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DC microgrid has attracted more and more attention due to its unique characteristics, but the development of a dc microgrid is also facing some challenges, such as interruption and isolation of short-circuit fault currents. Therefore, the dc circuit breaker plays an important role in ensuring the normal work of a dc microgrid. In this paper, a novel bidirectional dc solid-state circuit breaker is proposed to realize the bidirectional flow of energy, which ensures the higher operating efficiency of the dc microgrid. Compared with other solid-state circuit breakers, the proposed novel topology has simpler structure, common ground, and fewer components. First, this paper makes a detailed analysis of the different working stages of the circuit when the fault occurs in combination with simulation results of the circuit topology. Then, the standard of selecting the components through the calculation and analysis of the circuit is given in this paper. Finally, the performance of the novel bidirectional solid-state circuit breaker is verified by experiments. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editors discuss various reports within the issue including one on new emerging technologies in motion control systems, one on control of direct current (dc) motors, and one on linear actuator with high backdrivability.

The aim of this paper is to study the dynamics of a permanent magnet direct current (PMDC) brushed motor while two crucial space-domain effects, namely, slotting effect and commutation phenomenon are taken into account. Semi-analytical dynamic time-domain PMDC brushed motor model inclusive of these space-domain effects is used to analyze and investigate the dynamics. Pulse width modulation operated discontinuous current PMDC brushed motor drive with steady duty cycle is employed to acquire the bifurcation patterns. The chaotic nature due to slotting effect and commutation is identified from the bifurcation patterns and reported. It also has to be admired that such nonlinear dynamical behavior with the same set of parameters at similar operating condition has not been observed when a conventional PMDC brushed motor dynamic model is considered. These space-domain effects, namely, slotting effect and commutation also have a prominent impact to make some time-domain states of a PMDC brushed motor drive chaotic. The role of slotting effect and commutation phenomenon to analyze the chaos over time-domain state variables of the PMDC brushed motor drive is investigated and validated experimentally in this paper. [ABSTRACT FROM PUBLISHER]

A hybrid switched-capacitor/interleaved boost converter able to provide a high voltage gain and to cancel the input current ripple at a preselected duty cycle was recently presented in the literature. This paper extends that work by proposing an optimized switching strategy for operating conditions at other values of duty cycle. The switching strategy effect on the converter's voltage gain and input current ripple is quantified and compared against the performance under the conventional switching strategy. The approach is validated in the laboratory through hardware prototyping. [ABSTRACT FROM PUBLISHER]

This paper deals with the innovative “variable-voltage” bus concept for future more-electric aircraft platforms. Using new functionalities and opportunities offered by innovative sources actively controlled by power electronics, there is an opportunity for a significant increase of their output power (within powertrain capabilities, machine thermal limits, and power converter maximum current) to supply increased load demands under certain conditions. This paper investigates applications of this concept to satisfy power demands for the wing ice-protection system for business jet. The study includes a detailed study into the control challenges of the proposed approach and suggests corresponding theoretical solutions. Furthermore, the controller design aspects are explored with considerations to achieve stable operation. The concept is tested in simulation environment and validated through experimental studies. This paper might be of interest for the researchers and engineers focused on innovative solutions for future aircraft platforms. [ABSTRACT FROM AUTHOR]

In this paper, an improved postfault strategy is introduced for modular multilevel converters, which utilizes the remaining healthy capacity of the converter more efficiently and increases the line voltage amplitude under these circumstances. Previous works proposed to use the neutral point shift in ac side and dc side of the converter simultaneously. This paper adds a comprehensive and coherent formulation, deep analyzation, and experimental validation. The main improvement of the proposed method is maximizing the line voltage amplitude using the optimum dc component and the best neutral point position. Indeed, fewer restrictions in fault numbers, fewer restrictions in load power factor, and fewer nonsolution conditions are among other improvements of a combined dc and neutral-shift method. Comparative simulation and experimental results verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper proposes a hybrid static synchronous compensator (hybrid-STATCOM) in a three-phase power transmission system that has a wide compensation range and low dc-link voltage. Because of these prominent characteristics, the system costs can be greatly reduced. In this paper, the circuit configuration of hybrid-STATCOM is introduced first. Its V–I characteristic is then analyzed, discussed, and compared with traditional STATCOM and capacitive-coupled STATCOM (C-STATCOM). The system parameter design is then proposed on the basis of consideration of the reactive power compensation range and avoidance of the potential resonance problem. After that, a control strategy for hybrid-STATCOM is proposed to allow operation under different voltage and current conditions, such as unbalanced current, voltage dip, and voltage fault. Finally, simulation and experimental results are provided to verify the wide compensation range and low dc-link voltage characteristics and the good dynamic performance of the proposed hybrid-STATCOM. [ABSTRACT FROM PUBLISHER]

