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In this article, a current control algorithm is conceived for converting solar dc power into ac power of high-quality. This conversion is based only on a dc-ac converter, avoiding the use of two stage converter. The proposed algorithm is founded on tracking the maximum power point which ensures controlling the electrical power exchanged with the grid. To assure a good dynamics and reduce the control errors, a proportional resonant controllers are utilized and a self-tuning founded on particle swarm optimization is utilized. The proposal strategy goodness is verified by means of numerical simulations.

The paper deals with a full sliding mode approach for induction motor drives direct torque and flux control. The proposed solution is composed by an integral sliding-mode control and by a speed-sensorless sliding mode observer. The suggested Integral Sliding-Mode Direct Torque Control (ISMDTC) is formulated in the stator flux reference frame, without employing current regulators. The adopted sliding mode observer provides stator-flux and torque estimation, without rotor-speed adaptation. The numerical results show that the ISMDTC maintains the robustness of the sliding controller, reducing the chattering during the steady state. Moreover, it consents to achieve similar performance like more complex second-order SMC offering more simple implementation. The ISMDTC assures good accuracy and robustness to parameters deviations at low-speed operations as well as in all the operative range of the IM drives. For proving the goodness of the proposal the suggested control is also compared with a conventional DTC control.

The rapid growth of the electric vehicle market has stimulated the attention of power electronics and electric machine experts in order to find increasingly efficient solutions to the demands of this application. The constraints of space, weight, reliability, performance, and autonomy for the power train of the electric vehicle (EV) have increased the attention of scientific research in order to find more and more appropriate technological solutions. In this chapter, it proposes a focus on the main subsystems that make a zero-emission vehicle (ZEV), examining current features and topological configurations proposed in the literature. This analysis is preliminary to the various electric vehicle architectures proposed in the final paragraph. In particular, the electric drive represents the core of the electric vehicle propulsion. It is realized by different subsystems that have a single mission: ensure the requested power/energy based on the operating condition. Particular attention will be devoted to power subsystems, which are the fundamental elements to improving the performance of the ZEV.

The full electric/hybrid electric propulsion is nowadays a promising technology for the aircraft. The problems related to the electric flight are inherent the storage system sizing, the design of lightweight motors and the power electronics converters. In this paper is proposed a preliminary study about the on-board wiring system. In particular, the maximum power transferrable with the common aeronautic wires in a distributed propulsion system has been investigated.

This paper presents a sensorless control for a Doubly Fed Induction Generator (DFIG) in the context of grid-connected turbine-based wind generation systems. The paper proposes a full order adaptive observer able to track with excellent accuracy the DFIG rotor position even in presence of significant parameters deviations. The developed adaptive observer is coupled with a traditional stator flux based Field Oriented Control (FOC). The novel approach has been validated by an extensive numerical analysis.

This article proposes a space-vector modeling formalism and a harmonic balance control strategy for the voltage equalization of a double-star modular multilevel converter employed as an active interface between a three-phase ac grid and an HVdc grid. The balancing procedure is based on the control of the power flow through the converter legs, with the aim to stabilize the energy stored in each switching module to the same value. By properly exploiting both the space-vector/zero-component and the common/differential mode decompositions, the proposed approach is able to highlight all the viable interactions between the system voltages and currents. The harmonic balance control is then formulated in its most general form in order to effectively increase the degrees of freedom available to the voltage equalization while guaranteeing the hardware constraints and functional requirements of the converter. The resulting control algorithm, based on a subset of the harmonic control variables, has been implemented on a controller board and experimentally validated on a 1.5-kW test bench.

This study deals with direct torque control (DTC) of induction motor drives when a multilevel inverter is involved. The proposed control technique is finalised to reduce the amplitude of torque oscillations and to improve the motor dynamic response regardless of operating speed conditions while assuring a satisfactory input current distortion. This is achieved by using a special look-up-table built with a novel approach. The control itself is based on the application in each sampling interval of one or two voltage vectors suitably selected, in order to ensure low electromagnetic torque ripple in steady-state operations and fast dynamic behaviour in transient operations. In the algorithm, the converter voltage vectors selection is carried out by using a novel look-up table whose entry points are computed by means of a specialised relation, starting from the outputs of multilevel hysteresis controllers. The suggested method, suitable for a generic multilevel inverter, is detailed for a neutral point clamped three-level inverter. The proposed algorithm is validated by several numerical results, and compared with a DTC algorithm based on a traditional three-speed range look-up tables. Finally, some experimental results, obtained with a scale laboratory setup, are shown, discussed and compared with the numerical ones.

