The Nikola Tesla Institute of Electrical Engineering

In this study the modeling of a resistance spot welding (RSW) system is described. The numerical field solution based on the 3D finite formulation allows the calculation of the equivalent circuit parameters, with particular attention to the resistance of each part, and allow the estimation of external magnetic fields. Determining the equivalent circuit is crucial for the efficiency of the RSW, and assessing environmental field pollution is essential for protecting humans and equipment. Two different resistance spot welding operating conditions, AC and DC, are examined. To validate the proposed method, some experimental analyses are conducted on some parts of the RSW system and numerical and experimental results are compared.

Mineral oils may react with the copper conductors in oil impregnated transformers, leading both to the formation of dissolved and suspended copper in the oil and to the deposition of copper onto the paper surface of the solid insulation. Available standard methods for copper detection in mineral oil may not be fully effective in assessing the total copper content. Dissolved and suspended copper species in the oil are shown to affect oil's losses to a different extent.

In this paper modern methods and techniques for water purification are described. Good water quality and the rational use of water are an absolute need of the new millennium. The quality of life depends on water quality. Although it is an ecological priority to prohibit the contamination of water, it is a responsibility for environmental engineers and chemists to develop and apply effective methods for water purification. Beside well-known methods and techniques, it is necessary to create new methods for water purification and treatment. Water recycling is especially important. Plants with membrane separation technologies enable savings in fresh water with high efficiency. Methods for monitoring and controlling processes and water protection should be developed simultaneously with the development of techniques for the methods for purification.

This paper discusses the process of functional aging of Geiger-Muller counters. Two types of Geiger-Muller counter chambers were characterized in an experiment using a combined constant voltage. Chamber A had a coaxial geometry and chamber B had a plan-parallel geometry. The experimental results indicate that the aging process was faster in the case of chambers with a coaxial geometry. The results are explained based on the process of electrical discharges in gasses.

The increasing penetration of wind and solar power sources in power networks has led to the need for advanced converters' control technologies to support the integration of these renewable energy sources into the grid. Inverters play a critical role in power networks, and understanding the benefits and limitations of different inverter types can facilitate the effective management of renewable energy sources and contribute to a more stable and sustainable power system. With multiple inverters connected to the grid, different instability phenomena can occur (ranging from low to medium to high frequencies). The main objective of this paper is to explore the instability phenomena related to grid-following inverters in weak power networks with high penetration of wind and solar power sources. These inverters have electronic circuits, such as the phase-locked loop (PLL), that provide information about the phase angle of the grid voltage waveform. This paper provides insight into the advantages and disadvantages of this type of inverter control and its impact on grid stability and intermittency management of renewable energy sources. It will also discuss the challenges associated with stability and propose a possible improvement of the grid-following control approach by introducing additional control circuitry emulating synchronverter [1]. The presented simulation results demonstrate the quality of the proposed modification of the grid-following control approach.

Geomagnetically induced currents (GICs) can cause a range of adverse consequences related to the normal functioning of a power system, e.g. the saturation of power transformers, unusual flows of active and reactive power, voltage fluctuations, the appearance of the third harmonic, a shift of power frequency and erroneous operation of the protection relay. In the worst case, the above mentioned malfunctions may result in thermal destruction of the power transformers and heavy damage of parts or even the entire national power system. The same effect is particularly pronounced in high-voltage power facilities, whose neutral is effectively grounded, as is the case with substations rated voltage of 110 kV (owned by PE EPS), or 220 kV and 400 kV (owned by Joint Stock Company EMS). This paper gives a short overview of the problems related to the presence of GIC and possible preventive actions aimed at countering the potential adverse consequences caused by GIC.

The reliability of power transformers operation largely depends on the insulation system condition, especially on the condition of paper insulation. Determining the condition of paper insulation of transformers in use cannot be made by direct measurements. The most common way of assessing the paper insulation condition is by analyzing the content of 2-furfural dissolved in the insulating oil and by applying correlations between the degree of paper polymerization and the 2-furfural content in the insulating oil. The concentration of 2-furfural in the oil depends on the moisture content of the insulation system and the oxygen and acid content in the oil. In order to gain a better insight into the cumulative content of 2-furfural produced by paper degradation, excluding changes in the 2-furfural content caused by migration between paper and oil at different operating temperatures, dependence of the degree of polymerization of kraft paper to 2-FAL content in the oil, corrected to a reference temperature of 20°C, for a moisture content in cellulose of 1-1.5% and 3-4% is presented in this paper. This diagram has been verified on 4 transformers according to available data from the database of the Electrical Engineering Institute Nikola Tesla. In the future, this diagram should be completed for moisture content of 2.5% in cellulose insulation.

