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Integrated gate-commutated thyristors (IGCTs) have successfully demonstrated to be well-suited devices for an application in medium-voltage dc transformers based on LLC resonant converter topology. This appli- cation enables unusually high switching frequencies in kilohertz range thanks to the zero-voltage switching while maintaining the high conversion efficiency. Moreover, the clamp circuit that is an inseparable part of every hard-switched IGCT-based converter can be completely omitted—further lowering the converter complexity and cost. Nevertheless, without the clamp circuit, the effect of shoot through becomes fatal, considering that the IGCTs have a defined mode of failure in short circuit and the desaturation effect is practically nonexistent. To prevent the shoot through, this letter proposes a simple, yet effective, protection method based on anode-voltage measurement that can be implemented locally on the IGCT gate unit. The validity of the solution is demonstrated using a custom IGCT gate unit in a medium-voltage resonant test setup. [ABSTRACT FROM AUTHOR]

Recently, integrated gate-commutated thyristors (IGCTs) have shown a great potential for the soft-switching dc transformer applications based on LLC resonant converter. The zero-voltage switching that is present in this converter topology enables an operation of IGCTs without clamping circuit and with significantly higher switching frequencies, while maintaining the exceptionally low conduction losses. The high voltage and current ratings of the IGCTs make them a preferred choice for an efficient bulk power transmission. This article shows how gate units for IGCTs can be optimized for this soft-switching application. It presents a simplification of the driving topology and the design process for high switching frequencies. This results in low power consumption and a very small size of the gate unit. The prototype of the gate unit is built and experimentally validated in a 2.5 kV 0.75 kA resonant operation of the IGCT. [ABSTRACT FROM AUTHOR]

With a large deployment of power electronic converters in various critical applications, condition health monitoring is increasingly becoming important. In modular multilevel converter, submodule capacitors represent one of the critical components having a strong impact on the overall converter cost and performance. This article proposes a simple method for the submodule capacitance monitoring, deployed directly on the low-cost submodule controller and operating without disturbing or impacting the normal converter operation. Offline and real-time simulations, as well as experimental results, demonstrate the effectiveness of the proposed method in identifying changes in the submodule capacitance, which is of high importance for reliable converter operation. [ABSTRACT FROM AUTHOR]

Stacking of floating structures, known as submodules or cells, provides the modular multilevel converter with theoretically unlimited voltage scalability. However, such a convenience comes at a price of increased control complexity, especially in the domain of internal energy control. In other words, energies of the submodule clusters must be controlled to their setpoint values; otherwise, stable and high-performance operation of the converter cannot be ensured. So far, several approaches toward balancing of the modular multilevel converter internal energy, in both vertical and horizontal directions, have been proposed. Nevertheless, differences among them have never been analytically supported. In this article, three seemingly different energy balancing strategies are thoroughly explained, providing a framework for the theoretical comparison of different dynamic responses provided by them. All the results are verified on the large-scale hardware-in-the-loop platform, serving as a digital twin of a grid-connected 3.3-kVac/5-kVdc 1.25-MW converter being driven by the industrial ABB PEC800 controller. [ABSTRACT FROM AUTHOR]

The duty-cycle control is a popular option for current limiting in LLC resonant converters. Such current limiting is required to protect the converter either during the start-up or during an overload. Although this method proved to be effective, the switching and conduction losses of the converter are increased because of the modified current waveforms in the resonant tank. Hence, for the safe operation, it is vital to distribute the losses between the switches symmetrically. However, the conventional duty-cycle or phase-shift modulation methods do not provide this naturally. This letter elaborates on a method that was proposed for a different soft-switching topology more than a decade ago to solve this problem, and is adaptable for the emerging dc transformers. Furthermore, a simple implementation of the method is shown and the equal distribution of losses is validated experimentally. The VHDL code with the implementation of the modulator is attached to this letter available openly for reuse. [ABSTRACT FROM AUTHOR]

In the available literature, including reference books on the topic, the spectral analysis of voltage waveforms obtained through the use of pulse width modulation (PWM) relies on the assumption of an ideally flat voltage across the dc-link of a considered switching structure. Nevertheless, certain converter families (e.g., modular multilevel converter, characterized by the presence of distributed and floating dc-links), feature low-frequency oscillations of dc voltage within their switching stages. On these terms, the flat dc-link assumption cannot be considered valid from both theoretical and practical viewpoints. In case carrier-based modulation is employed, unless properly compensated through the modulation signal adjustments, the above mentioned voltage oscillations cause undesirable distortion of the converter currents. However, the very same adjustments of modulation signals introduce additional challenges in the spectral analysis of the voltage across the ac terminals of an observed switching stage. This article provides the ac terminals voltage analysis of a single half-bridge experiencing dc-link oscillations, which are compensated through the modulation signal generation process. PWM with the conventional triangular carrier is considered, while all the results are presented in a generalized form, making them easily extendable to cascaded structures. All of the presented results were experimentally validated. [ABSTRACT FROM AUTHOR]

