T. Scimone

Optimal design of energy storage systems for stand-alone hybrid wind/PV generators

A major shortcoming in the exploitation of renewable energy sources relies in the effective power availability, that depends from the variability of the primary energy resource over the time. Because of the variability and unpredictability of primary power both photovoltaic arrays and wind turbines cannot ensure the minimum level of power continuity required to supply in island mode a generic set of residential loads. Therefore, they must be complemented with suitable energy storage devices to form a hybrid power generation system. The design of a hybrid generation system including energy storage devices is a quite complex task. A probabilistic design approach is then proposed in this paper based on the LPSP index. Such an approach is also used to detect the most advantageous combination of wind turbines, PV plant and energy storage system, for a stand-alone generator aimed to supply a small set of domestic loads.

Optimal size selection of step-up transformers in PV plants

Step-up transformers are used to connect large PV plants to the utility network, their sizing being often accomplished only taking into account the PV plant peak power. However, a largely unpredictable power injection on the main grid is obtained if a too large rated power is selected, leading to grid instabilities. This may result in frequent plant shutdowns, while requiring a remarkable reserve power to be provided by conventional generation systems. On the other hand, a too small transformer lead to the creation of a bottleneck, preventing an optimal exploitation of the solar energy. This situation becomes more complex if the introduction of an energy storage system is considered. In the present paper a design technique is proposed to optimally select the step-up transformer, either on conventional PV plants, either on PV plants with energy storage. It is based on the evaluation of initial and operating costs. Moreover, the effects of induced network instabilities are also considered. Taking into account full life costs optimal solutions have been detected according to the network power control capabilities for a 2 MW PV plant.

A buck-boost based DC/AC converter for residential PV applications

The single stage architecture is often preferred to realize grid connected Photo-Voltaic generators for residential applications. In fact,by halving the power processing steps, the power losses can be considerably reduced if compared with those of a two stage configuration. However, single stage power conditioners are generally unable to boost the voltage of the PV modules, therefore, they feature a narrower input voltage range. A new approach to design a power conditioner for grid connected PV plants for residential applications is presented in this paper. It is based on the series connection between a buck-boost converter,and the PV array. According to such an approach, the buck-boost converter processes only a fraction of the inverter input power, while always operating in conditions of high efficiency. Experimental tests show that the proposed solution features an efficiency level very close to that of a single stage power conditioner, but owns a voltage boosting capability, similar to that of a two stage configuration.

A new approach to improve the current harmonic content on open-end winding AC motors supplied by multi-level inverters

In open-end winding AC motor drives, terminals of stator phases are normally connected between two inverters, normally, of the same type and size. In the present paper, an alternative configuration is studied where the machine is supplied exploiting two different converters: a main multi-level inverter and an auxiliary two-level inverter. The last operates as active power filter in order to eliminate undesired low-frequency voltage harmonics produced by the multi-level modulation process. Furthermore, the two level inverter is operated at a rather low voltage, thus it can be equipped with power MOSFETs. Consequently, a voltage PWM at a remarkably higher frequency is allowed, if compared with standard multilevel inverters. According to the proposed approach, the phase current harmonic content is remarkably reduced, the torque ripple is mitigated while the drive efficiency is increased. Numerical simulations and experimental results confirm the consistency of such new methodology. A particular emphasis is given to stator current harmonic content and power losses analysis as well as to specific implementation issues.

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.

THD and efficiency improvement in multi-level inverters through an open end winding configuration

A new approach, based on a current controlled Asymmetrical Hybrid Multilevel Inverter (AHMLI), is proposed to reduce power losses and output current distortion on Multi-Level Inverters (MLI) used in AC motor drives, STAT COM devices, Photovoltaic and Wind generators. A key feature of the proposed approach is that the AC machine (motor or transformer) is operated in an open-end winding configuration, being supplied on one end by a main Multi-Level Inverter and, on the other side, by an auxiliary Two-Level Inverter (TLI). The MLI controls the main active power stream; it operates at a low switching frequency and can be equipped with very low on-state voltage drop IGBT devices. The auxiliary TLI is instead operated according to a conventional high frequency two-level PWM technique and acts as an active power filter providing only a null-average power to the AC machine. As the DC bus voltage of the TLI is remarkably lower that that of the main MLI, the auxiliary inverter can be equipped with low switching losses IGBTs, or even Power MOS devices. Simulations and experimental results confirm that using the proposed approach, the phase current harmonic content is remarkably reduced, the efficiency is increased and in motor drive applications, the torque ripple is mitigated.

A NPC transformerless single phase inverter with inner voltage boosting capability

Thanks to their characteristic quiet rail switching mechanism, derivatives of the basic half bridge converter, such as the Neutral Point Clamped topology, are advantaged among single stage, single phase, inverter configurations for PV applications by very low ground currents. On the other hand, as the majority of single stage inverters, they are unable to boost the input voltage, therefore featuring a narrow operating range. Moreover, if compared with full bridge derived topologies, half bridge derivatives require a double input voltage. A new inverter configuration, based on a three level Neutral Point Clamped configuration, owning an inner voltage boosting capability is presented in this paper. No extra high frequency switching devices are introduced, thus featuring reduced power losses. Moreover, no reactive currents flow back to the DC bus, due to the inner AC side bypass and no common mode current filters are required, thanks to the quite rail switching operation.

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.

An Open-end Winding approach to the design of multi-level multi-motor drives

An Open-end Winding approach to the design of multi-motor drives exploiting multi-level inverters is presented in this paper. A key feature of the proposed approach is that a single multi-level inverter is tasked to provide the entire active power required by the system, while a two-level inverter acts as a power filter, providing a zero average active power. Moreover, a staircase voltage modulation technique is adopted on the multi-level inverter in order to minimize the switching losses, while the two-level inverter is PWM operated to accurately shape the motor currents. The proposed solution produces a much lower stator current distortion if compared to a conventional MLI switching at the fundamental frequency, while it requires less power switches and generates lower power losses than a PWM multi-level inverter featuring the same THD. Experimental results obtained on Multiple Motor Single Converter and Multiple Motor Multiple Converter systems are provided in order to confirm the consistence of the proposed approach in terms of efficiency maximization and motor voltage THD minimization.

Overvoltage mitigation in open-end winding AC motor drives

An over-voltage mitigation technique for open-end winding electrical drives with long power cables is proposed in this paper. In particular, a suitable dwell time is included in the switching patterns of the two inverters to minimize the over-voltage occurring at the stator terminals of this kind of systems. No passive RLC networks are required, thus avoiding additional costs and extra power losses. The proposed approach is first theoretically introduced, then validated by numerical simulations and experimental tests. Finally its effectiveness is compared with that of conventional passive RC filters.