The topic of the paper is related to the assessment of exposure of workers to electric and magnetic fields during maintenance works on distribution overhead power lines. The exposure of workers during maintenance work on 0.4 kV, 10 kV, 20 kV and 35 kV distribution overhead power lines is analysed in the paper. The testing was performed on several overhead power lines with different technical characteristics and configurations. The analysis is based on results of electric field strength and magnetic flux density obtained by both measurements and calculations. In order to show that there are no safety risks arising from the exposure of workers to electromagnetic field, the obtained results are compared to the action levels prescribed by the Directive 2013/35/EU.

This paper is a summary of broadband measurement values of radiofrequency radiation around GSM base stations in the vicinity of residential areas in Belgrade and 12 other cities in Serbia. It will be useful for determining non-ionizing radiation exposure levels of the general public in the future. The purpose of this paper is also an appropriate representation of basic information on the evaluation of measurement uncertainty.

Multi-channel system for remote power quality monitoring of distribution power stations in low and medium voltage supply networks is presented in the paper. The goal was to develop low-cost, space saving device that could simultaneously take measurements from all (usually eight or up to twelve) outgoing threephase feeders in distribution substation. In that case, we should perform at least 3 voltage measurements and 36 current measurements at the same time. In order to save space, but not to reduce the measurement accuracy, a data acquisition system is designed based on real-time multiprocessing with microcontroller and FPGA circuit. Voltage and current measurements and their corresponding high-order harmonics are calculated using fast FPGA circuit, while other calculations (power, power factor, voltage and current phase angles, etc.) are performed in microcontroller. Further savings are obtained using multichannel analog input modules with multiplexed inputs. Communication with supervising computer is done using GPRS modem or wireless network module depending of station location. Results obtained on the developed industrial prototype confirm proposed solution.

In this paper, a method for grounding impedance measurement is proposed. The method is experimentally validated in five substations rated voltage 35/10 kV. Compared to a standard measuring method, the basic benefits of the proposed method are: (a) the current probe is realised in the form of a ground drill (a drill-type electrode designed to be driven into the earth) instead of being used the grounding grid of a next substation, (b) proposed method does not use an overhead line (or a cable line) for purposes to connect remote current probe, (c) the proposed method requires a test signal source whose output power may be in the order 100 W (instead of 1 kW in standard method), and (d) measuring accuracy of the proposed method is the same as in the case when standard method is used.

Posle skoro četrdeset godina rada na dva sinhrona motora od 15,4MVA, 6kV, u PAP „Lisina“ u okviru „Vlasinskih hidroelektrana“,“ izvršena je zamena postojećih elektromašinskih sistema pobude novim statičkim sistemima pobude. Ova investicija je urađena u cilju povećanja energetske efikasnosti, pouzdanosti i raspoloživosti celokupnog postrojenja. U ovom radu su prikazani osnovni problemi i način njihovog rešavanja pri radu sinhronih motora, kao i njihov uticaj na konfigurisanje energetske i regulacione opreme u okviru sistema pobude. Dat je osvrt na metode pokretanja, samosinhronizaciju, održavanje statičke stabilnosti i način regulacije.

This paper addresses the problem of suppression of a systematic measurement error during the detection of a parasitic DC voltage component, superimposed to an AC voltage. As a sensor the toroidal transformer is employed. The origin of the error under scope is the fluctuation of ac voltage around its rated value. Two groups of experiments, based on the new even harmonics method for precise DC voltage offset detection, are performed, analyzed and results compared. The first one successfully resolves detection of parasitic DC components in low voltage distributive grids. The second one is focused on the DC offset voltage detection of the Fluke 6100A Electrical Power Standard. A comparative analysis of the influence of different sets of the even harmonics of transformer excitation current on error propagation, give the consistent results.

In this paper, a novel approximation of the prime counting function, based on modified Eulerian logarithmic integral, is going to be presented. Proposed approximation reduces the approximation error without increase of computational complexity when it is compared to approximation based on Eulerian logarithmic integral. Experimental results were used to support the claim. Combining proposed method with |Riemannian approximation of prime counting function it is possible to design the new approximation function that outperforms Riemannian approximation for all values that were analyzed.

