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This paper proposes a medium-frequency (MF) solid-state transformer (SST) based grid interface for high-power multipulse rectifiers. In the proposed approach, the low-frequency ac (grid) voltage is transformed to MF voltage via front-end converters. The three single-phase MF outputs from these converters are fed to the primary side of a three-phase, five-limb multiwinding isolation transformer (or alternately three one-phase transformers). The secondary side windings from the transformer are configured in a certain “zig-zag” arrangement, which are connected to the inputs of three-phase diode rectifier modules as in a typical multipulse system. The proposed topology can better than double the power density by reducing volume (or weight) of the required front-end isolation transformer while maintaining the same input current quality as a conventional low-frequency transformer based approach. Usage of an open-loop square wave pulse width modulation for the front-end converters makes this approach simple to implement. This paper explains the proposed concept taking an adjustable speed drive application's perspective using modeling, simulation, and experimental results from a scaled down laboratory prototype. [ABSTRACT FROM AUTHOR]

This paper presents a novel family of single-switch resonant dc–dc converters with low switch voltage stress. The single-switch resonant converter which has a ground-referenced switch is advantageous for implementing the gate drive circuit and operating at several-MHz switching frequency. However, the conventional ones mostly have high voltage stress on the switch, roughly 4–5 times the input voltage. In this paper, we propose the single-switch converter topologies derived from the drain-source impedance networks consisting of two inductors and two capacitors. The switch voltage of the proposed converters is shaped into a near trapezoid by designing the resonant networks to have the desired drain-source impedance. Furthermore, a simple and specific design scheme is presented here so that the peak switch voltage is lowered to 2.2–2.5 times the input voltage while zero voltage switching is achieved. Experimental results from a 20-W GaN-based prototype operating at 10 MHz demonstrate the feasibility of the proposed converter topologies and the design method. [ABSTRACT FROM AUTHOR]

Thanks to the favorable advantage of low cost, integrated three-port dc–dc converters with reduced switches have attracted extensive attention. In order to provide more new topologies, this paper aims to propose a programmable topology derivation method, which effectively simplifies the cumbersome process of the conventional combination method. Instead of the manual connection and examination, the proposed alternative can quickly and rigorously derive multiple viable integrated three-port dc–dc topologies from a great number of possible connections with the aid of computer program. Besides, generalized analysis is also accomplished, with which performance characteristics of all derived converters are simultaneously obtained and then a comprehensive comparison can be easily conducted to select a preferred one for the practical application. Finally, an example-specific application with one input and two outputs is given, with topology selection, design, and experimental results demonstrated in detail. [ABSTRACT FROM AUTHOR]

This paper proposes a novel dc power electronic transformer (DCPET) topology for locomotive, ac/dc hybrid grid, dc distribution grid, and other isolated medium-voltage and high-power applications. Compared with conventional PET topology, the proposed DCPET has fewer power semiconductor devices and high-frequency isolation transformers, which can improve the power density and reliability. Fault handling or redundancy design can be achieved to further improve the reliability when some dc–dc modules break down. Also, input voltage sharing control can be omitted to simplify the control system and improve the stability. Meanwhile, soft switching is guaranteed for all the switches, which is beneficial to increase switching frequency and improve power density. In this paper, the principle, evolution, and control of the proposed DCPET are respectively presented and studied in detail. Finally, a prototype of the proposed DCPET is built and the experimental results verify the validity and superiority of the proposed topology. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMCs) are being developed for the grid connection of offshore wind or tidal farms. In order to reduce the construction and maintenance costs, it is desirable to reduce the weight and volume of the system. In current MMC submodule designs, the reservoir capacitor usually accounts for over 50% of the total volume and 80% of weight. This paper presents a new design concept and control principle for a submodule using the stacked switched capacitor (SSC) architecture that can significantly reduce the capacitor size in an MMC. Practical considerations for a high-voltage high-power SSC-based MMC submodule are presented in this paper, through the design of a 21-level, 40-kV (pole-pole dc), 19.1-MW, grid-connected system, and the concept is demonstrated experimenttally on a scaled-down 400-V, 12.3-Apeak laboratory prototype submodule. It is shown that with the proposed SSC architecture, the total volume of capacitors in each submodule can be reduced by more than 40% without significantly increasing the power loss. [ABSTRACT FROM AUTHOR]

This paper is the continuation of Part I in which an online reference limitation strategy for shunt-connected converters is presented. This limitation strategy is successfully validated in this Part II of the paper by means of experimental results obtained in a laboratory setup. Two different experiment sets are carried out in order to check the two application scenarios described in Part I. On the one hand, the limitation strategy is implemented in the STATCOM application reducing the reactive power reference. On the other hand, the grid balancer functionality is implemented together with the new limitation strategy reducing the current references. In this last scenario, two different experiments are shown: 1) a switching-on of the converter; and 2) a voltage variation at the point of common coupling. In both functionalities, the corresponding reference limitations are activated due to overcurrent and overvoltage cases and the limitation due to the dc bus voltage oscillations is validated in the grid balancer functionality. [ABSTRACT FROM AUTHOR]

The noninverting buck–boost converter has attracted significant attention in the photovoltaic (PV) module integrated application, as it offers high efficiency while maintaining a low cost and a simple topology. When this converter is employed, special care must be taken at the transition between buck and boost operating modes, as a dead-zone in the voltage transfer function can cause abrupt perturbations in the controlled voltage, decreasing the regulation quality and ultimately lowering the power conversion efficiency. In this paper, a new dead-zone compensation technique is proposed with the scope of smoothing the transition between operating modes, eliminating the voltage ripple and improving the regulation performance, while maintaining high efficiency. The converter under analysis is studied together with its gate driving circuit, which is based on a bootstrap capacitor power supply for the high-side switches. The proposed dead-zone compensation technique is deduced by using the principle of maintaining the ideal voltage gain function across the converter operating range. The technique is analyzed, implemented and tested on a purposely built PV module integrated noninverting buck–boost converter. The experiments reveal a sensible improvement of the voltage regulation during mode transition, confirming the effectiveness of the proposed technique and its fitness for the PV application. [ABSTRACT FROM AUTHOR]

