PDIV Modelling for Rectangular Wire Turn-to-Turn Insulation of Inverter-Fed Motors through Thermal Ageing

This contribution develops the partial discharge inception voltage (PDIV)-FEM-based model based on Schumann’s streamer inception criterion (SCSIC) with respect to thermal ageing time (TAGT) or the insulation lifetime for the turn-to-turn insulation of coil winding made by rectangular insulated wires. The accelerated thermal ageing is performed at 250°C, which is 50°C higher than the thermal class, with ageing intervals of 156 and 312 hours. PDIV measurements are conducted at a constant temperature of 25°C, 40% relative humidity, and atmospheric pressure (1013 mbar). The measurements are done under AC 50 Hz excitations, following IEC 60034-18-41 guidelines for inverter-fed motor insulation system qualification. Furthermore, this study analyzes streamer inception parameters such as critical field line length (CFLL), effective ionization coefficient (α eff ), partial discharge (PD) inception field ( E discharge ), firing voltage (V firing ) and its correlation with PD charge amplitude as a function of TAGT or the insulation lifetime

Assessment of Edgewise Insulated Wire Bend Radius Impact on Dielectric Properties of Turn-to-Turn Insulation through Thermal Ageing

This study aims to evaluate the impact of the bending radius of edgewise insulated wires on dielectric properties such as partial discharge inception voltage (PDIV), partial discharge extinction voltage (PDEV), dielectric dissipation factor (DDF), and insulation capacitance (IC) through thermal ageing. The tests are performed at room temperature (20°C) and atmospheric pressure (1013 mbar) on unaged and thermally aged polytetrafluoroethylene (PTFE)-wrapped pairs of edgewise insulated wires, models of the turn-to-turn insulation. The accelerated thermal ageing is carried out at 250°C (i.e., 50°C higher than thermal class) for two different ageing periods: 156 and 312 hours. To manufacture a coil, it is generally demanded to shape 90-degree bends out of edgewise enamelled winding wires. Therefore, the obtained experimental results are helpful for the coil manufacturers, providing a clue how the bending radius can impact the dielectric properties of turn-to-turn insulation and introducing an optimum radius which can present better insulation performance

Fillet Radius Impact of Rectangular Insulated Wires on PDIV for Turn-to-Turn Insulation of Inverter-Fed Motors

This contribution elucidates the impact of the fillet radius, a geometric feature of rectangular insulated wires not commonly considered, on the partial discharge inception voltage (PDIV) in low-voltage machine turn-to-turn winding insulation. Initial PDIV tests involve edgewise-insulated wire samples with a reference fillet radius. These measurements are performed under constant conditions: 30°C temperature, 35% relative humidity, and atmospheric pressure (1000 mbar). These tests are carried out under AC 50 Hz excitations, following IEC 60034-18-41 guidelines for inverter-fed motor insulation system qualification. Subsequently, a probabilistic PDIV predictive model is developed based on Schumann’s streamer inception criterion (SCSIC). This expanded model then analyses and forecasts the impact of the fillet radius on PDIV and its associated dispersion level, within the context of the 2-parameter Weibull distribution and the given environmental conditions. Furthermore, a novel method is presented to understand partial discharge (PD) phenomenology and its destructive potential in rectangular insulated wires with varying fillet radii. This approach employs SCSIC-derived streamer inception parameters (SIPs): critical field line length (CFLL), air effective ionization coefficient (α_eff), PD inception field (Einc), and firing voltage (Vfiring). The developed probabilistic predictive model enables the selection of an optimal fillet radius value, facilitating the creation of a reliable insulation system with maximum PDIV and minimal PD-related damage

Modeling Humidity Impact on PDIV for Turn-to-Turn Insulation of Inverter-Fed Motors at Different Temperatures

This paper models the partial discharge inception voltage (PDIV) as a function of ambient humidity ( H ) at various temperatures ( Ts ) using Schumann’s streamer inception criterion (SCSIC) for turn-to-turn insulation, which is the most vulnerable part in inverter-fed motors’ insulation system. The Schumann constant (i.e., the natural logarithm of the critical electron number defining the Townsend-to-streamer discharge transition, K ) varies with H , showing distinct patterns at low and high Ts . The K equations are derived across a wide range of relative humidity (RH) levels (20%, 30%, 40%, 50%, 60%, 70%, 80%, and 90%) at four Ts (25°C, 40°C, 60°C, and 90°C). These equations can be used in finite element analysis software to predict PDIV under varying H with outstanding accuracy. Additionally, a novel approach is presented for partial discharge (PD) phenomenology under H variations at different Ts using SCSIC-derived streamer inception parameters (SIPs): critical field line length (CFLL), air effective ionization coefficient (α eff ), partial discharge (PD) inception field ( E inc ) and firing voltage ( V firing ). Notably, at high Ts (e.g., 90°C), a transition phase emerges concerning RH, leading to significant SIP changes due to the disappearance of a critical region (CritR) in α eff at specific electric field intensities. The developed humidity-dependent PDIV model supports insulation designers in achieving PD-free designs that account for H variations and sheds light on SIPs variations concerning H changes

PP/PP-HI/silica nanocomposites for HVDC cable insulation: Are silica clusters beneficial for space charge accumulation?

