P. Verde

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.

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.

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.

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.

The effects of voltage waveform factors on cable life estimation using measured distorted voltages

This work investigates expected life reduction of power cables under harmonic pollution, by processing the results of voltage harmonics measurements. Life estimates come from a life model developed by the authors in former studies, used here in its complete form, with all voltage waveform factors, i.e. peak, shape and rms factor. The application regards MV XLPE-insulated cables. The whole day duty cycle is split into rush hours and non-rush hours, each characterized by a different level of harmonic distortion. The results show that life is significantly reduced with respect to rated sinusoidal life, and that both shape and peak factors play a major role, shape factor during rush hours, peak factor during non-rush hours

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.

A Framework for Regulation of RMS Voltage and Short-Duration Under and Overvoltages

This paper proposes a general approach for voltage-quality regulation. The same framework covers voltage-amplitude variations, dips, and swells. The framework uses the difference between power-quality events and power-quality variations to introduce a ldquoresponsibility-sharing curverdquo distinguishing between normal operational voltage for which equipment should be immune and events that are subject to voltage-quality regulation. It is further proposed to use equipment immunity requirements as a basis for the responsibility-sharing curve.

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.