This work refers to the criterion values used to assess the state of the active part of the transformer based on the analysis of the effective value of the total vibration acceleration and the frequency spectrum. It was proved in the work that the criteria values should be differentiated for transformers of different rated power. Transformers with lower rated power are characterized by lower RMS values of vibration acceleration than units with higher rated power, which cannot remain without impact on the criteria values. Trend analysis of the total a_RMS values may reveal increasing defects before the currently applicable criteria values are exceeded. In addition, the influence of the position of the sensor on the frequency spectrum of the obtained signals was analyzed. It was proved that the sensors should be mounted in the middle of the transformer tank, between its lower part and the cover. The dependence of RMS value of vibration acceleration on no-load losses was also determined for transformers of different types but the same rated power. This relationship will not have a large share in the total vibrations of the transformer, due to slight changes in the value of a_RMS occurring for all analyzed units.

Rail is the most efficient and low carbon means of transport, but efforts are still being made to improve the energy efficiency of this sector. In Poland, one of the elements of the implementation of the energy transformation in rail is the “Green Railway” program, which assumes an increase in the share of renewable energy sources (RES) in the power supply structure of the sector to 50% in 2025 and 85% in 2030, and ultimately, to 100%. An increase in energy efficiency leads to a reduction in financial costs, and also contributes to improving the environment and, consequently, to enhancing the economic and social benefit through a cost–benefit analysis. Energy consumption in railway operations is characterized (unlike in construction) by being repetitive. This energy consumption is produced in four areas: in the movement of trains; in auxiliary systems in the trains; in auxiliary systems in the infrastructure (lighting consumption of tunnels or sections of track, point heating systems, the signaling and communication systems, etc.); and in stations, workshops and by other consumers. The aim of this article was to review modern technologies using renewable energy sources in rail transport for traction and non-traction customers.

This article presents information about harmonic distortion and resonance in distribution networks. The theory behind harmonics and resonance is presented. Examples from the literature and the results of power quality measurements, as well as the authors’ experiences connected with significant changes in harmonic distortions, are presented. The harmonic resonance phenomenon is explained, and the risk of resonance in a distribution system network is highlighted. Attention is paid to the connection of a new power plant to the network; however, other risks, e.g., those connected to network reconfiguration, are also highlighted. Further simulation case studies are presented to show interactions between volt/VAR control and harmonic distortion. Simulations consider a few scenarios: impact of voltage change on impedance characteristics and resulting harmonics amplitudes, the impact of a capacitor on impedance characteristics, and the impact of network expansion on harmonic distortion. The final part presents alternative, low-cost harmonics mitigation methods. The concept of the utilization of phase-shifting transformers for two twin-type 1 MW plants located next to each other is verified by on-site measurement. The concept of adapting the harmonics spectrum of new devices to impedance characteristics is presented. Finally, an alternative concept for active mitigation of harmonics under resonance conditions is provided. The concept is based on the reactive power correction in order to change the harmonics phase shift. A comparison of harmonic mitigation methods and general recommendations are provided. Further research is outlined.

Synchro-check (SC) relays are devices that control synchronization between different fragments of power system networks. Synchronism is monitored by controlling the voltage amplitude, angle, and frequency differences. The goal of monitoring is to avoid dangerous transients after the circuit breaker (CB) closing, which occur when the differences between the voltage, angle, and frequency at both sides of the CB are above the threshold. Although the idea of SC is straightforward, one needs to realize that blocking of the CB closing may cause even more severe issues, i.e., the deterioration of power system stability and safety. The conflicting goals of the switching operation result in many different interpretations, regarding the recommended settings and utilization of the SC relays in different locations of power system networks. The paper presents considerations connected with switching operations in power systems, in particular, in 110 kV networks. Different problems connected with switching operations are highlighted and described. Moreover, the switching-relevant parameters and switching constraints are provided. Further, the simulation results connected with switching operations are presented, the realistic possible voltage and angle differences before switching operations are provided, and the impacts of mitigation measures—renewables and energy storage—are estimated. Finally, the proposed procedures and algorithms of switching stress mitigation are provided.

W artykule opisano wybrane zagadnienia związane z sieciami elektroenergetycznych niskiego napięcia, do których są przyłączane przydomowe elektrownie fotowoltaiczne. Określono ogólne parametry techniczne sieci dystrybucyjnej celem przybliżenia problematyki związanej z ograniczeniami technicznymi sieci. Dokonano symulacji komputerowych celem zweryfikowania skuteczności zabezpieczeń nadprądowych w sieciach dystrybucyjnych, w których występują lokalne źródła energii

The article is the second part of the analysis of the impact of connecting PV sources to the low voltage distribution network on the risk of overloading the transformer station. Based on the simulations, the problem of overloading MV/LV transformers by connecting a large number of photovoltaic sources was characterized. The model was used to assess the degree of load of the distribution transformer for different values of its rated power. The results of the simulations and analyzes are presented in this article