Showing papers by "Johann W. Kolar published in 2012"

Efficiency-Optimized High-Current Dual Active Bridge Converter for Automotive Applications

Abstract: An efficiency-optimized modulation scheme and design method are developed for an existing hardware prototype of a bidirectional dual active bridge (DAB) dc/dc converter. The DAB being considered is used for an automotive application and is made up of a high-voltage port with port voltage V1, 240 V ≤ V1 ≤ 450 V, and a low-voltage port with port voltage V2, 11 V ≤ V2 ≤ 16 V; the rated output power is 2 kW. A much increased converter efficiency is achieved with the methods detailed in this paper: The average efficiency, calculated for different voltages V1 and V2, different power levels, and both directions of power transfer, rises from 89.6% (conventional phase shift modulation) to 93.5% (proposed modulation scheme). Measured efficiency values, obtained from the DAB hardware prototype, are used to verify the theoretical results.

Closed Form Solution for Minimum Conduction Loss Modulation of DAB Converters

Abstract: An optimal modulation scheme that enables minimum conduction and copper losses is presented for a bidirectional dual active bridge (DAB) dc-dc converter. The considered converter system is employed for an automotive application and comprises of a high voltage (HV) port with port voltage V1, 240V ≤ V1 ≤ 450 V, and a low voltage (LV) port with port voltage V2, 11 V ≤ V2 ≤ 16 V; the rated output power is 2 kW. The closed-form expressions for the optimal control parameters are derived and implementation details are presented in order to facilitate the direct application to a given DAB converter. The paper further details the properties of the presented modulation scheme with respect to switching losses. Experimental results confirm a considerable increase of the converter efficiency achieved with the proposed optimal modulation scheme, compared to the efficiency obtained with conventional phase shift modulation. The efficiency increase is most distinct at V1=450V and V2 = 11V with an increase from 78.6% to 90.6% at 1 kW output power and from 85.9% to 90.7% at rated output power as compared to conventional phase shift modulation.

Comparative Evaluation of Three-Phase AC–AC Matrix Converter and Voltage DC-Link Back-to-Back Converter Systems

Abstract: This paper introduces the methodology and the results of a comprehensive comparison of a direct matrix converter (MC), an indirect MC, and a voltage dc-link back-to-back converter for a 15-kW permanent magnet synchronous motor drive. The comparison involves the investigation of the passive components, including the EMI input filter, the required silicon chip area for a defined maximum admissible thermal loading of the power semiconductors, the total losses and/or achievable efficiency, a prediction of the resulting volume and weight of the passive components, and, finally, a tradeoff study between the efficiency, volume, and weight of the converters. Different performance indicators that ultimately allow a systematic determination of the application area of each converter topology are provided with this comparative evaluation.

Core Losses Under the DC Bias Condition Based on Steinmetz Parameters

Abstract: The calculation of core losses in inductive components is difficult and has not yet been entirely solved. In particular, it is impossible to predict the influence of a dc premagnetization on the losses without extensive measurements. For this paper, different materials have been tested to gain information on how core losses are influenced by a premagnetization. Measurements on molypermalloy powder, silicon steel, nanocrystalline material and ferrite cores have been performed. The Steinmetz premagnetization graph (SPG) that shows the dependency of the Steinmetz parameters ( , and ) on premagnetization is introduced. This permits the calculation of core losses under dc bias conditions. Such graphs are given for different materials and different operating temperatures. In addition, a detailed description of the test system is given, as high accuracy is crucial.

Towards a 99% Efficient Three-Phase Buck-Type PFC Rectifier for 400-V DC Distribution Systems

Abstract: In telecom applications, the vision for a total power conversion efficiency from the mains to the output of point-of-load (PoL) converters of 95% demands optimization of every conversion step, i.e., the power factor correction (PFC) rectifier front-end should show an outstanding efficiency in the range of 99%. For recently discussed 400-V dc distribution bus voltages, a buck-type PFC rectifier is a logical solution. In this paper, an efficiency-optimized, 98.8% efficient, 5-kW three-phase buck-type PFC rectifier with 400-V output is presented. Methods for calculating losses of all components are described and are used to optimize the converter design for efficiency at full load. Special attention is paid to semiconductor losses, which are shown to be dominant, with the parasitic device capacitance losses being a significant component. The calculation of these parasitic capacitance losses is treated in detail, and the charge-balance approach used is verified. A prototype of the proposed rectifier is constructed which verifies the accuracy of the models used for loss calculation and optimization.

