Showing papers on "Converters published in 2011"

Review of Nonisolated High-Step-Up DC/DC Converters in Photovoltaic Grid-Connected Applications

Abstract: The photovoltaic (PV) grid-connected power system in the residential applications is becoming a fast growing segment in the PV market due to the shortage of the fossil fuel energy and the great environmental pollution. A new research trend in the residential generation system is to employ the PV parallel-connected configuration rather than the series-connected configuration to satisfy the safety requirements and to make full use of the PV generated power. How to achieve high-step-up, low-cost, and high-efficiency dc/dc conversion is the major consideration due to the low PV output voltage with the parallel-connected structure. The limitations of the conventional boost converters in these applications are analyzed. Then, most of the topologies with high-step-up, low-cost, and high-efficiency performance are covered and classified into several categories. The advantages and disadvantages of these converters are discussed. Furthermore, a general conceptual circuit for high-step-up, low-cost, and high-efficiency dc/dc conversion is proposed to derive the next-generation topologies for the PV grid-connected power system. Finally, the major challenges of high-step-up, low-cost, and high-efficiency dc/dc converters are summarized. This paper would like to make a clear picture on the general law and framework for the next-generation nonisolated high-step-up dc/dc converters.

A Hybrid AC/DC Microgrid and Its Coordination Control

Abstract: This paper proposes a hybrid ac/dc micro grid to reduce the processes of multiple dc-ac-dc or ac-dc-ac conversions in an individual ac or dc grid. The hybrid grid consists of both ac and dc networks connected together by multi-bidirectional converters. AC sources and loads are connected to the ac network whereas dc sources and loads are tied to the dc network. Energy storage systems can be connected to dc or ac links. The proposed hybrid grid can operate in a grid-tied or autonomous mode. The coordination control algorithms are proposed for smooth power transfer between ac and dc links and for stable system operation under various generation and load conditions. Uncertainty and intermittent characteristics of wind speed, solar irradiation level, ambient temperature, and load are also considered in system control and operation. A small hybrid grid has been modeled and simulated using the Simulink in the MATLAB. The simulation results show that the system can maintain stable operation under the proposed coordination control schemes when the grid is switched from one operating condition to another.

Multiresonant Frequency-Locked Loop for Grid Synchronization of Power Converters Under Distorted Grid Conditions

Abstract: This paper presents a new multiresonant frequency-adaptive synchronization method for grid-connected power converters that allows estimating not only the positive- and negative-sequence components of the power signal at the fundamental frequency but also other sequence components at other harmonic frequencies. The proposed system is called MSOGI-FLL since it is based on both a harmonic decoupling network consisting of multiple second-order generalized integrators (MSOGIs) and a frequency-locked loop (FLL), which makes the system frequency adaptive. In this paper, the MSOGI-FLL is analyzed for single- and three-phase applications, deducing some key expressions regarding its stability and tuning. Moreover, the performance of the MSOGI-FLL is evaluated by both simulations and experiments to show its capability for detecting different harmonic components in a highly polluted grid scenario.

Filter-Based Active Damping of Voltage Source Converters With $LCL$ Filter

Abstract: Pulsewidth modulation (PWM) voltage source converters are becoming a popular interface to the power grid for many applications. Hence, issues related to the reduction of PWM harmonics injection in the power grid are becoming more relevant. The use of high-order filters like LCL filters is a standard solution to provide the proper attenuation of PWM carrier and sideband voltage harmonics. However, those grid filters introduce potentially unstable dynamics that should be properly damped either passively or actively. The second solution suffers from control and system complexity (a high number of sensors and a high-order controller), even if it is more attractive due to the absence of losses in the damping resistors and due to its flexibility. An interesting and straightforward active damping solution consists in plugging in, in cascade to the main controller, a filter that should damp the unstable dynamics. No more sensors are needed, but there are open issues such as preserving the bandwidth, robustness, and limited complexity. This paper provides a systematic approach to the design of filter-based active damping methods. The tuning procedures, performance, robustness, and limitations of the different solutions are discussed with theoretical analysis, selected simulation, and experimental results.

