Showing papers by "Johann W. Kolar published in 2019"

Applicability of Solid-State Transformers in Today’s and Future Distribution Grids

Abstract: Solid-state transformers (SSTs) are power electronic converters that provide isolation between a medium-voltage and a low-voltage (LV) system using medium-frequency transformers. The power electronic stages enable full-range control of the terminal voltages and currents and hence of the active and reactive power flows. Thus, SSTs are envisioned as key components of a smart grid. Various SST concepts have been proposed and analyzed in literature concerning technical aspects. However, several issues could potentially limit the applicability of SSTs in distribution grids. Therefore, this paper discusses four essential challenges in detail. It is found that SSTs are less efficient than low-frequency transformers (LFTs), yet their prospective prices are significantly higher. Furthermore, SSTs are not compatible with the protection schemes employed in today’s LV grids, i.e., they are not drop-in replacements for LFTs. The limited voltage control range typically required in distribution grids can be provided by competing solutions, which do not involve power electronics (e.g., LFTs with tap changers), or by hybrid transformers, where the comparably inefficient power electronic stage processes only a fraction of the total power. Finally, potential application scenarios of SSTs (ac-dc, dc-dc, weight/space limited applications) are discussed. All considerations are distilled into an applicability flowchart for SST technology.

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

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

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

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

All-silicon 99.35% efficient three-phase seven-level hybrid neutral point clamped/flying capacitor inverter

Abstract: With the increasing use of photovoltaic systems, a large demand for efficient, power-dense and lightweight grid-interface inverters is arising. Accordingly, new concepts like multi-level converters, which are able to reduce the converter losses while still keeping a low construction volume, have to be investigated. The hybrid seven-level topology analyzed in this paper comprises an active neutral point clamped stage, followed by a flying capacitor stage. Compared to a pure flying capacitor converter, the combination of these two stages allows to save more than half of the capacitor volume, while still having the same requirement for the output filter stage, and hence, the same output filter volume. Moreover, the topology employs low-voltage devices and ensures low conduction and switching losses, resulting in a higher efficiency. The principle of operation of the system is briefly reviewed, and based on a detailed component modeling, an efficiency vs. power density optimization is carried out, for which switching loss measurements of state-of-the-art 200 V semiconductors are performed. From the optimization, a high-efficiency design is selected and the practical hardware realization is discussed. The simulation and optimization results are then verified by realizing an all-silicon 99.35% efficient three-phase seven-level system, featuring a volumetric power density of 3.4 kW/dm 3 (55.9 W/in 3 ), a gravimetric power density of 3.2 kW/kg, and fulfilling CISPR Class A EMI requirements. Finally, it is shown that an all-silicon realization with next generation silicon switches can achieve 99.5% efficiency with the same hardware, and 99.6% with commercial state-of-the-art GaN switches.

99.3% Efficient Three-Phase Buck-Type All-SiC SWISS Rectifier for DC Distribution Systems

Abstract: DC power distribution systems for data centers, industrial applications, and residential areas are expected to provide higher efficiency, higher reliability, and lower cost compared to ac systems and have been an important research topic in recent years. In these applications, an efficient power factor correction (PFC) rectifier, supplying the dc distribution bus from the conventional three-phase ac mains, is typically required. This paper analyzes the three-phase, buck-type, unity power factor SWISS Rectifier for the realization of an ultrahigh-efficiency PFC rectifier stage with a 400-V rms line-to-line ac input voltage and a 400-V dc output voltage. It is shown that the mains current total harmonic distortion of the rectifier can be improved significantly by interleaving two converter output stages. Furthermore, the dc output filter is implemented using a current-compensated integrated common-mode coupled inductor, which ensures equal current sharing between the interleaved half bridges and provides common-mode electromagnetic interference (EMI) filter inductance. Based on a theoretical analysis of the coupled inductor's magnetic properties, the necessary equations and the design procedure for selecting semiconductors, magnetic cores, the number of turns, and the EMI filter are discussed. Based on these results, an ultrahigh-efficient 8-kW 4-kW $\cdot$ dm $^{-3}$ (66-W $\cdot$ in $^{-3}$ ) laboratory-scale prototype converter using 1.2-kV SiC MOSFETs is designed. Measurements taken on the prototype confirm a full power efficiency of $\text{{99.16}{\%}}$ and a peak efficiency of $\text{{99.26}{\%,}}$ as well as the compliance to CISPR 11 Class B conducted emission limits.

