Multiple single-active bridge (SAB) DC-DC converters are connected in the input-parallel and outputparallel (IPOP) configuration to achieve higher power output in many applications. Module shutdown, known as phase shedding, is used to improve light-load efficiency in the IPOP modular converter. The SAB is known for its simplicity and robustness, but it is only moderately efficient; therefore, larger snubber capacitance is used to lower switching and conduction losses in the SAB and improve efficiency. However, larger snubber capacitance increases minimum power and reduces load range in the SAB, resulting in the unattainability of certain load-range segments in the IPOP converter because capacitance increases when phase shedding is used. Hence, snubber capacitances across the modules are optimized to improve average efficiency while maintaining the existing load range. As a result, snubber capacitances differ across SAB modules. With nonidentical modules, nonuniform module power distribution is used in the IPOP converter for higher light-load efficiency with selective phase shedding. This increases the average IPOP converter efficiency from 92.5% to 94.1% across the load range of 260 W to 13.6 kW. Peak efficiency is also increased from 93% to 95.5%. In this article, we present a method to optimize the modular SAB IPOP system for high efficiency over a wide load range.

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Due to increasing levels of fault currents, fault current limiters (FCLs) are expected to play an important role in the protection of future power grids. Inductive saturable FCLs are particularly interesting due to their inherent reaction on the fault. Many different configurations have been proposed in literature. Being difficult or impossible to create accurate analytical models of some FCL configurations, the development of finite element (FE) models for inductive FCLs is required. This paper presents a 3D nonlinear transient FE modeling technique applied to two inductive FCLs, namely the so-called open-core and three-leg single-core FCLs. The models have been validated by comparing simulation results with lab measurements. Results show excellent agreement. The models constitute a valuable tool for design, optimization, and verification of inductive FCLs. @en

Progress in material technology in wide-bandgap semiconductors, such as Silicon Carbide (SiC) enables increasing the operation frequency of high-voltage (HV) generators and using less series connected devices for the same voltage rating of the HV-generators, e.g. for x-ray applications. These generators are frequently equipped with HV multipliers, such as the Cockcroft-Walton cascade, a resonant inverter and a high voltage transformer. Power conversion at high frequency usually helps reducing the system size and cost and allows better efficiency at smaller size and lower weight. At high frequency and high voltage, however, the influence of parasitic effects of the multiplier, namely parasitic capacitance, becomes highly relevant to the feeding circuit. Experience from existing circuits indicates, that the total parasitic capacitance at secondary side is not only related to the HV transformer and the junction capacitance of the diodes, but also to the extension of the electric AC field in the rectifier circuit, which reflects the structural parasitic capacitance. It can happen that a substantial amount of driving current is required only for inverting the voltage across the parasitic capacitance, which puts a limit on the maximum useful frequency. In general it is to be expected that parasitic capacitance will increase with denser packaging of the components. In this paper an approach for the determination of the equivalent structural parasitic capacitance is given that is based on a 3D finite element (FE) analysis of the energy in the electric AC fields in the cascade circuit. The method is subsequently used to investigate effect of material technologies on the equivalent structural parasitical capacitance. These investigated technologies are: 1) the effect of the increased blocking voltage and associated reduced number of series connected devices, 2) the effect of the permittivity of isolating media and 3) the effect of a grounded box around the rectifier. The capa- - citance obtained by field simulation is validated by small signal measurements.@en

Cooling for power electronics in leisure vessels has so far been based in the use of forced air, even though such vessels are surrounded by water which can ultimately be used to dump the heat developed by power electronics converters and electrical machines. Some of the reasons to avoid the use of sea water for cooling are the need to deal with biological-fouling, salt water, corrosion, and condensation. We propose to apply a new concept where pre-filtered seawater is used to cool down power electronic modules in order to significantly reduce the size of the cooling system. A new type of cold plate with direct flow of seawater has been designed considering the mentioned challenges. The cold plate will cool down a 5kW (10kW peak) power electronics module. Such module consists of a 3 kW 3-phase AC-DC rectifier, a 2 kW (10kW peak) full-bridge DC-DC converter and a 5 kW (10 kW peak) DC-AC single phase inverter.@en

