• Energy Research

Abstract Photovoltaic systems (PVSs) are among the most diffuse Distributed Generators based on renewable energy sources. PVSs contribute to the short-circuit currents during a fault, modifying the short-circuit capacity of the distribution systems. Then, the contribution of PVSs to the fault current must be adequately modeled to extend the traditional short-circuit analysis to distribution networks with PVSs. In this paper an analytical model based on the phase-coordinates approach is proposed to evaluate the fault contributions of three-phase PVSs connected to unbalanced distribution networks in presence of symmetrical and asymmetrical shunt short-circuits, with and without fault impedance. The model of the PVS takes into account the environmental conditions, the inverter control system, the maximum current that can flow through the inverter switching devices, the filter, the interface transformer and the self-protections imposed by the Standard IEEE 1547. Both a first cycles and a steady-state model of the PVS are developed accounting for inverter control systems equipped with VAr or LVRT control schemes or inverter control system without reactive power regulation. The model of the PVS is applied to a typical North American MV network and the results of the proposed approach are compared with time-domain simulations.

The problem of the evaluation of the buried cable thermal useful life in presence of harmonic distortion and taking into account the moisture migration is analysed. The evaluation is effected in nondeterministic scenario with a probabilistic approach. Numerical applications referred to medium voltage cables are presented and discussed in order to show the influence of the harmonics and moisture migration on the cable useful life.

The present paper deals with the relevant aspects related to EG and its influence on the quality of supply of the distribution networks, focusing on the impact of EG on harmonic distortion, voltage variations and dips. It is shown that assessment of the network power quality levels can be efficiently performed by means of suitable indices defined for either the entire system or portions of it. The usefulness of the approach is tested on a real case distribution network.

This paper focuses on automatic control systems for train application. The analysis starts examining the grades of automation in train service application, and focuses on the tasks, which are typical for Automatic Train Operation (ATO) systems. After the presentation of the analytical statement of the problem, a brief survey on the more recent proposals for designing ATO systems is illustrated. The survey gives evidence to the comparison among the alternative proposals for realizing real ATO systems, in terms of algorithm and performance index.

Discrete Fourier Transform (DFT) and high-performance signal-analysis techniques (i.e., Prony, ESPRIT, root-MUSIC methods) have been proposed to calculate Power Quality indices that characterize waveform distortions in power systems. DFT can be affected by spectral leakage problems that cause inaccuracies in the spectral component calculation while high-performance signal-analysis techniques usually involve onerous computational efforts. Recently, hybrid methods were proposed in the relevant literature which make it possible to calculate PQ indices with accuracy and more acceptable computational efforts. This paper proposes a new three-step hybrid method which applies alternatively DFT and Prony methods to sliding windows. This method does not suffer problems of spectral leakage and guarantees a significant reduction of computational efforts compared with the application of the only Prony method. Experiments are conducted and comparisons are made with traditional methods using both synthetic and measured waveforms. The results confirm the effectiveness and efficiency of the proposed scheme.

Some techniques are presented to obtain the probability density functions of phase-voltages in an unbalanced three-phase power system. The techniques are based on the Monte Carlo simulation applied to the non-linear and linear three-phase load flow equations, on convolution process and on the Pearson distributions. These methods are compared in terms of time execution and accuracy in the evaluation of phase-voltage and unbalance factor probability density functions and in particular regarding their 95/sup th/ percentiles, being these quantities the statistical measures of greatest interest in many international standards for power quality. Several numerical applications are presented and discussed with reference to the three phase unbalanced IEEE 13-bus distribution test system.

In this paper, we propose a new method for the accurate estimation of frequencies and magnitudes of harmonics and interharmonics caused on the supply-side currents by an adjustable-speed drive. The proposed method consists of a three-step procedure in which Estimation of Signal Parameters by Rotational Invariance Technique (ESPRIT) and Prony high-resolution methods are properly applied together with theoretical formulas. The method guarantees high accuracy with acceptable computational effort. Numerical applications were made on a hardware PWM adjustable-speed drive, and comparisons were made between the results of the proposed method, results obtained by applying the traditional IEC standard approach, and results obtained by another high-resolution method proposed in the relevant literature. The numerical applications and comparisons demonstrated the superior effectiveness and efficiency of the proposed method.