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Ferroresonance is a complex nonlinear electrical phenomenon that can cause significant dielectric and thermal stresses in power equipment, leading to potential faults, equipment damage, power supply interruptions, etc. Three-phase networks with isolated neutral point and single-phase isolated inductive voltage transformers are susceptible to ferroresonance, caused by the interaction of the transformer's nonlinear inductances and zero sequence capacitance of power network. The paper analyzes the possibility of ferroresonance occurrence in a real 6 kV power network, with an isolated neutral point, using the electromagnetic transients program ATP/EMTP. A detailed network model is developed, taking into account the nonlinearity of voltage and power transformers, which is utilized to analyze various configurations, switching operations, and faults in the network that could trigger ferroresonance. It is shown that, under certain circumstances, ferroresonance can occur, with faults clearing being the most common initiators of ferroresonance. The efficiency of different measures to suppress ferroresonance is also analyzed. The impact of the resistance values of voltage transformer's open delta winding dumping resistor on the suppression of ferroresonance occurrence, is considered.

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 paper analyzes the possibility of using the Heidler function in programs for electromagnetic transients. The parameters of the Heidler function are calculated for various standard waveforms, where greater accuracy is shown for certain waves than with the use of parameters defined by the standard. The calculated parameters for the selected waveforms are presented in the table and the possibility of using the table to estimate the certain waveform parameters not given in the table is shown. Possible ways of applying the Heidler function in programs for electromagnetic transients are presented.

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.

The paper presents the possibility of using high voltage tests within frame of acceptance tests in the power facilities of renewable energy sources. The practical problems that present them to the examiners on site and the ways to overcome them are described. Special emphasis is given to the testing of power cables, because these tests are particularly on site demanding. Realistic test results of busbars and cells of metal-enclosed switchgear, power cables and surge arresters are presented with interpretations, comments and recommendations.

The tower footing grounding system is of major importance for analysis of the lightning overvoltages on an overhead transmission line during backflashover and shielding failure. This paper presents one approach to wide-band modelling of tower footing grounding impedance based on impulse response measurement. The model derived is implemented in the simulation software ATP/EMTP. A general methodology is presented based on the Matrix Pencil Method (MPM) for rational fitting of time domain measured signals and determining frequency domain response. The MPM is used to estimate the poles and residues of the frequency response using time domain signals. The second part of the paper describes the process of implementation of the model capable of representing grounding structures over a wide band of frequencies in EMTP type software. At the end of the paper, it is shown how the described procedure was applied to one measured impulse response of the tower-footing grounding system. The obtained results show that the simulation results of the impulse response of the tower footing grounding systems model correspond very well to the measured impulse response. It has also been shown that MPM model filters the measured signals appropriately.

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.

The paper deals with the topic of the installation and connection of temporary field electrical installations by external contractors in power plants and large industrial plants. One of the possible solutions and a detailed description of the equipment are recommended, and a way of connecting and installing of a temporary field electrical installation is given. The distribution of a temporary field electrical installation containing connecting cables, a main distribution cabinet with associated protective devices, isolation and instrument transformers, electrical installations from the contractor's distribution cabinets to the other distribution cabinet and the junction box is described. The significance of the contractor's training was recognized and, on a characteristic example, an explanation of possible synchronization of still valid rules and new standards regarding the application of the protective measure - electrical separation was shown on a characteristic example. The boundaries of the contractor's responsibility regarding the correctness of electrical installations are clearly given. Finally, one way of running and connecting a temporary electrical installation is proposed in accordance with applicable laws, regulations and standards.

This paper demonstrates the procedure for estimating parameters of a dynamic metal-oxide surge arrester model by using a genetic algorithm, implemented in ATP/EMTP graphic preprocessor (ATPDraw) optimization module. The advantages of new ATPDraw options that allow optimization of electric circuit elements are shown. The optimization process is applied to two frequency-dependent MOSA models. At the end of the work, a comparison of results obtained before and after optimization is given.

The goal of this paper is to introduce a procedure for risk assessment of insulation failure due to storm surges. The simulations were carried out in ATP / EMTP software, which allowed the use of the Monte Carlo method for the statistical analysis of insulation coordination. The paper gives a detailed account of modelling of all the elements relevant to the analysis. At the same time, the capabilities of the programming language MODELS (allows the creation and implementation of new complex components within the ATP software) are discussed.