Showing papers presented at "European Conference on Power Electronics and Applications in 2020"

Validity of power cycling lifetime models for modules and extension to low temperature swings

Abstract: Various papers in power electronics contain a part of lifetime estimation depending on power cycles in application. The used model is often the CIPS08 lifetime model published at the conference CIPS 2008. In many applications, a lot of cycles with low temperature swings occur. The used model, however, is only valid for temperature swings above 40 K. For temperature swings < 30 K, there are strong deviations, since some materials are now approaching the elastic region. First experimental power cycling results are gained below 30 K temperature swing. Also an approximation for the reliability of low temperature swing is given in this paper.

New Grid-Connected Multilevel Boost Converter Topology with Inherent Capacitors Voltage Balancing Using Model Predictive Controller

Abstract: This paper presents a new grid-connected multilevel boost converter topology. The proposed multilevel boost converter includes several switched-capacitor units along with a developed H-bridge. The capability of voltage boosting, inherent voltage balancing of capacitors, reduction of power electronic elements, and voltage on switches are the merits of proposed topology compared to other topologies. All required mathematical analysis related to the suggested topology is presented. The proposed topology is verified by experimental results for grid-connected applications using model predictive controller.

Leakage Inductance Modelling of Transformers: Accurate and Fast Models to Scale the Leakage Inductance Per Unit Length

Abstract: Fast and accurate transformer leakage inductance models are crucial for optimisation-based design of galvanically isolated converters. Analytical models are rapidly executable and therefore specially suitable for such optimisations. These analytical leakage inductance models typically consist of two steps: First, acquire the leakage inductance per unit length and second, scale this value with a suitable length. In this paper, the term leakage length is introduced for the scaling length. It is shown that the leakage length depends on the magnetic energy distribution and the most influential factors are determined. Furthermore, two accurate and fast leakage length models for E-core and U-core transformers with concentric windings are proposed: The Empirically Corrected Axial Flux (ECAF) model is based on a compact modification of the known axial flux formula. The cut line (CL) model pursues a semi-analytical approach and achieves high accuracy at the cost of higher computational effort. The models are verified with more than 6000 FEM simulations and the error of both models is significantly lower than the error of the known axial flux formula.

Wide Bandwidth Current Sensor for Commutation Current Measurement in Fast Switching Power Electronics

Abstract: This paper presents a current sensor for commutation current measurement with a bandwidth from DC up to 315 MHz. The sensor is inserted in the commutation loop by a coaxial housing with a low insertion inductance of about 1.2 nH. The sensor is based on the HOKA principle using tunnel magnetoresistance sensors and a Rogowski-coil for low and high frequency current measurement respectively. The current sensor is tested in a half-bridge configuration equipped with gallium nitride transistors and compared to two state-of-the-art current sensors.

Implementation of Control Strategy for Step-down DC-DC Converter Based on Piezoelectric Resonator

Abstract: With the growth in demand for miniaturization in power electronics, the current solutions are starting to display their limits in dimensions, power densities and efficiency. To meet the previous demands, the new piezoelectric materials achieving high power densities and efficiency could be the solution to ensuring the requirements. The piezoelectric resonators (PRs) and the piezoelectric transformers (PTs) have been used previously. Unlike the PTs, the use of PRs in power electronics has not been fully explored, and their use has been limited to operating as switched capacitors. However, a new operating principle using PRs based on energy and electrical balance exhibits good performances in steady state. In this paper, our motivation is to investigate in the capability to control a dc-dc converter based on PRs using this operating principle. Indeed, this paper presents the control strategy of a new step-down DC-DC converter based on a piezoelectric resonator (PR), which is used as an energy storage element. The operating principle of the converter is also presented. Moreover, the control algorithm has been implemented in field programmable gate array (FPGA) to regulate the output voltage. The control principle is validated experimentally for input-output voltages 120 - 48 V, and achieving an efficiency up to 94% for large operating power range.

High Performance LQR Control of Modular Multilevel Converters with Simple Control Structure and Implementation

Abstract: In this paper, a novel feedback controller concept for grid-connected Modular Multilevel Converters (MMC) is proposed. The controller is based on the Linear Quadratic Regulator (LQR) method and shows outstanding control performance since it is a multi-input multi-output (MIMO) controller. The proposed controller achieves efficient energy balancing control with high bandwidth, such that the required margin for the module capacitance value for dynamic control is reduced and an excellent transient behavior can be accomplished. Alternatively, compact MMC designs can be realized with a reduced capacitance value by sacrificing some transient performance. The implementation requires a relatively low computational effort, and the simple control structure enables a straightforward tuning and time-delay compensation.