The paper deals with a high performance fast charger system for Zero Electric Vehicles (ZEVs). The proposed converter, which is built upon two interleaved three-leg two-level Voltage Source Converters (VSC), allows for a substantial increase of the total output power, thus leading to a high reliable topology. Furthermore, this approach allows a strong reduction of the output voltage ripple while exhibiting excellent dependability characteristics. The performance of the proposed DC/DC converter is validated by an extended numerical analysis, which confirms the effectiveness of both the control strategy and the system topology.

This paper presents an overview of the most recent state of the art in the field of high-speed electric machines fed through high-frequency converters. This type of systems is rapidly wide spreading in aeronautical and automotive applications, as well as microturbines. Each typology has its own advantages and downsides, which are analytically presented in this paper. Some types of high-speed electric machines require high-frequency voltage supply, highly stressing the dielectric materials of the winding insulation system. For this reason, in high-speed electric drives, premature failure may occur and a reduction of the total system reliability has been observed in the past years. Such issues are comprehensively investigated in this paper.

The paper deals with the control of three-phase voltage source rectifiers (VSRs) with an arbitrary number of voltage levels. By means of an approach borrowed from the “Direct Torque Control” of AC drives, the paper presents an enhanced direct power control (DPC) strategy for VSRs which directly controls both active (P) and reactive (Q) line power by considering the values of their time derivatives. This control strategy can be applied both to the Neutral Point Clamped and to Cascaded H-bridge multilevel converters. In every sampling interval the proposed control technique is able to select the most suitable converter voltage vector to obtain good dynamic response and to maintain P and Q within their respective reference bands. This result is achieved in the different operating conditions by means of a Dynamic Look-Up Table (D-LUT) of the converter voltage vectors, which are ordered on the basis of their influence on P and Q time derivatives. The mentioned D-LUT depends on the instantaneous operating point and is not predetermined. Some significant operating conditions are numerically investigated in order to show the capability of the modified DPC technique to ensure good values of power quality indexes for the grid. Experimental results on a laboratory setup based on a three-level VSR show good accordance with the simulation ones.

This paper proposes a new mathematical model of modular multilevel converters for battery electric vehicles with space-vectors enabling a critical analysis of cell balancing for the battery management system. In particular, the requirements for power balancing and the actual number of degrees of freedom of the control are investigated. The paper shows that the traditional approach of cell balancing is a special case of the proposed control methodology. Numerical analyses with Matlab/Simulink™ highlight the reasons of the slow response of the standard balancing technique for specific operating conditions of the battery electric vehicle. The paper suggests potential improvements that could be introduced through the proposed generalised approach.

In recent years the aerospace industry has made a growing effort to develop a quieter and more environmentally friendly aircraft. In particular, several research activities have been focused on innovative solutions aimed at the design/optimization of an on-board electric system fully compatible with this new approach. A first important step in the evolution towards an All Electric Aircraft (AEA) is the replacement of the hydraulic actuators with fully electric ones. The transition process is not easy to carry out, since weight, size and reliability represent highly critical issues for aircraft applications. In this context, the significant improvements in semiconductor technologies can be exploited as a critical means to overcome the constraints mentioned. Indeed, this work proposes a Silicon Carbide (SiC) based Power Factor Correction (PFC) converter, whose design and control have been tailored in order to properly supply a wide range of on-board Electro-Mechanical Actuators (EMA). In particular, while the adopted circuit topology allows for power factor correction and bi-directional power flow, the SiC technology, thanks to the higher efficiency with respect to other semiconductor-based technologies, leads to a significant reduction in the overall system weight/volume. Furthermore, to meet the strict requirements in terms of dynamic and steady state performance imposed by the application, a novel adaptive regulator is conceived. A reduced-scale laboratory prototype of the SiC-based converter (3 kVA) is realized in order to verify the effectiveness of the proposed design and control approach.

