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Lazic B.,EPS Distribution | Ivanovic B.,Electric Power Industry of Serbia
IET Conference Publications | Year: 2016

A connection of perspective 8 MW power plant Umka to 10 kV power distribution grid in Belgrade is presented in the paper. The connection is specific about two aspects: 8 MW is high power to be evacuated over 10 kV grid and power plant will be connected to 10 kV directly, without step-up transformer. Short circuits and load flow system analyses are carried out by using the computer simulation models of distribution system critical operation regimes in order to investigate connection criteria defined by distribution system grid code. Another contribution of power plant Umka should be strengthening of 10 kV distribution grid and elimination of voltage dips problems that have occurred for years in a factory Umka.

This paper describes a mathematical relation which is developed to estimate the occurrence of track mechanism failure in function on the mineral dust (SiO2) content, i.e. wear intensity. This relation is based on actual data of track-type machine (bulldozers) failures, the properties of rocks and measurements of wear intensity on the upper rollers of track mechanism. Failures of bulldozers were recorded during the period of 12 months on six open pits in Serbia, together with their location which is correlated rock type and SiO2 content. This enabled establishment of the reliability indicators using two-parameter Weibull distribution. Further on, correlation is interpreted based on the linearization model using the method of least square. This research has impact on proper management of track-type machines operating on lignite open pits, in the sense of predicting time to failures and cost of maintenance of these machines. This approach provided guidelines for the establishment of reliability centered maintenance model. © 2016 Polish Academy of Sciences Branch Lublin. All rights reserved.

Jankovic S.,University of Belgrade | Ivanovic B.,Electric Power Industry of Serbia
Electric Power Systems Research | Year: 2015

Abstract Load flow is the most used calculations in power system operation planning. Renewable resources have caused that system operator has to do power flow analysis and/or contingency analysis as fast as possible in order to predict next step in power system control. LU Decomposition of Jacobian matrix remains the most computationally expensive task during Newton-Raphson iterative method. Computational time appears to be critical issue when load flow calculation is performed on large power system load flow models. In this case, Jacobian matrix LU decomposition should not be performed in iterations in which convergence rate is not violated but performed in iterations in which convergence rate drop below specified level. In other words, modified Newton-Raphson Method which eliminates the repeated Jacobian matrix LU decomposition and generic Newton-Raphson method are combined depending on convergence rate. The paper presents application of proposed combined Newton-Raphson method which is based on convergence rate control. Comparison of combined, Shamanskii, generic, and modified Newton-Raphson methods is carried out taking into consideration computational time and number of iterations required to achieve convergence of load flow models of various dimensions. © 2015 Elsevier B.V.

Milanovic Z.,Electric Power Industry of Serbia | Stankovic K.,University of Belgrade | Vujisic M.,University of Belgrade | Radosavljevic R.,University of Belgrade | Osmokrovic P.,University of Belgrade
IEEE Transactions on Dielectrics and Electrical Insulation | Year: 2012

The possibility of generating a statistical sample of the pulse breakdown voltage random variable numerically is examined for arbitrary shaped pulses. Impulse characteristics are then determined on the basis of the generated statistical sample. Numerically generated statistical samples of the pulse breakdown voltage random variable are compared to the corresponding experimentally obtained statistical samples. Impulse characteristics obtained from the numerically generated statistical samples are compared to the corresponding impulse characteristics derived from the semi-empirical Area Law and the Time Enlargement Law. The set of impulse characteristics obtained in this way is compared to the results obtained experimentally for different shapes of the pulse voltage load. Gases used in the experimental and numerical models include SF6, N2 and Ar. Gas pressures range from 1×10 2 Pa to 6×10 2 Pa, and inter-electrode gaps from 0.1 to 10 mm. A homogenous electric field is considered. © 2012 IEEE.

Rajovic Z.,Electric Power Industry of Serbia | Vujisis M.,University of Belgrade | Stankovic K.,University of Belgrade | Osmokrovic P.,University of Belgrade
IEEE Transactions on Plasma Science | Year: 2013

This paper investigates the synergetic effect of the SF6-N 2 gas mixture, taking the effective temperature of the free electron Maxwell spectrum at the time of breakdown as an independent parameter. In this way, a direct link between a macroscopic variable (dc breakdown voltage of the mixture) and a fundamental microscopic variable (effective temperature) is established. Derivations are presented of expressions that relate the streamer mechanism breakdown voltage in an SF6-N2 gas mixture to the pd product (pressure × interelectrode distance), the percentage contribution of the N2 gas (χ), and the effective temperature of the spectrum of free electrons in the mixture (Td) at the time of breakdown (breakdown temperature). A new theoretical model for the dependence of the electron attachment coefficient (effective cross section) in the electronegative SF6 gas was used, which resulted in the final expression being different from the corresponding expressions obtained from other models. The obtained results were verified by experiments, under well controlled laboratory conditions. There was a high degree of agreement between the experimental and the theoretically calculated results. © 2013 IEEE.

