M. Loggini

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. >

The effects of series inductors for flicker reduction in electric power systems supplying arc furnaces

The effect of a series inductor on the reduction of voltage flicker in electric power systems supplying arc furnaces is assessed. The arc furnace is simulated as a voltage generator with variable amplitude during the melting process. The design of the inductor hinges on the fact that, when connected into the electric power system, it should not interfere with the production capacity of the plant itself. The use of a powerful simulation program (EMTP) makes it possible to study a large number of cases and therefore to determine the appropriate size of the series inductor for the purpose of reducing voltage flicker. The simultaneous presence of the series inductor with capacitors and/or filters, with the purpose of limiting harmonic pollution and improving the power factor, is also discussed.<<ETX>>

Comparison of approximate methods for estimate harmonic currents injected by AC/DC converters

This paper concerns a comparative study on available estimators of harmonic current amplitudes injected in the AC network by six-pulse AC/DC converters. Current estimates are reached by approximate methods that resort to simplified current waveforms, as well as by an exact procedure that carries out the harmonic decomposition of the actual waveform. It has been shown that approximate procedures assuming ripple-free DC current can provide amplitude values significantly different from actual ones. In particular, it has been ascertained that the rectangular waveshape does not always correspond to the worst case as far as harmonic current amplitudes are concerned. The approximate methods that take ripple into account are generally more accurate, but the algorithms required for current amplitudes estimates are not as easy to apply as those valid for ripple-free waveforms. These findings should be kept in mind when systems for harmonic compensation, particularly filters, are to be designed. Moreover, national and international standards, when based on ripple-free DC current assumptions, should be carefully considered. >

Voltage-Distortion Compensation in Electrical Plants Supplying Static Power Converters

Voltage distortion compensation is a problem that must often be tackled in electrical plants supplying nonlinear loads, such as static power converters, both to ensure reliable behavior of apparatus sensible to harmonics and to comply with standard limits. Analysis of the compensation system constituted by harmonic filters and protection reactance is carried out, to clarify their contribution to voltage distortion factor improvement and fundamental project criteria. To this end a study of the current commutation transient, cause of voltage notches and, hence, of voltage distortion, is performed in the presence of filters, and the distortion factor is calculated by means of two separate methods. Comparison between these methods, based on voltage notches and harmonic voltage drop, respectively, and determination of the expressions for estimating the overlap angle after filter insertion allow the influence of filters on the line and the converter to be analyzed.

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>>

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>>

Electrical properties for detection of thermal aging in XLPE cable models

The electrical behavior (DC conductivity and AC properties) of XLPE (cross-linked polyethylene) cable models aged under thermal stress is investigated as a function of aging time. Charging and discharging currents as well as the dielectric constant and loss factor of specimens aged at different times and temperatures were measured from 55 to 90 degrees C and 10/sup -2/ to 10/sup 5/ Hz AC properties. Significant variations of DC conductivity and AC losses with aging time and temperature are pointed out. Cable models thermally aged above 100 degrees C revealed a strong increase of DC electrical conductivity with aging time. The same behavior, i.e., increase with aging time at 100 and 130 degrees C, is exhibited by dielectric constant and loss factor. The observed increase of electrical conductivity and dielectric losses supports the build-up of thermooxidative processes in thermally-aged specimens.<<ETX>>

General criteria for the stochastic sum of measured harmonics at plant busses

Criteria are presented for constructing the probability density and cumulative functions of the amplitudes and phases of the harmonic currents and voltages at plant buses when the harmonic vs. time behavior in the plant branches is detected by on field measurements. In particular, the convolution and Monte Carlo methods are applied and the relevant results compared on the basis of simulated and experimental samples. The methods do not reply on assumptions pertinent to any kind of probability function. The significance of providing harmonic quantities associated with the occurrence probability is considered. Examples of applications based on real plant measurements and comparisons of the procedures used are included.<<ETX>>

Filters and protection reactance for distortion compensation in low voltage plants

Low-voltage electrical plants supplying static power converters are often affected by current and voltage harmonic distortion. The effectiveness of harmonic filters and protection reactance in compensating the distortion factor and, in general, nonactive power, is shown by theoretical and experimental results. Good agreement of voltage notch and distortion factor estimates, after filter and protection reaction insertion, with experimental data measured on a model of an electrical plant has been found. The strong contribution to voltage notch and distortion factor reduction of filters connected at the converter-side of protection reactance is pointed out. The final result is that well-designed filters and protection reactances, suitably inserted in the plant, can satisfactorily compensate both reactive and distortion power and lead to compliance with standard limits on harmonic distortion even for electrical plants with a low short-circuit ratio.<<ETX>>

A simplified harmonic simulation procedure

A simplified harmonic simulation approach suitable for implementation on general-purpose mathematics software for a personal computer is discussed. A set of procedures for simulating the performance of small industrial plants employing static power converters is presented. Harmonic current flow, voltage distortion levels, and harmonic stressing of system components (transformer, capacitor, and filters) are calculated. The advantages and disadvantages of a specific application developed in MathCAD are examined.<<ETX>>