Marine energy sources represent an attractive and inexhaustible reservoir able to contribute to the fulfillment of the world energy demand in accordance with climate/energy regulatory frameworks. Wave energy converter (WEC) integration into the main grid requires both the maximization of the harvested energy and the proper management of the generation variability. The present paper focuses on both these mentioned issues. More specifically, it presents an embedded point pivoted absorber (PPA) and its related control strategy aimed at maximizing the harvested energy. Experimental and numerical investigations have been carried out in a wave/towing tank facility in order to derive the design characteristics of the full-scale model and demonstrate the validity and effectiveness of the proposed control strategy.

In the context of grid connected photovoitaic (PV) generation systems, there are two paramount aspects regarding the Maximum Power Point Tracking (MPPT) of the photovoltaic units and the continuity of the service. The most diffused MPPT algorithms are based on either perturb and observe, or on an incremental conductance approach and need both PV current and voltage measurements. Several topology reconfigurable converters are also associated with the PV plants, guaranteeing fault-tolerant features. The generation continuity can also be assured by interleaved inverters, which keep the system operating at reduced maximum power in case of failure. In this paper, an evolution of a hysteresis based MPPT algorithm is presented, based on the measurement of only one voltage, together with a novel space vector modulation suitable for a two-channel three-phase grid connected interleaved inverter. The proposed MMPT algorithm and modulation technique are tested by means of several numerical analyses on a PV generation system of about 200 kW maximum power. The results testify the validity of the proposed strategies, showing good performance, even during a fault occurrence and in the presence of deep shading conditions.

The paper deals with ball screw actuators used for the electric steering of nose landing gears in civil aircrafts. Design options and performance analyses are carried out with reference to actuators composed by ball screws coupled with two multi-stage axial-flux permanent magnet motors, which are fed by two separate inverters. A redundant configuration of the drives is developed in order to increase the availability of the steering service. Numerical analyses are carried out with reference to some significant operating conditions.

In this paper the control of a grid-tied PV (Photovoltaic) Cascaded H-bridge inverter is discussed. The operation principle of the considered circuit is analyzed and a particular attention is paid to the different modulation techniques well-suited for this configuration. Moreover, some critical implementation issues are addressed, while a fair comparison between the proposed modulation scheme and the Phase Shifted PWM (Pulse Width Modulation) is carried out. The adopted control strategy is based upon a sorting algorithm in order to modulate only one power cell (i.e. H-bridge) per sorting cycle while the others are in fixed state. This technique allows to reduce power switching losses, thus increasing overall system efficiency. The numerical results reported in the paper permit to confirm the feasibility of the proposed modulation strategy and the better performance with respect to PS-PWM in terms of power efficiency.

This paper is focused on the design of a PMSM for the electric steering of a nose landing gear. The traditional hydraulic steering systems utilized in a nose landing gear are usually based on a rack and pinion solution with two hydraulic pistons at the external sides. The main idea proposed in the paper is the substitution of the hydraulic steering system with an electric steering system based on the coupling between an harmonic drive and a three phase double rotor axial flux permanent magnet machine fed by two different three phase inverter.

With reference to light railways transportation systems, in the paper a comparison is carried out between vehicles equipped with Induction Motor (IM) drives and Permanent Magnet (PM) brushless motor drives. The attention is particularly focused on the deceleration phase of the speed diagram, which consists of an interval of “coasting” followed by a braking interval. The analyses carried out aim to contribute in understanding possible disadvantageous aspects of PM drives if they are taken into consideration to replace traditional IM drives in propulsion systems. In order to comparatively evaluate energetic and dynamic performance, some numerical results are obtained and discussed with reference to different operating conditions of an effective case-study.