This article proposes a novel class of neural-network-inspired statistical data-driven models, especially derived for the purpose of design optimization of medium-frequency transformers. These models allow for an efficient [three to four orders of magnitude faster compared to a finite-element method (FEM)] yet sufficiently accurate (within 5–10% error relative to the FEM) and numerically stable estimation of the complex effects, with otherwise impractically high computational cost and/or convergence issues. The application of the proposed modeling framework is described in detail on two characteristic examples of the complex electromagnetic phenomena occurring within the medium-frequency transformers. The performance of the derived models is verified both with detailed FEM simulations and experimental results. [ABSTRACT FROM AUTHOR]

This paper discusses the technical challenges and tradeoffs tied to design of medium-frequency transformers for medium-voltage high-power applications and emerging solid-state transformers. A dedicated design optimization algorithm is used to generate the set of all feasible transformer designs for the given electrical requirements, originating from a modular dc–dc solid-state transformer, and taking into account different geometry ratios, materials, and operating frequencies. All possible design choice variations are compared in terms of maximum achievable efficiency, gravimetric and volumetric power densities, thus identifying design limits for a given set of input parameters. The impact of modularity and choice of operating frequency directly related to selected semiconductors, reveal interesting trends that can aid overall solid-state transformer design optimization. [ABSTRACT FROM AUTHOR]

Modular multilevel converters are increasingly being considered or used for various medium voltage applications. Multiple control methods have been proposed for the control of the direct to three-phase modular multilevel converter. They differ one from another in the way the capacitor voltage ripples are handled, i.e., either neglected, estimated, reconstructed by filtering or measured. This has implications on the performance level that can be obtained. This paper provides insights on the advantages and drawbacks of each control method, in inverter and rectifier mode, with a fair and thorough assessment supported by extensive simulations, with converter ratings that are realistic for medium voltage applications. Finally, this works highlights the impact of the higher dynamics for medium voltage direct current (dc) applications compared to high voltage dc ones on the choice of the control method. [ABSTRACT FROM AUTHOR]

Ferrite core materials are preferred for magnetic components in power electronic converters considering their low cost and high permeability. Thanks to their negligible electrical conductivity, core loss is dominated by the frequency-independent hysteresis effect in most operating conditions. In the applications of high switching frequency under a certain modulation scheme, however, the core loss of ferrite materials do exhibit obvious frequency dependence, known as relaxation effect, which is strongly influenced by the magnetic flux's rate of change and cannot be ascribed to material conductivity. In order to incorporate the core loss including both frequency-dependent and frequency-independent effects into system-level simulation environment of power converters so that the system efficiency can be pre-evaluated during the design phase as a reference for optimization, dynamic model that is physically intuitive and easy to parameterize is desired. This paper proposes an approach to model the core loss of ferrite materials combining both frequency-dependent relaxation effect and frequency-independent hysteresis, which can be flexibly applied for various operation conditions with good accuracy. [ABSTRACT FROM AUTHOR]

Cascaded H-Bridge converters in medium-voltage (MV) applications have all the dc link capacitors supplied from external source through a multiwinding phase-shift transformer. This type of transformers has a complex winding geometry, which leads to unbalanced leakage flux paths. The unbalance affects the dynamic behavior of the converters. This work proposes a modeling approach, which realistically captures the unbalance in the leakage flux path of phase-shift transformers, using permeance magnetic circuit. The model can be seamlessly integrated into system-level simulation of power electronic circuits. Taking advantage of the repetitive structure of the windings, the model requires very limited number of parameters, which can be easily obtained from the geometry data together with only a few experimental tests. The fidelity of the model is experimentally confirmed on a phase-shift transformer from a commercial MV drive system. [ABSTRACT FROM AUTHOR]

This paper describes a novel, model-based, and design optimization methodology for high-power medium-frequency transformers (MFTs) for medium-voltage high-power electronic applications, namely emerging solid-state transformers. Presented procedure enables the designer to interactively select the most optimal design in a simple and intuitive way, while inherently offering flexibility in terms of various design alternatives, depending on the component availability. The core of the design algorithm is explained in detail together with all of the relevant modeling and technical challenges associated with realization of a prototype. A $\text{100 kW}$ , $\text{10 kHz}$ MFT prototype has been designed, according to the presented algorithm, practically realized and thoroughly tested in order to verify the theoretical design. [ABSTRACT FROM AUTHOR]

