Luigi Battistelli

Optimal design of DC electrified railway stationary storage system

The improvement of the energy efficiency is a key issue for electrified railway systems in order to reach an increased competitiveness compared to other transportation technologies in terms of both costs and environmental pollution. An important step towards this direction has been made with the increasing development of storage technologies. The installation of stationary or on-board storage technologies, as well known, allows the recovery of the braking energy of the rolling stocks for increasing the energy efficiency of the whole system as well as improving the voltage profile. In the paper an optimization procedure is proposed for choosing in the planning stage the fundamental characteristics of a stationary storage device. The optimal parameters of the storage system can be determined by taking contemporaneously into account the energy saving aspects, substation current minimization and reduction of voltage drops along the feeder. The considered storage system is based upon a bidirectional dc-dc converter with ultracapacitors. A sliding mode control is implemented for tracking the optimal trajectory determined by the previously mentioned optimal technique. Numerical applications put in evidence the effectiveness of the proposed procedure.

2

A 2×25-kV 50 Hz traction power system was analyzed and modeled in the time domain in order to simulate short-circuit conditions and to attain a practical method to identify the short circuit behavior of the traction system. In particular, due to the difficulty in assessing the track-line parameters which mainly depend on changing environmental conditions, the possibility of neglecting the capacitive parameters was analyzed. A complete model of the 2×25-kV 50 Hz traction power system was developed and validated through comparison with the results of referenced models and experimental tests performed on real systems. Simulations, addressed to reduce the short-circuit modeling complexity, demonstrate that neglecting the capacitive effects between conductors and to ground does not affect the calculation accuracy for standard analyses. The obtained results show that the proposed model can be remarkably useful for the traction system design as well as for investigating the effects of short-circuit conditions on other circuits, such as telecom circuits, power-supply equipment, and signaling track circuits.

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In the paper a methodology for optimal controlling storage devices at the aim of improving the performances of an electrified mass transit system is proposed. A rational procedure is arranged for counteracting the unavoidable stochastic variables involved in the description of the characteristics of this complex system. The core of the control procedure is based upon an optimization technique which should result particularly effective where plant parameters exhibit random behavior. The potentiality of the procedure is shown with reference to the reduction of the network power fluctuations. In this case the control law can be expressed in an analytical way in terms of power mean value over the interest time cycle. Kalman filter is employed to dynamically estimate the mean value of the total required power. A numerical application is reported so to demonstrate the feasibility and the goodness of the proposed control strategy.

25-kV 50 Hz High-Speed Traction Power System: Short-Circuit Modeling

In the paper a new design methodology of storage devices for improving the performances of light transportation systems is proposed. A critical analysis is performed with respect to the stochastic variables involved in the description of the behavior of this complex system. Some considerations are performed in order to properly take into account various uncertainties at the aim of optimizing the size of storage devices and satisfying all the technical constraints. The core of the design procedure is a multivariable constrained optimization technique stochastically applied. In the numerical application, for sake of clarity, only the time displacement between two trains is considered as a stochastic variable described as a lognormal distribution function. The probabilistic properties of the output variables of the optimization procedure are then investigated.

Energy management of electrified mass transit systems with Energy Storage devices

In the paper, a detailed model for short circuit analysis of the AC 2times25 kV-50 Hz traction system has been derived and implemented. Two different methods have been used: the time domain and frequency domain approaches. The two approaches are discussed and numerical applications are reported in order to evidence the feasibility of the two procedures.

Generalized approach to design supercapacitor-based storage devices integrated into urban mass transit systems

One of the most important issues in DC electrified transportation system management is to perform, as far as possible, optimal operation in terms of economy, quality and reliability. In this paper a voltage control strategy is performed in order to minimize power losses and contemporaneously optimize the power sharing between feeding substations. The problem is formalized as a multiobjective optimization, this allowing to compare various solutions by simply varying the weights in the objective function. The control structure is designed according to a hierarchical architecture. For the traction load characteristics identification, an estimation algorithm is also described. In the last part of the paper a numerical application is presented, highlighting the feasibility and the goodness of the proposed strategies

Short Circuit Modelling and Simulation of 2×25 kV High Speed Railways

A methodological contribution is presented in the framework of safety and security studies, where it is of paramount importance to be able to statistically characterize very rare and uncertain events. For this purpose, the paper illustrates a Bayesian methodology for the estimation of a stochastic process characterizing the maximum value of a succession of random variables (RV), representing the successive values of a disturbance in time. This stochastic process, already proposed and applied in power systems by the authors, is a powerful mathematical tool very adequate for describing a safety index (SI) for any engineering system, as discussed in the paper, also with some references to electrical applications. The paper is focused upon a Bayesian estimation (BE) technique, applied for the first time at the best of authorspsila knowledge, in which a new probability density function (pdf) -the so-called ldquoNegative Exponential Betardquo pdf - is adopted for converting prior information about rare events probabilities into accident rate information. Such BE is both efficient and easy to implement, as shown also by means of numerical simulations. In particular, the superiority of the BE with respect to the ldquoclassicalrdquo Maximum Likelihood (ML) estimation methods, traditionally adopted in power system applications, is illustrated in terms of ldquorelative efficiencypsila. The ML estimates are outperformed by the BE, especially when few experimental data are available, as typically occurs when dealing with rare events affecting safety.

A comparison among various optimization control strategies for DC electrified transportation systems

The widespread use of advanced technologies and automation in AC and DC railway systems requires an accurate investigation of the Power Quality disturbances they suffer. In some cases, both railway systems operate in the same transmission network area, so that voltage unbalances and waveform distortions can be complicated by the interactions between the two systems. In this paper, a robust electronic compensation device is proposed in order to solve the interaction problems; the solution is based upon a two-phase inverter with a non-linear and variable structure control system. Several numerical applications give evidence of its rejection action against disturbances and power parameters variations.