The paper deals with acoustic noise in permanent magnet synchronous motors (PMSM) with tooth concentrated armature windings. In particular, the noise emitted by this kind of motors is investigated with reference to high-torque low-speed applications. In these cases, due to the high number of poles, the stator yoke is usually thin and it is characterized by low natural frequencies that could lead to undesired amplifications of mechanical resonance phenomena. The paper analyzes the effects of variations of stator yoke thickness on vibrations and noise emitted by the machines. The aim is to investigate the relation between noise emission and stator yoke thickness without any other variation of the machine configuration. The analysis is led by the mathematical model of PMSM, used to characterize analytically the time behavior of the radial forces that excite the machine vibrations; FEM analysis and numerical extrapolation of the machine acoustic response are performed by the software “Manatee”.

The paper concerns the analysis of urban electrical mini-bus power configurations based on two different energy storage systems. Full electrochemical battery power solution and hybrid power configuration based on battery and fuel cell are considered, modelled and analyzed. Performance analyses are carried out by modeling the two different powered mini-buses running along the same drive cycle experimentally tested in actual traffic conditions. The results are compared and discussed in term of performance.

The study deals with a novel torque derivative control of induction motor drives. The proposed technique is mainly finalised to improve the motor dynamic response regardless of speed operating conditions, ensuring satisfactory values of both motor currents distortion and torque ripple. The core of the discussed approach is a special look-up table, in which the converter voltage space vectors are sorted with respect to their influence on the torque derivative, taking also into account their influence on the flux magnitude. The table is indeed accessed by a desired torque derivative value, obtained by properly processing the output of hysteresis controllers. Moreover, a variable duty-cycle technique is implemented in order to ensure a satisfactory torque ripple in steady-state conditions. The proposed algorithm, suitable for a general multilevel inverter, is numerically tested on a three-level neutral point clamped inverter and its performance are compared with the ones obtained by a traditional direct torque control method based on three speed ranges look-up tables. The control is then validated experimentally by means of a reduced scale laboratory prototype fixing the same condition used for numerical analysis.

This paper deals with a Power Electronic Transformer (PET) topology for a 3-phase AC distribution grid. In the discussed topology, a Modular Multilevel Converter (MMC) and a Full-Bridge converter are employed for the medium voltage (MV) and the low voltage (LV) side, respectively. By using the space vector, approach a mathematical model for the MMC is presented and a grid-synchronous algorithm is implemented to easily control the power flow through the structure. The MV and LV side converters are linked through a High Frequency (HF) transformer, whose control strategy is a Dual-Active Phase-Shift Control (PSC) with Square Wave Modulation (SQM). This technique is combined with a predictive algorithm, which is able to keep each leg’s capacitors’ voltages balanced both in stationary and in transient conditions. The proposed algorithm is numerically validated in the Matlab/Simulink® environment.

In this paper a solution is proposed for contra-rotating propellers in electric unmanned aerial vehicles. It is based on a direct drive system where the contra-rotating blades are directly fixed on the rotors of an axial-flux permanent magnet motor. The latter one is constituted by two external rotors and a single internal stator divided in two different sides. The study is focused on a 6 slots-4 poles three-phase motor and on a 10 slots-4 poles five-phase motor, characterized by single-layer armature windings. For each motor configuration some sets of motor sizes are found by using an automatic calculus procedure and by imposing suitable electrical, thermal and mechanical constraints.

This work deals with an optimal control strategy for a three-phase grid-tied photovoltaic (PV) cascade H-bridge inverter. The main challenge for the control of this system topology derives from the inherent power imbalance among the phases as well as from the different power levels of the H-bridge cells in each phase leg. The power imbalance among the phases is compensated by the injection of a proper zero-sequence voltage. The power distribution among the cells of each phase could be affected by uneven irradiance and temperature conditions of PVGs (PV Generators). This latter issue is addressed by means of an adaptive modulation method especially suited to manage unequal dc sources, while assuring stable circuit operation and maximizing the power extraction through a dedicated MPPT (Maximum Power Point Tracking) algorithm. The digital controller is developed and tested in Matlab/Simulink environment integrated with XSG (Xilinx System Generator), thus allowing an easy transfer on FPGA embedded in dSPACE real-time platform. Simulations and experimental results prove the validity of the proposed design and control approach.

