1,721,116 research outputs found
Assessment of electric power quality: indices for identifying disturbing loads
No full textThe study aimed at defining parameters capable of assessing, in terms of distortion and unbalance, the quality
of the electric power drawn by the customer: Taking as a starting point the advantages and limitations of the
methods proposed so far in the literature, two new indices are introduced, useful for quantifying the disturbing
characteristics of a load. The efficiency of these indices has been assessed by computer simulations to assess
the applicability of the method to three-wire and four-wire three-phase systems in many realistic situations
Power quality monitoring in modern electric distribution systems
No full textIn electric power systems, there may be disturbances in the supply voltage and the load current drawn by a user. These disturbances happen when the frequency and amplitude are not equal to their corresponding nominal values, when there are non-sinusoidal waveforms, and when there is the presence of negative and zero-sequence components in three-phase systems. Power quality (PQ) is a generic expression used to characterize the disturbances. This paper offers an overview of the methods and instruments currently used in PQ measurements and some research activity to improve the
Hysteresis and eddy currents compensation in current transformers
No full textIn the paper a digital technique for improving the accuracy of instrument current transformers is presented. Since the exciting current can be considered as the main error source, its evaluation can allow the compensation of its detrimental effects to be obtained. The exciting current required by the transformer in every kind of steady state operation can be determined by simply acquiring the secondary current, provided that the examined CT has been preliminarily identified. A simple scalar model for the CTs magnetic core, taking into account saturation as well as hysteresis and eddy currents phenomena, has been implemented in a software compensation routine. This allows us to improve the accuracy in the reproduction of the primary current, in the case of both sinusoidal and distorted current waveforms (provided that DC components are not present).
Many experimental tests, under different practical situations, have been performed. The results clearly show that the proposed technique is able to significantly reduce, in comparison with traditional methods, the errors introduced by current transformers
Comparative analysis between active and passive current transducers in sinusoidal and distorted conditions
No full textComparative analysis between active and passive current transducers in sinusoidal and distorted conditions
No full textCurrent measurements are a critical item in the monitoring of modern electrical power plants, since the quantities to be measured often show high harmonic content. The current transducers should therefore guarantee a good accuracy even when they operate with distorted waveforms. In this paper, the behavior of some devices commonly used for current measurement is examined, considering either sinusoidal or distorted conditions. Traditional current transformers, transducers based on the Hall effect, and resistive shunts combined with isolation amplifiers have been considered. The comparison is performed by evaluating a synthetic error index, called composite error, which allows all the main causes of uncertainty (amplitude errors, time shifts, and distortion) to be taken into account.
The proposed investigation aims to find out the main characteristics that make each transducer suitable or unsuitable in different practical situations
A digital compensation method for improving current transformer accuracy
No full textThis paper deals with error compensation in current transformers. Usually the manufacturer tries to limit these errors by stating a transformer nominal ratio slightly different from theturns ratio. On the other hand techniques have been developed
that allow software current compensation. The proposed method requires acquisition of the instantaneous secondary current and its adjustment with magnetizing current, taking into account hysteresis effects. To do this, a preliminary procedure is performed for core and transformer parameter identification. The compensation method has been tested on a variety of instrument transformers, with sinusoidal primary current. Test results show, in all examined cases, an improvement in primary current reproduction accuracy, compared with that achieved using CTs nominal ratio, even for core partial saturation. A further development of the compensation technique is in progress, with a view to eliminating some restrictive hypothesis introduced in the present study
On the measurement of power quality indexes for harmonic distortion in the presence of capacitors
No full textOn the measurement of power-quality indexes for harmonic distortion in the presence of capacitors
No full textDifferent techniques have been recently proposed in the literature for the identification of the sources of harmonic distortion in electric-distribution systems. Each of them has its own advantages and limitations as far as reliability, accuracy, and practical usability are concerned. In this paper, these techniques are grouped into three main fundamental criteria in order to investigate their performance in the presence of capacitors. A case study is analyzed by means of both experimental tests and computer simulations. The results put in evidence some major problems that may concern the information provided by the considered methodologies, in virtue of the interaction of the capacitors with the other network components
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