P. Caramia

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.

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 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.

Site and System Indices for Power-Quality Characterization of Distribution Networks With Distributed Generation

The problem of developing a definition that adequately assesses power-quality (PQ) levels in the presence of distributed generation (DG) is addressed by using proper probabilistic indices for distribution networks. In the planning of new DG installations, these indices take the variation of PQ level into account and use weighting functions properly. They are useful in quantifying the impact of the installation of DG units because they can consider several PQ disturbances simultaneously. Also, they indicate how each PQ disturbance may affect the decision concerning the installation of DG units. Several tests on real distribution networks were performed and discussed in order to show the usefulness of these indices in evidencing the impact of DG on PQ levels. The analysis of the electrical distribution systems on the basis of the considered indices is valuable also to help decide the best allocation and size of distributed generators.

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.

Point estimate schemes for probabilistic load flow analysis of unbalanced electrical distribution systems with wind farms

In this paper, the point estimate method is applied to account for the uncertainties that affect the evaluation of the steady state operating conditions of an unbalanced three-phase power system in the presence of wind farms. The accuracy of the proposed technique is tested on the three-phase unbalanced IEEE 34-bus test system adequately modified to take into account the presence of wind farms; the results obtained by applying the Monte Carlo simulation are assumed as a reference. The main conclusion is that the point estimate method gives good solutions in terms of accuracy and computational efforts.

Probabilistic techniques for three-phase load flow analysis

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.

The GREAT project: Integer linear programming-based day-ahead optimal scheduling of a DC microgrid

Microgrids 1 are electrical subsystems which include small generation units, storage devices and loads, which can operate either in grid connected or islanding mode. This paper deals with a DC microgrid that will be realized in an Italian industrial facility of GETRA Group. In particular, in this paper an optimization model is formulated to solve the problem of the day-ahead optimal scheduling of the microgrid. The model aims to optimize storage devices and controllable loads operation minimizing the daily costs of imported energy from AC grid and satisfying operational constraints such as production process constraints. The optimization problem is formulated using the Integer Linear Programming (ILP) theory. Case studies are also investigated in order to show the feasibility and the effectiveness of the proposed approach.

Power quality degradation effects on PWM voltage source inverter with diode bridge rectifier

The paper describes the effects of power quality degradation on the AC input current response of three-phase PWM-VSI adjustable speed drive with diode bridge rectifier. The study accounts for different voltage nonideal conditions, showing the results obtained in presence of single and multiple disturbances, like harmonics, fluctuation and unbalance. The findings of the paper confirm that the main key factor in determining the severity of voltage harmonics relays in the peak value of voltage supply rather than only in its harmonic content. Further, the equivalent supply impedance parameters play a crucial role to assess the final input current distortion. Finally, fluctuation and imbalances in the power supply voltages generate not only unbalanced input current harmonics, but also additional interharmonic components that, in case of voltage fluctuations, are considerably increased.