In this article, a maximum power point tracking algorithm properly adapted for a grid­tied photovoltaic multi-level inverter is presented. The inverter structure is based on a single­phase cascaded H­bridge configuration, where each power cell is supplied by an individual photovoltaic panel. The dedicated maximum power point tracking algorithm is based on a suitable hysteresis band, which defines the proper boundaries for maximum power point tracking references to ensure both stable inverter operation and maximum photovoltaic power extraction. The inverter control method is a mixed staircase pulse-width modulation based on a sorting algorithm. Some experimental tests have been performed by using a laboratory prototype of a single­phase five­level photovoltaic cascaded H­bridge inverter, which is controlled by means of dSPACE real­time hardware platform (ds1006, dSPACE GmbH, Paderborn, Germany), where the control section is implemented on a field-programmable gate array (Xilinx Virtex5, Xilinx, Inc...

The paper deals with the efficiency evaluation of a micro-generation unit for energy recovery in water distribution networks. The analyzed system includes an electric motor-driven pump working as turbine/generator, connected to the electrical grid through a bi-directional power converter. The micro-generation unit is implemented in an experimental setup which simulates a real water network in a reduced scale. The unit was tested and analyzed in order to monitor some critical electrical quantities as well as to evaluate its generating efficiency. In the measurements campaign, various operating conditions were analyzed. The corresponding results are reported and discussed.

This paper presents a control technique for smoothing voltage oscillations in the sub-modules of Modular Multilevel Converter during start-up operations of high-power medium-voltage induction motor drive. At steady-state, the power fluctuations in each sub-module at fundamental and 2 nd harmonic frequencies generate capacitor voltage oscillations which are much higher as the current frequency decreases and tend to infinite at zero frequency. This is a problem in variable speed drives, where very low frequency values are needed during start-up. The control injects a homopolar component of the converter output voltage and an alternative high frequency circulating current, whose magnitude is much higher than the induction motor one during start-up operations. A space vector modulation technique is used in the control implementation. A numerical investigation is carried out with reference to some relevant cases-study in order to show the validity of the proposed control which allows an effective reduction of voltage oscillations on the dc-link capacitors of the different converter sub-modules.

The rail traffic in Europe is increasing in frequency. As a result of this, all the countries are updating their power supply system involving the use of high-power static converters. Moreover, the high required power implies the use of medium voltage (MV) supply chains (i.e. 15-25 kV) while the traction drives are operated at voltages of some kV. In order to adapt the two voltage levels, an on board transformer is typically used. Anyway, this transformer is characterized by high size and weight and limits the performances of the vehicle. A solution to this issue can be represented by the use of a power electronic transformer (PET) that is constituted by a double stage converter with an interposed high frequency transformer. The use of a high frequency transformer instead of the traditional at industrial frequency allows the reduction of size and weight with consequent advantages for vehicle performances. The paper presents a focus on circuital model, design and dimensioning of a PET. In particular, a topology of the converters and the relative control strategy are proposed. Moreover, the problem of core sizing is analysed and some guidelines are given. Finally, for chosen values of power and switching frequency, the sizing is executed and results of numerical simulations are given.

This work deals with the design of a three-phase grid-tied photovoltaic (PV) cascade H-bridge inverter for distributed power conversion. The power balancing among the phases must be properly addressed. In fact, an intra-phase power imbalance—arising from uneven irradiance and temperature conditions—generates a per-phase power imbalance. This latter can be compensated by the injection of a proper zero-sequence voltage, while the intra-phase balance is ensured by means of a hybrid modulation method which is able to guarantee the handling of unequal DC (Direct Current) sources, stable circuit operation, and maximization of PV power production. The digital controller is developed and tested in Matlab/Simulink environment integrated with XSG (Xilinx System Generator), thus allowing an easy transfer on a field-programmable gate array (FPGA) platform and accurately describing the behavior of a real hardware implementation. Thus, numerical results have been considered to prove the effectiveness of the proposed approach.

