposed system consists of an induction motor operated water pump, controlled by field-oriented control with artificial neural network current control technique. The maximum power point tracking as well as dc-link voltage, is regulated by three-phase voltage source inverter. The estimation of motor speed eliminates the use of a mechanical sensor and makes the system cheaper and robust. A new robust speed adaptive algorithm is presented, which is less dependent on parameters. A detailed study of various factors affecting the efficiency of the motor is given to improve the behavior of the induction motor drive for water pumping. The designed motor is tested on a developed prototype in the laboratory and its suitability is judged through various tests under steady-state and dynamic conditions of insolation variations. [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]

In this paper, a regeneration protection solution is proposed to address the dc-link overvoltage issue and the unequal power sharing in the parallel uninterruptible power supply (UPS) systems. First, a dc-link voltage protection control strategy is proposed to protect the inverter against the excessive dc-link voltage that may trigger the protection mechanism of the UPS system. In addition, an active power-sharing control strategy by regulating the virtual resistance is proposed to solve the circulating current caused by UPS regeneration issue. Finally, the feasibility of the proposed regeneration protection solution is verified by experimental results from the parallel UPS system prototype. [ABSTRACT FROM AUTHOR]

This paper focuses on a fully distributed peer-to-peer control scheme for voltage regulation and reactive power sharing of multiple inverter-based distributed energy resources (DERs) in ac microgrids. The proposed peer-to-peer control strategy is fully distributed enabled through broadcast gossip communication, where each DER unit only requires local voltage and current measurement from its own and some (but not all) nearby neighbors for the voltage and reactive power sharing control. Employing the broadcast gossip communication protocol with attractive scalability and reliability properties, the control inputs can be updated to restore the voltage magnitudes at the point of common coupling to a desired value ensuring an accurate reactive power sharing for each DER. Since the proposed distributed controllers are implemented on local DERs, the central controller and hierarchy are no longer required. Accordingly, the microgrid system stability is preserved in the peer-to-peer requirements of line switches, which in turn, enables a plug-and-play operation of DERs and their robustness against microgrid topology change scenarios. Simulation studies in a modified IEEE 34-bus test network demonstrate the effectiveness and applicability of the proposed control strategy. [ABSTRACT FROM AUTHOR]

This paper proposes a switching strategy for the Z-source inverter (ZSI), which has unequal short-through intervals to reduce the current ripple in the inductor of its impedance source. A size reduction of the inductor of the ZSI is one of the important issues in improving the power density of the ZSI, and the inductor current ripple reduction is required to achieve it. The proposed method can reduce the inductor current ripple by 27.8% compared with the conventional method that has equal short-through intervals without increasing the number of inverter switching. The proposed operation and ripple reduction were confirmed in experiments with a 3-kW-class laboratory prototype. Its result agreed to the calculated results, and it was confirmed that there is no increase in circuit losses caused by the application of the proposed method. [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]

In this paper, a new zero-sequence current suppression control strategy for the five-phase open-winding fault-tolerant fractional-slot concentrated-winding interior-permanent-magnet (FTFSCW-IPM) motor drive system with a common dc bus is proposed. By unequal distribution of the zero voltage vector, the proposed method can well suppress the zero-sequence current caused by the dead-time effect that the conventional eliminating method cannot be achieved. First, the dead-time effect to the zero-sequence current is discussed. Then the zero-axis controller is designed to obtain the reference common-mode voltage. Last, according to the principle of the unequal zero voltage vector distribution, the switching sequence of the two inverters can be calculated, and the maximum modulation index is also first deduced in detail. The proposed method is proved by the simulation and experimental results. [ABSTRACT FROM AUTHOR]

There is an increasing need to apply rigorous model-order-reduction techniques in the analysis of large-scale networks of inverter-based distributed generation resources due to the limitations of existing simulation tools. Various coherency-based aggregation techniques have long been used to construct reduced-order dynamic models of large-scale synchronous machine (SM) networks. Such techniques have the advantage of preserving the nonlinear nature of the dynamic model throughout the order-reduction process, enabling the efficient and accurate analysis of large-scale network dynamics during large disturbances such as fault events. This paper proposes the application of a rigorous coherency-based aggregation technique to the analysis of large-scale networks of grid-forming droop-controlled inverters. A rapid and powerful generalized eigenvalue perturbation technique for coherency identification, previously only applied to SM networks, is adapted to grid-forming droop-controlled inverter networks. The resulting reduced-order models are physically insightful and are capable of accurately reproducing the system response in the aftermath of large disturbances. For some networks, a rigorously-derived condition of coherency can be difficult to achieve, given the expected range of L–C–L filter impedances. To remedy this limitation, the potential for high-bandwidth inverter control to enforce the conditions that allow for coherency of droop-controlled inverters has been investigated and confirmed using a controller hardware-in-the loop testbed. Using this approach, the use of simple nonlinear aggregate inverter models to accurately model large sections of the inverter network can be more rigorously justified. [ABSTRACT FROM AUTHOR]

