Showing papers in "IEEE Journal of Emerging and Selected Topics in Power Electronics in 2019"

On the Inertia of Future More-Electronics Power Systems

Abstract: Inertia plays a vital role in maintaining the frequency stability of power systems. However, the increase of power electronics-based renewable generation can dramatically reduce the inertia levels of modern power systems. This issue has already challenged the control and stability of small-scale power systems. It will soon be faced by larger power systems as the trend of large-scale renewable integration continues. In view of the urgent demand for addressing the inertia concern, this paper presents a comprehensive review of inertia enhancement methods covering both proven techniques and emerging ones and also studies the effect of inertia on frequency control. Among those proven techniques, the inertia emulation by wind turbines has successfully demonstrated its effectiveness and will receive widespread adoptions. For the emerging techniques, the virtual inertia generated by the dc-link capacitors of power converters has a great potential due to its low cost. The same concept of inertia emulation can also be applied to ultracapacitors. In addition, batteries will serve as an alternative inertia supplier, and the relevant technical challenges as well as the solutions are discussed in this paper. In future power systems where most of the generators and loads are connected via power electronics, virtual synchronous machines will gradually take over the responsibility of inertia support. In general, it is concluded that advances in semiconductors and control promise to make power electronics an enabling technology for inertia control in future power systems.

99% Efficient 10 kV SiC-Based 7 kV/400 V DC Transformer for Future Data Centers

Abstract: The power supply chain of data centers from the medium voltage (MV) utility grid down to the chip-level voltage consists of many series connected power conversion stages and accordingly shows a relatively low efficiency. Solid-state transformers (SSTs) could improve the efficiency by substantially reducing the number of power conversion stages and/or directly interfacing the MV ac grid to a 400 V dc bus, from where server racks with a power consumption of several tens of kilowatts could be supplied by individual SSTs. The recent development of SiC MOSFETs with a blocking voltage of 10 kV enables the realization of a simple and, hence, highly reliable two-stage SST topology, consisting of an ac/dc power factor correction rectifier and a subsequent isolated dc/dc converter. In this context, an isolated 25 kW, 48 kHz, 7 kV to 400 V series resonant dc/dc converter based on 10 kV SiC MOSFETs is realized and tested in this paper. To achieve zero voltage switching of all MOSFETs, a special modulation scheme to actively control the amount of the switched magnetizing current on the MV- and low voltage-sides is implemented. Furthermore, the design of all main components and, especially, the electrical insulation of the employed medium-frequency transformer are discussed in detail. Calorimetric efficiency measurements show that a full-load efficiency of 99.0% is achieved, while the power density reaches 3.8 kW/L ( $63~\text {W}/\mathrm {in^{3}}$ ).

DC Microgrid Protection: A Comprehensive Review

Abstract: DC microgrids have attracted significant attention over the last decade in both academia and industry. DC microgrids have demonstrated superiority over AC microgrids with respect to reliability, efficiency, control simplicity, integration of renewable energy sources, and connection of dc loads. Despite these numerous advantages, designing and implementing an appropriate protection system for dc microgrids remains a significant challenge. The challenge stems from the rapid rise of dc fault current which must be extinguished in the absence of naturally occurring zero crossings, potentially leading to sustained arcs. In this paper, the challenges of DC microgrid protection are investigated from various aspects including, dc fault current characteristics, ground systems, fault detection methods, protective devices, and fault location methods. In each part, a comprehensive review has been carried out. Finally, future trends in the protection of DC microgrids are briefly discussed.

Adaptive Passivity-Based Control of dc–dc Buck Power Converter With Constant Power Load in DC Microgrid Systems

Abstract: This paper presents a robust nonlinear control strategy to solve the instability problem of dc–dc buck power converter with a constant power load in dc microgrid systems. Based on the passivity-based control (PBC), a nonlinear disturbance observer (NDO) is designed to improve the control robustness against both load and line variations, whereas the PBC guarantees the system stability due to its property of transient energy dissipation. By applying the disturbance estimation technique, NDO works in parallel with the PBC controller to compensate the disturbances through a feed-forward channel. This strategy ensures large signal stability as well as fast recovery performance of the system during disturbance/uncertainty as compared to other nonlinear control methods. Hardware-in-loop (HIL) experiment is performed on an OPAL-RT real-time simulator. MATLAB simulation and HIL results are provided to verify the proposed control strategy. Further validations are presented using a real hardware experiment to emphasize the robustness of the proposed controller.