In this paper a power converter based on a modular multilevel topology is discussed. The Modular Multilevel Converter (MMC) is one of the emerging technology for high- or medium-voltage transformerless power conversion. The paper presents a space vector modulation technique developed for a MMC with arbitrary number of voltage levels; the presented technique integrates the modulation of the homopolar component of the converter output voltage. A numerical investigation is carried out with reference to some relevant cases-study in order to show the validity of the proposed modulation technique which allows an effective control of the converter recirculating currents.

In this paper an analysis of a power converter based on a modular multilevel topology is discussed. A mathematical model of the MMC is developed in order to evaluate its dynamic behavior and to better understand the incidence of the circulating current phenomenon which could compromise converter performance in variable speed drive applications. In fact, if such problem is not properly addressed reliability problems arise.

A fast and effective control of three-phase PWM-Rectifiers is presented in the paper, with reference to multilevel converter topologies. The presented strategy con be considered a “Direct Power Control” (DPC) because it is devoted to the direct control of active and reactive line-power, using a technique very similar to the well-known “Direct torque control” (DTC) of AC drives. An interesting aspect is the extension of the control technique to a generic m-level Voltage Source Rectifier (VSR). The converter architecture (diode-clamped, cascaded H-bridge, ...) has no influence on the proposed method. The selection of the most suitable converter voltage space-vector to follow the actual power reference values is made in each sampling interval using double-band hysteresis controllers. The outputs of these controllers allow us to quickly identify the most suitable voltage among properly ordered lists of the converter vectors, which can be dynamically built-on. The procedure is validated by means of numerical investigations in some relevant cases-study, with specific reference to the very reduced values of current distortion and asymmetry that can be obtained for fixed average value of the switching frequency.

In the last years the interest towards Power Electronic Transformers (PET) is increasing. These new conversion apparatuses perform either voltage transformation or power quality functions, using power electronics both on primary and secondary sides of a transformer operating at medium frequency. In the technical literature are proposed several circuital configurations, with different control strategies. However these are not exhaustive yet, especially if is considered that PET can be utilized in a lot of different fields of application. Simplicity, effectiveness and high-resolution of the control are important aspects which can strongly improve performance of PETs. The paper presents an overview of the main topologies of converters for PET with a Medium Frequency (MF) transformer, and the control strategies are analyzed.

An high performance control technique of dc-brushless drives is presented in the paper, with the aim to considerably reduce current and torque ripples. The proposed control algorithms are very simple and are separately pointed out for “two phase on” and “three phase on” operations, i.e. either far or during the commutation of two consequent armature phases. They can be successfully used in those dc-brushless drives which uses permanent magnet motors with any arbitrary shape of the air-gap flux-density distribution generated by rotor magnets. A magnetically isotropic motor configuration can be considered with respect to the air-gap field generated by the armature m.m.f. The smoothing torque control procedure is a “current control technique” operating in two cascaded phases. As first step it evaluates the value of the instantaneous reference current able to reduce to zero the torque oscillations magnitude. Afterwards, a simple model-based predictive algorithm computes the voltage reference values for the PWM motor supplying inverter. As in many other applications, the voltage predictive control provides better performance than PI or hysteresis controller because more homogeneous results can be yield for low and high speed (or load) operating conditions. Some cases-study are numerically investigated in order to validate the proposed control technique.

This paper deals with the control of small-power wind turbine using fixed-pitch blades. A three-phase permanent magnet (PM) brushless generator is connected to the grid through a double stage converter: active rectifier (VSR) + d.c.-link + inverter (VSI). A very simple and effective control strategy of the generation set (PM generator +VSR) is presented; it is mainly aimed to extend the wind-turbines operation range to high wind-speed region, maintaining the generated electric power constant and equal to its “rated” value. By means of this technique, the cut-off speed can be considerably increased with respect to the usual values on the market, improving the energy production of the considered mini-generation plant during the years. The numerical analysis carried out in the paper shows that the VSR has to be “reasonably” oversized, while the PM generator ratings are not modified, because the overload operations do not produce any dangerous over-heating. The improvements linked to the proposed technique are more evident when the turbine blades are properly designed and when the wind speed frequently exceeds the ordinary values. Moreover, the proposed control algorithm is also able to improve the efficiency of the wind conversion system for wind speed values lower than the rated one.

