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During its operation, electrical equipment is constantly exposed to overvoltages of various waveforms, such as lightning discharges, switching operations, faults, etc. Therefore, adequate selection of overvoltage protection and equipment insulation withstand voltages are key factors in increasing system reliability. In general, insulation coordination is carried out by analyzing the insulation behavior for standard impulse and switching overvoltage waveforms. However, overvoltages that occur in operating conditions can significantly deviate from the standard waveforms of insulation withstand voltages, in terms of the front time, the duration of the wave, the existence of highfrequency components as result of reflections in the facility and other factors. This paper presents possible methods of assessing the behavior of equipment insulation for real voltage waves to which the insulation is exposed during exploitation and which deviate from standard waveforms, by analysis in the frequency domain. As there are different components in the overvoltage signal at different frequencies, which a different energy, the main goal of this paper is to analyze the insulation characteristics of a specific transformer from the point of view of the energy spectral density of the voltage wave that occurs at its terminals when an atmospheric discharge occurs. The analysis is based on the determination of the safety margin on the entire frequency spectrum of the overvoltage wave, that is, on the calculation of the so-called FDSF (Frequency Domain Severity Factor). The calculation results show that although the overvoltage amplitude is lower than the withstand voltage of the transformer insulation with an appropriate safety margin (the condition of the classic approach to insulation coordination is satisfied), the spectrum analysis of the voltage wave in the frequency domain shows that the insulation may still be compromised.

The procedure for determining the reduction factor for 110 kV feeding cable lines based on the measurement results is presented in this paper. The obtained results were compared with the values obtained by cable line modeling by ATP/EMTP software tool. The values obtained for reduction factor are compared, as well as the values of induced voltage in the cable sheath. The obtained results show a fairly good agreement between the simulation model and measurements. In the created model, the influence of soil resistivity and conductors arrangement on the values of the reduction factor were analyzed. It has been shown that their combined impact can be significant.

U radu je analizirana mogućnost upotrebe Hejdlerove funkcije u programima za elektromagnetne prelazne procese. Izračunati su parametri Hejdlerove funkcije za razne standardne talasne oblike, pri čemu je za određene talase pokazana veća tačnost nego upotrebom standardom definisanih parametara. Izračunati parametri za odabrane talasne oblike su predstavljeni tabelom i pokazana je mogućnost korišćenja tabele za procenu parametara određenih talasnih oblika koji nisu dati u tabeli. Prikazani su mogući načini primene Hejdlerove funkcije u programima za elektromagnetne prelazne procese.

The paper presents an approach to testing old cable, damaged during atmospheric discharges. A description of the cable is given and a series of tests in the field is proposed in order to evaluate the cable insulation condition. First, the results of the insulation resistance and capacity obtained by the DC test voltage are given and explained. The results of the insulation resistance and capacitance tests as well as the values of the dielectric loss factor tgδ, obtained at sine wave voltage, frequencies of 0.1Hz (VLF), were then interpreted. The example shows a good value of tgδ as well as a breakthrough in cable insulation, and the characteristic changes in the measured cable parameters during the test are highlighted. The advantage of estimating the insulation state by measuring the dielectric loss factor method is emphasized. Finally, the importance of systematic data collection for a particular cable type and the ability to determine the residual life with minimum insulation stress are highlighted.