A fully digital, nonlinear, wide-bandwidth current controller for dc–ac and dc–dc voltage source converters is presented in this paper. Exploiting oversampling, the controller mimics an analog hysteresis current controller, but it does not employ analog comparators, digital-to-analog converters, or any other analog signal pre- or postprocessing circuitry. Indeed, it fully virtualizes the hysteresis controller's operation and, based only on a nonlinear, efficient current error processing algorithm, drives the power converter at almost constant switching frequency. Overall, it offers the same excellent dynamic performance of the analog hysteresis controller and, at the same time, solves most of the related problems. Because the current error sample processing algorithm is inherently parallel in structure, the controller is suited for VHDL synthesis and field-programmable gate-array implementation, which guarantees flexibility and low cost, together with minimum computation and signal conversion delays. Its intended application areas include active filters, uninterruptible power supplies, microgrid distributed energy resource controllers, laboratory battery testers, and welding machines. [ABSTRACT FROM PUBLISHER]

In this paper, the model predictive current control is introduced in the semicontrolled open-winding permanent magnet synchronous motor (OW-PMSM) generation system with a common dc bus to suppress the zero-sequence current for the first time. Particularly, the basic voltage vectors of the semicontrolled OW-PMSM, which are synthesized by the three-phase full-controlled converter and the three-phase uncontrollable diode bridge, is analyzed and their influences on the zero-sequence current are investigated for the first time in this paper. Then the current-error-based cost function that includes dq-axis current error and zero-sequence current error is designed to evaluate all the voltage vectors. Moreover, to further improve the steady-state control performance of the whole generation system, a zero-sequence current suppression method based on the uncontrollable side voltage vector adjustment, which has the advantage of low computation burden, is developed. In this method, two voltage vectors of the controllable converter are applied during one control period. Based on the distribution character of the basic voltage vector in the semicontrollable OW-PMSM generation system, the first voltage vector is selected according to the three-phase current direction to adjust equivalent vector of the uncontrollable diode bridge, whereas for the second voltage vector, the selection method based deadbeat voltage vector sector is designed to reduce the range of the candidate voltage vector. In addition, the vector durations are also computed. Finally, simulation and experiment are carried out to validate the proposed control approaches. [ABSTRACT FROM PUBLISHER]

This paper presents an improved saliency-based position sensorless drive of an interior permanent-magnet synchronous machine (IPMSM) with a single current sensor, used for low cost applications. The phase current, reconstructed from a dc-link current, shows a reconstruction error resulted from the difference of the sampling point from that of a full current sensor drive. This reconstruction error results in degradation of the overall sensorless control performance. In this paper, we analyze the impact of the reconstruction error depending on the injection frequency for a high frequency voltage signal injection sensorless control. Based on this analysis, we propose an improved position sensorless drive that employs a new current reconstruction method. In order to minimize the reconstruction error, the proposed current reconstruction method predicts the dq-axis currents with the same sampling point as a full current sensor drive. Several experimental results are provided to verify the analysis of the reconstruction error and the proposed method's improved sensorless control performances. [ABSTRACT FROM PUBLISHER]

Microinverters require high voltage gain capability for interfacing the low dc voltage output of photovoltaic (PV) module to single-phase ac grid. A two-stage nonisolated inverter is proposed in this paper, with a first boost stage and a second traditional pulse width modulated grid-tied inverter stage. The proposed boost stage consists of a coupled inductor added with a voltage multiplier to achieve the required high voltage gain at high efficiency and to operate over a wide input voltage range. Among the coupled inductor topologies, adding a clamp circuit is a common solution to limit the voltage spike caused by leakage inductance. In the proposed converter, the resonance between the coupled inductor and voltage multiplier is used to address the issue of voltage spike, thereby reducing both component count and device voltage stress. Detailed analysis and design procedure of the proposed converter is presented in this paper. Experimental results of a 250-W microinverter are presented to validate the proposed converter. [ABSTRACT FROM PUBLISHER]

The stacked polyphase bridges converter has gained increasing interest in recent years due to its modular design and easy adaption to different power levels. The real-time control and dynamic data transmission of a stacked polyphase bridges converter consisting of a number of submodules are challenging tasks. For the reason that the distributed control architecture can maintain the benefit of modularity, such an approach dedicated for the stacked polyphase bridges converter is presented in this paper. Suitable communication techniques and synchronization techniques for the presented architecture are discussed. In addition, based on a dynamic model of the converter, the sensitiveness of the distributed control system to communication bandwidth and delay is studied on an experimental prototype. [ABSTRACT FROM PUBLISHER]

This paper focuses on the operation of a grid-connected modular multilevel converter (MMC) supplying a pulsed dc load. The goal is to achieve minimum ac power fluctuation despite the high power fluctuation present on the dc side. The MMC has been selected for its inherent ability to decouple ac and dc current controllers. However, if no additional provisions are taken, the pulsed load causes imbalance of cell capacitor voltages between upper and lower arms in each phase. This paper presents the theoretical analysis of the imbalance problem and proposes a simple arm-balancing controller to enable the operation of the converter under pulsed dc load. The effectiveness of the controller has been successfully verified on a 7 kW MMC experimental prototype with a 3 kA pulsed dc load. [ABSTRACT FROM AUTHOR]

