Search

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 power extracted from PV arrays is usually maximized using maximum power point tracking algorithms. One of the most widely used techniques is the perturb &amp, observe algorithm, which periodically perturbs the operating point of the PV array, sometime with an adaptive perturbation step, and compares the PV power before and after the perturbation. This paper analyses the most suitable perturbation step to optimize maximum power point tracking performance and suggests a design criterion to select the parameters of the controller. Using this proposed adaptive step, the MPPT perturb &amp, observe algorithm achieves an excellent dynamic response by adapting the perturbation step to the actual operating conditions of the PV array. The proposed algorithm has been validated and tested in a laboratory using a dual input inductor push-pull converter. This particular converter topology is an efficient interface to boost the low voltage of PV arrays and effectively control the power flow when input or output voltages are variable. The experimental results have proved the superiority of the proposed algorithm in comparison of traditional perturb &amp, observe and incremental conductance techniques.

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.

The paper deals with the vector control of two surface-mounted permanent-magnet (PM) synchronous-motors in configuration “single-inverter dual-motor”, operating also with unbalanced loads. Since this structure is justified by the objective of obtaining low cost drives, a low computation burden sensorless method is proposed in order to further reduce the system complexity. A very simple algorithm for position/speed estimation coupled with a hybrid adaptive Phase-Locked Loop is indeed sufficient to achieve satisfactory results also in heavy transient operations. Furthermore, a novel control strategy is conceived with reference to an “equivalent” motor, whose rotor position is the average of the separately evaluated positions of the two PM motors. The proposed control technique is validated by an extensive numerical campaign.

Over-temperature occurring in silicon solar panels, subject to partial shadowing, are reduced by means of a series connected voltage divider which sustains part of the reverse voltage developing across the shaded solar cells. The circuit is based on a power MOSFET which is driven by the output voltage of the solar panel. When the solar panel is uniformly illuminated the MOSFET is in the ON state and does not affect the operation of the module, while, when the solar panel is partially shaded, and the output voltage decreases, the MOSFET enters in the pinch-off operation mode, thus subtracting its voltage from the reverse voltage across the shaded cell. Experiments performed on a commercial solar panel showed one third of reduction of the reverse voltage across shaded solar cells, with a reduction of the hot spot temperature of about 25°C.

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.

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.

With reference to a strong variable parameters AC brushless machine, the paper deals with an online method of fault detection for closed loop controlled PMSM. The proposed technique exploits the monitoring of the motor d,q axes inductances via a Recursive Least Square Algorithm (RLS). The motor parameters, which have been pre-mapped with reference to the d,q axes current components, are compared to the results of the online identification procedure and their discrepancy is evaluated in real time. A fault indicator index is derived and its behavior conditioned in order to properly recognize any anomalous condition. The proposed method is tested via numerical simulations on the basis of a real PM brushless machine whose parameter have been evaluated in different operating conditions by means of several off-line experimental measurements.

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.

The diffusion of distributed electric generation sources is a growing phenomenon, encouraged by the changes in the electric energy market and the concerns about gas emission. Microgrids are composed by different types of generators, several loads and power converters necessary to interface all the microgrid components and the microgrid itself with the main AC grid. This paper presents a new flexible power converter capable of changing its topology by means of static switches so that it can provide different conversion functions using the same hardware. This proposal aims to reduce the number of components, enhancing compactness and simplicity, and resulting in lower equipment costs. The proposed converter is conceived for domestic microgrids, is connected to the AC grid and a renewable energy source (RES) and is configurable as a single-input dual-output (SIDO) converter, providing two different DC voltage levels. After showing all the possible configurations of this converter, the analysis focuses on SIDO, deriving its mathematical model and discussing a procedure to design its elements. The controllability of the SIDO is tested via numerical simulations showing good performance even during fast transient conditions.

In recent years lithium iron phosphate batteries are spreading because of their high energy and power density, long lifetime and high inherent safety. In technical literature different models have been proposed to characterize these batteries. These models have different complexity but give results of different accuracy. In this paper a comparison of three equivalent models is performed. The comparison is made taking into account the impedance of the cells as function of the working frequency. For this reason electrochemical impedance spectroscopy method is used. The real and imaginary parts of the impedance calculated at each frequency are compared with that resulting from experimental tests in order to understand which are the frequency limit of each model. From the analysis clearly results that the traditional simple models obtained with 2 or more RC branches cannot represent the behavior of these batteries in all the frequency range. On the contrary, the models presenting a Warburg impedance, even if more complex, can represent the frequency behavior of these batteries in a wider frequency range.

