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This paper has the purpose to investigate the variation of electric field inside a HVDC cable insulation, and consequent potentiality of partial discharge inception in a defect upon load current variation. Simulations carried out by COMSOL Multiphysics show that load variations can incept partial discharges during cable operation at a constant voltage, depending on insulating material characteristics, defect size and location, and, in particular, the temperature and field dependence of electrical conductivity. Different values for temperature and field dependency of electrical conductivity, as well as of thermal conductivity and heat capacity are considered and their effect on the field distribution inside cavities near inner and outer semiconducting layer analyzed. For this purpose, a new analytical model to predict the behavior of PD amplitude and repetition rate from cable energization to steady state, including load variation, is introduced. The likelihood of PD inception during load transients, and not only voltage transients as previously demonstrated, poses further attention and constraints to HVDC cable design.

Condition monitoring and maintenance in rotating machine are generally managed through expert evaluation of the diagnostic properties and relevant harmfulness of the underlying aging mechanisms. In order to prompt broad diffusion of condition monitoring systems, especially in large assets and in the rising field of electrification transport, however, ways to get rid of the time-consuming and expensive support of experts must be prompted. A straightforward solution is transiting towards automatic and unsupervised diagnostics and condition assessment methodologies. This paper proposes and applies algorithms for the automatic detection of partial discharges, which is the property most often associated to the fastest accelerated aging mechanisms in electrical insulation, including noise rejection, identification of the type of partial discharge sources and estimation of an health condition index in rotating machines fed by AC sinusoidal voltage. The goal is to obtain a self-assessment of the health condition by each rotating machine of an asset, which can thus interact with the asset or maintenance manager when needed, that is, when reliability is at risk.

Broadening the concept of smart grid to include bi-directional interactions between grid and asset electrical components as regards their health conditions would help optimizing maintenance and operation actions from asset/grid management side. However, as long as data processing from diagnostic tools requires long and costly expert time, there is no real chance that the knowledge and technology developed in the field of condition monitoring can become broadly available for grid smart management. This article presents algorithms able to automate the acquisition of health information from electrical asset components. Referring to partial discharges, which are associated with the most harmful insulation aging factor, an innovative technique able to reject noise and thus make automatic partial discharge recognition possible, is proposed and its validity assessed. The information from partial discharge measurement, and other diagnostic quantity monitored is translated into a simple and straightforward indication of component health. This assessment can be automatic, dynamic and continuously updating during operation, thus making feasible an effective time-based condition maintenance approach. The outcome of the proposed approach is to contribute keeping asset reliability at a specified level, minimizing disastrous outages and increasing the return of investment. This benefits also optimal electrical component operation and power flow.

Quality control, commissioning and diagnostic tests on insulation systems of electrical apparatus are necessary to reduce the failure risk under operation, eliminate infant mortality failures and reduce the impact of ageing on insulation system reliability. This study tries to establish a path which, based on the statistical–phenomenological modelling of the hazard rate, or bathtub curve, drives in the choice of test overvoltage and procedures. The choice of the test voltage and its waveform should be made carefully to improve reliability (reduce failure hazard) in any section of the bathtub curve. This may require, however, different choices depending on the section which corresponds to the planned test. For example, large overvoltages could be acceptable to remove the infant mortality to ensure the highest reliability of newly commissioned apparatus, but they can incept highly energetic, and, thus, fast degradation mechanisms on apparatus operating on the field for a period of time, that can worsen, rather than improving, the insulation system reliability. Resorting to partial discharge measurement during any overvoltage test seems to be the optimal solution to find the most appropriate overvoltage for any type of testing need, leveraging the Latin concept that ‘in medio stat virtus’.