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

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

Three-Phase Two-Third-PWM Buck-Boost Current Source Inverter System Employing Dual-Gate Monolithic Bidirectional GaN e-FETs

Abstract: Latest dual-gate (2G) monolithic bidirectional (MB) gallium nitride (GaN) enhancement-mode field effect transistors (e-FETs) enable a performance breakthrough of current DC-link inverters, e.g., in terms of power conversion efficiency, power density, cost and complexity. In fact, a single 2G MB GaN e-FET can replace the two anti-series connected conventional power semiconductors required in this inverter topology, realizing a four-quadrant (AC) switch with bidirectional voltage blocking capability and allowing controlled bidirectional current flow. Furthermore, as shown in this paper in case of three-phase (3-Φ) buck-boost (bB) current source inverter (CSI) systems comprising a DC-link current impressing buck-type DC/DC input stage and a subsequent boost-type 3-Φ current DC-link inverter output stage, a variable DC-link current control strategy, based on a Synergetic Control concept, can be applied to significantly reduce the switching losses occurring in the 3-Φ inverter. This strategy is denominated two-third pulse-width modulation (2/3-PWM), since by properly shaping the DC-link current with the input stage, the desired 3-Φ sinusoidal load phase currents can be generated by switching, in each switching period, only two out of the three phases of the output stage. Based on comprehensive circuit simulations and analytical calculations, a detailed explanation of the developed modulation and control schemes in different operating conditions is provided, and the reduction of losses enabled by 2/3-PWM is confirmed. Next, the seamless transition of the 3-Φ bB CSI system from 2/3-PWM to conventional 3/3-PWM is demonstrated. Finally, a 3.3 kW 3-Φ bB CSI system, applying 2/3-PWM and employing research samples of 2G MB GaN e-FETs in the 3-Φ inverter, is estimated to achieve an efficiency of 98.4% and a power density of 18 kW/dm(295 W/in) at a switching frequency of 140 kHz.

Performance Evaluation of Series-Compensated IPT Systems for Transcutaneous Energy Transfer

Abstract: Today's implantable mechanical circulatory support devices, such as left ventricular assist devices, still rely on a percutaneous driveline, which is a frequent cause of severe infections and which reduces the quality of life for the patients. Inductive power transfer (IPT) is therefore a promising technology to replace the driveline and, hence, reducing the likelihood of an infection. This paper focuses on the series–series compensated IPT system and provides an in-depth comparison of two operating modes, i.e., the operation at resonance and the operation above resonance, and highlights the advantages and disadvantages with respect to the requirements set by the application at hand. In addition, the paper presents the design and the realization of a fully functional transcutaneous energy transfer (TET) implant hardware prototype, which includes the IPT front-end, the control circuit, the backup battery and its charging converter, as well as the communication electronics in a boxed volume of only 10.3 cl. The experimental verification shows that overall dc–dc efficiencies of up to 90% can be achieved for both operating modes when transmitting 25–30 W from the external battery to the implant backup battery, each having a nominal voltage of 14.8 V, using TET coils with 70 mm diameter and 10 mm coil separation distance.

P $^{3}$ DCT—Partial-Power Pre-Regulated DC Transformer

Abstract: In this paper, a new approach to regulate the output voltage of a resonant, constant voltage transfer ratio 380 V/48 V isolated dc–dc converter is presented. Rather than applying variable frequency control to the resonant converter, which would result in reactive power processing and a more complicated electromagnetic compatibility filter design, the converter remains in its optimal operating point all time and an additional partial-power (PP) processing auxiliary converter is used to tightly regulate the output voltage. The PP converter, supplied through a tertiary winding of the resonant converter's transformer, regulates the output by adding or subtracting voltage from the dc input and has only a marginal impact on the overall efficiency of the dc–dc converter. The principal of operation is explained in detail including Sankey diagrams to illustrate the power processing of the converter and a feedback control system is proposed to tightly regulate the 48 V output voltage. A hardware demonstrator rated at 1.5 kW is implemented to cope with input voltage variations between 340 and 420 V and experimental results are provided showing that the output voltage can be kept within $\pm {1}{\%}$ of the nominal 48 V even under harsh input voltage and load transients. The realized dc–dc converter with PP pre-regulation features an overall efficiency of 97.7 $\%$ at rated power and a power density of 8.6 kWdm $^{-3}$ (141w/ $^{3}$ in).