Ferrite materials are widely used for magnetic cores in power electronic converters. The hysteresis effect of the material leads to power loss and harmonic distortion. In order to predict the behavior of the magnetic component in the system environment during the design phase, accurate system-level time-domain simulation is desired. This paper proposes an approach to model the frequency-independent magnetic hysteresis effect of ferrite core materials in magnetic circuits based on the permeance–capacitance analogy. The model is able to accurately reproduce the per-cycle energy loss and equivalent permeability of the hysteresis loops under excitation in a wide range of amplitudes. [ABSTRACT FROM AUTHOR]

In multiwinding transformers, different geometry of the individual windings leads to unbalanced leakage flux paths. The unbalance affects the behavior of the power electronic converters, where these transformers are used. This paper proposes an approach to model the leakage flux path of transformer with repetitive multiwinding structure using permeance magnetic circuit. The model is composed of lumped components, it can be seamlessly integrated into system-level simulation of power electronic circuits and achieve good accuracy in time-domain simulation. Taking advantage of the repetitive structure, the model requires very limited number of parameters, which can be easily obtained from the geometry information together with only a few experimental tests. The fidelity of the model is experimentally confirmed on a multiwinding transformer prototype connected to power electronic devices. [ABSTRACT FROM PUBLISHER]

During the design phase of a modular multilevel converter (MMC), an accurate loss evaluation of the submodule (SM) plays an important role. In this paper, a method based on the analytical description of the MMC key waveforms that allows to directly obtain the average semiconductor and capacitor losses that each SM will experience is introduced, under different operating conditions or control schemes. To verify the proposed concept, the results are compared with the losses obtained from a switched model with closed-loop control, where the analytical MMC key waveforms are approached in steady state. The proposed method provides a great flexibility and a significant reduction of the simulation/computational time otherwise needed to evaluate SM losses under various operating conditions. [ABSTRACT FROM PUBLISHER]

Recently, the world's first ever power electronic traction transformer (PETT) for a 15-kV 16.7-Hz railway grid has been newly developed, commissioned, and installed on the test locomotive, where the PETT is presently in use. The design and development of the PETT are described in two parts of this paper. The details related to the 1.2-MVA medium voltage (MV) PETT prototype, including the electrical design of the power circuit, the sizing and characterization of insulated-gate bipolar transistor (IGBT) devices, the design of the medium-frequency transformer, the cooling system design, insulation coordination, and protection, are presented in this part, while the preceding paper deals with the overall system requirements and control system design based on the low-voltage (LV) PETT prototype. The identical control hardware and software from the LV PETT prototype were used for the MV PETT prototype, thus reducing significantly commissioning time. The experimental results obtained during the testing are presented to demonstrate the performance of the developed MV PETT prototype. [ABSTRACT FROM PUBLISHER]

Medium voltage high-power applications are usually realized using high voltage semiconductors (3.3 kV and above) operated in the hard switching mode with low switching frequencies (several hundreds of hertz). However, for high-power dc–dc converters employing a transformer for galvanic isolation, it is attractive to increase the switching frequency so that the transformer size can be reduced. An increase of the switching frequency implies an increase of the switching losses, and this has to be mitigated somehow, usually by choice of resonant topologies or soft switching techniques. Main focus of the paper is on the operation of the insulated gate bipolar transistor (IGBT) within a high-power dc–dc LLC resonant converter, in order to explore interactions between semiconductor and circuit properties, which both must be simultaneously considered in order to achieve the best utilization of a high voltage power semiconductor operating at higher switching frequencies. For these purposes, switching properties of a standard 6.5 kV IGBT are compared with switching properties of two different optimized versions of a 6.5 kV IGBT. Experimental results are included to support theoretical considerations and findings. [ABSTRACT FROM AUTHOR]

Recently, a world's first ever power electronic traction transformer (PETT) for 15 kV, 16 ^\,2\/3Hz railway grid, has been newly developed, commissioned, and installed on the locomotive, where it is presently in use. This marks an important milestone in the traction world. The design and development of the PETT are described in this paper, where a low-voltage (LV) PETT prototype is presented. It has been designed for the purposes of control hardware and software commissioning, thus serving a role of an analogue simulator. In this paper, emphasis is placed on the overall system requirements, from where control system has been developed, implemented, and successfully commissioned. The development of a 1.2MVA medium-voltage PETT prototype will be reported separately in accompanying paper. [ABSTRACT FROM AUTHOR]