Model Predictive Control of High-Power Modular Multilevel Converters—An Overview

Abstract: Model predictive control (MPC) has emerged as a promising approach to control a modular multilevel converter (MMC). With the help of a cost function, the control objectives of an MMC are achieved easily by using an MPC approach. However, the MPC has several technical challenges and issues including the need of accurate system models, computational complexity, and variable switching frequency operation and weighting factor selection, when it comes to the control of an MMC. In the past few years, several research studies are conducted to address some of the challenges and issues in an MPC and developed several model predictive algorithms for an MMC. In this paper, the importance of each challenge and its impact on the system performance is discussed. Also, the MMC mathematical models used in the implementation of MPC are presented. Furthermore, some of the popular MPC algorithms are discussed briefly, and their features and performance are highlighted through case studies. Finally, summary and future trends of MPC for an MMC are presented.

High-Frequency Three-Phase Interleaved LLC Resonant Converter With GaN Devices and Integrated Planar Magnetics

Abstract: The LLC converter is deemed the most widely used topology as dc/dc converter in server and telecom applications. To increase the output power and reduce the input and output current ripples, three-phase interleaved LLC converter is becoming more and more popular. It has been demonstrated that three interleaved LLC converter can achieve further efficiency improvement at the 3-kW power level. However, the magnetic components for multiphase LLC converter are complex, bulky, and difficult to manufacture in a cost-effective manner. In this paper, a high-frequency gallium nitride (GaN)-based three-phase LLC converter is employed to address these aforementioned challenges. With GaN operating at 1 MHz, all magnetic components, namely, three inductors and three transformers, can be integrated into one common structure while all magnetic windings implemented in a compact four-layer PCB with 3-oz copper. The proposed structure can be easily manufactured cost-effectively in high quality. Furthermore, shielding techniques for full-bridge secondary have been investigated, and additional two-layer shielding has been integrated to reduce common-mode noise. A 1-MHz 3-kW 400 V/48 V three-phase LLC converter is demonstrated, and the peak efficiency of 97.7% and power density of 600 W/in3 (37 kW/L) are achieved.

99.1% Efficient 10 kV SiC-Based Medium-Voltage ZVS Bidirectional Single-Phase PFC AC/DC Stage

Abstract: Due to their extremely high energy demand, data centers are directly supplied from a medium-voltage (MV) grid. However, a significant part of this energy is dissipated in the power supply chain since the MV is reduced step-by-step through multiple power conversion stages down to the chip-voltage level. In order to increase the efficiency of the power supply chain, the number of conversion stages must be substantially reduced. In this context, solid-state transformers (SSTs) are considered as a possible solution, as they could directly interface the MV AC grid to a 400 V DC bus, whereby server racks with a power consumption of several tens of kilowatts could be directly supplied from an individual SST. With a focus on the lowest system complexity, the SST, ideally, should be built as simple two-stage system consisting of an MV AC/DC power factor correction (PFC) rectifier stage followed by an isolated DC/DC converter. Accordingly, this paper focuses on the design and realization of a 25 kW, 3.8 kV single-phase AC to 7 kV DC PFC rectifier unit based on the 10 kV SiC MOSFETs. By simply adding an $LC$ circuit between the switch nodes of the well-known full-bridge-based pulse width modulated AC/DC rectifier, the integrated triangular current-mode concept is implemented, which only internally superimposes a large triangular current ripple on the AC mains current and, therefore, enables zero-voltage switching over the entire AC mains period. Special attention is paid to the realization of the MV inductors and their electrical insulation, the AC-input $LCL$ filter to limit electromagnetic interference emissions, and the challenges arising due to cable resonances when connecting the SST to the MV grid via an MV cable. Despite the large insulation distances required for MV, the realized 25 kW MV PFC rectifier achieves an unprecedented power density of 3.28 kW/L (54 W/ $\mathrm {in}^{3}$ ) and a full-load efficiency of 99.1%, determined using a calorimetric measurement setup, which is discussed in detail in the Appendix.

