D. Lauria

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.

Two-Leg Three-Phase Inverter Control for STATCOM and SSSC Applications

Flexible ac transmission systems (FACTS) devices are attracting an increasing interest both in power system academic research and in electric utilities for their capabilities to improve steady-state performance as well as system stability. Several converter topologies for FACTS applications have been proposed in the recent literature, even if those based upon voltage source inverters (VSI) seem to be more attractive due to their intrinsic capability to rapidly respond to network changes such as perturbations subsequent to a fault and their property of being immune to resonance problem. In this paper, a new topology for inverter-based FACTS is proposed. This configuration, employing a two-leg three-phase inverter is employed for both series and parallel-connected reactive power compensators. The converter utilizes a modular topology for allowing a satisfaction of electronic components rating. A control strategy based on variable structure control technique with sliding mode is employed to track appropriate reference quantities. Design and control, as well as good tracking performances, are also verified through numerical simulations.

A PV AC-module based on coupled-inductors boost DC/AC converter

In this paper the design and the control of a PV (Photovoltaic) AC-module based on a double coupled-inductors boost topology are discussed. This circuital configuration allows obtaining of the high voltage amplification needed in individual panel applications. The operation principle of the proposed circuit is analyzed and a proper calibration of the PV panel simulation model is performed. The adopted control strategy is based upon sliding control technique in order to exploit its well-known properties of robustness. Furthermore, a Maximum Power Point Tracking (MPPT) approach based on a Incremental Conductance (IC) algorithm is used to track the actual maximum power point. Finally, the numerical results reported in the paper permit to confirm the feasibility of the proposed design and control strategy.

On the design and the control of a coupled-inductors boost dc-ac converter for an individual PV panel

In this paper the design and the control of an individual PV (Photovoltaic) panel dc-ac converter based on a double coupled-inductors boost topology are discussed. The operation principle of the proposed circuit is analyzed. A proper calibration of the PV panel simulation model is performed. An optimization procedure is proposed for choosing the fundamental parameters of both converter and control law based upon sliding control technique. Furthermore, a Maximum Power Point Tracking (MPPT) approach based on a Perturb & Observe (P&O) algorithm is used to track the actual maximum power point through successive approximations. Finally, the proposed inverter is employed as a grid inverter in a single PV panel application. The numerical results reported in the paper permit to confirm the feasibility of the proposed design and control strategy.

Energy management of electrified mass transit systems with Energy Storage devices

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.

Special transformers arrangement for AC railway systems

This paper deals with special transformer connections for mitigating the current and voltage unbalances generated by single-phase AC traction loads, as typically occurs for electric high speed railway loads supplied by the three phase power system. In fact they represent one of the main sources of power quality perturbation of the grid, with consequent denigration of its performance. The current unbalance reduces efficiency, productivity and profits at the generation, transmission and distribution of electric energy. Voltage asymmetry reduces efficiency, productivity and profits at the consumption and utilization level. Transformer connections for mitigating these perturbations are summarized and compared each other. These mainly consist on different types of connections of single-phase transformers of the substation to the three-phase primary section with phase-transformation methods. Finally, an advanced scheme for current balancing and power factor improving of two unbalanced loads is performed.

Negative Log‐Gamma distribution for data uncertainty modelling in reliability analysis of complex systems ‐ Methodology and robustness

In the paper, the problem of uncertain data in reliability analysis of complex systems is examined. The analysis is addressed to system reliability assessment with imprecise knowledge of component reliabilities, an item becoming more and more important for systems affected by considerable technological change. Starting from component uncertain data, a new method for the whole system reliability uncertainty description, based upon a Bayesian approach and not depending on the reliability model of each component, is proposed. The reliability value of each component is considered as a random variable described by a Negative Log‐Gamma distribution. The proposed methodology makes it possible to compute the features of system reliability uncertainty (i.e. reliability distribution, confidence intervals, etc.) as functions of component uncertain data, thus characterizing the propagation of uncertainty from the components to the system. Numerical applications, related to a test system, are presented to show the validity of the method and its “robustness”, i.e. it is shown that it yields satisfactory results also when component reliabilities are not Negative Log‐Gamma but Beta distributed.

Some Basic Properties of the Failure Rate of Redundant Reliability Systems in Industrial Electronics Applications

This paper discusses some common reliability architectures, such as “parallel” and “k out of n” systems, adopted to add redundancy in many modern industrial systems, such as parallel-inverter systems. The focus is on some crucial properties of the failure rate (FR) of such systems, motivated by the fact that, in applied literature, the system FR is often simply evaluated as the reciprocal of the “Mean Time To Failure” of the system. However, this relationship is valid if, and only if, the system has a “series” reliability architecture. This is indeed the only case in which also the system has a constant FR, i.e., an Exponential lifetime distribution. Instead, the system FR of redundant systems is a function of time, which can never be constant. It is simply shown indeed that the FR of a parallel system with constant FR components is an increasing, or “first increasing, then decreasing” function of time, eventually reaching the value of the smallest FR. These results are extended to k out of n reliability systems, and also to more general reliability models with nonconstant FR, such as the Weibull or the “bathtub” model.

Reliability aspects in wind farms design

Wind park design is not a straightforward task because it involves many heterogeneous aspects to handle in integrated and systemic way. Historically, the reliability issue has often been neglected in favor of economic issue in power systems design. By following the modern tendency of the power system literature, the reliability constraints have to be satisfied a priori, for sake of power system security and safety. For this reason, a rationale procedure is developed in the paper for suitably comparing various alternatives, at the aim of identifying the optimal candidate to be realized. In the work, a specific objective function is proposed in order to select the better wind farm configuration. It is constituted by some terms which basically compare the profits related to the economic trading in the deregulated electric market and the costs due to the investment, operation & management and to system unavailability. This objective function is accurately investigated as a function of the turbines number in order to derive the most convenient alternative, this implying also the optimal choice of the single wind generators size. The ranking coming out from this assessment is then compared with that one which establishes a preferability in terms of expected load not supplied (ELNS). A compromise choice, between the best alternatives provided by the two criteria has finally to be adopted. A simple numerical application is reported in the last part of the paper for testing the validity of the proposed approach.

Transient stability probability assessment and statistical estimation

Abstract In the paper, a general analytical method for the probabilistic evaluation of power system transient stability is discussed and a new statistical inference approach for this evaluation is proposed. In particular, the transient stability probability (TSP) is defined and evaluated by taking into account the random nature of both the system loads and the fault clearing times (FCT). The paper is focused upon the aspect of statistical estimation of the TSP—a topic generally neglected in literature—on the basis of the obvious consideration that the parameters affecting the TSP (e.g. mean value and variance of loads, FCTs, etc.) are not known, but must be estimated. New properties of point and interval estimations of the TSP are derived and, in particular, an efficient “lower confidence bound” for the TSP estimation is proposed, based upon a suitable Beta probability distribution. In order to show the feasibility of the proposed approach, a numerical application to the Cigre test network is illustrated. Moreover, extensive Monte Carlo simulations to evaluate the estimator efficiency are performed. In the final part of the paper, also a practical example of possible application to the optimization of system design is illustrated. The application of the method is illustrated and performed by using the potential energy boundary surface method, but the estimation results hold their validity irrespective of the method adopted for the transient stability problem formulation and resolution.