In dc microgrids, it is common that constant power loads (CPLs) exhibit negative incremental resistance. They degrade the stability margin of operation with their LC input filters and, thus, the whole system. In traditional active-damping methods, the extra stabilizing current is injected into CPLs to modify its input impedance to fulfill stability criteria. However, this injected current may cause undesirable performance of the loads, e.g., fluctuation in rotating speed of tightly regulated motors. There is always a compromise between stability margin and load performances. In order to overcome this drawback, this paper proposes a method that stabilizes the system from source-side converters, instead of from CPL themselves. In the proposed method, a virtual resistance is built in the source-side converter. This virtual resistor is effective around the resonant frequency of the LC input filter and thus can indirectly reduce its output impedance to fulfill Middlebrook's stability criterion. This paper also proves, both analytically and experimentally, that, in dc microgrids, resonant frequencies of LC filters of paralleled CPLs have to be different to maintain system stability. Simulation and experimental results are reported to verify the effectiveness of the proposed idea. [ABSTRACT FROM PUBLISHER]

This paper presents a theoretical analysis, an optimal design method, and experimental results for a wireless power transfer (WPT) system with an intermediate coil. The analytical expression of the dc voltage transfer function is presented and discussed. In a two-coil WPT system, which has low coupling coefficient, the intermediate coil boosts the apparent self-inductance and magnetizing inductance of the primary side at around the resonance frequency of the intermediate coil, so that the apparent coupling coefficient is compensated. The coupling coefficient makes the system efficiency increase and induces bifurcation phenomenon. From the analysis, this paper proposes an optimal design method using the second resonance frequency operation with the bifurcation phenomenon and presents design procedure for high efficiency. A prototype of the WPT system with the intermediate coil is implemented and experimented to verify the validity of the analysis and the proposed design method. The prototype operates at 100 kHz switching frequency and has an air gap between primary and secondary side of 200 mm. An overall system efficiency of 95.57% has been achieved at 6.6 kW of output power. [ABSTRACT FROM PUBLISHER]

The twenty-two papers in this special section focus on flexible control of power converters which serve as interfaces between the distributed generation (DG) units and the legacy alternating current (ac) grid or the ac or direct current (dc) microgrid (MG), is the key to realization of high penetration of renewable energy in a safe and stable fashion. When connected to the ac legacy grid, these power converters need to provide ancillary services such as frequency and voltage support, harmonic compensation, as well as synthetic inertia emulation. Another emerging solution is to interface the DG units with the ac legacy grid through an intermediate entity called anMG. MG can be based either on ac and dc architecture and can work in both stand-alone and grid-connected modes. Since it is responsible for multiple power converters, an MG has higher operational flexibility than individual units.However, due to a lack of stiff voltage reference source and natural inertia, control of MGs is generally more challenging than control of individual grid-connected power converters. [ABSTRACT FROM AUTHOR]

This paper presents a methodology to control a two-staged ac/dc converter, which is a cascade combination of a three-phase ac–dc boost power factor correction (PFC) rectifier and a phase-shifted full-bridge (PSFB) dc/dc converter. This paper explains the issues of instability and losing unity power factor operation, i.e., high reactive power transfer with conventional PI compensator due to the sensitive variation of output impedance of the PFC stage at different loads. In this paper, this instability issue is resolved by the proposed state-feedback control methodology, which shows that there exists a load-insensitive range of controller parameters, providing unity PFC. As an additional benefit of this control, total number of sensors are reduced by three (an input phase current, a dc/dc primary current, and an output dc voltage), in comparison to the conventional PI-based control. This reduction is achieved by introducing two augmented state variables followed by their reduced state observer design. A 6-kW laboratory prototype of the integrated three-phase PFC and PSFB dc/dc is developed and designed to validate the proposed control algorithm. The experimental results show a conversion efficiency of 95.4% at full load, input total harmonic distortion of 4.1%, power factor of 0.998, and output voltage ripple of ±1%. [ABSTRACT FROM PUBLISHER]

A sensorless stand-alone control scheme of a doubly fed induction generator (DFIG)-DC system is investigated in this paper. In this layout, the stator voltage is rectified by a diode bridge that is directly connected to a dc bus. The rotor-side voltage source inverter is the only controlled converter required in this system and is directly powered by the same dc bus created by the stator-side rectifier. DC voltage and stator frequency are regulated by two independent proportional-integral regulators that give the references for inner current controllers implementing field-oriented control. As it is capable of creating a stable and regulated dc bus, this system can be conveniently adopted to supply dc loads or to form a dc grid. Due to the constraint imposed by the stator diode bridge, the DFIG has to operate under a constant stator voltage, and the conventional stator field-oriented control implemented in stand-alone ac DFIG must be modified. This paper presents the control structure and the theoretical framework for the controller synthesis. Simulation and experimental validations on a small-scale rig are included. [ABSTRACT FROM PUBLISHER]