Dielectric polymer nanocomposites are considered as one of the most promising insulation material candidates for future capacitive energy storage applications, providing tailorability of charge trapping and transport properties at the nanometric level which is a key for increased dielectric performance of biaxially oriented polypropylene (BOPP) for metallized film capacitors in high-voltage direct current (HVDC) applications. In this study, a comprehensive investigation of morphology and dielectric performance of pilot-scale BOPP nanocomposites with hexamethyldisilazane (HMDS)-treated hydrophobic fumed silica nanoparticles was carried out, providing critical perspectives on the performance and challenges of PNCs for thin film capacitors also in a broader context. In non-oriented cast films, incorporation of nanosilica modified the crystallization kinetics and α/β-crystalline spherulitic morphology of polypropylene and reduced the accumulation of space charge under a DC electric field. The nanocomposites exhibited promising dispersion characteristics in the nano-scale, however, the low amount of micron-sized agglomerates inherently present in commercial fumed silica persisted in the compounds which can become critical for thin film applications. Subsequently, biaxial-stretching-induced morphology development and dielectric properties of silica-BOPP nanocomposites were evaluated, highlighting the role of precursor morphology and film processing in the silica-BOPP film morphology, defects and dielectric performance. Charge trapping and transport properties of silica-BOPP films were investigated by isothermal and thermally stimulated techniques under high DC electro-thermal stresses, indicating profound modification of the trap density of states brought about by nanosilica. This resulted in more homogeneous space charge distribution and reduced temperature- and field dependent DC conductivity at 100 °C in comparison to neat BOPP under moderate field stresses (

As mentioned in a recent editorial in this magazine [1], distributed generation (renewables), automotive and, in general, transport and industrial utilization are fast changing the structure of electrical assets as well as the typologies and design of electrical apparatus. This is due mostly to the more widespread use of renewable generation and the electrification of transport, including power electronic converters and inverters, which can provide higher voltage, lower losses and make the asset more efficient and power dense. In the long run, medium voltage (MV) transformers will tend to disappear and perhaps high voltage (HV) transformers may be endangered. The future typical industrial, transport and, likely, T&D assets will be made by hybrid networks where power will be delivered in AC or DC, depending on convenience, and most of the loads (rotating machines) will be controlled by power electronics.

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, 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 materials, which supposedly can withstand partial discharge activity. This paper investigates the possibility of using corona resistant magnet wires for electrical machines operating at reduced pressures, such as the actuators for primary control surfaces in the more electric aircraft. The results show that corona resistant insulations are characterized by an improved behavior at ground level but are not a viable option at reduced pressures.

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.

Reliability of DC electrical asset components has not been investigated as broadly as for sinusoidal AC supply. On the other hand, DC assets are more and more common, and they are forecasted to grow in number, power, and voltage in the near future. This is not only in transmission grids, but also in distribution/generation (renewables), industrial application and electrification transportation. To complicate the framework, it has to be recognized that DC asset components are not only subjected to DC steady-state voltage, but also to voltage and temperature transients, as those coming from energization, voltage polarity inversion, ripple, repetitive voltage impulses and load variations. The major issue to asset component reliability can come, in these conditions, from electrical insulation. While designing insulation under AC sinusoidal voltage is a century-long practice, with many feedbacks from field installations, the same does not hold for modulated sinusoidal (i.e., power electronics) and DC supply. Electrical stresses can be different in magnitude and distribution from AC to DC, and load variations in DC can contribute to electric stress variations much more than in AC. All of this may impact significantly on aging rate and reliability of electrical insulation.This paper investigates the difference between electric field distribution, and consequent aging mechanisms and rate, from AC sinusoidal to DC supply, considering, in particular, the real DC operating conditions during which voltage and load transients can occur frequently. The contribution of partial discharges to aging rate will be also taken into account, bringing to the derivation of a probabilistic life model that can allow reliability estimations in the design of DC insulation systems to be achieved.

The reliability of epoxy resins (and dielectrics in general) employed for HVDC applications is significantly affected by space charge accumulation in the insulating material. Several methods have been tested to limit the injection and accumulation of space charge. In this work, planar specimens featuring graphene coatings were tested. Measurements of space charge accumulation, conductivity and permittivity at different temperatures (from 30°C to 60°C) and fields (from 30 kV/mm to 50 kV/mm) were carried out on epoxy specimens with and without coatings. Results show accumulation of space charge for low fields and temperatures in the reference specimens, while a reduction can be noticed with a layer of graphene coating. On the other hand, at higher fields or temperatures, the effect is reversed.