Flux harmonic distortion factors (HDFs) of two space vector pulse width modulation (SVPWM) schemes, aimed at sinusoidal output voltage generation with five-phase voltage source inverters, have been evaluated in Part 1 of this paper. Analytical considerations and numerical integration have been applied in order to determine the switching properties of the considered SVPWM methods. It has been shown that the SVPWM based on four large vectors leads to a considerably higher overall per-phase flux HDF than the method based on two large and two medium space vectors. The purpose of the second part of the paper is to relate flux HDFs to the current ripple and current total harmonic distortion and thus explore further switching characteristics of the two SVPWM techniques. The applied approach is based on simulation and experimentation, in conjunction with the subsequent fast Fourier transform of the waveforms, so that all the results for the ripple are obtained using a methodology completely different from the one in Part 1. Relationships that correlate the current ripple and the flux HDFs are established, and it is further shown that the current ripple can be easily calculated from the flux HDF values of Part 1 if the relevant leakage inductances of the two planes are known. Hence, the complete theory of Part 1 is fully verified by both simulation and experimentation. It is also shown that, in certain cases, the SVPWM based on four large vectors may lead to a smaller current ripple despite a considerably higher flux HDF. [ABSTRACT FROM AUTHOR]

Multiphase variable-speed drives supplied from two-level voltage source inverters (VSIs) are, at present, contenders for numerous industrial applications. Various pulsewidth modulation (PWM) methods for multiphase VSIs, aimed at sinusoidal output voltage generation, have been developed using both carrier-based and space-vector approaches. An increased number of voltage space vectors, available for the synthesis of the output voltage, offer greater flexibility for the PWM scheme development, compared to the three-phase case. This paper presents comprehensive analytical analysis and comparison of switching ripple characteristics of two continuous space vector PWM (SVPWM) methods for a five-phase two-level VSI. Considered SVPWM schemes select a different set of four active space vectors per switching period in order to generate sinusoidal output voltages. This paper also presents the evaluation of the flux harmonic distortion factors of the SVPWM techniques. Correlation with corresponding current ripple and total harmonic distortion is established in Part 2 of this paper, where theoretical considerations are verified by simulations and through experimental investigation on a five-phase VSI-fed induction motor drive. [ABSTRACT FROM AUTHOR]

A multilevel multiphase space vector pulsewidth modulation (SVPWM) algorithm has been introduced recently, in which the reference is separated into an integer part and a fractional part. The fractional part is, in essence, a two-level multiphase space vector algorithm. This paper shows that, with appropriate adaptations, the fractional part of the general space vector multilevel multiphase PWM can be applied as a stand-alone PWM method in conjunction with two-level voltage-source converters with any number of phases. Simulation results of the five- and six-phase cases are shown, and the new algorithm is compared with another recent multifrequency SVPWM algorithm, which follows the standard approach of selecting the switching vectors and calculating their application times using dq planes. The experimental verification is provided using a five-phase two-motor series-connected induction motor drive, supplied from a custom-designed five-phase voltage-source inverter. [ABSTRACT FROM AUTHOR]

Multiphase variable-speed drives, supplied from two-level voltage-source inverters (VSIs), are currently considered for a number of industrial applications. Although various multiphase pulsewidth-modulated (PWM) schemes had been recently developed, very little has been reported with regard to the impact of these schemes on harmonic losses (current ripple). This paper presents a general analytical analysis and comparison of harmonic losses of two basic continuous PWM (CPWM) schemes aimed at sinusoidal output-voltage generation. The analysis utilizes the concept of multiple polygon connections, which exist for multiphase systems, and covers all multiphase topologies with an odd phase number and single neutral connection. General harmonic distortion factors (HDFs) of sinusoidal PWM (SPWM) and harmonic injection PWM (HIPWM) are determined and compared for various phase numbers. Theoretical findings are verified by means of simulations and experimentally using a five-phase induction motor drive. [ABSTRACT FROM AUTHOR]