This paper presents a technical solution for the replacement of the excitation system of the unit B1 in TPP 'Nikola Tesla B' as a part of the maintenance service in 2012. Since the generators of TPP 'Nikola Tesla B' have the greatest power in the power system of Serbia, it was necessary to achieve high reliability of the excitation system so that the process of producing electricity is not endangered Considering this, the implemented excitation system uses modern technology with redundancy both in the power and control blocks, which resulted in an increase in the hot reserve by 100%. In addition, it was necessary to adjust the excitation system to increased generator power and performance from 618MW to 667.5MW. In this paper, the main parameters of the excitation system are given: the power, the excitation system control, the thyristor ignition system, the event recorder system, the digital relay protection, as well as the measuring and signaling functions.

While in operation, the generators are exposed to various electrical, thermal, mechanical and chemical stresses, but also to pollutants that shorten their lifetime. All of these factors have detrimental impact on generator, especially to the characteristics of their insulation system. It should be kept in mind that the influence of these factors is interactive and simultaneous. Fast detection and fault repair are of great importance because they enable further exploitation of generator and reduce financial losses due to undelivered energy.

In this paper, we analyze the possibilities of the diagnosis of Parkinson's disease at an early stage, based on characteristics of the input-output curve. The input-output (IO) curve was analyzed in two ways: we analyzed the gain of the curve for low-level transcranial stimulation and we analyzed the overall 'quality' of the IO curve. The 'quality' of the curve calculation is based on basic concepts from quantum mechanics and calculation of Tsallis entropy.

U radu je prikazan postupak modelovanja elemenata elektroenergetskog sistema (EES) prilagođenog za simulacije naponskih prilika i tokova reaktivne snage i njegova upotreba. Cilj razvoja ovog modela ja da se postigne razuman komprimis između složenosti modela elemenata EES sa jedne strane i postignute preciznosti proračuna i tokova snaga i napona i brzine proračuna sa druge. Pri proračunima tokova reaktivnih snaga uzimaju u obzir korekcije zbog tokova aktivnih snage kroz grane kola. Tako dobijene<br> vrednosti su dovoljno tačne (greška manje od 1%) u slučajevima kada je ugao napona preko modela induktivnosti manji od 15 stepeni. Glavna prednost predloženog pristupa je da se rešenje direktno računa u jednom koraku, a ne korišćenjem iterativnog postupka. Zbog toga je predloženi model praktičan i numerički efikasan za prektične primene u mikroprocesorima i programabilnim logičkim kontrolerima koji u okviru opreme koja upravlja EES. Da bi se demonstrirala svestranost predloženog, modeli su korišćeni u simulacijama rada regulatora napona naponske zone. Kontrola napona je realizovana kroz dva kontrolna režima rada. Prvi reguliše ukupnu razmenu reaktivne snage sa susednim zonama i održava je na nuli sve dok je rezerva naponske margine pilot čvora zone dovoljna. U drugom režimu rada održava se željena naponsko reaktivna karakteristika pilot čvora sa zadatim statizmom (koji se takođe može menjati). Rezultati simulacije regulacije napona zone pri različitim promenama opterećenja unutar i izvan zone predstavljeni su i detaljno objašnjeni.<br>

The relay protection settings performed in the largest thermal powerplant (TE 'Nikola Tesla B') are reffered and explained in this paper. The first calculation step is related to the coordination of the maximum stator current limiter settings, the overcurrent protection with inverse characteristics settings and the permitted overload of the generator stator B1. In the second calculation step the settings of impedance generator protection are determined, and the methods and criteria according to which the calculations are done are described. Criteria used to provide the protection to fulfill the backup protection role in the event of malfunction of the main protection of the transmission system. are clarified. The calculation of all protection functions (32 functions) of generator B1 were performed in the project 'Coordination of relay protection blocks B1 and B2 with the system of excitation and power system protections -TENT B'.

In this paper proof of the Polignac's Conjecture for gaps bigger than four is going to be presented. The proof represents an extension of the proof of the twin prime conjecture. It will be shown that primes with a gap of size bigger than four could be obtained through two stage sieve process, and that will be used to obtain a reasonable estimation of their numbers.