Novel Three-Phase Two-Third-Modulated Buck-Boost Current Source Inverter System Employing Dual-Gate Monolithic Bidirectional GaN e-FETs

Abstract: The unprecedented characteristics of dual-gate (2G) monolithic bidirectional (MB) gallium nitride (GaN) enhancement-mode field-effect transistors (e-FETs) enable a potential performance breakthrough of current DC-link inverters, e.g. in terms of power conversion efficiency, power density, cost and complexity. In fact, a single 2G MB GaN e-FET can conveniently replace and outperform two anti-series connected conventional power semiconductors to realize the four-quadrant (AC) switch required in this circuit topology. Furthermore, a variable DC-link current control strategy can be applied to a three-phase (3-Φ) buck-boost (bB) current source inverter (CSI) system comprising a DC-link current impressing buck-type DC/DC input stage and a subsequent boost-type 3-Φ current DC-link inverter output stage to significantly reduce the occurring losses. The proposed strategy is denominated Two-Third Modulation, since by properly shaping the DC-link current with the input stage by means of a Synergetic Control structure, it allows to generate the desired 3-Φ sinusoidal load currents by switching only two out of the three phases of the output stage. Circuit simulations of the 3-Φ bB CSI system support the explanation of the analyzed concept and confirm the associated reduction of losses, for which analytical expressions are as well derived. Finally, the operation of new 2G MB GaN e-FET research samples is verified in a hardware prototype, taking the first step towards the practical realization of the described power converter.

New Synergetic Control of a 20kW Isolated VIENNA Rectifier Front-End EV Battery Charger

Abstract: EV chargers with output power levels in the range of tens of kW are typically employing a front-end three-phase boost-type PFC rectifier stage for sinusoidal input current and DC-link voltage control, and a series-connected isolated DC/DC converter controlling the actual output/charging current or voltage. This paper explores a new synergetic control of both converter stages, which utilizes the DC/DC converter also for varying the DC-link voltage with six times the mains frequency, such that the currents of two mains phases are shaped sinusoidally. Accordingly, two bridge legs of the rectifier stage can remain clamped in 60°-wide intervals of the mains cycle and the pulse width modulation (PWM) can be restricted to the phase carrying the lowest current, i.e., only one of the three bridge legs is operated with PWM, designated as 1/3-PWM. Furthermore, the DC-link voltage that is switched by the operating rectifier phase is kept to the minimum and the system features high efficiency and low EMI, but still maintains boost capability, i.e., the option of conventional PWM of all three rectifier bridge legs (thus denominated as 3/3-PWM), which is advantageous in case a wide input or output voltage range needs to be covered. The new control concept is derived starting from a conventional approach with constant DC-link voltage, and is verified by simulations for a three-level Vienna Rectifier front-end and two cascaded DC/DC modules supplied from the halves of the symmetrically partitioned DC-link voltage. First, the operating behavior of the system utilizing the proposed control is described analytically. Next, the performance improvement achievable with the proposed control scheme is comparatively evaluated for a 20kW system designed for operation in a wide mains voltage range (260-530V rms line-to-line) and an extremely wide DC output / battery voltage range (150-750V dc ), according to EV charging equipment supplier requirements of the State Grid Corp. of China. Finally, simulation results are presented which validate the operating principle of the proposed modulation and control scheme.

New 40kV / 300kVA Quasi-2-Level Operated 5-Level Flying Capacitor SiC “Super-Switch” IPM

Abstract: Emerging applications, e.g., traction systems and utility scale renewable energy systems, demand for Medium Voltage (MV)power electronic switches with blocking capabilities above 20kV, which cannot be provided with today's 10kV or 15kV SiC power semiconductors. Therefore, this work investigates different concepts for the realization of a MV half-bridge, i.e., a series connection of 10kV SiC MOSFETs, a JFET Super Cascode arrangement, and Modular Multilevel Converter (MMCs)and Flying Capacitor Converter (FCCs)topologies. The FCC topology features several advantages such as reduced switching losses, reduced chip area, lower $\mathrm{d}v/\mathrm{d}t$ of the switching transitions, and robust voltage balancing. Moreover, the volume of the flying capacitors can be reduced with Quasi-2-Level (Q2L) operation of the half-bridge, where the intermediate voltage levels are only used during very short time intervals, i.e., during the switching transitions. This paper analyzes the design, the switching behavior, and the voltage balancing of the Q2L-FCC bridge-leg and confirms its suitability as a versatile MV switch. Finally, the integration of a complete bridge-leg, including gate drivers, isolated cooling interfaces, measurements, and Q2L control into a 300 kVA/40 kV SiC Super-Switch Intelligent Power Module (SiC-SS-IPM)is presented.