A Modular SiC High-Frequency Solid-State Transformer for Medium-Voltage Applications: Design, Implementation, and Testing

Abstract: The advancements in wide-bandgap devices are enabling applications of power electronic converters coupled directly with medium-voltage (MV) grids that incorporate galvanic isolation within the converter using high-frequency solid-state transformers (SSTs). This paper presents analysis, design, and the characterization results of a 50-kVA modular soft-switched ac–ac SST using silicon carbide (SiC) MOSFET for MV (>6 kV ac) applications. The SST is comprised of two hard-switched ac-line interface bridges and a resonant dc–dc stage switched at approximately 180 kHz. To minimize the cost of switching elements per ampere and maximize the design flexibility, the design uses multiple discrete SiC devices of, readily available, 1700-V ratings. This paper covers the analysis of soft-switching operation, control architecture, converter design, and system-level integration.

Dynamic On-Resistance in GaN Power Devices: Mechanisms, Characterizations, and Modeling

Abstract: Gallium nitride (GaN) power devices enable power electronic systems with enhanced power density and efficiency. Dynamic on-resistance ( $R_{\mathrm{\scriptscriptstyle ON}}$ ) degradation (or current collapse), originating from buffer trapping, surface trapping and gate instability, has been regarded as a primary challenge for the lateral GaN-on-Si power devices. In this paper, we present an overview and discussion of the mechanisms, characterizations, modeling, and solutions for the degradation of dynamic $R_{\mathrm{\scriptscriptstyle ON}}$ in GaN power devices. The complex dynamics of acceptor/donor buffer traps and their impacts on dynamic $R_{\mathrm{\scriptscriptstyle ON}}$ have been analyzed and revealed by TCAD simulations and high-voltage back-gating measurements. The gate instability-induced dynamic $R_{\mathrm{\scriptscriptstyle ON}}$ increase in different GaN device technologies and the role of gate overdrive are also discussed. Wafer-level and board-level characterization techniques enabling accurate dynamic $R_{\mathrm{\scriptscriptstyle ON}}$ evaluation are reviewed. The dynamic $R_{\mathrm{\scriptscriptstyle ON}}$ performance of the state-of-the-art commercial GaN devices is presented, and a behavioral model with the dynamic $R_{\mathrm{\scriptscriptstyle ON}}$ degradation taken into consideration has been implemented for circuit analysis. The latest progress in GaN device technologies for enhanced dynamic performance is also reviewed and discussed.

Model Predictive Control of Six-Phase Induction Motor Drives Using Two Virtual Voltage Vectors

Abstract: Finite-control set model predictive control (FCS-MPC) has been successfully applied to three-phase electric drives and has proven to bring fast dynamics and high flexibility. The extension of FCS-MPC to regulate machines with more than three phases (i.e., multiphase) presents, however, additional challenges. Leaving aside the higher computational requirements, the appearance of additional degrees of freedom requires a simultaneous current tracking in different subspaces. Hence, the accuracy in the fundamental plane needs to be accomplished with no simultaneous excitement of the secondary planes in order to avoid unacceptable inefficiencies. This paper jointly tackles the performance improvement of fundamental and secondary planes following a two-step procedure: individual virtual voltage vectors (VVs) first ensure low circulating currents and the optimal combination of two VVs provides enhanced current tracking in the fundamental plane. Comparative experimental results confirm the satisfactory performance of the proposed strategy.

Modulated Model-Predictive Control With Optimized Overmodulation

Abstract: Finite-set model-predictive control (FS-MPC) has many advantages, such as a fast dynamic response and an intuitive implementation. For these reasons, it has been thoroughly researched during the last decade. However, the waveform produced by FS-MPC has a switching component whose spread spectrum remains a major disadvantage of the strategy. This paper discusses a modulated model-predictive control that guarantees a spectrum switching frequency in the linear modulation range and extends its optimized response to the overmodulation region. Due to the equivalent high gain of the predictive control and to the limit on the voltage actuation of the power converter, it is expected that the actuation voltage will enter the overmodulation region during the large reference changes or in response to load impacts. An optimized overmodulation strategy that converges toward the FS-MPC ’s response for large tracking errors is proposed for this situation. This technique seamlessly combines PWM’s good steady-state switching performance with FS-MPC ’s high dynamic response during large transients. The constant switching frequency is achieved by incorporating modulation of the predicted current vectors in the model-predictive control of the currents in a similar fashion as the conventional space-vector pulsewidth modulation is used to synthesize an arbitrary voltage reference. Experimental results showing the proposed strategy’s good steady-state switching performance, its FS-MPC -like transient response, and the seamless transition between modes of operation are presented for a permanent magnet synchronous machine drive.