Typical operating conditions of permanent magnet synchronous motors correspond to nonlinear relationships between fluxes and currents. The determination of equivalent motor parameters is a non trivial task and can be critical for the design of motor controllers, which is typically based on a mathematical model. In this paper an experimental method to determine the flux-current nonlinear maps and a model based approach to implement a predictive controller capable to deal with such nonlinearities is presented. Numerical results show that the optimization technique is able to provide the maximum torque per ampere tracking.

In the paper is discusses the project, the design and overall performance of a micro-cogenerator prototype of 2.0 kW of electric power and about 5.0 kW of thermal power, suitable for single off-grid applications and fuelled by gas produced from biomass anaerobic digestion. This unit is based on a previous prototype designed and developed at Istituto Motori CNR of Italy and it has been conceived specifically as a local individual off-grid energy platform, born to be mated with small biomass reactor, which need part of the heat produced by the internal combustion engine to have a stable and efficient behaviour. The adopted internal combustion engine maintains many technical solutions and constructive elements already experimented in previous prototypes, the unit has been modified in order to be fuelled by natural gas with partial methane content, or with LPG when necessary. The generation system is a permanent magnet generator fed by a low cost inverter which can offer very high efficiency and a reasonable bulk and weight and gives to possibility to include an electric energy storage to compensate service discontinuity. The proposed cogeneration unit has in cost reduction, durability, low noise/vibration and a good electric efficiency the most important achieved objectives. It is very flexible in terms of gas supply and in respect of the electric needs of a generic utility, representing a very smart way for energy conversion in many situations when electricity couldn't be easily produced because of fuel availability and a cheap way to reach the full utilization of fuel energy together with a significant CO2 reduction.

The paper presents a mathematical model of Permanent Magnet AC brushless machines (PMSM) having a number of stator poles-obtained via the distribution of the three-phase winding- different from the rotor one, realized by the displacement of the permanent magnets. For these so- called ‘Fractional-Slot’ motors (where few total slots generate high equivalent number of poles) the available mathematical models appear partial and/or incomplete, despite of numerous designing considerations made in literature. In this paper an analytical approach is developed to point out an instantaneous- values mathematical model with concentrated parameters, that can be suitable for control architecture systems when some simplification hypotheses are added. The model takes into account a general distribution of the armature magnetomotive force field, and the total harmonic content of the rotor flux density. The whole MMF harmonic content is then properly reduced and the result is compared to a traditional PMSM model

The paper focuses on an innovative energy management control strategy of a wayside supercapacitor (SC) applied in DC electrified light transit networks. In order to achieve minimum energy demand from the electric feeding substations, the target of the control is the optimal tracking of the storage device voltage subject to the minimization of the mean square of the supply currents and, consequently, the reduction of power losses along the line. Hence, optimization theory has been used for the analytical determination and implementation of real time control strategy. Therefore the proposed strategy has been verified and validated via numerical simulations with reference to realistic LRV operation and different headway among the operating vehicles. A comparison with a classical energy storage system (ESS) state of charge control has been also carried out, as well as the energy saving improvement has been highlighted and the energy storage system integration and challenges are addressed. The results validate the theoretical approach and prove that the suggested control strategy is quite effective, putting in evidence its potentiality and the possibility of an actual implementation.

Nowadays, fuel cells are the most promising source of energy for stationary applications. Unfortunately, the power density of fuel cells is not adequate for modern applications. A fuel cell - based hybrid power supply is required. The fuel cell is coupled with a high-energy density power source to improve performances of the composite power source. A power management stage is introduced between the two basic sources to control the power flow path from the fuel cell to the auxiliary source and the load. In order to avoid effects of a limited power density of the fuel cell on the whole power supply, sophisticated power management algorithms are implemented. Accurate modeling is required to test hybrid source performances as closely as possible to the actual working conditions since the simulation step, avoiding risks of experimental failure. In this paper, a fuel cell - battery hybrid power supply system for household appliances is presented. The power system is implemented in PSIM environment, the house model is implemented in MATLAB environment and MATLAB co-simulation is provided to test the whole composite system. Simulation results are shown to test the efficiency of the novel approach.

Power quality depends generally on the interaction of electrical power with electrical equipments. If electrical equipments operate correctly and reliably without being damaged or stressed, a suitable level of power quality is assured. On the other hand, if the electrical equipment malfunctions, is unreliable, or is damaged during normal usage, power quality is poor and probably the economical loss could be important like the technical one. In the scenario of the Distributed Generation, power quality issues will be moreover important because an higher dissemination of power conditioning equipment will be requested and this obviously increases the sources of vulnerability of the electrical system. In this paper fault tolerant power converters are considered as a viable solution of power quality problems and a suitable control algorithm of them is presented. The control proposed in the paper is based on the model of the power converter reformulated in terms of healthy leg binary variable and the paper shows how this control is able to save the aspect of power quality when the converter works in the linear range. The effectiveness of such an algorithm and of the fault tolerant power converters are finally verified by means of simulations.