To interconnect low-voltage solar photovoltaics (PV) with dc system, a dc–dc boost converter is required. To minimize the switching frequency oscillations in solar PV voltage, aluminum electrolytic capacitors (AECs) are connected between solar PV and the converter. Operational life of AECs depends on electrical and environmental parameters and is less than that of solar PV panels. Equivalent series resistance (ESR) of AEC increases with life, thereby increasing the voltage ripple across PV terminals. This results in less power extraction from solar PV. This paper proposes a method for health monitoring of AEC using the parameters measured for maximum power point tracking (MPPT) (PV voltage and current). The proposed technique is applicable for both continuous conduction mode and discontinuous conduction mode of operation of the converter. Effect of temperature on ESR is incorporated for accurate determination of the health of capacitor. Key advantage is that no additional current or voltage sensor is required to implement this technique, thereby offering a low-cost solution. Further, the proposed technique does not require significant processing capabilities and is implemented in the same digital controller used for MPPT. Detailed simulation studies are carried out and results are included in this paper. A laboratory prototype of dc–dc boost converter is developed for experimentation. Results are in agreement with that obtained from simulation studies. [ABSTRACT FROM AUTHOR]

In this paper, a new structure for symmetric and cascade multilevel converter is presented. This structure requires the least power electronic components, gate driver circuits, power diode, and dc voltage source. The number of output levels is high. Reduction of the number of power electronic switches against voltage levels is an important factor in multilevel converters due to safety in switches, circuit size, cost, installation area, and complexity in control. To maximize the number of output level in the proposed optimized structure in order to attain different goals, such as reduction of stress on switches, the result of comparing the proposed structure and other structures present. The proposed method is robust against the switching faults in multilevel converter for dc source and the equipment. The simulation and experimental results presented in this paper show the performance of the proposed method. [ABSTRACT FROM AUTHOR]

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

A novel position sensorless permanent-magnet synchronous motor (PMSM) drive that uses only a direct current (dc)-link sensor is presented in this paper. This concept is already known, but several new implementation details are shown in this paper. The proposed drive system is simple and can be implemented using low-cost control hardware. The dynamic performance of the proposed drive is evaluated and compared with the performance of position sensorless and full sensored drives. [ABSTRACT FROM PUBLISHER]

This paper addresses robust design of the active-power and dc-link control loops of power-synchronization control. Robustness is obtained by analytic gain selections, which give large enough stability margins. The proposed design allows robust stability irrespective of the grid strength and of the operating point, the latter with one exception. The proposed design is compared to design based on the principle virtual synchronous machine. Experiments show that the time-domain results correlate well with the frequency-domain results. [ABSTRACT FROM AUTHOR]

In this paper, a generic framework for a rectifier to continuously take part in the regulation of grid voltage and frequency is proposed. It can automatically change the power consumed to support the grid, without affecting the normal operation of the load. Such a rectifier has a built-in storage port, in addition to the normal ac and dc ports. The flexibility required by the ac port to support the grid is provided by the storage port without affecting the dc port. The grid support of the ac port is achieved through the recently proposed universal droop controller (UDC). A dc-bus voltage controller cascaded to the UDC is proposed to regulate the dc-bus voltage of the storage port within a wide range so as to provide the flexibility needed, while the dc-port voltage is maintained constant. An illustrative example is presented with the recently proposed $\theta$ -converter, which consists of three ports with only four switches. Experimental results are provided to verify the capability of the rectifier to regulate the grid voltage and frequency without affecting the load. [ABSTRACT FROM AUTHOR]

The aviation industry pursues the integration of fuel cells (FCs) to reduce pollutant emissions. Their connection to the on-board power supply leads to new requirements for electrical grid protection. The limited short-circuit current of a weight-optimized FC system cannot trip a standard fuse. Hence, in this paper, a new protection device is developed as an under/overvoltage primary protection. The proposed device consists of three basic elements: relay; differential amplifier; and current source. This paper includes a detailed description of the protections design process, which includes a requirement analysis, a detailed robustness consideration, and an experimental validation of a developed prototype. [ABSTRACT FROM AUTHOR]

The forced commutated controlled series capacitor (FCSC) rectifier has been recently proposed to power stand-alone variable-voltage variable-frequency generation systems, such as the full authority digital electronic control (FADEC) system, in more electric aircraft. The employment of this rectifier helps to meet increasing power demand requirements while maintaining low current harmonic levels without the need for bulky filters. The FCSC rectifier is controlled to maintain resonance with line frequency variation by inserting variable capacitive reactance in series with the variable-frequency generator's inductive reactance. Unity power factor operation with nearly pure sinusoidal converter input currents is thus achieved, for better utilization of the generator. The FCSC rectifier operation with a variable dc on-board load is analyzed in this paper, including a description of the principle modes of operation of the circuit. The enhancement in circuit's output voltage and output power achieved by replacing the conventional diode bridge rectifier with the FCSC rectifier is also demonstrated. Both rectifiers are analyzed numerically using Saber/Synopsys over a wide range of operating voltages and frequencies. Results are experimentally verified using 1-kW laboratory test rig. [ABSTRACT FROM AUTHOR]