This paper presents a new component within the flexible ac-transmission system (FACTS) family, called distributed power-flow controller (DPFC). The DPFC is derived from the unified power-flow controller (UPFC). The DPFC can be considered as a UPFC with an eliminated common dc link. The active power exchange between the shunt and series converters, which is through the common dc link in the UPFC, is now through the transmission lines at the third-harmonic frequency. The DPFC employs the distributed FACTS (D-FACTS) concept, which is to use multiple small-size single-phase converters instead of the one large-size three-phase series converter in the UPFC. The large number of series converters provides redundancy, thereby increasing the system reliability. As the D-FACTS converters are single-phase and floating with respect to the ground, there is no high-voltage isolation required between the phases. Accordingly, the cost of the DPFC system is lower than the UPFC. The DPFC has the same control capability as the UPFC, which comprises the adjustment of the line impedance, the transmission angle, and the bus voltage. The principle and analysis of the DPFC are presented in this paper and the corresponding experimental results that are carried out on a scaled prototype are also shown.@en

Nowadays Pulsed Electric Field (PEF) treatment of food needs to be performed prior to packaging, either hygienic or aseptic packaging is necessary. New techniques for PEF treatment after packaging can be considered when plastic conductive (film) electrodes can be integrated within the package, so that the package and the product can be treated as a whole. This paper describes a newly developed treatment chamber, which can be used to test the ability of any arbitrary plastic packaging film to be used as electrodes for PEF treatment. Tests with a flexible commercially available electrically conductive copolymer film showed that reduction of Lactobacillus plantarum by PEF was possible. This heat sealable film obeys the mechanical properties of a polymer; however it has an electrical conductivity of 0.75 S m¿ 1 and approximately 2.3% of the surface area is electrically conductive. The maximum obtained inactivation was 2.1 log10 with a specific energy of 17 J ml¿ 1. The microbial experiments gave a consistent outcome compared with finite element simulations and with models from literature. Further research to reach real pasteurisation levels is needed as well as issues concerning utilisation of these film electrodes.@en

Fault current limiters (FCLs) are expected to play an important role in protection of future power systems. Saturated-core FCLs are particularly interesting because of their supreme operational advantages: inherent reaction on a fault, inherent post-fault recovery and unrestricted duration of a limiting period. The testing of a lab-scale FCL prototype proved the principle of operation. The full-scale 10 kV/400 A FCL prototype is built in co-operation with industry partners. The initial low-voltage 400 V testing has been successful. Full-voltage 10 kV testing will be performed later this year.@en

Fault current limiters (FCLs) are expected to play an important role in the protection of future power grids. Inductive FCLs are particularly interesting due to their inherent reaction to a fault, but are not commercialized because of a too large amount of magnetic material and an induced overvoltage across dc windings. This paper introduces a new three-phase FCL with a common core and trifilar windings. With the new FCL topology, the phase windings are placed on a single core, resulting in significant reduction of the amount of required magnetic material. Furthermore, the phase windings are wound simultaneously, so that the flux coupling between the phases is increased considerably. It cancels out the magnetic field and reduces the FCL normal impedance significantly. The dc windings are used only to compensate for possible asymmetry in the system currents. Consequently, the number of dc turns and the induced overvoltage are considerably decreased. Testing of a scaled-down FCL lab prototype made it possible to verify the FE simulation results and demonstrated the principle of operation of the new topology. Experimental and simulation results matched very well.@en

Connection of Distributed Generation (DG) units to a distribution network will result in a local voltage increase. As there will be a maximum on the allowable voltage increase, this will limit the maximum allowable penetration level of DG. By reactive power compensation (by the DG unit itself) a significant increase in allowable penetration level can be achieved. In this paper this and several methods (overrating and generation curtailment) are investigated to achieve a further increase in allowable penetration level.@en