As Vehicle-to-Grid (V2G) concept convenes strong interests among automakers, electric utility owners, and policy makers, it is critical to investigate the potential impacts of V2G on the electric distribution system. Such study, together with other benefits of V2G, may encourage utility support for V2G commercialization. This paper focuses on a pressing technical issue associated with the penetration of V2G into the electric distribution network-the energy loss in the feeders. Mathematical models are formulated to investigate how V2G reactive power injection and different load patterns can impact energy loss on a distribution system. It is shown that loss reduction can be greatly influenced by reactive loading factor, the pattern of loading, the amount of V2G reactive power injection, and the position of V2G parking lot on the feeder segment. Useful results are obtained, with a promise that more than 95% energy loss reduction (relative to energy loss in the system without V2G installed) is possible by optimally locating a V2G parking lot along the feeder. The results in this study may serve as a nomogram to determine energy loss at the planning stage of installing a V2G parking lot on a distribution feeder segment.

The electricity market and environmental concerns, with wide utilization of renewable sources, have improved the diffusion of distributed generation units changing the operations of distribution grids from passive networks to microgrids. A microgrid includes a cluster of electrical loads, energy storage devices and microsources, which provide both power and heat to their local area. A microgrid has usually one connection point to the utility grid through power electronic converters placed at customers’ sites. This paper analyses a Reconfigurable Integrated Converter (RIC) used for a domestic microgrid with inputs from the AC mains and photovoltaic arrays, and two DC outputs at different voltage levels. A RIC as a dual-boost DC-DC converter is proposed, modelled and analysed in the paper. The advantages of such a topology in comparison with traditional boost converters are outlined. Reported simulations results give evidence on the controllability of this converter and the capability of achieving the desired voltage outputs with reduced ripple.

The paper discusses the concept and the overall performance of a 20 kW micro-cogeneration unit, suitable for distributed energy conversion and based on a 4-cylinder, supercharged methane fuelled, spark ignited, internal combustion engine (ICE). The latter is coupled to a three-phase PM-brushless electric generator, whose armature is controlled by an active front-end [12,14]. The ICE is derived from a widespread Diesel unit, which has been properly modified; it is chosen among many other engines as a reliable, very high efficiency, cost effective unit [6], suitable for energy conversion systems (such as residential or commercial applications). The paper starts by defining the criterion which led to the adoption of a high brake mean pressure, in order to produce the above mentioned electric power. After the description of the solutions adopted to optimize the ICE operation performance, a very simple and efficacious control strategy of the generator+converter set is presented. This technique is able to conveniently reduce the magnitude of shaft-torque ripple produced by a high brake mean pressure (and consequently cycle peak pressure), in order to reduce vibrations, noise and rotating speed oscillations. The control is based on assigned curves of ICE torque vs. shaft angular position. Some considerations are also made to define the right generator sizing, in order to allow an efficacious damping action, maintaining good efficiency levels of the generator+converter set. The effectiveness of the control is validated by a series of numerical results in significant operating conditions.

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 anisotropic permanent magnet (PM) brushless motors, the papers deals with a procedure for on-line identification of electrical parameters, based on the “recursive least square method”, designed for the cases of strong parameters variation depending on the operating condition. The identification procedure is superimposed to a conventional control diagram of PM AC-brushless drives (either torque-control or speed-control) and is really very simple. The results of this analysis can be used as identifiers of failure conditions in the drive; otherwise, during motor characterization on a test-bench, the method can be a simple instrument to map motor parameters in different operating conditions. The proposed technique is tested by means of numerical investigation on a motor whose electrical parameters have been off-line evaluated by experimental measurements and mapped in different operating conditions.

The paper deals with isotropic PM-brushless drives in configuration “single-inverter, dual-motor” operating with unbalanced load-torques. After a short review of some control techniques based on the classical control methodologies of single-motor PM drives, an innovative control algorithm is presented. It is suitable to minimize the resultant armature current needed to obtain an assigned resultant motor torque, whatever is the load unbalance. Simplified analytical expressions are given in order to quickly evaluate optimized reference currents with good approximation. From these reference values, a predictive feeding algorithm evaluates the reference voltage space-vector for the inverter supplying the two motors in parallel. Current and torque oscillation, torque/current ratio, dynamic response and stability are the mainly observed quantities. Effectiveness of proposed control technique is highlighted.