This paper proposes a capacitor voltage regulation method for the dual converter with a floating bridge for aerospace applications. This topology has previously been reported, but with a constrained voltage utilization factor due to the need for capacitor voltage regulations. In this paper, the effect of switching states on the voltage variation of capacitor is quantitatively modeled and an enhanced space vector modulation scheme with current feedback is proposed to achieve an active control of the floating capacitor voltages. This proposed method also allows further exploitation and utilization of converter voltage. The relationship between the allowed modulation index of dual converter and load power factor is obtained and expressed using a fitted polynomial equation. The advantages of the proposed method include boosted voltage utilization and superior performance in term of capacitor voltage balance. These advantages have been proven through simulation and experimental results on RL loads as well as with an open-end winding induction motor. The proposed modulation scheme can boost the converter voltage utilization by at least 10% while achieving full four-level operation. More importantly, the higher available voltage allows extending the constant torque region of the motor, the further beginning of field weakening operation could be postponed. [ABSTRACT FROM AUTHOR]

Direct modulation principle is widely used in modular multilevel converters (MMCs) for its simplicity and natural stability. However, it results in strong active and reactive power coupling. This paper carefully analyzes and identifies four power coupling paths and two power influence factors with the direct modulation principle. Full power decoupling of the MMC can be realized by cutting off the derived coupling paths and compensating the influence factors. However, this will further deteriorate the already poor damping of the MMC and significantly increase dc-bus current fluctuations. To solve this problem, the sources resulting in poor damping are identified and then a new closed-loop dc-bus current controller is incorporated into the control system. Finally, the proposed power decoupling strategy with the newly designed current controller realizes completely decoupled power regulation, avoids overload operation, and obviously improves the dynamic performance of the MMC. Furthermore, it increases the stability of the MMC system under all possible operation conditions. The proposed control strategy is confirmed by experimental results. [ABSTRACT FROM AUTHOR]

When fast voltage control is used in a power factor control, the ripple of the dc link causes current harmonics. This paper proposes a method to reduce the harmonic content of a fast voltage loop in a power factor correction, which can both produce low distortion during steady-state operation and fast recovery from a load transient. The performance is achieved by using a Takagi–Sugeno-type nonlinear controller for dc-link voltage control. The proposed nonlinear control is compared with linear proportional–integral (PI) control, which is tuned to meet the standardized regulations for the harmonic content in a 3-kW single phase power supply. The results show that the presented nonlinear voltage controller can maintain the dynamic performance of the linear control with reduced current harmonics. It is also shown that the used nonlinear control method requires only marginally more complex control algorithm compared to commonly used linear PI control. [ABSTRACT FROM AUTHOR]

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

This article exhibits an output voltage-tracking control algorithm incorporating a self-tuning algorithm and disturbance observers (DOBs) for dc–dc boost converters without tracking error integrators, considering model nonlinearities, parameter variations, and load uncertainties. The proposed algorithm is designed such that it forces the output voltage to track its target trajectory driven by a time-varying low-pass filter (LPF), despite model-plant mismatches. This paper makes two main contributions. First, it constructs the self-tuning algorithm by updating the closed-loop cut-off frequency in the time-varying LPF to improve the transient tracking performance. Second, it embeds first-order DOBs in the proportional-type nonlinear controller to achieve both performance recovery and the offset-free properties. The experimental verifications are carried out to illustrate the effectiveness of the closed-loop system driven by the proposed technique with a 3-kW prototype dc–dc boost converter; the 46% improvement of the tracking performance is observed, compared with the conventional feedback-linearization controller. [ABSTRACT FROM AUTHOR]

One of the future challenges in modular multilevel converters (MMCs) is how to size key components with compromised costs and design margins, while fulfilling specific reliability targets. It demands better thermal modeling compared to the state-of-the-arts in terms of both accuracy and simplicity. Different from two-level power converters, MMCs have inherent dc-bias in arm currents and the power device conduction time is affected by operational parameters. A time-wise thermal modeling for the power devices in MMCs is, therefore, an iteration process and time-consuming. This paper, thus, proposes a simple analytical thermal modeling method, which adopts equivalent periodic power loss profiles. More importantly, time-domain simulations are not required in the proposed method. Benchmarking of the proposed methods with the prior-art solutions is performed in terms of parameter sensitivity and model accuracy with a case study on a 30-MW MMC system. Experiments are carried out on a specifically designed scaled-down system to verify the electrothermal aspects. [ABSTRACT FROM AUTHOR]

Among the various topologies proposed in this literature, the modular multilevel converter (MMC) is considered the next generation of converters for medium- and high-voltage applications. This paper presents a comparison between two variants of MMCs for STATCOM applications: The double-star chopper cell-MMC and the single-delta bridge cell-MMC. These converter topologies are compared and benchmarked during positive and negative sequence compensation. The comparisons are supported by analytical results regarding minimum effective dc-link voltage, number of cells, current rating, energy storage requirements, and operation during unbalanced conditions. The dynamic behavior, power losses, and cost of both solutions are evaluated through simulation model of a 15 mega volt-ampère (MVA) MMC STATCOM. [ABSTRACT FROM AUTHOR]