This paper proposes an innovative transformer management system (TMS) for energy control of customer loads and photovoltaic (PV) systems to solve the problems of transformer overloading and overvoltage due to power injection by PV renewable energy. The operation status of distribution transformers is monitored by transformer terminal unit (TTU) and reported to the control master station via the hybrid communication system. The demand response of customer load control is activated to reduce the loading level of transformers for mitigation of overloading problem. For the residential customers with installation of roof top PV system, the TTU is applied to perform the power factor control of smart inverter to prevent the overvoltage problem. The hybrid communication system consisting of power line carrier, LoRa radio frequency, and optical fiber is used to provide two-way communication for data collection and controls of customer loads and PV system. According to the field testing of the proposed TMS in Taiwan Power Company, the adaptive energy control of customer loads by TTU is executed when the overloading problem of distribution transformer occurs. The TTU also performs the power factor control of PV smart inverters to solve the overvoltage problem due to too much of PV power being injected during the solar peak period. The overvoltage problem due to PV power injection has also been solved by TTU for control of PV smart inverters to perform the proper reactive power compensation. [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]

Artificial lift systems are generally applied in the petroleum industry to provide the energy to transfer the oil from the reservoir to the surface. Electrical submersible pump (ESP) system is an artificial lift method extensively used in recovery. The maintenance cost of ESP and associate equipment is very high; therefore, the maximum availability of the system components is continuously sought. The aim of this paper is at analyzing the availability and reliability of frequency converters applied to medium voltage (MV) multilevel cascaded H-bridge inverters topology for ESP. A method to increase availability of MV frequency drives is thus proposed, enabling fewer losses in the production and a greater return on the economic investment (ROI). [ABSTRACT FROM AUTHOR]

In this paper, a multipulse ac–dc converter fed multilevel inverter (MLI)-based indirect vector controlled induction motor drive (IVCIMD) is proposed for medium-power application. A 54-pulse ac–dc converter is utilized at the supply side to suppress the input current total harmonic distortion, as per the IEEE-519 standard. Mainly three separate isolated transformers are utilized and each transformer is configured as an 18-pulse ac–dc converter. Due to the phase shift in primary windings, these three transformers are reconfigured as a 54-pulse ac–dc converter. A 15-level binary-based cascaded MLI is utilized at the motor side to enhance the required performance of IVCIMD. A 15-level inverter is switched at fundamental frequency, which ensures the very low switching losses in the proposed IVCIMD. A modified nearest level modulation strategy is proposed to achieve fundamental switching frequency. The performance of the proposed IVCIMD is simulated at various operating conditions by utilizing Simulink/MATLAB platform. Simulated results are validated with experimental results on a developed prototype in the laboratory to show its usefulness for medium-power applications. [ABSTRACT FROM AUTHOR]

In this paper, we investigate rotor and stator designs of interior permanent synchronous motors for variable speed/load applications to reduce the carrier harmonic losses caused by pulsewidth modulated inverters. The rotor core and magnet shapes are determined by automatic optimization methods with electromagnetic field analysis, which considers the inverter carrier. Appropriate stator-winding configurations are also investigated by this analysis. It is clarified that the total carrier harmonic loss can be reduced by more than 20% by optimizing both the stator and rotor shapes, whereas the other important characteristics are not deteriorated. The mechanism of carrier loss reduction is also clarified. [ABSTRACT FROM AUTHOR]

Central inverters based on conventional topologies are the current preferred solution in solar farms because of their low cost and simplicity. However, such topologies have some disadvantages such as poor maximum power tracking, use of bulky filters, and low-frequency transformers. An interesting alternative in this case is the SiC-based cascaded multilevel converter (CMC), which provides a distributed maximum power point tracking control with reduced footprint and high flexibility. Each cell of a CMC usually has, as an intermediate stage, a solid-state transformer based on a dual-active-bridge dc–dc converter. Due to the unidirectional power flow characteristic of the photovoltaic application and aiming at further reduction in the converter volume, this work proposes a forward dual-active-bridge (F-DAB) topology, which reduces the number of active switches. This paper shows through analytical, simulation, and experimental results that the cell using an F-DAB is superior to other unidirectional topologies in three aspects: higher power density with commercially available power modules, reduced part count, and simplicity of control. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the variation in performance of variable speed induction motors fed by inverters with grades of core materials used for stators and rotors from both results of measurement and finite element analysis. Identical shaped stator/rotor cores are manufactured by using several kinds of electrical steel sheets from low to high grades. Then, the characteristics of these motors are measured. Finite element analysis is applied to these motors to understand the measured results by separating losses and torques according to origins. The measured motor characteristics are found to be in good agreement with the results of the analysis. The analysis reveals the variation in each loss component with electrical steel sheets. Finally, the strong and weak points of the application of high-grade electrical steel sheets to the variable speed induction motors are discussed. [ABSTRACT FROM AUTHOR]