Survey and Comparison of 1D/2D analytical Models of HF Losses in Litz Wire

Abstract: Litz wire (LW) is essential for reducing HF winding losses in medium frequency transformers and inductors. In order to predict the losses and optimise the parameters of LW, several analytical loss models have been presented. However, a detailed comparison of the models and the accuracy is missing. Therefore, this paper presents a comprehensive comparison of four loss models with respect to general mathematical approach, computational effort, as well as the impact of different parameters on the accuracy of the different models. Guidelines and insights for choosing the most suitable model are also provided.

A High Performance 48-to-8 V Multi-Resonant Switched-Capacitor Converter for Data Center Applications

Abstract: Resonant switched-capacitor (SC) converters are becoming increasingly attractive for high performance applications due to their efficient utilization of switches and use of high energy density capacitors. Multiphase SC converters can offer further improvements in performance, as they can achieve the same conversion ratio as traditional two-phase SC converters with fewer capacitors and switches, making them ideal for large conversion ratio applications. This paper presents theoretical analysis and experimental results for a resonant multi-phase SC converter comprising a cascaded series-parallel topology derived from the conventiona1 4-to-1 series-parallel converter. The proposed converter can achieve a 6-to-1 conversion ratio with the same number of switches and capacitors as the 4-to-1 series-parallel converter. A 48-to-8 V prototype designed for data center applications was built and tested with 40 A maximum output current. The prototype achieved 99.0% peak efficiency (98.5% including gate drive loss) and 2230 W/in3 power density, demonstrating one of the best in-class performances.

High-Frequency SiC-Based Medium Voltage Quasi-2-Level Flying Capacitor DC/DC Converter With Zero Voltage Switching

Abstract: This paper presents a medium voltage (1.5 kV) flying capacitor DC/DC converter rated at 30 kW. The fast switching 1.2 kV/11 mΩ SiC MOSFETs converter operates in quasi-2-level (Q2L) mode at high frequency (100 kHz) to obtain a low size of the flying capacitor (330 nF). Moreover, a novel switching pattern is introduced and ZVS at turn-on is also enabled. Such control method leads to lower switching losses, higher switching frequency and minimized inductor volume compared to more conventional topologies. Presented experimental results prove correct operation of the proposed converter and control method.

Compensation of the Radial and Circumferential Mode 0 Vibration of a Permanent Magnet Electric Machine based on an Experimental Characterisation

Abstract: This paper presents an approach how to compensate radial and circumferential mode 0 vibrations based on harmonic currents. First, the influence of harmonic currents and the mechanic transfer function are identified. Using these results, harmonic currents are derived which compensate the pre-existing vibrations.

Operation of an Externally Excited Synchronous Machine with a Hybrid Multilevel Inverter

Abstract: An innovative hybrid multilevel inverter is proposed which is able to deliver a separate DC voltage while a three-phase system is provided. The voltages are independently, but delivered by shared battery cells. The new system operates an electrically excited synchronous machine's rotor and stator current circuit in a vehicle drivetrain.

Enabling foil windings of medium-frequency transformers for high currents

Abstract: In foil windings of medium-frequency transformers rated for several hundred Amperes and operating at ten or several tens of kHz, parallel connection of foils is necessary to provide sufficient conductor cross-section. In this case, careful winding design is required to keep circulating currents among the foils under control. Such currents were investigated by means of an analytical model, a 2-d finite-element model, and measurements on a reduced-scale transformer for the case of two parallel connected foils. Simulations and measurements yield a consistent picture and show the potential of high extra losses in foil windings with unmitigated circulating currents. In particular, spiral windings of many turns may incur a circulating current that exceeds the useful net current by far if the inductance of the foil connections is small. Practically, it can be expected that the inductance of the foil connections leads to a noticeable reduction of the circulating current. This reduction, however, is not sufficient to bring the AC losses down to acceptable levels, such that it is recommended to transpose the foils between series-connected winding portions. Transposition largely cancels out the axial magnetic flux in the radial gaps between the parallel connected foils. The effectiveness of transposition in balancing the foil currents at frequencies up to 40 kHz is shown both theoretically and experimentally for aluminum foils of 0.2 mm thickness. With proper transposition, the extra AC losses due to circulating currents between foils can be reduced to practically negligible levels.

