Power electronics switching devices currently represent the dominant technology for supplying low voltage (LV) electric motors. The fast switching processes exert a different class of stress on dielectric insulating materials than standard sinusoidal excitations. Such stresses result in an increase in the dynamic activity of the working electric field, which in turn lead to an increased likelihood of partial discharges (PD). The stator design of low voltage motor is often in form of random-wound windings, where the magnet wires (copper or aluminum round wires coated with thin layer of insulation) form a common system of coils with not precisely defined mutual position of particular turns, resulting in various turn-to-turn and coil-to-coil voltage distributions. Pulse Width Modulated (PWM) voltage waveforms from modern electronic inverters are characterized by very short rise times and presence of repetitively occurring overvoltages that can significantly stress the insulation of feeding cables and motors. These factors influence the inception and dynamics of PD and processes of space charge accumulation in electrical insulation. In this paper investigations performed on round magnet wire twisted-pair samples representing LV motor random-wound winding elements are presented. Special attention was afforded to the twist configurations, observed breakdown voltage and PD activity. To describe the field conditions for the formation of PD in the turn-to-turn insulation system, the results of numerical simulations of electric field distributions for winding wires with different diameters, modeled using the COMSOL program, were analyzed. PD created in the insulating systems of model twisted-pair systems were registered and analyzed using the phase resolved partial discharge analysis (PRPDA) method.

The paper describes problem related to measurements and interpretation of partial discharges in insulation systems in presence of voltage harmonics, with special focus on epoxy resin based insulation. Laboratory testing system with controlled generation of periodic sinusoidal and harmonically modified voltages was used. Several month investigations of partial discharges in specially prepared epoxy resin samples have been done. Results of measurements of partial discharges linked to high local electric field obtained in point-to-plane electrodes configuration are presented. Due to increasing number of non-linear high power loads harmonic disturbances of supplying voltages are more often present. The investigations of its influence on phase resolved images of partial discharges are the aim of this paper. It was found the influence of harmonics on partial discharge intensity, mainly increase, but also distinct changes in phase-resolved images.

Progress in construction of electrical insulation systems needs new investigations of partial discharges (PD) mechanisms. Working conditions and exploitation stresses, i.e. overvoltages, harmonics and other disturbances have crucial influence on arising of PD in insulation systems. Their influence on partial discharge development is different from normal conditions at nominal sinusoidal voltage. In such a cases interpretation of measurement results (by classical or phase-resolved PD images method) could be more difficult. Paper presents influence of sinusoidal and non-sinusoidal testing voltages (triangular and trapezoidal) on partial discharge distributions, their parameters and changes with increasing of applied voltage. The test object was real insulation system (stator bar thermo-setting insulation system) with many internal PD sources. Presented results of measurements made for sinusoidal, triangular and trapezoidal voltages show the PD mechanism in these conditions. Slew-rate of applied voltage with fixed peak voltage value determines specific conditions for PD generation and has important consequences for phase location of discharges.

Currently, zero-emission targets require future global energy concepts to be based on renewable energy sources; therefore, huge investments are being made in bulky offshore wind parks worldwide. In this context, there is ongoing and enormous development and a need for HVDC submarine cables (both static and dynamic) to connect offshore wind farms. One of the basic problems when analyzing the operating conditions of HVDC cables is assessing the effects of the load current, which generates thermal and electric fields on the insulation systems in these cables. This article considers the problem of the influence of the thermal effect and space charges—the field effect—on the electrical conductivity of polymeric insulating materials and, thus, on the distribution of the electric field intensity in the cable insulation. An analytical methodology for joint analysis of the thermal-effect- and space-charge-related influence is presented. The critical value of the electric field intensity at which the electrical conductivity is significantly modified under coupled thermal–electric exposure is determined. Special focus is placed on the analysis of the coefficient representing the dependence of the electrical conductivity on the temperature in a much broader range than typically assumed. Hence, the intention of this paper is to highlight the limit values of the electric field strength under the simultaneous action of the space charge and temperature gradient. Recognizing the changes in the electric field intensity value in the insulation is of fundamental importance from the point of view of HVDC cable technology and construction.

Artykuł zawiera charakterystykę badań wyładowań niezupełnych wprowadzanych w kolejnych etapach produkcji kabli elektroenergetycznych o izolacji z papieru nasyconego i polietylenu, na wszystkich poziomach napięć znamionowych tj. średnich, wysokich i najwyższych. Zwrócono uwagę na prace CIGRE w tej dziedzinie i działalność Konferencji Wielkich Sieci Elektrycznych, których efektem był pierwszy projekt programu badań poprodukcyjnych kabli wysokiego napięcia opisany w Przeglądzie Elektrotechnicznym nr 1 w 1931 roku. W dalszej części niniejszego artykułu opisano metody pomiaru wyładowań niezupełnych realizowane w systemach pomiarowych zapewniających przetwarzanie przebiegów impulsowych wyładowań do postaci obrazów fazowych. Przedstawiono również wyniki badań laboratoryjnych oddziaływania wyładowań niezupełnych na materiały izolacyjne, w tym identyfikacje procesów erozyjnych w źródłach wyładowań i skutki migracji ładunków elektrycznych na powierzchniach granicznych w układach izolacyjnych.