Adding the auxiliary switches to the conventional H-bridge inverter is an effective way to eliminate the leakage current for transformerless PV systems, such as H5, H6, etc. Inspired by the newly developed embedded-switch H6 topology, a novel embedded-switch inverter (ESI) is proposed in this paper for three-phase transformerless PV systems. First, the operation principle and characteristics of the proposed ESI are analyzed. Second, the common-mode model of the three-phase ESI is established, based on which the main factors that affect the leakage current are discussed. The finding reveals that the switching states with conventional modulation strategy result in high-frequency common-mode voltage, which is the source of leakage current. In order to solve the problem, a new modulation strategy is presented for ESI to eliminate the high-frequency leakage current. Finally, the time-domain simulation and the experimental tests are carried out. The results verify the effectiveness of the proposed solution. [ABSTRACT FROM AUTHOR]

This paper addresses the inverter nonlinearity of a three-level T-type inverter. The main causes of the nonlinearity are pulse shaping, pulse skipping due to narrow pulses for the dead time, and voltage drop of the switching devices. Although the same causes have existed in two-level inverters, the pulse skipping phenomenon and results are quite different in the case of the three-level T-type inverter. The effects of pulse shaping and skipping have been investigated and discussed. In addition, the voltage drop of the switching devices has been considered in conjunction with the conduction path of the T-type inverter. Compensation methods based on the pulse width modulation (PWM) of T-type inverter to alleviate the inverter nonlinearity have been proposed from research results. The proposed methods could be easily implemented by adding appropriate offset voltages to the voltage references of the inverter in PWM. Furthermore, a neutral voltage balancing method of a three-level T-type inverter has been proposed in conjunction with the proposed PWM methods using offset voltage. Through extensive experimental tests, the fifth- and seventh-harmonic components of the current are conspicuously reduced along with the even harmonics. The neutral voltage of the inverter can be balanced effectively with the proposed offset voltage adjustments as well. [ABSTRACT FROM AUTHOR]

This paper proposes a multilevel series compensator (MSC) to deal with voltage sags/swells, harmonic compensation, or reactive power compensation. Such a device can be considered as a dynamic voltage restorer or a series active power filter (Series-APF). The MSC can improve the power quality of loads located in stiff systems. The configuration is based on three-phase bridge (TPB) converters connected by means of cascaded single-phase transformers. This arrangement permits the use of a single dc-link. A generalization for $K$ -stages in which $K$ -transformers are coupled with $K$-TPB converters is presented. The topology permits generating a high number of levels in the voltage waveforms with a low number of power switches in comparison with a classic topology. The multilevel waveforms are generated by the converters through a suitable pulsewidth modulation (PWM) strategy that takes into consideration the transformer turns ratios. Modularity and simple maintenance make the proposed MSC an attractive solution compared with some conventional configurations. Model, PWM strategy, and overall control are discussed in this paper. Simulation and experimental results are presented as well. [ABSTRACT FROM PUBLISHER]

Various magnetically coupled impedance source (MCIS) networks have been proposed in the literature for increasing voltage gain and modulation index simultaneously while reducing the number of passive components used in the converter. However, applications of such networks have been limited by their discontinuous currents drawn from the sources and/or high stresses experienced by their components. This paper thus proposes three new MCIS networks named, respectively, as quasi-Y-source, quasi-Γ-Z-source, and quasi-T-source or quasi-trans-Z-source networks. These new networks inherit all advantages of the existing MCIS networks. In addition, they demonstrate advantages like continuous input currents, reduced source stresses, and lower component ratings that are not achievable by other existing networks. Further, dc-current-blocking capacitors used in the networks help to avoid saturation of magnetic core. Derivations of all two-winding MCIS networks, including existing and new networks, from the generalized three-winding MCIS networks are then systematically illustrated, before comparing them. Operational principles, mathematical derivations, simulation, and experimental results of all studied networks have been presented in the paper. [ABSTRACT FROM PUBLISHER]

The two-switch cascade converter is the most appropriate solution for efficient high-power applications among the existing converter topologies. However, the cascade structure and the nonlinear dc gain of boost circuit are more likely to cause severe oscillations if the criterion for the selection of the operation mode is chosen improperly during step-up/step-down transition. A novel high-power input-parallel output-series buck-half-bridge (IPOSBHB) converter is proposed in this paper. The circuit structure, operating principles, and basic relations are analyzed. Comparisons between IPOSBHB and two-switch cascade converter are also elaborated on with respect to voltage and current stress, total volume of magnetic components, and efficiency. In addition, the inconformity of the transfer functions, the discontinuity caused by the switching time delay, and the disturbance of input voltage are analyzed. Methods for compensation are also proposed. The proposed converter can achieve satisfactory comprehensive performance when the step-up ratio is low. The combinational control strategy and the compensation methods proposed in this paper are easy to carry out in order to achieve effective control at steady state, smooth transition at mode change, and reduction of the adverse effect caused by the disturbance of input voltage. A 15-kW prototype is used to validate the proposed theory. [ABSTRACT FROM AUTHOR]