Control of an ac machine requires only two currents. It is, therefore, possible to achieve independent control of a number of three-phase machines using various inverter topologies with a reduced switch count. One such drive system comprises n three-phase induction machines supplied by a (2n + 1)-leg voltage source inverter (VSI). In such a configuration, one inverter leg is common for all machines. This paper presents a general pulsewidth modulation (PWM) method that is applicable for any number of machines n in the group. It is based on utilization of the standard three-phase modulators in conjunction with additional logic to generate modulation signals for all (2n + 1) legs of the inverter, and it enables an arbitrary distribution of the available dc bus voltage between the n machines. A developed PWM method has been implemented and tested in two-machine, three-machine, and four-machine drives, supplied from five-leg, seven-leg, and nine-leg VSIs, respectively. Experimental results are reported for open-loop V/f control, closed-loop current-controlled V/f mode of operation, and full vector control. It is shown that the nonideal nature of the VSI causes unsatisfactory operation of the drive system in the open-loop V/f mode, and that closed-loop current control is, therefore, preferred in this type of a drive system. It is believed that the four-machine drive has a good potential for industrial applications since, in certain cases, it enables a considerable saving in the required number of switches and installed inverter power. [ABSTRACT FROM AUTHOR]

Multiphase variable-speed drives, supplied from two-level voltage source inverters (VSIs), are nowadays considered for various industrial applications. Although numerous pulsewidth modulation (PWM) schemes for multiphase VSIs, aimed at sinusoidal output voltage generation, have been developed, no detailed analysis of the impact of these modulation schemes on the output current ripple has ever been reported. This paper presents a comprehensive analytical analysis and comparison of the output current ripple caused by the application of three different continuous PWM schemes, using a five-phase VSI as an example. Main properties of sinusoidal PWM, fifth harmonic injection PWM, and space vector PWM are elaborated and analyzed using the harmonic flux concept. Space vector theory is applied in the analysis. As a result, harmonic distortion factors are obtained for each PWM scheme. Theoretical considerations are verified by simulations and experimental investigation using a custom-designed five-phase VSI-fed induction motor drive. [ABSTRACT FROM AUTHOR]

Two-level multiphase voltage source inverters (VSIs) are typically used as the supply for multiphase machines. Such machines are often with concentrated winding in which case, certain low-order stator current harmonics are injected to provide torque enhancement. Also, a multimotor drive system can be realized by connecting a number of multiphase machines in series, using phase transposition while supplying the whole system from a single multiphase VSI. In both situations, it is important to know the limits of the inverter operation in the linear modulation region. This paper develops a simple method that enables analytical determination of the boundaries of the linear modulation region for all multiphase inverters with a prime number of phases. The limits are independent of the actual pulse width modulation (PWM) method utilized, and are equally applicable to both carrier-based and space vector PWM techniques. Theoretical considerations are verified experimentally using the five-phase and seven-phase VSI rigs. [ABSTRACT FROM AUTHOR]

Multiphase variable-speed drives, supplied from two-level voltage-source inverters (VSIs), are nowadays considered for various industrial applications. Depending on the drive structure and/or the motor design, the VSI is required to generate either sinusoidal voltages or voltages that contain a certain number of sinusoidal components ("multifrequency output voltages"). The existing space vector pulsewidth-modulation (SVPWM) schemes are based on selection of (n - 1) active space vectors (for odd phase numbers) within a switching period and they yield either sinusoidal voltage or sinusoidal fundamental voltage in combination with a limited amount of other harmonic terms. This paper develops a SVPWM scheme, which enables multifrequency output voltage generation with arbitrary values of various sinusoidal components in the output voltage. The method is based on initial selection of (n - 1)2 /2 active space vectors within a switching period, instead of the common (n - 1) active vectors. By properly arranging the sequence of the vector application, it is possible to provide an automatic postreduction of the number of applied active vectors to (n - 1), thus maintaining the same switching frequency as with the existing schemes while simultaneously avoiding the limiting on the generated sinusoidal output voltage components. Theoretical considerations are detailed using a five-phase VSI. The experimental verification is provided using a five-phase two-motor series-connected induction motor drive, supplied from a custom-designed five-phase DSP-controlled VSI. [ABSTRACT FROM AUTHOR]

This paper focuses on the analysis of a seven-phase voltage-source inverter for high-performance motor drives. The problem of the modulation strategy of the inverter is solved by combining the multiple space vector representation with traditional carrier-based pulsewidth modulation. This approach leads to the definition of a general modulation strategy that can be usefully utilized in multimotor drives, as well as in multiphase motor drives for improving the torque density. The inverter output voltage capability is investigated, showing that the proposed modulation strategy is able to fully exploit the dc input voltage either in sinusoidal or nonsinusoidal operating conditions. The results obtained in the analytical investigation are confirmed by experimental tests. [ABSTRACT FROM AUTHOR]