Analysis of the coupling between the outer and inner control loops of a Grid-forming Voltage Source Converter

Abstract: The question of grid forming control is very different depending on the connection to a low voltage or high voltage grid. In case of higher power application, the low switching frequency may induce some stability issues. This question has been studied and some solutions have been proposed through new inner current and voltage control tuning methods. However, the possible interactions between the inner and the outer controls have not been discussed yet. Actually, in large power system, the phasor modeling approximation is used in order to ease the analysis and reduce the time computations. It assumes a good decoupling between the control loops, which allows neglecting the inner loop dynamics. This paper investigates the effectiveness of this assumption by taking some examples of tuning methods proposed in the literature and showing the ability of each method to guarantee the decoupling between controllers. In this paper, small-signal analysis tool, participation factors and parametric sensitivities are used.

A High-Efficiency Wireless Power Transfer System for Unmanned Aerial Vehicle Considering Carbon Fiber Body

Abstract: The application of wireless power transfer in the unmanned aerial vehicles (UAV) is becoming more and more extensive. In order to meet the requirements of miniaturization and lightweight, the fuselage of UAV often uses carbon fiber materials and the influence on the magnetic field distribution and energy transmission cannot be ignored. In this paper, the eddy current loss of the carbon fiber fuselage is simulated and analyzed, and the effect of the aluminum shielding method is studied. The study found that by optimizing the parameters such as the size, thickness and position of the shielded aluminum ring, the total system loss will be better improved, and the weight is light, which will not affect flight. Finally, a 500 W prototype is established to validate the method.

Maintenance Scheduling in Power Electronic Converters Considering Wear-out Failures

Abstract: Power electronic converters are one of failure sources in energy systems, and hence drivers of downtime costs in power systems. Different approaches can be employed for converter reliability enhancement including design/control for reliability methods, condition monitoring and fault diagnosis, and maintenance strategies. This paper proposes optimal preventive maintenance strategies based on wear-out failure model of converter components. The proposed approaches employ two different performance measures at converter-level and system-level. The converter-level measures take into account planned and unplanned maintenance times or costs in a single unit or small-scale system. Moreover, the system-level measure considers not only maintenance times, but also energy losses and additional maintenance costs induced by aging of the converter components. The outcome is optimal replacement time of converter and its components, which depends on the employed performance measure. Optimal replacement scheduling is of importance for risk management and decision-making during planning of modern power electronic based power systems. The applicability of the proposed approaches is illustrated by numerical analysis in a photovoltaic system.

Dual Interleaved 3.6 kW LLC Converter Operating in Half-Bridge, Full-Bridge and Phase-Shift Mode as a Single-Stage Architecture of an Automotive On-Board DC-DC Converter

Abstract: On-board DC-DC converters are required to operate over a wide input and output voltage range depending on the state-of-charge of the input and output battery. Conventionally, the power transfer between these batteries is enabled by a two-stage converter concept where a galvanically-coupled DC-DC converter regulates the input voltage of the second-stage galvanically-isolated DC-DC converter. This paper presents a single-stage interleaved 3.6 kW LLC converter for this purpose. While LLC converters are usually not suitable for such a wide voltage range, this LLC converter is operated in full-bridge mode for large gains and in half-bridge mode for low gains. For intermediate gains and loads, the LLC makes use of phase-shift mode. To operate the interleaved LLCs at an equal switching frequency enabling output current ripple cancellation, again phase-shift mode is utilized to balance the output currents during full-bridge mode while asymmetrical duty-cycle mode is proposed for current balancing during half-bridge mode. This paper analyzes the converter design for these modes of operation. A 3.6 kW-prototype employing Si-superjunction MOSFETs achieves a power density of 2.1 kW/1. The maximum efficiency reaches 96.5 % while for most operating points it is kept well above 90 %.

Reducing the Energy Storage Requirements of Modular Multilevel Converters with Optimal Capacitor Voltage Trajectory Shaping

Abstract: The required module capacitance value is a driving factor of the volume and cost of modular multilevel converters (MMC). In order to minimize the required capacitance value, an optimal circulating current and common mode (CM) voltage injection strategy which keeps the conduction losses and die CM voltage as low as possible is proposed in this paper. Unlike previous methods from literature that only focus on minimizing the amplitude of energy fluctuation in the arm capacitors, the proposed optimization procedure is based on the optimal shaping of both the arm voltages and energies. As a result, the proposed optimization scheme achieves a further reduction in the required capacitance value of up to 42% compared to existing methods. The procedure is validated with closed loop simulation results covering the full operating range of an exemplary MMC.