Pwm control of dual active bridge: Comprehensive analysis and experimental verification

Abstract: The dual-active-bridge (DAB) topology is ideally suited for high-power dc-dc conversion, especially when bidirectional power transfer is required. However, it has the drawback of high circulating currents and hard switching at light loads, if wide variation in input and output is expected. To address these issues, this paper presents a comprehensive analysis and experimental results with pulsewidth-modulation (PWM) control of the DAB. The PWM control is in addition to phase-shift modulation between the two H-bridges. The analysis addresses PWM of one bridge at a time and of both bridges simultaneously. In the latter, five distinct modes arise based on the choice of PWM and load condition. The possibilities are analyzed for optimizing power density and efficiency for low-load operation. Finally, a composite scheme combining single and dual PWM is proposed that extends the soft-switching range down to zero-load condition, reduces rms and peak currents, and results in significant size reduction of the transformer. Experimental results are presented with a 10-kW prototype.

Platform architecture for solar, thermal and vibration energy combining with MPPT and single inductor

Abstract: A 0.35µm CMOS energy processor with multiple inputs from solar, thermal and vibration energy sources is presented. Dual-path architecture for energy harvesting is proposed that has up to 13% higher conversion efficiency compared to the conventional two stage storage-regulation architecture. To minimize the cost and form factor, a single inductor has been time shared for all converters. A novel low power maximum power point tracking (MPPT) scheme with 95% tracking efficiency is also introduced.

Design Techniques for Fully Integrated Switched-Capacitor DC-DC Converters

Abstract: This paper describes design techniques to maximize the efficiency and power density of fully integrated switched-capacitor (SC) DC-DC converters. Circuit design methods are proposed to enable simplified gate drivers while supporting multiple topologies (and hence output voltages). These methods are verified by a proof-of-concept converter prototype implemented in 0.374 mm2 of a 32 nm SOI process. The 32-phase interleaved converter can be configured into three topologies to support output voltages of 0.5 V-1.2 V from a 2 V input supply, and achieves 79.76% efficiency at an output power density of 0.86 W/mm2 .

Quasi-Z-Source-Based Isolated DC/DC Converters for Distributed Power Generation

Abstract: This paper presents new step-up dc/dc converter topologies intended for distributed power generation systems. The topologies contain a voltage-fed quasi-Z-source inverter with continuous input current on the primary side, a single-phase isolation transformer, and a voltage doubler rectifier (VDR). To increase the power density of the converter, a three-phase auxiliary ac link (a three-phase inverter and a three-phase isolation transformer) and a three-phase VDR are proposed to be implemented. This paper describes the operation principles of the proposed topologies and analyzes the theoretical and experimental results.

Grid-Filter Design for a Multimegawatt Medium-Voltage Voltage-Source Inverter

Abstract: This paper describes the design procedure and performance of an LCL grid filter for a medium-voltage neutral-point clamped converter to be adopted for a multimegawatt (multi-MW) wind turbine. The unique filter design challenges in this application are driven by a combination of the medium-voltage converter, a limited allowable switching frequency, component physical size and weight concerns, and the stringent limits for allowable injected current harmonics. Traditional design procedures of grid filters for lower power and higher switching frequency converters are not valid for a multi-MW filter connecting a medium-voltage converter switching at low frequency to the electric grid. This paper demonstrates a frequency-domain-model-based approach to determine the optimum filter parameters that provide the necessary performance under all operating conditions given the necessary design constraints. To achieve this goal, new concepts, such as virtual-harmonic content and virtual filter losses are introduced. Moreover, a new passive-damping technique that provides the necessary damping with low losses and very little degradation of the high-frequency attenuation is proposed.

Comprehensive Study of Single-Phase AC-DC Power Factor Corrected Converters With High-Frequency Isolation

Abstract: Solid-state switch mode AC-DC converters having high-frequency transformer isolation are developed in buck, boost, and buck-boost configurations with improved power quality in terms of reduced total harmonic distortion (THD) of input current, power-factor correction (PFC) at AC mains and precisely regulated and isolated DC output voltage feeding to loads from few Watts to several kW. This paper presents a comprehensive study on state of art of power factor corrected single-phase AC-DC converters configurations, control strategies, selection of components and design considerations, performance evaluation, power quality considerations, selection criteria and potential applications, latest trends, and future developments. Simulation results as well as comparative performance are presented and discussed for most of the proposed topologies.