Switching impulses superimposed onto rated DC voltage are a particularly-challenging kind of overvoltage which is applied to HVDC cable systems both in service and during qualification tests in the laboratory. This paper aims at a preliminary experimental study of the effect of Superimposed Switching Impulses on XLPE-insulated HVDC cables. It studies the effect of aging 0.15-mm XLPE specimens by applying a sequence of Superimposed Switching Impulses, defined according to CIGRÉ technical Brochure 496. As illustrated in this paper, only preliminary results have been obtained so far from these tests. Such results show an increase in the conductivity in the aged specimens compared to non-aged specimens. An interfacial relaxation phenomenon is also noticed at low frequencies; however, no significant dipolar polarization can be noticed under the above-mentioned aging conditions.

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. publishedVersion

The main fundamental mechanisms proposed for the aging of polymeric insulation to breakdown at the DC design field are reviewed. A correlation between the action of static mechanical stress affecting the whole insulation volume, and DC electric-mechanical stress providing local strain large enough to bring the polymer to plastic deformation is hypothesized. With the corollary that local plastic deformation leads to the formation of cavities, or crazes, of size large enough to trigger highly energetic phenomena and bring about insulation failure, an inter-relationship between mechanical and electrical life models can be obtained and supported by calculation and fitting to experimental results. The influence of space charge is discussed and it is argued that it would play a central role in DC ageing even if its presence is not a precondition.

Aging of insulation systems of rotating machines controlled by voltage source converters is an issue becoming of utmost importance due to the broader and broader application of speed-controlled rotating machines. Since one of the major concerns for machine reliability is the appearance of partial discharge (PD), this paper investigated how PD behave under multi-level inverter waveforms, and what is their impact on insulation life. PD patterns under sinusoidal and repetitive impulse voltage, considering two, three and five level inverters, are presented showing that PD phenomenology is deeply different, and its impact on insulation life changes depending on number of levels and voltage impulse rise time. This holds for both Type I and Type II insulation.

This study focuses on the influence of polar and unpolar surface functionalization of silica on the dielectric performance of PP/POE (polypropylene/poly(ethylene-co-octene)) nanocomposites. The silica fillers were surface-modified with unpoalr trimethylethoxysilane (TMES) or polar 3-aminopropyl triethoxysilane (APTES). Fourier-Transform Infrared Spectroscopy and Thermogravimetric analysis were performed and confirmed qualitatively and quantitatively the successful silica-silane modification. Silica/PP/POE nanocomposite films were prepared using a mini-extruder. Scanning Electron Microscope images showed good dispersion of the modified silica in the PP/POE matrix. However, the polar silica cluster size (300 nm) is slightly larger than the one of the unpolar silica (100 nm). Thermally Stimulated Depolarization Current (TSDC) characterization data showed that the polar silica introduced deeper charge traps than the unpolar one, which also showed higher trap density. The apparent conductivity measured during the TSDC poling phase indicated that the polar silica filled nanocomposites featured very fast polarization to reach the saturation state, while the unpolar silica had a rather slow polarization rate. From Pulsed Electroacoustic Analysis it was obvious that the space charge is suppressed by addition of the polar silica, but increased by the presence of the unpolar silica. acceptedVersion

Impulse voltage waveforms from power converters enhance the voltage stress on the turn insulation system of rotating machines. When the voltage stress exceeds a critical level, partial discharges can be incepted leading to premature failure of the machine insulation. Besides using thicker insulation, hybrid organic/inorganic insulation (Corona Resistant, CR) can be used. CR systems have proved to be successful to extend the lifetime of industrial drives. For aerospace applications as the More Electrical Aircraft, one should consider the challenging conditions posed by operation at reduced pressures. It has already been proven that the energetic content of Partial Discharges (PD) is greatly influenced by the reduction of pressure. However, the reliability of CR insulation systems in conditions representative of those experienced by systems used in the MEA have not been investigated so far. Therefore, electrical life tests at constant voltage and different pressures were performed on both non-CR and CR Grade 2 enameled wires. The results obtained show that CR insulation systems are not able to ensure the reliable operation of electrical drives used in aerospace application.