This paper proposes two coupled-inductors-based Z-source networks, called the modified series and tapped switched-coupled-inductors quasi-Z-source networks (mSSCL-qZSN and mTSCL-qZSN, respectively). These networks offer high voltage gains with less-than-unity winding turn ratios $(n < {\text{1)}}$ , resulting in a reduced inductive element size. Other advantages include a less number of active components and their lower ratings and a higher efficiency. In addition, a lower magnetic core size is required for the proposed ZSNs due to the lower peak magnetizing current. The performance principles and detailed comparative analysis are provided and confirmed through experiments on a 200-W dc–dc converter. [ABSTRACT FROM AUTHOR]

In this paper, a family of step-up resonant switched-capacitor converters (RSCs) with a continuously adjustable conversion ratio is proposed based on the Cuk dual-resonance core (DRC) featuring two resonant paths. By adopting the on-time fixed frequency modulation, the two resonant paths can conduct simultaneously and satisfy the charge balance of the flying capacitor during the overlapping period. As a result, the conversion ratio—upper bounded by two determined by the circuit configuration—can vary continuously and efficiently even at the light load condition (1/10 of the rated power). A comprehensive analysis—the operation principle, voltage-gain curve, and voltage/current stresses—about the DRC is given. In addition, the Cuk DRC was extended to the series–parallel switched-capacitor converter (SCC), Fibonacci SCC, and voltage doubler to form a family of step-up RSCs, whose voltage-gain curves of different topologies are derived. Moreover, a characteristic comparison between the Cuk DRC and the traditional “direct” and “indirect” resonant core is given. Then, a 48 to 62–94.6 V resonant core with peak efficiency 97.8% and maximum power 120 W under the open loop condition was built. The analyses were verified by both experimental and simulation results. [ABSTRACT FROM AUTHOR]

This paper proposes a voltage balancing method for stacked multicell converters (SMCs) based on phase-disposition pulse-width modulation (PD-PWM). In order to reduce the switching transitions of the power devices, only optimal transitions between consecutive voltage levels are used. Selection of the optimal transition sequence is performed by evaluating a cost function. Significant reductions in the switching transitions and power losses of the power devices are achieved as compared to the optimal-state voltage balancing method where nonoptimal transitions are allowed. Simulation and experimental results from a seven-level $3\times 2$ SMC verifies that the proposed PD-PWM voltage balancing method is robust under linear/nonlinear loads and transients. [ABSTRACT FROM AUTHOR]

The five-level nested neutral-point-clamped (5L-NNPC) converter is one of the most promising topologies for medium-voltage (2.3–7.2 kV) high-power applications such as medium-voltage drives, wind energy conversion systems, and grid-connected systems. The 5L-NNPC requires a fewer number of switching devices, freewheeling and clamping diodes, and flying capacitors compared to the existing five-level multilevel converters. In the 5L-NNPC topology, each flying capacitor voltage is regulated at one-fourth of the dc-bus voltage to obtain the five-level operation. Due to the lack of redundant switching states, it is difficult to control the flying capacitor voltages by using the pulse-width-modulation-based classical control methods. This paper proposes a model-predictive current control (MPCC) approach to control the flying capacitor voltages along with the output currents of the 5L-NNPC converter. The discrete-time model of 5L-NNPC is developed to implement the MPCC scheme. The simulation and experimental studies are conducted to verify the dynamic and steady-state performance of 5L-NNPC with the MPCC scheme. The performance of the proposed MPCC approach is compared with the conventional space-vector-modulation-based voltage-balancing approach. Furthermore, the flying capacitor voltage control capability of MPCC is verified at different load power factors. [ABSTRACT FROM AUTHOR]

This paper presents an overview of resonant and hybrid switched-capacitor converters with a particular focus on multimode operation. The multimode approach is discussed as a tool to affect high efficiency and power density across a wide load range, provide variable regulation, and, as a general framework, to conceptualize the advantages and opportunities of the approach compared to more traditional dc–dc converters. A general analysis is provided to quantify the advantage of hybrid and resonant converters that factors in the tradeoffs between size and loss in the magnetic component and voltage stress on the active devices. A broader set of operating modes is presented that spans continuous and discontinuous conduction and includes resonant, quasi-resonant, and more traditional buck- or boost-like operation. A prototype flying-capacitor multilevel converter is presented to demonstrate the variety of operating modes and motivate important considerations such as voltage balance on the flying capacitors. [ABSTRACT FROM AUTHOR]

The trend toward higher switching frequencies and higher power densities in dc–dc conversion led the designers to rediscover resonant topologies. However, the analysis of these converters is often limited to a particular operating mode and/or topology. This paper wants to fill the gap by presenting a state-space-based analysis for a broad class of series-resonant converters including voltage-fed as well as current-fed topologies, either unidirectional or bidirectional. The proposed solution consists of a mathematical framework that solves the trajectory of state variables in the resonant tank, as well as the main characteristic functions such as voltage conversion ratio and transferred power. The matrix approach permits a flexible and compact formulation and easy implementation in computational tools such as MATLAB. The theoretical framework is applied to both voltage-fed and current-fed resonant topologies: the first is used to show the advantages of the proposed model respect to the first harmonic approximation, while a detailed analysis in terms of operating mode boundaries, delivered power, and voltage gain is provided for the second. Simulations and measurements on a 300-W prototype of the current-fed topology are used to validate the correctness of the proposed solution. [ABSTRACT FROM AUTHOR]

