A. Cavallini

Arc-furnace model for the study of flicker compensation in electrical networks

This paper presents an arc-furnace model consisting of nonlinear, time varying resistance where time-variation laws of arc length are considered. One consists of a periodic, sinusoidal law, the other of a band-limited white-noise law. The arc-furnace model is implemented by EMTP, referring to actual electric-plant configurations. Simulations are reported where the values of flicker sensation and short-term flicker severity P/sub ST/, are determined according to UIE specifications. The results show that the model based on the sinusoidal time-variation law can be useful for worst-case approximations, while the model using white-noise law is able to fit flicker measurements made in electric plants supplying arc furnaces. The models are used to investigate the effect on flicker compensation of the insertion of series inductors at the supply side of the furnace transformer. It is shown that considerable reduction of P/sub ST/ is obtained at the point of common coupling by series inductor installation at constant furnace active power. >

Compensation strategies for shunt active-filter control

Compensation strategies for control of shunt active filters are compared in the paper. It is shown that the strategy based on unity-power factor control is appropriate when the supply voltage waveform of the plant where the active filter is connected shows significant distortion. As voltage distortion increases, this strategy provides compensated line current having lower harmonic distortion and RMS value with respect to the strategy generally used. This contributes to diminish the current and voltage distortion in networks. The unity-power factor compensation strategy conditions the current flowing in the plant where compensation is realized to fit the voltage waveform, thus reaching a unity power factor. Hence, the line current RMS value is minimum. The comparison of the strategies is performed by both Monte Carlo and ATP simulation. >

A new approach to the diagnosis of solid insulation systems based on PD signal inference

The authors have developed digital instrumentation that can measure the amplitude, shape, phase, and number of pulses per cycle. Examples are given of results obtained from an induction motor, a hydrogenerator, a polymeric cable system with a defective joint, and a high-voltage current transformer.

Stochastic evaluation of harmonics at network buses

Evaluation of harmonic distortion and power quality in electrical networks cannot neglect the stochastic approach, which takes into account the random nature of harmonics injected in the network by distorting loads. In this paper, procedures are proposed to evaluate the resultant probability density and cumulative functions of the sum of harmonic currents at buses of electrical networks. These procedures are applied to the results of long-term measurements performed on MV networks, which supply both residential and industrial loads. It is shown that the time series associated to real and imaginary components of harmonic currents are, in general, nonstationary, so that the statistical tools proposed in literature to evaluate the probability function of the magnitude of the sum of random vectors may give inaccurate results. On the contrary, the proposed procedures allow accurate estimates of the sum of harmonics, associated to given probabilities, to be achieved. >

The effect of voltage frequency on partial discharge activity

The knowledge of partial discharge (PD) as a physical phenomenon has made considerable progress in the last decade(s). These efforts have led to a validated statistical-mathematical model able to simulate the PD behavior in dielectric bounded spheroidal cavities. Recent studies demonstrated that an approach based on simulations can give both qualitative information on the PD process and a good estimation of the main PD quantities (PD magnitude, PD inception/extinction voltage, PD pattern). This theoretical operating procedure was until now restricted to discharges ignited at the power stress frequency of 50 (60) Hz. In this paper the authors describe the use of a procedure to simulate partial discharges in dielectric bounded spheroidal cavities at frequencies in the range of 0.1 Hz....1000 Hz. Information on the effect of voltage frequency on the PD process has been derived from the simulation results. The results were validated by measurements at a range of test frequencies on test specimens containing spherical cavities.

A deterministic/stochastic framework for power system harmonics modeling

A mixed deterministic/stochastic vector process able to describe the behavior of power system harmonics is discussed in this paper. The process consists of a framework where the recorded signal is decomposed in deterministic functions and a normalized gaussian vector. Validation resorts to measurements performed on medium voltage networks, showing that the model appears to be an effective tool for calculation of the statistical properties of the quantities which are usually involved in compensation-system design for electrical networks.

Modeling memory phenomena for partial discharge processes in insulation cavities

Partial discharge measurements performed at 0.1, 20, 50 and 300 Hz and at different voltage levels are compared with simulations based on models available in literature. It is shown that these models need to be improved to be able to fully extrapolate PD behavior at different frequencies. A solution based on a time-dependent increase of the work function when detrapping from negative charged surfaces is proposed and verified on the basis of experimental data.

Flicker and distortion compensation in electrical plants supplying arc-furnaces

A method for compensation of flicker and harmonic distortion in electrical plants supplying arc-furnaces, which consists of the insertion of series inductors and AC harmonic filters, is investigated. For this purpose, a three-phase model of an arc furnace, implemented by EMTP, is provided, where arc-length time variation obeys either sinusoidal or white-noise law. The model is applied to simulated cases and experimental results, showing that both short-term flicker severity, P/sub ST/, and total harmonic distortion, THD, decrease with the insertion a compensation system constituted of series inductor and harmonic filters. The proposed three-phase model proves to fit well the experimental data, thus constituting a useful tool for arc-furnace behavior inference.<<ETX>>

Long-term electrical performance and life model fitting of XLPE and EPR insulated cables

The results of life tests performed on cross-linked polyethylene (XLPE) and ethylene propylene rubber (EPR) cable models are presented and discussed. The performances of the two insulating materials when subjected to electrical stress are compared on the basis of characteristic parameters derived by the best-fitting life models. It is found that while EPR is well described by linear models, XLPE data fit curvi-linear life models, showing tendency to electrical threshold. Hence the fundamental parameter for XLPE characterization is the threshold value, while the voltage endurance coefficient should be used to characterize EPR. Considerations on test and data-processing methodologies, as well as insulation design, point out the criteria to be used for the two models. >

Evaluation of harmonic levels in electrical networks by statistical indexes

The problem of harmonic pollution in electrical networks is dealt with using statistical procedures. Due to the randomness of the operating conditions of nonlinear devices, a stochastic approach seems the most suitable for obtaining results meeting both technical and economical requirements. A comparative study is carried out on the effectiveness of semiempirical expressions which have been proposed to simplify the evaluation and forecasting of harmonic pollution levels in AC networks. It is found that the approximate model for the stochastic sum of harmonic currents at plant buses could be usefully employed for the design of network devices relevant to distortion and power factor compensation, such as filters and capacitors, as well as for the evaluation of immunity levels.<<ETX>>