P. Varilone

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.

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.

A Bayesian-Based Approach for a Short-Term Steady-State Forecast of a Smart Grid

Future distribution networks are undergoing radical changes, due to the high level of penetration of dispersed generation and information/communication technologies, evolving into the new concept of the Smart Grid. Dispersed generation systems, such as wind farms and photovoltaic power plants, require particular attention due to their incorporation of uncertain energy sources. Further and significant well-known uncertainties are introduced by the load demands. In this case, many new technical considerations must be addressed to take into account the impacts of these uncertainties on the planning and operation of distribution networks. This paper proposes novel Bayesian-based approaches to forecast the power production of wind and photovoltaic generators and phase load demands. These approaches are used in a probabilistic short-term steady-state analysis of a Smart Grid obtained by means of a probabilistic load flow performed using the Point Estimate Method. Numerical applications on a 30-busbar, low-voltage distribution test system with wind farms and photovoltaic power plants connected at different busbars are presented and discussed.

An integrated probabilistic harmonic index

Both amplitude and argument of voltage harmonics can affect the electrical component behavior. In the paper an integrated index taking into account both of them is proposed for inclusion in harmonic standards. This index, which refers to the well known peak value of distorted voltage, allows taking into account the main effect of distorted voltages on component life. The time varying nature of harmonics is also considered and, coherently, limits to be assigned to this index are proposed also in probabilistic scenarios.

Voltage Regulators and Capacitor Placement in Three-phase Distribution Systems with Non-linear and Unbalanced Loads

This paper investigates the problem of contemporaneously choosing optimal locations and sizes for both shunt capacitors and series voltage regulators in three-phase unbalanced distribution systems. The sizing and placement procedure not only minimizes the power losses along distribution feeders but also makes sure that both capacitors and series regulators will have the minimum possible impact on the harmonic distortion of bus voltages in the system. The fact that distribution systems can operate under unbalanced loading conditions means that the optimization will have to account for any unbalances in the system. A genetic algorithm, which successfully solves the above-described problem, is developed and tested. This paper presents the results of simulations for a 34-bus IEEE distribution test system.

Methods for Assessing the Robustness of Electrical Power Systems Against Voltage Dips

Electrical power system robustness is defined as the intrinsic capacity of an electrical power system to maintain assigned disturbance levels when external conditions change. This paper compares two methods for estimating voltage dip severity: the critical distance method and the fault position method. Their differences are not linked to the type of network but to the type of results which we are searching for. This paper proposes a graphical color visualization of the during fault voltage matrices that immediately correlates dip severity with colors defined by a proper scale. New robustness indices are also proposed to synthesize bus performance in terms of affected and exposed areas. Numerical applications refer to real transmission and real distribution systems.

The Inherent Structure Theory of Network for power quality issues

The Inherent Structure Theory of Network is a valuable tool to solve power quality problems. In fact, in the frame of this theory, the direct or inverse sequence impedance matrix at the fundamental frequency or the harmonic impedance matrices can be formulated in terms of their eigenvalues and eigenvectors; these expansions are useful to create a framework in which the power system response to power quality disturbances can be better understood. In this paper, after briefly outlining the theoretical background, the above theory is applied to solve some power quality problems, evidencing when the proposed tool can be more useful and what are the benefits deriving from its application. Numerical applications on a test system are presented and discussed to give evidence of the significance and potentiality of this new tool.

A global index for discrete voltage disturbances

The presence of the new liberalized markets has increased interest in power quality disturbances due to their economic value. Customers have more choices; then disturbances are becoming bigger issues and suppliers and customers should regulate them in the frame of power quality contracts. A single global power quality index can be very useful for characterizing the overall level of the voltage waveform quality; in particular, this index can provide global indications regarding different aspects of voltage quality and allows a strong reduction of the measurement data needed to cover all power quality disturbances, in this paper, a global power quality index (GPQI) for discrete disturbances is proposed, based on the discrete severity indicators (DSI). We present the results of a GPQI evaluation of an Italian distribution system on voltage dips, whose measured data are available on the website of the project QuEEN led by CESI Ricerca.

Decision theory criteria for capacitor placement in unbalanced distribution systems

In this paper, decision theory criteria are applied to select the optimal allocation and the sizing of capacitors in unbalanced systems with the presence of harmonic sources, also taking into account the uncertainties due to the presence of unbalanced loads. The decision theory is a valuable tool for distribution engineers that have to deal with financial costs and with uncertainty. The financial costs must include, in the most general case, at least initial investment cost, Joule losses costs, quality costs; the uncertainties should refer at least to non linear and linear load scenario, also including issues due to deregulation. A case study referred to the IEEE 13-node test feeder is presented and discussed