Assessment of Aging and Performance Degradation of Supercapacitors Integrated into a Modular Multilevel Converter

Abstract: The interest for modular multilevel converter (MMC) with energy storage systems (ESSs) has increased quickly over the last decades. Since an MMC has several hundreds of sub-modules (SMs) and ESS performances usually deteriorate quickly, the ability of the converter to provide the expected service during its lifetime must be investigated. In this work, supercapacitors are used as ESS and the reduction of their capacity is first calculated through lifetime simulation of the ESS using aging models. Then, simulations are used to analyze the available energy of the proposed converter considering dispersions for the initial parameters of ESSs and for the aging rate. Finally, the results are compared with different levels of redundancy and maintenance intervals to discuss the viability of the solution. For the depicted case study, results show that oversizing the ESSs without carrying any maintenance operations during the lifetime of the converter minimizes the total number of cells required.

Design, modelling, and test of a solid-state main breaker for hybrid DC circuit breaker

Abstract: Driven by the requirements of reducing air pollutant gas emissions and fuel consumption, the concepts of more electric ship and electric aircraft are attracting increasing attention. Medium voltage DC (MVDC) distribution architectures have been proposed as potential candidates to transmit and distribute energy from generators to motors in these applications. However, the low impedance in MVDC systems results in extremely fast propagation speed of fault currents. Therefore, it is necessary to interrupt the DC fault in a very short period. This paper investigates a solid-state circuit breaker with an ultrafast interruption speed as a main breaker for a hybrid DC circuit breaker. A simulation model of the hybrid circuit breaker is established using PLECS software to evaluate the performance of the main breaker. A 1 kV solid-state main breaker prototype based on series and parallel connected insulated gate bipolar transistors (IGBTs) is built. Series and parallel connection of IGBTs are implemented to increase the voltage and current level. The maximum voltage across the solid-state circuit breaker is limited to 1.8 kV during current interruption. The solid-state main breaker prototype is experimentally tested under dynamic current conditions. The solid-state main breaker prototype successfully interrupts current of 400 A within 300 microseconds and presents good voltage balancing as well as current sharing performance. The experimental results show good agreement with the simulation results.

Bidirectional Isolated Ripple Cancel Triple Active Bridge DC-DC Converter

Abstract: To achieve a highly reliable DC distribution microgrid network, a bidirectional isolated ripple cancel triple active bridge (TAB) converter is proposed in this paper. In a conventional full-bridge TAB converter, the DC-link capacitors suffer from high ripple current, which significantly reduces their lifetime. To solve this issue, the proposed converter can reduce the ripple current to nearly zero by adding the clamping capacitors as an internal ripple-cancellation circuit topology. In addition, the proposed converter inherits the advantages of the conventional full-bridge TAB converter such as a wide range of soft switching and bidirectional power conversion. This makes it easier to replace the conventional full-bridge TAB converters. Finally, a 1 kW prototype was built to demonstrate its feasibility. In the experiment, the ripple current reduced to nearly zero (0.16 A) under 400 V/400 V/400 V and rated power 1 kW operating conditions. The proposed converter could be used to make highly reliable and efficient DC distribution microgrid as a power router.

Separation of Magnetic Flux Density Trajectories into Subloops for the Prediction of Hysteresis Loss

Abstract: Splitting the magnetic flux density trajectory into subloops has been proposed in literature to extend core loss prediction to arbitrary waveforms. This paper evaluates the approach and demonstrates that the accommodation phenomenon as well as magnetic history hamper the estimation of hysteresis loss in the presence of minor loops.

Rapid Impedance Estimation Algorithm for Mitigation of Synchronization Instability of Paralleled Converters under Grid Faults

Abstract: Paralleled grid-connected converters operated as grid-following structures are vulnerable to transient synchronization instability during grid faults. This paper mathematically describes the instability phenomenon of paralleled converters and why it is more pronounced than for single-converter operation. Based on this model, instability can be averted by modifying each converter current reference depending on the external network impedance where asymptotic stability is proven. A rapid impedance estimation algorithm is presented, which can extract the network impedance based on the disturbance of the grid fault. This estimation is used to accurately adjust the converter current references in order to guarantee stability of all paralleled converters for any severity of the grid fault. The proposed control structure is verified in a detailed simulation study and through experimental tests, which demonstrate its potential and robustness.