Design and Experimental Analysis of a New Magnetically Levitated Tubular Linear Actuator

Abstract: The usage of tubular linear actuators (TLAs) in direct-drive systems, where linear reciprocal motion is needed, is beneficial compared to systems where a rotational actuator is used together with a mechanical transmission. Systems with TLAs are more compact, more dynamic, and more reliable. Today's TLAs commonly employ mechanical or air bearings, which either result in friction and wear due to contact, or a costly and bulky system due to the external pressurized air supply. These issues can be avoided with magnetic bearings (MBs). In the literature, it has been proposed to use two separate MBs on each axial side of the TLA, but this approach leads to a longer shaft and a more complex overall system due to additional power and control electronics for the MBs. Therefore, this paper proposes an integration of MBs into the TLA, resulting in a new, self-bearing (bearingsless) TLA. The proposed system is derived from the standard TLA, by changing its stator geometry. The principle of operation is explained and key design aspects are studied using finite element method (FEM). A prototype integrated into a test bench is built, and used for experimentally verifying the design of the novel actuator.

A 300 000-r/min Magnetically Levitated Reaction Wheel Demonstrator

Abstract: Magnetic bearings can be used in reaction wheel systems to avoid several drawbacks of ball bearings, such as limited lifetime due to mechanical friction and lubricant monitoring/sealing requirements. Therefore, this letter discusses an electrical machine topology with integrated reaction wheel and magnetic bearings. The slotless/ironless structure, together with a new electromagnetic arrangement resulting in a shorter rotor than in earlier works, enables efficient operation at very high speeds, which in turn enables the miniaturization of the system. Measurements taken on a demonstrator prototype at 300 000 r/min show a significant increase of the feasible operational speed range compared to the state-of-the-art reaction wheels for small satellites, which typically run below 10 000 r/min.

Endurance of Polymeric Insulation Foil Exposed to DC-Biased Medium-Frequency Rectangular Pulse Voltage Stress

Abstract: The endurance of polymeric insulation foil is investigated under a mixed medium-voltage stress (DC + medium-frequency rectangular pulse) by means of accelerated lifetime testing. A dedicated setup is used that allows us to selectively eliminate the known risk factors for premature insulation failure under medium-frequency pulse voltage stress: partial discharges (PDs) during pulse transitions, excessive dielectric heating, and systemic overvoltages. The obtained results on polyethylenterephtalat (PET) insulation foil suggest that the adequate consideration of these factors is sufficient for eliminating the adverse effects of the pulse modulation under the investigated conditions. Indeed, if all mentioned risk factors are eliminated, the time to failure observed under a pure DC stress is shorter than with a superimposed pulse (keeping the same peak voltage). There is then no indication of an additional detrimental “per pulse” degradation process (i.e., the time to failure is not dependent on pulse frequency). In contrast, when repetitive PDs are present, the lifetime under combined DC + rectangular pulse stress strongly decreases with increasing pulse switching frequency. PD erosion of the foil is quantified by means of confocal microscopy, and the applicability of the streamer criterion for predicting PD inception is discussed.

Ultra-High Bandwidth GaN-Based Class-D Power Amplifier for Testing of Three-Phase Mains Interfaces of Renewable Energy Systems

Abstract: AC power amplifiers are e.g. used to emulate the power grid for testing three-phase power electronics mains interfaces of renewable energy systems or measuring the inner impedance of power distribution grids. Typically, the output stages of the test systems are realized as analog amplifiers and thus achieve very high dynamics and a very high signal quality, but also have significant drawbacks, especially high losses and/or poor efficiencies, which leads to a large cooling volume and weight that dominates the full system power density. In addition, analog amplifiers cannot easily handle bidirectional power flow, i.e., power fed back from active loads such as renewable energy inverter systems is dissipated internally. Furthermore, future power distribution systems, e.g., of More Electric Aircraft, will have fundamental frequencies exceeding 1 kHz, thus ultra-high bandwidth (> 100 kHz) power amplifiers with multiple kW output power are required to emulate harmonic distortions and variations of voltage and/or frequency in such power grids. Currently available analog power amplifiers reach bandwidths of up to 30 kHz, which is too low for all desired applications. Switched-mode, i.e., class-D power amplifiers achieve a high efficiency, however, bandwidths of no more than 5 kHz are reached today due to the high required switching frequency (around a factor of 50 higher than the output bandwidth). However, as shown in this paper, novel wide bandgap semiconductor technology and suitable circuit techniques such as series interleaving (multi-level converter topologies) and parallel interleaving render a switched-mode realization with an effective switching frequency of 4.8 MHz and/or 100 kHz large signal bandwidth possible, while still keeping the switching losses at a moderate level. An analysis of designs with different numbers of voltage levels and interleaved branches is performed and shows that with a triple-interleaved three-level flying capacitor converter, where each switching cell operates at 800 kHz, the targeted effective switching frequency is reached and an efficiency above 95 % is feasible for both directions of power flow. Furthermore, a virtual prototype of the selected design is presented, showing that thanks to the high effective switching frequency, an extremely power-dense overall realization is possible (50 kW/dm3). Finally, simulations of the control behavior verify excellent control dynamics of the presented concept.