Many of the renewable energy resources, like offshore wind, utility PV farms, are located far away from the load centers, which requires dc power transmission system to efficiently deliver power. Voltage-source converter solution is required in many applications due to the stability and compensation requirement. This paper presents a new voltage source converter topology for medium to high voltage dc application, named modular embedded multilevel converter (MEMC). MEMC is based on a three-level topology structure and consists of series-connected insulated-gate bipolar transistor (IGBT) bridge stacks, like the popular modular multilevel converter (MMC) and series-connected thyristor or press-pack high voltage integrated gate-commutated thyristor or IGBT stacks. The basic operation principle, control solutions, and the methods to commutate the thyristors are explained in detail. Due to the three-level structure, the total number of IGBT stacks are reduced by half compared with MMC, leading to much lower energy storage, weight, volume, and system complexity. Also by replacing part of the IGBT stack in MMC with thyristor stack, both of the system conduction loss and switching loss can be further reduced. There are other topology variations of MEMC with a similar operational principle but different characteristics. [ABSTRACT FROM AUTHOR]

In this paper, novel topologies for symmetric, asymmetric, and cascade switched-diode multilevel converter are proposed, which can produce many levels with minimum number of power electronic switches, gate driver circuits, power diodes, and dc voltage sources. The number of required power electronic switches against required voltage levels is a very important factor in designing of multilevel converter, because switches define the reliability, circuit size, cost, installation area, and control complexity. For asymmetric and cascade converter, new algorithms for determination of dc voltage sources values are presented. To produce maximum number of levels at the output voltage, the proposed cascade topology is optimized for different goals, such as the minimization of the number of power electronic switches, gate driver circuits, power diodes, dc voltage sources, and blocking voltage on switches. Comparison of the results of various multilevel converters will be investigated to reflect the merits of the presented topologies. The operations of the proposed multilevel converters have been analyzed with the experimental and simulation results for different topologies. Verification of the analytical results is done using MATLAB simulation. [ABSTRACT FROM PUBLISHER]

A family of dimmable ac light-emitting-diode (LED) drivers fed from dc voltages, is presented in this paper based on zero-current-switching quasi-resonant converters. The proposed family of drivers is based on replacing the diode in the conventional converter topologies (i.e., buck, boost, or buck–boost) by a string of high-brightness LEDs (HB-LEDs). Hence, the HB-LED string will be working as the rectifier diode and the load, switching at the same frequency of the main switch. In this case, the output current, which is experimentally validated, shows a negative current peak due to the reverse-recovery effect of the HB-LEDs. In order to reduce the reverse-recovery effect on the HB-LEDs, the main switch of the proposed topologies is replaced with a full-wave resonant switch, which makes possible to reduce the di/dt during the turn-off of the HB-LED string, therefore the reverse-recovery effect is eliminated. Moreover, the dimming of the HB-LEDs is done by means of changing the switching frequency of the converter by varying the turn-off, while keeping a constant turn-on time. The proposed converters are suggested to be used in a postregulator stage of an ac–dc converter or as the interface between the low-voltage bus and the HB-LED string in a dc nanogrid. In order to validate the analysis, the proposed topologies have been experimentally tested on a constructed prototype with an output power of 7.5 W, that is able to achieve an electrical efficiency of 94.5% and a luminous efficacy of 110 lm/W at full load. Moreover, the analysis has been extended to a comparison in terms of reliability with the conventional topologies on a 700-h test. [ABSTRACT FROM AUTHOR]

This paper presents a new predictive power-factor-correction (PFC) controller for series-interleaved three-level boost (TLB) converters. Compared to the state-of-the-art TLB PFC controllers, where a two-cycle prediction and a detection of an operating region are necessary, the proposed controller achieves a low total harmonic distortion of the input current by using a single equation to predict the input current in all operating regions of the converter, in just one operating cycle. The average current control is achieved by sampling at the peak of the triangular carrier. The proposed PFC controller significantly reduces the distortion of the input ac current near the zero-crossing points, resulting in a low total harmonic distortion of the input current. The operation of the proposed controller was evaluated, and its stability and robustness to parameter changes was confirmed analytically. The controller operating principles were verified in simulations and validated by experiments on a medium-voltage 50-kW converter prototype. [ABSTRACT FROM AUTHOR]

Owing to low cost, small size, and low weight, transformerless inverters became prominent in single-phase grid connected photovoltaic (PV) systems. Key issues pertaining to these inverters include suppression of common mode (CM) leakage current and improvement of conversion efficiency. Achieving higher efficiency in single-phase grid-connected photovoltaic systems depends on the number of stages involved in feeding power to the grid, predominantly, if the PV array voltage is less than the peak value of the grid voltage. In this paper, an integrated dc–dc converter based grid-connected transformerless PV inverter is proposed which is aimed at maintaining high efficiency, even if the PV array voltage falls below the peak value of grid voltage (efficient operation at an extended input voltage range). A modulation strategy is discussed in order to minimize the flow of CM leakage current. Further, the efficiencies of certain transformerless inverter topologies are analyzed and compared with that of the proposed topology. Detailed simulation studies are carried out in MATLAB/Simulink environment to verify the analysis. Experimental results for a scaled down laboratory prototype are included as a proof-of-concept to validate the claims. [ABSTRACT FROM AUTHOR]

Five-level hybrid-clamped (5L-HC) converter is a newly proposed topology which is suitable for high-performance medium-voltage high-power conversions without switches directly connected in series. The critical issue of this converter is that two flying capacitor voltages of each phase and two neutral-point potentials of the dc-link need to be balanced. This paper presents a decoupled voltage balancing method for this 5L-HC converter based on modified phase-shifted pulse width modulation (PS-PWM) and optimal zero-sequence voltage injection. The voltages across the central dc-link capacitor and two flying capacitors are balanced first by adjusting the width of four PWM signals. Second, the relationship between the neutral-point currents and the output phase voltage is studied and the upper and lower dc-link capacitor voltages are balanced by zero-sequence voltage injection. Both steady-state and dynamic-state simulation and experimental results are presented to confirm the validity of this method. [ABSTRACT FROM AUTHOR]

