Salvatore De Caro

An active current ripple compensation technique in grid connected fuel cell applications

Fuel cell stacks represent a quite promising technology for the realization of small electrical generators for distributed generation. Key advantages of fuel cells over combustion based generators are the absence of polluting emissions and a high efficiency even on small size systems and partial loads. A typical application of distributed generators is in the residential field, where they are connected to single phase AC grids. In these cases a power unbalance is generated because the power drawn by the grid features a large ripple at twice the fundamental frequency, while the power produced by the stack is essentially constant. A large capacitor bank is then used to solve such a power unbalance, that however causes a ripple on the stack output current. This ripple is shortens the fuel cell lifetime, to worsen the stack efficiency and to reduce the available peak power. Active current ripple reduction techniques have been proposed in the past exploiting expensive active filters. An alternative approach is proposed in this paper where an active filter is integrated into the converter, without introducing extra power devices. Moreover, the auxiliary power supply is integrated in the main converter partially compensating the power unbalance. According to the proposed approach the size of the electrolytic capacitor bank can be largely reduced, thus improving the reliability, while lowering costs and size.

Active voltage ripple compensation in PV Systems for domestic uses

Photo Voltaic (PV) generators today find application in the domestic field, where the decentralized power conversion concept has proven to be more advantageous than the conventional string architecture. According to the decentralized power conversion concept, PV modules are connected to a single phase AC grid through a two stage converter composed of a DC/DC converter and an inverter. The DC/DC converter accomplishes the Maximum Power Point Tracking (MPPT) and boosts the voltage. It must also include a suitable capacitor bank to solve the unbalance between the DC power generated by the modules and the AC power drawn by the grid. Such a power unbalance in fact generates a voltage ripple that negatively affects the MPPT, forcing the system to work only around its optimal operating point. On the other hand, electrolytic capacitors are highly disliked in PV systems for reliability reasons. Therefore, active current ripple reduction techniques have been proposed in the past, exploiting active filters that increase the complexity and the cost of the system. An alternative approach is proposed in this paper where an active filter is integrated into a Double Interleaved Boost Converter, without introducing extra power devices. According to the proposed approach the size of the electrolytic capacitor bank can be largely reduced, thus improving the reliability and the efficiency of PV systems.

Current-Sharing Strategies for Fault-Tolerant AC Multidrives

Thanks to an inner redundancy, AC Multi-Drives can be made fault tolerant with minor modifications of their structure. However, some additional components are generally required in order to create emergency current paths in case of a fault. According to a recently presented approach the amount of additional components can be reduced by exploiting healthy drives to support faulty drives. In this case current and power sharing among drives assumes a key role. The paper deals with some new current sharing methodologies for AC multi-drive systems, where a suitable interaction among healthy and faulty drives is exploited, in order to cope with one or more faults. Specifically, three different current sharing control methods are presented and evaluated by means of simulations and experimental tests, taking into account the effects of operations in fault conditions both on healthy and faulty drives.

An Open-End Winding Motor Approach to Mitigate the Phase Voltage Distortion on Multilevel Inverters

An open-end winding machine configuration and a suitable control strategy for medium-voltage ac motor drives applications are proposed in this paper in order to reduce the distortion of phase voltages in multilevel inverters (MLIs). Differently than standard open-end winding configurations, where two inverters provide active power to both sides of the stator winding, a main MLI supplies in this case the machine on one side, while an auxiliary two-level inverter acts as active power filter on the other side. A high efficiency step modulation manages the MLI, while the auxiliary unit is pulse width modulation (PWM) operated. As the phase current harmonic content is improved, the torque ripple is reduced and the drive efficiency is increased. By exploiting the proposed approach, the apparent switching frequency of the system is that typical of a PWM inverter, although the main unit is operated according to a voltage step modulation strategy. Simulation and experimental results confirm the consistency of the proposed methodology.