This paper proposes a new dc–ac converter with a reduced number of semiconductor switches and gate drive and control circuit components. A salient feature of this inverter is its pure sinusoidal line-to-line voltages with no need for output filter. Also, the proposed inverter improves the voltage utilization factor of the input dc supply compared to the four-switch three-phase inverter (first best topology in the literature from the number of switches point of view) and standard six-switch inverter with sine pulsewidth modulation. The proposed inverter is capable of operating with a wide range of output voltages from zero to the full value of the dc input voltage by appropriately altering the instantaneous duty cycle. A three-phase universal inverter with only three power switch has been proposed in this paper for the first time ever, to our knowledge. For the purpose of providing a good compromise between fast transient response and stability, a model predictive controller is proposed. All the design expressions have been derived. A comparison with other dc–ac converters is given to show the merits of the proposed converter. Finally, satisfactory circuit operation is confirmed by experimental results from a laboratory prototype. [ABSTRACT FROM AUTHOR]

The severe torque ripple normally occurring in the doubly fed induction generator dc (DFIG-dc) system can cause premature failure of mechanical components and shorten the life of the drive train. This paper addresses the torque ripple issue by proposing a predictive direct torque control strategy, which delivers at the same time torque ripple suppression and minimization of losses. The existing control algorithms for torque ripple mitigation are mostly based on resonant controllers and repetitive control forcing the compensation signal either through the current chain or directly into the rotor voltage commands. All these techniques lead to structures with multiple controllers whose tuning is not straightforward. Furthermore, they are very sensitive to the operating frequency, making optimized operation with variable frequency highly challenging. Conversely, the proposed algorithm predicts directly the best rotor voltage space vector to minimize torque ripple and track a prescribed rotor flux amplitude to minimize losses, with no current control chain. As confirmed by simulations and experiments, the strategy allows large stator frequency variations as required by the optimal flux command for minimum losses, whilst ensuring effective torque ripple compensation. [ABSTRACT FROM AUTHOR]

Due to current sensor errors, tolerance of power switching devices, and asymmetry of PWM gating driving pulses, grid-connected inverters without isolation transformer usually have certain amount of dc components injected to the ac grid. Many efforts, such as using a blocking capacitor, a current dc component feedback control, and a voltage dc component feedback control have been introduced to try to suppress the dc injection to the grid. This paper proposes a novel control strategy of suppressing dc current injection to the grid for a three-phase inverter by accurately sensing the dc component of line voltages of three-phase inverter and adding a dc component control loop. A dc suppression control scheme is presented, and the suppression performance of the proposed dc rejection method is evaluated and compared with the traditional method. Finally, the control scheme is verified on a 20-kW three-phase grid-connected inverter. [ABSTRACT FROM AUTHOR]

This paper presents a new control method for cascaded connected H-bridge converter-based static compensators. These converters have: classically been commutated at fundamental line frequencies, but the evolution of power semiconductors has allowed the increase of switching frequencies and power ratings of these devices, permitting the use of pulsewidth modulation techniques. This paper mainly focuses on dc-bus voltage balancing problems and proposes a new control technique (individual voltage balancing strategy), which solves these balancing problems, maintaining the delivered reactive power equally distributed among all the H-bridges of the converter. [ABSTRACT FROM AUTHOR]

This paper presents a complete method that is used to balance dc link voltages in a cascaded H-Bridge (CHB) multilevel rectifier. Recently, such converters have been the subject of extensive research due to their suitability for high-power applications. One requirement in using a multilevel active rectifier at high levels of power is to limit the switching losses by reducing the switching frequency to a minimum. Another requirement for these converters is to ensure that individual dc link capacitor voltages for each cell of the converter are always balanced to ensure controllability and to limit stress on the converter cells. This paper presents a complete method in solving both of these problems using a selective-harmonic-elimination pulsewidth-modulation scheme. The scheme utilizes a simple controller to track each cell dc link capacitor voltage magnitude and accordingly biases the power flowing into each cell to ensure that the voltages across each cell capacitor converge. This is the case even when the loads attached to the individual cells are not balanced. The theory is supported by both simulated results from Saber and by experimental results from a seven-level CHB single-phase multilevel rectifier. [ABSTRACT FROM AUTHOR]

