Paolo Seri

Qualification of inverter-fed electrical machines through the procedure suggested in the IEC std. 60034-18-41 entails the application of thermal, mechanical, and ambient stresses, followed by measuring partial discharges (PD-proof test). For practical reasons, stress factors are applied sequentially, and measurements are conducted on the machine at room temperature. This procedure is simple but has two major limitations. The first is that testing will not activate synergies between stresses. The other one is that partial discharge inception voltage measurements under impulsive voltages sometimes provide overly optimistic results. This paper suggests an alternative procedure to conduct the qualification of inverter-fed machine insulation. The proposed procedure combines electrical and thermal stress, thus accounting for their synergies. As the applied electrical stress is like that experienced in service, partial discharges can be initiated during aging. This might lead to failure without the need for testing partial discharge.

Mathematical modelling is a fundamental instrument in the development of theoretical validation for experimental results and in their interpretation. The gist of this paper is the application of a special multi-valued function, called Lambert W function, to several aspects of electrical engineering. This multi-valued function serves the purpose of inverting a vast gamut of expressions involving exponential functions, and can be used in many models concerning electrical insulation issues, yielding lower computational times as opposed to other methods solving nonlinear problems. In particular, this paper presents the analytical calculation of parameters for capacitor field and equipotential lines, Paschen’s law inversion, a model for electric field distribution in power cables, the inversion of Schottky and Dissado, Montanari, Mazzanti model for cable life estimation.

HVDC cables are subject to several types of impulses superimposed on the rated DC voltage during their service lifetime. Temporary Overvoltages (TOVs) and Superimposed Switching impulses (SSIs) are considered some of the most challenging due to the relatively long impulse duration. This paper aims at investigating experimentally the effect of TOVs and SSIs on XLPE insulation for extruded HVDC cables. 0.15-mm-thick DC-XLPE specimens, aged by applying TOVs and SSIs, are characterized using dielectric analyzer and Fourier Transform InfraRed spectroscopy (FTIR) to detect the aging effects on the insulation. Results show an increase in the imaginary part of permittivity, ε", accompanied with the appearance of additional dipolar polarization losses peaks. The amplitude and frequency of the aforementioned peaks vary with the amplitude and the number of applied TOVs and SSIs. An increase in electrical conductivity is also noticed with aging. FTIR results show absorbance peaks in the aged specimens likely due to the intramolecular bonds rupture accompanied with the formation of aging products. In summary, SSIs and TOVs cause a noticeable reduction of insulating properties in XLPE specimens. The higher the peak of the transient, the greater the aging effect.

This paper proposes a circuital model that describes polarization and depolarization currents observed in thin samples of polymeric materials under the application of an average electric field up to 30 kV/mm. The model employs the least possible number of parameters to fit the experimental measurements. A single carrier with negative charge is considered. The polarization curves are modelled by a power law to account for transport assisted by shallow traps. A model for the deep traps with a Gaussian distribution of the density of states is describes the depolarization curves. Experimental measurements made on various materials with different charging and discharging times suggest that a portion of the deep traps is filled within few seconds, while the time needed to release the trapped charges is in the order of several hours. The agreement between experimental and simulated currents confirms the validity of the proposed model, that can be seen as a generalization of the extended Debye model.

HVDC cables are subject to several types of impulses superimposed on the rated DC voltage during their service lifetime. Temporary Overvoltages TOVs and Superimposed Switching impulses SSIs are considered some of the most challenging due to the relatively long impulse duration. This paper aims at investigating experimentally the effect of TOVs and SSIs on XLPE insulation for extruded HVDC cables. 0.15-mm-thick DC-XLPE specimens, aged by applying TOVs and SSIs, are characterized using dielectric analyzer and Fourier Transform InfraRed spectroscopy (FTIR) to detect the aging effects on the insulation. Results show an increase in the imaginary part of permittivity, ε”, accompanied with the appearance of additional dipolar polarization losses peaks. The amplitude and frequency of the aforementioned peaks vary with the amplitude and the number of applied TOVs and SSIs. An increase in electrical conductivity is also noticed with aging. FTIR results show absorbance peaks in the aged specimens likely due to the intramolecular bonds rupture accompanied with the formation of aging products.

This study, in two parts, investigates the design of high-voltage direct-current (HVdc) cables depending on electrothermal life modeling of the insulating material. The first part investigated the way in which different parameters affect the design of an HVdc cable in steady-state conditions, i.e., with known nominal voltage and ampacity. This article investigates the effect of electrical (i.e., voltage) transients, such as voltage polarity reversals (VPRs), in addition to thermal transients (such as load cycles of cable current) on the electrothermal life map of HVdc cables, and their influence on cable feasibility. An ad hoc MATLAB code has been developed for this study. The results show a considerable negative effect of electrical transients on the expected life. A more severe effect of electrical transients is noticed for materials with a lower value of the temperature coefficient of conductivity, due to a longer duration of transients. On the contrary, thermal transients associated with load cycles—which mostly imply current values equal to or lower than cable ampacity—imply insulation temperature values always lower than the maximum permissible temperature, and thus, they have a positive effect on the electrothermal life since the thermal stress applied on the cable insulation will be reduced, allowing for smaller cable geometries for a fixed voltage and ampacity rating. It must be noticed that only the electrothermal life is investigated in this article, and other types of stresses (e.g., mechanical stress) may also affect insulation life, but this is out of the scope of this study.

The development of more electrical transportation systems requires rotating machines having higher efficiency and specific power. To date, this is achieved by feeding them with rising supply voltages with higher frequencies. This generalized strategy is introducing several challenges in the correct design of the insulation that is necessary for a reliable system. A major issue to be dealt with are partial discharges (PDs), an aging mechanism that can bring insulation to failure in short times. A solution often proposed by manufacturers involves the use of the so-called corona resistant (CR) materials, which supposedly can withstand PD activity. This article investigates the possibility of using CR magnet wires for electrical machines operating at reduced pressures, such as the actuators for primary control surfaces in more electric aircraft. The results show that CR insulations are characterized by an improved behavior at ground level but are not a viable option at reduced pressures.