Extreme efficiency power electronics

Abstract: In this paper a generalized description and an overview of degrees of freedom and selected measures for efficiency improvement of power electronics converters is given. The background of all considerations is formed by single-phase PFC rectifier systems, but the concepts shown are fundamental and fully applicable for other converter systems. First, the influence of the main components of the losses of a converter on the efficiency characteristic over the output power is discussed. Subsequently, a detailed analysis of the possibilities, of minimizing the semiconductor losses, the losses of the passive components including the EMI filter, and the power requirements of auxiliary systems in the course of the design process are given. In this context also the technological boundaries that limit the maximum efficiency of a converter are clarified and the compromise that always has to be made between efficiency and power density is highlighted. Furthermore, a control procedure is discussed to maximize the efficiency in the partial load range and a resonant transition mode ZVS converter system is presented that allows to attain efficiencies significantly over 99% without the use of SiC semiconductors. In addition the accuracy of the input and output power measurements required for measuring highest efficiencies is clarified, whereby the advantage of a direct loss measurement by means of a calorimeter becomes immediately clear. Finally, results of measurements on a demonstrator of a CCM single-phase PFC rectifier system with 99.1% max. efficiency and η > 99% above half rated power, and on a resonant transition mode PFC rectifier system with ηmax = 99.3% and η > 99% above 15% rated power are presented.

Classification and comparative evaluation of PV panel integrated DC-DC converter concepts

Abstract: Strings of photovoltaic panels have a significantly reduced power output when mismatch between the panels, such as partial shading, occurs since integrated diodes are then partly bypassing the shaded panels. With the implementation of DC-DC converters on panel level, the maximum available power can be extracted from each panel regardless of any shading. In this paper, different concepts of PV panel integrated DC-DC converters are presented, comparative evaluation is given and the converter design process is shown for the buck-boost converter which is identified as the best suited concept. Furthermore, the results of high precision efficiency measurements of an experimental prototype are presented and compared to a commercial MIC.

Closed-loop IGBT gate drive featuring highly dynamic di/dt and dv/dt control

Abstract: In this paper, a closed-loop active IGBT gate drive providing highly dynamic di C /dt and dv CE /dt control is proposed By means of using only simple passive measurement circuits for the generation of the feedback signals and a single operational amplifier as PI-controller, high analog control bandwidth is achieved enabling the application even for switching times in the sub-microsecond range

Bearingless 300-W PMSM for Bioreactor Mixing

Abstract: This paper presents a novel exterior rotor topology of a bearingless brushless synchronous motor with rated power of 300 W. Owing to the large possible magnetic gap and the absence of mechanical bearings, this motor is especially qualified for high-purity and low-shear applications (e.g., bioreactor mixing). Both torque and magnetic bearing forces are created inside this disk-shaped motor using a sophisticated control (proportional-integral-differential vector control) with superimposed drive and bearing currents fed to the concentrated combined stator coils. Optimal design is derived based on an electromagnetic analysis using the three-dimensional finite element method (3D-FEM), and the simulation results are verified with a prototype mixer setup.

Milestones in Matrix Converter Research

Abstract: The Matrix Converter (MC) evolved from the forced commutated cycloconverters and has been extensively investigated for more than thirty years. In this publication, the milestones in research on MCs in academia and industry are reviewed and presented chronologically and thematically ordered. The major contributions in the fundamental topic areas such as the development of the topology, topological extensions, commutation, modulation, loss calculation, control, or filtering and EMC are compiled and then expanded with examples of the latest activities in the corresponding field of research. In addition, an overview of the publicly reported research on MCs in industry is provided and the development of the commercialized MCs is briefly summarized. This review concludes with a comparison of the MC with the Voltage DC-Link Back-to-Back Converter (V-BBC) and a discussion of the current status of the MC technology and its future potential. The key contributions in the development of the MC can be summarized as follows: the investigation of the basic MC concept by Venturini in 1980 [19], the development of the multi-step commutation by Burany and Oyama et al in 1989 [64,65] to solve the commutation problem of the bidirectional switches, and finally the space vector representation of the modulation by Huber and Boroyevich in [80] to enable a consistent mathematical description of the converter system from the source to the load. With these fundamental features, the MC (CMC) is operational and can be used as a direct ac-ac converter with variable voltage and frequency transformation capability. The world’s first commercial MC was presented by the Japanese drive manufacturer Yaskawa in 2005 with the product name “Varispeed AC”. This converter series is based on the CMC topology and is implemented with RB-IGBTs. One year later in 2006, Fuji Electric announced also a new MC product, the Frenic-MX, which, however, is currently not anymore part of Fuji’s product portfolio. Meanwhile, Yaskawa extended their matrix converter product line with a medium voltage MC series. Despite intensive research for the last three decades, MCs have until now only achieved low market penetration with Yaskawa Electric as the only drive manufacturer currently offering MCs as commercial products. The reason for the low usage of the MC technology in industry is mainly due to the intrinsic, physical limitations given by the MC concept, such Fig. 1. Basic three-phase Matrix Converter topologies: (a) single-stage half-bridge or Conventional (direct) Matrix Converter (CMC) and (b) two-stage Indirect Matrix Converter (IMC)