Operation and Control Methods of Modular Multilevel Converters in Unbalanced AC Grids: A Review

Abstract: High-voltage direct current (HVdc) transmission systems based on modular multilevel converters (MMCs) are a promising solution for efficient bulk power transmission over long distances. As in grid-connected converters, the occurrence of grid-side faults is rather common, leading to imbalances and distortions of the ac-side voltages. Therefore, operation and control of MMC-HVdc systems under grid imbalances become of great significance in order to satisfy grid codes and reliability requirements of the system. The aim of this paper is to provide a comprehensive review of operation and control methods applied to MMC-HVdc systems with a specific focus on unbalanced ac-grid conditions. The methods are classified based on their main control targets that include ac-side power control, control of the circulating current, and dc-side power ripple suppression control. Special attention is given to the comparison of the different control methods and specific requirements under certain operating conditions that include grid, load, or internal imbalances.

Analysis of Partial Power DC–DC Converters for Two-Stage Photovoltaic Systems

Abstract: Two-stage photovoltaic (PV) configurations have become increasingly popular due to the decoupling between the inverter dc-link voltage and the PV voltage, adding flexibility to extend the maximum power point tracking range. However, the additional dc–dc converter increases the power converter losses. The concept of partial power converters (PPCs), which reduce the amount of power handled by the dc stage, can mitigate this effect. However, the type of topology, its power and voltage rating, efficiency, and an operating range can vary significantly depending on the function (boosting or reducing voltage) and type of PV application and scale (micro-, string-, or multi-sting inverter). This paper analyzes the possible configuration of connections of PPC depending on the application and scale of the PV system and introduces a new buck-type PPC. Three solutions for practical PV systems are further elaborated, including experimental validation. Results show that the PPC concept greatly improves the overall PV system efficiency with the added benefit that the dc–dc stage power ratings achieved are only a fraction of the PV system, reducing size and cost of the power converter without affecting the system performance.

Current Stress Minimization of Dual-Active-Bridge DC–DC Converter Within the Whole Operating Range

Abstract: Current stress minimization is one of the most important challenges for the steady-state studies of dual-active-bridge (DAB) dc–dc converter. Triple-phase-shift (TPS) control can minimize the current stress to the utmost extent, and a segmented analytical method-based TPS (SA-TPS) control has been proposed in this paper to minimize the current stress of DAB within the whole operating range. The full model of DAB with TPS control was classified into 12 operating modes by a complete operating mode classification method. On the basis of the analytical expressions of transmission power and current stress in different modes, the current stress minimization issue was transferred to inequality constraints’ problems in different operating modes. The mathematical derivation process of the SA-TPS control method is described in detail, and the analytical expressions of current stress minimization results as a function of voltage conversion ratio and transmission power have been derived by solving the forward and reverse power transmission processes, respectively. The operating mode selection principle and the regularities of minimization results varying with transmission power and voltage conversion ratio have been explored. Experiments under different conditions have been implemented on a laboratory prototype to validate the correctness and effectiveness of the proposed SA-TPS control method.

Analytical Methodology for Loss Calculation of SiC MOSFETs

Abstract: For evaluating and optimizing the efficiency of power converters, the loss model of the power semiconductor devices is needed. The analytical loss model is favorable for its simplicity. However, the model accuracy needs to be improved. This paper proposes an analytical loss model for the commutation pair of silicon carbide (SiC) MOSFETs and SiC Schottky barrier diodes. Compared to the conventional loss calculation method, the proposed model is derived based on the conservation of energy, which considers the impact of the displacement currents on estimating the turn-on and turn-off losses. The modeling and extraction methodology of the model parameters are given, and the impact of operational conditions on the model parameters is studied. Using the proposed model, the losses can be calculated analytically and also the switching trajectories can be predicted with good consistency. A double pulse tester is built to verify the proposed model. The results show that an average modeling error is reduced from 20% using the conventional piecewise linear model, to around 10% using the proposed model under various operational conditions, and the time cost is at least 3000 times smaller compared to the existing SiC MOSFETs models.