Abstract High power density power electronics converters are increasingly demanded. Generally speaking, raised switching frequency improves the power density. However, dramatically increasing frequency on the present level still depends on the advance of power electronic components in material property, packaging technique, thermal management technique and etc. This paper surveys the recent material development of key components in the PWM power converters and then try to quantify the potential of improving the system power density by using advanced components. For illustrating this potential, power densities of air-cooled dual active bridge DC-DC converters (12V-360V, 1 kW) with three categories of components are analyzed and compared, namely state-of-art components, available advanced components and future components. The future components assume ten times better loss-related properties than those of the advanced components currently available.@en

Electrically conductive polymer composites consisting of a nonconductive polymer matrix and conductive fillers, such as carbon black, are widely used. This contribution describes a newly developed measurement setup that has been built to investigate the specific electrical properties of polymer composite films for pulsed conditions in the microsecond (10-6 s) range. For an industrially available volume conductive polymer film (Carbostat) the contact resistivity to copper has been investigated. Also, three methods for minimizing the contact resistivity, namely pressing, gluing, and wetting, have been compared for a wide range of applied current densities.@en

Abstract The increase of relatively small scale dispersed power generation (DG) is likely to impact the structure and operation of power generation and distribution systems. The current power system comprises multiple generators. Their intrinsic kinetic energy buffer (rotor inertia) plays an important role in short term system stability. Generators that are connected via power electronic power converters lack these kinetic buffers. Optimal power transfer from source to grid is often used as a control objective to optimize the energy yield. At power system level this approach may compromise stability. In this paper it is demonstrated that by emulation of rotor inertia, stability problems on a system level can be mitigated. For that purpose a short-term energy buffer is added to the system that can be controlled at a fast rate, thereby forming a so-called Virtual Synchronous Generator (VSG). Power system stability support then becomes an additional control objective of the DG. Maximum rotor angular speed deviation and maximum critical clearing time are used as performance indicators for the evaluation. Sizing of the short term buffer constitutes a second topic of this paper.@en

Abstract Installation of new generating units and increased penetration of distributed generators (DGs) in the power systems increase the total power transmitted in the power grid. In the case of a fault, the amount of power captured by a short circuit is enlarged, leading to higher peaks of the fault currents and, consequentially, to higher stresses to the power system components. Installations of fault current limiters (FCLs) are expected to prevent expensive upgrade and replacement of the components exposed to the over-stresses. Inductive FCL are particularly interesting due to their inherent reaction to a fault. However, some challenges are to be solved: excessive weight and induced over-voltage across the DC winding. The goal of this paper is to introduce a new configuration of the inductive FCL, where the amount of the required material has been reduced considerably and the induced over-voltage across the DC winding is decreased. Employment of one core per-phase instead of two reduces the amount of magnetic material. The core has three legs, where the middle leg is used as a shunt path for the AC flux. It enables a gap insertion in the AC magnetic circuit without influencing the DC magnetic circuit, i.e. amount of DC winding material. This means that a smaller core (less magnetic material) can be used for a same power level. The simulation results obtained from FCL models created in SaberDesigner are presented. They have been confirmed through the testing of the lab-scale prototype.@en

In this paper, the possibility of reaching high power densities in multikilowatt dc-dc converters with galvanic isolation is demonstrated and the main design issues are discussed. The issues related to converter topology, transformer design, and thermal management are addressed, and new conceptual solutions are proposed. Implementing zero-voltage-switching quasi-zero-current-switching topology, optimized transformer design with leakage layer, and thermal management based on conduction enhanced by heat pipes at critical places resulted in very high power density and efficiency. The power density reached by the converter prototype is 11.13 kW/L with water cooling and 6.6 kW/L with air cooling. In the same time, the measured efficiency exceeded 97% in a broad load range. The new design concepts are demonstrated on a 50-kW converter prototype that was successfully tested at full-load conditions.@en

