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The degree of complexity of the Integrated Power Systems (IPS) for All Electric Ships (AESs) promotes the need to review the design process from a dependability perspective. This paper suggests the application of dependability concepts and definitions to demonstrate adherence to requirements set forth by recent rules and regulations as regards to threat levels for shipboard electrical equipment. Guidelines on 1) how to properly decompose the IPS into conceptual subsystems; 2) how to define appropriate mathematical models suitable to investigate such sub-systems in a quantitative fashion using RBD (Reliability Block Diagram) and DRBD (Dynamic Reliability Block Diagram); 3) how to define the threat level the IPS (or the sub-system object of the study) should be tested against to prove its dependability are given. Such guidelines and investigation methods will be then applied on a part IPS of a recently built cruise vessel IPS, to illustrate their application to a real case study. A small sub-system will be decomposed according to guidelines, down to a level matching typical ship design practice, defining entities here called “atoms”. A model, showing interactions among atoms, will be prepared applying the RBD and DRBD techniques. Threats and associated performance levels will be derived from relevant international rules and regulations in force, and “translated” into operations with atoms and “propagation path lengths” within the model; acceptance criteria will, in this framework, be rendered as accept/reject conditions. Considerations regarding the adopted decomposition and applied techniques will be given in the conclusions.

PM brushless DC (BLDC) motors should be fed by square-wave currents synchronized with the back emfs to develop a constant torque with the required value. In practice, phase inductances and limited source voltage produce non ideal square-wave currents and unbalanced phase currents during commutations. Such phenomena affect the torque developed by the PM BLDC drives by deviating its value from the requested one and by giving rise to torque ripple. In this paper, an analytical study of the torque characteristics, namely motor torque and torque ripple, of PM BLDC drives is carried out as a function of the motor speed and is supported by experimental data. Torque characteristics are expressed in terms of a quantity distinctive of the motor, thus giving a generalized formulation to the results.

The integration of power systems has been successfully implemented onboard cruise ships built by Fincantieri in the last twenty years. The experience gained is now conveyed also in the application of this principle in the next naval vessels and in the ‘mega yachts’ projects. In this paper two philosophies of integrated power system designed by the company are presented. The next steps for innovative solutions that are actually studied by the shipbuilder further to new regulations requirements, energy saving aspects and utilization of PWM converters are evidenced. In this scenario an important role is played by the cooperation with the University of Trieste, whose research group is developing a software simulator of the integrated power system and propulsion drives. This paper focuses on two particular tasks of this study: modelling/simulation of propulsion drives and stability analysis and control systems involved into the regulation of electro-mechanic quantities of the integrated power system.

With direct torque control (DTC), the stator flux magnitude and the torque of an induction motor (IM) drive are controlled in a closed-loop way. The actual values of the controlled variables are estimated by processing the inverter dc-link voltage and the stator currents by means of the IM voltage model. Any error in the model parameter (the stator resistance) or in the voltage and current transduction degrades the drive performance. In this paper, a theoretical investigation of the effects of the various errors on a DTC IM drive is carried out. The investigation gives a proof of the drive instability due to some errors and provides, for the other errors, the amount of the deviation produced on the controlled variables in steady state. The theoretical findings are substantiated by experimental and simulation results.

In the field of All Electric Ship (AES) design, increasing importance is assigned to simulation tools. Shipyards adopt simulation tools to solve design problems and achieve improved efficiency and relevant cost savings in the electrical systems. The calculation of harmonic pollution is one of these design problems. The paper proposes a simplified approach for modeling the very sophisticated MV plant of an all-electric cruise-liner. Having assumed reasoned simplifying hypotheses, the MV plant has been simulated using EMTDC-PSCAD, software commonly used for the simulation of earth power systems. The main outcome is the high accordance of the results of a simulated measurement of Total Harmonic Distortion of Voltage (THDv) on the vessel main MV busbar, with the experimental data taken from real measurements carried out on board. This validates the proposed approach for this kind of design problem.

The switching characteristics of an inverter feeding an induction motor controlled with the direct torque control (DTC) technique are assessed in steady state. At first, the application share of the inverter voltage vectors for the stator flux covering half a sextant is defined and predicted. The prediction indicates that, under operation at fixed inverter dc link voltage and stator flux magnitude, the application share depends only on the supply frequency of the motor and, to a small extent, on the load. Afterwards, the inverter transitions and the corresponding phase commutations within a stator flux sextant are analyzed. The outcome of the analysis permits to compute the commutations of the inverter phases in one turn of the stator flux and, from them, the inverter switching frequency is obtained. Its value is influenced by the sampling interval and the control delay arising from the microprocessor implementation of DTC. For given sampling interval and control delay, it is shown that the inverter switching frequency depends on the same quantities as the application share of the inverter voltage vectors. Acomparison with the switching characteristics of an inverter controlled with the space vector modulation technique is carried out. At last, the paper discusses the sensitivity of the switching frequency of an inverter for DTC to the following quantities: inverter dc link voltage, sampling interval and control delay. Throughout the paper simulation and experimental results are given to confirm the theoretical findings.