EKF-Based Predictive Stabilization of Shipboard DC Microgrids With Uncertain Time-Varying Load

Abstract: The performance of dc shipboard power systems (SPSs) may degrade due to the negative impedance of constant power loads (CPLs) connected to dc microgrids (MGs). To control the dcSPS effectively, estimation of the instantaneous power flow to the time-varying uncertain CPLs is necessary. Furthermore, fast adaptive control is needed to deal with changes in the CPL power flow and quick stabilization of the dc MGs. Such a controller typically uses injection current from an energy storage system for actuation. Since measuring the CPLs’ powers require installing current sensors that are both costly and not optimal, an estimation of the CPLs’ powers should be employed. In this paper, an extended Kalman filter (EKF) is developed to estimate a time-varying power of uncertain CPLs in a dc MG based on measuring capacitor voltages. The estimated power is then used in a Takagi–Sugeno fuzzy-based model predictive controller (MPC) to manipulate the energy storage unit. The proposed approach is tested experimentally on a dc MG that feeds a single CPL. The experimental results show that the proposed MPC controller alongside the developed EKF improves the transient performance and the stability margin of the dc MGs used in the SPSs.

A New Generalized Multilevel Converter Topology Based on Cascaded Connection of Basic Units

Abstract: In this paper, a new single-phase multilevel converter (MLC) topology based on the cascade connection of novel basic units is presented. The proposed basic unit generates 17-level output voltage waveform and can be extended for higher voltage levels by using a simple cascade connection. Both the proposed basic unit and cascaded topologies are compared with other state-of-the-art MLC topologies. From the comparison results, it will be shown that the proposed topology has several advantages such as the reduced number of power electronic components, lesser number of dc sources, and blocking voltage. Moreover, the proposed MLC has reduced power losses and improved efficiency. The operability and feasibility of the proposed converter are validated through extensive simulation and experiments. Finally, the corresponding results affirming the predominance of the proposed topologies are presented.

Electrical Performance Advancement in SiC Power Module Package Design With Kelvin Drain Connection and Low Parasitic Inductance

Abstract: Silicon carbide (SiC) power modules are promising for high-power applications because of the high breakdown voltage, high operation temperature, low ON-resistance, and fast switching speed However, the large parasitic inductance in existing package designs results in compromised performance, ie, long blanking time in the desaturation protection scheme and large overvoltage spikes during the switching transient Consequently, the benefits of SiC devices are often not fully utilized in practical applications This paper deals with these two issues and aims at improving the electrical performance of the existing SiC module package Specifically, a package design with Kelvin drain-to-source connection is first proposed to minimize the blanking time More than 99% reduction of blanking time is achieved experimentally compared to the conventional package design Second, a low parasitic inductance package with double-side cooling is proposed to allow the fast switching speed of SiC devices without sacrificing the thermal performance A power loop inductance of 163 nH is realized from Q3D simulation Verified by the experiment, more than 60% reduction of power loop inductance is achieved in comparison to a previously designed baseline module At 0- $\Omega $ external gate resistance, the turn-off voltage spike is less than 9% of the dc-link voltage under the rated load condition

Load-Independent Voltage and Current Transfer Characteristics of High-Order Resonant Network in IPT System

Abstract: Load-independent output characteristics of an inductive power transfer (IPT) system are of increasing interest in electric vehicle and LED lighting applications. All compensation networks in the IPT system are actually high-order resonant circuits. In a high-order resonant network, there are multiple resonant frequencies to get load-independent voltage output and current output. It is critical to analyze the resonant conditions to achieve high efficiency in both load-independent voltage output and current output modes. This paper proposed a general modeling method for arbitrary high-order resonant networks to get both the load-independent voltage and current transfer characteristics. A high-order circuit can be modeled as a combination of an LC network, a multistage T-circuit, and/or multistage $\Pi $ -circuit in series. The proposed method is verified by applying to voltage-fed double-sided inductor–capacitor–capacitor (LCC), series–series (SS), S-SP, LCC-S, and current-fed CLC-LC compensation networks in the IPT system. The MATLAB simulation and the experimental prototype of a constant voltage-fed double-sided LCC compensated IPT system with up to 3.3-kW power transfer are built. The efficiency of the double-sided LCC compensated IPT system is up to 92.9% and 90.6% when the IPT system operates at resonant frequencies that achieve constant current output and constant voltage output, respectively, which are compliance with the frequency requirement by SAE J2954 standard.