22 kW EV Battery Charger Allowing Full Power Delivery in 3-Phase as well as l-Phase Operation

Abstract: A new universal front-end PFC rectifier topology of a battery charger for Electric Vehicles (EVs) is proposed, which allows fast charging at rated and/or full power level in case of I-phase (USA) as well as 3-phase (Europe) mains supply. For a conventional 3-phase PFC rectifier, only 1/3 of the rated power would be available in case of I-phase operation. The new topology is based on a two-level six-switch (2LB6) 3-phase PFC rectifier, which is extended with a diode bridge-leg and additional windings of the Common-Mode (CM) chokes of the EMI filter. The focus of the paper is on the analysis of the generated CM and Differential Mode (DM) EMI noise for both operating modes, i.e., I-phase and 3-phase operation, resulting in guidelines for EMI filter design. The suggested topology modifications are also applied to a three-level T- type (Vienna) PFC rectifier structure. The theoretical considerations are verified with simulations. Finally, a 22kW prototype of the new universal PFC rectifier is designed using $\eta-\rho$ Pareto optimization, targeting an efficiency of 98.4 %, a power density of 6.8kW/dm3, i.e., a volume of 3.24dm3, and an EMI filter that ensures compliance with the CISPR class B EMI standard. Compared to a volume-optimized design of a conventional 3-phase topology of same power rating and efficiency, but limited 1-phase power transfer capability, the additional volume required for the proposed extension is less than 0.5dm3(15%) and therefore clearly justified considering the enabled universal applicability of the converter system.

Origin and quantification of increased core loss in MnZn ferrite plates of a multi-gap inductor

Abstract: Inductors realized with high permeable MnZn ferrite require, unlike iron-powder cores with an inherent dis-tributed gap, a discrete air gap in the magnetic circuit to prevent saturation of the core material and/or tune the inductance value. This large discrete gap can be divided into several partial gaps in order to reduce the air gap stray field and consequently the proximity losses in the winding. The multi-gap core, realized by stacking several thin ferrite plates and inserting a non-magnetic spacer material between the plates, however, exhibits a substan-tial increase in core losses which cannot be explained from the intrinsic properties of the ferrite. In this paper, a comprehensive overview of the scientific literature regarding machining induced core losses in ferrite, dating back to the early 1970s, is provided which suggests that the observed excess core losses could be attributed to a deterioration of ferrite properties in the surface layer of the plates caused by mechanical stress exerted during machining. However, in a first experimental analysis no structural evidence for a deteriorated layer close to the surface is identified by means of Scanning Electron Microscopy (SEM). Therefore, in a next step, a new calorimetric measurement setup based on temperature rise monitoring is proposed in this paper in order to quantify and differentiate between core losses associated with the bulk and the surface of the ferrite plates and therefore to pinpoint the measured excess core loss to shallow layers of ferrite with deteriorated magnetic performance. Electrical measurement of the surface related core losses utilizing the widely accepted two-winding wattmeter method with reactive power compensation is outlined in the appendix but was not employed in this work due to comparably low measurement accuracy. By means of the proposed measurement technique, the bulk and surface core loss density of the MnZn ferrite material 3F4 from FerroxCube was determined for sinusoidal flux density amplitude varying from 75 mT up to 200 mT and excitation frequencies ranging from 200 kHz to 1 MHz. The measured core loss densities (W/cm 3 ) show good agreement with the Steinmetz model provided by the manufacturer validating the proposed calorimetric core loss measurement technique. The measured surface loss density (W/cm 2 ) can also be well predicted with a Steinmetz model, whereby the frequency exponent α in the surface is slightly smaller and the flux density exponent β is slightly larger compared to the Steinmetz parameter of the bulk ferrite. It is shown that the ratio between surface and bulk core losses of a composite core assembled from individual plates is only a function of plate thickness and does not depend on the actual cross section area. Critical plate thickness is then defined to be reached when the total power loss in the composite core has doubled compared to a solid (single-piece) core sample. This new quantity provides a very helpful figure for multi-gap inductor designs. Besides the deteriorated surface layers, several other mechanisms potentially contributing to increased core losses in multi-gap inductors were identified and are finally discussed in the appendix of this paper: flux crowding in the core due to tolerances and imperfections in machining and assembly; deterioration of ferrite properties due to pressure buildup in the stack of plates during the curing of the employed epoxy resin; ohmic loss in the ferrite associated with the current flowing in the conduction path provided by the low impedance of the ferrite material at high frequencies and the parasitic capacitance between winding and the ferrite core.