Power quality aspects are becoming more and more challenging in the AES field. Among them, voltage-THD requirements stated by ship-owners are becoming narrower and narrower, while traditional countermeasures to harmonic distortion do not seem to completely meet such demands any more. The paper presents the employment of MV active filters as a mean for solving the problem of on board voltage harmonic distortion. Points of strength of this proposal are discussed and its application on an all-electric cruise liner is described. Outstanding performance can be achieved as far as new power devices are made available by the market.

In the paper, a novel technique for the direct torque control (DTC) of an induction motor is proposed, which overcomes the trouble of high torque ripple afflicting the conventional DTC technique. With the novel technique, the inverter voltage vector selected from the switching table is applied for the time interval needed by the torque to reach the upper (or the lower) limit of the band, where the time interval is calculated from a suitable modeling of the torque dynamics. By this approach, the control system emulates the operation of a torque hysteresis controller of analog type since the application time of the inverter voltage vector is dictated by the allowed torque excursion and not by the sampling period. It is shown by experimental results that the technique yields a considerable reduction of the torque ripple. A further and ultimate reduction is obtained by compensating for the delay inherent in the discrete-time operation of the control system. The outcome is that the torque ripple of the motor is constrained within the hysteresis band of the torque controller, for a band of customary value. An ancillary merit of the technique is the almost full elimination of the average torque error inherent in the conventional technique. If the hysteresis band is shrunk, the torque ripple is bound to swing out the band limits. Under this circumstance, an extension of the technique is developed, which helps keep the torque ripple at minimum. To assess the characteristics of the proposed DTC technique, the following quantities: average torque error, rms value of the torque ripple, and inverter switching frequency are measured for different stator flux angular speeds and hysteresis bands of the torque and flux controllers. As a comparison, the same quantities are given for the conventional DTC technique.

At the present days, the AC motors is an industrial standard, not only in high power constant speed applications, but also in the medium and low power range. With the diffusion of induction motors some crucial problems emerged: (i) to control the AC drive in order to achieve high dynamic performances; (ii) to monitor the induction motor, with the aim to detect the most common faults. Several types of nonlinear controllers/observers were developed, to solve the control problem. In this paper we introduce a novel approach to fault diagnosis of an AC drive, controlled by a feedback linearizing controller together with a proper nonlinear observer. We propose to investigate the tracking errors, that are generated by this control scheme, with the aim to monitor the integrity of the motor itself. Simulation results on a model of a real AC motor are reported showing that the tracking errors can be used as fault symptoms.

While most of the problems related to the feed- back control of AC motors have been successfully solved, the condition monitoring and the detection of their most common faults is still an open problem. In this work a novel approach to the fault diagnosis of AC motors is introduced. The detection and the identification of a fault is performed through the analysis of the tracking errors (errors between reference and observed quantities) evaluated by a non- linear observer. The description of the diagnostic method will be given and extensive simulation results concerning different control schemes will be reported. Finally some experiments on a real induction motor drive will be described, that confirm the results obtained in the simulations.

Over the past decades, the growth of international cruise tourism has been constant. The forecasts made by the Ships and Maritime Equipment Association indicates an annual growth rate of about 7%, leading to the triplication of global cruise passengers in the future (from 19 million passengers in 2010 to more than 54 million in 2035). To cover the future market needs, roughly six or eight new cruise ship buildings per year are planned until 2031, along with an increase in ship size (up to 8,000 persons accommodated onboard). These two facts have led to a growing interest in the passengers' safety, which has been addressed by the International Maritime Organization (IMO) Maritime Safety Committee (MSC) in 2000, with the launch of an initiative for adequate international safety regulation for large passenger ships. The first issue that emerged from such an initiative was the difficulty in safely evacuating passengers toward lifeboats during emergencies (in particular, during a fire or flooding). The solution proposed by the MSC requires that future passenger ships be designed to increase their intrinsic survivability, exploiting the concept that the ship is its own best lifeboat (thus avoiding abandoning the ship as much as possible). Such a solution was formalized in December 2006 through a package of amendments to regulations dedicated to large passenger ships and published in the 82nd session of the MSC, i.e., the Resolution MSC.216(82), commonly known as safe return to port regulations (SRtP).