Advanced Single-Phase Nine-Level Converter for the Integration of Multiterminal DC Supplies

Abstract: This paper presents a new symmetrical single-phase nine-level dc–ac converter for the applications of distributed generation systems. The proposed multilevel inverter (MLI) utilizes four independent dc sources to generate the nine output voltage levels considering a reduced number of switching devices. The proposed inverter consists of main and auxiliary circuits. The main circuit is a single H-bridge circuit, which is responsible for output voltage polarity. The auxiliary circuit consists of a special combination of switching devices, which are used to synthesize the multilevel output voltage. The merits of the proposed MLI are: increased output voltage levels at reduced number of switching devices, low switching loss, low total harmonic distortion, low dv/dt stress on the switches, and hence, low ratings of the switching devices, and low cost. The effectiveness of the proposed MLI and its pulsewidth modulation switching pattern has been verified experimentally by using a laboratory prototype controlled by DSPACE-1103.

A High-Power-Density Power Factor Correction Front End Based on Seven-Level Flying Capacitor Multilevel Converter

Abstract: This paper presents a power factor correction (PFC) front end based on a seven-level flying capacitor multilevel (FCML) boost converter. Compared to the conventional two-level boost converter, the proposed seven-level FCML converter features the use of low-voltage-rated transistors, reduced voltage stress, and high effective switching frequency on the filter inductor. These characteristics of the FCML converter lead to drastic reduction in the filter inductor size while maintaining high efficiency and, therefore, significantly improve the power density of the PFC front end compared to conventional solutions. On the other hand, the small inductance imposes challenges on the PFC control. The dynamics of the seven-level FCML converter has been analyzed, and a feedforward control has been implemented to overcome these challenges. A hardware prototype is designed for universal ac input (90 to 265 Vac), 400-V dc output, and 1.5-kW power rating. Compared to existing solutions, the hardware prototype demonstrates improved efficiency and power density while maintaining high power factor and low THD. A power density of 219 W/in3 (490 W/in3 for the power stage) has been achieved, and a peak efficiency of 99.07% has been experimentally verified.

Evaluation of Power Processing in Series-Connected Partial-Power Converters

Abstract: This paper proposes an analytical methodology to evaluate the power processed by dc–dc converters operating as series voltage regulators, which provide an alternative to increase efficiency in photovoltaic systems. Via the analysis of both active and nonactive power processing, the proposed methodology allows to clearly distinguish among circuit topologies as truly partial-power processing (PPP) or just partial active power processing topologies. When an isolated dc–dc topology is connected in series as a voltage regulator, the overall processed power can be reduced, which reduces power losses and improves efficiency. Conversely, this paper also demonstrates that some series-regulator topologies may not actually reduce the proportion of nonactive processed power. To demonstrate the applications of the proposed methodology and to emphasize its significance, two well-known series-connected voltage regulators (flyback and full-bridge phase shift) and a full-power regulator (boost) were evaluated. The results indicate that the full-bridge series regulator can perform a true PPP, whereas the flyback series-regulator processes the same amount of power as that processed by conventional nonisolated boost converters and cannot be considered a partial-power topology. This study finding contradicts assertions in the literature that this topology achieves high-efficiency dc–dc conversion through PPP. To confirm the theoretical analysis, experimental results from three 750-W prototypes are presented alongside its simulations.

Autotuning Technique for the Cost Function Weight Factors in Model Predictive Control for Power Electronic Interfaces

Abstract: This paper presents an autotuning technique for the online selection of the cost function weight factors in model predictive control (MPC) The weight factors in the cost function with multiple control objectives directly affect the performance and robustness of the MPC The proposed method in this paper determines the optimum weight factors of the cost function for each sampling time; the optimization of the weight factors is done based on the prediction of the absolute tracking error of the control objectives and the corresponding constraints The proposed method eliminates the need of the trial-and-error approach to determine a fixed weight factor in the cost function The application considered is a capacitor-less static synchronous compensator based on the MPC of a direct matrix converter This technique compensates lagging power factor loads using inductive energy storage elements instead of electrolytic capacitors The result demonstrates that the proposed autotuning approach of cost function weights makes the control algorithm robust to parameter variation and other uncertainties in the system The proposed capacitor-less reactive power compensator based on the autotuned MPC cost function weight factor is verified experimentally

