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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.

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.

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.

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