Ileana-Diana Nicolae

Overvoltages are brief and significant increases in the voltage level in an electrical system. They can be caused by a variety of factors, but the most common are associated with atmospheric discharges (lightning). When lightning strikes a power line, the resulting shock wave can induce surges in electrical equipment that is connected to that line, including transformers. The authors develop an analytical method in order to determine the maximum values of the overvoltages propagating in the transformer windings in case of lightning impulse. It is considered that the transmitted overvoltages consist of an inductive component (magnetic dispersion is neglected, as well as the energy exchange between the capacitances and inductances of the energized winding) and a capacitive component (between the energized winding and the receiving winding; in this case, the emphasis is on the energy exchange between the series capacitance and the inductance of the series winding). The proposed method was applied on a TTUS—ONAN/ONAF 31.5/40 MVA, 110/5/6.6 kV power transformer, and the obtained results were validated by experimental tests. The percentage error between the results obtained by modeling and the results obtained from transformer testing was less than 2%.

Inrush current is still a persistent problem affecting the quality of the power system. This paper presents the theoretical aspects of this transient phenomenon, the simulation of the phenomenon for prediction purposes and its measurement in a test laboratory. In the operation of power transformers, there may also be cases where electrical phenomena affecting the operation of transformers may overlap. When power transformers are in operation, there may also be cases where electrical phenomena affecting their operation overlap. This paper includes a study describing such a situation, where the maximum value of the inrush current is amplified by the fact that the series resonance condition has been met. The intersection of these electrical phenomena resulted in internal and external electrical discharges that led to the overhaul of a 440 MVA transformer; the description of this situation is based on field data. The paper focuses on real information and situations that support the development of a maintenance management schedule based on accurate and up-to-date data. The information presented in this paper will be particularly useful to personnel specializing in power transformer design and/or those monitoring/operating high power transformers in the energy sector, but can also be a teaching aid for students interested in such transient phenomena.

This study presents issues related to electromagnetic pollution and the level of magnetic field radiation occurring around conductors used for electricity transmission and distribution. The fact that modeling and simulation are the most efficient methods of optimization, considering the cost–benefit ratio, was the premise of this work. This paper proposes the performance of a complex analysis, carried out in a comparative manner, which includes physical tests and simulations in the existing field around transmission and distribution cables used in transformer substations. In the first stage, the level of the magnetic field existing near the conductor carried by an electric current was tested (measured), and a virtual model was then designed to simulate the field in conditions similar to those of the test. The results obtained from the simulation were analyzed in comparison with those obtained by testing. The maximum permissible limits of exposure to an electromagnetic field, which are regulated by Government Decision HG 520/2016 of 20 July 2016 and Directive 2013/35/EU of the European Parliament and of the Council of 26 June 2013, were used as the reference to formulate conclusions for both situations considered. These comparisons were intended to determine the level of exposure to electromagnetic fields existing in places where electricity transmission/distribution conductors are located. Energy sustainability exists due to the versatile properties of the conductors, with the energy transmission and distribution network being functional regardless of the source of energy production.

Theoretical models, numerical simulations, and experimental tests for determining surges transferred between the windings of transformers subjected to lightning impulse are addressed. Considering the surges transmitted in the transformer windings have inductive and capacitive components, the proposed model is focused on five aspects: 1) voltage transmission by electromagnetic induction; 2) surge transferred in the transformer windings by capacitive coupling; 3) oscillating component of the voltage transferred between the windings; 4) voltage transferred between the primary and secondary windings; and 5) voltage transferred to the tertiary winding. The maximum transferred voltage is analytically determined. The simulation relies on the Math Script RT Module. Experimental tests were performed on two types of transformers. For the first one (type TTUS-ONAN, 50/67 MVA, 132/13.8/6.6 kV, Ynyn0d1 connection), the maximum voltage between the medium-voltage (MV) winding and ground, in case of a 200 V impulse applied to the high-voltage winding, is around 20.3 V; the voltage transferred to the tertiary winding reaches 8.1 V. For the second type (type TTUS-ONAN, 54/73 MVA, 110/15/6.6 kV, Ynyn0d1 connection), the voltage transferred to the MV winding reaches 21.1 V; the voltage transferred to the tertiary winding is 11.9 V. The laboratory tests validate the proposed analytical and simulation model.

