with the increasing application of power electronic components in driving rotating machines, the electrical stress factors were significantly changed. This paper deals with the investigation of lifetime characteristics for insulation of type II rotating machines performed by breakdown measurement of pulse voltage at the different waveforms and rise time. The results were compared with those gathered at power frequency, using different time scaling parameters (number of pulses, effective time). It is found that the effective time of stress voltage provides a clearer perception to study the lifetime behaviour than the number of pulses, especially for comparing the effect of the different applied voltage waveforms on acceleration aging. The results indicate that the rise time of the pulse voltage is considered the main influence on the life of insulations, it leads to insulation failure in a lower time. Additionally, the effect of combined stress (electrical, thermal, mechanical) was studied by using a specially developed sample, which included the curved parts of windings. It was found that the most critical part of such "coil-windings" are those curved parts because they are stressed not only by higher mechanical but also by larger electrical field strength.

Dielectric materials in their different types are an essential part of a power system. These materials always work under high electric stress which leads to their deterioration at a long run. Therefore, partial discharge (PD), which is a consequent of high electric stress, is an important tool for evaluating the high voltage insulation systems. In this paper, the properties of void discharges within different dielectric materials under sinusoidal AC voltage at power frequency were investigated experimentally and through modelling. The test object consists of a cylindrical void with different dimensions and positions in different insulating materials like Silicone Rubber, Ethylene Propylene Diene Monomer Rubber, and High-Density Polyethylene. The finite element analysis by using COMSOL Multiphysics software was used for modelling while the experimental work was done by using PD analyzer LDD-6. This paper includes studying the effect of changing the void position and dimension on the partial discharge properties. It is well known that the electric field intensity inside any void is related to the outside electric field according to the ratio of the permittivities. Therefore, the changing of void position in the uniform field should have no effect of the electric stress inside the void. However, the obtained results showed that the PD properties affected by different void positions. This may be due to increase the potential difference across the void as well as the inception voltage value when the void approaches to high voltage electrode, and vice versa, when it approaches to ground electrode. So that the partial discharge values increase when changing the void position from ground electrode to high voltage electrode. The change of PD values in high relative permittivity insulating material is higher than low relative permittivity materials at the same void dimension. The obtained results from the experimental work and simulating agree well for the case of a uniform field by using the electrode having the Rogowski profile. However, still much work has to be done for the case of a non-uniform field.

The increased application of HVDC components leads, at the same time, to forced usage of Partial Discharge (PD) measurement as an important tool for quality testing and reliability. Current issues are missing relations between the PD behaviour caused by insulation defects and the measured PD parameter as well as requirements for appropriate PD measuring systems. The adequate modelling of PD behaviour under DC conditions could improve that situation and might lead to deeper understanding of physical PD processes within the insulation. The paper deals with the modelling of partial discharge behaviour at DC condition by using a modified three capacitances (abc) model and compares it with its behaviour at AC applied voltage. The improved model has been used to study the effect of temperature, applied voltage, and void dimensions on partial discharge behaviours within a cylindrical void inside XLPE insulating material. The results show that under DC, the temperature rise will greatly affect the repetition rate, while the magnitude of the applied voltage is of higher influence in the case of AC. Also, the results show the magnitude of PD increases with higher void size, and the larger effect is the increase of the void radius.

It is well known that the presence of a void affects the performance of insulating material and its life time. The properties of partial discharge (PD) occurring at a void inside a solid insulating material are effected by the void conditions, mainly its dimension, position, temperature and gas pressure, which determine the electric field distributions within the void site and in turn influence the properties of partial discharge. In this paper, the PD properties of interest were inception electric field, maximum PD apparent charge and repetition rate. Composite and traditional insulating materials like Silicon Rubber, Ethylene Propylene Diene Monomer Rubber, high density polyethylene and Polyvinyl Chloride were used. Two different models depended on the classical abc and the finite element analysis model were used to represent a cylindrical void within homogeneous insulating materials. These models are used to study the effect of dimensions and positions of the void on the PD properties. The obtained results show that the repetition rate and the inception electric field depend on the void height. On the other hand, the maximum PD apparent charge depends on the radius of the void. Changing the void position has no significant effect on the PD properties.

Partial discharge (PD) is one of the major factors that damage dielectric materials in medium and high voltage components. Therefore, partial discharge evaluation is generally used to observe the status of electrical insulation in high and medium voltage components used in the performance appreciation of an insulation system. In this work, partial discharge behavior on solid- air interfaces has been investigated commonly characterized as surface discharge. The investigation was performed by PD measurement as well as by adequate modelling for different insulating materials as the Natural Rubber (NR), Silicone Rubber (SiR), Ethylene Propylene Diene Monomer Rubber (EPDM) and High- Density Polyethylene (HDPE). Through modeling the partial discharge process, a better conception of the phenomenon may be achieved. The model of different homogeneous insulating materials has developed using Finite Element Analysis (COMSOL) software in parallel with MATLAB programming function. The results of the study indicate that the repetition rate and the peak apparent charge increased with the insulating material relative permittivity increase while the inception voltage does not change significantly.

Insulation materials are critical for the reliability and performance of electrical power systems, particularly in high-voltage rotating machines. While failures can arise from thermal, mechanical, or electrical stress, they predominantly manifest as electrical breakdowns. Prior research has primarily concentrated on aging in straight winding sections, despite evidence indicating that failures frequently occur in the bending regions of turn insulation. This study explores the influence of high-frequency pulsed electrical stress on the lifetime behavior of Type II insulation systems used in high-voltage rotating machines. Practical samples, designed with geometric configurations and technology akin to that in rotating machines, were tested under conditions characterized by voltage slew rates (dv/dt) exceeding 10 kV/μs, with variations in frequency and waveform shape. The findings reveal that the rate of electrical aging remains consistent across differing pulse widths, risetimes, and polarities, displaying a similar lifetime exponent. Nonetheless, insulation durability is markedly more compromised under pulsed conditions. At the identical times-to-failure, the sinusoidal waveform necessitated nearly twice the applied peak voltage as the bipolar pulse waveform. This finding clearly suggests that pulsed excitation exacerbates insulation degradation more effectively due to the sharp rise times and high (dv/dt) rates imposing substantial electrical stress on dielectric materials.