Simplified Calculation of Parasitic Elements and Mutual Couplings of Wide-bandgap Power Semiconductor Modules

Abstract: This paper presents a simplified calculation of parasitic elements (LC) and mutual couplings between parasitics of wide-bandgap (WBG) power semiconductor modules, based on analytical equations and on 3D FEM. A simplified parallel plate capacitor is derived from stray fields of different plate surfaces. The simple structures e. g. two parallel round wires with different directions of current, are considered to calculate the parasitic inductance and the magnetic coupling. The analytical models are verified by ANSYS Q3D results. This method includes stray fields of capacitive and inductive parasitic structures based on a simplified geometric approach. The package of a SiC-MOSFET half-bridge power module is 3D-modeled and the parasitic elements are extracted. The analytical models are verified by numerical results. At last, the influence of parasitic elements and mutual couplings on the switching characteristics is analyzed.

Measurement of Dynamic On-State Resistance of High-Voltage GaN-HEMTs under Real Application Conditions

Abstract: GaN-HEMTs gain a lot of attention to power electronics engineers. However, they could exhibit increased on-state resistance due to charge trapping. This paper focusses on measuring this effect for high voltage devices. Measurements reveal an unexpected blocking voltage dependency of on-state resistance. Therefore, a second measurement setup serves as verification.

Combining multiple temperature-sensitive electrical parameters using artificial neural networks

Abstract: Temperature-Sensitive Electrical Parameters (TSEPs) are often discussed for on-line determination of the junction temperature of semiconductors, and as key parameters for condition monitoring. This paper focuses on the combination of several simultaneously captured TSEPs using Artificial Neural Networks (ANNs) to reduce cross-dependencies and improve accuracy.

Effect of Unipolar and Bipolar SPWM on the Lifetime of DC-link Capacitors in Single-Phase Voltage Source Inverters

Abstract: This paper explores the impact of different modulation techniques on the lifetime of DC-link capacitors for different modulation indices and varying operating power of voltage source inverter system. The conducted analysis show that the capacitor lifetime improves when using unipolar Sinusoidal pulsewidth modulation (SPWM), which introduces the highest lifetime for DC-link capacitors when a single unit voltage source inverter is used. Likewise, for two paralleled voltage source inverter unit' topology, both units using unipolar SPWM, having the same switching frequency produces the least thermal stress on the capacitor and consequently giving the highest lifetime.

Impact of grid-forming control on the internal energy of a modular multilevel converter

Abstract: This paper presents a comparative analysis regarding the dynamic behavior of a Modular Multi-level Converter (MMC) with grid-forming control either with or without controlling the MMC internal energy. It has been demonstrated that the internal energy of the MMC in low-level control interacts with the high-level control, which is performed by a grid-forming scheme. In case of controlling the internal energy of the MMC, this interaction is mitigated. Moreover, it has been shown that the dynamic behavior of a grid-forming controlled MMC with internal energy control is identical to an equivalent 2-level Voltage Source Converter (VSC).

Two-Layer Genetic Algorithm for the Charge Scheduling of Electric Vehicles

Abstract: The advent of Electric Vehicles (EV) may introduce disturbances to the operation of the Power Grid, due to the great demands of electric power that is required during the simultaneous charging of large EV fleets. Towards this end, novel approaches for the management of the EV charging have been proposed in recent literature. Nevertheless, the implementation of a framework that allows flexibility in the definition of the decision-making objectives, along with user-defined criteria is still a challenge. Towards addressing this challenge, a framework for the smart charging of EVs is presented in this paper. The smart charging is facilitated by a two-layer Genetic Algorithm that operates in Charging Stations with various types of chargers that are connected to multiple charging points in a resource-sharing manner. The benefits of the proposed approach are the fast optimisation time, the inclusion of user-defined criteria, and the extraction of feasible solutions considering the availability of the chargers in the station. The communications between the EV and the Charging Station are facilitated by the Open Platform Communications-Unified Architecture (OPC-UA) standard. The proposed algorithm succeeds into finding competitive solutions even during charging scenarios with conflicting criteria.

Efficiency Requirements for Passively Cooled Converters with Thermal Measurement Based 3D-FEM Simulation

Abstract: Passively cooled housings for modern power electronic converters with high power density require maximum heat dissipation over little surface area. To determine the power dissipation budget, this paper presents thermal measurement based convection modeling with temperature dependent heat transfer coefficients and the derivation of efficiency requirements for given housing dimensions.

Figures-of-Merit and current metric for the comparison of IGCTs and IGBTs in Modular Multilevel Converters

Abstract: IGCTs and IGBTs are compared in the case of a HVDC MMC. Specific figures of merit, and a current metric providing simple means to compare them, are introduced and discussed. Simulation results of a MMC model and figures of merit are shown to provide consistent results, proving that the proposed figures of merit are a very simple and fast way to select the best semiconductor switch. Furthermore, our analysis supports the growing interest in IGCTs for MMCs, as they are found to produce the lowest level of losses.