A Transformerless Medium-Voltage STATCOM Topology Based on Extended Modular Multilevel Converters

Abstract: A new transformerless four-leg topology is suggested for shunt compensation, the modular multilevel converters (MMC) based on the half-bridge converters, to achieve higher performance as a STATCOM in a distorted and unbalanced medium-voltage large-current (MV-LC) system. Further, an extended MMC (EMMC) is proposed in order to manage more accurate compensation for high-power applications. Both proposals can be controlled for various purposes such as reactive power and unbalance compensation, voltage regulation, and harmonic cancellation. Moreover, related control strategies are also suggested for both the MMC and the EMMC to ensure that the source-end three-phase currents are sinusoidal and balanced. Also, the dc-link capacitors of the half-bridge converters are regulated. One interesting application for the EMMC-based STATCOM could be the improvement in power quality and performance of the electrified railway traction power supply system. Both the MMC- and the EMMC-based STATCOM along with their proposed control strategies were simulated; further, to verify the suggestions, these proposals were also implemented on a 30-kVA modular laboratory prototype. Experiments and simulations confirm the predefined objectives.

Control of Electric Machine Drive Systems

Abstract: Preface. 1 Introduction. 1.1 Introduction. 1.2 Basics of Mechanics. 1.3 Torque Speed Curve of Typical Mechanical Loads. 2 Basic Structure and Modeling of Electric Machines and Power Converters. 2.1 Structure and Modeling of DC Machine. 2.2 Analysis of Steady-State Operation. 2.3 Analysis of Transient State of DC Machine. 2.4 Power Electronic Circuit to Drive DC Machine. 2.5 Rotating Magnetic Motive Force. 2.6 Steady-State Analysis of a Synchronous Machine. 2.7 Linear Electric Machine. 2.8 Capability Curve of Synchronous Machine. 2.9 Parameter Variation of Synchronous Machine. 2.10 Steady-State Analysis of Induction Machine. 2.11 Generator Operation of an Induction Machine. 2.12 Variation of Parameters of an Induction Machine. 2.13 Classification of Induction Machines According to Speed Torque Characteristics. 2.14 Quasi-Transient State Analysis. 2.15 Capability Curve of an Induction Machine. 2.16 Comparison of AC Machine and DC Machine. 2.17 Variable-Speed Control of Induction Machine Based on Steady-State Characteristics. 2.18 Modeling of Power Converters. 2.19 Parameter Conversion Using Per Unit Method. 3 Reference Frame Transformation and Transient State Analysis of Three-Phase AC Machines. 3.1 Complex Vector. 3.2 d q n Modeling of an Induction Machine Based on Complex Space Vector. 3.3 d q n Modeling of a Synchronous Machine Based on Complex Space Vector. 4 Design of Regulators for Electric Machines and Power Converters. 4.1 Active Damping. 4.2 Current Regulator. 4.3 Speed Regulator. 4.4 Position Regulator. 4.5 Detection of Phase Angle of AC Voltage. 4.6 Voltage Regulator. 5 Vector Control. 5.1 Instantaneous Torque Control. 5.2 Vector Control of Induction Machine. 5.3 Rotor Flux Linkage Estimator. 5.4 Flux Weakening Control. 6 Position/Speed Sensorless Control of AC Machines. 6.1 Sensorless Control of Induction Machine. 6.2 Sensorless Control of Surface-Mounted Permanent Magnet Synchronous Machine (SMPMSM). 6.3 Sensorless Control of Interior Permanent Magnet Synchronous Machine (IPMSM). 6.4 Sensorless Control Employing High-Frequency Signal Injection. 7 Practical Issues. 7.1 Output Voltage Distortion Due to Dead Time and Its Compensation. 7.2 Measurement of Phase Current. 7.3 Problems Due to Digital Signal Processing of Current Regulation Loop. Appendix A Measurement and Estimation of Parameters of Electric Machinery. A.1 Parameter Estimation. A.2 Parameter Estimation of Electric Machines Using Regulators of Drive System. A.3 Estimation of Mechanical Parameters. Appendix B d q Modeling Using Matrix Equations. B.1 Reference Frame and Transformation Matrix. B.2 d q Modeling of Induction Machine Using Transformation Matrix. B.3 d q Modeling of Synchronous Machine Using Transformation Matrix. Index.