Direct current voltages unbalance of respective power cells is an inherent and critical problem in a cascaded multilevel converter, which is widely used in the solid-state transformer. In this paper, an n -dimension space vector pulse width modulation is presented to address this issue in a three-level H-bridge-based cascaded multilevel converter. This algorithm utilizes the redundant switching pairs of the inner cell and the redundant vectors of mutual cells to balance capacitors’ and cells’ voltages. Meanwhile, a smooth vector transition strategy is included in this algorithm to avoid the level-skip phenomenon. The calculation processes are analyzed in one-cell, two-cells, and three-cells modes, respectively. It is simple, calculable, and flexible to extend in n-cells mode. Finally, the voltage balancing capacity of the inner cell and mutual cells with smooth level transition are verified by simulations and experiments not only at startup but also with unbalanced loads. [ABSTRACT FROM PUBLISHER]

This paper presents an optimized topology of 3-$\phi$ , multilevel inverter (MLI) configuration for five or higher level operation with reduced switch count. The proposed solution consists of two basic units (BUs) of multilevel converter and a T-structured 3-level inverter (3-LI). Both BUs are equally shared by the three phases maintaining symmetry among the phases. Moreover, for a higher level operation of the proposed MLI, the modifications required are at two BUs only and not at three legs of the inverter. It further helps in reducing the requirement of extra components for higher level operation compared to the conventional solutions. Furthermore, the topology also has the benefits of reduced switching and conduction loss. An algorithm is also devised to generate the switching pulses for the BUs and the 3-LI, using a carrier-based space vector modulation technique. Presented topology is compared with other existing topologies to prove its advantage. All the details regarding the operating modes and pulse width modulation techniques employed for the proposed system are given and supported by the simulation and experimental results. [ABSTRACT FROM AUTHOR]

This paper proposes an interleaved-stage ac–dc bidirectional power converter topology feasible to medium-voltage railway systems. It is based on an arrangement that uses input-series-output-parallel configuration with identical power modules. Each module consists of two interleaved legs employing coupled windings so that it is possible to connect a medium-frequency transformer. The topology conception is presented, followed by the adopted control and modulation technique. An estimation of semiconductor losses is also discussed, while a comparative analysis by means of simulation results validates the improved performance of the introduced approach if compared with similar solutions. The topology results are obtainedt through a medium-voltage high-power railway application, demonstrating the converter functionalities and performance under both steady-state and dynamic conditions. It is then possible to state that good performance in terms of efficiency is achieved even though the power stage presents higher component count if compared with the topology that presents the best result, thus making it suitable to high-power ac–dc machine drive applications. [ABSTRACT FROM AUTHOR]

The voltage balancing of the floating capacitors in modular multilevel converter (MMC) is of great importance for the safe operation. However, the existing voltage-balancing algorithms usually suffer from high switching frequency, leading to unnecessary switching losses and device costs. This paper proposes an adaptive capacitor voltage-balancing method for high-power MMCs with reduced switching frequency. The operational characteristics and switching process of the submodules (SMs) are analyzed in detail. And then the relationship between the switching frequency and the capacitor voltage deviation is derived. Based on the analysis, an adaptive method is proposed to make a trade-off between the switching losses and the balancing effect through a closed-loop control of the alternating number of the SMs. The calculation formulas of the switching frequency are also presented. PSCAD/EMTDC simulation and RT-LAB-based real-time control hardware in the loop (HIL) test results demonstrate the effectiveness of the proposed method under different operating conditions. [ABSTRACT FROM PUBLISHER]

This paper proposes a new \boldsymbol n -port hybrid dc circuit breaker for offshore multiterminal HVDC grid application. The \boldsymbol n-port dc circuit breaker can substitute \boldsymbol n-1 hybrid dc circuit breakers at a dc bus with \boldsymbol n-1 adjacent dc transmission lines. The system-level behavior of the proposed multiport hybrid dc circuit breaker is similar to other hybrid dc circuit breakers. The operation principles of the proposed multiport dc circuit breaker are introduced, analyzed, and compared to the existing solution in this work. The components ratings are compared to the existing solution, and the functionality of the proposed device is verified by simulation. [ABSTRACT FROM PUBLISHER]

This paper is devoted to the step-up dc/dc converter family with a cascaded quasi-Z-source network (qZS-network). The cascaded (two-stage) qZS-network could be derived by the adding of one diode, one inductor, and two capacitors to the traditional quasi-Z-source inverter (qZSI). The proposed cascaded qZSI inherits all the advantages of the traditional solution (voltage boost and buck functions in a single stage, continuous input current, and improved reliability). Moreover, as compared to the conventional qZSI, the proposed solution reduces the shoot-through duty cycle by over 30% at the same voltage boost factor. Theoretical analysis of the two-stage qZSI in the shoot-through and non-shoot-through operating modes is described. The proposed and traditional qZS-networks are compared. A prototype of a step-up dc/dc converter with the cascaded qZS-network was built to verify the theoretical assumptions. The experimental results are presented and analyzed. Finally, a further optimization method of the cascaded qZS-network is proposed, and some practical design issues are discussed. [ABSTRACT FROM PUBLISHER]