Coil design and operating frequency are key factors impacting the efficiency of the inductive power transfer system. This article presents the modeling of printed circuit board spiral coils, design optimization and analysis on coil geometry, and operating frequency for medium-voltage (MV) applications, such as auxiliary power supplies for MV converters. Analytic model considering the geometric distribution of the magnetic field is proposed to calculate with high accuracy equivalent electrical parameters of coils. For a defined design space and constraints, a large number of coils are designed to determine their performances and select optimal coil geometries. Selected coil designs are prototyped and experimentally verified under design-specific conditions. In addition, further characterization is performed considering variations of several parameters that can be encountered in practice. [ABSTRACT FROM AUTHOR]

The use of dc shipboard power distribution networks has been shown to offer several advantages over their ac counterparts. The reduction of the size and number of system components, the increased simplicity of operation and fuel efficiency of the system, and convenient integration of energy storage systems offer significant economical benefits to the ship operator. Solid-state bus-tie switches are widely regarded as being an indispensable component in dc shipboard power distribution networks, for their role as first line of defense against faults. Nevertheless, these devices are not limited exclusively to protection, but can provide additional functionality thanks to their design. This article shows how a solid-state bus-tie switch can be employed for soft-starting an unenergized section of a shipboard power distribution network, eliminating the need for additional components aimed specifically at this task. A technique is suggested and its performance is experimentally verified in a laboratory scaled-down model of a dc shipboard network. The additional functionality is achieved exclusively on a control basis, without modification of the switch topology initially intended purely for system protection. [ABSTRACT FROM AUTHOR]

This article introduces a thermally-compensated magnetic hysteresis model, capable of accurately determining core losses of ferrite materials while accounting for core temperature variations. The employed model is based on permeance-capacitance analogy and it captures the frequency-independent hysteresis effect with the help of the Preisach model. For validation purposes, a 100 kW, 10 kHz realized medium frequency transformer prototype is selected and modeled as a part of a full-bridge LLC resonant converter in a time-domain simulation environment. Conducted simulations show the ability of the thermally-compensated magnetic hysteresis model to impact the generated core losses for core temperatures of up to 120° C. Finally, a temperature-feedback loop within the transformer is successfully closed inside of a system-level simulation, allowing for a more precise determination of the core and winding steady-state temperatures. [ABSTRACT FROM AUTHOR]

Steadily increasing requirements for grid-connected converters, their growing unit power, and output voltage call for flexible high-power medium-voltage grid emulators. Four-quadrant cascaded H-bridge topology features high output voltage resolution and high effective switching frequency, which enable high-dynamic high-fidelity grid emulation. This article shows that despite nonidealities (turn ratios, phase-shift angles, and different stray inductances) in a typical phase-shifting multiwinding transformer, active front-ends of the cells can be effectively synchronized to their respective supply voltages using transformer primary-side voltage measurements and nameplate values. Consequently, cell input filters can be omitted, making the grid emulator more compact, cheaper, faster, and exhibiting more robust grid synchronization. Theoretical developments and designs are verified by means of high-fidelity time-domain simulations. [ABSTRACT FROM AUTHOR]

The need to increase the efficiency and reduce the operating cost of shipboard dc power distribution networks results in a desire to increase the operating voltage of such installations to the medium-voltage levels. Devices, such as solid state bus-tie switches, are an essential component of such systems, as they allow for system reconfigurations and prevent fault propagation across different sections of the system. This article presents a scalable solid-state bus-tie switch topology designed initially for low-voltage power-distribution networks, but easily scalable in terms of voltage and current. The use of a single active device greatly increases simplicity and reliability of the solution. The ability to extend the current and voltage ratings has been verified on the prototypes through extensive experimental tests in parallel and series connection, respectively. Several methods for fault detection are discussed and further verified experimentally. [ABSTRACT FROM AUTHOR]

The high-current delivered by a dedicated power supply is the condicio sine qua non to obtain the strong magnetic field produced by superconducting magnets. The end goal of such power supply is to control the current, very precisely, across this distinctive kind of load. To this extent, the topology and a specific mode of operation that fit the requirements is analyzed and presented in this article. To obtain a contemporary power supply design, a design optimization of the overall system with respect to efficiency and volume is carried out. Considering the modularity specification, the system is split into elementary building blocks and one optimally designed unit is practically implemented and verified against predictions from the design tool. In sum, this article provides the comprehensive design process for a modern 2-quadrant high-current low-voltage dc/dc power supply. [ABSTRACT FROM AUTHOR]

