Saša Mujović

The Impact of the Load Side Parameters on PC Cluster's Harmonics Emission

Harmonics current emission from personal computers (PCs) is of special interest to power quality researchers, due to their wide spread application and the fact that they are usually co ...

Multi-parameter mathematical model for determination of PC cluster total harmonic distortion of input current

Purpose – Low power devices with switched-mode power supply represent harmonic generating apparatus in widespread use nowadays. The influence of personal computers (PCs), which affect the supply voltage, is considered. Harmonic level due to simultaneous PC operation is quantified by the total harmonic distortion of input current (THD I ). The purpose of this paper is to propose a multi-parameter mathematical model for the THD I calculation. The model is convenient for practical engineering application. Design/methodology/approach – The model is derived using the measured and simulated data. The model coefficients are obtained in the least squares sense. Findings – Mathematical modeling of THD I is the least expensive and the most convenient solution for engineering application. The models proposed in the literature have many drawbacks, which motivated the authors to develop a more comprehensive solution. Grid stiffness, capacitance of PC power supply unit and PC cluster size represent the major parameters...

Characteristics of Overvoltage Protection with Cascade Application of Surge Protective Devices in Low-Voltage AC Power Circuits

Surge Protective Devices (SPDs) are widely used for protection of the equipment in low-voltage AC power circuits against wide variety of surges. Cascade application of SPDs starting at ...

Least Squares Modeling of Voltage Harmonic Distortion Due to PC Cluster Operation

Regarding the possibility of harmonic generation, personal computers (PCs) are very significant within the category of non-linear low power loads. In modern distribution systems, the number of PCs simultaneously connected to distribution grid constantly increases. This PC clustering can negatively affect the quality of supply. The paper deals with mathematical modeling of the voltage total harmonic distortion (THDU) caused by operation of PC cluster. The proposed model is based on the power quality measurements carried out in a computer centre and computer simulations. It takes into account the THDU dependence on the PC cluster size and grid stiffness. Model parameters are derived in the least squares sense. The influences of cable cross-section and pre-existing THDU of the phase voltage are also discussed. The accuracy of the proposed model is confirmed by additional measurements performed in a commercial building.

Zero-Sequence Voltage-based Method for Determination and Classification of Unloaded Overhead Line Operating Conditions at the Moment of Energization

Abstract Overhead line energization can be performed under normal or faulty conditions. The latter indicates an occurrence of a fault along the line that exists at the moment of energization. This can lead to significant over-voltages that could endanger proper line operation. The issue gets more complex to protective devices when it comes to high-resistance faults, which impair the ability of relays to react promptly and accurately. Consequently, installation of additional detection devices becomes necessary. This paper proposes a novel method that properly detects and classifies line operating conditions at the moment of energization. The method is designed to be useful for utilities and it can be considered as a low-cost, fast, and accurate detection and classification approach, suitable for dealing with both low-resistance and high-resistance faults. Through comprehensive mathematical modeling it was found that both normal and faulty conditions during line energization are accompanied by the zero-sequence voltages of specific characteristics. The differences between zero voltage sequences are reflected in harmonic content, magnitudes of dominant frequencies, and their phase angles in regard to supply voltage. These findings are taken as the methods detection and classification criteria. The validity of the proposed model is verified by simulations and by field measurements.

Simultaneous Operation of Personal Computers and Mathematical Assessment of Their Harmonic Impact on the Grid

In the smart grids environment, non-linear electronic devices are present to a great extent. Since these devices generate higher harmonics in the current spectrum and require high voltage quality in order to operate properly, the power quality issue becomes very important. This study deals with mathematical modelling of parameters of current and voltage harmonic distortion due to operation of PC cluster. The proposed models are developed using measurements and simulation results. Model parameters are derived in the least squares manner. The models are convenient for practical engineering application.

Application of EMD and STFT Methods in Analysis of Energization of an Unloaded Overhead Line Under Different Operating Conditions

Maintaining of voltage conditions recommended by EN 50160 Standard is ultimate aim related to power quality. This issue gets an additional importance in era of modern power systems and smart grid environments. Occurrence of voltage whose magnitude exceeds the highest voltage level recommended for proper electrical equipment operation is known as overvoltage. Switching operation such as line energization can cause this type of disturbance. The paper deals with switching overvoltages occurred during energization of an unloaded overhead transmission line under different operating conditions. Namely, this process can be performed under normal or fault conditions. The latter means that some kind of fault has occured along the line and exists at the moment of energization. Switching overvoltages have to be analyzed not only from the aspect of magnitude, but also from the aspect of the existing harmonics in the harmonic spectrum. Accordingly, the Empirical Mode Decomposition and Short time Fourier Transform were used. The obtained results were compared to the calculated ones. In this way, one may get characteristic harmonics values which occur during the energization process, what would make a worthwhile contribution related to recognition of operating conditions during energization. Also, a conclusion about applied methods efficiency is drawn.

Effects of Different Combination Wave Generator Design on Surge Protective Devices Characteristics in Cascade Protection Systems

Cascade application of surge protective devices (SPDs) in low-voltage ac power circuits is intended to ensure proper equipment protection against surges as well as to provide surge energy distribution between SPDs. In order to evaluate equipment response and SPDs performance, concept of representative surges, which can be used for surge testing purposes, is introduced by international standards. Combination wave surge is defined both by IEEE C62.41.2 standard and IEC 61000-4-5 standard as one of the representative surges. However, these standards specify different tolerances regarding short-circuit current waveforms undershoot delivered by combination wave generator (CWG). The aim of this paper is to investigate the influences of different CWGs regarding short-circuit current undershoot in cascade two-stage protection system. It will be shown that differences that appear during surge testing with application of different CWGs can lead to inappropriate selection of SPDs regarding energy absorption capability and/or protection level.

Power Quality-Based Classification and Modelling of Small Loads and Generators

The paper presents an overview of existing methods of classification of small loads and generators, giving a new approach to this issue. This new approach divides loads according to their consumed current-time waveform distortion and generators according to generated output voltage wave shape, i.e. power quality issues. For each of the selected categories an adequate mathematical (mathematical relation) and computer simulation model has been developed. The model is verified by results of laboratory measurements. The proposed approach is more suitable for practical application than previously proposed classifications as it facilitates obtaining information about the level of higher harmonics, without knowledge of the internal device structure. Further, it can be easily applied in practical laboratory setups to facilitate teaching at both undergraduate and postgraduate levels.