This study compares different polymer-nanofiller blends concerning their suitability for application as insulating thermoplastic composites for High Voltage Direct Current (HVDC) cable application. Two polymer blends, PP/EOC (polypropylene/ethylene-octene copolymer) and PP/PP-HI (polypropylene/ propylene - ethylene copolymer) and their nanocomposites filled with 2 wt.% of fumed silica modified with 3-aminopropyltriethoxysilane were studied. Morphology, thermal stability, crystallization behavior dynamic relaxation, conductivity, charge trap distribution and space charge behavior were studied respectively. The results showed that the comprehensive performance of the PP/PP-HI composite is better than the one of the PP/EOC composite due to better polymer miscibility and flexibility, as well as lower charging current density and space charge accumulation. Nanosilica addition improves the thermal stability and dielectric properties of both polymer blends. The filler acts as nucleating agent increasing the crystallization temperature, but decreasing the degree of crystallinity. Dynamic mechanical analysis results revealed three polymer relaxation transitions: PP glass transition ( $\beta$ ), weak crystal reorientation ( $\alpha 1$ ) and melting ( $\alpha 2$ ). The nanosilica introduced deep traps in the polymer blends and suppressed space charge accumulation, but slightly increased the conductivity. A hypothesis for the correlation of charge trap distribution and polymer chain transition peaks is developed: In unfilled PP/EOC and PP/PP-HI matrices, charges are mostly located at the crystalline-amorphous interface, whereas in the filled PP/EOC/silica and PP/PP-HI /silica composites, charges are mostly located at the nanosilica-polymer interface. Overall, the PP/PP-HI (55/45) nanocomposite with 2 wt.% modified silica and 0.3 wt.% of antioxidants making it $a$ promising material for PP based HVDC cable insulation application with $a$ reduced space charge accumulation and good mechanical properties.

The design and operation of DC insulation systems is more complex than for AC sinusoidal supply. The most subtle issues are how the electric field in DC can vary with operation and affect the partial discharge inception voltage and partial discharge phenomenology. During operation, a DC insulation system is indeed subjected to voltage and load transients, the former due to energization and voltage polarity inversion. This paper sheds some lights on such issues, going through another fundamental problem, that is, how to carry out effective partial discharge detection under DC supply, both in steady state and during voltage transients. An algorithm that can enable automatic and unsupervised partial discharge measurements in DC electrical apparatus is presented and validated based on partial discharge monitoring performed on test objects representing defective insulation systems. Identification of the source generating partial discharges, which is the basis for diagnostic and condition maintenance, is achieved without the support of experts. The impact of voltage transients on the life and reliability of DC insulation systems is modelled in the presence and absence of partial discharges. It is shown that, depending on the repetition rate, the presence of partial discharges, even if only during voltage transients, can dramatically affect the insulation life and cause premature insulation breakdown.

DC cables are supposed to withstand a number of operations involving supply-voltage peak-value time variations, such as energization, switch off and polarity inversion. It is known that voltage-supply time variations cause electric field transients in cable insulation that can last for a long time before steady-state conditions are achieved, and during this time the electric field distribution is not only driven by electrical conductivity, as in steady state, but also by permittivity, as in AC. This paper shows that highly energetic degradation phenomena, that is, partial discharges, PD, can occur during voltage transients that a DC cable has to withstand during life. This results in a cumulative damage which can cause premature insulation breakdown, at times well shorter than those expected based on a cable design which is PD free under steady-state conditions. Quantification of the damage caused by PD, based on aging and residual life models, is proposed in the paper, and it is shown that damage can be significant enough to pose a threat for cable life when operation transient rate is e.g., a few per day, and that the faster the slew rate of the supply voltage variation, the highest the cumulative damage due to PD.

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.