This paper introduces a novel loss model concept for performance evaluation and design optimization of power electronics converters based on MathCAD sheet. A dual active bridge (DAB) converter (12 V/360 V, 1 kW and 25 kHz) is used as the test platform. The practical methods for extracting useful component parameters used in loss model and for experimentally assessing losses to verify the loss model are elaborated. With the verified loss model, design optimizations of the DAB converter are done theoretically.@en

The separated interline power flow controller (S-IPFC) presented is a new concept for a FACTS device. The S-IPFC is an adapted version of the IPFC, which eliminates the common dc link of the IPFC and enable the separate installation of the converters. Without location constrain, more power lines can be equipped with the S-IPFC, which gives more control capability of the power flow control. Instead of the common dc link, the exchange active power between the converters is through the same ac transmission line at 3rd harmonic frequency. Every converter has its own dc capacitor to provide the dc voltage. This paper presents the basis theory of the S-IPFC, steady-state analysis, primary control loop and the corresponding simulation results.@en

Fault current limiters (FCLs) are expected to play an important role in the protection of the future power networks, since the increase of loads and expansion of the power networks lead to much higher short-circuit power. This paper presents the comparison of four different configurations of inductive FCL, with respect to the FCL weight (magnetic core and winding material) and losses during both the nominal and the fault state of operation. Two main challenges in the inductive FCL design are reduction of the material weight and reduction of the induced dc winding over-voltage during the fault period. So far, solutions (core configurations) proposed in the literature are: decoupling of the dc and the ac magnetic circuits to avoid high voltages across the dc winding during a fault and the so-called open- core configuration. The presented results reveal the merits and drawbacks of each of the configurations and compare them to the conventional inductive FCL design characteristics. The results are obtained through the simulations in SaberDesinger and by experiments.@en

The increasing penetration of variable speed wind turbines in the electricity grid will result in a reduction of the number of connected conventional power plants. This will lead to a reduction of inertia in the grid, as the rotational speed of a variable speed turbine and the grid are decoupled by a power electronic converter. A lower system inertia will result in larger and faster frequency deviations after occurrence of abrupt variations in generation and load. It is possible to implement an additional control loop in the power electronic converter of the turbine, which connects the turbine inertia directly to the grid. It is even possible, by control, to make a `virtual inertia¿ that is larger than the real inertia. During a frequency drop additional power can be released to the grid with this control. This behaviour will be called inertial response. The additional power is obtained from the kinetic energy that is stored in the rotating mass of the turbine. In this contribution two different control strategies are investigated and compared with each other. A type of control that is equal to the primary frequency control of large power plants shows the best results, both with respect to the power that is needed to limit the frequency drop and with respect to electrical and mechanical stress of the turbine. Keywords: Frequency control; Inertia; Stability; Wind power generation@en

The increasing penetration of variable-speed wind turbines in the electricity grid will result in a reduction of the number of connected conventional power plants. This will require changes in the way the grid frequency is controlled. In this letter, a method is proposed to let variable-speed wind turbines emulate inertia and support primary frequency control. The required power is obtained from the kinetic energy stored in the rotating mass of the turbine blades.@en

In this paper, a solution is described that makes it possible for wind turbines using doubly-fed induction generators to stay connected to the grid during grid faults. The key of the solution is to limit the high current in the rotor in order to protect the converter and to provide a bypass for this current via a set of resistors that are connected to the rotor windings. With these resistors, it is possible to ride through grid faults without disconnecting the turbine from the grid. Because the generator and converter stay connected, the synchronism of operation remains established during and after the fault and normal operation can be continued immediately after the fault has been cleared. An additional feature is that reactive power can be supplied to the grid during long dips in order to facilitate voltage restoration. A control strategy has been developed that takes care of the transition back to normal operation. Without special control action, large transients would occur.@en