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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.

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 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.

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.

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.

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.

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.

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.

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.

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 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.

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.