Weight optimisation of coreless axial-flux permanent magnet machines

Abstract: This study explores the upper limits in power-to-weight and torque-to-weight ratios of coreless axial-flux machines with permanent magnets. Moreover, it provides a comprehensive multifunctional optimisation procedure that is utilised for obtaining these limits. The procedure encompasses analytical analysis of electro-magnetic, thermal and structural (mechanical) aspects of axial-flux machines. Obtaining global minima is ensured by considering the whole machine design space, and mapping it into the performance space, where a Pareto front can be easily identified. From it, an optimal motor/generator for airborne wind turbines is identified. The design has a power-to-weight ratio of 6.4 kW/kg (19 Nm/kg at 3200 rpm) including structural (purely mechanical) parts, at an efficiency of 95%. This is a significantly higher ratio than the one in modern commercial machines or designs reported in the literature. Therefore, the resulting machine is manufactured and experimentally tested in order to verify the claimed limits and the optimisation methodology.

Energy Harvesting With Single-Sided Linear Induction Machines Featuring Secondary Conductive Coating

Abstract: With the continuously broadening application area of Internet of Things (IoT), robust energy harvesters and power conditioning units for supplying remote sensors and actuators are gaining importance. A single-sided linear induction machine (SLIM) with solid secondary operating in generator mode could meet these requirements as it can harvest electric energy from the kinetic energy of a moving conductive body (secondary). Therefore, the energy harvesting performance of SLIMs is studied in this paper, using analytical models, which are derived with special attention to the case where a conductive coating is applied on the mover—a simple and practical modification that is shown to increase the harvester performance significantly. Measurements are given for three different secondary materials and stator geometries. It is shown that 15 W of electrical power can be harvested with $\text{14}\%$ efficiency over an air gap of 0.5 mm from a copper-coated steel mover with a surface speed of 29 m/s, covering only $\text{17 cm}^{\text{2}}$ of the surface area.

New Series-Resonant Solid-State DC Transformer Providing Three Self-Stabilized Isolated Medium-Voltage Input Ports

Abstract: This work analyzes the properties of a new four-port, series-resonant, and bidirectional DC transformer topology in steady-state and transient conditions. The investigated topology evolves from the Integration of Three (3) Series-Resonant Converters (I3SRC), enables the realization of a converter with high efficiency (99.0%) and high power density (7.1 kW/dm3), due to the utilization of a three-phase High Frequency (HF) transformer and reduced capacitor currents in the secondary-side DC link, and features the use of a six-pack power module on the secondary side. The analysis is based on the results of detailed circuit simulations of an I3SRC, which takes the couplings between the phases, introduced by the three-phase transformer, into account and considers a DC transformer, i.e., DC-DC converter, with a total rated power of 15 kW, three primary-side DC voltages of 1.1 kV, and a secondary-side DC voltage of 700 V, which is a core part of a scaled demonstrator of a Solid-State Transformer (SST). The obtained findings clarify the self-stabilizing capabilities of the I3SRC for operation with Half-Cycle Discontinuous Conduction Mode (HC-DCM), even if the three isolated DC ports are subject to time-varying and substantially different power levels and different directions of energy transfer.