DC shipboard power systems have been considered as a promising solution for stricter environmental regulations on ships due to their main benefits in fuel savings with variable speed engines and easy integration of energy storage systems. In order to employ the dc solution in the shipboard power systems, the dc power systems have to be protected from a system fault with protection selectivity to minimize impacts of the fault or to avoid other undesirable situations in the system. For low-voltage dc shipboard power systems, a three-level protection has been proposed: 1) fast action (first)—bus separation by solid-state dc bus-tie switch; 2) medium action (second)—feeder protection by high-speed fuse; and 3) slow action (third)—generator–rectifier fault controls. This paper proposes a new method by means of additional bus capacitance added in main dc buses to help reliable operation of the three-level protection. The principle of the proposed method is introduced and the sizing of the additional bus capacitance is addressed in this paper. With the modeling of the dc shipboard power systems, the analyses of voltage drops for the bus separation failure and fault clearing time for the feeder protection are carried out to verify the proposed method. The results show that the proposed method not only mitigates the voltage drop for the bus separation failure, but also achieves the selectivity and the sensitivity for the feeder protection. [ABSTRACT FROM AUTHOR]

Medium-voltage-direct-current collection and distribution grids are being considered for emerging applications, and require enabling power electronic technologies. Medium-voltage high power DC--DC converters are one of the solutions that will play an important role in future energy systems. This study presents a novel bidirectional multiport power electronic converter, interfacing medium-voltage and low-voltage distribution grids while integrating distributed energy storage elements, such as batteries and super-capacitors. The multiport energy gateway concept is based on a modular input-series output-parallel topology includes medium frequency transformer for the galvanic isolation, and combines open-loop and closed-loop operated converter stages. Comprehensive converter design aspects and operating principles are presented and discussed considering representative cases of studies. The validity of the approaches is proved by simulation and experimental results obtained on a small scale prototype. [ABSTRACT FROM AUTHOR]

Modern grids are facing a massive integration of power electronics devices, usually associated to instability issues. In order to assess the likelihood and severity of harmonic instability in the high-frequency region, this paper develops a multivariable input-admittance model that accurately reflects the following aspects: 1) the discrete controller frequencies are defined inside a spectrum region limited by the Nyquist frequency and 2) the physical system aliases are transformed into lower frequency component inside the discrete controller. The proposed model shows that dynamic interactions are not theoretically band-limited; however, the control action tends to be strongly limited in a low-frequency range, due to the natural low-pass filter behavior of acquisition and modulation blocks. This is reflected in a reduced resistive part (either positive or negative) of the input-admittance in the high-frequency range. More specifically, considering the input-admittance passivity criterion, the excursions into the nonpassive area are very smooth at high frequencies, where the input-admittance is well-described by simply its inductive filter. Comprehensive experiments are conducted on a lab scale prototype, which includes measurements beyond the Nyquist frequency and alias identification. The experimental results well-match the theoretical model. [ABSTRACT FROM AUTHOR]

Dual active bridge (DAB) is a topology that is receiving more and more attention as a potential solution to interface dc grids of different voltage levels. From a system level, the implications of DABs on the stability of complex power systems are addressed in this paper. Dynamics modeling and stability assessment for a DAB implementation aimed to interface low-voltage energy resources with medium-voltage dc (MVdc) collection and distribution grids are presented. The DAB admittance is analytically derived and assessed in order to describe its dynamics and anticipate its behavior when integrated in a complex MVdc grid. The model considers the low-frequency range, mostly dominated by the controller action, and the high-frequency range, described by a nonlinear operation. The theoretical analysis is verified by hardware-in-the-loop emulation, with the controller running on a digital signal processor. The proposed implementation is proved to achieve passivity in the whole spectrum, which undoubtedly is a desired feature for massive power electronics integration in the future MVdc grids. [ABSTRACT FROM AUTHOR]

In the last thirty years, the integrated gate-commutated thyristor (IGCT) has been employed in medium voltage $\,\mathrm{M}\mathrm{W}$ power level converters and has demonstrated some of the lowest conduction losses of any actively controlled device in traditional hard-switched, sub- $\,\mathrm{k}\mathrm{Hz}$ applications. Extending the use of the device to medium frequency applications, such as dc transformers is possible through soft-switched operation, where low turn-off current and zero-voltage turn-on significantly reduce switching losses. For this purpose, this article explores the tradeoffs in the transition between zero-voltage and zero-current switching conditions of reverse conducting IGCTs. Switching conditions resulting in minimal switching loss are identified for standard commercial devices and engineering samples subjected to increased electron irradiation to reduce switching energy at the expense of on-state voltage. The operation of the IGCTs at the record frequency of 5 $\mathrm{k}$ $\mathrm{Hz}$ is thermally validated under load in the identified switching conditions. [ABSTRACT FROM AUTHOR]