Swiss rectifier — A novel three-phase buck-type PFC topology for Electric Vehicle battery charging

Abstract: This paper introduces a novel three-phase buck-type unity power factor rectifier appropriate for high power Electric Vehicle battery charging mains interfaces. The characteristics of the converter, named the Swiss Rectifier, including the principle of operation, modulation strategy, suitable control structure, and dimensioning equations are described in detail. Additionally, the proposed rectifier is compared to a conventional 6-switch buck-type ac-dc power conversion. According to the results, the Swiss Rectifier is the topology of choice for a buck-type PFC. Finally, the feasibility of the Swiss Rectifier concept for buck-type rectifier applications is demonstrated by means of a hardware prototype.

Comparative evaluation of control methods for Inductive Power Transfer

Abstract: Inductive Power Transfer (IPT) has recently been proposed for application with Electric or Hybrid Electric Vehicles (EV/HEV), where a highly efficient system operation is demanded for the high-power transfer. Due to the high requirements on the regulation of the output power and the output voltage, and due to the large variations of the magnetic coupling, the control of these systems is a challenging task. In this paper, the frequency characteristics of a series-parallel compensated IPT system are discussed. Different control methods found in the literature are analyzed and a comparative evaluation of the current and voltage stress in the transmission coils, the resonant capacitors, and the semiconductors in the primary-side inverter is presented. It is shown that the dual control method offers a number of advantages in the controllability and potentially lower losses compared to the frequency control which is commonly used.

Switching control strategy for full ZVS soft-switching operation of a Dual Active Bridge AC/DC converter

Abstract: A switching control strategy to enable Zero-Voltage-Switching (ZVS) over the entire input-voltage interval and the full power range of a single-stage Dual Active Bridge (DAB) AC/DC converter is proposed. The converter topology consists of a DAB DC/DC converter, receiving a rectified AC line voltage via a synchronous rectifier. The DAB comprises primary and secondary side full bridges, linked by a high-frequency isolation transformer and inductor. Using conventional control strategies, the soft-switching boundary conditions are exceeded at the higher voltage conversion ratios of the AC input interval. Recently we presented a novel pulse-width-modulation strategy to fully eliminate these boundaries, using a half bridge — full bridge DAB configuration. In this papers the analysis is extended towards a full bridge — full bridge DAB setup, providing more flexibility to minimize the component RMS currents and allowing increased performance (in terms of efficiency and volume). Experimental results are given to validate the theoretical analysis and practical feasibility of the proposed strategy.

Three-phase high power factor mains interface concepts for Electric Vehicle battery charging systems

Abstract: This paper discusses novel three-phase high power factor mains interfaces appropriate for Electric Vehicle (EV) battery charging systems. Initially, a highly efficient two-stage ac-dc system, consisting of a three-phase line-commuted rectifier combined with a three-phase shunt connected Active Power Filter (APF) and a group of interleaved dc-dc buck converters operating in Triangular Current Mode (TCM), is presented. In order to replace the costly APF circuit of the front-end converter, while maintaining PFC capability at the input and allowing similar operating conditions for the back-end dc-dc converter, a rectifier topology employing an active third harmonic current injection circuit is proposed. In addition, a novel three-phase buck-type PFC rectifier is introduced for EV charging systems. The characteristics of the presented EV systems, including the principle of operation, modulation strategy, suitable control structures, and dimensioning equations, are described in detail. Finally, a comprehensive comparison of the studied converters rated to 12kW is shown.

