Davide Fabiani

Polymeric HVDC Cable Design and Space Charge Accumulation. Part 1: Insulation/Semicon Interface

From theory and experiments, it can be deduced that materials for DC applications should not accumulate a large amount of space charge if accelerated degradation of the insulation system is to be avoided. Therefore, the characterization of DC insulation must take into account the evaluation of space charge accumulation. This cannot be done exhaustively without taking a system approach considering both the semiconductive material and the insulation, in particular, the properties of the semicon/insulation interface. The latter interface, in fact, plays a major role in space charge injection/accumulation in the insulation bulk. Having analyzed different semiconductive and insulating materials candidate for HVDC cable applications, the best solution to be exploited for HVDC cable design would be the combination showing a high threshold for space charge accumulation, a small rate of charge accumulation as a function of electric field and a small activation energy, i.e., a space charge amount less dependent on temperature. Therefore, space charge measurements will provide important information to cable material manufacturers with the aim of tailoring insulation and semicon specifically for HVDC application and, thus, improving the reliability of polymeric cables.

Fast and slow charge packets in polymeric materials under DC stress

The presence of slow space charge packets crossing the insulation thickness from one electrode to the other and causing significant electrical field distortion has been reported already in several papers. They are activated in general by very high dc fields or, in highly polluted materials, by relatively low fields and constitute an important ageing factor, concerning dc electrical stress. It has been observed, in fact, that such packets can cause accelerated breakdown of insulation. The development of fast systems for space charge measurements has allowed the presence of almost instant heterocharge to be observed close to electrodes in certain field and temperature conditions, especially in cable models. This has been explained often by the separation of ionic charge populations, even though such heterocharge appears also in materials, such as Polyethylene or crosslinked Polyethylene that represent the best extra-clean technologies. The measurements reported here use a high speed technique to investigate the build up of heterocharge in model cables that have been treated to remove volatile chemical species. They show that in fact the heterocharge is built up by many very small and very fast charge packets (i.e. charge packets having a high mobility), which are injected from both electrodes and cross the insulation in less than one second. Because the packet charge is unable to exit the counter-electrode at the same rate at which it arrives, hetero-charge is built up within just a few seconds from the beginning of the polarization. The mobility of these charges, depending significantly on temperature, is estimated through observation of charge packets as a function of time, and compared with that of the already-known slow packets, generally occurring at higher fields with respect to fast packets. The basis for the interpretation and modelling of such phenomena is discussed.

Fast charge packet dynamics in XLPE insulated cable models

This paper deals with the investigation of space charge accumulation dynamics in XLPE electrical insulation. In particular, very fast charge packets, i.e., having large mobility, are observed to be injected from both electrodes and to cross the insulation bulk, thereby contributing to heterocharge build-up at the opposing electrodes after just a few seconds from the beginning of polarization. The mobility of these charges, depending significantly on temperature, is estimated through observation of charge packets as a function of time.

Charging properties and time-temperature stability of innovative polymeric cellular ferroelectrets

After appropriate mechanical and electrical treatments, some cellular polymers become able to retain space charge for a long time, i.e. they acquire electret behavior. The electrical treatment consists of charging under high levels of DC electric field. The mechanical treatment, based on the application of stretching forces to cellular polymer slabs that were before expanded under pressurized gas, affects the cavity size and shape, and therefore also the effectiveness of the charging process itself. An investigation of charging mechanisms, as well as of mechanical treatment, is therefore fundamental for optimizing the ferro- and piezo-electret properties. The aim of this paper is to discuss the effect of the physical dimension of the cavities on the charging behavior of cellular ferroelectrets and to focus on the time-temperature stability for two families of polymeric cellular ferroelectrets based on polypropylene (PP) and on a cyclo-olefin copolymer (COC). Emphasis will be given to the stretching process and in particular to the expansion rate applied during the manufacturing process (which affects the radial dimension and the height of the cavities, respectively). Space-charge and partial-discharge measurements as a function of time and temperature are the main tools to infer the influence of the cavity size on charging and stability characteristics

Effect of oxide nanoparticles on thermal and mechanical properties of electrospun separators for lithium-ion batteries

This study reports the fabrication and characterization of poly(ethylene oxide) (PEO) and poly(vinylidenefluoride-cochlorotrifluoroethylene) (PVDF-CTFE) nanofibrous separators for lithium-ion batteries loaded with different amounts of fumedsilica and tin oxide nanoparticles. Membrane morphological characterization (SEM, TEM) showed the presence of good-quality nanofibres containing nanoparticles. Thermal degradation and membrane mechanical properties were also investigated, and a remarkable effect of nanoparticle addition on membrane mechanical properties was found. In particular, PEO membranes were strengthened by the addition of metal oxide, whereas PVDF-CTFE membranes acquired ductility.

Power electronics and electrical insulation systems - Part 3: Diagnostic properties

The paper presents that partial discharge detection is an effective tool to prevent premature breakdown of electrical equipment fed by power electronics. it is recommended that it be used as a quality control measure in the factory and as an online diagnostic procedure in service.

The Processing of Nanocomposites

Manufacturing techniques can have a significant impact on the dielectric properties of nanostructured materials, sometimes even larger than the effect of the nanofiller itself. Indeed, the choice of the best nanofiller to improve the electric behavior of the final nanocomposite is often frustrated by residual contaminants of the matrix-nanofiller compatibilization process, moisture absorption, nanofiller aggregation, etc. Therefore manufacturing techniques should be tuned to eliminate or limit the spurious effect of defects/contaminants to obtain the desired material properties. In the following, after a comprehensive presentation of the most common nanofillers and compatibilization treatment, attention is paid to the main processing techniques and purification procedures to minimize the effect of contaminants.

Nanodielectrics-examples of preparation and microstructure

When investigating the dielectric properties of nanodielectrics; the synthesis and processing of the materials are often neglected. It seems intuitive to think that the physical properties of the constituent parts of an insulation material directly lead to the physical properties of a composite material. However, as has been shown in the literature, nanodielectrics are more than just the sum of their parts. Compounding of dielectric nanocomposites is a complex endeavor, and it seems that nanodielectrics can only unleash their full potential when the filler material is distributed evenly throughout the host material. This article gives an overview of common preparation methods and discusses the influence of the preparation method on the microstructure.

Fast pulse-like conduction in XLPE based materials detected through charging current measurements

Recent papers have shown small, but repetitive, charge pulses of both polarities, travelling very fast in polyethylene and nanostructured epoxy resin specimens at moderate electric fields. This paper shows that such charge pulses can be detected also by means of charging current measurements, once the measuring system is properly modified in order to acquire small signals with high dynamics, through attenuation of the low-frequency content of the polarization current. These measurements are shown to be sometimes even more sensitive in detecting charge pulses than space charge measurrements, thus they can be used successfully to evaluate pulse repetition frequency and amplitude at such fields where these conduction mechanisms are incepted.

High mobility conduction in insulating polymers through fast soliton-like charge pulses

We present evidence for the existence of a new form of charge transport in insulating polymers which takes the form of charge pulses that move exceptionally quickly across the insulation in comparison to the charge carrier mobility typical of such materials. This phenomenon could be associated with electromechanical compression of the polymer (a minimum amount of charge is needed for this to occur) that triggers a discontinuous injection of charge at the electrodes and allows transport inside the bulk through chain displacement modes. These pulses behave as charged solitary waves, or solitons, with a speed that corresponds to a mobility of some orders of magnitude higher than that of independent carriers.