Carlo Concari

A Nine-Level Grid-Connected Converter Topology for Single-Phase Transformerless PV Systems

This paper presents a single-phase transformerless grid-connected photovoltaic converter based on two cascaded full bridges with different dc-link voltages. The converter can synthesize up to nine voltage levels with a single dc bus, since one of the full bridges is supplied by a flying capacitor. The multilevel output reduces harmonic distortion and electromagnetic interference. A suitable switching strategy is employed to regulate the flying-capacitor voltage, improve the efficiency (most devices switch at the grid frequency), and minimize the common-mode leakage current with the help of a novel dedicated circuit (transient circuit). Simulations and experiments confirm the feasibility and good performance of the proposed converter.

Differential Diagnosis Based on Multivariable Monitoring to Assess Induction Machine Rotor Conditions

Multivariable supervision systems for online monitoring of induction motors allow large versatility and diagnosis robustness. As regards rotor faults, the diagnostic procedure based on sideband current components may fail due to the presence of interbar currents that reduce the degree of rotor asymmetry and, thus, the amplitude of these spectral components. On the other hand, the interbar currents produce core vibrations in the axial direction; these vibrations can be detected using a suitable vibration sensor. In this paper, a differential fault analysis based on traditional motor current signature analysis (MCSA) and on radial and axial vibration monitoring is proposed to discern cases in which the presence of interbar currents decreases the sensitivity of MCSA. The features of stator currents and of radial and axial core vibration signals are investigated in order to increase the reliability of the diagnostic system. Moreover, to explore the possibility of obtaining further information, stray flux signals are taken into account.

Active Common-Mode Filter for Ground Leakage Current Reduction in Grid-Connected PV Converters Operating With Arbitrary Power Factor

This paper proposes a solution for reducing the ground leakage current in transformerless single-phase grid-connected photovoltaic converters. This is obtained with the introduction of an active common-mode filter able to compensate for variations of the output common-mode voltage of the power converter. The active common-mode filter is applied to a widespread and efficient full-bridge driven by a three-level pulse width modulation, allowing the power converter to operate with low ground leakage current and with an arbitrary power factor. After showing the desired voltage waveform for common-mode voltage compensation, this paper presents the design guidelines for the needed additional magnetic component together with the power loss considerations for all the devices added for the proposed solution. Experimental results show the performance of the proposed solution in terms of ground leakage current reduction, effectiveness of dead-time compensation, total harmonic distortion of the injected grid current, and power losses.

Thorough Understanding and Experimental Validation of Current Sideband Components in Induction Machines Rotor Monitoring

The MCSA diagnostic procedures for the rotor diagnosis of induction machines are mostly based on the sideband lines near the supply frequency line that appear in the input current spectrum. The left component (1-2s)f is the effect of rotor backward rotating field caused by the cage rotor asymmetry and it varies with the asymmetry degree and with the rotor current, i.e. with the machine load. The right component (1+2s)f is caused by the speed ripple created by the left component, being the combined machine-load inertia the main parameter that spreads the asymmetry effect into the two sideband components. Theoretically it can be shown that the sum of the two components is not affected by the speed ripple effect and therefore this sum may be used as an effective diagnostic index to state rotor conditions. The above results can be experimentally validated only once a suitable test set-up is realized that allows changing the inertia value. To this aim a test bed was designed so that the virtual inertia of the mechanical system can be chosen by the user. This allows to prove the theoretical claims and to obtain a thorough understanding about the effect of inertia on sideband components, improving the performance of rotor condition monitoring techniques

Mixed-Mode PWM for High-Performance Stepping Motors

This paper describes the realization of a high-performance drive based on stepping motors, with a specific reference to the optimal choice of pulsewidth modulation. An original modulation technique is proposed that allows, in conjunction with current feedback, to achieve high performances together with a reduced current ripple. A prototype digital signal processor-based drive was realized for a small-size stepping motor.