Design and Comparison of Permanent Magnet Self-Bearing Linear-Rotary Actuators

Abstract: Linear-rotary actuators (LiRAs) are electric machines that can perform linear and rotary movements. They are used in many different applications, for example, for pick-and-place robots, in packaging or sorting lines, or as gearbox actuators. A linear-rotary movement can be obtained with various combinations of linear and rotary machines, whereas depending on the specifications of the underlying application the most suitable actuator arrangement has to be identified. In order to simplify the selection of the appropriate actuator configuration, this paper first gives an overview of possible realization concepts of linear-rotary actuators, which are also suitable to implement magnetic bearings (MB). Afterwards, fundamental scaling laws concerning achievable axial forces and torques of linear and rotary machines with interior and exterior rotor arrangement are derived, enabling a qualitative comparison in order to figure out the most suitable actuator concept. In this context, it is important that the derivation also considers the machine-internal heat flow and the heat dissipation to the ambient, which finally leads to a maximum current density depending on the selected topology. All findings are verified by finite element method simulations. In order to show the applicability of the derived scaling laws, a design example is discussed.

Comparison of Bearingless Slice Motor Topologies for Pump Applications

Abstract: This paper compares two bearingless slice motor topologies, namely the temple and slotless designs, with respect to the achievable motor efficiency for pump applications. Stator losses, passive stiffness properties and the achievable torque are determined by means of 3D FEM simulations. Some basic pump design guidelines are introduced and the expected motor performance is evaluated for typical operating conditions encountered in medical applications. It is shown that the temple design outperforms the slotless design in all but low pressure applications. Two prototypes, one radial and one axial pump, are manufactured by means of stereolitograpy 3D printing. The hydraulic characteristics of the designs are measured using the temple and slotless motor for the radial and the axial pump, respectively. The radial pump exhibits significantly higher pressure-numbers, while the axial design yields higher flow-numbers.

Novel Single-Phase Buck+Boost PFC Rectifier with Integrated Series Power Pulsation Buffer

Abstract: This paper introduces a novel integrated series power pulsation buffer (iSPPB) concept for a single-phase AC-to-DC two-switch buck+boost PFC rectifier to compensate the fluctuating power mismatch between the AC input and the DC output. Accordingly the DC-link capacitance can be drastically reduced and an electrolytic-capacitor-Iess PFC rectifier system featuring a higher power density and an increased lifetime is obtained. The proposed iSPPB concept consists of a simple full-bridge circuit with a buffer capacitor and is placed in series with the buck-boost inductor thus no additional inductive component is required for the iSPPB realization. The basic operating principle of the buck+boost PFC rectifier with iSPPB is investigated and the characteristic waveforms are presented. As shown in the analysis due to the employment of the iSPPB also the maximum buck+boost inductor current is reduced which compared to the conventional rectifier system without iSPPB allows to further downsize the inductive component. Consequently the major drawbacks of conventional single-phase PFC rectifiers are eliminated. Furthermore the control structure is presented and the proper operation is verified based on a closed-loop circuit simulation. Finally the proposed buck+boost PFC rectifier with iSPPB and a conventional two-switch implementation employing electrolytic capacitors are quantitatively compared by means of simple performance indices.

Analysis of Low Frequency Grid Current Harmonics Caused by Load Power Pulsation in a 3-Phase PFC Rectifier

Abstract: This paper investigates implications of Low-Frequency (LF) phase current distortions that emanate from a grid-connected three-phase PFC rectifier, which powers a single-phase load inverter via a common DC-link. The presented study reveals the computation of the grid-side distortions that result for LF operation of the load inverter and determines the limits of the allowable distortion currents imposed by relevant standards (IEC 61000-3-11 and IEC 61000-3-12 with regard to harmonic currents and flicker, respectively). In the course of a design example, a PFC rectifier with a rated power of 20 kW and a DC-link voltage of 700 V is examined and it is found that, in case of a strong grid with an inner impedance of 21 m Ω, full compliance with IEC 61000 is achieved with a DC-link capacitance of 27 mF. However, in case of a weak grid with an assumed inner impedance of 212 m Ω, a very high DC-link capacitance exceeding 50 mF would be required. In this case, the presented design procedure is rather used to identify the frequency-dependent power limitation characteristic that enables the implementation of deratings in critical frequency ranges.