Spiric J.V.,I. Strele 6 27 | Stankovic S.S.,Electric Power Industry of Serbia | Docic M.B.,Electric Power Industry of Serbia | Popovic T.D.,Electric Power Industry of Serbia
International Journal of Electrical Power and Energy Systems | Year: 2014

High values of total percentage losses of electricity in Serbia's distribution network and a very small percentage of success in detecting electricity fraud indicate a need for a sophisticated means of detecting fraud perpetrators. In order for this task to be performed efficiently, the authors of the paper emphasize the need for a suitable and comprehensive use of the billing system or the database of the amounts of invoiced spent electricity in accounting periods and other relevant data regarding registered customers. For that purpose, they suggest using the rough set theory and give a general approach to its use. The point of the paper is forming a criterion for the estimation of accurate (suitable) discretization of original data. The criterion is based on the amount of lost not invoiced electricity due to electricity fraud. Based on consumption characteristics of detected fraud of customers whose measurement points were regularly (monthly) read, a list of the suspicious customers will be formed which will serve as the basis for sending expert teams to specified locations with the task to confirm or dismiss the fraud suspicion. © 2014 Elsevier Ltd. All rights reserved.

Popovic Z.N.,Electric Power Industry of Serbia | Popovic D.S.,University of Novi Sad
Electric Power Systems Research | Year: 2010

In this paper a multi-period planning problem, with arbitrarily defined planning goals, is formulated in terms of graph theory. The proposed formulation represents a multi-period planning problem as a weighted graph problem and thus decomposes original problem into a number of sequences (spanning paths) of static planning problems without loss of accuracy. This graph problem is solved using dynamic programming technique. The proposed dynamic programming algorithm guaranties that optimal solution of multi-period planning problems will be found efficiently, assuming that optimality of static planning problems is guarantied. Detailed numerical results and comparisons presented in the paper show that proposed approach could improve noticeably the quality of multi-period solutions. © 2010 Elsevier B.V. All rights reserved.

Kostic M.M.,Electric Power Industry of Serbia
International Review of Electrical Engineering | Year: 2010

The high-frequency stray load losses (LLr), at rated current, are a result of increase motor current from no-load value (I0) to rated value I=IN. They can be estimating by high frequency no-load losses due to MMF higher harmonics (PHF,f0). The basic idea is to divide the stray no-load losses in two components: first component, high frequency stray no-load losses due to the slot permeance harmonics (PHF,ξ0), is proportionate to the squared motor no-load voltage value; and component of high frequency no-load losses due to MMF higher harmonics (PHF,f0) is proportionate to the squared motor no-load current value. Also, it is shown that the high frequency no-load losses due to MMF higher harmonics (PHF,f0n) can be determined only by noload test with slip measurement by stroboscope method. Synchronous speed measurement test can be omitted. © 2010 Praise Worthy Prize S.r.l. - All rights reserved.

Gvozdenac D.D.,University of Novi Sad | Simic T.S.,Electric Power Industry of Serbia
Thermal Science | Year: 2012

This paper analyzes energy efficiency in Serbia. The analysis has been done on the basis of energy intensity indicators for Serbia and neighboring countries, and some other countries and regions. It relates to the period of some ten years and it is directed to the consideration of required interventions regarding the change of the National Energy Efficiency Policy. Regardless of constant attempts to improve and increase energy efficiency and to expand utilization of renewable energy sources, it seems that accomplished results are still very modest. The analysis of several energy indicators and their changes in the midterm period confirms this statement.

Popovic L.M.,Electric Power Industry of Serbia
Progress In Electromagnetics Research B | Year: 2014

The paper presents a method of determination of ground fault current distribution when HV (high voltage) substations are located in urban or suburban areas, or where many relevant data necessary for determination of this distribution are uncertain or completely unknown. The problem appears as a consequence of the fact that many of urban metal installations are situated under the surface of the ground and cannot be visually determined or veri-ed. On the basis of on-the-site measurements, the developed method enables compensating all de-ciencies of the relevant data about metal installations involved with the fluctuating magnet -eld appearing around and along a feeding power line during an unbalanced fault. The presented analytical procedure is based on the fact that two measurable quantities, currents in one phase conductor and in one neutral line conductor, cumulatively involve the inductive e®ects of all, known and unknown surrounding metal installations. Once, this quantity has been determined, the problem of determination of di®erent parts of a ground fault current becomes solvable by using a relatively simple calculation procedure. The presented quantitative analysis indicates at the bene-ts that can be obtained by taking into account the presence of surrounding metal installations.

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