The article discusses the proposal, specifications and overall performance of a 10 kW electric power range extender suitable for electric plug-in and series hybrid vehicles, based on a single cylinder, four stroke internal combustion engine, derived from a motorcycle engine, modified and developed at Istituto Motori CNR of Italy, coupled with a PM generator. This unit has been thought as a low-cost mobile recharging system for city cars and small commercial vehicles and it's based on the required characteristics of a modern electric city car to define the reference power for the range extender. In the paper is explained the ratio which leaded to the adoption of a single cylinder internal combustion engine, followed by details of the main design characteristics of the unit, which has been assembled just with off-the-shelf elements, in order to reduce the cost coming from new design and making. The use of the air cooling for the electric generator (a permanent magnet axial unit) permitted to simplify the whole system, still allowing an easy packaging of the same for the final application on the vehicle. As field test, a complete description of functioning of the internal combustion engine and the generator has been reported; then the two sub-systems have been coupled together and finally tested as a whole system with conventional unleaded gasoline.

With reference to medium-voltage grid-connection of large photovoltaic (PV) plants, the paper proposes and analyses a high performance solution based on a power electronic transformer (PET). It Includes dc-links and multilevel converters either in low-voltage or in high-voltage side, and a medium-frequency (MF) transformer. Together with the very reduced sizes of the whole conversion apparatus, an important feature of the proposed topology is the permitted presence of unbalanced voltages on the different dc-links corresponding to the partitions of the PV plant. This is enabled by the multilevel cascaded H-bridge configuration of the low-voltage side converter, in order to take into account the nonuniform distribution either of temperature or (mainly) of solar radiation on the different cells arrays, together with eventual different kind or age of used cells. another advantageous feature is the good level of power quality indexes on grid quantities, ensured by a multilevel NPC front-end: mainly low values of either harmonic distortion, or voltage and current dissymmetry. this occurs even if the commutation frequency of the switching devices of this converter is not high (1÷2 kHz). However, the paper is mainly focused on the control technique of the multilevel converters connected to the primary and secondary windings of the MF transformer, imposing multistep- or square-wave voltages. Some simple control algorithms are presented in order to maximize the power generated by every partition of the plant (also in unbalanced operations) together with the minimization of the joule losses of the central conversion unit in quasi-stationary operating conditions.

The paper deals with control methodologies of power electronic transformers (PET) which include a single-phase medium-frequency (MF) electromagnetic transformer. Reference is made to a PET configuration having dc-links either in high-voltage or in low-voltage sides. Both primary and secondary windings of the ferrite-core transformer are supplied by a voltage-source-inverter (VSI, on high-voltage side) and by a voltage-source-rectifier (VSR, on low-voltage side) which impose square-wave voltages. The proposed control strategy has the aim to define the optimised delay between primary and secondary voltages which allows us to obtain the power instantaneously demanded by the load, ensuring fast dynamic response and maximization of the power/current ratio. The control algorithm is based on algebraic equations derived from the mathematical model of the considered system by means of proper simplifications. The used predictive technique gives good results also in four quadrants operations. An application to a traction drive is analyzed.

An optimized control of PWM voltage source rectifiers (VSR) is presented. First, in a multi-input look-up table the different converter voltage-vectors are ordered with respect to their aptitude to increase (or decrease) the active power, depending on the angular position of the input voltage vector. A simple procedure allows the quick selection of the most suitable converter-voltage which ensures the correct slope sign of both active and reactive power, in order to satisfy the respective reference values. A predictive procedure evaluates the application time of the selected voltage vector within the actual sampling interval, in order to keep the instantaneous active power inside prefixed hysteresis bands. The proposed algorithm quickly evaluates also the voltage vector to be applied in the remaining part of the sampling interval. Numerical investigations in some heavy operating conditions are carried out at a fixed average value of the switching frequency, especially with reference to current distortion and asymmetry.