The paper presents several theories related to definitions of powers and power factors in non-sinusoidal and non-symmetrical regimes. The theories must meet some requirements: (a) to facilitate the measuring of power quantities by using acquired electrical waveforms; (b) to support the correct quantification of powers and power factors for a fair charge; (c) to support solutions for efficient compensation of non-sinusoidal and non-symmetrical regimes, simultaneous with the power factor compensation along the fundamental harmonic. Only theories meeting the above-mentioned requirements are approached. Aspects specific to power definitions are discussed and commented. Three theories rely on the Fourier decomposition of non-sinusoidal waveforms, valid only for steady signals, whilst the fourth relies on the Discrete Wavelet Transform (DWT) and can also be applied to unsteady signals. Dedicated original data acquisition systems were used to acquire experimental data for three case studies. Data were analysed with original software tools, based on the Fast Fourier Transform and Discrete Wavelet Transform, implementing the approached theories. Comparisons between results yielded for analogue quantities proved that the approached theories satisfy the requirements for which they were created, except for the fourth theory, which can be used only for compensation purposes.

The paper approaches some theories related to certain power components and power quality indices of active filtering. A brief review of the specialty literature tied to technical aspects of compensation based on active filtering is provided. Following that, the steps of a design created by the authors to perform active filtering with a shunt filter are provided. The developed filtering system was tested and experimental data were acquired on a test stand for different operational modes. The data acquired with and without filtering were compared in order to evaluate the filter efficiency. Data were analyzed and observations were made relative to the limits of one of the analyzed theories. It was concluded that another theory has to be used in order to define powers and indices for the quality of power/energy. The evaluation of compensation was made from a technical point of view, according to the standards in force for the domain of power/energy quality. It was noticed that electromagnetic interferences might occur between the active filter and the supplying network. Two indices are proposed by authors in order to provide a more accurate value for the active filtering efficiency considering the financial point of view. Examples for their utilization are provided in real cases.

In this article, issues related to electromagnetic compatibility and energy/power quality for a laptop are addressed. Power quality problems are outlined. Testing frameworks (developed by authors) are presented and discussed for a laptop when two different sampling frequencies are used: 80 kHz and 19.2 kHz. Waveforms corresponding to the voltage across the laptop’s terminals and current absorbed by it were acquired. Recordings and numerical processing, based on original software using Fast Fourier Transform, are discussed, and the analysis of waveforms in both cases is performed. Various power quality indices were calculated. Theoretical and practical aspects related to the determination of power quantities, power flows, and power factor are presented. Some conclusions concerning different ranges of frequencies are presented, along with some considerations for power quality problems. Tests for conducted emissions (single phase and neutral wire respectively) and for the radiated emissions (for horizontal and vertical polarizations) are presented. Comparisons to the emission standards were made. The immunity behavior for a laptop is discussed according to immunity standards. It is shown that new measures (e.g., active filters) are necessary for network protection when connecting a laptop to the network. Some considerations, based on the authors’ observations concerning new standards for power quality and electromagnetic compatibility, are outlined.

The paper deals with the evaluation of root mean square deviations and maximum absolute relative errors associated to the decomposition followed by recomposition based on Wavelet Packet Transform (WPT) of signals pol-luted with harmonics. Subtrees associated to sets of harmonics presenting practical interest for industrial applications are addressed. The study uses artificial signals generated through the superposition of perfect sinusoids with pairs of harmonics which proved to be related in an almost exclusive manner to pairs of nodes from the bottom level of a WPT tree. 4 parameters had to be considered when determining the maximum and minimum values of errors for each set: the clustered harmonics’ magnitudes and their phase-shifts relative to the component of fundamental frequency. The decomposition/recomposition are time-efficient due to an original system of flags labeling each node from the WPT tree. For each analyzed set of harmonics, 3d graphical representation of minimum and maximum errors along with the associated 3d graphical representation of the phase-shifts are provided. At the same time, per set limits of errors ranges were established and discussed while specific patterns were deduced for the context in which extreme errors appear (phase-shifts and harmonic magnitudes). The results were commented, and conclusions were drawn.