Open-Loop Control of Modular Multilevel Converters Using Estimation of Stored Energy

Abstract: The internal control of a modular multilevel converter aims to equalize and stabilize the submodule capacitor voltages independent of the loading conditions. It has been shown that a submodule selection mechanism, included in the modulator, can provide voltage sharing inside the converter arm. Several procedures for controlling the total stored energy in each converter arm exist. A new approach is described in this paper. It is based on estimation of the stored energy in the arms by combining the converter electromotive force reference, the measured alternating output current, and the known direct voltage. No feedback controllers are used. Experimental verification on a three-phase 10 kVA prototype is presented along with the description of the new procedure.

Switched-Inductor Quasi-Z-Source Inverter

Abstract: This paper deals with a new family of high boost voltage inverters called switched-inductor quasi-Z-source inverters (SL-qZSIs). The proposed SL-qZSI is based on the well-known qZSI topology and adds only one inductor and three diodes. In comparison to the SL-ZSI, for the same input and output voltages, the proposed SL-qZSI provides continuous input current, a common ground with the dc source, reduced the passive component count, reduced voltage stress on capacitors, lower shoot-through current, and lower current stress on inductors and diodes. In addition, the proposed SL-qZSI can suppress inrush current at startup, which might destroy the devices. This paper presents the operating principles, analysis, and simulation results, and compares them with those of the SL-ZSI. To verify the performance of the proposed converter, a laboratory prototype was constructed with 48 Vdc input and an ac output line-to-line voltage of 120 Vrms. The simulation and experimental results verified that the converter has high step-up inversion ability.

Direct Active and Reactive Power Regulation of Grid-Connected DC/AC Converters Using Sliding Mode Control Approach

Abstract: This paper proposes a new direct active and reactive power control (DPC) for the three-phase grid connected dc/ac converters. The proposed DPC strategy employs a nonlinear sliding mode control (SMC) scheme to directly calculate the required converter's control voltage so as to eliminate the instantaneous errors of active and reactive powers without involving any rotating coordinate transformations. Meanwhile, there are no extra current control loops involved, which simplifies the system design and enhances the transient performance. Constant converter switching frequency is achieved by using space vector modulation, which eases the design of the ac harmonic filter. Simulation and experimental results are provided and compared with those of the classic voltage-oriented vector control (VC) and conventional lookup table (LUT) DPC strategies. The proposed SMC-DPC is capable of providing enhanced transient performance similar to that of the LUT-DPC, and keeps the steady-state harmonic spectra at the same level as those of the VC scheme. The robustness of the proposed DPC to line inductance variations is also inspected during active and reactive power changes.

Multivariable-PI-Based $dq$ Current Control of Voltage Source Converters With Superior Axis Decoupling Capability

Abstract: This paper presents a linear direct-quadrature current control strategy for voltage source converters (VSCs) in a rotating reference frame (RRF). The described method is based on multivariable-proportional-integral (PI) regulators and provides fast dynamics and a zero steady-state error. Contrary to the well-known conventional PI-based control strategies in RRFs, the presented method provides practically decoupled axes with a superior disturbance rejection capability. Moreover, its implementation is relatively simple and does not impose excessive structural complexity compared to its conventional PI-based competitors. The method is applicable to both single- and three-phase systems and also to anisotropic three-phase systems, e.g., synchronous motors with different direct and quadrature impedances driven by VSCs. Implementing a three-phase test system, the performance of the presented method is experimentally evaluated.

High-Performance Motor Drives

Abstract: This article reviews the present state and trends in the development of key parts of controlled induction motor drive systems: converter topologies, modulation methods, as well as control and estimation techniques. Two- and multilevel voltage-source converters, current-source converters, and direct converters are described. The main part of all the produced electric energy is used to feed electric motors, and the conversion of electrical power into mechanical power involves motors ranges from less than 1 W up to several dozen megawatts.