This paper has the main purpose to assess the influence of nanofillers on the mobility of electrostatic surface charge buildup. Electro Hydro Dynamic actuators were produced and tested at a fixed voltage amplitude and frequency, using different thermoplastic dielectrics, some of which belonging to the family of nano-filled materials. The investigation focuses on the mobility of superficial electrostatic buildup, also experimenting the validity of an alternative high voltage potential measurement technique based on monitoring of multiple embedded electrodes. Results show that nanofillers have a profound effect on electrostatic charge buildup, strongly reducing their mobility.

Aerospace electrification has to go through significant innovations regarding adjustable speed drives, which are needed to increase specific power and effectiveness of propulsion and power control systems. This involves electrical insulation design with high-electric field and, therefore, increased risk of accelerated insulation aging. This risk becomes dramatic if highly-energetic degradation phenomena, such as partial discharges, PD, incept in (unavoidable) insulation defects. However measuring partial discharges, that is, the property associated with the fastest electrical degradation mechanism in organic insulation, has to face an increasing amount of issues compared to sinusoidal AC voltage supply. Referring to power electronics for rotating machines drives, the main difficulty lies with the extraction of partial discharge pulses from the noise produced by switch commutation, especially when the slew rate is fast and the operating voltage is high, compared to the threshold for partial discharge inception. Since partial discharges tend to occur mostly during voltage rise and fall times, thus simultaneously to switching noise, poor techniques for noise rejection and partial discharge identification may affect significantly any quality, commissioning and diagnostic test performed on electrical devices and apparatus supplied by power electronic waveforms. This paper presents a novel approach to partial discharge and noise separation, which can work also for very fast switch rise times, and increased operating voltage and frequency. Based on experimental testing, this paper shows that considering the time domain, rather than the frequency domain on which noise filtering is commonly based, and resorting to signal variance evaluation, partial discharge pulses can be extracted effectively from commutation noise.

Designing insulation systems for DC application is not straightforward as under AC, both because electric field can distribute inside the insulation in significantly different ways, and due to the effect on field distribution of voltage transients, such as energizations and the polarity inversions. During and after each voltage variation, the electric field in the insulation is mainly driven by permittivity, as in AC, while at steady-state the electric field profile depends on conductivity and, hence, on dielectric material and load. This can impact on aging phenomena and rate, thus on the electro-thermal life of an insulation system. It is, therefore, important to estimate how long it takes for the electric field to reach its steady state condition (i.e. the transient time) upon voltage-time variations. Different methods for estimating the electric field transient time are discussed in this paper, from conductivity and permittivity measurements at high or low fields, as a function of temperature, to partial discharge time evolution. Specimens made by polymeric materials having different conductivities, and containing artificial defects, are used for the experimental validation of those methods.

This paper is intended to contribute to research into the interpretation of PD measurements under DC conditions. A simple PD system comprising of a cylindrical air filled void surrounded by polypropylene based copolymers is considered. A DC ramp of 1 kV/s reaching 10 kV was applied to the system and PD activity recorded from the start of the ramp for 104 seconds. An equivalent simulation model was constructed and used to investigate the influence of different phenomena. It was demonstrated that surface conduction is likely to play a crucial role in the applied field dynamics, leading to the cessation of PD activity. It also appears that charge deployed by PDs must be undergoing rapid recombination in order to support the high repetition rate in the first 1000 s of the experiment. The future work required to develop a full PD activity model is discussed.

Starting from measurements performed on insulation models, estimation of partial discharge inception voltage, PDIV, and inference of the different behavior of PD for a MV cable models are presented and discussed. The estimations are validated through laboratory testing on a cable with an artificial defect. It is shown that PDIV under DC can become considerably higher or lower than that under AC, depending on temperature and material properties, which affect PD repetition rate as well. However, resorting to the model presented in this paper, measuring PD under AC will help to identify and validate the results of the PD DC measurements.

Current standards base the evaluation of partial discharge (PD) harmfulness on their amplitude and repetition rate. This work shows how, on the contrary, the non conventional environmental conditions introduced by aerospace applications (i.e. low-pressure atmosphere, higher frequency supplies) lead respectively to lower or unchanged PD apparent charges, which are however characterized by different mean energies, hence destructive potential, in a counter intuitive manner.