Novel AC-Coupled Gate Driver for Ultrafast Switching of Normally Off SiC JFETs

Abstract: Over the last years, more and more SiC power semiconductor switches have become available in order to prove their superior behavior. A very promising device is the 1200 V 30 A JFET manufactured by SemiSouth. It features a very low on-resistance per die area (2.8 mΩ-cm2), switching within 20 ns, normally off characteristic, high-temperature operation and has already been commercialized in contrast to many other SiC switches. To fully exploit the potential of the SiC normally off JFET, conventional gate drivers for unipolar devices must be adapted to this device due to its special requirements. During on-state, the gate voltage must not exceed 3 V, while a current of around 300 mA (depending on the desired on-resistance) must be fed into the gate; during switching operation, the transient gate-source voltage should be around ±15 V and the low threshold voltage of less than 0.7 V requires a high noise immunity which is a severe challenge as the device has a comparably low gate-source but high gate-drain capacitance. To meet these requirements, several concepts have been published recently. They deal with the challenges mentioned, but they still show certain limitations (e.g., frequency and duty cycle limitations or need for additional cooling due to high gate driver losses). In this paper, a novel gate driver consisting of only one standard gate driver IC, resistors, capacitors, and diodes is designed and experimentally validated. It supplies enough gate current for minimum on-resistance, allows fast switching operation, features a high noise immunity, and can be used for any duty cycle and typical switching frequencies without significant self-heating.

Optimization of Phase Change Material Heat Sinks for Low Duty Cycle High Peak Load Power Supplies

Abstract: A power electronic device's lifetime depends on its maximum operating temperature and the temperature swings it is subjected to Heat sinks employing phase change materials (PCMs) can be employed to achieve a temperature reduction, but only for a limited duration This makes such heat sinks appropriate for use in applications with high peak loads but with low duty cycles The heat sink is modeled using the thermal resistors and capacitors (RCs) network approach, and an optimization procedure for designing a hybrid air-cooled heat sink containing PCM is developed, yielding a maximum possible temperature reduction for a given application It is shown that air-cooled heat sinks employing pure PCMs are best suited for applications with pulses width lengths of several minutes with a period of several tens of minutes In order to achieve a faster response of the PCM, the concept of PCM-metal foam is explored and modeled Experimental data is presented which confirms the validity of the thermal RC network approach

A Three-Phase Delta Switch Rectifier for Use in Modern Aircraft

Abstract: In the course of the More Electric Aircraft program frequently active three-phase rectifiers in the power range of several kilowatts are required. It is shown that the three-phase -switch rectifier (comprising three -connected bidirectional switches) is well suited for this application. The system is analyzed using space vector calculus and a novel PWM current controller modulation concept is presented, where all three phases are controlled simultaneously; the analysis shows that the proposed concept yields optimal switching sequences. Analytical relationships for calculating the power components average and rms current ratings are derived to facilitate the rectifier design. A laboratory prototype with an output power of 5 kW is built and measurements taken from this prototype confirm the operation of the proposed current controller. Finally, initial EMI-measurements of the system are also presented.

Balancing Circuit for a 5-kV/50-ns Pulsed-Power Switch Based on SiC-JFET Super Cascode

Abstract: In many pulsed-power applications, there is a trend to modulators based on semiconductor technology For these modulators, high-voltage and high-current semiconductor switches are required in order to achieve a high pulsed power Therefore, often, high-power IGBT modules or IGCT devices are used Since these devices are based on bipolar technology, the switching speed is limited, and the switching losses are higher In contrast to bipolar devices, unipolar ones (eg, SiC JFETs) basically offer a better switching performance Moreover, these devices enable high blocking voltages in the case where wide-band-gap materials, for example, SiC, are used At the moment, SiC JFET devices with a blocking voltage of 12 kV per JFET are available Alternatively, the operating voltage could be increased by connecting N JFETs and a low-voltage MOSFET in series, resulting in a super cascode switch with a blocking voltage N times higher than the blocking voltage of a single JFET For the super cascode, auxiliary elements are required for achieving a statically and dynamically balanced voltage distribution in the cascode In this paper, a new balancing circuit, which results in faster switching transients and higher possible operating pulse currents, is presented and validated by measurement results

Design of a minimum weight dual active bridge converter for an Airborne Wind Turbine system

Abstract: The design procedure for a bidirectional DAB dc-dc converter, which provides a high dc voltage of 8kV to the tether of a 100 kW Airborne Wind Turbine (AWT) system, is presented. The maximum allowed weight of the dc-dc converter is 25 kg and, thus, the main challenge is the realization of a light-weight DAB converter. The investigated dc-dc converter is split up into 16 single DAB modules with a rated power of 6.25 kW and a dc port voltage of 2kV. Thus, a weight of less than 25 kg/16 = 1.56 kg needs to be achieved for a single DAB module. The design method used to obtain the minimum weight DAB converter, based on the evaluation of a power-to-weight ratio versus efficiency Pareto Front γ-η-Pareto Front) is presented in this paper. For this purpose the transient voltages and currents of the employed SiC JFETs during switching are discussed in detail, since the respective measurement results allow for a prediction of the switching losses. Moreover, the calculated transformer is realized and experimental results are used to verify the weight, the losses, and the cooling system performance of the transformer. For a single DAB module a weight of m = 1.43 kg, an efficiency of η = 97%, and a power-to-weight ratio of γ = 4.4kW/kg results.

