G. Carpinelli

Interharmonics: Theory and Modeling

Some of the most remarkable issues related to interharmonic theory and modeling are presented. Starting from the basic definitions and concepts, attention is first devoted to interharmonic sources. Then, the interharmonic assessment is considered with particular attention to the problem of the frequency resolution and of the computational burden associated with the analysis of periodic steady-state waveforms. Finally, modeling of different kinds of interharmonic sources and the extension of the classical models developed for power system harmonic analysis to include interharmonics are discussed. Numerical results for the issues presented are given with references to case studies constituted by popular schemes of adjustable speed drives.

Distributed generation siting and sizing under uncertainty

The necessity for flexible electric systems, changing regulatory and economic scenarios, energy savings and environmental impact are providing impetus to the development of distributed generation, which is predicted to play an increasing role in the future electric power system; with so much new distributed generation (DG) being installed, it is critical that the power system impacts be assessed accurately so that DG can be applied in a manner that avoids causing degradation of power quality, reliability and control of the utility system. Considering that uncertainties on DG power production are very relevant and different scenarios have to be taken into consideration, traditional deterministic planning techniques should not be used to take the right decisions. In this paper a three step procedure, based on genetic algorithms and decision theory, is applied to establish the best distributed generation siting and sizing on an MV distribution network, considering technical constraints, like feeder capacity limits, feeder voltage profile and three-phase short circuit currents in the network nodes. In the last part of the paper the procedure is applied to an actual distribution network.

Optimal Integration of Distributed Energy Storage Devices in Smart Grids

Energy storage is traditionally well established in the form of large scale pumped-hydro systems, but nowadays is finding increased attraction in medium and smaller scale systems. Such expansion is entirely complementary to the forecasted wider integration of intermittent renewable resources in future electrical distribution systems (Smart Grids). This paper is intended to offer a useful tool for analyzing potential advantages of distributed energy storages in Smart Grids with reference to both different possible conceivable regulatory schemes and services to be provided. The Smart Grid Operator is assumed to have the ownership and operation of the energy storage systems, and a new cost-based optimization strategy for their optimal placement, sizing and control is proposed. The need to quantify benefits of both the Smart Grid where the energy storage devices are included and the external interconnected grid is explored. Numerical applications to a Medium Voltage test Smart Grid show the advantages of using storage systems related to different options in terms of incentives and services to be provided.

Power Quality Indices in Liberalized Markets

DESCRIPTION Power Quality (PQ) indices are a powerful tool for quickly quantifying PQ disturbances. They also serve as the basis for illustrating the negative impact of electrical disturbances on components and for assessing compliance with the required standards and recommendations within a regulating framework. Within these pages lies a comprehensive overview of both the traditional PQ indices in use today and new indices likely to be used in the future.

Chaos-based modeling of DC arc furnaces for power quality issues

DC arc furnaces are more and more applied in large industrial systems and represent one of the major sources of perturbations for the feeding system. This paper deals with the problem of the dc arc modeling using three well-known chaotic attractors (Ro/spl uml/ssler, Chua, and Lorenz attractors). A new tuning procedure is adopted to determine the most adequate parameters of the attractors to model the dc arc furnace. Waveform distortions and voltage fluctuations indexes are calculated from the simulation results of a whole existing plant and compared with measured data. The proposed models of dc arc furnace can be used to assess the impact in terms of power quality of the dc arc furnace when planning new plants or evaluating the performances of compensating devices.

Analytical modeling for harmonic analysis of line current of VSI-fed drives

The ac/dc/ac double-stage converter, frequently used nowadays to provide ac adjustable frequency and magnitude voltages in the adjustable-speed drives, consists of a voltage source inverter (VSI) fed by a three-phase diode bridge rectifier; this rectifier generates high line current harmonics in the supply system. This paper presents a complete modelling of the VSI-fed drives, when energized by nonsinusoidal supply voltages, based on the analytical form of the current harmonics injected in the supply system; the equations of the current harmonics are obtained for both the continuous and discontinuous current modes and are derived by Fourier analysis of steady-state current time evolution. The high accuracy of the proposed models is demonstrated by comparison with the results of time-domain simulation. The modelling is useful to study the effects of supply voltage distortions on current harmonics injected in the supply system, both in continuous and discontinuous mode, with a very short computing time.

An approach to life estimation of electrical plant components in the presence of harmonic distortion

Harmonic distortion can affect significantly the reliability of electrical plant components by increasing the degradation rate of electrical insulation. This paper presents life models which enable estimation of failure times of the electrical insulation of the main components of industrial electrical plants, i.e. capacitors, cables, motors and transformers, when the prevailing aging stresses are voltage and temperature and harmonics give rise to considerable voltage and current distortion. The models are inserted in a probabilistic framework, which accounts for the inherently random nature of harmonics. It is shown that the degradation rate can become much larger than in the sinusoidal case, especially due to the contribution of harmonics to voltage peak magnification: also current distortion can play an important role. Hence, significant life reduction can be expected in the components which are subjected to harmonics.

Probabilistic three-phase load flow

Abstract In actual systems unavoidable uncertainties affect the steady-state operating conditions of a power system: these uncertainties are mainly due to changes of load demands and of generation system. Taking into account the uncertainties, it is most useful to introduce random variable and to apply probabilistic techniques of analysis. Up to now probabilistic power flows neglect the unbalances of the loads and of the other components and consider adequate single-phase model of the whole power system. In this paper the probabilistic power flow is extended to the three-phase field to evaluate the uncertainties which affect the steady-state operating conditions of an unbalanced power system. Both Monte Carlo procedure and a linearized form are proposed. A numerical application to a test system is also discussed.

Adaptive Prony Method for the Calculation of Power-Quality Indices in the Presence of Nonstationary Disturbance Waveforms

The presence of the new liberalized markets has increased the interest in power-quality (PQ) disturbances due to their economic effect. In particular, in the case of disturbances caused by a single event (such as a capacitor switching or voltage sag), the waveform assessment can be difficult due to the rapid variations in waveform spectral component characteristics; these difficulties require a suitable choice of signal-processing techniques for spectral analysis and, in particular, the resort to time-frequency representations. In this paper, the adaptive Prony method is proposed to calculate PQ indices based on a time-frequency analysis of waveforms. Numerical applications on a simulated transient due to capacitor switching, a measured voltage sag, and a test waveform are also presented and discussed in order to investigate the validity of the proposed method.

A review of single-objective optimization models for plug-in vehicles operation in smart grids part I: Theoretical aspects

Plug-in Electric Vehicles (PEVs) could become remarkable resources for improving quality and reliability of future grids, if adequate control of their onboard storage systems is provided. Hence, the problem of PEVs optimal control strategies is a crucial topic of research in modern power system operation. This is particularly true in the Smart Grid (SG) context, where the presence of bi-directional communications among distributed energy resources and customers makes it possible to obtain an optimized operation of the grid. In this paper a critical overview of some of the most significant single-objective optimization methodologies proposed for the optimal operation of PEVs in electrical distribution networks is presented in order to verify their feasibility in the context of SGs. These methodologies allow the performance of several services, which aim to meet both needs internal to the SG (for example, the losses minimization or the minimization of average voltage deviations) and external to it (for example, ancillary services to the electrical system to whom the SG is interconnected). This paper reports the theoretical aspects of the optimization models, whereas the companion paper “Part II: numerical applications to vehicle fleets” proposes the results of the analysed models.