Series-connected integrated gate-commutated thyristors (IGCTs) find application in hard-switched commercial power converters at the megawatt power levels. Expanding the application of these devices to medium-voltage dc–dc resonant converters is of great interest due to the IGCT's low conduction losses. This requires a deeper understanding of the device's behavior in series connection and under switching conditions different than those in hard-switched converters. In particular, the voltage sharing between series-connected IGCTs must be achieved with significantly reduced turn-off currents, typical for resonant converters. Relying on a versatile IGCT test setup, this article explores the operational performances of series-connected IGCT operation at very low turn-off current and with snubber capacitance in the range of a few tens of nanofarad. The results presented in this article are obtained from commercially available 4.5-kV 68-mm reverse-conducting IGCTs (with standard irradiation), as well as two customized engineering samples (high irradiated variants), optimized on their technology curve for high switching frequencies. The IGCTs are successfully operated in series connection at 5 kHz, with device voltage being effectively shared in resonant converter operation. [ABSTRACT FROM AUTHOR]

In the future, new aspects from decentralized generation using different dc voltage levels are expected to influence the general concept of power exchange. Converter are needed to adapt the voltage between low voltage (LV), medium voltage (MV), and high voltage (HV). In this paper, a galvanic isolated bidirectional dc/dc converter based on the modular multilevel converter is studied, with a large potential for secure and flexible dc power flow control. The use of medium frequency transformation allows savings in copper and iron. A fundamental frequency modulation method is introduced for the presented converter, that enables variable stepup or step-down between primary and secondary dc voltage in discrete steps. The balancing mechanism of the internal power storage components is explained and verified by simulation and experiment. [ABSTRACT FROM AUTHOR]

Power electronic transformer (PET), a converter technology that utilizes power semiconductors in combination with medium-frequency transformers, is considered a promising solution for certain applications requiring flexible galvanic isolation. Among are those where space occupied by bulky low-frequency transformers is of concern and/or where advanced power quality control features are needed. In this paper, the PET for a single-phase traction on-board application is discussed with emphasis on the efficiency improvements and reductions of energy consumption during the operation on the vehicle. Several control algorithms devised to improve efficiency under light-load conditions are tested on a low-voltage prototype of the PET, and experimental results are presented demonstrating the effectiveness of the proposed algorithms. [ABSTRACT FROM AUTHOR]

Rotor-flux-oriented control of multiphase induction machines with sinusoidal magnetomotive force (MMF) distribution can be realized theoretically by using only two current controllers in the synchronous reference frame. In practice, however, this is inadequate, as shown in this paper using experimental results collected from five-phase induction motor drive laboratory prototypes. Various asymmetries of the machine and nonideal nature of the inverter necessitate an appropriate modification of the current control scheme in the rotating reference frame. Such a modified current control scheme is developed in this paper, and its ability to provide operation of the machine with a practically perfectly sinusoidal and balanced system of five-phase currents is demonstrated experimentally. [ABSTRACT FROM AUTHOR]

Utilization of the integrated gate-commutated thyristor as a semiconductor switch in the series resonant converter for isolated medium-voltage dc–dc conversion offers an opportunity for high conversion efficiency while operating at the high switching frequency. Low conduction losses of the switch as well as decreased switching losses due to zero-voltage turn-on and low-current turn-off in the subresonant operating regime are reflected in the efficiency increase of the converter. This article explores the switching behavior of the semiconductor device under low currents while giving insight into the achievable turn-off energy losses and duration of the turn-off transients, as information required during the design process of series resonant converter. Experimental measurements confirm trends observed with simulation results related to turn-off delay process, providing further understanding of the switching losses and achievable performances under resonant mode of operation. [ABSTRACT FROM AUTHOR]

This paper presents a design methodology and its experimental validation for the input-admittance passivity compliance of LCL grid-connected converters. The designs of the LCL filter parameters and the discrete controller are addressed systematically, and suitable design guidelines are provided. The controller design is developed in the Z-domain, with capacitor-voltage-based active damping used as a degree of freedom to compensate for system delay effects. The role of resistive components in the circuit, which have inherent dissipative properties, is also discussed. As an outcome of the design, a passive input-admittance shaping is obtained. The theoretical development is further verified in a low-scale prototype supplied from a controllable grid simulator. For the sake of generality, different combinations of resonant to sampling frequency are tested. Experimental results fully prove the input-admittance passivity compliance. [ABSTRACT FROM AUTHOR]