Multilevel converters can reach medium-voltage operation increasing the efficiency of high-power applications. Among the existing multilevel converter topologies, the modular multilevel converter (MMC) provides the advantages of high modularity, availability, and high power quality. Moreover, the main advantage compared to cascaded multilevel converters is the lack of an input transformer which results in a reduction of cooling requirements, size, and cost. One of the drawbacks of this topology when used as an ac–ac converter is the input and output frequency components in the control loop, resulting in a more complex controller design. In this paper, a single-phase ac–ac MMC predictive control approach is proposed. The controller minimizes the input, output, and circulating current errors and balances the dc voltages. Experimental results show the performance of the proposed predictive control scheme. [ABSTRACT FROM PUBLISHER]

This paper presents a matrix-based nonisolated three-phase ac–dc converter with a current-doubler rectifier (CDR) circuit. Buck-type rectifiers are normally used for step-down ac-to-dc conversion. However, for three-phase buck rectifiers, the lower bound of rectified dc voltage is limited as the converter is severely underutilized by operating at lower modulation index. Moreover, at lower modulation index, the rms values of current increases contributing to higher conduction losses. However, by using a matrix (3 × 1) topology followed by a CDR, the desired dc output voltage can be reduced by half. The matrix topology directly converts three-phase line frequency ac voltages into intermediate high-frequency ac voltage which is subsequently, rectified using a CDR to obtain the required output dc voltage. A modified space vector modulation based modulation scheme especially suited for the proposed converter is presented for superior input power quality with reduced power loss. Comprehensive analysis and design of the proposed converter is carried out followed by simulation and laboratory-based experimental tests. Subsequently, the loss analysis of the proposed converter is carried out and a comparative evaluation of the proposed converter with the traditional six-switch buck rectifier is provided to demonstrate the suitability of the proposed converter for large step-down voltage gain. Digital implementation of the proposed modulation scheme is carried out at 40-kHz switching frequency. A hardware prototype of 500 W is developed to validate the theoretical and simulation results. [ABSTRACT FROM AUTHOR]

In this paper, a single-phase converter consisting of two legs with four switches, called the $\theta$-converter, is proposed. It has a common ac and dc ground, which reduces common mode currents and removes the need for an isolation transformer, and two capacitors: one across the whole dc bus and the other across the output. The dc bus capacitor provides a direct path for the double-frequency ripple current inherently existing in single-phase converters to return continuously so the output capacitor can be sized very small, only to filter out switching ripples. Moreover, the dc bus capacitor is intentionally designed to store the system ripple energy with large voltage ripples, which reduces its capacitance. Hence, the total capacitance needed and the output voltage ripples are reduced at the same time. This makes it cost-effective to use highly reliable film capacitors instead of bulky and vulnerable electrolytic capacitors. Because of the removed isolation transformers and bulky electrolytic capacitors, the power density and system reliability are improved. In order to properly operate the converter, two independent controllers are designed for the two legs, respectively, to achieve the desired functions and other normal objectives, such as the unity power factor. Experimental results are presented to demonstrate the high performance of the proposed converter. [ABSTRACT FROM AUTHOR]

Differential-mode buck-boost inverters have merits such as reduced switch number, ability to provide voltages higher or lower than the input voltage magnitude, improved efficiency, reduced cost and size, and increased power density, especially in low-power applications. There are five buck-boost inverters that can provide flexible output voltage without the need of a large electrolytic input side capacitor, which degrades the reliability of inverters. The continuous input current of these inverters is appropriate for maximum power point tracking operation in photovoltaic and fuel cells applications. Three of the five inverters can be isolated with high-frequency-link transformers where the common-mode leakage current can be mitigated. However, the performance and control of such converters have not been discussed in detail. In this paper, the five possible single-stage three-phase differential-mode buck-boost inverters with continuous input current are investigated and compared in terms of total losses, maximum ripple current, total harmonic distortion, and device and passive element ratings. In addition, the possible methods are presented for eliminating the input third-order harmonic current, resulting from the stored energy in the passive elements, as well as the output second-order harmonic currents. The ability for isolating the input and output sides of the inverters with a small–high frequency transformers is discussed. A changeable-terminal 2.5-kW bidirectional inverter is used to validate the design flexibility of the inverter topologies, when digital signal processor-controlled. [ABSTRACT FROM AUTHOR]

This paper presents a new formulation of a finite control set model-predictive control (FCS-MPC) algorithm, allowing to determine the best commutation instant in order to optimize the converter current tracking. This algorithm, mixing properties of FCS-MPC method and time resolution related to pulse width modulation schemes, is illustrated in a parallel flying capacitor converter. The parallel association of classical commutation cells is a good solution for increasing the output current driven by the converters, while the standard semiconductor technology is used. The control strategy used with this type of connection must be able to ensure not only a good distribution of the output current among the different legs, but also the balancing of the floating capacitor voltages. [ABSTRACT FROM PUBLISHER]

One important test for the formation and grading of a lithium-ion battery is to confirm the performance of the battery while discharging it down to zero volts. In this paper, a novel soft-switching charge/discharge converter with the zero voltage discharge function is proposed. The proposed converter is able to discharge a battery until its voltage reaches zero volts. The phase-shift full-bridge method is used to charge the battery and the current-fed push–pull method with bidirectional switches is used for the discharge. Zero voltage switching (ZVS) of the switches is achieved at turn-on during the charge operation. ZVS turn-on and zero current switching turn-off of the switches are achieved during the discharge operation. The charge/discharge controllers are designed taking into account the internal impedance of the battery. The performance of the system is verified by the experiments using lithium-ion batteries. [ABSTRACT FROM AUTHOR]