Analysis of Power Consumption and Linearity in Capacitive Digital-to-Analog Converters Used in Successive Approximation ADCs

Abstract: Successive-approximation analog-to-digital converters (SA-ADCs) are widely used in ultra-low-power applications. In this paper, the power consumption and the linearity of capacitive-array digital-to-analog converters (DACs) employed in SA-ADCs are analyzed. Specifically, closed-form formulas for the power consumption as well as the standard deviation of INL and DNL for three commonly-used radix-2 architectures including the effect of parasitic capacitances are presented and the structures are compared. The proposed analysis can be employed in choosing the best architecture and optimizing it in both hand calculations and computer-aided-design tools. Measurement results of previously published works as well as simulation results of a 10-bit 10 kS/s SA-ADC confirm the accuracy of the proposed equations. It will be shown that, in spite of what commonly is assumed, although the total capacitance and the power consumption of those architectures employing attenuating capacitors seem to be smaller than conventional binary-weighted structures, the linearity requirements impose much larger unit capacitance to the structure such that the entire power consumption is larger.

Switched-coupled inductor cell for DC-DC converters with very large conversion ratio

Abstract: By replacing the inductor in basic converters by a coupled-inductor, and adding one diode, new converters with a small count of elements and a high conversion ratio are obtained. The additional diode helps to circulate the leakage inductance energy to the load in a non-oscillatory manner. This diode conducts for a short period of time. The transistor turns on/off with soft-switching. The diodes turn on with zero-voltage switching and turn off with zero-current-switching, their reverse-recovery problem is alleviated. A switching-coupled inductor boost converter is studied in detail, and a family of DC-DC converters (boost, buck-boost, C-uk, Sepic, Zeta) based on the new switching cell is then presented. The dependence of the voltage gain on the magnetic-coupling coefficient and turns ratio of the coupled inductor is studied. The influence of the load value on the voltage conversion ratio is discussed based on an exact analysis, which avoids assumptions of ideal elements. Computer simulation and experimental results confirmed the theoretical expectations.

A Low-Voltage Ride-Through Technique for Grid-Connected Converters of Distributed Energy Resources

Abstract: With more and more distributed energy resources (DERs) being installed, the utility requires these generation systems and their interface converters to remain grid connected during voltage sags to ensure the operating stability of the ac power system. These low-voltage ride-through (LVRT) requirements also suggest that the DER generation system injects real power and reactive power to support grid voltages. In this paper, a positive- and negative-sequence current injection method is proposed to meet the LVRT requirement. The proposed method imposes a predefined ampere constraint in its current injection to reduce the risk of overcurrent during the LVRT operation. Its operation principle and control method are explained and analyzed. Experimental results are presented to validate the effectiveness of the proposed method. Comparisons of the proposed method and other LVRT techniques are also presented.

Review of high power isolated bi-directional DC-DC converters for PHEV/EV DC charging infrastructure

Abstract: PHEV/EV DC charging infrastructure attracts more and more attention recently. High power isolated bi-directional DC-DC converters provide galvanic isolation, V2G capability and reduce the cost and footprint of the system. Maintaining high power efficiency in wide vehicle battery pack voltage range is required. Three full bridge based high power bi-directional DC-DC converters are conceptually designed for this application and their advantages and disadvantages are addressed. Experimental test bench is built and efficiency evaluation for bi-directional operation is reported.

Grid-Interfacing Converter Systems With Enhanced Voltage Quality for Microgrid Application—Concept and Implementation

Abstract: Grid-interfacing converter systems with enhanced voltage quality are proposed for microgrid applications in this paper. By adapting the conventional series-parallel structure, a group of grid-interfacing system topologies are proposed for the purpose of interfacing local generation/microgrid to the grid, or interconnecting microgrids. The functionality of the proposed systems is also reconfigured in order to ease the control design and to improve overall system performance, differing from existing series-parallel structure-based systems. Experiments with a concrete laboratory system are given to detail the proposed concepts and to demonstrate the practical implementations. Two three-phase four-leg inverters, together with dc microsources and nonlinear loads, are employed to construct a general series-parallel grid-interfacing system. Through the introduction of multilevel control objectives, it is illustrated that the proposed system could ride through voltage disturbances and continue the power transfer between the local generation and the grid, while a high-quality voltage is maintained for the local loads. The system also shows the possibility to achieve auxiliary functions such as voltage unbalance correction and harmonic current compensation. The main design aspects of the controllers are specified, and the entire system is effectively validated on a laboratory setup.