The open-loop control scheme presented in part I of this paper for a dual five-level-inverter-fed induction motor (IM) drive with two dc power supplies maintains dc-link capacitor voltage balancing and common-mode voltage (CMV) elimination throughout the operating range of the drive. The operating limitation of the proposed open-loop control scheme to take corrective action toward the existing unbalance in the dc-link-capacitor voltages is also pointed out in part I of this paper. As a solution to this, a simple closed-loop control scheme, which is based only on the switching-state redundancy, is proposed in this part of the paper. The proposed closed-loop control scheme not only prevents further unbalancing of capacitor voltages but also takes corrective actions to bring back the capacitor voltages in the balanced state. The proposed closed-loop scheme achieves dc-link capacitor voltage balancing and elimination of CMV together in the complete modulation range, including overmodulation of up to the 24-step operation. The proposed control scheme does not affect the output fundamental voltage generated by the inverter, as it effectively utilizes only the availability of redundant switching states of the inverter, and does not call for additional power circuit hardware. The scheme is presented with the simulation studies and experimentally verified with a 1.5-kW open-end winding IM drive. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to develop a model and propose control strategies to mitigate dc-link current ripple for current-source grid-connected converter (CSGCC) under unbalanced grid conditions. Based on the model of instant active power under unbalanced grid conditions, this paper proposes the optimized negative-sequence current references for eliminating the double-frequency oscillations on active power at ac side of CSGCC. Both the single CSGCC and the paralleled CSGCCs are considered in this paper. In order to track the asymmetric current references more accurately while maintaining LC resonance damping, the hybrid current controller is proposed by combining the closed-loop fundamental current controller under double synchronous frames and the closed-loop high-bandwidth harmonic current controller under stationary frame. The design of the hybrid current controller is analyzed. Finally, the experimental results are shown to verify that the control strategies can mitigate the dc-link current ripple effectively, and also provide good LC resonance damping performance for grid currents. [ABSTRACT FROM PUBLISHER]

DC-link capacitors in power electronic converters are a major constraint on improvement of power density as well as reliability. Evaluation of the dc-link capacitors in terms of power loss, ageing, and failure rate will play an important role in design stages of the next-generation power converters. This paper proposes a new evaluation circuit for dc-link capacitors used in a high-power three-phase inverter, which is intended for testing power loss, failure rate, ageing, and so on. The evaluation circuit produces a practical ripple current waveform and a dc bias voltage into a capacitor under test, in which the ripple current is equivalent to that generated by the three-phase inverter on the dc link. The evaluation circuit employs a full-scale current-rating and downscaled voltage-rating inverter for producing the ripple current, so that the power rating of the evaluation circuit is much smaller than that of a full-scale current-rating and full-scale voltage-rating inverter. Theoretical analysis and simulated results verify the effectiveness of new evaluation circuit. [ABSTRACT FROM PUBLISHER]

This paper proposes an effective test bed for a 380-V dc distribution system using isolated power converters. The proposed test-bed system is composed of a grid-interactive ac–dc converter for regulating the dc-bus voltage, a bidirectional converter for the battery power interface, a renewable energy simulator, dc home appliances modified from conventional ac components, a dc distribution panel board, and its monitoring system. This paper discusses three isolated power converters, i.e., a bidirectional ac–dc converter, a bidirectional dc–dc converter, and a unidirectional dc–dc converter for the effective power interface of a dc bus. These isolated power converters are designed using a dual-active-bridge converter and the resonant topologies of $CLLC$ and $LLC$. The proposed test-bed system was implemented using a 5-kW bidirectional ac–dc prototype converter, a 3-kW bidirectional dc–dc prototype converter, and a 3-kW unidirectional dc–dc prototype converter. Finally, the performance of the test-bed system has been verified using practical experiments of load variations and bidirectional power flow, employing the prototype converters. [ABSTRACT FROM PUBLISHER]

In this paper, a simplified pulsewidth modulation with switching constraint control scheme is proposed to eliminate the circulating currents for bidirectional paralleled ac/dc converters in grid-tied system. The proposed control scheme can reduce the circulating currents and does not need additional current sensors and communication device among paralleled converters compared with conventional methods. Therefore, the paralleled system cost can be reduced. In addition, the current shaping and sharing between the paralleled converters can be well accomplished so that the overall performance of the paralleled converter system can be raised. Furthermore, the dc, ac, and self-generated circulating currents are clearly analyzed by graphics, and the synchronous circulating currents are first explored. Finally, a prototype system is constructed, and the proposed control scheme is implemented using a Spartan-3E XC3S250E FPGA. Both simulation and experimental results verify the validity of the proposed theory and control scheme. [ABSTRACT FROM PUBLISHER]

This paper presents a promising solution to the problem of the bad environmental impacts of the induction-motor-based water pumps. Due to the low wire-to-water efficiency of the current residential water heaters (around 30%), a high energy cost is paid by every household in North America and even worldwide. The contribution of this paper comes in twofolds. The first is the development of a mapping algorithm that relates the pressure head $H$ and the water flow rate $Q$ to a corresponding torque and speed under the chart of best efficiency point, which was programmed to the controller. The second is the development of a cost-effective brushless direct-current (BLDC) motor through an extensive numerical analysis technique. The proposed motor solution designs the BLDC with minimal pulsating torque by comparing two different designs. The new controller is based on a modified field-oriented control. The controller printed circuit board is of two layers to offer a cost-effective solution. The proposed pump eliminates the need of a battery for the controller as the control voltage is fed directly from the line together with the motor. The motor simulation model and the controller design procedure are presented with simulation results of two possible prototypes. An experimental prototype for a 150-W residential pump was built, and preliminary results are given to highlight the merits of the work. [ABSTRACT FROM PUBLISHER]