Modulation Scheme Optimization for a Dual Three-Phase Active Bridge (D3AB) PFC Rectifier Topology

Abstract: The recently presented Dual Three-Phase Active Bridge (D3AB) converter integrates a three-phase Power Factor Corrected (PFC) rectifier and a DC–DC converter stage with galvanic isolation, originating from the Dual Active Bridge converter, in a single converter unit, and accordingly features low complexity, high efficiency, and high power density. This paper proposes a multi-objective optimization procedure that considers the objectives of low RMS transformer currents and/or switching losses in order to identify Pareto-optimal low-loss modulation schemes for the D3AB rectifier that meet given specifications, prevent power pulsation, and facilitate interleaved operation. From the obtained results, which are verified by means of detailed circuit simulations, an improved modulation scheme is selected, which, compared to conventional modulation, reduces the primary-side switching losses by 54% and the total converter losses by 11%. Due to the general nature of the presented method, the application to other three-phase rectifier/inverter topologies is directly feasible.

Transformator für einen Dreiport-Spannungswandler, Dreiport-Spannungswandler und Verfahren zum Übertragen von elektrischer Energie

Abstract: Die vorliegende Erfindung schafft eine galvanisch getrennte Ubertragung elektrischer Energie zwischen drei Spannungssystemen. Hierzu ist ein Transformator vorgesehen, welcher insgesamt funf Wicklungen umfasst. Durch gezieltes Ansteuern der einzelnen Wicklungen kann dabei die Ubertragung zwischen den einzelnen Spannungssystemen gesteuert werden.

Three-phase buck–boost PFC rectifier with common-mode free output voltage and low semiconductor blocking voltage stress

Abstract: Three-phase buck-boost power-factor correction (PFC) rectifiers are characterised by a unity power-factor mains behaviour and/or sinusoidal input currents and are providing a wide-output voltage range. In this study, an extension of a state-of-the-art three-phase buck-boost PFC rectifier topology is proposed. The DC output of the new topology does not suffer from a high-frequency common-mode voltage with respect to the (grounded) mains star point, which alleviates electromagnetic interference concerns. Also, the blocking voltage requirements of the AC-side switches are reduced significantly (almost by a factor of two for a 400 V line-to-line mains and a 400 V DC output), which facilitates a broad selection of cost-effective power semiconductors for the system's realisation. The rectifier can be controlled with a simple feedback system and the concept is especially suitable for low-power applications. A 1 kW hardware demonstrator is employed to verify the results of theoretical considerations. The system seamlessly operates in the buck and boost regime and achieves conversion efficiencies of 95.3% and mains current total harmonic distortion figures in the range of 1-5%.

Speed detecting device and speed detecting method

Abstract: A speed detecting device (1) includes a magnetic flux producing unit (2) for producing a magnetic flux changing at a predetermined frequency so that a magnetization pattern (7) is formed on a principal surface (6a) of a relative moving body (6) as the relative moving body (6) moves or rotates. This flux producing unit may be a rotor having permanent magnets, whose rotational speed is controlled, a coil which is excited at a controlled frequency, or a permanent magnet coupled to a voice coil which is excited at a controlled frequency.A magnetic flux detecting unit (3) is arranged to detect an induced voltage, induced based on a magnetic flux produced by the magnetization pattern (7).A magnetic flux frequency control unit (4) is arranged for controlling the predetermined frequency such that the induced voltage detected by the magnetic flux detecting unit (3) becomes zero, and a speed estimating unit (5) is arranged for estimating a moving or rotating speed of the relative moving body (6) in a state where the induced voltage detected by the magnetic flux detecting unit (3) is zero, based on the predetermined frequency controlled by the magnetic flux frequency control unit (4).Alternatively, a rotor 8 having permanent magnets 8a may be driven by a magnetic flux produced by eddy currents generated in the moving body, to rotate passively at a speed with is lower than the speed of the moving body. This rotor speed will be measured. Additionally a voltage induced in a magnetic flux detecting unit 3 will be measured. With such an arrangement, it is possible to determine the speed of the moving body, based on a table or an arithmetic formula expressing a relationship between rotor speed, induced voltage and the speed of the moving body.

Scaling and design of miniature high-speed bearingless slice motors

Abstract: Recent years have shown a development of electrical drive systems toward high rotational speeds to increase the power density. Applications such as optical systems benefit from rotational speeds at which conventional ball bearings suffer from high losses, excessive wear, and decreased reliability. In such cases, magnetic bearings offer an interesting alternative. This work presents a universally applicable design procedure for miniature bearingless slice motors intended for rotational speeds of several hundred thousand revolutions per minute. Design trade-offs are illustrated and facilitate the selection of Pareto-optimal implementations. An exemplary motor prototype for rotational speeds of up to 760 000 rpm with a rotor diameter of 4 mm and a suitable inverter featuring an FPGA-based controller are demonstrated briefly.