Vector Control of Single-Phase Voltage-Source Converters Based on Fictive-Axis Emulation

Abstract: This paper presents an alternative way for the current regulation of single-phase voltage-source dc-ac converters in direct-quadrature (dq) synchronous reference frames. In a dq reference frame, ac (time varying) quantities appear as dc (time invariant) ones, allowing the controller to be designed the same as dc-dc converters, presenting infinite control gain at the steady-state operating point to achieve zero steady-state error. The common approach is to create a set of imaginary quantities orthogonal to those of the real single-phase system so as to obtain dc quantities by means of a stationary-frame to rotating-frame transformation. The orthogonal imaginary quantities in common approaches are obtained by phase shifting the real components by a quarter of the fundamental period. The introduction of such delay in the system deteriorates the dynamic response, which becomes slower and oscillatory. In the proposed approach of this paper, the orthogonal quantities are generated by an imaginary system called fictive axis, which runs concurrently with the real one. The proposed approach, which is referred to as fictive-axis emulation, effectively improves the poor dynamics of the conventional approaches while not adding excessive complexity to the controller structure.

Common-Mode Circulating Current Control of Paralleled Interleaved Three-Phase Two-Level Voltage-Source Converters With Discontinuous Space-Vector Modulation

Abstract: This paper presents a control method to limit the common-mode (CM) circulating current between paralleled three-phase two-level voltage-source converters (VSCs) with discontinuous space-vector pulsewidth modulation (DPWM) and interleaved switching cycles. This CM circulating current can be separated into two separate components based on their frequency; the high-frequency component, close to the switching frequency, can be effectively limited by means of passive components; the low-frequency component, close to the fundamental frequency, embodies the jumping CM circulating current observed in parallel VSCs. This is the main reason why it is usually recommended not to implement discontinuous and interleaving PWM together. The origin of this low-frequency circulating current is analyzed in detail, and based on this, a method to eliminate its presence is proposed by impeding the simultaneous use of different zero vectors between the converters. This control method only requires six additional switching actions per line cycle, presenting a minimum impact on the converter thermal design. The analysis and the feasibility of the control method are verified by simulation and experimental results.

Newly-Constructed Simplified Single-Phase Multistring Multilevel Inverter Topology for Distributed Energy Resources

Abstract: In the microgrid system, the distributed energy resource (DER)-based single-phase inverter is usually adopted. In order to reduce conversion losses, the key is to save costs and size by removing any kind of transformer as well as reducing the power devices. The objective of this letter is to study a novel five-level multistring inverter topology for DERs-based dc/ac conversion system. In this study, a high step-up converter is introduced as a front-end stage to improve the conversion efficiency of conventional boost converters and to stabilize the output dc voltage of various DERs such as photovoltaic and fuel cell modules for use with the simplified multilevel inverter. The simplified multilevel inverter requires only six active switches instead of the eight required in the conventional cascaded H-bridge multilevel inverter. In addition, two active switches are operated at the line frequency. The studied multistring inverter topology offers strong advantages such as improved output waveforms, smaller filter size, and lower electromagnetic interference and total harmonics distortion. Simulation and experimental results show the effectiveness of the proposed solution.

A Family of Three-Port Half-Bridge Converters for a Stand-Alone Renewable Power System

Abstract: A systematic method to generate three-port half-bridge converters (TPHBCs) interfacing a renewable source, a storage battery, and a load is proposed for a stand-alone renewable power system application. Allowing dc bias current in the transformer, the primary circuit of a half-bridge converter can function as a synchronous rectification buck converter, by which a power flow path can be configured between the renewable source and the battery, which is connected in parallel with one of the dividing capacitor. To make the voltage on any two of the three ports independently regulated, a postregulation and a synchronous regulation with various implementations are proposed. As a result, a family of TPHBCs with merits of simple topologies and control, reduced number of devices, and single-stage power conversion between any two of the three ports is presented. A TPHBC with a synchronous regulation is given as an example to verify the proposed approach and experimental results are also given to validate the TPHBC operation. The topology generation idea is further extended and some novel TPCs are derived for different applications.