Common Mode Voltage Reduction in a Single-Phase Quasi Z-Source Inverter for Transformerless Grid-Connected Solar PV Applications

Abstract: The quasi-Z-source inverter (qZSI) is becoming a popular inverter topology that can buck or boost input voltage without a dc–dc converter and hence can be used in transformerless configuration Due to its single-stage conversion, the qZSI can be used as an efficient transformerless grid-tie inverter However, the common mode current is a major problem in transformerless topologies due to the absence of galvanic isolation This paper proposes a modified pulsewidth modulation (PWM) technique to control the qZSI, along with two extra semiconductor switches, to reduce the common mode current The proposed method offers an efficient solution for grid integration of solar photovoltaic systems The proposed topology has the following features: 1) uses phase-leg shoot-through for boosting the dc voltage to the required level which eliminates the additional dc–dc converter; 2) eliminates the PWM deadtime and provides freewheeling through additionally connected switches; 3) minimizes the common mode current by modifying the PWM and adding additional switches at the output side of inverter; and 4) avoids the conduction of body diode of H-bridge which has poor reverse recovery characteristics The proposed topology can efficiently control the reactive power, and the suitable PWM scheme is also reported Simulation results have been performed with the Standard Test conditions of the photovoltaic panel Experimental results for a single-phase 500-W prototype are presented to validate the proposed PWM scheme for the qZSI topology

Investigating Impact of Emerging Medium-Voltage SiC MOSFETs on Medium-Voltage High-Power Industrial Motor Drives

Abstract: The SiC MOSFETs are becoming game-changing devices in the field of power electronics, enabling higher temperatures, power densities, and efficiencies. However, at higher voltages than 1.7 kV, these semiconductors are at early stages of development and yet not commercialized. Based on the characterization results of the state-of-the-art 3.3-kV SiC MOSFETs, for the first time, this paper investigates the design and comparison of topologies commercially used for medium-voltage (MV) drives in 4.16–13.8-kV voltage range in the presence of MV SiC MOSFETs. For this purpose, the cascaded H-bridge, modular multilevel converter, and five-level active neutral point clamped (5-L ANPC) topologies are targeted. Design is carried out at 4.16-, 6.9-, and 13.8-kV voltages (4.16 and 6.9 kV in the case of 5-L ANPC) and 3- and 5-MVA power ratings using commercial Si IGBTs as well as latest generation noncommercial 3.3-kV SiC MOSFETs, in order to enable investigation of impact from the emerging MV SiC MOSFETs on motor drive system. Selection of several voltage and power levels is to elucidate behavior of converters at a different voltage and power rating and determine the best option for given operating point. Based on design data, comparisons are done among the mentioned topologies from different points of view including efficiency, passive component requirement, semiconductor utilization, power density, low-speed operation capability, fault containment, and parts count. Experimental results on an H-bridge cell made with 3.3-kV SiC MOSFETs are brought to verify converter modeling in MATLAB environment as well as the conveyed thermal calculations.

On the Origin of the $C_{\text{oss}}$ -Losses in Soft-Switching GaN-on-Si Power HEMTs

Abstract: The unprecedented performance potential of gallium nitride-on-silicon (GaN-on-Si) high electron mobility transistors (HEMTs) is seen as the key enabler for the design of power converters featuring extreme power density figures, as demanded in next-generation power electronics applications. However, unexpected loss mechanisms, i.e. dynamic $ {R}_{\text {ds,on}}$ phenomena and $ {C}_{\text {oss}}$ -losses, are appearing in currently available GaN transistors and are compromising their operation. In this paper, measurements of $ {C}_{\text {oss}}$ -losses are performed in a dedicated calorimetric measurement setup and, through a systematic approach, the root cause of the loss mechanism is potentially identified. Afterward, with the essential support of a manufacturer of power semiconductors, a novel transistor, featuring an enhanced multilayer III-N buffer, is developed according to the acquired knowledge. A significant reduction in terms of $ {C}_{\text {oss}}$ -losses, i.e. of soft-switching losses, and the absence of dynamic $ {R}_{\text {ds,on}}$ phenomena are verified experimentally on the new device. These achievements enable a significant performance improvement for future soft-switching power converters featuring GaN-on-Si HEMTs.

