Typically, in laboratory and diagnostics tests focused on measuring partial discharges, a pure sinusoidal voltage waveform is assumed. However, in practice the spectral content of the applied voltage is rarely so ideal and additional spectral components have a significant impact on the discharge behavior in electrical insulation systems. In this paper the influence of voltage harmonics on partial discharge behavior and pattern evolution is analyzed. The presented experiments were conducted on a transformer installed in a municipal network and on specimens representing defects in a transformer insulation system, in the form of a micro blade protrusion on a winding conductor and an embedded gas inclusion. The experimental results showed that various harmonic compositions superimposed on the fundamental sinusoidal waveform have a significant impact on PD intensity and maximum charge. In consequence, PD pattern phase and amplitude distributions are distorted and calculated statistical parameters are changed. The rough parameters reflecting the waveform spectral quality, such as THD, are rather superficial and do not provide a unique and reliable signature of the applied voltage in terms of phase angle variations. In certain environments, neglecting harmonic content in the voltage may lead to a misleading interpretation and assessment of partial discharge severity. A complete knowledge of harmonic content is essential and critical in conducting a proper assessment of the impact of PD, including acceptability criteria and compatibility with the standards.

The fast switching in PWM drive applications exerts complex transient stresses on the cable and motors, creating conditions for different partial discharge form initiation. The mechanism of surface PD on dielectric materials (ethylene-propylene rubber and thermosetting insulation) under semi square voltage has been investigated. In the rod-plane electrode arrangement positive polarity PWM-like pulses with variable rise time (800 ns and longer), and frequency (0.1 Hz to 2 kHz) have been used. Experiments show correlation of PD inception voltage and its time range within a semi-square voltage period with slew rate of applied voltage and its switching frequency. Partial discharges appear not only at rising/falling slope of the voltage pulses but also during the voltage plateau, even when the rod electrode potential becomes zero. The main process taking place during surface PD action on insulating materials is the interaction of space charge near the rod electrode with the polarized dielectric. As a result it was revealed that in such conditions PD can occur within a whole period. This phenomenon is relevant to understand the destructive processes in insulating materials under PWM-like pulses action.

This paper describes a novel approach to understanding partial discharge (PD) dynamics and mapping of the discharge channels in a void. During long term PD exposure, discharges develop inside channels within a void, leading to a variation in the discharge area on the void wall as well as a clustering effect. In this paper, the development of a method for identifying localized changes in the dielectric structure, through the detection of dispersed channels with reduced resistivity which occur due to multichannel discharges, is described. Discharge spots in selected ranges of surface resistivity, occurring due to the energetic impact of PD, are mapped. This mapping shows both the distribution of local centers on the surface of the specimen, characterized by a reduced value of surface resistivity. Furthermore, during aging tests this surface resistivity exhibited a progressive reduction by four orders of magnitude, justifying the occurrence of an effective discharge area in the void source. This additionally impacts PD pattern modulation and can be a possible initiation of the formation of cross channels in the bulk isolation which, in the long term, can lead to complete discharge.

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.

This paper describes the influence of AC harmonics in HVDC systems on one of the main indicators of high voltage insulation deterioration, partial discharge. HVDC systems may be subject to various disturbances including harmonics and ripples in the form of pulsating voltages. Experiments, performed on model void specimen at DC voltage with selected superimposed harmonics, are described. A novel approach for recording partial discharges at DC, based on synchronization with selected superimposed harmonics, is proposed. In this way individual partial discharge pulses, occurring during a given measurement time, are accumulated with respect to the phase angles of certain harmonics. The partial discharge inception threshold is analyzed for cases where the DC level is held constant and the AC magnitude and harmonic frequency are varied and for cases where the DC level is varied and the AC magnitude is held constant. In addition, a long term aging test where insulating paper and resin impregnated paper was subjected to pure DC voltage and DC containing 12th harmonic 600 Hz, is described. The aging test shows that the superimposed harmonic content resulted in an amplification of partial discharge presence. By mapping the discharge spots occurring on the void surface, it is possible to quantify the level of erosion in both cases. It was observed that in case of harmonic crest voltage being below partial discharge PD inception level the DC component is crucial, however when harmonics magnitude is above the inception voltage the role of AC is dominant relative to DC. Thus, the presence of harmonics may result in a strong acceleration of the deterioration of insulating materials subjected to DC voltage.

A novel investigation into the behavior of the interface between an insulating material and a conductor subjected to sinusoidal stimulus is presented. Both copper and aluminum wires are used in the windings of electrical machines and also as power cable conductors. During in-service operation, partial discharges (PD) may be initiated in the insulation systems. The paper presents experimental results showing the interaction of different kinds of conductors and dielectric materials on the mechanism of partial discharges in the insulation system. The dedicated test setup is designed to perform experiments with various electrodes, with special focus on copper. Two kinds of specially prepared insulation samples were tested, ethylene propylene rubber (EPR), cross-linked polyethylene (XLPE). These insulating materials were subjected to a sinusoidal stimulus at predefined time intervals and were exposed to PD action. The paper comprises the results of PD measurements and microstructural observations performed by means of scanning electron microscope (SEM) coupled with energy dispersive analysis (EDS). The morphological and elemental analysis reveals the interaction of conductor on polymeric material degradation. Two zones were distinguished, migration and erosion. The migration of atoms of electrode material into the dielectric substrate due to PD activity was observed.