Flying-capacitor converters (FCCs), like most multilevel converter topologies, require a balancing mechanism of the capacitor voltages. FCCs feature natural voltage balancing when a special modulation technique is used. The classic methods, such as phase-shifted pulse width modulation (PS-PWM), result in very slow balancing for some duty-ratio ranges. Previous work has shown that for a single-leg five-level FCC, one time constant is infinite for a zero desired output voltage. In this paper, a modified PS-PWM scheme for a single-leg five-level FCC is presented, which results in faster balancing over the total duty-ratio range. The modified PS-PWM scheme is studied, resulting in an averaged voltage-balancing model. This model is verified using simulations and experiments. The modified PS-PWM scheme solves the slow-balancing problems of the normal PS-PWM method for odd-level FCCs, while maintaining the passive control property, and it provides a self-precharge capability. [ABSTRACT FROM AUTHOR]

A novel interleaved boost converter with zero-voltage switching (ZVS) and zero-current switching (ZCS) characteristics is proposed in this paper. By using the interleaved approach, this topology not only decreases the current stress of the main circuit device but also reduces the ripple of the input current and output voltage. Moreover, by establishing the common soft-switching module, the soft-switching interleaved converter can greatly reduce the size and cost. The main switches can achieve the characteristics of ZVS and ZCS simultaneously to reduce the switching loss and improve the efficiency with a wide range of load. This topology has two operational conditions depending on the situation of the duty cycle. A driving circuit is designed for the proposed topology to determine the two conditions automatically. The operational principle, theoretical analysis, and design method of the proposed converter are presented. Finally, simulations and experimental results are used to verify the feasibility and exactness of the proposed converter. [ABSTRACT FROM AUTHOR]

A new single-switch high step-up dc–dc converter is proposed, in this paper, for fuel cell vehicles. The proposed topology utilizes an L2C3D2 network to obtain high voltage gain and reduce the voltage stress on the power switch. Additionally, the proposed converter has a universal input voltage in order to suit the soft output characteristics of the fuel cell. Comprehensive analyses of the steady-state operation in continuous conduction mode and discontinuous conduction mode, and design considerations of the proposed converter are given. Finally, a 400 V/1.6 kW scaled-down prototype is developed to validate the effectiveness and feasibility of the proposed converter. [ABSTRACT FROM AUTHOR]

In this paper, an isolated high step-up ratio dc-dc converter aimed to be used in interface systems between low-voltage renewable energy sources, such as photovoltaic panels and fuel cells and the utility grid, is presented. The converter is based on the active clamp flyback topology with a voltage multiplier at the transformer secondary side. Such configuration, while naturally clamping the rectifier diode voltages thus avoiding the use of dissipative snubber circuits, allows the reduction of the circulating current during the active clamp operation due to the resonance involving the transformer leakage inductances and the diode parasitic capacitances. Experimental results taken from a 300-W-rated prototype are reported, showing the absence of parasitic oscillations after diodes and switch transitions and high efficiency, in agreement with the theoretical expectations. [ABSTRACT FROM AUTHOR]

Flyback topology has been widely used in low-power applications due to its simplicity, wide input voltage range, low cost, and low standby power. However, conventional over current protection (OCP) control scheme in flyback converters cannot meet the high precision constant current (CC) requirement in battery charger and LED applications. The proposed adaptive high-precision OCP control scheme can be widely used in ac/dc flyback converters to improve their CC accuracy. It consists of an OCP-Comp circuit and a Vpump circuit to detect the primary current in current cycle and adjust it in next cycle adaptively. The proposed OCP control scheme has been implemented in a controller IC adding no extra pin and achieves a significant improvement in output current accuracy compared to conventional OCP schemes. Test results show that the output current variation of the proposed prototype is less than ±0.6% when the input line voltage changes from 85 to 265 Vac. [ABSTRACT FROM AUTHOR]

A topology approach is proposed to derive forward converters suitable for wide voltage range applications. A unified forward switching cell concept from the core principle of forward conversion is proposed with three cells given, named as forward positive cell, forward negative cell, and forward composite cell. All cells consist of active switch, diode, transformer, and comprehensive demagnetizing voltage source. By utilizing different subcircuit to balance the voltage of the comprehensive demagnetizing voltage source, various types of forward switching cells, including RCD type, active-clamping type, LCDD type, flyback-integrated type, and independent-circuit type, are deduced from the traditional forward converters with topology derivation rules given. Families of forward converters, including existing ones and novel ones, are constructed as examples. The forward converters built in such a way illustrates features such as recycle of leakage and magnetizing energy, low voltage stress on devices, etc. The proposed method is extended to flyback converter family to further justify the feasibility. An input-series interleaved RCD forward converter and an active-clamping forward converter are analyzed in detail and verified with experimental results. [ABSTRACT FROM AUTHOR]

This paper presents a novel topology for the generation of high voltage pulses that uses both slow and fast solid-state power switches. This topology includes diode-capacitor units in parallel with commutation circuits connected to a positive buck-boost converter. This enables the generation of a range of high output voltages with a given number of capacitors. The advantages of this topology are the use of slow switches and a reduced number of diodes in comparison with conventional Marx generator. Simulations performed for single and repetitive pulse generation and experimental tests of a prototype hardware verify the proposed topology. [ABSTRACT FROM AUTHOR]

This study develops a generalized additional voltage pumping solution (GAVPS) for a series of high-step-up converters, whose topologies can be modeled by a generalized architecture. The newly designed converters can achieve additional voltage pumping by only applying an additional diode and capacitor to the conventional high step-up converter structure. Integrating the proposed solution into the existing high-step-up converters, five typical case studies with corresponding analyses and comparisons are conducted and presented, which verifies the proposed voltage pumping strategy. Additionally, five prototypes are built in this study, and the experimental results further validate the effectiveness of the proposed GAVPS solution. The proposed strategy in theory will work synergistically with all types of dc–dc converters, which demonstrates its unparalleled significance in expanding its applicability in contemporary power converter industries. [ABSTRACT FROM AUTHOR]