New potential High Voltage Direct Current (HVDC) cable insulation materials based on nanocomposites are developed in this study. The nanocomposites are produced by blending of polypropylene (PP), propylene-ethylene copolymer (PP–HI) and a modified fumed silica (A-silica) in a concentration of 1 and 2 wt %. The A-silica is successfully modified with (3-aminopropyl)triethoxysilane (APTES) via a solvent-free method, as proven by infrared spectroscopy, thermogravimetry and transmission electron microscope mapping. A-silica in the polymer matrix acts as a nucleating agent resulting in an increase of the crystallization temperature of the polymers and a smaller crystal size. Moreover, the silica addition modified the crystals morphology of the unfilled PP/PP-HI blend. The composite containing A-silica with 2 wt% contains bigger-size silica clusters than the composite filled with 1 wt%. The composite with the higher A-silica concentration shows lower space charge accumulation and a lower charge current value. Besides, much deeper traps and lower trap density are observed in the composite with 2 wt% A-silica addition compared to the one with a lower concentration. Surprisingly, the presence of silica clusters with dimensions of more than 200 nm exhibit a positive effect on reducing the space charge accumulation. However, the real cause of this improvement might be due to change of the electron distribution stemming from the amine-amine hydrogen bond formation, or the change of the chain mobility due to the presence of occluded polymer macromolecules constrained inside the high structure silica clusters. Both phenomena may lead to a higher energetic barrier of charge de-trapping, thus increasing the depth of the charge traps

Modelling of supply voltage frequency effect on partial discharge repetition rate and charge amplitude from AC to DC at room temperature

This paper has the purpose to derive an analytical model ('continuum' model) able to describe the behavior of partial discharge (PD) repetition rate and amplitude, occurring in a cavity embedded in polymeric insulation, as a function of the frequency of the supply voltage, going from AC power supply frequency, 50-60 Hz, to DC. In the range between DC and 50-60 Hz focus is made on data coming from tests under AC sinewave with very low frequency (VLF) such as 0.1 Hz and 0.01 Hz, which are commonly used for cable testing. It is shown that the proposed 'continuum' model can provide reasonably good fit to the experimental results obtained in the range DC to 60 Hz, regarding PD repetition rate and amplitude. To reach such result, the equivalent circuit is modified from that commonly used and made by fully-capacitive or resistive components, in order to take into account the change of polarization mechanisms which, depending on dielectric material, may play a non-negligible role to establish the repetition rate from low frequency to DC power supply. In addition, the residual voltage after a PD event has to vary with frequency to reach good fitting. Also, it is shown that PD amplitude under DC and VLF can be lower than under AC 50-60 Hz due to the delay time of the firing electron, thus experimental PD amplitude varies with frequency depending on material and defect typology and location

Temperature Impact on PDIV for Turn-to-Turn Insulation of Inverter-Fed Motors: from Ground Level to Cruising Altitude

Considering the turn-to-turn insulation as the most susceptible part to the failure of the electrical machine’s insulation system in inverter-fed motors, this paper aims to elucidate the effect of temperature ( T ) on the streamer inception parameters (SIPs), including the Schumann constant ( K ), critical field line length (CFLL), air effective ionization coefficient (α eff ), and streamer inception field ( E inc ). Particular attention is also given to extending the partial discharge inception (PDIV) modelling at cruising altitude (CA), equivalent to the air pressure of 200 mbar, as a function of T based on the Schumann streamer inception criterion (SCSIC), compared with the results obtained at 1000 mbar, i.e., ground level (GL). Furthermore, the ratios of SIPs and PDIV values at CA to those attained at GL are analysed as a function of T . The investigation findings serve as a prescription for electrical machine designers to enhance the insulating design of electrical machines used in More Electric Aircraft (MEA) applications

A Contribution to Everlasting Electrical Insulation for DC Voltage: PD-Phobic Materials

This paper focuses on the design of insulating materials candidate from DC-supply applications, proposing viable solutions that may increase the defect-tolerance of an insulation system and avoid the presence of highly energetic phenomena, specifically partial discharges, which can cause premature insulation breakdown. It is shown that, in principle, polymeric materials for DC insulation can be structured, possibly through nanotechnology, to avoid partial discharge inception in steady state even in UHVDC cables and high-field design insulation systems, which would exclude partial discharge degradation from the aging factors that can affect insulation reliability and life. This approach may provide basic tools to design DC insulation having electro-thermal life and reliability of virtually unlimited extent, thus of so-called PD-phobic materials

An Approach to Noise Rejection and Partial Discharge Separation in DC Cable Testing, during Steady State and Voltage Polarity Inversion Transients

Quality control, commissioning and operation of DC cable systems (and, in general, electrical apparatus) require PD testing and monitoring technologies that are effective in rejecting noise and identifying partial discharges and their source, both under DC and AC power supply. Such technologies are not fully available at present, so that this paper presents a contribution to make them feasible. A new algorithm is suggested that may be effective to separate partial discharges from noise both under steady-state DC and during voltage transients, through an automatic and unsupervised approach which looks very promising for laboratory and on-field testing and monitoring of partial discharges in DC cable systems