Reliability Assessment of Power MOSFETs Working in Avalanche Mode Based on a Thermal Strain Direct Measurement Approach

A strong demand of even more compact and reliable power electronic devices has powered in the last years the development of advanced device design techniques. A key role in these techniques is played by the reliability assessment, a procedure that estimates the expected lifetime of power devices according to given mission profiles. The reliability assessment of a low voltage MOSFET working in avalanche mode is faced in this paper through a new experimental approach based on the Coffin Manson law and a direct measurement of the thermal strain over the Source Aluminum layer. The consistence of the proposed technique is evaluated by comparing obtained estimations of the progressive increment of the on state resistance with estimations carried out from other reliability models and endurance tests results. The described approach can be usefully applied to assess the reliability of MOSFETs in applications typical of the automotive field were power devices are tasked to operate in avalanche mode, such as: brake pump drivers, electromagnetic valve control, direct high-pressure injection, starter-alternator and active suspension systems.

A solar AC module with active filter capabilities

Small photo-Voltaic generators for residential applications are rapidly spreading all over the world in an effort to reduce polluting emissions. In this kind of applications the AC solar module concept has recently proven to be more advantageous than the conventional string architecture. According to the AC solar module concept, single PV modules are directly connected to the AC grid through a DC/DC converter and an inverter. However, in this case a periodical power unbalance is generated because the power drawn by a single-phase residential grid features a large ripple at twice the fundamental frequency, while PV modules generate a DC power. Such a power unbalance affects the Maximum Power Point Tracking and forces the system to operate under suboptimal conditions and is faced with the introduction of bulky electrolytic capacitors. In this paper a alternative approach is presented enabling a standard DC/DC converter to also act as a power ripple damper, without the introduction of extra power devices. A hybrid PWM/unfolder approach has been also developed to reduce the switching power losses of the inverter connecting the DC/DC converter to the grid.

Full-frame infrared thermal imaging of power electronics devices by means of multiple time-delayed measurements

The paper presents a method for mapping the temperature distribution of very fast transient events (i.e. having a bandwidth of 10 kHz or more) by means of a standard infrared camera working at 25 Hz frame rate with 320 × 256 pixels full frame. The proposed method is based on triggering multiple time-delayed acquisitions of the observed thermal phenomenon, which must be periodic, by means of a very precise and stable programmable digital micro-controller and by reconstructing the time domain IR sequence using the frames acquired at each trigger event. The measurement accuracy of the reconstruction process has been assessed by using a laser-cut planar resistor cyclically heated by means of a pulsed electrical current. A practical application to investigate the temperature distribution over the source metal of a power MOSFET device is finally presented.

Pulse Counting Sensorless Detection of the Shaft Speed and Position of DC Motor Based Electromechanical Actuators

Some of DC actuators used in home automation, office automation, medical equipment and automotive systems require a position sensor. In low power applications, the introduction of such a transducer remarkably increases the whole system cost, which justifies the development of sensorless position estimation techniques. The well-known AC motor drive sensorless techniques exploiting the fundamental component of the back electromotive force cannot be used on DC motor drives. In addition, the sophisticated approaches based on current or voltage signal injection cannot be used. Therefore, an effective and inexpensive sensorless position estimation technique suitable for DC motors is presented in this paper. This technique exploits the periodic pulses of the armature current caused by commutation. It is based on a simple pulse counting algorithm, suitable for coping with the rather large variability of the pulse frequency and it leads to the realization of a sensorless position control system for low cost, medium performance systems, like those in the field of automotive applications.

Reliability assessment on power MOSFETs working in energy absorption mode

The on state resistance of power MOSFETs tasked to perform repetitive avalanche operations is subject to modifications caused by the growth of voids and cracks in the source metallization. Endurance tests are the traditional way to monitor these changes in order to assess the device reliability. However, they are very time expensive, requiring even months of uninterrupted operations. An interesting alternative consists in the assessment of the reliability through a suitable model, but no standard techniques have been developed up to now to accomplish this task. A possible approach is followed in this paper exploiting a dynamic analysis of the temperature distribution over the source metal. Coupling the thermodynamic analysis with a reliability model, carried out from the Coffin-Manson law, the device degradation over the time can be estimated and the level of reliability as well. The consistence of the obtained reliability prediction is confirmed by comparison with results of endurance tests. The described approach can be usefully applied to assess the reliability of MOSFETs in a large set of applications in the automotive field.