The paper deals with active voltage source rectifiers (PWM-VSR) having multilevel cascaded H-bridge configuration. With reference to operations with unsymmetrical supply voltages and/or unbalanced three-phase loads, some optimized control strategies are presented and compared. Particular attention is devoted to the most effective control algorithm in terms of distortion of the input-currents and limitation of the line-current unbalancing. The algorithms are represented by a set of algebraic equations able to generate in a very simple way the actual reference values for the dynamic control of VSRs. Some heavy transient operations are analyzed in order to show the validity of the proposed techniques and the capability to limit negative influence on the line power-quality indexes by means of optimized control of cascaded H-bridge active front-ends.

The paper deals with some problems concerning fault analysis in multilevel V.S.I. converters. With reference to cascaded H-bridge topology, first the possible conditions of "single fault" are classified by analyzing the modification of output voltages when a generic power device is damaged ("short-circuited" or "definitely open"). Then a procedure for on-line quick identification of the corrupted device is explained. It is based only on output voltages and currents measurements and does not require additional sensors. The minimum number of measurements allowing fault identification is linked to the number of converter levels. Finally, failure adapted techniques of converter modulation are presented and simulation results are discussed with reference to some significant cases. These techniques have the aim to minimize the influence of the different faults on the service of the whole drive.

In the paper a mathematical model of multilevel PWM-rectifiers is pointed out. The model formulation is generalized to "m" voltage levels and is unified for neutral-point-clamped, cascaded-H-bridge, and flying-capacitors topologies. A modulation technique is described for active front-ends. Specific solutions are proposed for the different regions of the complex plane correspondent to the m-level structure of the converter. Instead of complex look-up tables, very simple relations are given in order to identify the region and to select the most effective modulation pattern. Some opportunities to conveniently use the intrinsic redundancy of the converter topology are described.

With reference to multilevel converters in cascaded H-bridge configuration, the paper presents a technique suitable to quickly identify fault conditions during normal operations. First a modulation technique generalized to m-level converters is synthetically illustrated in order to evidence the presence of different modulation patterns and the corresponding criteria of selection in normal operating conditions. Then, all the "single- fault" conditions are classified, evidencing symmetrical situations, and some consequences on the converter voltage- vector star are analysed. On this basis, a procedure is presented for on-line identification of the corrupted switching device and of the converter module where it is located. No additional sensors are requested, because the procedure is based only on the values of phase-voltages and currents already detected for the closed loop converter control. The fault identification is carried out in a few modulation intervals, also thanks to modifications automatically introduced in the modulation algorithm exploiting redundancies of different converter voltage vectors. Numerical results are presented with reference to some significant fault conditions in cascaded H-bridge inverter having an assigned number of voltage levels.

The paper is devoted to the investigation of a three-phase multilevel H-bridge PWM-Rectifier operating as a first stage of a back-to-back converter in high-power ac drives. First, a predictive control technique is described with reference to a 5-level converter. Starting from properly defined reference values, the algorithm pointed out allows the direct evaluation of the duty-cycles of all the voltage vectors involved in the VSR modulation. By means of the evaluated switching functions, at the end of every sampling interval the line-currents assume values practically equal to the imposed ones. Some unrestrictive hypotheses are introduced in order to efficaciously simplify the analytical expressions. As a second step, in the paper a specially designed modulation technique is shown. In addition to low values of dc-link voltage oscillations and of line-current distortion in steady-state operations together with high values of the power-factor, this modulation technique allows to keep balanced the different capacitor voltages, by properly using the intrinsic redundancy of the converter topology. Numerical results are presented with reference to some significant operative conditions in order to validate the theoretical considerations.

The paper focuses on an energy management strategy for an electric vehicles powered with three power sources (PEM fuel cells, LiFePO4 battery and supercapacitors). The aim of the proposed strategy is to split the power demand between the different sources maximizing the driving range and the performances of the vehicle and increasing the life cycle of the battery and of the PEM fuel cell. To accomplish load sharing an energetic model of the overall system based on experimentally validated models of the PEM Fuel Cell, LiFePO4 battery and supercapacitors is formulated and an optimal control problem with inequality constraints is resolved using the particle swarm optimization technique (PSO) to search the global near-optimum at each sample interval. The results show that the proposed strategy guarantees energy saving and assures failsafe operations for the battery and the PEM Fuel Cell unaffecting the drivability of the vehicle and increasing autonomy.