A Novel Model Predictive Control Strategy to Eliminate Zero-Sequence Circulating Current in Paralleled Three-Level Inverters

Abstract: The main contribution of this paper is the proposal of a novel finite control set model predictive control (MPC)-based zero-sequence circulating current (ZSCC) elimination strategy for parallel operating three-level inverters without any modification or extra hardware on the three-level inverters. An equivalent model of the ZSCC is first developed, and the voltage differences of common-mode voltages (CMVs) among paralleled inverters as well as those of the neutral point potentials (NPPs) are proved to be the exciting sources of the ZSCC. With the analysis, an MPC-based zero CMV method (ZCMV-MPC) is presented to reduce the difference of CMVs among the paralleled inverters, meanwhile, an active NPP perturbation-based ZSCC feedback control method is proposed to further eliminate the ZSCC, that may be caused by dead-time effects and the asymmetries of both hardware and control parameters. With the proposed method, the ZSCC between paralleled inverters can be eliminated effectively, and both grid current tracking and NPP balance control can also achieve satisfactory performances. Simulation and experimental results supported the theoretical study and verified the effectiveness of the proposed scheme.

Frequency and Voltage Tuning of Series–Series Compensated Wireless Power Transfer System to Sustain Rated Power Under Various Conditions

Abstract: In a wireless power transfer (WPT) system, voltage tuning and frequency tuning are two major methods to control the output power. For the design of a WPT system at the rated power, it is crucial to understand the characteristics of a WPT system with a constant-power load (CPL) under voltage tuning and frequency tuning. The model of a series–series compensated WPT system with a CPL is established based on the Thevenin equivalent circuits. The conditions to achieve the maximum coil-to-coil efficiency for the two tuning methods are obtained. For frequency tuning, the tuning zone above the resonant frequency is selected because of its wide tuning range and zero-voltage switching. The maximum output power at different operating frequencies is derived, and the required inverter dc voltage and the tuning range for a target output power can be obtained accordingly. With a strong coupling, the maximum efficiency of frequency tuning can be as high as that of voltage tuning; while with a weak coupling, the efficiency of frequency tuning is much smaller than that of voltage tuning. The model with a CPL is helpful for the design and optimization of a WPT system with the rated power.

Hybrid Modulation of Parallel-Series $LLC$ Resonant Converter and Phase Shift Full-Bridge Converter for a Dual-Output DC–DC Converter

Abstract: A novel pulse frequency and phase shift hybrid modulated dual-output dc–dc converter is proposed in this paper. The proposed converter is composed of hybrid modulation of an input-parallel-output-series LLC resonant converter and a phase shift full bridge (PSFB) converter. The output voltage of the LLC resonant converter is modulated by the switching frequency, whereas the output voltage of the PSFB converter is modulated by the phase angle; therefore, the two output voltages are regulated independently and free from cross regulation. The series inductor of the PSFB converter does not need to be specially designed to help achieving the soft switching operation, and the duty cycle loss could be mitigated. Zero-voltage switching can be achieved on the power switches in the whole load range. Finally, a 1.2-kW 400-input 200–400-V and 48-V output prototype is built and tested to verify the effectiveness of the proposed converter.

Analysis and Control of a Novel Modular-Based Energy Router for DC Microgrid Cluster

Abstract: Energy router is one of the key elements for power electronic-based DC micro-grid (DCMG) cluster system. Traditional ac/dc converter and solid-state transformer can act as an energy router, but their functions and interfaces are restricted. In this paper, a novel modular-based energy router (MBER) for DCMG cluster has been proposed to extend the functions of energy router. Each module of MBER is composed of an ac/dc converter and an isolated dual-active-bridge converter with high-frequency transformers. The power multidirectional exchange mechanism between ac grid and DCMG cluster is shown. Then, the operation mechanism and the operation modes of MBER are analyzed. Considering the operation range of MBER is limited by the operation modes and the dc voltage of each module, a dc voltage adjustment strategy and the control method have been proposed to expand the operation range of MBER. Finally, the simulation and experimental results are presented to validate the proposed topology and control methods.