This work deals with the design and implementation of a new charger for a battery-operated electric vehicle (EV) with power factor improvement at the front end. In the proposed configuration, the conventional diode converter at the source end of existing EV battery charger is eliminated with the modified Landsman power factor correction (PFC) converter. The PFC converter is cascaded to a flyback isolated converter, which yields the EV battery control to charge it, first in constant current mode then switching to constant voltage mode. The proposed PFC converter is controlled using single sensed entity to achieve the robust regulation of dc-link voltage as well as to ensure the unity power factor operation. The proposed topology offers improved power quality, low device stress, and low input and output current ripple with low input current harmonics when compared to the conventional one. Moreover, to demonstrate the conformity of the proposed charger to an IEC 61000-3-2 standard, a prototype is built and tested to charge a 48 V EV battery of 100 Ah capacity, under transients in input voltage. The performance of the charger is found satisfactory for all the cases. [ABSTRACT FROM AUTHOR]

The five-level modular neutral-point-clamped (5L-MNPC) converter is realized by connecting two three-level neutral-point-clamped modules in series. For five-level operation, each dc-bus capacitor voltage of the modular neutral-point-clamped topology should be regulated at one-fourth of the net dc-bus voltage. In this letter, a simple voltage-balancing approach based on the carrier-based modulation scheme is proposed to achieve the above objectives. The proposed approach uses the redundancy switching states along with the current direction and the instantaneous value of the dc-bus capacitor voltage to achieve the voltage balancing in the 5L-MNPC topology. The dynamic performance of the 5L-MNPC topology with the proposed voltage-balancing approach is verified experimentally. [ABSTRACT FROM AUTHOR]

This letter presents a single-phase cascaded transformer based multilevel inverter with a modified carrier-based level shift sinusoidal pulse width modulation (LS-SPWM) technique. The developed topology has two bridges with individual low frequency transformers. The bridges can generate quasi-square waveform and pulse width modulated waveform independently and energized the two transformers whose secondary terminals are cascaded to attain 19-level output voltage waveform across the load. The anticipated configuration has the least number of components to reduce the cost and enhance the reliability of the converter for medium power applications with inbuilt isolation. Furthermore, this letter presents the most common LS-SPWM technique with a new carrier to enhance the fundamental magnitude and shifts the dominant harmonics into three times of the traditional strategy for the same modulation indices. The performance of the proposed topology is validated with experimental results. [ABSTRACT FROM AUTHOR]

A family of two-phase interleaved LLC (iLLC) resonant converter with hybrid rectifier is proposed for wide output voltage range applications. The primary sides of the two LLC converters are in parallel, and the connection of the secondary windings in the two LLC converters can be regulated by the hybrid rectifier according to the output voltage. Variable frequency control is employed to regulate the output voltage and the secondary windings are in series when the output voltage is high. Fixed-frequency phase-shift control is adopted to regulate the configuration of the secondary windings as well as the output voltage when the output voltage is low. The output voltage range is extended by adaptively changing the configuration of the hybrid rectifier, which results in reduced switching frequency range, circulating current, and conduction losses of the LLC resonant tank. Zero voltage switching and zero current switching are achieved for all the active switches and diodes, respectively, within the entire operation range. The operation principles are analyzed and a 3.5 kW prototype with 400 V input voltage and 150–500 V output voltage is built and tested to evaluate the feasibility of the proposed method. [ABSTRACT FROM AUTHOR]

Arm voltage and submodule (SM) capacitor voltage balancing is a key factor for the safe and reliable operation of modular multilevel converters (MMCs). The arm voltage balancing is achieved through a zero-sequence voltage controller in carrier pulse-width modulation (CPWM). In this study, a dual space-vector pulse-width modulation (SVPWM) technique is proposed for an MMC, which eliminates the external controller for arm voltage balancing. In this approach, the three-phase top and bottom arms are independently controlled using SVPWM. In addition, the capacitor voltage balancing can be achieved using redundant switching vectors. However, this will increase the computational load on the space-vector modulator. Therefore, an external capacitor voltage-balancing approach is proposed to minimize the computational complexity. The proposed approach uses the direction of load current instead of the arm current in SM selection process. As such, the required number of current sensors is reduced to 50% in a three-phase system. The proposed modulation and voltage-balancing approach are simulated and experimentally verified on the MMC system with three-level flying capacitor (3L-FC) SMs. Simulation and experimental results show the successful balancing of the arm voltage and SM capacitors voltage. [ABSTRACT FROM PUBLISHER]

A nine-switch power converter having two sets of output terminals was recently proposed in place of the traditional back-to-back power converter that uses 12 switches in total. The nine-switch converter has already been proven to have certain advantages, in addition to its component saving topological feature. Despite these advantages, the nine-switch converter has so far found limited applications due to its many perceived performance tradeoffs like requiring an oversized dc-link capacitor, limited amplitude sharing, and constrained phase shift between its two sets of output terminals. Instead of accepting these tradeoffs as limitations, a nine-switch power conditioner is proposed here that virtually “converts” most of these topological short comings into interesting performance advantages. Aiming further to reduce its switching losses, an appropriate discontinuous modulation scheme is proposed and studied here in detail to doubly ensure that maximal reduction of commutations is achieved. With an appropriately designed control scheme then incorporated, the nine-switch converter is shown to favorably raise the overall power quality in experiment, hence justifying its role as a power conditioner at a reduced semiconductor cost. [ABSTRACT FROM AUTHOR]