EV battery state of charge: neural network based estimation

Different electric vehicles (EV) types have been developed with the aim of solving pollution problems caused by the emission of gasoline-powered engines. Environmental considerations promote the adoption of EV for urban transportation. As it is well known one of the weakest points of electric vehicle is the battery system. Vehicle autonomy and therefore accurate detection of battery state of charge are among the main drawbacks that prevent the spread of electric vehicles in the consumer market. This paper deals with the analysis of battery state of charge: performances of a few sizes of batteries are analyzed and their state of charge is estimated with a neural network (NN) based system. The obtained results have been used to design a lithium-ion battery pack suitable for electric vehicles. The proposed system presents high capability of energy recovering in braking conditions, together with charge equalization, over and under voltage protection. Moreover a neural network based estimation of battery state of charge has been implemented in order to optimize autonomy instead of performances or vice-versa depending on journey.

Vibrations, currents and stray flux signals to asses induction motors rotor conditions

Multi-variable monitoring systems for on-line monitoring of induction motors allow larger versatility and robustness. In case of rotor faults, the diagnostic procedure based on sideband current components may fail for the presence of inter-bar currents that reduce the amplitudes of these components. On the other hand the inter-bar currents modify the core vibration; therefore the analysis of both line currents and core vibration signals may increase the effectiveness of the diagnostic procedure. In this paper the features of stator currents and core vibration signals are investigated in order to achieve an accurate and reliable diagnostic system. Moreover stray flux signals are considered to explore the possibility of obtaining further information

Toward Practical Quantification of Induction Drive Mixed Eccentricity

As far as industrial applications of induction drives are concerned, one of the most critical issues is the statement of acceptable mixed eccentricity degree and of other mechanical abnormal conditions. These troubles cause characteristic spectral components in several drive quantities, but for the sake of economy, monitoring the electrical quantities involved in drive control is preferable. Among them, current signals (i.e., motor current signature analysis) are the signals of choice for assessing the machine conditions, although no analytical link between the current signature and the eccentricity degree through machine parameters has ever been presented. Each trouble produces spectral lines at typical frequencies in the stator current: monitoring them is a usual procedure, but the threshold to discern acceptable intrinsic defects from alarming situations is hardly defined. This paper presents the formulation of a mixed eccentricity index developed through the interaction between static and dynamic eccentricities on stator windings and rotor cage. The index can be computed by a no- or mild-load test without spectral resolution problems, and it does not require any preliminary knowledge of machine health conditions.

Different Procedures for the Diagnosis of Rotor Fault in Closed Loop Induction Motors Drives

This paper deals with different procedures that can be applied to diagnose the severity degree of rotor faults in closed loop controlled induction machines. A solution based on field oriented control (FOC) was investigated in order to validate the robustness of the diagnostic procedures towards the regulator parameters. Different diagnostic procedures were investigated, that rely on different diagnostic indexes. Specifically a few electric quantities are considered as possible fault alert signals: the spectral analysis of manipulated and controlled variables, the voltage homopolar components (voltage zero sequence), and the spectral analysis of motor input current due to a high frequency voltage injection. It turns out that the last quantity is a simple and effective index able to detect and quantify the fault severity degree. The procedures based on the above mentioned quantities are validated by experiments made with a DSP based current controlled voltage source inverter (CCVSI) supplying an induction machine with three rotors in different health conditions.

Performance Evaluation of a Three-Level ANPC Photovoltaic Grid-Connected Inverter With 650-V SiC Devices and Optimized PWM

Photovoltaic (PV) energy conversion has been on the spotlight of scientific research on renewable energy for several years. In recent years, the bulk of the research on PV has focused on transformerless grid-connected inverters, more efficient than traditional line transformer-based ones, but more critical from a power quality point of view, especially in terms of ground leakage current. Neutral-point-clamped (NPC) inverters have recently gained interest due to their intrinsically low ground leakage current and high efficiency, especially for MOSFET-based topologies. This paper presents an active NPC (ANPC) topology equipped with 650-V silicon carbide (SiC) MOSFETs, with a new modulation strategy that allows to reap the benefits of the wide-bandgap devices. An efficiency improvement is obtained due to the parallel operation of two devices during the freewheeling intervals. Simulations and experimental results confirm the effectiveness of the proposed converter.