Modeling and experimental analysis of a Coupling Inductor employed in a high performance AC power source

Abstract: A large number of papers regarding Coupling Inductors (CIs) for interleaved parallel connected converter stages is available in literature However, to the knowledge of the authors, a comprehensive summary of the basics and a description of the modeling and the experimental analysis, as well as the conditions which must be satisfied to avoid saturation of the CI's core are still missing This paper intends to close this gap partially based on research conducted on a multi-phase 3-level voltage source converter which is intended to be employed as a high performance AC power source The main focus of the investigations is on CIs with high coupling factors k ≈ 1 Besides a detailed CI literature review, the focus is on the modeling of the component itself and on the analysis of the requirements for the targeted application The separation of the coupling inductor currents and bridge-leg output voltages into longitudinal and transverse components is introduced to illustratively explain the CI's behavior A complete investigation to avoid saturation in all operating conditions is conducted Finally, the derived theoretical analysis and their conclusions are successfully verified by extensive measurements on a phase leg of a 10 kW 3-level voltage source converter prototype

Imposed Sinusoidal Source and Load Currents for an Indirect Matrix Converter

Abstract: A new strategy for indirect matrix converters which allows an optimal control of source and load currents is presented in this paper. This method uses the commutation state of the converter in the subsequent sampling time according to an optimization algorithm given by a simple cost functional and the discrete system model. The control goals are regulation of output current according to an arbitrary reference and also a good tracking of the source current to its reference which is imposed to have a sinusoidal waveform with low distortion. Experimental results support the theoretical development.

Novel Hybrid Modulation Schemes Significantly Extending the Reactive Power Control Range of All Matrix Converter Topologies With Low Computational Effort

Abstract: A novel approach based on indirect modulation, which significantly extends the reactive power control range for three-phase ac-ac matrix converters (MCs; applicable to all matrix topologies) and which is implementable with lowest computational effort, is proposed. This new method denoted as hybrid modulation facilitates the formation of reactive input current also for purely reactive load. The derivation of the modulation schemes, which rely on a decoupling of the output voltage and the reactive input current formation, is described in detail. Furthermore, the operating limits, i.e., the maximum reactive input current that could be formed for the given output voltage amplitude and load current amplitude, are determined. Finally, all theoretical considerations are verified by measurements taken on a 6.5-kW Very Sparse MC.

Comparative evaluation of soft-switching, bidirectional, isolated AC/DC converter topologies

Abstract: For realizing bidirectional and isolated AC/DC converters, soft-switching techniques/topologies seem to be a favourable choice as they enable a further loss and volume reduction of the system. Contrary to the traditional dual-stage approach, using a power factor corrector (PFC) stage in series with a DC/DC isolation stage, we showed recently that the same functionality can be achieved under full soft-switching operation using a single-stage dual active bridge (DAB) AC/DC converter. This paper investigates the performance of this single-stage approach by comparing it with a state-of-the-art conventional dual-stage concept (both soft-switching converters), where a bidirectional interleaved triangular current mode (TCM) PFC rectifier was chosen in combination with a DAB DC/DC converter. The advantages and drawbacks of each concept are discussed in detail, focusing on the impact of the utilized semiconductor technology and silicon area on the converter efficiency. Furthermore, a comprehensive comparison of power density is allowed by the analytical models that correlate the component losses with their respective volume.

Voltage source converter (VSC) with neutral-point-clamped (NPC) topology and method for operating such voltage source converter

Abstract: A Voltage Source Converter (VSC) (10) with Neutral-Point-Clamped (NPC) topology with one or more phases, comprises an intermediate DC circuit having at least a first and a second capacitance connected in series between a positive terminal and a negative terminal, providing a central tap terminal between both capacitances, and at least one sub-circuit for generating one phase of an alternating voltage, each sub-circuit comprising an AC terminal for supplying a pulsed voltage; a circuit arrangement of the form of a conventional NPC converter, with a first series connection of at least two switches between said AC terminal and said positive terminal, a second series connection of at least two switches between said AC terminal said negative terminal, and switchable connections from said central tap terminal to the centers of both two-switch series connections; and additional first and second auxiliary switches assigned to said two-switch series connections.