Power factor correction (PFC) is an essential part of ac/dc converters in order to improve the quality of the current drawn from the utility grid. The PFC closed-loop control system requires a precise measurement of the boost inductor current in order to tightly shape the input current. Current sensors are widely used in the PFC closed-loop control system to measure the boost inductor current. Current sensors introduce delay and noise to the control circuitry. Also, they significantly contribute to the overall cost of the converter. Therefore, they make the implementation of the PFC converter complicated and costly. Current sensorless control techniques can offer a cost-effective solution for various applications. The unique structure of the boost PFC converter makes it challenging to robustly estimate the inductor current due to the nonlinear structure of the converter. Also, it is shown in this paper that the system loses observability at some singular operating points, which makes the observer design more challenging. In addition, the load value is unknown in most applications. Thus, the observer should be able to estimate the inductor current in the presence of uncertainties in the load and other parameters. This paper presents an adaptive nonlinear observer for the boost PFC, which is able to accurately estimate the inductor current. The adaptive structure of the converter allows the robust and reliable performance of the observer in the presence of parameter uncertainties, particularly load variations. Also, an auxiliary compensation is integrated into the observer to circumvent the singular operating points and provide a precise estimation for the entire range of operation. Experimental results are presented to verify the feasibility of the proposed sensorless control approach. [ABSTRACT FROM PUBLISHER]

This paper presents a modulation strategy for symmetrical cascaded multilevel converters that simultaneously balances the individual DC voltages and reduces switching losses by eliminating redundant or unnecessary commutations. The voltage regulation is based on a 1-bit digital signal indicating whether the DC link voltage of each H-bridge is above or below the set point. The system command identifies commutations that can be suppressed without causing output voltage distortion, thus reducing converter losses. The voltage balancing is reached by postponing each switching commutation for a short lapse of time. Simulation and experimental results for an active filter application validate the proposed methods. [ABSTRACT FROM PUBLISHER]

This paper introduces a switched-capacitor dc combiner suitable for energy harvesting applications. Since the dc voltage is small, addition of the voltages from different sources can boost the final output voltage with self-startup capability. This approach is valid for any number of dc voltage sources. To prove the energy harvesting principle, three sources are selected. The design of the three-input dc combiner, with no closed-loop controller, is fabricated in 0.13-\mu\textm CMOS technology and the chip area is 1.5 × 1.5 \textmm^2. With integrated tunable thyristor-based differential oscillators, the chip can combine power from all three inputs. Manual frequency modulation is used to deliver the maximum power to the load. The system has proven to be functional even if one or two of the sources is/are weak or not available. Finally, for indoor conditions, the power from three different solar cells is combined where 70 $\mu$ W is harvested with an open-circuit voltage of 1.4 V and the short-circuit current of 160 $\mu$A. [ABSTRACT FROM PUBLISHER]

In the hybrid ac–dc microgrid, the ac and dc subgrids are connected by bidirectional power converters (BPCs) that play an important role in the load power sharing and power interaction between the ac and dc subgrids. The coordination control and circulating current suppression for the parallel BPC system are very challenging. In this paper, a decentralized coordination control method is proposed for parallel three-phase BPCs, which can suppress the circulating currents, realize proper power interaction, and achieve overall load power sharing in both the grid-connected mode and the islanded mode. The performance of the proposed control methods is verified by the real-time hardware-in-loop tests. [ABSTRACT FROM PUBLISHER]

This paper proposes an improved dc transformer (NDCT hereinafter) based on switched capacitor with reduced switches for the integration of low-voltage dc energy storage systems and medium-voltage dc power distribution grid. The topology, phase-shift (PS) shoot-through and multiple PS modulations, and voltage, current, and power characterizations, as well as commutation characterization and control strategy, are proposed and analyzed comprehensively. Compared with the traditional schemes, the proposed NDCT not only can enhance reliability and flexibility but can increase voltage adjustment and power transfer capabilities as well. Moreover, the current ripple performance and soft-switching performance are improved, and the number and capacity of switches are reduced, thereby increasing the efficiency and cost of the system. Finally, an NDCT prototype is built. Experimental results verify the correctness and effectively of the proposed solution. [ABSTRACT FROM PUBLISHER]

This paper presents a sinusoidal current control scheme for the doubly fed induction generator (DFIG)-DC system to reduce the harmonic current losses of the stator and rotor windings. The DFIG-DC system can be controlled as an open-winding system if the DFIG is equivalent in the Г circuit. The stator is connected to the dc link through the diode bridge and the rotor is connected to the same dc link through the rotor-side converter. A phase-locked loop based on stator flux is applied for estimating not only the stator frequency but also the angle of stator flux linkage. The sinusoidal stator current can be achieved by injecting appropriate harmonic voltages to the rotor side. The rotor harmonic voltages are produced by applying the stator voltage directly without employing a series of resonant controllers. Since the voltage of the diode bridge depends on the direction of the flowing current, the stator voltage of DFIG can be expressed by the stator current avoiding the stator voltage sensors. In this way, the harmonic current losses of the stator and rotor windings can be greatly reduced, which is beneficial for system efficiency. Furthermore, the torque ripple and power ripple can also be reduced, which is a good operation condition of the DFIG-DC system. The proposed control method is validated through experiments. [ABSTRACT FROM PUBLISHER]