Phase-Shifted Full-Bridge Series-Resonant DC-DC Converters for Wide Load Variations

Abstract: This paper presents the design of a phase-shifted full-bridge series resonant converter (PS-FB SRC). The proposed FB SRC features a novel two-mode operation. It is operated in series resonant mode at normal loads. The switching frequency is varied to regulate the output voltage. The fixed-frequency phase-shifted pulse width modulation, on the other hand, is used to adjust the effective duty cycle and regulate the output voltage at light loads. The proposed converter exhibits high conversion efficiency for wide-range load conditions. The relationships among the voltage gain, the switching frequency, and the effective duty cycle are discussed and analyzed. Finally, a 48-V/42-A prototype is implemented. Experiments are conducted to verify the theoretical analysis.

Multiple-Module High-Gain High-Voltage DC–DC Transformers for Offshore Wind Energy Systems

Abstract: Renewable energy sources, such as offshore wind farms, require high voltage gains in order to interface with power transmission networks. These conversions are normally made using bulky, complex, and costly transformers and high-voltage ac-dc converters with unnecessary bidirectional power flow capability. Multiple modules of single-switch single-inductor dc-dc converters can reach high gains without transformers in these applications due to low semiconductor conduction loss in high-power devices. This paper describes a new approach for high-gain high-voltage dc-dc converters using multiple modules of single-switch single-inductor transformerless converters. Results for low-voltage experimental prototypes show gains of up to 29 p.u. and demonstrate the potential of the approach as high-gain dc-dc converters for offshore wind farms. This paper then demonstrates the viability of multiple-module converters compared to a conventional high-voltage dc converter and a theoretical full-bridge converter due to fewer devices and valves, comparable isolation levels, and ease of interleaving for increased reliability.

Control and Design of a Modular Multilevel Cascade BTB System Using Bidirectional Isolated DC/DC Converters

Abstract: This paper discusses the control and design of the 6.6-kV back-to-back (BTB) system combining bidirectional isolated dc/dc converters and modular multilevel cascade pulsewidth modulation (PWM) converters. The system consists of multiple converter cells connected in cascade per phase at both front ends. Each converter cell consists of a bidirectional isolated medium-frequency dc/dc converter and two voltage-source H-bridge (single-phase full-bridge) PWM converters. Extremely low-voltage steps bring a significant reduction in harmonics and electromagnetic interference emissions to the BTB system. This paper designs, constructs, and tests a single-phase downscaled BTB system rated at 120 V and 3.3 kW to verify the viability and effectiveness, leading to the actual system.

Single-Switch Cell Voltage Equalizer Using Multistacked Buck-Boost Converters Operating in Discontinuous Conduction Mode for Series-Connected Energy Storage Cells

Abstract: The cell voltage imbalance of series-connected energy storage cells, such as supercapacitors (SCs) and lithium-ion cells, causes premature deterioration and a decrease in the available energies of the cells. Various equalization techniques have been developed for diminishing such imbalances. However, since the number of switches, sensors, and/or multiwinding transformers present in conventional equalizers is directly proportional to the number of series connections of the cells, the circuit complexity and cost of the equalizers are prone to increase with the number of series connections. In this paper, single-switch cell voltage equalizers using multistacked buck-boost converters, such as the single-ended primary inductor converter (SEPIC), Zeta, and Cuk converters, are proposed. These equalizers consist of passive components and a single switch, significantly reducing the complexity of the circuit when compared with that of conventional equalizers. In addition, when the proposed equalizers operate in discontinuous conduction mode, feedback control is not required to limit currents flowing through cells and circuit components. The proposed equalizers are compared with conventional topologies in terms of the number of active and passive components required. Operating analyses were conducted under both cell-voltage-balanced and -imbalanced conditions. Experimental equalization tests were performed for four series-connected SCs using the SEPIC-based single-switch equalizer. The energies of the series-connected SCs were preferentially redistributed by the equalizer to the cell(s) having the lowest voltage, resulting in the elimination of the cell voltage imbalance and subsequent uniformity of the cell voltages.