Soft-switching techniques for medium-voltage isolated bidirectional DC/DC converters in solid state transformers

Abstract: Soft switching techniques are very attractive and often mandatory requirements in medium-voltage and medium-frequency applications such as solid state transformers. The effectiveness of these soft switching techniques is tightly related to the dynamic behavior of the internal stored charge in the utilized semiconductor devices. For this reason, this paper analyzes the behavior of the internal charge dynamics in high-voltage semiconductors, giving a clear base to understand the previously proposed zero-current-switching techniques for IGBT-based resonant dual-active-bridges. From these previous approaches, the two main concepts that allow switching loss reduction in high-voltage semiconductors are identified: 1) shaping of the conducted current in order to achieve a high recombination time in the previously conducting semiconductors and 2) achieving ZVS in the turning-on device. The means to implement these techniques in a triangular current mode dual-active-bridge converter together with the benefits of the proposed approaches are analyzed and experimentally verified with a 1:7 kV IGBT-based NPC bridge. Additionally, the impact of the modified currents in the converter's performance are quantified in order to determine the benefits of the introduced concepts in the overall converter.

Modeling and comparison of machine and converter losses for PWM and PAM in high-speed drives

Abstract: The interaction of the machine and the converter is becoming increasingly important, especially for high-speed drives, mainly due to the effect of the converter modulation on the machine losses. The allocation of the losses to different components of the drive system needs to be known in order to choose the ideal machine and modulation match. In this paper, individual models are introduced for calculating the rotor, copper, core and inverter losses, taking the modulation type into account. These models are developed considering two typical high-speed permanent-magnet synchronous motor topologies (slotted and slotless machines) driven by PulseAmplitude-Modulation (PAM) and Pulse-Width-Modulation (PWM) converters. The models are applied to two off-the-shelf machines and a converter operating both with PAM and PWM. The test bench used to experimentally verify the models is also described and the model results are compared to the measurements. The results show that PAM produces a higher overall efficiency for the high-speed machines considered in this work. However, PWM can be used to move the losses from the rotor to the converter at the expense of decreasing the overall drive efficiency. The possible benefits of these results are discussed.

Capacitor voltage balancing in modular multilevel converters

Abstract: A universal capacitor voltage control method for converters built from series connected modules is presented. It fully exploits both the circulating currents and the common-mode voltage without affecting the phase current control. The controllability of the capacitor voltages in various such converters is investigated. It is found that the nonzero branch currents and terminal voltages are necessary for capacitor voltage balancing. (5 pages)

Integration of Leakage Inductance in Tape Wound Core Transformers for Dual Active Bridge Converters

Abstract: Dual Active Bridge (DAB) converters are nowadays used in applications such as automotive and general energy storage interfaces, where efficient and compact isolated bidirectional DC/DC converters are required. The heart of this converter is a transformer which, in some designs, may include the required inductance to shape the current and control the energy transfer. This paper shows that for these designs, tape wound cores, usually made of amorphous or nanocrystalline materials, are not the best core option due to leakage flux which is orthogonal to the lamination layers. This flux increases losses in these types of cores. Solutions are proposed to overcome this significant drawback. Analysis developed in this paper is validated by experimental results, which show that core losses due to orthogonal flux in a tape wound core transformer can be reduced more than 6 times if an adapted leakage layer is used instead of a regular one.

Comprehensive comparative evaluation of single- and multi-stage three-phase power converters for photovoltaic applications

Abstract: For utility-scale photovoltaic (PV) power plants, the trend goes towards larger installations with reduced levelized costs of electricity. Further cost reductions can be achieved by modular and redundant inverter topologies, which enable higher reliabilities and a better overall system availability. In this paper, a comparative evaluation of selected three-phase single-and multi-stage voltage source inverters with rated power of 50kW for modular utility-scale PV plants is presented. Based on detailed loss, volume and thermal models, the inverter systems are designed according to a variety of constraints and standards covering a wide range of practical issues, such as grid codes, EMI requirements and lifetime considerations. Finally, for different operational switching frequencies, the optimal topologies are identified by means of an analysis regarding achievable efficiencies, power density and required semiconductor chip areas.