This paper presents a new least-mean-square (LMS) algorithm based deadtime compensation method to suppress the current distortion in permanent-magnet synchronous motor (PMSM) field-oriented control (FOC) drives. Compared to conventional average value compensations, the proposed method is robust to switching device parameter variations thanks to the online adaptation capability of the LMS algorithm. Similarly, the disturbance observer compensators are also immune to switching device parameter variations; however, varying motor parameters degrade their compensation performance. Without prior knowledge of switching device or motor parameters, the proposed method can directly reduce the deadtime current harmonics by generating compensation voltage references. In addition, the proposed method is easy to implement since it does not require voltage errors estimation or current harmonics extraction, which are necessary for disturbance observer and adaptive filter based methods. The proposed method is tested on a 2.5-kW voltage-source inverter PMSM drive controlled by an FOC algorithm. Its effectiveness is validated by both experimental results and spectrum analysis. [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]

Inverter dead-time is known to make open-loop induction motor drives oscillatory, while operating at light-loads in particular. While dead-time compensation schemes exist, precise compensation of the same is often challenging as it involves factors such as device and driver delays and switching transition times, which are difficult to be quantified accurately. Hence, the effect of dead-time could be under or overcompensated. This paper presents a comprehensive analysis of the impact of such undercompensation and overcompensation on the stability of a 100-kW induction motor drive, along with experimental investigations of the same. The effects of undercompensation and overcompensation on the inverter output voltage are analyzed on a switching-cycle average basis. Further, small-signal models are proposed for an open-loop drive, when it is undercompensated and overcompensated. Stability analysis of the above motor drive is reported, which predicts oscillatory behavior with both under and overcompensation, over certain fundamental frequency ranges, respectively. Further, two different types of oscillatory behavior, characterized by different oscillating frequencies, are observed for the cases of under and overcompensation. The analyses are supported by simulations. Experimental investigations carried out also support the analyses and simulation results. [ABSTRACT FROM AUTHOR]

This paper proposes a zero-voltage ride-through (ZVRT) method and an LCL filter design method to meet the fault ride-through (FRT) requirements for a single-phase grid-connected inverter with a minimized LCL filter. A high inverter output current overshoot occurs at a grid voltage sag and a grid voltage recovery, when a low-inductance LCL filter design is employed in order to minimize the filter volume. In order to suppress the current overshoot, a high-speed gate-block operation and the design method of the minimized LCL filter are proposed. In particular, the filter design considers the delay time in the gate-block operation in order to clarify the reduction limitation of the inductance in the LCL filter. As a result, the maximum inverter output current overshoot is suppressed to 146% by the high-speed gate-block operation and the designed LCL filter, where the impedances of an interconnected inductor and a filter inductor are 1.0% and 0.78% of the inverter normalized impedance, respectively. Therefore, it is possible to meet the FRT requirements with the proposed ZVRT method even with the minimized LCL filter. [ABSTRACT FROM AUTHOR]

Nowadays, a multilevel converter represents a good idea for commercial and industrial applications, where small size and cost, low maintenance, reliability in a wide range of environments, and good efficiency are required. Furthermore, the multilevel converter can provide control for both maximum power tracking and output power factor reducing the filter weight and volume. The proposed power converter topology, called five-level E-type inverter (5L E-Type Inverter), for grid-connected applications is presented and analyzed in this paper. The performance of the three-phase 5L E-Type Inverter has been theoretically investigated. The thermal models of the power semiconductor have been created taking into account the parameters provided by the manufactures and employing multidimensional lookup tables in the Plexim/PLECS environment. The design and selection of passive components have been described in order to obtain amazing performances in terms of weight, volume, and efficiency and power quality. New control strategy, named concurrent control, for 5L E-Type Inverter has been implemented and validated in the LabVIEW development environment. The peak efficiency of the converter above 98.3% and the total harmonic total harmonic distortion current (THDi) less than 3% have been achieved. [ABSTRACT FROM AUTHOR]

In this paper, we propose a simple driving method that uses only one three-phase inverter for an actively controlled 2-DOF bearingless motor. This motor has a six-slot/six-pole structure, including two separated windings for the single-phase six-pole motor and the three-phase four-pole suspension. The stator core has a staggered-tooth structure to produce starting torque. The single-phase motor winding is connected between the neutral point of the three-phase suspension winding and the midpoint of the dc power sources. The radial suspension force is calculated analytically and verified using the finite-element method. In addition, we introduce a ripple compensation method for the suspension force. A prototype of a bearingless permanent magnet motor with 2-DOF active suspension was built and tested. The results of experiments demonstrated that the rotor, which was driven at speeds up to 8000 r/min, could be levitated successfully, using only one three-phase inverter. [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]